Essential Requirements Of Fibre Forming Polymers
Polymer Fundamentals:
Fiber Applications
Polymer science addresses the
chemistry and physics of large, chain-like molecules. As with the molecules
themselves, this technical pursuit is diverse and complicated. The following
discussion provides an introduction to the manufacture and use of synthetic
organic polymers for those with some knowledge of basic science. More
advanced tutorial information on polymers is contained in the links
displayed at the end.
What is a Polymer?
The term �polymer� is derived from the Greek �poly�, meaning
�many�, and �mer�, meaning �parts� � thus polymers are substances made of
�many parts�. In most cases the parts are small molecules which react
together hundreds, or thousands, or millions of times. A molecule used in
producing a polymer is a �monomer� � mono is Greek for single, thus a
monomer is a �single part�. A polymer made entirely from molecules of one
monomer is referred to as a �homopolymer�. Chains that contain two or more
different repeating monomers are �copolymers�.
The resulting molecules may be long, straight chains, or they may be
branched, with small chains extending out from the molecular �backbone�. The
branches also may grow until they join with other branches to form a huge,
three-dimensional matrix. Variants of these molecular shapes are among the
most important factors in determining the properties of the polymers
created.
The size of polymer molecules is important. This is usually expressed in
terms of molecular weight. Since a polymeric material contains many chains
with the same repeating units, but with different chain lengths, average
molecular weight must be used. In general, higher molecular weights lead to
higher strength. But as polymer chains get bigger, their solutions, or
melts, become more viscous and difficult to process.
Proteins and Carbohydrates
Life as we know it could not exist without polymers. Proteins,
with large numbers of amino acids joined by amide linkages, perform a wide
variety of vital roles in plants and animals. Carbohydrates, with chains
made up of repeating units derived from simple sugars, are among the most
plentiful compounds in plants and animals. Both of these natural polymers
are important for fibers. Proteins are the basis for wool, silk and other
animal-derived filaments. Cellulose as a carbohydrate occurs as cotton,
linen and other vegetable fibers. The properties of these fibers are limited
by the form provided in their natural state. Some, like linen and silk, are
difficult to isolate from their sources, which makes them scarce and
expensive. There are, of course, many other sources of proteins and
cellulose, such as wood pulp, but natural polymers tend to be very difficult
to work with and form into fibers or other useful structures. The
inter-chain forces tend to be strong because of the large number of polar
groups in the molecular chains. Thus, natural polymers usually have melting
points that are so high that they degrade before they liquefy.
The most useful molecules for fibers are long chains with few branches
and a very regular, extended structure. Thus, cellulose is a good
fiber-former. It has few side chains or linkages between the sugar units
forcing its chains into extended configurations. However, starches, which
contain the same basic sugar units, do not form useful fibers because their
chains are branched and coiled into almost spherical configurations.
Synthetic Polymers
Synthetic polymers offer more possibilities, since they can be
designed with molecular structures that impart properties for desired end
uses. Many of these polymers are capable of dissolving or melting, allowing
them to be extruded into the long, thin filaments needed to make most
textile products. Synthetic polymer fibers can be made with regular
structures that allow the chains to pack together tightly, a characteristic
that gives filaments good strength. Thus, filaments can be made from some
synthetic polymers that are much lighter and stronger than steel.
Bullet-proof vests are made from synthetic fibers.
There are two basic chemical processes for the creation of synthetic
polymers from small molecules (1) condensation, or step-growth
polymerization, and (2) addition, or chain-growth polymerization.
Step-Growth Polymerization
In step-growth polymerization, monomers with two reactive ends
join to form dimers (two �parts� joined together), then �trimers� (three
�parts�), and so on. However, since each of the newly formed oligomers
(short chains containing only a few parts) also has two reactive ends, they
can join together; so a dimer and a trimer would form a pentamer (five
repeating �parts�). In this way the chains may quickly great length achieve
large size. This form of step-growth polymerization is used for the
manufacture of two of the most important classes of polymers used for
textile fibers,
polyamide (commonly known as nylon), and
polyester.
There are many different commercial versions of polyester in a wide
variety of applications, including plastics, coatings, films, paints, and
countless other products. The polymer usually used for textile fibers is
poly(ethylene terephthalate), or PET, which is formed by reacting ethylene
glycol with either terephthalic acid or dimethyl terephthalate. Antimony
oxide is usually added as a catalyst, and high vacuum is used to remove the
water or methanol byproducts. High temperature (>250oC) is
necessary to provide the energy for the reaction, and to keep the resultant
polymer in a molten state.
PET molecules are regular and straight, so their inter-chain forces are
strong � but not strong enough to prevent melting. Thus, PET is a
�thermoplastic� material; that is, it can be melted and then solidified to
form specific products. Since its melting point is high, it does not soften
or melt at temperatures normally encountered in laundering or drying.
Another important property of PET is its Tg, or �glass transition
temperature�. When a polymer is above its glass transition temperature, it
is easy to change its shape. Below its Tg, the material is
dimensionally stable and it resists changes in shape. This property is very
important for textile applications because it allows some fibers, and the
fabrics made from them, to be texturized or heat-set into a given shape.
This can provide bulk to the yarn, or wrinkle resistance to the fabric.
These set-in shapes remain permanent as long as the polymer is not heated
above its Tg. Because its chains are closely packed and its ester
groups do not form good hydrogen bonds, polyesters are also hydrophobic
(i.e., they do not absorb water). This property also requires special dyeing
techniques.
