[11.0] Plastics

v1.3.0 / chapter 11 of 15 / 01 jun 16 / greg goebel

* Along with traditional materials, in modern times we are heavily dependent of plastics, used for everything from throwaway bags to wings for combat aircraft. Plastics are cheap, lightweight, strong, often attractive, and can be synthesized with a wide range of properties. This chapter provides a survey of plastics technology.




* Plastics are based on polymers, long-chain organic molecules made up of repeating fundamental molecular elements, or "monomers". People have been using natural organic polymers for centuries in the form of waxes and shellacs -- as well as fabrics and ropes, which are based on a plant polymer named "cellulose". By the early 19th century natural rubber, based on a polymer now known as "isoprene" and tapped from rubber trees, was in widespread use.

Eventually, inventors learned to improve the properties of natural polymers. Natural rubber was sensitive to temperature, becoming sticky and smelly in hot weather, and brittle in cold weather. In 1834, two inventors, Friedrich Ludersdorf of Germany and Nathaniel Hayward of the US, independently discovered that adding sulfur to raw rubber helped prevent the material from becoming sticky.

In 1839, the American inventor Charles Goodyear (1800:1860) was experimenting with the sulfur treatment of natural rubber when, according to legend, he dropped a piece of sulfur-treated rubber on a stove. The rubber seemed to have improved properties, and Goodyear followed up with further experiments, developing a process known as "vulcanization" that involved cooking the rubber with sulfur.

Compared to untreated natural rubber, Goodyear's "vulcanized rubber" was stronger; more resistant to abrasion; more elastic; much less sensitive to temperature; impermeable to gases; and highly resistant to chemicals and electric current. Vulcanization still remains an important industrial process for the manufacture of rubber in both natural and artificial forms. Vulcanization creates sulfur bonds that link separate rubber polymers together, improving the material's structural integrity and its other properties.

Goodyear, incidentally, was never able to achieve real commercial success with his vulcanized rubber, dying penniless. He did become something of an American folk hero, however, and so the Goodyear tire company adopted his name as a tribute and promotional measure. Goodyear himself never had anything to do with the company.



* All Goodyear had done with vulcanization was improve the properties of a natural polymer. The next logical step was to use a natural polymer, cellulose, as the basis for a new material.

Inventors were particularly interested in developing synthetic substitutes for natural materials that were expensive and in short supply, since that meant a profitable market to exploit. Ivory was a particularly attractive target for a synthetic replacement. In 1856, an Englishman named Alexander Parkes (1831:1890) developed a "synthetic ivory" named "pyroxlin", which he marketed under the trade name "Parkesine", and which won a bronze medal at the 1862 World's Fair in London. Parkesine was made from cellulose treated with nitric acid and a solvent. The output of the process hardened into a hard, ivory-like material that could be molded when heated.

However, Parkes was not able to scale up the process to an industrial level, while products made from Parkesine quickly warped and cracked after a short time. An American printer and amateur inventor named John Wesley Hyatt (1837:1920) took up where Parkes left off. Parkes had failed for lack of a proper solvent, but Hyatt discovered that camphor would do the job nicely. Hyatt was something of an industrial genius who understood what could be done with such a shapeable, or "plastic", material, and proceeded to design much of the basic industrial machinery needed to produce quality plastic materials in quantity. Since cellulose was the main constituent used in the synthesis of his new material, Hyatt named it "celluloid". It was introduced in 1863.

One of the first products were dental pieces. Sets of false teeth built around celluloid proved cheaper than existing rubber dentures. However, celluloid dentures tended to soften when hot, making tea drinking tricky, and the camphor taste proved to be hard to eliminate. Celluloid's real breakthrough products were waterproof shirt collars, cuffs, and the false shirt fronts known as "dickies" -- whose inclination to pop up at awkward times became a stock joke in silent-movie comedies. Such celluloid items didn't wilt and didn't stain easily, and Hyatt sold them by trainloads. Corsets made with celluloid stays also proved popular, since perspiration didn't rust the stays as it did metal stays.

