Wood: Answering the Call

Is there anything that can be done to make wood a more stable product?

 

Is there any way to make some woods such as Ash, Aspen, Beech, Birch and Poplar more useful to the Architect and Designer?

 

Can you make wood more durable and retain color consistent throughout the wood for Commercial Projects?

 

Is there a natural way to make woods more insect repellent and less susceptible to rot?

 

Based on research that was started in Finland and France in the 1990’s there is a technology that helps answer all of the above questions.

Researchers found that when you take the basic building blocks of wood – Cellulose, Hemicellulose, and Lignin and super heat them there are some chemical reactions that take place that alter the behavior of wood.

If we look at the Cellulose molecule you will see that there are open chemical bonds of Hydrogen and Oxygen in the molecule structure.

Cellulose Molecule

These open bonds are free to bond with another Hydrogen atom and create H2O or attract water. This is why wood is water loving or “hygroscopic”, it will readily attach to water.

Research has found that if you heat wood to higher temperatures another chemical process will happen. This process causes at least one of the –OH groups to be replaced by an –O- group (Esterification). Once this process occurs it changes the chemistry of the wood. The result is that the wood does not as readily bond with water changing the hygroscopic nature of the wood making it more stable.

So what is the change in the properties of the wood?

  • Repels moisture. Equilibrium moisture content is lowered by 50 – 90% compared to untreated wood
  • Stabilizes the wood. Moisture related swelling and shrinkage is reduced by 5-15 times. Cracking and checking of the wood is also diminished.
  • Color. Beautiful even brown color, consistent throughout the wood to the very center of the board.
  • Green. 100% chemical Free. Process is done with heat and not chemicals. Absolutely environmentally friendly.
    • This is the feature that has caused the heat-treated wood market to grow by 30% in Europe. Europe started to ban chemical treatment of woods and Canada and the USA followed in 2004 in such sectors as children’s playground production, and decks for waterfront homes. In 2004 the United States restricted the use of chromates arsenate-treated woods. Concerns over possible health risks of the chemical treatment has left heat treatment as a viable alternative.

 

Essential durability. 25 year life cycle in outdoor applications.

 

Different Species

 

THE PROCESS

 

Phase 1-The basic process to heat treat lumber is to cook the wood using a balance of heat and steam (to 200 degrees F.)  in a special kiln in a low oxygen environment to prevent the wood from burning. Just heat and steam are the only elements used no chemicals. The moisture content of the wood decreases to nearly zero.

Phase 2 – Once drying has taken place the temperature inside the kiln is increased to 365 to 400 degrees F. The temperature remains constant for 2-3 hours depending upon the degree of treatment.

Phase 3 – Cooling and moisture conditioning. The final stage is to lower the temperature by using water spray systems; when the temperature has reached 175 – 200 degrees, re-moisturizing takes place to bring the wood moisture content to a usable level, 6-9%.

Another advantage to this process is the darker color caused by the heat treating. This allows woods such as Ash, Beech, Aspen, Beech, Birch, and Poplar to take on a different look with a natural brown tone. A pleasing and more natural look than staining.  American Ash has the potential to be a lower cost alternative for Ipe in outdoor living areas and the stability. These Softer woods can be used for exterior decking and siding with the added durability of the heat treatment.

 

Different Temperatures

 

MAJOR FEATURES OF HEAT -TREATED WOOD

 

Major Features

 

The following information was obtained from extensive research that was done in Europe, United States, and Canada*

Wood Properties Expanded
Parameter Effect Comments
Equilibrium Moisture Content Reduced 50%
Dimensional Stability Swelling and shrinkage reduced 3-5 times both radial and tangential and up to 15 times for hardwoods. This depends on Relative Humidity. Cells in softwood species differ in chemical composition than hardwood.
Color Attractive Golden Brown Appearance and even color at all depths Color is affected by temperature and time based on the process
Appearance Attractive even color at all depths Color based on treatment temperature
Density Lower at 5-10% MC Improve cost efficiency of shipping
Resin Almost fully removed Softwood species
Brinell Hardness Increases as the temperature increases Most species become harder but this is relative to the species
Permeability Reduced up to 5 times depending on treatment temperature Surface of the wood is not porous but solid and the chemical composition of the wood changes
Thermal conductivity Decreased by 10 – 30% Lower water content and structural changes
Fire Resistance About the same as non-treated wood
Resistance to insect attack Significantly reduced Especially for wood beetles
Mold and Fungal Resistance Absent at high temperature treatment
Rot Resistance Improved However should not be saturated with water
Durability Reduced fungus attack Based on EN 113 & ENV 807 standards showed significantly improved level of resistance to fungi
UV Resistance It will start to grey without coating and small shakes can occur Apply a pigment based surface protection to prevent color changes and surface shakes
Ground Contact Not recommended to be used in deep ground applications where structural performance is necessary
Emissions Lower than kiln dried material The smoke-like smell comes out of thermos treated wood and disappears almost totally with time or after coating
Environmental Purity Made without the insertion of chemicals At least the same durability of chemically treated lumber
General Strength Reduced proportionally with density reduction Heat treated has a slightly lower density and correlated lower strength
Bending Strength Lower 1-30% Especially for lumber with knots- Should not be used for load bearing structural usage
Splitting strength Reduced 20 – 40 % Depends on treatment temperature. Hardwoods are better than softwoods
Screw holding Less Pre-drilling narrower pre holes helps. Stainless Steel screws for outdoors
Sawing Performance differs Use sharp blades to prevent tearing. Use the highest possible number of teeth per blade. Carbide tips preferred
Gluing and finishing Longer dry time for water based glues and finishes Due to less water absorption. PVA requires longer compression time and drying time. High solids glues perform better or chemically hardening glues such as polyurethane or PVA + hardener or melamine resin
Chamfering Avoid sharp edges Due to the brittle nature of the material
Planning Easier due to lack of resin
Sanding Finer dust Use efficient dust collection systems

Research in North America.

  • In 1955 Forest Products Laboratory in Madison, Wisconsin in cooperation with University of Wisconsin published report 1621 providing testing data for a so called Staybwood. This report was based on research going back to 1920-30-40th in Germany by Mr. Stamm and Hansen, who later were associated with the University of Wisconsin. For their test the Heat -treated wood was produced utilizing hot-melted metals at temperatures up to 300°C. The main result from this report is a proof that heat-treated wood becomes substantially more dimensionally stable, while losing weight and hardness.
  • Louisiana Forest Products Laboratory conducted test on the ability of Heat -treated wood to withstand termite attacks in 2002. The softwood, heat-treated with hot oil, showed no additional resistance to termite attacks compared to untreated softwood.
  • Now the University of Wisconsin Forest Laboratory was deemed as most knowledgeable and having accumulated substantial information as related to heat-treated wood and processes. This lab has a long standing interest in the heat-treated wood.
  • Forintek Canada Corp. was involved in tests for heat-treated wood. The tests were devoted to abrasion resistance, impact and static bending, dimensional stability and resistance to fungal decay.

In 2008 Westwood Corp. has performed a series of experiments on physical properties of Thermotreated hardwoods and softwoods. The series of structural tests and getting of the US builder codes for Heat -treated decks applications are expected in 2010.