IFAM study: the tension between the circular economy and life cycle assessment concerning adhesive bonding technology

After 20 months of WFH and online meetings, Afera’s Valencia Conference, which took place at the end of September, welcomed 100 refreshed adhesive tape industry professionals who benefitted from 14 presentations and an Industry panel debate focused around circularity. Day 1 was moderated by Afera Marketing Chairman Rupert Kichler, who is also marketing director at Avery Dennison Performance Tapes. Andreas Groß, head of workforce training and technology transfer at Fraunhofer IFAM, delivered the second presentation of the day, contributing this article on the tension between the circular economy and life cycle assessment concerning adhesive bonding technology according to a study by Fraunhofer IFAM:

Adhesive bonding technology in the area of tension between circular economy and life cycle assessment - A study by Fraunhofer IFAM
by Andreas Groß, Bernd Mayer, Katharina Haag, Fraunhofer IFAM

The use of adhesive bonding supports or enables the implementation of a comprehensive circular economy and thus has positive effects on life cycle assessment
The effectiveness of the circular economy results from the fact that adhesively bonded joints can be used safely and then released again in a controlled manner. The technological potential of adhesive bonding can bring about an improvement in the life cycle assessment when considered holistically across all product life cycle phases from manufacture to utilisation and disposal of adhesively bonded products. The key factor here is the unique ability of adhesive bonding to securely bond different material combinations over the long term while retaining product-relevant material properties and to integrate additional functions into the adhesively bonded product that go beyond mere joining. Thus, adhesive bonding enables eco-balance-effective construction methods.

Fig. 1: The potential of adhesive bonding technology to become the no. 1 joining technology
of the 21th century

This unique selling point of adhesive bonding technology supports the Ecodesign Directive
It takes a holistic view of a product's life cycle with a focus on greenhouse gas emissions. The holistic approach must already be included in the adhesive bonding technology product planning and design phase, which influences 80% of the product’s impact. For the LCA-relevant connection of energy consumption, material efficiency and resource efficiency, the Ecodesign Directive classifies material and energy as a resource. This allows energy consumption and material efficiency to be transferred to resource efficiency and adhesively bonded products to be considered holistically. In rail vehicle construction, for example, where adhesive bonding technology is used comparatively widely, the driving operation, i.e. the product life cycle phase "utilisation", is the determining environmental factor compared to the product life cycle phases "manufacture" and "disposal": the adhesive bonding technology use of lightweight materials reduces energy consumption and thus the specific CO2-equivalent emissions per passenger or tonne kilometre and thus supports this industrial sector, which is demanding from the point of view of safety and long-term durability, in an eco-balance effective manner.

The life cycle assessment (LCA / EN ISO 14040 - 44) enables a holistic view of adhesively bonded products with regard to ecological improvements
It is currently the most comprehensive assessment method for environmental impacts. An indispensable prerequisite for this is the quality, timeliness and availability of the underlying data: FAIR (findable/accessible/interoperable/reusable). The product-related harmonisation approach of the environmental footprint of products and services ("Product Environmental Footprint - PEF") initiated by the European Commission makes comparative environmental statements on functionally identical products comprehensible. This requires the timely definition of product group-specific PEF Category Rules (PEFCR) for end products manufactured using adhesive bonding technology. Due to the low proportion of adhesives in the end product, it is not advisable to focus on adhesives, as the positive effects of adhesives in the end product would be disregarded. Nevertheless, extensive data sets are available in Model Environmental Declarations (Model EPDs) of adhesives and sealants, e.g. for the construction sector.

