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 layers of wood are summarized in Fig. 4. This representation, based on the cumulative evidence to date. shows the expected alterations of the “normal” surface depicted in Fig. 3; 1) lignin and extractives are hydrolyzed and removed; 2) the lignin and poll- 365 R. A. Young, M. Fujita and B. H. River

Fig. 4. Schematic depiction of the effect of 40% nitric acid on the surface chemistry of wood

saccharides are oxidized and nitrated; and 3) the entire surface is swollen and plasticized to a limited extent with the nitric acid. These changes suggest alternate approaches to wood bonding.

Alternate approaches to wood bonding

A new wood bending approach which evolved from previous studies on bonding by wood surface activation was the direct use of wood surfaces as reactive sub­ strates. The basic concept is that the induced functional groups can react with each ether, with wood components or gap-filling agents in essentially the same way as derivatization reactions are carried out with isolated wood polymers. Therefore. in this study, an attempt was made to bond wood by surface derivatization. Many derivatization reactions of isolated wood components, such as cellulose, hemicelluloses, and lignin, involve a pretreatment or co-reaction with caustic soda. The sodium hydroxide both swells the wood components and, especially in the case of cellulose, forms a much more reactive intermediate (alkali cellulose). Thus, wood surfaces were treated with sodium hydroxide to form a reactive “alkali wood” surface for further derivatization reactions. The first derivative attempted was the xanthate formed by reaction of alkali activated wood hydroxyl groups with gaseous carbon disulfide as shown below.

When cellulose is reacted in a similar manner, the derivative is soluble in aqueous alkali and referred to as viscose; the raw material for subsequent regeneration to rayon fibers and cellophane films. A similar reaction at wood surfaces is thought conducive to melding the swollen wood faces under compression. Possible regeneration of the wood hydroxyl groups in situ was also considered possible due to the natural acidic character of wood. A xanthation derivatization is also desireable because it is very easy to carry out; the alkali activated wood is simply exposed to carbon disulfide vapors in a closed container at room temperature. Wood panels were activated with sodium hydroxide, xanthated, dried and pressed together without further treatment. A strong bond was initially obtained New approaches to wood bonding 369

but the panels spontaneously separated after a few days under ambient conditions. In further experiments the panels were additionally sprayed with an aqueous sodium hydroxide solution, after xanthation and drying, to remoisten the wood surfaces. In this case, a more permanent bond resulted as shown in Table 1. Distilled water was also used to re-moisten wood surfaces but this treatment did not cause the xanthated panels to bond. An important observation was that panels treated with only sodium hydroxide bonded and gave better dry strengths than those of the xanthated boards, although the bonds were not water resistant. Since the xanthation time of the panels did not enhance the bond strength, xanthate bonding probably did not occur and it appears that the majority of the bond strength was due to the action of the sodium hydroxide activation. Another approach to xanthate bonding is coupling of the introduced xanthate functional groups at the wood surfaces. This reaction has been used to crosslink starch and cellulose molecules and results in a very stable linkage (Russell et al. 1962). The anticipated reaction for wood bonding by this mechanism is illustrated below.

Both zinc chloride and ferric chloride have been tested as crosslinking agents for xanthated wood as shown in the above equations. The results of the mechanical tests on the xanthate crosslinked, bonded wood panels are shown in Table 2. Zinc chloride gave better shear strengths than ferric chloride. This is probably due to the difference in the crosslinking mechanism for the two compounds. Zinc from zinc chloride forms a crosslink between two xanthate groups. while ferric chloride only oxidatively catalyzes the crosslinking reaction of xanthate groups without the iron being incorporated into the crosslink.

Table 1. Shear strength of panels bonded with xanthation and sodium hydroxide activation

Xanthation NaOH conc. Shear strength,psi time for activation h dry wet

0 3N NaOH 2754 ± 219 17 ± 6 2 1198 ± 372 – 0 5N NaOH 2290 ± 118 22 ± 6 1/2 2317 ± 124 – 1 2374 ± 216 – 2 1399 ± 172 – 3 2409 ± 116 – 370 R. A. Young, M. Fujita and B. H. River

Table 2. Shear strength of panels bonded by coupling of xanthate groups Coupling agent Xanthation Shear strength, psi time h dry wet

