Making rubber articles; plastics and their types
The word plastic comes from Greek «mouldable» and Latin «mass». One of the world’s first plastics, parkesine (an early form of celluloid), was patented in 1856 by the English inventor Alexander Parkes. By their reaction to heat, plastics divide into thermoplastics and thermosets. Thermoplastics (polyethylene, polyvinyl chloride, polystyrene) soften and change shape when heated, so their waste can be reprocessed. The molecules of thermosets form a network, so they cannot be remelted. Polyethylene is waxy and impermeable to water and air: thin film is used for packing and covering greenhouses, the thick type for water pipes and containers. Polyvinyl chloride (PVC), when flexible, suits wire insulation and hoses, when rigid, pipes. Polystyrene is clear and brittle, and its foam type keeps heat. Organic glass (polymethyl methacrylate) is clear but scratches easily, while Teflon as a pan coating helps cook without fat. Plastic is cheap and convenient but breaks down very slowly in nature, so sorting and recycling it matters. When plastic burns it gives off toxic gases, so it must never be burned or held near fire.
Sorting plastics: clean, dry plastic containers (bottles, packs, cups) brought to class are grouped by the marks or labels on them and put into the recycling box. No burning or heating is done and sharp-edged pieces are not brought in.