Page 286 - Handbook of Plastics Technologies

ELASTOMERS

4.78 CHAPTER 4

bonding between rubber molecules and filler particles is necessary. Others believe that no
chemical bonding is necessary. Others feel that the truth is somewhere in between these
two views. It is possible that weaker-than-primary-chemical bonding occurs. If the elas-
tomer molecules are bonded in such a fashion that bonds are easily broken during defor-
mation but rapidly reform during the same process, significant energy dissipation will
occur (and heat will evolve) with an increase of the energy to rupture the rubber. The re-
straint of the polymer chains due to even the weak bonding would cause increases in stiff-
ness and modulus, over and above that due to the presence of a significant volume of
nondeformable material. The bonding of rubber to filler explains the fact that reinforcing
fillers give steeper stress-strain curves at elongations greater than about 100 to 200 per-
cent. In the absence of any bonding, elastomer would be peeled away from the filler parti-
cle surfaces in a dewetting process. One measure of reinforcement is the ratio of stress at
300 percent tensile strain to that at 100 percent tensile strain, the ratio being higher for
well reinforced elastomers. The measurements are made during the standard stress-strain
test.
The changes that occur during strain give rise to what is known as the Mullins effect or
Mullins softening. When a filler-reinforced vulcanizate is prestretched and then relaxed,
the force to stretch it again is less than for the unprestretched sample, but only up the strain
used in the prestretch. At higher strains, the stress-strain curve resumes that of the unpre-
stretched specimen. This implies breakage or movement of rubber-filler bonds during
stretching. After aging, the prestretched sample reverts to the nonprestretched state.
Particle Size, Surface Area, and Structure. Fillers with primary particle sizes
greater than 10 µm act as flaws, which can initiate rupture during flexing, bending, or
stretching. Fillers with primary particle sizes between 1 and 10 µm are diluents, usually
having only small effects on vulcanizate properties. Semireinforcing fillers, with primary
particle sizes ranging between 0.1 and 1 µm (between 100 and 1000 nm), can improve the
strength of vulcanizates and increase modulus and hardness. Fillers of primary particle
sizes ranging between 10 and 100 nm greatly improve strength, tearing resistance, wearing
resistance, and other qualities. The reason for using the phrase, “primary particle size,” is
that, in many cases (e.g., silica or carbon black fillers), during manufacture, first essen-
tially spherical primary particles are formed, which coalesce into aggregates. The aggre-
gates are in the form of compact structures, chains, or branched chains of high shape
factor, i.e., high structure. Various types of fillers are classified according to type and pri-
mary particle size in Fig. 4.26. Types of structures formed from aggregated primary parti-
cles are illustrated by Fig. 4.27.
Fillers that have very small primary particle sizes have high surface area per unit
weight of filler. Fillers that have high surface area have larger amounts of contact area
available for interaction and bonding with the elastomeric matrix polymer.
The average particle diameter of a filler sample can be directly determined by electron
microscopy where, for example, 1000 to 1500 single particles are measured under magni-
fications from 50 to 75,000. Particle-size distributions are then determined.
From the average primary particle diameter, a theoretical total surface area can be cal-
culated, assuming spherical particle shapes. This does not account for the aggregate struc-
ture or porosity. Another method to determine surface area (per 100 g of filler) is by
measuring the absorption of a gas of small atoms, e.g., nitrogen. The gas penetrates the
finest crevices. According to the nitrogen adsorption method developed by Brunauer, Em-
2
mett and Teller, one obtains the so-called BET-value of surface area, expressed as m /g.
Structure can be thought of as degree of difference from a spherical shape. It is similar
to shape factor. High structure aggregates are in the form of chains, branched chains, and
so on. We note that some of this aggregate structure can break down during processing due
to the development of high stress on the structures.

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