ABSTRACT

In most countries only phenolic adhesives are considered suitable for the manufacture of exterior grade wood products, particularly if exposure to extreme climatic conditions is anticipated (Pizzi et al. 1981). Due to the high cost of synthetic adhesives, there have been several attempts to replace a portion of the petroleum-derived compounds with phenolic-type compounds obtained from renewable resources. Principal among these efforts is the development of adhesives from tannin (Pizzi and Scharfetter 1978, Pizzi 1983). Tannin-based adhesives have in the past been heavily fortified with urea, urea-formaldehyde, phenol-formaldehyde (PF), and resorcinol-formaldehyde with encouraging results (Pizzi 1977, 1983, 2008; Pizzi and Roux 1978; Pizzi and Scharfetter 1978). However, very little work has been done with Acacia catechu and Uncaria gambier tannins. Tannins contained in vegetable extracts are polymers of flavonoid units or polyflavonoids fairly well diffused in various tree barks, wood, cones, and fruits. Tannins are broadly divided into hydrolyzable and condensed tannins. The former are simple mixtures of pyrogallols, gallic acid, and ellagic acid, all of which are simple phenols with low reactivity toward formaldehyde. However, they constitute only 5 to 10 percent of a total industrial production of tannin extracts manufactured in the world. The main tree species exploited to obtain these hydrolyzable or gallic tannins is chestnut (Castanea) (Pizzi 1980).

Condensed tannins are the polymerization of monomeric flavan-3-ol or flavan-3,4-diol precursors (Figs. 1 and 2). The structure and formation of condensed tannins are the topic of many speculative theories that have been summarized and criticized by many researchers. The “catechin hypothesis” was introduced by Freudenberg and De Lama (1958) and suggests that the complex polymeric structures of many of the condensed tannins are primarily derived by polycondensation of precursors of a flavonoid type.

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The generic term catechin was first used by Freudenberg and De Lama (1958) to describe the colorless crystalline substances commonly located in plant tissues in association with the condensed tannins. Since the catechins were readily converted in vitro to amorphous tannin-like materials, the researchers regarded them as direct precursors of the tannins in nature.

Later work showed that the catechins are derivatives of the basic flavan-3-ol structure, and these compounds are currently referred to as flavan-3-ols to avoid confusion with trivial names (Haslam 1966). The first and perhaps most widely distributed compounds of this class to be isolated from natural sources were (+)-catechin from U. gambier and (−)-epicatechin from A. catechu; these are diastereoisomers of the 5,7,3′,4′-tetrahydroxyflavan-3-ol structure (Figs. 3 and 4)

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The tannins of U. gambier and A. catechu belong to the phloroglucinolic type. These types of tannins are difficult to handle because of their high molecular weight and extremely high reactivity toward formaldehyde (Hemingway and McGraw 1976, Schroeder 1976, Rossouw 1978). This reactivity causes premature curing, and the residual active centers become too far apart for formaldehyde molecules to bridge, resulting in incomplete cross-linking. This shortcoming results in brittleness, poor wood penetration, and poor wet strength (Sowunmi et al. 1996). This also prevents the formation of tannin-resols, tannin resins carrying methylol reactive groups, because the methylol reactive groups will condense with other tannin phenolic nuclei in a very short time. Thus resol resins, which dominate synthetic PF technology, are not a feasible alternative for tannins. In short, tannin-resols are not stable, and their shelf life is far too short to be of industrial or practical significance. This leads to tannins being added to PF resins to overcome the short pot life of tannin-formaldehyde resins.

The aim of this study therefore was to improve the adhesive properties of U. gambier and A. catechu tannin adhesives. Specifically, the objective was to modify the resulting structure with resol-type PF resin to reduce the tendency for premature curing and encourage a high degree of cross-linking.

Materials and Methods

Chemicals

The chemicals used in this study included phenol (Qualigens, laboratory reagent [LR] grade), formalin (37% to 41%; Ranbaxy, LR grade), sodium hydroxide pellets (Ranbaxy, LR grade), hide powder (E. Merck AG), chrome alum (s d fine-chem Limited, LR grade), kaolin (CDH, LR grade), hydrochloric acid (Ranbaxy, LR grade), and methanol (Ranbaxy, LR grade).