5% FeCl3 1 697 ± 221 0 3 1151 ± 402 –

a 1544 ± 319 – 5% ZnCl2 1 2 2928 ± 177 0 3 1740 ± 407 – 3 2851 ± 380 –

72% ZnCl2 2 2892 ± 103 – 3 3511 ± 189 31 ± 11

a 1 ml of ZnCl2 solution was applied to both sides

Table 3. Shear strength of panels bonded with various agents Base Shear strength, psi

dry wet 1N NaOH 1684 ± 220 42 ± 30 3N NaOH 2154 ± 219 17 ± 6 5N NaOH 2920 ± 118 22 ± 6

50% ZnCl2 2234 ± 248 9 ± 7

72% ZnCl2 2624 ± 78 3 ± 3

30% NH3 (aq.) 731 ± 248 0

Zinc chloride is also known to decrystallize cellulose and at a concentration of 72% the decrystallization reaches a maximum (Young, Miller 1975). Therefore, zinc chloride could also function like sodium hydroxide and extensively swell the wood surface to give better contact and bond strength. Two concentrations of zinc chloride were evaluated as shown in Table 2. Although the xanthated panels bonded with 72% zinc chloride had quite high dry strength, they were not water resistant. The fact that sodium hydroxide alone can bond wood suggested further experi­ ments with base activation. In addition to sodium hydroxide, zinc chloride as an activating agent rather than a crosslinking agent and ammonium hydroxide were used as activating agents. Zinc chloride is known to decrystallize cellulose, as mentioned previously, and ammonium hydroxide is a strong swelling agent for wood. These agents were expected to perform a similar function to that of sodium hydroxide and bond the panels by promoting intimate contact. As shown in Table 3, the resulting panels also had reasonable dry strength but negligible water resistance. Although the panels bonded by base activation were not water resistant, the dry strength of panels was high and the sodium hydroxide did not appear to degrade the wood surface as much as many of the acids tested in previous studies (Zavarin 1984). Therefore, further experiments were performed to improve the water New approaches to wood bonding 371 resistance of the bonds. For this purpose, additional gap-filling agents were applied in combination with sodium hydroxide activation. Starch was the first material evaluated and the results are shown in Table 4. Starch solutions with sodium hydroxide activation gave quite high dry strength but no wet strength. Lignin and wood degradation products were also tested as gap-filling agents with the results shown in Table 5. Because sodium hydroxide can pulp wood, the degradation products were thought to be important for bond formation. Thus, sugar maple flakes were pulped with 5N sodium hydroxide and the spent liquor was used as a gap-filling agent. Again reasonable dry strength was obtained but no wet strength (Table 5). A combination of lignin, starch, and sucrose was also used to simulate wood components for bonding. This combination also gave very high dry strength but no water resistant bonds. Bonding wood by activation with alkali was also attempted by Stofko (1974); but further studies on alkali treatments were apparently abandoned due to the poor water resistance of the bond. These results suggested that only secondary bonds with low water resistance develop with sodium hydroxide activation both with and without the introduction of gap-filling agents. Sodium hydroxide is a good swelling agent for cellulose and

Table 4. Shear strength of panels bonded with starch solution and sodium hydroxide activation a

Concentration Shear strength, psi of starch solution % dry wet

5 3100 ± 150 – 10 2787 ± 261 0 15 3174 ± 199 – 20 3352 ± 81 0 25 2978 ± 300 – 30 2907 ± 258 0 a Bonding conditions NaOH conc. for activation: 5N 2.5 g of starch solution was applied on one side.

Table 5. Shear strength of panels bonded with carbohydrate and lignin with sodium hydroxide activation

Gap-filling agent Shear strength, psi

dry wet

Control a 3446 ± 134 32 ± 34 Spent liquor of 5 N NaOH pulping 3239 ± 106 14 ± 11 9% lignin-starch-sucrose in 0.1 N NaOHb 3477 ± 132 60 ± 52 20% lignin-starch-sucrose in 0.1 N NaOH 3709 ± 179 0 20% lignin-starch-sucrose in 5 N NaOH 3507 ± 194 0 a Bonded with sodium hydroxide only b Lignin: starch: sucrose = 1 : 1 : 1 (by weight); percentage is total solids by weight 372 R. A.Young, M. Fujita and B. H. River

5 Normal corresponds to about 17% solution, which is very close to the merceriza­ tion concentration of cellulose. Sodium hydroxide at this concentration will also pulp wood. Therefore, activation by sodium hydroxide probably swells the wood surface extensively, or even disintegrates it to some extent, and cleaves some of the hydrogen bonds of cellulose. Then, when pressed at high temperature, the wood surfaces recombine by hydrogen bonding and other secondary forces as water evaporates. This mechanism explains why panels bonded with sodium hydroxide activation had high dry strength and no water resistance. Hydrogen bonding can impart high dry strength but the bonds are very susceptible to water.

