Dynamic network based on eugenol-derived epoxy as promising sustainable thermoset materials
Graphical abstract
Introduction
Nowadays, the entire value chain associated to polymers, in particular thermosetting polymers, is facing a rough scrutiny for its negative impacts in the economy and the environment. The global commitment to tackle these negative impacts involve that all plastic must be reused and/or recycled. Therefore, an efficient physical reprocessing strategy is necessary in order to stop the waste disposal into the environment, the consumption of non-renewable resources and CO2 emissions. As a result, contribute achieving the EU Product Policy Framework of the Circular Economy targets.
Compared with non-crosslinked thermoplastics, thermosetting polymers present a large range of engineered advantages. Thermosetting polymers are nowadays widely used as adhesives, electrical castings, flooring and paving applications as well as matrices for structural performing composites and foams. They globally represent 15–20% of plastics manufacture [1]. Among them, epoxy resins are of great economic importance as they represent around 70% of thermosets global market without including polyurethanes [2]. The development of more sustainable thermosetting epoxy materials with an improved LCA is a strong challenge nowadays. Then, to improve the environmental impact from cradle to grave, the properties and the behavior of this type of materials, two main ways can be underway and coupled (i) Developing more sustainable approaches during the synthesis (ii) Developing materials with an improved ends of life, in order that they can be reshaped, repaired or recycled after usage or damage.
For instance, dynamic covalent networks [3], [4] such as vitrimers [5], [6], [7], [8], [9], [10], [11], [12], [13], [14] have been recently established as an innovative and new class of polymer that are characterized by a crosslinked network that when it is heated at a certain temperature it can flow without depolymerization. This behavior is due to the occurrence of thermally activated crosslink exchange reactions of the network in an associative manner without a decrease on the crosslink density, retaining the 3D cross-linked structure. Vitrimers are able to undergo a rearrangement of the initial network topology by the breaking of a covalently crosslinked bond followed by the instant creation of a new covalent bond in other position of the polymer chain. In this context, this type of dynamic crosslinked system allows the material to flow at high temperatures and to be reprocessed.
It has been found that one effective strategy to confer reprocessing capabilities to a fossil-based thermosetting polymer is to introduce disulfide-containing amines within the polymer network [15], [16], [17], [18], [19], [20], [21], [22]. The proposed strategy allowed that the thermosetting polymer undergo a network rearrangement while the 3D structure is maintained without a sudden drop in viscosity. Hence, in terms of applications this strategy is more adequate than the reversible and dissociative approach of e.g, the well-known Diels-Alder reaction. This highlighted disadvantage is because through the Diels-Alder strategy the crosslinked network depolymerizes and as result an undesirable loss of the polymer stability is obtained [23], [24], [25].
Besides, most of the studies on polymeric vitrimers and dynamic networks have been performed from fossil-based epoxies that have hazardous effects on human health and eco-toxicity. Therefore, the exploitation of renewable resources, such as plant oils [26], is necessary and timely from both scientific and industrial points of view, with the sustainable development of epoxidized precursors in line with the criteria established by the new EU chemical regulations, RoHS and REACh [2]. These epoxidized precursors can be used in the synthesis of partially or fully biobased thermosetting polymers with improve thermomechanical properties. For instance, aromatic epoxidized oils such as cardanol [27], [28], soybean [29] and eugenol [30], [31], [32], [33] have been reported as proper renewable substitutes of DGEBA. Nevertheless, chemically cross-linked plant-derived epoxy resins are also nonrecyclable.
On the basis of literature research, recently, an eugenol-derived epoxy (Eu-EP) has been reported [31] with reshaping, self-healing, and shape memory properties prepared by reacting Eu-EP with succinic anhydride at various ratios in presence of zinc-based catalysts. On the other hand, a metal-catalyzed ESO-based vitrimer was prepared using fumaropimaric acid as a curing agent and zinc acetylacetonate as the transesterification catalyst [34]. Besides, a biobased thermosetting vitrimer from isosorbide-derived epoxy and aromatic diamine containing disulfide bonds has been reported [35]. Nevertheless, these two latter biobased vitrimers did not exhibit thermomechanical properties that are comparable or superior to the well-established fossil-based DGEBA. Hence, the investigation of environmentally friendly alternatives for the development of biobased epoxy with dynamic networks that are in continuous advancement still remains a challenge for a more sustainable polymer industry.
The objective of the current work is the development of a partially biobased aromatic material from eugenol that can be reshapeable, self-healable and recyclable by means of associative dynamic chemistry without catalysts on agreement with principles for a green chemistry. Thermosetting polymers based on synthetized tri(epoxized-eugenyl) phosphate (TEEP) were prepared. Epoxidized eugenol was selected as the thermosetting matrix to confer high modulus and thermal properties compared with fossil-based DGEBA [33], [36]. A disulfide-containing amine was employed as a crosslinker.
Section snippets
Materials
Eugenol (99%), phosphorus oxychloride (99%), triethylamine (99%), 3-chloroperbenzoic acid (77%), 4-aminophenyl disulfide, DSA (98%) were purchased from Sigma Aldrich France. 3,3′-diaminodiphenyl sulfone, DDS (98%) was purchased from Alfa Aesar.
Synthesis of trieugenylphosphate (TEP)
First, a mixture of eugenol (61 mmol) and triethylamine (61 mmol) in ethyl acetate (100 mL) was prepared and phosphorus oxychloride (20.33 mmol) was added drop by drop at temperatures between 0 and 5 °C. The mixture was maintained between 0 and 5 °C
Results and discussion
TEEP:DSA and TEEP:DDS partially biobased epoxy systems were prepared through a two-step synthesis pathway. First, a trieugenylphosphate (TEP) was synthesized by treating eugenol with phosphorus oxychloride. Afterwards, the allylic groups of TEP were epoxidized with m-CPBA to obtain tri(epoxized-eugenyl)phosphate (TEEP). The physicochemical characterization of TEP and TEEP has been previously reported by Caillol et al. [30], corroborating the disappearance of OH groups of eugenol by the reaction
Conclusions
In this work we successfully synthesized a sustainable dynamic thermoset with high renewable carbon content based on epoxidized eugenol oil crosslinked with an aromatic diamine containing disulfide functional groups. The dynamic eugenol-derived epoxy, TEEP:DSA, revealed comparable stiffness and higher Tg values than that fossil-based DGEBA. Thermal stress relaxation of TEEP:DSA confirmed the associative crosslink exchange reactions by means of disulfide bonds rearrangement at high temperatures.
Data Availability Statement
The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.
CRediT authorship contribution statement
Connie Ocando: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Writing - original draft, Visualization, Writing - review & editing. Yvan Ecochard: Methodology, Investigation, Writing - review & editing. Mélanie Decostanzi: Methodology, Investigation, Writing - review & editing, Writing - review & editing. Sylvain Caillol: Resources, Supervision, Writing - review & editing. Luc Avérous: Resources, Conceptualization, Supervision, Writing - review & editing, Funding
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.
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