Abstract

Borate preservatives are used in a wide range of applications to provide protection against fungal and insect attack. One such application is the treatment of framing lumber used in interior construction. Pressure treatment of framing lumber with borates has become accepted practice, and research has demonstrated that these treatments result in deep penetration of boron into the wood (Morrell and Lebow 1991; Morris et al. 1996, 1997; Baker et al. 2001; Lebow et al. 2005). Borate formulations may also be spray applied to framing lumber during the construction process. Some degree of boron penetration (diffusion) into the framing lumber is expected from these treatments, but the extent of penetration is poorly quantified. Much of the past research on boron diffusion following nonpressure treatments has been conducted with green lumber and typically involved an immersion period followed by a controlled diffusion period (Smith and Williams 1969, Fowlie et al. 1988, Barnes et al. 1993, Puettmann and Schmidt 1997, Wang et al. 2007). The conditions for boron diffusion are likely to be less favorable when borate solutions are sprayed onto framing lumber. It is typically recommended that framing lumber be dried to below 19 percent moisture content (Bergman 2010) to minimize concerns with shrinkage and other moisture-related problems. Although measured moisture contents often exceed 19 percent (Garrahan et al. 1991), the moisture content of framing lumber is still likely to be substantially lower than that of green lumber treated by traditional immersion/diffusion processes. The concentration of boron applied to the wood surface with a spray application may also be lower than that applied by an immersion treatment. In some cases, additives such as glycol are used to thicken the borate solutions and potentially allow higher surface loadings, but the benefit of these additives is also poorly quantified.

A recent evaluation of the ability of surface-applied borates to penetrate framing lumber found that diffusion can occur in wood maintained at 18 to 20 percent moisture content (Lebow et al. 2010), suggesting that diffusion may be possible in some framing applications. The finding of diffusion occurring at 18 to 20 percent moisture content conflicts with the assumption that diffusion will cease at moisture contents below the fiber saturation point (Becker 1976). However, Schoeman et al. (1998) reported up to 4 and 6 mm of boron penetration in southern pine specimens dipped in a 5 percent disodium octaborate tetrahydrate (DOT) solution and maintained in conditions that produced approximately 20 and 25 percent moisture content, respectively. Based on specimens exposed in other conditions, they concluded that some diffusion could occur at wood moisture contents as low as 15 percent. Morrell and Freitag (1995) also observed borate diffusion in wood with 20 percent moisture content, although in that study additional moisture was provided because the borate solutions were added to shallow wells machined into the wood. In a study of the use of coatings to minimize boron depletion from treated wood exposed outdoors, researchers concluded that boron leaching and thus diffusion occurred at moisture contents of 18 to 20 percent (Peylo and Willeitner 1995). In contrast, a study of diffusion of boric acid from adhesive found that although diffusion did occur in specimens conditioned at 100 percent relative humidity (RH) and 25°C, it was minimal at 88 percent RH and 25°C (Lesar et al. 2011). Moisture content was not reported in that study, but these conditions might have been expected to produce equilibrium moisture contents of approximately 19 and 25 percent, respectively. Researchers in New Zealand also reported limited boron penetration when a glycol-borate solution was brushed onto the exterior surface of multiple stud framing units but noted that penetration in studs and nail-laminated lintels could be improved with a combination of brushing and internal injection applications (Page and Singh 2011, Simpson et al. 2012). Wood moisture content was not reported in those studies.

The moisture content groups evaluated in Lebow et al. (2010) were achieved by pressure treating the wood with water and then gradually drying to the target moisture content. Although this approach may simulate a situation where framing lumber is installed at elevated moisture contents and equilibrates in place, it less closely simulates the adsorption that occurs when dry framing lumber is exposed to high humidity, such as in a crawl space. Wood typically equilibrates to a slightly lower moisture content during adsorption than desorption, and it is possible that at low moisture contents, even slight moisture content differences could have an effect on boron diffusion.

