Circular Economy in Adhesives and Sealants

Circular economy refers to an economy comprising industrial processes and economic activities that employ a systems-based approach and are designed to be restorative or regenerative by nature. This approach aims to ensure that resources maintain their highest values for as long as possible and to eliminate waste through superior design of materials, products, and systems (including business models). Circular economy follows certain key principles such as designing out waste and pollution, keeping products and materials in use, and regenerating natural systems. Circular economy represents a revolutionary shift from the model in which raw materials are extracted from the ground, converted into products, and then completely discarded after use. A circular economy reduces material consumption, redesigns materials to be less resource-intensive, and recaptures "waste" as a resource for producing new materials and products. It encompasses fundamental concepts such as the use of renewable energy and non-toxic substances, prevention of natural resource depletion, use rather than ownership, and design of products to enable longer periods of use, repair, and reuse. When examining these elements in the context of the Adhesives and Sealants Industry, we observe that most manufacturers are progressing toward commitments to use 100 percent renewable energy in their own operations. Similarly, there is increasing emphasis on the use of renewable, bio-based materials such as starch, vegetable oils, proteins, lignin, and natural resins.

Design Logic – Key Elements

Although we are often unaware, adhesives are present in nearly every product we use. From the electronic and mobile devices in our hands to the airtight windows in our homes, from doors and building facades to vehicles and aircraft used for transportation, from toys and hobby products to boats and shoes—the manufacture of countless different products is made possible thanks to adhesives. Since adhesives and sealants are used as components and enable the production of much larger volumes of products with significantly greater environmental footprints, it is more rational to focus on the recycling and circularity of the products and systems manufactured using adhesives rather than on recycling the adhesives themselves. For this reason, apart from the manufacturing process of adhesives, where most companies strive for zero-waste-site status in their production facilities, elements such as high-value reuse and recycling do not apply to adhesives. Additionally, there are few examples of supply chain collaboration between new and established supply chains. When greater emphasis is placed on bio-based materials rather than traditional fossil-based raw materials, the emergence of new supply chain networks is inevitable. This certainly represents an opportunity to establish efficient and interconnected networks that are collaborative and regenerative toward the environment. Therefore, perhaps the most relevant component through which the Adhesives Industry can make an impact is assisting in product redesign to enable circularity.

Demountable Design Approach

If we examine any literature on the design of waste and pollution, and particularly on "demountable design," we will always encounter recommendations to avoid adhesives and use mechanical fasteners instead. However, this approach can be said to be oversimplified and would not be applicable in most cases when the entire product life cycle is considered. For example, adhesives positively impact the economy by providing end products with greater durability, light weight, and compact characteristics. These properties contribute to other principles of circular economy, such as enabling compact design, keeping products and materials in use for longer periods, or reducing the time and energy required for assembly. For instance, adhesives used in building segments contribute positively to environmental factors by preventing heat loss through better sealing performance. Completely eliminating adhesives is certainly not rational, but it is an undeniable fact that new-generation adhesives designed to adapt more quickly to circular economy principles must be developed. For this reason, in each of the main applications, solutions can be explored that prevent adhesives from being perceived as a barrier in terms of demountable design and enable easy disassembly at the end of the product's useful life. Principal Application Areas Adhesives Used in the Construction Sector As the construction industry looks at "material passports" to document the materials entering a building and ensure recovery of materials at the end of their useful life, "demountable design" and "modularity" are increasingly demanded in recent times. Providing solutions to enable such design is a subject that the Adhesives and Sealants Industry must work on. Adhesives for Labels Removable adhesives that enable glass and plastic bottles to be easily recycled without excessive effort or contamination are an extremely important area, as fast-moving consumer goods (FMCG) are responsible for a large portion of waste generation resulting from packaging that requires labeling.

Recyclable Paper Adhesives

As online shopping continues to grow exponentially, there will be significant demand for resealable and reusable packaging. Adhesives can enter an efficient recycling path. Adhesives in Automotive and Bus Body Manufacture With the trend toward electric and autonomous vehicles, digitalization and demand for lightweight materials will increase. This will likely increase adhesive usage in manufacturing. At the same time, adhesives must develop to provide properties such as on-demand bond breakage to ensure recovery of materials "at end of life."

