Growing global environmental concerns and the desire to limit the dependency on non-renewable mineral resources has stimulated the development of cost-effective and renewable adhesives as alternatives to conventional glues, sealants, and other bonding compounds.
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Owing to their non-toxicity, and biodegradability, several biomaterials, such as starch, cellulose, and lignin, show huge potential as a renewable feedstock for sustainable adhesive applications.
Currently, the majority of adhesives suitable for industrial applications are primarily derived from petroleum resources, such as ethylene-vinyl acetate, block-copolymers of styrene and butadiene or isoprene, polyesters, polyamides, polyurethanes, and polyolefins. Polymer-based adhesives can be generally classified into water-based adhesives, solvent-based adhesives, reactive adhesives, and hot melt adhesives depending on the curing or solidification/drying process.
Emerging Eco-Friendly Adhesives
In recent years, there has been an increasing drive to improve the sustainability of chemical manufacturing processes and products. Growing health and environmental awareness of customers together with the more stringent regulations and the looming shortage and price volatility of fossil-based feedstock has accelerated the transformation toward a circular and resource-efficient industry.
In the field of adhesives, this shift has manifested itself most notably as a switch from solvent- to water-based or high-solid adhesives, and in the renewed interest in traditional natural adhesive materials such as polysaccharides, proteins, and vegetable oils.
Biopolymers are naturally occurring or man-made polymers based on renewable raw materials such as agricultural feedstock. Such feedstock includes corn starch, soy protein, vegetable oils, as well as materials, such as cellulose and lignin, that are extracted from biomass and can be further refined into biopolymers.
Utilization of Wood-Derived Biopolymers
Cellulose is the most abundant biopolymer on the planet with an estimated annual production of around 1.5 × 1012 tons. Although cellulose meets most of the requirements needed for adhesive applications, due to its complex semicrystalline structure featuring hydrogen bonding and hydrophobic interactions, cellulose cannot be melted or easily dissolved in common industrial solvents.
Nevertheless, several cellulose derivatives, such as trimethylsilyl cellulose and silylated cellulose compounds, have been synthesized with increased solubility in a range of organic solvents resulting in highly flexible and biodegradable film-forming adhesives or sealants.
After cellulose, lignin is the second most abundant constituent of plant biomass. Lignin is a nonlinear low molecular weight phenolic biopolymer that is obtained as a byproduct of the lignocellulosic biorefining process and is a valuable renewable resource for bio-based materials. The mechanical and chemical properties of the materials can be easily modified by grafting reactions. For example, by introducing reactive epoxy functional groups into its structure, lignin can establish a renewable epoxy network. Such modified lignins can replace conventional bisphenol A components in epoxy resin adhesive compounds.
Starch-Based Adhesives and Sealants
Starch is a polysaccharide very similar to cellulose but with different configurations of the intramolecular bonds permitting the macromolecular chains to form extended ribbon-like conformations. This conformational dissimilarity makes starch less crystalline and more easily solubilized than cellulose.
Several biodegradable composites with excellent adhesion, high robustness, and outstanding elastic properties based on corn starch and polydimethylsiloxane have been developed by incorporating large amounts of unmodified corn starch (exceeding 80 vol%) in acetoxy-polyorganosiloxane matrix. Additionally, corn starch granules have also shown excellent compatibility with polysiloxane chemistry.
Regardless of the starch concentrations used, all the developed bio-elastomers have hydrophobic surfaces with a low friction coefficient and much less water uptake capacity than thermoplastic starch, which makes them suitable for adhesive and sealant applications. Such starch-based compounds are biocompatible and can biodegrade rapidly even in an aquatic environment, thus avoiding one of the main drawbacks of standard silicone adhesives.
The Advantages of Vegetable Oils
The advantage of vegetable oils, such as availability, low toxicity, low cost, and biodegradability among others, have already brought them into the spotlight of chemical manufacturing. Like lignin-derivatives, the properties of adhesives derived from vegetable oils can be tuned by the introduction of various functional groups.
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For example, an adhesive obtained from acrylated methyl oleate (a by-product from the production of biodiesel) exhibited typical pressure-sensitive adhesive properties, very similar to commercial products. Adhesive gels with tunable viscoelastic properties that have carboxyl groups as adhesion enhancers were synthesized by polycondensation of dimer fatty acid and diols together with maleinized triglycerides.
Recently, researchers synthesized the first completely bio-based two-component structural epoxy adhesive derived from cashew nut shell oil. The bio-based epoxy exhibited comparable thermal and mechanical properties to classical epoxides.
Combining nanocrystalline cellulose (as a cross-linking agent) and furfurylamine (a bio-based chemical produced from agricultural byproducts) resulted in an adhesive with a strength exceeding 20 MPa when applied to aluminum substrates (comparable strength to that of petroleum-based adhesives), thus making it highly suitable for automotive applications.
These developments demonstrate that the technological processes of obtaining monomers and polymers from sustainable feedstock can replace petroleum products in adhesive manufacturing. The current trend of increasing the market share of sustainable bio-based adhesives is supported by R&D efforts to improve their mechanical and chemical properties as well as to develop novel special-purpose adhesives.
References and Further Reading
Tenorio-Alfonso, A., et al. (2020) A Review of the Sustainable Approaches in the Production of Bio-based Polyurethanes and Their Applications in the Adhesive Field. J Polym Environ 28, 749–774. Available at: https://link.springer.com/article/10.1007/s10924-020-01659-1
Marques, A.C., et al. (2020) Review on Adhesives and Surface Treatments for Structural Applications: Recent Developments on Sustainability and Implementation for Metal and Composite Substrates. Materials, 13(24), 5590. Available at: https://www.mdpi.com/1996-1944/13/24/5590
Colarossi, J. (2019) Sustainable Adhesives of the Future Won’t Stick Around. [Online] Boston University. Available at: https://www.bu.edu/articles/2019/sustainable-adhesives
Magalhães, S., et al. (2019) Brief Overview on Bio-Based Adhesives and Sealants. Polymers, 11(10), 1685. Available at: https://www.mdpi.com/2073-4360/11/10/1685
Hemmilä, V., et al. (2017) Development of sustainable bio-adhesives for engineered wood panels – A Review. RSC Adv., 7, 38604-38630. Available at: https://pubs.rsc.org/en/content/articlelanding/2017/RA/C7RA06598A