siurvey of industrial chemestry - chapter 17
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Chapter 17
Fibers
1.
HISTORY, ECONO M ICS,
AND
TYPES
OF
FIBERS
Fibers have been used for thousands of years to make various textiles.
For centuries certain natural products have been known to make excellent
fibers.
Probably
the first
synthetic
fiber
experiment came with
the
work
of
Christian Schonbein,
who
made cellulose trinitrate
in 1846. After
breaking
a
flask of
nitric
and
sulfuric acids,
he
wiped
up the
mess with
a
cotton apron
and
hung
it in
front
of the
stove
to
dry. Cellulose trinitrate
was
developed
by
Hilaire
de
Chardonnet
as a
substitute
for
silk
in
1891,
but it was
very
flammable and was soon nicknam ed mother-in-law silk," being an
appropriate gift
for
only disliked persons. W hen rayon came along
in
1892,
Chardonnet silk" was soon forgotten. Then the entirely synthetic fibers
came, with the pioneering work of W. Carothers at Du Pont synthesizing the
nylons in 1929-30. Com mercialization occurred in 1938. Polyesters were
made by
Whinfield
and
Dixon
in the
U.K.
in
1941. They were
commercialized in 1950.
It is important to understand the different types of fibers. Classes are best
differentiated
based
on
both
the
origin
of the fiber and its
structure.
The
structure
and
chemistry
of
many
of
these polymers
was
discussed earlier
in
Chapter
14.
Table
17.1
contains
a
list
of the
three important types
of
fibers—natural, cellulosic,
and noncellulosic—as
well
as a
list
of
specific
polymers
as
examples
of
each type.
The
ones marked with
an
asterisk
are
the most important.
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Table 17.1 Types of fibers
Natural
From plant sources—all
are
c ellulose polymers
a. *
Co tton
(from
the cotton plant)
b. Flax (from
blueflowers)—used
to m ake linen
c. Jute (from
an
East
Indian plant)—used
fo r
burlap
and
twine
2. From animal sources—all are proteins
a.
Silk (from the silkworm)—mostly glycine and alanine
b.
*Wool (from sheep)—complex m ixture
of
amino acids
c.
Mohair
(from
the Angora
goat)
3.
From inorgan ic sources
a.
Asbestos—mostly calcium and magnesium silicates
b. Glass—silicon
dioxide
Cellulosic
These
fibers are
also called semisynthetic since
the
natural cellulose
is
modified
in
some way chemically.
1.
*Rayon (regenerated cellulose)
2. *C ellulo se acetate
3. Ce llulose triacetate
Noncel lu los i c
These are entirely synthetic, made from polym erization of small mo lecules.
1.
*N ylons 6 and 6,6
2.
*Polyester— poly(ethylene terephthalate)
mostly
3. *Polyolefms— polypropylene mostly
4. Acrylic—polyacrylonitrile
5. Polyurethane
Referring back to Fig. 16.1, we see that the value of U.S. shipments for
cellulosic and
noncellulosic
fibers,
though quite small compared
to
plastics,
is still a big industry. W hile Plastics M aterials and Resins (NA ICS 3 25211 )
in 1998
was
$44.9 billion, N oncellulosic Fibers (NA ICS 325222)
was
$10.5
billion and Cellulosic Fibers (NAICS 325221) was $1.5 billion. These tw o
fibers together have a $12.0 billion value, which is 3% of Chemical
Manufacturing.
W e must also remember that many of these fibers are sold
outside the chemical industry, such as in Textile Mill Products, Apparel, and
Furniture,
all
large segments
of the
economy.
The
importance
of
fibers
is
obvious.
In
1920
U.S. per
capita
use was
30 Ib/yr, whereas
in
1990
it was 66
Ib/yr. From 1920 to 1970 the most important fiber by far was
cotton.
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Year
Figure 17.1 U.S. production of fibers. Source: Chemical and Engineering News,
Chemical and
Economics
Handbook,
and the
U.S. Department
of
Agriculture/Foreign
Agriculture Service)
However, synthetic fibers (cellulosic and noncellulosic) increased much
m ore rapidly in importance, w ith cellulosics boom ing between W orld W ars I
and
II and noncellulosics dominating after World War II, w hi le all that time
cotton showed only a steady pace in com parison. The m ore recent
competition between the various fibers in the United States is given in Fig.
