Plastics

From Storax to Polystyrene

Chemical Institute of Canada — Fri, 04 Mar 2011 01:13:55 GMT

Most chemists will not have heard of storax, yet it plays an important role in chemical history. It isn’t a specific compound, it is a resin produced by a number of tropical trees of the family Styracaceae in response to an injury to their bark. It has a long history of use in perfumes because of its lingering fragrance and its ability to slow the evaporation of other compounds that contribute to the overall scent. This “fixative” effect allows a perfume to keep the original fragrance for a longer time. Like other fixatives, such as sandalwood, patchouli, frankincense, benzoin, cedarwood, ambergris, musk and castor oil, storax is a complex mixture of compounds. Cinnamic acid, alpha-pinene, ethyl cinnamate and vanillin are just some that have been isolated. But in terms of historical impact, perhaps the most interesting compound found in storax is styrene.

In 1839, Eduard Simon, a German apothecary was attempting to separate the components of storax obtained from the Liquiambar orientalis tree by distillation. One of the fractions he collected was an oily substance that seemed to be a single compound. Simon named it styrol and stored it in a bottle. Much to his surprise, a few days later he noted that his styrol had changed from an oil into a hard translucent mass. Since he hadn’t added anything to the sample, he figured that it must have reacted with oxygen, and dubbed the new material “styrol oxide.” As it turned out, he was wrong. August Wilhelm Hofmann, one of the leading lights of German chemistry, showed that the styrol transformation also occurred in the absence of oxygen.

A solution to the mystery was proposed in 1866 by the brilliant French chemist, Marcelin Berthelot. The molecules of styrol, Berthelot suggested, must have joined together to form a long chain. Just three years earlier, he had delivered a landmark lecture to the Chemical Society of Paris in which he introduced the novel idea that small molecules could link together to form giant molecules, or polymers. But Berthelot did more than theorize. He carried out experiments to show that ethylene molecules could be joined together to form a new substance he called “polyethylene,” surely the first time that term was ever used. The history of polymer chemistry can be said to have begun with Berthelot’s pioneering work.

Simon’s “styrol” was eventually identified as styrene, or “vinylbenzene,” if we want to use proper chemical terminology, and his “styrol oxide” was actually polystyrene. Neither Simon nor Berthelot found a practical use for polystyrene, but by the 1930s German chemists did. They discovered that polystyrene could be cast into virtually any shape by pouring the molten substance into moulds and that it could also be extruded into sheets or films. Today, polystyrene is used to make a myriad items ranging from clear plastic glasses, laboratory equipment and cases for compact discs, to smoke detectors and disposable razors.

The most common use for polystyrene, however, is to produce “expanded polystyrene.” That’s the foamy stuff of coffee cups, insulation materials and those packing peanuts used to cushion electronic equipment. The “expansion” is produced by blowing pentane or carbon dioxide into the melted plastic. Pentane is a liquid, but quickly evaporates to yield a vapour and carbon dioxide of course is a gas. These gases attempt to escape from the molten polystyrene but they get trapped as the material cools, forming pockets. The result is polystyrene foam. “Styrofoam” is the Dow Chemical Company’s trade name for its version of foamed polystyrene. Of course, the styrene needed to make polystyrene is no longer obtained from the distillation of storax. It is now produced on a gigantic scale from benzene and ethylene derived from petroleum.

While polystyrene is a very useful substance, it is overused. We need computer housings and insulation material, but do we need all those disposable razors, cutlery and coffee cups? Polystyrene can be recycled but it is not always economical to do so, and it often ends up in landfills or being incinerated. That needs to change. We have to conserve our petroleum resources and our environment.

Polycarbonate is Polyfunctional

Chemical Institute of Canada — Wed, 17 Nov 2010 02:45:06 GMT

You really have to do something major to have a street named after you. You can thrill the world with music, you can make an impact in politics, you can become a star athlete, or like Daniel Fox, you can invent a plastic. Dan Fox Drive in Pittsfield Massachusetts is a tribute to the man who gave the world polycarbonate, a plastic that was to profoundly alter our lives.

Dr. Fox graduated with a PhD from the University of Oklahoma in 1952 and soon found a job as a research chemist with General Electric in Schenectady, N.Y. At the time G.E. researchers were looking for novel materials to be used as insulation for electric wires but were repeatedly stymied. Every material they tried deteriorated when exposed to water. It was at this point that Fox remembered a curious substance he had encountered in graduate school.

