November 21, 2019
By Randy Clark
Wood use in buildings dates as far back as the Stone Age. In 9000 BC, the Neolithic Long House, a timber home in Britain, was one of the largest structures in the world. Since then, and through several ears, wood has been used in a variety of buildings. Even with the introduction of other materials, wood is still one of the most important and viable options.
While wood is one of the most sustainable materials, it is susceptible to degradation if not treated to endure long-term exposure risks like harsh climate conditions and attacks by insects and fungus.
While people have been preserving wood since ancient times—soaking bridge timbers in olive oil or using tar for ships during the Roman age—the commercialization of treated wood began with the railway industry using creosote, a distillate of coal tar that has been in use for more than 150 years.
“Large-scale commercialization of wood preservation in North America was associated with the development of transportation and communication technologies in the 19th century,” said Phil Evans, a professor of wood science at the University of British Columbia (UBC) in Vancouver. “At that time, railway companies were using untreated ties and having to replace them every five or so years.”
Using a wood preservative extended the life of the ties, reducing labour and material costs and the destruction of natural forests.
Types of wood modification
Modern times have brought advances in wood preservation in the form of modification processes, such as heat, densification, and polymer treatments, which are used to control its degradation and improve its ability to resist water, swelling, and biological agents like bugs and fungi. These processes do not use biocides.
Wood modification treatments using heat or non-biocidal chemicals have been researched for more than 100 years, but have only been on the commercial market for the last 15.
Thermally modified wood
“The concept of thermally modified wood dates back to 1915 as a result of research at the United States Forest Products Lab (FPL), but the process was first commercialized in the last decades of the 20th century in Finland,” said Evans.
Thermal modification involves heating the wood to a high temperature, up to 190 to 210 C (374 to 410 F), in a kiln under controlled conditions. The heat degrades the hygroscopic components of the wood, making it less susceptible to reabsorption of moisture and decay, and offers dimensional stability. Oxygen can be excluded from the process to reduce oxidation of the wood. Chemicals and pressure are not involved with this method; only heat and water vapour.
There are two process options for thermal modification of softwoods (e.g. pines) involving different temperatures: 190 or 212 C (374 or 414 F), depending on the end use of the thermally modified wood. Regardless, the process turns the wood to a darker brown and leaves a burnt odour.
Thermally modified wood is used indoors and for above-ground applications such as decking, exterior siding, fences, furniture, panelling, and pallets. It is not resistant to termites and cannot be used for ground contact applications.
Currently, in North America, there is not an approved test standard to effectively evaluate thermally modified wood to demonstrate it is fit for purpose or longevity from either the International Code Council (ICC), Wood and Door Manufacturers Association (WDMA), or the American Wood Protection Association (AWPA). According to a 2006 blog post on FPInnovations, the Québec Forest Industry Council (QFIQ), FPInnovations, and Cecobois are collaborating with eastern Canadian manufacturers to set up a thermally modified wood classification standard based on four major categories to address fungal degradation, dimensional stability, and colour grade. At this time, nothing has been formalized. Further, no specific code approvals have been provided by the Canadian Wood Council (CWC). In this author’s experience, the significant amount of variables in manufacturing thermally modified wood make it difficult to create code and standard approvals.
In simple terms, wood preservatives, or biocides, such as copper, ammonia, arsenic, and organic pesticides are forced into the wood using a pressure treatment. The wood is first dried—by stacking outside or by kiln drying, steam conditioning, boultonizing, and in some cases incising—so it is receptive to the preservative.
According to Wood Preservation Canada (WPC), preservatives work by penetrating the wood, neutralizing the food supply within the material, and providing protection outlasting the product’s useful life. These treatments have a lifespan of 10 to 20 years. Just like other treatments, this product’s lifespan depends on variables such as climate, finishing (coatings), maintenance, and proper installation. Pressure-treated wood is also one of the more affordable preserved wood products on the market, and the most widely used.
