The object of this invention is to fix effectively and simultaneously, in one operation, a compound of copper and arsenic in wood, strawboards, palms and other materials containing cellulose and lignin so that when the latter, after drying, are exposed to moisture or running water, not only very little of any copper or arsenic is washed out of the material, but their relative and individual degrees of leachability can be controlled within wide limits.
In this connection it may be mentioned that the British patent specification No. 2972/1912 states that metallic salts can be fixed in wood by the addition of water soluble chromium compounds. This claim has not been borne out to any considerable extent when zinc chloride or sodium fluoride are used as the impregnating chemicals in my experiments. As far as I am aware, there has been no proposal made or mixture put on the market consisting mainly or solely of copper, arsenic and chromium, especially in the proportions I have found to be most suitable for wood preservation purposes. It is not only the toxicity of copper and arsenic against vegetable and animal organisms that is taken advantage of in this invention but also their optimum toxicity properties corresponding. to certain ranges in a high degree of fixation in wood. The latter fact coupled with the experimental fact that in the presence of chromium salts, copper commences to be fixed in wood much earlier than arsenic, tends to establish certain optimum properties between copper and arsenic salts in solution. Though containing copper, the preservative is neither decomposed by nor is it corrosive to steel and iron.
It has been proposed to use other organic and/or inorganic salts in ad-ixture with chromic acid or chromium salts with or without arsenic compounds. By way of example, I may be allowed to refer (1) to the Indian specification No. 17,229 of 1930, in which it is proposed to use 45 I parts of K2Cr207, 50 parts of NaF and 5 parts of dinitro-phenol with or without arsenic compounds, and (2) to the Indian specification No.
S18,580/1932 in which it is proposed to use solely or almost solely arsenic and chromium salts.
So far as I am aware, it has hereto not been proposed to use chromium compounds with copper and arsenic compounds as main wood preserving ingredients, especially in a certain rela-. tive proportion and to fix simultaneously both the metallic radicals of copper and arsenic. By 55 the expression "main ingredients", it is meant that the copper, arsenic and chromium compounds together should not be less than 80%.
According to my invention, I propose to use chromium in the form of water-soluble chromium compounds along with water soluble copper and arsenic compounds as the main ingredients of the wood preservative.
If trivalent arsenic compounds are used, chromates or dichromates cannot be used in solution with them owing to the reducing action of the latter; chromic acid can, however, be employed, in which case the damage due to the corroding action on iron or steel vessels used in commercial wood preservation is enhanced. According to an important feature of my invention, chromium constitutes between 45 per cent to 90 per cent of the total copper, arsenic and chromium (calculated in terms of the elements) present in solution. Within the above range, I have detected certain novel technical effects never observed before with regard to the simultaneous fixation of copper and arsenic in wood as will be evident from the description of the invention given below. The facts on which my present invention is based can be understood from the following table in which are given the data, with only one species of wood, and with one proportion of copper and arsenic, indicating the phenomenon, which forms the basis of my invention.
No. of experiment 1--2--------3 ---...
6-........ 7----8 ----9 --10.----. 12.......
13-----14----.. 15--...... 16 ------17.---18 .---.-19. --20 --21--22 .......
23--24--Copper sulphate (CuSO4, 5120) 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 Arsenic pentoxide (AssOs, 2H20) 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 Potassium dichromate (K2Cr2Or) 0.5 0.5 0.6 1.0 1.0 1.0 1.5 1.5 1.5 2.0 2.0 2.0 2.5 2.5 2.5 3.0 3.0 3.0 3.5 3.5 3.6 4.0 4.0 4.0 Percentage of chemical (estimated on the original quantity injected) leached out after 20,000 shakes in 100 c. c. 30 of water Copper I sulphate (CuSO,) 10.5 11.0 12.1 5.3 5.1 5.2 Traces Traces Traces Nil Nil Nil Nil Nil Nil Nil Nil Nil Nil Nil Nil Nil Nil Nil Arsenic pentoxide (AsOs) 32.2 35.1 39.0 24.7 24.4 25.0 40 19.4 18.1 17.5 15.6 12.4 11.6 7.6 45 7.6 4.6 4.4 4.4 2.5 2.5 2.6 2.8 Traces Traces Traces shown in the above table were carried out as follows:Small pieces of air-dried semul wood (Bombax malabaricum) were impregnated with wood preservative solution containing the chemicals, as 60 shOwn in the table, under test: in each case, a known concentration of either or both of the chemicals was used. As the amount of the solution absorbed by the wood piece in each case was known by weighing the piece before and after the impregnation, the amount of arsenic, copper and/or chromium compounds injected into the wood could be calculated.
