Materials Horizons: Communication
Materials Horizons: Communication
Materials Horizons: Communication
Horizons
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DOI: 10.1039/d2mh01211a
rsc.li/materials-horizons
Introduction
based on high density crosslinked polymer networks traditionally
High performance thermoset plastics and polymers deliver assembled using non-reversible covalent bonding. However, their
lightweight, durable and mechanically robust materials for a covalent network structure renders them single-use, and non-
wide range of industrial applications.1 These include high recyclable in their current form. Epoxy-resins are one of the
durability coatings,2,3 structural composites4,5 (some of which leading thermoset technologies for industrial applications, due
are used in the renewable energy sector), lightweight compo- to their excellent mechanical strength, versatility and robustness
nents for aerospace and automotive application, and structural with over 323 000 tonnes of epoxy resin produced in Europe per
adhesives.6,7 Whilst focus is dominated on the challenge of annum.10
plastic recycling of commoditised (low-cost) thermoplastic The increasing focus on sustainable development goals11
materials used in bottles, packaging and films,8 the significant and Net Zero Strategy,12 (including sustainable and recyclable
environmental and scientific challenge of recyclable thermoset materials), highlights a major limitation of epoxy-resin tech-
materials has received proportionately less attention. Thermoset nology, in the limited opportunity for chemical or mechanical
polymers, which make up B20% of all polymeric materials,9 are recycling.13,14 This issue is highlighted in applications such as
wind turbine blades, where it is estimated that B250 000
a
Department of Applied Sciences, Northumbria University, Newcastle upon Tyne,
tonnes of epoxy thermoset polymers reside, that are destined
NE1 8ST, UK. E-mail: matthew.unthank@northumbria.ac.uk
b
Department of Chemistry, University of Cambridge, Cambridge, CB2 1EW, UK
for landfill or incineration, once the wind turbines have
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/ reached the end of their useful life.10 Without lightweight,
10.1039/d2mh01211a structural epoxy-thermosets renewable wind-energy would not
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be possible, yet their inherent ‘almost indestructible’ molecular ester (2d) was added according to Table S2 (see ESI† for details).
structure renders them unsuitable for conventional recycling The reaction mixture was stirred at 55 1C for 1 hour and
processes.4 45 minutes under nitrogen whilst viscosity slowly increased,
The development of new materials, that have mechanical or before drawing down onto a non-stick PTFE sheet using a
chemical recyclability designed into their molecular structure, 400 mm draw-down bar to control film thickness. The resulting
offer an opportunity to replace existing single-use materials films were allowed to cure at room temperature for 16 hours
with more sustainable alternatives. Dynamic materials, such as under nitrogen, followed by a post cure schedule ramp at 80
covalent adaptive networks (CANs), offer a potential solution to (1 hour), 120 (1 hour) then 170 1C (1 hour, total 3 hours thermal
this issue,15–20 but the creation of such materials would need to cure). After cooling to room temperature, the resulting poly-
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be based on readily available high-performance building meric films were removed from the PTFE sheet to give dioxa-
Open Access Article. Published on 16 December 2022. Downloaded on 3/16/2023 5:44:38 AM.
blocks, if a rapid pathway to impact and market acceptance is zaborocane crosslinked epoxy-amine polymers (D0–D100) or
to be realised.13 Further to this, such materials would need to viscous liquid films (D0–D40).
be designed in a way that offered versatile material perfor-
mance, with tuneable properties spanning hard, high Tg glassy Epoxy-amine (control) preparation
materials to more flexible adhesives.21–23 If the new class of 1,4-Butanediol glycidyl ether (4.00 g, 0.020 moles) and phenyl
materials could be both mechanically recycled (thermally repro- glycidyl ether (1.49 g, 0.010 moles) were combined in a round
cessed) and chemically recycled (i.e. molecule recovery and bottom flask under nitrogen with stirring. To the reaction
separation from mixed material), then this would represent a mixture, the appropriate mass of n-hexylamine (HA), cyclo-
significant contribution to this important area of polymer hexylamine, 1,6-hexanediamine and 4,4 0 -MDH was added
chemistry. according to Tables S3–S5 (see ESI† for details). The reaction
The research in this report describes a scalable and versatile mixture was stirred at 55 1C for 1 hour and 45 minutes under
approach to tackle this challenge. The approach exploits widely nitrogen whilst viscosity slowly increased, before drawing down
used and structurally diverse epoxy resins, polymerised with onto a non-stick PTFE sheet using a 400 mm draw-down bar to
equally diverse aliphatic amines.22 When crosslinked with control film thickness. The resulting film was allowed to cure at
diboronic acid esters (to form thermoset materials), a range room temperature for 16 hours under nitrogen, followed by a
of tailored material types are accessible, with tuneable physical post cure schedule ramp at 80 (1 hour), 120 (1 hour) then 170 1C
properties. Unlike ‘traditional’ epoxy-amine thermosets, this (1 hour, total 3 hours thermal cure). After cooling to room
approach allows the polymer backbone to be based on epoxy- temperature, the resulting polymeric films were removed from
amine polymerisation, generating b-amino diol groups, which the PTFE sheet to give solid sheets of epoxy-amine polymer
are subsequently complexed with boronic esters24–26 to form (A60–A100) or viscous liquid films (A0–A40).
