Лако-красочные материалы — производство

As the cost of cold-setting exterior-grade adhesives based on resorcinol is very high due to the high cost of resorcinol itself, the tendency to decrease the amount of resorcinol while maintaining unaltered the performance of the adhesive, when brought to its ultimate conclusion leads to the concept of exterior cold-setting phenolic adhesives of zero level resorcinol. As alkaline PF resins do not have an ambient temperature rate of reaction that is even vaguely sufficient to set and harden the adhesive to a sufficient level, some modifications need to be introduced to overcome in this regard the lack of resorcinol. This can be done in several ways: (i) by using standard PF thermosetting resol resins and hardening them by increasing the glue line temperature by radiofre­quency in fingerjointing and glulam manufacture. The system is expensive and needs considerably higher capital outlay and more careful handling of both the equipment and of the joint, for results that are certainly not particularly exciting. (ii) By using resins in which the PF resol of adhesives of type 2 above is terminated by the terminal grafting of a resorcinol substitute, for example a natural polyflavonoid tannin [26], this system being truly cold setting and yielding relatively good results but at best just on the inferior limit of the standard requirements [26]. (iii) By using self-neutralizing acid­setting PF resols. The term ‘‘acid-setting’’ when used in the presence of a lignocellulosic substrate makes wood technologists shudder, conjuring visions of extensive acid — induced substrate degradation and early exterior joint failure. And this is indeed the case! In reality some exterior aminoplastic resins do harden in the moderately acid range without any major substrate degradation problems. PF resins, however, while hardening very rapidly under acid conditions, do need very acid conditions to give a hardened strong network, and this elevated acidity is not really acceptable as regards long term durability of the substrate. The damage due to the acid hydrolysis of

cellulose and other wood carbohydrates is particularly aggravated and compounded by the long term effect of the glue line remaining acid after resin hardening. However, the main negative effect due to acid-induced degradation of the substrate has been over­come by using acid-setting PF resins containing no resorcinol but hardened by the use of a self-neutralizing catalyst [27]. According to this principle the adhesive first becomes acid to allow the PF resin to cure and after hardening the hardened glue line self — neutralizes in very short time [27]. The greater majority of the effects of substrate degradation are then avoided and very strong and durable exterior wood joints are produced [27]. The system works well in radiofrequency cured joints, yielding much better results than the alkaline resols of point (i) above, and can work well under purely cold-setting conditions [27] (Fig. 2).

Other rapid setting adhesive systems not containing resorcinol are those based on melamine-urea-formaldehyde resins and on one-component polyurethanes, described in Chap. 32 and Chap. 34, respectively [28,29].