17.1.
N ylon was originally the most im portant synthetic (195 0-197 1) but
polyester now leads the market (197 -present). For a few years (1970-1980)
acrylics were third in production, but since 1980 polyolefms have been
rapidly increasing. Po lyolefm s are now second o nly to polyester in synthetic
fiber prod uction. Cotton, being an agricultural crop, certainly dem onstrates
its
variable production w ith factors such as weather and the econom y. It is
an
up-and-do wn industry m uch m ore so than the synthetics.
The student should also review Chap ter 1, Table
1.16,
where the top
polym er production is given numerically. Overall a 1.8% per year growth
was recorded for 1990-2000 in synthetic fibers. A net decrease in the
cellulosics
of 3.6% per year shows their dim inishing importance. A crylics
also decreased 3.9% ann ually in this period. The rising star is po lyole fm s,
which
increased 5.8% per year in the past decade.
otton
Polyester
Polyolefms
Nylon
crylics
i
o
o
o
u
n
d
s
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U.S. price trends show that cotton and polyester are the most popular for
good
reason: they
are the
cheapest. N ylon
and
acrylic have
had
price
increases
over the last few years, part of which may be due to improvements
and
safeguards needed to manufacture their precursors acrylonitrile,
butadiene, and
benzene, which
are on
carcinogen lists.
2. PROPERTIES
OF
FIBERS
It
is important to understand some common terms used in this industry
before
studying
fiber
properties
or
individual fibers.
The term fiber
refers
to
a one-dimensional structure, something that is very long and thin, with a
length at least 100 x its diameter. Fibers can be either staple fibers or
monofilaments.
Staple fibers are bundles of parallel short fibers.
Monofilaments are extruded long lengths of synthetic fibers. Monofilaments
can be used as sing le, large diameter fibers (such as in fishing line) or can be
bundled
and
twisted
and
used
in
applications similar
to
staple fibers.
Fabrics are
two-dimensional materials made from
fibers.
Their primary
purpose is to
cover things
and
they
are
commonly used
in
clothes,
carpets,
curtains,
and upholstery. The motive for covering may be aesthetic, therm al,
or acoustic. Fabrics are made out of yarns or twisted bundles of fibers. The
spinning
of
yarns can occur in two ways: staple
fibers
can be twisted into a
thread
( spun yarn )
or
monofilaments
can be
twisted into
a
similar usable
thread
( filament yarn" or continuous filament yarn"). A ll these definitions
are important in order to understand the conversation of the fiber industry.
The tensile properties of fibers are not usually expressed in terms of
tensile
strength (lb/in
2
or kg/cm
2
). The strength of a fiber is more
often
denoted
by
tenacity.
Tenacity (or
breaking tenacity)
is the
breaking strength
of a fiber or yarn in force per unit denier (Ib/denier or
g/denier).
A denier is
the weight in grams of 9,000 m of fiber at 7O
0
F and 65% relative humidity.
A denier
defines the
linear density
of a fiber
since
it
depends
on the
density
and the diameter of a fiber. To give you an idea of common values of
deniers for fibers, most commercially useful fibers are
1-15
den ier, yarns are
15-1600 den ier, and monofilament (when used singly) can be anywhere from
15 denier on up. Table 17.2 lists common values of tenacity for various
fibers and
compares these values
to
tensile strength. Tenacities
can be
converted into tensile strength
in
pounds
per
inch square
by the
following
formula:
tensile strength (lb/in
2
)
=
tenacity
(g/denier) x
density (g/cm
2
)
x 12,791
Note that tenacity values fo r m ost fibers range from
1-8
g/denier.
