Enhanced wood preservative composition

A wood preservative composition comprising at least one biocide, such as an iodopropargyl compound or a triazole compound, in combination with at least one antioxidant, such as a hindered phenol, a flavonoid compound, or a naturally occurring polyphenol derived from a woody plant, is useful as a cost-effective and environmentally safe wood preservative. The invention also provides a method for the use of such composition and compositions so treated.

BACKGROUND OF THE INVENTION
 Hardwoods constitute over one-third of the U.S. timber resource. However,
 with the exception of cross-ties, hardwoods are rarely treated for
 exterior use applications, and demand for treated hardwood products has
 until recently been low. In view of the projected softwood timber shortage
 and relative abundance of hardwoods, expanded use of treated hardwoods for
 both composite and solid wood products is expected. The problem is that
 replacing softwoods with hardwoods is not straightforward since most wood
 preservatives, including second generation biocides, are considerably less
 effective when used to treat hardwoods. (Nicholas, D. D., Proc. of the
 Northern Hardwood Resource: Management and Potential Conference, Houghton,
 Mich., Aug. 18-20 (1986); Preston, A. F., et al., Proc. Am. Wood
 Preservers' Assn., 79, 207 (1983)). This disparity is attributable to the
 considerably higher toxic threshold values obtained when treated hardwoods
 are attacked by white- and soft-rot fungi as compared to softwoods treated
 with the same biocide and exposed to brown-rot fungi. (Nicholas, supra.)
 Accordingly, the object of the present invention is to provide wood
 preservative systems that are effective in protecting hardwoods and
 providing greater efficacy for softwoods.
 An apparent solution to the problem of preserving hardwoods is to use
 substantially higher biocide levels, but this approach leads to higher
 costs and increased environmental risks. A more attractive solution would
 be to increase the efficacy of biocides for treating hardwoods.
 In addition, more environmentally benign preservatives to treat softwoods
 are needed, since all major wood preservatives used today to protect
 softwoods have perceived environmental problems.
 DESCRIPTION OF RELATED ART
 Prior to the present invention, antioxidants have been used in wood
 treatments of various kinds to stabilize the mixture from chemical
 decomposition or as a color stabilizer. For example, U.S. Pat. No.
 1,168,062 discloses the use of oxidation inhibitors as an additive in
 oil-in-water emulsions for use in wood preservatives which contain
 pentachlorophenol as the active ingredient. U.S. Pat. No. 3,889,020
 provides di-tert-butyl cresol (also called butylated hydroxytoluene, or
 BHT) as a stabilizer for pentachlorophenol-based preservatives wherein the
 cresol is intended to improve the surface color of treated poles.
 U.K. Patent Application GB 2,025,769A discloses the use of antioxidants,
 selected from such compound classes as sulfites, hydrosulfides, hydrazines
 and thiosemicarbazides. The purpose of the antioxidant there is to
 stabilize biocides from decomposition. U.S. Pat. No. 3,881,940 describes a
 composition containing an antioxidant stabilizer such as
 di-tert-butylcresol in a biocide comprising a heavy metal oxide and
 pentachlorophenol. The antioxidant served to prevent discoloration of wood
 and to prevent sludge formation during treating steps. U.S. Pat. No.
 4,400,298 teaches the combination of dithiocarbamate and a borate with an
 antioxidant stabilizing agent, e.g., potassium metabisulfite, for the
 prevention of fungal decay in wood. U.S. Pat. No. 4,783,221 describes wood
 preservatives containing an isothiazolone and metal salts of carboxylic
 acids, to which various additives are added including antioxidants. U.S.
 Pat. No. 5,462,589 discloses a synergistic wood preservative composition
 comprising copper and organic derivatives, of which antioxidants are
 recited as possible additives.
 Various biocides have also been used to preserve wood. U.S. Pat. No.
 4,977,186 teaches a composition for preserving wood comprising
 iodopropargyl biocides with a pyrethroid-type insecticide. U.S. Pat.
 No.5,634,967 teaches a composition for preserving wood comprising a
 synergistically effective amount of a cuprammonium compound and a triazole
 biocide.
 U.S. Pat. No. 5,730,907 (U.S. '907) teaches the combination of an
 antioxidant with three organic biocides such as a quaternary ammonium
 compound, e.g., didecyldimethyl ammonium chloride (DDAC), an
 isothiazolone, or an isophthalonitrile gave an enhanced effect in
 protecting a hardwood against a white-rot fungus, Irpex lateus. The patent
 also provides a method for the use of such composition and compositions so
 treated.
