Engineering Thermoplastics Questions and Answers

Answer: C

The polymer sensitive to moisture is Polyamide 6.

Nylon 6 contains polar amide groups (-CO-NH-) that form hydrogen bonds with water. It can absorb about 2 to 3 percent water from the air and up to about 9 percent when saturated, which plasticises it, lowers its stiffness and changes its dimensions. It must be dried before moulding to avoid hydrolysis and voids.

Polyethylene, polystyrene and polybutadiene are hydrocarbons with no polar groups and absorb almost no water.

Answer: D

Most engineering polymers have an HDT above 80°C.

The heat distortion (deflection) temperature is the temperature at which a standard bar deflects by a set amount under a set load. Engineering plastics such as nylons, polycarbonate, acetal, PBT and PET are expected to carry load at elevated service temperatures, and their HDTs typically range from about 80 to well over 100°C.

An HDT of only 0 or 20°C would mean a part could not bear load at room temperature, and -80°C is a glass transition region typical of rubbers, not engineering thermoplastics.

Answer: D

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

• Expanded polystyrene is a light closed-cell foam used for sound and heat insulation in walls.
• PEEK keeps its strength and wear resistance at high temperature, which suits it to bearings.
• Polycarbonate is transparent and very tough, the material of safety glazing.
• PBT, a moulding polyester with good electrical properties, is used for housings such as TV cabinets.

Each other option misplaces polycarbonate, PEEK or expanded polystyrene, for example putting the foam in a TV cabinet or using PBT for safety glass.

Answer: A

The reason given by the key is higher molecular rigidity.

Polyamide 6 has an amide group after every five methylene units, regularly spaced along an otherwise simple chain. These groups make the chain stiffer and let neighbouring chains line up and hydrogen bond in a regular pattern, so the crystals melt at about 220 °C.

A thermoplastic polyurethane links its urethane groups through an extra oxygen (-NH-CO-O-), which rotates easily, and it contains long flexible soft segments of polyether or polyester. Its chains are therefore more flexible, pack less regularly and melt lower.

Lower rigidity would lower the melting point, not raise it. Branching disrupts packing and also lowers it. Both polymers are thermoplastics, so differences in crosslinking are not what separates their melting points.