In the future, adhesively bonded products will have to be considered over all phases of their product life cycle
From an ecological point of view, design for the environment ("Design for Environment" - DfE) based on FAIR data concepts including repair, maintenance or renovation should be used. Here, the PEF would offer the advantage of incorporating the data sets collected in the life cycle assessments and EPDs. Safety and technical data sheets of the adhesives should be supplemented with information for the product life cycle phase "disposal". When assessing the circularity of an adhesively bonded product, the adhesive manufacturer should be involved as an advisor from the beginning, but the responsibility of the decision rests with the adhesive user. With regard to new challenges initiated by circular economy, intensive communication as well as close networking of adhesive bonders, recyclers and customers as part of the "adhesively bonding" system along the life cycle of adhesively bonded products is necessary. The technical flexibility, willingness to adapt and innovative capacity required for this on the part of adhesive manufacturers has already been demonstrated in the past with many new requirements such as the reduction or replacement of substances recognised as critical (including solvents, certain monomers, migrating substances, plasticisers or biocides). Industrial adhesive development has verifiably the potential to develop and offer technically suitable solutions within given legislative and ecological boundary conditions, to meet these new requirements with adequate industry-specific solutions and to arrive at optimised products. The ability of adhesive bonding technology and adhesive development to produce adhesively bonded joints in line with life cycle assessments has already been demonstrated many times by the players involved in the fields of R&D, the adhesive industry and adhesive application.

Adhesive bonding technology and eco-design do not contradict each other
In terms of controlled longevity, the control of product integrity during the product life cycle phase "utilisation" is linked to the targeted separation of materials in the product life cycle phase "disposal". The latter is a prerequisite for recycling and deconstruction as well as for repair. The detachment of an adhesively bonded joint is caused by at least one external trigger that does not occur in the normal use of the adhesively bonded joint and therefore does not affect the safe use of the adhesively bonded product.

Fig. 2: The importance of the holistic approach

Adhesive bonding technology and repair are also not contradictory
The repair of windscreens in means of transport has long been state of the art. The repair of mobile phone screens can now also be carried out by skilled laymen. In general, adhesive bonding is probably the most frequently used joining method in the repair sector. Since objects of daily use can thus be used for longer, adhesive bonding technology contributes to the conservation of resources. In the case of safety-relevant components, adhesive bonding technology is used in accordance with detailed repair instructions.

Adhesive bonding technology and recycling are not contradictory either
As in the case of reparability, the development potential lies in already taking detachability into account in product design. As a rule, a combination of local heat and mechanical (peeling) loading is likely to be used for debonding. In the case of a very heterogeneous mixture of different products, the separation of the materials will only be possible after shredding. The adhesives - as well as lacquers and various impurities - remain on the separated materials, which is not a problem with materials that are hot to recycle, such as metal or glass. In the case of plastics, however, adhesions lead to a loss of quality, but this can be regarded as uncritical in the case of heterogeneous product mixtures, which always contain aged mixed plastics including adhering adhesives and varnishes. Successful examples are the recycling of cars, mobile phones and soft packaging.

Fig. 3: Adhesive bonding and repair

Adhesive bonding technology and resource efficiency are also not contradictory
Adhesively bonding can increase the longevity of products. For example, adhesives prevent contact corrosion in metal bonding. The extension of the product life cycle phase "utilisation" is achieved through improved ageing resistance, for example through surface pre-treatment to increase adhesion strength. In addition, adapted requirement profiles and optimised design and production specifications avoid resource-intensive "over-engineering". Optimisation of adhesive bonding technology, for example through improved application technology that applies the exact amount of adhesive required and thus avoids costly reworking, can also contribute to increasing resource efficiency.

In addition, adhesives lead to material savings, even in areas as diverse as food packaging (shelf-life extension) and transport construction (lightweight construction). The development of alternative energy sources is just as inconceivable without adhesive bonding (sealing of solar cells, joining of rotor blades in wind turbines) as electromobility (construction of magnetic cores for electric motors from electro-packaging sheets, sealing of battery cells, thermal management of batteries with thermally conductive adhesives) and fuel cells (hermetic sealing, joining of bipolar plates). Furthermore, adhesive bonding technology can also save secondary packaging material with currently available adhesives. However, this still requires development work.

The "adhesive" itself can also contribute to resource efficiency
With regard to circular economy-friendly adhesive formulations, recyclates can be suitable raw materials for adhesives. Furthermore, bio-based adhesives are already used today in certain mass applications. However, many necessary property profiles of synthetic adhesives cannot yet be figured by bio-based adhesives, which still limits their applicability. Nevertheless, adhesives are an ideal product group for the use of renewable raw materials. The moderate quantities are sufficient so that the raw material synthesis could be economically feasible and the ecological impact large enough. The development of renewable raw materials for widespread utilisation in adhesives is well on its way but also requires enormous research to reach the level of fully synthetic adhesives. One example with already relatively good results is the so-called BioDur system with up to 99% use of renewable raw materials. Degradable adhesives based on various renewable materials such as starch, cellulose or proteins have long been used industrially, for example, in food labelling.

