Treatment of wood and wood-based materials

Details Treatment of wood and wood-based materials Treatment of wood and wood-based materials

1991-01-18

1994-07-19

Beck, Shrive

Stock material or miscellaneous articles

Composite

Of carbohydrate

427254, 4272552, 427297, 4273933, 427317, B32B 2104

Patent

active

053308476

DESCRIPTION:

BRIEF SUMMARY
This invention is concerned with methods for the preservative treatment of timber and wood-based products e.g. wood-based boards, to offer protection against rot, insect attack or to impart flame or fire resistance. The invention also embraces apparatus suitable for carrying out the method and materials treated by the process and/or in such apparatus.
Many organo-boron compounds are gases or low boiling point liquids. When arranged to contact with timber or wood-based products, certain of these compounds hydrolyse with the wood moisture to release the boron as boric acid in the timber. For example, trimethyl borate (TMB) is believed to react with moisture in wood to form boric acid by the reaction:
Thus, according to this reaction, the organo-boron compound trimethyl borate is capable of hydrolysing to boric acid reaction product and other reaction product, which is methanol in this case. Of the two reaction products, methanol has the lower boiling point about 64.7.degree. C. at atmospheric pressure.
Trimethyl borate boils at about 68.5.degree. to 69.degree. C. at atmospheric pressure. In previously known treatments, application of the vapour at high temperature required both the treatment vessel and the timber to be heated to prevent condensation of the vapour. Wood moisture content also affected the quantity of trimethyl borate converted to boric acid.
Proposed treatments at working moisture contents of wood have been found to be largely ineffective for bulk timber because of incomplete penetration of the TMB beyond a surface layer of the timber. Reduction of wood moisture content was found to increase penetration but full penetration was only found at reduced moisture contents which were below practical, working moisture contents. Timber dried to such levels can suffer problems such as warping or splitting, which Would render such timber of little commercial value.
An object of the present invention is to obviate or mitigate the aforesaid disadvantages, and to provide a treatment and apparatus also suitable for wood-based board materials.
According to one aspect of the present invention, there is provided a method of treating timber or wood based board comprising exposing said timber or board to vapour derived from a mixture comprising an organo-boron compound and a second compound, said compounds being capable of forming a positive azeotrope if mixed in suitable molar proportions, said organo-boron compound hydrolysing to boric acid reaction product in said timber or board and other reaction product, the vapour exposure being effected at a temperature which, under the treatment conditions selected, is greater than or equal to the boiling point of the mixture used, but below the boiling point of said other reaction product.
The treatment temperature, under the selected treatment conditions of e.g. reduced initial pressure, wood or board type, moisture content, desired level of boric acid penetration, is therefore most preferably capable of generating vapour from the mixture but of suppressing vaporisation of the other reaction product, being suppression of vaporisation of methanol reaction product in the case that TMB is used as the organo-boron compound. It has been found that commercially available positively azeotropic liquid mixture of TMB/methanol performs well in the present invention, comprising approximately equi-molar proportions of these two compounds, and having a boiling point lower than both individual compounds.
The molar proportion of the second compound may vary and it is preferred to use mixtures whereby the molar amount of second compound is from 10% to 90%, more preferably at or near the azeotropic molar percentage.
According to an embodiment of the present invention there is provided a method of treating timber or wood based board comprising exposing the timber or wood based board to the vapour of a positive azeotrope of a liquid organo-boron compound, which compound is hydrolysable to boric acid, with a second liquid, said treatment being effected at a temperature above the boiling

REFERENCES:
patent: 3342629 (1967-09-01), Martin
patent: 4012507 (1977-03-01), Knoepfler et al.
patent: 4354316 (1982-10-01), Schroeder
patent: 4678686 (1987-07-01), Park
patent: 5024861 (1991-06-01), Vinden et al.

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