Elsevier

Polymer Testing

Volume 18, Issue 7, October 1999, Pages 483-493
Polymer Testing

Material Properties
On the percolative behavior of carbon black-rubber interlinked systems

https://doi.org/10.1016/S0142-9418(98)00035-XGet rights and content

Abstract

A series of SBR—loaded with high abrasion furnace (HAF) carbon black and vulcanized in contact with NBR have been produced on a laboratory scale, and their dielectric properties were investigated. The dielectric properties of these interlinked systems are described in the frequency range between 102 and 105 Hz as a function of frequency, carbon black concentration and vulcanization time. Relatively high values of the dielectric constant were obtained when measuring conductive samples and interlinked samples with small initial vulcanization time. A percolative model gives a suitable explanation, within the experimental error of the studied systems.

Introduction

Rubber is an increasingly important engineering material and an improved understanding of its mechanical properties and, particularly, of its electrical behavior is required. In practice, rubber vulcanizates almost always contain fine-particle fillers of which the carbon blacks are the most important. Such fillers enhance viscoelastic response to deformation and increased electrical conductivity and dielectric constant [1].

The strength of adhesion between two elastomer layers vulcanized in contact is important for various practical 2, 3, 4applications: tyres, conveyor belts,.... etc. Previous studies have examined the relationship between the strength of adhesion and the density of chemical interlinking at the interface.

The electrical conduction in polymer-carbon black composites has been extensively reported, but only a minor part being devoted to a.c. conductivities [5]. The insulator-conductor transition in these composite systems is often sharp and characterized by a critical carbon black loading, the threshold percolation concentration.

It is to be noted that the application of the percolative theory to rubber-carbon black composites is relatively recent 6, 7, 8.

The complex geometry and structure of carbon black aggregates and the agglomeration phenomenon reduce the threshold percolation concentration and enhance the conductivity of the system. Other characteristics of fillers such as high microporosity, low volatile content and high surface area also raise the resultant composite conductivity.

The present work describes the behavior of styrene butadiene rubber loaded with different concentrations of HAF (high abrasion furnace) black and interlinked by sulphur bridges with acrylonitrile butadiene rubber NBR as far as their dielectric properties as a function of frequency from 102 to 105 Hz are concerned. The effect of time of vulcanization and interlinked processes on the threshold percolative concentration were studied.

Section snippets

Experimental

The mix formulations are given in Table 1. For SBR and NBR the crosslinking agent is sulphur.

Flat sheets, about 2 mm thick, were prepared in all cases by a molding process, between films of polyethylene terephthalte (PET).

The elastomer sheets were partially crosslinked by heating for a time t1 at the vulcanization temperature of 150°C and at a pressure of 4.5 MPa, between electrically heated platens. The protective films were then removed and two sheets, crosslinked to the same degree, were

Results and discussions

Electrical conductivity is important in many rubber and plastic compounds including antistatic applications, wire and cable shielding and shielding against electromagnetic interference (EMI). Elastomers and plastics are insulators to which conductivity is imparted by addition of a finely divided or colloidal fillers of high intrinsic conductivity, such as carbon black.

At low loading of the HAF black in the SBR host material (single layer), the conductivity of the composite is essentially that

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