Friday, July 17, 2009

The Reactions of Polyisocyanates

The important reactions are those between polyisocyanates and substances containing active hydrogen atoms, addition polymers either linier or cross linked being formed. Recently homopolymers of polyisocyanates have been described.

There are three classes of compound containing active hydrogen atoms that are of interest in their reaction with polyisocyanates, namely compounds containing hydroxyl groups (polyols such as polyhydric alcohols, polyesters, polyesteramides, polyethers and castrol oil), carboxylic acids, and amines. The addition polymers formed with any of these are loosely refered to as polyurethanes, but this term is correctly applied to the product of reaction and amines (and also water) reacting to form polyureas.

In adhesive applications it is the polyurethanes that are of most importance. The hydroxyl containing component generally used a saturated polyester although unsaturated polyester have also been used, in one patent, for example, a metal to metal adhesive based on triphenyl methane 4,4,4-triisocyanate and the unsaturated polyester from maleic acid and diethylene glycol is claimed. A typical saturated polyester is polyethylene adipate, among many others the esters from phthalic unhydride, trimethylolpropane and 1,4-butandiol have been used or described.

In the preparation of the polyester, the ratio of alcohol to acid should be considerably in excess of equimolecular in order to ensure polyester chains terminated almost entirely by hydroxyl groups. Such hydroxyl groups in their reaction with isocyanate groups do not split off any other substances, whereas carboxyl groups split off carbon dioxide – as also does water. It is difficult to eliminate these sources of carbon dioxide totally from the polyester. The evolution of carbon dioxide creates bubbles in the glue layer that can be a source of weakness in the point – a shortcoming that has probably had some effect in limiting the development of polyurethane adhesive. With further reference to this disadvantage, it has been reported that when glued joints were prepared in an atmosphere of 190 psi nitrogen no bubble of carbon dioxide were formed and the bond strengths were 10-20% higher than when joints were prepared in atmosphere pressure; but their preparation in dry air would on feels, also produce bubble free glue films.

Whilst polyesters with hydroxyl end group such as polypropylene glycol are of great importance in the manufacture of polyurethane foams, they are of mine interest in adhesive systems; one of the reasons may be the readiness with which they foam, presumably due to traces of water. The opinion has, however, been expressed that the formation of bubble free films from solventless polyester may become possible through progress made with catalyst that, being specific for the polyurethane reaction, do not catalyze the reaction with carboxyl groups or water.