melt transformation coextrusion polypropylene and polyethylene) a thesis
TRANSCRIPT
]MELT TRANSFORMATION COEXTRUSION ...-,.--
POLYPROPYLENE AND POLYETHYLENE)
A Thesis Presented to
The Faculty of the College of Engineering
and Technology
Ohio University
In Partial Fulfilment
of the Requirements for the Degree
Master of Science
by
Craig L. Shoemaker - -.-.
August, 1984
Acknowledgment
I wish to express my sincere thanks to Dr. John
Collier, my advisor, for all his help and cooperation in
this investigation. I would also like to thank Dr. Richard
Mayer for his help using the Apple computer, the National
Science Foundation for their funds used to purchase the twin
screw extruder, and Leistritz for the donation of the
microprocessor on the extruder.
I would like to extend special thanks to my father,
Richard E. shoemaker, and fiancee, Elizabeth Hurlbut, for
all their support and help during the writing and editing of
this manuscript.
ABSTRACT
Shoemaker, Craig Leroy. M.S. August, 1984 Chemical Engineering
Melt Transformation Coextrusion of Polypropylene and - - Polyethylene. (146pp. )
Director of Thesis: Dr. John R. Collier
The melt transformation coextrusion process was studied
in this investigation. The main focus of this investigation
was to utilize the melt transformation coextrusion process
by using a twin screw plasticating extruder to obtain higher
die pressures, similar to the pressures used in the melt
transformation extrusion process, so coextruded polymer
samples with enhanced thermal and mechanical properties
could be produced at higher production rates.
A multimanifold die, with variable draw ratio inserts,
was used. A single screw plasticating extruder was used for
processing the polyethylene shell polymer while a twin screw
counterrotating plasticating extruder was used to process
the polypropylene core polymer. The extrudate was
coextruded downward into a water bath and then through a
take-up device.
The results of this investigation showed that
coextrusion did dramatically increase the production rate,
from a few centimeters per minute up to 45 feet per minute,
of the melt transformation extrusion process. The samples
produced had oriented polypropylene cores which had melting 0
points elevated by as much as 5 C when tested by
differential scanning calorimetry (DSC). When tested, the
mechanical properties showed increased elastic modulus,
yield strength, and ultimate tensile strength. When the
samples, cooled in liquid nitrogen, were fractured the cores
displayed layered surfaces when viewed by a scanning
electron microscope.
The polyethylene shell layers exhibited depressed 0
melting points, lowered by as much as 2.7 C, when tested by
DSC. The shell layer gave the coextruded samples properties
that enabled them to be greatly elongated, up to 990%,
before failure when tensile tested. It was also found that
the die pressure and the length of the melt conditioning
pipe were important factors in determining the degree of
orientation of the samples.
TABLE OF CONTENTS
Page
I. INTRODUCTION
11. Literature Review . A. Orientation of Polymers .
1. Polymer Orientation . . 2. Methods of Orientation
3. Melt Transformation Extrusion . B. Coextrusion Methods .
1. General Overview
2. Effects on Orientation
3. Flow Patterns . 4. Stability of Flow
5. Theoretical Flow
111. EXPERIMENTAL . A. Equipment . B. Experimental Procedure
IV. RESULTS .
V. DISCUSSION OF RESULTS
A. Discussion . B. Limitations
VI. CONCLUSIONS
V I I. RECOMMENDATIONS
VIII. BIBLIOGRAPHY .
IX. APPENDICES
A. Experimental Data
B. Percent Crystallinity.
C. Material Stock properties .
LIST OF FIGURES
F i g u r e 1: S p h e r u l i t e Polymer C r y s t a l S t r u c t u r e . F i g u r e 2: Necked C r y s t a l l i n e Polymer S t r u c t u r e . F i g u r e 3: E x t e n d e d C h a i n C r y s t a l S t r u c t u r e . F i g u r e 4: F r e e E n e r g y v s T e m p e r a t u r e o f
P o l y e t h y l e n e C r y s t a l s and Melts . F i g u r e 5: Nemat ic L i q u i d C r y s t a l S t r u c t u r e . F i g u r e 6: P r e s s u r e v s T e m p e r a t u r e o f a
P r e s s u r e I n d u c e d L i q u i d C r y s t a l P o l y m e r .
F i g u r e 7: F r e e E n e r g y v s T e m p e r a t u r e o f a L i q u i d C r y s t a l Forming Polymer Below t h e C r i t i c a l P r e s s u r e ( P < PC)
F i g u r e 8: F r e e E n e r g y v s T e m p e r a t u r e o f a L i q u i d C r y s t a l Forming Polymer Above t h e C r i t i c a l P r e s s u r e ( P > P C )
F i g u r e 9: E f f e c t o f S h e a r and E l o n g a t i o n a l Flow on C r y s t a l S t r u c t u r e
F i g u r e 10: E l o n g a t i o n a l Flow F i e l d i n t h e M e l t T r a n s f o r m a t i o n E x t r u s i o n D i e .
F i g u r e 11: M u l t i m a n i f o l d D i e . F i g u r e 12: F e e d b l o c k D i e . F i g u r e 13: Two L a y e r I n s u l a t e d M u l t i m a n i f o l d D i e . F i g u r e 14: Types o f Polymer C o e x t r u s i o n L a y e r s . F i g u r e 15: V i s u a l V e l o c i t y F i e l d . F i g u r e 16: I n t e r f a c e S t a b i l i t y and I n s t a b i l i t y . F i g u r e 17: D i e C h a n n e l V e l o c i t y P r o f i l e
F i g u r e 18: C o e x t r u s i o n D i e - S i d e 1
F i g u r e 19: C o e x t r u s i o n D i e - S i d e 2 . F i g u r e 20: C o e x t r u s i o n D i e I n s e r t s
Page
. 6
. 8
. 1 0
List of Figures (continued)
Page
Figure 21: Equipment Setup - Top View . . 74
Figure 22: Equipment Setup - Side view . . 75
Figure 23: Connector for the Twin Screw . 77
Figure 24: Twin Screw Extruder Barrel Assembly . . 79
Figure 25: Volumetric Flow Rate vs Die Pressure ; Runs A - D . . 86
Figure 26: Volumetric Flow Rate vs Die Pressure ; Runs F - K . . . 87
Figure 27: Elevation of the ~elting Point Temperature of the Polypropylene Core vs Die Pressure ; Runs A - D . . 88
Figure 28: Elevation of the Melting Point Temperature of the Polypropylene Core vs Die Pressure ; Runs F - K . . 89
Figure 29: Depression of the Melting Point Temperature of the Polyethylene Shell layer vs Die Pressure ; Runs A - D . . 91
Figure 30: Depression of the Melting Point Temperature of the Polyethylene Shell layer vs Die Pressure ; Runs F - K . . 92
Figure 31: Percent Crystallinity of the Polypropylene Core vs Die Pressure ; Runs A - D . 93
Figure 32: . Percent Crystallinity of the Polypropylene Core vs Die Pressure ; Runs F - K . 94
Figure 33: Elastic Modulus vs Die Pressure Runs A - D . 95
Figure 34: Elastic Modulus vs Die Pressure Runs F - K . 96
Figure 35: Yield Strength vs Die Pressure Runs A - D . 98
Figure 36: Yield Strength vs Die Pressure Runs F - K . 99
L i s t o f F i g u r e s ( c o n t i n u e d )
Page
F i g u r e 3 7 : U l t i m a t e T e n s i l e S t r e n g t h v s . D i e P r e s s u r e ; Runs A - D . . 100
F i g u r e 3 8 : U l t i m a t e T e n s i l e S t r e n g t h v s D i e P r e s s u r e ; Runs F - K . . 1 0 1
F i g u r e 39: T o r n Edge o f Run J ( m a g n i f i e d 1 4 . 5 ~ ) . . 103
F i g u r e 40: S u r f a c e o f Run F ( m a g n i f i e d 1 4 . 5 ~ ) . 103
F i g u r e 41: T h i n S l i c e o f Run J C r a c k e d w i t h S h e l l Layer H o l d i n g t h e Sample T o g e t h e r
( m a g n i f i e d 29x) . 104
F i g u r e 42: Torn e d g e o f Run L ( m a g n i f i e d 29x) . 104
F i g u r e 43: SEM of Run K ( m a g n i f i e d 1000x) F r a c t u r e d S u r f a c e ( u s i n g L i q u i d N i t r o g e n ) . 1 0 5
F i g u r e 44: SEM of Run J ( m a g n i f i e d 2 0 0 ~ ) T e n s i l e T e s t F a i l u r e S u r f a c e . 1 0 5
F i g u r e 45: SEM o f Run J ( m a g n i f i e d 1 0 0 0 ~ ) F r a c t r u e d End S u r f a c e ( u s i n g L i q u i d N i t r o g e n ) . 106
F i g u r e 46: T y p i c a l DSC Scan . . 1 4 3
LIST OF TABLES
Table 1:
Table 2:
Table 3:
Table 4:
Table 5:
Table 6:
Table 7:
Table 8:
Elastic Modulus Values . Comparison of Coextrusion Methods . Characteristics of Coextrusion Methods . Coextrusion Equipment Suppliers
~er'cent Elongation to Failure Compared to the Core to Shell Volume Ratio
Experimental Data . Material Properties - Polypropylene
Material Properties - Polyethylene .
Page
4
. 35
. 36
. 44
I. INTRODUCTION
For sometime engineers have sought lightweight
strong construction materials to replace metals. Polymers
have made great strides in this respect because of their low
density and modest strength. processing methods have been
developed to increase some polymers' strengths dramatically.
One of these methods developed at Ohio university is
the Melt Transformation Extrusion (MTE) (1) process. This
process has produced polymer samples with tensile moduli
equal to that of aluminum, but the process production rate
has been slow (usually a couple of centimeters per minute).
To make this a viable industrial process, some way of
increasing the production rate is needed.
The main problem with increasing the production rate is
the frictional drag in the die between the solidified
polymer and the die wall. Baked on fluoropolymer coatings
have been applied to the die surfaces and have helped
increase the production rate some, but not sufficiently, and
sample surface quality. However, these coatings do not last
long and must be reapplied by dismantling the die, cleaning
it, recoating it, and then assembling it again, a tedious
and time consuming process. Therefore, other alternatives
are being sought to replace the coating.
It was decided to try coextrusion to increase the
production rate of the MTE process. A uniaxial die was
retrofitted with inserts to allow a second polymer melt to
enter along the walls of the die. This work was first done
~y Mario Perez (2). He was able to show that Melt Transfor-
mation Coextrusion (MTC) did work in increasing the
production rate dramatically. However, because of equipment
limitations he was able to build sufficient pressure to
operate at the lower end of the original MTE process
pressure conditions. Thus, his samples did not have as high
a degree of orientation as did the original MTE samples.
Therefore, the thrust of this research was to try to
operate at higher pressures using a twin screw extruder for
the core material and show that MTC produces similar
orientation results to the MTE process. The effect of the
skin layer on the properties of the core material were
analysed when the samples were tensile tested.
11. LITERATURE REVIEW
The following is a review of the research done in the
field of coextrusion of oriented polymers.
11. A. Orientation of Polvmers
This section details the advances made in orienting
polymers by various methods. Among these advances in
particular, are the effects of pressure and converging flow
geometry in the orientation process.
I. A. 1. Polymer Orientation
Polymers are widely used in many applications because
they offer advantages over other materials such as metals
and ceramics. These include low density, ease in molding
into complex shapes, chemical inertness, and coloring
possiblities. Their major weakness is their low tensile
strength. Table 1 (3-4) lists the elastic moduli of some
high strength materials. Note that the bulk polymers tend
to have low modulus values.
Two techniques for increasing polymer moduli are
orientation and reinforcement. - Reinforcement with glass
fibers or fillers will increase the modulus some, but will
Table 1: Elastic Modulus Values ( 3 - 4 )
Glass
Kevlar
Material
Bulk Polymers
High-Density Polyethylene
Polypropylene
Aluminum Alloys
S tee1
High-Density Polyethylene
Polyvinyl Alcohol
Carbon Filaments
Approximate Modulus - 1 0 2
x 10 dynes/cm
1 - 7
1
42
< 70
7 0 - 8 5
130
240 - 500
Condition of
the material
Isotropic
Isotropic
Theoretical
----
Fiber
Fiber
----
Theoretical
Theoretical
Theoretical
5
also increase the cost of the polymer material. Orientation
is another technique of increasing the modulus by modifying
the polymers' chain structure when it is processed. It is
possible to calculate the maximum theoretical modulus for
polymers, even though only a small fraction of this value
has been obtained experimentally (4).
Most polymers are made up of crystalline and amorphous
(glassy) regions. Depending on the relative amounts of each
phase present, a polymer' is either amorphous, semi-crystall-
ine, or crystalline. If two different polymers are orient-
ed, one amorphous and the other semi-crystalline (or
crystalline), the amorphous polymer will show little
increase in elastic modulus when oriented, while the semi-
crystalline (or crystalline) polymer's modulus will increase
dramatically. This is due to the fact that amorphous
polymers fail by chains separating and pulling past one
another. When oriented, the chains are stretched out, but
there is little increase in difficulty of pulling the chains
past one another. Thus, orientation has little effect on
amorphous polymers (4).
In semi-crystalline polymers, failure is caused by
chain scission (breaking the main chain bonds), because the
crystal regions hold the chains more rigidly than amorphous
polymers. The crystalline structure of a quiescently cooled
polymer is made up of small crystal structures
(spherulites), Figure 1 (5), which are tied together by tie
F i g u r e 1: S p h e r u l i t e Polymer C r y s t a l S t r u c t u r e ( 5 )
7
molecules that bear the load applied and determine the
polymer's strength. When a semi-crystalline polymer is
oriented, its crystal structure changes and also its
mechanical properties and behavior.
If a sample of semi-crystalline polymer is deformed,
the spherulite structure begins to deform slightly
(necking) . As more stress is appli.ed, the folded chain
lamellae radiating out from the center of the spherulite
begin to slip past one another. At this stage, orientation
of the crystals takes place in the direction that the stress
is being applied. This necking causes a decrease in cross-
sectional area, a high degree of orientation, and a fibril
structure ( 6 ) .