Mechanisms of base activation

Sodium hydroxide is known to have additional effects on cellulose and wood which could have important implications for wood bonding. The first effect is to improve the wetting properties. Increases in alkalinity in aqueous solutions reduce surface tension. remove extractives, and increase the ability of the adhesive to dissolve extractive materials on the wood surface (Wellons 1980; Hancock 1963). The wettability of the wood surface would be particularly enhanced for alkaline- based adhesive systems. A second important effect is to change the submicroscopic structure of the system of capillaries in the wood surface. This has been demonstrated by the Russian investigators. Erinsh and Odintsov (1967). They determined the internal area and volume of submicroscopic capillaries of wood by sorption of n-hexane vapors at 20 °C. They found that sodium hydroxide solutions caused great changes in the submicroscopic capillaries in wood, even at 0.3% sodium hydroxide concen­ tration. Figure 5 shows a plot of their data for the effect of sodium hydroxide on the changes in volume of different groups of submicroscopic capillaries in birch. At low degrees of swelling a major part of the total volume is occupied by capillaries of radii less than 25 A; but after swelling in a 10% sodium hydroxide solution, the majority of capillaries have radii from 25-80 A. The dimensions of the capillaries increase to an average radius of 45 Å. After treatment of wood with a 5-10% alkali solution, the volume of capillaries less than 19 Å decreases to less than 10% of the total volume of capillaries. The internal surface area reaches a maximum of 350-400 m2/g and the volume of the submicroscopic capillaries reaches 0.8 cm3/g at 10% alkali concentration. Another important observation by Erinsh and Odintsov (1967) was that to achieve an equal degree of swelling of fir wood to that observed for birch it was necessary to treat the wood with an alkali solution 30-40 times higher in concentration. This may reflect differences in the quantity of cellulose or lignin in the cell wall or possibly the greater liability to alkali of hardwood versus softwood lignin. Sawabe and Mori (1981) in Japan also found that sodium hydroxide solutions had a significant effect on the pore structure of holocellulose samples. Based on nitrogen absorption measurements, these investigators demonstrated that the majority of the pores in holocellulose from Japanese cedar had diameters between New approaches to wood bonding 373

Fig. 5. Effect of aqueous sodium hydroxide on the volume of submicroscopic capillaries in birch according to Erinsh and Odintsov (1967)

20-60Å with two peaks (22 Å and 32 Å) in the distribution curve. When the holo­ cellulose was treated with sodium hydroxide, the distribution shifted to a larger diameter range with the interstices of the microfibrils up to twice the size. The number of large pores, above 100 A in diameter, increased according to sodium hydroxide concentration. While the pore size increased with increasing sodium hydroxide concentrations. the number of pores decreased with increasing alkali concentration. This suggests that the change in the pore structure by the sodium hydroxide treatment is caused mainly by unification of adjacent pores and expansion of smaller pores. It was therefore assumed that the two peaks observed in the distribution curve for alkali treated holocellulose at 40-50 Å and 70 Å originated from unification of adjacent intermicrofibrillar pores. These results are consistent with the earlier work of Erinsh and Odintsov (1967) with wood. The dramatic effect of sodium hydroxide solutions on wood submicroscopic structure could produce significant positive interaction with an adhesive in wood bonding. The larger micropore structure could provide interlocking sites for polymeric adhesives and increased surface area for reaction at the adhesive­ adherend interface. In addition, the sodium hydroxide plasticizes the wood surface to some extent which would enhance the contact between opposing wood faces. Indeed, Sugihara (1972) found that when wood powder was treated with a 5% sodium hydroxide solution. the material could be molded with heat (175 °C) and pressure (3000 psi) to a product with a flexural rigidity of 5-6 kg/mm2 (7100-8500 psi). Sugihara attributed the moldability to formation of alkali lignin. The postulated character of the wood surface after treatment with a concentrat­ ed solution of sodium hydroxide is shown in Fig. 6. The wood is swollen with openings and pores created at the surface. Both lignin and extractives have been removed or displaced from the outer wood layers. The cellulose is occluded with alkali and the microfibrillar structure is disrupted. The formation of alkali cellu- 314 R. A.Young, M. Fujita and B. H. River

