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

Подождите немного. Документ загружается.

Unsaturated bond The double- or even triple-covalent bond joining two atoms together in an

organic molecule. When a single covalent bond replaces the unsaturated bond, polymerization

can occur.

Viscoelasticity The deformation of a material by elastic deformation and viscous ﬂow of the

material when stress is applied.

Vulcanization Cross-linking elastomer chains by introducing sulfur or other chemicals.

PROBLEMS

Section 16-1 Classification of Polymers

16-1 What are linear and branched polymers? Can

thermoplastics be branched?

16-2 Deﬁne (a) a thermoplastic, (b) thermosetting

plastics, (c) elastomers, and (d) thermoplastic

elastomers.

16-3 What electrical and optical applications are poly-

mers used for? Explain using examples.

16-4 What are the major advantages associated with

plastics compared to ceramics, glasses, and met-

allic materials?

Sections 16-2 Addition and Condensation

Polymerization

16-5 What do the terms condensation polymerization,

addition polymerization, initiator, and terminator

mean?

Section 16-3 Degree of Polymerization

Section 16-4 Typical Thermoplastics

16-6 Explain why low-density polyethylene is good to

make grocery bags, however, super high molec-

ular weight polyethylene must be used where

strength and very high wear resistance is needed.

16-7 The molecular weight of polymethyl meth-

acrylate (see Table 16-3) is 250,000 g/mol. If all

of the polymer chains are the same length,

(a) calculate the degree of polymerization, and

(b) the number of chains in 1 g of the polymer.

16-8 Calculate (a) the degree of polymerization if

polytetraﬂuoroethylene (see Table 16-3) is 7500.

(b) If all of the polymer chains are the same

length, calculate

(i) the molecular weight of the chains, and

(ii) the total number of chains in 1000 g of the

polymer.

16-9 A polyethylene rope weighs 15.12 g/cm. If each

chain contains 7000 repeat units,

(a) calculate the number of polyethylene chains

in a 3-m length of rope, and

(b) the total length of chains in the rope, as-

suming that carbon atoms in each chain are

approximately 0.15 nm apart.

16-10 Analysis of a sample of polyacrylonitrile (see

Table 16-3) shows that there are six lengths of

chains, with the following number of chains of

each length. Determine

(a) the weight average molecular weight and

degree of polymerization, and

(b) the number average molecular weight and

degree of polymerization.

Number of Chains

Mean Molecular Weight of

Chains (g/mol)

10,000 3,000

18,000 6,000

17,000 9,000

15,000 12,000

9,000 15,000

4,000 18,000

Section 16-5 Structure–Property Relationships

in Thermoplastics

Section 16-6 Effect of Temperature

on Thermoplastics

Section 16-7 Mechanical Properties

of Thermoplastics

16-11 Explain what the following terms mean: de-

composition temperature, heat distortion tem-

perature, glass temperature, and melting tem-

perature. Why is it that thermoplastics do not

have a ﬁxed melting or glass temperature?

16-12 Using Table 16-5, plot the relationship between

the glass temperatures and the melting tem-

peratures of the addition thermoplastics. What

is the approximate relationship between these

two critical temperatures? Do the condensation

CHAPTER 16 Polymers540

thermoplastics and the elastomers also follow

the same relationship?

16-13 List the addition polymers in Table 16-5 that

might be good candidates for making the

bracket that holds the rearview mirror onto

the outside of an automobile, assuming that

temperatures frequently fall below zero degrees

Celsius. Explain your choices.

16-14 Based on Table 16-5, which of the elastomers

might be suited for use as a gasket in a pump

for liquid CO

at 78



C? Explain.

16-15 How do the glass temperatures of polyethylene,

polypropylene, and polymethyl methacrylate

compare? Explain their di¤erences, based on the

structure of the monomer.

16-16 Which of the addition polymers in Table 16-5

are used in their leathery condition at room

temperature? How is this condition expected

to a¤ect their mechanical properties compared

with those of glassy polymers?

16-17 What factors inﬂuence the crystallinity of poly-

mers? Explain the development and role of

crystallinity in PET and nylon.

16-18 Describe the relative tendencies of the following

polymers to crystallize. Explain your answer.

