Part II Engineering Materials
7.6 GUIDE TO PROCESSING CERAMICS
The processing of ceramics can be divided into two basic categories: molten ceramics and particulate ceramics. The major category of molten ceramics is glassworking (Chapter 12). Particulate ceramics include traditional and new ceramics; their processing methods constitute most of the rest of the shaping technologies for ceramics (Chapter 17).
Cermets, such as cemented carbides, are a special case because they are metal matrix composites (Section 17.3). Table 7.7 provides a guide to the processing of ceramic materials and the elements carbon, silicon, and boron.
TABLE 7.7 Guide to the processing of ceramic materials and the elements carbon, silicon, and boron.
Material Chapter or Section Material Chapter or Section
Glass Chapter 12 Synthetic diamonds Section 23.2.6
Glass fibers Section 12.2.3 Silicon Section 35.2
Particulate ceramics Chapter 17 Carbon fibers Section 15.1.2
Cermets Section 17.3 Boron fibers Section 15.1.2
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REFERENCES
[1] Carter, C. B., and Norton, M. G. Ceramic Materials:
Science and Engineering. Springer, New York, 2007.
[2] Chiang, Y-M., Birnie, III, D. P., and Kingery, W. D.
Physical Ceramics. John Wiley & Sons, Inc., New York, 1997.
[3] Engineered Materials Handbook, Vol. 4, Ceramics and Glasses. ASM International, Materials Park, Ohio, 1991.
[4] Flinn, R. A., and Trojan, P. K. Engineering Materials and Their Applications, 5th ed. John Wiley & Sons, Inc., New York, 1995.
[5] Hlavac, J. The Technology of Glass and Ceramics.
Elsevier Scientific Publishing Company, New York, 1983.
[6] Kingery, W. D., Bowen, H. K., and Uhlmann, D. R.
Introduction to Ceramics, 2nd ed. John Wiley &
Sons, Inc., New York, 1995.
[7] Kirchner, H. P. Strengthening of Ceramics. Marcel Dekker, Inc., New York, 1979.
[8] Richerson, D. W. Ceramics—Applications in Man-ufacturing. Society of Manufacturing Engineers, Dearborn, Michigan, 1989.
[9] Richerson, D. W. Modern Ceramic Engineering:
Properties, Processing, and Use in Design, 3rd ed.
CRC Taylor & Francis, Boca Raton, Florida, 2006.
[10] Scholes, S. R., and Greene, C. H. Modern Glass Practice, 7th ed. CBI Publishing Company, Boston, 1993.
[11] Schwarzkopf, P., and Kieffer, R. Cemented Carbides.
The Macmillan Company, New York, 1960.
[12] Singer, F., and Singer, S. S. Industrial Ceramics.
Chemical Publishing Company, New York, 1963.
[13] Somiya, S. (ed.). Advanced Technical Ceramics.
Academic Press, San Diego, California,1989.
REVIEW QUESTIONS
7.1. What is a ceramic?
7.2. What are the four most common elements in the Earth’s crust?
7.3. What is the difference between the traditional ceramics and the new ceramics?
7.4. What is the feature that distinguishes glass from the traditional and new ceramics?
7.5. What are the general mechanical properties of ceramic materials?
7.6. What are the general physical properties of ceramic materials?
7.7. What type of atomic bonding characterizes the ceramics?
7.8. What do bauxite and corundum have in common?
7.9. What is clay, as used in making ceramic products?
7.10. What is glazing, as applied to ceramics?
7.11. What does the term refractory mean?
7.12. What are some of the principal applications of cemented carbides, such as WC–Co?
7.13. What is one of the important applications of tita-nium nitride, as mentioned in the text?
7.14. What are the elements in the ceramic material Sialon?
7.15. Define glass.
7.16. What is the primary mineral in glass products?
7.17. What are some of the functions of the ingredients that are added to glass in addition to silica? Name at least three.
7.18. What does the term devitrification mean?
7.19. What is graphite?
MULTIPLE CHOICE QUIZ
There are 17 correct answers in the following multiple choice questions (some questions have multiple answers that are correct). To attain a perfect score on the quiz, all correct answers must be given. Each correct answer is worth 1 point. Each omitted answer or wrong answer reduces the score by 1 point, and each additional answer beyond the correct number of answers reduces the score by 1 point. Percentage score on the quiz is based on the total number of correct answers.
7.1. Which one of the following is the most common element in the Earth’s crust: (a) aluminum, (b) calcium, (c) iron, (d) oxygen, or (e) silicon?
7.2. Glass products are based primarily on which one of the following minerals: (a) alumina, (b) corundum, (c) feldspar, (d) kaolinite, or (e) silica?
7.3. Which of the following contains significant amounts of aluminum oxide (three correct answers): (a) alumina, (b) bauxite, (c) corundum, (d) feldspar, (e) kaolinite, (f) quartz, (g) sandstone, and (h) silica?
