RECYCLING THERMOPLASTIC EVA
(POLYETHYLENE-CO-VINYL ACETATE) WITH
IMPROVED PROPERTIES
by
HAOCHEN GUO
Submitted in partial fulfillment of the requirements for the
degree of Master of Science
Thesis Advisor: Prof. Ica Manas-Zloczower
Department of Macromolecular Science and Engineering
CASE WESTERN RESERVE UNIVERSITY
May, 2020
CASE WESTERN RESERVE UNIVERSITY
SCHOOL OF GRADUATE STUDIES
We hereby approve the thesis of
HAOCHEN GUO
candidate for the Master of Science degree
Committee Chair
Ica-Manas Zloczower
Committee Member
David Schiraldi
Committee Member
Joao Maia
Date of Defense
3/27/2017
*We also certify that written approval has been obtained for any proprietary material contained therein. Table of Contents
Acknowledgement ...... ix
Abstract ...... x
1. Introduction ...... 1
2. Experimental ...... 7
2.1. Materials...... 7
2.2. Preparation of EVA vitrimer...... 8
2.3 Preparation of EVA vitrimer-CNS nanocomposite ...... 9
2.3. Proton Nuclear Magnetic Resonance (NMR) for simulation of the reaction of
triethyl borate and methyl acetate...... 10
2.4. Characterizations...... 11
2.4.1. Fourier-transform infrared spectroscopy and In-situ dynamic Fourier-transform
infrared spectroscopy...... 11
2.4.2. Dynamic Mechanical Analysis (DMA)...... 11
2.4.3. Differential scanning calorimetry (DSC)...... 11
2.4.4. Thermogravimetric analysis (TGA)...... 12
2.4.5. Mechanical Characterization...... 12
2.4.6. Rheological Analysis...... 12
2.4.7 UV aging resistance analysis...... 12
2.4.8 Optical microscopy...... 13
iii
3. Results and discussion ...... 13
3.1. Dynamic crosslinking for recycling EVA...... 13
3.2. Thermal and thermal-mechanical properties of EVA vitrimers with different
crosslinking density...... 18
3.3. Mechanical properties and reprocessablity of EVA vitrimers...... 20
3.4. Analysis of UV-aging for EVA vitrimers...... 24
3.5 EVA-CNS nanocomposites -Morphology ...... 25
3.6 EVA-CNS nanocomposites electrical conductivity ...... 28
3.7 Thermo-mechanical properties of segregated structure vEVA-(EVA-xCNS)
nanocomposites ...... 32
4. Conclusion ...... 33
Reference ...... 35
iv
List of Figures
Figure1.1 Ethylene- co- vinyl acetate applications in 2014.[1] ...... 1
Figure1.2 Global cumulative plastic waste generation and disposal.[2] ...... 3
Figure1.3 Brick-Wall structure...... 6
Figure3.1 Schematic of crosslinking between thermoplastic EVA and triethyl borate...... 13
Figure3.2 a) samples (Thermoplastic EVA(left) EVA vitrimer (right)) before testing b) samples (Thermoplastic EVA(right) EVA vitrimer (left)) immersed in
THF for 5 days at room temperature...... 14
Figure3.3 FTIR of EVA-V-1, EVA-V-2, EVA-V-3, and thermoplastic EVA (Neat).
...... 15
...... 16
Figure3.4 Swelling ratio versus crosslinking density...... 16
Figure3.5 NMR result after transesterification (triethyl borate, methyl acetate, trimethyl borate, and ethyl acetate)...... 17
Figure3.6 In-situ FTIR of the C-O bond a) Neat EVA (Thermoplastic) b) EVA-V-
1...... 18
Figure3.7 The storage modulus a) and tan(δ) of thermoplastic EVA and EVA vitrimers as functions of temperature from -50 to 200 °C...... 19
Figure3.8 DSC of exothermic peaks a) and endothermic peaks b) for thermoplastic
EVA and EVA vitrimers...... 20
v
Figure3.9 Complex viscosity vs angular frequency at 200 oC...... 22
Figure3.10 EVA-V-3 processed multiple times by extrusion and compression
molding, and EVA-V-3 compression film after five times reprocessing...... 23
Figure3.11 Comparison of Young’s modulus a) and storage modulus b) before and
after recycling for thermoplastic EVA and EVA vitrimers...... 24
Figure3.12 Tensile Modulus a), Storage Modulus b) and Elongation at break c) of
thermoplastic EVA and dynamic crosslinked EVA before and after UV-exposure.
...... 25
Figure3.13 Morphology for pv(EVA-0.3CNS) (left) and vEVA-(EVA-0.3CNS)
(right) ...... 26
Figure 3.14 Morphology comparison for vEVA-(EVA-xCNS) with different CNS loading...... 27
Figure3.15 Complex viscosity for EVA vitrimer and thermoplastic EVA-CNS composite at different CNS content. (left) The viscosity ratio between EVA vitrimer and thermoplastic EVA-CNS at different CNS content. (right) ...... 28
Figure3.16 Electrical conductivities of vEVA-(EVA-xCNS), pv(EVA-xCNS), and vEVA-xCNS...... 29
Figure3.17 Electrical conductivities at four different points in one single film of vEVA-0.7CNS...... 30
Figure3.18 C-O bond peak for pv(EVA-0CNS), pv(EVA-0.3CNS) and pv(EVA-
1CNS)...... 31
vi
Figure3.19 Elongation at break of vEVA-(EVA-xCNS), pv(EVA-xCNS), and vEVA-xCNS (left) and Young’s Modulus of vEVA-(EVA-xCNS), pv(EVA- xCNS), and vEVA-xCNS(right)...... 32
Figure3.20 Storage modulus for vEVA-(EVA-0CNS), vEVA-(EVA-0.1CNS), vEVA-(EVA-0.3CNS), vEVA-(EVA-0.5CNS) and vEVA-(EVA-0.7CNS)...... 33
vii
List of Tables
Table2.1 Catalyst and crosslinker content for EVA-V-1, EVA-V-2, EVA-V-3.
...... 9
Table 3.1 Approximate molar amounts of small molecules for the model reaction ...... 17
Table3.2 Young’s modulus, tensile strength, and elongation at break of thermoplastic EVA and EVA vitrimers...... 21
viii
Acknowledgement
I would like to thank Professors Ica Manas- Zloczower for being my advisor and help me with my research in the past two years. I would like to thank Professor Maia and
Professor Schiraldi for the time and effort in serving on my committee. I would also like
to thank Dr. Patel, Dr. Yue, soon to be Dr. Rohm, YY and the rest of the mixing group to
help me with my research. Also, I would like to thank my girlfriend, Dr. Yuan, for always
being with me and help me every little thing.
ix
Recycling Thermoplastic EVA (Polyethylene-Co-Vinyl
Acetate) With Improved Properties
Abstract
by
HAOCHEN GUO
The low properties of recycled polymers associated with the high cost of recycling
hinders development of the thermoplastic recycling industry. Dynamic crosslinking of
recycled thermoplastics with the formation of vitrimers enables superior mechanical
properties, good reprocessability as well as superior chemical and environmental
resistance. Herein, a poly (ethylene-vinyl acetate) (EVA) vitrimer was formed by crosslinking with triethyl borate with the catalyst (bis(acetylacetonato) dioxo-molybdenum
(VI)). The resultant EVA vitrimer shows enhanced thermal stability and mechanical
properties with up to two times improvement in Young’ modulus and storage modulus by
comparison with the thermoplastic EVA. Moreover, 90 % of Young’s modulus was
maintained in the EVA vitrimer after five times of recycling, whereas only 72 % can be
maintained for recycling the thermoplastic EVA. This dynamic crosslinked EVA also
exhibits superior UV and solvent resistance, which helps to extend the service time of the
recycled material. This work introduces a facile and efficient method to recycle and reuse
EVA with low property loss. It has potential to enable the production of high-performance
EVA from EVA waste for different applications.