Fig. 6. Schematic depiction of the effect of aqueous sodium hydroxide on the surface chemistry of wood

lose and similar addition compounds with the wood polymers makes the surface much more reactive. Alkaline solutions appear to affect the wood surfaces to a greater extent than acidic solutions based on the earlier work by Erinsh and Odintsov (1967). Although the same patterns of change of the capillary system were observed by the Russian workers, a much greater concentration (30%) of acid (sulfuric) was necessary to realize an effect on birch similar to the effect of sodium hydroxide. as shown in Fig. 7. A 20% solution of sulfuric acid has no effect on the submicroscopic structure of the capillaries. With fir wood, the elementary capillaries appeared only after the acid concentration was increased to 55%. Likewise, Sugihara (1972) found that when he attempted to mold wood powder treated with sulfuric acid rather than alkali as described above, the flexural strength of the product was only 2-3 kg/mm2 (2800-4300 psi) versus 5-6 kg/mm2 (7100-8500 psi) for the sodium hydroxide treated wood powder.

Enhancement of bonding with base activation

Since hydrogen bonds seemed to be mainly responsible for wood bond formation with sodium hydroxide activation alone, as previously mentioned, a compound which could crosslink adjacent hydroxyl groups should improve water resistance. Therefore formaldehyde, a well-known crosslinking agent, was selected for incor­ poration in the adhesive system. A lignin preparation was also included as a reactive gap-filling agent. The formaldehyde could, of course, also methylolate the lignin as shown in Fig. 8; and this might further enhance the reactivity of the lignin for adhesive bonding as described by previous investigators (Enkvist 1975; Dolenko, Clarke 1978). The bonding results using formalin are shown in Table 6. Although a slight increase in the wet strength was obtained, these values were not spectacular. The molar ratio of lignin and formaldehyde, the open assembly time, and the pot life did not affect the bond strength very much, although longer pot life increased both dry and wet strengths slightly. Longer pot life would further methylolate the lignin. The most discouraging result was that the lignin-formaldehyde solution without New approaches to wood bonding 375

Fig. 7. Effect of aqueous sulfuric acid on the volume of submicroscopic capillaries in birch according to Erinsh and Odintsov (1967)

Fig. 8a and b. Reaction of lignin with formaldehyde; a Lederer-Manassa reaction; b Tollen’s reaction

sodium hydroxide activation gave much better results than were obtained with 5 N sodium hydroxide activation as shown in Table 6. Remarkable and unexpected improvement in wet strength, as well as in dry strength, was obtained by using 3N sodium hydroxide solution for activation as shown in Table 7. The strength of saturated specimens nearly equaled the strength requirement for joints at 16 percent moisture content in the standard for structural 376 R. A. Young. M. Fujita and B. H. River

Table 6. Shear strength of panels bonded with lignin-formaldehyde solution with 5 N sodium hydroxide activation

Molar ratio Shear strength, psi (lignin : HCHO : NaOH) a dry wet

Controls Activation alone 3239 ± 325 89 ± 30 Activation + lignin solution 3091 ± 333 101 ± 69 No activation 1 : 1.5 : 0.3 2928 ± 325 1048 ± 434 NaOH activation 1 : 1: 0.3 3428 ± 177 86± 45 1:1.5:0.3 3360 ± 220 334 ± 230 (30 min open assembly time) 3249 ± 272 221 ± 238 (24 hour pot life at room temp.) 3484 ± 197 462 ± 238 1 : 2.5 : 0.3 2775 ± 207 256 ± 127 (30 min open assembly time) 3188 ± 332 807 ± 313 1 : 3.5 : 0.3 2800 ± 204 223 ± 137 a 36% solid content (lignin + formaldehyde)