While the elastic modulus in the necked region of the
polymer has increased about 10 fold, it is still an order of
magnitude away from its theoretical strength. Even though
the chains are almost fully oriented they still contain
chain folds, Figure 2 ( 6 ) , which cause the modulus to be
less than the theoretical maximum value.
The spherulite structure (Figure 1) is thus deformed to
produce an oriented necked-down fibril structure (Figure 2).
The modulus of this necked structure could be increased by
either decreasing the number of chain fold defects or
increasing the number of tie molecules.
Another highly oriented crystal structure that can be
produced is an extended chain crystal structure . This can
F i g u r e 2: Necked C r y s t a l l i n e Polymer S t r u c t u r e ( 6 )
TIE MOLECULES
CHAIN FOLD DEFECTS
LAMELLAE BLOCK
ORIENTATION DIRECTION
9
be produced by crystallization of a polymer melt under a
high elongational flow field (7) or high pressure. In this
structure some of the polymer chains are completely extended
which has been seen to lead to their failing by shear. This
type of structure is shown in Figure 3 (8). This extended
chain crystal structure gives modulus values closer to the
theoretical value than does the necked crystal structure.
This is because the polymer crystal structure has fewer
chain fold defects and more extended chain crystals.
The polymer molecules have a tendency to fold back when
they are crystallized, because their free energy barrier is
reduced when they fold. This free energy barrier also
exists for the melting of these extended chain crystals and
causes their observed melting point increase (super heat)
( 9 ) Thus, work continues on ways of orienting polymers to
increase their observed moduli to reach the theoretical
values. It is this search that has lead polymer scientists
to various methods of producing high modulus polymers by
processing techniques rather than using reinforcements.
F i g u r e 3 : E x t e n d e d C h a i n C r y s t a l S t r G c t u r e ( 8 )
t
EXTENDED CHAIN CRY"'
FOLDS
MORE TIE ? i / , MOLECULES
11. A. 2. Methods of Orientation
The methods for producing high modulus oriented
polymers fall into several categories. Among them are cold
drawing (10), hydrostatic extrusion (11-14), solid state
extrusion ( 1 5 - 8 , solution spinning (19) , and melt
transformation extrusion (20).
The common feature that all these methods have is a
high elongational flow field. A measure of the severity of
this elongational flow field is the draw ratio. The draw
ratio (DR) is defined as the ratio of the cross-sectional
area of the initial sample to the cross-sectional area of
the final sample. Thus greater DR values indicate higher
elongational flow fields. Work done by Takayonagi (21)
indicated that the DR was able to correlate the results of
different high orientation methods. Thus, highly oriented
samples processed at the same DR's by different methods had
the same mechanical properties.
Cold drawing has been greatly studied. Cold drawing is
the deforming of a sample by pulling it through a die. It
is a very temperature sensitive operation. Most drawing
operations go up to a DR of approximately 10. However, by
varying the quench temperature before drawing, DR's of up to
35 have been attained (10). >
~ydrostatic extrusion has been used to produce highly
oriented, ultra high molecular weight polymers such as
polyethylene and polytetrafluoroethylene. Extrusion of this
type is done at high pressures with a fluid to transfer the
pressure to the polymer billet indirectly. This reduces the
pressure required to extrude polymers, as it does in metal
extrusion.
Hydrostatic extrusion of polyethylene was studied by
Davis (11). He was able to show a correlation between
extrusion pressure and material properties. One problem
which was encountered by Davis was the sticking of the
polymer to the die during extrusion. Several things were
done to alleviate this problem including coating the die
with a spray of polytetraf luoroethylene. With this method,
DR's of up to 10 were used, but above this DR, problems with
the billet sticking and then extruding suddenly at an
increased pressure were again encountered.
Some work is also being done on tubular and non-
circular hydrostatic extrusion of polyethylene at ~ e e d s
University (12). Again, direct correlation between elastic
modulus and extrusion DR was found.
Hydrostatic extrusion of polypropylene has been done,
as well (13). A stick-slip problem also developed with this
extrusion operation around the critical DR of polypropylene
of about 6.3. Above this ratio, a helical fracture pattern
in the extrudate's surface was observed. his reduced the
quality and strength of the samples.
The hydrostatic extrusion pressure was found to be
directly related to the DR at a constant processing
temperature. Differential scanning calorimetry (DSC)
measurements of the samples showed no change in melting
point temperature (164 degrees Celsius), but the area under
the (DSC) curve , which is related to the heat of fusion,
varied depending on the extrusion DR and temperature. The
relative heat of fusion is a measure of the increase in
crystallinity after extrusion (14). The relative heat of
fusion of a sample increases with an increasing DR.
A variation of hydrostatic extrusion has been developed
by Porter et al. 1 5 - 1 8 . This technique is called solid
state extrusion and has produced very high modulus polymer
samples using an Instron Capillary Rheometer (18) . The
process is very temperature sensitive and requires that the
polymer also be extruded above a critical shear rate. This
produces extreme crystal orientation with at least 10
percent of the molecules being extended chain crystals (18),
see Figure 3. Morphological crystal differences have been
reported between samples hydrostatically extruded and those
produced by solid state extrusion. This indicates that the
chain orientation is not the same in the samples produced by
the two methods (22).
Work on achieving the same pressure and temperature
conditions as those found in the solid state extrusion
process using a heated rolling mill was done at Battelle
14
Labs (23) . They were able to produce highly oriented
polyethylene tapes at much higher production rates than the
solid state or hydrostatic extrusion methods. It was, in
fact, the linear velocity of the rollers that determined the
extrusion rate (24) . A technique for producing ultra high modulus
polyethylene from a 5 percent solution of polyethylene in
xylene has been presented (19). A critical stirring speed
was needed to produce Talyor vortices which generated a
elongational flow field. This formed elongated crystals of
polyethylene.
The final method of producing ultra high modulus
oriented polymers was developed at Ohio university by
Collier et al. (20). This process is known as Melt
Transformation Extrusion and is discussed thoroughly in the
next section. This technique involves using a extruder to
melt pressurize a polymer, and then to condition this melt
for a period of time in a reservior before extruding it
through a specially designed die.
These are just some of the methods used to produce
highly oriented polymers. As mentioned earlier, they all
have one feature in common which is an elongational flow
field used to produce the orientation in the polymer.
However, the samples produce by these methods do vary in
shape and form with some being small diameter fibers and
others producing flat sheets. Some methods produce either
fibers or sheets and are more versatile because of this
fact. Most of these methods are also limited in their
production rates. While the heated roller has advantages in
production speed, it is limited more to producing tape like
samples as compared to the other methods which can produce
sheets and fibers. However, work on increasing the
production rates of solid state and melt transformation
extrusions does show promising results.
11. A. 3. Melt Transformation Extrusion
The Melt Transformation Extrusion (MTE) process
developed at Ohio University by Collier et al. (20), has
several advantages over solid state extrusion. First, lower
extrusion pressures are needed in the MTE process, usually
less than 380 Atm., while solid state extrusion requires at
least 2000 Atm. Second, solid state extrusion processes
have limited production rates, while in the MTE process,
higher production rates are usually found. Third, the MTE
process is continous, as an extruder supplies polymer to the
die, while the solid state extrusion process is a batch
process using a capillary rheometer, which must be reloaded
when empty, to supply polymer to the die. Fourth and
finally, the MTE process produces slightly higher modulus
polymer samples than the solid state extrusion process at
equivalent draw ratio values (25).
The samples produced by the MTE process show very
similar characteristics to those produced by solid state
extrusion. These include a high degree of transparency, a
fibrous structure shown by Scanning electron microscopy
(SEM), a 8 - 10 degree Celsius elevation in melting point
using Differential Scanning Calorimetery (DSC), a fibrous
fracture failure observed in tensile tests, and an increased
degree of crystallinity.
The polymers most used for such orientation methods as
17
MTE and solid state extrusion are semi-crystalline ones.
That is, they contain both crystalline and amorphous regions
in their solid state. As discussed earlier, the polymers
that contain crystalline regions exhibit the greatest
increase in strength when oriented. These semi-crystalline
polymers include polyethylene (PE) , polypropylene (PP) , and
polyoxymethylene (POM) to name a few. While work is being
done on the orientation of many different polymers, the one
most studied to date is polyethylene.
It has been shown that the crystallization of polymer
molecules can be enhanced by either pressure or elongational
flow, or a combination of the two. Pressure bomb
experiments by Wunderlich et al. (26-31) have shown that
pressures greater than 3500 Atm. can increase the
crystallization rate of polyethylene from the melted state.
Crystallization under quiescent conditions with high
pressure produced extended chain crystal structures in the
polyethylene. As the pressure was increased, the relative
volume of amorphous polyethylene decreased while the
crystalline polyethylene volume increased. Thus, pressure
is very important in determining the crystal structure and
the relative amounts of crystal and amorphous (non-
crystalline) regions in solidified polymers cooled from a
melt.
Flow of polymer solutions or melts can also change the
morphology of the crystal regions in a polymer. The "Shisk-
18
Kebab" morphology reported by Keller and Machin (32) has
been obtained by flow-induced crystallization of
polyethylene solutions ( 3 3 ) . Research done by George and
Tucker ( 3 4 ) showed that their Shisk-Kebab structure came
from the elongational flow field, but they were not able to
rule out the possibility of simple shearing flow as playing
a part in producing the Shisk-Kebab structures.
Work on polymer melts has also shown that flow-induced
crystallization can greatly accelerate the crystallization
rate. Research done on flow-induced crystallization rates
of dilute solutions ( 3 5 ) shows that once the polymer
molecule is extended above 4 0 percent of its length the
resulting fibrous crystal nucleation exceeds that in the
quiescent solution by several orders of magnitude. The
increase in crystallization rate can be shown by the fact
that polyethylene can be crystallized above its normal
melting point by shearing the melt ( 3 6 ) .
Figure 4 ( 3 7 ) shows the free energy-temperature curves
for oriented and unoriented polyethylene crystals and melts.
L* and L are the curves for the oriented melt and unoriented
melt respectively, while C1 and C11 are the curves of the
extended chain and folded chain crystals respectively. Tm
is the melting point of the original polyethylene resin, 0
Tm is the theoretical melting point of extended chain * crystals, and Tm is the estimated transition temperature
of the sheared melt.
Figure 4 : Free Energy vs Temperature of Polyethylene Crystals and Melts (37)
TEMPERATURE
Figure 4 shows that the extended chain crystal struc-
ture is more stable at higher temperatures than the folded
chain crystal structure. The original polyethylene, resin
melts at 133 degrees C., while the extended chain crystal
structure should melt at 145.5 degrees C, and an oriented
melt should crystallize, when under a sheared condition
(flow-induced crystallization), at 160 degrees C, almost 27
degrees C above the original melting point of the resin.
Thus extended chain crystals are more stable at higher
temperatures, and melts can be crystallized far above their
quiescent melting point by flow-induced crystallization.
The free energy barrier is retained by the oriented
crystals as shown by the super heating effect observed by
researchers (38). Super heating is the increase in melting
point temperature measured by a DSC that occurs in a polymer
when it is oriented by some method. It is this flow-induced
crystallization rate enhancement that has produced much of
the interest in increasing the crystallization rate of
polymers during their processing (39-40) . With this knowledge, the melt transformation extrusion
(MTE) process will now be discussed. The MTE process
combines both pressure and flow-induced crystallization
effects to produce highly oriented samples. The pressure
effect has been seen in the fact that the higher the
pressure used to extrude the polymer, the stronger and more
transparent the sample becomes (41) .
21
As the research on the MTE process progressed it was
found that a melt-conditioner was needed to make the process
continuous (42). In order to obtain highly oriented samples
the polymer melt had to be under the effects of the pressure
and temperature conditions of the pipe between the extruder
and die for at least 15 minutes. This pipe became known as
the "conditioning zone" because the polymer had to be
"conditioned" for the 15 minutes before being extruded
through the die in order to produce highly oriented samples.
It is this fact that has lead Dr. Collier and others- to try
to determine why this conditioning time is needed and what
changes, if any, the polymer melt goes through during this
time . Dr. Collier suspects that either a liquid crystal form-
ation (43) or an unsteady state cooling is a possible reason
for the melt condintioning time. The time needed for the
cooling of the polymer melt from the extruder temperature to
90 percent of the die temperature is about 10 - 15 minutes
(44) . However, recent work by J. Ghosh (45) has shown a
birefringence change in the polymer melt in the die during
flow and nonflow conditions indicating possible liquid
crystal formation.
The onset of .the birefringence enhancement for the
flowing polymer melt case occurs at about 1700 psig and
continues up to 4500 psig. It has been noted that the
pressure necessary to start the enhancement of the
22
mechanical properties, melting point elevation (super heat
effect), and optical properties (transparency) of the
polymer samples produced is about 2000 psig. This
enhancement also increases with higher melt conditioning
pressures (44).
Thus, the polymer melt may change from an isotropic
melt to an anisotropic liquid crystal melt. The simplest
liquid crystal form is a nematic phase which contains only
molecular-orientation, see Figure 5 (46). This type of
liquid crystal form has the chains of the molecules aligned,
but the ends are not aligned. This is a possible form since
it has less order than other liquid crystal forms and it
tends to be optically active with a strong birefringence
(46)
Liquid crystal behavior has been studied before . in
other polymers (47). These liquid crystal melts or
solutions exhibit flow properties of liquids but optical
properties of crystal structures because of the ordered
state of the molecules. While some polymers exhibit liquid
crystal forms at atomspheric pressures other polymers might
exhibit these at higher pressures. Figure 6 is a
temperature versus pressure curve for a polymer that forms a
liquid crystal above a critical pressure PC. Figures 7-8
are free energy versus temperature curves for a polymer that
exhibits a liquid crystal phase above a critical pressure
(PC) . Figure 7 shows the polymer under the effects of a
F i g u r e 5 : Nemat ic L i q u i d C r y s t a l S t r u c t u r e ( 4 6 )
F i g u r e 6 : P r e s s u r e v s T e m p e r a t u r e of a P r e s s u r e I n d u c e d L i q u i d C r y s t a l Polymer
LIQUID
TEMPERATURE
F i g u r e 7 : F r e e E n e r g y v s T e m p e r a t u r e o f a L i q u i d C r y s t a l Forming Polymer Below t h e C r i t i c a l P r e s s u r e ( P < PC)
I I I .
