Flame retardant eugenol-based thiol-ene polymer networks with high mechanical strength and transparency
Graphical abstract
Introduction
With the increasing concern on energy crisis and associated environmental issues caused by the rapid depletion of nonrenewable fossil resources, the development of sustainable bio-based polymer materials have gained widespread attention. Many efforts have been made to transformation and application of various biomass raw materials, such as polysaccharide, cellulose, lignin, protein, and plant oil [1], [2]. Eugenol, a renewable, low-toxicity, and easily available plant essential oil, is mainly produced from clove oil or prepared by allylation of guaiacol as a lignin derivative, and it is a resource for the preparation of bio-based polymers [3]. Owing to the well-defined molecular structure and reactive functional groups of eugenol, various eugenol bio-based polymer materials have been developed, such as acrylic resins [4], [5], polyester resin [6], epoxy resin [7], [8], polybenzoxazine resin [9], [10], bismaleimide resin [11], and cyanate ester resin [12]. However, similar to most bio-based polymer materials, eugenol-based materials inherently possess relatively high flammability, posing a fire risk in household or industrial applications. This disadvantage limits the wide application of eugenol-based polymer materials.
A potential solution is to introduce the flame retardants into the bio-based polymers. In recent decades, halogen-free flame retardants, including phosphorus- [13], [14], nitrogen- [15], [16], silicon- [17], [18], and boron-containing [19], [20] compounds, flame retardant nanomaterials [21], [22], [23] and intumescent flame retardants (IFRs) [24], [25], [26] have been widely applied in synthetic polymers because of their low-toxicity and environment friendly characteristics [27], [28], providing sufficient experience for the development of flame retardant bio-based polymer materials. Generally, a large amount of flame retardant must be added into the materials to ensure the effect of flame retardancy, which may cause physical and chemical compatibility issues. These problems can be solved by the molecular structure design of flame retardants with reactive functionality to act as both a monomer and an additive [29], [30]. Cyclophosphazene has an inorganic ring composed of alternating phosphorus and nitrogen atoms, which can act simultaneously as acid and gas resources for IFRs [27], [31]. It has a rigid ring can be easily substituted to yield different functional cyclophosphazene derivatives that serve as reactive flame retardants [32], [33], rendering it the most attractive IFRs for developing high-performance flame retardant polymer materials.
Photo-induced thiol–ene reaction as an efficient polymerization method has recently drawn increased interest because of its several advantages, such as mild reaction condition, relatively high reaction rate, reduced oxygen inhibition and precise spatiotemporal controllability, as well as low shrinkage and uniform crosslinked structure of resultant polymers [34], [35], [36]. In this work, flame retardant eugenol-based thiol-ene polymer networks (HEP-SH) were established by the copolymerization of a synthesized hexa-eugenol substituted cyclophosphazene monomer (HEP) with multi-thiol monomers via photo induced thiol-ene polymerization (Scheme 1). Four HEP-SH polymer networks (HEP-TEGDT, HEP-TTMP, HEP-PETMP, HEP-TEMPIC) were prepared by varying the number of thiol groups or the backbone of thiol monomers. The photopolymerization kinetics, thermal stability, flame retardancy, mechanical properties, and transparency of the HEP-SH networks were systemically investigated. Apart from the renewable eugenol raw material and the effectiveness and facile operation of thiol-ene photopolymerization, the chemical incorporation of cyclophosphazene into this network effectively lowered the inherent flammability and improved the mechanical properties of the resultant materials. Compared with previously reported bio-based thiol-ene networks [37], [38], [39], the HEP-PETMP and HEP-TEMPIC networks exhibited excellent flame retardancy, high transparency, and high mechanical strength simultaneously. The combination of properties, greatly enhanced the potential of these materials for application in industries in which fire resistance, high mechanical strength and transparency are in high demand. These areas include optics, coatings, electronical devices and so on.
Section snippets
Materials
Eugenol, hexachlorocyclotriphosphazene (HCP), 3-mercaptopropionic acid, p-toluenesulfonic acid, 1,3,5-tris(2-hydroxyethyl)cyanuric acid (THEIC) and anhydrous potassium carbonate (K2CO3) were purchased from Energy Chemical, China. Acetonitrile, toluene, ethanol and acetone were purchased from Greagent, China. 2,2′-(ethylenedioxy)diethanethi-ol (TEGDT), trimethylolpropane tris(3-mercaptopropionate) (TTMP), pentaerythritol tetrakis(3-mercaptopropionate) (PETMP) and 2,
Synthesis and characterization of HEP
The reactive flame retardant bio-based monomer HEP was synthesized by the nucleophilic substitution reaction of eugenol with hexachlorocyclotriphosphazene (HCP) in the presence of potassium carbonate, and excess eugenol was used to ensure the complete substitution of P-Cl groups in HCP (Scheme S1). The structure of HEP was characterized by FTIR, NMR and HRMS spectroscopy. In the FTIR spectrum (Fig. S1a), peaks at 952 cm−1 are attributed to P-O-Ph deformation vibration, peaks at 1638, 995 and
Conclusions
In this work, a hexa-eugenol substituted cyclophosphazene monomer (HEP) was successfully synthesized and identified, and then copolymerized with multi-thiol monomers (PETMP, TTMP, TEGDT and TEMPIC) to build polymer networks by thiol-ene photopolymerization. Among the resultant HEP-SH networks, HEP-PETMP and HEP-TEMPIC possess excellent intumescent flame retardancy, as determined from the results of LOI, UL-94 vertical burning test, and cone calorimetry test. In addition, HEP-TEMPIC shows high
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China (31741022) and the Natural Science Foundation of Guangdong Province (2018A030310146).
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