When organic polymers are reinforced with glass fibers or minerals, the interface, or interphase region, between the polymer and the inorganic substrate is involved in a complex interplay of physical and chemical factors. These factors are related to adhesion, physical strength, coefficient of expansion, concentration gradients and retention of product properties.
A very destructive force affecting adhesion is migration of water to the hydrophilic surface of the inorganic reinforcement. Water attacks the interface, destroying the bond between the polymer and reinforcement, but a “true” coupling agent creates a water-resistant bond at the interface between the inorganic and organic materials.
Silane coupling agents have the unique chemical and physical properties not only to enhance bond strength but also, more importantly, to prevent de-bonding at the interface during composite aging and use.
The coupling agent provides a stable bond between two otherwise poorly bonding surfaces. Figure 3 shows (via an SEM of the fracture surface) the difference in adhesion between a silica-filled epoxy resin with silane vs. without silane.
With silane, the epoxy coating on the silica particles is apparent; without silane, clean silica particles can be seen in the epoxy matrix.In composites, a substantial increase in flexural strength is possible through the use of the right silane coupling agent.
Silane coupling agents also increase the bond strength of coatings and adhesives as well as their resistance to humidity and other adverse environmental conditions.
Other benefits silane coupling agents can provide include:
- Better wetting of inorganic substrates
- Lower viscosities during compounding
- Smoother surfaces of composites
- Less catalyst inhibition of thermoset composites
- Clearer reinforced plastics