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joined together to make long chains that result in amaterial with a useful blend of properties. Usinganother Greek word poly which means many, thelong chain of mers forms a polymer. The monomersare held together in a polymer chain by very strongattractive forces between molecules. Much weakerforces hold the polymer chains together. The polymerchains can be constructed in many ways. Some simpli-fied examples of the way polymers are built are shownin Figures 1.1-1.5:

MONOMERS: A, B, CExamples of monomers are ethylene, styrene, vinyl

chloride and propylene.

Figure 1.1

HOMOPOLYMERS: A-A-A-A-A-A-A-A-AHomopolymers are polymers constructed from

joining like monomers.

Some examples of polymers built this way are poly-ethylene, polystyrene and PVC.

Figure 1.2

COPOLYMERSCopolymers are polymers constructed from two dif-

ferent monomers.

ALTERNATING TYPES: A-A-A-B-A-A-A-B-A-A-A-BSome examples of alternating copolymers are ethyl-

ene-acrylic and ethylene-ethyl acrylate.

Figure 1.3

GRAFT TYPES: A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-l l l l lB B B B B

l I lB B BI lB B

IB

Some examples of grafted copolymers are styrene-butadiene, styrene-acrylonitrile and some acetals.

Figure 1.4

TERPOLYMERS: A-A-A-B-C-C-A-A-A-B-C-CTerpolymers are polymers constructed from three

different materials.

An example of a terpolymer is acrylonitrile-butadi-ene-styrene (ABS).

Figure 1.5

Introduction to Plastics HOW PLASTICS ARE MADE 3

4 HOW PLASTICS ARE MADE Introduction to Plastics

The two monomers in a copolymer are combinedduring the chemical reaction of polymerization.Materials called alloys are manufactured by the sim-ple mixing of two or more polymers with a resultingblending of properties which are often better than eitherindividual material. There is no chemical reaction in thisprocess. Some examples of alloys are polyphenyleneoxide-high-impact styrene, polycarbonate-ABS andABS-PVC.

Molecular WeightIt is important for the chemist to know how long the

polymer chains are in a material. Changing the length ofthe chains in a thermoplastic material will change itsfinal properties and how easily it can be shaped when itis melted.

The repeating unit or molecular group in thehomopolymer is A- (Figure 1.2), the group of moleculesin the copolymer A-B- (Figure 1.3), and in the terpoly-mer A-B-C- (Figure 1.5). The number of repeating unitsin the polymer chain is called the degree of polymer-ization. If the repeating unit has a molecular weight(the combined weight of all of the molecules in therepeating unit) of 60 and the chain or polymer has 1,000repeating units, then the polymer has an average mol-ecular weight of 60 1,000 = 60,000. The molecularweight is a way of measuring the length of the polymerchains in a given material.

The molecular weight of plastics is usually between10,000 and 1,000,000. It becomes increasingly difficult toform or mold the plastic with the application of heatand pressure as the molecular weight increases. A mol-ecular weight of about 200,000 is about the maximumfor a polymer to still permit reasonable processability.Some higher molecular weight materials, like ultra-high-molecular-weight-polyethylene (UHMW-PE), thathas a molecular weight from 3,000,000 to 6,000,000, canbe cast using processes specifically designed to shape it.

Crystalline and Amorphous MaterialsPolymers are often described as being either crys-

talline or amorphous when it is actually more accurateto describe plastics by their degree of crystallinity.Polymers cannot be 100 percent crystalline, otherwisethey would not be able to melt due to the highly orga-nized structure. Therefore, most polymers are consid-ered semi-crystalline materials with a maximum of 80percent crystallinity.

Amorphous materials have no patterned orderbetween the molecules and can be likened to a bowl ofwet spaghetti. Amorphous materials include atacticpolymers since the molecular structure does not gener-

ally result in crystallization. Examples of these types ofamorphous plastics include polystyrene, PVC and atac-tic polypropylene. Presence of polar groups, such as acarbonyl group CO, in vinyl type polymers also restrictcrystallization. Polyvinyl acetate, all polyacrylates andpolymethacrylates are examples of carbonyl groupsbeing present and the resulting polymers being amor-phous. Polyacrylonitrile is an exception to this since car-bonyl groups are present in their structure but they alsocrystallize. Even amorphous materials can have adegree of crystallinity with the formation of crystallitesthroughout their structure. The degree of crystallinity isan inherent characteristic of each polymer but may alsobe affected or controlled by processes such as polymer-ization and molding.

