Chemistry for Everyone edited by
Products of Chemistry
George B. Kauffman
The Influence of Solidification Techniques on the History of Material Culture
California State University Fresno, CA 93740
Norman E. Shank Department of Natural Sciences, Messiah College, Grantham, PA 17027;
[email protected] From the beginning of human history people have invented ways to make the things they need from materials found in their environment. These things must be made, at least in part, from solid materials so that they will maintain their useful shape. One approach has been to start with solid raw materials such as stones, soil, wood, or fibers. The raw material can be cut to change its shape or to reduce its size. Several pieces of raw material can be joined together to enlarge their size. Thus by a combination of cutting and joining a solid raw material, the desired shape can be achieved. Wooden houses and cloth garments are made in this way. This “cut– join” technology has been employed ever since someone first cut a stick and joined it to a stone to make a tool. However, material cultures that depend entirely on cut–join technology are quite limited in the kinds of artifacts they can produce. Another approach is to start with a fluid raw material, which has one distinct advantage over a solid, namely, that it can be shaped much more easily. But this is of little value, since the shape is just as easily lost. However, if after shaping, the fluid can be solidified into a durable solid, it becomes a very useful material for the manufacture of artifacts. Thus by a combination of shaping and solidifying a fluid raw material, the desired artifact can be made. Cement houses and glass bottles are made in this way. “Shape–solidify” technology makes possible the rapid mass production of an item once a mold has been fashioned or an extruder has been assembled. Since the making of the first pottery, most significant innovations in the use of raw materials have come about because of a discovery of some new way to make artifacts by shape– solidify technology. This article is a historical survey of the discovery of fluid raw materials that can be solidified into durable solids. Each discovery of a new material opened up an entire range of possibilities for the development of manufacturing. Since most manufactured items used today require some fluid-tosolid conversion for their production, this survey will outline the development of our modern material culture. The survey will also illustrate the difference between physical and chemical changes. This has direct implications regarding which artifacts can be recycled and which cannot. The information in this article is useful for introducing the field of materials science in lower-level chemistry and physical science courses. The approach is to integrate the information around previous studies of physical and chemical change and the transition from the liquid to the solid state. It is easy to point out examples during the class period, since most materials in the classroom and the school building underwent a fluid-to-solid conversion in their manufacture. Historical information helps students appreciate more fully the benefits of our modern material culture.
Definitions and Principles In this paper the term “artifact” refers to anything that is made by humans, directly or indirectly, from raw materials found in the environment. It may be small and simple, like a pin, or large and complex, like a building. The term “solid” refers to anything that keeps its shape if left on a shelf for a long period of time. It may be flexible. This includes stone, wood, leather, rubber, and cloth. The term “fluid” refers to anything that is fluid enough to be poured or soft enough to be molded. This includes molten metal, wet cement, and wet clay. The most critical and often most difficult step in the shape–solidify method of manufacture is the solidification. There are three basic ways this is usually achieved: 1. Cooling a hot liquid whose freezing point is above room temperature. 2. Removing water from a slurry of solid particles. 3. Producing a chemical reaction whose product is a solid.
The first two processes are physical changes and therefore easily reversible. If the material is used in pure form, it can easily be recycled and formed into a new shape. By contrast, the third process is a chemical change and is not easily reversible. These materials usually cannot be recycled in the sense of returning the material to the fluid state and reforming it into a new shape. Solidification by Cooling This technology works with a material that is a durable solid at room temperature. The material is made fluid by heating it to a sufficiently high temperature. This presents challenges both in achieving the high temperature and in handling the hot material. After shaping, solidification is automatic when the material is allowed to return to room temperature. However, it is often important to control the rate of cooling so that undesirable strains do not develop in the resulting solid. Three major classes of materials are used in this way: metals, glass, and thermoplastics.