There are also many important classes of synthetic polyamides (nylons)
and they have a wide variety of commercial uses. These are usually
distinguished from each other by names based on the number of carbon atoms
contained in their monomer units. As with polyesters, polyamides are formed
by step-growth polymerization of monomers possessing two reactive groups.
Here, the reactive functions are acids and amines. The monomers used may
have their two reactive functions of the same chemical type (both acids, or
both amines), or of different types. Thus, nylon 6,6 � a very common fiber
polymer � is made by reacting molecules of adipic acid (containing six
carbons in a chain, with an acid function at each end) with hexamethylene
diamine (also six carbon atoms, with amine functions at each end). In
another variant the diamine contains ten carbons atoms, the product
designated nylon 6,10.
The other common polyamide fiber polymer is nylon 6. Its monomer has six
carbons in the chain, with an amine at one end and an acid at the other.
Thus only one form of monomer is needed to conduct the reaction. Commercial
production of nylon 6 makes use of caprolactam, a derivative which provides
the same result.
As with the polyesters, nylons have regular structures to allow good
inter-chain forces that impart high strength. Both nylon 6 and nylon 6,6
have melting points similar to PET but they have a
lower Tg
Also, since the amide functions in nylon chains are good at hydrogen
bonding, nylons can be penetrated by water molecules. This allows them to be
dyed from aqueous media, unlike their polyester counterparts.
In addition to nylon, there is another commercially important group of
synthetic polyamides. These are the
aramids, which
contain aromatic rings as part of their polymer chain backbone. Due to the
stability of their aromatic structures and their conjugated amide linkages,
the aramids are characterized by exceptionally high strength and thermal
stability. Their usefulness for common textile applications is limited by
their high melting points and by their insolubility in common solvents. They
are expensive to fabricate, and they carry an intrinsic color that ranges
from light yellow to deep gold.
Other step-growth polymers � the polyurethanes � are produced by the
reaction of polyols and polyisocyanates. For fiber purposes, this class of
linear polymers is formed from glycols and diisocyanates. Usually, the
reactions are carried out to form block copolymers containing at least two
different chemical structures � one rigid, and the other flexible. The
flexible segments stretch, while the rigid sections act as molecular anchors
to allow the material to recover its original shape when the stretching
force is removed. Varying the properties of the segments, and the ratio of
flexible to rigid segments controls the amount of stretch. Fibers made in
this way are classified as
spandex and
they are used widely in apparel where stretch is desirable.
Chain-Growth Polymerization
Chain-growth polymerization occurs when an activated site on a
chemical, such as a free radical or ion, adds to a double bond, producing a
new bond and a new by activated location. That location then attacks another
double bond, adding another unit to the chain, and a new reactive end. The
process may be repeated thousands, or millions, of times, to produce very
large molecules. This is usually a high energy process and the intermediate
species are so reactive that, in addition to attacking available monomer,
they also may attack other chains, producing highly branched structures.
Since these branches prevent the molecules from forming regular structures
with other molecules, their inter-chain forces are weak. The resulting
polymers tend to be low-melting and waxy.
The breakthrough in making chain-growth polymers useful for fibers and
for most commercial plastics came with the development of special selective
catalysts that drive the production of long, straight polymer chains from
monomers containing basic carbon-to-carbon double bonds.
Ethylene and propylene form the simplest chain-growth polymers. Since
their polymer chains contain no polar groups, these polyolefins must rely on
close contact between the molecular chains for strength. Thus, the physical
characteristics of polyethylene are very sensitive to even a small number of
chain branches. Very straight chains of polyethylene can form strong
crystalline structures which exhibit exceptional strength. Protective
fabrics made from this type of highly structured polyethylene are virtually
impossible to penetrate or cut.
Polypropylene is more complicated. Even without chain branching, each
monomer unit adds one methyl group pendant to the chain. The arrangement of
these side groups is described as the �tacticity� of the polymer. A random
arrangement is considered �atactic�, or without tacticity. Regular
arrangement with all side groups on one side of the chain is �isotactic�,
and a regular alternating structure is �syndiotactic�. Polypropylene
molecules can only pack closely in an isotactic arrangement. Synthesis of
these polymers was a major challenge, but several stereoselective catalysts
are now available, and high-density polypropylene has become a commodity
product. Fibers made from it are lightweight, hydrophobic and highly
crystalline. Their resistance to wetting gives them good moisture wicking
and anti-staining properties. This also makes them virtually undyeable,
except when the dye is applied to the polymer in its molten state � a
process know as �solution dyeing�.
By contrast, the pendant nitrile functions in polyacrylonitrile are
sufficiently polar to produce very strong inter-chain forces. Pure
homopolymers from acrylonitrile are non-thermoplastic and difficult to
dissolve or dye. Thus, for most commercial acrylonitrile polymers, small
amounts of other monomers with bulky side chains are introduced to force the
chains apart, to reduce the inter-chain forces. Common co-monomers for these
fiber applications include vinyl chloride, vinyl acetate, acrylic acid, and
methyl acrylate.
Specialty fiber polymers
There are also a number of complex, specialty fiber polymers with
methods of synthesis that are not easily classified. These materials are
occasionally used in high performance materials where the complex structures
impart exceptional strength, thermal stability, electrical conductivity, and
others desirable properties. They include (polybenzimidazole)and
Their
chemistry is beyond the scope of this introductory discussion.
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