Carbohydrate-modified resins

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. 

In an earlier paper (2), we determined that carbohydrates could replace a significant portion of the phenol-formaldehyde resin used for bonding plywood veneer. Carbohydrates from renewable resources such as wood can replace up to 50% of the phenol and formaldehyde in resins formulated under basic conditions without significant loss of bond quality. Two-ply, Douglas-fir-veneer panels bonded with these carbohydrate-modified resins have shear strengths approximately equivalent to those for panels bonded with unmodified phenol-formaldehyde resin. 

Water extraction studies of the carbohydrate modified resins in the earlier study indicate that a portion of the carbohydrate is apparently incorporated into the final cured resin. The absolute amount of modifier incorporated into the cured resin increases with the amount initially added during resin formulation. IR studies indicated that the carbohydrate derivative is probably incorporated into the resin via ether linkages. 

Color of Bond Line. Two-ply veener panels bonded with carbohydrate-modified resins formulated under neutral conditions had bond lines that are extremely light colored in contrast to the dark red-black color characteristic of resins cured under basic conditions. The color ranges from a light yellow-tan with unmodified and xylose-modified resins to a medium tan with the prehy-drolyzate modified resins. These resins would therefore be suited for bonding wood used for decorative purposes. 

Table III lists a number of selected references that describe the formulation of resins or adhesives at each intersection in Figure 1. PF, UF, UF modified with phenolics, and PF modified with nitrogenous compounds (e.g., urea) have not been included, because they do not contain carbohydrates and because they are in common use. The resin and adhesive systems that have been investigated most recently are those formed by the combination of carbohydrates with PF, both with and without the addition of a nitrogenous compound. Our attempts at the Forest Products Laboratory to use carbohydrate modified PF to bond wood are discussed in Chapter 25.

Carbohydrate-Modified Phenol-Formaldehyde Resins Formulated at Neutral Conditions... 

CONNER ET AL. Carbohydrate-Modified Phenol—Formaldehyde Resins 359... 

Viscosity of Neutral Resins. The neutral resins, both unmodified and carbohydrate-modified, were very viscous. The resins have a consistency of taffy candy. This might present problems in their utilization with present commercial equipment, although equipment for applying foamed resins might be suitable. 

Carbohydrate-phenolic-based resins can be modified to change their physical and chemical properties, and faster curing adhesives can be made from these modified resins. However, the nature of the research presented here is exploratory, and much remains to be done. In particular, the molecular structure of these resins needs to be defined. 

Photolabile linkers play an important role in solid-phase organic synthesis (SPOS) due to their stability under both acidic and basic conditions. The ONb photolabile linker was modified to improve cleavage rates and yields Fmoc-Tos-OFI was released in 87% yield after 23 h (Scheme 4) [24]. Specifically, the primary alcohol was changed to a secondary benzylic alcohol and the attachment to the resin was through an alkyl chain as opposed to an amide function. Linker 20 was used for the production of carboxylic acids or carbohydrates. A second example... 

The substitution of selected and possibly modified carbohydrates for part or all of the phenol in the synthesis of phenol-formaldehyde resins. 

Choice of the Lignin Modification Reaction. The phenolysis reaction was selected as a means of modifying the structure and reactivity of the ammonium lignin sulfonate for three main practical reasons. First, because this lignin derivative is soluble in (and will ultimately be used in conjunction with) liquid phenol itself second, because unreacted phenol, unlike other reaction solvents, would not have to be removed from the phenolated product after reaction and before conversion to the adhesive resin and third, because lignins and carbohydrates are known to react with phenols under acidic conditions (6,7). 

Historically, several adhesives have been derived from natural carbohydrate polymers (1,4-6). In a few cases, they have been utilized because of their own particular adhesive quality. However, natural carbohydrate polymers are usually utilized as modifiers for more costly synthetic resins, especially as thickeners, collodial stabilizers, and flow controllers. Table II lists examples of the use of natural gums in adhesives (7-40). 

Monosaccharides (e.g., glucose), disaccharides (e.g., sucrose), and oligosaccharides can be obtained readily from natural sources, either directly or by hydrolysis of natural carbohydrate polymers. These can be used to either modify synthetic adhesive resins or to replace them altogether. In addition, reactive derivatives could be synthesized from these compounds and used to formulate adhesive polymers. 

Modified Synthetic Adhesives. Phenol-formaldehyde (68) and urea-formaldehyde (69) are important synthetic adhesives. Phenol-formaldehyde adhesives (PF) find a variety of applications including bonded abrasives, foundry applications, fiber bonding, and wood bonding. Urea-formaldehyde adhesive resins (UF) are used generally to bond wood products. I will illustrate the modification of synthetic adhesives with carbohydrates using both these general types of adhesives. 

Polymeric carbohydrates of an undetermined degree of polymerization have also been used to modify synthetic adhesive resins. In particular, cellulosic papermill sludges have been used to modify PF and UF resins (127). A carbohydrate polymer was reported to be an excellent extender and modifier for polyvinyl alcohol adhesives (128). 

Incorporation of Carbohydrate into Cured Resin. Several methods were used to determine whether the carbohydrate component is chemically incorporated into the final cured resin. These methods were water extraction of the modifier from the cured resin, IR spectroscopy, and isolation of reaction products formed between a carbohydrate and a model compound that contained a phenolic methylol group. 

Extract ability. About 60 to 70% of the total modifier added is extractable from resins cured under basic conditions and modified with alditols and methyl glycosides (2). In contrast, only about 0 to 20% of the xylose and prehydrolysate is extractable from samples of cured resin modified with 0.6 moles of either modifier per mole of phenol. Approximately 20 to 30% of xylitol (II), methyl xyloside (III), or glucose (IV) is extractable from neutral resins modified with these carbohydrates, indicating that neutral resins incorporate the carbohydrate more effectively than resins formulated and cured under basic conditions. In addition, free reducing sugars can be used directly. 

In another method for isolating the carbohydrate fraction of wood, chlorine dioxide is used, instead of chlorine, and the lignin derivatives formed are extracted out with pyridine-water. This method was first reported by Schmidt and Graumann. Several years later, Jayme modified the procedure by using sodium chlorite and acetic acid in place of the explosive, chlorine dioxide gas. Jayme s method was subsequently modified by Wise and coworkers, " and, in this form, has been used extensively in the characterization of wood. The procedure may be applied directly to air-dried wood without previous extraction, except in the case of resinous woods, where a preliminary treatment with alcohol and ether is desirable. 

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