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Plastics and Properties Important in Extrusion

Chapter 4

Professor Joe Greene

CSU, CHICO

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Chapter 4 Objectives

• Topics– Main types of plastics– Flow properties– Thermal properties– Help

• Select appropriate machines for extrusion

• Set proper processing conditions

• Analyze extrusion probelms

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Polymer Chains

• Average Molecular Weight– Polymers are made up of many molecular weights or a

distribution of chain lengths.• The polymer is comprised of a bag of worms of the same

repeating unit, ethylene (C2H4) with different lengths; some longer than others.

• Example, – Polyethylene -(C2H4)-1000 has some chains (worms) with 1001 repeating

ethylene units, some with 1010 ethylene units, some with 999 repeating units, and some with 990 repeating units.

– The average number of repeating units or chain length is 1000 repeating ethylene units for a molecular weight of 28*1000 or 28,000 g/mole .

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Main Type of Plastics

• Polymers are carbon-based materials made up of very long molecules

• Polymers– Thermoplastic: Melt and flow upon heating

• Can be reheated and flow again

• When cooled behaves as a solid

• Very suitable for recycling

– Thermoset: React and cross-link (set-up) upon heating • Can be heated only once.

• Material is not easily recycled

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Amorphous and Crystalline Plastics

• Thermoplastics are further classified based upon molecular arrangement of polymer chains– Amorphous: (without shape)

• Polymer chains are random arrangement

– Crystalline• Polymer chains form regular pattern

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• Amorphous- Molecular structure is incapable of forming regular order (crystallizing) with molecules or portions of molecules regularly stacked in crystal-like fashion.

• A - morphous (with-out shape)

• Molecular arrangement is randomly twisted, kinked, and coiled

States of Thermoplastic Polymers

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• Crystalline- Molecular structure forms regular order (crystals) with molecules or portions of molecules regularly stacked in crystal-like fashion.

• Very high crystallinity is rarely achieved in bulk polymers

• Most crystalline polymers are semi-crystalline because regions are crystalline and regions are amorphous

• Molecular arrangement is arranged in a ordered state

States of Thermoplastic Polymers

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Factors Affecting Crystallinity

• Cooling Rate from mold temperatures

• Barrel temperatures

• Injection Pressures

• Drawing rate and fiber spinning: Manufacturing of thermoplastic fibers causes Crystallinity

• Application of tensile stress for crystallization of rubber

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Types of Polymers• Amorphous and Semi-Crystalline Materials

• LDPE Crystalline• HDPE Crystalline • PP Crystalline • PET Crystalline • PBT Crystalline• Polyamides Crystalline• PMO Crystalline• PEEK Crystalline• PPS Crystalline • PTFE Crystalline• LCP (Kevlar) Crystalline

• PVC Amorphous • PS Amorphous • Acrylics Amorphous• ABS Amorphous• Polycarbonate Amorphous • PhenoxyAmorphous• PPO Amorphous • SAN Amorphous • Polyacrylates Amorphous

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Liquid Crystalline Plastics (LCPs)

• The molecules of LCPs are rod-like structures organized in large parallel domains, not only in the solid state but also in the melt state.

Mechanical PropertiesPEEK LCP Polyester Nylon 6,6

Density, g/cc 1.30-1.32 1.35 - 1.40 1.13-1.15

Tensile Strength,psi

10,000 – 15,000 16,000 – 27,000 14,000

Tensile Modulus,psi

500K 1,400K - 2,800K 230K – 550K

TensileElongation, %

30% - 150% 1.3%-4.5% 15%-80%

Impact Strengthft-lb/in

0.6 – 2.2 2.4 - 10 0.55 – 1.0

Hardness R120 R124 R120

CLTE10-6 mm/mm/C

40 - 47 25-30 80

HDT 264 psi 320 F 356F -671F 180F

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Elastomers• Elastomers are materials capable of large elastic

deformations with elastic elongation > 200%– Conventional: vulcanizable

• polyisoprene, polybutadiene, polychloroprene, polyisobutylene

– Thermoset elastomers: cross-linking reaction• polyurethane, silicone

– Thermoplastic elastomers: physical linking• olefinic, TPO• urethane, TPU• etherester, TPE• copolyester, TPE• styrenic, TPR

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Flow Behavior of Plastic Melts• Viscosity

– Defined as the material’s resistance to flow

– Most important property of plastics for processing

– Low viscosity materials flow easily: e.g. water, syrup, olive oil

– High viscosity materials flow very slowly when heated: most plastics, e.g., LDPE, HDPE, PP, PS, PU, Nylon, PET, PBT, etc.

