Old-new glue for plywood, composites

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Old-new glue for plywood, composites
Illustrated ad of interior doorway and open wooden door with a woman inside room applying lipstick
Is newer always prettier? After World War II, cheap synthetic glues led to an explosion of flush doors, among hundreds of new products. In 1981, Germany and Denmark limited release of formaldehyde from composite woods.
When plywood came into industrial use a century ago, the criss-crossing layers of wood were glued with a soybean derivative. Then, in the 1930s (long before tofu sprouted in the American diet), strong, synthetic glues, derived from natural gas or oil, started to shoulder aside the soy stuff. Cheap and water-resistant, these urea formaldehyde glues helped plywood, particle board and similar composite wood products dominate the furniture and building industries. Formaldehyde was also used in clothing, paint, paper, wall covering and roll insulation. Many of these products released formaldehyde (CHOH) but inside the average house, the bulk of the exposure came from plywood and its plural planar progeny. In 1991, the U.S. Environmental Protection Agency branded formaldehyde a probable human carcinogen. The compound quickly dissolves in mucus membranes in the nose, throat and lungs, causing irritation and triggering asthma attacks. Inside the home, formaldehyde can be released from composite wood in furniture, cabinets, sub-floors and wall panels. The notorious FEMA trailers that sparked so many health complaints after Hurricane Katrina were chock-a-block with cheap, formaldehyde-glued structures.
ight African American kids, three on bikes, playing in front of row of six new trailers and two cars
Photo: FEMA
After Hurricane Katrina, “FEMA trailer” residents blamed health problems on formaldehyde released from composite wood products in the trailers. A new soybean-based glue could eliminate these problems after it is adopted more widely by manufacturers of particle board.

Legumes to the rescue!

Glass of white soymilk with a handful of white-ish soybeans at its base
Although manufacturers have taken steps to reduce formaldehyde releases, a more comprehensive solution could reside in the plant that was originally used to glue plywood — the soybean. Charles Frihart, a chemist at the U.S. Department of Agriculture Forest Products Laboratory, described the evolution of a soy super glue to the American Chemical Society meeting in Boston yesterday. “Biological materials were used as adhesives until petroleum became very cheap after World War II,” he told us, “and the synthetics displaced agricultural, natural material.”
Soybean is chockfull of proteins. You can drink soy milk, or extract good glue from soy flour.
Photo: LinasD
The soy-story is developing into a key advance for “green chemistry,” the quest to reduce toxic burdens from the factory to the disposal site. Until a few years ago, soy glue was less water-resistant than synthetics, Frihart says. Then, Kaiching Li of Oregon State University discovered how to increase water resistance by “cross-linking” the strands in soy-based adhesive.

All formaldehyde, all the time!

Formaldehyde is all around us. Because it is released by natural wood, nobody claims to make “formaldehyde-free” plywood. But no-added-formaldehyde glues slash formaldehyde releases.

More links = more power!

Long ago, cross-linking became the key to converting soft, sticky natural rubber into a useful product. “Rubber was never very useful because it would soften in the heat, until they developed a way to crosslink it,” says Frihart. Charles Goodyear invented vulcanizing, a process for cross-linking rubber, in about 1840, laying the groundwork for the bicycle and then the automobile. Invention is usually a group activity these days, and Frihart points to cooperation between scientists at the Forest Products Lab and Columbia Forest Products, Inc., which is already making products with soy-based glues. A key driver for the new glue came from California, which deemed formaldehyde-based glues an indoor health hazard. Before it banned the use of formaldehyde in indoor products, however, California needed to know that an alternative technology was available. Once the cross-linked soy glue was proven to work, the state enacted a low-formaldehyde standard that, in effect, went national. “California is the largest market and people are not going make a separate product for California,” Frihart says. The cross-linking invention is already reducing indoor formaldehyde at the top of the furniture market, where plywood is used instead of particle board, Frihart says. “More than 50 percent of interior plywood used for cabinets and furniture is now being made with the improved soy-based adhesive.”
Machine with opposing faces has just finished squeezing 2 narrow strips of wood.
Researchers at Forest Products Laboratory in Madison, Wis. test the strength of soy glues: Two rams compress and heat the joint, then clamps at each end of the wooden strips pull until the joint breaks.

Practical for particle board?

Particle board remains the basis for most cheaper furniture, and while some particle board already uses formaldehyde-free glue, “We want to make it better, with higher strength, especially in wet conditions,” Frihart says. Charles Goodyear did not have to integrate vulcanization into a rubber industry – which barely existed — but today, a new adhesive “has to fit into the way the product is being made commercially,” Frihart says, “and we continue to work on modifying the adhesive to fit better individual plants.” Frihart says the soy story echoes a larger, but behind-the-scenes evolution in materials. “Because we better understand materials, and how to manipulate them, we keep figuring out ways to make them work better, even if they are not labeled new-and-improved.”

– David Tenenbaum

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Terry Devitt, editor; S.V. Medaris, designer/illustrator; Jenny Seifert, project assistant; David J. Tenenbaum, feature writer; Amy Toburen, content development executive

Bibliography

  1. Formaldehyde in the Indoor Environment, Tunga Salthammer, Sibel Mentese, and Rainer Marutzky, Chem. Rev. 2010, 110, 2536-2572.
  2. Soy protein beats urea formaldehyde!
  3. U.S. Forest Products Lab.
  4. Glue goes green.
  5. Better bonding with beans.
  6. Formaldehyde and cancer risk.
  7. Formaldehyde and childhood asthma.
  8. History of plywood.
  9. Introduction to green chemistry.
  10. Soybeans in sustainable building.
  11. Blue mussels: the other bio-based glue.
  12. Vulcanization.
  13. FEMA trailers may be making residents sick.
  14. Banned FEMA trailers get a second life.
  15. FEMA trailer study.
  16. More green chemistry: spider silk.
  17. Another environmental hazard: mercury pollution.
  18. Asthma.
  19. Air hazards.
  20. Rebuilding after Katrina.
  21. Salvage logging.