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Ta ble 17.2 Ph ysic al
Properties of
Typical Fibers
Polymer
Cellulose
Cotton
Rayon
High-tenacity
rayon
Cellulose
diacetate
Cellulose
triacetate
Proteins
Silk
Wool
Nylon 6,6
Polyester
Polyacrylonitrile
(acrylic)
Polyurethane (Spandex)
Polypropylene (polyolefm)
Asbestos
Glass
Tenacity
(g/denier)
2.1-6.3
1.5-2.4
3.0-5.0
1.1-1.4
1.2-1.4
2.8-5.2
1.0-1.7
4.5-6.0
4.4-5.0
2.3-2.6
0.7
7.0
1.3
7.7
Tensile Elongation
Strength
(%)
(kg/cm
2
)
3000-9000
2000-3000
5000-6000
1000-1500
1000-1500
3000-6000
1000-2000
4000-6000
5000-6000
2000-3000
630
5600
2100
2100
3-10
15-30
9-20
25-45
25-40
13-31
20-50
26
19-23
20-28
575
25
25
3
Source:
Seymour/Carraher
In
general, you should recall
that
the tensile strength and modulus of
fibers
must be much higher than that fo r plastics and their elongation must
be m uch lower. Synthetic fibers are usually stretched and oriented
uniaxially to increase their degree of parallel chains and increase their
strength and modulus.
What makes a polym er a good fiber? This is not an easy question to
answer. If one fact can be singled out as be ing important, it would be that all
fibrous polymers must have strong intermolecular forces of one type or
another. Usu ally this means hydrogen bonding or dipole-dipole interactions,
shown
on the
following page
for
various types
of fibers.
Besides the high tenacity, a number of other properties are considered
necessary for most fiber applications. A lthough no one polymer is superior
in all of these categories, the list in Table 17.3 represents ideals for polymers
being screened as fibers.
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Table 17.3 Ideal Properties of Polymers for Fiber App lications
1. High tensile strength, tenacity, and modulus
2. Low elongation
3. Proper
T
g
and
T
m
.
A low
T
8
aids
in
easy orientation
of the fiber. The
T
m
should
be
above 20O
0
C
to
accept ironing
(as a
textile)
but
below 3 0O
0
C
to be
spinnable.
4.
Stable
to
chemicals, sunlight,
and
heat
5. Nonflammable
6.
Dyeable
7.
Resilient (elastic) with
a
high
flex
life
(flexible lifetime)
8. No static electrical build-up
9.
Hydrophilic (adsorbs water
and
sweat easily)
10. Warm
or
cool
to the
touch
as
desired
11. Good "drape" (flexible
on the
body)
and
"hand" (feel
on the
body)
12.
No
shrinking
or
creasing except where intentional
13. Resistance
to
wear after repeated washing
and
ironing
hydrogen bonding:
dipole-dipole:
cellulosics proteins,
nylons
polyesters acrylics
3.
IMPORTANT
FIBER S
A
study of the fiber industry is not complete without some knowledge of
the
characteristics
of
individual
fibers.
Since each
is so
different
it is
difficult
to
generalize
or
com pare directly. This section presents
a
summary
of each important fiber, including pertinent information on their
manufacture, properties, uses, and current economics in a brief, informal and
concise manner.
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(a pure form of cellulose)
1.
Fewer processing steps, cheaper than wool
2. Good hand
3.
W ears hard and long
4. Easily dyed
via
free hydroxy groups
5. A bsorbs water but dries easily. If preshrunk, it is stable to washing
and ironing more than other fibers.
6. Hydrophilic—cool and comfortable. Comfort never matched by
synthetics. Used especially
in
towels
and
drying cloths.
7. Disadvantages: creases easily, requiring frequent ironing;
agricultural variables in growing the plant; brown lung disease in
workers
in
mills;
waste
about
10% in
harvest; variable strength;
unpredictable
price
3.1.2 Wool
(protein, mostly keratin, a complex mixture of amino acids)
1. Processing requ ires 20 stages, therefore very expensive
2. Good resilience (elasticity) because orientation changes
a-keratin
(helical protein) into p-keratin (zigzag). Example: woolen carpet
recovers even after years of heavy
furniture.
3. Good warmth
due to
natural crimp (many folds
and
waves) which
retains air, therefore a good insulator
4. Hydrophilic—absorbs perspiration away from body and is
comfortable, not sweaty
5.
Dyed easily since
it has
acidic
and
basic groups
in the
am ino acid
6.