Table 7. Shear strength of panels bonded with lignin-formaldehyde solution with 3N sodium hydroxide activation a

Molar ratio Shear strength, psi (lignin : HCHO : NaOH)b dry wet

Controls Activation alone 3359 ± 280 211 ± 60 Activation + lignin solution 3297 ± 225 58 ± 45 Activation + HCHO (no lignin) 2248 ± 181 400 ± 248 No activation 1 : 1.5 : 0.3c 2928 ± 325 1048 ± 434 NaOH activation and lignin-formaldehyde solution 1: 1.5:0.3 4120 ± 172 1343 ± 179 1 : 2.5: 0.3 4112 ± 137 1635 ± 79 NaOH activation and methylolated lignin 1 : 1.5 : 0.3 3856 ± 257 1737 ± 75 1 : 2.5: 0.3 4233 ± 159 1750 ± 60 Laminated timber standard (Amer. Nat. Stand. Inst. 1983) 2100 1820 a 30 min open assembly time was used in all cases b 36% solid content (lignin + formaldehyde) c Molar ratio d The highest moisture content in the standard is 16%. No requirement is given for saturated specimens New approaches to wood bonding 377 glued laminated timber (American National Standards Institute 1983). Further improvement in wet strength was also obtained by increasing the methylolation of lignin. The lignin-formaldehyde solutions were reacted at 80 °C for 1 hour by the method of Gamo (1983) to improve the methylolation. The lignin appeared to react with formaldehyde even at room temperature. As shown in Table 7, activa­ tion with sodium hydroxide, lignin, and formaldehyde are essential for durable bonds. If any of these three factors were not applied, durable bonds were not ob­ tained. The selection of the proper sodium hydroxide concentration to give the most desireable pore structure to the wood surface appears to be a critical factor for durable wood bonds. Durability tests were also performed on joints bonded with methylolated lignin and 3N NaOH surface activation with results shown in Table 8. A vacuum pressure soak test (VPS) and a boiling test were done following ASTM specifica­ tion D3110 (ASTM 1983) except for the smaller specimen size and some minor procedural changes. The bonded specimens survived all these tests. As shown in Table 8 even after the boiling test, the strength did not decrease substantially. The dry strength and the wet strength after VPS treatment are quite comparable to those of phenolic bonded maple panels (Millet, Gillespie 1978). The specimens generally showed very high percentages of wood failure 80%), although shallow wood failure was dominant in some specimens. Eighty percent wood failure exceeds the requirement of Specification D3110 for wet-use adhesives for nonstructural glued lumber products as well as Specification D2559 for wet-use adhesives for structural laminated wood products (ASTM 1983a) and ANSI A190.1 for structural glued laminated timbers (American National Standards Institute 1983). This result is all the more remarkable when one considers it was achieved with such a dense, strong wood as hard maple. The water-resistant bond was obtained by the addition of formaldehyde and the reduction of the concentration of sodium hydroxide from 5N to 3N. As

Table 8. Durability of panels bonded with methylolated lignin and 3N sodium hydroxide activation Test condition Shear strength, psi

L:F:NaOH L:F:NaOH 1:1.5:0.3a 1 :2.5 :0.3a

Dry 3856 ± 257 4233 ± 159 24 h cold water soaking 1737 ± 75 1750 ± 60 VPS 1297 ± 135 1415 ± 51 VPS + drying + VPS 1058 ± 181 1404 ± 74 Boiling + drying + boiling 1077 ± 264 1151 ± 178 Phenolic-bonded maple Dry 3990 ± 415 VPS 1490 ± 96 a Molar ratio of lignin, formaldehyde, and NaOH. 36% solid content (lignin + formalde­ hyde) b Values are taken from Millett and Gillespie (1978) 378 R. A. Young, M. Fujita and B. H. River mentioned previously, the wood surface is extensively swelled and some of the linkages between the components in the solid wood are cleaved by sodium hydroxide activation. A gap-filling agent, such as the lignin-formaldehyde solution, can then penetrate deeper inside the wood surface and the possibility of the reac­ tion between wood hydroxyl groups and formaldehyde and lignin molecules is increased. Methylolation of lignin provided the crosslink between lignin molecules as well as additional hydroxyl groups, and made it more likely that a reaction could occur with the wood surface. It appears that if the concentration of sodium hydroxide is too high (5N), the solution does not properly penetrate the wood microfibrillar structure and create the desireable pore structure at the wood surface. Further work is underway to better understand and optimize this new adhesive system.

