Ch 07

Chapter 7

Polymers: Structure, General Properties, and Applications QUALITATIVE PROBLEMS 7.18 What characteristics of polymers make them attractive for clothing? A number of characteristics make polymers attractive for clothing, including: thermal resistance, ability to be formed into fibers, low cost, low elastic modulus and very high wear resistance. 7.19 Do polymers strain harden more than metals or vice versa? Explain. In general, polymers strain harden more than metals. This can be seen in Fig. 7.13 on p. 180, where the polymer tension test specimen has a stable neck region, and the necked region grows as the tension test continues. A metal would neck and fracture. 7.20 Inspect various plastic components in an automobile, and state whether they are made of thermoplastic materials or of thermosetting plastics. By the student. Some typical parts that are thermoplastics are dashboard trim, cup holders, plastic fasteners, seat components, radio and dashboard knobs, carpeting, and seat-belt holders. Thermosets can be found as steering wheels, battery casings, structural parts in door frames, and car bodies in some models. (See also Sections 7.6 and 7.7 on pp. 183-186.) 7.21 Give applications for which flammability of plastics would be of major importance. By the student. A major concern is their use in aircraft and the subsequent effects during fires and crash landing. In these circumstances it is not only the flammability of the polymers that is a concern, but also the toxicity of combustion byproducts. The same concerns hold for use of polymers inside buildings, where it is essential to contain and prevent the spreading of fires. There are numerous applications where flammable polymers cannot be used, such as pressure vessels or containers for flammable materials, and cooking implements. (See also Table 7.3 on p. 183.)

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Polymers: Structure, General Properties, and Applications

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7.22 What characteristics make polymers advantageous for applications such as gears? What characteristics are drawbacks in such applications? By the student. The advantages include the low friction of polymers, even when not lubricated, and the high adhesive and abrasive wear resistance (see Section 33.5). In addition, polymers have good damping characteristics so that sound and impact forces are not as severe with plastic gears. Plastics used for gears also have manufacturing characteristics that allow the production of tooth profiles with superior surface finish (Chapter 19). The main drawbacks to polymers used in gears are associated with their low stiffness, especially at elevated temperature. Also, polymers have lower strength than metals, especially the steels used in gears, so the loads that can be transferred for an equivalent sized gear is much lower. 7.23 What properties do elastomers have that thermoplastics in general do not have? Elastomers (Section 7.9 starting on p. 188) are capable of returning to their original shape after being stretched, while thermoplastics cannot do so. In addition, there is a pronounced hysteresis loop in the loading and unloading of an elastomer, making these materials very valuable in vibration damping applications. 7.24 Do you think that the substitution of plastics for metals in products traditionally made of metal may be viewed negatively by the public at large? If so, why? By the student. The public may negatively view the substitution of plastics for metals if they consider plastics as a cheap material (rather than cost effective), are lower in strength (even if the strength is suitable for the particular application), and not as hard or durable. For some applications, there is an environmental perception polymers cannot be recycled as easily. 7.25 Is it possible for a material to have a hysteresis behavior that is the opposite of that shown in Fig. 7.14, so that the two arrows run counterclockwise? Explain. If the arrows in Fig. 7.14 on p. 189 were counterclockwise, the material would have a hysteresis gain, rather than a loss. This would mean that the energy put into the material is lower than the energy recovered during unloading, which is not possible. 7.26 Observe the behavior of the specimen shown in Fig. 7.13, and state whether the material has a high or a low strain-rate sensitivity exponent, m (see Section 2.2.7). The m value represents the strain-rate sensitivity of a material (see Section 2.2.7 on p. 63-64). The material in Fig. 7.13 on p. 180 elongates considerably by the orientation of molecules; thus the material would be expected to have a high strain-rate sensitivity, i.e., a high m value. This is equivalent to diffuse necking, as opposed to localized necking commonly observed with most metals in tension tests. 7.27 Add more to the applications column in Table 7.3. By the student. Some examples are:

© 2014 Pearson Education, Inc. Upper Saddle River, NJ. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction ,storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to : Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.


