EFFICACY OF ALTERNATIVE COPPER-BASED PRESERVATIVES IN 2 PROTECTING DECKING FROM BIODEGRADATION

1  DOI:10.4067/S0718-221X2020005XXXXXX 1 EFFICACY OF ALTERNATIVE COPPER-BASED PRESERVATIVES IN 2 PROTECTING DECKING FROM BIODEGRADATION 3 4 Stan T. Lebow, Katie M. Ohno, Patricia K. Lebow, Michael H. West 5 1 USDA, Forest Service, Forest Products Laboratory, Madison, WI, USA. 6 2 Delta Research, Senatobia, MS, USA, deceased. 7 8 Corresponding author: stan.lebow@usda.gov 9 Received: December 10, 2019. 10 Accepted: February 19, 2020. 11 Posted online: February 20, 2020 12 13 ABSTRACT 14 15 The above-ground performance of decking treated with two alternative copper-based 16 preservative formulations is being evaluated at a test site near Madison, Wisconsin, USA. 17 Southern pine sapwood lumber specimens (38 mm by 140 mm by 910 mm) were pressure treated 18 with 0,93 %, 1,40 % or 2,34 % (oxide basis) actives concentrations of a boron-copper 19 formulation (BC) composed of 7,2 % copper hydroxide and 92,8 % sodium tetraborate 20 decahydrate. Similar specimens were pressure-treated with 0,66 % or 1,32 % actives 21 concentrations of a copper-zinc formulation (CZDP) composed of 18 % copper (CuO basis), 12 22 % zinc (ZnO basis), 14 % dimethylcocoamine and 56 % propanoic acid. In both cases untreated 23 specimens and specimens treated with a 1% concentration of chromated copper arsenate Type C 24 (CCA-C) were included for comparison. The specimens were installed on racks approximately 25 760 mm above the ground and periodically evaluated for extent of fungal decay and surface 26 microbial growth. After 18 years in test specimens treated with the lowest solution concentration 27 of BC (0,93 %) suffered substantial degradation and all but three replicates have failed. Obvious 28 decay has not yet been detected in specimens treated to the highest BC concentration (2,34 %), 29 but decay is suspected in one of these specimens. Decking specimens treated with CZDP 30 exhibited no evidence of decay until year 17 when a fruiting body was observed on one specimen 31 treated with a 0,66 % solution concentration. There has been no evidence of decay in specimens 32 treated with 1,32 % CZDP or in either set of specimens treated with 1 % CCA-C. Both BC and 33 CZDP-treated specimens were at least as effective as 1 % CCA-C in minimizing noticeable 34 surface microbial growth. These decking studies confirm that relatively low copper 35 concentrations can provide substantial protection for decking exposed in a moderate climate, and 36 that the CZDP formulation is potentially more effective than the BC formulation. However, 37 caution is warranted in extrapolating these findings to more severe climates and to construction 38 designs that are more likely to trap moisture. 39 40

% zinc (ZnO basis), 14 % dimethylcocoamine and 56 % propanoic acid. In both cases untreated 23 specimens and specimens treated with a 1% concentration of chromated copper arsenate Type C 24 (CCA-C) were included for comparison. The specimens were installed on racks approximately 25 760 mm above the ground and periodically evaluated for extent of fungal decay and surface 26 microbial growth. After 18 years in test specimens treated with the lowest solution concentration 27 of BC (0,93 %) suffered substantial degradation and all but three replicates have failed. Obvious 28 decay has not yet been detected in specimens treated to the highest BC concentration (2,34 %), 29 but decay is suspected in one of these specimens. Decking specimens treated with CZDP 30 exhibited no evidence of decay until year 17 when a fruiting body was observed on one specimen 31 treated with a 0,66 % solution concentration. There has been no evidence of decay in specimens 32 treated with 1,32 % CZDP or in either set of specimens treated with 1 % CCA-C. Both BC and

47
There is continued interest in development of economical, effective and low toxicity wood 48 preservative formulations, particularly for residential applications where the majority of treated 49 wood is used above-ground. Copper has been used in wood preservatives for over a century and 50 remains a common component in current formulations (Freeman and McIntyre 2008). Unlike 51 carbon-based preservatives, copper is not biodegraded and retains is efficacy for long periods. 52 However, copper is not effective against all types of fungi. Some mold/stain fungi can grow on 53 copper-treated wood, and certain types of decay fungi are classified as "copper-tolerant" (Ohno 54 et al. 2015(Ohno 54 et al. , 2016. These fungi can sporadically cause severe and rapid damage in wood treated 55 with copper, and thus commercial copper-based preservatives typically include a co-biocide such 56 as arsenic, quaternary ammonium compounds, triazoles, or naphthenic acids to provide 57 additional protection. Zinc has also been widely used as a component of wood preservative 58 formulations. Zinc chloride was commonly used for pressure-treatment of railroad ties in the 59 early 1900's but provided only a moderate increase in durability. Zinc-meta-arsenite was used 60 for treatment of poles and timbers in the late 1920's through the 1930's and was at one time 61 standardized by the American Wood Protection Association (Lebow and Anthony 2012).

