Silane Coupling Agents: Techno Brief

Coupling agents are adhesion promoters that are used to provide a stable bond by reducing the interfacial tension between the fibrous or particulate inorganic component and the organic matrix polymer in reinforced and filled plastics. This Coupling Agents guide is designed to help you understand more about the use of silanes as coupling agents in reinforced plastics, focusing on the benefits and main applications.

Silane Coupling Agent Benefits in Thermoplastics

Silane coupling agents are a critical component of fiberglass-reinforced thermoplastics. 

Silane coupling agents have proven valuable in polyamide (PA), and polybutylene terephtalate (PBT) composites using glass reinforcements.

Most important benefits imparted to glass-reinforced thermoplastics coupled with silanes are:

  • Improved electrical properties
  • Increased mechanical strength of the composites
  • Improved resistance to moisture attack at the interface, leading to more durable composites.

Polyamide Glass Fiber

Polyamides are formed in reversible reactions between organic acids and amines, giving "polyimides" which are reactive with other amines at high temperature by virtue of that reversibility. An amino or chloropropyl functional silane on the surface of the glass lends itself to coordination with the amidegroups and possibly coupling of polyamides to the glass surface, silica, mica an so on.

PBT Glass Beads

PBT is intrinsically less polar and less reactive than PA. Consequently, silanes are less effective in improving the physical strengths of reinforced composites. Z-6040, however, is effective in improving the physical properties of a glass bead-filled PBT, as shown in the table below.

System Flexural Strength
Flexural Modulus
(105 psi)
Tensile Strength
Dry Wet* Dry Wet* Dry Wet*
Unfilled resin 12900 13000 3.14 3.00 7300 7300
Filled with 35% glass beads, Untreated 10800 10100 5.83 4.04 5600 4800
Filled with 35% glass beads, 0.25% Z-6040 14900 14400 6.07 5.38 8000 7900
* After 16 hours immersion in water at 50°C
Table 1 : Performance of silane coupling agents in glass bead reinforced PBT

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