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New Approaches to Wood Bonding a Base-Activated Lignin Adhesive System

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New Approaches to Wood Bonding a Base-Activated Lignin Adhesive System Wood Science Wood Sci. Technol. 19:363-381 (1985) andTechnology Springer-Verlag 1985 New approaches to wood bonding A base-activated lignin adhesive system R. A. Young, M. Fujita and B. H. River, Madison, USA Abstract. Current knowledge of wood surface characteristics and surface modification are briefly reviewed and the postulated effects of chemical activation are summarized. It was found that aqueous sodium hydroxide can effectively activate wood surfaces to give strong dry autohesive bonds. but only low wet strength was obtained. However, excellent dry and wet wood bond strengths, equivalent to phenol-formaldehyde bonded samples. were obtained when methylolated lignin was used in combination with 3N sodium hydroxide activation. Several mechanisms of base activation are suggested, including enhanced wood surface contact and reactivity. Introduction The large old-growth forests around the world are being rapidly depleted with a concomitant decrease in solid structural timber. Thus the future of the wood industry almost certainly lies with the greater use of reconstituted wood products. This trend is already obvious in Europe, particularly in the use of particleboard which has risen dramatically in recent years (Stier, Buongiorno 1982). The effect is also evident in the burgeoning of flakeboard products in the United States. As the industry shifts more towards reconstituted products such as particle­ board and flakeboard there will be an increasingly greater need for adhesives to convert second-growth timber into serviceable products and structural members. The source of the adhesives for the wood industry has changed dramatically over the past 75 years. Adhesives from renewable resources were common in the 1930’s but the shift to petrochemically-based adhesives came as a result of the decreased cost of petrochemicals and improved durability or water resistance. The current heavy dependence of the wood industry on petrochemically derived adhesives is exemplified by the fact that over 40% of the thermosetting resins used in the U.S. annually go into wood products of some type (White 1979; Skeist 1962). Both small and large wood products industries, dependent on petrochemical and natural gas derived chemical feedstocks, have become alarmed at possible dramatic changes in the supply and cost of traditional fossil sources of organic chemicals for adhesives. Thus both political and economic factors could have sudden immediate effects on the wood adhesives industry. 364 R. A. Young. M. Fujita and B. H. River Obviously there is a very great need for low-cost adhesives of reliable supply and durability for wood-based products. It would be highly desirable to again obtain adhesives from renewable resources while maintaining the high water resis­ tance and durability we have come to expect from petrochemically based adhesives. A very intriguing concept is the possible use of wood process residues and wastes as the source of the renewable adhesives for the wood industry. There have been efforts to make the wood process industries energy self-sufficient (Rowell, Hajny, Young 1982); it would also be very timely to make this industry materials self- sufficient as well. New methods and approaches to wood bonding are therefore under investigation. Surface characteristics of wood One approach is to try to better understand the nature of the wood surface as it is prepared in the factory for bonding. Wood has one of the most heterogeneous surfaces in the range of bonded materials. This is because each small section is very variable in both physical and chemical characteristics. Whereas a metal surface can be planed microscopically smooth and oxide layers removed, the planing of a wood surface still results in an extremely rough surface on the microscopic scale. This is shown in the micrographs in Fig. 1 and depicted graphically in Fig. 2. Vessel elements, tracheids, ray cells and resin canals, when cut through during surface preparation, create openings or porosity at the wood surface. In a positive way, this adds to surface area and provides regions for penetration and interlocking of the adhesive; but negatively, air can be entrapped and good contact between opposite wood faces is difficult to achieve because of the surface roughness. Additional anatomic variability is due to springwood and summerwood variation and concomitant density differences. Apparently springwood bonds more efficiently; probably due to better deformability characteristics (Marian, Stumbo 1962a. 1962b). During machining, the summerwood is also preferentially sheared off due to the greater compressibility of the springwood (Marian, Stumbo, Maxey 1958). Recent