x
A self-segregated structure EVA vitrimer-CNS composite was fabricated with thermoplastic EVA-CNS and EVA vitrimer through melt-mixing. The resultant system displays a self-segregated structure with more than two times higher elongation at break by comparison with the original EVA vitrimer. Moreover, two other methods – filler/vitrimer melt mixing and post vitrimer reaction of the EVA/CNS were used to fabricate EVA vitrimer-CNS composites. The self-segregated structure EVA vitrimer-CNS composite has superior electrical conductivity and similar Young’s modulus compared with the other two sets composites. The proposed method of obtaining a segregated filler network in a vitrimer composite has potential to enable high electrical conductivity and high flexibility making it promising for different applications.
xi
xii
1. Introduction
Ethylene-vinyl acetate (EVA), is the copolymer of ethylene and vinyl acetate. The weight
percent of vinyl acetate usually varies from 10 to 40%, with the remainder being ethylene.
The properties and application of EVA can by varied by tuning the content of vinyl acetate.
The EVA with low VA content, under 4%, is usually referred as low-density polyethylene.
Low VA content EVA has similar properties with low-density polyethylene and has improved softness and flexibility. The EVA with medium VA content, up to 30%, is considered as a thermoplastic elastomer. Most of EVAs in this VA content range have good low-temperature properties and toughness. EVA with a high proportion of VA, higher than
60%, is a rubber-like material in terms of toughness and flexibility. EVA is a versatile material with good clarity, superior properties at low-temperature, as well as reasonable resistance to UV radiation.
Figure1.1 Ethylene- co- vinyl acetate applications in 2014.[1]
1
Currently, over 5 billion tons of polymer waste are generated annually, but only less than
10% are recycled. [2] Among them, EVA is a versatile copolymer material, hitting 7.29
billion dollars for the global market and is anticipated to reach 11.4 billion dollars in
2023.[3] Along with population and per capita income rises, the consumption of EVA film in the packaging industry in the past few years has been increasing. EVA film has been the largest application in packaging, agricultural film, greenhouse film as well as pharmaceutical usage. Most of these high-end uses for EVA are non-renewal, which could cause severe environmental impact for decades. [4]
There are two major processing methods for plastic waste disposal recycling -
incinerating and reprocessing. The shortages of nowadays recycling industry are low cost-
performance, bad properties of recycled material[5, 6] and toxic chemicals[4, 7, 8], etc.
Incinerating polymer waster is a relatively simple and common way to manage polymer
waste, however, this method is relatively expensive, energy-consuming, environmentally
unfriendly as well as generating toxic chemicals. Reprocessing polymer waste always
compromises recycled properties due to the decrease in molecular weight by mechanical
or thermal degradation during reprocessing [9]. The recycled thermoplastics become weaker
and end up as unusable after multiple reprocessing procedures, which makes reprocessing
recycled polymer waste low efficient and costly.
2
Figure1.2 Global cumulative plastic waste generation and disposal.[2]
Recently, vitrimers, a new type of polymers with dynamic covalent bonding have
attracted lots of attention for their ability to be processed by conventional processing
methods despite being cross-linked.[10, 11] Thermoset vitrimers such as epoxy[10, 12] and
polyurethane[13, 14] can be reprocessed at high temperature by the formation of covalent
adaptive networks.
Such dynamic networks show excellent welding,[12, 15, 16] self-healing,[17, 18] and shape
memory performance[15, 19] and have been used also for composite fabrication.[19-21]
Catalysts have been vectored into existing permanent networks and converted them into vitrimers through a process coined as vitrimerization allowing for thermoset recycling.[22]
Vitrimer networks can also be achieved by cross-linking thermoplastic polymers containing active functional groups with dynamic cross-linkers.[11, 23] Thermoplastic-based vitrimers have superior reprocessability with fewer additives in comparison with thermoset
3
vitrimers. Thermoset vitrimers can mostly undergo up to three times reprocessing[10, 16, 19,
24] with over 10 wt % catalyst. For thermoplastic vitrimer systems, the amount of catalyst
can be decreased to as little as 0.166 wt % (in the current work) with more than five times
reprocessing. Thermoplastic vitrimers show improved mechanical performance and
enhanced resistance to environmental factors, such as humidity, UV, heat, and chemical
solvent corrosion in comparison with conventional thermoplastics[11] due to the formation
of cross-linked networks.[25, 26] The network helps to stabilize the molecular architecture
during reprocessing and prevents the breakage of polymer chains.[5, 27]
The mechanical properties of the elastomer matrix can improve even more with
additional nanofillers. Using adequate fillers, elastomer nanocomposites can become
electrically conductive with applications for EMI shielding,[28] piezoresistive behavior,[29-
31] etc. Therefore, EVA vitrimer composites were considered to further expand the potential value of recycled EVA.
Carbon nanostructures (CNS) are rod-like nano-carbon materials. CNS have a much higher aspect ratio (up to 132000000:1) compared with any other commercial nano-carbon fillers, which endows CNS with exceptional mechanical and electrical properties.32 CNS
have extraordinary performance in terms of mechanical, electrical as well as thermal
conductivity. CNS have an electrical conductivity as high as 106 S/m and a thermal
conductivity at room temperature up to 3500W/mK [33]based on the wall area. Moreover,
the elastic modulus can approach nearly 1 TPa and the tensile strength 100GPa which is
over 10 fold higher than any available industrial fibers. [34]
There are two ways to prepare EVA nanocomposites, and each of them will lead to
different dispersion degree. (1) Solvent mixing, where the polymer matrix is dissolved and
4
incorporated with nanofiller. The solvent will further have to be removed by either
evaporation, centrifugation or filtration. (2) Melt mixing, where nanofiller is incorporated
into the molten polymer matrix in an extruder or mixer. Melt mixing is conducive to a
lower degree of filler dispersion compared with solvent mixing however is preferred by
most industry manufacturing.
The main challenge for preparing high-quality nanocomposites is to find ways to
improve efficiency for filler functions based on processing conditions. Good dispersion is
needed for controllable and consistent physical properties as well as high filler efficiency.
Vitrimer is silica-like polymer[10], and the processing is more welding-like rather than flow-like because of high viscosity due to high crosslinking density.[35, 36] High viscosity
and difficult reprocessability tend to lead to insufficient mixing for fillers. All in all, it is
extremely difficult to fabricate well dispersed high-quality EVA vitrimer based composite
by traditional non-solvent dispersion methods.
Serval strategies have been proposed to control the filler distribution to fabricate high-
quality nanocomposites.[30, 37] Among them, the filler segregated structure is suitable for
polymer-based composites. Ke et al[30, 31], came up with a novel way to restrictively locate
CNS around TPU particles by solvent dispersion of CNS and then coat the well dispersed
CNS onto thermoplastic polyurethane (TPU) fine particles. After compression molding, a
brick-wall structure (Figure 1.3) was created. Segregated structure TPU-CNS achieves high electrical conductivity, superior piezoresistive sensitivity as well as ultralow percolation threshold. The segregated structure usually comprises two components: a
segregated phase and a dispersive phase. The segregated phase is thermal-reprocessable
and the dispersive phase is locally coated around the segregated phase. This method
5
requires the assistance of a solvent to disperse and coat filler onto polymer fine particles.
