Wood Science and Technology 29 (1995) 243 251 �9 Springer-Vedag 1995

Chemical modification of wood by non-formaldehyde cross-linking reagents Part 3. Mechanism of dimensional stabilization by glyoxal treatment and effect of the addition of glycol

R. Yasuda, K. Minato

Summary The probability of bond between wood components and glyoxal was examined by means of a mechanical method, infrared (IR) spectrometry, and solid state 13C-nuclear magnetic resonance (NMR) spectrometry. The successive fixation of a compressed wood by the glyoxal treatment suggested the formation of cross-linkings between wood components and/or wood structures. The IR spectra showed that ester bond as well as ether bond was formed between wood components and glyoxal. The existence of linkages between glyoxal and cellulose was indicated also from the NMR spectra. The addition of glycol to the glyoxal solution was investigated from the viewpoint of stabilizing effect of the linkages. When 0.2 mole ratio of glycol was added to 5-10% glyoxal solution, weight gain and antiswelling efficiency (ASE) were largest, however the addition of excessive amount of glycol did not advance further the weight gain and ASE. When an appropriate amount of glycol was added to the impregnation solution, both weight and ASE did not largely reduce even by the repeated hot water soaking. By the treatment without glycol, the dimensional stability after water soaking was attributed to only restraint of the swelling. On the other hand, when the glycol was added, the dimensional stability was developed not only by the restraint of the swelling but also by the buckling effect.

Introduction In previous papers, we reported that when wood was treated with non-formaldehyde reagents such as glyoxal and glutaraldehyde under the sulfur dioxide (SO2) catalyst, antiswelling efficiency (ASE) reached ca. 70%, and loss tangent decreased largely without the decrease of dynamic Young's modulus (Yasuda, Minato 1994a). Moreover, it became apparent that the creep deformation of treated specimen was fairly restrained, and that the amount of adsorbed water was reduced because of the restraint of swelling (Yasuda et al. 1994b). The changes of physical properties by these treatments were similar to those by formaldehyde treatment (formalization), and these results suggested that the cross-links are formed also by the treatments with non-formaldehyde reagents.

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Received 15 April 1994

Miss Rie Yasuda (graduate student), Dr. Kazuya Minato Department of Wood Science and Technology, Faculty of Agriculture, Kyoto University, Sakyo-ku, Kyotoe 606-01, Japan

We would like to thank Associate Professor Dr. Umezawa, Wood Research Institute, Kyoto University, for his invaluable support in NMR analysis. Thanks are also due to Dr. Inoue, Wood Research Institute, Kyoto University, for his preparing the wood specimens.

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However, Weaver et al. (1960) reported that these linkages were unstable, and we also observed that the excessive washing caused the decreases of ASE and restraining effect of creep deformation (Yasuda et al. 1994b).

The elusion of reagents is very serious for cotton fabrics which are subjected to washing repeatedly. In the finishing of cotton fabrics, it was reported that glycol, e.g. ethylene glycol and diethylene glycol, added to the reaction system stabilized the bonding, and that the wrinkle resistance and antishrink properties, retention of strength, and elimination of discoloration were imparted (Welch, Danna 1982; Welch 1983; Welch 1984; Welch, Peters 1987). Welch (1983) speculated that one mole of glycol formed a cyclic compound with one mole of glyoxal which linked to cellulose, because the addition of equivalent mole of glycol to glyoxal was the most effective. Nakano (1994) treated the wood specimens impregnated in 40% aqueous glyoxal solution which contains glycol and aluminum sulfate catalyse. From the weight gain data, he concluded that glyoxal hardly reacted to wood without glycol.

In this study, we intend to know whether the cross-linking is formed or not between wood components of glyoxal. At first, the effect of the glyoxal treatment on the recovery of wood specimen deformed by compression was investigated. Inoue et al. (1992) reported that the compression deformation of wood specimens was permanently fixed by formalization. They thought that the fixation was attributed to the formaldehyde cross-linkings between wood components. Inversely, if the cross-linking is formed, the deformation of compressed wood should be fixed also by the glyoxal treatment. The possibility of reaction between wood components and glyoxal was pursued also by means of infrared (IR) absorption spectrometry and solid state ~3C-nuclear magnetic resonance (NMR) spectrometry.

