Method for pressure treating wood

The present invention is a process for pressure treating wood and includes wood which has been pressure treated by the process. The process comprises infusion into the wood of a solution in water of an anhydride or the analogous acid of an anhydride, followed by removal of moisture from the wood and the infusion of the wood with a molten waxy solid comprising hydrocarbon paraffins or saturated fatty acids. The molten waxy solid then solidifies, filling all voids in the wood.

FIELD OF THE INVENTION
 The invention relates to a method for the pressure treatment of wood and to
 wood treated by the disclosed method. In the pressure treatment of wood,
 various active ingredients such as fungicides or other wood preservatives
 are impregnated deeply into wood through the application of pressure. A
 well known example of such pressure treated wood is wood intended for
 outdoor use in fences or decks and impregnated with preservatives to
 prevent deterioration of the wood through the action of the elements or
 from insects or microbes.
 BACKGROUND OF THE INVENTION
 Although there are various pressure treatment methods for impregnating wood
 with active ingredients, the Bethel process is one of the better known. In
 the Bethel process, wood is dried so that its moisture content is
 substantially reduced. The wood is placed in a vacuum chamber to draw air
 from the wood. A mixture of water and active ingredients is then injected
 into the chamber under pressure. Pressures up to 250 pounds per square
 inch (psi) can be applied. The pressure is removed so that the wood is
 again subject to atmospheric pressure. The wood then is transferred to a
 kiln and dried to reduce the moisture content thereby leaving the active
 ingredient infused and imbedded in the wood.
 The prior art of pressure treatment of wood has proven unsatisfactory in
 several respects. Most pressure treated wood is used outdoors and is
 exposed frequently to water, which is able to seep into the prior art
 pressure treated wood. The movement of water in and out of the wood causes
 two things to occur. First, the water dissolves any water soluble active
 ingredients and extracts those ingredients from the wood, thereby reducing
 the beneficial properties the ingredients may have imparted, such as rot
 prevention or flame retardance. Second, the water causes dimensional
 instability of the wood, which can take the form of splitting and cracking
 upon freezing.
 An effective active ingredient commonly used for the pressure treatment of
 wood is Copper Chrome Arsenate (CCA), a heavy metal. The possibility of
 leaching has caused some persons to criticize the use of CCA.
 The problem of leaching of active ingredients from pressure treated wood is
 recognized in the prior art, and attempts have been made to address the
 problem. One prior art attempt at a solution is to use polymeric binders
 to secure particles of an active ingredient to the wood. These polymeric
 binders typically use aminoplast curing agents that have the undesirable
 characteristic of generating formaldehyde. Formaldehyde has various
 undesirable characteristics, such as generating odors. Formaldehyde also
 is a suspected carcinogen.
 SUMMARY OF THE INVENTION
 It is an object of the present invention to provide a method for pressure
 treating wood wherein the active ingredients are secured within and to the
 wood so that the active ingredients cannot leach from the wood and thereby
 decrease the active life of the pressure treatment. It is a further object
 of the invention to provide pressure treated wood within which and to
 which active ingredients are secured so that the active ingredients may
 not leach from the wood. It is a further object of the invention to
 improve the dimensional stability of pressure treated wood to reduce
 maintenance and to increase the service life of the wood. It is a further
 object of the invention to provide a machined treated wood by-product for
 use in particle board or oriented strand board to impart the beneficial
 properties of the pressure treated wood to the particle board and to the
 strand board.
 These objectives are accomplished and the deficiencies of the prior art
 rectified by infusing or injecting into wood a reactive solid in a water
 solution using familiar pressure treatment techniques. The reactive solid
 comprises a chemical compound selected from the class comprising
 anhydrides and the analogous acids of anhydrides. The reactive solid
 solution may include active ingredients such as fungicides, mildewcides,
 bactericides, flame retardants, colorants, and water repellants.
 The reactive solid reacts with and forms chemical bonds to and within the
 cellulosic structure of the wood. The wood treated with the reactive solid
 has improved dimensional stability and resistance to swelling on contact
 with water. In tests on southern yellow pine, infusion with anhydrides
 reduced swelling due to absorption of water to less than three percent.
