AIR1-CT92-0682 Modification of Wood with Environmentally Acceptable Chemicals to Improve its Durability and Dimensional Stability

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AIR Cluster VIII - Wood Products : Wood (Lignocellulose)

	Proposal No:	AIR1-CT92-0682
	Date Prepared:	November 1999
	Source:	Final Project Report Summary

Introduction

Though one of the most commonly used building materials in the world, wood exhibits some disadvantages compared with other materials, such as biodegradability, flammability and changing dimensions with varying moisture content. These properties of wood reflect the cell wall chemistry, in particular to the hydroxyl functions and oxygen containing groups of cellulose, hemicellulose and lignin. It is known that lignocellulosic materials such as wood may be upgraded in terms of their resistance to fungi and insect attack as well as their dimensional stability using the same treatment whereby the cell walls are chemically modified. This can be done with chemicals that are able to react with the -OH groups. Most knowledge in this field of research has been accumulated in relation to acetylation, but other chemicals also show potential in this area.

Objectives

The first objective was to search for novel reactants with the ability to modify the susceptible -OH groups of the wood cell wall. Chemicals with good potentials on laboratory scale were used for durability tests, tests on the effect of modification treatments on mechanical strength, tests on the effect of treatment on wood finishing and weathering performance and glueability tests.

The second objective was to concentrate on the improvement of the technology to acetylate solid wood using acetic anhydride without a catalyst. The degree of acetylation was recorded as weight gain (WPG) due to the reaction. The aim of this part of the project was to investigate the parameters for a proper acetylation, using both conventional heating and microwave heating. Areas investigated included the impregnation procedure, reacting or heating phase, removal and regaining of non-reacted chemicals. Material properties such as durability, dimensional stability, painting and further processing of the material were then examined on wood produced in a laboratory acetylation plant.

Activities

Various types of reaction, selected on the basis of a literature search, were studied at the laboratory scale. These included esterification, epoxidation, acrylation, isocyanate treatment, silylation and oxidation of wood. The reaction conditions, technological properties and decay resistance of the chemically modified wood were investigated in detail. Modification reactions that gave substantial improvement in properties, in particular esterification of wood using 1,2 cyclohexane dicarboxylic anhydride and acrylation with N-hydroxymethylacrylamide, were selected for further testing according to European Standard test methods.

A study of the impregnation step in an acetylation process showed that pine and birch were easily impregnated. Ash, beech and maple showed to be not completely penetrated when impregnated at 10 bar, however, the uptake was sufficient for the samples to be acetylated provided that the initial moisture content was not too high. For end grain sealed spruce wood, impregnated at 12 bar, only a minor penetration was obtained. However, diffusion of acetic anhydride will to some extent contribute to an acetylation of areas within the wood not primarily impregnated with anhydride.

Conventional heating

Various parameters of an acetylation process with different wood species were studied on laboratory scale to get a better understanding of their influence on the degree of acetylation and as a tool for further work with larger wood samples. Most of the acetylation took place during heating up of the timber. An optimum temperature appeared to be 120 - 130°C. A moisture content of 16-18%, which is quite common for joinery has an effect on the amount of acetic anhydride to be used (due to hydrolysis), but could still result in sufficient degrees of acetylation. Dilution of acetic anhydride with acetic acid, the by-product of hydrolysis and reaction with the wood, considerably diminished the degree of acetylation. Based on these experiments a laboratory acetylation plant was built for samples up to 1.5 m. Several tests were done to compare the achievable degree of acetylation to various process parameters, wood species and assortments. Increasing dimensions did not always decrease the degree of acetylation. Poplar proved to be acetylated best. Spruce, which is usually very difficult to impregnate, proved to be impregnated better with acetic anhydride compared to water. Large spruce samples showed a gradient of good acetylation in the outer layer and poor acetylation in of the inner parts. Pine poles and poplar planks had a high degree of acetylation throughout the whole sample.

Microwave heating

The possibility to use microwave energy as the source of heat in the acetylation process was investigated, in aim to reduce reaction time, achieve an efficient removal of excess chemicals after reaction and to obtain a uniform distribution of acetyl groups within the acetylated wood. Studies of microwave absorbing properties showed a levelling of heat within the wood during acetylation, promoting a uniform heating pattern. The penetration depth of microwaves into a wood sample impregnated with acetic anhydride was shown to be about 10 cm, indicating that acetylation using microwave heating could at least be performed on solid wood in dimensions of approximately 20 by 20 cm (tangential x radial). Microwave acetylation trials showed that variation in acetyl content both within and between samples was less than 2%, which implies a high degree of reproducibility in the process. Generally, a somewhat higher acetyl content was obtained in the middle of a microwave acetylated wood sample than in the outer part of it. If water-leaching before oven-drying at 90°C is used as a method for removal of the last traces of acetic acid in the wood after acetylation, 25 to 30% of the acetyl groups formed during acetylation were split off by hydrolysis of ester bonds, compared with air-drying of the wood for a week at room temperature.

Microwave heating could be used for removal of excess acetic anhydride and acetic acid under vacuum. Pine wood samples acetylated for 3 hours at 130°C followed by a vacuum step for 2 hours at 130°C, showed an acetyl content of 19%. The content of residual chemicals was about 5%. No difference in strength properties and dimensional stability of pine and spruce were obtained when acetylation by microwave heating was compared with acetylation by conventional heating.

Material properties

Resistance of acetylated wood to fungi decay was tested in laboratory tests. A WPG of 13% was sufficient to give full protection decay by brown and white rot. Degradation by soft rot fungi could be prevented with a weight gain of 10%. Acetylation could not prevent discoloration of the wood by blue stain. A considerably increased biological resistance of acetylated pine wood was obtained, when tested in field. The decay ratings were in the same range or lower than ratings obtained for CCA preservative treated wood with the highest retention level tested (9 to 10 kg/m³). Testing of acetylated wood in marine environments showed almost no preserving effect against attack from marine borers. However, a slight decrease in attack with increase in acetyl content could be observed. Performance of paints and stains was much better on acetylated pine compared to untreated samples. This was caused by the improved dimensional stability by acetylation which was up to 80% reduction in swelling and shrinkage. Acetylated wood proved to be resistant against UV discoloration.

Acetylated products and costs

A few cubic meters of wood was acetylated for the production of several test products which were used for field trials. They included acetylated claddings, acetylated planks for wood in ground, fresh water and salt water contact, wood for garden use and acetylated doors and windows. Preliminary results of these tests showed that acetylation contributes to a considerable improvement of performance of acetylated products in practice. An acetylated beech door for example proved to be more dimensionally stable than a door made of dimensionally stable tropical hardwood.