Metals Copper was the first metal to be extracted from its ore, sometime around 4000 B.C.E. (1, 2). By 3000 B.C.E. bronze was being made by alloying tin with copper to make a more useful metal. When the Hebrews left Egypt about 1450 B.C.E., six metals—bronze, tin, lead, iron, gold, and silver— were in use in the Middle East (3). After 1200 B.C.E., iron replaced bronze as the most important metal in middle eastern culture. However, at that time the best iron artifacts were made from wrought iron, not cast iron, because the temperatures
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needed to melt low-carbon iron were not attainable (4 ). Aluminum became a major manufacturing material only after the Hall–Heroult process for the electrolytic reduction of bauxite was invented in 1886. Two challenges needed to be overcome to develop the usefulness of metals. First, most metals are found as ores in which the metal atoms exist as positive ions. These must be reduced to the neutral metal atoms. Early metal workers used charcoal for this purpose. Second, metals must be heated above their melting point to make them liquid for casting. This requires very high temperatures, especially for the harder, more useful metals. For example, copper melts at 1083 °C. The melting point can be lowered by alloying with another metal. Early bronze had about 10% tin in copper, and it melted around 1000 °C. Solid metals are malleable and ductile, which makes them especially versatile materials for manufacturing artifacts. Liquid metal is often cast in the simple shape of a bar or ingot. The detailed artifact is then produced by bending, beating, forging, rolling, or drawing. Since the solidification of molten metals is a physical process, metals can be recycled by melting and recasting them.
Glass Glass has been used from very early times. Glass found in Egypt has been dated around 2500 B.C.E. The practical use of glass increased greatly after the development of glass blowing around the first century B.C.E. (5, 6 ). Glass vessels and windows were made in Roman times (7). Early glass was made from a mixture of sand, sodium carbonate or plant ashes, and lime. The sodium carbonate or ashes lowered the melting point of the mixture to a workable level, and the lime made the glass stable in the presence of water (8, 9). Modern glasses have many formulations. Soda-lime glass for windows and glass bottles is about 70% SiO2, 15% Na2O, and 8% CaO, plus minor ingredients. “Pyrex”-type glass for laboratory use has about 16% B2O3 instead of CaO and less Na2O (10). At room temperature glass is an amorphous solid; that is, the atoms are not arranged in a regular crystal structure, and the solid does not have a precise melting point. As glass is heated, it gradually softens until, around 1000 °C for sodalime glass, it reaches a viscosity low enough to be easily worked into the desired shape. Bottles can be made by blowing a bubble of glass into a mold. Plate glass can be made by continuously pouring a ribbon of molten glass onto molten tin. Molten soda-lime glass is solidified by cooling below about 500 °C. It is then cooled slowly to room temperature in an annealing process to relieve strains in the solid glass. This physical process is reversible so glass can be recycled by melting and reshaping it. Thermoplastics Thermoplastics are solid carbon compounds that can be softened, enough to be molded, by heating and then solidified by cooling. Long before modern synthetic thermoplastics were invented, a few natural thermoplastics were molded to make artifacts. The following are some examples. The Romans used beeswax to make candles (11). Keratin, a strong fibrous protein obtained from horns, hooves, or tortoiseshell, was molded into artifacts before the 13th century (12). Keratins are linear polymers of amino acids with 1134
an α-helix structure. During the last century gutta percha, obtained from palaquium trees, was used to produce a wide variety of artifacts, including sheathing for the first submarine telegraph cable (13). Gutta percha is a polymer of trans-isoprene, (–CH2–C(CH3)=CH–CH2–)n. Asphalt is a thermoplastic that was used as a mortar as early as 3800 B.C.E. (14). While it is seldom used alone today to make artifacts, when mixed with other materials it is used extensively as a roofing material and a pavement for roads. The first synthetic thermoplastics, Parkesine (1862) and Celluloid (1869), were derivatives of naturally occurring cellulose (15, 16 ). Completely synthetic thermoplastics were first produced around the beginning of the 20th century. Those in common use today are produced by the polymerization of small organic molecules (17 ). Addition polymers are usually made by the polymerization of a single alkene. nH2C=CHX → [–CH2–CHX–]n Examples include polyethylene (X = H), polystyrene (X = C6H5), and polyvinyl chloride (X = Cl). Condensation polymers are made by the polymerization of two different molecules in an alternating sequence, with the elimination of a small molecule such as water. For example, nylon (18) is produced by the reaction nNH2(CH2)6NH2 + nHOOC(CH2)4COOH → H [–NH(CH2)6NH–CO(CH2)4CO–]n OH + (2n – 1)H2O Block copolymers occur when a long sequence of one monomer alternates with a long sequence of another monomer. These can be made to have elastic properties (19). For example, the styrene–butadiene–styrene block copolymer (SnBnSn) has the abbreviated structural formula –(–CH2–CH[C6H5]–)n(–CH2CH=CHCH2–)n(–CH2–CH[C6H5]–)n–