 The objectives of the present invention may be further accomplished by
 infusing or injecting into the wood a waxy solid having a melting point
 above the temperature to which the wood will be exposed in use. Suitable
 waxy solids comprise paraffinic hydrocarbons or a saturated fatty acid.
 Infusion or injection of the waxy solid is accomplished by heating the
 waxy solid and the wood to a temperature above the melting point of the
 waxy solid and then using familiar pressure treatment techniques to infuse
 the waxy solid into the wood. The waxy solid may be mixed with an active
 ingredient. The waxy solid solidifies in the wood, forming a barrier to
 water and preventing leaching of the active ingredients.
 The finished wood product differs from other solid-filled wood products
 such as the wood-polymer composites in that no sensitizing or dangerous
 ingredients such as acrylated monomers and peroxide or Vaso catalysts
 typically associated with this type of product are used.
 Wood may be pressure treated by infusion either with the reactive solid
 solution or with the waxy solid. Best results are obtained by using both
 techniques and by first infusing the wood with the reactive solid solution
 followed by infusion with the waxy solid.

DETAILED DESCRIPTION
 In describing a preferred embodiment of the invention, specific terminology
 will be selected for the sake of clarity. However, the invention is not
 intended to be limited to the specific terms so selected, and it is to be
 understood that each specific term includes all technical equivalents that
 operate in a similar manner to accomplish a similar purpose.
 In the preferred embodiment and as illustrated in FIG. 1, kiln-dried wood 6
 with a moisture content of less than 20% and preferably less than 10% is
 placed in a vacuum chamber 2. Air 4 is evacuated from the vacuum chamber 2
 to remove air 8 from the wood 6.
 As shown by FIG. 2, a solution of a reactive solid and water 10 is
 introduced into the evacuated vacuum chamber 2. The reactive solid
 comprises an anhydride. In the preferred embodiment, the anhydride is
 maleic anhydride, phthalic anhydride, or a mixture of the two at a ratio
 of six parts maleic anhydride to one part of phthalic anhydride (although
 a range of one to twenty-five parts maleic anhydride to one part phthalic
 anhydride may be used for specific applications). The analogous acid which
 would result from hydration of an anhydride may be substituted for the
 anhydride in the solution.
 One or more active ingredients may be mixed with the anhydride solution.
 Because the solution is water based, the active ingredients must be water
 soluble, water dispersed or emulsified. In the preferred embodiment, the
 active ingredient is boric acid.
 In the preferred embodiment, the reactive anhydride comprises up to
 twenty-five percent by weight of the reactive anhydride solution, boric
 acid comprises up to three percent of the solution by weight and water
 comprises the balance of the solution. Certain Lewis acids; namely
 SnCI.sub.4, may be added as co-promoters.
 As shown by FIG. 2, air 4 is pressurized within the vacuum chamber 2,
 thereby infusing the reactive solid solution 10 into the wood 6. In the
 preferred embodiment, pressure of the air 4 may be increased up to 250
 psi.
 Wood 6 is removed from the vacuum chamber 2, shown in FIG. 2, and is placed
 in a kiln 12, as shown by FIG. 3. The temperature of the wood 6 is raised
 in the kiln to remove excess moisture 13. In the preferred embodiment, the
 wood is heated and dried for about 24 to 48 hours at a range of
 temperatures between approximately 170-190 degrees F.
 As shown by FIG. 4, the heated wood 6 then is placed in a vacuum chamber 2
 and air 4 is evacuated from the chamber to remove air 8 from the wood 6.
 One or more molten waxy solids 14, FIG. 5, is introduced into the
 evacuated vacuum chamber 2, and the air 4 within the vacuum chamber is
 pressurized thereby infusing the waxy solid into the wood. In the
 preferred embodiment, the pressure may be increased by up to 250 psi.
