CA1048407A - Wood treating process - Google Patents

Abstract

ABSTRACT

An improved process for treating wood which has been impregnated with a solution of a treating agent in a suitable solvent. After the impregnation step, the wood is steamed at an elevated temperature and superatmospheric pressure, thus causing the solvent to condense on the sur-face of the wood. The pressure is then reduced to atmos-pheric pressure and the steaming continued until essentially all of the solvent is removed from the wood. By this pro-cess, deposits of treating agent crystals and wood exudates are largely removed from the surface of the wood.

Description

~)484g~7 The treatment of wood and wood products to im-part one or more desirable properties to the wood, such as resistance to microbial attack, by impregnation of the wood with treating agents dissolved in a solvent is well known. Many wood products for use in industrial applica-tions have been impregnated with a petroleum oil as the solvent for the treating agents. Water had been employed as the solvent for water-soluble copper salts, and organic solvents such as methylene chlorid~ and liquid petroleum gases have been used as solvents for many agents. The use of water and organic solvents has become more widely employed in recent years as the power companies have gone to natural appearing poles and poles that can be painted.
Similarly, dimensional lumber has been treated with agents from water and organic solvents, as in many situations paint or other finishes are desirably applied. However, each of these latter treatments suffers from the disadvantage that as the water or solvent is removed from the wood the treat-ing agent and wood exudates are brought to the surface.
; 20 These surface deposits have conventionally been removed by brushing the wood after treatment. It would therefore be advantageous if a process could be provided which would not require the after-treatment brushing.
The disadvantages of the prior processes men-tioned above have been substantially overcome by the pres-ent invention, wherein, in a method for treating wood by impregnation with one or more wood treating agents selec-ted from the group consisting of antimicrobials, fire re-tardants, bulking agents, dimensional stabilizers, and 17,144-F -1-11~)484~7 coloring agents, wherein the treating is accomplished by a pressure impregnation with a solution of the wood treat-ing agent(s) in a volatile, water-immiscible, low boiling organic solvent and recovery of the solvent is accomplished by steaming after impregnation, the improvement which con-sists of maintaining during steaming a solvent-rich atmos-phere in contact with the wood, thereby to condense sol-vent on the surface of the wood to wash the surface free of treating agent and wood exudates.
In practicing the process of the present inven-tion, wood is impregnated with a solution of a treating agent or agents by immersing the wood in the solution in a pressure cylinder and applying pressure to cause the desired penetration of the solution into the wood. Fol-lowing impregnation, the cylinder is drained of solution which has not entered the wood and solvent removal from the wood is commenced. In a preferred embodiment of the present invention, solvent removal is accomplished by subjecting the impregnated wood to steam under superat-mospheric pressure up to 100 psig and temperatures up to 240F (ca 115.5C). The steam and its accompanying sol-vent vapors are vented at such a rate that the solvent vapors build-up in the cylinder to a sufficient quantity that under the pressure within the cylinder solvent vapors condense on the wood surface and flush the surface free of crystalline agent and wood exudates. When a major portion of the solvent retained in the wood has been re-noved in this manner, the cylinder is vented to lower the pressure to atmospheric pressure and steaming is 17,144-F -2-10484(~7 continued to remove the remainder of the solvent in the cylinder. The cylinder is then ready to open to the at-mosphere. The wood removed after this treatment is clean, free of agent at the surface and has the natural appear-ance (color) of wood.
It is to be understood that other solvents vola-tile at below 240F (115.5C) may be employed, including water. Best results have been obtained when methylene chloride is the solvent.
Substantially any treating agent can be employed which is soluble in the solvent. For example, antimicro-bial agents such as polychlorinated phenols, particularly pentachlorophenol, fire retardant chemicals, bulking agents and dimensional stabilizers can be used. Dyes may also be employed, either alone or in combination with one or more of the aforementioned agents.
The pressure impregnation is advantageously conducted at ambient temperature to about 40C (104F) under superatmospheric pressures up to about 150 psig for from 1/4 to about 8 hours. The steaming and solvent recovery cycle of the present invention is advantageously ; carried out at temperatures of from about 100C (212F) to about 240F (115.5C) under superatmospheric pressures up to about 100 psig for from about 2 to about 16 hours.
The steps of the process of the present inven-tion are broadly set forth below:
1. Charge a cylinder, which is equipped with heating coils at the bottom, with wood products and seal cylinder.

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2. Fill the cylinder with treating solution.

3. Apply pressure to treating solution to cause required amount of solution to enter wood.

4. Release pressure and drain the cylinder without permitting air to enter.

5. Introduce steam into the cylinder or intro-duce water into cylinder and activate the heating coils to heat water to generate steam in the cylinder. Thus, the cylinder and contents are heated and produce a pres-sure above atmospheric pressure, thereby to cause the solvent vapor, or at least a part of it, to condense on the wood during the heating of the wood. (The wood will not achieve the temperature of the surrounding atmosphere until the major portion of the solvent has been removed from the wood).

6. Vent the cylinder when it is at the desired pressure to remove excess vapors resulting from the sol-vent being vaporized from the wood.

7. Lower the pressure in the cylinder to at-mospheric when the cylinder vapor temperature reaches the desired temperature, not higher than 240F (115.5C).

8. Continue steaming at atmospheric pressure ; to remove solvent vapors from the cylinder.

9. Cool cylinder, drain cylinder condensate, open to atmosphere and remove wood.
These steps differ from those described in United States Patent 3,685,959, issued to Dunn et al., August 22, 1972, in that the steaming of the wood after impregnation is done under pressure to insure the solvent b 17,144-F -4-~)4~407 vapors will condense on the wood during the early stages of solvent removal to wash the wood surface free of pre-servative and wood exudates brought to the surface during the removal of the solvent. The accumulation of preserva-tive and wood exudates are found to occur during the early stages of the solvent recovery from the wood nearest the surface. It appears that after these materials are brought to the surface, there is an equilibration from the interior and less material is brought to the surface as the solvent content of the wood is reduced. Thus, the present inven-tion provides major washing during the inital stages of solvent removal and lesser washing as the system heats up and the final traces of solvent are being removed.
In a representative example employing the steps above, wood was found to be substantially free of wood preservative, in this case pentachlorophenol, and wood exudates. In a comparative example wherein steaming was carried out at atmospheric pressure it resulted in the surface of the treated wood having an accumulation of pentachlorophenol crystals and wood exudate, both of which must be removed by brushing before the wood can be used.

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Claims (4)

1. In a metnod for treating wood by impregna-tion with one or more wood treating agents selected from the group consisting of antimicrobials, fire retardants, bulking agents, dimensional stabilizers, and coloring agents, wherein the treating is accomplished by a pres-sure impregnation with a solution of the wood treating agent(s) in a volatile, water-immiscible, low boiling organic solvent and recovery of the solvent is accom-plished by steaming after impregnation, the improvement which consists of maintaining during steaming a solvent-which atmosphere in contact with the wood, thereby to condense solvent on the surface of the wood to wash the surface free of treating agent and wood exudates.

2. The process as in Claim 1 wherein the steam-ing step is carried out at a temperature of up to 240°F
(115.5°C) and a pressure of up to 100 psig.

3. The process as in Claim 1 wherein the treat-ing agent is pentachlorophenol.

4. The process as in Claim 1 wherein the sol-vent is methylene chloride.