In an era of glass and steel construction, wood may seem old-school. But now researchers say they have given timber a makeover to produce a material that is not only sturdy, but also transparent and able to store and release heat.
The researchers say the material could be used in the construction of energy-efficient homes, and that they hope to develop a biodegradable version to increase its eco-friendly credentials as an alternative to plastic, glass or even cement.
“We prepared a material that is multifunctional – it can transmit light very well and also it can store heat. We combined these two functions in a single material,” said Céline Montanari of the KTH Royal Institute of Technology in Stockholm. Montanari is presenting the work at the spring national meeting of the American Chemical Society in Orlando.
To produce the material, the team built on previous work in which they took balsa wood and removed its lignin – a component of wood that gives it strength and colour. Acrylic, which is non-biodegradable and water-repellent, was introduced into the remaining tissues where it filled both the tiny pores left by the removal of lignin and the hollow vessels that carried water in the tree. That, said Montanari, not only helped maintain the wood’s structure but also restored its strength and improved its optical properties. The upshot was a frosted-looking wood-based material.
In the latest work the acrylic was mixed with another substance called polyethylene glycol, which permeates wood well. Crucially, polyethylene glycol also has another feature: when it is heated it absorbs energy and melts, but when temperatures fall it hardens, releasing energy in the process. The team say this property means their wood-based material, which goes from semi-transparent to transparent when warmed, could be used to make buildings more energy-efficient, with energy captured from the sun during the day released later into the interior.
“If you take 100g of this transparent wood material with the [polyethylene glycol] inside, it can absorb up to 8,000J of heat, which corresponds to basically what a 1W [bulb] could produce in two hours,” said Montanari, adding that different types of polyethylene glycol melt at different temperatures so the team could tweak the transparent wood’s properties to its application.
Materials containing substances that can store and release heat in such a way are not a new idea in the construction industry, with various types on offer as a novel form of insulation to reduce energy consumption. But the team say their approach is different because it uses a natural material that reduces the need for oil-based materials and related CO2 emissions.
However, Montanari said there was plenty of work still to do – including replacing the acrylic with a biodegradable alternative for some applications, scaling up production of the material, and carrying out computer models of buildings to see how transparent wood compares with glass.
Mark Miodownik, professor of materials and society at University College London, who was not involved in the research, cautioned against engineering the material to be biodegradable, saying it would make the wood less environmentally sustainable, not more.
“We need construction materials to be carbon sinks and so they need to be recyclable and reusable, not biodegradable,” he said, adding that one possibility would be for the new type of timber to be recovered from buildings and used in new projects, similar to the way steel is used in “modular construction”.
While Miodownik added that the material appeared to be a “solution looking for a problem”, he said such an approach has precedent, noting that it is “how many materials have got invented in the past”.
