ACETYLATION AND THE DEVELOPMENT OF NEW PRODUCTS FROM RADIATA PINE

Transcription

1 ACETYLATION AND THE DEVELOPMENT OF NEW PRODUCTS FROM RADIATA PINE D.V. Plackett, R.M. Rowell*, and E.A. Close, Forest Research Institute (*US Forest Products Laboratory, Madison, Wisconsin) ABSTRACT Wood may be chemically modified by impregnating it with chemicals that can react with chemical functional groups in the wood structure. In a joint project with the United States Forest Products Laboratory at Madison, Wisconsin, flakes of New Zealand radiata pine sapwood and heartwood acetylated by treatment with acetic anhydride followed by heat-curing, were used to produce samples of phenol formaldehyde resin-bonded board. Flakeboards made from acetylated sapwood or heartwood flakes showed similar rates and extents of swelling in liquid water and in humidity tests. Flakeboards made from acetylated flakes swelled much more slowly and to a lesser extent in liquid water tests and in humidity tests than untreated control boards. Following the initial study on acetylation of flakes, the fundamental chemical changes involved in the acetylation of radiata pine were studied using Fourier Transform infra-red (FTIR) spectroscopy. This work was undertaken on cellulose, lignin, and hemicellulose separately and the results support the conclusions drawn by researchers elsewhere examining acetylation of other wood species. INTRODUCTION The properties of wood and wood products can potentially be improved by chemical modification in which chemicals react with chemical functional groups in the wood cell wall. For example, hydroxyl groups in lignin and holocellulose can be acetylated to create wood-acetate bonds by a number of means. This technology and other chemical modification methods have been summarised by Rowell (1983) and many aspects of the fundamental chemistry and properties of modified wood have been investigated. The potential application of acetylation technology to New Zealand radiata pine was studied initially in a joint project with the United States Forest Products Laboratory (Rowell and Plackett 1988). The wider implications for possible chemical modification of composite wood products were discussed by Rowell (1990). In this paper, the results of the joint study in which flakeboard was used as the test material are summarised; and work undertaken at the Forest Research Institute (FRI) in which Fourier Transform infra-red (FTIR) spectroscopy has been used to study the reaction of acetic anhydride with radiata pine is then discussed. The objective of the work with flakeboard was to demonstrate the dimensional stability gained by acetylation and to examine any differences between the reactivity of sapwood and heartwood. MATERIALS AND METHOD Acetylation of radiata pine flakes Material used for the flakeboard study was Pinus radiata sawn timber (100 x 100 mm), obtained from the green chain at the Waipa Sawmill, Rotorua. The timber was sorted into sapwood with an average basic density of 446 kg/m 3 and a moisture content of 109%, and heartwood with an average basic density of 389 kg/m 3 and a moisture content of 28%. Individual pieces were then cut into shorter lengths, converted into flakes using a Bezner disc-type flaker, and then screened to exclude flakes less than 26 mm in length. Flakes were air-dried and then oven-dried for 12 hours at 105 C before use. Flake size was approximately (thickness x length x width, in mm) 1 x 40 x random. Flake width varied from 10 to 20 mm. Oven-dried flakes were acetylated by dipping in acetic anhydride for 1 minute and draining for 3 minutes at 25 C. Curing took place in a stainless steel reactor at 120 C for various lengths of time followed by vacuum (750 mm Hg) for 2 hours at 120 C. Flakes were then oven-dried at 105 C for 12 hours. The weight percentage gain (WPG) due to acetylation was calculated based on the weight of oven-dried (o.d.) unreacted flakes. Acetyl content was determined both before and after leaching by gas chromatography following deacetylation with sodium hydroxide solution Flakeboards were prepared with a target density of 640 kg/m 3 and a resin solids content of 6% (based on oven-dried flakes). The adhesive used was a 43.5% aqueous solution of a phenol formaldehyderesin. The mat moisture content was 12-13% and the pressing time was 10 minutes at 177 C. Duplicate samples (50 x 50 mm) were cut from untreated and acetylated flakeboards and immersed in water at 20 C to record thickness swelling. Cyclic water soaking tests were also run in which samples were immersed in water for 5 days followed by oven-drying at 105 C for 2 days. The procedure was repeated for six cycles of water immersion followed by oven-drying.