Packaging Adhesives

Lamination adhesives hold multi-layer packaging together. Packaging of fast-moving consumer goods is probably the most difficult problem to solve in terms of circularity. In all of the above cases, the greatest challenge appears to be the need for on-demand bond breakage of products using these adhesives at the end of their useful life. While separation of low-strength adhesives is relatively easy, separation of high-strength structural adhesives can be quite difficult. The maximum service temperature of the adhesive is important, as bonded parts must be heated above this temperature for easy separation. New Developments Promoting Reversible Adhesive Systems Some on-demand bond-breaking approaches have been commercially available for some time, such as, for example, a concept in wall decoration products and reusable fasteners used in the automotive industry to secure interior components with demountability in mind. It remains to be seen whether similar concepts can be expanded or whether new innovations will emerge based on market needs. To date, new developments in on-demand bond breaking have typically been based on three types of approaches. • Use of reversible or reprocessable adhesive systems, • Separation of bonded adhesive joints using electric current, • On-demand bond breaking using reactive fillers. Use of Reversible or Reprocessable Adhesive Systems The most commonly used approaches for reversible adhesives presented in recent literature are based on retro-Diels-Alder concepts. For example, Sandia National Laboratories (Albuquerque, N.M., USA) developed a reversible adhesive that responds to temperature changes. However, this reattachment capability is finite. Researchers at the Adolphe Merkle Institute developed a polymer-based material that can be bonded and separated on demand while using ultraviolet light. According to the researchers, these new materials are created by a mechanism known as supramolecular assembly. The supramolecular structure consists of smaller molecules assembled into longer, polymer-like chains by using metal ions or hydrogen bonding motifs to bind small components together and create a "semi-crosslinked" network structure. When exposed to intense ultraviolet light, supramolecular structures break down, resulting in a "liquid-like" material state. As soon as the stimulus is removed, supramolecular structures reshape themselves. Electrically Induced Bond Separation Electrically induced bond separation of adhesives is a technique in which adhesives can be released on demand with the help of an applied electrical potential. The technology is based on a chemical reaction at the interface between the adhesive layer and an anodic adhesive bonded to the positive electrode of a DC power source. Depending on the potential applied to the adhesive joint, the lower layers can be separated within seconds. Several such adhesive formulations are patented, and some are now commercially available. On-Demand Bond Breaking Using Reactive Fillers One methodology to achieve controlled adhesive separation is the activation of functional fillers within the adhesive or primer. These additives can be nanoparticles or microcapsules that can be activated by external energy sources such as heat (thermal and induction), electrical potential, electromagnetic energy, etc. In the literature, there are various approaches, such as Ciardiello and colleagues using metallic (iron oxide) nanoparticles embedded in a hot-melt adhesive (HMA) to separate plastic joints bonded with adhesive, or the American Chemistry Council's Plastics Division (ACC-PD) and Michigan State University (MSU) Composite Vehicle Research Center developing reversible adhesive bonds using iron oxide nanoparticles in various thermoplastic adhesives that can be separated and re-bonded multiple times via electromagnetic energy. The work of Byungsun Lee and colleagues used vaporizable polymeric nanocapsules in a thin adhesive film to generate gas bubbles through thermal stimuli and demonstrated a bond-breaking effect. This technique using polymeric nanocapsules is expected to be applicable to advanced adhesive thin films used in displays and semiconductors due to its ability to control adhesive strength while preserving initial film properties. Another technique developed for separation of adhesive bonds involves embedding microparticles (microcapsules) into the adhesive layer. These microparticles can be thermally expanding particles (TEPs) or blowing agents that can be activated at a specific temperature to mechanically separate substrates. (In Japan, this was created by Sakurai and others for bonding plywood sheets. Subsequently, Ishikawa and others modified and advanced the technique of bonding wallpaper over plywood or gypsum boards in construction applications). This innovative idea was extended to structural adhesives for recycling purposes by Nishiyama and colleagues. Simply heating the joint above 100°C enables easy separation of the bonded materials. A technique called INDAR Inside®, for resolving structural adhesive bonds through thermal activation of additives, was developed and patented by Rescoll Technology Center (France). Researchers described this technique as follows: "The Rescoll process involves reformulation of new adhesives or commercial adhesives. When heated to a specific temperature, the additives begin to decompose and release gases that migrate to the interfaces through diffusion from the bulk adhesive, creating localized stresses that lead to joint separation." One advantage of this technology is that cleaning of parts is easy due to interfacial failure. The advantage of the bond-breaking technologies presented in this section is that they can be applied with heat-cured, high-strength adhesives. However, the main disadvantage of this approach is that microparticles can impede initial bonding, leading to weak joints. Additionally, there are some concerns about accidental triggering and the types of fillers and gases involved. Conclusion With the developments outlined above, good progress is being made in the area of "on-demand bond breaking" to overcome reversibility, which is the greatest obstacle to circularity with respect to the adhesives industry. Like any other solution, success depends only on the suitability of the solutions to the application in question and their commercial viability. With mandatory regulatory pressures such as the European Green Deal and other regulations worldwide, these solutions are likely to become commercially viable and perhaps become mainstream in the near future.   References: https://www.adhesivesmag.com/articles/96662-the-circular-economy-and-the-adhesives-and-sealants-industry http://www.am-institute.ch/research/projects/debond-on-demand-adhesives https://www.researchgate.net/publication/331707237_Debonding_on_Demand_of_Adhesively_Bonded_Joints_A_Critical_Review https://chemicalsinourlife.echa.europa.eu/guest-corner/-/asset_publisher/vcrOSpI91ebF/blog/chemical-innovations-for-a-circular-economy https://www.researchgate.net/publication/324849804_Polymeric_nanocapsules_containing_methylcyclohexane_for_improving_thermally_induced_debonding_ of_thin_adhesive_films_Research_Article https://www.circularity-gap.world/updates-collection/circle-economy-launches-cgr2020-in-davos https://adhesives.specialchem.com/tech-library/article/circular-economy-adhesives-sealants?src=asnews&utm_source=news&utm_medium=industry&utm_ campaign=news210727