A global team of scientists has developed a new type of nontoxic, water-resistant adhesive for plywood by combining glucose with citric acid.
According to the team from China’s Southwest Forestry University and Guizhou University, and France’s University of Lorraine, previous studies have shown that solutions of sucrose–a two-unit sugar made of glucose and fructose–and citric acid form a natural and water-resistant wood glue.
However, a zinc chloride catalyst is a must if you want to decrease the energy consumption for the plywood curing. Unfortunately, this also reduces the adhesive’s strength.
This led Southwest Forestry’s Hong Lei and colleagues to discover if pure glucose and citric acid could produce a strong adhesive with a less energy-intensive curing process.
As part of their research, the scientists heated solutions of glucose and varying amounts of citric acid into a sticky liquid that they applied onto poplar veneers. They then stacked three veneers and pressed them into a single sheet at 392°F for six minutes.
This done, the scientists cut the sheets into smaller pieces for strength tests and found that under pressures greater than 101 psi, the plywood samples all broke along the wood fibers and not at the glued seams.
According to Lei, these results satisfy the standard requirement for plywood in China.
When the plywood samples were soaked in hot or boiling water, only the ones made with citric acid to glucose ratios above 0.6 had adhesive strengths that satisfied the standard requirement.
Lei and his colleagues attribute this to the increase in ester links between citric acid and wood, which increased the wood-binding strength and water-resistance–something the wood products industry would surely be interested in.
In order to make plywood, wood manufacturers currently glue together thin layers of wood and then cure the material under pressure and heat, creating large, flexible panels.
One of the most common adhesives is a urea-formaldehyde resin because it’s inexpensive and bonds strongly to wood.
However, formaldehyde emissions from plywood with this type of resin have raised health and environmental concerns.
Image and content: ACS Applied Materials & Interfaces