Vitrimer bead foams: Cell density control by cell splitting in weld-compression molding
Graphical abstract
Introduction
Polyolefin foams have been extensively used in many applications such as packaging, cushioning, agriculture, construction, electronic apparatus, and automotive production [1,2]. Currently, conventional polyolefin foams have been gradually banned or restricted for disposable packaging products in many countries due to their inherent non-biodegradability [[3], [4], [5]]. Bio-based polymeric foams like expanded polylactide (EPLA) seem to be a solution, but it still takes several months to be biodegraded in landfills [[6], [7], [8]]. Therefore, Upcycling or repurposing post-consumer polyolefin foams and even biodegradable polymer foams is an urgent demand [[9], [10], [11]].
It is well known that many polyolefins with a linear chain structure have poor foamability because of their low melt strength, which strongly limits their applications in harsh environments [12]. The introduction of crosslinking networks into polyolefins could significantly improve its melt strength. However, conventional covalent crosslinks such as peroxide crosslinking make them difficult to recycle and re-use under mild conditions [13]. In contrast, when polymer chains are crosslinked by the dynamic covalent bonds, the obtained materials, known as vitrimers, not only exhibit physical performances comparable to thermosets, but can flow and relax completely owing to the exchangeable links can disassociate under external stimuli [14]. The flowability enables the dynamic crosslinked polymers to be reprocessed like thermoplastics. Therefore, incorporating dynamic crosslinking networks represents a promising strategy for designing and recycling polymers [15]. To date, chemistries for dynamic covalent networks have capitalized on the reversible formation of various transesterifications [[16], [17], [18]], dioxaborolane metathesis [19], vinylogous urethanes [20], disulfides [21], olefin metathesis [22], hemiaminals [23], thioaminals [24], diketoenamine exchange [25], and trans-carbonation exchange [26]. Moreover, the non-covalent interactions involving hydrogen bonding [27], π-π stacking [28], and host-guest recognition [29] are also options for dynamic crosslinking networks.
Inspired by the excellent recyclability of bulk polymer vitrimers via the rearrangement of topology networks, the dynamic crosslinking strategy is being applied to recycle polymer products like polymeric foams [17,30,31]. In our previous work, we have demonstrated that ethylene-vinyl acetate copolymer (EVA) vitrimer foams could be recycled and reprocessed repeatedly by two different approaches, namely re-joining and re-foaming [17]. Like vitrimers, the essential target in recycling vitrimer foams is to make the mechanical properties of recycled products close to the original one. The reaction kinetics of dynamic covalent chemistry during the re-joining of snipped foam pieces have proved critical in tuning mechanical performances and reprocessing windows [17]. These findings provide significant enlightenments to mold and recycle polymer bead foam products.
In this work, we will illustrate another unique feature of the vitrimer foam, i.e., the cell density of the vitrimer bead foams can also be tuned during the re-joining process of recycling through a cell splitting process. Because bead foam is one type of the most widely used polymeric foams, which have advantages of higher expansion rate, better dimensional control ability, and devisable complex three-dimensional shape [3], we will illustrate this feature using vitrimer bead foam with a dynamically crosslinked polyolefin elastomer (POE) vitrimer. The weld-compression molding with a controlled compression level was employed to adjust the cell density. In this way, the re-use of vitrimer foams is not limited to the recovery of the mechanical properties of the original foams. Instead, the mechanical properties of the recycled foams can be tuned over a wide range.
Section snippets
Materials
Ethylene-octene copolymer (POE, SK/8705) and poly (ethylene octene) grafted with glycidyl methacrylate (POE-g-GMA) were kindly provided by Shanghai Sunny New Technology Development Co., Ltd. The grafted ratio of GMA in POE-g-GMA was about 2–3 wt%, according to the supplier. 1,2,4-Benzenetricarboxylic acid (BTCA, 99%) and 1,5,7-triazabicyclo [4.4.0] dec-5-ene (TBD, 98%) were purchased from Adamas company. Chemical foaming agent (azodicarbonamide, AC-9000) and zinc oxide (ZnO, 99.9%) were
Synthesis of POE vitrimers
Transesterification is a classical reversible reaction, which has been widely applied to construct vitrimers. However, linear poly(ethylene-octene) copolymer (POE) is difficult to crosslink dynamically due to a lake of reactive functional groups. Therefore, grafting chemical modification is needed. Herein, functional ethylene-octene copolymer bearing pendant epoxy groups (POE-g-GMA) was selected as the bulk polymer to react with a tri-functional acid (1,2,4-benzenetricarboxylic acid, BTCA) in
Conclusions
In this study, we suggest a convenient pathway toward dynamic crosslinked polyolefin elastomer (POE) based on the classic transesterification by reactive extrusion, using commercially available POE-g-GMA as the matrix, BTCA as dynamic cross-linker, and TBD as the catalyst. The dynamic behaviors and mechanical performance of POE vitrimers could be regulated by varying the amount of BTCA cross-linker, and POE vitrimers were endowed with reprocessability. Dynamic networks in POE vitrimers enable
CRediT authorship contribution statement
Lin Cheng: Experiments, Formal analysis, Writing – original draft. Benke Li: Formal analysis. Sijun Liu: Supervision. Wei Yu: Supervision, Writing – review & editing, Reviewing and Editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors are grateful for the financial support from the National Natural Science Foundation of China (No. 21790344and 51625303).
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