Preparation of PF resin, using 5, 8, and 11 percent alkali as the catalyst

One hundred grams of molten phenol (60°C to 65°C) was charged into the 500-mL round-bottom flask followed by 120 g of formalin (37% to 41% concentration) and 5 g of sodium hydroxide dissolved in 10 mL of distilled water. The condensation reaction was carried out for 30 minutes at a refluxing temperature of approximately 100°C. When the viscosity of the resin syrup was about 100 cps and the water tolerance was about 1:5, the resin was discharged from the round bottom flask and cooled to room temperature by keeping the container in cold circulating water (George 1977).

The procedure for the manufacture of PF resin using 8 and 11 percent alkali concentration was similar to that described for the 5 percent alkali concentration, except for the increased amount of sodium hydroxide.

Preparation of phenol-tannin-formaldehyde (PTF) resin at 5, 8, and 11 percent alkali concentration

The adhesive formulation was based on a constant molar ratio of phenol-tannin with formaldehyde. The molar ratio (P+T):F was 1:1.2. The replacement of phenol by tannin was successfully achieved up to 50 percent, above which gel formation of resins was observed.

Required amounts of molten phenol (grams) and formaldehyde (grams) were mixed together in a beaker. Tannins of U. gambier and A. catechu taken separately (grams, oven dry) were added into the phenol-formaldehyde mixture and stirred for 15 to 20 minutes at room temperature until the tannin was completely incorporated (Table 1). The reactants were transferred into a 500-mL round-bottom flask with aqueous sodium hydroxide (5%, 8%, and 11% to that of phenol and tannin) and experiments were carried out for 30 minutes at the refluxing temperature (100°C). The physicochemical properties of PF resin prepared after substitution of phenol with tannins of U. gambier using 5, 8, and 11 percent sodium hydroxide as the catalyst are reported in Tables 2 and 3.

Table 1. Partial substitution of phenol with tannins of Uncaria gambier and Acacia catechu in phenol-formaldehyde resin.

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Table 2. Physicochemical properties of phenol-formaldehyde (PF) resin with phenol partially substituted with Uncaria gambier tannin.^a

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Table 3. Physicochemical properties of phenol-formaldehyde (PF) resin with phenol partially substituted with Acacia catechu tannin.^a

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Testing

Eighteen test specimens of 2.54 by 15.24 cm were cut along the grain of face veneers from each plywood panel and used for glue shear strength testing (Anonymous 1983, 1989) in dry, water resistance, and mycological environments (Table 4). The results are shown in Tables 5 and 6.

Table 4. Basic gluing conditions for different resins prepared with Uncaria gambier and Acacia catechu tannins.^a

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Table 5. Test results of glue shear strength of plywood made using phenol-formaldehyde (PF) and phenol-tannin-formaldehyde resins (Uncaria gambier).^a

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Table 6. Test results of glue shear strength of plywood made using phenol-formaldehyde (PF) and phenol-tannin-formaldehyde resins (Acacia catechu).^a

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In the water resistance test, the specimens were kept in boiling water for 8 hours. They were then cooled by submerging in cold water at room temperature and then tested for glue shear strength. For the mycological test, the specimens were kept in a porcelain dish filled with a 25-mm layer of sawdust obtained from the sapwood of a perishable timber like semul (Bombax ceiba) in its natural condition. The sawdust was moistened with 1 liter of water containing 15 g of sucrose (cane sugar was normally used), so that it was damp but not so wet that free water could be squeezed from it by hand pressure. To attain this condition with dry sawdust, it is generally necessary to add water at three times the mass of the sawdust. The dish was sealed against the glass with screws so that the atmosphere around the test ply samples remained saturated. The dish and contents were maintained at a temperature of 27°C ± 2°C for a period of 3 weeks, after which specimens were tested at room temperature for glue shear strength.

Results and Discussion

PTF adhesives

Properties of PTF adhesives prepared using different molar ratios of reactants and alkali concentrations are reported in Table 1; viscosity, pH, gelation time, and specific gravity of U. gambier and A. catechu tannins are reported in Tables 2 and 3, respectively.