(a) branched polyethylene versus linear poly-

ethylene,

(b) polyethylene versus polyethylene-polyprop-

ylene copolymer,

propylene, and

(d) polymethyl methacrylate versus acetal

(polyoxymethylene).

16-19 Explain the meaning of these terms: creep,

stress relaxation, crazing, blushing, environ-

mental stress cracking, and aging of polymers.

16-20 A stress of 17 MPa is applied to a polymer serv-

ing as a fastener in a complex assembly. At a

constant strain, the stress drops to 16.6 MPa af-

ter 100 h. If the stress on the part must remain

above 14.5 MPa in order for the part to function

properly, determine the life of the assembly.

16-21 A stress of 7 MPa is applied to a polymer that

operates at a constant strain; after six months,

the stress drops to 5.9 MPa. For a particular ap-

plication, a part made of the same polymer must

maintain a stress of 6.2 MPa after 12 months.

What should be the original stress applied to the

polymer for this application?

16-22 Data for the rupture time of polyethylene are

shown in Figure 16-17. At an applied stress of

4.8 MPa, the ﬁgure indicates that the polymer

ruptures in 0.2 h at 90



C but survives 10,000 h

at 65



C. Assuming that the rupture time is

related to the viscosity, calculate the activation

energy for the viscosity of the polyethylene and

estimate the rupture time at 23



16-23 For each of the following pairs, recommend

the one that will most likely have the better

impact properties at 25



C. Explain each of your

choices.

(a) polyethylene versus polystyrene,

(b) low-density polyethylene versus high-den-

sity polyethylene, and

ﬂuoroethylene.

Section 16-8 Elastomers (Rubbers)

16-24 The polymer ABS can be produced with vary-

ing amounts of styrene, butadiene, and acryl-

onitrile monomers, which are present in the

form of two copolymers: BS rubber and SAN.

(a) How would you adjust the composition of

ABS if you wanted to obtain good impact

properties?

(b) How would you adjust the composition

if you wanted to obtain good ductility at

room temperature?

you wanted to obtain good strength at

room temperature?

16-25 Figure 16-22 on the next page shows the stress-

strain curve for an elastomer. From the curve,

calculate and plot the modulus of elasticity ver-

sus strain and explain the results.

Applied stress (MPa)

1.4

2.8

4.2

6.9

13.8

27.6

Figure 16-17 (Repeated for Problem 16-22)

The effect of temperature on the stress-rupture

behavior of high-density polyethylene.

Problems 541

Section 16-9 Thermosetting Polymers

16-26 Explain the term ‘‘thermosetting polymer’’.

Why can’t a thermosetting polymer be pro-

duced using just adipic acid and ethylene glycol?

16-27 Explain why the degree of polymerization is

not usually used to characterize thermosetting

polymers.

16-28 Defend or contradict the choice to use the fol-

lowing materials as hot-melt adhesives for an

application in which the assembled part is sub-

jected to impact-type loading:

(a) polyethylene,

(b) polystyrene,

(d) polyacrylonitrile, and

(e) polybutadiene.

16-29 Compare and contrast properties of thermo-

plastics, thermosetting materials, and elastomers.

Figure 16-22 (Repeated for Problem 16-25)

The stress-strain curve for an elastomer. Virtually all

of the deformation is elastic; therefore, the modulus

of elasticity varies as the strain changes.

CHAPTER 16 Polymers542

Composites: Teamwork and

Synergy in Materials

Have You Ever Wondered?

9 What are some of the naturally occurring composite materials?

9 Why is abalone shell, made primarily of calcium carbonate, so much stronger than chalk, which is

also made of calcium carbonate?

9 What sporting gear applications make use of composites?

9 Why does the new 787 Dreamliner plane have 50% composite materials?

Composites are produced when two or more mate-

rials or phases are used together to give a combi-

nation of properties that cannot be attained other-

wise. Composite materials may be selected to give

unusual combinations of stiffness, strength,

weight, high-temperature performance, corrosion

resistance, hardness, or conductivity. Composites

highlight how different materials can work in

synergy. Abalone shell, wood, bone, and teeth

are examples of naturally occurring composites.

Microstructures of selected composites are shown

in Figure 17-1. An example of a material that is

a composite at a macro-scale would be steel-

reinforced concrete. Composites that are at a

543

micro-scale include such materials as carbon or

glass-fiber reinforced plastics (CFRP or GFRP).

