plastic science

Material science — plastics

Canadian industry and researchers were exploiting the properties of plastics well before Dustin Hoffman's character had his plastics epiphany in the The Graduate. In 1946, the Canadian National Research Council (NRC) produced the world's first moulded-plastic furniture prototype - three years before Charles Eames' more famous designs were revealed to the world (1).

1946 plastic chair designed by James Donahue

All plastics are polymers: poly from the Greek polus, meaning many, and mer from the Greek meros, meaning parts; therefore, polymer means many parts. The basic building block of the typical long-chain polymer is called a monomer (mono is the Greek word for one).

Plastics with two-dimensional molecular structures are termed thermoplastics and can be heat softened and recycled. Two thermoplastics commonly used in the furniture industry are polypropylene (Robin Day's polychair) and eco-questionable PVC (polyvinyl chloride), used for panel edging and foils.

Plastics with three-dimensional internal structures, termed themosets, will not resoften with heat once formed. Polymers in this category play a largely supportive role in furniture manufacturing - polyurethane (adhesives and upholstery foam), phenolics (adhesives) and polyester (finishes, fabrics). Polymers can also be combined with each other (similar to alloys in metals) or with other materials such as glass fibre - Eero Saarinen's tulip chair is a classic example.

Plastics are much less environmentally friendly than wood (a natural polymer). Their monomers are often toxic; plastics production consumes high levels of energy; most polymers are currently derived from non-renewable oil; and thermosets are difficult to recycle. With the exception of PVC and some thermosets, most of these problems can be minimized with containment technology, stringent eco-labelling/recycling and research into bio-alternatives for monomer production. Designers and manufacturers should think carefully before using polymers and only for applications where the positive properties outweigh the negative. For example, the light weight and high strength of plastics can get the job done with far less material.

To some extent these issues are not a problem for SMEs as they lack the finances needed to install thermoplastic injection-moulding equipment to produce large-scale plastic furniture. Where thermoplastics can play an important role is in the fabrication of small components from sheet stock. Compared to plastic, a bracket fabricated from steel is less expensive but heavier and prone to oxidation; aluminum is lighter but more expensive and an energy-consuming glutton. And wood, no matter how eco-friendly, cannot offer the transparency of plastics. (see plastic fabrication)

Examples of plastics available in sheet form that can be worked with common woodworking tools or manipulated with low-cost heating devices are acrylonitile butadiene styrene (ABS), polycarbonate (PC), glycol-modified polyethylene teraphylate (PETG), polylypropylene (PP) and polymethyl methacrylate (PMMA). The following chart compares Young's modulus (stiffness) and production energy of the five thermoplastics; MDF was included to show the difference between wood/plastic panels.

Chart: Young's modulus v production energy

(1) Modern Furniture in Canada 1920 to 1970, by Virginia Wright.
(2) Production energy is the energy required to make 1 kg of manufactured materials from its feedstock. Figures shown are primarily for moulding grades of thermoplastic. When using sheet materials always refer to the manufacturer's specifications. Chart data courtesy of Ansys/Granta Design.

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