2,2,2-Trifluoroethyl methacrylate-graft-natural rubber: Synthesis and application as compatibilizer in natural rubber/fluoroelastomer blends

https://doi.org/10.1016/j.matchemphys.2013.02.019Get rights and content

Abstract

The incompatibility and immiscibility of natural rubber (NR)/fluoroelastomers (FKM) blends were improved by incorporation of a graft copolymer synthesized from the free radical graft copolymerization of 2,2,2-trifluoroethyl methacrylate (TFEM) onto NR initiated by benzoyl peroxide via a melt-mixing process. The grafting properties were investigated as functions of the initiator and monomer concentrations, reaction temperature and time. At the optimal conditions, the obtained graft NR (GNR) purified by Soxhlet extraction contained a maximum grafting efficiency of 1.34% with 49.1% monomer conversion. The structure of the purified GNR was analyzed using attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) and nuclear magnetic resonance spectroscopy (1H NMR and 19F NMR). The gross GNR was then applied as the compatibilizer for NR/FKM vulcanizates (20/80 (w/w)) cured by peroxide vulcanization. The addition of GNR at 15 parts per hundred of rubber (phr) gave a vulcanizate with the highest tensile strength (9.93 MPa), some 5.31-fold higher than that of the incompatibilized one (1.87 MPa). This is likely to be due to the higher degree of homogeneity of the constituent phases in the GNR-compatibilized blends, as observed by scanning electron microscopy (SEM). The GNR-compatibilized NR/FKM vulcanizates were also resistant to gasohol E85 (85% (w/w) of ethanol).

Highlights

► 2,2,2-Trifluoroethyl methacrylate could be grafted on NR via melt-mixing process. ► Effect of grafting parameters on grafting efficiency and gel content was observed. ► NR/FKM containing graft NR had higher tensile strength than uncompatibilized ones. ► The compatibilized NR/FKM vulcanizates had higher gasohol-swelling resistance.

Introduction

With the rapid decrease in the amount of non-renewable fossil fuel reserves, which are mostly being consumed by the increase in the population and energy requirement for transportation and industrial sections, the demand for renewable and sustainable alternative energy fuels, such as biodiesel and gasohol (blend of ethanol and gasoline), has increased [1], [2]. In the partial replacement of gasoline by gasohol, the addition of ethanol enhances the polarity of the conventional gasoline. Thus, the rubber materials used in fuel transmission lines and engine parts are required to have a higher resistivity against this more polar fuel component. For this purpose, fluoroelastomers (FKMs) are viewed as an excellent choice among the other general rubbers since they have carbon–fluorine bonds that provide a higher polarity and stability, including a superior thermal and fluid resistance. Thus, they are appropriate for use in automotive parts as well as some other applications that are operated under relatively more severe environments [3], [4]. However, FKMs also have some drawbacks. For example, some of them are crystalline, difficult to be cured and require specific vulcanizing agents [5], as well as having poor solubility in common organic solvents that results in difficulty in their utilization [6]. Moreover, the price of FKMs is significantly higher than that of general elastomers depending on the grade of FKMs.

Natural rubber (NR) is one of the outstanding bio-based elastomers with excellent mechanical and dynamic properties that can be applied in engineering parts in many industries. It is also claimed to be a low cost and sustainably renewable material that is readily produced in tropical countries. However, NR has poor resistance to thermal and oxidative degradation due to its high level of unsaturated carbon–carbon double bonds (Cdouble bondC). In addition, NR cannot be used in the presence of oil and hydrocarbons or other non-polar solvents [7]. To improve the resistance of NR to heat, oxidation and non-polar solvents and to decrease the production cost of using only FKM, the blending of NR with FKM is expected to give finished products with more desirable properties at an economically viable cost. However, blends of NR and FKM result in phase separation due to their different polarities and optimal vulcanization systems, and this causes inferior properties in the finished products [8]. Thus, the addition of a compatibilizer is necessary to improve the interfacial adhesion between these rubberic phases to provide better mechanical properties for the blends.

