Askeland D.R., Fulay P.P. Essentials of Materials Science & Engineering

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Phenolics Phenolics, the most commonly used thermosets, are often used as adhe-

sives, coatings, laminates, and molded components for electrical or motor applications.

Bakelite

is one of the common phenolic thermosets. A condensation reaction

joining phenol and formaldehyde molecules produces the initial linear phenolic resin.

This process continues until a linear phenol-formaldehyde chain is formed. How-

ever, phenol is trifunctional. After the chain has formed, there is a third location on

each phenol ring that provides a site for cross-linking with the adjacent chains.

Amines Amino resins, produced by combining urea or melamine monomers with

formaldehyde, are similar to the phenolics. The monomers are joined by a form-

aldehyde link to produce linear chains. Excess formaldehyde provides the cross-linking

needed to give strong, rigid polymers suitable for adhesives, laminates, molding mate-

rials for cookware, and electrical hardware such as circuit breakers, switches, outlets,

and wall plates.

Urethanes Dependi ng on the degree of cross-linking, the urethanes behave as ther-

mosetting polymers, thermoplastics, or elastomers. These polymers ﬁnd application as

ﬁbers, coatings, and foams for furniture, mattresses, and insulation.

Polyesters Polyester s form chains from acid and alcohol molecules by a conde nsation

reaction, giving water as a byproduct. When these chains contain unsaturated bonds, a

styrene molecule may provide cross-linking. Polyesters are used as molding or casting

materials for a variety of electrical applications, decorative la minates, boats and other

marine equipment, and as a matrix for composites such as ﬁberglass.

Epoxies Epoxies are thermosetting polymers formed from molecules containing a

tight CaaOaaC ring. During polymerization, the CaaOaaC rings are opened and the

bonds are rearranged to join the molecules. The most common of the commercial ep-

oxies is based on bisphenol A, to which have been added two epoxide units. These

molecules are polymerized to produce chains and then co-reacted with curing agents

that provide cross-linking. Epoxies are used as adhesives, rigid molded parts for elec-

trical applications, automotive components, circuit boards, sporting goods and a matrix

for high-performance ﬁber-reinforced composite materials for aerospace.

Polyimides Polyimides display a ring structure that contains a nitrogen atom. One

special group, the bismaleimides (BMI), is important in the aircraft and aerospace

industry. They can operate continuously at temperatures of 175



C and do not decom-

pose until reaching 460



Interpenetrating Polymer Networks Some special polymer materials can be produced

when linear thermoplastic chains are intertwined through a thermosetting framework,

forming interpenetrating polymer networks. For example, nylon, acetal, and poly-

propylene chains can penetrate into a cross-linked silicone thermoset. In more advanced

systems, two interpenetrating thermosetting framework structures can be produced.

16-10 Adhesives

Adhesives are polymers used to join other polymers, metals, ceramics, composites, or

combinations of these materials. The adhesives are used for a variety of applications.

The most critical of these are the ‘‘structural adhesives,’’ which ﬁnd use in the auto-

motive, aerospace, appliance, electronics, construction, and sporting equipme nt areas.

CHAPTER 16 Polymers530

Chemically Reactive Adhesives These adhesives include polyurethane, epoxy, sili-

cone, phenolics, anaerobics, and polyimides. One-component systems consist of a single

polymer resin cured by exposure to moisture, heat, or—in the case of anaerobics—the

absence of oxygen. Two-component systems (such as epoxies) cure when two resins are

combined.

Evaporation or Diffusion Adhesives The adhesive is dissolved in either an organic

solvent or water and is applied to the surfaces to be joined. When the carrier evapo-

rates, the remaining polymer provides the bond. Water-base adhesives are preferred

from the standpoint of environmental and safety considerations. The polymer may be

completely dissolved in water or may consist of latex, or a stable dispersion of polymer

in water. A number of elastomers, vinyls, and acrylics are used.

Hot-Melt Adhesives These thermoplastics and thermoplastic elastomers melt when

heated. On cooling, the polymer solidiﬁes and joins the materials. Typical melting

temperatures of commercial hot-melts are about 80



C to 110



C, which limits the ele-

vated-temperature use of these adhesives. High-performance hot-melts, such as poly-

amides and polyesters, can be used up to 200



Pressure-Sensitive Adhesives These adhesives are primarily elastomers or elastomer

copolymers produced as ﬁlms or coatings. Pressure is required to cause the polymer to

stick to the substrate. They are used to produce electrical and packaging tapes, labels,

ﬂoor tiles, wall coverings, and wood-grained textured ﬁlms.