Celluloid proved extremely versatile in its field of application, providing a cheap and attractive replacement for ivory, tortoise-shell, and bone. Not only was celluloid cheaper in itself, but products that had been made with such traditional materials could now be molded in large batches, instead of produced by expensive hand craftsmanship. Some of the 19th-century items made with cellulose were beautifully designed and made. For example, celluloid combs made to tie up the long tresses of hair fashionable at the time are now jewel-like museum pieces. Such pretty trinkets were no longer only for the rich.

Celluloid could also be used in entirely new applications. Hyatt figured out how to fabricate the material in a strip format for movie film. By the year 1900, movie film was a major market for celluloid. However, celluloid still tended to yellow and crack over time, and it had another, more dangerous defect: it burned very easily and spectacularly, unsurprising given that nitric acid and cellulose are also used to synthesize smokeless powder. Ping-pong balls, one of the few products still made with celluloid, sizzle and burn if set on fire, and Hyatt liked to tell stories about celluloid billiard balls exploding when struck very hard. These stories might have had a basis in fact, since the billiard balls were often made of celluloid and covered with paints based on another, even more flammable, nitrocellulose product known as "collodion". The paints might have acted as primer to set the rest of the ball off with a bang.

* Cellulose was also used to produce cloth. While the men who developed celluloid were interested in replacing ivory, those who developed the new fibers were interested in replacing another expensive material, silk. In 1884, a French chemist, Hilaire de Charbonnet, the Comte de Chardonnay, introduced a cellulose-based fabric that became known as "Chardonnay silk". It was an attractive cloth, but like celluloid it was very flammable, a property completely unacceptable in clothing. After some ghastly accidents, Chardonnay silk was taken off the market.

In 1894, three British inventors, Charles Cross (1855:1835), Edward Bevan (1856:1921), and Clayton Beadle, patented a new "artificial silk" or "art silk" that was much safer. The three men sold the rights for the new fabric to the French Courtald company, a major silk manufacturer, who put it into production in 1905, using cellulose from wood pulp as the "feedstock" material. Art silk became well known under the trade name "rayon", and was produced in great quantities through the 1930s, when it was supplanted by better artificial fabrics. It still remains in production today, often in blends with other natural and artificial fibers. It is cheap and feels smooth on the skin, though it is weak when wet, and creases easily. It could also be produced in a transparent sheet form known as "cellophane".



* The limitations of celluloid led to the next major advance, known as "phenolic" or "phenol-formaldehyde" plastics. A chemist named Leo Hendrik Baekeland (1863:1944), a Belgian-born American living in New York state, was searching for an insulating shellac to coat wires in electric motors and generators. Baekeland found that mixtures of phenol (C6H5OH) and formaldehyde (HCOH) formed a sticky mass when mixed together and heated, and the mass became extremely hard if allowed to cool and dry.

He continued his investigations and found that the material could be mixed with fine sawdust, asbestos, or slate dust to create "composite" materials with a range of useful properties. Most of these compositions were strong and fire-resistant. The only problem was that the material tended to foam during synthesis, and the resulting product was of unacceptable quality. Baekeland built pressure vessels force out the bubbles and produce a smooth, uniform product. He publicly announced his discovery in 1909, naming it "bakelite". It was originally used for electrical and mechanical parts, finally coming into widespread use in consumer goods in the 1920s.

Bakelite was the first true plastic. It was a purely synthetic material, not based on any material or molecule found in nature. It was also the first "thermoset" plastic. Conventional "thermoplastics" can be molded and then melted again, but thermoset plastics form bonds between polymers when "cured", creating a tangled matrix that cannot be undone without destroying the plastic. Thermoplastics are tough and temperature-resistant. Bakelite was cheap, strong, and durable. It was molded into thousands of forms, such as radios, telephones, clocks, and of course billiard balls. Phenolic plastics are still in widespread use. For example, electronic circuit boards are made of sheets of paper or cloth impregnated with phenolic resin.