 For ecological and economic reasons, it is desirable to minimize the amount
 of biocide used to achieve a preservative effect. Accordingly, the
 addition of antioxidants in accord with the present invention should
 reduce the preservative retention levels required, and consequently,
 greatly improve the economics of both hardwood and softwood preservative
 systems, and thereby provide a more environmentally benign approach to the
 preservation of wood.
 SUMMARY OF THE INVENTION
 We have found the unexpected utility of adding antioxidants to selected
 commercial biocides to protect hardwoods and softwoods from fungal
 decomposition. Selected antioxidants significantly increase the activity
 and effectiveness of biocides to treat hardwoods and softwoods.
 Accordingly, the present invention provides a wood preservative composition
 comprising (a) at least one biocide, i.e., an iodopropargyl compound or a
 triazole compound, and (b) at least one antioxidant. The composition is
 especially effective when the antioxidant is a hindered phenol derivative.
 In addition to being highly effective, the wood preservative composition
 is environmentally safe and inexpensive to apply.
 None of the above biocides are taught by the U.S. '907 patent. The
 antioxidant and biocides combinations used in the present invention are
 effective against white-rot fungi, such as Trametes versicolor, to protect
 hardwoods, and brown-rot fungi, such as Gloeophyllum trabeum, to protect
 softwoods. Thus, the present invention greatly increases the utility of
 both hardwoods and softwoods in applications where the products are
 susceptible to biodeterioration. For example, to protect softwood lumber,
 which is used in residential construction and is susceptible to brown-rot
 fungi attacks.
 The antioxidant alone, such as butylated hydroxy toluene (BHT), has no
 effectiveness against either of the above fungi. The combination of an
 antioxidant and the foregoing organic biocides give greater efficacy than
 either component alone with a variety of woods, such as the hardwoods
 sweetgum and aspen, and the softwood southern yellow pine. Thus, the
 combination of the antioxidants and biocides of the present invention is
 synergistic against fungi.
 Of particular practical importance is the discovery that the low-cost
 antioxidant BHT, which is commonly used as a food additive, when combined
 with a biocide of the invention has an enhanced and synergistic biocidal
 effect with minimal environmental effects.
 DETAILED DESCRIPTION
 In one embodiment, the present invention provides a wood preservative
 composition comprising an effective amount of at least one biocide and an
 effective amount of at least one antioxidant; wherein the biocide is an
 iodopropargyl compound having the structure:
 ##STR1##
 wherein R.sub.1 is butyl, hexyl, cyclohexyl, or phenyl.
 Preferably, R.sub.1 is butyl (iodopropynylbutyl carbamate or IPBC).
 In another embodiment, the present invention also provides a wood
 preservative composition comprising an effective amount of at least one
 biocide and an effective amount of at least one antioxidant; wherein the
 biocide is a triazole compound containing a triazole group having the
 structure:
 ##STR2##
 Advantageously, the triazole compounds have the following formula:
 ##STR3##
 wherein R.sub.2 is a branched or straight chain C.sub.1-5 alkyl group
 (e.g., tert-butyl) and R.sub.3 is an unsubstituted phenyl group or a
 substituted phenyl group having one or more substituents, i.e., halogen
 (e.g., chlorine, fluorine or bromine), C.sub.1-3 alkyl (e.g., methyl),
 C.sub.1-3 alkoxy (e.g., methoxy), phenyl, or nitro group.
 Particularly preferred is a triazole compound of Formula A wherein R.sub.2
 is tert-butyl and R.sub.3 is 4-chlorophenyl (alpha-[2-(4-chlorophenyl)
 ethyl] -alpha(1, 1 -dimethylethyl)-1H-1,2,4-triazole-1 -ethanol, commonly
 known as Tebuconazole).
 Alternatively, the triazole compounds have the following formula:
 ##STR4##
 wherein R.sub.4 is an unsubstituted phenyl group or a substituted phenyl
 group having one or more substituents, i.e., halogen (e.g., chlorine,
 fluorine or bromine), C.sub.1-3 alkyl (e.g., methyl), C.sub.1-3 alkoxy
 (e.g., methoxy), or nitro group; and R.sub.5 is hydrogen or a branched or
 straight chain C.sub.1-5 alkyl group (e.g., propyl).