One of the most talked about and debated preservatives in North America is chromated copper arsenate (CCA). It contains inorganic arsenic, chromium, and copper. Inorganic pentavalent arsenate, one of the primary active ingredients in CCA, is a naturally occurring element people come into contact with every day through food, water, air, and soil. Since the 1940s, wood has been treated with CCA for use on outdoor residential structures, such as decks, fences, and children’s play equipment, to protect against fungi, termites, and other insects.
The United States (2003) and Canada (2004) voluntarily phased out CCA’s use in residential areas due to arsenic exposure. Today, it is used for certain commercial, industrial, and agricultural applications with some residential exceptions (wood shakes, shingles, and permanent wood foundations) and is registered for use in Canada under Health Canada’s Pest Management Regulatory Agency (PMRA).
“The most commonly used preservation treatments in Canada today for residential and commercial applications are alkaline copper quarternary (ACQ), copper azole (CA), and micronized copper azole (MCA),” said Martin Tauvette, executive director at WPC, a group representing manufacturers and suppliers of pressure-treated wood throughout Canada.
Choosing the proper preservative-treated wood depends on the intended use. All preservatives are registered and regulated by PMRA. The following are the most common pressure-treated wood products used throughout Canada.
Alkaline copper quarternary
A water-based wood preservative, ACQ’s main ingredient is copper oxide and a quaternary or ‘quats’ ammonia compound. Developed in the 1990s as an alternative to CCA, ACQ adds protection from fungi and insect attack as well as decay. It is used for exterior residential applications such as decks, patios, landscaping, gazebos, fencing, decking, and other wood structures. ACQ types—B, C, and D—offer different ammonia properties or quat compounds depending on the wood species being preserved.
The active ingredients in copper azole are copper and an organic biocide such as tebuconazole. Widely used on field crops, tebuconazole offers fungicidal protection and works as an active co-biocide with copper. CA is used on exterior residential applications like decks, patios, landscaping, fencing, walkways, and boardwalks.
Micronized copper azole
The most recently developed wood preservative, MCA uses micronized copper (nanoparticles and micron-sized particles of copper carbonate) and micronized tebuconazole together to use on exterior residential applications above- or in-ground and in freshwater contact.
Irrespective of the wood preservative used or specified, when working with pressure-treated wood, it is advisable to follow the safe practices listed on the end tag or refer to proper association guidelines.
Modified wood undergoes a “non-biocidal modification to improve its properties, such as strength and hardness and resistance to water, swelling, and biological agents,” as described by Evans.
The two chemically modified wood processes that have been commercialized in the last 15 years are furfurylation and acetylation. Where preservatives add a biocide to wood, furfurylation and acetylation chemically change the molecular structure of the material. Both of the options take fast-growing softwood or hardwood and through a proprietary process, the wood becomes harder, decay resistant, and in some instances, insect resistant and dimensionally stable. Even more attractive is that both processes are non-toxic and offer sustainable benefits.
Furfurylation has been studied for more than 50 years and is beginning to achieve market acceptance. The process uses furfuryl alcohol and catalyst(s) to penetrate and fill the wood cell walls or lumens with a resin-like plastic. The wood is then heated to just above 100 C (212 F) to harden the resin and finally dried.
While it can be used for interior applications, wood treated by furfurylation is typically employed for exterior applications and one manufacturer offers a 30-year warranty for decking and cladding products only. ICC has approved it for above-ground use only in decking applications. Canadian Standards Association (CAN/CSA) 080, Wood Preservation, is used to evaluate the durability of wood. Currently, there are no Canadian standards for chemically modified wood.
Acetylation, studied for more than 90 years, was first commercialized in 1997 in Europe. The process chemically alters the wood’s free hydroxyls into stable acetyl groups. Acetyl groups are naturally present in all wood species, which means nothing toxic is added. The altered cell structure of the wood makes it an unrecognizable food source for insects, including termites, and prevents fungal decay.
Only one manufacturer has successfully commercialized acetylated solid wood. It is warrantied for 50 years above ground and 25 years below ground, and swelling and shrinkage are reduced by 75 per cent or more. The manufacturer also meets ICC requirements for above-ground or ground-contact applications requiring protection against fungal decay or termites as described by the code.
