Лако-красочные материалы — производство

The fundamental problem of lignin, slowing of the pressing time obtainable with PF resins, was partly eliminated by Sellers (1990) [19] and by Calve (1990) [20], who first reacted lignin with formaldehyde in a reactor for a few hours. A methylolated lignin (ML) equivalent to a PF resol was obtained. As in this case the reactivity of the methylol groups of lignin introduced depends on the reactivity of phenolic nuclei available for reaction, mixing with a synthetic PF resin ensures that the reactivity of the PF resin is not impaired. In this manner up to 30% ML could be used to substitute for the PF adhesive, with no drop in performance and pressing times. In plywood industrial plant trials with such PF — ML systems, Sellers and Calve; both obtained excellent results. It is believed that at least one Canadian plywood mill is using such a system industrially today. As plywood pressing time is not the really critical variable in a plywood mill, this system did not itself prove suitable for application to particleboard mills, where the shortness of the pressing time that can be obtained is the determining variable.

Attempts were made to use more reactive lignins, such as bagasse (sugarcane waste) lignins, which present 0.7 to 0.9 of a reactive position for each phenylpropane unit, using the same approach. Although good particleboard could be obtained with a mixture of 67% methylolated bagasse lignin (MBL) and 33% PF resin, these could be obtained only at pressing times of 37 to 50 s/mm, still far too long to be of any interest to a particleboard mill [21]. Thus, for particleboard, the low reactivity of lignins toward formaldehyde and the limited number of sites available for reaction with formaldehyde on most aromatic nuclei of the phenylpropane units of lignin are clearly the limiting factors to utilization of this material.

It then became clear that a different but equally or more efficient cross-linking route to be employed in parallel to formaldehyde cross-linking had to be used if feasible pressing times for particleboard mills were to be achieved. Two parallel approaches toward this end have proved successful. First, MBL and methylolated kraft lignin (MKL) were reacted in water with diisocyanate according to a new reaction and the mechanism observed for PF resins [22]. Combinations of polymeric MDI (4,40-diphenylmethanediisocyanate), syn­thetic PF, and MLs yielded particleboard with full exterior-grade properties at pressing times as fast as 20 s/mm when using up to 55% MBL [23]. Pressing times using MKL were also faster but still too slow [23]. Second, as a consequence of the elucidation of PF a-set acceleration mechanisms [24], pressing times as short as 7.5 s/mm for MBL and 10 s/mm for MKL were obtained, at a lignin content of the resin as high as 65% of total adhesive [25]. These pressing times are faster than for synthetic phenolic resins and almost of the same order of magnitude as for UF resins. Industrial plant trials have been held for this system, which appears for the first time to have eliminated the main problem of lignin in wood adhesives for particleboard, that is, the problem of too long pressing times.