– Units are Pascal-seconds (Metric= N/m2-sec), Poise (English=lb/ft2-sec)

– Viscosity can be reduce by • flowing faster (increasing shear rate)

• increasing temperatureMaterial Viscosity (Pascal-second) Viscosity (Poise)Air 0.00001 0.0001Water 0.001 0.01Olive Oil 0.1 1Plastic melts 100 to 1,000,000 10 to 100,000Pitch 1,000,000,000 100,000,000

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Melt Index• Melt index test

– Measures the flow of a material at a temperature and under a load or weight.

– Procedure (ASTM D 1238)• Set the temperature per the material type.• Add plastic pellets to chamber. Pack with rod.• Place mass (5Kg) on top of rod.• Wait for the flow to stabilize and flow at constant

rate.• Start stop watch• Measure the flow in a 10 minute interval• Repeat as necessary

Mass

Temp

Plastic

PlasticResin

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Melt Index and Viscosity• Melt index for common materials

Material Temp Mass• Polyethylene 190°C 10 kg

• Nylon 235°C 1 kg

• Polystyrene 200°C 5 kg

• Melt Index is indication of Viscosity

• Viscosity is resistance to flow

• Melt index flow properties– High melt index = high flow = low viscosity– Low melt index = low flow = high viscosity

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Melt Index and Molecular Weight • Melt Index is indication of length of polymer chains• Molecular Weight is a measurement of the length of polymer chains• Melt index MW properties

– High melt index = high flow = short chains

– Low melt index = low flow = long chains

• Table 3.1 Melt Index and Molecular Weight of PS Mn Melt Index* (g/10min)

• 100,000 10.00

• 150,000 0.30

• 250,000 0.05

* T=200°C with mass =5 kg

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Stresses, Pressure, Velocity, and Basic Laws

• Stresses: force per unit area– Normal Stress: Acts perpendicularly to the surface: F/A

• Extension

• Compression

– Shear Stress, : Acts tangentially to the surface: F/A• Very important when studying viscous fluids

• For a given rate of deformation, measured by the time derivative d /dt of a small angle of deformation , the shear stress is directly proportional to the viscosity of the fluid

FCross SectionalArea A

A

FA

F

Deformed Shape

F

= µd /dt

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Some Greek Letters• Alpha: • beta: • gamma: • delta: • epsilon: • zeta: • eta: • theta: • iota: • kappa: • lamda: • mu:

• Nu: • xi: • omicron:

• pi: • rho: • sigma: • tau: • upsilon: • phi:• chi: • psi: • omega:

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Effect of Shearing• Shear flows are present in plastic processing

– In shear flow (tangential flow), layers of the plastic move at different velocities.

– Rate of shearing is called the shear rate• shear rate = velocity/thickness

• Thin gaps = high shear rates

• High flow rates = high shear rates

Fluid

Wall

Wall

Before: Wall at Rest

Fluid

Wall

Wall

After: Top Wall Set in motion induces shear stress

F

Velocity, v

shear rate = v/H

H

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Viscosity• Viscosity is defined as a fluid’s resistance to flow under an applied

shear stress, Fig 2.2

• The fluid is ideally confined in a small gap of thickness h between one plate that is stationary and another that is moving at a velocity, V

• Velocity is u = (y/h)V• Shear stress is tangential Force per unit area,

= F/A

Stationary, u=0

Moving, u=V V

xy

Y= 0

Y= hP

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Viscosity• For Newtonian fluids, Shear stress is proportional to velocity

gradient.

• The proportional constant, , is called viscosity of the fluid and has dimensions

• Viscosity has units of Pa-s or poise (lbm/ft hr) or cP

• Viscosity of a fluid may be determined by observing the pressure drop of a fluid when it flows at a known rate in a tube.

dy

duyx

LT

M

Ln shear rate,

Ln

0.01 0.1 1 10 100

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Viscosity• For non-Newtonian fluids (plastics), Shear stress is proportional to

velocity gradient and the viscosity function.

• Viscosity has units of Pa-s or poise (lbm/ft hr) or cP

• Viscosity of a fluid may be determined by observing the pressure drop of a fluid when it flows at a known rate in a tube. Measured in– Cone-and-plate viscometer

– Capillary viscometer

– Brookfield viscometer

dy

duyx

Ln shear rate,

Ln

0.01 0.1 1 10 100

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Viscosity• Kinematic viscosity, , is the ratio of viscosity and density• Viscosities of many liquids vary exponentially with

temperature and are independent of pressure• where, T is absolute T, a and b

• units are in centipoise, cP

Tbae ln

Ln shear rate,

Ln

0.01 0.1 1 10 100

T=400

T=300

T=200

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Viscosity Models• Models are needed to predict the viscosity over a

range of shear rates.

• Power Law Models (Moldflow First order)

• Moldflow second order model

• Moldflow matrix data

• Ellis model

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Viscosity Models• Models are needed to predict the viscosity over a range

of shear rates.

• Power Law Models (Moldflow First order) where m and n are constants.

