Trends in Silane Surface Treatments

Organosilanes have been around for quite some time. They have provided effective primers and coupling agents for composites, adhesives, and sealants for many years. Because of the bifunctionality of these materials and the variety of end-groups that are possible, they are not easy to specify. (The recent SpecialChem4Adhesives article describes guidelines for selecting the appropriate organosilane for adhesives and sealants.) However, these properties are also what provide organosilanes with a great deal of versatility, and this versatility has lead to applications that were not well known a decade ago.

One example of the modern use of silanes is as a glass treatment for double sided pressure sensitive tapes having a foam carrier.¹ These tapes are being used increasingly as glazing materials for window installations. Glass is hydrophilic (water loving) and may lead to performance issues over time in humid or wet environments due to water vapor undercutting the bond line and interfering with normal adhesion of the foamed tape. The silane primer treats the glass surface creating a hydrophobic surface that will act to protect the bond line.

Conventional organosilanes rely on the hydrolysis of their Si-O-R groups and subsequent condensation for their coupling with inorganic surfaces. Human health and environmental concerns are leading to the development of new products with less hydrolysis / condensation by products. These include hydrolyzed, lower alkoxy-containing intermediates or solventless products. Prehydrolyzed silanes under well-controlled conditions, water based silane solutions, solid carrier supported silanes that could be added during extrusion processes, and plasma surface treatments in the presence of silanes are among the approaches being investigated to reduce VOC issues.

Novel silane based metal pretreatments have also been developed as cost effective alternatives to the chromating processes. The new process is a simple dip process, is non-carcinogenic, and has outperformed chromate systems under different test conditions. The surface treatment process provides superior corrosion and adhesion performance.²

Sustainable biocomposites consisting of agriculturally grown fibers and either petroleum based or biobased resin matrices represent yet another use of modern silane technology. Poor fiber/matrix interfacial adhesion can negatively affect the physical properties of the resulting composites due to surface incompatibility between the hydrophilic natural fibers and non-polar polymers. A variety of silanes (mostly trialkoxysilanes) have been found to be effective coupling agents that promote interface adhesion and improve the properties of these composites.³ The silane is considered to modify the highly hydrophilic nature of the surfaces of biofibers such as cellulose, jute, hemp, etc. Once treated these fibers have improved compounding properties and efficiently transfer load to the surrounding resin matrix.

Because silanes require a monomolecular thickness, they provide good primers and adhesion promoters for electrically conductive adhesives. For example, the use of a silane coupling agent in an electrically conductive epoxy adhesive was found to provide a significant improvement in electrical conductivity as well as lap shear strength increase before and after service aging.⁴ Similar advantages of silane coupling agents were also found in improving the dielectric properties of barium titanate epoxy composites.

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References

1. Silane Glass Treatment AP115, 3M Company.
2. Subramanian, V. and Ooij, W.J., "Silane Based Metal Pretreatments as Alternatives to Chromating", Surface Engineering, Vol. 15, No. 2, 1999, pp. 169-192.
3. Xie, C., et. al., "Silane Coupling Agents Used for Natural Fiber / Polymer Composites: A Review", Composites Part A: Applied Science and Manufacturing, Vol. 41, No. 7, 2010, pp. 806-819.
4. Tan, F., et. al., "Effects of Coupling Agents on the Properties of Epoxy Based Electrically Conductive Adhesives", Int. J. of Adhesion and Adhesives, Vol. 26, No. 6, 2006, pp. 406-413.