Radical induced cationic frontal polymerization for preparation of epoxy composites

Abstract

Epoxy composites are usually prepared from two component formulations that have limited storage stability and require time and energy consuming post curing. Radical induced cationic frontal polymerization (RICFP) circumvents these drawbacks and allows the fast on demand curing of these materials. In this study, the most suitable reactive diluents for preparing composites based on highly viscous bisphenol-A-diglycidylether were identified. The results revealed that 1,6-hexanediol diglycidylether (HDDGE) and (3-ethyloxetan-3-yl)methanol (EOM) were most effective reactive diluents regarding viscosity and reactivity. Composite production with various types of fillers were successfully evaluated regarding their frontal polymerization parameters. It was possible to show that high filler contents of up to e.g. 74 vol% glass microspheres and 40 vol% short carbon fibres were able to be processed. In comparison with thermal curing techniques, the epoxy-based fibre reinforced polymer composites prepared by RICFP showed similar values in tensile measurement, but the manufacturing process was facilitated significantly.

Introduction

The frontal polymerization (FP) is a self-sustaining reaction which is driven by the exothermic heat of polymerization [1]. With advantages of fast curing and energy efficiency, this method is widely investigated for various type of monomers like (meth)acrylates, acrylamides, dicyclopentadiene, and several other monomers [2], [3], [4], [5], [6]. Recently, a combination of the frontal polymerization and radical induced cationic polymerization (RICP) established a new method for curing of epoxy resins, called radical induced cationic frontal polymerization (RICFP) [7], [8], [9].

Epoxy resins are one of the most important functional monomers in applications like flooring, casting formulations, coatings, adhesives and also composites, which are commonly cured by thermal methods or photopolymerization. The thermal curing is a bulk curing technique in which a mixture of epoxy monomers and different hardeners like amines and anhydrides are reacted at elevated temperature [10]. However, this technique has several disadvantages most important that the formulations have very limited potlife after mixing and an oven as big as the specimen is required. Curing in the oven is quite time consuming and also energy consumption is high. On the other hand, the photocuring of epoxy resins can only be used for thin coatings up to now, since the light necessary to start the curing process has a very limited depth of penetration. Nevertheless, a formulation containing monomers and photoinitiators benefits from their long potlife (if stored dark) and fast curing [10], [11]. The RICFP we used is a method which overcomes the above mentioned disadvantages and combines the advantages of the two techniques in bulk curing: long potlife, fast and energy efficient processing.

The successful RICFP for bisphenol-A-diglycidylether (BADGE) using antimonate-based iodonium salt (I-Sb) as a photoacid generator (PAG) and benzopinacol (TPED) as a radical thermal initiator (RTI) was reported in previous works [9], [12]. The promising results encourage to investigate the possibility of RICFP to cure epoxy-based reinforced polymer composites. The RICFP of highly filled composites is very crucial, since fillers can improve the properties of the final product while reducing cost. In RICFP, the released heat from a cationic ring opening polymerization plays an important role in the propagation front. This heat may be affected by the presence of inorganic fillers, which can be followed through the frontal polymerization parameters. On the one hand, it is well known that adding fillers resulted in decrease of frontal velocity and frontal temperature due to their heat uptake. Klikovits [13] revealed that the frontal velocity went down from 7.5 to 6.8 cm/min, while adding of up to 3 wt% of nano SiO₂. Nason [14] also reported a similar behavior in study using calcium carbonate, kaolin, and a triacrylate monomer. Mariani [15] prepared epoxy montmorillonite nanocomposites by the frontal polymerization. Adding 10 wt% montmorillonite, beside the decrease of frontal velocity, the frontal temperature was reduced to 188 °C, compared to the specimens containing no filler with a temperature of 220 °C. On the other hand, the presence of conductive fillers can accelerate the frontal polymerization. Goli et al [16] reported that the presence of continuous conductive element likes copper and stain steel wire resulted in a higher frontal velocity, compared to those with the pure monomers. In a study of dicyclopentadiene-based fibre reinforced polymer composite, Robertson et al [17] illustrated that due to high thermal conductivity of carbon woven, the frontal velocity of the composites was significantly higher (9.8 cm/min) than those of the pure monomers (7.5 cm/min).

Considering epoxy-based reinforced polymer composite, RICFP bears challenges regarding high viscosity and low reactivity of highly filled BADGE type monomers. This is not an issue in the conventional curing techniques, because a large amount of hardeners and diluents are commonly used with these BADGE-type monomers [11]. In order to overcome these challenges, reactive diluents can be used not only to decrease the viscosity, but also increase the reactivity of the formulation. Some potential diluents are aliphatic epoxy monomers which show very low viscosity and increase the reactivity of the system significantly [18], [19]. They could also increase the released exothermal heat and therefore lead to faster frontal polymerization, compared to neat BADGE [9]. Other diluents like oxetane-based monomers are considered due to their lower toxicity and easy handling [20], [21], [22].

In this article, RICFP has been investigated with BADGE as the monomer, TPED as the radical thermal initiator and bis (4-tert-butylphenyl) iodonium tetrakis (perfluoro-tert-butyloxy) aluminate (I-Al) as the photoacid generator. I-Al is a recently developed photoacid generator which exhibits very high reactivity even with a low used concentration, compared to I-Sb [23]. Effects of reactive diluents on the formulation viscosity, RICFP parameters, and mechanical properties of the polymer have been studied. RICFP for filled composites has been investigated with a variety of common fillers which are usually used for epoxy resins. For instance, glass microspheres are applied to obtain lightweight and insulation materials [24], [25]. Graphite and aluminum, are added to increase the thermal and electric conductivities [26], [27], [28]. Short carbon fibres help to improve mechanical properties of the product [29]. Mica is used for insulation application [30]. In addition, with success of RICFP for fibres reinforced epoxy composites [31], [32], woven carbon fibres have been chosen as reinforcement in this study to compare performance of RICFP with the thermal curing technique.