62
Chromated zinc chloride was standardized by the AWPA until 1992, when it was removed from 63 the standards for lack of use. Combinations of zinc and copper have also been evaluated (Rak 64 and Unligl 1978) and a formulation of ammoniacal copper zinc arsenate (ACZA) is currently 65 standardized for treatment of poles, piles and timbers in the United States (AWPA 2019). In 66 general, zinc is a less effective fungicide than copper, but when used at sufficient concentrations 67 it provides adequate protection, especially for applications out of ground contact (Barnes et al. 68 2004; DeGroot and Stroukoff 1998). Zinc solutions have the advantages of being colorless, and in causing less corrosion to metal fasteners than copper-based formulations. Boron is a third 70 element commonly used in wood preservatives. Borates have relatively low toxicity to humans 71 and the environment and are effective in preventing attack by decay fungi and termites (Ahmed 72 et al. 2004;Drysdale 2004, Manning 2008. Unfortunately, borates are not chemically bound to 73 the wood, and thus can be leached out of the wood in external exposures. This disadvantage can 74 be partially overcome by selection of appropriate end-use applications, and by the incorporation 75 of a less-leachable co-biocide. It should be noted that in some applications the solubility of boron 76 can also be an advantage because it allows the boron to diffuse more deeply into untreated wood.

77
Treatment by diffusion plays a key role when boron is applied externally to the ground line area  and is effective in protecting wood against decay and termites in laboratory and covered ground 91 proximity tests Lebow et al. 2005b;Lebow et al. 2006;Woodward et al. 2002). However,

92
Maderas-Cienc Tecnol 22(3):2020 Ahead of Print: Accepted Authors Version 4 laboratory leaching tests also indicated that 45 % -55 % of the boron and 5 % -6 % of copper 93 was depleted from specimens during exposure to 750 mm of simulated rainfall (Lebow et al. 94 2009). This suggests that boron could be largely depleted from wood exposed to precipitation 95 after a few years in areas with moderate rainfall or even more rapidly in wet climates.

97
Other aspects of adapting BC to use as a pressure treatment preservative, particularly for wood 98 exposed outdoors, have not been fully evaluated. Permanence of the boron and copper in the 99 treated wood is a consideration and it is expected that the type of outdoor exposure (above-100 ground versus ground-contact) will affect both efficacy and permanence. Stake tests installed in fungi, and there is some evidence that there is less risk of degradation by copper-tolerant fungi in 128 treated wood used above ground (Choi et al. 2002). This paper reports on continuing research to 129 evaluate the potential use of BC and CZDP treatments for above-ground applications such as 130 decking. Pressure-treated decking specimens were exposed at a test site near Madison,

131
Wisconsin and were periodically evaluated during exposure for either 18 years (BC) or 17 years 132 (CZDP).

134
Decking specimens (0,91 m in length) were prepared from 38 mm by 140 mm (2 in. by 6 in. 135 nominal) southern pine lumber free of heartwood, large knots or other large defects. The 136 specimens were then conditioned to constant weight in a room maintained at 74° F (23° C) and 137 65 % relative humidity prior to pressure treatment. This procedure was followed for both the BC and CZDP decking evaluations, but the studies were initiated a year apart. Twelve or ten 139 replicate specimens per treatment group were used for the BC and CZDP evaluations, 140 respectively.

142
Three BC solution concentrations (0,93 %, 1,40 % or 2,34 %, oxide basis) were used for pressure 143 treatment (see Table 1 for borate and copper concentrations). An additional set of specimens 144 was treated with a formulation of 1,4 % BC plus 10 % sodium silicate to evaluate whether the 145 silicate would impart water repellency, potentially minimizing checking and/or slowing the 146 leaching of borate from the wood. Two CZDP solution concentrations (0,66 % and 1,32 %, oxide 147 basis) were evaluated (Table 2). A set of specimens treated with a 1 % chromated copper 148 arsenate (CCA-C) solution was also included in each test to serve as a positive control (Tables 1,   149 2).