With an increasing molar ratio of tannin in PF resin, the viscosity of adhesives increased sharply as with a simultaneous increase of catalyst from 5 to 11 percent in both cases. Adhesives made from A. catechu tannin had greater viscosity than those made from U. gambier tannin (Figs. 5 and 6). This may have been due to A. catechu tannin extract having more nontannins (22.04%) compared with U. gambier tannin extract (20.08%). The presence of nontannin compounds such as hydrocolloid gums, which are hydrophilic and highly branched polysaccharides, in A. catechu tannin extract tends to promote high solution viscosities and affects the properties of tannin-based adhesives, particularly their moisture resistance (Sowunmi et al. 1996). The pH of the adhesives prepared using both tannins remained alkaline yet it decreased with increasing catalyst and molar ratio of tannin. This was due to the acidic nature of tannins used. The effect of viscosity was clearly reflected in the gelation time of resin prepared from both of the tannins; the higher the viscosity, the lower the gelation time. Resins with more viscosity have higher polymerization because less time is required for complete polymerization during the determination of gelation time. The specific gravity of the resins prepared using both the tannins showed an increasing trend with an increase in molar ratio of tannins in PF as well as the catalyst. However, it was insignificant compared with the specific gravity of the control resin.

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Statistical analysis

The data of various parameters, dry testing failing load (DTFL), boiling water resistance failing load (BWRFL), and mycological testing failing load (MTFL), were subjected to analysis of variance (ANOVA) for 5, 8, and 11 percent alkali concentrations. From this analysis, we found a significant difference for 5 and 8 percent alkali concentrations from the standard (control) for all parameters in all effects. However, for 11 percent alkali concentration, the glue bond quality of treated plywood was on par with the standard for DTFL and BWRFL; for MTFL it was significantly different from the standard (Table 7). To evaluate the differences among alkali concentrations, all data were pooled excluding the performance of standard and analyzed. From this analysis it was observed that all major effects (i.e., alkali concentration, species, molar ratio), all possible two-factor interactions, and three-factor interactions were significantly different for all three parameters except the main effect of species for the BWR test (Table 8).

Table 7. Mean values for comparison between control and treated samples.^a

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Table 8. ANOVA for different parameters.^a

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Conclusions

We observed that U. gambier tannin can be substituted for up to 50 percent of the phenol using 8 and 11 percent sodium hydroxide as catalyst, and in the case of A. catechu, tannin can be substituted for up to 30 percent of the phenol using 8 and 11 percent sodium hydroxide as the catalyst for making BWR grade plywood meeting the requirements of IS 303 (Anonymous 1989) in all respects (i.e., glue shear strength in dry, water resistance, and mycological tests). We prepared PTF resin at 5, 8, and 11 percent alkali concentrations. The effect of pH on the rate of curing and polymerization of phenolic resin is well known. It is also widely accepted that at alkaline pH, the curing rate of phenolic nuclei as nucleophiles is strengthened by ionization of the phenol to form phenolate ions. PF resins are used as thermosetting wood adhesives, an application for which only PF resins of very high alkalinity, generally at pH values between 10 and 13, are used to impart faster reactivity and shorter pressing time to the adhesive. Further, it is also observed that U. gambier tannin is a better partial substitute of phenol compared with A. catechu tannin. This may be because of the presence of more nontannin compounds in A. catechu such as hydrocolloid gums that promote high solution viscosity and poor moisture resistance of tannin-based adhesives (Sowunmi et al. 1996).

Resin from both of these tannins with 8 percent alkali used as the catalyst results in glue bond quality of the plywood meeting the Indian Standard; thus, these tannins can be used for making resin. The use of 11 percent sodium hydroxide as the catalyst will increase the cost of resin; however, the statistical analysis of the results showed that 8 and 11 percent sodium hydroxide can be used as the catalyst. In the case of U. gambier tannin, up to 50 percent of the phenol can be replaced, and in the case of A. catechu tannin, up to 30 percent of the phenol can be replaced. Both yield comparable results for plywood meeting the IS 300 requirement. However, if 11 percent sodium hydroxide is used as the catalyst (with both tannins), results are comparable to the control and better than those with resin prepared using 8 percent sodium hydroxide as the catalyst. As shown in Table 7, results of treated plywood (both tannins) are comparable to those of the control at 8 and 11 percent sodium hydroxide using all parameters. Consequently, tannins of U. gambier and A. catechu are successful partial substitutes for phenol in PF resin for making three-ply S. robusta boards.