Design requirements Mechanical strength Functional and decorative Housings and hollow shapes Functional and transparent Wear resistance

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Applications Hangers, cables Electrical outlets, light switches Pens, electrical plugs Cassette holders, food containers Rope, car seats

7.28 Discuss the significance of the glass-transition temperature, Tg , in engineering applications. In engineering applications, where thermoplastics would be expected to carry a load (structural members), the material should have a glass-transition temperature higher than the maximum temperature to which it would be subjected in service. If it didn’t, the plastic would soften and eventually fail. 7.29 Describe how a rechargeable lithium battery works. See Case Study 7.2 on p. 181 for a discussion. Students should be encouraged to supplement this information with a literature or internet search. 7.30 Explain how cross-linking improves the strength of polymers. Cross-linked polymers (p. 174) have additional bonds linking adjacent chains together. The strength is increased because these cross-linking bonds give additional resistance to material flow; these bonds must be broken before the molecules can slide past one another. 7.31 Describe the methods by which the optical properties of polymers can be altered. Optical properties can be altered by additives which can alter the color or translucence of the plastic. Additives can either be dyes or pigments; they impart color to the plastic. Also, stress whitening makes the plastic appear lighter in color or more opaque. As stated in Section 7.2.2 on p. 177, optical properties are also affected by the degree of crystallinity of the polymer. 7.32 How can polymers be made to conduct electricity? Explain. Polymers can be made to conduct electricity. As stated on pp. 180-181, there are electrically conductive polymers such as polyacetylene, polyaniline, and polythiophene. Other polymers can be made more conductive by doping them with metal particles or whiskers. If continuous wire reinforcement is present, the polymer can be directionally conductive; it can be conductive in a plane if a mesh reinforcement is used. An electroless nickel plating of a polymer part can make it conductive. 7.33 Explain the reasons for which elastomers were developed. Elastomers were developed to obtain a material that could undergo a large amount of deformation without failure. They provide high friction and nonskid surfaces, shock and vibration isolation, and protection against corrosion. 7.34 Give several examples of plastic products or components in which creep and stress relaxation would be important considerations. By the student. Obviously, it is in high-temperature, low-stress situations where creep is important. Low temperature, high stress circumstances are where stress relaxation



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is important. As an example of the creep’s importance, polymers used in cookware as for pot handles must have creep resistance. As an example of stress relaxation, seat cushions will deform to provide a uniform stress distribution and provide comfort for the occupant. 7.35 Describe your opinions regarding the recycling of plastics versus the development of plastics that are biodegradable. By the student. Some arguments that may be made are that recycling actually has a cost associated with it, for example, in the fuel which must be consumed and other costs involved in collecting the material to be recycled. Also, the properties of the recycled polymer my be inferior compared to the virgin polymer. Biodegradable plastics have drawbacks as well; it is difficult to design them to degrade in the intended time frame, and they may have more failures in service, as a result. They can be much more expensive than polymers that are not biodegradable. 7.36 Explain how you would go about determining the hardness of plastics. Many of the hardness tests described in Section 2.6 on p. 67 are not suitable for polymers. Inelastic recovery of the surface makes tests such as Brinell and Knoop tests difficult to assess. The depth of penetration as measured in a Rockwell test will be time dependent because of stress relaxation. Consequently, durometer testing (p. 70) is an appropriate approach. There are variations of the hardness tests shown that are suitable for polymers; an Internet search will result in descriptions quickly. 7.37 Compare the values of the elastic modulus, given in Table 7.1, to the values for metals given in Chapters 2, 5, and 6. Comment on your observations. Individual comparisons can be made, but the obvious trend is that metals have an elastic modulus of at least an order of magnitude higher than those for polymers, even for the softest of metals. Consider the following plot showing the elastic modulus of various materials and note that the moduli for polymers are far lower than those for metals (or ceramics).



