The wood preservation process involves impregnating the wood with chemicals that protect the wood from biological deterioration and to delay combustion due to fire. The most common process includes pressure-treatment in which the chemical is carried into the wood by a carrier fluid under pressurized conditions. Treatment chemicals utilized during the wood preserving process are separated into four major categories (AWPI, 1994). These categories include: waterborne preservatives including CCA, oilborne preservative including pentachlorophenol, creosote, and fire-retardants. The purpose of the first three chemicals is to prolong the useful life of wood products by protecting them against insect and fungal attack. Wood exposed to the outdoor atmosphere and in direct contact with soil and water are more prone to decay and therefore usually require treatment. The fourth preservative, fire-retardants, delay the combustion process when wood is subjected to fire.
Fire-retardants represent the smallest fraction of the wood treatment market. Formulations for fire retardants include ammonium salts, borates, phosphates, bromides, and antimony oxides. Creosote is a heavy black-brown liquid produced by condensing vapors from heated carbon-rich sources, such as coal or wood. The resultant preservative is sometimes mixed with tar oils and petroleum oils. Oilborne preservatives utilize oil to carry the treatment chemical into wood. Oilborne preservatives include copper naphthenate, zinc naphthenate, and pentachlorophenol. The most common of these is pentachlorophenol which is a crystalline aromatic compound. Both pentachlorophenol and creosote impart a dark color to the wood, have an odor, and result in an oily surface which is difficult to paint. Wood treated with either chemical is flammable and contact with skin may cause irritation. Most common uses for creosote treated wood include railroad and bridge ties. Pentachlorophenol is used to treat utility poles and crossarms (Milton, 1995). Neither pentachlorophenol- nor creosote-treated wood should be used inside residences.
Waterborne preservatives utilize water as the carrier fluid during the treating process. The water is evaporated from the wood shortly after treatment leaving behind the treatment chemicals. The most common waterborne chemicals are metal oxides. These chemicals include chromated copper arsenate (CCA), acid copper chromate (ACC), ammoniacal copper arsenate (ACA), chromated zinc chloride (CZC), and ammoniacal copper zinc arsenate (ACZA). The most common of these waterborne preservatives is CCA which represents over 90% of the U.S. waterborne preservative market (AWPI, 1996). CCA is composed of the oxides or salts of chromium, copper, and arsenic. The copper in the wood serves as the fungicide whereas the arsenic protects the wood against insects. The chromium fixes the copper and arsenic to the wood. CCA can be separated into Type "A", "B", or "C" depending upon the relative proportions of metals (table I-1). The relative proportions range from 35-65%, 16-45%, 18-20% for chromium, arsenic, and copper, respectively. The amount of CCA utilized to treat the wood or retention level depends upon the particular application for the wood product (table I-2). Low retention values (0.25 lb/ft3) are permissible for plywood, lumber, and timbers if the wood is used for above ground applications. Higher retention values are required for load bearing wood components such as pilings, structural poles, and columns. The highest retention levels (0.8 and 2.5 lb/ft3) are required for wood components which are used for foundations or saltwater applications.
The primary advantages
in the use of CCA-treated wood are that it produces no odor or vapor and its
surface can be easily painted. At low retention values it does not change the
general appearance of the wood, maintaining the aesthetic quality of natural
wood. The wood is suitable for use indoors and is generally used for interior
parts of a wood structure in contact with the floor. Drawbacks of the wood are
a strong green color at high retention values. It should not be used in applications
where it is in contact with food or drinking water. CCA is used to treat primarily
lumber, timbers, posts, and plywood. Its use in treating other products, such
as poles and pilings, has seen relative increases as well.
|CCA-Type A||CCA-Type B||CCA-Type C|
|Chromium as CrO3||65.5%||35.3%||47.5%|
|Copper as CuO||18.1%||19.6%||18.5%|
|Arsenic as As2O5||16.4%||45.1%||34.0%|
Table I-1: Composition of CCA-Type A,B, and C (AWPA, 1996)
|Above ground: lumber, timbers, and plywood||0.25|
|Ground/Freshwater contact: lumber, timbers, plywood||0.40|
water splash, wood foundations: lumber, timbers, and plywood
|Foundation/Freshwater: pilings and columns||0.80|
|Salt water immersion: pilings and columns||2.50|
Table I-2: Retention Requirements for CCA-Treated Wood (AWPA, 1996)