Many of new interfaces have been created between the two phases which decrease the system mechanical properties. The challenge to optimize and scale-up segregated structure composites is how to eliminate the solvent and also improve the interactions at the interfaces.
Figure1.3 Brick-Wall structure.
The self-segregated structure was conceived to optimize the segregated structure processing and improve interfacial strength. Self-segregated structures were constructed with the same components in both phases. The lower viscous phase is continuously wetting the high viscous phase and serves as “glue” to weld the high viscous particles together [38].
Yang, et al successfully fabricated POE/CNT composites using crosslinked POE and
POE/CNT.[39] Ming, et al developed a partial cured PDMS and fabricated ultralow
electrical percolation threshold PDMS/CNT composites with good interfacial interaction.
They also fabricated PLLA/CNT composites using PLLA with different crystallinity
rates.[40, 41]
In this work, EVA was crosslinked with dynamic covalent B-O bonds to form a dynamic
network. This EVA vitrimer can be reprocessed multiple times at high temperature by
compounding, extrusion and compression molding. The crosslinked network helps to
improve the thermal stability and chemical resistance of the recycled EVA. Moreover, the
6
EVA vitrimer exhibits enhanced mechanical properties in comparison with thermoplastic
EVA and has negligible loss of mechanical properties upon multiple times recycling. The
dynamic crosslinked EVA also shows superior properties at high temperature and enhanced
UV resistance allowing for extended service time by comparison with the thermoplastic
EVA.
Segregated structure EVA vitrimer/EVA-CNS composites were fabricated with high
electrical conductivity and superior elongation at break. Thermoplastic EVA was mixed
with CNS to form the continuous phase. The cured EVA vitrimer forms the segregated
phase. The final EVA vitrimer/EVA–CNS composite shows a self-segregated structure
with high conductivity and improved elongation at break. Specifically, the EVA vitrimer
composite with segregated structure has nearly 100 times higher electrical conductivity
than the regular melt mixed EVA vitrimer-CNS composite, and nearly 50% higher
elongation at break. The thermal mechanical properties for the self-segregated EVA
vitrimer-CNS composites at different nanofiller content indicate better matrix/filler
interfacial strength than for the homogenously dispersed EVA vitrimer-CNS composites.
2. Experimental
2.1. Materials.
A commercial Poly (Ethylene-co-Vinyl Acetate), Ultrathene UE624000 (Lyondell
Basell Industries N.V., North America), with vinyl acetate content of 18 wt. % was kindly
provided by Lyondell Basell. Methyl acetate, triethyl borate (99 %),
bis(acetylacetonate)dioxo-molybdenum(VI) were purchased from Sigma-Aldrich. Ethyl
7 acetate, tetrahydrofuran (THF) were purchased from Fisher Chemical. Chloroform-d, 99.8
% atom D (CDCL3) (D enrichment>= 99.75 %), containing 1 v/v % tetramethylsilane
(TMS) was purchased from Acros Organics. Carbon nanostructures (flake, 70 μm long, 10
μm thick and ca. 9 nm nanotube diameters) were provided by Applied Nanostructured
Solutions LLC (Lockheed Martin Corporation). EVA was dried in an oven at 75 o C overnight before use, to eliminate hydrolytic degradation. All other materials were used as received without further purification unless denoted otherwise.
2.2. Preparation of EVA vitrimer.
EVA was crosslinked in two steps. First, thermoplastic EVA and catalyst bis(acetylacetonate)dioxo-molybdenum(VI) were melt-mixed at 100 ºC, 50 rpm for 15 min using an internal mixer (Haake Polylab OS system, Thermo Fisher Scientific, USA).
Various amounts of triethyl borate were added afterward as crosslinker for another 10 min mixing. The product obtained was then cut into small pieces and cured in an oven at 160
ºC for 10 hours. EVA vitrimer networks with different crosslinking ratios were prepared by controlling the triethyl borate to acetate group ratio. For a fully crosslinked EVA, the molar ratio triethyl borate and vinyl acetate is 1:3 (B-O: C=O is 1:1, i.e. 50 g EVA, 9 g/0.105 mol vinyl acetate, fully react with 5.145 g/0.035 mol, triethyl borate), and it is labeled EVA-V-3. EVA-V-1 and EVA-V-2 represent 33.3 % crosslinked EVA (1.715 g triethyl borate/50 g EVA), and 66.6 % crosslinked EVA (3.43 g triethyl borate/50 g EVA), respectively. Table 2.1 lists the amounts of catalyst and crosslinker used in the different formulations. The crosslinker was used in excess of 5 % to the stoichiometric amount. The catalyst was used at a constant molar ratio with the crosslinker as 2.5×10-4 g catalyst/0.035 mol crosslinker. The crosslinked EVAs were compression molded (10 cm×10 cm×~0.1
8
cm) at 200 ºC for 20 min under the constant pressure of 10 MPa. For the rheological test,
the crosslinked EVAs were compression molded into a disk (radius of 1.25 cm, and
thickness of ~0.1 cm).
Table2.1 Catalyst and crosslinker content for EVA-V-1, EVA-V-2, EVA-V-3.
Modified EVA Catalyst content(g) Crosslinker (triethyl borate) content (g)
EVA-V-1 0.083 1.715
EVA-V-2 0.166 3.43
EVA-V-3 0.250 5.145
EVA neat samples, EVA-V-1, and EVA-V-3 samples were reprocessed for 5 cycles through the extruder (Haake Polylab OS system, Thermo Fisher Scientific, USA). For each cycle, the polymer was extruded at 60 rpm and 200 ºC for 5 min and then cooled down to room temperature for 30 min. The samples obtained by extrusion were then compression molded (10 cm×10 cm×~0.1 cm) at 200 ºC for 20 min under the constant pressure of 10
MPa.
2.3 Preparation of EVA vitrimer-CNS nanocomposite
EVA vitrimer-CNS nanocomposites with different filler loadings were prepared in three different ways : 1) EVA vitrimer-CNS fabricated by melt mixing EVA vitrimer with CNS in a batch-mixer (Haake Polylab OS system, Thermo Fisher Scientific, USA) at 200 oC at
25 rpm for 10 min for two times, and were marked as vEVA-xCNS where “v” represents
vitrimer and “x” indicates the nanofiller content. 2) EVA vitrimer-CNS composites were
also prepared following the procedure: First, thermoplastic EVA and CNS were melt mixed
two times for 10 min each at 100 oC and 110 oC, 50 rpm in the batch-mixer. Then, EVA-
CNS and catalyst were compounded using the batch mixer at 100 oC, 50 rpm for 15 min.
The resulting mixture was blended with triethyl borate in the batch mixer at 100 oC and 50
9
rpm for another 10 min. Finally, the compounded complex was cured in the oven at 150
oC for 8 hours. The amount of crosslinker and catalyst was the same as for EVA-V-3
(calculated based on the total amount of EVA vitrimer) and was marked as pv(EVA-xCNS) where “pv” stands for post vitrimer reaction processing and “x” indicates the nanofiller content. (3) EVA vitrimer-EVA-CNS composites were prepared in three steps using two
components: EVA vitrimer and thermoplastic EVA-CNS composite. First, thermoplastic
EVA-CNS composites were prepared through melt mixing with two times mixing for 10 min each at 100 oC and then 110 oC, at 50 rpm in the batch mixer. The EVA vitrimer was
prepared using the same amounts and method as EVA-V-3 at 150 oC. Finally, EVA
vitrimer and thermoplastic EVA-CNS composite were melt-mixed in the batch mixer at
130 oC and 50rpm for 10 min at a constant mass ratio: 9:1. The CNS content was calculated
based on the ratio of CNS to the mass of the total product, for example, 1wt% EVA
vitrimer-CNS for this method comes from 10wt% thermoplastic EVA-CNS diluted into the
EVA vitrimer matrix under 9:1 mass ratio. The EVA vitrimer-CNS fabricated by this method was named as vEVA-(EVA-xCNS) where “v” stands for vitrimer and x indicates the filler content.
