Elsevier

Polymer

Volume 232, 12 October 2021, 124159
Polymer

Vitrimer bead foams: Cell density control by cell splitting in weld-compression molding

https://doi.org/10.1016/j.polymer.2021.124159Get rights and content

Highlights

  • POE vitrimers and POE vitrimer foams were prepared via classical transesterification.

  • Two approaches were suggested for reprocessing and recycling POE vitrimer bead foams by weld-compression molding and re-foaming.

  • The cell density of the vitrimer bead foams could be tuned through a cell splitting process.

  • The mechanical properties of the recycled foams can be controlled over a wide range.

Abstract

The fabrication of weldable and recyclable polyolefin bead foams remains a great challenge. Here, we report a convenient pathway to transform polyolefin elastomer (POE) into vitrimers based on classical ester-hydroxyl transesterification through reactive extrusion. The dynamic behaviors and mechanical properties of POE vitrimers could be readily regulated by varying the dosage of the cross-linker. POE vitrimer bead foams are prepared by a simple and feasible free foaming technology. We suggest two approaches to reprocess and recycle POE vitrimer bead foams by weld-compression molding and re-foaming. POE vitrimer bead foams could be well bonded with adjustable cell density and performance through weld-compression molding strategy. The dynamic transesterification can not only promote the fusion of foam beads, but also induce the fusion of contact buckling cell walls under compression. This investigation would motivate further sustainable development in the designing and recycling of polymer bead foams.

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).

References (49)

  • A. Zych et al.

    Polyethylene vitrimers via silyl ether exchange reaction

    Polymer

    (2020)
  • F. Caffy et al.

    Transformation of polyethylene into a vitrimer by nitroxide radical coupling of a bis–dioxaborolane

    Polym. Chem.

    (2019)
  • F. Yang et al.

    Highly elastic, strong, and reprocessable cross-linked polyolefin elastomers enabled by boronic ester bonds

    Polym. Chem.

    (2020)
  • J.J. Jiang et al.

    Evolution of ordered structure of TPU in high-elastic state and their influences on the autoclave foaming of TPU and inter-bead bonding of expanded TPU beads

    Polymer

    (2021)
  • C.B. Ge et al.

    Steam-chest molding of expanded thermoplastic polyurethane bead foams and their mechanical properties

    Chem. Eng. Sci.

    (2017)
  • M. Nofar et al.

    A novel technology to manufacture biodegradable polylactide bead foam products

    Mater. Des.

    (2015)
  • Y. Guo et al.

    Critical processing parameters for foamed bead manufacturing in a lab-scale autoclave system

    Chem. Eng. J.

    (2013)
  • O.S. Alimi et al.

    Microplastics and nanoplastics in aquatic environments: aggregation, deposition, and enhanced contaminant transport

    Environ. Sci. Technol.

    (2018)
  • R. Geyer et al.

    Production, use, and fate of all plastics ever made

    Sci. Adv.

    (2017)
  • D. Vethaak et al.

    Microplastics and human health

    Science

    (2021)
  • I.A. Kane et al.

    Seafloor microplastic hotspots controlled by deep-sea circulation

    Science

    (2020)
  • J.M. Eagan et al.

    Combining polyethylene and polypropylene: enhanced performance with PE/iPP multiblock polymers

    Science

    (2017)
  • D.K. Hyslop et al.

    Functional nitroxyls for use in delayed-onset polyolefin crosslinking

    Macromolecules

    (2012)
  • A. Rahimi et al.

    Chemical recycling of waste plastics for new materials production

    Nat. Rev. Chem.

    (2017)
  • Cited by (9)

    View all citing articles on Scopus
    View full text