PLASTICS

Plastics play a large part in the design of modern electronic devices, but the choice of plastic is not a simple matter. Plastics come in many forms with their own strengths and weaknesses depending on their chemical structure.

A short list of common plastics:-

Polycarbonate (PC), Poly(methyl methacrylate) (PMMA), Acrylonitrile butadiene styrene (ABS), Polyvinyl chloride (PVC), Rubber.

 

POLYCARBONATE (PC)

Polycarbonates belong to a group of thermoplastic polymers. They are easily moulded and transformed so these plastics are widely used in the chemical industry. There properties, temperature resistance, impact resistance and optical properties make them an ideal material for manufacturing products that fall between the commodity plastics and engineering plastics. Unlike other plastics Polycarbonates do not have a unique plastic identification code but are identified as Other.7.
The glass transition temperature of polycarbonate is 150°C (320°F) and reaches its flow temperature at 300°C (572°F). The non-Newtonian fluid flow makes it more difficult to use in the injection moulding process compared to other thermoplastics. To make stress and strain free products the tooling used in the injection moulding process must be above 80°C (176°F). Lower molecular mass grades are easier to mould but have less strength. The higher molecular mass grades are tougher but more difficult to process. Polycarbonates are used in many applications such as household wares, laboratory equipment, where high impact resistance, temperature resistance and optical properties are required.
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POLY (METHYL METHACRYLATE) (PMMA)
 

Poly (methyl methacrylate) is a transparent thermoplastic which chemically is the synthetic polymer of  methyl methacrylate. It is sold under many trade names, such as Policril, Plexiglas, Gavrieli, Vitroflex, Limacryl, R-Cast, Per-Clax, Perspex, Plazcryl, Acrylex, Acrylite, Acrylplast, Altuglas, Polycast, Oroglass, Optix and Lucite but is most commonly referred to as acrylic glass, acrylic, perspex or plexiglas.

It is used as an alternative to glass in situations where safety is important. PMMA is in direct competition with polycarbonate (PC). Although the cost of production of Perspex is low, it is very brittle when under load stresses. One of its main drawbacks is, that at a temperature of 460°C it burns, giving off a cocktail of undesirable gasses and compounds (carbon dioxide, carbon monoxide, water and formaldehyde). PMMA is processed at 240°C to 250°C. All the normal processes can be used; injection moulding, compression moulding and extrusion. PMMA sheets are produced by cell casting with the polymerization and moulding steps taking place at the same time. Due to its high molecular mass, the strength of the material is higher than the moulding grades. To increase the strength of PMMA, rubber toughening is used to improve its brittleness under applied loads.

PMMA can be joined using cyanoacrylate cement (superglue), by using heat to melt the edges together or by using a solvent such as bichloromethane (trichloromethane) which dissolves the edges of the joint, fuses and sets.
Image courtesy of:- http://histper.co.uk/tcs/poly%20methyl.html
 
ACRYLONITRILE BUTADIENE STYRENE (ABS)
ABS is a widely used thermoplastic which is light and rigid and easily moulded into a variety of products such as piping, musical instruments, golf club heads, car body parts, wheel trims, electrical enclosures, protective safety equipment, children’s toys and Lego bricks. Finely ground ABS is used as a colorant in some tattoo inks.
ABS is a copolymer produced by polymerizing styrene and acrylonitrile in the presence of polybutadiene. There are varying proportions of ingredients used to make ABS of differing properties. Typical proportions are 15% to 35% acrylonitrile, 5% to 30% butadiene, and 40% to 60% styrene. The resulting structure of ABS is a long chain of polybutadiene with a criss-cross of shorter chains of poly (styrene-co-acrylonitrile). The polarized chains from the neighbouring nitrile groups attract each other and bind the chains together; this makes ABS stronger than pure polystyrene. The rubbery substance, butadiene, provides resilience at low temperatures the normal working range for the majority of applications is -25°C to +60°C, its mechanical properties changing with temperature. The properties of ABS can be changed by using rubber toughening, with the inclusion of fine particles of elastomer within the matrix.
ABS has the advantage over many thermoplastics by combining the strength and rigidity of acrylonitrile and styrene polymers with the toughness of polybutadiene rubber. It is impact resistant and tough. There are ways to improve these properties by using different percentage mixes as mentioned above. To improve impact resistance the polybutadiene component is increased with relation to styrene and acrylonitrile. By changing the compounds in this way ABS can be prepared in many different grades. The two main grades are; ABS for extrusion and ABS for injection moulding. Generally ABS has a useful working temperature range of -40°C to +100°C.
Image courtesy of:- http://www.archplastics.com/us_california_plastic_distributor/abs_sheet_plastic.html
 
POLYVINYL CHLORIDE (PVC)
A thermoplastic polymer. It is a vinyl polymer constructed of repeating vinyl groups (ethenyls) with one hydrogen atom being replaced with a chlorine group. Widely used in the construction industry because of the cheapness of its production, and ease of assembly. It can be made more flexible and softer with the addition of plasticizers, the most common being phthalates. It is used in the production of clothing and upholstery, flexible hosing and tubing, flooring and roofing membranes, and electrical cable insulation. Also used in figurines and inflatable waterbeds.
Image courtesy of:- http://www.sheetplastics.co.uk/
 
 
 
RUBBER

Natural rubber is an elastomer (elastic hydrocarbon polymer) that is derived from a milky colloidal suspension or latex, found in the sap of numerous plants. The purified form of natural rubber is the chemical polymeric isoprene (polyisoprene) which can now be produced synthetically.

Since World War II, synthetic rubber has supplanted natural rubber as a major source for the rubber manufacturing industry. By way the largest manufacturing consumers of rubber are the tyre and tubing industry, accounting for 56% of the market with the remaining 44% being general rubber goods (GRG).

Discounting the tyre and tubing industry, the other major uses of rubber are door and window profiles, hoses, belts, matting, flooring, and dampeners (anti-vibration mounts). Rubber is also used as an adhesive in many industries and products with the paper and carpet manufacturers being the largest consumers.

Rubber is also used for producing seals and gaskets which are used in many situations for the prevention of water entering an enclosure which may contain electrical apparatus. The enclosure is usually injection moulded in two parts with one part having a groove to accept a rubber seal and the other part having a flange which clamps the rubber seal between the parts to form a water-tight enclosure. Image courtesy of:- http://www.open2.net/historyandthearts/discover_science/rubber_p.html