Chemical treatment of wood involves impregnating or modifying wood with various chemical substances to enhance its durability, stability, and resistance to degradation by biological agents like fungi and insects, as well as environmental factors such as moisture and UV radiation. This process extends the service life of wood products, making them suitable for a wider range of applications, especially in outdoor or high-moisture environments.
The primary goals of chemical wood treatment include:
- Preventing decay: Protecting against wood-destroying fungi and bacteria.
- Controlling insect infestation: Deterring termites, borers, and other wood-eating insects.
- Improving dimensional stability: Reducing swelling and shrinking due to moisture fluctuations.
- Enhancing fire resistance: Making wood less combustible.
- Increasing strength and hardness: Though less common for standard treatments.
Types of Chemical Wood Treatments
Chemical treatments can broadly be categorized into wood preservation and chemical modification, each employing different mechanisms and chemicals to achieve desired properties.
1. Chemical Wood Preservation
This involves applying or impregnating wood with biocidal chemicals designed to kill or repel organisms that degrade wood. The chemicals are typically forced into the wood under pressure or applied through dipping or brushing.
Common types of wood preservatives include:
- Copper-based preservatives: These are among the most widely used due to copper's effectiveness against fungi and insects.
- Chromated Copper Arsenate (CCA): Historically very popular for its broad-spectrum protection, CCA is now restricted in residential applications in many regions due to environmental and health concerns regarding arsenic. It's still used in some industrial applications.
- Alkaline Copper Quaternary (ACQ): A lead-free and arsenic-free alternative, ACQ uses copper as the primary fungicide and insecticide, combined with a quaternary ammonium compound.
- Copper Azole (CA): Another popular copper-based alternative that uses azole compounds to enhance fungal protection.
- Creosote: A tar-based product primarily used for industrial applications like railroad ties, utility poles, and marine pilings due to its excellent water repellency and protection against decay and insects. It has a distinctive odor and can leach, limiting its use in residential settings.
- Borates: Boron compounds are effective against fungi and insects, including termites. They are often used for interior framing or wood not exposed to direct wetting, as they can leach out in continuous contact with water. They are typically applied by diffusion or pressure treatment.
- Light Organic Solvent Preservatives (LOSP): These contain fungicides and insecticides dissolved in organic solvents. They penetrate wood well and cause minimal swelling, often used for joinery and non-ground contact applications.
Application Methods for Preservatives:
| Method | Description | Typical Application |
|---|---|---|
| Pressure Treatment | Wood is placed in a sealed cylinder, and chemicals are forced into the wood cells under vacuum and pressure. Achieves deep penetration. | Decking, fences, utility poles, structural timber |
| Non-Pressure Treatment | ||
| - Dipping | Wood is submerged in a chemical solution for a short period. | Siding, joinery, sapwood treatment |
| - Brushing/Spraying | Chemical is applied to the surface. Offers limited penetration. | End-grain sealing, remedial treatments |
| - Diffusion | Green (wet) wood is soaked in a high-concentration solution, allowing chemicals to diffuse into the wood. | Freshly cut timber, sapwood |
2. Chemical Wood Modification
This approach involves altering the chemical structure of the wood itself, or introducing non-biocidal chemicals into the cell walls, to fundamentally change its properties rather than just adding a protective layer. These modifications can significantly improve dimensional stability, hardness, and resistance to biological degradation without relying solely on toxic biocides.
Key chemical modification techniques include:
- Acetylation: This process treats wood with acetic anhydride in the presence of a strong acid catalyst. The acetic anhydride reacts with hydroxyl groups in the wood's cell walls, converting them into more stable acetyl groups. This modification considerably reduces the hygroscopicity (ability to absorb moisture) of wood and increases its resistance to fungi by making the cell walls less digestible for microorganisms. For this treatment to be successful, the wood must be completely impregnated with the chemical, ensuring thorough modification throughout the material. Acetylated wood offers enhanced dimensional stability and decay resistance without the use of traditional heavy metal biocides.
- Furfurylation: Wood is impregnated with furfuryl alcohol, which then polymerizes within the wood cell walls, making the wood denser, harder, and more dimensionally stable. This also enhances decay resistance.
- Silane Treatment: Involves impregnating wood with silane compounds that react with the wood's cell walls, forming chemical bonds that enhance water repellency and dimensional stability.
- Resin Impregnation: Wood is infused with thermosetting resins (like phenolic resins) that cure within the wood structure, improving density, hardness, and stability.
These advanced treatments offer environmentally friendlier alternatives to traditional preservation methods, providing long-term performance benefits for wood in demanding applications.