The effects of the addition of glycol to the impregnation solution were also examined from the dimensional changes on water soaking, and the mechanism of dimensional stabilization was discussed.

Experimental

Materials and reagents Specimens of sitka spruce (Picea sitchensis Carr.) were used. They were 30 mm(T) • 30 mm(R) • 5 mm(L) for the determination of dimensional change, and 30 mm(T) • 20 mm(T) • 30 mm(L) for the recovery test of compressed wood. Wood meal of sitka spruce (150-355 ~tm) and whatman cellulose powder (CF- 11) were subjected to the IR and NMR spectrometries.

Reagent grade 40% aqueous solution of glyoxal, ethylene glycol and diethylene glycol were used. The catalyst was obtained from a commercial SO2 bomb.

Glyoxal treatment of compressed wood and recovery test The specimens, impregnated in 10% aqueous solution of glyoxal with occasional evacuation, were compressed to about 50 % of its original dimension in radial direction, and then they were dried under the restraint of deformation. Three pieces of specimen, 5 mm in longitudinal direction, were cut from the center of the compressed specimen. These specimens were cured at 120 ~ for 1-24 h in a 3.5 liter glass vessel under the catalysis of SO2. The concentration of catalyst was 4.0 x 10 -s mol/dm 3. After the oven-dried dimensions were measured, the specimens were subjected to the recovery test.

The recovery test was carried out by soaking the specimens into water at room temperature for 6 h (evacuated for initial 1 h), oven-drying, and followed by the

measurement of the dimension in radial direction. Then the specimens were boiled in water for 1 h, and their dimensions were measured once more in oven-dried state. The recovery was defined as follows:

Recovery (%) = { (TR -- Tc ) / (To -- Tc ) } x 100,

where To is the oven-dried dimension before treatment, Tc, the oven-dried dimension after compression, and TR, the oven-dried dimension after recovery treatment.

Glyoxal treatment with the addition of glycol The glyoxal treatment was fundamentally carried out according to the previous paper (Yasuda, Minato 1994a). The 0.1 to 2 mole ratio of glycol to glyoxal was added into 5 or 10% aqueous solution of glyoxal. The air-dried specimens were cured at 120 ~ for 24 h under the catalysis of 4.0 x 10 -3 mol/dm 3 of SO2.

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Repeated water soaking test The treated specimens were soaked for 5 days in water at room temperature with occasional evacuation and exchange of water. Once oven-dried, they were soaked to hot water (70 ~ for 3 h while frequently exchanging the water. The cycle of oven-drying and hot water soaking was repeated three times, and at every wet and dry states, the dimensions in radial and tangential directions were determined. The ASE was obtained according to the previous definition (Yasuda, Minato 1994a).

Analyses by IR and solid state 13C-NMR spectrometries The IR spectra were obtained by means of Shimadzu FTIR-8100M with KBr method. The solid state 13C-NMR spectra were obtained at room temperature by Varian XL-200 spectrometer (50.309 MHz) fitted with Doty Scientific CP-MAS Probe. Contact time and repetition time were 1000 kisec and 2.065 sec, respectively.

Results and discussion

Recovery test of compressed wood Figure 1 shows the recovery of compressed wood treated with glyoxal against the curing time. While the recovery of untreated specimen was 45 % after cold water soaking, that of specimen treated with glyoxal decreased with the elongation of curing time, and became nearly 0% on and after 3 h. However, Inoue et al. (1992) pointed that the recovery force of compressed wood decreases also by the hydrolytic degradation of wood components. Actually, when the compressed wood was heated under the existence of SO2 alone, the recovery decreased with increasing of curing time, and after 24 h any difference was not observed between glyoxal treatment and heating with SO2 alone. On the other hand, when the specimens were cured for short period, the recovery of glyoxal-treated specimen was clearly lower than that of simply heated specimen. This result indicates that the fixation of compressed wood with glyoxal treatment is not always due to only decrease of recovery force by degradation. Namely, it can be concluded that the fixation of deformation by the glyoxal treatment is attributed to some linkages between wood components.

