The feasibility of a chemical reaction has been considered with respect to polyvinyl formal and also polyvinyl butyral. In the work reported on polyvinyl formal. It is considered that under the influence of phosphoric acid and at a temperature of 150 oC for 30 minutes, hydroxyl group of the polyvinyl formal apparently crosslink by co-condensations. It is noteworthy that these conditions also favor homo-condensations of the formal. In the work reported on polyvinyl butyral. It is claimed that reaction occurs with phenolic resins. If the temperature exceeds 170 oC, and no mention is made of acidic conditions.
The question of a reaction taking place under the midly alkaline conditions that normally prevail in the curing of the two polymer systems is thus left undetermined. Whether or not any effective chemical reaction does take place. It would appear that the improved adhesion obtained with the conjoint adhesive is predominantly due to a physical effect. Obvious advantages result from the flexibilising of the P.F rein by the acetal, reducing shrinkage stresses on curing and enhancing the ability of the adhesive to accommodate strain and absorb shock.
At this point it is interesting to compare as on the table, the mean failing loads of a series of metal to metal and metal to wood joints made:
- With a phenolic resin alone
- With polyvinyl formal alone and
- with the two polymer adhesive
The component ratio was 0.75 part phenolic resin to 1 part polyvinyl formal, and the test speciment were 0.036 inch BS 1.72 aluminum alloy and B.S 1204 beech. Wood to metal joints were made with 1 inch wide strips and 1 inch overlap, and metal to metal joints with 1 inch wide strips and 1/2 inch overlap.
| Failing Stress (psi) | |||
|---|---|---|---|
| Phenolic Resin | Polyvinyl Formal | Two Polymer Adhesive | |
| Wood to metal | 500 | 710 | 850 |
| Metal to metal | 1310 | 2880 | 4400 |