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Heat treatment processes have been used on wood for more than a century and have become a common way of elevating properties such as durability. These processes take place in heated environments without oxygen and the heating process changes the internal chemical makeup of the wood, and therefore its physical properties.
Given how much wood is used today, there are always innovations taking place to improve heat treatment processes. In recent times, nanoparticles have been added to wood products to improve the heat treatment process. One of the most recent investigations has been to impregnate pine wood with nanosilver to see if the heat treatment benefits fall in line with similar studies where nanosilver has been used on other wood types.
Heat Treatment Processes for Wood
Heat treatment processes are ultimately used to improve the overall properties of the wood, particularly the physiomechanical properties that lead to longer wood lifetimes. The second reason is that the heat treatment process tends to remove any sugars from the wood, removing the food source for fungi and other micro-organisms, so heat treatment processes are used to improve the wood’s resistance to organism/fungi as well.
However, there are a few issues with traditional heat treatment methods. While the wood is made more durable (increased dimensional stability) and resistant to invading species, treating the wood and altering the chemical makeup of the wood can make it a lot more flexible, prone to bending, and it can reduce the impact strength of the wood.
This breakdown is caused by the lignin and hemicellulose molecules (molecules that bind cellulose together) in the wood becoming modified, causing the structural effects of cellulose to be changed as well. This creates wood with higher flexibilities. However, at mild temperatures, it is thought that the cellulose molecules change from a semi-crystalline state into a crystalline form, making them a lot more rigid. This rigidity then negates the negative aspects of many high-temperature heat treatment processes, and the addition of nanoparticles into the wood is one way to do this.
The Effect of Nanosilver on Heat Treatment Processes
Nanomaterials have been trialed for a while in heat treatment processes. Over the years, several different inorganic nanomaterials ranging from copper nanoparticles to zinc nanoparticles have been integrated into different woods. A team of researchers has used silver nanoparticles in pine woods to see if there are beneficial effects, but this is not the first time the team has used silver nanoparticles.
In a previous study, the researchers investigated how the thermally modified beech wood heat process could be improved. These are not the only studies to date, and other studies have looked at how the effects of nanosilver can be used to improve the heat treatment processes of tropical wood species. All the studies using nanosilver have so far shown increased durability of the wood and a large resistance against fungi and other micro-organisms.
The fungi and micro-organism resistance has been increased over the standard heat treatment process so far because the choice of nanoparticles to date are those that have strong anti-microbial resistance properties. The addition of these nanoparticles provides an extra layer of microbial/fungi protection because you not only get the food-removal properties (removal of sugar molecules) from the heat process, but you also get the microbial-killing properties that nanoparticles such as copper or silver offers. It could be seen as a dual-protection approach against microbes and fungi.
The recent study builds on the beneficial fungicidal and durability effects found in other studies that used nanosilver and applied it to pine woods. One of the key benefits of using silver nanoparticles in this study is that mild temperatures could be used to perform the heat treatment process, and this meant that the heat treatments could be performed at 145 °C instead of at elevated temperatures.
The researchers impregnated the wood with a 400 ppm nanosilver and tested the properties of the wood after heat treatment cycles at 185 °C, 165 °C, and 145 °C. At the lower temperature of 145 °C, there was a reduced thermal degradation of the lignin and hemicellulose molecules, so the natural polymer-like internal structure did not break down so easily. Moreover, the lignin and hemicellulose also formed extra chemical bonds at these temperatures, and hornification took place. These factors improved the physical properties of the wood.
The ability for the wood to be treated at lower temperatures was due to the impregnation of silver nanoparticles increasing the thermal conductivity of the wood. Because the heat spread easier through the wood’s molecular structure, higher heats were not needed to see beneficial treatment effects.
Once the temperature was elevated, the negative effects often observed in heat treatment processes also materialized. At 165 °C, the presence of hornification offset and compensated for some of the mass loss observed due to the higher heat. However, at 185 °C, the temperature was too high for any molecular compensation to occur, and the high heat degraded the cellular polymers in the cell wall and had a negative effect on both the physical and mechanical properties of the wood.
The inclusion of silver nanoparticles enabled the wood to be treated at much lower temperatures and this, in turn, improved the mechanical and physical properties of the wood, negating some of the issues of many heat treatment processes.
The addition of silver nanoparticles increased the durability and fungicidal/microbial resistance properties of the wood and enabled the heat treatment to be performed at lower temperatures than normal.
In terms of properties, this enabled some of the negative effects of heat treatments (reducing rigidity and impact strength) to be overcome. The lower temperatures of the heating processes enabled the cellulose to become more crystalline and facilitated the formation of new chemical bonds in the structure, promoting better structural properties in the wood compared to the untreated wood.
References and Further Reading
Taghiyari H. R. et al. (2020) Heat Treatment of Pine Wood: Possible Effect of Impregnation with Silver Nanosuspension, Forests, https://doi.org/10.3390/f11040466