These composites offer significant gains in spe-

cific strengths and are finding increasing uses

in airplanes, electronic components, automotives

and sporting equipment. For example, the latest

Boeing 787 Dreamliner plane has 50% composite

materials. The resultant lightweight structure will

cause 20% increase in its fuel efficiency.

As mentioned in Chapters 12 and 13, dis-

persion-strengthened (like steels) and precip-

itation-hardened alloys are examples of traditional

materials that are nanocomposites. In a nano-

composite, the dispersed phase consists of nano-

scale particles and is distributed in a matrix

phase. Essentially, the same concept has been

applied in developing what are described as hybrid

organic-inorganic nanocomposites. These are ma-

terials in which the molecular, or microstructure,

of the composites consists of an inorganic part or

block and an organic block. The idea is not too

different from the formation of block copolymers

(Chapter 16). These are often made using the

sol-gel process. These and other functional com-

posites can provide unusual combinations of elec-

tronic, magnetic, or optical properties. For exam-

ple, a porous dielectric material prepared using

phase-separated inorgani c glasses exhibits a di-

electric constant that is lower than that for the

same material with no porosity. A transparent pol-

ymer that is responsive to magnetic field can also

be prepared using composites. Space shuttle tiles

made from silica fibers are lightweight because

they consists of air and silica fibers and exhibit a

very small thermal conductivity. The two phases

in these examples would be ceramic and air.

Many glass-ceramics are nano-scale composites

of different ceramic phases. Many plastics can be

considered composites as well. For example,

Dylark

is a composite of maleic anhydride-styr-

ene copolymer. It contains carbon black for stiff-

ness and protection against ultraviolet rays. It also

Figure 17-1 Some examples of composite materials: (a) plywood is a laminar composite of

layers of wood veneer, (b) abalone shell, (c) fiberglass is a fiber-reinforced composite

containing stiff, strong glass fibers in a softer polymer matrix ( 175), and (d) concrete is a

particulate composite containing coarse sand or gravel in a cement matrix (reduced 50%).

C HA P T E R 1 7 Composites: Teamwork and Synergy in Materials544

contains glass fibers for increased Young’s mod-

ulus and rubber for toughness. Silver-filled ep-

oxies provide thermal conductivities higher than

those for expoxies. Some dielectric materials are

made using multiple phases such that the overall

dielectric properties of interest (e.g., the dielec-

tric constant) do not change appreciably with

temperature (within a certain range). Similarly,

many composites are prepared using magnetic

and optical materials. Some composite structures

may consist of different materials arranged in dif-

ferent layers. This le ads to what are known as

functionally graded materials and structur es. For

example, yttria stabilized zirconia (YSZ) coating

on a turbine blade will have other layers in be-

tween that provide bonding with the turbine blade

material. The YSZ coating itself is made using a

plasma spray or other technique and contains

certain levels of porosity which are essential for

providing protection against high temperatures.

Similarly, different coatings on glass are examples

of composite structures. Thus, the concept of us-

ing composites is a generic one and can be ap-

plied at the macro, micro, and nano length-scales.

In composites, the properties and volume

fractions of individual phases are important. The

connectivity of phases is also very important.

Usually the matrix phase is the continuous phase

and the other phase is said to be the dispersed

phase. Thus, terms such as ‘‘metal-matrix’’

indicate a metallic material used to form the

continuous phase. Connectivity describes how

the two or more phases are connected in the

composite. Newnham has described a con-

nectivity model for describing connectivities

for functional composites. Composites are often

classified based on the shape or nature of

the dispersed phase (e.g., particle-reinforced,

whisker-reinforced, or fiber-reinforced compo-

sites). Whiskers are like fibers; but their length is

much smaller. The bonding between the par-

ticles, whiskers, or fibers and the matrix is also

very important. In structur al composites, poly-

meric molecules known as ‘‘coupling agents’’ are

used. These molecules form bonds with the dis-

persed phase and become integrated into the

continuous matrix phase as well.

