5036
Ind. Eng. Chem. Res. 2001, 40, 5036-5039
RESEARCH NOTES Cocondensation of NaOH-Catalyzed Liquefied Wood Wastes, Phenol, and Formaldehyde for the Production of Resol-Type Adhesives M. Hakkı Alma* and M. Altay Bas¸ tu 1 rk Department of Industrial Engineering of Forestry, Faculty of Forestry, University of Kahramanmaras¸ Su¨ tc¸ u¨ Imam, Kahramanmaras¸ 46060, Turkey
N. Shiraishi Department of Wood Science & Technology, Faculty of Agriculture, Kyoto University, Kyoto 606, Japan
In this study, wood (Betula maximowiczina Regel) wastes were liquefied with phenol in the presence of NaOH at an elevated temperature of 250 °C. The liquefied wood was then resinified with formaldehyde, and the resulting resol-type resin adhesives were applied for the production of plywood. The results indicated that almost all of the resol-type resin adhesives prepared met the Japanese Industrial Standard as far as dry shear adhesive strengths of plywoods were concerned. Furthermore, boiling water-resistant resol-type adhesives could be prepared from the resinification of NaOH-catalyzed liquefied wood with formaldehyde at a formaldehyde/phenol ratio of at least 2.0 and from the addition of an appropriate cross-linking agent such as polymeric diphenylmethane diisocyanate to the resol. Introduction More recently, several attempts have been made to prepare adhesives from a variety of lignocellulosic materials such as lignin,1,2 spent sulfite liquor,3 untreated wood,4,5 wood wastes chemically modifed,6 tannin,7,8 and tree barks9 to find out a new alternative to phenol largely consumed in the production of phenol/ formaldehyde resins. However, very few researchers have studied the making of resol-type aqueous phenol resin adhesives from the whole untreated wood or the modified wood wastes. It was reported that wood wastes based adhesives and moldings4,5,10 were successfully prepared by resinification of liquefied wood wastes prepared by phenolysis of untreated/treated wood particles with various inorganic acid catalysts at mild temperatures (80-150 °C) or without any catalyst at elevated temperatures (200250 °C). It has been known that, in the presence of phenol at 250 °C, cellulose and hemicellulose undergo transglycosylation to form (hydroxymethyl)furfural compound in high yield.11,12 As shown in Figure 1, the furfural thus formed has been postulated to condense with phenol and formaldehyde through a methylene bridge when resol was produced.13 Moreover, because of the cleavage of lignin in the presence of phenol at 250 °C, a variety of phenolic compounds such as guaiacol, coniferyl alcohol, vanilin, etc., were determined. For example, guaiacol formed from the cleavage of guaiacylglycerol-b-guaiacyl ether (GG), used as a model component of lignin, was deter* To whom correspondence should be addressed. Tel: +90 344 2237666-351. Fax: +90 344 2237409. E-mail: alma@ ksu.edu.tr.
Figure 1. Scheme for the reaction of (hydroxymethyl)furfural with phenol and formaldehyde.
mined to condense with phenol and formaldehyde (Figure 2).14 One of our studies15 on the phenolysis of wood wastes in the presence of a variety of alkalis for making novolak-type resins showed that using a small amount of NaOH as the catalyst in the liquefaction process of wood was found to be much more effective compared to
10.1021/ie000858x CCC: $20.00 © 2001 American Chemical Society Published on Web 10/24/2001
Ind. Eng. Chem. Res., Vol. 40, No. 22, 2001 5037
Figure 2. Scheme for the reaction of guaiacol with phenol and formaldehyde.
using a noncatalyst system in view of achieving enough low unreacted wood part. Therefore, the aim of this study was to make new types of resol resin adhesives from wood wastes liquefied with phenol in the presence of NaOH catalyst at an elevated temperature of 250 °C and to characterize them by applying for making plywoods.