Thermoplastics are composed of very long chain molecules. The chains may be linear or branched. However, there are no chemical bonds from one molecule to another. The only forces between molecules are much weaker intermolecular forces. Because the molecules have a high molecular weight, these intermolecular forces are strong enough to make the materials solid at room temperature. As the temperature is raised, the molecules have enough energy to overcome the intermolecular forces and move about in a liquid state. Lowering the temperature returns the material to its solid form. Because the process is reversible, thermoplastics can be recycled. Solidification by Removal of Water This technology works with a material that is a durable solid at room temperature. The material is pulverized if necessary and mixed with water to make a fluid slurry or soft paste. After shaping, the material is solidified by removing the water. This may be done by evaporation, perhaps assisted by heating. Another method is to absorb the water into some porous substance, which may be the mold itself. It is important that the solid particles stick tightly together. A nonvolatile adhesive can be added to the water if necessary. Two major classes of materials are used in this way: clay and wood pulp.
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Clay When clay is mixed with water, a soft, easily molded material is produced, as anyone knows who has made mud pies or been stuck in mud. When the water evaporates, the clay particles stick together and a rigid solid is formed. However, this solid has limited usefulness because it is easily scratched and softens in the presence of water. It can be made harder and waterproofed by heating to a high temperature. The secondary heating produces a chemical reaction, which will be considered later. This section considers only the use of dry, unfired clay. Clay is used by birds to build nests, by termites to build homes, and by wasps to build cells for eggs. Humans have built homes with clay since the oldest city known—Jericho, about 8000 B.C.E. (20). Adobe construction using sun-dried clay bricks is especially suited to arid climates but is also used in many other parts of the world. With a stone foundation and an overhanging roof, adobe buildings can last for centuries. Today at least a third of the world’s population lives in houses made from unbaked earth. Clay is essentially a hydrated aluminum silicate. Other cations are usually present, and the crystal structure may vary so that a variety of clays exist. Clay particles are attracted by hydrogen bonding to water molecules, which lie between them (21). When enough water is present, the clay particles can slide past each other, but when water is removed, they stick together. Since the addition or removal of water is a reversible process, unbaked clay can be recycled. Wood Pulp Wood pulp or other finely shredded plant fibers can be mixed with water to form a slurry. This is shaped into sheets, which are dried to produce paper or fiberboard. In the year 105 C.E., Ts’ai Lun of China patented a process for making individual sheets of paper from a slurry by lifting out the fibers with a screen (22). The process was a closely guarded secret until Arabs captured the city of Samarkand in 751. They took the process to North Africa, and from there to Europe. In 1799 Louis Robert patented the first machine to produce a continuous strip of paper (23). The essence of paper is the network of plant fibers. Fillers such as clay can be added to give a glossy surface. Fiberboards are similar to paper, only thicker. Rollers are used to squeeze out most of the water before the final drying (24 ). Paper can be recycled by adding water and mixing to reconstitute the pulp slurry. Solidification by Chemical Reaction This technology works with a material that is a fluid at room temperature. After shaping, solidification takes place when a chemical reaction occurs within the fluid that converts it to a durable solid. In some cases the chemical reaction is initiated by heating. In other cases the reaction is spontaneous, and shaping must be accomplished in a limited time before the material sets. Three major classes of materials are used in this way: clay, inorganic cements, and thermoset polymers.
