Incorporation into phenol-formaldehyde

Fast pyrolysis of biomass produces a phenol-rich oil suitable for incorporation into phenol-formaldehyde (PF) resins. To evaluate the reactivity and network formation characteristics of the compounds typically found in these oils, a series of phenolic model compounds was reacted with formaldehyde under conditions typically used to prepare PF resins. This study indicates that the substituted phenolics commonly found in pyrolysis oils are more reactive than phenol. It also showed that the network formation process for tiiese complex phenolic mixtures follows the predictions of the simple statistical approaches developed by Flory (23) and Stockmayer (24). These results show that the substituted phenolics commonly found in pyrolysis oils will be chemically bonded into the polymer network and that under the proper reaction conditions a highly stable network will be formed. 

Since rosin can react with formaldehyde in a similar way as phenol, rosin components have also been used in the preparation of the corresponding rosin-phenol-formaldehyde resins with molecular weights and solubility that are adequate for their incorporation into printing inks. The incorporation of rosin components into phenol-formaldehyde prepolymers can occur through esterification or methylol condensation at one of the unsaturated carbons (Fig. 4.11). 

Two-Stage Resins. The ratio of formaldehyde to phenol is low enough to prevent the thermosetting reaction from occurring during manufacture of the resin. At this point the resin is termed novolac resin. Subsequently, hexamethylenetetramine is incorporated into the material to act as a source of chemical cross-links during the molding operation (and conversion to the thermoset or cured state). 

Special resoles are obtained with amine catalysts, which affect chemical and physical properties because amine is incorporated into the resin. For example, the reaction of phenol, formaldehyde, and dimethylamine is essentially quantitative (28). 

Phenol-formaldehyde adhesives are produced by a condensation polymerization reaction between phenol and formaldehyde. The phenolics used for exterior particleboard are made at a formalde-hyde/phenol ratio greater than 1.0 i.e., they are classified as resoles and additional formaldehyde is not required to complete the curing reaction to a highly cross-linked network structure. Many characteristics can be incorporated into the adhesives by changes in the F/P ratio, condensation pH, and condensation time. The reactive solids content is normally between kO and 50 percent since the stability and viscosity are adversely affected at higher solids. 

In this study, up to about 50% of the phenol-formaldehyde was replaced with carbohydrates and the modified resins used to bond wood veneer panels. The carbohydrate modified resins were formulated and cured under neutral conditions. The resins bond wood with acceptable dry- and wet-shear strengths, and wood failures. Reducing as well as nonreducing carbohydrates can be used as modifiers. The carbohydrate modifiers are being incorporated into the resin via ether linkages between the hydroxyls of the carbohydrate and methylol groups in the phenol-formaldehyde resin. The resins formulated under neutral conditions are very light in color. 

Matsumoto and coworkers incorporated poly(A -(hydroxyphenyl)maleimides) into composite materials with phenol-formaldehyde resins. Poly(iV-(4-hydroxyphenyl)malei-mide) has been shown to form miscible blends with phenolic resins and, after crosslinking, produces composites with good thermal and chemical stability. The hardening or crosslinking agent most commonly used is hexamethylenetetramine (HMTA), to form an insolnble and infusible three-dimensional polymeric network (Scheme 29). 

A large number of commercially important condensation polymers are employed as homopolymers. These include those polymers that depend on crystallinity for their major applications, such as rylons and fiber-forming polyesters, and the bulk of such important thermosetting materials like phenolics and urea-formaldehyde resins. In many applications, condensation polymers are used as copolymers. For example, fast-setting phenolic adhesives are resorcinol-modified, while melamine has sometimes been incorporated into the urea-formaldehyde resin structure to enhance its stability. Copolyesters find application in a fairly broad spectrum of end uses. 

Regardless of the fact that numerous investigations exist about the possibility of incorporating the furan heterocycle into wood adhesive formulations, their industrial exploitation is still modest. The first suggestion concerning the use of 1 in partial substitution of formaldehyde in phenolic resins was put forward in 1958 by Baxter and Redfern [18] who proposed that the furfural units were incorporated into the polymer skeleton following condensation reactions such as ... 

Under all reaction conditions, the reactivity of all three model compounds was higher than that of phenol (see Figure 2). Under the higher reaction temperature with higher levels of NaOH/P molar ratio and F/P molar ratio, for example, the reaction of 2-methoxy-4-methylphenol, 2-methylphenol, and 4-methylcatechol with formaldehyde was 3.5, 2.7, and 6.0 times faster than that of phenol. These results indicate that the pyrolysis oils will be highly reactive under traditional PF resin synthesis conditions and is likely to be chemically incorporated into the PF network. 

Phenolic resins can also be formulated from lignin. High grade lignin can be either directly blended with the phenol resin or incorporated into the resin at high ratios by condensation or derivatiza-tion with phenol or formaldehyde (39). 

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