The amounts of free phenol remaining after the phenolation and resinification processes were measured by using a high-performance liquid chromatograph (HPLC; Shimadzu LC-10A series) equipped with a SPD10A UV-vis detector and a Shimadzu shim-pack CLCODS (M) column (4.6 mm i.d. × 150 mm). The chromatographic areas were visualized under UV light (254 nm). Measurements were performed at 40 °C and a flow rate of 1 mL/min using a methanol/water solution (1/2, v/v) as the mobile phase. Additionally, pure phenol was used as a standard solution. For the measurements, the mixture without unreacted wood residue, obtained after the phenolation stage, and resol prepared after resinification was injected into the HPLC apparatus in the amount of 10 mL. Then, the amounts of the unreacted free phenol after both the phenolation and resinification stages were determined in grams, and the percent of free phenol after phenolation (FPhI) and the percent of free phenol after resinification (FPhII) were determined by the following equations:
FPhI (%) )
FPhI × 100 PhT
(2)
FPhII (%) )
FPhII × 100 FPhI
(3)
Experimental Section Monarch birch (Betula maximowiczina Regel) wood meal (20-80 mesh), “guaranteed reagent-grade” phenol (organic solvent), a 37% formaldehyde (formalin) solution, 40% aqueous NaOH and methanol, and polymeric 4,4′-diphenylmethane diisocyanate (MDI) were purchased from Nacalai Chemical Co. First of all, a mixture of oven-dried wood meal (10 g), phenol (4.3, 6.7, and 10.0 g), and water (7 g, 70%, based on wood meal by weight) was first heated mainly in the presence of NaOH (0.35 g, 5%, based on water added by weight) in a closed pressure-proof tube placed in an oil bath at 250 °C for 1 h, and then the reaction temperature was allowed to decrease to about 100 °C under ambient conditions. The resulting reaction mixture was dissolved with methanol and filtrated with a glass-fiber filter (Toyo GA100). The methanol-insoluble parts, the so-called “unreacted wood residues”, were oven-dried at 103 ( 2 °C in an oven and weighed. Finally, the percent of unreacted wood residue (UWr) was determined by the following equation:
UWr (%) )
Wr × 100 W0
(1)
where W0 is the oven-dried amount of starting wood material (g) and Wr is the oven-dried amount of unreacted wood residues determined after stage I (g). However, the methanol-soluble parts were evaporated at 50 °C under vacuum in order to remove the methanol. Subsequently, a 37% formaldehyde solution was added to a methanol-free soluble part/mixture (i.e., phenolated wood, unreacted phenol, water, and NaOH) at different molar ratios based on unreacted free phenol, and the pH of the mixture was adjusted to 11 by dropwise addition of a 40% aqueous NaOH solution. The soobtained mixture was heated in a three-necked flask furnished with a mechanical stirrer, thermometer, and condenser oil bath at 100 °C for 1.5, 3.0, and 6.0 h. The adhesive eventually obtained was a black viscous liquid containing a small amount of unreacted wood particles (for some cases), water, wood components reacted with phenol, reacted or unreacted phenol, and formaldehyde.
