The Chemistry of Plastics Rudolph D. Deanin Plastics Engineering Department, University of Lowell, Lowell, MA 01854 Humankind has used conventional structural materials for millenia: steel and aluminum for large buildings, autos, airplanes, and packaging; stone and concrete for construction; glass for windows and packaging; wood for buildings and packaging; leather and natural fibers for clothing and upholstery; and paper for writing and packaging. ~~~i~~ the east 115 "vears., chemists have bee,, learning how to synthesize polymers-organic compounds of very high molecular weight-and engineers have been learning how to fabricate them into useful plastic products, sometimes replacing conventional structural materials but often permitting t h e development of new useful products t h a t were not even possible before. From 1869 to 1932, U S . plastics production grew t o 100 million pounds. Since 1932 it has grown t o 40 billion pounds and is currently growing over 10% per year. T h e reason ~ l a s t i c sare Pawing in importance is because they offer a number of technical advantages over conventional structural materials:
-
-
1. Processability. Plastics are easily shaped in the liquid state and
then solidified rapidly into finished whereas tional materials are difficult to shape and many small parts must can be converted be assembled into a finished oroduct.. dasties . into complete products of great complexity in a single simple processing step such as extrusion or injection molding.
2. Light weight. Plastic products weigh much less than metal or glass, only one-half to one-tenth as much, making them much
easier to transport and to store. 3. Toughness. Plastics can be made as clear as glass hut are much
4.
6.
6.
7.
8.
more resistant to breakage, and, when they break, they do not shatter. Lubricity. Plasticgearsand bearings havemueh less friction then metals, so they can run with little or no messy lubrication and run much longer before wearing out. f l a r n e . r r t a r d o n c ~ . \Vhilp organic cmqm~ndsusunllg du I,urn, the flammahilny varles greatly nith their ipec~fictrmpoiitim Many plastics are more flame-retardant than wood, paint, and paper and can he used in place of them to improve fire safety. Insulation. Plastics are mueh better thermal insulators than metals or ceramics, particularly when they are expanded into foams. They are much better electrical insulators than metals, textiles, or . paper, and lighter in weight than ceramics. . Appearance. Whereas conventionalmaterials are very limited in appearance, plastics are readily produced in a full range of colors, from transparent to translucent and opaque, . . and a great variety of surfacesand textures. Weather resistance. Whereas steel rusts, and woad swells, shrinks, and rats, plastics are much more resistant to weathering, and the best grades can withstand many decades outdoors with no observable change
Volume 64
Number 1
January 1987
45
9. Chemical resistance. Plastics are
much more resistant to inorganic chemical environments than most metal, wood, or paper products. Resistance to organic chemicals depends upon the specific plastic structure and can usually be obtained by choosing the proper polymer.
Synthesis of plastics generally begins with three raw materials: petroleum, salt, and air. During the past 40 years, petroleum chemists have learned to convert these economically into small reactive organic molecules called monomers. These are then ioined toeether hv. nolvmerization reactions, . . in on(, or more stages, to iorm giant pdgmer mulecules that ha\.e the structural strcnrth required in end ~ r o d u c t s'I'rn . the leading ones are described lx.lm\. Pdyrthgl~meis made in two strps from petroleum, C,H,,+,
-
CH,=CHz
-
-CH,-CH,-CH,-CHz-
and is most widely used in packaging film and bottles for milk and bleach. Polypropylene is also made in two steps from petroleum, CnH2012
- CH2=FH -
is mixed with liquid styrene monomer and organic peroxide initiator. This is impregnated into glass fiber mat and molded into end products such as automobile body parts, boat hulls, building panels, large storage tanks, furniture, and appliance bodies. In this case, the low-molecular-weight liquid intermediates provide easy molding, and the peroxide and heat polymerize them into cross-linked polymers of infinite molecular weight to deliver the strength of the end products. Polyurethanes require several intermediate synthetic steps, but the key polymerization reaction between liquid monomers HO-R-OH
t O=C=N-R'-N=C=O
-
-CH2CH-CH2-CH-
CHs
1
CH3
I
CH3
i ~ n dis must widely used in carpeting, rope, packaging cunlainerr, nutomubile hntteries and panelling, and small appliances. Polystyrene is made in four steps from petroleum,
isvery easily carried out to produce a great range of products ranging from flexible foam for upholstery and clothing and rigid foam for thermal insulation in buildings and appliances, t o molded front-ends for autos, high-strength rubber ~ r o d u c t sand elastic aarments. and high-durabilitv. paints . and adhtiives. The polymrr chrmist produces d l this \,arb r t s simuly hs his choicr uf R and t
