The go-to materials for building home furniture, décor and floors are composite wood products in large sheets. But the glues and resins holding together particleboard, fibreboard, and plywood usually contain formaldehyde and could release this probable carcinogen into the air. To develop a non-toxic adhesive, researchers reporting in ACS Applied Materials & Interfaces have combined glucose and citric acid—sugar and an orange juice ingredient—into a strong, water-resistant wood glue for plywood.
To make plywood, manufacturers 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 firmly to the wood. However, formaldehyde emissions from plywood with this resin type have raised health and environmental concerns. 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. But a zinc chloride catalyst is required to decrease the energy consumption for the plywood curing, which also reduces the adhesive's strength. So, Hong Lei and colleagues wanted to see if pure glucose and citric acid could produce a strong bond with a less energy-intensive curing process.
The researchers heated solutions of glucose and varying amounts of citric acid into a sticky liquid that they applied onto poplar veneers. Then they stacked three veneers and pressed them into a single sheet at 392 F for six minutes. The team 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 fibres and not at the glued seams. These results satisfy the standard requirement for plywood in China. When the plywood samples were soaked in hot water, only those made with citric acid to glucose ratios above 0.6 had adhesive strengths that satisfied the standard requirement. The researchers attribute these results to increased ester links between citric acid and wood, which increased the wood-binding strength and water resistance. The researchers say that citric acid-glucose adhesives hold promise for the wood products industry.