Reinforcement of Butyl with Carbon Black. I. Effect of Oxidized Blacks on Stress-Strain Properties and Bound Rubber
Abstract The increased reinforcing capacity of oxidized blacks in butyl is illustrated, and the results are discussed in terms of the relationships which exist between polymer and black. Oxygen functionality on the black, it is shown, is the critical factor controlling the extent to which butyl is reinforced by the black. The lesser effects of decreased aggregate structure in the black are also demonstrated. These results are obtained using blacks which, though similar to attrited blacks, derive both their physical and chemical properties without the use of comminution. A much more unambiguous approach to the questions of black structure and oxygen content is therefore provided. In butyl, the unique sensitivity of the stress-strain curve to reinforcing effects is attributed to low unsaturation in the polymer. This sensitivity is used to qualify the nature of the reinforcement which is obtained. With oxidized blacks, true reinforcement is pictured as a stiffening effect which, starting with the gum vulcanizate, shifts the stress-strain curve without essentially changing its shape. The result is a “reinforced gum” which, it is suggested, derives its physical characteristics through the bonding of carbon black in the rubber network. With untreated black, carbon black is pictured as being enmeshed or entangled in an independently formed rubber network. Changes in the shape of the stress-strain curve are therefore attributed to steric restrictions which this arrangement imposes on the initial movement and subsequent orientation of network chains when the sample is extended.