C------- ---+ TteL TM 7S-N
TEMPERATURE
F i g u r e 8 : F r e e E n e r g y v s T e m p e r a t u r e of a ~ i q u i d C r y s t a l Forming Polymer Above t h e C r i t i c a l P r e s s u r e ( P > P C )
I
* * * I
I * * I *
- * - - - - -..-*-.----+--L---- -U_j T8-N Tu TWL
TEMPERATURE
pressure less than the critical transition pressure PC,
while Figure 8 shows the same polymer above the critical
transition pressure PC. This could be an explanation of the
time needed in the melt conditioner. That is, the 15 minutes
may be the time necessary to change the unoriented melt into
a liquid crystal phase melt which does have some
orientation. This type of transition may be what 3. Ghosh
found with the change in birefringence at a pressure of
about 1700 psig using polypropylene in a specially designed
die (45).
The work of Ide and Ophir (47) showed that simple
shearing flow will not orient liquid crystals with stable
domains, while elongational flow will orient liquid crystal
melts. Figure 9 is a representation of the types of orient-
ation produced by quiescent, simple shearing flow, and
elongational flow in isotropic and liquid crystal melts or
solutions -(47). Thus, if a elongational flow field is
applied, it will orient a liquid crystal melt more
effectively than simple shearing flow.
The general process of the MTE is as follows. A polymer
melt is first extruded into a conditioning pipe where it is
maintained at specific temperature and pressure conditions.
This may produce an oriented (possibly a liquid crystal)
melt which is then accelerated using an elongational flow
field in a specially designed die, see Figure 10 (48) . Then
a steep temperature gradient is imposed at the tip of the
Figure 9 : Effect of Shear and Elongational Flow in Isotropic and Liquid Crystal Phase Melts (47)
QUI ESCENT NO FLOW
t I i 1 iISOTROPIC /
MELT
I
I ' LIQUID I i ' CRYSTAL I
MELT 1 1
Figure 10 : Elongational Flow Field in the Melt Transformation Extrusion Die (48)
28
die to quickly crystallize the oriented melt using flow-
induced crystallization which locks in the orientation.
Thus, the MTE process produces ultra high modulus poly-
mer samples which also have enhanced optical and thermal
properties. The optical properties are enhanced by the
fibril crystal structure which is achieved in the oriented
polymer crystals. The diameter of the fibrils is small
enough so that they do not interfere with the light passing
through them. Thus, the highly oriented sample is much more
transparent than the original opaque polymer resin (43).
The thermal properties of the oriented sample are also
increased. Differential Scanning Calorimetry (DSC) shows
that the highly oriented samples have a melting point
increase of up to 10 degrees Celsius over the unoriented
original polymer resin. To explain this the following
qualitative derivation is shown (49).
The symbols are defined as follows:
Sf = entropy of the melt at the melting temperature
OSf = the change in entropy between final and initial states
Tm = melting temperature
AHf = the change in the heat of fusion at Tm
ecc = extended chain crystal morphology (solid)
rc = random folded chain crystal structure (solid)
m = melted polymer liquid (unoriented)
om = melted polymer liquid (oriented)
2 9
The entropy of the random unoriented crystal structure
is greater than that of the extended chain crystals, because
they have more disorder.
Sfrc > Sfecc (1
The crystallization temperature of the two crystal
structures can be defined as follows.
Tmecc = AHfecc /bSfecc (2)
Tmrc = A H ~ K C / ASfrc ( 3 )
The heat of fusion is assumed to be approximately equal
for both structures.
Thus,
so that
where
A Hfecc = bHfrc
(Tmecc) (ASfecc) = (Tmrc) (hSfrc)
Tmecc / Tmr = AS£ rc / A Sfecc
BSfrc = ( Sfrc - Sfm ) ( 7 )
ASfecc = ( Sfecc - Sfom ) . ( 8 )
The entropy of most systems is not affected much by
flow, but in polymer systems, the entropy of a melt is very
flow dependent, especially elongational flow where the
molecules are oriented by being accelerated in the flow
30
direction (49). Since the oriented polymer is crystallized
from an oriented flowing polymer melt, its change in entropy
(AS£) is less than the unoriented folded chain crystal
structure produced from a quiescent melt.
Thus,
Sfm >> Sfom > Sfrc > Sfecc
such that
( Sfrc - Sfm ) > ( Sfecc - Sfom ) ,
SO
and
Subsituting into (6) :
Thus,
Tmecc / Tmrc > 1.
Tmecc > Tmrc.
If the melting point elevation ( superheating effect)
is defined as
Tmpe = Tmecc - Tmrc, (15)
then the sample with more extended chain crystal structures
will have a higher melting point elevation than one where
the chains are not as fully extended.
The enhanced thermal properties of the highly-oriented
31
polyethylene can also be shown by annealing work done by
Porter et a1 . (50) . Their work showed that the temperature
stability range for properties such as elastic modulus,
sample dimensions, and transparency were increased as the
extrusion draw ratio (EDR) increased.
Annealing can also affect the folded chain crystals in
the sample without affecting the above mentioned properties.
It is also shown that the extended chain crystalline
structure becomes more stable at higher EDRs. It is the
extended chain structure which gives the highly oriented
samples their improved high temperature range stability
(50). Since the melt transformation process samples exhibit
many of these same characteristics, their thermal properties
should also respond the same as those of Porter.
Thus the melt transformation extrusion process is
capable of producing high elastic modulus, transparent,
thermally enhanced polymer samples. The only drawback to
the MTE process has been its limited production rates. It
has been shown that coextrusion does increase the production
rate. It hopefully will be s,hown that highly oriented
samples can be produced using melt transformation
coextrusion, too.
11. B. Coextrusion Methods
This section deals with coextrusion and its
applications to the specific case of increasing polymer
processing production rates.
11. B. 1. General Overview
Coextrusion is a process that combines two or more
materials into a multilayered extruded product. The tech-
niques for producing and the applications of multilayer
polymer films and sheets continue to expand. There are two
main types of extrusion dies for producing coextruded sheets
and films. The first is the multimanifold type. The
schematic for a multimanifold die is shown in Figure 11
(51). The two polymer melts enter in separate manifolds and
can be controlled by restrictor bars. This type of die
works well with polymers that do not have closely matched
viscosities and flow properties. It does not work well if
more than three layers are needed, because the required dies
become complex to design and hard to adjust.
The second type of extrusion die is the feedblock type,
shown in Figure 12 (51). In this type of set up, a regular
die can be used with the feedblock for joining the polymer
melt streams after they leave the extruders and before they
Figure 11 : Multimanifold Die (51)
RESTRICTOR BAR
Figure 12 : Feedblock Die (51)
FEEDBLOCK
enter the die. Thus the polymers come in contact with each
other before they enter the die. This type of system has
low equipment costs and can handle many different layers but
does not work well with polymers having widely varing
viscosities.
Table 2 (51) compares the two type of coextrusion
methods for various applications. Note that the feedblock
method is more versitile than the multimanifold die method
except for coextruding polymers with widely different
viscosities. However, the multimanifold die does have
other advantages, too. Table 3 ( 5 2 ) lists some
characteristics of the two methods for coextruding polymers.
Note that multimanifold dies also have the ability to keep
the polymers separate from each other for a longer period of
time before combining them. Thus, heat sensitive polymer
resins can be handled at different temperatures until they
are combined by using a multimanifold die with insulation
between the two halves as shown in Figure 13 ( 5 2 ) .
One of the major concerns when coextruding polymers is
their chemical compatibilty. Poor adhesion between layers
is usually very deterimental to the products' properties.
There are three ways to alleviate this problem. The first
is to use an adhesive layer, usually a thermoplastic, to
hold the two layers together, see Figure 14 (C). The second
is to blend a compatible layer, usually a copolymer, for the
Table 2: Comparison of Coextrusion Methods (51)
Feedblock Die
Method
. good
excellent
excellent
good
excellent
poor
excellent
Requirement
Application to existing dies
Suitability for > 3 layers
Ability to change layer thickness
Coextrude thin surface layers
Coextrude thin adhesive layers
Coextrude resins with widely different viscosities
Ease of die adjustment
Multi- Manifold Die Method
poor
poor
fair
good
fair
good
fair
Table 3 : Characteristics of Coextrusion Methods (52)
Feedblock Die
Lower
5,7,&more
Simpler
10%
dies >40"
Superior
------
Characteristic
Cost
Number of layers
Complexity
Layer uniformity
Thin surface layers
Degradable core material
Degradable skin
Multimanifold Die
Higher
3 , 4
Complex
5 %
dies < 40"
------
Superior
Figure 13 : Two Layer Insulated Multimanifold Die (52)
BARS
Figure 14 : Types of Polymer Coextrusion Layers (53)
POLYMERS I j POLYMERS
. l
DISSIMILAR INHERENT m1N8
CB3
DISSIMILAR AOHESfVE LAYER NEEDED
CF3
38
middle layer which will act as an adhesive material, see
Figure 14 (D) (53) . The third way of coextruding noncompatable polymers
without adhesives is to extrude them such that they inter-
lock. Work is being done in Japan to coextrude such dis-
similar resins as nylon and polyolefins using this method.
The two resins are interlocked like a zipper. The shape of
this interlocking pattern can be varied by this method. The
peeling strengths of such bonds are equal or greater than
the adhesives they are replacing. These films are
reported to have higher impact strengths than the
conventionally prepared films they replaced (54).
Another problem encountered when coextruding two
polymers is surface layer uniformity, because in most
products a uniform skin layer is very important. Color
quality and strength can be sacrificed if the skin layer is
not uniform and thin. This problem can usually be overcome
by better die design of manifolds or feedblocks, by
choosing the right polymer viscosities, or by optimizing
conditions such as die temperature and polymer melt
properties.
Coextrusion has an almost unlimited number of possible
layer structures, see Figure 14 (53). These include resins
which are compatible with each other (B) and (D) to ones
that need an adhesive layer (C) and (F). It is also
possible to extrude seven - nine - or eleven layer sheets.
39
The ability to extrude multilayered polymer products
has lead to their use in the packaging industry. Both wax
coated paper milk cartons and tin plated steel cans have
combined the different properties of two materials to make
effective packaging containers, and this technology has been
applied to polymer containers, too. Different polymers
handle gases and moisture differently. This can be applied
such that strength, moisture, and gases are retained in the
final product. Examples of the selective properties of
different polymers are the following: acrylonitrile has good
oxygen.permeability depending on the moisture content, while
polyvinyl alcohol has good gas barrier characteristics when
dry but deteriorate when moist. If either one of these
polymers is to be used as a package material, a moisture
barrier such as a polyolefin should be placed between the
product and the barrier layer to keep it from getting moist
and losing its properties.
One method of multilayer extrusion of bottles involves
laminating two or more polymers together, conditioning their
adhesion to each other, and then thermoforming them into a
liner. The scrap from the thermoforming is then delaminated
and recycled. The liner is then preformed with another
polymer layer and blow molded into its final shape ( 5 5 ) .
Oxygen barrier bottles are being coextruded using tech-
nology from Japan. The bottles have five layers extruded as
in Figure 14 (F) . The inner layer is ethylene-vinylalcohol
(EVAL) and the skin layers are polypropylene (PP) or high
density polyethylene (HDPE) held together by adhesive
layers (56). The bottles are used for oxygen-sensitive
foods such as barbecue sauce, catsup, mayonnaise, salad
dressing, and tomato based products ( 5 7 ) . A new approach to sheet coextrusion has been developed
by Composite Containers Company. This feedblock design is
able to combine a much wider range of dissimilar viscosity
polymers. This feedblock has backflow entry of the skin
layer polymers. Thus, the skin layer polymer is picked up
by,the main stream polymer at the rate it is being extruded.
This reduces interfacial turbulence, layer thickness
varations, imperfect bonding, and marblizing . Thus
polymers with viscosity ratios greater than 3:l can be
processed with this feedblock as well as polymers that are
not very compatible at their interfaces ( 5 8 ) .
Shrink films are also coextruded. These contain selec-
tively oriented and nonoriented layers of ethylene-propylene
copolymer, and two heat sealing outer layers. These films
have better characteristics, such as tear resistance and
strength, than the PVC and PE shrink films that they
replace.
Coextruded barrier sheet films are just starting to'be
used in this country. They are widely used in Europe for
soft cheeses, fruit drinks, butter, and yogurt thermoform
packs. while in this country most coextruded sheet goes
into decorative products, this may change as FDA mandated
dairy-products packaging regulations go into effect as well
as vapor-transmission minimum standards for packages (58).
Coextruded foam-core ABS pipes are now being produced
for drain waste vent (DWV) applications. This technology
combines foam extrusion with coextrusion to produce cheap
light weight ABS pipes that can compete in cost and
performance with current DWV pipes. This process uses two
extruders with a feedblock to produce a three layer foam
core pipe. The foam core amounts for about 25% of the total
weight of the pipe. Thus, the core extruder is smaller than
the skin layer extruder (59).
Work is also continuing in die design for pipe
coextrusion. This has lead to spiderless dies which produce
better quality pipe. The spiderless dies do not have any
supports through the polymer flow channel for the mandrel of
the die. Therefore, there are not any knit lines where the
polymers were separated and then joined back together that
can cause weaknesses in the pipe. These spiderless dies are
almost turbulent free, because these spiderlegs can cause
turbulence in the polymer stream. This die can also be used
more universally than most pipe dies for different combina-
tions of polymers (60). These dies also provide more
uniform layer thickness and higher throughput rates. In
this die, three layers can be controlled by three separate
extruders that control the thickness of the layers by their
outputs (61) . There are also some new ways of coextruding polymers.
A paper by Iwakura and Fugimura (62) dealing with
coextrusion using only a one barrel extruder is described.
Since there is a slight tendency for the two polymers to mix
while they are in the barrel, the uses of this type of
extruder barrel are limited. It can be applied in processes
where fillers, reinforcements, pigments, or foams are used,
si'nce a little intermixing of components is tolerable. This
mixing effect also tends to physically adhere incompatible
polymers to some degree without using adhesives.
