Abstract:
Wood products impregnated with from 30 to 80% of a polymerizable monomer selected from the group consisting of hexanediol diacrylate and hexanediol dimethacrylate have excellent indent resistance. The wood products are impregnated with the monomer solution by placing the wood under a vacuum to remove air, contacting the wood with a monomer solution containing a polymerizable hexanediol diacrylate and/or hexanediol dimethacrylate monomer and from 0.5 to about 2 percent of a thermally activated free radical source as a polymerization initiator for a period sufficient to achieve the desired polymer loading. The wood is then heated under pressure to polymerize or cure the monomers.

Description:
This application is a continuation application of U.S. Ser. No. 08/074,772, filed Jun. 9, 1993 now abandoned. 
    
    
     TECHNICAL FIELD 
     The present invention relates to hardened and fire retardant wood products and a method of chemically treating wood to increase its hardness and fire retardant properties. 
     BACKGROUND OF THE INVENTION 
     Wood possesses many desirable physical properties which make it an ideal material for many different applications. As compared to other building materials, wood is generally structurally strong, may be finished to a pleasing appearance and is easily shaped. Though a desirable material, wood has several physical properties which have limited its use in a number of different applications. These properties include the relative softness of wood, its typical non-uniform density and flammability. 
     Due to wood being an anisotropic material, i.e. the properties of wood are different in all three directions in which a tree grows, the specific gravity of wood may vary from 0.3 to over 1 depending on a number of factors. These factors include the particular species of wood, whether the wood is latewood (grown in the summer and early fall), summerwood (grown in the spring), heartwood or sapwood, the extractable content, mineral content, and other factors. Since the hardness of a wood surface is directly related to its specific gravity, the variability in specific gravity results in a wide variability in surface hardness. If a floor were made of oak, for example, the more dense latewood may have a specific gravity of approximately 0.7 whereas the less dense earlywood could have a specific gravity of approximately 0.5. The range of specific gravities is even wider in other species of wood 
     Uniformity in hardness is especially desirable for wood that is used as a flooring material since soft spots in wood flooring are particularly vulnerable to permanent indentation resulting from the application of localized pressure. For example, a localized pressure of 100 pounds per square inch (psi) may result in an indentation in the earlywood of most wood species. 
     The pressure exerted on the surface of a wood floor varies with the size and weight of object in contact with the floor. For example, a person weighing 150 pounds wearing shoes with a 0.5 inch by 0.5 inch (normal high heal), may exert a pressure of up to 600 psi on the surface beneath the heel. The same 150 pound person wearing shoes with 0.25 inch by 0.25 inch (stiletto) heels may exert up to approximately 2400 psi on the surface beneath the heel, resulting in a permanent indentation. An exposed nail in a shoe heel with a much smaller surface area, can cause even more damage to a wood floor. For example, the same 150 pound person with an exposed nail having a diameter of 0.5 min. can exert a specific pressure of 387,000 psi beneath the nail. 
     Several approaches have been taken in the past in attempts to harden wood surfaces. These attempts have included the application of surface coatings such as varnishes and impregnation of the wood with various materials. When hardness is achieved through a surface coating the ultimate hardness of this system is still controlled by the hardness of the wood under the coating. When indentation occurs on this surface, it may be more visible in a surface coated system than an uncoated surface due to the change in the refraction of the glossy coating. There have been attempts to harden wood surfaces by impregnating the wood with various materials such as methyl methacrylate with varying degrees of success. In the case of methyl methacrylate, however, the amount of polymer absorbed by the wood is limited. Moreover, since methyl methacrylate is a relatively volatile material, it may be lost through evaporation. 
