Лако-красочные материалы — производство

The properties of polyurethane adhesives depend on the isocyanates, polyols and chain extenders used (these may be short-chain amines or alcohols).

Primary TID was used as an isocyanate source for many years, but was later found (due to its high volatility) to cause allergic reactions. To overcome this problem, oligomers with TID end-caps were made available. Other diisocyanates used are based upon methylene dianiline, and its dimers and trimers.

The most widely used polyols employed in polyurethane adhesives fall into three main classes: polyesters, polyethers, and polybutadiene polyols. Polyester polyols are usually based upon caprolactone and are manufactured with a variety of molecular weights. The molecular weights of polyether polyols are generally less than 10 000, and they are usually based on tetrahydrofuran or propylene oxide. Butadiene polyols are short-chain butadiene polymers with hydroxyl end groups. Castor oil is a low — priced polyol that is often used together with low-molecular-weight polyethers; however, due to its low molecular mass and high functionality it yields high-strength, but sometimes brittle, products.

Chain extenders are part of the mechanism which provides an improvement in the toughness of polyurethane-based adhesives. For this, materials such as ethylene glycol, 1,4-butanediol, 1,4-bis(hydroxymethyl)cyclohexane or ethylene diamine can be added. The chain extender reacts with the isocyanate, which then phase — separates into domains enriched in isocyanate/chain extender blocks or ‘hard segments’; the polyol phase then separates into ‘soft segments’. This phase — separated structure is the source of the extraordinary toughness displayed by polyurethane adhesives [34].

Unlike the case with other adhesives, the addition of fillers and rheological additives is a ‘must’ in the formulation of polyurethane adhesives. For cost-reduction purposes, carbonates and silicates such as talc, silica or clay are added (up to 60%). In addition, the rheological and mechanical properties ofthe adhesive can be improved by means of active fillers such as carbon black or pyrogenic silica (up to a maximum of 20%). The use of plasticizers (mostly based upon phthalates, phosphates or aromatic oils, up to a maximum of30%) allows the processing viscosity and swelling behavior of the finished adhesive to be controlled. Molecular sieve desiccants are often employed to dry the added fillers, plasticizers and polyols.

A typical formulation for a two-part polyurethane adhesive based on polyether and castor oil contains about 33% polyols, 10% castor oil, 2% rheological additive, 50%

filler, and 5% desiccant. If, for the formulation, this represents 100 parts of part A, then about 40 parts of polyisocyanate are added as part B [35].

A typical formulation of a moisture-setting, one-part polyurethane adhesive contains 49.3% isocyanate-terminated prepolymer, 49.3% filler, 1% polyol, 0.27% rheological additive, and 0.13% high-boiling solvent [35].

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