2.3. Proton Nuclear Magnetic Resonance (NMR) for simulation of the reaction of triethyl borate and methyl acetate.
A mixture of methyl acetate (7.4 g/0.1 mol), triethyl borate (4.82 g/0.033 mol), bis(acetylacetonate)dioxo-molybdenum (VI) (2.4×10-2 g, /~7.5×10-5 mol), was transferred
into a high- pressure reactor at 150 ºC for 24 hours and then quenched by dry ice. The catalyst was removed by centrifugation at 3000 rpm for 15 minutes at room temperature.
CDCl3 with TMS was chosen as solvents for internal reference. The product of the reaction
10
was mixed in CDCl3 and then transferred into the NMR-tube. 1H NMR spectroscopy was
performed on a 500 MHz Bruker Ascend Avance III HDTM at room temperature (25 ºC).
2.4. Characterizations.
2.4.1. Fourier-transform infrared spectroscopy and In-situ dynamic
Fourier-transform infrared spectroscopy.
ATR-FTIR (Cary 680 FTIR Spectrometer, Agilent) was performed at room temperature.
Real-time transmission infrared spectroscopy (Nicolet™ iS50 FTIR Spectrometer, Thermo
Fisher Scientific) was performed at 160 ºC for 30 min. A film with a thickness of 0.1 cm
was placed under the diamond ATR element with a scan ranging from 400-4000 cm-1 to
record the transesterification reaction details on cured EVA vitrimer and the resultant
signals were averaged over 32 times’ recording results. A film (0.1 cm×0.1 cm×0.01 cm)
was placed on the hot plate element of Real-time FTIR. Real-time FTIR signals were
recorded every 2 min with a scan range from 400-4000 cm-1. The hot plate was heated to
160 ºC for the first two minutes and then maintained at 160 ºC for another 28 min.
2.4.2. Dynamic Mechanical Analysis (DMA).
Prepared samples of rectangular shape were tested under a “tension film” mode with a
DMA (Q800 TA Instruments, USA). A temperature sweep from -50 to 200 ºC was
performed with a heating ramp of 5 ºC/min at a frequency of 1 HZ. A strain of 0.5 %, a
preload force of 0.01 N and a force track of 125 % were used.
2.4.3. Differential scanning calorimetry (DSC).
DSC measurements were performed using a DSC Q100 differential scanning calorimeter
from TA Instruments with Tzero aluminum pans. Each sample was heated from -50 to 200
°C at a heating rate of 5 °C/min under constant nitrogen flow. The glass transition
11
temperature, Tg, was taken as the midpoint of the drop in heat capacity during the heating
cycle. The melting temperature, Tm, was taken as the minimum endo peak during the
heating cycle.
2.4.4. Thermogravimetric analysis (TGA).
The thermal stability of dynamic crosslinked EVA was investigated by TGA using the
TA Instrument Q500 with an aluminum pan. The samples were about 10 mg each and were exposed from room temperature to 600 ºC with a heating rate of 10 ºC/min under constant nitrogen flow.
2.4.5. Mechanical Characterization.
Uniaxial tensile testing was done using an MTS instrument (Model 2525-806, MTS
System Corporation, MN, USA). Five specimens were tested for each sample using ASTM
D1708. The UV-exposed neat EVA sample was tested on a Zwick Roell tensile machine
(mode Z0.5) at room temperature using similar dog-bone shaped specimens (this sample is too weak to be tested using MTS, all other samples were tested on MTS). A loading rate of 10 mm/min was used for both test machines.
2.4.6. Rheological Analysis.
A frequency sweep was performed using an ARES G2 oscillatory rheometer (TA
Instruments, USA) at 200 ºC with 25 mm parallel plates geometry under a frequency range
of 0.01 to 100 rad/s in the linear viscoelastic regime (strain amplitude γ=0.1).
A frequency sweep for EVA vitrimer-CNS composite was performed using an ARES G2
oscillatory rheometer at 130 oC with 25 mm parallel plates geometry under a frequency
range 0.01 to 100 rad/s in the linear viscoelastic regime (strain amplitude γ=0.08).
2.4.7 UV aging resistance analysis.
12
The EVA neat sample and EVA-V-1 sample were exposed to UV (QUV-340 accelerated tester running in ASTM G154 cycle4) for 15 days. A UV light lamp that emits light between 280 and 400 nm was used with an irradiance level of 1.55 W/m2 at 340 nm. The
temperature in the exposure chamber is around 70 ºC.
2.4.8 Optical microscopy.
The dispersion of CNS in the EVA vitrimer matrix was observed by an optical microscope equipped with a camera (Olympus BX51TF, Tokyo, Japan). Before analysis,
thin sheets were cut by a microtome (LEICA EM FC6) at −80 °C under liquid nitrogen.
3. Results and discussion
3.1. Dynamic crosslinking for recycling EVA.
EVA vitrimers were obtained by crosslinking the thermoplastic EVA with triethyl borate via the transesterification reaction between borate ester and vinyl acetate as shown in
Figure 3.1 By introducing triethyl borate into the system, each ethoxy group replaces one ester group of vinyl acetate and then forms a new boron-centered three-dimensional (3D)
crosslinking network using bis(acetylacetonate)dioxo-molybdenum (VI) as the catalyst.
The reaction is thermally triggered and forms dynamic B-O bonds that enable the recycling of the EVA vitrimer at high temperatures. The solubility test in Figure 3.2 shows that the
EVA vitrimer is insoluble in tetrahydrofuran due to the formation of a crosslinked network, whereas the thermoplastic EVA can be easily dissolved in the solvent.
Figure3.1 Schematic of crosslinking between thermoplastic EVA and triethyl borate.
13
Figure3.2 a) samples (Thermoplastic EVA(left) EVA vitrimer (right)) before testing b)
samples (Thermoplastic EVA(right) EVA vitrimer (left)) immersed in THF for 5 days at
room temperature.
Different degrees of crosslinking in the EVA vitrimers were obtained by varying the
content of the triethyl borate added to the system. The presence of the B-O bond is
supported by Fourier transform infrared (FTIR) spectra as shown in Figure 3.3. All spectra
-1 [42] were normalized by the CH2- intensity of polyethylene at 721 cm . The bands at around
780 and 1200 cm-1 can be attributed to the characteristic peak of the B-O, and they show
an increasing trend from EVA-V-2 to EVA-V-3 sample. The small amount of B-O in the
EVA-V-1 makes it hard to be detected. Also, as the crosslinking ratio increases, the intensity of the 1730 cm-1 peak which is attributed to carbonyl stretch (C=O) decreases
because more vinyl acetate reacted with the borate. Also, the peak of 1240 cm-1 that
originally comes from the vinyl acetate of thermoplastic EVA decreases.
14
The swelling test results (Figure 3.4) also support that the EVA-V-3 has the highest crosslinking density and EVA-V-1 has the lowest in agreement with the FTIR and DMA results.
Figure3.3 FTIR of EVA-V-1, EVA-V-2, EVA-V-3, and thermoplastic EVA (Neat).
15
Figure3.4 Swelling ratio versus crosslinking density.
