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Adhesive science can help optimize use of timber resources

Virginia Polytechnic Institute And State University : 20 March, 2002  (Technical Article)
The North American supply of large trees and timbers has diminished from harvest and environmental regulation, but the demand continues to rise. Chip Frazier, associate professor of wood science and forest products at Virginia Tech, is working to advance technology used in composite materials, which he says offer us an ability to use our timber resources more efficiently.
'In order to optimize efficiency in the use of our timber resources, we have to learn as much as we can about the adhesive bonding that allows the use of composite materials,' Frazier says. 'Adhesion is one key to meeting the building needs of society.'

'Composites are made from trees that were previously considered undesirable because of their small size or natural defects. But when these trees are reduced to small particles, slivers, or strands, and then reconsolidated with adhesive, they can be as strong or even stronger than solid wood,' Frazier says.

In fact, according to Frazier, the design strength of composites is greater than the design strength of solid wood.

'Composite materials have no major flaws like knots, for example, because knots are chipped up and redistributed throughout the composite. Consequently, composite materials have more uniform properties, which is why the design strengths are often higher than for solid wood,' he says.

Frazier and his group of graduate students and postdoctoral research associates are investigating the molecular nature of adhesion, focusing on durability. They study the chemistry of adhesives under conditions that simulate conditions to which the material will be exposed. They also focus on the durability of novel adhesive systems.

'We want to know what impacts durability, what molecular phenomena lead to enhanced durability,' he says.

Early composite technology began in the 1920s with products like plywood. The field has grown slowly over the years, but has really taken off in the past 20 years. In many places where you formerly would have seen plywood, they now use Oriented Strand Board. It's often used instead of plywood for structural sheathing, sub-floor sheathing, roofing, etc. It's less expensive to produce than plywood, which is made from solid wood sheets, or veneers. OSB is made from smaller flakes and strands, so it can be made from alternative sources that aren't appropriate for plywood. That's one reason why it's cheaper, and a big reason why resource utilization is improved.

A grant from the Virginia Agricultural Experiment Station is helping to support Frazier's work to understand the performance of the adhesives used to make wood-based composites. He and his associates are studying the molecular structures and motions in the adhesive 'interphase,' that area in the bondline that is neither wood nor adhesive, but an intimate combination of both. They then will correlate that molecular information with durability.

Based on the research, they may recommend the use of different types of adhesives, or different processing conditions such as how the adhesive is applied, how much is used, and at what temperature the work is done. Or they may recommend that adhesive be formulated in a different way.

'Traditionally, people who are trained in wood science aren't very knowledgeable about chemistry, and vice versa,' he says. 'But our students receive training in wood science and polymer science. This makes them valuable to both industries by bridging the gap.'
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