Clay As noted above, clay mixed with water is easily molded, and dries to a rigid solid. However, the solid scratches easily
and softens again in the presence of water. A much harder, waterproof solid can be formed by heating the dry clay to a high temperature—a process called firing. Fired pottery from Japan has been radiocarbon dated to about 10,700 B.C.E. (25). Pottery was made in Asia Minor before 6500 B.C.E. (26 ). Fired bricks were used in the Middle East (27), and ceramic tiles were used in Egypt (28) during the third millennium B.C.E. Today fired clay is used to make many artifacts, including electrical insulators. Firing produces several chemical reactions, depending on the temperature. These can be illustrated with a typical clay, kaolinite, Al2Si2O5(OH)4, which is a constituent of most pottery. All water used for softening is baked out below 200 °C. At a temperature around 550 °C the chemically combined water of hydration is driven off to produce metakaolin, Al2Si2O7. At 980 °C another reaction begins to separate SiO2 from the metakaolin. The reaction goes further around 1050 °C to give the overall reaction 3Al2Si2O7 → Al6Si2O13 + 4SiO2 The solid maintains its rigid shape during the entire process, and the final product mixture is much harder than dried clay (29). The chemical reactions above 900 °C are completely irreversible, so that fired clay cannot be made soft again with water. Thus it is not possible to recycle artifacts by returning them to the fluid state.
Inorganic Cements Inorganic cements are the binding agents in materials such as poured concrete, brick mortar, and plasters. Several materials are included here. All are compounds of calcium. When the powdered material is mixed with water, a paste is formed that is easily shaped or spread. In every case the solidification process involves one or more chemical reactions. As a result, the solid cannot be softened and reworked simply by adding more water. As early as 2200 B.C.E. gypsum plaster was used by Egyptians in pyramids (30), and it has been used extensively ever since. Plasterboard, used for the internal walls of many modern buildings, is made from gypsum plaster layered between two sheets of heavy paper. The plaster is made by mixing water with a powder which is either (CaSO4)2⭈H2O or CaSO4 or some mixture of both. (Plaster of Paris is pure (CaSO4)2⭈H2O.) In a short time the paste becomes hard, not because it dries out, but because the water combines chemically with the powder to produce solid gypsum, CaSO4⭈2H2O. This reaction can only be reversed by heating the calcium sulfate hydrate above 128 °C. Around 3000 B.C.E. in Egypt (31) and 2500 B.C.E. in Mesopotamia (32), lime, Ca(OH)2, was used as a mortar. When mixed with sand, lime mortar solidifies not only by water evaporation, but also by chemical reaction with atmospheric CO2 and with the sand. 3Ca(OH)2 + CO2 + SiO2 → CaCO3 + Ca2SiO4 + 3H2O The Romans made a major improvement by mixing lime with volcanic ash to make a cement that set even under water. They were the first to use poured concrete in building construction (33). Some of their structures still stand today. The ash is an example of a pozzolana—any siliceous material that reacts with Ca(OH)2 and water to form a cement that sets under water (34, 35).
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The next major improvement came in 1824 when Joseph Aspdin of England patented Portland cement, made by heating limestone and clay together. This powder is mixed with sand, or sand and small stones, to make the common cement now used so extensively. Its main ingredients (36 ) are 3CaO⭈SiO2, 2CaO⭈SiO2, 3CaO⭈Al2O3, and 4CaO⭈Al2O3⭈Fe2O3. All these anhydrous compounds react chemically with water to form solid hydrates. For example: 2(3CaO⭈SiO2) + 7H2O → 3CaO⭈2SiO2⭈4H2O + 3Ca(OH)2 Wet cement does not dry out. Instead, the water combines with the powder to become part of the solid product. Therefore cement will set under water and remain stable under water.
