THE ROLE OF THE

Seal and Gasket Design 121
within the molecule, with the bonds being stretched beyond their minimum energy
capacity, but a thermal component is also involved. Again, rotation of a molecule
plays a role because, between cross-linked sites, different geometries can occur as
the molecule rotates. The molecular interactions at room temperature store kinetic
energy as oscillations occur. When the structure is strained, this energy is released
in the form of heat and a decrease in entropy occurs. We can see this relation in the
following equation:
S= kBInΩ (8.1)
Here, S is entropy, k B is the Boltzmann constant (1.38066 × 10 –23 ), and Ω is the
number of microstates that the molecule can assume per macrostate. If we consider
entropy as the inability of a system’s energy to do work, we can see that entropy will
decrease as the chain is stretched [6]. Likewise, as the chain is relaxed, entropy will
increase in an endothermic process.
Natural rubber shows a strong dependence on temperature; it has behavioral
characteristics in three regions: glassy region,
transition region, and rubber region [7]. At lower
temperature, the rubber will be crystalline and,
as the rubber’s temperature increases, it enters
the transition phase where it becomes leather-like
[7]. Eventually, the material will enter the rubber
phase with the sheer modulus decreasing in each
phase [7].
As natural rubber is vulcanized, the disulfide
bonds shown in Figure 8.11 shorten the chains of
the rubber and increase the rate at which the chain
will contract. The greater the number of disulfide
bonds, the greater is the hardness of the natural
rubber. Hardness affects the seal’s ability to compress
as well as its performance in thermal cycling events.
As with all rubbers, natural rubber (NR) is greatly affected by the type of filler
utilized, amount of filler, and shape of the filler. The filler content contributes to the
Payne and Mullins effects. The Payne effect is exhibited in seals and gaskets with
carbon black filler. A. R. Payne studied this effect, which is seen during cyclic loading
conditions upon a rubber material [8]. In this effect, as the strain gets larger, the storage
modulus decreases. The effect is seen at approximately 0.1% strain amplitude [8].
That is, at over 0.1% strain amplitude, the ability of a rubber to store energy decreases
rapidly. At 20%, a lower limit comes into play [8]. The Payne effect is not observed
in samples without filler. Likewise, the Mullins effect (named for Leonard Mullins)
is the stress–strain phenomenon that can be considered softening of the stress–strain
behavior when a load is beyond that which has been previously reached.
In 2005, nearly 8.6 million tons of natural rubber were produced. Of this amount,
94% was from Thailand, Malaysia, and Indonesia. The total world consumption of
rubber is around 18 million tons per year with about 20% SBR (styrene butadiene
R
S
FIgure 8.11 Diagram of disulfide
bond.
S
R