Key Words: Borates; Cellulose; Composites; Gypsum; Insulation; Mold; Sick building syndrome; Wallboard; Wood.

Introduction

Molds are fungi that grow superficially on materials, but generally do not decay or weaken the strength of such materials. On wood products for example, this means they are considered completely differently to the basidiomycetes. Carbon sources utilized by molds are simply residual sugars and stored carbohydrates of said materials. Although mold fungi do not cause decay, the disfigurement and discoloration of materials can be of aesthetic and economic importance. Generally, they are blue-black or blue-gray but can be brownish or purple, depending on the fungus responsible (Eaton and Hale 1993). 

Moisture content is probably the most important factor in determining the rate and extent of mold infestation. Construction materials going into a new structure are generally dry and so would not suffer from such fungal colonization. If the materials remain dry, this will remain the case, however, should very high humidity, condensation, penetrating or rising dampness or other forms of moisture ingress occur over long periods, the materials can become susceptible to mold growth, as well as other fungi. 

In the event of poor design or maintenance, mold growth can be found on many domestic and commercial construction materials in service. These include timber, wood composites, gypsum wallboard, and insulation products. Previously it could have been argued that control of the non-structurally impacting mold fungi was unimportant. However, specific growth of a number of these organisms has now been associated with negative health effects. 

Recently, poor moisture management in homes, hotels, and portable school buildings has been linked specifically to the growth of toxic mold fungi. In a survey of homes in twenty-four U.S. cities, nearly 50% of them had moisture problems (Jacobs et al., 1999). The majority of the molds found in homes are Cladosporium, Penicillium, and Alternaria, which are now known to cause chronic sinus infections, respiratory infections, and asthma. A potentially lethal mold, Stachybotrys atra, although not as common was also found (Mann 1999). Stachybotrys atra produces airborne toxins that can cause inflammation and injury in the gastrointestinal and pulmonary tissues in children and adults. An outbreak of pulmonary hemosiderosis (bleeding lung disease) killed ten children in Ohio and another 60 infants nationwide before it was traced to toxic mold growth resulting from inadequate home ventilation and humidity controls; with at least 28 more children killed since the initial outbreak (Jacobs et al., 1999). 

This study reviews the presence of borates in some construction materials and evaluates their ability to control mold growth. It would be of commercial value if in addition to the primary purpose of incorporating borates into construction materials, some performance against mold in houses was achieved as well. Wood Products: Historically, the earliest use of borates against wood infesting fungi was in the development of preventatives against mold and sapstain in sawn lumber. The extensive American studies in the 1920’s and 1930’s were brought together by Scheffer and Lindgren (1940) and at that time they suggested that borates were the most effective means of protecting timber from sapstain. Further specific efficacy against staining fungi (e.g. Ceratocystis spp., Aureobasidium spp.) has also been shown in many other studies (Da Costa, 1953; Orman, 1954; McQuire, 1959; Fenton, 1962; Urbanik, 1965; Miller et al.; 1989; Byrne, 1990; Amburgey, 1990; Forsyth & Amburgey, 1992; Laks et al.; 1993 and Edlund, 1995). A table of borate retentions required to give protection of lumber against some of these organisms (Lloyd, 1996) is given in Table 1. 

Table 1. Toxic Threshold for Some Sapstain/Mold Fungi 

Today, timber to be used in construction can be treated to control the growth of decay fungi and wood destroying insects. It does not normally receive a treatment to provide mold control in service, although much green lumber is treated at the sawmill to prevent mold and sapstain prior to, and during drying. In timber treated with borates, the basidiomycete decay fungi are fully controlled (Dickinson & Murphy, 1989) and this is probably true in other cellulosic materials as well. Lumber receiving commercial treatment with borates in the United States is treated with disodium octaborate tetrahydrate (DOT-Na2B8O13 4H2O). It receives a borate retention of 0.17 pcf B2O3 (
0.9% DOT) in accordance with AWPA C31 and ICBO evaluation report - 4890 to control decay, beetles, and native termites or 0.28 pcf B2O3 (
1.5% DOT) to protect against Formosan subterranean termites. Wood composites on the other hand are treated with zinc borate (ZB-2ZnO 3B2O3 3.5H2O) and generally meet a retention of 0.75% ZB to control decay and termites (including the Formosan termite) in accordance with AWPA recommendations. 