The bond exchange reaction of the B-O bond has been discussed in several papers,[6, 25-
27] but the transesterification of the borate system was surprisingly “forgotten”. To further
demonstrate that the borate-crosslinked EVA is a dynamic network, a customized small
molecular simulation reaction was designed. In this reaction, triethyl borate and methyl
acetate were chosen as the reagents to represent the transesterification between EVA and
triethyl borate. The molar amounts of the reagents are listed in Table 3.1. Triethyl borate
and methyl acetate were reacted in a high-pressure reactor at 150 ºC for 24 hours to ensure the reaction reached equilibrium [43], then the system was quenched immediately by dry ice and the catalyst was removed by centrifugation. The NMR results indicate all the “reagents” triethyl borate, methyl acetate and the “products” trimethyl borate and ethyl acetate remain in the system after 24 hours of transesterification reaction and the
16 ratios of the four chemical stays around 1:1:1:1 (Figure 3.5).
Figure3.5 NMR result after transesterification (triethyl borate, methyl acetate, trimethyl
borate, and ethyl acetate).
Table 3.1 Approximate molar amounts of small molecules for the model reaction
Small Molecules Initial amount (/mol) Product yield(/mol) Type
Methyl acetate 0.1 ~0.05 Reagent
Triethyl borate 0.033 ~0.0165 Reagent
Ethyl acetate 0 ~0.05 Product
Trimethyl borate 0 ~0.0165 Product
The dynamic characteristic of the system was also studied by real-time FTIR. FTIR spectra were taken every 2 minutes for a total of 30 minutes at 160 ºC on both EVA vitrimer and thermoplastic EVA. The intensity changes for the C-O bond are shown in Figure 3.6.
17
Thermoplastic EVA shows no change in the peak intensity whereas for the EVA-V-1 the
intensity changes in the first 10 minutes due to transesterification. After 15 min, a new
equilibrium was established.
Figure3.6 In-situ FTIR of the C-O bond a) Neat EVA (Thermoplastic) b) EVA-V-1.
3.2. Thermal and thermal-mechanical properties of EVA vitrimers with different crosslinking density.
Figure 3.7 presents the storage modulus E’ and tan(δ) of the neat EVA and the dynamic crosslinked EVAs with different crosslinking ratios as a function of temperature.
Thermoplastic EVA shows typical thermoplastic behavior with the storage modulus decreasing nearly linearly before Tg, and continuing to decrease after Tg to reach a
negligible value upon melting. Unlike thermoplastic EVA, a clear rubbery plateau was
observed for all EVA vitrimers after the melting temperature. (Figure 3.7a)According to
[44, 45] Ferry et al, the rubbery plateau modulus, Gp, is inversely proportional to the molecular
weight between successive crosslinks, Me:
(1) 𝜌𝜌𝜌𝜌𝜌𝜌 𝐺𝐺𝐺𝐺 ≅ �𝑀𝑀𝑀𝑀
18
where Gp is the storage modulus of the rubbery plateau at temperature T, and ρ is the polymer density. A higher rubbery plateau modulus resultant from lower molecular weight between crosslinks is consistent with a higher crosslinking density. Therefore, EVA-V-3 has the highest crosslinking degree whereas EVA-V-1 has the lowest.
Figure3.7 The storage modulus a) and tan(δ) of thermoplastic EVA and EVA vitrimers
as functions of temperature from -50 to 200 °C.
Figure 3.7b shows the tan(δ) peak of DMA for thermoplastic EVA and EVA vitrimers.
Unlike thermoplastic EVA with a broader tan(δ) peak due to the broad range of chain movement from different segments, EVA vitrimers all exhibit a narrower tan(δ) peak, indicating the increased molecular structure uniformity by crosslinking. However, the crystallinity of the EVA decreases upon crosslinking as shown in Figure 3.8. The decreased crystallinity enables chain movement at a lower temperature, therefore, reduced tan(δ) peaks correlating to the polymer glass transition temperature were observed for the
EVA vitrimers.
19
Figure3.8 DSC of exothermic peaks a) and endothermic peaks b) for thermoplastic EVA
and EVA vitrimers.
The melting behavior of the thermoplastic EVA and EVA vitrimers was studied by DSC
(Figure 3.8). Thermoplastic EVA shows the highest intensity of the melting peak indicating the highest crystallinity. By increasing the crosslinking density of the EVA vitrimers, the melting peak intensity decreases which are probably due to the restricted chain mobility upon crosslinking. Therefore, it is harder for the chains to rearrange and crystallize. Besides, the melting temperature also decreases upon crosslinking.
Thermoplastic EVA has Tm of 87 ºC, whereas for the EVA vitrimers lower Tm were observed, EVA-V-1(83 ºC), EVA-V-2(76 ºC), EVA-V-3(67 ºC).
3.3. Mechanical properties and reprocessablity of EVA vitrimers.
Mechanical properties of thermoplastic EVA, EVA vitrimers, recycled EVA and recycled EVA vitrimers were evaluated by uniaxial tensile test. EVA vitrimers show increased Young’s modulus and decreased elongation at break compared with the thermoplastic EVA, which is common for crosslinked materials.[46] By introducing
20
dynamic crosslinking, the EVA-V-3 sample shows an increase of more than twofold in
Young’s modulus (57 MPa) by comparison with the thermoplastic EVA (26 MPa). The elongation at break decreases in all vitrimers especially with increasing crosslink density.
Table3.2 Young’s modulus, tensile strength, and elongation at break of thermoplastic
EVA and EVA vitrimers. Sample Name Young’s Modulus(MPa) Tensile Strength(MPa) Elongation at break (%)
Thermoplastic EVA 26±4 15±4 946±96
EVA-V-1 42±2 13±4 336±170
EVA-V-2 56±3 10±1 141±32
EVA-V-3 57±3 7±0.8 78±13
21
While EVA vitrimers have a higher viscosity than the thermoplastic EVA, as shown in
Figure 3.9, they maintain good reprocessability. Figure 3.10 illustrates that EVA vitrimers can be recycled by various processing methods (compression, extrusion) for multiple times.
Figure3.9 Complex viscosity vs angular frequency at 200 oC.
22
Figure3.10 EVA-V-3 processed multiple times by extrusion and compression molding,
and EVA-V-3 compression film after five times reprocessing.
Mechanical properties of recycled thermoplastic EVA and recycled EVA vitrimers were evaluated by uniaxial tensile testing, and the results are shown in Figure 3.11(a). In general
Young’s modulus and elongation at break of thermoplastic EVA decrease upon multiple
reprocessing.[47] Recycled thermoplastics experience both mechanical and thermal
degradations during each reprocessing procedure.[48, 49]
By introducing dynamic crosslinking, less loss in mechanical properties upon multiple reprocessing is expected. The thermoplastic EVA can maintain only 72 % of the original
modulus after five times recycling whereas the EVA-V-1 retains more than 90 % of the modulus upon five times recycling and the EVA-V-3 retains close to 90 % of the original modulus upon recycling.
The viscoelastic properties of thermoplastic EVA and EVA vitrimers were studied by
DMA before and after recycling. The results in Figure 3.11(b) indicate that at all temperatures the storage modulus is higher for the vitrimers and upon recycling the properties are better preserved for the vitrimers. However, the fully crosslinked EVA-V-3 shows reduced properties upon 5 times recycling at high temperatures most likely due to the temperature and humidity sensitivity of the B-O bond. [50, 51]
23
Figure3.11 Comparison of Young’s modulus a) and storage modulus b) before and after
recycling for thermoplastic EVA and EVA vitrimers.
