Recycling of multilayer packaging using a reversible cross‐linking adhesive

Abstract

Plastic‐based multilayer packaging has an important function on the packaging market, but is currently not recyclable as the polymer layers used are usually thermodynamically immiscible. This work therefore follows the approach to prepare separable multilayer packaging using a packaging adhesive modified with thermally unstable adducts, and proposes a corresponding recycling process. For this purpose, typical multilayer structures (polyethylene (PE)// polyethylene terephtalate (PET), PET//aluminum, and PE//aluminum) were prepared by curing furan‐/maleimide‐functionalized polyurethane (PU)‐prepolymers with a three‐functional cross‐linking agent. Adhesions of up to over 3N per 15 mm test specimen were measured or substrate failures of PET films were observed. However, heating in dimethylsulfoxide, the retro‐Diels–Alder reaction takes place and the cross‐linked adhesive turns thermoplastic and dissolves in the solvent. Thus, the laminate separates and the pure PE, PET, and aluminum foils can be recovered without any PU residue.

Recyclable Multilayer Packaging by Means of Thermoreversibly Crosslinking Adhesive in the Context of Food Law

“…However, the delamination speed could be influenced by a smaller film piece size, as this increases the surface area of the cut edges through which the solvent can penetrate the adhesive. Similarly in the previous work, [9] the PE-containing laminates PE-PET and PE-aluminum are separated faster than the PET-aluminum laminate, since the diffusion coefficient of PET is significantly higher than that of PE [36], especially since the melting point of PE is almost reached at 105 • C.…”

Re-usable thermally reversible crosslinked adhesives from robust polyester and poly(ester urethane) Diels–Alder networks

“…Retention of properties after multiple re-bonding and re-use cycles without using solvents contrasts with the reported single debonding of polyurethane Diels-Alder networks using hot dimethyl sulfoxide. 38 The results also differ to acrylic hetero Diels-Alder networks based on thioesters where a single bond cleavage was permanent since a Lewis acid catalyst was necessary for network formation at ambient temperature. 39 Naito et al 34 reported thermo-resettable acrylic Diels-Alder but the approach is not sustainable since furan functional acrylic polymers were applied from toxic chloroform solution and the toxic BMI monomer was used as a crosslinker.…”

“…The Diels-Alder reaction between maleimide and furan groups has also been used to make reversible non-isocyanate polyurethanes 36,37 although no detail has been reported on their material properties. A very recent study 38 has highlighted that flexible packaging films bonded with crosslinked polyurethane adhesives can be separated effectively, however this requires immersion of the bonded laminates in hot dimethyl sulfoxide to dissociate the maleimide-furan adducts. Hetero Diels-Alder reactions between cyclopentadiene and thioesters in acrylic networks have also been used for reversible adhesion.…”

An Efficiently Reworkable Thermosetting Adhesive Based on Photoreversible [4+4] Cycloaddition Reaction of Epoxy‐Based Prepolymer with Four Anthracene End Groups

“…Adhesives, which act on the surface of the two substrates and adhere firmly them together, [ 1 ] are playing an important role in industrial production and many aspects of daily life. [ 2 ] A variety of thermoplastic and thermosetting polymers can be used to prepare adhesives, such as, acrylic esters, [ 1–11 ] epoxy resins, [ 12–15 ] polyurethanes, [ 16,17 ] thiol‐ene networks, [ 18,19 ] azo‐benzene derivatives, [ 20,21 ] etc. Usually, thermosetting adhesives provide excellent bonding strength and stability, due to their permanently cross‐linked polymer networks.…”

“…The heat‐induced method is suitable to a wide range of substrates, and the reversible reaction can carry out in a thick sample. However, heat‐induced reworkable adhesives, [ 2,12–17,22–24 ] obtaining based on the reversible Diels–Alder (DA) bond, [ 2,16,17,23 ] disulfide bond and ester bond, [ 12–15 ] which need heat energy that may also act on those parts that are uninjured and unwanted. Therefore, redundant reactions, which have no contribution to requirements, are also triggered during thermal treatment, resulting in not only a waste of energy but also deformation or pyrolysis of the adhesives which do not need to be removed.…”