Thermoset Polymers Thermosetting polymers are composed of long-chain molecules in which the chains are chemically cross-linked. Thus the entire solid is effectively one giant molecule. If heat is added, the molecules are not free to move about, so the material remains solid even at high temperature. In 1838 Charles Goodyear (1800–1860) made a thermoset polymer by reacting sulfur with natural latex obtained from the rubber tree. Natural latex is a polymer of cis-isoprene, (–CH2–C(CH3)=CH–CH2–)n. Sulfur forms cross-links between the long rubber molecules (vulcanization), producing a material suitable for today’s automobile tires (37). In 1907 Leo Baekeland (1863–1944) invented the first completely synthetic thermoset by reacting phenol with formaldehyde to form a three-dimensional network structure:
+
CH2
OH
O OH
OH
CH2
CH2
CH2
CH2 HO
CH2
CH2
CH2
OH
The product, Bakelite, is a good electrical insulator and is used to make many artifacts (38). Many thermosets in current use are similar to the condensation polymers described above. However, one of the monomers must be at least trifunctional so that cross-linking can occur. Bakelite is one example of a phenolic resin in which phenol, C6H5OH, is the trifunctional monomer. Alkyd resins are polyesters in which an organic acid, HOOC–R–COOH, is condensed with a trialcohol such as glycerol, HO–CH2–CH(OH)–CH2–OH. Today, thermosets are often mixed with fibers of glass or graphite to produce strong, light composite materials. Urea formaldehydes are mixed with sawdust to make particle board. In urea formaldehyde, one molecule of urea, H2NCONH2, can condense with up to four molecules of formaldehyde, H2CO. Products of Chemistry articles in this Journal have featured a number of materials that harden by chemical reaction (39–41). Manufacture with thermosets begins with a fluid mixture in which molecules are held together only by intermolecular forces. The fluid is shaped, and a chemical reaction is initiated 1136
within the mixture which forms cross-links between the molecules. In some cases the cross-linking reaction is initiated by heating; in others, by the mixing of the precursor chemicals. The product of the cross-linking reaction is a network of chemically bonded atoms that is solid even at high temperatures. (At very high temperatures the material decomposes or chars but does not melt.) Because this chemical reaction is irreversible, the material cannot be made fluid again for reshaping. Conclusion The production of artifacts ultimately relies on the use of materials found in the environment: stone, sand, and clay, and plant and animal materials. Cultures that use only the solid forms of the materials are limited to cut–join technology. However, the fluidizing of many of these materials and the discovery of solidifying processes has opened the way to shape–solidify technology and greatly expanded the kinds of artifacts that can be made. This shape–solidify technology has made possible the material culture of today. Literature Cited 1. Raymond, R. Out of the Fiery Furnace: The Impact of Metals on the History of Mankind; Pennsylvania State University Press: University Park, 1986; pp 9–26, 50–63, 224–225. 2. Knauth, P. The Emergence of Man: The Metalsmiths; Time-Life Books: New York, 1974; Chapter 2. 3. Numbers 31:22; New International Version of the Holy Bible; Zondervan: Grand Rapids, MI, 1988. 4. Sass, S. The Substance of Civilization: Materials and Human History from the Stone Age to the Age of Silicon; Arcade: New York, 1998; pp 85–86. 5. The New Encyclopedia Britannica; Encyclopedia Britannica, Inc.: Chicago, 1998; Vol. 5, p 297. 6. Fine, G. J. J. Chem. Educ. 1991, 68, 765–768. 7. Doremus, R. Glass Science, 2nd ed.; Wiley: New York, 1994; pp 1–8. 8. Polak, A. Glass: Its Traditions and Its Makers; Putnam’s: New York, 1975; p 11. 9. Sass, S. Op. cit.; pp 102–103. 10. Rawson, H. Glasses and Their Applications; Institute of Metals: London, 1991; pp 1–6. 11. Funk & Wagnalls New Encyclopedia; Funk & Wagnalls, Inc.: New York, 1979; Vol. 5, p 100. 12. Williamson, C. J. In The Development of Plastics; Mossman, S. T. I.; Morris, P. J. T., Eds.; Royal Society of Chemistry: Cambridge, 1994; pp 3–5. 13. Mossman, S. In The Plastics Age: From Bakelite to Beanbags and Beyond; Sparke, P., Ed.; Overlook Press: Woodstock, NY, 1992; pp 17–18. 14. Encyclopedia Americana; Grolier: Danbury, CT, 1996; Vol. 2, p 516. 15. Seymour, R. B.; Kauffman, G. B. J. Chem. Educ. 1992, 69, 311–314. 16. Seymour, R. B.; Carraher, C. E. Giant Molecules: Essential Materials for Everyday Living and Problem Solving; Wiley: New York, 1990; Chapter 12. 17. Strong, A. B. Plastics: Materials and Processing; Prentice-Hall: Englewood Cliffs, NJ, 1996; pp 153–191.