Gypsum

Gypsum wallboard or ‘drywall’ is a major component of construction. It is not usually a structural component and is often thought to be an inert material, not specifically prone to biological attack. However, gypsum products have been proven to be susceptible to mold, decay, and termite damage (Fogel and Lloyd, 2000). Organisms can use gypsum as a substrate to grow on or in. Gypsum has also been shown to specifically support fungal growth in other commodities that can lead to structural problems. Gypsum products are an ideal substrate to support mold growth under the right conditions, with both a high paper and starch content, in addition to their high moisture holding capacity and surface area. In North America, lighter weight boards are now being produced by incorporating foam. Lightweight boards are beneficial for ease of handling, shipping costs, and energy savings, but disadvantages are also seen. Decreasing the gypsum content in the board leads to over drying at the board surfaces and a decrease in the strength of the bond between the gypsum and the paper, which results in peeling. Problems with sagging can also be seen in ceiling applications of lightweight boards. Borates are used by some manufacturers to overcome these problems (McBroom and Vizel, 1999; Hancock and Mu 1994). They successfully raise the calcination temperature of the gypsum, protecting the board from over drying. This is accomplished by the water-soluble borate infiltrating the surfaces and edges of the board, creating a concentrated layer that increases the density of the surface, and develops a harder outer edge of the board. This dense layer also improves the adhesion of the paper backing (McBroom and Vizel, 1999; Hancock and Mu 1994). The gypsum also forms larger and thicker crystals, contributing to an increase in the rigor of the boards, and decreasing the likelihood of sagging (McBroom and Vizel, 1999). Borates also accelerate the curing of gypsum boards, prevent wrinkles from forming on the surface of the boards (McBroom and Vizel, 1999; Hancock and Mu 1994), and increase the fire retardancy of the boards (Mitsui Toatsu Chemicals Inc. 1992). All this can be accomplished without increasing the overall weight of the board. Currently, manufacturers adding borates target a retention of between 0.1% and 0.3% boric acid overall, which tends to give a higher surface retention due to water movement during drying. 

Insulation Materials:

Thermal and sound insulation materials are very important in the construction of homes as well as within the building industry in general. Insulation can be made from various materials including synthetic foams, mineral or glass wool, and natural fibers such as cellulose insulation (Grinda and Kerner-Gang 1982). Cellulose insulation is made of recycled newsprint milled into fibrous form with a high thermal insulation value and low cost. However, cellulose insulation is prone to two types of combustion: flaming and smoldering combustion. Fire retardants are added to control these (Siddiqui, 1989).

Borates are a well known fire retardant in cellulose insulation, that prevent flaming combustion, suppress after glow, and improve char formation, which tends to dehydrate the cellulose, forming protective glazes at the burning surface. Borates are also reported to give protection of cellulose insulation (Siddiqui, 1989) against fungal and bacterial growth, that cellulose insulation would be expected to support. Cellulose insulation is a good carbon source, has high moisture holding capacity, and a very large surface area for mold growth. 

A number of studies have been conducted on the biological resistance of insulation materials. Grinda and Kerner-Gang (1982) tested insulation materials’ resistance to mold fungi and wood-damaging basidiomycetes. Their study concluded that mineral insulating boards and granular volcanic rock cannot be utilized by mold fungi but can be overgrown by them, while the foams they tested contained components that could be utilized by the fungi. This showed that untreated insulation materials (even those considered inert) could be affected and overgrown with mold. Viitanen (1991) studied the influence of insulation materials on wood biodeterioration. He concluded that insulation materials do influence growth of mold and decay fungi in contact with wood. At high humidity, boron compounds added to the cellulose fiber diffused into the wood and prevented growth of brown rot fungi. It was also shown that mineral wool is destroyed by decay fungi, and at the same time actively supports surrounding timber decay. 

In most countries cellulose insulation materials are already treated with borates at high retentions to impart flame retardancy. Typical commercial retentions range from 15% to 25% borate by weight (a combination of boric acid and/or borax) in order to meet national fire standards (Bower, 1978; Siddiqui, 1989). Phosphates or sulfates are sometimes used as partial replacements for some of the borate, but probably should not be considered in warm humid environments for corrosion and fiber deterioration (Winandy, 2000). Ammonium sulfate, the most common replacement for borates is acidic, can corrode copper, may release ammonia gas under certain conditions of pH and temperature, and promotes fungal growth (Bower, 1978). 

Superficial treatment of timber:

Superficial treatment of timber is extensively carried out at some mills to prevent mold and sap stain growth. Fungi attack soon after timber has been felled and given suitable conditions for growth they develop very quickly. Preventative treatment is therefore carried out as soon as possible, normally within twenty-four hours of conversion. The only types of treatment, which have proved sufficiently quick and simple, are the use of a chemical dip or spray and these have been developed all over the world. 

While under normal good practices borates alone can provide some control of mold and staining organisms in many circumstances, in very severe mold and sap stain hazard situations it has been demonstrated that the use of other anti-sap stain chemicals in conjunction with borates can enhance their performance significantly (Lloyd, 1996). Specific organic fungicides have proven to give the greatest benefits when used in conjunction with borates. Good examples include formulations using didecyl dimethyl ammonium chloride (DDAC), 3-iodo-2-propynylbutylcarbamate (IPBC) and azole products. 