* After the First World War, improvements in chemical technology led to an explosion in new forms of plastics. Among the earliest examples in the wave of new plastics were "polystyrene (PS)" and "polyvinyl chloride (PVC)", developed by the I.G. Farben company of Germany.

Polystyrene is a rigid, brittle plastic that is now used to make plastic model kits, disposable eating utensils, and similar nicknacks. It would also be the basis for one of the most popular "foamed" plastics, under the name "styrene foam" or "styrofoam". Foam plastics can be synthesized in an "open cell" form, in which the foam bubbles are interconnected, as in an absorbent sponge, or "closed cell" form, in which all the bubbles are distinct, like tiny balloons, as in gas-filled foam insulation and flotation devices.

styrene monomer, polystyrene polymer

PVC has side chains incorporating chlorine molecules, which form strong bonds. PVC in its normal form is stiff, strong, heat and weather-resistant, and is now used for making pipe, gutters, house siding, enclosures for computers and other electronics gear, and compact-disk media. PVC can also be softened with chemical processing, and in this form it is now used for shrink-wrap, food packaging, and raingear.

vinyl chloride, polyvinyl chloride polymer

* The real star of the plastics industry in the 1930s was "polyamide (PA)", far better known by its trade name, "nylon". Nylon was the first purely synthetic fiber, introduced by Du Pont Corporation at the 1939 World's Fair in New York City.

In 1927, Du Pont had begun a secret development project designated "Fiber66", under the direction of a Harvard chemist named Wallace Carothers (1896:1937). Carothers had been hired to perform pure research, and not only investigated new materials, but worked to understand their molecular structure and how it related to material properties. He took some of the first steps on the road to "molecular design" of materials. His work led to the discovery of synthetic nylon fiber, which was very strong but still very flexible. The first application was for bristles for toothbrushes. However, Du Pont's real target was silk, particularly silk stockings.

nylon polymer

It took Du Pont twelve years and $27 million USD to refine nylon and develop the industrial processes for its manufacture in bulk. With such a major investment, it was no surprise that Du Pont spared little expense to promote nylon after its introduction, creating a public sensation that was called "nylon mania".

Nylon mania came to an abrupt stop at the end of 1941, when America entered World War II. The production capacity that had been built up to produce nylon stockings, or just "nylons", for American women, was taken over to manufacture parachutes for aircrew and paratroopers. After the war ended, Du Pont went back to selling nylon to the public, engaging in another promotional campaign in 1946 that resulted in an even bigger craze, triggering off "nylon riots". Nylon still remains an important plastic, and not just for use in fabrics. In its bulk form it is very wear-resistant, and so is used to build gears, bearings, bushings, and other mechanical parts.



* Another plastic that was critical to the war effort was "synthetic rubber", which was produced in a variety of forms. Practical synthetic rubber grew out of studies published in 1930 written independently by Carothers and the German scientist Hermann Staudinger (1881:1965). These studies led in 1931 to one of the first successful synthetic rubbers, known as "neoprene". Neoprene is highly resistant to heat and chemicals such as oil and gasoline, and is used in fuel hoses and as an insulating material in machinery.

In 1935, German chemists synthesized the first of a series of synthetic rubbers known as "Buna rubbers". These were "copolymers", meaning that their polymers were made up from not one but two monomers, in alternating sequence. One such Buna rubber, known as "GR-S (Government Rubber Styrene)", is a copolymer of butadiene and styrene, became the basis for US synthetic rubber production during World War II.

Worldwide natural rubber supplies were limited, and by mid-1942 most of the rubber-producing regions were under Japanese control. Military trucks needed rubber for tires, and rubber was used in almost every other war machine. The US government launched a major effort to ramp up synthetic rubber production, and by 1944 a total of 50 factories were manufacturing it, pouring out a volume of the material twice that of the world's natural rubber production before the beginning of the war. However, even today synthetic rubbers are only a complement to natural rubber in the manufacture of items such as tires, with synthetics, particularly GR-S, being used to extend the supply of natural rubber and to provide properties not available with natural rubber on its own. Nobody has yet developed a synthetic that can perfectly replace natural rubber at a competitive cost.