In this chapter, we will primarily focus on

composites used in structural or mechanical ap-

plications. Composites can be placed into three

categories—particulate, fiber, and laminar—

based on the shapes of the materials (Figure

17-1). Concrete, a mixture of cement and gravel,

is a particulate composite; fiberglass, containing

glass fibers embedded in a polymer, is a fiber-

reinforced composite; and plywood, having al-

ternating layers of wood veneer, is a laminar

composite. If the reinforcing particles are uni-

formly distributed, particulate composites have

isotropic properties; fiber composites may be ei-

ther isotropic or anisotropic; laminar composites

always display anisotropic behavior.

17-1 Dispersion-Strengthened Composites

A special group of dispersion-strengthened nanocomposite materials containing par-

ticles 10 to 250 nm in diameter is classiﬁed as particulate composites. These dispersoids,

usually a metallic oxide, are introduced into the matrix by means other than traditional

phase transformations (Chapters 12 and 13). Even though the small particles are not

coherent with the matrix, they block the movement of dislocations and produce a pro-

nounced strengthening e¤ect.

At room temperature, the dispersion-strengthened composites may be weaker than

traditional age-hardened alloys, which contain a coherent precipitate. However, be-

cause the composites do not catastrophically soften by overaging, overtempering, grain

17-1 Dispersion-Strengthened Composites 545

growth, or coarsening of the dispersed phase, the strength of the composite decreases

only gradually with increasing temperature (Figure 17-2). Furthermore, their creep

resistance is superior to that of metals and alloys.

The dispersoid must have a low solubili ty in the matrix and must not chemically

react with the matrix, but a small amount of solubility may help improve the bonding

between the dispersant and the matrix. Copper oxide (Cu

O) dissolves in copper at

high temperatures; thus, the Cu

O-Cu system would not be e¤ective. However, Al

does not dissolve in aluminum; the Al

-Al system does give an e¤ective dispersion-

strengthened material.

Examples of Dispersion-Strengthened Composites Table 17-1 lists some materials

of interest. Perhaps the classic example is the sintered aluminum powder (SAP) com-

posite. SAP has an aluminum matrix strengthened by up to 14% Al

. The composite

is formed by powder metallurgy. In one method, aluminum and alumina powders

are blended, compacted at high pressures, and sintered. In a second technique, the alu-

minum powder is treated to add a continuous-oxide ﬁlm on each particle. When the

powder is compacted, the oxide ﬁlm fractures into tiny ﬂakes that are surrounded by

the aluminum metal during sintering.

Figure 17-2

Comparison of the yield strength of

dispersion-strengthened sintered

aluminum powder (SAP) composite

with that of two conventional two-

phase high-strength aluminum alloys.

The composite has benefits above

about 300



C. A fiber-reinforced

aluminum composite is shown for

comparison.

TABLE 17-1 9 Examples and applications of selected dispersion-

strengthened composites

System Applications

Ag-CdO Electrical contact materials

Al-Al

Possible use in nuclear reactors

Be-BeO Aerospace and nuclear reactors

Co-ThO

Possible creep-resistant magnetic materials

Ni-20% Cr-ThO

Turbine engine components

Pb-PbO Battery grids

Pt-ThO

Filaments, electrical components

W-ThO

, ZrO

Filaments, heaters

C HA P T E R 1 7 Composites: Teamwork and Synergy in Materials546

Another important group of dispersion-strengthened composites includes thoria-

dispersed (ThO

) metals such as TD-nickel (Figure 17-3). TD-nickel can be produced

by internal oxidation. Thorium is present in nickel as an alloying element. After a

powder compact is made, oxygen is allowed to di¤use into the metal, react with

the thorium, and produce thoria (ThO

). The following example illustrates calculations

related to a dispersion-strengthened composite.

Figure 17-3

Electron micrograph of TD-nickel. The

dispersed ThO

particles have a diameter of

300 nm or less (2000). (From Oxide

Dispersion Strengthening, p. 714, Gordon and

Breach, 1968. > AIME.)