Fox had been working with guiacol, a compound that was found in creosote, the black oily guck that builds up inside chimney flues as a result of incomplete burning of wood or coal. There was interest in guiacol because of its potential to be converted into compounds that had applications in the pharmaceutical and food industries. Guiacol itself had antiseptic properties and also served as a potential raw material for the synthesis of vanillin, the major flavour component of the vanilla bean. As part of his research, Dr. Fox synthesized a number of guiacol derivatives, one of which was guiacol carbonate. Interestingly, this compound resisted breakdown, even in boiling water. At the time this didn’t seem to have any great importance, but now at G.E. Fox was searching for just such a material.

Guiacol carbonate was water resistant all right, but it was a simple molecule that could not be formulated into any sort of a plastic. But Fox understood how carbonates were synthesized and realized that if he started with the right ones he could link them together into long chains, in other words, into “polycarbonates.” There was a good chance that such a polymer would have the properties he desired. Alas, when he mixed his chemicals, all he got was a glob of a material that was so hard he couldn’t even remove his stirring rod. This was certainly not going to be any sort of insulating material. But it was interesting enough to keep around the lab. The blob of glob became a curiosity, sometimes used to drive nails, sometimes thrown down stairs in futile attempts to shatter it. No luck with that.

Hmmm, Fox thought, a plastic that doesn’t break ought to be patented! And in 1955, just two years after he had come across the novel material, Dr. Fox applied for a patent. As is routine with any new patent application, he carried out a patent search and discovered much to his amazement that the German chemical company Bayer had applied for a polycarbonate patent the same year! Apparently Dr. Hermann Schnell at Bayer had independently come up with a plastic almost identical to Fox’s. Since neither patent had yet been granted, the two companies held discussions and forged a working agreement. Whichever company was granted legal priority, it would allow the other one to manufacture polycarbonate as long as appropriate royalties were paid.

As it turned out, this worked in G.E’s favour. The patent was awarded to Bayer since the company was able to document that Schnell had invented polycarbonate a week before Fox gave birth to his discovery. Polycarbonate, it seems, had two legitimate fathers! It wasn’t long before G.E found a use for the new plastic. At the time, the company had a monstrous problem with electrical meter covers then made of glass. Rambunctious teenagers, stones, and glass meter covers were not a good mix, as G.E. found out. But meter covers made of polycarbonate could readily withstand the teenagers’ assaults! Lexan, as General Electric named its polycarbonate, was off and flying. Before long, it was really flying. Into outer space!

Astronauts helmets had to be able to withstand impact at both high and low temperatures, and the visors had to have exceptional clarity. Polycarbonate fit the bill. That “one small step for man, one giant leap for mankind” could not have been taken without polycarbonate. The plastic also leaped into car headlight assemblies, safety glasses, bullet resistant shields, fighter plane canopies, skylights, and “unbreakable” bottles. A polycarbonate tunnel allowed visitors at SeaWorld to walk through a shark tank. Many viewed the predators through lightweight, shatterproof plastic eyeglasses made of polycarbonate.

Then along came the electronic age, essentially made possible by polycarbonate. Computer casings, cell phones, and perhaps most importantly, compact discs and DVDs are all made of this remarkable plastic. It’s the only one that meets the strength, weight and optical purity characteristics needed for compact audio and video discs. Our life would truly be different without this plastic. In more ways than one.

The raw material needed to make polycarbonate is bisphenol A (BPA)! The very same compound that has made headlines because of its potential toxicity. Traces of BPA show up in our blood and urine, which is no surprise given that it is impossible to keep a chemical that is produced in such massive amounts from escaping into the environment to some small extent. But most of our BPA exposure actually comes from food, basically through the leaching of trace amounts from the epoxy resin that lines the inside of food cans. Like polycarbonate, it is formulated with bisphenol A.

The canning industry is frantically searching for a replacement, even though any effect of BPA on humans is controversial, to say the least. Contrary to some claims, BPA does not build up in the body, the amount we take in is equal to the amount we eliminate in the urine. Still, replacing the epoxy resin in can linings is a reasonable application of the precautionary principle. But clamoring for the total elimination of BPA is unrealistic, unscientific and unnecessary. Unless, that is, we’re ready to forget about our hockey and bicycle helmets, laptops, CDs, DVDs and cell phones.

