Лако-красочные материалы — производство

The characteristics of PF resins and the reactive chemical groups they present render them particularly suitable for the preparation of binders by coreaction with other resins. This is still a relatively young field, and the most interesting and relevant co-resins that are being used or explored in this respect are the aminoplastic resins, in particular urea-formalde­hyde (UF) and melamine-formaldehyde (MF) (the copolymerization with the latter being a somewhat older use), and the diisocyanates.

While MF resins have been known for a long time to be able to form true copoly­mers with PF resins, this has not been the case for UF resins. Until quite recently, copolymerization between PF and UF resins or urea was not thought to be likely [53], the system curing as a polymer blend only. However, applications of this type have been shown to be useful also [31,45,54]. Such a deduction was based on the lack of detection of any methylene bridge between the phenolic nuclei and the amido group of urea. Recently, PUF resins of two different types and for two different purposes have been shown to be able to copolymerize. First, Tomita [54] has shown that copolymerization between PF and urea resins occurs under acid conditions, the driving force of this work being the aim to produce a PUF copolymer in which the methylol groups are on the amido group of urea, and thus able to cure rapidly at very mild acidic pH values as a UF resin while the resin retains a level of water resistance. Second, in the alkaline pH range PF resins were shown to be able to copolymerize rapidly with urea, doubling the PF linear degree of polymer­ization while presenting full water resistance of the cured resin [45]. This latter approach was extended to copolymerize fairly high molar amounts of urea and to form a true PUF of excellent exterior performance, much lower cost, and much faster curing and pressing time while still curing at the alkaline pH characteristic of true PF resins [31,32]. The PUF resin showed higher strength of the finished network and increasing hardened strength when the proportion of urea is progressively increased. In this reported work the propor­tion of urea capable of copolymerizing to form the PUF was shown to depend on the formaldehyde to phenol molar ratio of the resin; within certain limits, the higher the molar ratio the higher the proportion of urea capable of copolymerizing. The upper proportion of urea was limited by the relative increase in viscosity and related pot-life shortening of the resin and no other factors [31,32].

Both behaviors are easily explained by the relative rates of PF condensation and of urea hydroxymethylation and subsequent combination. Thus in Fig. 6 the relative gel times of PF and UF resins are shown, as well as taking in consideration the relative rate constants of PF autocondensation and urea hydroxymethylation and self-condensa­tion, indicating quite clearly in which pH ranges copolymerization is possible and with which species [55]. Figure 6 shows that urea and PF resin, with little free formaldehyde or methylol ureas, will easily copolymerize at pH higher than 7; in Fig. 6, PF and UF resins copolymerize in the pH range 6-9. The type of reactions and mechanisms involved have been worked out and summarized in a scheme (shown on page 555) indicating all the main reactions occurring in the formation of PUF resins [31] even taking in consideration that the urea hydroxymethylation reaction is an equilibrium and that reversible reactions do occur.

PF resols in water solution have been shown [56] to react rapidly and readily with polymeric 4,40-diphenylmethane diisocyanate (MDI) with minimal deactivation of the

isocyanate groups by water. This peculiar behavior is based on the much faster rate of reaction of the isocyanate group with the PF methylol groups (hydroxybenzyl alcohol groups) than with water [56]. Such adhesives are now used industrially to a limited extent, for bonding difficult to glue hardwood veneer species into exterior-grade plywood [57], and present exceptional adhesion, ease of bonding, adequately long pot life, and very high adhesive strength. There is now more industrial interest in using these adhesives for particleboard and for wood panels other than plywood since it has been perceived that they are an excellent and viable alternative to both pure isocyanates as well as pure phenolics. The reactions which bring network cross-linking of these copolymers (propor­tions between 5 and 30% of isocyanate on PF resin solids are used when the two are mixed in the glue mix and coreacted in the hot press while pressing the wood panel; 1-2% are used when they are prereacted, but this latter approach is not as yet used industrially and might present some problems) are:

(i) The formation of urethane bridges derived by the reaction of the isocyanate group with the methylol group of the PF resin [33,56], which is perhaps one of the two dominant reactions introducing better dissipation of energy at the interface and hence contributing to strength improvement; (ii) the classical formation of methylene bridges in the PF resin only [33,56,57]; (iii) the formation of polyurea networks due to the reaction of the poly­meric polyisocyanate with the water carrier of the PF resin [33,56,57]; and (iv) the reaction of the PF methylol group to form methylene bridges on the aromatic nuclei of polymeric MDI [56].

This latter reaction was also observed to occur between the methylol groups of lignin and the aromatic nuclei of the polymeric MDI which had been proven to be able to penetrate the wood cellular walls [58].