Some of the most recent advances in coextrusion deal
with new die designs. . A die designed by the Cloeren Company
employs both feedblock and multimanifold technology in the
same die. The die can coextrude material with melt flows as
dissimilar as 80 and 0.5, extrude barrier layers 1 mil thick
with 40 mil outer layers, rearrange material layers into a
new product structure in 25 minutes without removing the die
from the machine, and use heat sensitive resins next to
polyesters, polycarbonates, nylons and other high heat outer
layers. The new feedblock has three adjustments which can
be made on each layer to improve velocity matches and change
and match the viscosity of each layer going through the feed
block. This then feeds the multimanifold die which has
43
movable vanes to change the angle at which the feedblock
layers come in contact with the multimanifold die layers.
This die is reported to handle viscosity differences up to
400:l (63).
In October of 1983 there was a coextrusion conference
in West Germany. Some of the topics included 9-layer sheet
products and how key problems were solved for this, feed-
block dies that can handle 100:l viscosity ratios and thick-
ness ratios up to 200: 1 with layer uniformities of 2.5%, 5-
layer coatings of oriented polypropylene with high aroma
barrier capability, and new techniques in blow molding (64) i
Many companies are getting into the coextrusion field.
While some companies are in many areas of coextrusion die
design, some are very specific in their development (65).
Table 4 (65) lists some of the major equipment suppliers in
the coextrusion area. Most of the suppliers deal with cast
film, flat sheet, or blow film molding. There are fewer
suppliers of pipe and web coatings.
These are just some of the applications and uses of
coextruded polymer products as cited from current
literature. As new techniques for coextruding more widely
varying polymers begin to emerge, the uses and types of
coextruded products should increase dramatically. It is
this process which has dramatically increased the production
rate of the MTE process.
Table 4 : Coextrusion Equipment Suppliers (65)
F = Feedblock M = ~ultimanifold * = In Development
(continued)
Table 4 : Coextrusion Equipment Suppliers (cont.)
(continued)
= In
Pipe
F
*
M
M
*
*
Web Coats
F,M
*
F = Feedblock
Company
General Engineering
Gloucester
B.F.Goodrich
HPM
Johnson
Killion
Gerd Lester
Mecha Design
Mod. Plastics Machinery
NRM
Nelmor
Plastics Equiq. & Access.
Development
Profiles & Tubing
F
*
M
M
M
*
F,M
M =
Blown Film
M
M
M
M
M
Wire& Cable
M
*
M
M
M
Cast Film
F,M
F,M
*
M
Multimanifold
Flat Sheet
-----
F,M
FIM
F,M
*
M
F
F
-----
Table 4 : Coextrusion Equipment Suppliers (cont.)
F = Feedblock M = Multimanifold * = In Development
Profiles & Tubing
*
M
Company
Reifenhauser
John Royle
Sano Design
Sheldahl
Sterling
Welex
Werner & Phleiderer
Wilmington
Wire& Cable
M
M
M
Web Coats
F
Pipe
x
Blown Film
M
M
F,M
M
Cast Film
M
F
Flat Sheet
----- M
F
F
-----
11. B. 2. Effects on Orientation
Much of the work done on coextrusion production rate
enhancement of high orientation polymer processes has been
done by Porter et al. (66-69). This work has been done with
the solid state extrusion process.
This technique involves cutting a polymer billet
lengthwise and inserting another polymer film strip in
between the halves. This is then inserted into an Instron
capillary rheometer and extruded (66). The results indicate
that the elongational flow field which was observed produced
the same results at the same draw ratio (DR) as the normal
solid state extrusion process, but the solid state
coextrusion process operated at lower pressures, higher
production rates, and smaller die entrance angles.
This technique has been used to extrude high' density
polyethylene (HDPE) (66), polystyrene (PS) (67), ultra high
molecular weight polyethylene (UHMPE) (68) I and
polyethyleneterephthalate (PET) (69). Using the PET
coextrusion method, Porter et al. showed that the bulk PET
had the same overall chain extension as the PET at the
surface of the film. This indicates that the elongational
flow field is very effective in the orientation of the
polymer chains in the solid state coextrusion process.
This elongational flow field has been studied by using
quartz dies to visually photograph the flow field using an
48
Instron capillary rheometer (70). Solid state extrusion of
PET with powdered aluminum as the tracer was used in the
experimentation. Figure 15 (70) is the results of their
experiment showing the velocities and relative positions in .
the die. Velocities are in centimeters per second for a 90
degree entrance angle die. Note the increase in velocities
as the polymer approaches the die exit. This produces the
elongational flow field. This elongational flow is what
produces the extended chain crystal structure in the
extruded sample.
It has been shown that dies with wider angles of
convergence produce greater extensional velocities and yield
greater orientation than small angle dies, but these wide
angle dies are more sensitive to melt fracture and extrudate
distortions as the polymer melt is extruded (71). Thus, it
is very possible that coextrusion can avoid this hinderance
by reducing the friction between the polymer melt and the
die surface at higher die entrance angles. This would give
an effective elongational flow field at lower flow rates,
because the higher angle dies are more effective in
producing elongational flow at lower flow rates than small
angle dies. The lower flow rates would also increase the
effectiveness of the temperature gradient necessary to
freeze the orientation in the polymer, because the effect of
the lower flow rate gives more time for heat
transfer between the die and polymer. Thus, coextrusion
F i g u r e 15 : V i s u a l V e l o c i t y F i e l d ( 7 0 ) V e l o c i t y i n cm/sec
90 D e g r e e E n t r a n c e Angle i n D i e PET a t 285 d e g r e e s C e l s i u s
Q = 0.015 cc/sec
50
should be an effective method to use to increase the MTE
process production rate.
11. B. 3. Flow Patterns
Stratified two phase flow of fluids has been studied by
several researchers. Some of the first interest in it came
from the oil pipeline industry. It was found that adding
small amounts of water to crude oil flowing in pipelines
reduced the pressure necessary to make the oil flow, thereby
reducing pumping costs considerably ( 7 2 ) . The mechanics of two phase polymer melt flows has also
been studied. It has been noted that one of the polymers
tends to encapsulate the other one. So, if the polymers are
extruded, and there is a difference between their second
normal stresses, the polymer melt with the larger second
normal stress value will tend to encapsulate the other
polymer ( 7 3 ) .
White et al. ( 7 3 ) also made an experimental study using
polyethylene and polystyrene extruded from a capillary rheo-
meter. The interface between the two polymer melts was
distorted with the polyethylene trying to encapsulate or
surround the more viscous polystyrene polymer.
It has been shown by others ( 7 4 - 7 5 ) that the less
viscous polymer will tend to surround the more viscous
polymer. If the viscosity of the two polymers are the same
and there is a difference in their elasticity, then the more
elastic melt will try to wrap around the less elastic melt
( 7 4 ) . . However, any differences in the polymer viscosities
will usually outweigh any effect of the elasticity (75),
though the elastic ratio will tend to affect the surface
of the interface between the polymers, causing a coextruded
polymer combination with a higher elastic ratio difference
to produce a rougher surface.
The pressure gradients and velocity profiles have also
been studied for stratified polymer melt flows. Pressure
gradient studies done by Han and Shetty (76) showed that
when the less viscous polymer melt is flowing concentrically
around the more viscous polymer melt, the pressure gradient
for the system falls below those of either of the separate
polymer melts. They also observed that the velocity
gradient was discontinuous across the interface but the
shear stress was continuous across it indicating that there
was no slipping of the polymer melts at their interface.
This reduction in the pressure gradient can also
produce interesting effects. Since the viscosity of polymer
melts is dependent on the shear rate, it is possible to have
the viscosities of the two melts change enough such that one
may be more viscous than the other melt closer to the wall
while near the center of the flow it has a lower viscosity
compared to the other melt (77). In some experiments,
polymer melts were placed in a pipe such that one half the
pipe was filled with one melt and the ofher half with a
different melt, the interface initially being a straight
line through the center of the pipe. The different shaped
interfaces produced were found to be the results of
differences in viscosity and, to a slight degree,
differences. in elasticity of the different polymer melts
( 7 8 )
The velocity profile also tends to be more blunt for
concentrical coextruded melts than in single phase flows.
If the average velocity <v> of the polymer flow is known,
then the maximum velocity at the center of the flow is about
1.8 times <v>, while in single phase flow, it is more than 3
times <v> (79). This tends to make the coextrusion flow
more plug shaped than in the single phase flow.
Thus, coextrusion of two polymer melts can, if properly
done, reduce the pressure gradient and provide less shearing
type flow than single phase flow. While the viscosity
difference between the polymer melts reduces the pressure
gradient, any differences in elasticity can affect the
interface between the two melts and make it rough and
uneven. Thus, in general, multiphase flow should help the
MTE process and therefore increase its production rate and
reduce the resistence to flow in the die.
11. B. 4. Stabilitv of Flow
One of the most important problems in coextrusion is
the stability of the interface between the two polymer melt
streams. If the interface is rough, it could produce uneven
thicknesses of the polymer layers which might reduce the
final product's strength and properties.
The types of interface are shown in Figure 16 (80).
The severe instability interface produces very poor
extrudates and tends to cause very uneven or surge type
layers where one polymer comes out and then the other, which
leads to a wave like interface. Instability has been found
to occur at or above a critical interfacial shear stress in
most systems. This critical shear stress varies with
temperature, polymer combinations, and length to diameter
ratio of the die (74).
The variables controlling interface stability were
found to be skin layer temperature and viscosity, the ratio
of skin to core layer thickness, total extrusion rate, and
die gap (80). Thus, stable interfacial flow can be obtained
by increasing the skin layer thickness, reducing the total
extrusion rate through the die, reducing the gap of the die,
or by lowering the viscosity of the skin 'layer polymer.
This interfacial flow instability has been shown to occur at
shear stresses less than those needed to cause melt fracture
between the die wall and polymer melt.
Figure 16 : Interface Stability and Instability (80)
INSTABILITY
5 6
This instability of the interface between polymer melts
has also been reported by Han and Shetty (81) using
polyethylene and polystyrene. They found definite
correlation between the onset of interfacial instability and
wall shear stress, skin to core layer thickness ratio,
interfacial shear stress, normal stresses, and viscosity of
the polymer melts.
Other earlier work by Southern and Ballman indicates
that the interfacial instability is caused by differences in
the elasticities of the polymer melts (82). The greater the
difference in elasticity the more jagged the interface
between the polymer melts becomes. In fact, if the
extrusion rate becomes very large, the extrudate exhibits
distortions. This is where the less viscous polymer
extrudes faster and then depletes itself such that the more
viscous melt extrudes all at once. This produces a surging
flow of wave like layers at the polymers' interface (83).
There has been some interest lately in geometry
design of dies and feedblocks to reduce interfacial
instabilities (84). Research has also continued in
predicting interfacial instabilities as a function of
viscosity, normal stress, and layer thickness (85). .This
area of coextrusion is still under intense study since
interfacial stability is very critical to the final
products' performance and quality.
11. B. 5. Theoretical Flow
Because the flow rates and velocity profiles are
important in determining the flow behavior of the
coextrusion process, the calculations for determining the
velocity profiles and flow rates of the polymer are derived
below. Since much of the literature results indicate that
most polymer-polymer interfaces do not slip ,unless very
widely varying polymer combinations are chosen, it will be
assumed that there is no slipping at the interface. Figure
16 (86) shows the velocity profile for this case. The
following assumptions are used in the derivation:
A) Both polymer melts obey a power law function as the relationship for their shear dependent viscosity.
B) No interfacial or wall instabilities exist.
C) The melt flows have isothermal and isotropic characteristics.
D) Steady state flow has already been reached.
E) No slippage of the polymer melts at the wall or their interface occurs.
F) The width and length of the die gap is much larger than the height of the gap so that parallel plate geometry can be assumed.
G) There are no effects of bouyancy (density differences) or gravity on the polymer melt flowing system.
H) Symmetrical two phase flow has been obtained.
I) The lower viscosity melt is flowing around the higher viscosity polymer.
F i g u r e 17 : D i e C h a n n e l V e l o c i t y P r o f i l e ( 8 6 )
INTERFACE
. - - - )flJsIoN Z / ------.
,/' DIRECTION
.---.- -- FLUID L -.----- * .-,,.--.-- 1 -r\
W A L WIDTH CWI
The following symbols are defined as follows:
Fluid A = skin layer polymer melt
Fluid B = core layer polymer melt
P = pressure
T = shear stress
X,Y,Z = directions (see Figure 17)
K = viscosity of the polymer melt i i
M ,n = constants of the power law viscosity equation for i i
polymer melt i
V = velocity
Q = flow rate of polymer i i
Thus from the Z-component equation of motion,
/ b y - (16) - 6 p / & Z = S T YZ
Let
E = - 6 P / s z = [ ( P 'P ) / b z ] * 0 1
T ~ U S , (16) becomes
The value of c can be found using the boundary condition at 1
the center of flow.
Therefore,
Thus,
For fluid A, the power law can be written as follows.
Subsituting (22) into (21),
Solving for V and integrating yields z,a
1 /na (l/na) +l v =- (E/Ka) [na/(na+l) I (Y) +c . (24) z,a 2
Using the no slip boundary condition at the wall,
Then
Thus, the velocity distribution becomes
for fluid A.
The volumetric flow rate per half channel width (w) is
Integration yields
l/na 2 Q /W= (E/Ka) [ (nah )/(2na+l)] {l- (l/h) [1+2na)/(l+na) 1 a
(l/na) +2 +[na/(l+na) ( d /h) 1 1 . (28)
The value Q is the volumetric flow rate for the flow a
of fluid A in half the total channel. So Q = 2 Q . a total a
Now, for the same calculations using fluid B,
Solving for V , z,b
l/nb (l/nb) +l v =- (E/K~) [nb/(nb+l)l (Y) +C (30 z,b 3
using the boundary condition at the interface where the two
velocity equations must be continuous. Then
Thus,
So the velocity distribution in fluid B becomes
The volumetric flow rate is again
Subsituting (33) into (34) and integrating yields
Thus, the total volumetric flow rate in the entire die
gap (2h) becomes
Q = 24 + 2Q total a b
where Q is the total volumetric flow rate in the entire total
die gap. Thus, the velocity distribution and flow rates can
be determined for the two phase flow found in coextrusion
using the assumptions originally stated.
111. EXPERIMENTAL
This section describes the procedures and equipment
used during the research sequence.
111. A. Equipment
The following is a list of equipment used in this
investigation.
1. Die:
A uniaxial ribbon die was used for this investigation.
The split type die was originally designed by Pandya (87).