 Particularly preferred is a triazole compound of Formula B wherein R.sub.4
 is 2,4-dichlorophenyl and R.sub.5 is propyl
 ((1-[[2-(2,4-dichlorophenyl)-4-propyl-1,
 3-dioxolan-2-yl]methyl]-1H-1,2,4-triazole, commonly known as
 Propiconazole).
 The antioxidants used in the present invention are hindered phenolics
 having the structure:
 ##STR5##
 wherein R.sub.6, R.sub.7, and R.sub.8 are the same or different and are
 selected from hydrogen, halogen, methoxy, a C.sub.2-12 alkoxy, or a
 C.sub.1-12 alkyl group.
 The preferred hindered phenols used as antioxidants in the invention
 include the phenol shown above wherein R.sub.6 is methoxy or methyl, and
 R.sub.7 and R.sub.8 are both tert-butyl (butylated hydroxy toluene (BHT)
 or butylated hydroxy anisole (BHA)); and the phenol wherein R.sub.6 is
 hydrogen and R.sub.7 and R.sub.8 are both tert-butyl.
 Other antioxidants which may be used include dimers, trimers, or tetramers
 of the hindered phenols having the basic structure above, such as
 tetrakis[methylene(3, 5-di-tert-butyl-4-hydroxy hydro-cinnamate)] or
 4,4'-methylenebis (2,6-di-tert-butylphenol).
 Another class of antioxidants which are useful in the present invention are
 polyphenols, which include flavonoids and other naturally occurring
 polyphenols.
 The flavonoids have the following structure:
 ##STR6##
 wherein R.sub.9, R.sub.10, R.sub.11, R.sub.12, and R.sub.13 are the same or
 different and are hydrogen, hydroxyl, or a C.sub.1-12 alkoxy group; and
 wherein the dashed line represents a single or a double bond.
 The preferred flavonoid includes compounds wherein R.sub.9 and R.sub.11 are
 both hydroxyl and R.sub.10, R.sub.12, and R.sub.13 are all hydrogen, and
 wherein the dashed line signifies a double bond (quercetin). Examples of
 the preferred flavonoids are chrysin, luteolin, myrcetin, hespertin and
 rhamnetin.
 The naturally occurring polyphenols are those derived from woody plants.
 These antioxidants include, but are not limited to, tannins (or their
 isolated derivatives) and lignins (or their isolated derivatives), for
 example, kraft pulping lignin, lignin sulfonates, organosolve lignin,
 autohydrolysis lignin, acid-hydrolyzed lignin, and steam-exploded lignin.
 Tannins include quebracho, chestnuts, wattle, Pinus spp. bark condensed
 tannins, and the ellagitannins of chestnuts, oaks, and eucalyptus.
 Examples of the preferred tannins and their derivatives are quebracho,
 wattle, Pinus spp. bark condensed tannins, chestnuts and oaks. Examples of
 the preferred lignins and their derivatives are kraft, lignin sulfonates
 and organosolve lignin.
 The wood preservative composition disclosed herein may further comprise a
 liquid carrier medium such as a solvent and a suspending agent. Preferably
 the liquid carrier medium is a solvent such as water, ketones (e.g.,
 acetone), alcohols (e.g. methanol, ethanol), esters (e.g. ethyl acetate),
 aromatic hydrocarbons (e.g. toluene), paraffinic hydrocarbons (e.g.
 hexanes and mineral spirits), or halogenated hydrocarbons (e.g. methylene
 chloride). The suspending agent may be a foam or gel. The composition may
 alternatively be applied as a solution in miscible mixtures of solvents or
 as an emulsion in multiphasic media by conventional means known in the
 art.
 The composition of the invention may be provided not only as a diluted use
 solution but also as concentrates, emulsions, or suspensions of biocide.
 The composition may also be formulated without added solvent or diluent as
 a powder or pellets.
 If desired, conventional additives such as stabilizers, buffers,
 water-repellents, insecticides, pigments, odorants, coloring agents,
 surfactants, emulsifiers, flame-retardant compositions, and other
 additives may be added to the treating solution. The amount of such
 additives may vary over a range from about 0.01% to about 7% and
 preferably from about 0.01% to about 5% by weight.
