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 Thermoset Composites

Silane coupling agents are a critical component of glass-reinforced thermosets.

Methacrylate functional silanes, Z-6030, were first developed for the reinforcement of polyesters. More recently, epoxy functional, Z-6040, vinyl-benzyl-amino functional silanes such as, Z-6032, have proven valuable in epoxy, polyester and other thermoset resin composites using glass reinforcements.
Most important benefits imparted to glass-reinforced thermosets coupled with silanes are:
  1. Epoxy Glass Laminates
  2. Polyester Glass Laminates
  3. Epoxy Glass Beads
  4. Other thermoset

Epoxy Glass Laminates

Obtain Flexural Strength with SilanesGlass-epoxy laminates are the standard for high performance electronics, such as printed circuit boards. Silane coupling agents are used as a finish on woven glass fabric to improve physical properties, especially the wet strength, of the composite.
Circuit-Board manufacturing is a very high technology application, and requires excellent coupling agent technology to provide the flaw-free benefits that are required. Dow Corning has developed unique technology for this application. Dow Corning Z-6032 and modified products based on this technology have provided the quality and benefits required, as shown in the table below.

Epoxy- Fiberglass Laminate Flexural Strength
Dry Wet*
Without silane 68 340 38 270
With Z-6032
(Cationic Styryl)
97 800 72 460

* After 72 hours immersion in boiling water

Table 1 : Performance of silane coupling agents as a fiberglass finish in epoxy laminate

Polyester Glass Laminates

Polyesters with various glass fiber reinforcements comprise a large volume, diverse market. Glass products treated with silane coupling agents Z-6040, Z-6030 and Z-6032 are ideally suited to this use. The effect of the organic structure of the coupling agent on improving the flexural strength of a glass-reinforced unsaturated polyester composite is shown in the figure below.

* After 72 hours immersion in boiling water

Figure 1 : Performance silanes coupling agents as a fiberglass finish in polyester laminates

The vinylbenzyl-functional silane coupling agent (Dow Corning Z-6032) gives greater improvement in the flexural strength of a glass-reinforced epoxy system than does the epoxy-functional silane coupling agent (Dow Corning Z-6040). More significantly, the retention of strength after aging for 72 hours in boiling water is significantly better with any of these silanes than if no silane coupling agent is used, but Z-6030 (Methacryloxy silane) and Z-6032 give better retention of flexural strength. These types of effects are what are generally expected from the use of silane coupling agents.

Epoxy Glass Beads

Silanes are highly effective coupling agents for glass beads reinforced epoxy resins.

Other thermosets

  • Vinyl-Unsaturated Types - Peroxide Curable
    Glass fiber products with finishes incorporating methacrylate (Z-6030) or vinyl-benzyl-amino (Z-6032) functional silanes are recommended, along with a vinyl-functional type (Z-6300/Z-6518), depending on the reactivity of the resin unsaturation. Free acid or other functionality in resins may permit use of other silanes such as Z-6040 or Z-6020 in some cases.

  • Alkyd and Allyl Resins
    These two types chiefly find use in the electrical field in glass-filled molding compounds and some fabric laminates (allyl) where optimum electrical properties are needed.

  • Vinyl Ester Resins
    These specialty resins with a structure similar to epoxy resins contain unsaturation enabling a radical-initiated cure.They find use in filament-wound and fabric-reinforced chemical processingcomponents. Z-6032 is particularly recommended for this type of resin.

  • Polybutadiene Resins
    Polymerization of butadiene by the "1,2 mechanism" gives many pendant vinyl groups which are cured by peroxides. The all-hydrocarbon stucture imparts low loss/electrical properties, making it valuable in glass laminates and molding compounds, filled composites, and encapsulating resins for the electrical and electronic industries. Z-6032 and Z-6300/Z-6518 are recommended for these resins.

  • Condensation Resins
    Condensation resins commonly cure through condensation of a methylol group with an active hydrogen, generating water as byproduct. Amine functional groups like those available in Z-6020 can also participate in such condensation reactions. Other silanes such as the epoxy functional (Z-6040), and vinyl-benzyl-amine (Z-6032) silanes are also effective in improving adhesion of condensation resins.On the basis of cost vs. performance, however, aminosilanes such as Z-6020 or Z-6011 are most commonly used.

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