Questions about PUR and epoxy for woodworking
November 30, 2021 | 5:03 pm CST
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Q. What is a PUR adhesive and how does it work with wood?

A. Polyurethane (PUR) adhesives are a class of adhesives. The chemistry and final properties vary from one product to another. For specifics, check the label or discuss your needs with a knowledgeable manufacturer’s rep. Maybe check several companies Polyurethane adhesives are usually formed by the reaction of various isocyanates with polyols. When curing, small molecules combine into large molecules that attach to the wood.
PURs are good gap-filling adhesives as they maintain strength with small gaps in the joint. Super smooth and tight-fitting joints are not essential. Joints can have small unevenness. This, however, is no excuse for being sloppy.

Sometimes wood and water do not mix well. Most PURs do not add any water, compared to the PVA adhesives we have used for over 50 years. No sunken joints with PUR. Some PURs are inelastic (hard) when cured, but others are elastic (flexible).

Most PUR is a single mixture in a bottle, tube or bucket, but some PURs are a two-part adhesive that you have to mix before use. Often the single mix is cured with the presence of moisture in the wood, meaning wetter wood will cure faster. The more uniform the wood’s moisture content in a production situation, the better, because curing time will be uniform.

Some PURs are hot-melt types and are called HM PURs. HM PUR glue guns are very hot. I believe HM PURs have great undiscovered uses within our industry. HM PURs are one-part formulations that combine the initial curing speed of a hot melt adhesive (PUR) with the strength of a structural adhesive. The bond forms in two stages. Stage 1: When the adhesive cools down, and it solidifies like an ordinary hot melt, with reasonable strength already. Stage 2: The moisture-curing reaction proceeds over 24 to 48 hours to reach final strength. HM PURs are resistant to temperature extremes and usually flexible enough to be strong even when the wood moves from MC changes.

PURs are generally high-strength adhesives, so the cured wood joint is 50 percent or stronger than the wood itself when correct procedures are followed. If there are no gaps in the joint, PVAs can do as well, but gaps are often the cause of failures with PVAs, especially in the wintertime. The strongest PURs are two-part adhesives.

PUR adhesive when handled and as it cures can be a health hazard (may be carcinogenic; may cause respiratory allergies), so adequate ventilation is essential. Avoid skin contact. Use appropriate safety equipment; consult with the manufacturer. (If you use this at home, my advice is to avoid using it when children are present nearby.)

Cured PUR can be sanded and can be painted. PUR is weatherproof if used on outside projects. Some PURs are suitable for food contact.

Working time (time between spreading and applying pressure) varies with different formulations. When assembling the joint, squeeze out should be removed as soon as possible. One client of mine claimed that quick removal allowed them to stain without any issues.

FINAL ADVICE: Most good PUR adhesives for wood seem to be expensive, but when the cost is prorated over many pieces of furniture or cabinets, the adhesive cost is actually small. Repairing a poor glue joint when a product is in use is very expensive, so using proper adhesive and proper procedures is worth the expense.

Q. We tested epoxy glue in some joints, including miters in a door. Everything was made perfectly and fit tightly. The pressure was good. Yet, we had failures at low-stress levels. I thought you said epoxy was really strong. Help, please.

A. Indeed, epoxy is strong, but the two parts must be combined in the correct proportions and mixed well. Then, to cure, the epoxy chemicals generate heat. Where we often have trouble is that our joints are too perfect, so the excess epoxy spread in the joint is squeezed out and there is not enough epoxy left to generate the needed heat. In many applications, so that a thick glue line is not obvious, the joint will be "hollow ground." This means that the exposed edges look tight, but inside there is a gap to hold plenty of epoxy for heating and curing. Unlike many adhesives, epoxy can bridge a small gap without losing strength. Remember to wait at least 24 hours to achieve a full cure.

Incidentally, I would consider testing a hot-melt PUR adhesive for a miter, with a little hollow grinding and maybe a spline. Hot-melt applicators are hotter than hobby glue guns. Although it takes a day to develop full strength, within a minute or so, the joint will have considerable strength. I also have seen the squeeze-out (which is a good thing to see with most adhesives except epoxy) cleaned off and there was no effect on finishing.

Q. When using the pin meter with insulated needles to check the moisture content of kiln-dried lumber, can we drive them into the edge of lumber? This would help us check incoming lumber before unloading from the truck.

A. Yes indeed, this is a valid technique if the moisture content (MC) of the lumber is uniform, shell to core, which is usually a good assumption for most kiln-dried lumber. Uniformity is an essential requirement for lumber to be manufactured into cabinets, furniture, millwork, and flooring.
When buying KD lumber, check the incoming lumber’s MC. Appreciate that often a typical kiln contains 10,000 pieces of lumber. The experienced operator selects 10 or 12 pieces of lumber as samples that are monitored to assure the entire load is dry. An unusual piece of wood, equipment failure, or moisture measurement error (rare) can occur.

Some meters tell you that the needles should run with the grain, meaning up and down the lumber on the face, while other meters say to run the needles across the grain, meaning in the width direction on the face. At low MCs, like 7% MC, the orientation of the pins is not a factor.

Moisture meters do not measure moisture directly, but instead they measure an electrical property of the wood, and then this electrical property is calibrated to MC. Buy a meter made in the USA. I have seen many foreign-made meters that are not accurately calibrated. You should make it clear to your supplier what meter (brand, model, needle type) you will use and that they will accept those results.

Q. A quick question: What is the best way to dry 8/4 red oak?

A. If you mean "The Fastest Way," vacuum drying is the best. As vacuum drying is somewhat new technology in the U.S., make sure the operator has successfully dried 8/4 red oak before.

If you mean the most expensive way, with high quality (if the kiln has modern controls), then kiln-dry it green from the saw in about 75 to 80 days. I usually figure that kiln-drying hardwoods will make a gross profit of $25 per day per MBF, so an 80-day drying cycle means you need to charge 25×80 = $2,000 per MBF for drying. If you charge less or sell the wood without a $2,000 premium over green prices, then you could have made more profit by drying several loads of 4/4, air-dried red oak.

If "best" means the lowest cost with high quality, then shed air drying (a shed with plastic mesh burlap walls for the first month or two, followed by shed drying with the mesh removed). Then after 180 days of shed drying with at least 60 days being summertime, go into the kiln for 30 days.

Always, the most critical time to prevent checking and honeycomb is from green to about 50% MC, which is the first 1/3 of the drying cycle.

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About the author
Gene Wengert

Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 35 years. He is extension specialist emeritus at the University of Wisconsin-Madison.