As a result of the growing need for safer, more environmentally friendly compounds and the use of renewable sources, to reduce the depletion of natural resources, it became necessary to search for bio-based alternatives to phenol-formaldehyde resins. They are synthetic polymers that can be found in the wood-processing industry, various insulating foams, in coatings and as a material for electronic housings. The compounds used for synthesis are derived from fossil resources, namely phenol from petroleum via the cumene process and formaldehyde from methanol. Phenol is toxic, corrosive in high concentrations, harmful to both human health and the environment, while formaldehyde irritates the respiratory tract, is classified as a carcinogen, and is released from the final polymer in the form of vapors. The thesis describes possible candidates for replacing both phenol and formaldehyde with compounds from renewable sources and the effect of substitution on material properties in different subsequent applications. Bio-oil and similar renewable sources are not the most suitable options for replacing phenol, because of their heterogeneous composition and the presence of impurities. Substitutes for both substances, preparation route and the extent and effectiveness of substitution in individual applications are presented. I found that it is not possible to identify a single universally suitable substitute compound for phenol or formaldehyde and make a general assessment. A change in the properties of the final material can be both positive for one application and negative for another. For example, reducing the crosslink density is beneficial for coatings and foams, because it increases their flexibility, but at the same time it is harmful for composites. In some applications, the material has improved so much after replacement that it has surpassed conventional phenol-formaldehyde resins, especially in terms of higher heat resistance when they use aromatic dialdehydes instead of formaldehyde. It is important to be aware that the renewable nature of the raw materials does not necessarily mean a better impact on the environment and health, as more energy may be required for preparation, or other volatile compounds may be released from the final product. An example of this is the use of glyoxal based resins in plywood, where acetaldehyde releases from the product and therefore it is not necessarily a safer alternative to formaldehyde. It was found that tannin is suitable to use instead of phenolic glue, because it is already commercially available and meets the requirements necessary for use in wood-based panels. In coatings, cardanol was found to be the best choice instead of phenol. Among all of the formaldehyde substitutes I covered, furfural and aromatic dialdehydes had the greatest potential.
|