Though the recovery of untreated specimen after boiling was nearly 80%, that of glyoxal treated specimen became to be undectable after curing for 24 h. The recovery of the specimen treated with glyoxal was extremely lower than that heated with SO2 alone for 1-8 h, and the resultant recovery after boiling also suggests the formation of linkages

I

5 10 15 20 25 Curing time (h)

80

60

4O 8

20

0 0

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Fig. 1. Recovery after cold water soaking (solid symbols) and boiling (open symbols). O �9 Glyoxal, A �9 SO 2 alone

with glyoxal. However, for the specimens cured for 1 to 3 h, the recovery after boiling increased more largely than that after soaking in cold water. Therefore, the bonds which restrain the recovery are probably unstable to the drastic water soaking condition, and this result was analogous to that of creep test reported previously (Yasuda et al. 1994b).

IR s p e c t r o m e t r y In Fig. 2, IR spectrum of wood meal treated with glyoxal was shown along with those of untreated wood meal and various blank samples. It was reported that the ether bond was formed between cellulose and glyoxal (Head 1958). Because the absorbance at 1510 cm ~, which is due to the benzene ring of lignin (Jones 1949; Harrington et al. 1964), should not change by the treatment, the absorbance ratio of 1160 cm-~ which is due to ether bond to 1510 cm-~ was calculated. The ratio for treated sample was about 1.4 times larger than that for untreated. In the spectra of wood samples only impregnated with glyoxal or heated with SO2 alone, the absorbance ratio did not differ significantly from that for untreated. Further, the polymeric glyoxal heated only with SO2 was also analyzed, because the glyoxal polymerized in wood meal may cause the ether bond after curing. However, any evidence of ether bond was not observed. From these results, it can be concluded that the absorption at 1160 cm-~ refers to the ether bond formation between wood components and glyoxal.

The absorption around 1735 cm-~ which is due to ester bond increased clearly after glyoxal treatment, and the absorbance ratio of 1735 cm -~ to 1510 cm x for the treated sample was about 1.9 times larger than that for untreated. The absorbance ratios for blank samples were also same degree as that for untreated, and the absorbance was not found in the spectrum of heated polymeric glyoxal. These results suggest that ester as well as ether bonds are formed between wood components and glyoxal.

Sol id state ~3C-NMR spec trometry Figure 3 shows the solid-state ~3C-NMR spectra of untreated and glyoxal-treated cellulose. The signal around 104 110 ppm which is due to C1 carbon of the

(a)

i

l l i i l l I i i l I I I I I I 4 0 0 0 2 0 0 0 1 5 0 0 1 0 0 0

Wave number (em-1)

Fig. 2a-e. The IR spectra of untreated wood meal (a), wood meal treated with glyoxal (b), wood meal impregnated with glyoxal (c), wood meal heated at 120 ~ for 24 h with SO2 (d) and polymeric glyoxal heated at 120 ~ for 24 h with SO2 (e)

247

1 I I I I 120 100 80 60 40

ppm

Fig. 3a and b. The solid state 13C-NMR spectra of untreated cellulose (a) and cellulose treated with glyoxal (b)

248

I . e " 4O

-~ e 2 0

0 i I i I

i 10

O I i I

0 1 2 Mole ratio

Fig. 4. Dependence of uptake after impregnation, weight gain and ASE on mole ratio of glycol added to glyoxal. (3 A 5% glyoxal solution, �9 �9 10% glyoxal solution, (3 �9 Ethylene glycol, A �9 Diethylene glycol

anhydroglucose repeat unit (Haw et al. 1984) was split into two clearer peaks after glyoxal treatment. The splitting which is supposed to be due to the increase of crystallinity of cellulose (Maciel et al. 1982) or the change of crystalline form (Tanahashi et al. 1989) was not found in the spectra of blank cellulose samples only impregnated with glyoxal as well as heated with SO2 alone. At any rate, the change of crystallinity of cellulose is probably resulting from the reaction between glyoxal and hydroxyl group in amorphous region.

