16.7 Plastic Foams

Plastic foams are manufactured from thermoplastic and thermoset resins in various forms. The main pitfalls in joining plastic foam are (1) causing the foam to swell or collapse by contacting it with a solvent or monomer, and (2) having the adhesive change the properties of the foam through its absorption into the foam. Adhesion is usually not a serious problem because of the porous nature of the foam.

It should be noted that there are closed cell and open cell foams. Adhesives may spread or wick deeply into the open cell variety, thereby affecting the resulting mechanical properties of the foam and perhaps even weakening the foam. When bonding foam to another less porous substrate, the adhesive should be applied to the non-foam substrate to minimize the wicking and ingress of the adhesive into the body of the foam. With the closed cell variety, the adhesive cannot wick deeply into the foam, but usually the foam's skin must be machined or abraded to allow for some surface roughness for the adhesive to mechanically attach.

There are also low surface energy foams, such as polyethylene, that require either surface treatment or special adhesives for bonding. Fortunately, extremely strong bonds are generally not required because the foam has a relatively low cohesive strength. Therefore, simple cleaning is generally the only surface preparation required. The surface treatments that are recommended are those that are described in the previous section for the parent plastics. However, the possibility of wicking of the...