Polymers and Plastics Questions and Answers

Answer: C

Wood flour, calcium carbonate, mica or glass fibre displaces expensive resin and may stiffen, toughen or improve the dimensional stability. It generally reduces the transparency and the impact resistance of an unreinforced grade.

Answer: D

A measured charge is squeezed in a heated mould and cures there, which suits a material that sets irreversibly. A thermoplastic would have to be cooled before the mould could be opened, which makes the cycle uneconomic.

Answer: A

Correct answer: melted, forced into a cold mould and solidified

The cycle suits a thermoplastic exactly: the granules are melted in the barrel, injected under pressure into a cooled mould, allowed to solidify and ejected, and the process repeats every few seconds. Nothing is changed chemically, so scrap and sprues can be reground and used again. A thermoset requires a heated mould and a cure time instead, which is why it is compression or transfer moulded.

Answer: B

Correct answer: continuous uniform cross-section

Extrusion pushes the melt continuously through a shaped die, so the product has whatever cross-section the die defines and any length the process is allowed to run - pipe, sheet, film, rod, cable sheathing and window profile. It cannot make a shape that varies along its length, which is what injection moulding is for, nor a hollow closed vessel, which is blow moulding.

Answer: C

Correct answer: plasticised in a separate chamber and injected

In compression moulding the charge is placed in the open cavity and the mould closes on it. In transfer moulding it is softened in a separate pot and then forced through runners into a closed cavity, which gives better dimensional control, permits thin sections and delicate inserts, and reduces flash. Both cure a thermoset in a heated mould; only the way the material reaches the cavity differs.

Answer: D

Correct answer: isoprene

Natural rubber is cis-1,4-polyisoprene, tapped from Hevea brasiliensis. The all-cis configuration is what prevents the chains from packing and crystallising at rest, leaving them coiled and free to be extended - the basis of its elasticity. The trans isomer of the same polymer is gutta percha, a hard and non-elastic material, which shows how much the geometry matters.

Answer: A

The occasional cross-link stops the chains sliding permanently past one another, so the rubber recovers its shape instead of flowing. Strength, elasticity and resistance to solvents and temperature all improve together.

Answer: B

Correct answer: harder and less elastic

Sulphur creates cross-links between the chains, and the more cross-links the less the chains can move relative to one another. A few per cent gives the soft elastic rubber of a tyre; thirty per cent or more gives ebonite, a hard rigid thermoset with no elasticity at all. The whole range from soft rubber to hard vulcanite is a matter of cross-link density.

Answer: C

Correct answer: abrasion resistance

SBR wears more slowly than natural rubber against a road surface, which is the property a tread must have, and it is cheaper and more consistent in supply. Natural rubber retains advantages in resilience, heat build-up and tear strength, which is why truck and aircraft tyres still use a large proportion of it while car treads are predominantly SBR. Neither is oil resistant, which is nitrile rubber’s province.

Answer: D

Correct answer: chloroprene

Neoprene is polychloroprene, and the chlorine atom on the chain is responsible for its virtues: it reduces the reactivity of the remaining double bonds towards oxygen and ozone, giving good weathering resistance, and it raises the polarity of the chain, giving resistance to hydrocarbon oils. It is also flame retarding, which natural rubber is not.