Effect of the addition of glycol to the glyoxal treatment In Fig. 4, the uptake after impregnation, weight gain after curing and sequential soaking in cold water, and ASE were plotted against mole ratio of glycol added to glyoxal. The uptake increased with the increasing of mole ratio of glycol to glyoxal in the range of the experiments. Especially, when dimethylene glycol was added, the uptake was larger than ethylene glycol at the same mole ratio. This is resulting from the difference of molecular weight between the two glycols.

The weight gain after cold water soaking showed maximum value, when 0.2 mole ratio of glycol was added; they were 14% (ethylene glycol) and 16% (diethylene glycol) with 10% glyoxal solution. The addition of excessive amount of the glycol resulted in rather decrease of the weight gain.

While the ASE was 70% with 10% glyoxal solution without glycol, it attained maximum values (77%) by the addition of 0.2 mole ratio of any glycol. However, above that the ASE dropped with increasing of mole ratio of glycol added to the solution. When 2 mole ratio of diethylene glycol was added, the ASE was nothing but about 10 %.

2~ t

v

249

o r d ~ e a o ~ e ~ o d d 6 ~ N ~ e ~ ~

Mole ratio

Fig. 5. Changes of weight gain and ASE by hot water soaking. EG: Ethylene glycol, DEG: Diethylene glycol, [] Decrease by soaking, �9 Remained after soaking

Similar results were obtained for 5% glyoxal solution, although the degree of decrease was less than that of 10% solution. The ASE did not depend on the uptake after impregnation, but on weight gain after treatment. In cotton fabrics, high performance was attained by the addition of equivalent mole of glycol to glyoxal, whereas the large amount of glycol did not always increase the weight gain nor improve the dimensional stability of wood specimen.

Durability of bond to water soaking In Fig. 5, the weight gain and ASE of the specimens treated with 10% glyoxal solution are shown before and after 3 times of water soaking at 70 ~ Without glycol, the weight gain decreased from over 10% after cold water soaking to 3% after hot water soaking. On the other hand, when the ethylene glycol or diethylene glycol was added, the weight loss by hot water soaking reduced with increasing of the mole ratio of glycol added. The remaining weight gain after hot water soaking was maximum with addition of 0.2 mole ratio of glycol. The final weight gain was larger by the addition of diethylene glycol than that of ethylene glycol.

Though the ASE was estimated over 70% after cold water soaking, it decreased to 30% by hot water soaking for the specimen treated with glycol. The decrease of ASE by hot water soaking slowed down with increasing mole ratio of the glycol added. Final ASE values of over 50% were kept by the addition of 0.2 mole ratio of any glycols. It can be said that the additives are contributed to stabilize the bonding against soaking to some extent.

The mechanism of the dimensional stabilization was clarified by pursuing the volumetric changes in each water soaking step (Fig. 6). The volumetric swelling was calculated on the basis of the oven-dried volume before impregnation. The plots were connected by a zigzag line only for the specimen treated without glycol, and further those in wet state and in oven-dried state were connected directly by an individual line.

25o

10 , . . . . , , ;

t - O , i , , i , i i T

to ~ is ~ . 10 ~ . ,

i ." - / '., : i

0 I I i I i I I I I �9

- i + + " I - " - r " r

Fig. 6. Dependence of volumetric swelling on the repeated water soaking. Mole ratio of glycol to glyoxal: �9 0, (3 0.2, �9 0.5, A 1

Irrespective of the addition of glycol to the impregnation solution, the volume in wet state did not recover the value before treatment even by the repeated water soakings. However, the final volume in wet state increased with increasing mole ratio of glycol added. This shows that the swelling is restrained by some structure formed with glyoxat, and that such structure is considerably proof against the water soakings. The fact that the wet volume recovered more largely when higher mole ratio of glycol was added may be explained by the excessive swollen of the specimen at which the bonds were formed. This is reasonable also from the result that the wet volume became larger by the addition of bulky diethylene glycol.

