Circular Economy in Adhesives & Sealants Industry: What it Means Today?

TAGS:  Sustainability / Natural Adhesives    

Circular economy is one of the key enablers for the adhesives and sealants industry to move towards a more sustainable future. It is based on the key principles of mimicry from nature where nothing is wasted, and the system is infinitely regenerative. While we are a long way from emulating the perfection of nature, a circular economy provides hope for a much more efficient model for:

• Use of natural resources,
• Reduction in greenhouse gas emissions &
• Better preservation of nature.

The circular economy follows some basic principles of designing out waste & pollution, keeping products/materials in use, and regenerating natural systems. There are several elements to the implementation of circular economy as listed below:

• Use of renewable energy,
• Use of non-toxic substances & no depletion of natural resources,
• Focus on new revenue models – paying for use instead of ownership,
• Move towards product redesign – Designing products to enable reuse, repair & longer use,
• Promote supply chain collaboration – new alliances between companies in new or established production chains,
• Enable high-value reuse & recycling.

If we view the above elements in the context of the Adhesives & Sealants Industry, we see that most of the manufacturers are moving towards a commitment of use of 100% renewable energy in their own operations. Similarly, there is an increased emphasis on the use of renewable, bio-based materials such as starch, vegetable oils, proteins, lignin, and natural resins.


Let's explore more about circular economy & the latest developments in the field of “debonding-on-demand”  adhesives & sealants. 

Circular by Design – The Key Element

While we are all familiar with adhesives since our childhood experience of art & craft, we hardly realize the importance of their use in almost everything around us. From the mobile devices in our hands to airtight windows in our homes, glass facades in buildings, to things that we use for transportation – cars, buses & airplanes – adhesives are everywhere.

Because adhesives and sealants are used as components and enable the production of much larger volume products having a much bigger environmental footprint, it is important to focus on recycling and circularity of the products and systems manufactured using adhesives rather than recycling of adhesives themselves.¹


Hence, elements like high value reuse & recycling for adhesives themselves do not apply here, except for the production process of adhesives, where most companies are striving for zero-landfill status in their manufacturing plants. Also, we are yet to see examples of supply chain collaborations between new and established supply chains. Though, with more emphasis on bio-based materials instead of traditional fossil-based raw materials, new supply chain networks are bound to emerge. This is definitely, an opportunity for establishing efficient & interdependent networks that are collaborative & regenerative towards the environment.

Hence, perhaps the most relevant component where the Adhesives Industry can make an impact is to help in product redesign to enable circularity.

» Meet the Needs of Circular Economy with Debonding-on-command Structural Adhesives

Design for Disassembly – Promote Debonding on Demand

If we look at any literature on designing out waste and pollution and specifically on “design for disassembly”, we will invariably find advise to avoid adhesives and use mechanical fasteners instead. However, this approach might be too simplistic and might not consider other factors that might be in favor of adhesives when we take the whole life cycle into consideration.

For example, adhesives positively impact the economy by imparting features to the end products to make them more durable, lightweight & compact. These features contribute to the other principles of circular economy, like enabling compact design, keeping products and materials in use for a longer time or reducing time & energy required for assembly. For example:

• Adhesives used in building segments contribute positively to environmental factors by preventing heat loss through better sealing performance.
• In transportation sector, they contribute positively by making vehicles lightweight and reducing noise, vibration & harshness.

While it is clear that it is not a good idea to eliminate adhesives altogether, it does make sense to come up with a new generation of adhesives that enable faster adaption of the circular economy. Hence, several areas can be explored in each of the main applications where adhesives are perceived to be a barrier in terms of the design of disassembly and come up with solutions that enable easy disassembly at the end of the product life.

MAJOR APPLICATION AREAS

Adhesives for Labels – Removable adhesives that enable easy recycling of glass and plastic bottles without too much effort or contamination. This is an important area as FMCG goods are mainly responsible for bulk of the waste creation due to packaging that must be labelled.

Adhesives for Recyclable Paper – As online shopping continues to grow exponentially, there will be a huge demand for re-closable/reusable packaging. Adhesives might come in the way of efficient recycling.

Adhesives in Construction – With the construction industry looking at “material passports” to document materials going into a building to enable recovery of materials at the end of life, there is a push for “design for disassembly” and “modularity”. This requires response from the Adhesives & Sealants industry to provide solutions to enable such design.

Adhesives in Automotive & Bus Body Building – With the trend towards electric and autonomous vehicles there will be increased digitalization and demand for lightweight materials. This is likely to increase use of adhesives in manufacturing. At the same time, the adhesives will need to evolve to provide features like – de-bond on demand to enable “end of life” recovery of materials.

Adhesives in Packaging – Laminating adhesives keep together multilayer packaging. Packaging of FMCG goods is the biggest issue in terms of pollution and probably the toughest challenge to solve in terms of circularity.

