A rubber compound is not one material. It is a polymer plus a handful of other ingredients, mixed in a set order and then cured under heat and pressure. Two compounds made from the same polymer can behave very differently, because the other ingredients decide hardness, cure speed, heat resistance, compression set and fluid resistance.
What "phr" means
Recipes are written in phr, parts per hundred parts of rubber, by weight. The polymer is always 100. Everything else is expressed against it. A recipe that lists 30 phr of carbon black has 30 g of black for every 100 g of polymer.
This makes recipes easy to scale and to compare, because the polymer is the fixed reference. The total of all phr is the batch weight per 100 of rubber, and the polymer is therefore only about half to two-thirds of a typical compound by weight. To get a percentage by weight, divide an ingredient by the total phr.
The ingredient groups
Polymer. The backbone of the material, such as FKM, HNBR or FEPM. It sets the ceiling for heat, fluid and low-temperature performance. No other ingredient can lift a compound much above what its polymer allows.
Fillers. Carbon blacks, silica, mineral powders. Fillers are usually present at many times the level of the chemical additives. They set hardness and stiffness, tear and tensile strength, and have a strong effect on compression set and processing. Fluoro-rubbers often use low-reinforcing medium-thermal black (N990) because it gives low compression set and good flow. HNBR uses a wider range, from N990 up to reinforcing grades such as N330 or N550.
Curatives. The chemicals that join polymer chains into a network: bisphenol and an accelerator for many FKM grades, a peroxide for HNBR, FEPM and peroxide-curable FKM. Without them the material stays soft and flows like putty.
Coagents. Reactive co-chemicals such as TAIC that help a peroxide build a tight, stable network. They are small in weight but have a large effect on compression set.
Acid acceptors and activators. Metal oxides and hydroxides such as magnesium oxide, calcium hydroxide and zinc oxide. They neutralise acidic by-products during cure and heat ageing, and in bisphenol-cured FKM they take part in the cure chemistry.
Plasticisers and processing aids. Esters, waxes and soaps. They make the stock easier to mix and mould, help release from the tool, and in HNBR improve low-temperature flexibility. Too much can cost heat resistance or cause bloom.
Antioxidants. Used mostly in HNBR and other hydrocarbon rubbers. They slow heat ageing but can interfere with peroxide cure, so they are chosen with care.
How to read a recipe quickly
Ask four questions of any formulation:
- What is the polymer, and which cure system does it need? A cure-incorporated FKM gum already contains curative and needs only acid acceptors, while a base gum needs a separate curative.
- How much filler is there, and which type? A 30 phr N990 compound and a 60 phr N550 compound are different materials even at the same hardness.
- What is the acid acceptor package? In bisphenol-cured FKM it usually includes magnesium oxide and calcium hydroxide.
- Is there a post-cure? Fluoro-rubbers are almost always post-cured in an oven. The post-cure schedule is part of the recipe.
A recipe alone does not describe the product. The mixing procedure, the batch size, the cure temperature and the post-cure all change the result. Two plants running the same phr list can get different compression set.
Sources and further reading
- P. A. Ciullo and N. Hewitt, The Rubber Formulary, Noyes Publications / William Andrew, 1999. A reference that collects supplier formulations.
- ASTM D1418 for rubber abbreviations; ASTM D1765 for carbon black designations.