Conductive Adhesives A polymer adhesive may contain a ﬁller material such as silver,

copper, or aluminum ﬂakes or powders to provide electrical and thermal conductivity.

In some cases, thermal conductivity is desired but electrical conductivity is not wanted;

alumina, boron nitride, and silica may be used as ﬁllers to provide this combination of

properties.

16-11 Polymer Processing and Recycling

There are a number of methods for producing polymer shapes, including molding,

extrusion, and manufacture of ﬁlms and ﬁbers. The techniques used to form the poly-

mers depend to a large extent on the nature of the polymer—in particular, whether it is

thermoplastic or thermosetting. The greatest variety of techniques are used to form the

thermoplastics. The polymer is heated to near or above the melting temperature so that

it becomes rubbery or liquid. The polymer is then formed in a mold or die to produce

the required shape. Thermoplastic elastomers can be formed in the same manner. In

these processes, scrap can be easily recycled and waste is minimized. Fewer forming

techniques are used for the thermosetting polymers because, once cross-linking has

occurred, the thermosetting polymers are no longer capable of being form ed. Elas-

tomers are processed in high-shear equipment such as a Banbury mixer. Carbon black

and other additives are added. The heating from viscoelastic deformation can begin to

cross-link the material prematurely. After the mixing step, a curing agent (e.g., zinc

oxide) is added. The material discharged from the mixer is pliable and is processed

using a short extruder, molded using a two-roll mill, or applied on parts by dip coating.

This processing of elastomers is known as compounding of rubber.

The following are some of the techniques mainly used for processing of polymers;

most of these, you will note, apply only to thermoplastics.

16-11 Polymer Processing and Recycling 531

Extrusion This is the most widely used technique for processing thermoplastics.

Extrusion can serve two purposes. First, it provides a way to form certain simple shapes

continuously (Figure 16-24). Second, extrusion provides an excellent mixer for additives

(e.g., carbon black, ﬁllers, etc.) when processing polymers (Figure 16-25) that ultimately

may be processed using some other process. A screw mechanism consisting of one or

Figure 16-24 Schematic of an extruder used for polymer processing. (Source: Strong, A.

Brent, Plastics: Materials and Processing, 2nd, > 2000. Electronically reproduced by

permission of Pearson Education, Inc., Upper Saddle River, New Jersey. )

Figure 16-25 One technique by which polymer films (used in the manufacture of garbage

bags, for example) can be produced. The film is extruded in the form of a bag, which is

separated by air pressure until the polymer cools. (Source: Strong, A. Brent, Plastics: Materials

and Processing, 2nd, > 2000. Electronically reproduced by permission of Pearson Education,

Inc., Upper Saddle River, New Jersey.)

CHAPTER 16 Polymers532

a pair of screws (twin screw) forces heated thermoplastic (either solid or liquid) and

additives through a die opening to produce solid shapes, ﬁlms, sheets, tubes, pipes, and

even plastic bags (Figure 16-24). An industrial extruder can be up to 18 m to 21 m long,

60 cm in diameter, and consist of di¤erent heating or cooling zones. Since thermo-

plastics show shear thinning behavior and are viscoelastic, the control of both temper-

ature and viscosity is critical in polymer extrusion. One special extrusion process for

producing ﬁlms is illustrated in Figure 16-25. Extrusion also can be used to coat wires

and cables with either thermoplastics or elastomers.

Blow Molding A hollow preform of a thermoplastic called a parison is introduced

into a die by gas pressure and expanded against the walls of the die (Figure 16-26). This

process is used to produce plastic bottles, containers, automotive fuel tanks, and other

hollow shapes.

Injection Molding Thermoplastics heated above the melting temperature using an

extruder are forced into a closed die to produce a molding. This process is similar to

die casting of molten metals. A plunger or a special screw mechanism applies pressure

Figure 16-26 Typical forming processes for thermoplastic: (a) extrusion, (b) blow molding,

16-11 Polymer Processing and Recycling 533

to force the hot polymer into the die. A wide variety of products, ranging from cups,

combs, and gears to garbage cans, can be produced in this manner.

Thermoforming Thermoplastic polymer sheets heated to the plastic region can be

formed over a die to produce such diverse products as egg cartons and decorative

panels. The forming can be done using matching dies, a vacuum, or air pressure.

