Chemistry Reference
In-Depth Information
filler particles, can adversely affect the electrical properties of the elasto-
mer. Ex situ generated titania particles have also been used, particularly
particles derivitized with trifunctional siloxanes to make them highly
hydrophobic.
111
Carbon black provides some reinforcement, but can in-
terfere with some types of peroxide cures. Also, its electrical conductivity
can severely compromise the electrical properties of the material.
104, 107
In some cases, silane coupling agents are used to improve the bonding
between the reinforcing phase and the polymer.
112-114
These molecules
typically have the structure X
3
SiY, with X chosen to interact strongly
with one phase, and Y with the other. For example, if X is an alkoxy group
it can hydrolyze and react with OH groups on the surface of a filler par-
ticle.
2
Similarly, if Y is a vinyl group it can be polymerized into the or-
ganic matrix being reinforced, providing enhanced filler-matrix
bonding.
2
It is also possible to form reinforcing filler particles in situ, for
example by the sol-gel hydrolysis and condensation of precursors such as
organosilicates.
115-116
Not all fillers are designed to improve mechanical properties. Extend-
ing fillers, for example, reduce the cost of the compounded elastomer.
Nonreinforcing fillers are exemplified by kaolin, diatomaceous earth, and
minerals such as calcium carbonate. Coloring agents can be either organic
or inorganic, but the former can adversely affect heat stability. Examples
of suitable inorganic colorants are oxides and salts of iron, chromium,
cobalt, titanium, and cadmium. Some not only provide color but can also
have some beneficial heat-aging effects.
Processing aids are particularly important in the case of elastomers
that contain highly reinforcing silicas, since these fillers adsorb polymer
chains so strongly to their surfaces that premature gelation can occur.
These additives have a softening or plasticizing effect, thus ameliorating
the occurrence of this complication.
104,
107
The nature of the curing (cross-linking) agents introduced depends
on the particular chemical reaction chosen for generating the cross
links.
117-121
In the case of end-linking reactions, the end groups are gen-
erally either hydroxyl or vinyl units. In the former case, the end-linking
agent may be TEOS [Si(OC
2
H
5
)
4
], which reacts by a condensation reac-
tion, with a stannous salt used as catalyst. In the latter case, the end-
linking agent can be an oligomeric siloxane that contains reactive Si-H
groups, with the H atoms adding to the double bonds in the polymeric
siloxane. Platinum salts are catalysts for this type of addition curing
reaction.
117-120, 122
Other functional groups at the ends or along the
chains can serve the same purpose.
17, 123 -13 4
In an interesting reversal of
roles, a polysiloxane was used to cross link cellulose acetate.
135