If m = , and n = 1, for a Newtonian fluid,

you get the Newtonian viscosity, .

• For polymer melts n is between 0 and 1 and is the slope of the viscosity shear rate curve.

• To find constants, take logarithms of both sides, and find slope and intercept of line

1 nm

mn lnln1ln

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Shear Thinning or Pseudoplastic Behavior• Viscosity changes when the shear rate changes

– Higher shear rates = lower viscosity– Results in shear thinning behavior– Behavior results from polymers made up of long entangles chains. The

degree of entanglement determines the viscosity– High shear rates reduce the number of entanglements and reduce the

viscosity.– Power Law fluid: viscosity is a straight line in log-log scale.

• Consistency index: viscosity at shear rate = 1.0• Power law index, n: slope of log viscosity and log shear rate

– Newtonian fluid (water) has constant viscosity• Consistency index = 1• Power law index, n =0

Logviscosity

Log shear rate

Power law approximation

Actual

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Effect of Temperature on Viscosity• When temperature increases = viscosity reduces

• Temperature varies from one plastic to another– Amorphous plastics melt easier with temperature.

• Temperature coefficient ranges from 5 to 20%,

• Viscosity changes 5 to 20% for each degree C change in Temp

• Barrel changes in Temperature has larger effects

– Semicrystalline plastics melts slower due to molecular structure

• Temperature coefficient ranges from 2 to 3%

Viscosity

Temperature

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Viscous Heat Generation• When a plastic is sheared, heat is generated.

– Amount of viscous heat generation is determined by product of viscosity and shear rate squared.

– Higher the viscosity = higher viscous heat generation– Higher the shear rate = higher viscous heat generation– Shear rate is a stronger source of heat generation– Care should be taken for most plastics not to heat the

barrel too hot due to viscous heat generation

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Thermal Properties• Important is determining how a plastic behaves in an

extruder. Allows for – selection of appropriate machine selection– setting correct process conditions– analysis of process problems

• Important thermal properties– thermal conductivity– specific heat– thermal stability and induction time– Density– Melting point and glass transition

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Thermal Conductivity• Most important thermal property

– Ability of material to conduct heat– Plastics have low thermal conductivity = insulators– Thermal conductivity determines how fast a plastic can be

processed.– Non-uniform plastic temperatures are likely to occur.– Long times are needed to equalize temperatures

• Channel is 20 mm in diameter, it may take 5 to 10 minutes for temperatures to equalize

• Typical residence is 30 seconds.• Results in high temperature melt stream persists all through the die and

causes non-uniform flow at the die exit and a local thick spot in extruded product.

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Specific Heat and Enthalpy• Specific Heat

– The amount of heat necessary to increase the temperature of a material by one degree.

– Most cases, the specific heat of semi-crystalline plastics are higher than amorphous plastics.

– The amount of heat necessary to raise the temperature of a material from a base temperature to a higher temperature is determined by the enthalpy differences between two temperatures.

• If you know the starting temperature (room T) and the ending temperature (die exit) then we can determine the energy required to heat plastic material.

• Enthalpy to heat of PVC from Room T to 175C is 150 kW.hr/kg or for 100 kg/hr (220lbs/hr) the minimum power is 5 kW (6.7 HP)

• LDPE is much higher enthalpy than PVC, or it takes more energy to heat up and cool down than PVC

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Specific Heat and Enthalpy

• Specific Heat– The amount of heat necessary to increase the temperature of a material

by one degree.– Most cases, the specific heat of semi-crystalline plastics are higher than

amorphous plastics.– If an amount of heat is added Q, to bring about an increase in

temperature, T. – Determines the amount of heat required to melt a material and thus the

amount that has to be removed during injection molding.

• The specific heat capacity is the heat capacity per unit mass of material.– Measured under constant pressure, Cp, or constant volume, Cv.– Cp is more common due to high pressures under Cv

VV

PP dT

dQC

dT

dQC

;

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Specific Heat and Enthalpy• Specific Heat Capacity

– Heat capacity per unit mass of material

– Cp is more common than Cv due to excessive pressures for Cv

– Specific Heat of plastics is higher than that of metals

– TableMaterial Specific Heat Capacity

(J/(kgK))ABS 1250-1700Acetal 1500PA66 1700PC 1300Polyethylene 2300PP 1900PS 1300PVC 800-1200Steel (AISI1020)

460

Steel (AISIP20)

460

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Thermal Stability and Induction Time• Plastics degrade in plastic processing.

– Variables are:• temperature

• length of time plastic is exposed to heat (residence time)

– Plastics degrade when exposed to high temperatures• high temperature = more degradation

• degradation results in loss of mechanical and optical properties

• oxygen presence can cause further degradation

– Induction time is a measure of thermal stability.• Time at elevated temperature that a plastic can survive without measurable

degradation.

