What Is Treated Wood? The Copperific Truth Behind Green Wood

Last week we introduced the subject of corrosion in the fasteners used in wood construction. Homeowners have an enormous choice of lumber to use in their projects. We all know the feeling of being in the big box store and looking at the seemingly endless choices of lumber stacked in huge piles. And why is it colored that weird green? Let’s take a few minutes to talk about treated wood.

Many Types of Wood Preservatives Are Now in Use

Going back again to Forest Products Laboratory Researcher Sam Zelinka’s Guide for Materials Selection and Design for Metals Used in Contact with Copper-Treated Wood, “Wood preservatives are chemicals that are injected into the wood to help the wood resist attack by decay fungi, mold, and/or termites. Waterborne wood preservatives are used in most cases where the wood may be in contact with humans or will be painted. While many different formulations of waterborne preservative treatments have been developed, only a few of these have been used commercially. Most of the commercial treatments contain cupric ions [copper molecules], which give treated wood its characteristic greenish-brown coloration.”

As we mentioned last week, 2004 ushered in major changes with treated wood when Environmental Protection Agency regulations restricted the use of chromated copper arsenate (CCA) in the United States. The European Union and Australasia made similar changes in their regulations at about the same time. This was a significant change, as CCA had dominated the U.S. preservative market for many years.

Zelinka tells us that “CCA can still be used in certain situations, specifically wood used in highway construction (excluding pedestrian bridges or hand railings).” Since the regulation change, alternatives to CCA have been introduced, and these alternatives now dominate the market.

FPL’s Stan Lebow has summarized alternatives to CCA in many publications, particularly Alternatives to chromated copper arsenate (CCA) for residential construction and the Wood Preservation chapter of the Wood Handbook.

Several alternatives with different formulas are now available. Zelinka says, “Although the formulations of . . . wood preservatives are different from each other, they all have a higher percentage of copper than CCA.” This is important, as the corrosion mechanism has to do with reducing cupric ions in the preservative. In 2007, Zelinka and others found in Direct current testing to measure corrosiveness of wood preservatives that chromates and arsenates in CCA act as corrosion inhibitors.

Many of the post-2004 preservatives have been standardized by the American Wood Protection Association. Additionally, several commercially important preservatives have been introduced to the market by ICC-ES (ICC Evaluation Services) evaluation reports.

According to Zelinka, “These preservatives include “micronized” formulations . . . which have various trade names. In these formulations, soluble copper is not injected into the wood; rather solid copper, copper oxide, or copper carbonate is ground into submicron particles or “micronized” and suspended in solution prior to injection. Several different formulations of these preservatives are covered by different ICC-ES evaluation reports. These formulations differ in the listed uses, required retentions, and have slight differences in the formulations, but in general require less copper than the nonmicronized counterparts.”

From left to right are examples of different treated wood: micronized copper quaternary (MCQ), didecyldimethylammonium carbonate (DDAC), and alkaline copper quaternary (ACQ-D). Cupric ions from the wood preservative causes the dark coloration of the wood. Excess copper has deposited on the MCQ (green splotches) and the ACQ (along the end grain).

From left to right are examples of different treated wood: micronized copper quaternary (MCQ), didecyldimethylammonium carbonate (DDAC), and alkaline copper quaternary (ACQ-D). Cupric ions from the wood preservative causes the dark coloration of the wood. Excess copper has deposited on the MCQ (green splotches) and the ACQ (along the end grain).

Behold the many choices available to the homeowner. Armed with knowledge, that deck you build next summer can be beautiful and will last a long time.

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