Graft copolymers, one type of compatibilizer, contain segments with identical structures or with specific reactive interactions that can promote the interfacial adhesion and reduce the interfacial tension among the immiscible rubber components in the blends [9], [10]. The graft copolymerization of various fluorinated monomers onto polymeric substrates has been reported previously, such as the grafting of either 2,2,3,3,4,4,5,5,6,6,7,7,8,8,9,9,-hexadecafluoro-nonyl ester [11] or heptadecafluorooctylethyl acrylate [12] onto polypropylene via a melt mixing process, the grafting of 2,3,4,5,6-pentafluorostyrene on polybutadiene via solution polymerization in the presence of tetrahydrofuran (THF) [13] and the grafting of 2,2,3,3,4,4,4-heptafluorobutyl acrylate on NR latex film by UV-induced graft copolymerization [14]. The grafting of these fluorinated monomers increased the impact strength and thermal stability [11], including the hydrophobicity of the polymeric backbones [14]. Guo et al. [15] reported that the graft copolymer of 2,2,2-trifluoroethyl methacrylate (TFEM) onto silicone rubber enhanced the compatibility between FKM and silicone rubber in the blends and resulted in improved mechanical properties and oil resistance of the FKM/silicone rubber vulcanizates.

Due to the ease of performing radical polymerization of TFEM by various methods [16], it is possible to use it as a grafting monomer for the graft copolymerization onto the NR backbone. Although the graft copolymerization is normally carried out by the solution polymerization or melt-mixing processes, the latter one is more advantageous for commercial applications as it does not require either large quantities of solvents or a separation process [17].

Thus, this research focused on the preparation of poly(TFEM)-graft-NR (PTFEM-g-NR) via the melt-mixing process. The effects of the initiator concentration, TFEM–monomer content, reaction temperature and time on the degree of TFEM–monomer conversion, grafting efficiency (GE) and gel content were evaluated. The gross graft NR (GNR) at various contents was then applied as a compatibilizer for NR/FKM vulcanizates at 20/80 (w/w). The tensile properties, morphology and resistance to thermal aging and swelling in gasohol E85 of the resulting vulcanizates were examined.

Section snippets

Materials

For graft copolymerization, solid NR (STR-5L) was supplied by the Innovation Co., Ltd. (Thailand). TFEM was received from Sigma–Aldrich (USA). Benzoyl peroxide (BPO) was purchased from Panreac (Spain). Petroleum ether, methanol, ethanol and THF were all obtained from J.T. Baker (USA). Nitrogen gas (N2, 99% purity) was manufactured by Thai Industrial Gas Co., Ltd. (Thailand). Hexane and acetone were received from Honeywell Burdick & Jackson (USA) and Qrec (New Zealand), respectively.

Proposed reaction mechanism of graft copolymerization of TFEM onto NR

The reaction mechanism of graft copolymerization of NR with TFEM via free radical grafting copolymerization is proposed in Eqs. (9), (10), (11). The explanation for this reaction was adapted from the graft copolymerization of 2,3,4,5,6-pentafluorostyrene onto polybutadiene proposed by Paz-Pazos and Pugh [13]. The abundance of BPO radicals (R1radical dot) was generated by homolytic thermal decomposition (Eq. (9)) and attached to the NR backbone by addition or abstraction of an allylic hydrogen atom from NR

Conclusions

The graft copolymer of TFEM onto NR was prepared by the melt-mixing process. A maximum %GE of 1.34% with a 82.5% gel content was achieved when 40 phr of TFEM was initiated by 2 phr of BPO at 90 °C for 30 min. When GNR was applied as the compatibilizer for NR/FKM vulcanizates, the GNR containing 0.48% GE with a 50.8% gel content (obtained at 20 phr TFEM, 2 phr BPO at 90 °C for 30 min) was selected as optimal and the DCP curing system was found to be appropriate for vulcanization of the 20/80

Acknowledgments

The authors gratefully acknowledge the Thailand Research Fund and the Commission on the Higher Education for fiscal year 2010–2012 (MRG5380157), the Ratchadapiseksompoch Fund, Chulalongkorn University, and the Thai Government Stimulus Package 2 (TKK 2555) under the Project for Establishment of Comprehensive Center for Innovative Food, Health Products and Agriculture (PERFECTA) for financial support. The authors also wish to express their thanks to Miss Yaowaluk Chankeaw and Miss Wanwilai

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