1012

1011

Metals

Polymers

83

Ceramics Carbides Alumina

Steels Cast iron Brass, bronze Aluminum Zinc alloys Magnesium alloys Babbitts

Graphite

Modulus of elasticity, E, Pa

1010 Phenol formaldehyde Acetal Nylon

109

Polyethylene

108

107 Natural rubber

106

7.38 Why is there so much variation in the stiffness of products made of polymers? Explain. Table 7.1 on 170 shows a wide range of stiffness; for example, for polyethylene the change can be 1400%. This is mainly due to the varying degree of polymerization and crystallinity, and the number of crosslinks, if any, present, as well as the effects of the reinforcements. Stiffness will increase with any of these variables. 7.39 Explain why thermoplastics are easier to recycle than thermosets. Thermoplastics are processed by melting or plasticizing polymer pellets or powders and then forming them into desired shape. If a polymer’s chemistry can be identified, then a formed polymer can be cut into small shapes (such as pellets or particles) and fabricated as is done with so-called virgin polymers. There is some degradation of mechanical properties and a measurable loss of molecular weight, but if properly sorted, these drawbacks can be minimized. It is difficult to recycle thermosets because it is impossible to break down a thermosetting resin into its mer components. Thus, the manufacturing strategies for the original polymer and for its recycled counterparts have to be different. Furthermore, the recycled thermoset cannot be chopped up and melted as thermoplastics would. 7.40 Give an example where crazing is desirable. By the student. As one example, plastic strips with adhesive on one side are commonly used as labels, where a die with a letter or number is pushed into the plastic (embossing, see p. 417), locally crazing it to form a visible imprint. 7.41 Describe the principle behind shrink wrapping. Shrink wrap consists of branched thermoplastics. When deformed above their glasstransition temperature, the branches attain a preferred orientation, similar to the effect of combing hair. The plastic is then quickly lowered in temperature, preventing stress



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relaxation. When the sheet is then wrapped around an object and then heated, the plastic relieves the stresses and shrinks around the object. 7.42 List and explain some environmental pros and cons of using plastic shopping bags versus paper bags. By the student. Some advantages of plastic bags over paper bags: trees are not consumed to make the plastic bags; plastic bags can be recycled; since they use less energy during manufacture than paper bags. Some environmental advantages of paper bags over plastic bags: paper is biodegradable; it is a renewable resource; trees filter the atmosphere and remove carbon dioxide and carbon monoxide, so by manufacturing bags and then burying them or disposing of them in landfills, the atmosphere is actually cleaned. 7.43 List the characteristics required of a polymer for (a) a bucket, (b) a golf ball, (c) an automobile dashboard, (d) clothing, (e) flooring, and (f ) fishing nets. By the student. For example: i. Some of the characteristics needed for a polymer insert in a total hip replacement are that it be biocompatible; not dissolve or warp in the presence of bodily fluids; support the loads developed during normal walking, sitting, and standing; not wear excessively; provide low friction. Cost is not as imperative as other applications, given the high cost of surgery for hip replacement. ii. For a golf ball, abrasion resistance is important, as well as impact strength and toughness. The polymer needs to have a stiffness consistent with most golf balls in order to ensure that the velocity and spin achieved off the club is repeatable. The polymer must be coatable, so that it can be made into a bright color. Cost is also important. iii. An automobile-dashboard polymer needs to be formable into the desired (and quite demanding) shape. It also has to be available in a range of desired colors, and should have reasonable manufacturing cost. iv. The polymer in clothing needs to be produced into very small-diameter fibers and in continuous lengths. The fibers need to be sufficiently flexible so that they can be woven into cloth and withstand normal wear and tear. The polymer needs to have low elastic modulus but sufficient strength so that the cloth feels soft but doesn’t tear easily. It must also be inexpensive. v. Laminated flooring needs to have high hardness to resist abrasion, and should be made with proper decoration and texture, and easy to clean. Laminated flooring also should have high friction so that people can walk on the surface without slipping. vi. Fishing nets must be strong, to efficiently capture fish without tearing. There is an interesting feature in some new fishing nets, intended to address the problem when a net is lost at sea, and floats in the ocean continuing to catch fish. Some novel fishing nets are degradable so that prolonged exposure to saline solutions causes them to lose strength and become gels; these then fall apart and do not cause uncontrolled devastation to an ecosystem. 7.44 How can you tell whether a part is made of a thermoplastic or a thermoset? By the student. There are several nondestructive and destructive tests (see Sections 36.10 and 36.11, respectively) that can be performed. Tension tests will demonstrate