In all the above cases the biggest issue seems to be the requirement of debonding on demand at the end of the life of the products that use these adhesives. While low strength adhesives are relatively easy to debond, high strength structural adhesives can be quite challenging to debond. The maximum service temperature of the adhesive is important as the bonded parts need to be heated above this temperature for easy debonding.³

New Developments Promoting Reversible Adhesives Systems

Some approaches of debonding on demand does exist commercially for some time now. For example, as a concept in wall decoration products as well as re-closable fasteners used in the automotive industry for fixing parts in vehicle interiors with removability in mind. Whether similar concepts can be extended, or new innovations can emerge based on the market needs remains to be seen.

New developments in the area of debonding on demand so far are typically based on 3 types of approaches.

1. Using Reversible or Reworkable Adhesive Systems
2. Debonding of Adhesive Joints using Electrical Currents
3. Debonding on Demand Using Reactive Fillers

Using Reversible or Reworkable Adhesive Systems

The most 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) have developed a reversible adhesive that responds to temperature change. However, this re-bonding capability is finite.³

Researchers at the Adolphe Merkle Institute have developed a polymer-based material that can bond and de-bond-on-demand when using ultraviolet light. The new materials were created by a mechanism known as a supramolecular assembly. The supramolecular assembly is composed of smaller molecules, which are assembled into longer, polymer-like chains using metal ions or hydrogen bonding motifs to link the small components together and create a “quasi-crosslinked” network structure. When exposed to intense ultraviolet light, the supramolecular structures disassemble, resulting in a “liquid-like” material state. The supramolecular assemblies reform again as soon as the stimulus is removed.²

Electrically Induced Debonding

Electrically induced debonding of adhesives is a technique in which adhesives can be released on-demand with the help of an applied electric potential. “The technology is based on a chemical reaction in the interface between the adhesive layer and the anodic adherend connected to the positive electrode of a DC power supply. Depending on the applied potential to the adhesive joint, the substrates can be debonded in seconds. Several such adhesive formulations have been patented & some are now available commercially”.³

Debonding on Demand Using Reactive Fillers

One of the methodologies for achieving a controllable adhesive debonding is by activating functional fillers in the adhesive or in the primer. These additives can be nanoparticles or microcapsules which can be activated by external energy sources, such as: heat (thermal and induction), electric potential, electromagnetic energy, etc.

Several approaches are available in the literature like by Ciardiello et al. used metallic (iron oxide) nanoparticles, embedded in a hot-melt adhesive (HMA) in order to de-bond adhesively bonded plastic joints or by American Chemistry Council’s Plastics Division (ACC-PD) and Michigan State University (MSU) Composite Vehicle Research Center that used used iron oxide nanoparticles in a variety of thermoplastic adhesives to develop reversible adhesive joints that can be debonded and re-bonded multiple times, using electromagnetic energy.

Work from Lee et al. used evaporable polymeric nanocapsules in an adhesive thin film to form gas bubbles through thermal stimulus and showed a debonding effect. This technique using the polymeric nanocapsule is expected to be applicable to advanced adhesive thin films used in the display and semiconductor, owing to its capability of controlling the adhesive strength while maintaining initial film properties.

Another technique developed for adhesive debonding involves imbedding microparticles (microcapsules) in the adhesive layer. These microparticles might be thermally expanding particles (TEPs) or blowing agents, which can be activated at a certain temperature to induce mechanical separation of the substrates. (Originated in Japan by Sakurai et al. to bond plywood boards. Later, Ishikawa et al. modified and improved the technique for bonding wallpaper on plywood or plasterboards in construction fields). “This innovative idea has been extended to structural adhesives for recycling purposed by Nishiyama and co-workers. The simple heating of the joint over 100°C leads to an easy separation of the bonded materials.”³

A technique for debonding of structural adhesives by thermal activation of additives, called INDAR Inside^®, was developed and patented by Rescoll Technological Center (France). The Rescoll process involves formulating new adhesives or reformulating commercial adhesives. “Upon heating at a specified temperature, the additives start to decompose and release gases which migrate by diffusion from the bulk adhesive to interfaces generating local stresses leading to debonding of the joint.”³


The advantage of the debonding technologies presented in this section is that they are feasible with thermosetting high strength adhesives. However, the main disadvantage of this approach is that the microparticles may interfere with the initial adhesion, resulting in weak joints. There are also some concerns about accidental triggering and about the filler types and gases involve.³

Conclusion

With the above developments, there seems to be good progress in the field of “debonding-on-demand” to overcome the biggest barrier - reversibility - towards circularity concerning the adhesives industry. Like any other solution, the success however depends on suitability of the solutions for the application in question and the commercial viability of the solutions. With requisite regulatory push like the European Green Deal & other regulations around the world these solutions will become commercially viable and maybe mainstream soon. With the world being only 8.6% circular and still moving in the wrong direction in terms of circularity⁵, this is a much-needed change!

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