Lignin-phenolic resin

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

The bond performances of lignin-phenolic resin systems were studied through a series of experiments, each designed to elucidate a facet of the problem. The resin preparation and panel fabrication procedures were, however, maintained as uniformly as possible. Thus, unless otherwise specified, the experimental procedures described below were used in the study. 

Blending of Lignin-Phenolic Resin System. The methylolated lignin was blended with the phenolic resins at a solid weight ratio of 75/25 and at room temperature. The mixture was mechanically stirred for 15 minutes. The gel time, pH, and solids content were determined for each resin system. 

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. 

Experiment 3. Effect of Phenolic Resin Solid Content on Strength Properties of Lignin-Phenolic Resin System. Past experience has shown that the optimum solids content of typical phenolic resin formulations is between 40% to 50%. 

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 ... 

CH20/Phenol Ratio Methylolated Lignin/Phenolic Resin Ratio Viscosity pH Gel Time... 

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. 

Average physical and mechanical properties of the flakeboards are summarized in Table I. On the average, panels bonded with the lignin/phenolic resin... 

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

Altogether, there have been only a few studies published dealing with the copolymerization behavior of distinct phenols, and usually the characterization of the copolymers was not fully examined. An early study of copolymerizations between different phenols and anifines can be found, wherein the copolymer compositions were characterized by elemental analysis [78]. In addition, monomeric phenols have been copolymerized with phenol polymers. This procedure offers, for example, an interesting way to turn fignin, a polymeric by-product from the pulp and paper industry, into a technical material. Lignin was reacted with phenol in an HRP-catalyzed copolymerization to produce lignin phenolic resins [117]. 

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. 

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. 

The efficiency of ethylene glycol-water as a delignifying solvent has been demonstrated by Gast and Puls (10). Results showed that sufficiently delignified pulps could be obtained. Also, the lignins produced showed promising results as extenders in phenolic resin adhesives. 

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. 

COOK SELLERS Organosolv Lignin-Modified Phenolic Resins 333... 

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. 

Cure Rate Dependence on pH for the Phenolysis Lignin Resins. Cure behavior at different pH s of the resins was measured at 140°C, which is the usual hot-pressing temperature of phenolic resins. Relative rigidity change curves of LP-B at different pH s are illustrated in Figure 5. Cure advances faster as the pH of the resin increases. When the pH is 11.9, LP-B provides faster cure than the phenolic resin. A similar tendency has been found for LP-C. These findings clearly demonstrate that increasing pH of the resins improves cure rate. 

Figure 4. Cure behavior of phenolated steam explosion lignin-based resins.

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