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the difference: a pronounced plasticity is indicative of a thermoplastic. Exposure to high temperatures is another test: the presence of a glass-transition temperature is indicative of a thermoplastic. The shape of the part is often a clue; for example, thin films (roughly as thick as a sheet of paper) must be made of thermoplastics because they are blown from extruded tubing. 7.45 As you know, there are plastic paper clips available in various colors. Why are there no plastic staples? Paperclips must function completely in the elastic range. Because of their nature, staples require plastic deformation, with little or no elastic recovery. Thermoplastics will recover dramatically, making them unsuitable as staples, and thermosets will readily fracture before beginning to achieving the desired shape of a staple. 7.46 By incorporating small amounts of a blowing agent, it is possible to manufacture hollow polymer fibers with gas cores. List possible applications for such fibers. By the student. If a polymer has a gas core, it has clear benefits in that the fiber has lower density; if used for clothing, the cloth will be lighter for the same thickness. Also, since gases have lower thermal conductivity than polymers, a fiber with a gas core will have better thermal insulation properties. This means that very thin layers of the polymer fiber will be suitable for cold-weather clothing. 7.47 In injection-molding operations (Section 19.3), it is common practice to remove the part from its runner, to place the runner into a shredder, and to recycle the resultant pellets. List the concerns you would have in using such recycled pellets as opposed to so-called virgin pellets. The main concerns about shredded polymers are that the properties of the polymer may be deteriorated as a result of shredding. This can happen if dirt or contaminants get into the polymer, or if there is so much shredding that the molecular weight of the polymer is reduced. Also, there is concern that lubricants present in the system may contaminate the polymer, and for critical applications, there is the additional concern that wear particles from the shredder may end up in the polymer. Obviously, one cannot tolerate, for example, metal particles in pacifiers for infants or in food packaging. 7.48 From an environmental standpoint, do you feel it is best to incorporate polymers or metals into designs? Explain. By the student. This is a difficult problem, considering that some metals are recycled more than some polymers. Reliability of the material, the suitability of applications, and the carbon footprint over the entire lifecycle need to be considered in the answer.

QUANTITATIVE PROBLEMS 7.49 Calculate the areas under the stress–strain curve (toughness) for the materials shown in Fig. 7.11, plot them as a function of temperature, and describe your observations.



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By the student. The area under each curve in Fig. 7.11 on p. 179 is estimated by adding the areas under the initial elastic region and the flat regions under the curve. The points and a plot are as follows: Toughness (MJ/m3)

-25 0 25 50 65 80

140 635 760 730 520 270

Toughness (MJ/m3)

800 Temperature (°C)

700 600 500 400 300 200 100 -40

-20

0

20

40

60

80

100

Temperature (°C)

Note that the toughness is a maximum somewhere around 25◦ C. As temperature increases, the polymer begins to soften and melt; consequently its toughness will approach zero. 7.50 Note in Fig. 7.11 that, as expected, the elastic modulus of the polymer decreases as temperature increases. Using the stress–strain curves in the figure, make a plot of the modulus of elasticity versus the temperature. Comment on the shape of the curve. By the student. The curve is as follows. Note that the shape is very nearly linear.

Elastic modulus (psi x 103)

250 200 150 100 50 -40 -20

0

20 40 60 Temperature (°C)

80

100

7.51 A rectangular cantilever beam 75 mm high, 20 mm wide, and 1 m long is subjected to a concentrated load of 50 kg at its end. From Table 7.1, select three unreinforced and three reinforced materials and calculate the maximum deflection of the beam in each case. Then select aluminum and steel for the same beam dimensions, calculate the maximum deflection, and compare the results.



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The formula for maximum deflection, d, for a cantilevered beam is d=

P L3 3EI

where P is the load (50 kg or 490 N), L is the beam length (1.0 m), E is the elastic modulus, and I is the moment of inertia = bh3 /12, where b = 0.02 m and h = 0.075 m; hence, I = 7.031 × 10−7 m4 . Substituting into the equation above, d=

P L3 (490)(1.0)3 1 1 = = (2.323 × 108 N/m) −7 3EI 3(7.031 × 10 ) E E

As samples of calculations, the following have been taken from Table 7.1 on p. 172, using mean values when a range is given for Young’s modulus: Material

Young’s modulus (GPa) ABS 2.1 Acetal 2.45 Epoxy 10.25 ABS, reinforced 7.5 Acetal, reinforced 10 Epoxy, reinforced 36.5 Aluminum1 70 Steel1 195 Note: 1. From Table 2.2 on p. 58.

