REBOUND RESILIENCE

An apparatus for the determination of resilience by a rebound method. Also called Lupke Impact Resiliometer. See Resilience. The relevant standard is BS 903-A 8, Method B, Method for determination of rebound resilience. 

The ratio of the energy given up on recovery from deformation to the energy required to produce the deformation, expressed as a percentage. See Heat Build-Up, Hysteresis and Rebound Resilience. 

Disclosed are ethylene-vinyl acetate/ethylene-styrene interpolymer blends, which are useful in preparing foams exhibiting improved compression set resistance, dimensional stability and rebound resilience at similar foam densities than known foam systems, e.g. EVA. The crosslinked foams are particularly useful in fabricating footwear and gaskets. 

Before considering particular test methods, it is useful to survey the principles and terms used in dynamic testing. There are basically two classes of dynamic motion, free vibration in which the test piece is set into oscillation and the amplitude allowed to decay due to damping in the system, and forced vibration in which the oscillation is maintained by external means. These are illustrated in Figure 9.1 together with a subdivision of forced vibration in which the test piece is subjected to a series of half-cycles. The two classes could be sub-divided in a number of ways, for example forced vibration machines may operate at resonance or away from resonance. Wave propagation (e.g. ultrasonics) is a form of forced vibration method and rebound resilience is a simple unforced method consisting of one half-cycle. The most common type of free vibration apparatus is the torsion pendulum. 

Figure 9-6. Rebound resilience apparatus, (a) Lupke pendulum (b) Schob pendulum (c) Dunlop pendulum (d) Goodyear-Healey pendulum (e) tripsometer (m = off-centre mass) (f)...

The most straightforward way to measure the effect of low temperatures on recovery is by means of a compression set or tension set test. Tests in compression are favoured and a method has been standardised internationally. The procedure is essentially the same as set measurements at normal or elevated temperatures and has been discussed in Chapter 10, Section 3.1. As the recovery of the rubber becomes more sluggish with reduction of temperature the dynamic loss tangent becomes larger and the resilience lower (see Chapter 9), and these parameters are sensitive measures of the effects of low temperatures. Procedures have not been standardized, but rebound resilience tests are inherently simple and quite commonly carried out as a function of temperature. It is found that resilience becomes a minimum when the rubber is in its most leathery state and rises again as the rubber becomes hard and brittle. 

Resilience is one of the most common rebound tests. It is fundamentally a deformation of the material for half a cycle. The rebound resilience basically is the ratio of the indentor after to before impact expressed as a percentage. There are a large number of methods for this test, including the following. 

According to the British Standard, BS903, PartA8, the rebound resilience is defined as "The proportion of the applied energy usefully returned after an impact"... 

The resilience of a polymer will be high (i.e. tan <5 is small) in temperature regions where no mechanical damping peaks are found. This applies in particular to rubbery networks (T Tg), which therefore possess a high resilience. Various rubbers behave quite differently at room temperature the rebound resilience is for natural rubber, butadiene-styrene rubber and butyl rubber high, medium and low, respectively. In practice this means that tyres for cars must have medium rebound resilience high rebound resilience causes bumping on the road, whereas low rebound resilience causes a tyre to become very hot. 

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