The pieces after impregnation were dried and shaken in bottles containing 100 c. c. of distilled water in each case. The amount of arsenic, copper and chromium compounds washed out in each case into the water was determined by analysis, and is noted in the table as a percentage of the original amount injected into the wood piece. The bottles were shaken in a special shaking machine 20,000 times.
It may be added that the weight of solution absorbed by the wood pieces was about 50 per cent to 90 per cent of their untreated weight.
It will be seen from the results of the experiments indicated in the table that copper is fixed to the wood corresponding to a relatively low proportion of chromium, whereas for the flxation of arsenic in wood, a relatively higher proportion of chromium salts appears to be necessary. Also, when a mixture of copper, arsenic and chromium salts are injected into wood, the chromium is used for the fixation of copper before any considerable fixation of arsenic can take place, during which process a further fixation of copper proceeds. The practical result of such a phenomenon is that if the chromium required for fixing copper as well as arsenic is slightly less than what it should be, the copper is not leached out, but there is a danger of a considerable proportion of the arsenic being washed out of the wood or the like. Again, my experiments have shown that copper cannot be fixed in wood to the same very high degree as arsenic without considerably reducing Its toxicity so that the arsenic present should be as little as is necessary to resist fungus attack.
According to my experiments, the copper radical is fixed in wood more efficiently than arsenic for a certain quantity of chromium present and as wood-eating insects such as white ants contain more acid than fungi the copper will still be effective against such organisms, even against fungi, while arsenic will mostly cope with fungi against which it is very toxic even with a high degree of fixation in wood. The chemical basis for such an affinity for copper in preference to arsenic by wood in the presence of chromium salts is not understood, but this is a fact new to knowledge. On such a fact is built the best combination of copper and arsenic salts proposed in the invention. A useful formula for general conditions is as follows:-.75% CuSO4 5H20, .25% As2 s052H0, 1.25% K2Cr207.
My present invention is based on the fact that when soluble copper and arsenic compounds, in conjunction with chromium salts in aqueous solution, are used for impregnating different timbers, there is a certain critical ratio between, firstly, copper and chromium and, secondly, between arsenic and chromium above which only arsenic is washed out, or neither arsenic nor copper is washed out. In the case of hardwoods, containing gums, oleo-resins, a higher percentage of chromium salts are, in general, necessary for the fixation of copper and arsenic in wood; I have made experiments with two coniferous and four hardwoods in this connection. Instead of employing equal proportions of copper sulphate and arsenic pentoxide, I have made experiments to test the fixation of copper and arsenic when the copper varies between 8% and 40%, and the arsenic varies between 8% and 50% of the total copper, arsenic and chromium per cent in solution-as elements. It may be noted that as regards toxicity to wood destroying fungi, copper and arsenic are to a large extent supplementary.
If copper sulphate and arsenic pentoxide are used, generally very good results are obtained when copper sulphate (crystals) is used in twice '5 to four times as much as arsenic pentoxide. In other words, there are certain fungi like Lenzites sp. to which arsenic is not very sensitive while copper is sensitive. There are again fungi like Coniophora sp. to which arensic is very sensi- 3J tive, but copper is not quite so toxic. Again, in the case of insects, especially termites, copper appears to be almost as toxic as or even better than arsenic, especially when wood is protected from fungus attack by an efficient fungicide L5 like arsenic.
In practice, wood may be preserved by the injection, under pressure (without having any effect on the steel or on the preservative) of the composite solution in closed steel vessels, or dipping in open vessels or by brush or spray painting of the solution.
I claim:1. A wood preservative consisting of about 3 parts by weight of a copper sulphate, about 1 43 part by weight of a water-soluble arsenate, and about 3-5 parts by weight of an alkali dichromate.
2. A wood preservative consisting of about 3 parts by weight of copper sulphate, about 1 5o part by weight of arsenic pentoxide, and about 3-5 parts by weight of potassium dichromate.
3. A wood preservative consisting mainly of a copper sulphate, a water-soluble arsenate and a water-soluble chromium compound, the quantity of the copper sulphate being approximately from one to four times that of the arsenate, and the chromium content varying between approximately 45% and 90% of the total copper, arsenic, and chromium present, calculated as elements. 6jo SONTI KAMESAM.