In this work, the interest is in providing performance of materials in service in buildings, rather than at the sawmill. However this previous experience led to the testing of borates (and borates in combination with an organic fungicide) as possible remedial or in-situ applied products for mold infested buildings. 

Materials and Methods

Wood Products

Four different types of wood products (treated and untreated) were examined: Southern Yellow Pine solid lumber (split approximately half and half into sapwood and heartwood), aspen oriented strand board obtained from Michigan Technological University, oriented strand board obtained from a commercial source, and Douglas Fir plywood obtained from a local retail outlet. All OSB was bonded with pMDI resin and the Douglas fir was probably bonded with phenol formaldehyde. Bearing the latter in mind, all samples were evaporatively aged for one month to ensure no remaining free formaldehyde, which would have prevented fungal growth. All boards were cut into 3 x 4-inch panels. Borate retentions determined by analysis have been shown in Table 2, and all samples were tested in duplicate. 

Table 2. Average borate retentions.

Gypsum Products:

The gypsum boards were manufactured by taking 800g of oretch stucco and adding 575ml of distilled water that contained 0.16g of sodium chloride, 4g of starch and the required borate loading. Diluted alpha foamer mixed with water was added to the stucco mixture and stirred. The contents were poured into a 0.5-inch board press that was lined with a 14 x 14 inch gypsum paper envelop. This was placed into an oven at 138°C for 45 minutes. The boards were trimmed and placed into a 40°C oven overnight. Three by 4- inch panels of boards containing 0.0, 0.1, 0.3, and 1.0% boric acid retentions were tested. 

Superficially Treated Timber:

Dry southern yellow pine sapwood, was cut into 0.75 x 3 x 4-inch panels and subsequently treated in various solutions by completely immersing each sample for one second, successively 3 times. This multiple dipping technique was chosen to give good even cover and combined with the high surface area to volume ratios of the samples would give at least the same overall retention as could likely be achieved in a heavy remedial spray type operation. 

Combinations of 0.0, 1.0, 5.0, or 10.0% DOT and 0.0, 0.1, 0.5, 1.0, 1.5, or 2.0% DDAC solution concentrations were tested, and all samples were tested in duplicate.

Incubation:

A modified version of ASTM: D 3273 – 94 was used to test the various products resistance to mold growth. The experimental chamber consisted of a plastic container (23.5 x 12.25 x 16.25 inches), tilted on its side (to give a 45° roof angle to prevent condensate dripping on the samples), filled with 2-3 inches of water. The water was heated to 32.5 +/- 1°C, using a thermostatically controlled heating coil. A plastic bowl (11.5 x 13.5 x 5.25 inches) was filled to 3 inches from the top with a mixture of damp potting and outside unsterilized soil to act as an inoculum source, and then covered with plastic mesh. The bowl was then floated in the plastic container. The boards were placed on the plastic mesh above the inoculum source. The timber and wood composite tests ran for a duration of 4 weeks and the blocks were flipped over after 2 weeks. The gypsum board samples were examined after 6 weeks. 

The decision was taken to use an unsterilized soil approach as this provides a more natural mixed culture condition and gives the samples the opportunity to select the most suitable organism for growth on that substrate and the organism least susceptible to the treatments. It also gives some opportunity for biological succession where substrate modification is required by one organism before another can become established. The alternative testing approach would have been to use a specific list of fungi known to cause health problems in a monoculture test. 

Results

Wood Products:

Overgrowth of mold in terms of visible discoloration was scored and results are shown in Table 3. It was noted that of the untreated materials, the Southern Yellow Pine sapwood was most susceptible (Plate 1), the Douglas Fir (Plate 2) was quite susceptible, and the aspen OSB (Plate 2) was least susceptible. It was also noted that the papered and primed single surfaces of the commercial samples were resistant to mold growth (Plate 3). Of the treated materials, greater protection was achieved with greater zinc borate retention, and zinc borate was found to give better performance than disodium octaborate tetrahydrate.

Table 3. Severity of mold growth on boards

Gypsum Products:

The amount of mold observed on the gypsum samples decreased with the increase in borate retention (Table 4). Gypsum boards treated with 0.3% boric acid showed sparse mold growth, which was significantly lower than boards treated with no borate. The best result was seen on the 1.0% boric acid containing boards, which exhibited little to no growth after the six-week period (Plate 4). 

Table 4. Mold growth on gypsum boards containing borates.