 The amount of the biocidal composition used in the composition and method
 of the invention is a "biocidal effective amount," i.e., an amount
 effective to inhibit the growth of, or kill, one or more organisms that
 cause wood rot Such organisms include but are not limited to, Trametes
 versicolor (T versicolor) and Gloeophyllum trabeum (G. trabeum). In the
 wood preservative composition of the invention, the weight ratio of the
 biocide to the antioxidant is from about 0.001:1 to about 3:1, preferably
 from about 0.05:1 to about 1:1 The biocide may be present in an amount of
 from about 0.01 to about 10% by weight and the antioxidant in an amount of
 from about 0.10 to about 12% by weight.
 The wood is impregnated with the composition of the present invention by
 pressure-treating the wood with about 1 gallon of treating solution for
 each board-foot of lumber. About 20% to 50% of the solution is absorbed by
 the wood. Contact times of about 2 hours are typically used. Generally,
 1,000 board feet of lumber requires about 1,000 gallons of treating
 solution which is administered during a contact period of between about 1
 and about 6 hours.
 In yet another embodiment, the present invention also provides a method for
 preserving wood against destructive fungi which comprises contacting wood
 with a biocidally effective concentration of the wood preservative
 composition as defined above in a carrier liquid. Accordingly, the method
 permits preserving wood against at least one of the above fungi.
 Treatments of wood, e.g., lumber, timber, etc., can be carried out by
 conventional techniques including, but not limited to, dipping, spraying,
 brushing, pressure impregnation, and vacuum treatment. The length of the
 treatment time required will vary according to the treatment conditions,
 the selection of which is well known to those skilled in the art.
 In yet another embodiment, the present invention also provides wood
 preserved against destructive fungi by the method which comprises
 contacting the wood with a biocidally effective concentration of the wood
 preservative composition as defined above.
 Throughout this application, various references are cited within
 parentheses. These publications are hereby incorporated by reference to
 more fully describe the state of the art to which this invention pertains.
 This invention will be better understood from the Examples which follow.
 However, one skilled in the art will readily appreciate that the specific
 methods and results discussed herein are merely illustrative of the
 invention as described more fully in the claims which follow thereafter.
 Unless otherwise indicated, all parts and percentages in the Examples and
 the present specification are by weight.
 To show that free radical scavengers (primary antioxidants) will increase
 the efficacy of commercial biocides used as wood preservatives, two tests
 are performed: (1) agar-block test as shown in Example 1; and (2)
 soil-block test as shown in Example 2 These tests are defined in the
 Examples below.
 The wood used in the agar- and soil-block tests includes aspen sapwood and
 southern yellow pine sapwood, respectively, selected according to AWPA
 standard E 10-91. The white-rot fungi examined was Trametes versicolor
 (ATCC# 12679) and the brown-rotter was Gloeophyllum trabeum (ATCC# 11539).
 Samples treated with the antioxidant alone, or biocide alone at four
 different retention levels, were tested and the degree of fungi attack
 were compared with samples treated with the combination of antioxidant and
 biocide at the same retention levels. Two sets of untreated samples were
 also tested in each experimental set as controls, i.e., control set and
 container control. Throughout this experiment, the term "treated samples"
 means samples exposed to either biocide alone or biocide mixed with
 antioxidants, as opposed to the term "untreated samples".
 Each experimental set of a given retention level of biocide and/or
 antioxidant was performed with five replicate wood samples. These treated
 samples were placed into two cups, which contained the appropriate agar or
 soil, for incubation. Three samples were placed into one cup and the
 remaining two samples were placed into the second cup along with an
 untreated sample. The untreated sample placed into the second cup is
 called a "container control". Thus each set consists of five replicate
 treated samples of a given retention level of biocide and/or antioxidant
 and one additional untreated sample, which serves as a control. As a
 separate experiment, five untreated samples were placed into two cups as
 described above to provide the average strength loss data of the untreated
 samples. These five untreated samples are called the "control set".
 The average retentions are reported in pounds of preservative per cubic
 foot of wood (pcf), as is normal in the wood preservative industry. The
 biocide effectiveness is determined by percent compression strength loss
 of the sample, as tabulated below. In certain cases, the average
 retentions represent the toxic threshold value of such biocide.
