Cell Wall Impregnation with Polymers

The only treatments that are likely to be viable commercially are aqueous delivery systems. Of the systems described in this chapter, furfurylation is the most advanced commercially and appears to show great promise. There has also been the recent introduction of the DMDHEU-based modified wood Belmadur on the market by BASF. At the present time, no other systems appear to offer any immediate prospects for commercial exploitation. The use of silicone treatments has apparently received little attention, which is very surprising due to the ready commercial availability of these systems for masonry treatment. Whether this apparent lack of activity is due to an oversight, represents a lack of real potential or perhaps is due to commercial sensitivity will become clearer in the future. However, silicone treatments are confined to the wood surface only and are not capable of penetrating the cell wall, and would therefore provide little improvement in dimensional stability. Similarly, no significant improvement in biological durability would be expected, since the relatively thin envelope of the treatment would be breached easily. However, the use of silicones in combination with other treatments that may be teachable in service (e.g. borates) would be an area well worth exploring. 

Impregnation modification is an area of research that is relatively unexplored compared to other wood modification methods and there are undoubtedly many other systems that remain to be studied in the future. 

Wood Modification Chemical, Thermal and Other Processes C. Hill 2006 John Wiley Sons, Ltd 

The Finnish ThermoWood Association was formed in December 2000. The main roles of the association are quality control of the material, product classification and R D activities. Although ThermoWood production is primarily located in Finland, there is limited production in Austria and Estonia a small-scale production facility was also due to start producing in Canada at the end of 2004. 

ThermoWood is produced by a heat-treatment process in the presence of steam, and is thus a hygrothermal treatment. The steam acts as a blanket to limit the oxidative degradation of wood, and there are also additional reactions occurring as a result of the presence 

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Nakagami and Yokota (1983) impregnated wood with a solution containing methacrylic acid, trifluoracetic acid and sulphuric acid, to form a covalent bond with the cell wall polymers. The methacrylic-reacted wood was then impregnated with styrene, or methylmethacrylate, to form cross-links with the reacted cell wall polymers. Improved dimensional stability was obtained, although degradation of the wood was also observed. 

A solution of styrene in methanol to impregnate wood samples, followed by polymerization, was used by Furuno and Goto (1979). Penetration of the monomer into the cell wall was determined by solvent extraction of samples after polymerization. This removed lumen located polymer, whilst leaving the cell wall bound polymer in place. This showed that the concentration of cell wall bound polymer increased in proportion to the monomer content in methanol, up to a maximum of 80% of the monomer in the solvent. No cell wall penetration was observed for treatment with neat monomer. This was also found for bulking of the wood, as determined from external dimensions of the samples. Improvements in ASE were obtained as a result of the presence of cell wall bound polymer. To achieve similar ASE values with lumen located polymer required very high polymer loadings. 

It is also possible first to react the polymers that still remain in the cell wall with a simple reactive chemical and then to follow this with impregnation of a polymerizable monomer (15). The simple bonded bulking chemical provides dimensional stability, and the polymerized monomer provides strength (i6). 

Wood polymer composites are produced by impregnating the wood with different monomers, for example, styrene (St), methylmethacrylate (MMA), acrylonitrile (AN), and unsaturated polyester (UPE). These monomers can transfer into homopolymer or grafted on the wood cell wall during polymerization process, which can be initiated by y-radiation, high temperature, or chemical catalysts. 

If the concentration of low-molecular liquids (solvents) in the polymers surpasses their compatibility limit, they isolate and form spherical arrangements with a size of 10-20 pm in the polymer structure. When the solid phase volume exceeds that of the liquid, the formed structures are of the closed-pore kind and the liquid phase is distributed within the solid phase as local spherical inclusions [122]. As soon as the liquid phase content surpasses that of the solid, a new honeycomb structure with communicating cavities is formed whose solid phase builds up thin walls that separate the cells. This feature is to a greater extent typical of tough and crystallizable polymers. This is also relevant for systems like PE-MO where honeycomb structures with a pore size of up to several micrometers can be formed under certain conditions (Fig. 4.22) [123]. Such porous structures are perfect for the impregnation of modified additives, e.g. Cl. 

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