     All of the components of wood, cellulose, hemicelluloses, lignin, and extractables undergo thermal degradation. If enough heat and oxygen are provided, wood will burn. Thus, in order to meet the demands of many applications, wood must be treated with a fire retardant. 
     Although various methods have been proposed to treat wood to promote hardness or to provide fire retardancy, in general the prior art methods present numerous deficiencies. In some cases the treatment does not give uniform hardness, in other cases the treatment does not provide a surface hard enough to be used for flooring and similar applications. Some treatments do not provide for staining in combination with treatment with hardening chemicals. Other treatments change the natural appearance of the wood. Where the treated wood is used as a veneer, some treatments permit bleed through of the adhesive used to adhere the veneer to the core material. In still other cases, a fire retardant can not be used in combination with the chemicals used to harden the wood, or the treatment process is too expensive, complicated and time consuming. In yet other cases, the chemicals used to treat the wood have relatively low vapor pressures, resulting in the loss of chemicals during the treatment process and the generation of potentially noxious fumes. 
     Thus, there exists a need for a practical and economical method of treating wood to improve its hardness that also permits treatment to improve fire retardancy and provides for staining of the wood surface. 
     SUMMARY OF THE INVENTION 
     The present invention relates to the production of a hardened, fire retardant treated wood product for applications such as flooring where uniform hardness is desirable. In one embodiment of the invention, a wood product is produced by: 
     (1) impregnating wood, under vacuum, with a treating solution containing hexanediol diacrylate (HDDA) or hexanediol dimethacrylate (HDDMA) monomers and a free radical source as a polymerization initiator alone or in combination with a chlorinated flame or fire retardant with or without an organic dye or stain and draining excess treating solution from the wood; 
     (2) heating the impregnated wood product under pressure to promote polymerization or curing of the monomers. 
     In the practice of the present invention, wood is impregnated with compounds having the following formulas: ##STR1## to produce wood products having superior hardness and indent resistance. 
     In one embodiment of the present invention, wood product in the form of veneer is impregnated with a solution of hexanediol diacrylate or hexanediol dimethacrylate monomer and a polymerization initiator alone, or in combination with a fire retardant and/or a stain or dye for a predetermined period of time. The veneer is removed from the treating solution, excess monomer is removed and the veneer is heated under pressure to cure or polymerize the monomers. Wood veneers treated in this way demonstrate greatly improved hardness and fire retardancy. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of the process of the present invention; and 
     FIGS. 2 and 3 are cross sectional views of laminated wood products incorporating layers of wood treated in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the practice of the present invention, the wood product is first placed under vacuum for a predetermined period to remove air. As used herein, the term &#34;wood product&#34; refers to products produced from solid wood such as veneers, boards, dowels and similar items shaped from solid wood. Preferably, the wood products to be treated have a moisture content below 20 percent by weight. Impregnation of the wood product is achieved more readily when the wood is dry, but complete dryness is not essential for the process. 
     As schematically illustrated in FIG. 1, the wood product is subject to a vacuum of from about 5 mm. Hg. to about 15 mm. Hg. for a period of from about 1 to 30 minutes. The amount of vacuum as well as the period during which the wood product is subjected to the vacuum can, however, be varied above and below these limits. 
     While still under vacuum, the wood product is then immersed in a solution of hexanediol diacrylate (HDDA) or hexanediol dimethacrylate (HDDMA) monomer and a polymerization initiator alone, or in combination with a flame or fire retardant such as chlorinated wax. A thermally activated polymerization initiator suitable for use in the practice of the present invention is 2,2&#39;-Azobis-(2-methylbutyronitrile), commercially available from Dupont under the trade designation Vazo 67. Additionally, a dye, stain or other colorant can be added to the solution in an amount of 3 to 10 percent by volume. 