 "Threshold" is defined by the American Wood Preservers' Association
 Standards as the minimum amount of preservative that is effective in
 preventing significant wood decay, under the conditions of the test, by a
 particular fungus. This amount of preservative, expressed in kg/m.sup.3 or
 pcf, is referred to as the "threshold retention". To establish the
 threshold value, the wood must be treated with a high enough concentration
 of biocide, to completely inhibit fungal degradation. As explained
 previously, the object of this experiment is not to increase the
 concentration of the biocide but to increase its efficiency. Thus, for the
 purposes of this experiment, only four different concentrations of the
 antioxidant/biocide combinations were run against both white-rot and
 brown-rot fungi, and as a result, the toxic threshold may not be reached.
 In these cases, the toxic threshold value of the preservative is higher
 than the tested highest concentration of the biocide without the
 antioxidant and appears to be lower than or equal to the tested lowest
 concentration of the biocide with the antioxidant. Even when the toxic
 threshold is not reached, the data set forth below show that the
 antioxidant/biocide combination at a given concentration is more effective
 than the biocide alone at the same concentration.

EXAMPLE 1
 Agar Block Test
 Wood sticks of aspen sapwood (Populus spp.) were cut into thin 10 block
 samples of 5 mm.times.19 mm.times.19 mm. These 10 samples were paired up
 into 5 matching-paired samples and treated with 5% of BHT alone (see Table
 I). Additional aspen samples were treated with one of the following
 biocides: Propiconazole (see Table IIa), Tebuconazole (see Table IIb), or
 IPBC (see Table III), with and without 5% added BHT antioxidant. The
 solvent used was toluene. In addition, two sets of untreated controls,
 labeled the container controls and the control set as described above,
 were also run. One sample of each of the five treated matching-paired
 samples were exposed to the white-rot fungus T. versicolor in the agar
 block test described by (Archer, K, et al., For. Prod. J 45(1) 86-89,
 (1995)). The incubation period was four weeks. The samples were then
 saturated with water to above their fiber saturation point before their
 degree of white rot attacks was determined. The degree of attack was
 determined by measuring the compression strength of each of the five
 samples exposed to the fungus relative to the compression strength of its
 matched and treated, but not fungus-exposed, paired sample. The biocide
 effectiveness is then determined and the results are reported in percent
 compression strength loss of the samples in the following tables.
 The results for the antioxidant (5% BHT) alone in the agar-block test are
 shown in Table I below.
 TABLE I
 Antioxidant/Average Retention (pcf) Avg % Strength Loss .+-. Std. Dev.
 BHT/1 .507 176.85 .+-. 18.07
 Control Set 87.97 .+-. 6.82
 Container Control 95.58 .+-. 3.18
 The data in Table I clearly show that the use of 5% BHT provides
 essentially no protection against fungal attack as compared to the
 untreated control samples, with all sets losing 70% or more strength.
 The results for Propiconazole and Tebuconazole are shown in Tables II(a)
 and II(b) below, with and without an antioxidant. The symbol "***"
 signifies the approximate toxic threshold values, as defined above, for
 these biocides with and without an antioxidant. A definite toxic threshold
 value was not established for the Propiconazole/BHT combination, but the
 toxic threshold value appears to be less than or equal to 0.004 pcf.
 TABLE II(a)
 Biocide/Avg Ret. (pcf);
 Antioxidant/Avg Ret. (pcf) Avg % Strength Loss .+-. Std. Dev.
 Propiconazole/0.049; None 1.89 .+-. 4.36
 Propiconazole/0.023; None 1.38 .+-. 6.38 ***
 Propiconazole/0.012; None 42.40 .+-. 18.77
 Propiconazole/0.004; None 89.95 .+-. 3.55
 Propiconazole/0.044; BHT/1.825 1.96 .+-. 3.05
 Propiconazole/0.022; BHT/1.855 -1.37 .+-. 3.05
 Propiconazole/0.011; BHT/1.756 9.43 .+-. 16.63
 Propiconazole/0.004; BHT/1.830 -2.73 .+-. 5.71
 Control Set 97.04 .+-. 3.14
 Container Control 75.10 .+-. 21.01
 TABLE II(a)
 Biocide/Avg Ret. (pcf);
 Antioxidant/Avg Ret. (pcf) Avg % Strength Loss .+-. Std. Dev.
