Plastics are based on giant molecules (polymers) which have a structure so ordered that they can be shaped at elevated temperatures and pressures, ie, these long-chain polymers exhibit "plastic flow" when heated. They are often modified with other materials (eg, plasticizers, fillers, stabilizers) before being processed in the molten state. Certain "thermosetting" polymers are subject to irrevocable chemical changes (curing or cross-linking) so that once shaped they are infusible. The "thermoplastic" types (notably polyethylene, polyvinyl chloride, polypropylene, polystyrene) can be recycled because they do not lose their ability to flow when remelted.
The forming of plastics into film, pipe, bottles and a myriad of molded shapes is estimated to employ 70 600 people (1997) in Canada. In 1997, this industry generated shipments valued at $11.8 billion. The plastics-processing industry accounts for 0.5% of national gross domestic product (GDP), 0.5% of total national employment and 3.9% of manufacturing employment. Some of the major processors include Leco Inc, Toronto; Union Carbide Canada Ltd, Toronto; and Canadian General-Tower Ltd, Cambridge, Ontario. This manufacturing activity has defied easy classification because the output (from bathtubs to wire and cable insulation) is more often than not an integral part of another industrial operation. Where the plastic part is a component of a larger assembly (eg, automobiles, TV sets), the same blurring of industrial categories occurs. Many producers of durable goods operate plastics-processing equipment on their own premises. In contrast to the manufacture of the resins, usually undertaken by multinationals strong in polymer technology, the fabricating of plastics products is often guided by entrepreneurs. The most successful of these are generally members of the Society of Plastics Engineers (SPE) and the Society of Plastics Industry (SPI).
The equipment, whether the property of over 1400 small and medium-sized independent establishments or "captive" to a particular assembly line, has increased in sophistication and size over the last century. In 1881 a Toronto cabinetmaker began laminating cellulose nitrate (ie, celluloid) sheet onto piano and organ keyboards. The advent of phenolformaldehyde polymers in 1909-1910 (products in which Lawrence Redmond, had a pioneering role) paved the way for compression-molding presses.
Not until thermoplastic cellulose acetate, initially a material employed for coatings and fibres, was recognized as an excellent molding compound, did processors accept the injection-molding machine. The injection system entails melting the polymer and forcing the molten material under high pressure into closed-mold cavities. In 1931 French Ivory Products of Toronto, making cellulose acetate caps for toothpaste tubes, operated the first injection-molding machine in North America. The molding of cellulosics had limited application but, in the next 30 years, processors learned to work with new materials: nylon, polyvinylchloride, polystyrene, polyethylene and polypropylene came out of the laboratory before 1960.
Today a mold is designed to exploit the particular properties of a wide range of thermoplastics. Cost is of major importance; therefore, most injection-molded products are formed from so-called "commodity" resins (eg, ethylene, styrene, propylene, vinyl chloride). For certain high-performance parts requiring toughness, special electrical properties, or resistance to elevated temperatures or similar "hostile" environments, the molder is likely to choose one of several engineering resins (eg, nylon, polyphenylene oxide, polycarbonate, acetals, terephthalates).
Polyethylene is the most ubiquitous of thermoplastics. It is popular because it can be extruded through a shaping die to form a thin film. Film products such as milk pouches, bread bags, grocery sacks and garbage bags have transformed the flexible packaging industry. Vinyl films have a place in meat packaging and household wraps; polypropylene film is displacing paper and cellulose film for many types of overwraps.
The weathering characteristics of polyvinyl chloride have made it a popular material for other extrusions as well. Vinyl pipe is gaining ascendancy over metal, asbestos, clay and concrete for water and sewer lines, electrical conduits and ducts. Vinyl competes with aluminum and wood for house siding. Vinyl extruded "profiles" make excellent sashes and thermal breaks for windows.
Other plastics vie for a part of the pipe market. Most homes are now equipped with drain, waste and vent pipes extruded from a styrenic compound. Industrial effluent pipe is normally extruded from polyethylene. Farmland is often reclaimed with the help of polyethylene (or polypropylene) corrugated drain tubing. Either extrusion or injection molding is a necessary step in another plastics process, in which a parison (round hollow tube) is formed for subsequent blow molding. The parison is put between 2 halves of a mold and expanded with air pressure into a blown part. Bottles, drums and other hollow containers are made economically by this process. Polyethylene bottles are favoured for packaging detergents, bleaches and a wide range of other products. Vinyl bottles most often hold hair shampoos.