Frequently Asked Questions
The surface of glass is made up of a variety of different oxides of which the most common generic component is the silicon-oxygen bond (Si-O). Such surfaces are described as being polar in type and will therefore bond readily to polar adhesives such as polyurethanes, silicones, epoxies and acrylics. However, it is known that Si-O bonds are susceptible to moisture attack, leading to degradation and eventual failure. In view of this, problems may arise in applications where heat, moisture or polar solvents are present.
In the case of acrylic- or polyurethane-based adhesives, specific grades of material are required, otherwise their strong initial bonds to glass surfaces will reduce with time in the presence of moisture. In contrast, silicone adhesives, particularly the moisture curable types, show modest initial bond strengths, which are largely retained in normal service conditions.
To increase the long-term durability of adhesively bonded glass joints for hot and humid applications, the following may be adopted:
- Surface pre-treatment of the glass with appropriate silane chemicals (often called coupling agents) that protect the Si-O bonds
- Use of an inherently hydrophobic adhesive (i.e. fluorine-containing epoxy systems)
- Use of silicone-based adhesives that are hydrophobic and are not affected by moisture
- Use of adhesives (containing bulky functional groups) which can protect the Si-O bond from moisture attack
- Implementation of appropriate design measures to limit/prevent the exposure of Si-O bonds to moisture.
There are other factors that should be considered when selecting an adhesive for glass, the main ones being:
- Surface condition (presence of contamination, roughness, etc) and the method of surface pre-treatment (degreasing, abrasion, etching)
- Matching the coefficient of thermal expansion of the materials to be bonded (e.g. bonding metal to glass with a very rigid adhesive can result in catastrophic failure due to very high stresses developing at the bond line where temperatures change rapidly)
- Selection of appropriate dispensing methods (consider manufacturing volume, type of adhesive, accuracy of placement, complexity of product, type of cure, etc)
- Selection of appropriate curing regimes (room temperature, heat, UV, etc)
- Temperature and UV resistance when in service
- Colour or transparency, if required.
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