Deflection (m) 0.111 0.0948 0.0227 0.0310 0.0232 0.0636 0.00331 0.00119

ABS Acetal Epoxy ABS, reinforced 1m

Acetal, reinforced

75mm

Epoxy, reinforced 50 kg

Aluminum Steel 0

0.030

0.060

0.090

0.1200

Deflection (m)

7.52 Estimate the number of molecules in a typical automobile tire, then estimate the number of atoms in the tire. The number of molecules in a car tire is one because of the extensive cross linking occurring during the vulcanization process (see p. 189). Tire sizes vary, as do specific



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chemical makeups. If 80% of the molecules are hydrogen and the remainder carbon, the average atomic weight is 3.208 (1.0079 for hydrogen and 12.011 for carbon, as obtained from a periodic table of the elements). Recall from introductory physics courses that this means one mole of hydrogen weighs 1.0079 g. Therefore, assuming a weight of 10 kg for the tire, there are approximately 3000 moles in a tire, or 1.9 × 1027 atoms. 7.53 Using strength and density data, determine the minimum weight of a 1-mlong tension member that must support a load of 5000 N, if it is manufactured from (a) high molecular- weight polyethylene, (b) polyester, (c) rigid PVC, (d) ABS, (e) polystyrene, and (e) reinforced nylon. Refer to the derivation given in Prob. 5.41. The area needed is given by σy = F/A, or A = F/σy . The volume is V = AL = F L/σy , and the weight is W = ρgV = ρgF L/σy . In this case, L = 1.0 m, and F = 5000 N. The numbers here were obtained from a polymer handbook and are representative of the particular polymer, but the student may use approximate numbers from Table 3.1 on p. 89 and Table 7.1 on p. 170. Also note that the calculations are based on ultimate strength, not yield strength as was done with metals. The results are shown below: Material HMW polyethylene Polyester Rigid PVC ABS Polystyrene Reinforced nylon

Density (kg/m3 ) 950 1270 1400 1030 1000 1130

UTS (MPa) 24 50 41-52 30-52 32-56 90

Required weight (N) 1.94 1.24 1.67-1.31 1.68-0.97 1.53-0.87 0.61

7.54 Plot the following for any five polymers described in this chapter: (a) UTS versus density and (b) elastic modulus versus UTS. Where appropriate, plot a range of values. By the student. There are a large number of polymers that can be considered; see Table 7.1 on p. 170 for UTS and elastic modulus information. However, density information for particular polymers will require a literature or internet search.

SYNTHESIS, DESIGN AND PROJECTS 7.55 Conduct an Internet search, and describe differential scanning calorimetry. What does this technique measure? By the student. Differential scanning calorimetry applies heat to a sample, and carefully measures the temperature change, comparing this change to a standard sample. 7.56 Describe the design considerations involved in replacing a metal beverage container with one made of plastic. By the student. Note that the beverage can must be nontoxic and have sufficient strength (from low temperatures in the refrigerator to hot summer temperatures) to prevent from



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rupturing under internal pressure or buckling under a load. Because beverage cans are produced in the range of millions per day, the processing would have to be simple and highly reliable. Additional considerations include chilling characteristics, labeling, and aesthetics and feel. 7.57 Assume that you are manufacturing a product in which all of the gears are made of metal. A salesperson visits you and asks you to consider replacing some of these metal gears with plastic ones. Make a list of the questions that you would raise before making a decision. By the student. Some of the questions to be asked are: Will the plastic retain its required strength, stiffness, and tolerances if temperature rises during its normal use? How acceptable is the wear resistance and fatigue life of the gears? Is it compatible with metal gears that it may mesh with? What are its frictional characteristics? Is the plastic gear affected adversely by any lubricants present? Will the supplier be able to meet the quality demands? How much cost savings are involved? Various other questions can be raised by the student. 7.58 Assume you work for a company that produces polymer gears. You have arranged to meet with a potential new customer, who currently uses gears made of metal. Make a list of the benefits that plastic gears present, and prepare a presentation for the meeting. This is an open-ended question, with many possible answers. Another good option is to assign one-half of a class to this problem, and the others to Problem 7.57. Some of the benefits: • By using non-standard addenda and dedenda, the contact ratio can be made to be above 1. • Polymers are naturally lubricious and when properly designed can lead to less chance of pitting failure. • The polymer gears can be mass produced and are less expensive. • The polymer gears damp vibrations better. • The polymer gears are quieter. 7.59 Sections 7.6 and 7.7 list several plastics and their applications. Rearrange this information, making a table of products (gears, helmets, luggage, electrical parts, etc.) that shows the types of plastic that can be used to make these products. By the student. Some examples are: • • • • • •