7.4. Which of the following ceramics are commonly used as abrasives in grinding wheels (two best answers):
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(a) aluminum oxide, (b) calcium oxide, (c) carbon monoxide, (d) silicon carbide, and (e) silicon dioxide?
7.5. Which one of the following is generally the most porous of the clay-based pottery ware:
(a) china, (b) earthenware, (c) porcelain, or (d) stoneware?
7.6. Which one of the following is fired at the highest temperatures: (a) china, (b) earthenware, (c) por-celain, or (d) stoneware?
7.7. Which one of the following comes closest to express-ing the chemical composition of clay: (a) Al2O3, (b) Al2(Si2O5)(OH)4, (c) 3AL2O3–2SiO2, (d) MgO, or (e) SiO2?
7.8. Glass ceramics are polycrystalline ceramic struc-tures that have been transformed into the glassy state: (a) true or (b) false?
7.9. Which one of the following materials is closest to diamond in hardness: (a) aluminum oxide, (b) car-bon dioxide, (c) cubic boron nitride, (d) silicon dioxide, or (e) tungsten carbide?
7.10. Which of the following best characterizes the struc-ture of glass-ceramics: (a) 95% polycrystalline, (b) 95% vitreous, or (c) 50% polycrystalline?
7.11. Properties and characteristics of the glass-ceramics include which of the following (two best answers):
(a) efficiency in processing, (b) electrical conductor, (c) high-thermal expansion, and (d) strong, relative to other glasses?
7.12. Diamond is the hardest material known: (a) true or (b) false?
7.13. Synthetic diamonds date to (a) ancient times, (b) 1800s, (c) 1950s, or (d) 1980.
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8 POLYMERS
Chapter Contents
8.1 Fundamentals of Polymer Science and Technology
8.1.1 Polymerization
8.1.2 Polymer Structures and Copolymers 8.1.3 Crystallinity
8.1.4 Thermal Behavior of Polymers 8.1.5 Additives
8.2 Thermoplastic Polymers
8.2.1 Properties of Thermoplastic Polymers 8.2.2 Important Commercial Thermoplastics 8.3 Thermosetting Polymers
8.3.1 General Properties and Characteristics 8.3.2 Important Thermosetting Polymers 8.4 Elastomers
8.4.1 Characteristics of Elastomers 8.4.2 Natural Rubber
8.4.3 Synthetic Rubbers
8.5 Polymer Recycling and Biodegradability 8.5.1 Polymer Recycling
8.5.2 Biodegradable Polymers 8.6 Guide to the Processing of Polymers
Of the three basic types of materials, polymers are the newest and at the same time the oldest known to man. Polymers form the living organisms and vital processes of all life on Earth. To ancient man, biological polymers were the source of food, shelter, and many of his implements. However, our interest in this chapter is in polymers other than biological. With the exception of natural rubber, nearly all of the polymeric materials used in engineering today are synthetic. The mate-rials themselves are made by chemical processing, and most of the products are made by solidification processes.
A polymer is a compound consisting of long-chain molecules, each molecule made up of repeating units con-nected together. There may be thousands, even millions of units in a single polymer molecule. The word is derived from the Greek words poly, meaning many, and meros (reduced to mer), meaning part. Most polymers are based on carbon and are therefore considered organic chemicals.
Polymers can be separated into plastics and rubbers. As engineering materials, they are relatively new compared to metals and ceramics, dating only from around the mid-1800s (Historical Note 8.1). For our purposes in covering polymers as a technical subject, it is appropriate to divide them into the following three categories, where (1) and (2) are plastics and (3) is the rubber category:
1. Thermoplastic polymers, also called thermoplastics (TP), are solid materials at room temperature, but they become viscous liquids when heated to temperatures of only a few hundred degrees. This characteristic allows them to be easily and economically shaped into products. They can be subjected to this heating and cooling cycle repeatedly without significant degradation of the polymer.
2. Thermosetting polymers, or thermosets (TS), cannot toler-ate repetoler-ated heating cycles as thermoplastics can. When initially heated, they soften and flow for molding, but the elevated temperatures also produce a chemical reaction 153
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that hardens the material into an infusible solid. If reheated, thermosetting polymers degrade and char rather than soften.
3. Elastomers are the rubbers. Elastomers (E) are polymers that exhibit extreme elastic extensibility when subjected to relatively low mechanical stress. Some elastomers can be stretched by a factor of 10 and yet completely recover to their original shape.
Although their properties are quite different from thermosets, they have a similar molecular structure that is different from the thermoplastics.
Thermoplastics are commercially the most important of the three types, constituting around 70% of the tonnage of all synthetic polymers produced. Thermosets and elastomers share the remaining 30% about evenly, with a slight edge for the former. Common TP polymers include polyethylene, polyvinylchloride, polypropylene, polystyrene, and nylon.
Examples of TS polymers are phenolics, epoxies, and certain polyesters. The most common example given for elastomers is natural (vulcanized) rubber; however, synthetic rubbers exceed the tonnage of natural rubber.