 Propiconazole/0.049; None 1.89 .+-. 4.36
 Propiconazole/0.023; None 1.38 .+-. 6.38 ***
 Propiconazole/0.012; None 42.40 .+-. 18.77
 Propiconazole/0.004; None 89.95 .+-. 3.55
 Propiconazole/0.044; BHT/1.825 1.96 .+-. 3.05
 Propiconazole/0.022; BHT/1.855 -1.37 .+-. 3.05
 Propiconazole/0.011; BHT/1.756 9.43 .+-. 16.63
 Propiconazole/0.004; BHT/1.830 -2.73 .+-. 5.71
 Control Set 97.04 .+-. 3.14
 Container Control 75.10 .+-. 21.01
 The data in Tables II(a) and II(b) clearly show that the addition of small
 amounts of BHT produced a substantial increase in the activity of both
 biocides (less strength loss). For example, adding as little as 1.867 pcf
 BHT reduced the toxic threshold values of Tebuconazole from about 0.023
 pcf to about 0.011 pcf. Since BHT alone (see Table I) has no activity
 against wood strength loss and Tebuconazole alone was less active against
 wood strength loss than the combination of Tebuconazole and BHT, this
 shows that the combination of biocide and antioxidant is synergistic.
 The results for IPBC are shown in Table III below, with and without an
 antioxidant. The symbol "***" signifies the toxic threshold values for
 IPBC in each set of runs with and without an antioxidant.
 TABLE III
 Biocide/Avg Ret. (pcf);
 Antioxidant/Avg Ret. (pcf) Avg % Strength Loss .+-. Std. Dev.
 IPBC/0.052; None 0.061 .+-. 1.82
 IPBC/0.027; None 3.062 .+-. 6.90 ***
 IPBC/0.012; None 41.188 .+-. 33.30
 IPBC/0.004; None 77.696 .+-. 12.31
 IPBC/0.051; BHT/1.973 3.060 .+-. 3.60
 IPBC/0.027; BHT/1.922 0.503 .+-. 6.95
 IPBC/0.012; BHT/1.936 2.086 .+-. 2.19 ***
 IPBC/0.004; BHT/1.886 40.12 .+-. 52.16
 Control Set 86.36 .+-. 1.98
 Container Controls 95.06 .+-. 4.76
 The data in Table III clearly show that the addition of small amounts of
 BHT produced a substantial increase in the activity of IPBC (less strength
 loss). For example, adding as little as 1.886 pcf BHT reduced the toxic
 threshold values of IPBC from about 0.027 pcf to about 0.012 pcf Since BHT
 alone (see Table I) has no activity against wood strength loss and IPBC
 alone has less activity against wood strength loss than the combination of
 IPBC and BHT, this again shows that the combination of biocide and
 antioxidant is clearly synergistic.
 EXAMPLE 2
 Soil Block Test
 Wood sticks of southern yellow pine (Pinus spp.) sapwood blocks, cut in the
 same manner as described in Example 1, were treated with 5% of the
 antioxidant BHT alone (see Table IV). Additional southern yellow pine
 samples were treated with one of the following biocides: Propiconazole
 (see Table Va), Tebuconazole (see Table Vb), and IPBC (see Table VI), with
 and without 5% added BHT. In addition, two sets of untreated controls,
 labeled the container controls and the control set as described above were
 also run. One sample of each of the five treated matching-paired samples
 were exposed to the brown-rot fungus G. trabeum in the soil-block test
 described by (Archer, K., et al, For. Prod J. 45(1), 86-89, (1995)). The
 incubation period was four weeks. The samples were then saturated with
 water to above their fiber saturation point before the degree of brown rot
 degradation was determined. The degree of attack was determined as
 described in example 1.
 The results for the antioxidant (5% BHT) alone in the soil-block test are
 shown in Table IV below.
 TABLE IV
 Antioxidant/Average Retention (pcf) Avg % Strength Loss .+-. Std. Dev.
 5% BHT/1.597 93.27 .+-. 1.35
 Untreated Control 96.53 .+-. 0.65
 Container Controls 96.25 .+-. 0.90
 The data in Table IV clearly show that the use of 5% BHT provides
 essentially no protection against fungal attack as compared to the
 untreated control samples, with all sets losing 90% or more strength.
 The results for Propiconazole and Tebuconazole are shown in Tables V(a) and
 V(b) below, respectively, with and without an antioxidant. The
 Tebuconazole samples were incubated for a total of 47 days because of
 limited fungicidal degradation at 25 days. No toxic threshold values for
 Propiconazole were established with or without BHT, which signifies that
 the toxic threshold value for Propiconazole is greater than 0.044 pcf.