     The temperature of the monomer solution can be from 20 degrees up to 60 degrees C. Preferably, the wood is immersed in the solution for a period (soak time) sufficient to allow the polymer to absorb between about 30% to about 80% of monomer solution, based upon the weight of the untreated wood. In the case of veneers the soak time is preferably from about 5 to about 40 minutes. The soak time may, however, be varied above and below these limits depending upon the particular species of wood and the shape of the wood product. 
     Finally, the wood product is removed from the monomer solution and then heated under pressure to cure or polymerize the monomer. The temperature of the press can be from 90 to 180 degrees C. The duration of time in the press (cure time) for complete polymerization depends primarily on the temperature of the press and typically ranges from about 3 to about 10 minutes in the case of veneers. Again, the cure time may vary above and below these limits depending upon the particular species of wood and the shape of the wood product. 
     Depending on the type and size of the wood product to be treated, and the total processing to a final product, different process alternatives can be adopted. Wood in the form of veneers is a typical wood product amenable to treatment with the process of the present invention. Thicker wood materials can also be used in this process if the vacuum and atmospheric soaking times are extended. 
     In some applications, such as multiple layer laminated wood veneers, it may be desirable to further increase resistance to fire. If this is the case, fire retardancy can be further improved by treating the wood product with an aqueous solution of water soluble inorganic fire retardants, such as boric acid, diammonium phosphate, or ammonium borate. 
     As illustrated in FIG. 2, in one embodiment of the present invention, a veneer 1 may be treated with an inorganic fire retardant such as diammonium phosphate prior to treatment with the monomer solution. The veneer may then be laminated to additional layers 2-5 of veneer. In another embodiment of the present invention, as illustrated in FIG. 3, a layer of veneer 7 treated with an inorganic fire retardant such as diammonium phosphate may be placed under a top ply 6 that has been treated with the monomer solution. Additionally, treatment with a fire retardant such as a chlorinated wax may be combined with the monomer treatment. 
     The treated wood product of the present invention exhibits greatly improved indent resistance as compared to untreated wood of the same type. As used herein, &#34;indent resistance&#34; is measured by the depth of an indentation formed when a wood product is tested with a Gardner impact apparatus with a indent force of 50 inch-lbs. Thus, lower values reflect higher indent resistance. 
     The present invention is believed to have several advantages over the prior art in treating wood products. The present invention provides: (1) a harder wood product than is presently used for flooring which may be treated to enhance its fire retardancy; (2) a wood product with uniform hardness throughout its surface; (3) with the addition of a dye or stain, a uniform color throughout the surface of the wood product; and (4) a practical, economical method of hardening a wood product which uses less volatile chemicals and may be accomplished in less time than prior art methods. 
     The invention will be further described with respect to the following examples; however, the scope of the invention is not to be limited thereby. 
     EXAMPLE 1 
     Northern Red Oak veneers were placed in a vacuum chamber and subjected to a vacuum of 9 mm Hg. for 15 minutes. The veneers of this and the following examples ranged from approximately 0.10 to 0.25 inches in thickness. While maintaining the vacuum, monomer was introduced into the chamber to cover the veneers. The vacuum was then released to return the pressure to atmospheric. The veneers were kept in the monomer for 30 minutes before they were removed and cured in a heated press at 100° to 120° C. and 200 psi for 3 to 4 minutes. The monomers used were hexanediol diacrylate (HDDA), hexanediol dimethacrylate (HDDMA) and methyl methacrylate (MMA). Each monomer contained 0.5% Vazo 67 as a polymerization initiator. Specimens were treated separately with each of the monomer solutions and indent resistance of the veneers was determined using a Gardner impact apparatus. An indent force of 50 inch-lbs was used to conduct the tests. Each test was repeated approximately 25 times. The results of treatment were as follows: 
     