3.4. Analysis of UV-aging for EVA vitrimers.
Introducing a crosslinked network into the thermoplastic EVA enables increased
resistance to multiple environmental factors [52]including UV-exposure.[53] Thermoplastic
EVA is widely used for photovoltaic materials, in shoe soles, ropes, thus UV irradiation is
inevitable for most EVA products. UV irradiation decreases thermoplastic EVA properties
by initiating a reaction of the ester group.[54] The C=O group is susceptible to undergo various types of photochemical activated reactions,[55] weakening the thermoplastic EVA
properties and the higher the VA content the more severe the degradation.[53] In this work,
we simulate UV irradiation corresponding to approximately 3 years of natural sunlight UV.
The thermoplastic EVA sample shrinks around 30 % on the thickness and can barely hold
the original shape upon UV exposure, set aside further use. In contrast, the dynamic
crosslinked EVA vitrimers hold the original shape and maintain most of the mechanical
properties upon exposure.
24
Tensile testing and dynamic mechanical analysis were used to characterize the
mechanical properties of the aged thermoplastic EVA and EVA vitrimers. The Young’s
modulus of the aged thermoplastic EVA decreases from 26 MPa to 5 MPa, and the
elongation at break decreases from 946 % to 40 %. The aged crosslinked EVA vitrimers
maintain nearly 45 % of the initial elongation at break and 80 % of the modulus. The
storage modulus of the UV exposed EVA for 15 days decreases to 10 % of the original
virgin thermoplastic EVA (Figure 3.12). The storage modulus of the UV exposed EVA-
V-1 remains higher than that of the thermoplastic EVA at all temperatures.
Figure3.12 Tensile Modulus a), Storage Modulus b) and Elongation at break c) of
thermoplastic EVA and dynamic crosslinked EVA before and after UV-exposure.
3.5 EVA-CNS nanocomposites -Morphology
The pv(EVA-0.3CNS) and vEVA-(EVA-0.3CNS) have different morphologies as shown in the Figure 3.13. The pv(EVA-0.3CNS) has a more evenly distributed morphology obtained through the homogenously mixing of EVA and CNS. Post-vitrimer
25
reaction does not induce any phase separation. Also, morphologies of pv(EVA-xCNS) do
not change by varying the filler content.
Figure3.13 Morphology for pv(EVA-0.3CNS) (left) and vEVA-(EVA-0.3CNS) (right)
By contrast, the vEVA-(EVA-0.3CNS) has a phase-separated morphology. The white
chunks are EVA vitrimer and the continuous-like darker phase is comprised in majority of
thermoplastic EVA-CNS nanocomposite and some well-mixed two phases. The
morphology of vEVA-(EVA-xCNS) changes with different filler content. Generally
speaking, the vEVA-(EVA-1/3/5CNS) have different morphologies than the vEVA-(EVA-
7CNS). The white chunks (EVA vitrimer) were surrounded by a continuous (darker) phase
for vEVA-(EVA-1/3/5CNS). More evenly sized white chunks were obtained with increased CNS content, but most of the continuous phase is still around the white chunks
regardless of CNS content. A segregated structure formed for the vEVA-(EVA-1/3/5CNS)
composites, whereas the vEVA-(EVA-7CNS) has a more evenly dispersed morphology
26 rather than a “segregated” one. Most of EVA vitrimer either breaks up or elongates and can more evenly mixed with the dark phase instead of being coated by it.
Figure 3.14 Morphology comparison for vEVA-(EVA-xCNS) with different CNS
loading.
At high temperature, the EVA vitrimer does not really flow but is rather welded. Leibler also found that vitrimer behaves more “silica-like” at high temperature. EVA vitrimer is chemically welded at high temperature, and stays solid-like at relatively low temperatures.
Full stress relaxation can be observed for EVA-V-3 at 75 oC. We hypothesize that if the temperature is high enough and no thermal degradation occurs, the vitrimer will tend to
27
behave more “molten-like” because of the “infinite reaction rate”. At the relatively high
temperature (130 oC in this work), the EVA vitrimer can be ruptured into small domains
(the white chunks) and then coated by the thermoplastic EVA-CNS phase. The relatively
low viscosity of the EVA-CNS phase can help the coating procedure [38] thus allowing the
formation of a self-segregated structure.
In the OM picture, most of the white chunks (EVA vitrimer) got torn apart into smaller
ones and the darker phase seems to surround the white chunks. When increasing the CNS
concentration in the thermoplastic EVA, the viscosity ratio between the two phases gets
closer to 1 and the morphology changes to a more uniformly blended system (Figure 3.15).
Figure3.15 Complex viscosity for EVA vitrimer and thermoplastic EVA-CNS composite
at different CNS content. (left) The viscosity ratio between EVA vitrimer and
thermoplastic EVA-CNS at different CNS content. (right)
3.6 EVA-CNS nanocomposites electrical conductivity
Figure 3.16 shows the electrical conductivity behavior of the pv(EVA-xCNS), vEVA- xCNS, and vEVA-(EVA-xCNS) at different filler loading. The electrical conductivity of
28 all three systems increases with increasing the CNS loading. However, the vEVA-(EVA- xCNS) have much higher electrical conductivity than the pv(EVA-xCNS) and vEVA- xCNS at all filler loadings. These results indicate that the conductive properties of EVA vitrimer-CNS composites can be dramatically improved by the presence of segregated structures. It is worth mentioning that the vEVA-0.7CNS has huge variations (Figure 3.17) in electrical conductivity due to the poor dispersion of large amounts of CNS in the highly viscous EVA vitrimer even at 200oC.
Figure3.16 Electrical conductivities of vEVA-(EVA-xCNS), pv(EVA-xCNS), and
vEVA-xCNS.
29
Figure3.17 Electrical conductivities at four different points in one single film of vEVA-
0.7CNS.
Tensile testing was used to characterize the mechanical properties of pv(EVA-xCNS),
vEVA-xCNS, and vEVA-(EVA-xCNS). vEVA(EVA-xCNS) has higher elongation at
break than pv(EVA-xCNS) and vEVA- xCNS at the same CNS content. vEVA-xCNS has
the highest crosslinking degree among these three composites, because these systems were
fabricated using directly the EVA vitrimer. The elongation at break for vEVA-xCNS decreases with increasing the CNS content due to poor filler dispersion. The pv(EVA- xCNS)s have lower crosslinking density than the EVA vitrimer as shown by the FTIR spectra (Figure 3.18). The peak attributed to C-O bond is highest for the system with 1wt%
CNS. The vEVA-(EVA-xCNS) systems show higher elongation at break than the vEVA- xCNS or pv(EVA-xCNS) due to the presence of the thermoplastic EVA in the blend. It is worth mentioning that pv(EVA-1CNS) shows abnormal mechanical properties (higher elongation at break and lower Young’s modulus) due to its low degree of crosslinking as affected by the high viscosity of the system prior to crosslinking.
30
Figure3.18 C-O bond peak for pv(EVA-0CNS), pv(EVA-0.3CNS) and pv(EVA-1CNS).
pv(EVA-xCNS), vEVA-xCNS, and vEVA-(EVA-xCNS) have similar Young’s modulus at the same filler content, but overall the pv(EVA-xCNS) has the highest (Figure 3.19).
Theoretically, pv(EVA-xCNS) and vEVA-xCNS should have similar crosslinking densities, however the filler dispersion affects the mechanical properties. CNS dispersion in the thermoplastic EVA is better than in the vitrimer EVA. Thus, the vEVA-xCNS systems show more variation in the mechanical properties primarily due to improper filler dispersion.
31
Figure3.19 Elongation at break of vEVA-(EVA-xCNS), pv(EVA-xCNS), and vEVA-
xCNS (left) and Young’s Modulus of vEVA-(EVA-xCNS), pv(EVA-xCNS), and vEVA-
xCNS(right).