Calendaring In a calendar, molten plastic is poured into a set of rolls with a small

opening. The rolls, which may be embossed with a pattern, squeeze out a thin sheet of

the polymer—often, polyvinyl chloride. Typical products include vinyl ﬂoor tile and

shower curtains.

Spinning Filaments, ﬁbers, and yarns may be produced by spinning. The molten

thermoplastic polymer is forced through a die containing many tiny holes. The die,

called a spinnerette, can rotate and produce a yarn. For some materials, including

nylon, the ﬁber may subsequently be stretched to align the chains parallel to the axis of

the ﬁber; this process increases the strength of the ﬁbers.

Casting Many polymers can be cast into molds and permitted to solidify. The molds

may be plate glass for producing individual thick plastic sheets or moving stainless steel

belts for continuous casting of thinner sheets. Rotational molding is a special casting

process in which molten polymer is poured into a mold rotating about two axes. Cen-

trifugal action forces the polymer against the walls of the mold, producing a thin shape

such as a camper top.

Compression Molding Thermoset moldings are most often formed by placing the solid

material before cross-l inking into a heated die (Figure 16-27a). Application of high

Figure 16-27 Typical forming processes for thermosetting polymers: (a) compression molding

and (b) transfer molding.

CHAPTER 16 Polymers534

pressure and temperature causes the polymer to melt, ﬁll the die, and immediately begin

to harden. Small electrical housings as well as fenders, hoods, and side panels for au-

tomobiles can be produced by this process.

Transfer Molding A double chamber is used in the transfer molding of thermosetting

polymers. The polymer is heated under pressure in one chamber. After melting, the

polymer is injected into the adjoining die cavity. This process permits some of the ad-

vantages of injection molding to be used for thermosetting polymers (Figure 16-27b).

Reaction Injection Molding (RIM) Thermosetting polymers in the form of liquid res-

ins are ﬁrst injected into a mixer and then directly into a heated mold to produce a

shape. Forming and curing occur simultaneously in the mold. In reinforced-reaction

injection molding (RRIM), a reinforcing material consisting of particles or short ﬁbers

is introduced into the mold cavity and is impregnated by the liquid resins to produce a

composite material. Automotive bumpers, fenders, and furniture parts are made using

this process.

Foams Foamed products can be produced in polystyrene, urethanes, polymethyl

methacrylate, and a number of other polymers. The polymer is produced in the form of

tiny beads, often containing a blowing agent such as pentane. During the pre-expansion

process, the bead increases in diameter by as many as 50 times. The pre-expanded beads

are then injected into a die, with the indiv idual beads fusing together, using steam, to

form exceptionally lightweight products with densities of perhaps only 0.02 g/cm

Expandable polystyrene (EPS) cups, packaging, and insulation are some of the appli-

cations for foams. Engine blocks for many automobiles are made using a patter n made

from expanded polystyrene beads.

EXAMPLE 16-10

Insulation Boards for Houses

You want to design a material that can be used for making insulation boards

that are approximately 120 cm wide and 240 cm tall. The material must provide

good thermal insulation. What material would you choose?

SOLUTION

Glasses tend to be good insulators of heat. However, they will be heavy, more

expensive, and prone to fracture. Polymers are lightweight, can be produced

inexpensively, and they can be good thermal insulators. We can use foamed

polystyrene since the air contained in the beads adds signiﬁcantly to their

e¤ectiveness as thermal insulators. For better mechanical properties, we may

want to produce foams that have relatively high density (compared to foams

that are used to make co¤ee cups). Finally, from a safety viewpoint, we want

to be sure that some ﬁre and ﬂame retardants are added to the foams. Such

panels are made using expanded polystyrene beads containing pentane. A

molding process is used to make the foams. The sheets can be cut into required

sizes using a heated metal wire.

16-11 Polymer Processing and Recycling 535

EXAMPLE 16-11 Thermoset Polymers by Compression Molding

Thermosets are commonly processed using a process known as compression

molding. This process is similar to sintering of ceramics in that resin (like ce-

ramic powder) is fed into mold cavities, that depend upon the shape of the

object being made, and then compressed between platens. The resin is then

heated while being pressed. Under the application of heat and pressure the

resin crosslinks (Figure 16-27). Additional pressure is needed if the component

being made is deeper (thicker). The force required to form a part of certain size

by compression molding is given by:

F ¼ðAÞ½ðP

Þþðr  d

Þ

In this, F: force, A: projected area of the part which is exposed to pressure, P

is the cavity pressure (usually experimentally developed) necessary for a given

thermoset, r: depth factor, i.e., pressure per unit length needed beyond

a certain depth d

(Ref. A.B. Strong, Plastics: Materials and Processing, Second

Edn., Publisher Prentice Hall). Calculate the force needed to form a thermoset

shown below. Assume that for a thin part (i.e., less than 3 cm thick part) the

value of P

is 25 MPa, r is 2 MPa/cm beyond a depth of 3 cm. Assume the

projected width and length are 10 cm and 20 cm, respectively. Assume that

the depth of the part is 7 cm.

