Chapter 31 Processes Used to Condition Wood Materials

Objectives Moisture and temperature resistance conditioning Microwave drying Making wood easier to form Acetylation for toughness and impact resistance Chemicals for moisture resistance

Radio Frequency Dielectric Heating Radio frequency (RF) drying is also called microwave heating. Moisture is vaporized through induced heat. RF heating also improves impact resistance and quality of wood. RF process is most effective with hardwoods such as oak, walnut, or ash.

Wood Plastic Composition (WPC) To make WPC wood, the wood material is placed in a vacuum chamber and all of the air is removed. A plastic is then introduced and is drawn by vacuum into the cells of the wood. After the wood is filled with plastic, the work piece is bombarded with gamma rays from radioactive isotopes. The exposure to radiation generates heat and hastens the polymerization. When the work piece cures, the wood has been transformed into a material with many of the characteristics of plastic.

Impreg With impreg, a fiber penetrating thermoset resin is used. Impreg is usually made using green veneers to facilitate resin pickup. Once the wood is impregnated with resin, it is dried. After drying, the veneer is cured by heating at high temperature. Often veneers are laminated together to make products such as handles for knives and other kitchen utensils.

Stabilizing Wood Stabilizing wood involves injecting dyes into the material while it is being pressure treated. The advantage of this process is that the resulting product has the look and feel of wood. Compreg: Compreg is similar to impreg, but the wood is compressed prior to curing. Acetylated Wood: Acetic anhydride is introduced in a pressure cylinder with dry wood to acetylate the wood.

Plasticized Wood Wood can be made supple (plasticized) by exposing it to ammonia. After the material is exposed to ammonia, it can be formed and then dried. The anhydrous ammonia process requires keeping the wood in a form only for a brief period of time, and spring back is minimal. With the liquid ammonia process, the cooled wood stock is soaked in a tank for a period of time (one to 3 hours for ¾” stock). With the gaseous ammonia process the work piece is cooled and placed in an airtight vacuum chamber. Once the sample has been exposed to anhydrous ammonia gas long enough for it to be plasticized, the gas is removed from the chamber. The stock is then placed in a form for a brief period of time to set it in the desired shape.

Staypack Wood preconditioned between 30 % and 65 % relative humidity is compressed and heated. The pressure and heat reduces internal stresses and creates a new type of wood with improved tensile and flex properties. Both veneers and solid wood can be used to make Staypack. Typical application is tool handles.

Pressure Treated Wood All industries in the United States must comply with federal and state regulations. Primary concerns address pesticide regulation, waste management, human and environmental exposure, and availability of forest resources. National standards are published by AWPA (American Wood Preservers Association). Legislation regulating the use and disposal of pressure treated wood is shown in fig 31.5 page 407. Treated woods need to be disposed of through trash collection methods or by burial, not be burning.

Cautions when Handling Pressure Treated Wood Treated woods should not be used where it will contact food, humans, or animal feed. Treated woods should never be used as mulch, cutting boards, counter tops, animal bedding, or structures and containers for storing human or animal food and drinking water. Avoid prolonged inhalation of sawdust from treated wood. When possible cut treated wood outside to minimize accumulation of airborne sawdust.

Polyethylene Glycol (PEG) Polyethylene glycol is used in any application where the wood is wet and needs to be dried gradually without cracking. After parts are treated with PEG they are stacked using spacers between boards (Stickered) and dried in a well ventilated heated room.

Summary Radio frequency (RF) drying is also called microwave heating. To make Wood Plastic Composition (WPC) wood, the wood material is placed in a vacuum chamber and all of the air is removed. A plastic is then introduced and is drawn by vacuum into the cells of the wood. Wood can be made supple (plasticized) by exposing it to ammonia. National standards are published by AWPA (American Wood Preservers Association). Legislation regulating the use and disposal of pressure treated wood is shown in fig 31.5 page 407. Treated woods need to be disposed of through trash collection methods or by burial, not be burning. Polyethylene glycol is used in any application where the wood is wet and needs to be dried gradually without cracking.

Home Work 1. Who publishes national standards for pressure treated wood? 2. How should treated woods be disposed? 3. In what applications is polyethylene glycol used?