Historical Note 8.1 History of polymers
C
ertainly one of the milestones in the history of polymers was Charles Goodyear’s discovery of vulcan-ization of rubber in 1839 (Historical Note 8.2). In 1851, his brother Nelson patented hard rubber, called ebonite, which in reality is a thermosetting polymer. It was used for many years for combs, battery cases, and dental prostheses.At the 1862 International Exhibition in London, an English chemist Alexander Parkes demonstrated the possibilities of the first thermoplastic, a form of cellulose nitrate (cellulose is a natural polymer in wood and cotton). He called it Parkesine and described it as a replacement for ivory and tortoiseshell. The material became commercially important due to the efforts of American John W. Hyatt, Jr., who combined cellulose nitrate and camphor (which acts as a plasticizer) together with heat and pressure to form the product he called Celluloid. His patent was issued in 1870. Celluloid plastic was transparent, and the applications subsequently developed for it included photographic and motion picture film and windshields for carriages and early motorcars.
Several additional products based on cellulose were developed around the turn of the last century. Cellulose fibers, called Rayon, were first produced around 1890.
Packaging film, called Cellophane, was first marketed around 1910. Cellulose acetate was adopted as the base for photographic film around the same time. This material was to become an important thermoplastic for injection molding during the next several decades.
The first synthetic plastic was developed in the early 1900s by the Belgian-born American chemist L. H.
Baekeland. It involved the reaction and polymerization
of phenol and formaldehyde to form what its inventor called Bakelite. This thermosetting resin is still
commercially important today. It was followed by other similar polymers: urea-formaldehyde in 1918 and melamineformaldehyde in 1939.
The late 1920s and 1930s saw the development of a number of thermoplastics of major importance today.
A Russian I. Ostromislensky had patented polyvinyl-chloride in 1912, but it was first commercialized in 1927 as a wall covering. Around the same time, polystyrene was first produced in Germany. In England, fundamental research was started in 1932 that led to the synthesis of polyethylene; the first production plant came on line just before the outbreak of World War II. This was low density polyethylene. Finally, a major research program initiated in 1928 under the direction of W. Carothers at DuPont in the United States led to the synthesis of the polyamide nylon; it was commercialized in the late 1930s. Its initial use was in ladies’ hosiery; subsequent applications during the war included low-friction bearings and wire insulation. Similar efforts in Germany provided an alternative form of nylon in 1939.
Several important special-purpose polymers were developed in the 1940s: fluorocarbons (Teflon), silicones, and polyurethanes in 1943; epoxy resins in 1947, and acrylonitrile-butadiene-styrene copolymer (ABS) in 1948. During the 1950s: polyester fibers in 1950; and polypropylene, polycarbonate, and high-density polyethylene in 1957. Thermoplastic elastomers were first developed in the 1960s. The ensuing years have witnessed a tremendous growth in the use of plastics.
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Although the classification of polymers into the TP, TS, and E categories suits our purposes for organizing the topic in this chapter, we should note that the three types sometimes overlap. Certain polymers that are normally thermoplastic can be made into thermosets. Some polymers can be either thermosets or elastomers (we indicated that their molecular structures are similar). And some elastomers are thermoplastic. However, these are exceptions to the general classification scheme.
The growth in applications of synthetic polymers is truly impressive. On a volumetric basis, current annual usage of polymers exceeds that of metals. There are several reasons for the commercial and technological importance of polymers:
å Plastics can be formed by molding into intricate part geometries, usually with no further processing required. They are very compatible with net shape processing.
å Plastics possess an attractive list of properties for many engineering applications where strength is not a factor: (1) low density relative to metals and ceramics; (2) good strength-to-weight ratios for certain (but not all) polymers; (3) high corrosion resist-ance; and (4) low electrical and thermal conductivity.
å On a volumetric basis, polymers are cost-competitive with metals.
å On a volumetric basis, polymers generally require less energy to produce than metals.
This is generally true because the temperatures for working these materials are much lower than for metals.
å Certain plastics are translucent and/or transparent, which makes them competitive with glass in some applications.
å Polymers are widely used in composite materials (Chapter 9).
On the negative side, polymers in general have the following limitations: (1) strength is low relative to metals and ceramics; (2) modulus of elasticity or stiffness is also low—in the case of elastomers, of course, this may be a desirable characteristic; (3) service temperatures are limited to only a few hundred degrees because of the softening of thermoplastic polymers or degradation of thermosetting polymers and elastomers; (4) some polymers degrade when subjected to sunlight and other forms of radiation; and (5) plastics exhibit viscoelastic properties (Section 3.5), which can be a distinct limitation in load bearing applications.
In this chapter we examine the technology of polymeric materials. The first section is devoted to an introductory discussion of polymer science and technology. Subsequent sections survey the three basic categories of polymers: thermoplastics, thermosets, and elastomers.