3.7 Thermo-mechanical properties of segregated structure vEVA-(EVA-xCNS) nanocomposites
The storage modulus generally increases with filler content (Figure 3.20), primarily at temperatures above Tm. All vEVA-(EVA-xCNS) systems should have the same
crosslinking density because of the constant two-phase ratio; however, the CNS improves
the network strength. It is worth mentioning that the storage modulus of 0.5wt% CNS and
0.7wt%CNS may not be accurate due to possible errors in the sample preparation.
32
Figure3.20 Storage modulus for vEVA-(EVA-0CNS), vEVA-(EVA-0.1CNS), vEVA-
(EVA-0.3CNS), vEVA-(EVA-0.5CNS) and vEVA-(EVA-0.7CNS).
4. Conclusion
This work demonstrates a new method for recycling thermoplastic EVA. Reprocessing
with triethyl borate and catalyst results in vitrimer-like crosslinked EVAs, which can
maintain melt flowing ability for processing and show superior mechanical and thermal
properties. The crosslinked EVAs display good processability and better ability to retain thermomechanical properties after multiple reprocessing cycles. Furthermore, the
33 crosslinked EVAs also possess improved UV aging and chemical resistance, which could both benefit the reuse of the recycled EVA.
A novel way for the fabrication of EVA vitrimer-CNS nanocomposites was proposed and it shows promise. The two-step processing method for the fabrication of a self- segregated structure generates electrically conductive composites with higher elongation at break and hundred times improvement in electrical conductivity by comparison with homogenously dispersed composites at the same CNS content.
34
Reference
1. SpecialChem, Global Ethylene Vinyl Acetate (EVA) Market to Reach USD 11.41 Bn in 2023: TMR. Adhesive Industry NewsLetter 2016. 2. Geyer, R.; Jambeck, J. R.; Law, K. L. Production, use, and fate of all plastics ever made. Science Advances 2017, 3, e1700782. 3. Team, E. Ethylene-Vinyl Acetate(EVA): Production, Market, Price and its Properties. 4. Hopewell, J.; Dvorak, R.; Kosior, E. Plastics recycling: challenges and opportunities. Philosophical Transactions of the Royal Society B: Biological Sciences 2009, 364, 2115-2126. 5. Aurrekoetxea, J.; Sarrionandia, M. A.; Urrutibeascoa, I.; Maspoch, M. L. Effects of recycling on the microstructure and the mechanical properties of isotactic polypropylene. Journal of Materials Science 2001, 36, 2607-2613. 6. Torres, N.; Robin, J. J.; Boutevin, B. Study of thermal and mechanical properties of virgin and recycled poly(ethylene terephthalate) before and after injection molding. European Polymer Journal 2000, 36, 2075-2080. 7. Huang, S. J. Polymer Waste Management–Biodegradation, Incineration, and Recycling. Journal of Macromolecular Science, Part A 1995, 32, 593-597. 8. Salmiaton, A.; Garforth, A. Waste catalysts for waste polymer. Waste Management 2007, 27, 1891-1896. 9. Guo, H.; Yue, L.; Rui, G.; Manas-Zloczower, I. Recycling Poly(ethylene-vinyl acetate) with Improved Properties through Dynamic Cross-Linking. Macromolecules 2020, 53, 458-464. 10. Montarnal, D.; Capelot, M.; Tournilhac, F.; Leibler, L. Silica-Like Malleable Materials from Permanent Organic Networks. Science 2011, 334, 965-968. 11. Röttger, M.; Domenech, T.; Van Der Weegen, R.; Breuillac, A.; Nicolaÿ, R.; Leibler, L. High-performance vitrimers from commodity thermoplastics through dioxaborolane metathesis. Science 2017, 356, 62-65. 12. Capelot, M.; Montarnal, D.; Tournilhac, F.; Leibler, L. Metal-Catalyzed Transesterification for Healing and Assembling of Thermosets. Journal of the American Chemical Society 2012, 134, 7664-7667. 13. Denissen, W.; Rivero, G.; Nicolaÿ, R.; Leibler, L.; Winne, J. M.; Du Prez, F. E. Vinylogous Urethane Vitrimers. Advanced Functional Materials 2015, 25, 2451-2457. 14. Fortman, D. J.; Brutman, J. P.; Cramer, C. J.; Hillmyer, M. A.; Dichtel, W. R. Mechanically Activated, Catalyst-Free Polyhydroxyurethane Vitrimers. Journal of the American Chemical Society 2015, 137, 14019-14022. 15. Pei, Z.; Yang, Y.; Chen, Q.; Wei, Y.; Ji, Y. Regional Shape Control of Strategically Assembled Multishape Memory Vitrimers. 2015, 28, 156-160. 16. Yang, Y.; Pei, Z.; Zhang, X.; Tao, L.; Wei, Y.; Ji, Y. Carbon nanotube–vitrimer composite for facile and efficient photo-welding of epoxy. Chemcal Science 2014, 5, 3486- 3492. 17. Han, J.; Liu, T.; Hao, C.; Zhang, S.; Guo, B.; Zhang, J. A Catalyst-Free Epoxy Vitrimer System Based on Multifunctional Hyperbranched Polymer. Macromolecules 2018, 51, 6789-6799.
35
18. Liu, T.; Hao, C.; Zhang, S.; Yang, X.; Wang, L.; Han, J.; Li, Y.; Xin, J.; Zhang, J. A Self-Healable High Glass Transition Temperature Bioepoxy Material Based on Vitrimer Chemistry. Macromolecules 2018, 51, 5577-5585. 19. Yang, Z.; Wang, Q.; Wang, T. Dual-Triggered and Thermally Reconfigurable Shape Memory Graphene-Vitrimer Composites. ACS applied materials & interfaces 2016, 8, 21691-21699. 20. Denissen, W.; De Baere, I.; Van Paepegem, W.; Leibler, L.; Winne, J.; Du Prez, F. E. Vinylogous Urea Vitrimers and Their Application in Fiber Reinforced Composites. Macromolecules 2018, 51, 2054-2064. 21. Tran, T. N.; Rawstron, E.; Bourgeat-Lami, E.; Montarnal, D. Formation of Cross- Linked Films from Immiscible Precursors through Sintering of Vitrimer Nanoparticles. ACS Macro letters 2018, 7, 376-380. 22. Yue, L.; Bonab, V. S.; Yuan, D.; Patel, A.; Karimkhani, V.; Manas‐Zloczower, I. Vitrimerization: A Novel Concept to Reprocess and Recycle Thermoset Waste via Dynamic Chemistry. Global Challenges 2019, 1800076. 23. Brutman, J. P.; Delgado, P. A.; Hillmyer, M. A. Polylactide Vitrimers. ACS Macro letters 2014, 3, 607-610. 24. Chabert, E.; Vial, J.; Cauchois, J.-P.; Mihaluta, M.; Tournilhac, F. Multiple welding of long fiber epoxy vitrimer composites. Soft Matter 2016, 12, 4838-4845. 25. Jentsch, A.; Eichhorn, K. J.; Voit, B. Influence of typical stabilizers on the aging behavior of EVA foils for photovoltaic applications during artificial UV-weathering. Polymer Testing 2015, 44, 242-247. 26. Gu, S.; Cai, R.; Yan, Y. Self-crosslinking for dimensionally stable and solvent- resistant quaternary phosphonium based hydroxide exchange membranes. Chemical Communications 2011, 47, 2856-2858. 