The die was then converted by Perez (88) for use in
coextrusion. The die is made of 416 stainless steel with a
deformation ratio (DR) of 64:l possible. The ribbon
thickness and the DR can be changed by changing the inserts
which determine the final thickness of the ribbon. The die
is shown in Figures 18 and 19, and the inserts in Figure 20.
The die is held together by nine 3.5" x .5" bolts with
13 threads per inch. These are made of the same material as
the die, 416 stainless steel. The inserts are made of 416
stainless steel and are held in the die by two hexagonal
socket head bolts 1" x .25" with 20 threads per inch. These
are made of hardened carbon steel.
Figure 18 : Coextrusion Die - Side 1
: * : L....., I e .
I . .......... .
f e . * ...... I i . 3 8 I . I ! I :
I b .. 8 f ... i . .* . *'
i b * ' ...... : a+, ;
I I .. @ * .* * .. , ..a i * . m * . *
i t . -a
I I
i • I
. i f
. * * e
I . i......] i I 1 1 .
e
Z * I *
i :..........-.. i I . . *. f *. e .
i * e . . e . . . I I
e I
i .I
e * e ........ I ,...... a:..... 4
Figure 19 : Coextrusion Die - Side 2
Figure 20 : Coextrusion Die Inserts
2. Differential Scanning Calorimeter (DSC):
The DSC is a Perkin Elmer DSC-1B with a chart recorder,
model 56-1003, also made by Perkin Elmer Instruments,
Norwalk, Connecticut. The DSC was used to determine the
melting point elevation and crystallinity of the samples.
3. Extruders :
One single screw extruder with a 3/4 inch diameter
barrel, manufactured by Barbender Instruments Incorporated,
South Hackensack, New Jersey, was used. The extruder is a
model 200 and is supplied with a screw having a length to
diameter ratio of 20:l and a compression ratio of 4:l. The
extruder is also supplied with a 1.5 horsepower 220 volt
motor, model PL-V180, with a maximum speed of 2400 rpm, a
gear reduction box with a reduction ratio of 20.4:1, and a
torque arm mechanism. The speed controller for the motor is
a Fincor model 2,402 MKII DC motor controller manufactured by
Fincor Incom International Incorporated, York, Pennsylvania.
This extruder was used for the shell side polymer since a
lower volumetric flow rate was needed for this polymer.
The other extruder is a twin screw counterrotating
laboratory extruder LSM 30.34 with a micromatex 16.1
microprocessor distributed by American Leistritz Extruder
Corporation, Somerville, New Jersey. The extruder is
supplied with two 34 millimeter diameter screws with a
length to diameter ratio of 22.94:l and a flight depth of 4
millimeters. There are two optional feed or vacuum port
sections at 7.06 and 14.12 times the diameter of the screws
from the feed end. There are six heating zones down the
length of the barrel. The sections are interchangable and
provide flexibility when using the extruder for different
purposes and polymers.
The motor supplied with the extruder is a Braumuller
D.C. motor, type GNAF 112-MV, 220 volt DC, with a power
rating of 11.2 kilowatts. The extruder also has an optional
Siemen-Hinsch multistaged water sealed vacuum degassing
plant, model PL-25007. The vacuum pump can pull 20 CFM
suction at 50 Torr vacuum. The pump is powered by a Loher
3-phase delta 240 volt motor, model AL90 LA-2. There is
also a small feeder, model GLD-75, for suppling polymer to
the extruder at a constant rate. The feeder is supplied
with a permanent worm-gear motor, type NUP 420 EF, with a
maximum speed of 200 rpm and a power consumption of 0.05
kilowatt, 180 volts DC.
4. Heaters :
The seven heaters for the Leistritz twin screw extruder
were provided by the American Leistritz Extruder
Corporation. Four of them are rated at 700, two at 520, and
one at 400 watts, the 400 watt heater being at the end
adapter plate, and all run on 230 volts and are made by
69
ERGE. The six main heaters are also provided with cooling
fans which are rated at one watt power, 230 volts, and are
by EBM. The cooling fans cool the heating zone if the
temperature rises above the set point or if the set point is
lowered.
The two heaters on the single screw extruder were
provided by Brabender Instruments Company. They are rated
at 800 watts apiece and operate on 240 volts.
All other heaters used for the piping, connections,
tees, couplers, elbows, and the die were recommended and
supplied by the Industrial Heater Company, New York City,
New York. The heaters range from 300 to 850 watts depending
on the specific use of the heater and the power density
needed for the specific design. All heaters are mica
insulated resistance heaters which were covered with
fiberglass pipe insulation after they were installed to
reduce heat loss to the room.
5. Microscopic Pictures :
Magnified pictures of the extrudate were taken using a
Wild stero-microscope, model MSA, with a Wild camera, model
MP5II. A Wild semi-photomat, model MPS 15, and a varying
intensity Wild-Leitz light source were also used to
illuminate the samples. All the above equipment was made by
Wild-Heerbrugg Incorporated, Switzerland. Pictures were
also taken using a scanning electron microscope.
6. Pipes :
The stainless steel piping and fittings used in this
investigation were provided by Autoclave Engineers
Incorporated. The pipe is for medium pressure duty with a
maximum working pressure of 10,000 psi at 200 degrees
Fahrenheit. The pipe threads are left handed threads with
16 threads per inch. The outside diameter of the piping is
one inch with an inside diameter 0.678 inches. Stainless
steel elbows and tees were also used to change direction of
the piping and to incorporate the die in the system. All
threaded joints were treated with antiseize spray to make
removal of the joints at a later time easier.
7. Temperature Controllers :
All the heaters on the Leistritz twin screw extruder
were controlled by the microprocessor, Leistritz Micromatex
16.1, manufactured by American Leistritz Extruder
Corporation, Somerville, New Jersey. The Barbender single
screw extruder's two heaters and the die heater were
regulated by three Guardsmen Controllers, model JPC,
manufactured by West Instruments Corporation and provided by
Barbender Instruments Incorporated, South Hackensack, New
Jersey. Two of the controllers controlled the extruder
heaters and the other controller maintained the die heater.
71
The other heaters ( on the piping, fittings, and
couplers) were regulated by four Love proportional
controllers, model 52, manufactured by Love Controls
Corporation, Wheeling, Illinois. While the above mentioned
controllers regulated more than one heater, only one
thermocouple was used to measure the temperature. Thus, the
thermocouple measured the temperature at the end of the
heating section that the controller was regulating.
8. Tensile Testing :
Tensile tests of the samples were done on an Instron
Universal Testing Instrument, Floor model TT-D, manufactured
by Instron, Canton, Massachusetts. The load cell is a GR
type, model D 30-20, with a maximum capacity of 20,000
pounds. Samples were trimmed using a TensileKit model 10-68
cutter manufactured by Tensile Kit Engineering a division of
Sieburg Industries Incorporated. The cutter is powered by a
Sieburg motor, model T536 ,115 volt, 0.8 amps.
9. Transducers - and Pressure Measurement Equipment :
Four Dynisco melt pressure transducers were used to
measure the pressure at four points in the system. These
include models TPT 432 A-15 M 6/18, TPT 432 A-10 M 6/18, PT4
62 E-5 M 6/18, and TDA 463 -1/2- 5 C 15/46. These were
located at the ends of both extruders, at the die, and at
the pipe suppling the shell side polymer. All transducers
are manufactured by Dynisco, Norwood, Massachusetts.
Three of these were contected to three pressure
indicators, and the fourth to the microprocesser. Two of
the pressure indicators were made by Dynisco, one a model ER
4 7 8 A2 and the other a model ER 4 7 8 Al. The other pressure
indicator is a West pressure indicator, model IS-86, and
supplied by Barbender Instruments. The last melt pressure
transducer was connected to the Leistritz ~icromatex 16.1
microprocesser on the Leistritz twin screw extruder which
displayed the pressure digitally.
10. Water Tank and Take-up Device : --- A galvanized steel water tank was used to quench the
samples and provided the temperature gradient at the end of
the die. It measures 36.5 inches long, 15 inches wide and
24 inches deep. The tank is supported by a steel base'so it
sits 15 inches above the floor. An aluminum rack with
rollers was also used to direct the polymer sample coming
from the die. Because the galvanized coating was chipped in
some places, and the aluminum rack was present in the water
bath, corrosion had weakened some of the welds along the
seams of the tank. A polyester coating was applied to the
tank to keep it from leaking and corroding further. The
water ,was Athens city tap water and was kept at room
temperature.
A take-up device was used to keep tension on the
extrudate as it came out of the water tank. The take-up
device was made by Brabender Instruments Inc. and is type
G7. It is powered by a Bodine Electric Company DC motor
rated at 1/20 H P . The motor is controlled by a Bodine DC
motor controller, type A S H 400/DC. This device had two
three inch rollers, one stainless steel and the other a hard
polymer, which turned and kept tension on the polymer
extrudate. The device had a variable speed motor which
could be changed to regulate the roller's speed.
The final set up of the equipment is shown in Figures
21 and 22. Figure 21 is a top view of the equipment set up
while Figure 22 is a side view of the equipment set up.
Only the major equipment components are shown in these
figures. The heaters and wires have been omitted, but the
location and distances between components is given.
Figure 21 : Equipment Set Up - Top View
,,ye ' r . CORE SIDE
f , EXTRUDER /? WELL. SIDE CTWIN SCREW3 \
i : EXTRUDER '. [ 1 [SINGLE SCREW3
\
CONDITIONING
'7.5" LENGTHS
TAKE-UP DEVICE -,,-L/,7>
Z
7 5
Figure 22 : Equipment Set Up - Side View
111. B. Ex~erimental Procedure
The initial thrust of this investigation was to build
pressure in the multimanifold coextrusion die. This was to
be done by using the new twin screw counterrotating extruder
manufactered by Leistritz. First, the unit was wired and
plumbed by Ohio University maintance personel. The next
step was to hook up the medium pressure pipe used to connect
to the die to the extruder. This was originally to be done
by machining a new plate to replace the die plate on the
extruder. However, after consulting with the machinist, it
was decided that a sma.11 connector could be welded to the
existing die plate.
The original die hole was enlarged, and a connector was
made which adapted to the one inch pipe. This connector was
then welded on the die plate, see Figure 23. The die plate
was then bolted on the extruder and the piping was screwed
into connector.
It became apparent early in this investigation that the
screw and heater configuration was very critical in
extruding good polymer samples. The first problem
encountered was that of the polymer melt flowing back
through the extruder's vent port. This problem was remedied
by moving the vent port to the third heating zone from the
end of the extruder. The extruder's barrel is divided into
F i g u r e 23 : C o n n e c t o r f o r t h e T w i n Screw
j - ! I--1- I[, ,; ;:;;;-7 f....'
r
78
s e v e n z o n e s . The f i r s t zone is t h e f e e d z o n e , which is where
t h e polymer p e l l e t s e n t e r t h e b a r r e l . I t is c o o l e d by
w a t e r .
The n e x t s i x z o n e s a r e h e a t e d z o n e s . T h e s e a r e
i n d i v i d u a l l y c o n t r o l l e d by t h e m i c r p r o c e s s o r . The v e n t p o r t
was l o c a t e d a t zone f i v e , F i g u r e 2 4 . The o t h e r ways t o h e l p
remedy t h i s p rob lem of t h e m e l t f l o w i n g o u t of t h e v e n t p o r t
were t o lower t h e t e m p e r a t u r e i n t h e l a s t two z o n e s ,
i n c r e a s e t h e e x t r u d e r s p e e d , and l o w e r t h e d o s i n g s p e e d
(po lymer f e e d i n p u t ) . Using t h e s e methods , p r e s s u r e s of up
t o 4500 p s i were o b t a i n e d a t t h e end of t h e e x t r u d e r .
The h e a t e r s e c t i o n s a r e changed b y f i r s t h e a t i n g up t h e
e x t r u d e r and t h e n r u n n i n g t h e e x t r u d e r w i t h o u t a n y p i p e i n
t h e c o n n e c t o r a t t h e end of t h e e x t r u d e r . A f t e r t h i s , t h e
end p l a t e is u n b o l t e d and removed f rom t h e e x t r u d e r w h i l e i t
is s t i l l h o t . Then t h e s e c t i o n s a r e removed and changed i n
p o s i t i o n . C a r e mus t be t a k e n t o make s u r e t h e a l i g n m e n t
k e y s and s p a c e r s a r e c o r r e c t l y p l a c e d when r e i n s t a l l i n g t h e
h e a t e r z o n e s . The end p l a t e is t h e n b o l t e d b a c k o n , and
when t h e b a r r e l c o o l s , t h e f o u r main b o l t s a r e t i g h t e n e d
f i n g e r t i g h t . T h e s e a r e n o t o v e r - t i g h t e n e d , a s t h e b a r r e l
e x p a n d s more t h a n t h e b o l t s when i t is h e a t e d up t o
o p e r a t i n g c o n d i t i o n s .
The n e x t p r o b l e m e n c o u n t e r e d was g a s e n t r a p m e n t i n t h e
polymer m e l t a s i t came o u t of t h e e x t r u d e r . T h i s was '
remedied by c h a n g i n g t h e screw c o n f i g u r a t i o n i n t h e
F i g u r e 24 : Twin Screw E x t r u d e r B a r r e l Assembly Segmented barrel LSM 30.34
material disarge part \ tie rod plate
barrel element with heating band '- '
HEATING ZONES
80
extruder. The screws in the twin screw extruder are made in
sections and are place on a hexagonal shaft. Thus, the
actual screw segments are only 120 centimeters long, while
the shaft runs the entire length of the extruder.
To change the screw configuration, the extruder must
first be heated up to operating temperature and the die
plate removed so the end of the screws show. Then the
extruder is run so the polymer comes out of the screws,
leaving them with only a thin layer of polymer melt on them.
Then the extruder is shut off, and the screws pushed forward
out of the extruder by being carefully pried on in the vent
port. Once the screws are out of the extruder, the heaters
are turned off and the screws cooled before being changed.
The screws were changed such that the shearing ring
which forces the polymer melt through small slits on the
ring was in heater zone 3. This acted as a vacuum seal when
the vacuum pump was used. The next screw, in zone 4, was a
single helix screw, one screw flight per turn, and was
placed in the vacuum port opening. The last two screws were
triple helix screws. These have three flights per turn and
are the close meshing type which allows for pressure build
UP
Once .the screws were reassembled, the extruder was
heated up, and the screws were placed in the barrel together
so their ends lined up. Then the screws were rotated such
that their shafts were aligned with openings on the drive
shaft. The extruder motor was turned on slowly to make sure
the screws were aligned correctly in the extruder. Then the
die plate was bolted back on the extruder.
