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The Benefits of Silica Filler While Minimizing the Processing Hassles


Dilip Dhupia

Application Engineer, India

How Alpha’s Premier™ RPA can provide qualitative estimates of silanization.

Modern tire treads and other high-performance rubber are often reinforced by silica, which helps provide lower rolling resistance (providing fuel savings) as well as better wet traction in comparison with carbon black-reinforced treads. However, the addition of silica in rubber compounds leads to processing issues due to:

  • Incompatibility between highly polar silica and nonpolar raw rubber. 
  • Greater affinity between silanol groups leading to the formation of a string filler-tiller network, often referred to as flocculation, resulting in poor dispersion and distribution.
  • Poor filler-rubber interaction

This is due to difference of surface chemistry between silica and carbon black:


Inorganic surface chemistry
Inorganic surface chemistry
Organic surface chemistry

As a result, silica aggregates tend to flocculate because of their poor compatibility with rubber. Flocculation can occur during compound storage, as well as at the beginning of vulcanization. To minimize silica flocculation, improve mixing, and obtain mixing consistency, sulfur-containing silane coupling agents are added to the recipe. The mixing of silica with silane in rubber produces a chemical reaction, called silanization.

How to Remove Flocculation Out of Your Compound

In a silanization reaction, silanes chemically bond with silica during mixing, and then attach to rubber at vulcanization, which as a positive effect on the resulting compound properties in terms of improving filler-to-rubber reaction. The degree of silanization achieved during mixing is one of the key factors to control the quality of processing, rubber reinforcement and other compound properties.

Authors of the paper “Silanization characterization and compound properties of silica-filled rubber containing a blocked mercapto silane” Chenchy J. Lin, W. Michael York, Russell J. Cody; developed a feasible and reliable method to quantify silanization using a Premier RPA (Rubber process analyzer) from Alpha Technologies. The method was based on the fact that the degree of filler network developed is inversely proportional to the exposed silica surface coverage by a silane, therefore the amount of silane reacted with the exposed silica surface can be calculated. in their paper, an indirect method was demonstrated using rheology, which may be more convenient and practical for typical tire plants. Specifically, the authors propose to measure a silane’s capability to suppress silica filler network formation (flocculation) and to correlate those measurements to the resultant compound properties. 

The authors evaluated three types of silane coupling agents and adapted the test methods to shop floor Quality Control environments, making some modifications to make it user friendly and to create a test that can be conducted after each stage of mixing. The test sequence involves conditioning of the sample and determination of storage modulus before and after sample conditioning step. Delta of storage modulus after different mixing stage is being used to calculate the silanization. 

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