Experimental

Materials The sapwood of sugar maple (Acer saccharum Marsh.) was used throughout the investigation. Boards were selected according to ASTM standard D-905 (1983) from which 5/16 × 6 × 4 inches panels were cut for the bonding studies. One face of each panel was jointed in order to give a smooth surface. The sample panels were stored in a room conditioned at 72 °F and 44% relative humidity which gave an 8% equilibrium moisture content of the wood. After bonding the panels were re-condi­ tioned. Nine parallel laminated modified shear blocks described by Strickler (1968) were cut from each bonded panel. All the chemicals used in this study were obtained from commercial sources and used without further modification. The lignin was a commercial kraft lignin from Westvaco Corp. (Indulin AT). Methods. The panels were pressed in a 6x6-inch preheated hydraulic press (Carver Laboratory Press). Aluminium cauls were used to prevent direct contami­ nation of the press platens. Generally, three panels were made for each condition. Three shear blocks from each panel were selected randomly and used for the wet strength test, thus 18 shear blocks were used for the dry strength test and 9 shear blocks for the wet strength test. Shear strength was measured according to ASTM Standard D-905 (1983). For the wet strength test, shear blocks were immersed in cold tap water for 24 hours and the strength of the shear blocks was measured while they were wet. For some samples, a vacuum pressure soak test (VPS), a VPS + drying + VPS series, and a boiling test (i.e., 4-hour boiling + drying + 4-hour boiling) were per­ formed based on Method D3110 (ASTM 1983b) except the size and the type of the test specimens were different than the standard. For the VPS test, a vacuum of 25 inches of mercury was drawn and maintained for 30 minutes, followed imme­ diately with application of 65-70psi of pressure for 30 minutes. For the VPS + drying + VPS series and the boiling test, the weight of the specimens was measured before immersion in water. After the VPS or 4-hour boiling, the specimens were dried in an oven at 150 °F with sufficient air circulation until the specimens returned to their original weight (~ 8% m.c. OD basis). The drying of New approaches to wood bonding 379 the specimens took 5% hours. All the strength measurements after VPS or boil treatment were done while the specimens were wet. Xanthation of panels followed a procedure similar to that of Young (1977). Thus, after 2 g of sodium hydroxide solution were sprayed on each panel with a hand-held atomizer, the panels were placed on a tray in a desiccator over a solu­ tion of carbon disulfide. The desiccator was then evacuated for several minutes to increase the gas pressure of carbon disulfide and kept in this state for a desired period. After xanthation, the panels were dried in vacuo overnight to remove excess carbon disulfide. Before the xanthated panels were placed in the press, 1 g of sodium hydroxide solution was again sprayed on each panel or 1.5 ml of cross- linking agent for xanthate groups was smeared on one xanthated panel surface. The panels were then pressed together at 100 °C and 300 psi for 1 hour. For bonding with zinc chloride or ammonium hydroxide, 2 ml of reagent were smeared on the panels and dried overnight under hood ventilation. Then, 1 ml of zinc chloride or 2 ml of ammonium hydroxide was again smeared on the panels before pressing at 100 °C and 300 psi for 1 hour. The starch solution was made by heating with 0.1N sodium hydroxide at 100 °C until the starch dissolved. Before bonding each panel was sprayed with sodium hydroxide and dried overnight under hood ventilation. Then 2.5 g of this solution was smeared on one panel surface. Panels were also bonded with a liquor obtained by pulping sugar maple flakes with 5N sodium hydroxide for 2 hours at 170 °C. This spent liquor as well as lignin-starch-sucrose solutions were used as gap-filling agents for wood bonding. Sodium hydroxide solution (2 g) was first sprayed on the panels for activation. After the sodium hydroxide had soaked into the panels, 2 g of the spent liquor or lignin solutions were smeared on one panel surface. The panels were then pressed at 100 °C and 300 psi for 1 hour. Lignin-formaldehyde solutions were made just prior to bonding because form­ aldehyde seemed to react with lignin even at room temperature. The molecular weight of the lignin was assumed to be 180. Methylolation of lignin was generally done with formalin (37% by weight formaldehyde solution) at 80°C for 1 hour under solvent reflux (Gamo 1983). Two grams of the lignin-formaldehyde solution or methylolated lignin was applied to each panel activated with sodium hydroxide and the panels were pressed at 150 °C and 300 psi for 30 minutes.