Disadvantages: expensive; retains water
by
hydrogen bonding with
washing, causing shrinkage; many people allergic
to
protein
3 1 Natural
3.1.1 Cotton
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3.2
Cellulosic
3.2.1 Rayon
(regenerated cellulose from wood pulp, especially higher molecular
weight
"alpha"
fraction not
soluble
in
18%
caustic)
1.
Manufacture
a. Steeping 1 hr in 18% caustic gives "soda"
cellulose,
(C
6
H
10
Os)
n
+ NaOH
^ [(C
6
H
10
Os)
2
-NaOH]
n
where some
C—OH are
converted into
C— OTsIa
+
b. Reaction with CS
2
,
about
1
xanthate
for
each
tw o
glucose units, soluble
in 6%
NaOH
for
sp inning, called
"viscose rayon"
c.
Ripening—slow
hydrolysis
viscose —different molecular weight than starting cellulose,
has some xanthate groups
d.
Spinning—ZnSO
4
, H
2
SO
4
bath.
H
2
SO
4
neutralizes N aOH of viscose solution. ZnSO
4
gives
interm ediate w hich decomposes mo re slowly.
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2. Properties
Dyeable
(free
hydroxy groups), hydrophilic (comfortable), stable,
low
price,
poor wash
and
wear characteristics
3. Uses
Apparel, 31%; home furnishings (curtains, draperies, upholstery,
mattress ticking), 24%; nonwovens (medical and surgical, wipes
and towels, fabric softeners), 33%; miscellaneous, 12%.
4.
Economics
Peak production in 1950s, 1960s. Declined by 5.6%/yr in 1970-
1980,
4.9% /yr
in
1980-1990,
and
3.6%/yr
in
1990-2000.
3.2.2 Acetate
1. Manufacture
a.
Esterification
b.
Ripening with M g(OA c)
2
, H
2
O
.
Mg(OAc)
2
cellulose sulfoacetate
——
>
[C
6
H
7
O
2
(OH)O^
5
(AcO)
2
Os]
n
+
MgSO
4
Usually
the primary carbon has the OH, secondary carbons the
OA c. Ce llulose triacetate
has
three
OA c
groups.
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2. Properties
Lower tenacity than any other
fiber
because the bulky OAc group
keeps molecules far apart
Free OH more easily dyed and more hydrop hilic than triacetate
Random acetate groups make
it
less crystalline than triacetate.
So
triacetate
is
better
for ironing. But triacetate gives
stiffer
fabrics
with
inferior drape
and
hand.
Both are softer than rayon but not so strong, have poor crease
resistance,
and are not
colorfast.
3. Uses
Cigarette filters, 61%; yarn (especially
for
apparel, curtains,
draperies, bedspreads, quilt covers), 39%.
4. Economics
Down 4.5%/yr from 1970-1980, 4.2%/yr from 1980-90, and 3.6%/yr
from 1990-2000
3 3 Noncellulosic
3.3.1 Nylon 6 and 6,6
nylon
6
nylon
6,6
1.
Manufacture
Nylon 6,6
developed
by W. H.
Carothers
of Du
Pont
in
1930s.
Adipic acid, HMDA, 280-30O
0
C, 2-3 hr, vacuum. Trace of
acetic acid terminates chains with acid groups and controls
molecular w eight.
Nylon 6 developed by Paul Schlak in Germany in 1940.
Caprolactam plus heat plus water as a catalyst.
Fibers are melt spun (no solvent) while still above T
m
(=
265-27O
0
C
for nylon 6,6 and 215-22O
0
C fo r nylon 6). Extruded through a
spinerette.
Fibers oriented
by
stretching
to 4 x
original length
by
cold drawing
(two pulleys at different speeds) or by spin drawing as it is being
cooled
2.
Properties
Strongest and hardest wearing of all fibers
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Figure
17.2
Pilling chambers to test, by
rapid
rotation, the tendency of a
fabric
to
form
pills as in repeated washings and use. (Courtesy of Du Pont)
Heat stable
Dyeable
Disadvantages: hydrophobic
("cold and
clammy"), degraded
by
UV, yellows with
age,
poor hand, tends
to "pill"
(gentle rubb ing
forms
small nodules
or
pills, where surface fibers
are
raised,
see
Fig.