For in-place applications, borates are generally available as powdered solids, such as DOT, that are then dissolved in water or as liquids formulated with glycol to aid borate solubility. Lebow et al. (2010) did not find any significant difference in penetration between two glycol-borate formulations and a solution of DOT in water. This latter finding conflicts with previous research (Vinden et al. 1990, Puettmann and Williams 1992, Wang et al. 2007) and the commonly held assumption that the thickened borates allow for greater subsequent boron diffusion. Two aspects of the Lebow et al. (2010) methodology may have contributed to lack of observed glycol effect on boron penetration. First, the DOT concentration in all treatments was limited to 15 percent to ensure DOT solubility. In contrast, thickened glycol-borates are often applied commercially at a 23 percent DOT concentration. Second, the formulations in Lebow et al. (2010) were applied by briefly dipping the specimens in each liquid. This dipping technique was used to reduce variability in application of the formulations but may have resulted in greater solution absorption than typically occurs in spray applications. This greater solution absorption may have helped to obscure differences between the formulations.

The objective of this follow-up study was to further evaluate boron penetration from surface-applied DOT and glycol-borate formulations applied to framing lumber during construction. In this follow-up study, the target moisture content in all but one set of samples was achieved by adsorption rather than desorption, and the DOT concentration of the thickened glycol-borates was increased to the strongest concentration currently used commercially. The borate solutions were also applied by spraying rather than by brief immersion to more closely simulate commercial practice.

Materials and Methods

Data analysis

The penetration measurements were analyzed as randomized block designs with nested repeated measurements over the diffusion period using the mixed procedure in SAS (Version 9.2, SAS Institute Inc., Cary, North Carolina). Unstructured covariance matrices were assumed for both penetration depth and cross-section percent based on likelihood ratio tests and other model criteria, thus allowing for some heterogeneity and unequal correlations between repeat measurements. Within each diffusion period, initial overall tests for any differences were significant (<0.05). These were followed by the pairwise means comparisons within each diffusion period (Table 1). The change in extent of penetration over time was also of interest in this study. Because the statistical analysis did not detect interaction between treatment groups and diffusion period, the penetration measurements for all six treatment groups were combined for an overall test of changes in diffusion rates (Table 2).

Table 1. Summary of penetration measurements and moisture content for each diffusion period.^a

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Table 2. Statistical tests to determine if the extent of boron diffusion into the specimens increased over time.

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The boron assay concentrations were analyzed as a split-block design (also known as a strip plot) with factors for treatment and zone using the glimmix procedure in SAS version 9.2. Variability appeared to increase with increasing concentrations and models that allowed for this heterogeneity were investigated. In particular, the model based on logarithmic concentrations appeared to fit the data the best (i.e., concentrations appeared lognormally distributed). With this model, comparisons of treatment medians within each zone were performed (this model compares means on the transformed scale and then transforms back to the original scale to give comparisons of medians on the original scale). Within each assay zone, initial overall tests for any differences were significant (<0.05). These were followed by the pairwise medians comparisons within each zone (using the simulation method for multiple comparison adjustments).

Results and Discussion

Boron penetration increased during the diffusion period for all treatment groups (Table 1; Fig 1). Statistical analysis confirmed that both depth of penetration and percent cross section penetrated increased significantly between 6 and 13 weeks and again between 13 and 26 weeks (Table 2). As shown in Table 2, the rate of diffusion (millimeters per week or percentage of cross section per week) did not statistically decrease during the 13- to 26-week period, indicating that diffusion would be expected to continue under these wood moisture content conditions. There was no consistent or obvious difference in radial versus tangential penetration, although on average penetration was slightly greater on the wide than on the narrow faces.

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Possible diffusion penetration differences as a function of formulation and concentration warrant closer examination. One of the objectives of this study was to evaluate the effect of the glycol in formulations GB1 and GB2 on diffusion during storage. This effect can be evaluated by comparing the penetration of GB1 and GB2 at 15 percent DOT to that of the nonglycol 15 percent DOT at equivalent moisture content. As shown in Table 1, boron penetration in specimens treated with 15 percent GB1 or 15 percent GB2 was not significantly greater than in those treated with 15 percent DOT at any time period. This suggests that with equivalent solution concentrations, the glycol provided little benefit in increasing penetration.