studies have shown that damage to the S2 layers of the cell walls near the wood surface during surface preparation (i.e. planing) may be a more impor­ tant factor for good bond strength. Substantially intact S2 layers in knife-planed samples retain considerable strength; while rupturing and cracking of the S2 layers in abrasive planed samples significantly decreased bond quality (Murmanis, River, Stewart 1983). Much less is known about the chemical characteristics of wood surfaces. Zavarian (1984) has recently reviewed the present state of knowledge of wood surface chemistry. It is safe to say that the chemical state of wood surfaces is as variable as the physical state. Variable percentages of lignin, hemicelluloses, cellulose and extractives are present depending on how the wood is cut; transverse, tangential or radial. The cut may expose middle lamella, cell wall layers (i.e. the S3 layer), and/or ray cells which would also result in different composition of the wood surface. The middle lamella is known to be composed mainly of lignin with New approaches to wood bonding 365 Fig. 1a-c. Microscopic view of wood surface texture; a Douglas fir. 200×; b Douglas fir earlywood, 470×; c Maple with vessel element exposed at surface, 470× 366 R. A.Young. M. Fujita and B. H. River Fig. 2. Schematic depiction of the physical structure of wood surfaces Fig. 3. Schematic depiction of the chemical nature of wood surface layers some pectins, while the cell wall layers contain lignin, cellulose and hemicelluloses in various proportions (Young 1982). The resin ducts in softwoods are especially rich in extractives. External factors not inherent to wood properties can also affect wood surfaces. Surface preparation can cause changes of the wood surface due to heat generation during sawing or planing and cause deposition of traces of metals. Exposure to air and light can cause surface oxidation, dust and water of condensation can be accumulated during storage and fatty acids can be deposited from the air (Zavarin 1984). New approaches to wood bonding 367 It has been postulated that significant changes occur at the wood surface due to migration of extractives. There is a large amount of literature devoted to the migration of extractives and their effect on wood bonding (Hancock 1963; Troughton, Chow 1971; Dougal et al. 1980). We have demonstrated in our labora­ tory that the surface of wood is quite hydrophobic, based on ESCA measurements on the surface of maple panels (Young et al. 1982). Other studies have shown that the surface extractives can be removed by solvent extraction or dilute alkali for improvement of bond quality (Hancock 1963). Based on previous work, a reasonable representation of the surface layers of wood is given in Fig. 3. A variable structure is shown with all the important chemical components represented. A hydrophobic surface layer of extractives or fatty acids is apparent in the pictorial depiction. The characteristics and reactivity of the wood face can be significantly altered by surface modification. Wood surface modification Recent work on surface modification of wood has demonstrated the potential of bonding wood without adhesives through simple “activation” (Zavarin 1984: Kelley et al. 1983; Stofko, Zavarin 1984). Surface treatment to improve adhesion has appeared in the literature since 1939 (Tischer). Emerson (1953, 1963) patented a process for nitric acid pretreatment of wood before bonding with furfural and urea or lignosulfonates. More recently Stofko (1974) and Zavarin (Stofko, Zavarin 1974) demonstrated that surface activation could be achieved with a wide variety of oxidizing reagents. In our work (Kelley et al. 1983; Rammon et al. 1982), it was found that nitric acid produced the strongest dry autohesive bonds through surface treatment of solid maple panels. This work included an intensive study of the possible mechanisms of nitric acid activation of wood surfaces. The following conclusions were drawn from this investigation: 1. Oxidation of the wood surface to primarily carboxyl groups is an important aspect of wood activation for surface bonding. 2. Nitration of the wood is an integral part of the activation process (with nitric acid) and nitro compounds may be important to wood bonding. 3. Both hydrolysis of wood polymers and condensation of lignin probably occur with nitric acid treatment. 4. Nitric acid activation appears to occur in two stages: Stage I: Lignin is primarily oxidized, nitrated, and hydrolyzed at room tempera­ ture and xylan is extensively hydrolyzed. Stage II: The polysaccharides are further oxidized and hydrolyzed and some additional modification of lignin takes place at higher temperatures (above 100 °C). The hardwood hemicellulose, xylan. appears to be the most severely altered through oxidation and hydrolysis. The results of 40% nitric acid treatment on the surface
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