27. Imbernon, L.; Norvez, S. From landfilling to vitrimer chemistry in rubber life cycle. European Polymer Journal 2016, 82, 347-376. 28. Yuan, D.; Guo, H.; Ke, K.; Manas-Zloczower, I. Recyclable conductive epoxy composites with segregated filler network structure for EMI shielding and strain sensing. Composites Part A: Applied Science and Manufacturing 2020, 132, 105837. 29. Yuan, D.; Delpierre, S.; Ke, K.; Raquez, J. M.; Dubois, P.; Manas-Zloczower, I. Biomimetic Water-Responsive Self-Healing Epoxy with Tunable Properties. ACS applied materials & interfaces 2019, 11, 17853-17862. 30. Ke, K.; Solouki Bonab, V.; Yuan, D.; Manas-Zloczower, I. Piezoresistive thermoplastic polyurethane nanocomposites with carbon nanostructures. Carbon 2018, 139, 52-58. 31. Sang, Z.; Ke, K.; Manas‐Zloczower, I. Design Strategy for Porous Composites Aimed at Pressure Sensor Application. Small 2019, 1903487. 32. Harris, P. J. F. Carbon nanotube composites. International Materials Reviews 2004, 49, 31-43. 33. Balandin, A. A. Thermal properties of graphene and nanostructured carbon materials. Nature Materials 2011, 10, 569-581. 34. Michael F. L. De Volder, S. H. T., Ray H. Baughman, A. John Hart. Carbon Nanotubes: Present and Future Commercial Applications. Science 2013, 339, 535-539. 35. Yan, P.; Zhao, W.; Fu, X.; Liu, Z.; Kong, W.; Zhou, C.; Lei, J. Multifunctional polyurethane-vitrimers completely based on transcarbamoylation of carbamates:
36
thermally-induced dual-shape memory effect and self-welding. RSC Advances 2017, 7, 26858-26866. 36. Shi, Q.; Yu, K.; Kuang, X.; Mu, X.; Dunn, C. K.; Dunn, M. L.; Wang, T.; Jerry Qi, H. Recyclable 3D printing of vitrimer epoxy. Materials Horizons 2017, 4, 598-607. 37. Pang, H.; Bao, Y.; Xu, L.; Yan, D.-X.; Zhang, W.-Q.; Wang, J.-H.; Li, Z.-M. Double-segregated carbon nanotube–polymer conductive composites as candidates for liquid sensing materials. Journal of Materials Chemistry A 2013, 1, 4177-4181. 38. Wang, M.; Zhang, K.; Dai, X.-X.; Li, Y.; Guo, J.; Liu, H.; Li, G.-H.; Tan, Y.-J.; Zeng, J.-B.; Guo, Z. Enhanced electrical conductivity and piezoresistive sensing in multi- wall carbon nanotubes/polydimethylsiloxane nanocomposites via the construction of a self-segregated structure. Nanoscale 2017, 9, 11017-11026. 39. Li, T.; Ma, L.-F.; Bao, R.-Y.; Qi, G.-Q.; Yang, W.; Xie, B.-H.; Yang, M.-B. A new approach to construct segregated structures in thermoplastic polyolefin elastomers towards improved conductive and mechanical properties. Journal of Materials Chemistry A 2015, 3, 5482-5490. 40. Li, J.; Tan, Y.-J.; Chen, Y.-F.; Wu, H.; Guo, S.; Wang, M. Constructing multiple interfaces in polydimethylsiloxane/multi-walled carbon nanotubes nanocomposites by the incorporation of cotton fibers for high-performance electromagnetic interference shielding and mechanical enhancement. Applied Surface Science 2019, 466, 657-665. 41. Zhang, K.; Li, G.-H.; Feng, L.-M.; Wang, N.; Guo, J.; Sun, K.; Yu, K.-X.; Zeng, J.-B.; Li, T.; Guo, Z.; Wang, M. Ultralow percolation threshold and enhanced electromagnetic interference shielding in poly(l-lactide)/multi-walled carbon nanotube nanocomposites with electrically conductive segregated networks. Journal of Materials Chemistry C 2017, 5, 9359-9369. 42. Lei, J.; Gao, J.; Zhou, R.; Zhang, B.; Wang, J. Photografting of acrylic acid on high density polyethylene powder in vapour phase. Polymer International 2000, 49, 1492-1495. 43. Adachi, K.; Toyomura, S.; Miyakuni, Y.; Yamazaki, S.; Honda, K.; Hirano, T. Dioxomolybdenum(VI) and dioxotungsten(VI) complexes: efficient catalytic activity for crosslinking reaction in ethylene-vinyl acetate copolymer/alkoxysilane composites. Polymers for Advanced Technologies 2015, 26, 597-605. 44. James F. Sanders, J. D. F. Dynamic Mechanical Properties of Cross-Linked Rubbers. VII. Butyl Rubber Networks. Macromolecules 1974, 7, 681-684. 45. Liu, J.; Sue, H.-J.; Thompson, Z. J.; Bates, F. S.; Dettloff, M.; Jacob, G.; Verghese, N.; Pham, H. Effect of crosslink density on fracture behavior of model epoxies containing block copolymer nanoparticles. Polymer 2009, 50, 4683-4689. 46. Calvet, D.; Wong, J. Y.; Giasson, S. Rheological Monitoring of Polyacrylamide Gelation: Importance of Cross-Link Density and Temperature. Macromolecules 2004, 37, 7762-7771. 47. Bernardo, C. A.; Cunha, A. M.; Oliveira, M. J. The recycling of thermoplastics: Prediction of the properties of mixtures of virgin and reprocessed polyolefins. Polymer Engineering & Science 1996, 36, 511-519. 48. Oliveira, T. A.; Oliveira, R. R.; Barbosa, R.; Azevedo, J. B.; Alves, T. S. Effect of reprocessing cycles on the degradation of PP/PBAT-thermoplastic starch blends. Carbohydrate Polymers 2017, 168, 52-60.
37
49. Ammala, A.; Bateman, S.; Dean, K.; Petinakis, E.; Sangwan, P.; Wong, S.; Yuan, Q.; Yu, L.; Patrick, C.; Leong, K. H. An overview of degradable and biodegradable polyolefins. Progress in Polymer Science 2011, 36, 1015-1049. 50. Cromwell, O. R.; Chung, J.; Guan, Z. Malleable and Self-Healing Covalent Polymer Networks through Tunable Dynamic Boronic Ester Bonds. Journal of the American Chemical Society 2015, 137, 6492-6495. 51. Ogden, W. A.; Guan, Z. Recyclable, Strong, and Highly Malleable Thermosets Based on Boroxine Networks. Journal of the American Chemical Society 2018, 140, 6217- 6220. 52. Lange, R. F. M.; Luo, Y.; Polo, R.; Zahnd, J. The lamination of (multi)crystalline and thin film based photovoltaic modules. Progress in Photovoltaics 2011, 19, 127-133. 53. Jin, J.; Chen, S.; Zhang, J. UV aging behaviour of ethylene-vinyl acetate copolymers (EVA) with different vinyl acetate contents. Polymer Degradation and Stability 2010, 95, 725-732. 54. Angelika Beinert, C. P., Ines Durr, Michael D. Kempe, Gunter Reiter, Karl-Anders WeiB, The influence of the additive composition on degradation induced changes in poly(ethylene-co-vinyl acetate) during photochemical aging. In European PV solar Energy Conference and Exhibition, Amsterdam, The Netherlands, 2014. 55. Allen, N. S.; Edge, M.; Mohammadian, M.; Jones, K. Physicochemical aspects of the environmental degradation of poly(ethylene terephthalate). Polymer Degradation and Stability 1994, 43, 229-237.
38