Adhesive bonding technology supports the circular economy and eco-design
In the case of adhesively bonded products, this can be done by adhesively or cohesively influencing the adhesively bonded joint. Since, according to the current state of science and technology, influencing adhesively bonded joints, especially those relevant to safety, still requires considerable fundamental research, cohesive influencing is preferred. Cohesive disassembly strategies based on local stimuli such as heating and mechanical overload are particularly targeted. With chemically curing adhesives, softening of the polymeric adhesive layer can also be achieved by swelling. Another possibility is the degradation of the polymer network. The user of the adhesively bonded product as well as the recycler must be aware of the disassembly option provided by the manufacturer; this also applies analogously to repair processes. Digital information sources make it technically possible to pass on these specifications. In the mechanical recycling of plastic parts, it is helpful if the adhesive is as compatible as possible with the plastics. As a necessary supplement to mechanical recycling and as a preferred alternative to purely energetic recycling, chemical (raw material) recycling opens up a forward-looking perspective for adhesively bonded joints with plastic joining partners - including those that may still be contaminated with adhesive and paint.

Disassembly processes of adhesively bonded products for raw material recovery or repair will be key focal points of the development process and product testing in the future
From microelectronics to large structures in the building industry, requirements for disassembly should be listed in future specifications for product designs that are compatible with circular economy and ecobalance, and triggers or constructive measures for improving the economic efficiency of disassembly processes should already be taken into account in this early phase of product development. In the case of a high number of variants, small dimensions and high quantities, dismantling (shredding) can be a variant of the disassembly process, which enables sorting by material type. Large component dimensions with a high dead weight of the sorted components favour the use of mechanical detaching of adhesively bonded joints. Accessibility for disassembly aids must be ensured by design. Power-intensive processes can be mechanised or supported by automatic machines or robots. In this context, however, the construction of product-specific disassembly lines is largely untested. Detaching by heat input is a means of dismantling adhesively bonded products, both in combination with mechanical detaching and independently. Disassembly through media influence is particularly suitable for aqueous adhesive dispersions and is already being used successfully in handicraft (e.g. dissolving wallpaper) as well as for mass-produced products (e.g. removing labels from deposit bottles).

Adhesive bonding technology has long been an indispensable, eco-balance-relevant factor for the realisation of lightweight construction through multi-material construction. In the future, adhesive bonding technology will also be a key to ensuring that products can be repaired and recycled. Accordingly, adhesive bonding does not prevent the disassembly of products, but offers a promising possibility for detaching the joined joints.

Fig. 4: Users, repairers and recyclers must be informed about the dismantling option
provided by the adhesive manufacturer

Digitalisation supports decisive decisions in the use of adhesive bonding technology
Digital tools and the availability of material- and process-related data along the entirety of the process chain within the framework of FAIR data concepts will in future allow developers of adhesively bonded joints, with detailed knowledge of the material, to identify environmentally friendly optima in terms of material, cost and energy efficiency across the entire life cycle as part of a holistic approach. Cyber-physical systems digitise product and process information and pass it on along production processes. Digital twins, for example, enable the selection of an optimally suitable disassembly process. Adhesively bonded products can be fitted with an RFID chip that can be used to retrieve all the data on the materials and adhesives for disassembly. The high experimental verification effort required for the process integration of adhesive bonding technology into a production process can also be reduced with the help of suitably calibrated, digital material and component models. Verifiable service life predictions that reduce maintenance costs, lead to resource-saving replacement of wear parts and significantly increase the safety gain require the development of structural health monitoring (SHM) systems. The necessary sensor technology and the ways in which this information can be stored and read out are the subject of current research activities.

Fig. 5: Process steps and players in quality-assured adhesive bonding production

The view "from material to safe product" combines the aspects of safety and long-term stability with the requirements of a circular economy in the sense of sustainability of products. In this area of conflict, adhesive bonding technology has the necessary technological as well as ecological capabilities and thus offers the potential to become the leading joining technology of the 21st century.

For more information, read the Adhesion Magazine article on this subject here.

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