SOLUTION

The total projected area for the cavity is 20  10 cm ¼ 200 cm

. In this case,

there is only one cavity that is in the mold that will be ﬁlled with the resin. The

depth of the part is 7 cm; this is 4 cm beyond what is considered thin here.

Thus, the total force necessary will be:

F ¼ðAÞ½ðP

Þþðr  d

Þ

F ¼ð200 cm

Þ½25 MPa þð2 MPa=cm ð7  3ÞcmÞ

F ¼ 200 cm

½33 MPa

F ¼ð200  10

4

Þ33 MN=m

¼ 0:66 MN ¼ 660 kN

CHAPTER 16 Polymers536

In practice, it can be assumed that a factor of safety of 1.3 is necessary. This

means, we should have equipment that can deliver a necessary force of 660 þ

198 kN. Thus, a force of 858 kN or approximately 900 kN will be required.

Recycling of Plastics Recycling is a very important issue and a full discussion of the

entire process is outside the scope of this book. But recycling plays an important role in

our everyday lives. Material is recycled in many ways. For example, part of the poly-

mer that is scrap from a manufacturing process (known as regrind) is used by recycling

plants. The recycling of thermoplastics is relatively easy and practiced widely. Note that

many of the everyday plastic products you encounter (bags, soda bottles, yogurt con-

tainers, etc.) have numbers stamped on them. For PET products (recycling symbol

‘‘PETE’’, because of trademark issues), the number is 1. For HDPE vinyl (recycling

symbol V), LDPE, PP, and PS the numbers are 2, 3, 4, 5 and 6, respectively. Other

plastics are marked number 7.

Thermosets and elastomers are more di‰cult to recycle, although they can still be

used. For example, tires can be shredded and used to make safer playground surfaces or

roads.

Despite enormous recycling e¤orts, a large portion of the materials in a landﬁll to-

day are plastics (the largest is that of paper). Given the limited amount of petroleum,

the threat of global warming, and a need for a cleaner and safer environment, careful

use and recycling makes sense for all materials.

SUMMARY

V Polymers are made from large macromolecules produced by the joining of smaller

molecules, called monomers, using addition or condensation polymerization re-

actions. Plastics are materials that are based on polymeric compounds, and they

contain many other additives that improve their prop erties. Compared with most

metals and ceramics, plastics have low strength, sti¤ness, and melting temperatures;

however, they also have a low density and good chemical resistance. Plastics are

used in a most diverse number of technologies.

V Thermoplastics have chains that are not chemically bonded to each other, permit-

ting the material to be easily formed into useful shapes, to have good ductility, and

to be economically recycled. Thermoplastics can have an amorphous structure,

which provides low stren gth and good ductility when the ambient temperature is

above the glass temperature. The polymers are more rigid and brittle when the tem-

perature falls below the glass temperature. Many thermoplastics can partially crys-

tallize during cooling or by application of a stress. This increases their strength.

V The thermoplastic chains can be made more rigid and stronger by using nonsym-

metrical monomers that increase the bonding strength between the chains and

make it more di‰cult for the chains to disentangle when stress is applied. In addi-

tion, many monomers produce more rigid chains containing atoms or groups of

atoms other than carbon; this structure also produces high-strength thermoplastics.

V Elastomers are thermoplastics or lightly cross-linked thermosets that exhibit greater

than 200% elastic deformation. Chains are eventually cross-linked using vulcan-

Summary 537

ization. The cross-linking makes it possible to obtain very large elastic deformatio ns

without permanent plastic deformation. Increasing the number of cross-links in-

creases the sti¤ness and reduces the amount of elastic deformation of the elastomers.

V Thermoplastic elastomers combine features of both thermoplastics and elastomers.

At high temperatures, these polymers behave as thermoplastics and are plastically

formed into shapes; at low temperatures, they behave as elastomers.

V Thermosetting polymers are highly cross-linked into a three-dimensional network

structure. Typically, high glass temperatures, good strength, and brittle behavior

are found. Once cross-linking occurs, these polymers cannot be easily recycled.

