Commodity Thermoplastics Questions and Answers

Answer: B

The match is P-2, Q-4, R-1, S-3.

• Injection moulding feeds the melt from the nozzle to the cavities through a sprue and runner system.
• Twin screw extruders use kneading blocks on the screws to mix and disperse fillers and additives.
• Blow moulding inflates a tube of molten plastic, the parison, inside a mould.
• Reaction injection moulding meets two liquid reactants (as for polyurethane) at high pressure in a mixing head just before the mould.

The other options swap these tools between processes, for example giving a parison mould to injection moulding or kneading blocks to blow moulding.

Answer: A

The most common and cheapest film is Polyethylene.

Low-density polyethylene film is inexpensive, tough, flexible, heat-sealable and a good barrier to water vapour, which makes it the most widely used packaging film for bread, produce, frozen foods and many other solid foods.

Polypropylene film (especially oriented PP) is clearer and stiffer and is used for snack packs, but costs more. Polystyrene is mostly used for rigid trays and cups rather than film, and PVC film, used as cling wrap, is more expensive and raises concerns about plasticiser migration.

Answer: B

A plastic bucket is made by injection moulding.

A bucket is an open, thick-walled article with a defined shape, a rim and often moulded lugs for the handle. Injection moulding forces molten thermoplastic (usually polypropylene or polyethylene) into a closed mould, which reproduces such detail precisely in a short cycle, ideal for mass production.

Blow moulding makes hollow articles with narrow necks such as bottles. Extrusion makes continuous profiles such as pipes and sheet. Compression moulding is used mainly for thermosets and is too slow for buckets.

Answer: D

The match is P-4; Q-3; R-1; S-2.

• A plastic egg container is a thin shaped sheet, made by thermoforming: a heated sheet is drawn over a mould.
• A water tank is large, hollow and seamless, made by rotational moulding, where powder is tumbled in a heated rotating mould.
• A chair is a solid, detailed, rigid part, made by injection moulding.
• A cable sheath is a continuous profile, applied by extrusion (crosshead extrusion over the wire).

The other options give water tanks to injection moulding, which cannot make such large hollow vessels, or cables to a batch process.

Answer: D

Polystyrene swells in benzene.

A polymer swells or dissolves in a solvent with a similar solubility parameter, 'like dissolves like'. Polystyrene is a non-polar hydrocarbon with phenyl side groups, and benzene is a non-polar aromatic hydrocarbon with a closely matching solubility parameter, so it penetrates the chains and swells the solid.

Dilute NaOH, a weak HCl solution of pH 6 and distilled water are all aqueous and highly polar. Water's solubility parameter is far from that of polystyrene, and polystyrene has no groups that react with dilute acid or base, so none of them swells it.

Answer: C

Blend with ethylene propylene rubber.

Ordinary high impact polystyrene is toughened with polybutadiene-type rubber, whose backbone double bonds are attacked by oxygen, ozone and ultraviolet light, so the material yellows and embrittles outdoors. Ethylene propylene rubber has a saturated backbone with no such double bonds, so a polystyrene toughened with it keeps its impact strength on weathering.

Styrene butadiene rubber, natural rubber and nitrile rubber all contain backbone unsaturation from butadiene or isoprene, so they share the weakness that causes the problem.

Answer: D

The discontinuous operation is thermoforming.

Thermoforming works cycle by cycle: a heated sheet is clamped, drawn onto a mould by vacuum or pressure, cooled and trimmed, and then the next sheet is loaded. Each part is made in a separate cycle, as in moulding.

Calendering, extrusion and film blowing are continuous: material is fed and product emerges without interruption as a sheet, a profile or a film tube.

Answer: B

The match is P-3; Q-4; R-2; S-1.

• Bottles are hollow parts made by blow moulding, here injection blow moulding.
• Blister packs are formed from heated plastic sheet: thermoforming.
• Fibre-reinforced electric poles are long constant-section composite profiles: pultrusion, which pulls resin-wetted fibres through a heated die.
• Sewage pipes are continuous profiles: extrusion.

The other options assign pultrusion to bottles or sewage pipes, or extrusion to the reinforced poles, which need continuous fibre reinforcement that plain extrusion does not provide.

Answer: A

The correct match is P-4; Q-3; R-2; S-1.

• Feed zone (R): under the hopper, it receives the charge and conveys the solid pellets forward (2).
• Compression zone (Q): the channel depth decreases, compacting the material while heat from the barrel heaters and shear melts it (3).
• Metering zone (P): a shallow, constant-depth section that pumps the melt at a steady rate set by the screw flights, giving uniform output to the die (4).
• Working zone (S): in the paper's scheme, the section where high shear works and mixes the melt to make it uniform (1).

The other options swap these roles; for example, a feed zone cannot melt or mix the charge, because the polymer there is still solid pellets being conveyed.

Answer: A

Thermoforming is carried out at \(T_g \lt T \lt T_m\).

In thermoforming a sheet is heated until it is soft and rubbery, then drawn over a mould by vacuum, pressure or a plug. For a semi-crystalline polymer this rubbery window lies above the glass transition, where the amorphous regions are mobile, and below the melting point, where the crystallites still hold the sheet together so it does not sag or flow.

Below \(T_g\) the sheet is too stiff to form, and above \(T_m\) it is a melt that cannot support itself. The other options mix up these limits or are not valid ranges.