* Synthetic rubber would also play an important part in the space race and nuclear arms race. Solid-fuel rockets used during World War II used nitrocellulose explosives for propellants, but it was impractical and dangerous to make such rockets very big. During the war, California Institute of Technology (CalTech) researchers came up with a new solid fuel, based on asphalt fuel mixed with an oxidizer, such as potassium or ammonium perchlorate, plus aluminum powder, which burns very hot. This new solid fuel burned more slowly and evenly than nitrocellulose explosives, and was much less dangerous to store and use, though it tended to flow slowly out of the rocket in storage and the rockets using it had to be stockpiled nose-down.

After the war, the CalTech researchers began to investigate the use of synthetic rubbers instead of asphalt as the fuel in the mixture. By the mid-1950s, large missiles were being built using solid fuels based on synthetic rubber, mixed with ammonium perchlorate and high proportions of aluminum powder. Such solid fuels could be cast into large, uniform blocks that had no cracks or other defects that would cause nonuniform burning. Ultimately, most large military rockets and missiles would use solid fuels based on synthetic rubbers, and they would also play a significant part in the civilian space effort.



* Other plastics emerged in the prewar period, though some wouldn't come into widespread use until after the war. By 1936, American, British, and German companies were producing "polymethyl methacrylate (PMMA)", better known as "acrylic". Although acrylics are now well-known for the use in paints and synthetic fibers, such as "fake furs", in their bulk form they are actually very hard and more transparent than glass, and are sold as glass replacements under trade names such as "plexiglas" and "lucite". Plexiglas was used to build aircraft canopies during the war, and it is also now used as a marble replacement for countertops.

Another important plastic, "polyethylene (PE)", sometimes known as "polythene", was discovered in 1933 by the Reginald Gibson and Eric Fawcett at the British industrial giant Imperial Chemical Industries (ICI). This material evolved into two forms, "low density polyethylene (LDPE)", and "high density polyethylene (HDPE)".

ethylene monomer, polyethylene polymer

PEs are cheap, flexible, durable, and chemically resistant. LDPE is used to make films and packaging materials, while HDPE is used to containers, pipes, and automotive fittings. While PE has poor resistance to chemical attack, it was found later that a PE container could be made much more robust by exposing it to fluorine gas, which modified the surface layer of the container into the much tougher "polyfluoroethylene".

Polyethylene would lead after the war to an improved material, "polypropylene (PP)", which was discovered in the early 1950s. As mentioned earlier it is common in modern science and technology that the same inventions will pop up in different places at about the same time, but polypropylene was an extreme case of this phenomenon, being separately invented about nine times. It was a patent attorney's dream scenario, and litigation wasn't resolved until 1989. Polypropylene managed to survive the legal process, and two American chemists working for Philips Petroleum of the Netherlands, J. Paul Hogan (1919:2012) and Robert Banks (1921:1989), are now generally credited as the "official" inventors of the material. Polypropylene is similar to its ancestor, polyethylene, and shares polyethylene's low cost, but it is much more robust. It is used in everything from plastic bottles to carpets to plastic furniture, and is very heavily used in automobiles.

propylene monomer, polypropylene polymer

* Polyurethane was invented by Friedrich Bayer & Company of Germany in 1937, and would come into use after the war in blown form for mattresses, furniture padding, and thermal insulation. It is also used in non-blown form for sports wear as "lycra".

* In 1939, I.G. Farben Industrie of Germany filed a patent for "polyepoxide" or "epoxy". Epoxies are a class of thermoset plastic that form cross-links and "cure" when a catalyzing agent, or "hardener", is added. After the war they would come into wide use for coatings, "super glues", and composite materials. Composites using epoxy as a matrix include "fiberglass", where the structural element is glass fiber, and "carbon-epoxy composites", in which the structural element is carbon fiber. Fiberglass is now often used to build sport boats, while carbon-epoxy composites are an increasingly important structural element in aircraft, as they are lightweight, strong, heat-resistant, and can be molded into aerodynamically streamlined contours.