Crystalline materials exhibit areas of highly orga-nized and tightly packed molecules. These areas of crys-tallinity are called spherulites and can be varied inshape and size with amorphous areas between the crys-tallites. The length of polymers contributes to their abil-ity to crystallize as the chains pack closely together, aswell as overlapping and aligning the atoms of the mol-ecules in a repeating lattice structure. Polymers with abackbone of carbon and oxygen, such as acetals, readilycrystallize. Plastic materials such as nylon, and otherpolyamides, crystallize due to the parallel chains and strong hydrogen bonds of the carbonyl and amine groups. Polyethylene is crystalline because the chains are highly regular and easily aligned.Polytetrafluoroethylene (PTFE) is also highly symmet-ric, like polyethylene, with fluorine atoms replacing allthe hydrogens along the carbon backbone. It, too, ishighly crystalline. Isomer structures also affect thedegree of crystallinity. As the atactic stereochemistryresulted in amorphous polymers, those that are isotacticand syndiotactic result in crystalline structures formingas chains align to form crystallites. These stereospecificforms of polypropylene are those which are preferablefor structural applications due to their degree of crys-tallinity.

The degree of crystallinity affects many polymericproperties. In turn, other characteristics and processesaffect the degree of crystallinity. Molecular weight willaffect the crystallinity of polymers. The higher the mol-ecular weight the lower the degree of crystallinity, andthe areas of crystallites are more imperfect. The degreeof crystallinity is also dependent on the time availablefor crystallization to occur. Processors can use this totheir advantage by quenching or annealing to controlthe time for crystallization to occur. Highly branchedpolymers tend to have lower degrees of crystallinity, asis easily seen in the difference between branched low-density polyethylene (LDPE) and the more crystallinehigh-density polyethylene (HDPE). LDPE is very flexi-ble, less dense, and more transparent than HDPE. Thisis an excellent example that the same polymer can have

varied degrees of crystallinity. Stress can also result inincreased crystallinity as polymer chains align orientingthe crystallites. Drawing fibers, the direction of extru-sion, and gate placement will also affect the orientationof polymers and therefore the crystallites of the mater-ial. This allows the processor to maximize the effectsand benefits of the inherent crystallinity of the polymerbeing used in an application. The chart below describesother general effects due to the degree of crystallinity ina polymer.

COMMON CHARACTERISTICS OF CRYSTALLINE AND AMORPHOUS PLASTICS

Higher Percent Crystalline Higher Percent Amorphous Higher heat resistance Lower heat resistance Sharper melting point Gradual softening/melting More opaque point Greater shrinkage upon More translucent/transparent

cooling Lower shrinkage upon Reduced low temperature cooling

toughness Greater low temperature Higher dimensional stability toughness Lower creep Lower dimensional stability

Higher creep

Thermoplastic and Thermoset MaterialsThe terms thermoset and thermoplastic have been

traditionally used to describe the different types of plas-tic materials. A thermoset allows only one chance to liq-uefy and shape it. These materials can be cured or poly-merized using heat and pressure, or as with epoxies, achemical reaction started by a chemical initiator.

A thermoplastic can be melted and shaped severaltimes. The thermoplastic materials are either crystallineor amorphous. Advances in chemistry have made thedistinction between crystalline and amorphous lessclear, since some materials like nylon are formulatedboth as a crystalline material and as an amorphousmaterial.

Again, the advances in chemistry make it possiblefor a chemist to construct a material to be either ther-moset or thermoplastic. The main difference betweenthe two classes of materials is whether the polymerchains remain linear and separate after molding (like

spaghetti) or whether they undergo a chemical changeand form a network (like a net) by cross-linking.Generally, a cross-linked material is a thermoset andcannot be reshaped. Due to recent advances in polymerchemistry, the exceptions to this rule are continuallygrowing. These materials are actually cross-linked ther-moplastics with the cross-linking occurring either dur-ing the processing or during the annealing cycle. Thelinear materials are thermoplastic and are chemicallyunchanged during molding (except for possible degra-dation) and can be reshaped again and again.

As previously discussed, cross-linking can be initi-ated by heat, chemical agents, irradiation or a combina-tion of these. Theoretically, any linear plastic can bemade into a cross-linked plastic with some modificationto the molecule so that the cross-links form in orderlypositions to maximize properties. It is conceivable that,in time, all materials could be available in both linearand cross-linked formulations.

The formulation of a material, cross-linked or linear,will determine the processes that can be used to suc-cessfully shape the material. Generally, cross-linkedmaterials (thermosets) demonstrate better properties,such as improved resistance to heat, less creep and bet-ter chemical resistance than their linear counterpart;however, they will generally require a more complexprocess to produce a part, rod, sheet or tube.

SOME EXAMPLES OF THE VARIOUS TYPES OF MATERIALS

Linear Thermoplastics PVC Nylon Acrylic Polycarbonate ABS

Thermoplastics That Can Be Cross-linked After Processing PEEK Polyamide-imide UHMW-PE

Thermosets Phenolics Epoxies Melamines

Introduction to Plastics HOW PLASTICS ARE MADE 5