Rubber compounders are often challenged to improve overall dispersion quality and batch consistency, while simultaneously reducing the compound mixing time and increasing mixer output to save costs.
Selecting the right carbon blacks can improve the speed of dispersion and ease of mixing while also keeping costs down. SPHERON® 5000 and SPHERON 6000 carbon blacks have a unique morphology among our product portfolio. Low-to-medium viscosity rubber compounds typically develop less mixing shear and can benefit most from the good intrinsic dispersibility of these two SPHERON carbon black products.
There are two main factors that affect carbon black morphology during dispersion. The first is surface area, which is inversely relative to carbon black particle size. Surface area defines how much surface is available for interaction. The second is aggregate structure, which is relative to the volume of a polymer a carbon black aggregate will absorb.
In addition to surface area and structure, there are three other critical factors that affect dispersion and eventual extrudate surface finish:
- Tint. Lower tint at a given surface area delivers easier dispersion.
- Pellet properties. Softer pellets prove easier to break down and disperse in rubber compounding; however, carbon black pellets need to be hard enough to be conveyable.
- Impurities. Carbon black is essentially a pure elemental carbon; however, the product does contain very small amounts of material-like "grit" and inorganic contaminants from the manufacturing process.
SPHERON 5000 and SPHERON 6000 carbon blacks have an improved balance in the critical parameters affecting dispersion compared to standard ASTM carbon blacks. The chart below shows the number of defect counts measured on approximately 50 centimeters² of rubber strip surface. Our SPHERON carbon black products contain a much lower number of defect counts than the American Society for Testing and Materials (ASTM) carbon black products.
SPHERON® Carbon Black versus ASTM products
The image above represents approximately 1.2 centimeters² of the measured, extruded rubber strip surfaces. This visual impression of the superior surface finish observed in SPHERON carbon black products showcases the difference in quality between our carbon black products and ASTM carbon black products in an ethylene propylene diene monomer (EPDM) rubber test formulation. The carbon black PHR loadings were adjusted for 65 shore hardness and the same mixing cycle was employed. Rubber strips were extruded for each of the five resulting test compounds.