Introduction of bio-based materials in adhesives and renewable polymers
Renewable polymers: There are various methods through which renewable materials can be incorporated into laminate adhesives. The most straightforward approach is to use natural products, namely bio-based polymers such as proteins that already possess adhesive properties.
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The second option is to use monomers that can be derived from renewable sources, and to combine them to create polymers similar to synthetic adhesives. Ultimately, bio-based materials can be introduced as additives in the formulation of synthetic adhesives.
Biopolymer adhesives have historically been used as longer-lasting adhesives compared to synthetic polymers, but they have been replaced in many applications due to the cheaper production or superior characteristics of their synthetic counterparts. One example of an adhesive used on the back of stamps is historically based on natural gums like gum arabic, but it has now largely been replaced by polyvinyl acetate due to its adhesion and water resistance.
Proteins, natural rubber, and polysaccharides, particularly starch, natural gums, and cellulose, are all renewable polymers that have been used as adhesives in the past. When plywood adhesive was first used for construction in the 19th century, protein-based casein bio-adhesives were employed. These resins have now been replaced by synthetic phenolic resins that offer high adhesion strength and water resistance.
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Animal glue, derived from collagen obtained by burning animal connective tissue, has been in use for over 3000 years. Due to its solubility in water, it is now only utilized in specialized applications such as conservation and musical instrument making. Aside from the historical accuracy that necessitates its use, its brittleness is also beneficial as it allows for breaking without damaging the bonded wood, making it ideal for repair and reassembly.
In many fixed applications, solvent-based adhesives are used because water damages paper; however, in stick adhesives, the required amount of water is low enough not to cause issues. Pressure-sensitive adhesives for office labels often still contain natural rubber, which has good adhesive and shear resistance properties and is cheaper than synthetic alternatives.
These materials serve as a good basis for renewable adhesives. However, their structure—and consequently their properties—are compatible with their original natural environment. If they are to be used in various fields, especially where high performance is required, modifications to these structures are essential. For instance, due to the presence of numerous polar groups in proteins and polysaccharides, water resistance is often the main barrier that needs to be overcome.
A biopolymer with relatively different properties compared to proteins, natural rubber, and polysaccharides is lignin. Unlike the aforementioned bio-based polymers, it consists of a dense cross-linked aromatic network that has little compatibility with most solvents and decomposes without melting.