• Longer induction time = better thermal stability

• Measured with TGA (thermogravimetric analyzer), TMA

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Thermal Conductivity• Most important thermal property

– Ability of material to conduct heat– Plastics have low thermal conductivity = insulators– Thermal conductivity determines how fast a plastic can be

processed.– Non-uniform plastic temperatures are likely to occur.

• Where, k is the thermal conductivity of a material at temperature T.

• K is a function of temperature, degree of crystallinity, and level of orientation

– Amorphous materials have k values from 0.13 to 0.26 J/(msK)– Semi-crystalline can have higher values

dx

dTkA

dt

dQ

QT+TT

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Thermal Stability and Induction Time• Plastics degrade in plastic processing.

– Variables are:• temperature

• length of time plastic is exposed to heat (residence time)

– Plastics degrade when exposed to high temperatures• high temperature = more degradation

• degradation results in loss of mechanical and optical properties

• oxygen presence can cause further degradation

– Induction time is a measure of thermal stability.• Time at elevated temperature that a plastic can survive without measurable

degradation.

• Longer induction time = better thermal stabilty

• Measured with TGA (thermogravimetric analyzer), TMA

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Thermal Stability and Induction Time• Plastics degrade in plastic processing.

– Induction time measured at several temperatures, it can be plotted against temperature. Fig 4.13

• The induction time decreases exponentially with temperature

• The induction time for HDPE is much longer than EAA

– Thermal stability can be improved by adding stabilizers• All plastics, especially PVC which could be otherwise made.

HDPE

EAAInductionTime(min)

Temperature (degrees C)

Reciprocal Temp (K-1).1

1

10. 260 240 220 200

.0018 .0020 .0022

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Density• Density is mass divided by the volume (g/cc or lb/ft3)• Density of most plastics are from 0.9 g/cc to 1.4 g/cc_ • Table 4.2• Specific volume is volume per unit mass or (density)-1

• Density or specific volume is affected by temperature and pressure.– The mobility of the plastic molecules increases with higher temperatures

(Fig 4.14) for HDPE. PVT diagram very important!!– Specific volume increases with increasing temperature– Specific volume decrease with increasing pressure.– Specific volume increases rapidly as plastic approaches the melt T.– At melting point the slope changes abruptly and the volume increases more

slowly.

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Melting Point• Melting point is the temperature at which the crystallites melt.

– Amorphous plastics do not have crystallites and thus do not have a melting point.

– Semi-crystalline plastics have a melting point and are processed 50 C above their melting points. Table 4.3

• Glass Transition Point– Point between the glassy state (hard) of plastics and the rubbery state

(soft and ductile).• When the Tg is above room temperature the plastic is hard and brittle at

room temperature, e.g., PS• When the Tg is below room temperature, the plastic is soft and flexible at

room temperature, e.g., HDPE

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Thermodynamic Relationships

• Expansivity and Compressibility– Equation of state relates the three important process

variables, PVT• Pressure, Temperature, and Specific Volume.

• A Change in one variable affects the other two

• Given any two variables, the third can be determined

– where g is some function determined experimentally.

• Reference: MFGT242 Polymer Flow Analysis Book

0,ˆ, TVpf

TpfV ,ˆ

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Thermodynamic Relationships• Coefficient of volume expansion of material, , is defined as:

• where the partial differential expression is the instantaneous change in volume with a change in Temperature at constant pressure

• Expansivity of the material with units K-1

• Isothermal Compressibility, , is defined as:

• where the partial differential expression is the instantaneous change in volume with a change in pressure at constant temperature

• negative sign indicated that the volume decreases with increasing pressure• isothermal compressibility has units m2/N

pT

V

V

ˆ1

Tp

V

V

ˆ

ˆ1

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PVT Data for Flow Analysis• PVT data is essential for

– packing phase and the filling phase.– Warpage and shrinkage calculations

• Data is obtained experimentally and curve fit to get regression parameters

• For semi-crystalline materials the data falls into three area;– Low temperature– Transition– High temperature

Temperature, C100 200

SpecificVolume,cm3/g

1.04

1.20

1.40Polypropylene

Pressure, MPa020

60100

160

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PVT Data for Flow Analysis• Data is obtained experimentally and curve fit to get

regression parameters

• For amorphous there is not a sudden transition region from melt to solid. There are three general regions– Low temperature– Transition – High temperature

Temperature, C

100 200

SpecificVolume,cm3/g

1.04

1.20

1.40Polystyrene

Pressure, MPa020

60100

160

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PVT Data for Flow Analysis• The equations fitted to experimental data in previous

PVT Figures 2.11 and 2.12 are: – Note: All coefficients are found with regression analysis– Low Temperature region

– High Temperature Region

– Transition Region

paTaeapa

Ta

pa

aV 76

53

2

4

1ˆ

pa

Ta

pa

aV

3

2

4

1ˆ

Tbbp 21