Another objective of this study was to determine if increasing the DOT concentration from 15 to 23 percent, the concentration often used in commercial glycol-borate applications, would increase subsequent diffusion penetration. According to Fick's first law of diffusion, increased surface concentrations should result in increased diffusion under steady-state conditions (Ra et al. 2001). As shown in Table 1, the higher solution concentration resulted in significantly greater penetration in some but not all cases. For example, after 6 weeks of diffusion, the depth of penetration (mm) was greater for the 23 percent solutions than for any of the 15 percent solutions except the DOT-HiLo–treated specimens. However, after 6 weeks, the percentage of cross section penetrated by specimens treated with GB2‐23 percent was not significantly greater than those of specimens treated with the 15 percent solutions. Similar mixed results were observed for the 13- and 26-week diffusion periods. After 26 weeks, specimens treated with GB1‐23 percent had significantly greater depth of penetration (mm) than those treated with 15 percent concentrations of GB2 or DOT, but there was no significant difference between GB1‐23 percent and GB1‐15 percent. Diffusion penetration from the GB1 and GB2 formulations was similar. No significant difference in either depth of penetration or percent cross section penetrated was observed when GB1 and GB2 were compared at equivalent DOT concentrations and diffusion periods. Although the findings on the effect of solution concentration are not clear-cut, they do indicate that the primary benefit of the GB formulations is that they allow use of higher solution concentrations, which in turn deposit a greater concentration of boron on the wood surface to drive the diffusion process.

In general, the greatest penetration occurred with the DOT-HiLo specimens (specimens that were dried down from a higher moisture content), and this difference was often statistically significant. This increased diffusion is likely a result of the slightly higher moisture content that these specimens maintained throughout the diffusion period (Table 1). The higher moisture content in these specimens is likely a result of sorption hysteresis; the amount of water absorbed from a dry condition to equilibrium with any relative humidity is less than the amount retained in the process of drying to equilibrium from a wetter condition. Slightly greater penetration of both DOT and GB formulations was also observed in the earlier study in which all specimens were brought to equilibrium from higher moisture contents (Lebow et al. 2010).

The assay of boron retention revealed that the 23 percent DOT glycol-borate formulations did deliver significantly greater concentrations to the wood surface than the 15 percent DOT formulations (Fig. 2). Although this effect was most obvious in the outer assay zone, the GB1‐23 percent formulation also produced significantly higher boron concentrations in the 7- to 12-mm assay zone than did the DOT-15 percent formulation or the 15 percent GB1 or GB2 formulations. There was no significant difference in boron retention between the 23 percent GB1 and GB2 formulations in any assay zone or at either concentration. The GB1‐23 percent formulation did appear to yield slightly greater boron retentions than the GB2‐23 percent formulation, but this difference was not statistically significant.

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There was also little significant difference in boron concentration in the specimens treated with the three 15 percent formulations, although one of the glycol-thickened 15 percent formulations (GB1) did yield significantly greater boron in the outer assay zone than the nonthickened 15 percent DOT formulation. This finding could indicate that the more viscous glycol formulation allowed more solution to remain on the wood surface during spray applications. However, specimens sprayed with the glycol-thickened 15 percent DOT formulations did not appear to have greater weight gains than those sprayed with the nonglycol 15 percent DOT solution. The average weight gain of specimens sprayed with 15 percent GB1 and GB2 was 10.3 and 10.2 g, respectively, while the average weight gain of specimens sprayed with nonglycol 15 percent DOT was 10.5 g. The DOT-HiLo specimens had an average weight gain of 11.3 g, while average weight gains of specimens sprayed with the GB1 and GB2‐23 percent DOT solutions were 10.9 and 11.5 g, respectively. With the possible exception of the DOT-HiLo specimens, the treatment group weight gains do not appear to correspond with the trends seen in either penetration or retention.

As noted for boron penetration, the additional moisture in the DOT-HiLo specimens appeared to benefit diffusion. These specimens had significantly greater boron concentrations than specimens treated with the nonglycol-thickened 15 percent DOT solution in every assay zone. They also had greater boron concentrations than the 15 percent glycol-borate solutions in the 7- to 12-mm assay zone. In general, the boron assay results further indicate that surface concentration and wood moisture content are the keys to boron diffusion and that the primary benefit of the glycol formulations is that they allow use of more concentrated treatment solutions.