17.2)
3. Uses
Carpets and rugs, 74%; industrial and other (tire cord and
fabric,
rope and cord, belting and hose, sewing thread), 16%; apparel
(especially hosiery, anklets,
and
socks), 10%
4. Economics
Production increased
5.7%/yr
from 1970-1980, only 1.2%/yr from
1980-1990, and decreased 0.2%/yr
from
1990-2000.
The carpet and rug market has increased dramatically in recent
years.
Nylon 6,6 is two
thirds
of the
U.S. market, nylon
6 one
third.
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3.3.2 Polyester, Poly ethylene terephthalate),
PET
1. Manufacture
Developed by Whinfield and Dixon in the U.K . Originally mad e by
transesterification of DMT and
ethylene glycol
in a
1:2.4 ratio,
distillation of the methanol, then polymerization at
200-29O
0
C
in
vacuo with SbOa as catalyst.
In
early
1960s
pure
TA
began
to be
used with
excess
ethylene glycol
at
25O
0
C,
60 psi to
form
an oligomer with n = 1-6, followed by
polymerization
as in the DMT
method.
N ow
both methods
are
used.
M elt spin like nylon, T
m
= 250-265
0
C
Orientation above T
g
o f
8O
0
C
to 300-400%
2.
Properties
Stable in repeated laundering
Complete wrinkle resistance
Blends com patibly with other fibers, especially cotton
Can vary from low tenacity, high elongation to high tenacity, low
elongation by orientation
Disadvantages: stiff
fibers
(aromatic rings), poor drape except with
cotton blends, hydrophobic, pilling, static charge
buildup,
absorbs oils
and
greases easily (stains)
3. Uses
Clothing
(suits, pants, shirts,
and
dresses either nonblended
or
blended with other fibers such as cotton), 50%; home
furnishings (carpets, pillows, bedspreads, hose, sewing thread,
draperies, sheets, pillowcases), 20%; industrial (tire cords),
30%
4.
Economics
Production increased
10.5%/yr from
1970-1980, decreased 2.2%/yr
from
1980-1990,
and
increased 1.9%/yr from 1990-2000.
3.3.3 Polyolefin
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Figure
17.3 Spinning of fibers for use in carpets. (Courtesy of Du Pont)
1. Manufacture
Mostly isotactic polypropylene homopolymer,
but
some copolymer
with polyethylene,
and
some HDPE
A
high molecular weight
is needed, 170,000-300,000, for
good
mechanical strength because of weak intermolecular forces,
only
van der Waals.
Fiber
produced by melt spinning
2. Properties
Low
density (0.91),
the
lowest
of all
commercial
fibers,
meaning
lightness and high cover
Outstanding chem ical resistance, serving industrial filtration
High abrasion resistance for floor covering, upholstery, and hosiery
Insensitive to water, giving dimensional stability and
fabric dryness
in
contact with the skin
Resistance to mildew, microorganisms, and insects
Good insulator and electrical properties
Lowest thermal conductivity of commercial
fibers,
giving high
thermal insulation
in
textiles
No
skin irritation
or
sensitization, non-toxic
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Disadvantages: not easily dyeable, low resistance to oxidation—but
additives help, easily soiled, poor
fo r
ironing, poor
for dry
cleaning
3. Uses
Industrial, including rope, twine, conveyor belts, carpet backing,
tarpaulin, awnings, cable, bristles; home textile applications,
including floor covering, upholstery fabric, wall covering,
blankets; apparel, especially sportswear and hosiery
4. Economics
Growth rate large, 11%/yr from 1970-1980, 9%/yr from 1980-1990,
and 5.8%/yr from 1990-2000
Surpassed acrylics
in
1980, nylon
in
1998,
and is
catching
up to
polyester
Suggested
eadings
Kent,
Riegel s Handbook of Industrial
Chemistry, pp. 735-799.
Wittcoff and Reuben, Industrial Organic Chemicals in Perspective. Part
Two:
Technology,
Formulation,
and
Use,
pp.
104-125.