The pressure treatment method with preservatives only penetrates the envelope or perimeter of the board, often only a few millimetres deep. These treatments do not modify the cells of the wood like chemically modified wood processes.
Acetylated wood has been used on a number of projects in some of the most extreme applications, such as a water-submerged pedestrian bridge and marine docks including canal linings in the Netherlands, as well as coastal siding, outdoor furniture and cabinetry, windows, doors, and cladding on commercial buildings.
Chemically modified wood uses raw material sourced from forests certified by the Forest Stewardship Council (FSC). It is a sustainable alternative to tropical hardwood.
Future of wood technology
Several new wood product technologies are currently being explored. It is, however, a long process to create new products. For example, acetylation of wood was first attempted in the 1920s. The United States FPL studied and worked with it in the 1940s. It was not commercialized until the late 1990s.
Cross-laminated timber (CLT) for tall buildings is exciting, yet only a small market in comparison to other materials.
“CLT is a minor product on a global scale,” said Evans. “CLT tall buildings are attracting a lot of attention with about 20-plus buildings in existence compared to 76 skyscrapers erected in one year in an industrializing China using primarily concrete and steel.”
The majority of new commercial wood technologies—CLT, furfurylation, and acetylation—have come from the European Union (EU). Companies there have the funds to invest in pilot plants, a key step in taking laboratory research to the marketplace.
An International Building Code (IBC) change from 2018 says fire-treated wood must be impregnated and no longer be a surface coating. While states have a grandfather period of up to three years to adopt new codes, this is a game changer for the wood industry. Pressure from insurance companies and alternative product providers has driven homeowners, in particular, to products offering fire treatment or fire resistance. Additionally, California requires roofing and decking products must be fire-treated or fire-resistant to comply with Chapter 7A of the California Building Code (CBC). These code changes will drive development of new fire-retardant treated wood products.
The National Building Code of Canada (NBC) requires all fire-retardant treated wood (FRTW) to be produced through a pressure impregnation by a licensed treater in accordance to CAN/CSA 080.
Thermally modified wood products are testing a hybrid by adding preservatives, resins, and/or additives to its process. This may push the need for thermally modified wood manufacturers to meet standards and approvals by AWPA and CSA in the future.
Charred wood produced using the Shou Sugi Ban method, a Japanese tradition employed to preserve the wood by charring it for a durable and dramatic look, has taken off. The process provides some resistance to fire, insects, and decay. It has been used on acetylated, furfurylated, western red cedar, cypress, white oak, walnut, beetle-killed pine, and ash wood. Primarily employed for exterior cladding and decking and some interior applications on both commercial and residential structures, its use makes for a stunning design on any building.
Acetylated medium-density fibreboard (MDF) made for exterior applications including façade cladding, fascia and soffit panels, outdoor kitchen cabinets, garage and exterior doors, window components, and interior wet rooms is new to the North American market. Manufacturers are known to offer 50-year warranties above ground on this product type.
The UBC wood science research group recently tested an acetylated fibreboard panel against a regular fibreboard panel for one year. Two sheep structures were built from the two fibreboard panels and documented on social media for one year. The acetylated fibreboard performed flawlessly compared to the untreated board. According to Evans, there was significantly less weathering-induced discolouration and swelling of the acetylated fibreboard sheep compared to sheep made from unmodified fibreboard.
Wood’s renewable properties, design esthetics, and durability make it an attractive material to build and create long-lasting structures. While commoditized wood-like framing timbers and oriented strand board (OSB) will continue to drive wood use in residential and some commercial construction, new technologies will inspired its use in construction projects in innovative and unique ways.
Randy Clark is head of sales and technical Service, North America, Accsys Group. Clark works closely with customers to support and develop sales of Accoya acetylated wood products. With more than 30 years’ experience in the lumber industry, Clark has a rich background in developing new products, providing technical support, and conducting academic research projects. He can be reached at firstname.lastname@example.org.
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