The multimanifold die was then cleaned. The inside of
the die was cleaned by scraping and sanding lightly with 600
grit sandpaper. Care was taken not to scratch or damage the
mating surfaces of the die during cleaning. Once the die
was cleaned, the inserts were placed in the die and
tightened moderately using an Allen wrench.
The die was assembled and the bolts tightened to 95
ft .-lbs torque. The pressure - temperature probe was
installed in the die. Telfon tape was used to make removal
of the probe easier. The probe was bent slightly to allow
the shell side piping to fit in the die. The shell side
piping was then installed and the joints tightened to 100
ft.-lbs torque. All joints were treated with anti-seize
spray. Otherwise, removal would become very difficult.
The shell side piping was attached to the single screw
extruder. A U-bend was placed in the shell line piping to
allow adjustment of the die to fit the twin screw extruder
by only a straight pipe. The melt conditioning zone was
installed between the die and'the twin screw core extruder.
The heaters were then installed on the piping and fittings.
Most of this was covered by fiberglass insulation.
After this, initial setup runs were made, and the
82
system was not dismantled unless the die inserts or melt
conditioner pipe length were changed. When the system was
first heated up, all the joints were checked for torque in
case any polymer was in the threads. The joint would
tighten up more when hot.
The heaters were turned on and heated up in steps of
about 50 degrees Celius. When the temperatures reached the
set points, approximately 5 minutes were allowed for the
temperatures to equilibrate. Then the set points were moved
up about 50 degrees again, and the same procedure was
repeated. This was done until all the temperature
controllers were at their desired set points. Approximately
45 minutes were allowed for the system to reach thermal
equilibrium. The seven temperature settings on the twin
screw extruder were also set after it was turned on. The
general profile used in this investigation was: zones 1,2 - 0 0 0
195C, zone 3 - 185 C, zones 4,5,6 - 170 C, and zone 7 0
(die plate) - 170 C. These temperatures were set and
controlled by the microprocessor on the extruder.
Once thermal equilbrium was reached, the shell side
extruder was started. It was adjusted to the proper speed
and remained there throughout the run. After the polymer
was flowing out of the die, the twin screw extruder was
started, and the core material was joined with the skin
layer.
Once the polymer was flowing out of the die, it was
lead through the water bath, around the rollers, and through
the take-up device. The take-up device was then started. It
kept the line speed and tension constant as the polymer came
out of the water bath. When the flow stablizied, the water
level in the bath was raised till it just barely touched the
tip of the die. If the water level was raised any higher,
the surface quality of the extrudate decreased rapidly,
leading to blow outs and plugging of the die.
With this, the run was started, and data was taken.
Most of the runs lasted about 25 minutes, after which the
core extruder and the shell extruder were turned off. The
heaters were shut off after there was a zero pressure
reading in the die and pipes. The data was recorded, and
the samples made were rolled up. This is how the
multimanifold die runs were made.
The Differential Scanning Calorimetry (DSC) was used on
the samples to determine the percent crystallinity and
melting point changes in the core and shell layers. The
samples were cut out using a leather punch, weighed to the
nearest tenth of a milligram, and sealed in a DSC sample
pan. A known sample of Indium was used to standardize the
DSC. Then the samples were scanned twice. The' first scan
was to pick up the elevated core and depressed shell melting
point temperatures. The second scan was to find the regular
melting point, of the two layers and to find the volume
84
ratio of the shell to core. V
The samples were tested at a scan rate of 20 C per
minute and a range of 8 millivolts. The chart recorder was
set at a full range value of 10 millivolts and a chart speed
of 40 millimeters per minute. An Apple computer with a
graphics board was used to determine the areas under the
curves for the shell and core layers. From this, the
percent crystallinity was calculated as well as the core to
shell volume ratio.
Tensile tests were run on the samples to determine
elastic modulus, yield strength, and ultimate tensile
strength. The samples were cut to be one half inch wide and
two inches in length. The Instron grips were set at an
initial position three inches apart. The pull speed was set
at 0.5 inches per minute, and the chart speed was set at 2
inches per minute with a full scale reading of 500 pounds.
The samples were tested until fracture occured.
Microscopic pictures were taken of some of the samples.
These were taken using fast 400 speed black and white film.
The magnification varied depending on the picture and sample
size. Scanning electron microscopy (SEM) was perform on
several samples that had been fractured in liquid nitrogen.
IV. RESULTS
The first runs (A-D) were made using a die gap of 1/32
of an inch. However, the pressures built up in the die were
fairly low, so the die gap was changed to 1/64 of an inch
to increase the pressure in the die. Thus, the results are
divided into two groups. The first four runs (A-D) were
made using a die gap of 1/32 of an inch and a melt
conditioner length of 35.5 inches. The second set of runs
(F-K) was made using a die gap of 1/64 of an inch and a melt
conditioner length of 21.5 inches. One run, (L), was made
using a die gap of 1/64 of an inch and a melt conditioning
length of 12 inches. Run E was made using a feedblock die,
and the results were not used in this investigation.
Figure 25 contains a graph of the volumetric flow rate
of the polymer out of the die as a function of pressure in
the die for runs (A-D). Figure 26 is the graph of
volumetric flow rate versus die pressure for runs (F-K).
The flow rates were calculated using the samples' actual
width and thickness and the line speed during the run. The
pressure recorded was the average pressure in the die during
the run.
Figures 27 and 28 are the results of the differential
scanning calorimeter (DSC). These figures contain the
elevation of the melting point of the polypropylene core as
Figure 25 : Volumetric Flow Rate versus Die Pressure
Runs A
- D; Die Gap 1/32 in.;
Melt Conditioner Length 35.5
inches.
750
1000
1250
DI E
P
RE
SS
UR
E
C P
SI I
m 74 3 m I a, O a l z m 4 L l h C u o 4 * -74
Figure 27 :
leva ti on
of the Melting Point Temperature
m
of the Polypropylene Core versus Die Pressure
00
Runs A
- D
U
Die Gap 1/32 in.; Melt Conditioner
Length 35.5 inches.
"13.5
750
1000
1250
1500
DIE
PR
ES
SU
RE
C
PS
I I
rn I U C a , r l m
. d U k a , U O
a function of die pressure. These were found using the
differences in melting points of the polypropylene core
between the first and second DSC scans. The melting point 0
of the polypropylene was found to be 163 C in the unoriented
state.
Figures 29 and 30 are the graphs of the depression in
the melting point of the polyethylene shell layer as a
function of die pressure. The depression in melting point
of the polyethylene shell layer was determined between the
first and second scans on the DSC.
After the DSC scans, the area under the curves for the
shell and core polymers were measured using an Apple Comp-
uter. From this, the percent crystallinity of the polyprop-
ylene core was determined. It was assumed that the heat of
fusion of the polypropylene was 62.6 cal. per gram of 100
percent crystalline polypropylene. Figures 31 and 32 are
the results of these calculations. They contain the curve
of percent crystallinity of the core as a function of die
pressure for runs (A-D) and (F-K) respectively.
Next, the tensile tests were run on the samples. These
were used to determine their elastic moduli, yield
strengths, and ultimate tensile strengths. The samples were
tested at 0.5 inches per minute cross head speed and at room
temperature. Figures 33 and .34 show the resulting elastic
moduli as a function of die pressure for the two sets of
runs.
U A - a r m 4 4I= * U r l m Z rl m e-m
U t3 a, cuc
w c m a , o a , \ d
c h 0s a -4 u m m a , u rn h a , d a, LI 0 .* a!% D a, n a, c
* * u
t 0 ' U ) - cu ; - I n
C 3,) 113HS- 'ld 3NI113W 30 N O I S S ~ U ~ ~ U
urn g . C M crn -+a, Oa, 0 + U C 04 U
U - I M U C ma, 4.4 C h l a, .ria x m 4JIJh ,-i '- 4 a,,-im x 4 c C
a, 9-4s a , r C P : JJ C W W D l +' a c
a, \a, W C r l I J 0 a, 4 a
C h (P O C C3 .4 +' m a a, m h -4 a,& a U 0 a 04 a, a a,
C * * u
Figure 31 : P
ercent Crystallinity of the
m
Polypropylene Core versus Die Pressure
Runs A
- D
Die Gap 1/32 in.;
Melt Conditioner
U "
b Length 35.5 inches.
DIE
PR
ES
SU
RE
C
PS
I 3
Figure 32 :
Percent Crystallinity of the
Polypropylene Core versus Die Pressure
Runs F -
K Die Gap 1/64 in.; Melt Conditioner
Length 21.5 inches.
28 t -
--
--
--- I----
-._-l_
_.ll---"
---L
I.-.--
- t
t t
1000
1500
2000
2500
DIE
PR
ES
SU
RE
[P
SI
I
0 V)
a, ua , -4 C a 4 J u
ar f c V) a, -4
9 7
Figures 3 5 and 3 6 contain the plots of the yield
strength as a function of die pressure. Figures 37 and 38
show the curves of ultimate tensile strength as a function
of die pressure for runs (A-D) and (F-K) respectively.
Table 6 show the results of the tensile test elongation
of the sample runs as compared to their core to shell volume 4
ratio. The core to shell volume ratios were determined from
the DSC results.
Figures 39 - 4 2 are pictures taken of the samples using
the Wild microscope. Figures 39 and 4 0 are a torn edge and
a surface view of run J and F respectively. The extrusion
direction is parallel to the torn edge. Figure 4 1 is a thin
slice of run J which was bend until it cracked leaving only
the outside shell layer holding it together. The extrusion
direction is facing directly upward, perpendicular to the
picture's surface. Figure 4 2 is a torn edge of run L, with
its extrusion direction parallel to the crack.
The results of the Scanning Electron Microscope are
shown in Figures 4 3 - 4 5 . Figures 4 3 and 4 5 are surfaces
fractured in liquid nitrogen. Figure 4 4 is a tensile test
failure sample showing the failure edge.
a, u u a, 9 c m 0 m -4
a, 4J u .4
a a * C V)
a, o a , .d U .G a u
a 4 J c m d .d 3 l a , m = L O u 4 a, * - m > m * m
c c C 9.4 .G U P : 0 F cum c m c a, \ a , u d G i 4J m a
m 5 C3 d 8 .d .d
w a
C I S d I H13N3t l lS 013 I A
Fig
ur
e
36
: Y
ield
S
tre
ng
th v
er
su
s D
ie
Pr
es
su
re
R
un
s F
-
K
Di
e G
ap
1/6
4
in.;
M
el
t C
on
dit
ion
er
Le
ng
th
21
.5
inc
he
s.
DIE
PR
ES
SU
RE
[P
SI
1
U a, c 0
-rl
Jz 4J CI .rl
C7-l '-a c a , em a , u o a , u 3 CJJz 4J [I) U m m Q 4 J e
a, -I .cl a , u l a, -In, z m -rl 4 . m a , * -LO I = + [I) * m a a c c B 3 . d J z
m f f i .u 0 ' 2 N f J l 4 J m m e
\ a , E a , dcl
.rl > 4J 4
a a
3 C3
aJ . rl Q
a, rl -4 a w l a,
d e c z m .d h m a , *--I c .4 m * c u a a c c E 3 . d C
m f f i C, a 3 d ' t n u r n w c a & \ a , Ea, r lc l .d >
Table 5 : Percent Elongation to Failure Compared to the Core to Shell Volume Ratio
RUN *
A
B
C
D
F**
G
H
I
J
K
L
* Run E was omitted from this table because it was not used in this investigation.
Percent Elongation at Failure
* * Run F produced poor samples and they were not testable.
Core : Shell Volume Ratio
10 3
Figure 39 : Torn Edge of Run J (magnified 1 4 . 5 ~ )
Figure 40 : Surface of Run F (magnified 14.5~)
F i g u r e 4 1 : T h i n S l i c e of r u n J C r a c k e d w i t h t h e S h e l l Layer H o l d i n g t h e Sample T o g e t h e r ( m a g n i f i e d 2 9 x )
F i g u r e 42 : T o r n Edge of r u n L ( m a g n i f i e d 2 9 x )
Figure 43 : SEM of Run K (magnified 1000x) Fractured Surface (using Liquid Nitrogen)
Figure 44 : SEM of Run J (magnified 200x) Tensile Test Failure Surface
F i g u r e 45 : SEM of Run J ( m a g n i f i e d 1000x) F r a c t u r e d End S u r f a c e ( u s i n g L i q u i d N i t r o g e n )
V. DISCUSSION OF RESULTS
The following is a discussion of the results obtained
during this investigation and their limitations.
V. A. Discussion
The first set of data, runs A,B,C,D, were made using a
melt conditioner pipe length of 35.5 inches and a
multimanifold die with a die gap of 1/32 on an inch. With
this set-up, the pressure in the die reached a maximum of
1500 psi during run D in Table 6. Different die pressures
seemed to control the samples' thermal and mechanical
properties. The temperature profile of the pipes leading to
the die and the temperature of the die also affected the
samples' properties somewhat. The temperatures did not seem
to have as much of an effect on the properties of the
samples as did the various die pressures.
The temperature profile used in this investigation was
k e p t constant w i t h only one exception: in run D the d i e
temperature and shell temperature profile were changed
independently. It was noted that the shell temperature had
little effect on the die pressure. This was because the
ratio of the core polymer to the shell polymer was so large
that there was no measureable change in the die pressure
when the shell temperature was lowered or raised and the
core temperature was kept constant. Changes in the die
temperature (core temperature) had more effect on the die
pressure than changes in the shell temperature. If the die 0
temperature was lowered below 160 C, the polymer froze and
plugged the die. Thus, the die temperature was arbitrarily 0
set at 163 C. The data recorded in the Appendix (Table 6)
is the average temperature recorded during the runs. This
temperature data varied slightly from the set temperature. 0
The melt conditioning temperature was set at 165 C but
the temperature varied somewhat from the set temperature
also. However, if the melt conditioning temperature was
lowered, the pressure drop in the melt conditioning pipe
would increase and the die pressure would decrease. If the
melt conditioning temperature was increased, the die
pressure would decrease and the line speed would increase.