Summary and conclusions

The use of sodium hydroxide solutions for autohesive bonding of maple panels has been demonstrated in this work. It is probable that only a limited amount of secondary hydrogen bonds are formed since very little wet strength is obtained. However. excellent wood bond strength was obtained when methylolated lignin was used as a reactive gap-filling polymer. The dry strength surpassed, and the wet strength was equivalent to published values for conventional phenol-formaldehyde adhesives. Methylolated lignin has been previously successfully applied as an adhesive, but in these cases, the addition of a quantity of phenol-formaldehyde resin or isocyanate was necessary to obtain a durable wood bond. New approaches to wood bonding 38 1

Murmanis, L.; River, B. H.; Stewart, H. 1983: Microscopy of abrasive-planed and knife- planed surfaces in wood-adhesive bonds. Wood Fiber Sci. 15: 102-115 Rammon, R. M.; Kelley, S. S.; Young, R. A.; Gillespie, R. H. 1982: Bond formation by wood surface reactions. Part II. Chemical mechanisms of nitric acid activation. J. Adhesion 14: 257-281 Rowell, R. M.; Hajny, G. J.; Young, R. A. 1982: Energy and chemicals from wood. In: Young, R. A. (Ed.): Introduction to Forest Science, pp. 451 -469. New York John Wiley & Sons Russell, C. R.; Buchanan, R. A.; Rist, C. E.; Hofreiter, B. T.; Ernst, A. J. 1962: Cereal pulps. I. Preparation and application of cross-linked cereal xanthates in paper products. Tappi 45: 557-566 Sawabe, O.; Mori, K. 1981: Effects of treatment with sodium hydroxide on pore structure in holocellulose. Mokuzai Gakkaishi 27 409-413 Skeist, I. 1962: Handbook of adhesives. New York: Reinhold Press Stier, J. C.; Buongiorno, J. 1982: The forest products economy. In: Young, R. A. (Ed.): Intro­ duction to Forest Science. pp. 471-495. New York: John Wiley & Sons Stofko, J. 1974 The autohesion of wood. Ph.D. Thesis. University of California. Berkeley Stofko, J.; Zavarin, E. 1977: Method of bonding solid lignocellulosic material and resulting product. U.S. Patent No. 4,007,312 Strickler, M. D. 1968: Specimen designs for accelerated test. Forest Prod. J. 18: 84-90 Sugihara, M. 1972a: Molding materials from wood powder. I. Treating of wood powder with dilute sulfuric acid at room temperature. Tech. Rept. Kansai Univ. No. 13: 39-46 Sugihara, M. 1972b: Molding materials from wood powder. II. Treating of wood powder with sulfuric acid at high temperature. Tech. Rept. Kansai Univ. No. 13: 47-51 Sugihara, M. 1972c: Molding materials from wood powder. III. Treating of wood with dilute alkali solution under high temperature and pressure. Tech. Rept. Kansai Univ. No. 13: 52-57 Tischer, F. Y. 1939: Method of drying veneer. U.S. Patent No. 2,177,160 Troughton. G. E.: Chow. S.-Z. 1971: Migration of fatty acids to white spruce veneer surface during drying: Relevance to theories of inactivation. Wood Sci. 3: 129-133 Wellons, J. D. 1980: Wettability and gluability of Douglas-fir veneer. Forest Prod. J. 30: 53-55 White. J. L. 1979: Growing dependency of wood products on adhesives and other chemicals. Forest Prod. J. 29: 14 - 20 Young, R. A. 1977: Modification of high yield fibers by the xanthate method. J. Agr. Food Chem. 25: 738-742 Young, R. A.: Miller, B. 1975: Formation and properties of blended nonwovens produced by cellulose-cellulose bonding. In: A. F. Turbak (Ed.): Cellulose technology research. Am. Chem. Soc. Symp. Series 10, pp. 160-171.Washington. D. C.: American Chem. Soc. Young, R. A.: Rammon, R. M.; Kelley, S. S.: Gillespie. R. H. 1982: Bond formation by wood surface reactions. Part I. Surface analysis by ESCA. Wood Sci. 14: 110-119 Zavarin, E. 1984: Activation of wood surface and nonconventional bonding. In: R. M. Rowell (Ed.): Chemistry of solid wood. Washington, D.C.: American Chem. Soc.