* Two chemists named Rex Whinfield and James Dickson, working at a small English company with the quaint name of the "Calico Printer's Association" in Manchester, developed "polyethylene terephthalate (PET or PETE)" in 1941, and it would be used for synthetic fibers in the postwar era, with names such as "polyester", "dacron", and "terylene". PET is more impermeable than other low-cost plastics and so is a popular material for making bottles for Coke and other "fizzy drinks", since carbonation tends to attack other plastics; and for acidic drinks such as fruit or vegetable juices. PET is also strong and abrasion resistant, and is used for making mechanical parts, food trays, and other items that have to endure abuse. PET films, tradenamed "mylar", are used to make recording tape.

* One of the most impressive plastics used in the war, and a top secret, was "polytetrafluoroethylene (PTFE)", better known as "teflon", which could be deposited on metal surfaces as a scratchproof and corrosion-resistant, low-friction protective coating. The polyfluoroethylene surface layer created by exposing a polyethylene container to fluorine gas is very similar to teflon.

A Du Pont chemist name Roy Plunkett (1910:1994) discovered teflon by accident in 1938. During the war, it was used in gaseous-diffusion processes to refine uranium for the atomic bomb, as the process was highly corrosive. By the early 1960s, teflon "non-stick" frying pans were a hot consumer item. One of the questions that quickly come up about teflon is: since nothing sticks to it, how does it stick to the pan? The answer is that the pan is treated with acid so that it ends up surfaced with tiny holes; the teflon flows into the holes when it is laid down, and is effectively "clothespinned" to the pan by a multitude of tiny "clothespins".

tetrafluorethylene monomer, teflon polymer

Teflon was later used to synthesize the miracle fabric "GoreTex", which can be used to build raingear that in principle "breathes" to keep the wearer's moisture from building up. GoreTex is also used for surgical implants; teflon strand is used to make dental floss; and teflon mixed with fluorine compounds is used to make "decoy" flares dropped by aircraft to distract heat-seeking missiles.

* After the war, the new plastics that had been developed entered the consumer mainstream in a flood. New manufacturing were developed, using various forming, molding, casting, and extrusion processes, to churn out plastic products in vast quantities. American consumers enthusiastically adopted the endless range of colorful, cheap, and durable plastic gimmicks being produced for new suburban home life.

One of the most visible parts of this plastics invasion was Earl Tupper's (1907:1983) "Tupperware", a complete line of sealable polyethylene food containers that Tupper cleverly promoted through a network of housewives who sold Tupperware as a means of bringing some money. The tupperware line of products was well thought out and highly effective, greatly reducing spoilage of foods in storage. Thin-film "plastic wrap" that could be purchased in rolls also helped keep food fresh.

Another prominent element in 1950s homes was "formica", a plastic laminate that was used to surface furniture and cabinetry. Formica was durable and attractive. It was particularly useful in kitchens, as it did not absorb stains from food preparation, such as blood or grease, and so was easy to keep clean. With formica, a very attractive and well-built table could be built using low-cost and lightweight plywood with formica covering, instead of expensive and heavy hardwoods like oak or mahogany.

Composite materials like fiberglass came into use for building boats and, in some cases, cars. Polyurethane foam was used to fill mattresses, and styrofoam was used to line ice coolers and make float toys.

* Plastics continue to be improved. General Electric introduced "lexan", a high-impact "polycarbonate" plastic with structural similarities to nylon, in the 1970s. Du Pont developed "kevlar", an extremely strong synthetic fiber that was best-known for its use in bullet-proof vests and combat helmets. Kevlar was so remarkable that Du Pont officials actually had to release statements to deny rumors that the company had received the recipe for it from space aliens.

One of the most potentially important new developments in plastic are circuits out of plastics is conductive polymers. Electronic circuitry fabricated using plastics or other materials that could be simply printed on a substrate could be incredibly cheap, opening the door to throwaway electronic devices that would cost pennies, or to applications hardly dreamed of now. So far, electronic devices made with such materials have not been acceptable for production, but prototypes of devices based on such technologies have been demonstrated.