                       TABLE 1______________________________________Indentation Resistance of Oak Veneers Treated With DifferentMonomers       Polymer load                 Average IndentationMonomer     (%)       (inch)______________________________________HDDA        46-48     0.014-0.018HDDMA       43-45     0.015-0.026MMA         13-22     0.022-0.026______________________________________ 
    
     The foregoing example illustrates the superior results obtained through the use of HDDA or HDDMA as compared to MMA. As illustrated in Example 1, polymer absorption was approximately twice as great for HDDA and HDDMA as opposed to MMA under the same process conditions. Indent resistance was also greater for the veneers containing HDDA and HDDMA than for the veneers containing MMA. 
     EXAMPLE 2 
     Northern Red Oak veneers were treated separately with HDDA and HDDMA by the method described in Example 1, except the method was varied to obtain a range of polymer loadings. The veneers were tested for indent resistance and the results are shown in Table 2. 
     
                       TABLE 2______________________________________Indention Resistance of Treated Oak Veneers to Different PolymerLoadingsPolymer Load  Average Indentation(%)           (inch)______________________________________0             0.04016            0.03033            0.02643            0.02048            0.018______________________________________ 
    
     The foregoing example demonstrates the relationship between polymer loading and indent resistance. As polymer loading increases, indent resistance also increases. 
     EXAMPLE 3 
     Northern Red Oak veneers were treated separately with HDDA and HDDMA, including 0.5% Vazo 67 as a polymerization initiator, by the method described in Example 1 to a treatment weight gain of 48 percent. The indentation resistance of earlywood and latewood was measured. The results are shown in Table 3 below: 
     
                       TABLE 3______________________________________Indentation Resistance of Earlywood and Latewood of Oak Veneers         Average IndentionType of Wood  (inch)______________________________________Earlywood     0.019-0.025Latewood      0.018-0.022______________________________________ 
    
     The foregoing example illustrates the uniform degree of hardness imparted to wood products treated in accordance with the present invention. 
     EXAMPLE 4 
     Northern Red Oak veneers were treated separately with HDDA and HDDMA, including 0.5% Vazo 67 as a polymerization initiator, as in Example 1, except the vacuum time was varied to determine the effect of vacuum time on polymer loading. The veneers were also allowed to soak for varying periods to determine the effect of soak time on polymer loading. The samples were tested for polymer loading and the results are set forth in Table 4 below: 
     
                       TABLE 4______________________________________Vacuum Time and Soak Time vs. Polymer LoadingVacuum Time   Soak Time                  Polymer Loading(minutes)     (minutes)                  (%)______________________________________0             30       165             30       3510            30       4315            30       4815            5        3715            10       4215            15       4215            20       4515            30       5330            15       4830            300      56______________________________________ 
    
     EXAMPLE 5 
     Northern Red Oak veneers were treated separately with HDDA and HDDMA, including 0.5% Vazo 67 as a polymerization initiator, and placed in a heated press at 100° C. and 200 psi for varying amounts of time to determine the effect of heating time on the polymerization of the monomer. To determine whether polymerization occurred, a thermocouple was placed in the press with the sample. Since the polymerization process is exothermic, a temperature rise above the press temperature indicated that the polymerization reaction was occurring. The results are set forth in Table 5 below: 
     
                       TABLE 5______________________________________Time required to polymerize monomer in oak veneers.Time(min)               Polymerized______________________________________0                   no1                   no2                   no3                   no4                   yes5                   yes______________________________________ 
    
     EXAMPLE 6 
     Northern Red Oak veneers were placed in a vacuum chamber subjected to a vacuum of 9 mm Hg. for 15 minutes. While maintaining the vacuum, monomer was introduced into the chamber to cover the veneers. The vacuum was then released to return the pressure to atmospheric conditions. The veneers were kept in the monomer for 30 minutes before they were removed and cured in a press at 100 to 120° C. and 200 psi for 3 to 4 minutes. The monomer solutions used were HDDA and HDDMA with varying amounts of a chlorinated wax. Each monomer contained 0.5% Vazo 67 as a polymerization initiator. Specimens were treated separately with each of the monomer solutions and indent resistance of the veneers was determined using a Gardner impact apparatus. An indent force of 50 inch-lbs was used. The test results are set forth in Table 6 below: 
     
                       TABLE 6______________________________________Indentation Resistance of Oak Veneers Treated With Solutions ofHDDA and HDDMA With Chlorinated WaxRatio of HDDA or HDDMA             Average Indentationto Chlorinated Wax             (inches)______________________________________85:15             0.02175:25             0.01770:30             0.019______________________________________ 
    
     The foregoing example illustrates that a flame retarding agent, such as a chlorinated wax, may be incorporated into the monomer solution without adversely affecting the ability of the solution to harden wood. 
     EXAMPLE 7 
     Northern Red Oak veneers treated as in Examples 1 and 6 were laminated to untreated veneers to form 5 ply boards, the treated veneer being the top veneer. The boards were then tested for flame spread in accordance with ASTM E162. The results are set forth in Table 7 below: 
     
                       TABLE 7______________________________________Flame Spread on Polymer Treated Oak Veneers.MonomerTreatment        Flame Spread (%)______________________________________HDDA             118HDDMA            114HDDA or HDDMA with            9415% Chlorinated WaxHDDMA with 25%   82Chlorinated WaxHDDA or HDDMA with            8530% Chlorinated Wax______________________________________ 
    
