Two-step process helps wood and plywood to withstand combustion

Wood is a wonder material. Its renewability, availability, ease of manufacture, and good mechanical properties have seen it used in construction projects for millennia. Recent efforts to reduce the volume of concrete and steel in our urban environments has seen wood’s popularity surge further. 92% of all newly-built homes in the US are now wood-framed, and engineered wood products like cross-laminated timber (CLT) are being used to build towering ‘plyscrapers’ that can reach an impressive 25 stories.

While CLT has a high fire resistance thanks to its dense structure and design, natural wood still comes with a significant fire risk. As the authors of a new paper write, of the ~1.35 million fires that occurred in the U.S. in 2021, “…. home structure fires were responsible for 75% of all civilian fire deaths, 76% of fire injuries, and 55% of property loss, despite making up only 25% of the total number of fires.” That same team, led by chemists at Texas A&M University, has now developed a simple, UV-cured treatment that significantly increases the flame retardancy of natural wood and plywood, without negatively impacting its mechanical properties.

Their research, published in Polymer Degradation and Stability [DOI: 10.1016/j.polymdegradstab.2023.110467], focuses on producing an intumescence-based flame retardant. By combining particular chemical groups together, such retardants protect wood by forming an expanding char on its outer surface in the presence of extreme heat. This char then acts a physical barrier that insulates the underlying substrate from the heat and mass transfer that would propagate combustion.

For their coating, they made a solution of polyethylenimine (PEI) of different molecular weights and a phosphate-containing methacrylate (HMP), and mixed it with a photoinitiator, TPO (diphenyl(2,4,6 trimethylbenzoyl)phosphine oxide), in a 4:1 mass ratio. Samples of wood and plywood were then immersed in the PEI/HMP/TPO solution to coat them thoroughly. After rinsing in DI water, each sample was then cured under a UV lamp. The resulting coated samples appeared glossy and smooth, compared to uncoated wood samples.

To test the effectiveness of the flame-retardant, coated and uncoated wood samples underwent flame tests, followed by SEM analysis. After burning, small, hollow bubbles were observed across the coated wood surface, which the authors attributed to the intumescent mechanism of the treatment. When exposed to flame exposure, poly(HMP) thermally degrades, releasing phosphoric acid that dehydrates the carbon-rich wood substrate, and produces a protective char layer. As the PEI degrades, it releases inert gases (e.g., nitrogen and ammonia) which causes the char to expand, adding further protection for the underlying wood. They saw similar features on the coated plywood samples. Other than that, the appearance of the coated wood was largely unchanged. In contrast, the uncoated samples were destroyed by the flame test.

Energy dispersive X-ray spectroscopy analysis showed that the coating was not only present on the surface of the wood, but had made its way throughout the bulk of the material. Thermogravimetric analysis confirmed the formation of a protective char, as well as a slower rate of degradation, on the treated wood. Cone calorimetry revealed that the total heat

release, average heat release rate, and maximum average rate of heat emission were decreased for both coated samples. For plywood, those metrics decreased by 19%, 14%, and 28%, respectively. For wood, the decreases enabled by the coating were 17%, 41%, and 39%, respectively.

Three-pointing bending tests carried out with coated and uncoated wood revealed “statistically insignificant changes in flexural strength for both natural wood and plywood”, alongside minor increases in the flexural moduli. The authors write that this is “… a promising result as flame retardant treatments generally impair the mechanical properties of wood”, and suggest that when the treatment is applied to much larger pieces of wood, the impact on its mechanical properties would be negligible.

The authors say that their work “…demonstrates a significant step toward effective and environmentally benign protection of wood.”


Sarah G. Fisher, Danixa Rodriguez-Melendez, Ethan T. Iverson, Thomas J. Kolibaba, Jaime C. Grunlan. “Fire protection of wood with an environmentally benign UV-cured polyelectrolyte complex,” Polymer Degradation and Stability 215 (2023) 110467. DOI: 10.1016/j.polymdegradstab.2023.110467