Лако-красочные материалы — производство

Recently another step forward has been taken in the formulation of PRF adhesives of lower resorcinol content. Liquid resorcinol or PRF resins appear to be mostly linear [4]. The original concept in ‘‘branching’’ erroneously maintained that if a chemical molecule capable of extensively branching (three or more effective reaction sites with an aldehyde) is used after, before, or during, but particularly during or after, the preparation of the PF resin, the polymer in the branched PRF adhesive has (1) higher molecular weight than in normal PRF adhesives where branching is not present and (2) higher viscosity in water or water/solvent solutions of the same composition and of the same resin solids content (concentration). It also needs a much lower resorcinol amount on the total phenol to present the same performance as normal linear PRF adhesives. This can be explained schematically and is shown in Formula 10. With n > 1 and an integer number and comparable to, similar to, or equal to n in formula 9 for the production of PRF resins.

When comparing linear and branched resins, for every n molecules of phenol used a minimum of 2 molecules of resorcinol are used in the case of a normal, traditional linear PRF adhesive, whereas only 1 molecule of resorcinol for n molecules of phenol is used in the case of a ‘‘branched’’ PRF adhesive. The amount of resorcinol has then been halved or approximately halved in the case of the branched PRF resin. A second effect caused by the branching is a noticeable increase in the degree of polymerization of the resin. This causes a considerable increase in the viscosity of the liquid adhesive solution. Because PRF adhesives must be used within fairly narrow viscosity limits, to return the viscosity of the liquid PRF adhesive to within these limits, the resin solids content in the adhesive must be lowered considerably, with a consequent further decrease in the total liquid resin of the amount of resorcinol and of the other materials, except solvents and

resorcinol — CH2-£~ phenol—resorcinol resorcinol — CH2-t-phenol-CH24^resorcinol resorcinol — CH2-f’Phenol~CH2 resorcinol n in integer numbers

water. This decreases the cost of the resin further without decreasing its performance. Thus, to conclude, the decrease in resorcinol by branching of the resin is based on two effects:

1. A decrease in resorcinol percentage in the polymer itself, hence in the resin solids, due to the decrease in the number of the PF terminal sites onto which resorcinol is grafted during PRF manufacture.

2. An increase in molecular weight of the resin, which by the need to decrease the percentage of resin solids content to a workable viscosity decreases the percen­tage of resorcinol on liquid resin (not on resin solids).

It is clear that in a certain sense a branched PRF will behave as a more advanced, almost precured phenolic resin. While the first effect described is a definite advance on the road to better engineered PRF resins, the second effect can also be obtained with more advanced (reactionwise) linear resins. The contribution of the second effect to the decrease in resorcinol is not less marked than that of the first effect. It is, however, the second effect that accounts for the difference in behavior between branched and linear PRF adhesives.

Branching molecules which can be used could be resorcinol, melamine, urea and others [23]. Urea is the favorite one, because it is much cheaper than the others and needs to be added in only 1.5 to 2% of total resin. When urea is used as a brancher, the adhesive assumes an intense and unusual (for resorcinol resins) blue color, after a few days, hence its nickname, ‘‘blueglue.’’ However, later work [24,25] has shown that tridimensional branching has very little to do with the improved performance of these low-resorcinol content adhesives, with tridimensionally branched molecules contributing, at best, no more than 8 to 9% of the total strength [24,25]. In reality, addition of urea causes the reaction as foreseen, but not in three points of branching but rather only in two sites of the branching molecule. This is equivalent to saying that most of the resin doubles linearly in molecular weight and degree of polymerization, while the final effect, good performance at half the resin resorcinol content, is maintained [24,25]. This effect is based on the relative reactivity towards phenolic methylols of urea and of unreacted phenol sites and thus while the macro effect is as wanted, at the molecular level it is only a kinetic effect due to the different relative reactivities of urea and phenol under the reaction conditions used. Thus

resorcinol — CH2 — phenol — CH2]n—resorcinol

resorcinol — CH2 — phenol — CH2]n—resorcinol

resorcinol — CH2 — phenol —CH2]n—urea — CH2 + phenol — CH2]n—resorcinol

halving of the resorcinol content is still obtained, but between 90 and 100% of the poly­mers in the resin are still linear. It is noticeable that the same degree of polymerization and ‘‘doubling’’ effect cannot be obtained by lengthening the reaction time of a PF resin without urea addition [24,25].

These liquid resins then work at a resorcinol content of only 9 to 11%, hence considerably lower than that of traditional PRF resins. These resins can also be used with good results for honeymoon fast-setting adhesives in PRF-tannin systems, thus further decreasing the total content of resorcinol in the total resin system at a level as low as 5 to 6%.