HISTORY OF POLYMERS

Since most chemists and chemical engineers are now involved in some phase of polymer science or technology, some have called this the polymer age. Actually, we have always lived in a polymer age. The ancient Greeks classified all matter as animal, vegetable, and mineral. Minerals were emphasized by the alchemists, but medieval artisans emphasized animal and vegetable matter. All are largely polymers and are important to life as we know it. The word polymer is derived from the Greek poly and meros, meaning many and parts, respectively. Some scientists prefer to use the word macromolecule, or large molecule, instead of polymer. Others maintain that naturally occurring polymers, or biopolymers, and synthetic polymers should be studied in different courses.

However, the same principles apply to all polymers. If one discounts the end uses, the differences between all polymers, including plastics, fibers, and elastomers or rubbers, are determined primarily by the intermolecular and intramolecular forces between the molecules and within the individual molecule, respectively, and by the functional groups present. In addition to being the basis of life itself, protein is used as a source of amino acids and energy. The ancients degraded or depolymerized the protein in tough meat by aging and cooking, and they denatured egg albumin by heating or adding vinegar to the eggs. Early humans learned how to process, dye, and weave the natural proteinaceous fibers of wool and silk and the carbohydrate fibers of flax and cotton. Early South American civilizations such as the Aztecs used natural rubber (Hevea brasiliensis) for making elastic articles and for waterproofing fabrics.

Until Wo¨hler synthesized urea from inorganic compounds in 1828, there had been little progress in organic chemistry since the alchemists emphasized the transmutation of base metals to gold and believed in a vital force theory. Despite this essential breakthrough, little progress was made in understanding organic chemistry until the 1850s, when Kekule´ developed the presently accepted technique for writing structural formulas. However, polymer scientists displayed a talent for making empirical discoveries before the science was developed. Charles Goodyear grew up in poverty. He was a Connecticut Yankee born in 1800. He began work in his father’s farm implement business. Later he moved to Philadelphia where he opened a retail hardware store that soon went bankrupt. He then turned to being an inventor.

As a child he had noticed the magic material that formed a rubber bottle he had found. He visited the Roxbury India Rubber Company to try and interest them in his efforts to improve the properties of rubber, but they assured him that there was no need to do so. He started his experiments with a malodorous gum from South America in debtor’s prison. In a small cottage on the grounds of the prison, he blended the gum, the raw rubber called hevea rubber, with anything he could find, e.g., ink, soup, caster oil. While rubberbased products were available, they were either sticky or became sticky in the summer heat. He found that treatment of the raw rubber with nitric acid allowed the material to resist heat and not to adhere to itself.

This success attracted backers who helped form a rubber company. After some effort he obtained a contract to supply the U.S. Post Office with 150 rubber mailbags. He made the bags and stored them in a hot room while he and his family went away. When they returned they found the bags in a corner of the room, joined together as a mass. The nitric acid treatment was sufficient to prevent surface stickiness, but the internal rubber remained tacky and susceptible to heat. While doing experiments in 1839 at a Massachusetts rubber factory he accidently dropped a lump of rubber mixed with sulfur on the hot stove. The rubber did not melt but rather charred. He had discovered vulcanization, the secret that was to make rubber a commercial success. While he had discovered vulcanization, it would take several years of ongoing experimentation before the process was really commercially useful. During this time he and his family were nearly penniless.

Although he patented the process, it was too easily copied and pirated, so that he was not able to profit fully from his invention and years of hard work. Even so, he was able to develop a number of items. Charles Goodyear and his brother Nelson transformed natural rubber, hevea rubber, from a heat-“softenable” thermoplastic to a less heat-sensitive product through the creation of crosslinks between the individual polyisoprene chain-like molecules using sulfur as the crosslinking agent. Thermoplastics are two-dimensional molecules that may be softened by heating. Thermosets are materials that are three-dimensional networks that cannot be reshaped by heating. Rather than melting, thermosets degrade. As the amount of sulfur was increased, the rubber became harder becoming a hard rubber-like (ebonite) material. The spring of 1851 found the construction of a remarkable building on the lawns of London’s Hyde Park.

The building was designed by a maker of greenhouses so it was not unexpected that it had a greenhouse look. This Crystal Palace was to house almost 14,000 exhibitors from all over the world. It was the chance for exhibitors to show their wares. Charles Goodyear, then 50 years old, used this opportunity to show off his over two decades worth of rubber-related products

It was generally recognized by the leading organic chemists of the nineteenth century that phenol would condense with formaldehyde. Since they did not recognize the concept of functionality, Baeyer, Michael, and Kleeberg produced useless crosslinked goos, gunks, and messes and then returned to their research on reactions of monofunctional reactants. However, by the use of a large excess of phenol, Smith, Luft; and Blumer were able to obtain a hard yet meltable thermoplastic material. With his $750,000 Baekeland set up a lab next to his home.

He then sought to solve the problem of making the hard material made from phenol and formaldehyde soluble. After many failures, he thought about circumventing the problem by placing the reactants in a mold of the desired shape and allowing them to form the intractable solid material. After much effort he found the conditions under which a hard, clear solid could be made—Bakelite was discovered. Bakelite could be worked, was resistant to acids and organic liquids, stood up well to heat and electrical charge, and could be dyed to give colorful products. It was used to make bowling balls, phonograph records, telephone housings, gears, and cookware. His materials also made excellent billiard balls. Bakelite also acted as a binder for sawdust, textiles, and paper, forming a wide range of composites including Formica laminates, many of which are still used. It was also used as an adhesive giving us plywood.

While there is no evidence that Baekeland recognized what polymers were, he appeared to have a grasp on functionality and how to “use” functionality to produce thermoplastic materials that could later be converted to thermosets. Through control of the ratio of phenol to formaldehyde he was able to form a material that was a thermoplastic. He coined the term A-stage resole resin to describe this thermoplastic. This A-stage resole resin was converted to a thermoset crosslink, C-stage Bakelite, by additional heating. Baekeland also prepared thermoplastic resins called novolacs by the condensation of phenol with a lesser amount of formaldehyde under acidic conditions. The thermoplastic novolacs were converted to thermosets by addition of more formaldehyde. While other polymers had been synthesized in the laboratory, Bakelite was the first truly synthetic plastic. The “recipes” used today differ little from the ones developed by Baekeland, showing his ingenuity and knowledge of the chemistry of the condensation of the trifunctional phenol and difunctional formaldehyde.