These conditions produced samples with somewhat inferior
thermal and mechanical properties indicating that the lower
temperature was better for the melt conditioning pipe. 0
Therefore, 165 C seemed to be the optimum temperature for
the melt conditioner.
Run E is not in Appendix Data Table 6 since a
feedblock was used to combine the two polymer melt streams
before they entered the die. The results of the coextruded
samples produced by this die were not compatible with the
10 9
multimanifold die data so they were not included in Table 6.
The shell polymer worked so well in reducing the resistance
to flow that only 600 psi pressure was produced in the die.
The lubricating effect of the shell polymer was most
pronounced when a run was tried by first starting the core
extruder and building the die pressure up to about 2000 psi.
However, when the shell layer extruder was started the
pressure immediately dropped back to only 600 psi. This
indicated that the shell layer was very effective in
reducing the high shearing rate in the die due to its
lubricating effect.
Run E was made using a uniaxial die with only a 6:l die
reduction ratio. The die opening was 1/2 of an inch wide
and 1/16 of an inch thick. The multimanifold die, which was
used for the other runs, had a much higher die reduction
ratio (32: 1 and 64: 1) which contributed to the higher die
pressures in those runs. It would have been better to have
had a higher die reduction ratio for run E so a higher die
pressure could have been obtained.
The shell layer lubricated the die wall and reduced the
die wall friction, this reduced the tendency of the core
polymer to form "dead zones" in the die's converging
section. The core polymer tended to follow closer to the
wall's contour because most of the shearing flow was in the
thin shell layer. Therefore the shell layer allowed the use
of higher die reduction ratios which produced higher
elongational flow velocities.
The second set of data contained runs F through K and
were made using the multimanifold die with a die gap of 1/64
of an inch and a melt conditioning length of 21.5 inches.
These specifications were used in an attempt to build more
pressure in the die. The smaller die gap increased the
resistance to flow which in turn increased the pressure
required to make the polymer flow from the die at a given
volumetric flow rate.
The shorter melt conditioning pipe was used in an
attempt to reduce the pressure drop in the pipe from the
core extruder to the die. It was noted in the first data
set, runs A - D, that the difference in die pressure and
extruder pressure was quite significant. For example, in
run D (Table 6) the die pressure was 1500 psi, while the
pressure at the end of the extruder was 2200 psi. This
indicated a large pressure drop in the melt conditioning
pipe connecting the extruder to the die. The maximum die
pressure reached during runs F - K was 2800 psi in run J.
The core extruder pressure during this run was 4200 psi.
Run L was an attempt to get higher die pressures by
shortening the melt conditioning pipe to 12 inches in
length. This was tried because the new twin screw extruder
had better control, compared to the single screw extruder,
of the polymer melt's temperature when it was extruded. The
111 0
polymer melt was extruded at 170 C, which is normally
considered a good temperature to have the melt conditioner
set, in the melt transformation extrusion process. Since
the melt conditioner was only a reservoir for the polymer
melt, little cooling was needed in the conditioning zone so
a shorter pipe was used to decrease the residence time in
the melt conditioning pipe and reduce the pressure drop.
The pressure drop in the melt conditioning pipe was
large because the core polymer melt was close tb its melting
point and its viscosity was very high. Thus a large
pressure drop in the pipe was needed to make the melt flow
through the pipe. While the core extruder in run L operated
at 4600 psi, the die pressure reached only 2950 psi. More
importantly, the thermal and mechanical properties of the
extrudate were not nearly as good as those of runs J and K
which were made at 2800 and 2750 psi die pressure and the
same die temperature (see Table 6). This indicated that the
main function of the melt conditioning pipe was not to cool
the polymer melt but rather to provide a residence time
necessary for the melt to be conditioned before it was
extruded through the die.
The results of run L indicated that melt conditioner
pipe should be longer and the pipe was a conditioning zone
where the core polymer melt was transformed, probably, to an
ordered liquid crystal polymer,melt. Run L also indicated
that the shorter pipe had little effect on reducing the
pressure drop in the melt conditioning pipe.
Figures 25 and 26 are the plots of volumetric flow rate
versus die pressure for runs A - D and F - K respectively.
Both graphs indicated that the flow rate was not linearly
related to the die pressure. Note, however, for Figure 26
that the line starts to increase almost asymptotically at
about 2800 psi. his indicated that any increase in flow
rate would not increase the pressure in the die but would
only increase the line speed of the polymer extrudate coming
out of the die.
The die pressure can also be related to the core to
shell volume ratios. For example, run K operated at a die
pressure of 2750 psi and had a core to shell volume ratio of
192:l while run J operated at 2800 psi and had a core to
shell volume ratio of 384:l. However,run K had a higher
volumetric flow rate indicating that the slightly thicker
shell layer reduced the resistance to flow in the die and
allowed a higher line speed at a slightly lower die
pressure. Thus, the shell layer thickness had an effect on
line speed and die pressure during the coextrusion process.
In coextrusion operations, such as the melt
transformation coextrusion process, it was better to change
the die gap rather than increase the flow rate in an attempt
to increase the die pressure after the asymptotical pressure
value had been approached. An increase in flow rate only
served to increase the line speed of the process with little
effect on the die pressure and served to lessen the time the
melt spent in the melt conditioning pipe.
The results of the differential scanning calorimeter
(DSC) were used in Figures 27 -32. The elevation in melting
point temperature of the polypropylene core was shown as a
function of die pressure in Figures 27 and 28 for the two
sets of runs. Figure 27 showed a slight elevation in
melting temperature, probably, due to elongational flow of
the polymer melt, and the curve then leveled off when the
die pressure was between 1000 psi and 1500 psi.
However, Figure 28, indicated an elevation in melting
point temperature that began at a die pressure of 1500 psi,
and continued to increase up to the final die pressure of
2800 psi. Thus, the thermal enhanced properties of the
polypropylene core seemed to start around 1500 psi die
pressure and increased steadily. The elevation in melting
point temperature of the polypropylene core indicated that
the polymer molecules were more oriented in the coextruded
samples than in the bulk unoriented recrystallized state.
The thermal properties of the polyethylene shell layer
were also affected by the melt transformation coextrusion
process. Figures 29 and 30 show the depression in melting
point temperature of the shell layer as a function of die
pressure. The depression in shell melting temperature
increased in Figure 29, for runs A - D, over the pressure
range 600 - 1500 psi die pressure with the maximum
depression at 1500 psi pressure.
However, in Figure 30 (runs F - K) the depression
increased steadily upward to about 2500 psi die pressure.
Above that pressure there seemed to be a slight lowering of
the shell melting point temperature. This could have been
due to the fact that the shell material was starting to
become less disordered. The polyethylene shell was far 0
above its melting point (35 C above) in the die and
therefore probably would not form a liquid crystal melt
(37). The more probable explanation for this phenomenon was
that the data point at 2500 psi was in error and there was a
steady increase in the depression of the shell layer melting
temperature in this die pressure range.
The depression of the melting point was very unique
since this indicated that shell layer was more disordered in
the coextruded sample than in the bulk state. This was
probably caused by the high shearing rate in the shell layer
as it lubricated the solidifying core polymer leaving the
die.
The final results of the DSC scans were used in Figures
31 and 32. These contained the graphs of percent
crystallinity of the polypropylene core versus die pressure.
Both figures showed a slight upward trend in crystallinity
as the die pressure increased. This indicated the core
115
became more crystalline as the die pressure increased the
orientation of the molecules.
The results of the tensile tests were used in Figures
33 - 38 and in Table 5. Figures 33 and 34 contain the plots
of initial elastic modulus as a function of die pressure.
Figure 33 was for runs A - D, while the results of runs F -
G were used in Figure 34. The results showed an increase in
elastic modulus of the samples that were processed with
greater die pressures. This indicated that the samples
produced at the higher die pressures had more orientation
just as those produced by higher melt conditioning pressures
did in the melt transformation extrusion process. This
orientation of the molecules gave them a higher elastic
modulus because some of the molecules were extented so the
sample became stiffer, thus a higher elastic modulus was
realized ( 4 ) . Figures 35 and 36 were the yield strength versus die
pressure curves for the two data sets. Note that the yield
strength was greater for the first set of runs ( A - D) than
for the second runs (F - K ) . Since there were slight
variations in the sample thicknesses in the same run, the
thinner samples were affected more when subjected to the
tensile testing force. The melt conditioner's length was
also a factor and may have contributed to the lower yield
strengths of runs F - K .
Since the thickness of the coextruded samples were not
uniform, this indicated that the samples' cores were
swelling as they were being extruded, which might decrease
some of the orientation of the polymer molecules; that the
shell layers were swelling; or that the die lips were
separating. It was also possible that the cryst,alline
growth front may have approached the tip of the die (in runs
J,K,and L) since a slight decrease in transparency of the .
sample was detected as it exited the die. This indicated
that the heat transfer of the hot polymer to the cooler die
lips was not great enough at the higher line speeds to
completely solidify the core polymer in the die.
Both data sets showed an increasing value of yield
strength with an increase in die pressure. This indicated
that the more oriented polymer molecules, produced by higher
die pressures, resisted deformation to a greater extent.
The final two Figures (37 and 38) are the graphs
plotting the ultimate tensile strength versus the die
pressure. Once again the samples of the first set of data
(runs A - D) tested for tensile strength had higher values
than the second set (runs F - K). The important result here
was the increasing tensile strength with an increase in die
pressure. This also indicated an increase in orientation of
the molecules as the die pressure was increased. Since some
of the molecules were initially extented and there were more
tie molecules between the crystals, the ultimate tensile
117
strength was increased as more of the molecules were
oriented in the sample by the higher die pressures (4).
The final results of the tensile tests were used in
Table 5. These were the percent elongation at failure
compared to the core to shell volume ratio. This produced
some interesting results. The shell layer did not have to
be very thick to increase the elongation of the sample
dramatically.
Figure 44 was a scanning electron micrograph of a
tensile test sample from run J. The outside shell layer was
P peeled back in this picture while the failed core material
was behind this. The core failure seemed to exhibit a more
brittle type fracture than the shell layer which showed a
more elastic failure. The shell layer seemed to have
snapped away from the fractured edge after the core layer
failed.
The coextruded samples tested basically showed an
increased elastic modulus with a high percent elongation
before failure. The area under the stress-strain curve
indicated the high energy necessary to break the samples.
These were rigid tough samples with very thin shell layers
that had a greater elastic modulus and displayed
considerable elongation before failure.
It was the shell layers that gave the samples the
property of high elongation before failure. When the
samples were physically torn in the extrusion direction they
s e p a r a t e d f a i r l y r e a d i l y , y e t , i t c o u l d b e s e e n ( i n F i g u r e s
39 - 4 0 ) t h a t t h e s h e l l l a y e r was t h e b i n d i n g a g e n t t h a t
t e n d e d t o h o l d t h e c o r e m a t e r i a l t o g e t h e r . T h e s e p i c t u r e s
showed t h e t o r n s u r f a c e s which had l i t t l e p l a t e l i k e
p r o j e c t i o n s w h i c h a p p e a r e d on t h e s u r f a c e s o f t h e s e v e r e d
s a m p l e . T h i s i n d i c a t e d t h a t t h e s h e l l l a y e r was h e l p i n g t o
h o l d t h e s a m p l e t o g e t h e r .
When t h e s a m p l e s were s e v e r e d i n t h e e x t r u s i o n
d i r e c t i o n , i t was n o t i c e d t h a t t h e y t o r e i n s m a l l s e c t i o n s
f a i r l y e a s i l y and t h e n s t o p p e d u n t i l a d d i t i o n a l f o r c e was
a p p l i e d . T h i s c o u l d b e s e e n i n F i g u r e 39 w h e r e l i t t l e
p r o j e c t i o n s i n t h e s h e l l l a y e r were p e r i o d i c a l l y p r o t r u d i n g
from t h e s a m p l e 1 s s u r f a c e . Where t h e s e p r o j e c t i o n s
o c c u r r e d , more e n e r g y was r e q u i r e d t o t e a r t h e s a m p l e
l o n g i t u d i n a l l y .
F i g u r e 4 1 was a v e r y good i n d i c a t i o n o f j u s t how w e l l
t h e s h e l l l a y e r h o l d s t h e s a m p l e t o g e t h e r . The s a m p l e i n
F i g u r e 4 1 was a t h i n s l i c e c u t c r o s s s e c t i o n a l l y f rom r u n J .
The s a m p l e was t h e n b e n t u n t i l i t c r a c k e d . However, o n e
s h e l l l a y e r r e m a i n e d i n t a c t and h e l d t h e two p i - e c e s
t o g e t h e r . S i n c e t h e s h e l l l a y e r was s o t h i n t h e r e was n o t
much p o l y e t h y l e n e t o h o l d t h e s a m p l e t o g e t h e r . So i f a more
c r a c k r e s i s t a n t s a m p l e were r e q u i r e d , a t h i c k e r s h e l l l a y e r
would b e n e e d e d . T h u s , t h e c o e x t r u d e d s a m p l e s had less
t e n d e n c y t o c r a c k and t e a r which t e n d e d t o overcome t h e
core1 s brittle behavior.
The greater resistance to cracking and tearing seemed
to be characteristic of most of the samples tested
irregardless of the thickness of the shell layer. However,
there probably was a point where the shell layer could
become too thin and would reduce the elongational properties
of the sample.
The orientation of the polypropylene core is also
evident in Figures 43 and 45. These are scanning electron
micrographs of runs K and J respectively. Figure 43 was the
surface produced when the sample was cracked after being
cooled in liquid nitrogen. The extrusion direction was
illustrated horizontally in the picture. The upper
fractured surface showed a layered texture which was
indicative of highly oriented samples (1,41).
Figure 45 was a fractured surface produced by cracking
the sample that was cooled in liquid nitrogen. . However,
this picture is the end view of the sample with the
extrusion direction perpendicular to the picture's surface.
Here again a layered texture was observed. Only in this
case, the ends of the oriented layers were protruding
outward indicating a high degree of orientation in the
extrusion direction. Thus the scanning electron micrographs
indicated that the melt transformation coextrusion process
produced highly oriented samples.
The coextrusion process also tended to reduce the
surging die pressure noticed when melt transformation
extrusion process was used. The shell layer produced a more
constant flow rate from the die by lubricating the
solidifying core polymer as it was extruded from the die.