and
where FPhI is the amount of phenol remaining after the phenolation stage (g) and FPhII is the amount of phenol remaining in the resol made (g). Moreover, the conversion of free phenol remaining after resinification to condensed products was expressed as resol yield (RY) and determined by the following equation:
RY (%) )
FPhI - FPhII × 100 FPhI
(4)
The resol was mixed with a mixed filler consisting of nut hull and wheat flour (10%, based on resol). The final viscosities of all of the resols with filler were adjusted to about 6 P by using a Brookfield viscometer. Indeed, the final viscosity of the water-soluble phenolic adhesives is required to be between 1 and 10 P according to Japanese Industrial Standards (JIS) K-6852. Furthermore, to improve the curing behavior of the new resoltype adhesive, the prescribed amounts of MDI (10 parts) and water (4 parts) were added to the resol. Three-layer plywoods were prepared by following Japan Agricultural Standards concerning the preparation of structural plywood. Edge-grained Monarch birch (Betula maximowiczina Regel) veneers [125 (longitudinal) × 125 (radial) × 2 (tangential) mm] were bonded together. The veneers were dried at an ambient temperature under vacuum to the moisture content of 4%. Both sides of the core veneer were bonded with about 30 g of each resol. Face and back veneers were bonded to the core veneer in perpendicular directions to each other. The curing was performed at 140 °C under 12 kg/cm2 for 5 min. The bond strength was determined by the tensileshear test following the Japanese Industrial Standard for phenol resin adhesives to evaluate the dry adhesion strength. On the other hand, for evaluation of the waterproof adhesive bond strength, the plywoods were sub-
5038
Ind. Eng. Chem. Res., Vol. 40, No. 22, 2001
Table 1. Preparation of Resol-Type Aqueous Phenol Resin Adhesives from NaOH-Catalyzed Liquefied Wood Wastes and Their Characterizations reaction conditionsa
UWrb (%)
W/Phf (w/w) 7/3 3.04 6/4 1.64 5/5 1.79 6/4g 18.37 6/4h 6/4i 5/5i
FPhIc FPhIId (%) (%) 77.48 75.80. 76.17 80.20
13.16 10.21 9.09 10.65 10.21 10.21 9.09
RYe (%) 66.98 89.00 90.20 89.40
shear strength (N/mm2) dry boil-dry-boil 0.93 1.41 1.32 1.29 1.38 1.79 1.77
1.56 1.43
a General reaction conditions. For liquefaction of wood: temperature, 250 °C; time, 1 h; wood/phenol ratio, 6/4 (w/w). For resinification: formaldehyde/phenol molar ratio, 1.5; temperature, 82 °C; time: 3 h. b UWr, unreacted wood residue. c FPhI, free phenol remaining after wood liquefaction. d FPhII, free phenol remaining in resol. e RY, resol yield. f W/Ph, wood/phenol ratio. g Liquefaction without catalyst. h Without removal of unreacted wood residue. i Composition of adhesives: resol (100 parts) + pMDI (10 parts) + water (4 parts).
jected to a “boil-dry-boil” cycling test. That is, the plywoods were first immersed in boiling water for 4 h, dried in an air-circulated oven at 60 °C for 20 h, again placed in boiling water for 4 h, cooled in water (about 20 °C), and tested in wet conditions for their shear adhesive strength by following the Japanese Industrial Standard as described above. Results and Discussion Table 1 indicates the percent of unreacted wood residue, the resol yields, the percent of free phenol remaining after the phenolation and resinification, and the shear adhesive strengths for the plywood bonded with the resol-type resin adhesives made from NaOHcatalyzed liquefied wood wastes and phenol solutions as functions of various reaction conditions. As is shown in this table, at the phenolation conditions studied, the percent of unreacted wood residue decreases slightly when the wood/phenol ratio is decreased from 7/3 to 6/4 and does not differ significantly with further increment. Moreover, the amount of unreacted wood residue (18.37%) determined without using a catalyst is found to be considerably larger than those determined by using NaOH as the catalyst under the same conditions. As also depicted in the same table is the amount of free phenol remaining after liquefaction of wood ranges between 75.80 and 80.20%, revealing that approximately 80% of the starting phenol remains unreacted. So, it was intended to evaluate these huge amounts of free phenol in the production of resol-type adhesive resin. On the other hand, as listed in Table 1, after in situ resinification the percent of free phenol (about 80%) remaining after liquefaction is largely reduced to a level as low as 10% at the conditions employed, suggesting that decomposed wood substances arising from the liquefaction process may hamper phenol/formaldehyde reaction. The free phenol amounts remaining in the produced resol do not vary remarkably with decreasing wood/phenol ratio. As is also indicated from the same table, the yields of the resol show an increase with decreasing wood/ phenol ratio along with increases in the starting phenol content in the wood liquefaction stage and the formaldehyde contents in the resinification medium. Further-
Table 2. Relationship between Some Resinification Parameters and the Percents of Resol Yield and Free Phenol Remaining in the Resol resinification conditionsa F/Ph (mol/mol) 1.0 1.5 2.0 2.3 temperature (°C) 68 82 97 time (h) 1.5 3.0 6.0
RY (%)
FPhII (%)
54.82 89.00 95.80 106.40
18.12 10.21 1.12 trace 11.53 10.21 10.03 14.87 10.21 9.88
a General reaction conditions. For liquefaction of wood: temperature, 250 °C; time, 1 h; wood/phenol ratio, 6/4 (w/w). For resinification: formaldehyde/phenol molar ratio, 1.5; temperature, 82 °C; time, 3 h.
