Using PEG for dimensional stability

      The discovery and use of PEG for stabilizing previously unseasoned lumber. 1998.

by Professor Gene Wengert

I read an article in a magazine several years ago, which describes soaking lumber in a solution of PEG in order to acheive dimensional stability after drying. I cannot remember any more about the article than that, but I hope that you might know something about this subject.

Years ago, some researchers discovered that if green wood was soaked in a solution of polyethylene glycol-1000 (PEG-1000, where the number stands for the molecular weight of ethylene glycol--many molecules of PEG are tied together in a string to make PEG-1000; ethylene glycol-1 is antifreeze), the chemical gradually moved into the wood. This chemical ended up bulking the wood so the wood couldn't shrink when it was dried. So, PEG 1000 became a way of stabilizing green, never-dried-before, wood.

It has been used for many items, including walnut and maple gunstocks, coffee tables, and water soaked wooden artifacts. The wood itself must be porous enough, when green, to allow the PEG-1000 to diffuse into the wood.

PEG works well, although the chemical is expensive. There is some slow bleeding of the PEG out of the wood (it is waxy) in warm weather, potentially leading to finishing problems. However, cleaning the surface before finishing and then applying polyurethane works very well.

My favorite publication on PEG and how to use it is "How PEG helps the hobbyist who works with wood" by H. Mitchell, US Forest Products Lab, One Gifford Pinchot Drive, Madison, WI 53705. Many magazines have published information on PEG too. A very good write-up is included in Bruce Hoadley's book UNDERSTANDING WOOD; see Chapter 11 (1980 edition).

Professor Gene Wengert is Extension Specialist in Wood Processing at the Department of Forestry, University of Wisconsin-Madison.

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The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor A:
There seems to be some confusion on the role of PEG (Polyethylene Glycol) in protecting dimensional stability of green wood. The real benefit of using PEG is that it's a surfactant, meaning that it breaks the surface tension of water. By soaking green wood in PEG it essentially displaces enough water in the wood to evenly disperse and effectively break the inter-cellular surface tension in the wood, which allows it to breathe the water content at a more even rate. The water content in the wood will still fluctuate with the ambient relative humidity; but will do so more evenly throughout the piece instead of at different rates which causes dimensional instability.

PEG with higher molecular weight has longer polymer chains and thereby less easily absorbed into the smaller spaces in the wood, but is more effective in bulking the wood. Lower MW PEG is much less effective at bulking but has more effective penetration. A two step process is often recommended with a lower MW PEG followed by a higher.

Proper drying and cleaning before and after finishing is important as the PEG (particularly lower MW PEG) can come out later, particularly if exposed to warmer temperatures.

As also noted earlier PEG is fairly expensive and not particularly easy to find, especially the various MW desired. If you search the web, you can find it though.

The upside, whatever isn't lost in the process can be reused indefinitely, just store properly and test for proper solution ratio and add PEG/water as necessary to achieve desired ratio again. There are other downsides. PEG is corrosive to most metals other than stainless steel, so when using steel fasteners/brackets proper precautions must be used. PEG has other downsides and some great upsides. It all depends on what species and size/shape you're working with and what the purpose/product of the finished piece will be.

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