Modification

With chemically modified wood means wood which by treatment with chemicals or otherwise changed in their chemical structure so that it has become more resistant to wood-destroying organisms. Many of the methods that have been tried over the years have never had any great commercial importance. It is common that a particular modification seeks an attribute improvement, but that it also gets altered properties in other respects. For this reason, the description here is based on a sample method with actual application and describes the effects. Of the method described is currently the only treatment that occurs in any commercially interesting scale.

Heat treatment (thermal modification)

Heat treatment has been developed in particular in Finland in the late 1990s. This means that the wood is heated in an oxygen-free atmosphere in a closed vessel at temperatures of 180-240°C for 10-40 hours. During the treatment, changes occur in wood chemistry, which means that it gets an improved efficacy against fungi but also affecting the wood's technical / physical properties to different degrees. The wood has a brownish color, which relatively quickly turns into a grayish tone in outdoor exposure. It is more brittle than untreated wood, and the strength is reduced significantly. It should not be used in load-bearing structures. Heat-treated wood has a higher dimensional stability and less hygroscopic than untreated wood.

For heat-treated wood there is no official classification or certification, but it produced for basically two purposes, namely indoor and outdoor above ground. When used indoors, such as floors, bathroom and sauna as well as a panel, resistance is of minor importance, while consideration of dimensional stability and beautiful color are given prominence. For outdoor use, it is the increased resistance that is of primary interest to take advantage of. Heat-treated wood is used outdoors as timber facade, in fences, windows and garden furniture.

Today there are five competing processes in Europe:

  • Thermo Wood process (steam) Finland
  • Plato process (steam, drying and heat setting) Holland
  • IWT method (vapor pressure) France
  • NOW process (hot gas) France
  • Natural holzschutz (oil bath) Germany

Acetylation

The wood is pressure impregnated with acetic anhydride which may react with the wood at 120-130°C. Residual chemicals (mostly vinegar) is driven off by one of the following processes:

  • vacuum-drying in microwave reactor (Swedish process)
  • water leaching followed by conventional drying (Japanese process)
  • conventional vacuum drying followed by after impregnating resin solution and further drying / curing (Dutch process).

Treatment with phenolic resin

Wood veneer sheet can be impregnated with phenol-formaldehyde resin solution, with the water dried off, and veneer sheets are stacked in bundles and hot pressed at high pressure at 150°C. This gives rise to a very hard, dimensionally stable and resistant sheet material (compared to bakelite and Perstorp plate and laminate floor where strength layer consists of phenolic resin-impregnated paper). Products have been developed commercially by this method since the 1940s. It was used, for example during the 2nd World War, largely for the propeller blades for fighter aircraft. Today, however production is very modest.

Treatment with furfuryl alcohol

In the "furfurylation" pressure impregnation in FA solution (aqueous solution of FA and catalysts), the wood is dried in a vacuum drying and the resin is cured into the wood at 100°C. This gives rise to a mahogany like, harder, and more resistant form-stable wood. Unlike melamine resin, as biomass waste produces furfuryl alcohol (usually from corn and sugar cane but also Birch wood chips) and is thus a renewable raw material. Production of furfuryl alloyed woo is currently in Norway and Lithuania.

Treatment with melamine

The wood is pressure impregnated with MMF solution (methylated melamine-formaldehyde resin solution), water is dried off and the resin is cured into the wood at 120-130°C. This gives rise to a harder, more resistant form-stable and wood materials (compare Perstorp plate and laminate floor where the surface layer composed of melamine resin-impregnated paper). The development has been carried out mainly in Germany, Holland and Denmark. Test production of parquet flooring, chairs and window has been carried out and market trials, involving approximately 1,000 companies, have been performed.

Comparison of modified pine with untreated

Modification MethodDimension stabilityResistanceMechanical propertiesColorchange ring
BiologicalFireUVHardnessFlexural strengthToughnessStiffness stabilityAfter treatment
in soilabove ground
Heat treatment ++ + ++ + - unchanged - - + dark-staining
Acetylation +++ +++ +++ unchanged ++ slightly higher unchanged unchanged +++ unchanged
Phenolic resin (high level) +++ +++ +++ ++ + +++ ++ - ++ dark-staining
Furfurylation (low level) ++ ++ ++ + + + - ++ dark-staining
Furfurylation (high level) +++ +++ +++ ++ +++ + - +++ dark-staining
Melamine resin (low level) ++ ++ ++ ++ + + + - + unchanged

(+ = Positive change, - = negative change compared with untreated sapwood. Greater the number of plus and minus the greater change)

Product use

Modification MethodModification chemicalEx. of commercial product namesCurrent usage areasEv. future usage areas
Heat treatment - Thermo Wood ®, Bitus-Thermal, Plato ® Wood, Nature Conservation-holtz ®, Retified wood ® Facades, patios, garden furniture, sauna Facades, patios, garden furniture, sauna
Acetylation Acetic anhydride - Speaker Drivers Windows, doors, sheet materials, barge boards
Fenolhartsbehandl. (high level) Phenol-formaldehyde resin Compreg ®, Impreg ® Knife handles -
Furfurylation (low level) FA Supervisor Wood Brown ® Terraces Patios, windows, doors, garden furniture
Furfurylation (high level) FA Black Visor Wood ® Parquet Parquet flooring, marine structures
Melaminhartsbehandl. (low level) Methylated melamine-formaldehyde resin - - Parquet flooring, windows

Modified wood, product names and uses. Methods and products is constantly evolving, so product names sometimes disappear and others to come and product characteristics should be verified with the manufacturer.


 
European Wood (in China)
C412, Beijing Lufthansa Center
50 Liangmaqiao Road, Chaoyang District
Beijing, P.R. China 100125
T +86 10 6462 2066, F +86 10 6462 2067
info@europeanwood.org.cn
Sino-European Wood Center
Room 202, Engineering Department,
Taoliyuan Hou, Xuhui Campus of Jiaotong University,
No.655 Panyu Road, Xu Hui District, Shanghai
Tel/Fax:021-53098550
info@europeanwood.org.cn