Water-Resistant Three-Layer Plywood Using Glucose and Citric Acid

The most widely used materials for building decoration, home furniture and flooring are composite wood products available in large sheets.

Citric acid and glucose produce a wood glue that creates a strong, water-resistant three-ply plywood. Image credit: adapted from ACS Applied Materials & Interfaces, 2022.

However, resins and glues that are intact with fiberboard, particleboard, and plywood typically contain formaldehyde and could release this carcinogen into the air.

To design a harmless adhesive, scientists report in Applied materials and ACS interfaces have combined glucose and citric acid – sugar and an ingredient in orange juice – into a strong, water-resistant wood glue for plywood.

To create plywood, manufacturers glue together thin layers of wood and further harden the material under heat and pressure, creating huge, flexible panels. One of the general adhesives is a urea-formaldehyde resin because it is inexpensive and bonds firmly to wood.

But formaldehyde emissions from plywood with this type of resin have increased health and environmental concerns. Previous studies have shown that solutions of sucrose, a two-unit sugar made up of fructose and glucose, and citric acid form a natural, water-resistant wood glue.

However, a zinc chloride catalyst is required to reduce energy consumption for curing plywood, which also decreases adhesive strength. Therefore, Hong Lei and coworkers wanted to know if pure citric acid and glucose could produce a strong adhesive with a less energy-intensive curing process.

Glucose solutions and varying amounts of citric acid were heated by the researchers in a sticky liquid that was applied to poplar veneers. Additionally, they stacked three veneers, then pressed them into a single sheet at 392°F for 6 minutes.

The sheets were then cut into smaller pieces for strength testing, and under pressures above 101 psi, the plywood samples all broke along the wood fibers and not at the glued joints. These results meet the standard need for plywood in China.

When soaking plywood samples in hot or boiling water, only those created with citric acid to glucose ratios greater than 0.6 exhibited bond strengths that met standard requirements.

Scientists attribute these results to the increase in ester bonds between citric acid and wood, thereby increasing the water resistance and bond strength of the wood. According to the researchers, adhesives based on citric acid and glucose have proven effective for the wood products industry.

The study authors acknowledge financial support from the National Natural Science Foundation of China, the Yunnan Provincial Natural Science Foundation, Project 111, and the National Research Agency (ANR).

Journal reference:

Li, C. et al. (2022) Fully bio-based adhesive based on glucose and citric acid for high performance plywood. Interfaces of applied materials ACS. doi.org/10.1021/acsami.2c02859.

Source: https://www.acs.org/

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