Lignin-formaldehyde resins

Chen (29) found that the amount of sulfuric acid directly determines the hardening time in the acid condensation of spent sulfite liquors used in plywood and veneers. However, in general the adhesives based purely on acid condensed lignins have often been found to be an uneconomic and qualitatively inferior alternative to adhesives based on synthetic polymers and phenol or lignin-formaldehyde resins. 

By far the preponderance of the 3400 kt of current worldwide phenolic resin production is in the form of phenol-formaldehyde (PF) reaction products. Phenol and formaldehyde are currently two of the most available monomers on earth. About 6000 kt of phenol and 10,000 kt of formaldehyde (100% basis) were produced in 1998 [55,56]. The organic raw materials for synthesis of phenol and formaldehyde are cumene (derived from benzene and propylene) and methanol, respectively. These materials are, in turn, obtained from petroleum and natural gas at relatively low cost ([57], pp. 10-26 [58], pp. 1-30). Cost is one of the most important advantages of phenolics in most applications. It is critical to the acceptance of phenolics for wood panel manufacture. With the exception of urea-formaldehyde resins, PF resins are the lowest cost thermosetting resins available. In addition to its synthesis from low cost monomers, phenolic resin costs are often further reduced by extension with fillers such as clays, chalk, rags, wood flours, nutshell flours, grain flours, starches, lignins, tannins, and various other low eost materials. Often these fillers and extenders improve the performance of the phenolic for a particular use while reducing cost. 

Recent work has concentrated on the use of ALCELL lignin as a substitute for phenol-formaldehyde resins in wood adhesives, particularly wafer-board. Some of the results obtained when a PF resin (Bakelite 9111) was replaced with different levels of hardwood ALCELL lignin in waferboard manufacture will be briefly discussed below. Table III shows the conditions used for waferboard manufacture. 

It has been demonstrated that red oak OSL could be used to replace 35% to 40% of the phenol (or phenolic resin solids) in phenol-formaldehyde resins used to laminate maple wood and to bond southern pine flake boards (wafer-board and/or strandboard) without adversely affecting the physical bond properties. While this pulping process and by-product lignin do not commercially exist at this time in the United States, lignins from such processes are projected to cost 40% to 50% less than phenol as a polymer raw material. 

Cure Rate of the Phenolated SEL Resins. 13C NMR spectra of the phenolated SEL formaldehyde-treated resins revealed the formation of methylol groups. A similar cure reaction to resole type phenolic resins is expected to occur with the phenolated lignin-based resins. Since cure rate normally determines production capacity of a board mill, it is important that new types of adhesives have at least the same cure rate as the conventional phenolic adhesives. Cure analysis of resins has usually been examined by... 

Relative rigidity vs. temperature curves of LP s are shown in Figure 4 in comparison with a commercial phenolic resin. The pH of these resins was previously adjusted to around 10.8. The phenolic resin is fully cured at around 75°C. By contrast, the curves of the three lignin-based resins exhibit slower cure as compared to the phenolic resin. The retardation increases as the charge ratio of formaldehyde increases. Some retardation had already been found, but neglected, for the phenolated lignin/phenol-formaldehyde resins (12). In this study, the neat phenolated SEL was used for resin preparation. It can be concluded that phenolated steam explosion lignin-based resins have an intrinsically retarded cure behavior as compared to phenolic resin at the same pH. 

Table II gives a number of derived parameters used to assess the lignin s suitability as a binder. For a full binder, lignins required a minimum of three sites per average molecule for formaldehyde grafting. None of the lignins studied approach this level. At least twice the number of sites found is required for steam exploded lignins and more for the kraft lignin. PF resins have an average of eight sites per molecule, a much higher density than projected, so PF resins are excellent wood binders.

Early research and development work has indicated that the structure of Eucalin Lignin should make it ideally suited for use as a chemical intermediate. Research in South Africa has indicated that Eucalin Lignin ranks between softwood and bagasse lignin in terms of the number of reactive sites with formaldehyde (under acidic conditions), and that it allows a high percentage of substitution of phenol in phenol-formaldehyde resins. 

The reaction of lignin with formaldehyde is of practical interest not only for the potential use of lignin as reactive extender in phenol-formaldehyde resins but also for wood pulping. 

The institution of fractionated or chemically modified lignin for part or all of the phenol in the synthesis of phenol-formaldehyde resins. This would particularly include the newer forms of lignin recovered with minimum structural alteration and also those representing virtually complete depolymerization to phenylpropane units. 

Effects of Phenol-Formaldehyde Copolymer on Gluebond Performance of Lignin-Phenolic Resin Systems... 

Experiment 2. Effect of Molar Ratio of Sodium Hydroxide to Phenol of Phenolic Resin on Strength Properties of Lignin-Phenolic Resin Adhesives. Sodium hydroxide has been the predominant chemical used as a catalyst in resol resin technology. Through variation in the amounts of the catalyst and the method of catalyst addition, a wide variety of resin systems can be formulated. This experiment examined the properties of phenolic resins formulated with various sodium hydroxide/phenol ratios and their effects on the bond properties of structural flakeboards made with lignin-phenolic resin adhesive systems. Variables for resin preparation were four molar ratios of sodium hydroxide/phenol (i.e., 0.2, 0.45,0.7, and 0.95). The formaldehyde/phenol ratio and solids content were fixed at 3/1 and 42%, respectively. 

The most popular is the 40% solids content family, since it seems to give a little better cost/performance relationship. This experiment examined the effect of phenolic resin solids on the properties of structural flakeboards bonded with the lignin-phenolic resin system. The variables were three levels of phenolic resin solids content - 39%, 46%, and 54%. The formaldehyde/phenol ratio and NaOH/phenol ratio were 3/1 and 0.7/1, respectively. 

Formaldehyde to Phenol Ratio. The effect of formaldehyde/phenol ratio on pH, viscosity, and gel time of the lignin-phenolic resin system is summarized as follows ... 

It is noted that the gel time decreased as the formaldehyde/phenol ratio of the phenolic resin in the system increased. Furthermore, the gel time curve of the lignin-phenolic system was, in general, similar to that of phenolic resins measured in the previous section. The similarity of the gel time curves (Figure 3) may indicate that the phenolic resin plays the major role in affecting the cure speed of the lignin-phenolic resin system even though the phenolic resin consisted of only 25% by weight in the system. 

Figure 3. Gel time curves of phenolic and lignin-phenolic resins as affected by formaldehyde/phenol ratio.

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