Uses of Borates in Wood Treatment and Preservation

History

Wood is a natural organic material and as such can be degraded by biological organisms: bacteria, fungi and insects.

While wood preservation can be traced as far back as Biblical times, it has been suggested that during the reign of Alexander the Great (350 B.C.) bridges were built using wood soaked in olive oil. In the 1500s termites were controlled using a mercury chloride and arsenic oxide compound. Pressure impregnation came to the world of wood preservation in the early 1800s using creosote oil.

Many other preservation technologies in the past and continuing to present day have been documented. However, it is believed borate’s value began in the 1800s.

In 1877, Dr. Hugo Zerener in Germany developed a patent composed of a mixture of water glass, sodium chloride, boric acid and diatomite for impregnation of wood against attack by Serpula lacrymans, decay fungi. Admittedly it is not clear if the value of boric acid then was more for pH versus preservative value. Later in 1913, Dr. Karl Heinrich Wolman in Germany developed a chromium boron wood preservative. This was later enhanced by Dr. Sonti Kamesan (1939-1945) in India who developed a water soluble compound composed of copper-chromium-boron. The value of borates as a standalone for wood destroying insects developed in both New Zealand and Australia in the 1930s and became commercialized in 1949.

Over the 60+ years boron-based compounds have found their way into other formulations globally that have significantly helped to establish borate efficacy in the world of wood treatment.

Borate Benefits

Kills wood destroying insects
Bacteriocide/fungicide to control “dry rot”
Anticorrosive in some formulations
Fire retardant in some wood applications depending on borate loading and type used

Borate’s functionality in wood

Borate compounds {i.e. borax, boric acid, Etidot 67 (disodium octaborate tetrahydrate)} convert to boric acid when they come in contact with wood whose pH is 4 – 5. In solution boric acid works as a weak Lewis acid which accepts an hydroxyl (OH-) to form a tetrahydroxyborate ion. The efficacy of borate compounds depends on the quantity of borate used regardless of the borate being mixed.

Borates take advantage of the moisture content in the wood to penetrate deeper. Thus kiln-dried dimensional lumber having a moisture content of @ 9% will allow borates to penetrate marginally beyond the surface of the wood. However in fresh cut timber, where the moisture content can be 35% or higher by weight, the borate penetration will be deeper. Since borates offer varying degrees of solubility they provide different levels of concentration in the wood.

The depth of penetration is also due to:

borate concentration,
formulation package used,
number of treatments applied,
surrounding temperature
age, moisture and species of wood

The species is important since borates will penetrate farther in soft wood species (i.e. pine, spruce, fir, etc.) versus hard wood species (i.e. hickory, oak, elm, etc.).

However with all the above benefits it should be mentioned that borates will also leach from the wood if there is a moisture source on the exterior of the wood. That is why borate compounds are not labeled for use in the ground. The potential for constant availability of water will change the direction of the borate treatment in the soil around the structure.

The reverse leaching process will slow considerably as the concentration of borate in the wood structure is reduced. Research has shown that the efficacy of wood after years of water exposure will still be useful for decay fungi and wood destroying insects.

Inorganic borates do not break down and contain no organic volatile impurities. While water will evaporate from the borate solution, borates are stable within the wood assuming no external moisture source exists to leach the borate mineral.

The toxicity and thus the preservation of the wood is due to the complexing of the tetrahydroxyborate with polyols (oxidized co-enzymes and other compounds) in wood attacking both decay fungi and wood destroying insects such as termites. The following is a brief review of target organisms.

Decay Fungi

Decay fungi are usually classified into two groups: brown rot and white (yellow) rot which is sometimes miss-classified as dry rot. Brown rot breaks down the hemicellulose and cellulose of wood. There is a hydrogen peroxide process that helps to decompose the wood. A damp enviroment must exist in the beginning for the decay fungi to grow. Once decay has set in, the wood becomes dry, crumbles to the touch, discolors and shinks.

White rot (sometimes yellow) breaks down lignin and/or cellulose. The wood is often soft, spongy and/or stringy, plus moist to the touch and takes on on a white appearance. The mode of action for borates is not well understood and generally thought to disrupt cellular production of enzymes that allows fungi to extract nutrients from the wood. It has been reported that borate applications can be efficatious against decay fungi in a matter of days depending on borate concentration, wood moisture content and number of applications.