The oven-dried volume of the specimen treated without glycol recovered that before impregnation after 3 times of hot water soaking. On the other hand, when glycol was added, the oven-dried volume was larger than that before impregnation even after water soakings. These results show that before water soaking the restraint of swelling and bulking effect resulted in high ASE value, but disappearance of the latter effect by soakings lead the lowering of ASE. As stated by Welch (1983) and Nakano (1994), the glycol stabilized the bond between wood components with glyoxal, and maintained the bulking effect.

Conclusion We conducted some experiments to find the evidence of linkages with glyoxal. The reduced recovery of deformat ion given to wood by compression suggested the existence of some linkage between wood components . The IR spectra indicated the format ion of bonds, which was identified to be ester as well ether. Also from solid state ~3C-NMR spectra, some interact ion between cellulose and glyoxal was supposed. Large a m o u n t of glycol added to the glyoxal solut ion did no t always contr ibute to further increase of weight gain and ASE. However, the addi t ion of appropriate a m o u n t (about 0.2 mole ratio) of glycol to glyoxal stabilized the bond, and reduced the elusion by water soaking.

References Harrington, K. ]4 Higgins, H. G.; Michell, A. ]. 1964: Infrared spectra of Eucalyptus regnans F. Muell. and Pinus radiata D. Don. Holzforschung 18:108 133 Haw, ]. P.; Maciel, G. E.; Schroeder, H. A. 1984:Carbon-13 nuclear magnetic resonance spectrometric study of wood and wood pulping with cross polarization and magnetic-angle spinning. Anal. Chem. 56:1323 1329 Head, F. S. H. 1958: The reactions of cellulose with glyoxal. ]. Textile Inst. 49:T345-T356 Inoue, M.; Morooka, T.; Norimoto, M.; Rowell, R. M.; Egawa, G. 1992: Permanent fixation of compressive deformation of wood. (II) Mechanisms of permanent fixation. Bull. New Zealand FRI 176:181-189 ]ones, E. ]. 1949: The infrared spectrum of native spruce lignin. Tappi 32(4): 167-170 Maciel, G. E.; Kolodziejski, W. L.; Bertran, M. S.; Dale B. E. 1982: 13C NMR and order in cellulose. Macromolecules 15:686-687 Nakano, T. 1994: Reaction of glyoxal and glyoxal/glycol with wood. Wood Science and Technology 28:23-33 Tanahashi, M.; Goto, T.; Horii, F.; Hirai, A.; Higuchi, T. 1989: Characterization of steam-exploded wood III. Transformation of cellulose crystals and changes of crystallinity. Mokuzai Gakkaishi 35(7): 654-662 Weaver, 1. W.; Nielson, ]. F.; Goldstein, I. S. 1960. Dimensional stabilization of wood with aldehyde & Related compounds. Forest Products ]. 10:306 310 Welch, C. M. 1983: Glyoxal as a formaldehyde-free durable press reagent for mild curing applications. Textile Res. ]. 53:181-186 Welch, C. M. 1984: Glyoxal-glycol mild cure process. Formaldehyde-free durable press finishing of cotton. Textile Chem. Color. 16(12): 265-270 Welch, C. M.; Danna, G. F. 1982: Glyoxal as a non-nitrogeneous formaldehyde-free durable-press reagent for cotton. Textile Res. ]. 52:149 157 Welch, C. M.; Peters, ]. G. 1987: Low, medium, and high temperature catalysts for formaldehyde-free durable press finishing by the glyoxal-glycol process. Textile Res. ]. 57:351-356 Yasuda, IL; Minato, K. 1994a: Chemical modification of wood by non-formaldehyde cross-linking reagents. Part 1: Improvement of dimensional stability and acoustic properties. Wood Sci. Technol. 28:101 110 Yasuda, R.; Minato, K.; Norimoto, M. 1994b: Chemical modification of wood by non- formaldehyde cross-linking reagents. Part 2: Moisture adsorption and creep properties. Wood Sci. Technol. 28: in press

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