V Manufacturing processes used for polymers depend on their behavior. Processes

such as extrusion, injection molding, thermoforming, casting, drawing, and spin-

ning are made possible by the viscoelastic behavior of the thermoplastics. The non-

reversible behavior of bonding in thermosetting polymers limits their processing to

fewer techniques, such as compression molding, transfer molding and reaction-

injection molding.

GLOSSARY Addition polymerization Process by which polymer chains are built up by adding monomers

together without creating a byproduct.

Aging Slow degradation of polymers as a result of exposure to low levels of heat, oxygen, bac-

teria, or ultraviolet rays.

Aramids Polyamide polymers containing aromatic groups of atoms in the linear chain.

Blushing A thermoplastic bent repeatedly leads to small volumes of the material crystallizing;

this leads to voids that ultimately cause the material to fail.

Branched polymer Any polymer consisting of chains that consist of a main chain and secondary

chains that branch o¤ from the main chain.

Compounding Processing of elastomers in device known as a Banbury mixer followed by form-

ing using extrusion, molding, or dip coating.

Condensation polymerization A polymerization mechanism in which a small molecule (e.g.,

water, methanol, etc.) is condensed out as a byproduct.

Copolymer An addition polymer produced by joining more than one type of monomer.

Crazing Localized plastic deformation in a polymer. A craze may lead to the formation of

cracks in the material.

Cross-linking Attaching chains of polymers together to produce a three-dimensional network

polymer.

Degree of polymerization The average molecular weight of the polymer divided by the mole-

cular weight of the monomer.

Diene A group of monomers that contain two double-covalent bonds. These monomers are of-

ten used in producing elastomers.

Elastomers These are polymers (thermoplastics or lightly cross-linked thermosets) that have an

elastic deformation > 200%.

CHAPTER 16 Polymers538

Geometric isomer A molecule that has the same composition as, but a structure di¤erent from,

a second molecule.

Glass temperature (T

) The temperature range below which the amorphous polymer assumes a

rigid glassy structure.

Heat-deflection temperature The temperature at which a polymer will deform a given amount

under a standard load (also called distortion temperature).

Heat-degradation temperature The temperature above which a polymer burns, chars, or de-

composes.

Interpenetrating polymer networks Polymer structures produced by intertwining two separate

polymer structures or networks.

Linear polymer Any polymer in which molecules are in the form of spaghetti-like chains.

Liquid-crystalline polymers Exceptionally sti¤ polymer chains that act as rigid rods, even above

their melting point.

Mer A unit group of atoms and molecules that deﬁnes a characteristic arrangement for a poly-

mer. A polymer can be thought o¤ as a material made by combining several mers or units.

Monomer The molecule from which a polymer is produced.

Oligomer Low molecular weight molecules, these may contain two (dimers) or three (trimers)

mers.

Parison A hot glob of soft or molten polymer that is blown or formed into a useful shape.

Plastic A predominantly polymeric material made with some additives.

Polymer Polymers are materials made from giant (or macromolecular), chain-like molecules

having average molecular weights from 10,000 to more than 1,000,000 g/mol built by the joining

of many mers or units by chemical bonds. Polymers are usually, but not always, carbon based.

Relaxation time A property of a polymer that is related to the rate at which stress relaxation

occurs.

Repeat unit The repeating structural unit from which a polymer is built. Also called a mer.

Spinnerette An extrusion die containing many small openings through which hot or molten

polymer is forced to produce ﬁlaments. Rotation of the spinnerette twists the ﬁlaments into a yarn.

Stress-induced crystallization The process of forming crystals by the application of an external

stress. Typically, a signiﬁcant fraction of many amorphous plastics can be crystallized in this

fashion, making them stronger.

Stress relaxation A reduction of the stress acting on a material over a period of time at a con-

stant strain due to viscoelastic deformation.

Tacticity Describes the location in the polymer chain of atoms or atom groups in nonsym-

metrical monomers.

Thermoplastic elastomers Polymers that behave as thermoplastics at high temperatures, but as

elastomers at lower temperatures.

Thermoplastics Linear or branched polymers in which chains of molecules are not inter-

connected to one another.

Thermosetting polymers Polymers that are heavily cross-linked to produce a strong three

dimensional network structure.

Glossary 539

Askeland D.R., Fulay P.P. Essentials of Materials Science &amp; Engineering

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Askeland D.R., Fulay P.P. Essentials of Materials Science & Engineering