Abstract
There is a strong need to develop and implement appropriate alternatives to replace formaldehyde-based adhesive systems, such as phenol–formaldehyde, in the industry of wood-based panels (WBPs). This is due to the toxicity and volatility of formaldehyde and restrictions on its use associated with some formaldehyde-based adhesives. Additionally, the current pressure to reduce the dependence on polymeric materials, including adhesives, from petrochemical-based sources has led to increased interest in bio-based adhesives, which, in some cases, already provide acceptable properties to the end-product. Among the potential raw materials for good-quality, renewable-based adhesive formulations, this paper highlights tannins, lignin, and protein sources. However, regarding renewable sources, specific features must be considered, such as their lower reactivity than certain petrochemical-based sources and, therefore, higher production costs, resource availability issues, and the need for toxicological investigations on alternative systems, to compare them to conventional systems. As a result, further research is highly encouraged to develop viable formaldehyde-free adhesive systems based on renewable sources, either at the technical or economical level. Moreover, herein, we also showcase the present market of WBPs, highlighting the obstacles that the alternative and new bio-based adhesives must overcome.
Highlights
As a result of new technologies, living standards across the globe have increased, with a corresponding increase in the demand for new production methods and feedstocks to sustain this growth
The typical adhesives used in the production of wood panels are mainly petrochemical-based thermosets, such as phenol– formaldehyde (PF) resins, urea–formaldehyde (UF) and melamine–urea–formaldehyde (MUF) resins, and polymeric methylene diphenyl diisocyanate adhesive systems
UF adhesives are mainly used for an expected indoor-use panel, whilst the addition of melamine lowers the adhesive’s hydrolysis susceptibility, which leads to wood panels with better water and weather resistance [21], which may expand the uses for the manufactured panels
Summary
As a result of new technologies, living standards across the globe have increased, with a corresponding increase in the demand for new production methods and feedstocks to sustain this growth. Each of these products has a wide range of applications, with most of them used in the construction and furniture industries and significant use in decoration and packaging, exemplified in Figure 2 for OSB panels Adhesives such as the petroleum-based UF resin are the preferred adhesives in the WBP industry due to their excellent adhesion performance, even if they have comparatively lower water resistance. The use of traditional adhesives, mainly formaldehyde-based ones, is favoured in the WBP manufacturing industry due to their relative low curing temperatures, excellent adhesion properties, excellent flexibility of application, low cost, and water resistance They entail some significant drawbacks, such as the possibility of the release of volatile organic compounds (VOCs) and formaldehyde vapours, which pose a danger to human health, being known carcinogenic compounds [6], as well as causing other chronic illnesses. Some biomass sources, such as soy, tannins, organosolv/kraft lignin, and cottonseed meal, among others, have been studied as feedstocks to produce bio-based adhesives [20]
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