The results of this study support the earlier finding (Lebow et al. 2010) that boron diffusion can occur in wood with wood moisture contents in the range of 18 to 20 percent. The results further indicate that diffusion can proceed at these relatively low moisture contents when the wood is initially dry but then exposed to wetting or humid conditions during and after construction. It is likely that concept of fiber saturation point as an abrupt transition for boron diffusion is overly simplistic. As noted by Siau (1995), both free and bound water may be present over a range of moisture contents below the fiber saturation point. More recently, researchers using magnetic resonance imaging to determine the distribution of liquid water in sugar maple reported the coexistence of liquid and bound water at moisture contents below the fiber saturation point (Hernandez and Caceres 2010). Mass movement of bound water through the cell structure has also been discussed as a mechanism of moisture movement below the fiber saturation point (Choong 1963). Although most wooden members in most structures have less than 20 percent moisture content, moisture contents of 20 percent or greater have been reported in some framing members (Glass and TenWolde 2007). Temperature and humidity conditions similar to those evaluated in this study can occur in the vented crawl spaces of air-conditioned homes during warm, humid weather.

A further consideration with the surface-applied borate treatments is their ability to deliver a sufficient concentration of boron to prevent biological attack. There have been numerous reports on the concentration of boron needed to prevent colonization by decay fungi, as summarized in an article by Freitag and Morrell (2005). Depending on the fungus evaluated and the methodology used, threshold concentrations have been reported to vary from as low as 0.13 kg/m³ (as B₂O₃) to as high as 2.6 kg/m³ B₂O₃. In their article, which reported on a method intended to simulate wood used aboveground, Frietag and Morrell concluded that the threshold retention was in the range of 0.22 to 0.24 kg/m³ (as B₂O₃). If so, even the center portion of the specimens evaluated in this study would meet the efficacy threshold (Fig. 2). Subsequently, Lesar et al. (2009) evaluated the sensitivity of six decay fungi to borates and reported that retentions between 0.2 and 0.8 kg/m³ (as B₂O₃) are effective in agar-block tests while noting that higher retentions might be needed for soil-block tests. Other studies have reported higher threshold concentrations, and an earlier review reported the threshold as approximately 1.1 kg/m³ B₂O₃ (Drysdale 1994). Only the outer assay zone of the specimens in this study would exceed this more conservative assumption of fungal toxicity threshold. However, spray-applied treatment solutions, such as those used in this study, are typically more concentrated than those needed for pressure treatment, and Kartal et al. (2004) reported that solution concentrations as low as 0.5 percent DOT protected wood against attack by both decay fungi and Formosan subterranean termites.

Borates are less effective in preventing growth of mold fungi than decay fungi, although their efficacy can be enhanced with the use of co-biocides (Fogel and Lloyd 2002, Clausen and Yang 2004, Micales-Glaeser et al. 2004). Toxic thresholds for mold and sapstain fungi have been reported to range from 0.06 to 1.1 kg/m³ (as B₂O₃; Fogel and Lloyd 2002), but because molds colonize primarily the wood surface, retentions expressed on the basis of wood volume may be less relevant for mold fungi than for decay fungi. Application of a 5 percent DOT solution to wood surfaces was found to be ineffective in preventing mold growth (Clausen and Yang 2004), while applications of 8.5 or 15 percent DOT solutions was reported to inhibit or partially inhibit mold growth, depending on the fungal and wood species (Micales-Glaeser et al. 2004). Based on these findings, the 15 to 23 percent DOT concentrations applied to wood surfaces during spray treatment of framing lumber might also be expected to provide partial protection against mold growth.