Superficially Treated Timber:

Overgrowth of mold in terms of visible discoloration was scored and results are shown in Table 5 as well as in Plates 5 and 6. Treatment solution concentrations above 5% DOT and 0.5% DDAC were found to give some performance against mold growth. The optimum performance was gained with a combined treatment of 10% DOT with either 0.5 or 1% DDAC.

Table 5. Mold Growth Found on DOT/DDAC Treated Pine

Discussion and Conclusion

With such limiting indicative studies, it is of course difficult to make categorical recommendations. However, in all of the studies conducted the results indicate that the addition of borates to construction materials decreases the growth of mold fungi. It was found that the higher the borate concentration the lower the ability of the mold fungi to infest a material. However, it can be seen that the type of construction material (wood type, gypsum, and insulation), borate type, and borate loading are all important in determining the product’s performance against mold growth. 

Results from the timber and wood composites revealed that the addition of borates to the products does give a significant improvement in their mold performance. The ability of zinc borate to control mold appears better than disodium octaborate tetrahydrate, and this is consistent with the data produced by Laks et al. (1993) on freshly sawn lumber. 

It was also of interest to note the treated Southern yellow pine boards showed more mold growth on the heartwood side of the boards compared to the sapwood side. A possible explanation of this occurrence is that treatment solutions can more readily penetrate the sapwood portion of a board compared to the heartwood portion. This leaves the heartwood portion unprotected and therefore more susceptible to infestation, the opposite of untreated material. 

Results obtained from the gypsum study again showed that the higher the boron concentration the lower the ability of mold fungi to infest construction materials. The current commercially favored retention of 0.3% boric acid added to some gypsum products gives reasonable control of mold fungi. 

No mold testing of cellulose insulation was carried out in this study. However, from the review of the literature on cellulose insulation and the very high borate retentions used for flame retardancy it is clear that significant performance against mold growth can be concluded for these materials when treated with borates alone. The results obtained with the much lower retentions of borates in solid wood and wood composites, which are likely to perform similarly, also corroborate such a conclusion. It is also clear from the literaturethat cellulose treated with ammonium phosphate or ammonium sulfate alternatives would actually exacerbate the potential mold situation (Bower, 1978). This occurs due to both the additional nutrients that these flame retardants supply, but also probably because of their greater hygroscopicity. A difficulty then arises in suggesting the performance of mixed flame retardant types, where for example phosphates and borates are used in conjunction. Clearly these are likely to perform less effectively than the stand-alone borates, but do merit some specific testing of their own. 

The superficially treated timber demonstrated that high and appropriate combinations of DOT and DDAC, could achieve significant mold control. These are used effectively in short-term prophylactic control of mold on timber (e.g. freshly sawn lumber prior to drying). However, full control/prevention was not achieved and would not likely be achieved in service/practice, in for example damp buildings. Claims for DOT/DDAC combinations could perhaps be made as a short-term measure during the repair and drying of structures. This would of course also prevent the probable fungal decay and insect attack that would otherwise occur in such situations and so could still be worthwhile. It must also be noted that in an infected structure with active mold growth, applications of liquids by spray are highly likely to exacerbate human sensitivity/exposure due to the opening and disturbance of the structure and the dispersal of mold spores to air. Occupants would therefore be best advised to take temporary accommodation until work was completed. 

In future studies, work should try to more accurately determine the toxic threshold of borates in these products against mold fungi (by testing a greater number of retentions around the indicated toxic thresholds), and investigate other ways of improving the mold resistance. Further work, if considered, would also need to include specific testing with at least Stachybotrys atra now that some mold performance has been demonstrated. 

Since some products are already manufactured with borates, consumers of these materials can be advised that they are receiving greater benefits in terms of biodeterioration control and mold growth control than they realize. Table 6 summarizes the performance of the materials at their current commercial retention against mold growth. As a general rule however, if manufactures wish to make biocidal claims they need to obtain the support of the original borate supplier and obtain EPA pesticidal registration. In addition to this, those involved with tackling the problems of mold infestation in houses, should be made aware of the specific borate containing products as a possible part of an integrated control strategy, but should realize that the use of borate containing materials alone cannot be considered a complete preventative against mold.

Table 6. Summary of borate performance in construction materials

References

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Appendix: Photographs of mold growth on construction materials.

Plate 1. Mold growth on Southern Yellow Pine sapwood.

Plate 2. Mold growth on untreated Douglas Fir plywood and oriented strand board.

Plate 3. Mold growth on OSB pMDI provided by a commercial source.

Plate 4. Mold growth on gypsum boards containing various amounts of boric acid.

Plate 5. Mold growth on aged pine surfaced of samples treated with DOT and/or DDAC.

Plate 6. Mold growth on freshly cut surfaces of samples treated with DOT and/or DDAC.