2 PLACKETT, ROWELL, AND CLOSE The effect of exposure to high relative humidity was determined by exposing samples to 30% relative humidity for 21 days and then transferring to a room at 90% relative humidity for a further 21 days. This procedure was repeated for four cycles of 30% to 90% relative humidity and thickness changes recorded. FTIR spectroscopy of acetylated radiata pine FTIR spectroscopy has potential as a method to obtain qualitative and quantitative information about chemical modification of wood as an adjunct to measurement of chemical add-on or WPG. RESULTS AND DISCUSSION Acetylation of radiata pine flakes Increase in WPG due to acetylation of radiata pine flakes is shown in Figure 1. There was little difference between heartwood and sapwood flakes in the degree of acetylation for each reaction time. The effect of soaking samples in water is shown in Figure 2. Bothsapwoodandheartwoodflakeboardsmadefromacetylated flakes swelled at about the same rate and to the same extent during the 5-day water exposure test. Final increase in thickness Infra-red spectroscopy is based on the absorption of infra-red energy by vibrating molecules. The absorption is characteristic for various chemical functional groups (e.g., O-H stretching vibration of an alcohol occurs at about 3600 cm -1 and the symmetric stretching vibration of the carbonyl group in an aldehyde or ketone occurs in the vicinity of cm -1 ). In the work on acetylated radiata pine, samples for infra-red spectroscopy were prepared by sanding radiata pine wood surfaces to obtain a dust that was then mixed with an infra-red neutral matrix (KBr powder). Alternative methods forpreparing wood samples for FTIR spectroscopy have been described in the literature (e.g., Michell 1988). Figure 1. Degree of acetylation of pine flakes as a function of reaction time (120 C). = Sapwood; = Heartwood. In a parallel study designed to assess the reactivity of individual components of radiata pine wood, milled wood lignin, a commercial cellulose (Whatman CC31), and a hemicellulose (xylan) obtained using the Hamilton extraction procedure (Beelik et al.), were acetylated and the FTIR spectra recorded. Comparison was made with the FTIR spectra of unacetylated material. The procedure for obtaining milled wood lignin involved ball-milling clean, dry radiata pine chips for 4 to 5 days and then washing the resulting powder in acetone/water. The dried powder was dissolved in acetic acid, reprecipitated by addition of water, and then dissolved in a dichloroethane/ethanol solution. Further reprecipitation was achieved by adding diethyl ether to the solution. The resulting precipitate was dissolved in petroleum ether and the pure milled wood lignin obtained by evaporation of the solvent, Figure 2. Rate of swelling in liquid water of pine flakeboards made from control or acetylated flakes. = Sapwood control; = Sapwood acetylated (21.7 WPG); Heartwood control; x = Heartwood acetylated (19.1 WPG)

