State whether the following is true or false.
Atactic polypropylene is a useful commodity plastic.
Answer: A
The random arrangement of methyl groups prevents crystallisation, so atactic material is a soft tacky solid. Only the isotactic form made with a stereospecific catalyst has useful properties. Tacticity decides whether the polymer crystallises.
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Arrange the stages of making a moulded plastic article in order.
1. melt and shape it in a moulding machine
2. polymerise the monomer to resin
3. cool the article and trim it
4. compound the resin with additives
Answer: B
Compounding must follow polymerisation and precede moulding, since the additives have to be distributed through the melt. Cooling sets the shape and is the last step in the cycle. Cycle time is dominated by the cooling stage.
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Plasticisers are added to polyvinyl chloride in order to
Answer: C
Correct answer: lower its glass transition point
A plasticiser is a small molecule that inserts itself between the polymer chains, increasing their separation and letting them move past one another more readily. That lowers the glass transition temperature from about 80 degrees for rigid PVC to well below room temperature, turning a rigid pipe material into flexible sheet, cable insulation and upholstery - the same polymer in two quite different guises.
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Consider the following statements about plastics processing.
1. Injection moulding suits complex three dimensional articles.
2. Extrusion suits continuous profiles such as pipe and film.
3. Blow moulding is used for solid rods.
Answer: D
The first two are correct and describe the two main processes. The third is wrong, since blow moulding makes hollow articles such as bottles by inflating a parison against a mould. Each process suits a different geometry of article.
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A parison in blow moulding is
Answer: A
It is extruded or injected and then expanded by air pressure to take the shape of the cavity. The wall thickness of the finished article depends on how the parison is programmed. Bottles are the most familiar product of the process.
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State whether the following is true or false.
Recycling of thermosets by remelting is straightforward.
Answer: B
The crosslinked network cannot be melted, so thermosets are ground for use as filler or are burned for energy recovery. Only thermoplastics can be genuinely reprocessed into new articles. Grinding them for filler is the usual alternative.
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