EXAMPLE 17-1

TD-Nickel Composite

Suppose 2 wt% ThO

is added to nickel. Each ThO

particle has a diameter of

1000 A. How many particles are present in each cubic centimeter?

SOLUTION

The densities of ThO

and nickel are 9.69 and 8.9 g/cm

, respectively. The

volume fraction is:

ThO

9:69

8:9

¼ 0:0184

Therefore, there is 0.0184 cm

of ThO

per cm

of composite. The volume of

each ThO

sphere is:

ThO

pð0:5  10

5

cmÞ

¼ 0:52  10

15

Concentration of ThO

particles ¼

0:0184

0:52  10

15

¼ 35:4  10

particles=cm

17-2 Particulate Composites

The particulate composites contain large amounts of coarse particles that do not e¤ec-

tively block slip. The particulate composites are designed to produce unusual combi-

nations of properties rather than to improve strength.

17-2 Particulate Composites 547

Rule of Mixtures Certain properties of a particulate composite depend only on the rel-

ative amounts and properties of the individual constituents. The rule of mixtures can ac-

curately predict these properties. The density of a particulate composite, for example, is

ðf

 r

Þ¼f

þ f

þþf

ð17-1Þ

where r

is the density of the composite, r

; r

; ...; r

are the densities of each con-

stituent in the composite, and f

; f

; ...; f

are the volume fractions of each constituent.

Note that the connectivity of di¤erent phases (i.e., how the dispersed phase is arranged

with respect to the continuous phase) is also very important for many properties.

Cemented Carbides Cemented carbides,orcermets, contain hard ceramic particles

dispersed in a metallic matrix (Chapter 1). Tungsten carbide (WC) inserts used for

cutting tools in machining operations are typical of this group. Tungsten carbide is a

hard, sti¤, high-melti ng-temperature ceramic. To improve toughness, tungsten carbide

particles are combined with cobalt powder and pressed into powder compacts. The

compacts are heated above the melting temperatu re of the cobalt. The liquid cobalt

surrounds each of the solid tungsten carbide particles (Figure 17-4). After solidiﬁcation,

the cobalt serves as the binder for tungsten carbide and provides good impact resist-

ance. Other carbides, such as TaC and TiC, may also be included in the cermet. The

following example illustrates the calculation of density for cemented carbide.

Figure 17-4

Microstructure of tungsten carbide—20% cobalt-

cemented carbide (1300). (From Metals

Handbook, Vol. 7, 8th Ed., American Society for

Metals, 1972.)

EXAMPLE 17-2 Cemented Carbides

A cemented carbide cutting tool used for machining contains 75 wt% WC,

15 wt% TiC, 5 wt% TaC, and 5 wt% Co. Estimate the density of the composite.

SOLUTION

First, we must convert the weight percentages to volume fractions. The den-

sities of the components of the composite are:

¼ 15:77 g=cm

TiC

¼ 4:94 g=cm

TaC

¼ 14:5g=cm

¼ 8:90 g=cm

C HA P T E R 1 7 Composites: Teamwork and Synergy in Materials548

75=15:77

75=15:77 þ 15=4:94 þ 5=14:5 þ 5=8:9

4:76

8:70

¼ 0:547

TiC

15=4:94

8:70

¼ 0:349

TaC

5=14:5

8:70

¼ 0:040

5=8:90

8:70

¼ 0:064

From the rule of mixtures, the density of the composite is

ðf

Þ¼ð0:547Þð15:77Þþð0:349Þð4:94Þþð0:040Þð14:5Þ

þð0:064Þð8:9Þ

¼ 11:50 g=cm

Abrasives Grinding and cutting wheels are formed from alumina (Al

), silicon car-

bide (SiC), and cubic boron nitride (CBN). To provide toughness, the abrasive particles

are bonded by a glass or polymer matrix. Diamond abrasives are typically bonded

with a metal matrix. As the hard particles wear, they fracture or pull out of the matrix,

exposing new cutting surfaces.

Electrical Contacts Materials used for electrical contacts in switches and relays must

have a good combination of wear resistance and electrical conductivity. Otherwise, the

contacts erode, causing poor contact and arcing. Tungsten-reinforced silver provides

this combination of characteristics. A tungsten powder compact is made using con-

ventional powder metallurgical processes (Figure 17-5) to produce high interconnected

Figure 17-5 The steps in producing a silver-tungsten electrical composite: (a) Tungsten

powders are pressed, (b) a low-density compact is produced, (c) sintering joins the tungsten

powders, and (d) liquid silver is infiltrated into the pores between the particles.

17-2 Particulate Composites 549

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

Подождите немного. Документ загружается.

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