Page 350 - Handbook of Plastics Technologies

PLASTICS ADDITIVES

5.30 CHAPTER 5

5.3.2.6 Fatty Esters and Amides. These have been offered as low-cost coupling agents.
Their main service appears to be as dispersing agents.
5.3.2.7 Polypropylene/Acid Grafts. Grafts have been made with maleic anhydride or
acrylic acid. These are useful for example in coupling talc and mica into polypropylene.

5.3.3 Application Techniques
Most often, the filler and coupling agent are slurried in water, the coupling agent hydro-
lyzes Si-OR + H O → Si-OH and reacts with the Si-OH on the filler surface to form Si-O-
2
Si filler-to-coupling agent bonds. Alternatively, the coupling agent can be dry-blended
with the filler by tumbling at controlled humidity, but this requires more skill. In-situ treat-
ment is based on adding both filler and coupling agent to the molten polymer; this wastes
some coupling agent, but it eliminates the cost of a separate pretreatment step. Perhaps
most promising is vapor-phase application of the coupling agent, as is done in the manu-
facture of glass fiber, where the coupling agent acts first to protect the glass fibers and later
to bond them to the polymer matrix.

5.3.4 Coupling Agent Theory
There are a variety of theories to explain the action of coupling agents. Primary covalent
bonding is quite probable when organosilanes are used in thermosetting plastics. Second-
ary attractions are more likely when coupling agents are used in thermoplastics. Interpen-
etrating polymer networks (IPNs) may be postulated when the organosilane extends into
the polymer matrix. This concept may be broadened to consider the formation of a gradu-
ally modulated interphase rather than a sharp monomolecular interface. Morphology of
the coupling agent layer has been studied by electron microscopy, and some researchers
believe the coupling agent accumulates in tiny hills on the glass fiber surface, and these
hills act like the pins in a mechanical assembly, preventing the fiber from pulling out of the
polymer matrix. Coupling agent may create friction between the fiber and the polymer ma-
trix, increasing the stress needed to pull the fibers out of the matrix.

5.3.5 Practical Benefits of Coupling Agents
Coupling agent theory and salesmanship are often more optimistic than practical results. It
is important to be realistic about practical benefits outlined below.
5.3.5.1 Protection of Glass Fibers. This is definitely produced by vapor phase treat-
ment with organosilanes. They coat the glass fibers and keep them from scratching and
weakening each other. Since they provide coupling later, this is a double benefit.
5.3.5.2 Dispersion. Dispersion of fillers in liquid systems is faster and more complete.
This optimizes mechanical and optical properties. It is most evident in coatings, but it is
also important in plastics.

5.3.5.3 Lower Viscosity. Lower viscosity is often reported in liquid systems. Again, this
is most evident in coatings, but it may also be important in plastics.
5.3.5.4 Mold Wear. This is a distinct problem, especially in glass-fiber-reinforced ther-
moplastics. Compounders often claim that their proprietary coupling agents reduce mold
wear, but molders remain rather skeptical.

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