* Although plastics have had a remarkable impact on our culture, it has become increasingly obvious that there is a price to be paid for their use. The first controversy arose in the late 1950s and early 1960s. There were a number of incidents where small children suffocated after crawling into plastic bags used by launderers to cover clothing. The plastics industry managed to fend off trouble by launching a massive public-education campaign.

By the late 1960s, plastics were increasingly seen as a symbol of an outdated 1950s consumer culture. The term "plastic" became an insult, used to describe someone thought of as soulless. At the end of the 1960s, the Beatles would even sing of "Polyethylene Pam", a "go-getter" who would do anything to get ahead. That was partly just a fashion statement, since plastics remained in widespread use anyway, and in many cases were much more effective and environmentally benign than the alternatives. However, this led to a problem as well, since the consumption of massive amounts of plastic goods created an equally massive problem with litter and waste disposal.

Plastic was almost too good, since it was durable and degraded very slowly. In some cases burning it could release toxic fumes. There were also the problems that manufacturing plastics often created large quantities of nasty chemical pollutants, and depleted the Earth's bounded supply of fossil fuels. By the 1990s, plastic recycling programs were common in the United States and elsewhere. Thermoplastics can be remelted and reused, and thermoset plastics can be ground up and used as filler, though the purity of the material tends to degrade with each reuse cycle. There are methods by which plastics can be broken back down to a feedstock state, and also work on thermosets that can be more conveniently recycled.

Products such as automobiles are now being increasingly designed to make recycling of their large plastic parts easier. To assist recycling of disposable items, the Plastic Bottle Institute of the Society of the Plastics Industry devised a now-familiar scheme to mark plastic bottles by plastic type. A recyclable plastic container using this scheme is marked with a triangle with three "chasing arrows" inside of it, which enclose a number giving the plastic type:

Unfortunately, recycling plastics proved troublesome. The biggest problem with plastics recycling is that it is difficult to automate the sorting of plastic waste, and so it is labor-intensive. While containers are usually made from a single type and color of plastic, making them relatively easy to sort, a consumer gadget like a cellphone may have many small parts consisting of over a dozen different types and colors of plastics. Styrofoam is particularly difficult to recycle, since it's so light -- it's hardly worth the bother to haul around -- and its nature as a foam means it can't be recycled, except by rendering it down to a feedstock form. There has been a push to phase out styrofoam containers, replacing them with containers made of more benign materials.

Recycling plastics tends to be unprofitable. For this reason, the percentage of plastics recycled in the US has traditionally been small, but technology has been continuously improving and the level of recycling is rising accordingly. Research has been done on "biodegradable" plastics that break down with exposure to sunlight. Starch can be mixed with plastic to allow it to degrade more easily, but it still doesn't lead to complete breakdown of the plastic. Some researchers have actually genetically engineered bacteria that synthesize a completely biodegradable plastic. So far, these plastics have proven too costly and limited for general use, and critics have pointed out that they only real problem they address is roadside litter, which is regarded as a secondary issue. When such plastic materials are dumped into landfills, they can become "mummified" and persist for decades, even if they are supposed to be biodegradable.

* As a footnote to the subject of plastics, World War II not only saw the introduction of modern plastics, the conflict also saw the development of a set of materials called "silicones" that have the properties of oils, resins, or rubbery plastics, but which are much more chemically inert and tolerant of heat. A silicone consists of a chain of alternating silicon and oxygen atoms, with an organic group, such as a methyl group, attached to the silicon atoms. Silicone oils have short chains, while silicone resins and rubbers are long-chain polymers.

Since silicone oils remain viscous at very low temperatures and do not become too thin at high temperatures, they are often used as lubricants in aerospace applications. Silicone rubbers are useful for high-temperature insulation, and silicones are also used for waterproofing fabrics and paper. Silicone putties are used as waterproof household putties, particularly in the bathroom, and as medical prosthetics -- most famously (or infamously) for breast implants.