Reinforcing Fillers

Title: Filler Reinforced Rubber
FORMAT: Afternoon Technical DiscussionMeeting
Venue: IOM, LONDON
Date:14th Dec 2001

ACHIEVING SILANE COUPLING, FILLER DISPERSION & PROCESSABILITY IN SILICA REINFORCED PASSENGER TYRE "GREEN TYRE" TREAD COMPOUNDS

By Christopher R. Stone M.I.M., Tyre & SDE Consulting

The environmental and technical problems associated with the manufacture of tyres with these tread compounds are highlighted. These are then set against the principal technical and ecological benefits that these compounds confer to the tyres. The tyre manufacturers presumably believe that these make tackling the problems worth while, even to the extent of using multi-stage mixing cycles utilising variable rotor speeds. They are even changing their mixers from high throughput tangential rotor types to the lower throughput intermeshing rotor types.

In order to develop an understanding of the results of visco-elastic dynamic tests on uncured compound; two experiments are described. One uses a series of three compounds designed to have an equal degree of silane coupling but varying silica dispersion, the other uses a series of three compounds designed to have equal silica dispersion but varying degrees of silane coupling. The visco-elastic dynamic test results of tests on the uncured compound are compared with actual measurements of filler dispersion and certain physical properties, both on uncured and cured compound. This data is used to show that a rapid visco-elastic test on the uncured compound can be used to rank compounds for filler dispersion, silane coupling and silane degradation. This method of testing is then used to evaluate variations in mixing technique and additives.

The problem of mixing these silane coupled silica reinforced compounds is that the mixer is being used both as a mixer and a chemical reactor at the same time. To further complicate the matter, these two processes require conditions that are mutually opposed. To investigate the mixing cycle a series of three mixes were mixed with their masterbatches and remills mixed to equal energy input at three different rotor speeds. The highest speed gave the best silica dispersion but the temperatures achieved caused silane degradation, leading to a high viscosity and a short scorch time. The lower speeds gave less silane degradation and thus better scorch times but inadequate silica dispersion and less than satisfactory silane coupling, leading to inadequate compound properties.

A mix cycle with the masterbatch and remill both starting with a high rotor speed and then changing to a low rotor speed was mixed and compared with the other mixes. The high speed portion achieves the adequate silica dispersion and develops a high enough temperature (14WC) to initiate silane coupling. The low speed portion adds nothing to silica dispersion but maintains the temperature at a high enough temperature for enough time to ensure adequate silane coupling without causing the temperature to exceed 16TC at which temperature, silane degradation starts to occur. This mixing technique is shown to give compound with both good processability and good physical properties.

Published work states that to achieve the best silane coupling when mixing these silica reinforced S SBR/BR passenger tread compounds it is necessary to use a special mixing procedure. In this procedure only the polymers, filler, silane and oil are mixed in the first stage followed by a second stage mix when the zinc oxide, stearic acid, antidegradant and wax are added. Data is shown comparing regular mixing with special mixing. Unfortunately, it shows that whilst this mixing technique does indeed gives the best silane coupling, poor filler dispersion and silane degradation result.
It also highlights difficulties in the special mixing process and the poor processing properties, mostly due to the silane degradation and the poor silica dispersion. However, some properties are improved because of the improved silane coupling.

Mixes were prepared, using the special mixing process, without process additives and with a special fatty acid ester process additive in the first stage both with and without a specific, surfactant type, process additive in the second stage. The first stage process additive was specially developed not to adversely affect silane coupling. The process additive replacing stearic acid considerably improves the poor mixing at the start of the remill that is due to the lubricating effect of the other additives.

BREAKDOWN AND REFORMATION PROCESSES IN CARBON BLACK ELASTOMERS

By Ken Yamaguchi, Bridgestone Corporation on leave of absence at Queen Mary, University of LONDON

The structure of the carbon black in an elastomer material is altered under deformation. The changes to the structure are related to changes in the static and the dynamic behaviour during loading and are clearly important in understanding effects such as cyclic stress softening. However, traditional techniques that are used to examine changes in the structure of materials are difficult to utilise with carbon black filled elastomers and so alternative techniques have to be adopted. In this work the structural changes are examined using elastomers that are filled with carbon black above the percolation threshold using electrical conductivity tests. This technique allows the significant breakdown, orientation and reformation processes to be identified and discussed.

THE USE OF SILICA IN TYRE COMPOUNDING

By Dr Malcolm Evans, Kumho Tyres

Abstract: This talk is intended to provide an overview of the reasons why silica, as an elastomeric filler, has found greater interest over the past decade with compounders in their search to improve tyre performance.
After a short introduction to the tyre industry, the presentation describes the many apparently conflicting requirements placed on tyres from various customers. A novel route to solving the conflict was found in what has become known as the green revolution; novel polymers together with silica in combination with new effective coupling agents.

The chemistry behind the materials is briefly described together with some of the new challenges now facing the engineers who develop the formulations and those who try to understand their processing problems. An explanation of how compound properties can influence tyre performance is used to conclude how silica may have achieved the desired effects of breaking the 'Magic Triangle'.

ELECTRON TOMOGRAPHY AND VOLUME AND SURFACE AREA MEASUREMENTS OF INDIVIDUAL CARBON BLACK AGGREGATES

By: Charles Herd, Eden Hunt, Deepak Tandon, and Tyler Gruber Technology Laboratory, Columbian Chemicals Company, 1800 West Oak Commons Court, Marietta, Georgia, 30062, USA

The three-dimensional morphology of carbon black aggregates has been little studied and only recently with advanced computer power has it become more easily accessible. Electron tomography has been employed to generate three-dimensional volumes of individual carbon black aggregates that consist of voxels, which can then be manipulated on the computer for viewing and quantitative measurements. Four different aggregate shape types, spheroidal (Type 1), ellipsoidal (Type 2), linear (Type 3) and branched (Type 4) were characterised and volume and surface area measurements performed. The results showed that carbon black aggregates are anisotropic in nature with the z dimension generally shorter than the x and y dimensions. Also, the volume and surface area measurements of the individual aggregates were found to be in the range expected for the carbon black grade analysed