     EXAMPLE 8 
     Northern Red Oak veneers, treated as in Example 6, were laminated to 4 additional layers of red oak veneer and tested for flame spread. In the first test, the top ply was first treated with diammonium phosphate followed by the monomer treatment. In the second test, the top ply was monomer treated and the second ply was treated with diammonium phosphate. The boards were then tested for flame spread in accordance with ASTM E162. The results are set forth in Table 8 below: 
     
                       TABLE 8______________________________________Flame Spread on Polymer Treated Oak Veneers.MonomerTreatment         Flame Spread (%)______________________________________Top veneer treated with             85diammonium phosphatefollowed by treatment withHDDA or HDDMA with30% Chlorinated WaxTop veneer treated with             55HDDA or HDDMA with30% Chlorinated Wax,second veneer treated withdiammonium phosphate______________________________________ 
    
     EXAMPLE 9 
     Northern Red Oak veneers were treated separately with mixtures of HDDA and HDDMA with 30% chlorinated wax as in Example 6. The veneers were then tested for indentation resistance of earlywood and latewood. The results are set forth in Table 9 below: 
     
                       TABLE 9______________________________________Indentation Resistance of Earlywood and Latewood of Oak Veneers         Average IndentionType of Wood  (inch)______________________________________Earlywood     0.019-0.026Latewood      0.016-0.022______________________________________ 
    
     EXAMPLE 10 
     Northern Red Oak veneers were treated separately with HDDA and HDDMA, including 0.5% Vazo 67 as a polymerization initiator and 30% chlorinated wax, using varying vacuum times to determine the effect of vacuum time on polymer loading. The veneers were also allowed to soak for varying periods to determine the effect of soak time on polymer loading. The samples were tested for polymer loading and the results are set forth in Table 10 below: 
     
                       TABLE 10______________________________________Vacuum Time and Soak Time vs. Polymer LoadingVacuum Time   Soak Time                  Polymer Loading(minutes)     (minutes)                  (%)______________________________________0             30       175             30       3810            30       4315            30       4815            5        3815            10       4415            15       4415            20       4515            30       57______________________________________ 
    
     EXAMPLE 11 
     Northern Red Oak veneers were treated separately with HDDA and HDDMA, including 0.5% Vazo 67 as a polymerization initiator and 30% chlorinated wax, and placed in a heated press at 100° C. and 200 psi for varying amounts of time to determine the effect of varying heating times on the polymerization of the monomer. To determine whether polymerization occurred, a thermocouple was placed in the press with the sample. Since the polymerization process is exothermic, a temperature rise above the press temperature indicated that the polymerization reaction was occurring. Alternatively, visual inspection of the surface of the veneer for liquid monomer also indicated whether polymerization had occurred. The test results are set forth in Table 11 below: 
     
                       TABLE 11______________________________________Time required to polymerize monomer in oak veneers.Time(min)               Polymerized______________________________________0                   no1                   no2                   no3                   no4                   yes5                   yes______________________________________ 
    
     EXAMPLE 12 
     Several hardwood and softwood veneers were treated as described in Example 1 with and without 30 percent chlorinated wax, with and without a 3 percent addition of a colorant. The monomer solution contained 0.5% Vazo 67 as a polymerization initiator. Specimens were treated separately and following polymerization, indent resistance of the veneers was determined using a Gardner impact apparatus. An indent force of 50 inch-lbs was used. The results are shown in Table 12. 
     
                       TABLE 12______________________________________Indentation Resistance of Different Wood Treated Veneers                 Average    Polymer      Indentation                           UntreatedWood     Load (%)     (inch)    Control______________________________________Pecan    70           0.012     0.028Ash      65           0.014     0.030Cherry   70           0.011     --Maple    60           0.010     0.025Mahogany 70           0.014     --Pine     70           0.015     0.039Spruce   70           0.017     0.039______________________________________ 
    
     While the particular embodiments of the present invention have been described, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention. The following claims are intended to cover all such modifications that are within the scope of the invention.