When joining dissimilar materials, solvent bonds are rarely strong and durable, so the use of adhesives is often recommended for this purpose. The adhesive selected must be compatible with each material involved. If the materials being joined expand and contract at different rates, a flexible bond may be required. Take this into account when selecting the adhesive formulation. If expansion and contraction are a major concern, consider the use of mechanical fastening.
Adhesive Characteristics
A variety of adhesives are available for joining dissimilar plastic materials. As a result, it is difficult to make general observations. Unlike solvents, which evaporate, an adhesive layer remains a functional part of the finished assembly. Hence, the performance and appearance of the finished part may depend primarily on the characteristics of the adhesive layer. Several characteristics to consider when selecting an adhesive are:
- Chemical compatibility with the parts being joined
- Aesthetics of the finished joint
- Expansion/contraction with temperature changes
- Brittleness/rigidity/flexibility
- Weatherability, if required
- Durability/service life
- Adhesive strength (adhesion to the plastic)Â
- Cohesive strength (resistance to internal tearing)
- End-use requirements
- Suitability for food contact
Adhesive-Bonding Procedure
Joined surfaces must fit well without forcing and have no visible gaps. The surfaces to be bonded should be smooth but not polished. Sand the surfaces to be joined with a 120 grit or finer paper. Diamond-wheel polishers, jointer/planers, or other mechanical devices can produce excellent results. However, soft-polishing wheels or flame polishing are not recommended as these can round the edges, causing gaps and improper fit.
Adhesive Shrinkage
Some adhesives with a volatile component may shrink while curing. To compensate for this, cut the joint on an angle, providing space for the joint to be slightly overfilled to compensate for shrinkage. Consult your adhesive suppliers’ literature for specific information on shrinkage.
