16.5 PLASTIC FOAMS

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

Note 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 foam is bonded to another less porous substrate, the adhesive could be applied to the nonfoam 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 chemical compounds into the foam...