Wood has clear advantage for energy efficient buildings

Wood that has been treated to make it transparent could be a promising material for energy efficient buildings, even replacing glass in windows or roofs, according to researchers [Qiu et al., Composites Science & Technology 172 (2019) 43-48, https://doi.org/10.1016/j.compscitech.2019.01.005].

The transparency of glass is a unique advantage, but its high thermal conductivity is less than ideal in terms of energy efficiency. Wood, by comparison, possesses excellent thermal insulation and mechanical properties ideal for structural applications but is not transparent. Recently, however, scientists have devised a means of rendering wood see-through by removing lignin from natural wood and filling the voids with a transparent polymer. But residual lignin and resin in transparent wood ages when exposed to ultraviolet (UV) light, becoming discolored. Near infrared (NIR) light also passes easily through transparent wood, which is not ideal for maintaining indoor temperatures at a constant level.

“Our work has improved the environmental durability and heat retaining properties of transparent wood by adding antimony-doped tin oxide (ATO) nanoparticles,” explain Yonggui Wang and Yanjun Xie of the Key Laboratory of Bio-based Materials Science and Technology (Ministry of Education) at Northeast Forestry University in China.

Since ATO is a well-known thermal insulation and UV shielding material, the researchers added it in the form of nanoparticles dispersed in a polymer with a matching refractive index, namely poly(methyl methacrylate) (PMMA) to poplar wood delignified by treatment with sodium chlorite.

“After addition of modified ATO nanoparticles, both the optical and mechanical performance of transparent wood are improved,” point out Wang and Xie.

With just 0.3% ATO nanoparticles, the transparent wood retains 70% transmittance to visible light but has a very low transmittance in the infrared. Moreover, its thermal conductivity is not be increased by the addition of ATO.

“The ATO/TW fabricated in this study shows high transparency, excellent NIR heat shielding, and ultraviolet shielding,” say Wang and Xie. “Moreover, the incorporation of modified ATO nanoparticles enhances the interfacial bonding among the compounds, improving the fracture strength of ATO/transparent wood.”

As it is difficult to remove all the lignin from wood, UV exposure makes aging and discoloration inevitable. But the addition of ATO nanoparticles can mitigate this problem by absorbing UV and enhancing NIR absorption.

Better compositing strategies, moreover, such as depositing ATO on the cells walls of delignified wood before polymer infiltration, could compensate for the negative effect of ATO nanoparticles on the transparency of ATO/transparent wood.

“Although ATO/transparent wood exhibits potential as an energy-saving material for windows and transparent roofs, for example, it might take quite a while before it is available for practical applications,” caution the researchers. “For example, the delignification and impregnation for large sized pieces of wood may be a challenge.”