Effect of electron beam-cross-linked gels on the rheological properties of raw natural rubber

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Abstract

Electron beam (EB)-cross-linked natural rubber (NR) gels were prepared from latex and characterized by various techniques. The addition of a small amount of these gels to raw NR was found to reduce the apparent shear viscosity and die swell remarkably. This effect was further enhanced with the addition of butyl acrylate as a sensitizer. The apparent shear viscosity first decreased up to 8 phr of gel loading and then increased. However, the percent die swell value decreased steadily upon gel loading. These were explained by calculating principal normal stress difference, the activation energy of melt flow and characteristics of EB-cross-linked gels. These effects were also reflected in the changes of mechanical and dynamic mechanical properties of gel-filled raw NR. Tailoring of the above properties could be done with the help of these gels.

Introduction

Gels can be regarded as particles formed by the joining together of large polymer molecules into a cross-linked three-dimensional structure, by virtue of either chemical or physical bonding. They exhibit properties entirely different from the parent polymer chains due to their restricted mobility and interaction. In rubber industry, it is a fairly common practice to add gel to rubber to improve its processability to a marked extent. The advantages of this procedure were reported by Hofman (1964). The cross-linking of rubber latex can be performed by various methods, e.g., sulfur vulcanization and radiation curing, etc. Electron beam (EB) radiation cross-linking is a novel method of introducing cross-links into rubber matrix. There are a large number of studies on the EB cross-linking of rubber and rubber latex (Jayasuriya et al., 2001; Chirinos et al., 2003; Abad et al., 2002). Extensive work on EB cross-linking of various types of rubber was reported from our laboratory (Banik and Bhowmick, 1999; SenMajumder and Bhowmick, 1999; Chattopadhyay et al., 2001; Vijayabaskar et al., 2004). Conventional sulfur vulcanization of natural rubber (NR) latex produces a small amount of nitroso amine, which is toxic in nature. However, radiation-vulcanized rubber latex products are extremely safe due to the absence of such toxic entities. The low toxicity and fewer amounts of extractable proteins are the advantages of the EB technology (Ratnam et al., 1999). Nowadays radiation vulcanization technique of NR latex is extensively used to manufacture dipped articles like surgical gloves, balloons, etc (Chmielewski et al., 2005). In NR latex, cross-linking by EB irradiation is normally done in the presence of sensitizers, which in most cases are acrylates, namely butyl acrylate (Makuuchi and Hagiwara, 1984; Makuuchi et al., 1984). The amount of gel or cross-links formed in the latex is dependent on the radiation dose as well as on the nature of the sensitizer and their concentration (Haque et al., 1996). The mechanism of butyl acrylate sensitization was studied in detail by Sabharwal et al. (1998). However, there is no literature available which describes the effect of EB-cross-linked gels on the processability of raw rubber.

Several researchers have reported the effect of gel on raw rubber. The flow characteristics of raw nitrile rubber containing divinyl benzene-cross-linked gel particles were studied (Nakajima and Collins, 1978). They found that the viscosity as well as the die swell decreased with increasing gel content at intermediate and high shear rates. However, they also pointed out that there exists a crossover at lower shear rates where viscosity is markedly increased on addition of a gel in the form of discrete particles. Montes and Ponce-Velez (1983) investigated the effect of gel on the extrusion behavior of Guayule rubber. It was observed that the viscosity increased on incorporation of a gel, which was more evident at lower shear rates. Bhowmick et al. (1986) studied the influence of gels on the crystallization, stress relaxation and orientation properties of NR. Campbell and Fuller (1984) observed that the gel produced a slight decrease in the rate of relaxation in raw NR. The effect of gel on green strength of NR was reported by Kawahara et al. (2002). Understanding the effect of various types of gel on raw rubber properties, especially on the rheological properties, is of great commercial importance, since it may lead to the advent of even superior processing grades of rubber than the existing ones.

There is considerable contradiction prevailing in the current literature about the effect of gel on raw rubber viscosity as stated above. The exact nature of the influence of gels on the raw rubber properties has not been investigated systematically in terms of the gel size, shape and degree of cross-linking of the gel till date. In addition, the improvement in various properties upon incorporation of gels shows different trends for different rubbers and is largely dependent on both the type and the size of the gel and the respective elastomers in to which they are incorporated. In our present work, NR latex was cross-linked by EB irradiation in the presence and absence of butyl acrylate to form cross-linked gelled latex. These latex gels were then mixed with raw NR latex at 2, 4, 8 and 16 phr concentrations, respectively, in the latex stage and finally dried. Their effect on raw NR properties, with particular reference to capillary rheometry, was investigated in detail.

Section snippets

Materials

High ammonia centrifuged NR latex having 60% dry rubber content (DRC) was obtained from the Rubber Board, Kottayam, India. Butyl acrylate (density 896 kg m−3 at 20 °C) was procured from Loba Chemie, Mumbai, India, and was used as received. All other chemicals of LR grade and doubly distilled water used were obtained from indigenous sources.

Preparation of latex sample for irradiation

NR latex was diluted to 50% by adding 1% aqueous ammonia solution and was stabilized by adding 0.2 phr of 10% KOH solution. For the preparation of the samples

Effect of radiation dose on gel content and cross-link density

Table 2 presents the values of gel fraction and cross-link density, respectively, for an NR latex containing 0 and 3 phr butyl acrylate at different radiation doses. The trend of variation in gel fraction with irradiation dose is similar for both. The gel content increases with irradiation dose before displaying an almost constant value at a higher radiation dose. The butyl acrylate-sensitized latex showed a marginally higher gel content at any given dose compared to their unmodified

Conclusions

In the present investigation, EB-cross-linked gels were prepared by EB irradiation of an NR latex with and without a butyl acrylate sensitizer. They were characterized by various methods. The rheological, mechanical and dynamic mechanical properties of the raw NR and NR containing unmodified and modified EB-cross-linked gels were investigated. From the above study, the following conclusions are drawn:

  • 1.

    The apparent shear viscosity decreases with increasing shear rate for the raw NR and raw NR

Acknowledgments

We would like to acknowledge the financial assistance provided by DAE, BRNS, Mumbai vide Sanction no. 2005/35/4/BRNS/516 dated 08-06-2005. We are also thankful to Dr. K.S. Sharma and Mr. M. Asaddulla, RTDS, BARC, for their help in carrying out electron beam irradiation.

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