How to Read a Rubber Formula Sheet (phr Explained)
What phr means, why every number is relative to 100 parts of rubber, and how to convert a formula into a real batch weight.
Formulas are written in phr — parts per hundred rubber. Every ingredient is expressed relative to 100 parts of polymer, not as a percentage. This is the single most common source of confusion when a buyer compares two suppliers' data sheets.
Worked example
A compound reads: SBR 1502 — 100, N330 carbon black — 50, aromatic oil — 10, zinc oxide — 5, stearic acid — 2, sulphur — 1.8, CBS — 1.2, 6PPD — 2. Total phr = 172.
To make 500 kg of compound, the polymer charge is 500 ÷ 1.72 ≈ 291 kg, and every other ingredient is scaled by the same factor. Batch weights are never the phr number itself.
What to check before you accept a formula
- Is the polymer grade named? "SBR" alone is not a specification. SBR 1502 and SBR 1712 differ in oil extension and filler tolerance. - Are the accelerators named? The accelerator choice sets scorch safety and cure rate. - Is the antidegradant level stated? Below roughly 1.5 phr of an antiozonant, outdoor cracking resistance drops noticeably in sidewall-type applications. - Does the total make sense? Totals between 150 and 200 phr are typical for filled general-purpose compounds; much higher usually signals a heavily extended, lower-cost formulation. ## Need the documents or today's price? Send us the grade, quantity and destination port — we return a quotation with COA and MSDS attached. Email manager@fipolymer.com or use the inquiry form on this site.
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