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MODIFICATION OF WOOD WITH ENVIRONMENTAL ACCEPTABLE CHEMICALS TO IMPROVE ITS DURABILITY AND DIMENSIONAL STABILITY

Objective

The first objective of this project was to search for novel reactants with 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 of the project concentrated on the improvement of the technology to acetylate solid wood with uncatalysed acetic anhydride. The degree of acetylation is recorded as weight gain (WPG) due to the reaction. The aim of this part was to investigate the parameters for a proper acetylation process, both with conventional heating and microwave heating. This 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 had to be examined on wood produced in a laboratory acetylation plant.

Though one of the most commonly used building materials in the word, wood exhibits some disadvantages compared with other materials. These include biodegradability, flammability and changing dimensions with varying moisture content. These properties of wood are a result of the cell wall chemistry, in particular to the hydroxyl functions and other oxygen containing groups on the molecules of cellulose, hemicellulose and lignin. Under appropriate reaction conditions the susceptible hydroxyl groups may be transformed by reactive chemicals into less hydrophilic functions, thus improving the biological and technological properties of suchlike chemically modified wood.
Impregnation
A study of the impregnation step in an acetylation process showed that pine and birch were easily impregnated. Ash, beech and maple were shown not to be completely penetrated when impregnated even 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 low degree of penetration was obtained. However, diffusion of acetic anhydride partially contribute to the acetylation of areas within the wood, where not directly impregnated with anhydride.

Conventional heating
Various parameters relating to the acetylation process, with different wood species, were studied at a 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 120-1 30°C. A moisture content of 16-18%, which is quite common for joinery had an effect (due to hydrolysis) on the amount of acetic anhydride that had to be used , but could still result in a 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.

Acetylation
On the basis of these experiments a laboratory acetylation plant was built for samples up to 1.5 m. Several tests were carried out to compare the achievable degree of acetylation under various process parameters, wood species and sample size. 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 of using microwave energy as the source of heat in the acetylation process was investigated, in order 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 that an equilibrium was reached within the wood during acetylation, promoting a uniform heating pattern. The penetration depth of microwaves into a wood sample impregnate 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 (twidth and depth). Microwave acetylation trials showed that variation in acetyl content both within and between samples was less than 2%, which indicated a high degree of reproducibility in the process. Generally, a somewhat higher acetyl content were 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 was 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 against decay by brown and white rot. Degradation by soft rot fungi could be prevented with a weight gain due to acetylation 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.

Coating
Performance of paints and stains was much better on acetylated pine as compared to untreated samples. This was due to the improved dimensional stability of the material following acetylation, which resulted in to 80% reduction in swelling and shrinkage. Acetylated wood proved to resistant against discoloration.

Performance
A few cubic meters of wood were 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 test showed that acetylation can provide a considerable improvement of performance in practice. An acetylated beech door for example proved to be more dimensionally stable than a door made of tropical hardwood.
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 are liable to the cell wall chemistry, in particular to the hydroxyl functions and oxygen containing groups of cellulose, hemicellulose and lignin. Lignocellulosic material such as wood is known to be strongly upgraded biologically (resistance to fungi and insect attack) and in the same treatment technologically (dimensional stability) when the cell walls are chemically modified. This is known to be successfully done with chemicals that are able to react with the OH -groups. Most knowledge in this field of research has been built up with acetylation of lignocellulosic material, but other chemicals have potential too.

Activities

Various types of reaction, selected on the basis of existing literature, were studied at the laboratory scale. These reactions included esterification, expoxidation, 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 affording substantial improvement in properties, in particular esterification of wood using 1,2 cyclohexame dicarboxylic anhydride and acrylation with N-hydroxymethylacrylamide, were selected for further testing according to European Standard test methods.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

STICHTING HOUT RESEARCH - SHR
Address
P.o.box 1568
3600 BN Maarssen
Netherlands

Participants (2)

CHALMERS UNIVERSITY OF TECHNOLOGY
Sweden
Address

41296 Gotheborg
RIJKSUNIVERSITEIT GENT
Belgium
Address
St.pietersnieuwstraat 25
9000 Gent