Adhesives' Role in the Circular Economy Model

Sustainable Environment-Friendly Adhesives in the Circular Economy Model Adhesives are widely used in modern industries including electronics, biotechnology, aviation, automotive, white goods, furniture and composites. The negative environmental impacts of petrochemical sources and their expected depletion in coming years, along with excessive dependence on them, have inspired rapid development of bio-based materials, with the importance of transition to recyclable and reusable materials being emphasized at every opportunity. In this context, the transition to strong, durable, recyclable, reusable, environment-friendly and sustainable adhesives will facilitate economic decoupling from fossil fuels and contribute to the circular economy by preventing waste accumulation in the environment. Sustainable environment-friendly adhesives hold an important place in producing high-value products from waste raw materials, positioning themselves as candidates for establishing a distinct area within the Circular Economy Model. Converting low-value or valueless industrial waste into technological products creating exceptionally high value will be an indispensable topic of the Circular Economy Model in coming years.

What is a Sustainable Environment-Friendly Adhesive?

Sustainability is defined as protecting natural resources while creating economic benefits and ensuring social equity through all processes without harming the environment (Balbay et al., 2021). Circular economy is a production and consumption model involving the longest possible sharing, leasing, reuse, repair, renewal and recycling of existing materials and products. This way, product life cycle duration is extended while these products generate more value by being returned to the economy. While the linear economy applies the take-use-dispose model, the circular economy is based on prolonged use of all resources (energy, raw materials, water, etc.) (Figure 1) (Balbay et al., 2021). [caption id="attachment_158709" align="aligncenter"] Figure 1. Comparison of Linear Economy and Circular Economy (dongusel.csb.gov.tr)[/caption] In areas where adhesives are used, the chemical substance that adheres to materials being bonded together and provides the bonding process is called an adhesive. Important combinations of secondary chemical bonds (sometimes primary) between adhesive and bonded material, as well as sometimes a degree of mechanical interlocking (at micron level for smooth surfaces), enable adhesive surface attachment (Figure 2) (Bangash et al., 2023). [caption id="attachment_158710" align="aligncenter"] Figure 2. Adhesion mechanism[/caption] Classical adhesives are adhesives produced using especially petroleum-derived raw materials and consuming significant amounts of energy during production. For this reason, greater carbon emissions and waste are generated during production, and the VOC (volatile organic carbon) content of the produced adhesives is quite high (Packham, 2009). Environment-friendly adhesives, on the other hand, are solvent-free and biodegradable adhesives produced using newly developed methods with renewable and naturally sourced raw materials while consuming less energy (Packham, 2009).

Sustainable adhesives are generally evaluated in three groups:

1) Solvent-free adhesives: Today, many adhesives are produced with increased solid content and reduced solvent use. However, other than other methods, 100% solids content adhesives such as hot melt adhesives contain no solvent. Hot melts are widely used in all industries including the automotive sector. 2) Water-based adhesives: Water-based adhesives have been developed using water as a carrier instead of solvent. This reduces the product's weight while eliminating VOC emissions and minimizing fire risk during production. 3) Bio-based adhesives: Adhesives produced from a mixture of natural materials such as starch, vegetable oils, various resins, lignin and proteins. These materials guarantee reduced carbon footprint of products (Sustainable Adhesives, 2023). Sustainable environment-friendly adhesives are produced from renewable natural resources using new methods consuming less energy, in water-based and biodegradable, compostable, recyclable/reusable forms (Figure 3) (Allasia et al., 2022). [caption id="attachment_158711" align="aligncenter"] Figure 3. Sustainable environment-friendly adhesive production process (Allasia et al., 2022).[/caption] General formulation of bio-based adhesives (Boussetta et al., 2023); Starch + Tannin + Chitin → Bio-adhesive

Studies Conducted Using Adhesives

1) Carbon Foam Production

Carbon foams are highly porous, 3D and well-bonded honeycomb structures. They are new-generation macro-porous carbon-based materials and have attracted significant interest in many high-technology applications in recent years (Balbay and Acıkgoz, 2022). Properties: • Lightweight, • High Thermal Stability, • High Thermal and Electrical Conductivity, • Low Density, High Porosity, • High Compressive Strength, • Fire Resistance, • Excellent Electromagnetic Properties, • Sound Absorption. A new method for carbon foam production has been defined by mixing only organic substances with high volatile matter content with water-based commercial acrylic resins and curing them. In this method, carbon foams are produced at 600°C in 30 minutes. Not only presenting a new technique for carbon foam production, but also providing a promising method for producing technological products—carbon foams—from high organic content waste for environmental improvement purposes (Figure 4) (Balbay and Acıkgoz, 2022). [caption id="attachment_158712" align="aligncenter"] Figure 4. Carbon foam production via SENAY process (Balbay and Acıkgoz, 2022)[/caption] [caption id="attachment_158713" align="aligncenter"] Figure 5. FTIR spectrum, SEM and TEM images of carbon foam[/caption]

2) Sound Absorber Production

One of the greatest environmental problems of our time is noise pollution. Commonly used porous sound absorption materials are acoustic absorption foam and fibrous sound absorbers. One of the most effective and important physical parameters in acoustic absorption is porosity. In sound-absorbing porous materials, increased sound absorption has been observed with increased volumetric density and porosity and decreased flow resistance (Balbay et al., 2023). Green composite sound insulation materials produced using agricultural waste orange peels and water-based commercial acrylic resins as binder (Figure 6) were found to have sound insulation potential comparable to commercial sound absorbers. The developed porous sound insulation material exhibited performance similar to commercial sound insulation materials at low frequencies (Figure 7). The material's internal voids have been shown to be capable of absorbing sound energy (Figure 8) (Balbay et al., 2023). [caption id="attachment_158714" align="aligncenter"] Figure 6. Sound absorber production process[/caption] [caption id="attachment_158715" align="aligncenter"] Figure 7. Comparison of produced sound absorber and commercial sound absorbers[/caption] [caption id="attachment_158716" align="aligncenter"] Figure 8. FTIR spectrum and SEM image of produced sound absorbers[/caption]   Sources Balbay, S., Sarihan, A., Avsar, E. (2021). "Circular Economy / Industrial Sustainability" Approach in the World and in Turkey, European Journal of Science and Technology, (27), 557-569 https://dongusel.csb.gov.tr/hakkinda-i-105778 Bangash, M. K., Jabbar, M., Nawab, Y., Shaker K. (2023). Joining of composites using rapid curing resin systems, Rapid Cure Composites, https://doi.org/10.1016/B978-0-323-98337-2.00002-X Packham, D. E. (2009). Adhesive technology and sustainability. International Journal of Adhesion and Adhesives, 29(3), 248–252. doi:10.1016/j. ijadhadh.2008.06 https://www.adhesivesandcoatings.com/adhesives/ about-adhesives/sustainability-of-adhesives-andsealants/#tab-id-2 Allasia, M. Aguirre, M., Gugliotta, L.M., Minari, R J., Leiza, J. R. (2022). High biobased content waterborne latexes   Associate Professor Şenay Balbay Bilecik Şeyh Edebali University Graduate Education Institute Interdisciplinary Master's Programme in Industrial Sustainability