Gears: Acetals, nylons, polyesters. Helmets: ABS, cellulosics, polycarbonates. Luggage: ABS, polyethylene, polypropylene, polyesters. Electrical parts: Fluorocarbons, nylons, polyethylenes, alkyds, urea, epoxies. Lenses: Acrylics, polycarbonates. Pipes and tubing: Acetals, ABS, cellulosics, nylon, polyethylene, polypropylene, PVC.



90

7.60 Make a list of products or parts that currently are not made of plastics, and offer possible reasons why they are not. By the student. Some examples are: Gas-turbine components: plastics do not possess the necessary strength at high temperatures. Keys: thermal expansion of the keys may not allow them to fit properly in keyholes, and wear resistance of plastics is generally less than that for metals. 7.61 Review the three curves shown in Fig. 7.10, and give some applications for each type of behavior. Explain your choices. By the student. Some examples are: • Rigid and brittle: Handles, because they should not flex significantly; heat resistance, coupled with rigidity, is also useful for cookware handles. • Tough and ductile: Helmets, because these plastics can dissipate the energy from impact without fracturing. • Soft and flexible: Beverage bottles, because they can deform when dropped and regain their shape and not break, unlike glass bottles. 7.62 Repeat Problem 7.61 for the curves shown in Fig. 7.12. By the student. Some examples are: • Low-density polyethylene: The impact strength at low temperatures makes them useful for applications such as nonbreakable food containers. • High-impact polypropylene: The high impact strength at a range of temperatures allows it to be used in automotive trim so that, in a collision, the trim will not crack and may not have to be replaced. • Polyvinyl chloride (PVC): It can be either flexible or rigid, and either type can be used for tubing. Since it is not particularly strong or impact resistant, its use must be limited to low-pressure tubing. It is also very water resistant. • Polymethylmethacrylate: It has moderate strength, good optical properties, and is weather resistance; Note that the main drawback to this material is low impact resistance. These properties makes it useful for lighting fixtures (that by their nature do not require high impact resistance). 7.63 In order to use a steel or aluminum container for an acidic liquid, such as tomato sauce, a polymeric barrier is usually placed between the container and its contents. Describe possible methods of producing such a barrier. The most common method is to (a) dissolve a thermosetting polymer in a chemical liquid carrier, usually a ketone, (b) then spraying it onto the can interior, and (c) boiled off, leaving an adherent polymer coating. A less common approach is to laminate or coat the inside surface of the sheet stock with a metallic materials. 7.64 Conduct a study of plastics used for some products. Measure the hardness and stiffness of these plastics. (For example, dog chew toys use plastics with a range of properties.) Describe your observations. By the student. The numbers will reflect the values given in Table 7.1 on p. 170. 7.65 Add a column to Table 7.1 that describes the appearance of these plastics, including available colors and opaqueness.



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By the student. Note that most plastics can be made opaque, but only a few (such as acrylics and polycarbonates) are transparent. Most are available in more than one color, especially thermoplastics such as polyethylene and ABS. 7.66 With Table 7.3 as a guide, inspect various products, both in a typical kitchen and in an automobile, and describe the types of plastics that were used or could be used in making their individual components. By the student. A wide variety of answers are acceptable for all types of applications. It can be especially beneficial if students attempt to identify the particular polymer, either by examining product literature, performing an Internet search, or simply identifying the symbol used for recycling the polymer.


Ch 07

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