Лако-красочные материалы — производство

The analysis of these resins is difficult when unknown products, particularly fully cured have to be tested for UF and MF resins. Widmer [50] offers a method for the identification of UF and MF resins in technical products. This involves preparing crystalline products of urea and melamine and identifying them under the microscope. Melamine (in the form of melamine crystals) and urea (in the form of long, crystalline needles of urea dixanthate) can be seen. This method allows one to distinguish between urea and melamine even in a cured adhesive joint.

Quantitative determination of MF resins is also rather difficult. A method was developed by Widmer [50] for the quantitative determination of melamine in MF condensation products. In this method the resins are destroyed under pressure by aminolysis leaving the melamine intact. This is then converted to melamine picrate, which is easily crystallized and weighed. The Widmer method makes it possible to determine quantitatively the presence of urea and melamine in intermediate condensation products and in cured UF and MF resins (even when they have been mixed). Estimations are seldom in error by more than a few percent.

Hirt et al. [51] have published an effective and rapid method for the detection of melamine by ultraviolet spectrophotometry. This method can be used for products containing MF. It makes use of the strong absorption of the melamine ion at 235 nm. The resin is extracted from comminuted MF samples by hydrolyzing to melamine by boiling under reflux in 0.1 N hydrochloric acid. Stafford [52] also gives a method for the identification of melamine in wet-strength paper.

Uncured MF resin analyses are carried out by gel permeation chromatography (GPC) and 13C NMR. GPC was an inconvenient method for MF resins, although it has become much more accepted for this purpose in recent years. Dimethylformamide, dimethylsulfoxide, or salt solutions are generally used as solvents with a differential refractometer as a detector. Derivatives of the resin, such as those obtained by silylation, are generally used to decrease molecular association by hydrogen bonding. 13C NMR is a more convenient technique, and the chemical shifts of the different structural groups in the resin can easily and readily be identified [52,61]. This method is also quite convenient in comparing MF and MUF resin structures obtained by different manufacturing methodologies [52-61].

More recently effective equations correlating the results obtained by liquid-phase 13C NMR of the liquid MF and MUF resin before hardening with the strength and degree of cross-linking of the resin in the hardened state, hence of the IB strength and formalde­hyde emission of boards bonded with them were developed and reported by two different groups [41,62-64]. One set of equations applied to different formulations can be used to determine the chemical characteristics of the resin without knowing anything of its manufacturing parameters, procedures or molar ratio [63,64] while a different set of equations from a different group requires the previous knowledge of the molar ratio of the resin [62]. They are both very useful tools for the characterization of such resins.

Recently, thermomechanical analysis (TMA) has been used to characterize the performance of polycondensation resins, including MF and MUF adhesives, by cor­relating the deflection in bending of a beech wood joint bonded with a thermosetting resin with the IB strength the same resin can give when used to prepare a wood particleboard under given, industrially significant conditions [15,18,48] according to the equation IB = a (1/f + b, where f is the deflection obtained by TMA in three-point bending and a and b are coefficients characteristic of the type of resin used [65-67]. The experiments can be performed on a preprepared joint in isothermal mode or starting with the liquid glue between the veneer substrates in nonisothermal mode and hence following the hardening of the resin in situ within the joint. What is measured is the narrowest deflection of the cured composite joint at whatever temperature this is achieved, this being correlated through a factor with the inverse of the modulus of the bonded joint, as well as the increase of modulus as a function of time or temperature characterizing in situ the kinetic performance of a particular MUF or MF resin. The system can also give the characteristics of the resin-hardened network by calculation of both the energy of adhesion of interaction with the substrate as well as the level of tightness of the hardened network through the average size of segments between cross-linking nodes (and entanglement nodes) [68-70].

MF and today also MUF resins produce high quality plywood and particleboard because their adhesive joints are boilproof. Considerable discussion has occurred and many investigations have been carried out on the weather resistance of MF and MUF adhesives. Many authors uphold the good weather resistance of the more recently developed MF and MUF adhesives, especially those in which small amounts of phenol (PMUF or MUPF) have been incorporated. The more general trend, however, is to consider the wood products manufactured with these resins as capable of resistance to limited weather and water exposure only, such as in flooring applications, rather than being capable of true exterior-grade weather resistance for which phenolic adhesives are preferred. Figure 11 shows the different behavior of traditional older generation MF-bonded and PF-bonded particleboard in a series of wet-dry cycles. Whereas PF-bonded boards initially deteriorate rapidly then stabilize to a constant swelling value, MF particleboards have slower initial deterioration but never stabilize and continue deteriorating with time and additional wet-dry cycles. This indicates that MF-bonded

Figure 11 Typical trends of irreversible thickness swelling characteristics of wood particleboard bonded with MF and PF adhesives during a wet-dry cycle test.

wood is not completely impervious to further water attack, indicating the fundamental susceptibility of the aminoplastic bond to water. The rate of deterioration, and therefore bond hydrolysis, increases as the temperature increases. Considering the insolubility of melamine in cold water, this is quite understandable. Recent developments in MUF resins (as indicated in this chapter) have, however, introduced a question mark over this rather conservative definition of the weather resistance of MF and MUF resins in relation to phenolics, and clear indications exist that with wise formulation MUFs can indeed perform as fully fledged, heavy-duty exterior adhesives capable of competing successfully with both phenolic and diisocyanate resins.