3 4. WOOD MODIFICATION was about 7% which compares with 50 to 55% for control boards. Repeated water soaking/oven-drying tests (Figure 3) also gave similar results with acetylated sapwood and heartwood material with considerable reduction in thickness swelling when compared with unacetylated control material. The increase in final swelling from 7% toabout20% when comparing continuous water soaking (Figure 2) with a water soaking/oven-drying regime is probably attributable to the opening up of a greater surface area of the board to water contact as a result of the drying process. The thickness change at 30% and 90% relative humidity of boards made from control and acetylated flakes is illustrated in Figure 4. Boards made from unacetylated sapwood or heartwood flakes swelled to about the same extent with a maximum of about 28%. Boards made from acetylated sapwoodor heartwood flakes swelled much less than control boards with a maximum swelling of about 6%. FTIR spectroscopy of acetylated radiata pine A schematic diagram of an FTIR spectrometer is shown in Figure 5. The FTIR spectra of radiata pine and individual wood components are shown in Figure 6. The broad hydroxyl peak between 3500 and 3200 cm -1 is reduced in several of the spectra indicating that varying proportions of the hydroxyl groups have been replaced by acetyl groups. Similarly, the absorption due to the carbonyl group introduced during acetylation occurs at about 1750 cm -1. The reductions in intensity of hydroxyl group absorption bands in acetylated cellulose, xylan, and milled wood lignin suggest variations in the availability of hydroxyl groups for acetylation in the order of milled wood lignin ~ xylan > cellulose. These results are in agreement with research undertaken elsewhere with other wood species (Beelik et al.) in which the wood components are extracted after chemical modification of the substrate material. Figure 3. Changes in thickness in repeated water swelling test of pine flakeboards made from control or acetylated flakes. = Sapwood control; = Sapwood acetylated (21.7 WPG); Heartwood control; x = Heartwood acetylated (19.1 WPG). Figure 4. Changes in thickness at 30 and 90% RH of pine flakeboards made from control or acetylated flakes. = Sapwood control; = Sapwood acetylated (21.7 WE); Heartwood control; x = Heartwood acetylated (19.1 WPG)

4 PLACKETT, ROWELL, AND CLOSE Figure 5. Schematic diagram of an FTIR spectrometer. CONCLUSIONS The observations are significant because lignin, cellulose, or hemicelluloses are each thought to act as triggers for specific degradation processes (Rowell 1990) and knowledge of their availability for modification could potentially allow development of treatments designed to protect or enhance particular wood A study undertaken jointly with the United States Forest Products Laboratory demonstrated that there was no difference in reactivity of radiata pine sapwood or heartwood flakes during acetylation using acetic anhydride. Flakeboards made from acetylated sapwood or heartwood flakes performed similarly on exposure properties. to water or high relative humidity. Acetylated flakeboards

5 4. WOOD MODlFlCATlON swelled much more slowly and to a lesser extent in water and in humidity tests than untreated control boards. FTIR spectroscopy of acetylated milled wood lignin, cellulose, and hemicellulose suggested that hydroxyl groups in milled wood lignin were more available for acetylation than hydroxyl groups in a hemicellulose (xylan) or cellulose. FUTURE RESEARCH A programme to evaluate new wood modification techniques and potential new products from radiata pine has been established at FRI. Involvement in the international network of scientists involved in modification research is seen as a key element of this programme. In addition to the investigation of alternative treatment methods and the physicalandchemical properties of chemically modified radiata pine, the use of instrumental techniques such as FTIR and scanning electron microscopy to provide fundamental information about wood modification reactions will be assessed. Figure 6. FTIR spectra of acetylated and untreated P. radiata wood, cellulose, xylan, and milled wood lignin. REFERENCES Beelik. A.; Conca. R.J.; Hamilton, J.K.; Partlow, E.V. 1967: Selective extraction of hemicelluloses from softwoods. Tappi, 50 (3): Michell, A.J. 1988: A note on a technique for obtaining infrared spectra of treated wood surfaces. Wood and Fibre Science, 20 (2): Rowell. R.M. 1982: Distribution ofacetyl groups in Southern Pine reacted with acetic anhydride. Wood Science, 15 (2): Rowell. R.M Chemical modification of wood: Its application to composite wood products. Pp in Proceedings of the Composite Wood Products Symposium, Rotorua. New Zealand, November, Ministry of Forestry, FRI Bulletin No Rowell, R.M.; Plackett. D.V. 1988: Dimensional stability of flakeboards made from acetylated Pinus radiata heartwood or sapwood flakes. New Zealand Journal of Forestry Science, 18 (1): Rowell. R.M. 1983: Chemical modification of wood. Commonwealth Forestry Bureau, Forest Products Abstracts Review Article. 6(12): In: Burton, R.J., Tarlton, G.L., eds. Proceedings of the composite wood products symposium; 1988 November 9-10; Rotorua, New Zealand New Zealand: Forest Research Institute: 68-72; FRI Bulletin No Printed on recycled paper