Review on impregnation issues in laminates manufacture: opportunities and risks of phenol substitution by lignins or other natural phenols in resins

摘要

The resins used in impregnation of the core layers of Kraft paper-based decorative laminates are basically synthetized from phenol and formaldehyde (PF). The increasing costs of phenol force laminates manufacturers to search for alternatives or partial substitution products to reduce consumption of this chemical. Preferably, such PF alternative substitutes are environmentally friendly bio-based phenolic compounds, such as lignin, tannins, or cashew nut shell liquid. However, substitution raises some technological issues in the impregnation process due to the chemical variability of these natural raw materials and their different properties from phenol. These differences do not only have an influence on the technological behavior of the modified resins during impregnation and laminate pressing, but also require inclusion of new or modified processes such as biomass treatment (down-streaming, modification of natural polyphenols, activation, etc.) or novel synthetic procedures. Moreover, the use of resins containing natural compounds in the laminates manufacture can result in products with minor mechanical properties. This article provides a general overview of the most promising candidates of such bio-based materials and deals with the most important issues when it comes to their incorporation into PF resins. Due to their abundance on Earth, much knowledge of lignin-based materials has already been gained and uses of lignin in PF resins have been studied for many decades. Other natural polyphenols that are less frequently considered for impregnation are covered as well, as they do also possess some potential for PF substitution.