If the solidifying core polymer were to drag against the die
wall, greater friction would have occurred and would have
required a higher die pressure which in turn would have
produce a surging flow as observed in the melt
transformation extrusion process.
V. B. Limitations
The results of this investigation are limited to a
multimanifold die of the type used in this study. The two
most important limitations placed on the results are the
length of the melt conditioning pipe and the die pressure
used during the run. The length of the melt conditioner is
an important factor since the residence time the polymer
melt is affected by the pressure and temperature depends on
the volumetric flow rate and the length of pipe used. One
should be careful not to use data from these results without
taking these two factors into account.
These results are for the coextrusion of polyethylene
as the shell layer and polypropylene as the core. The
polyethylene shell layer was processed using a single screw
extruder while the polypropylene core polymer was processed
using a twin screw counterrotating extruder. Since shear
history can be important when dealing with polymers, the
processing equipment used during this investigation would
also be a limiting factor on the results (5).
Another restriction on these results is the core to
shell volume ratios used during this investigation. These
varied quite significantly during this investigation which
is cause for concern when comparing runs with different core
to shell volume ratios. In runs (A -D) the order of
magnitude of the core to shell volume ratio was the same.
However, runs (F - L) varied by two orders of magnitude, in an attempt to raise the die pressure.
Another limitation on the results is the take-up device
used during this investigation. This device kept the line
speed constant and provided tension on the extrudate when it
came out of the die. However, the effect of this tension on
the samples' quality and performance was not known. The
tension contributed to the fact that there was some
variation in the thickness of the sample in the same run.
Even though the line speed was constant, a change in
volumetric flow rate resulted in a-change in thickness or
width of the sample. The variations in width were not as
pronounced as the variations in thickness.
These results are also restricted by the temperature
profile of the various pipes leading to the die. This
profile was set arbitrarily after the test runs were made to
determine the best temperatures to use. Temperature
profiles used in the melt transformation extrusion process
were also considered when setting the final temperature
profile in the melt transformation coextrusion process.
VI . CONCLUSIONS
The conclusions listed below were drawn from the
results and observations recorded during this investigation.
A. The die reduction ratio, the die pressure, and the
melt conditioning pipe length were all important in
determining the sample's enhanced properties produced by
melt transformation coextrusion.
B. The length of the melt conditioning should be
longer when running melt transformation coextrusion compared
to melt transformation extrusion. Since the extrusion rate
was increased and the residence time in the melt conditioner
should remain constant, a longer melt conditioning pipe was
necessary when running at higher production rates.
C. The major pressure decrease in the melt
transformation extrusion process was in the melt
conditioning pipe between the core extruder and the die.
D. The melt conditioning pipe did not function as an
unsteady state cooling region, but rather provided residence
time necessary for the melt to be "conditioned" before it
was extruded through the die.
E. Shortening the melt conditioning pipe did not
increase the die pressure enough to overcome the reduction
in the sample's quality caused by the reduced time the
polymer melt spent in the melt conditioning pipe.
F. The shell layer acted as a lubricant in the die and
reduced the die pressure and increased the line speed during
the melt transformation coextrusion process.
G. Higher die pressures were obtained in the melt
transformation coextrusion process by either increasing the
volumetric flow rate or decreasing the die gap. However,
increasing the flow rate eventually lead to only an
increased line speed without much change in die pressure for
a given die gap.
H. Dies with high reduction ratios were used for melt
transformation coextrusion. The shell layer tended to
encapsulate and lubricate the solidifying core which reduced
the pressure necessary for the polymers to be extruded from
the die.
I. The shell layer was effective in reducing surface
cracks and tears in the core layer even at high core to
shell volume ratios.
J. Even though the thin shell layer provided only a
small percent of the total volume of the, coextruded sample,
it was effective in increasing the sample's elongation
before failure during the tensile testing. This indicated
that the shell layer did tend to encapsulate the core layer.
K. . Coextrusion increased the melt transformation
extrusion production rate dramatically. The samples
produced, when tested, showed an increase in the core's
thermal and mechanical properties as well as a more
crystalline oriented layered structure.
VII. RECOMMENDATIONS
The following suggestions are recommended for further
study.
A. Decrease the pressure drop in the melt conditioning
pipe between the core extruder and the die. This could be
done by increasing the diameter of the melt conditioning
pipe or using a less viscous polymer melt.
B. Increase the die pressure by either using smaller
inserts for the multimanifold die or by reducing the
pressure drop between the core extruder and the die.
C. Increase the flow resistance of the solidifying
core polymer after it has entered the land section of the
die.. This could be done either by using other shell layer
polymers with higher viscosities, or by a die with an
internally cooled -land section which is maintained sligthly
above the shell layer's melting point. Thus, the core layer
would solidify in this section but the shell layer would
only become more viscous and would increase the die
pressure. This cooling section might be several inches long
to accommodate the lesser temperature gradient imposed by
this method of cooling the die's land section.
127
D. Use a gear pump to increase the die pressure in an
attempt to produce polymer samples with greater enhanced
properties. A longer melt conditioning pipe may be. needed
to cool and condition the sheared and heated polymer melt as
it leaves the gear pump.
E. Use inserts having greater angles of convergence
and higher reduction ratios to obtain higher die pressures
and increased elongational flow velocities of the polymer
melt in the melt transformation coextrusion process.
F. Use a fiber die with a small die opening and higher
reduction ratio to further reduce the flow rate of the
polymer and increase the die pressure. This would also
increase the residence time the polymer spends in the melt
conditioning pipe since the volumetric flow rate would be
decreased.
G. Design coextrusion dies with small die gap openings
so as to have high reduction ratios that would increase the
die pressure, and with large cooling (land) sections to
accommodate the increased extrudate flow rate.
H. Studying the effects that aging have on the
coextruded polymer samples. Particularly since exposure to
certain chemicals, heat, and sunlight will damage some
polymers. In selecting shell layer polymers, their
resistance to environmental factors should be considered.
Since the shell layer would be capable of protecting the
high strength core in some applications.
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APPENDICES
Appendix A : Experimental Data
Table 6: Experimental Data
Multimanifold Die: Uniaxial, Coextrusion. Polypropylene in the Core, Polyethylene as the Skin Layer. Temperature profile of the Shell:180-165-165-160 (Degrees Celius)
RUN * Melt Conditioner Temperature
0
( C)
Draw Ratio
Die Temperature
0
* Run E was not included in this table since it was not related to this investigation.
Die Pressure
( C) (psi)
T a b l e 6 : C o n t i n u e d
* R e s u l t s o f Run F p r o d u c e d v e r y p o o r s a m p l e s w h i c h d i d n o t show a n y c o r e p o l y m e r i n t h e s a m p l e . T h u s , t h e r e s u l t s of t h e DSC a n d t e n s i l e tests a r e n o t shown i n t h i s t a b l e .
RUN
A
B
C
D
F*
G
H
I
J
K
L
L e n g t h o f t h e M e l t
C o n d i t i o n e r ( i n . )
3 5 . 5
3 5 . 5
3 5 . 5
3 5 . 5
2 1 . 5
2 1 . 5
2 1 . 5
2 1 . 5
2 1 . 5
2 1 . 5
1 2 . 0
C o r e E x t r u d e r P r e s s u r e
( p s i )
1000
1500
2000
2200
1 6 5 0
2250
2900
3700
4200
4500
4600
D i e Gap
( i n )
.03125
.03125
.03125
.03 1 2 5
.01563
.01563
.01563
.01563
.01563
.01563
.01563
Sample T h i c k n e s s
( i n . )
0 .0368
0 .0433
0 .0482
0.06 1 0
------
0.0138
0 .0302
0 .0262
0 .0230
0 .0243
0.0259
Sample Wid th
( i n . )
0 .938
0.950
1 .00
1 .00
----
0.875
0.938
1 .00
1 .00
1 .00
1 . 0 0
T a b l e 6 : C o n t i n u e d
R U N
A
B
C
D
F
G
H
I
J
K
L
V o l u m e t r i c F low R a t e
3 ( i n /min )
2 .35
3 .62
6 .44
9 .44
----
0 .27
1 . 3 5
3 .32
' 1 2 . 3
8 .84
8 .96
C o r e : S h e l l Volume
R a t i o
2 2 : l
18 : 1
3 8 : l
2 6 : l
----
3 : l
1 5 : 1
126 : 1
1 9 3 : l
384 : 1
128 : 1
Change i n M.P. C o r e
o ( C)
+ 0 . 9 5 - + . I 3
+2 .53 +.50 - +2 .50 - +.50
+2.67 - +.58
----------
+ 3 . 8 3 - +.29
+4.67 - + 2 . 1
- +4.00 +.50
+4 .73 - + . 2 5
+5.00 - +.30
- + 3 . 8 3 + .29
Change i n M.P. S h e l l
o ( C)
-1.40 - +.29
-1 .33 - +.29
-1 .83 - +.29
-1 .93 - + . I 2
----------
-1 .77 - + . 7 5
-2.25 - + 1 . 1
- -2 .75 + . 3 5
-2 .33 - +.29
-2 .30 - +.42
- -2 .17 + .58
T a b l e 6 : C o n t i n u e d
R U N
A
B
C
D
F
G
H
I
J
K
L
Modulus - 5
x 1 0 ( p s i )
0 . 9 5 - + .13
1 . 2 5 - +. 16
1 . 2 9 - +.08
1 . 4 6 - +.18
---------
1 . 2 4 - + .08
1 .30 - +.04
1 . 3 8 - + .03
1 . 4 2 - +.04
1 . 4 5 - +.07
1 . 2 8 - +. 28
Y i e l d S t r e n g t h
( p s i )
3950 - +100
4680 - +850
4740 - +350
5150 - +680
---------
3750 - +260
3610 - +310
4160 - + I 9 0
4310 - + 80
4290 - +390
3290 - +280
U l t i m a t e T e n s i l e S t r e n g t h
( p s i )
4060 - + 270
4420 - + 440
5110 - + 50
4870 - + I 2 1 0
----------
2800 - + 260
3510 - + 370
3920 - + 660
4240 - + 820
3460 - + 800
3660 - + 260
P e r c e n t C r y s t a l l i n i t y
o f t h e C o r e ( 0 5
3 0 . 6 - + 0 . 4
3 2 . 3 - + 3 . 5
3 3 . 0 - + 1 . 6
3 2 . 1 - + 0 . 6
-------------
3 4 . 8 - + 5 . 7
3 2 . 4 - + 2 . 5
3 2 . 0 - + 3 . 2
3 3 . 8 - + 3 . 4
3 2 . 3 - + 1 . 6
3 2 . 5 - + . 83
T a b l e 6 : C o n t i n u e d
L i n e Speed
( f t ./rnin)
5.67
7 .33
11 .13
12 .90
0.92
1 . 8 8
3 .97
10 .56
44.58
30 .33
29.67
RUN
A
B
C
D
F
G
H
I
J
K
L
P e r c e n t longa at ion a t F a i l u r e
( % I
850-870
720-870
860-870
2-900
-------
220-400
11-460
720-760
450-930
30-910
860-990
Appendix B : Percent Crystallinity
The percent crystallinity of the polyproylene core was
calculated from the differential scanning calorimeter (DSC)
results. The sample was obtained by using a leather punch
to cut it out. It was then weighted to the nearest tenth of
a milligram and sealed in the sample pan. The pan was then
placed in the DSC and two scans of the sample were made.
The results of these scans are represented in Figure
47. This represents a typical DSC scan of a sample. The
change in melting point temperature is shown by the
differences in the two peaks. The percent crystallinity was
' calculated using the areas under the curves. The area under
the curve was determined using an Apple computer with a
graphics board. A 2.5 mg sample of ~ndium was scanned and
the area under its curve was determined. The heat latent of
fusion of Indium is 6.2 cal/gram. Thus, the calories per
unit area on the graph was determined.
Several samples of polypropylene were scanned to
determine the average latent heat of fusion of the stock
polymer. Using this average the stock polypropylene had a
latent heat of fusion of 19.2 cal/gram. The accepted latent
heat of fusion of 100 percent crystalline polypropylene is
62.6 callgram (89) . Thus the stock polypropylene was 30.7
Fig
ur
e 4
6 : T
yp
ic
al
D
SC
S
ca
n
I PO
LYPR
OPY
LEN
E IN
ITIA
L M
ELT
ING
PO
INT
[O
RIE
NT
ED
POLY
PRO
PYLE
NE
SEC
ON
D M
EL
TIN
G P
OIN
T [
RE
CR
YS
TA
LL
IZE
D1
PO
LYE
THY
LEN
E I
NIT
IAL
ME
LTIN
G P
OI
PO
LYE
THY
LEN
E S
ECO
ND
ME
LTIN
G P
OIN
T
TEM
PE
RA
TUR
E
t " K
I
crystalline.
The area under the polypropylene curve on the graph
from the second (recrystallized) scan enabled the mass .of
the coextruded pol.ypropylene to be determined. Using the
area under the curve for the first scan and dividing by the
value for a pure crystalline structure, a percent
crystallinity was determined for the oriented sample. The
percent crystallinity listed in the experimental data
section (Table 6) is the result of the above calculations.
Appendix C : Material Properties
Table 7 : Physical Properties - Polypropylene
* - The average Melting Point of the Polypropylene in these 0
experiments was 164 C.
Polypropylene was Marlex HGZ-050-02 manufactured by the Phillips 66 Chemical Company. Lot No. 2011454.
Value
56.5
5000
225,000
17 0
Unit
lb. cu. ft.
psi
psi
Deg. C
Property
Density
Tensile Strength at
Yield (2"/min)
Flexural Modulus
* Melting Point
ASTM
D792
D638 TYPe I Spec.
D790
D2117
Table 8 : Physical Properties - Polyethylene
The average melting point of the polyethylene was 130
deg . C.
Polyethylene was ~lathorfS type I11 manufactured by the E.I. Dupont de Nemours Inc.
Property
Density
Tensile Strength at 73 deg. F
Flexural Modulus
Stiffness at 73 deg. F
Elongation at 73 deg. F
ASTM
D 79 2
D412
D790
D 74 7
D638
Unit
lb. cu. ft.
psi
psi
psi
%
Value
58.7-60.2
3300
120,000
90,000
600 - 1000