 TABLE V(a)
 Biocide/Avg Ret. (pcf);
 Antioxidant/Avg Ret. (pcf) Avg % Strength Loss .+-. Std. Dev.
 Propiconazole/0.042; None 30.82 .+-. 6.67
 Propiconazole/0.021; None 69.90 .+-. 16.80
 Propiconazole/0.011; None 93.92 .+-. 1.50
 Propiconazole/0.004; None 95.53 .+-. 0.77
 Propiconazole/0.044; BHT/1.82 12.69 .+-. 5.14
 Propiconazole/0.021; BHT/1.78 30.55 .+-. 6.18
 Propiconazole/0.011; BHT/1.82 41.14 .+-. 2.81
 Propiconazole/0.004; BHT/1.79 39.10 .+-. 2.23
 Control Set 96.67 .+-. 0.50
 Container Control 98.35 .+-. 2.12
 TABLE V(a)
 Biocide/Avg Ret. (pcf);
 Antioxidant/Avg Ret. (pcf) Avg % Strength Loss .+-. Std. Dev.
 Propiconazole/0.042; None 30.82 .+-. 6.67
 Propiconazole/0.021; None 69.90 .+-. 16.80
 Propiconazole/0.011; None 93.92 .+-. 1.50
 Propiconazole/0.004; None 95.53 .+-. 0.77
 Propiconazole/0.044; BHT/1.82 12.69 .+-. 5.14
 Propiconazole/0.021; BHT/1.78 30.55 .+-. 6.18
 Propiconazole/0.011; BHT/1.82 41.14 .+-. 2.81
 Propiconazole/0.004; BHT/1.79 39.10 .+-. 2.23
 Control Set 96.67 .+-. 0.50
 Container Control 98.35 .+-. 2.12
 The data in Tables V(a) and V(b) clearly show that the addition of small
 amounts of BHT produced a substantial increase in the activity of both
 biocides (less strength loss). For example, adding as little as 1.6 pcf
 BHT reduced the toxic threshold values of Tebuconazole from about 0.020
 pcf to less than or equal to 0.003 pcf. Since BHT alone (see Table I) has
 no activity against wood strength loss and Tebuconazole alone has less
 activity against wood strength loss than the combination of Tebuconazole
 and BHT, this clearly shows that the combination of biocide and
 antioxidant is synergistic.
 The results for IPBC are shown in Table VI below, with and without an
 antioxidant. The symbol "***" signifies the toxic threshold values for
 IPBC with and without an antioxidant. A toxic threshold value was not
 established for IPBC alone, which signifies that without BHT, the toxic
 threshold value of IPBC is greater than 0.045 pcf.
 TABLE VI
 Biocide/Avg Ret. (pcf);
 Antioxidant/Avg Ret. (pcf) Avg % Strength Loss .+-. Std. Dev.
 IPBC/0.045; None 48.708 .+-. 31.26
 IPBC/0.024; None 56.347 .+-. 19.62
 IPBC/0.010; None 66.841 .+-. 21.23
 IPBC/0.003; None 74.919 .+-. 31.75
 IPBC/0.044; BHT/1.689 4.991 .+-. 5.94 ***
 IPBC/0.023; BHT/1.611 8.435 .+-. 4.17
 IPBC/0.010; BHT/1.712 68.592 .+-. 24.38
 IPBC/0.003; BHT/1.662 87.68 .+-. 3.26
 Untreated Control 91.56 .+-. 18.87
 Container Controls 87.20 .+-. 19.67
 The data in Table VI clearly show that the addition of small amounts of BHT
 produced a substantial increase in the activity of IPBC (less strength
 loss). For example, adding as little as 1.611 pcf BHT reduced the toxic
 threshold values of IPBC from greater than 0.045 pcf to about 0.044 pcf.
 Since BHT alone (see Table I) has no activity against wood strength loss
 and IPBC alone has less activity against wood strength loss than the
 combination of IPBC and BHT, this clearly shows that the combination of
 biocide and antioxidant is synergistic.
 Discussion
 These experiments show that the activity of low concentrations of biocides
 used as hardwood and softwood preservatives can be significantly increased
 by adding antioxidants by the methods disclosed herein. Specifically, the
 addition of an antioxidant to biocides results in a significantly more
 active wood preservative.
 Variations of the present invention will suggest themselves to those
 skilled in the art, and are within the scope of the following claims.