Recycling Plastics

Chemical Institute of Canada — Mon, 19 Apr 2010 20:15:38 GMT

Recycling is easy to advocate, it is harder to do. In theory, paper, plastics, glass and metals can all be recycled but in practice, well that’s a different story. Aluminum, for example, is readily recycled. The metal can be melted down and used to make aluminum’s properties being affected. But such is not the case for plastics. Take, for example, polyethylene terephthalate, the plastic used to make those ubiquitous water and soft drink bottles. You can recognize this plastic by the number one recycling logo printed on the bottom. Yes, PET, as the plastic is abbreviated, can be recycled, although the bottles cannot be refilled. Plastic is not as inert as glass and there is no way to know what someone may have stored in an empty plastic bottle. Maybe some cleaning agent, or gasoline that could have been absorbed by the plastic. So the bottles cannot be refilled, but they can be recycled. How? The plastic is mechanically ground up, melted, usually mixed with virgin plastic, and formed into fabrics, carpeting, packaging and plastic lumber. But not made into new bottles. The problem is that the process by which plastics are made involves the stringing together of small molecules, called monomers, into long chains known as polymers. This is accomplished by the use of catalysts that are usually metal oxides or hydroxides. A small residue of these always stays behind in the plastic and causes a problem when the plastic is melted down and reformed into a novel form. The embedded catalyst weakens the plastic so that it isn’t suitable for a bottle, although it is fine for other uses.

A more attractive idea would be to break the plastic down into its original monomer components and sue these to make a polymer again. This can be done, but it requires high temperatures and pressures, which in turn make the process prohibitively expensive. The search is therefore on to fin a way to break the plastic down into its original components in a more environmentally friendly fashion. And new researchers at IBM and Stanford University may have come up with such a process. It all hinges on “organocatalysts,” an exciting area of research. Unlike most industrial catalysts, organic catalysts do not contain metal atoms and usually work under mild conditions. Plastic bottles can be chopped up, placed in a solution with the catalysts, and have their polymeric components be broken down into monomers. These monomers can be immediately reacted again with organic catalysts to form new polymers. If the process can be made to work on a large scale, it would go a long way towards making the recycling of bottles efficient and economic. Of course all of this is contingent on people placing bottles in the recycling bin in the first place, which many just don’t do. Most of the 13 billion plastic bottles disposed of each year globally end up in landfills. Of course it would be bar better to cut down on the use of these bottles in the first place. A glass and a tap make a very nice environmentally friendly combo.

On Behalf of Plastics

Chemical Institute of Canada — Mon, 11 Jan 2010 21:52:03 GMT

Just imagine a world without plastics. Although they are now a part of our daily lives, plastics are a relatively recent arrival on the chemical scene. These synthetic materials only began to enter our lives in a major fashion some seventy years ago. But they quickly captured peoples' imagination. Plastic Man, who could extend his arm to catch criminals, became an immediate hero! Of course, he was fictional. Today, though, we have real materials which before the Second World War could only be dreamed of. The raw material for most plastics derives from petroleum. But it is the ingenuity of the modern chemist that really makes plastics possible. The secret lies in taking the molecules derived from petroleum and linking them together to make the giant molecules, or "polymers," that characterize all plastics. The earliest plastics were actually just modifications of some naturally occurring polymers such as cotton. Treating cotton with a mixture of nitric and sulfuric acids yielded the first "miracle material," christened "celluloid." It could be molded to make all kinds of interesting objects ranging from combs, to billiard balls, to washable collars. The only problem with celluloid was its high flammability. Today, just about the only use for celluloid is in the making of ping-pong balls! No other plastic can quite match celluloid's bounce.

The first plastic that was not a modification of a natural substance but was made completely from scratch was "Bakelite," synthesized by Leo Bakeland in 1906. He found that mixing phenol and formaldehyde yielded a hard, useful synthetic. Early radios and black telephones were made of Bakelite. The pivotal moment in the development of modern plastics, however, occurred in the 1930's with the synthesis of nylon by the American chemist Wallace Carrothers. Combining hexamethylene diamine and sebacyl chloride instantly forms nylon, which can be drawn out like a fiber. Today we jump out of planes with nylon, scrape our windshields with it and even fasten our shoes with it. Velcro! Little nylon hoops and hooks which make life easier on earth and in space. There are some 250 meters of velcro tape used on each Space Shuttle mission to prevent objects from floating around. Other plastics we use regularly include polyvinyl chloride for vinyl raincoats, vinyl car seats, vinyl water pipes and vinyl balls. Polyethylene gives us bags to put our groceries in, hula hoops for exercise and plastic joints to replace a damaged hip. Polycarbonates like Lexan are super hard and give us roller blade wheels and almost unbreakable glassware. Some polymers, like the polyesters, can either be drawn into fibers or molded into various shapes such as bottles. Because polyester is so much lighter than glass, shipping costs are reduced. The polyester can even be recycled into clothing! It is an environmentally friendly material. True, not everything about plastics is rosy. Small amounts of their component chemicals can leach out and end up in the environment. Some of these have hormone like properties and have been accused of impairing our health. While there may be something to this, there is just no doubt that the benefits of plastics greatly outweigh any of the problems they present. If you don’t believe me, just try to go through a day without using some sort of plastic. I’ll wager you can’t.