Polymer and Plastic Industries Questions and Answers

Answer: C

Phenol formaldehyde is a thermoset that cures to a crosslinked network. The other three all soften on heating and can be recycled by remelting. Curing produces a network that spans the whole article. Only the first three can be remelted.

Answer: D

It governs the useful temperature range of an amorphous polymer and is quite distinct from the melting point of any crystalline regions. Polystyrene is brittle below its transition and rubbery above. The transition is not a true phase change.

Answer: A

Both are true and the reason gives the reason there are two: a wholly amorphous polymer shows only a glass transition, which is why polystyrene has no melting point in the ordinary sense. Crystallinity in a polymer is never complete.

Answer: B

The first two are correct, and their ratio is the polydispersity index. The third holds only for a hypothetical monodisperse sample, which real polymerisation never produces. A narrower distribution generally processes better.

Answer: C

Correct answer: 2.5

\[ \mathrm{PDI} = \frac{M_w}{M_n} = \frac{125{,}000}{50{,}000} = 2.5. \] The index is dimensionless, being a ratio of two molecular weights, so the options quoted in the tens of thousands cannot be right on units alone. It is always at least 1, which would mean every chain the same length, and a free radical polymerisation typically gives 2 to 5.

Answer: D

Chain entanglement gives strength but also raises the melt viscosity steeply. The grade chosen for a product is always a compromise between the properties wanted and the moulding process to be used. Melt flow index is the usual practical measure.

Answer: A

Polyethylene terephthalate is a polyester made by condensation with loss of water. The other three all form by addition across a vinyl double bond. The by-product distinguishes the two mechanisms. Bottle grade resin is the familiar example.

Answer: B

Branching reduces crystallinity and therefore density, stiffness and melting point. Both are made from ethylene, and the difference is entirely one of process and catalyst. Density and stiffness follow from the crystallinity.

Answer: C

Both are true and the reason gives the connection: Ziegler Natta and metallocene catalysts make linear chains at modest pressure, which is why the two grades come from quite different plants. Pressures above a thousand bar are used in the older route.

Answer: D

Isotactic polypropylene, which is crystalline and useful, cannot be made by free radical polymerisation at all. The discovery opened up a whole class of commodity plastics. Metallocene catalysts refined the idea further.