The minimum protective threshold for termite protection with borates also has not been precisely established. Previous researchers have reported effective borate (as B₂O₃) concentrations ranging from below 0.7 to over 7.0 kg/m³ (Drysdale 1994, Peters and Fitzgerald 2006). Much of this variability arises from differences in test methods, wood species, and termite species. Laboratory tests generally indicate efficacy at retentions of around 1.4 kg/m³, while some field tests indicate that higher retentions are needed to ensure protection (Peters and Fitzgerald 2006). A recent study in which treated specimens were weathered (leached) down to a range of boron concentrations before exposure to termites concluded that the threshold of effectiveness was about 1.12 kg/m³ (Lake and McIntyre 2007). In this study, the concentrations in the outer assay zone were generally above this threshold, but those in the inner and center assay zones were not (Fig. 2). Current pressure treatment standards for framing lumber specify boron retention of 2.7 kg/m³ (as B₂O₃) for most applications and a higher retention (4.5 kg/m³) for locations with Formosan subterranean termites (AWPA 2011c). The assay zone for these pressure treatment retentions is 0 to 15 mm from the wood surface. Building codes require pressure treatment for sill plates but typically do not require preservative treatment (pressure or nonpressure) for other framing members. Clearly, none of the surface-applied treatments evaluated in this study achieved the AWPA-specified pressure treatment boron retentions. There is also some indication that formulations with glycol may be effective at lower concentrations than those without glycol. When evaluated by oral toxicity, Tokoro and Su (1993) found that a glycol-borate formulation was toxic at a lower boron concentration than DOT or boric acid formulations. However, Grace and Yamamoto (1992) concluded that although ethylene glycol does exhibit a low level of toxicity toward termites, the DOT component is the primary contributor to the efficacy of the glycol-borate formulations.

The role of preservative penetration and retention in providing protection of framing members is less clear than for exterior members. Penetration is necessary in exterior members because drying checks and saw cuts break the outer layer of treatment and expose untreated wood to decay and termite attack. In framing lumber, breaks in the treated shell are also likely to occur when the framing is cut to install electrical, plumbing, and other utilities. However, if sufficient moisture is present to support fungal growth and if the area of moisture extends to the wood surface, boron from the surface may diffuse into the previously unprotected area. While the nearly ubiquitous presence of airborne fungal spores ensures contact with exposed wood surfaces, it is less likely that foraging termites will encounter small breaks in the treated shell within a much larger surface treated with high concentrations of borates. Even if fabrication during construction exposes some untreated inner surfaces, in many cases the termites would still be required to traverse over surfaces coated with high boron concentrations to reach the vulnerable wood. Although the level of deterrence provided by a borate-treated surface is not well understood, one study has reported that a borate application to concrete substantially decreased the length and viability of termite mud tubes across the treated surface (Smith and Lloyd 2004). Subsequent research indicated that although one type of glycol-borate surface treatment did deter termites from crossing treated wood to attack untreated wood, termites were able to construct mud tubes across sill plates pressure treated with borates (Wu et al. 2007).

Conclusions

Boron penetration by diffusion was observed for all solutions evaluated in this study, even at moisture contents well below the fiber saturation point. Diffusion continued between the 13- and 26-week samplings, indicating that this diffusion was not solely attributable to water uptake from the spray application. This finding indicates that spray-applied borates have potential for diffusion into framing lumber under some conditions. There was no significant difference in penetration between the powdered DOT and glycol-borate solutions when they were applied at the same DOT solution concentration (15%). However, in some cases, the glycol-borate solutions applied at the 23 percent DOT concentration produced significantly greater penetration than the solutions applied at the 15 percent concentration. Thus, the glycol did not appear to aid diffusion but did provide benefit by allowing the use of more concentrated treatment solutions. The results also indicate that even slight differences in moisture content may cause significant differences in the extent of penetration for wood below the fiber saturation point and that this effect may be greater than that of solution concentration. The assay results revealed that the glycol-borate solutions applied at the 23 percent DOT concentration yielded significantly greater boron retentions in the outer assay zone. Although these retentions are below those specified for pressure treatment, they did exceed many of the published values of retentions needed to prevent attack by subterranean termites. Retentions in the inner assay zones appeared less likely to provide protection against termites, but the role of penetration in providing protection of framing members from termite attack is less clear than for exterior members. The retentions resulting from the 15 percent solution concentrations were marginal in comparison to reported termite threshold retentions. Both the 15 and the 23 percent DOT solutions yielded retentions in the outer assay zone greater than most published values of threshold retentions needed to prevent fungal decay. Inner retentions were more marginal, but greater inward diffusion of boron might be expected at moisture contents needed to sustain growth by decay fungi.