Dimensional stability mechanisms of binderless boards by heat or steam treatment: a review

Binderless composite panels are manufactured from wood or non-wood lignocellulosic particles as well as fibers bonded together without using any adhesives. They have recently attracted increasing attention due to their numerous advantages, such as being eco-friendly and the possibility to use agricultural wastes, as well as resulting in the low production cost given that adhesives are not required in the production process. However, low dimensional stability and internal bond strength of the final products are two of the challenges commonly faced. Several studies have been conducted to develop a technique that will allow binderless boards with high dimensional stability. Some techniques or manufacturing processes include hot-pressing, steam or steam-exploded pretreatment, followed by the hot-pressing process, and steam-injection hot-pressing. Since the heat or steam treatment used during the manufacture of compression wood and that of binderless boards are almost same, it is presumed that their dimensional stability mechanisms are also similar. There are a number of ideas offered to explain the mechanisms of self-bonding on binderless boards. These include the thermal softening of the cell wall matrix as well as degradation or decomposition of the chemical components of lignocellulosic materials generating stable free radicals and reactive sites, which then will form cross-linking between polymers in the cell wall. Meanwhile, internal stress relaxation is still infrequently used as a mechanism for dimensional stability in binderless boards. On the other hand, it has been demonstrated that internal stress relaxation plays a significant role in the dimensional stability of compression wood. Therefore, it is suggested that the low-thickness swelling in binderless boards is, among others, caused by internal stress relaxation stored in the cell wall. Therefore, this study aims to review and collect data to clarify the above assumption. It is essential to verify the self-bonding mechanisms of binderless boards to further enhance their performance and promote industrialization.