Elsevier

Polymer Testing

Volume 28, Issue 3, May 2009, Pages 235-242
Polymer Testing

Material Properties
Comparison of cure, mechanical, electric properties of EPDM filled with Sm2O3 treated by different coupling agents

https://doi.org/10.1016/j.polymertesting.2008.11.015Get rights and content

Abstract

Composites based on ethylene–propylene–diene rubber (EPDM) were prepared. EPDM was reinforced with 100 phr Sm2O3 treated with coupling agents: stearic acid (SA), isopropyl tri(dioctylphosphate) titanate (NDZ102), bis-[-3-(triethoxysilyl)propyl]tetrasulfide (KH845-4), and N-β-(aminoethyl)-γ-aminopropylmethyldimethoxysilane (SG-Si602), respectively. Cure, mechanical and electrical properties of the composites were investigated. It was found that carboxyl in coupling agents could retard EPDM cure while amino groups, Pdouble bondO bonds and S atoms could accelerate EPDM cure. Amino groups enhanced composite mechanical properties by forming additional rigid C–C linkages, whilst S atoms boosted composite mechanical properties by generating flexible S–C linkages. Pdouble bondO bonds might be subject to cleavage during vulcanization and form flexible P–C linkage. Thus, composites with NDZ102 and KH845-4 treated filler exhibited better mechanical properties than that with SG-Si602 treated filler. In addition, treatment of filler could reduce composite electrical properties due to interfacial improvement.

Introduction

Ethylene–propylene–diene rubber (EPDM) consisting of ethylene, propylene and unsaturated diene, is one of the popular synthetic rubbers. Due to its saturated backbone, EPDM possesses excellent resistance to heat and oxidation [1], whilst the non-polar structure endows EPDM with excellent electrical resistivity and resistance to polar solvents [2]. Thereby, it has broad application to thermoplastic vulcanizates, electrical insulation, waterproof rolls and so on. In addition, EPDM can not only vulcanize in a peroxide-cured system but also in sulfur-cured systems due to unsaturated diene. Earlier reports showed that rubber vulcanized by sulfur could accommodate more stress and exhibit higher elongation compared with peroxide-cured rubber [3], and that tensile strength and elongation at break of rubber with a mixed-cured system were higher than those of sulfur and peroxide-cured systems [4].

Commonly used fillers for EPDM include carbon black, silica, clay and fiber [5], [6], [7], [8]. The surface functional groups of a filler could influence the cure properties of EPDM and finally affect the overall properties. Earlier reports observed that channel blacks with lots of oxygen functional groups, which were reported to be acid, would retard cure, whilst furnace blacks featuring a slightly alkali characteristic because of low oxygen content would accelerate vulcanization [5], [9]. Pongdhorn et al. concluded that sulfuric atoms on the filler surface introduced by surface treatment would affect cure properties in three sulfur-cured systems, conventional, semi-efficient and efficient system [6].

Sm2O3, a rare earth oxide, with special shell structure, prominent physical, chemical, electrical and magnetic properties is often utilized in surface engineering [10]. So far, there have been few reports about the direct addition of rare earth oxide into a rubber matrix to strengthen mechanical properties. Therefore, it is interesting to incorporate Sm2O3 into EPDM. Considering the poor adhesion between Sm2O3 and EPDM and strong agglomeration among Sm2O3 particles, coupling agents were often selected to modify the surface of particles, since chemical treatment of filler surface has become the most successful method to improve rubber–filler and reduce filler–filler interactions.

Recently, much emphasis has been given to the application of coupling agents which serve, to some degree, to couple a filler to the rubber molecule on a “like-to-like” basis [6], [11], [12]. Generally, coupling agents possess two functionally active end groups, an alkoxy group which is capable of reacting with the hydroxyl group on the surface of common fillers, with the rest of the molecule having functional groups or special atoms, such as amino groups, double bonds and sulfuric atoms, which are compatible with rubber or can participate in vulcanization, leading to strong physical or chemical linkage between coupling agents and rubber molecules [13], [14], [15]. As a consequence, the coupling agent acts as a bridge between filler and rubber that enhances the rubber–filler interaction and the degree of reinforcement.

In this work, Sm2O3 particles were treated with four types of coupling agents containing carboxyl, amino group, P and S atoms to compare the cure, mechanical and electric properties of EPDM containing the treated Sm2O3. The main aim is to investigate the correlation between the functional groups in the coupling agents and properties of composites. The properties of interest included cure characteristics, crosslink density, mechanical properties, morphology of fractured surface by scanning electron microscopy and electrical properties, including dielectric constant, loss and strength, surface and volume resistivity.

Section snippets

Materials

The rubber used in this study was ethylene–propylene–diene monomer (EPDM J-4045) containing 5-ethylidene-2-norbornene (ENB) as diene, which was manufactured by Jilin Petrochem., SINOPEC. The EPDM consisted of 52.0 wt% ethylene, 40.3 wt% propylene and 7.7 wt% ENB. Compounding ingredients, such as dicumyl peroxide (DCP), zinc oxide, stearic acid, 2-mercapto benzimidazole (antioxidant MB) and polymerized 2, 2, 4-trimethyl-1, 2-dihydroquinoline (antioxidant RD) were of reagent grade. The

FT-IR analysis

From Fig. 1, it is observed that the four coupling agents all absorbed bands at 2917 cm−1, 2850 cm−1 and 1464 cm−1 assigned to asymmetric stretching vibration of methylene, symmetric stretching vibration of methylene, and methylene scissoring vibration, respectively [18]. In addition, the characteristic peaks of SA at 1701 cm−1 and 935 cm−1 are assigned to Cdouble bondO stretching vibration and O–H transforming vibration of –COOH, respectively [18]. As for NDZ102, characteristic absorption peak appears at 1050 

Conclusions

The surface treatment of Sm2O3 with coupling agents has significant influence on cure, mechanical and crosslinking measurement of EPDM composites with Sm2O3. The reason is that the treatment could result in some functional groups being attached to filler surface. It was found that acidic carboxyl groups could make DCP partly decompose into ions and then retard cure, whereas, the amino groups which are alkali could make DCP decompose more radicals and then enhance crosslink density. The presence

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