What Is phr in a Formulation? Why Doesn't Compound Tonnage Equal Raw Rubber Tonnage?

Meaning of phr

All elastomers in a formulation are typically normalized to 100 phr. For example, natural rubber 70 phr + SBR 30 phr gives a total of 100 phr elastomer; after adding carbon black 50 phr, oil 10 phr, zinc oxide 5 phr, etc., the final compound mass significantly exceeds 100 parts.

Beyond the Three Basic Formulation Components

Carbon black is not simply a pigment, but one of the most important reinforcing fillers; its particle size, specific surface area, structure and dispersion state alter tensile strength, modulus, tear resistance, abrasion resistance, hardness, heat build-up and dynamic properties. The crosslink density and crosslink type of the cure system jointly determine hardness, modulus, elongation, heat resistance, fatigue resistance and permanent set.

Note the abbreviation conflict: CV in a cure system stands for Conventional Vulcanization, which is completely different from the CV in constant-viscosity grade CV60.

Practical Implications

Any conversion from 'compound output → raw rubber purchase volume' must first be based on knowledge of formulation structure and NR content; finished rubber article performance is determined jointly by raw rubber, blended rubbers, fillers, oils, cure system, anti-degradant system and processing history, so the raw rubber grade cannot be regarded as the sole determinant of finished product performance.