There are reports that mineral borates such as Ulexite and Colemanite provide efficacy against fungal growth based on overseas research.

Target wood decay fungi (partial list):

Brown rot

Coniophora sp.
Coriolus sp.
Gleoophyllum sp.
Lentinus sp.
Serpula sp.

White rot

Trametes sp.
Schizophyllum sp.

Wood destroying insects

The borate mode of action appears to disrupt the digestive process of insects causing them to starve by killing the bacteria that allows the insects to digest cellulose. Specific organisms such as termites require a higher concentration of borate (2%) by weight of wood than decay fungi for borates to be efficatious. Using the same criteria of borate concentration, wood moisture content and number of applications, it could take weeks or longer to kill these organisms.

Target wood destroying insects include (partial list):

Lyctis sp. (powder post beetles)
Hylotrupes sp. (old house borers)
Coptotermes sp. (subterranean termites)
Zootermopsis sp. (damp wood termites)
Incisitermes sp. (dry wood termites)
Camponotus sp. (carpenter ants)

Borates and other wood preserving compounds

Ammoniacal Copper Quinolate with boron (ACQ-B) and Copper Azole with boron (CBA)are compounds that have found wide use as an alternative to Chromated Copper Arsenate (CCA), has previously been used for the residential market. Borate acts as an anticorrosive and preservative value. These compounds are typically pressure impregnated into dimensional lumber.

Wood Applications

Borate’s value has found use in (partial list)

dimensional lumber
engineered wood (i.e. orient strand board, wood plastic composite, etc.)
furniture
log homes
millwork (i.e. doors & windows)
plywood
railroad ties
siding
utility poles.

Methods and Uses of Borate Application in Wood Treatment

Pressure impregnation – The process developed in the 19th century is still used today. Dimensional lumber is loaded into a pressure vessel, sealed and flooded with a water soluble mixture of borates and sometimes other addtives. The liquid under high pressure is forced into the wood. Later the wood is removed, dried and transported.

Dip Diffusion – This techique is largely used by log home manufacturers. The freshly cut logs that have been debarked and the cambrium layer removed have a considerable amount of moisture (35 – 45%) available. This allows the hot borate solution whose concentration is from 10 – 25% to penetrate the log more completely. The process involves logs placed for long periods of time in a liquid tank and later removed and wrapped for weeks at a time so the borate can diffuse into the log. Multiple dip treatments are not unusual with further wrapping to insure a complete penetration of the log.

Topical or Surface application – This approach is used for homes and other structures that have previously been built. The application can provide some protection but the level of wood penetration is minimal. Check and read the labeled instructions and have the proper EPA registration number for such uses.

Amorphous boron rod – Fused borate rods are typically composed of borates and used primarily for maintenance. A hole is drilled into the heartwood of a utility pole or other structure which would be suseptible to continual moisture. The rod is placed inside the drill hole and a plastic cap is placed to cover the hole. Over time the rod diffuses into the wood structure due to moisture and is replaced as needed.

Engineered Wood – As wood conservation moves forward another type of wood composite has been developed to replace dimensional lumber for the home building market. This wood composite can be used for floor joist or joinery (window & door frames). Depending on the composition used (wood chips or wood saw dust), the wood particles are fused with a resin system and placed under high heat and pressure to form a large board. The laminate or particle board formed can be treated during the production process with various borates including zinc borate. Zinc borates are preferred in wood chip applications due to their ability to dissolve slowly and their compatibility with the resin systems used.

Mineral borates such as Colemanite have received favorable attention in this application due in part to their lower solubility compared to refined borates and to their cost advantage. Still other refined borates (due to their potentially higher borate concentration in wood) can also be considered a fire retardant in such wood composites as particle board.

As a reminder, with all the aforementioned applications above an EPA registered product is required.

American Borate Company Products:

Refined

Borax 10 Mol
Etibor 48 (Borax 5 Mol)
Boric acid
Etidot 67 (disodium octaborate tetrahydrate)
Zinc borate

Mineral

It is further advised to check with American Borate Company’s (ABC) representative to determine if there is an opportunity to sub-register under ABC’s data. This has the potential of speeding up the process of sub-registering private label products when product is purchased from ABC.

Note: ABC does offer an EPA registered product specific to boric acid. Ask about registration and labeling availability for other ABC borate compounds.