Surface migration of carboxylic acid in styrene–butadiene rubber and its tribological consequences

Abstract

Surface migration of the carboxylic acids (C 10 H 21 COOH, C 11 H 23 COOH, C 12 H 25 COOH, C 15 H 31 COOH, C 16 H 33 COOH, and C 17 H 35 COOH) in styrene-butadiene rubber (SBR) matrix was investigated together with its influence on tribological properties of the vulcanisates. Macro-and microtribological properties were correlated with properties that come from carboxylic acid addition to the samples. A plastifying effect was characterized by a decrease of the rubber glass transition temperature measured by the DSC method. Thickness of an acid bloom was directly determined from an indentation experiment and estimated from the DSC curve as well as from FTIR reflection spectra. Morphology of the bloom was studied using an AFM technique to characterize topography as well as microroughness of the bloom surface. Macrofriction was studied using block-on-ring tribometer. Microfriction was examined with a Nano Test apparatus, applying ball-on-plate contact. Despite correlation found between plastification, morphology, and bloom thickness on the one hand, and tribologic properties of the rubber on the other hand, influence of the chain length of carboxylic acids on properties of the samples still needs further qualitative investigation. Tribological properties of SBR modified with carboxylic acids are the result of balance of two opposite effects: plastification of rubber-leading to an increase of friction and lubrication-which causes a decrease of friction of the rubber. Apart from plastifying and lubrication effects microroughness also plays an important role in friction.

Tribological modification of metal counterface by rubber

“…It is a consequence of limited solubility of low molecular weight components of rubber mix or products of their reactions, especially sulphur, carboxylic acids or their metal salts, respectively, in hydrocarbon matrix [5]. Together with a gradient structure of polymer materials [6] it facilitates tribochemical reactions, creating underestimated potential for modification of the surface of metal counterface.…”

Surface segregation of carboxylic acids in styrene-butadiene rubber — effect of the bloom on friction

“…Each rubber mix was cured with 1 phr of dicumyl peroxide (DCP, 98% by wt purity, Merck-Schuhardt, Germany). A lower amount of DCP was applied this time in order to obtain more elastic material -closer to typical rubber than previously tested [17]. Specimens were vulcanised in a steel mould at 160 • C during time t max (ISO 3417) exceeded by 10 min to ensure that the process was complete.…”

“…In our previous work [17,18,26,27], it was found that macroscale friction behaviour of rubber modified with low molecular weight additives is a function of two factors: (1) plasticization of rubber matrix, which causes an increase of friction force and (2) lubrication by a bloom, which is responsible for a decrease of friction force. If bloom on the surface of rubber is not thick enough to lubricate effectively, plasticization plays a dominant role in tribological behaviour, which results in increase of friction coefficient.…”

“…A relatively low number of papers refers to the effect of blooms on tribological properties of rubbers [15,16]. From our previous work, it follows that the bloom of low molecular weight additives can act as a lubricant, causing decrease of friction coefficient of vulcanisates [17,18]. In this paper, we present the results of studies concerning the effect of bloom formation conditions on its morphology.…”

A study of fire retardant blooming in HIPS by molecular modeling

“…The blooming process, frequently called additive surface migration, was thoroughly investigated experimentally and computationally 1–7…”