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Chemistry for Everyone 18. Kauffman, G. B. J. Chem. Educ. 1988, 65, 803–808. 19. Seymour, R. B.; Kauffman, G. B. J. Chem. Educ. 1992, 69, 967–970. 20. Dethier, J. Down to Earth: Adobe Architecture; Facts on File: New York, 1982; pp 7–21. 21. Brady, N. The Nature and Properties of Soils; Macmillan: New York, 1990; p 208. 22. Schlosser, L. B. In Paper—Art and Technology; Long, P.; Levering, R., Eds.; World Print Council: San Francisco, 1979; pp 3–5. 23. Biermann, C. J. Handbook of Pulping and Papermaking, 2nd ed.; Academic: New York, 1996; p 1. 24. Encyclopedia Americana; Grolier: Danbury, CT, 1996; Vol. 11, p 155. 25. Aikens, C. M. In The Emergence of Pottery: Technology and Innovation in Ancient Societies; Barnett, W. K.; Hoopes, J. W., Eds.; Smithsonian Institution Press: Washington, DC, 1995; pp 11–13. 26. Cooper, E. A History of World Pottery; Larousse: New York, 1981; p 16. 27. The New Encyclopedia Britannica; Encyclopedia Britannica, Inc.: Chicago, 1998, Vol. 2, p 509. 28. Encyclopedia Americana; Grolier: Danbury, CT, 1996; Vol. 26, p 744.
29. Lawrence, W. G.; West, R. R. Ceramic Science for the Potter; Chilton: Radnor, PA, 1982; pp 36–38. 30. Coburn, A.; Dudley, E.; Spence, R. Gypsum Plaster: Its Manufacture and Use; Intermediate Technology Publications: London, 1989; pp 2–10, 38. 31. Encyclopedia Americana; Grolier: Danbury, CT, 1996; Vol. 7, pp 507–508. 32. Encyclopedia Americana; Grolier: Danbury, CT, 1996, Vol. 17, p 487. 33. Sass, S. Op. cit.; pp 128–131. 34. Hall, C. J. Chem. Educ. 1976, 53, 222. 35. Gupta, J. S. In Lime and Other Alternative Cements; Hill, N.; Holmes, S.; Mather, D., Eds.; Intermediate Technology Publications: London, 1992; p 191. 36. Young, J. F. In Instructional Modules in Cement Science; Roy, D. M., Ed.; Pennsylvania State University Press: University Park, PA, 1985; pp 5–6. 37. Seymour, R. B.; Carraher, C. E. Op. cit.; pp 224–229. 38. Strong, A. B. Op. cit.; pp 211–219. 39. Seymour, R. B.; Kauffman, G. B. J. Chem. Educ. 1992, 69, 909–910. 40. Seymour, R. B.; Kauffman, G. B. J. Chem. Educ. 1992, 69, 646–647. 41. Kauffman, G. B. J. Chem. Educ. 1993, 70, 887–893.
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