dioxazaborocane crosslinks. Whilst epoxy resins have been
chemically modified previously to create dynamic or reversible Nuclear magnetic resonance (NMR)
thermoset materials27–32 such materials rarely benefit from NMR analyses were acquired at 25 1C using a JEOL ECS400
the high Tg or rapid dynamic exchange kinetics described in Delta spectrometer at frequencies of 399.78 MHz for 1H-NMR,
our approach, both desirable properties for recyclable 100.53 MHz for 13C-NMR and 128.28 MHz for 11B-NMR. All
materials.33–36 Other research to exploit b-amino diol function- chemical shifts are quoted as parts per million (ppm) relative to
alities (from epoxy-amine polymerisation) have been based on a tetramethylsilane (TMS, d = 0 ppm) as an internal standard in
modification of a pre-prepared polymeric material, and not either deuterated chloroform (CDCl3) or deuterated dimethyl
directly from a single stage synthetic process.37–39 Further to sulfoxide (DMSO-d6). OH group analyses were confirmed by
this, an efficient process for the chemical or mechanical doping samples with a drop of deuterated water (D2O).
13
recycling of these materials has not been demonstrated. C-NMR assignment was confirmed by DEPT analysis. The
spectral data is recorded as chemical shift (d), relative integral,
multiplicity (s = singlet, br = broad, d = doublet, t = triplet,
Materials and methods q = quartet, quin = quintet, sext = sextet, dd = doublet of
All materials were purchased from commercial vendors listed doublets, m = multiplet) and coupling constant (J = Hz).
in the supplementary information (Table S1, ESI†) and used as
Fourier transformed infrared spectroscopy (FTIR)
received. Methods for the preparation of all thermoset materi-
als used in this study can be found in the ESI.† Infrared spectroscopy was performed on a Bruker alpha
Platinum-ATR and the output data analysed in OPUS software.
Absorption maxima are expressed in wavenumbers (cm1).
Epoxy-amine-dioxazaborocane preparation
1,4-Butanediol glycidyl ether (BGE, 4.00 g, 0.020 moles), phenyl Gel permeation chromatography (GPC)
glycidyl ether (PGE, 1.49 g, 0.010 moles) and n-hexylamine (HA, Samples were dissolved in THF (2 mg mL1) and filtered
2.50 g, 0.025 moles) were combined in a round bottom flask through 0.2 mm nylon filters. Samples were analysed using an
under nitrogen with stirring. To the reaction mixture the Agilent 1260 infinity II system equipped with a RI and visco-
appropriate mass of 1,4-phenylenediboronic acid tetrabutyl metry detector, fitted with PLgel MiniMIX-E and PLgel
890 | Mater. Horiz., 2023, 10, 889–898 This journal is © The Royal Society of Chemistry 2023
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MiniMIX-D columns in sequence, using a THF mobile phase starting materials, such as epoxy resins and aliphatic amines.
and a flow rate of 0.6 mL min1. Analysis was performed To explore this concept, the preparation of suitable boronic
against a calibration curve of polystyrene standards (EasiVial esters that combined miscibility (in general purpose epoxy
PS-M supplied by Agilent). resins), stability and chemical reactivity (with a b-amino diol)
was required. A range of mono-boronic esters (1a–d) were
Dynamic mechanical thermal analysis (DMTA) prepared from readily available starting materials (Fig. 2)42
DMTA analyses were conducted for network polymers on a and their physical and chemical reactivity properties were
PerkinElmer DMA 8000 and analysed in Pyris software (version studied.
To explore the relative reactivity of the selected boronic
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materials.
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(Fig. 9a), the D100 sample has fully dissolved after 24 hours. In
all cases, the control epoxy-amine A100 remains undissolved, as
expected for a ‘traditional’ epoxy thermoset. The same result
was achieved using pinacol (2,3-dimethyl-2,3-butanediol) to
disassemble the dioxazaborocane D100 polymer network, lib-
erating the epoxy-amine polymer, with concomitant formation
of di-pinacol boronic ester 2b (Fig. 7, method b). 1H-NMR
analyses support this, with characteristic dioxazaborocane sig-
nals (Fig. 8, boronic ester model complex, 4.2–4.8 ppm) absent
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Fig. 7 Chemical recycling of epoxy-amine-dioxazaborocane D100 via either (a) phenylboronic ester or (b) 2,3-dimethyl-2,3-butanediol (pinacol)
methodologies.
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Fig. 11 (a) Ultimate tensile strength and (b) Young’s modulus, data for
D60–D100 and A360–A3100 materials.
896 | Mater. Horiz., 2023, 10, 889–898 This journal is © The Royal Society of Chemistry 2023
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