Elastomers Overview Questions and Answers

Answer: C

The hysteresis loss is 1.96 J.

The energy lost in the impact is the difference between the potential energy at the drop height and at the rebound height. It is dissipated as heat by internal friction in the rubber, which is what hysteresis means:

\[ \begin{aligned} \Delta E &= mg(h_{1} - h_{2}) = 0.1 \times 9.8 \times (10 - 8) \\ &= 1.96\ \text{J} \end{aligned} \]

9.80 J is the full potential energy at 10 m, and 19.60 J doubles it. 0.98 J would be the loss for a 1 m drop in height. The rebound resilience here is 8/10 = 80 percent.

Answer: D

Rubbers are known for low intermolecular forces.

An elastomer needs long, flexible chains that can uncoil when stretched and recoil when released. That requires weak forces between the chains, so the chains move freely at room temperature: rubbers are amorphous and well above their glass transition temperature (natural rubber's T_g is about -70 °C).

High intermolecular forces, a high T_g or crystallinity all restrict chain mobility and give rigid plastics or fibres instead. Strongly polar, hydrogen-bonded nylons, for instance, crystallise into fibres.

Answer: C

Elasticity comes from no change in enthalpy, but change in the entropy.

In an ideal rubber the chains are freely jointed and stretching does not alter bond lengths or intermolecular energies, so the internal energy (enthalpy) does not change. What changes is the number of available chain conformations: stretching aligns the chains and lowers their entropy, and the restoring force \(f = -T(\partial S/\partial L)_{T}\) is purely entropic. That is why a stretched rubber band warms up and why its stiffness rises with temperature.

Enthalpy-driven elasticity is the behaviour of metals and crystals, where bonds are stretched; the options with an enthalpy change, or with neither change, describe that case or no elasticity at all.

Answer: C

The 4 is the time in minutes after starting the motor when the measurement is taken.

In the notation ML(1+4) 100 °C, M is Mooney, L the large rotor, 1 the preheating time in minutes, 4 the running time in minutes after the rotor is started, at which the reading is taken, and 100 °C the test temperature. 60 is the Mooney viscosity in Mooney units.

So 4 is not the preheating time (that is the 1), not the number of samples, and not a shear rate: the Mooney viscometer runs its rotor at a fixed speed (2 rpm) that the notation does not state.

Answer: D

Nitrile rubber is a copolymer of butadiene and acrylonitrile.

Nitrile butadiene rubber (NBR) combines the flexibility of polybutadiene with the polarity of acrylonitrile units, which gives it its resistance to oils and fuels; it is used for seals, hoses and gloves.

Styrene and butadiene make SBR, styrene and acrylonitrile make SAN, and styrene with isoprene forms block copolymers such as SIS thermoplastic elastomers, not nitrile rubber.