 The waxy solid 14, FIG. 5, comprises paraffinic hydrocarbons, saturated
 fatty acids or a mixture of the two. The waxy solid must have a melting
 point above the maximum temperature to which the wood will be subjected in
 use. The paraffinic hydrocarbons and saturated fatty acids having this
 characteristic are well known to those of ordinary skill in the organic
 chemical arts. Suitable saturated fatty acids include those comprised of
 twelve carbon chains or higher, their esters, alcohols and transition
 metal salts. In the preferred embodiment the waxy solid comprises stearic
 acid (C-18 fatty acid), methyl stearate (ester of C-18 fatty acid),
 Stearyl alcohol (alcohol of C-18 fatty acid), zinc stearate (zinc salt of
 C-18 fatty acid), palmitic acid (C-16 fatty acid), myristic acid (C-14
 fatty acid), lauric acid (C-12 fatty acid) or paraffin (solid
 hydrocarbon).
 Paraffin is as effective for pressure treatment of wood 6 as the saturated
 fatty acids. Paraffin has the disadvantage that it is usually derived from
 petroleum products whereas the fatty acids are derived form renewable
 resources such as natural oils and fats. It therefore may be more
 desirable to use the fatty acids.
 The waxy solid 14, FIG. 5, may include one or more active ingredients. In
 the preferred embodiment, the active ingredients are oil soluble and are
 dissolved in the waxy solid. Suitable oil soluble active ingredients are
 well known in the wood treatment art.
 The step of infusing the waxy solid 14, FIG. 5, into the wood 6 must take
 place at an elevated temperature to ensure that the waxy solid remains in
 a liquid state and at low enough viscosity to ensure effective infusion.
 The temperature depends on the waxy solid selected. For stearic acid, that
 temperature preferably approximately 160 degrees F. For zinc stearate
 blended with stearic acid, the temperature preferably is approximately 220
 degrees F.
 The waxy solid preferably comprises 90% or more of the waxy solid-active
 ingredient mixture. Preferred embodiments of the waxy solid-active
 ingredient mixture are listed in Tables 1 through 6 below.
 TABLE 1
 Component Weight %
 Stearic Acid 95-100
 Active Ingredients 5-0
 100
 TABLE 1
 Component Weight %
 Stearic Acid 95-100
 Active Ingredients 5-0
 100
 TABLE 3
 Component Weight %
 Palmitic Acid 95-100
 Active Ingredients 5-0
 100
 TABLE 3
 Component Weight %
 Palmitic Acid 95-100
 Active Ingredients 5-0
 100
 TABLE 3
 Component Weight %
 Palmitic Acid 95-100
 Active Ingredients 5-0
 100
 TABLE 3
 Component Weight %
 Palmitic Acid 95-100
 Active Ingredients 5-0
 100
 In the preferred embodiment, all of the voids of the wood 6, FIG. 5, are
 filled by infusion with the waxy solids-active ingredient mixture 14.
 After infusion of the wood 6, the wood 6 is removed from the vacuum
 chamber 2 and allowed to cool. The waxy solid-active ingredient mixture 14
 will solidify to the center of the wood 6. The pressure treated wood 6 is
 then ready for use.
 There are many variations of this process that reduce the process time in
 the vacuum or kiln or reduce the amount of penetration of the treatment
 solution, as by application of a partial as opposed to a full vacuum. The
 alternatives which reduce penetration of the wood decrease manufacturing
 costs but result in decreased performance by the pressure treated wood.
 In the preferred embodiment, wood 6 is infused first with an anhydride
 solution 10 including active ingredients and subsequently infused with a
 waxy solid mixture 14 including active ingredients. Beneficial results
 also can be obtained by infusing the wood 6 only with the anhydride
 solution 10 or only with the waxy solid mixture 14.
 All solutions and mixtures have an indefinite shelf life and can be
 recycled. All byproducts generated from machining operations of the wood
 pressure treated with both the anhydride solution and the waxy solid can
 be used for particle board, oriented strand board or other wood composite
 products resulting in improved properties.
 Although the invention has been described with reference to the preferred
 embodiments, workers skilled in the art to which the invention pertains
 will recognize that changes may be made in form and detail without
 departing from the spirit and scope of the invention.