more, a noncatalyzed process also gives a resol yield similar to that of the NaOH-catalyzed liquefaction process. As one can see from Table 1, except for the wood/ phenol ratio of 7/3, all of the plywood specimens prepared by the resol-type resin adhesive from NaOHcatalyzed liquefied wood wastes show dry shear adhesive strengths of more than 10 kgf/cm2, which is over the Japanese Industrial Standards requirement for dry shear adhesive strength of 12 kgf/cm2. Also, the dry shear adhesive strengths of the plywood bonded with the resol-type resin adhesive prepared from NaOHcatalyzed liquefied wood are found to be similar to those of ones bonded with the resol-type adhesive from a noncatalyzed liquefied wood. Furthermore, as is also shown in the same table, both dry and wet shear adhesive strengths of the plywoods bonded with the NaOH-catalyzed liquefied wood-based resol-type resin adhesive can easily be enhanced by the further addition of a pMDI (10%, based on resol by weight) cross-linking agent, which exceeds satisfaction of the Japanese Industrial Standards requirements (dry shear adhesive strength, 12 kgf/cm2; wet shear adhesive strength, 10 kgf/cm2). This performance is likely because of the fast reaction of isocyanate groups (-NdCdO-) in pMDI with phenolic hydroxyl groups, methylol groups (-CH2OH) on phenol/formaldehyde resin, methylolated lignin most probably produced during the wood liquefaction, and hydroxyl groups (-OH) of decomposed wood components.16 On the other hand, it is important to notice that in comparison to the strength of the resol without unreacted wood residue, the strength of the resol including unreacted wood residue is relatively smaller. This is likely due to the “false viscosity” of the resol containing the wood particles. The wood particles make the solution viscosity of the resol higher. However, this type of resol has a lower molecular weight than the resol without wood particles and overpenetrates the wood substrate, thus resulting in a “starved” glue line. Table 2 presents the relationship among some resinification parameters and the percents of resol yield and free phenol remaining in the resol obtained. As illustrated in Table 2, the resol yield as well as the effectiveness of the adhesive increases with increasing formaldehyde/phenol molar ratio. In contrast, free phe-
Ind. Eng. Chem. Res., Vol. 40, No. 22, 2001 5039
strength was concerned, it was feasible to produce acceptable resol-type adhesives from wood wastes liquefied with phenol in the presence of NaOH as a catalyst for wood-based panel industries. However, it was possible to make boiling water-resistant resol-type resin adhesives prepared with the combination of a NaOHcatalyzed liquefied wood/phenol solution and formaldehyde at a formaldehyde/phenol ratio of around 2.0 and above. It was also found that the adhesiveness could be further enhanced by the addition of the appropriate cross-linking agent, e.g., polymeric MDI. Literature Cited Figure 3. Relationship between the formaldehyde/phenol molar ratio and the shear adhesive strength.