151
All treatments were conducted using a full-cell pressure process. The initial vacuum was 152 maintained at -75 kPa (gauge) for 30 min; the pressure was maintained at 1,03 MPa (gauge) for 1 153 hour. The 2,34 % BC treatment was conducted with a solution heated to approximately 50 ºC to 154 improve solubility, while the remaining BC, CZDP and CCA treatments were conducted at room 155 temperature. Each specimen was weighed before and after treatment to determine solution 156 uptake and allow calculation of uptake retention (Tables 3, 4). It was noted that the sodium 157 silicate interfered with solution uptake, causing a lower preservative retention in the 1,4 % BC 158 plus sodium silicate specimens.    The specimens were periodically evaluated for decay and surface microbial growth 172 (mold/mildew). Decay ratings were assigned by visually inspecting all sides of the specimens as 173 well as gently prodding the end-grain and other areas for evidence of softening. A five-point 174 rating scale (4, 3, 2, 1, 0) was used to express the extent of decay. In our experience the 175 commonly used 10, 9, 8, 7, 6, 4, 0 rating criteria described for many AWPA evaluation standards 176 (AWPA 2019) is not readily applicable to above-ground decking tests in moderate climates. The

177
AWPA ratings correspond to percent of cross-section decayed, but because decay in decking 178 specimens develops internally the percent of cross-section affected cannot be determined without 179 destroying the specimen. We adapted the decay rating system to correspond to examination of 180 the outside of the specimens. ratings were assigned as 4 (no evidence of decay); 3 (decay 181 suspected); 2 (obvious decay); 1 (severe decay); or 0 (easily broken along or across the grain).

182
The presence of even a single decay fungus fruiting body on any surface was considered obvious 183 decay.

184
The presence of surface mold or mildew did not influence the decay rating unless it was 185 accompanied by other signs of decay. However, because surface appearance can be an important 186 component of how consumers view decking durability, the non-decayed specimens were also 187 visually evaluated for surface microbial growth. Only the upper surface that would be visible to 188 the consumer was evaluated for appearance. The microbial growth evaluation was a binary "yes" 189 or "no" response to the question of whether a consumer might find that extent of growth 190 noticeable and objectionable. Both area of microbial coverage and visibility (generally darkness) 191 were considered. It is recognized that this is a subjective assessment.  (Table 3). Not surprisingly, no evidence of decay has 205 been observed in the specimens treated with the 1 % CCA-C solution. Surface microbial growth was poorly correlated with preservative type or retention. All 210 specimens exhibited some degree of mold growth, with frequency increasing over time ( Figure   211 2). Specimens treated with the highest (2,34 %) BC concentration appeared to have slightly less 212 surface growth than those treated with 1 % CCA at 18 years but both treatments did allow 213 substantial growth. The sodium silicate addition to BC appeared to lessen the extent of surface 214 discoloration to some extent but this was difficult to evaluate in later stages of the study because Although the CZDP treated decking specimens reported in this study have been relatively decay 254 resistant, stake specimens exposed in Mississippi (Lebow et al. 2012) were less durable (Table   255 5). At least one failure occurred in 4 or 5 years in stakes treated with equivalent CZDP 256 formulations, although it did take several more years for decay to be observed in 25 % of the 257 stakes treated with the 1,32 % CZDP concentration. It is likely that early failures in CZDP 258 treated stakes were caused by a copper-tolerant fungus which has been frequently observed at the 259 Mississippi test site (Lebow et al. 2012). That copper-tolerant fungus has not been reported in 260 the Wisconsin decking specimens, perhaps because copper-tolerant fungi are thought to be less 261 capable of degrading copper-treated wood above-ground (Choi et al. 2002;Ohno et al. 2015).  The laboratory leaching study indicates that much of the boron would have been depleted from 265 the BC decking within a few years, leaving copper as the only biocide. The BC specimens appear 266 to confirm that relatively low copper concentrations can provide substantial protection for 267 decking exposed in a moderate climate. Although studies have indicated that residential decks  However, it should be noted that the specimen configuration in these decking tests did not 273 incorporate moisture-trapping features that might be encountered in some types of construction.

274
The decking specimens are also exposed in an open area with full sun which likely promotes 275 drying after rainfall events and minimized accumulation of leaf litter or other organic detritus 276 that might contribute to the decay hazard. Thus, wood in use above-ground might be subjected to 277 decay hazards more severe than those in this study. The results also highlight the great difference in above ground and ground contact decay hazard. This is perhaps most evident with the BC 279 formulation which failed rapidly in ground contact at all retentions.

280
The CZDP formulation appeared to have the potential to be more effective than the BC