(Received April 24, 1984)

R. A. Young and M. Fujita Department of Forestry, University of Wisconsin, Madison, WI 53706

B. H. River USDA Forest Products Laboratory P.O. Box 5130 Madison, WI 53705 USA 380 R. A. Young, M. Fujita and B. H. River

The mechanism of base activation appears to be a combination of factors in­ cluding the formation of a larger pore structural network at the surface of the wood. modification and removal of lignin and extractives. formation of alkali addi­ tion products of greater reactivity and plasticization of the wood surface. This total effect in combination with a reactive methylolated lignin produces excellent wood bonds. Only kraft lignin was evaluated in this work: it is possible that more reactive organosolv lignins could result in even better wood bonds. Additional work is planned for further optimization of this promising adhesive system.

References

Anonymous 1983: American national standard for wood products - structural glued laminated timber. ANSI/AITC A190.1. American Institute of Timber Construction, Englewood, Colorado: American National Standards Institute Anonymous 1983a: Standard specification for adhesives for structural laminated wood products for use under exterior (wet use) exposure conditions. Designation D2559-82. Annual Book of ASTM Standards. Vol. 15.06. Philadelphia: American Society for Testing and Materials Anonymous 1983b: Standard specification for adhesives used in non-structural glued lumber products. Designation D3110-82. Annual Book of ASTM Standards. Vol. 15.06. Phila­ delphia: American Society of Testing and Materials Dolenko, A. J.; Clarke, M. R. 1978: Resin binders from kraft lignin. Forest Prod. J. 28: 41-46 Dougal, E. F.; Krahmer, R. L.; Wellons, J. D.; Kanarek, P. 1980: Glueline characteristics and bond durability of southeast Asian species after solvent extraction and planing of veneers. Forest Prod. J. 30: 48-53 Emerson, R. W. 1953: Molding compositions and method of making same. U.S. Patent No. 2,764,569 Emerson, R. W. 1963: Lignocellulosic molding compositions. U.S. Patent No. 3,097,177 Enkvist, T. V. E. 1975: Kraft or soda black liquor adhesive and procedures for making the same. U.S. Patent No. 3,864,291 Erinsh, P. P.; Odintsov, P. N. 1967: Changes in submicroscopic structure of wood caused by its plasticization with aqueous solutions of sodium hydroxide and sulfuric acid. In: Darzinsh, T. A. (Ed.): Modification of wood, pp. 22-32, Kiga: Academy of Sciences of the Latvian SSR Institute of the Chemistry of Wood Gamo, M. 1983: Wood adhesives from natural sources. Paper presented at the International Union of Forestry Research Organizations, All-Division 5 Conferences, Madison, Wis­ consin. USA Hancock, W. V. 1963: Effect of heat treatment on the surface of Douglas-Fir veneer. Forest Prod. J. 13: 8 1 -88 Kelley, S. S.; Young. R. A.; Rammon, R. M.; Gillespie, R. H. 1983: Bond formation by wood surface reactions: Part III - Parameters affecting the bond strength of solid wood panels. Forest Prod. J. 33: 21-27 Marian, J. E.; Stumbo, D. A. 1962a: Adhesion in wood. Part I. Physical factors. Holzfor­ schung 22: 134-148 Marian, J. E.; Stumbo, D. A. 1962b: Adhesion in wood. Part II. Physico-chemical surface phenomena and the thermodynamic approach to adhesion. Holzforschung 22: 168-180 Marian, J. E.; Stumbo, D. A.; Maxey, C. W. 1958: Surface texture of wood as related to glue strength. Forest Prod. J. 8: 345 - 351 Millett, M. A.; Gillespie, R. H. 1978: Precision of the rate-process method for predicting bondline durability. Report prepared by Forest Products Laboratory for U.S. Dept. of Housing and Urban Development. Washington, D.C.: Nat Tech. Inf. Service No. PB 80-121866