nol in the resol goes down briskly when the formaldehyde/phenol ratio is increased and then is found to be negligible at the 2.3 ratio. As is also listed in the same table, the resinification temperature does not affect the percent of free phenol remaining in the resol. However, the resinification time is determined to be significantly effective on the percent of free phenol remaining in the resol; that is, the percent of free phenol decreases with increasing reaction time. Figure 3 demonstrates the relationship between the formaldehyde/phenol molar ratio and the shear adhesive strength. As shown in this figure, the dry shear adhesive strength increases clearly when the formaldehyde/ phenol molar ratio is increased from 1.0 to 2.0 and then levels off with further increments. Also, the shear adhesive strengths determined for all of the formaldehyde/phenol molar ratios used are found to be acceptable as far as Japanese Industrial Standards specifications for dry shear adhesive strength are concerned. Also, it is obvious from the boil-dry-boil cycling test of the resol that the shear adhesive strengths of the plywood bonded using the NaOH-catalyzed liquefied wood-based resol-type adhesives comply with waterproof requirements of Japanese Industrial Standards specification when the formaldehyde/phenol molar ratio is around 2.0 or above for the conditions used. Moreover, it is evident from Figure 3 that the plywood specimens bonded with the new resol prepared using a formaldehyde/phenol ratio of less than 2.0 are delaminated when the boil-dry-boil cycling test is applied, thus revealing that the influence of the formaldehyde ratio on the adhesive strength is very tremendous. Conclusions The study on using the liquefied wood wastes to prepare phenol/formaldehyde plywood adhesives indicated that wood wastes were easily liquefied with phenol by using NaOH as the catalyst for both phenolation and resinification processes at 250 °C. As far as dry adhesive
(1) Stephanou, A.; Pizzi, A. Rapid Curing Lignin-Based Exterior Wood Adhesive. Holzforshung 1993, 47, 439. (2) Glasser, W. G. Potential Role of Lignin in Tomorrow’s Wood Utilization Technologies. For. Prod. J. 1981, 31, 24. (3) Trosa, A.; Pizzi, A. Wood Liquefaction Catalyzed by PhenolSulfonic Acid at Atmospheric Pressure and the Resulting WaterSoluble Phenolic Resin. Holz Roh- Werkst. 1998, 56, 229. (4) Chung, Y. H. Adhesive and Tropical Wood Composites; Forest Product Society: Madison, WI, 1994. (5) Ono, H. K.; Sudo, K. Phenolated WoodsA Source of Wood Adhesives. International Symposium on Chemical Modification of Wood, Kyoto, Japan, 1993; pp 35-37. (6) Kishi, H.; Shiraishi, N. Wood-Phenol Adhesives Prepared from Carboxymethylated Wood II. Mokuzai Gakkaishi 1986, 32, 520. (7) Gama, M. Adhesives from Natural Raw Material. J. Appl. Sci. Symp. 1984, 40, 101. (8) Marcos, A. E.; Melissia, G. D. B.; Anthony, H. C. Resol Resins Prepared with Tannin Liquefied in Phenol. Holzforshung 1995, 49, 146. (9) Ayla, C. Pinus Brutia Tannin Adhesives. J. Appl. Polym. Sci., Appl. Polym. Symp. 1984, 40, 69. (10) Alma, M. H.; Maldas, D.; Shiraishi, N. Resinification of NaOH-catalyzed Phenolated Wood-Phenol Mixture with Formalin for Making Molding Materials. J. Adhes. Sci. Technol. 2001, in press. (11) Yamada, T.; Ono, H.; Ohara, S.; Yamaguchi, A. Characterization of the products resulting from direct liquefaction of Cellulose I. Mokuzai Gakkaishi 1996, 42, 1098. (12) Bandzala, J.; Kokta, B. V. Optimization and Fundameentals of High-Yield Pulping with Ethanol. Wood Sci. Technol. 1995, 29, 467. (13) Pu, S. Wood Liquefaction by Thermo-Organosolvolysis: Ph.D. Dissertation, Kyoto University, Kyoto, Japan, 1994. (14) Lin, L. Characterization of Phenolated Wood and Study on the liquefaction Mechanism of Lignin. Ph.D. Dissertation, Kyoto University, Kyoto, Japan, 1996. (15) Alma, M. H.; Maldas, D.; Shiraishi, N. Liquefaction of Several Biomass Wastes into Phenol in the Presence of Various Alkalies and Metallic Salts as Cataysts. J. Polym. Eng. 1997, 18, 163. (16) Pizzi, A. Advanced Wood Adhesives Technology; Merckel Decker Inc.: New York, 1994.
Resubmitted for review April 23, 2001 Revised manuscript received July 30, 2001 Accepted August 15, 2001 IE000858X
