A TSR20 specification sets laboratory limits for dirt, ash, volatile matter, nitrogen, initial plasticity (Po) and plasticity retention index (PRI). Under ISO 2000, TSR20 allows up to 0.16% dirt, 0.80% ash, 0.80% volatile matter and 0.60% nitrogen, with Po of at least 30 and PRI of at least 40. Mooney viscosity is not part of the standard grade, but many buyers add it to the contract.
This guide explains what each parameter tells a compounder and where to tighten the limits. For grade, packing and supply details, see our page on TSR20 natural rubber from Liberia.
The ISO 2000 limits for TSR20
| Parameter | TSR20 limit | Test method | What it measures |
|---|---|---|---|
| Dirt | 0.16% max | ISO 249 | Foreign particles retained on a 45 µm sieve |
| Ash | 0.80% max | ISO 247 | Mineral content |
| Volatile matter | 0.80% max | ISO 248 | Mostly moisture |
| Nitrogen | 0.60% max | ISO 1656 | Protein content |
| Po | 30 min | ISO 2007 | Initial plasticity, a guide to molecular weight |
| PRI | 40 min | ISO 2930 | Resistance to oxidation during heating |
| Mooney ML(1+4) 100 °C | Not specified | ISO 289 | Viscosity for processing |
Older national schemes, such as SMR and SIR, set TSR20 dirt at 0.20%. If your contract cites a national scheme rather than ISO 2000, check which limit applies.
Dirt
Dirt is the share of particles that do not pass a 45 µm sieve after the rubber is dissolved. In TSR20 it comes mostly from bark, sand and leaves picked up with cup lump and tree lace in the field.
Dirt matters because hard particles act as starting points for cracks. In a tyre carcass or a dynamic part such as an engine mount, they reduce fatigue life. Buyers making high fatigue products often ask for dirt well below the maximum, for example 0.10%, or move to TSR10.
Ash
Ash is the mineral residue left after burning the sample at high temperature. High ash usually means sand or soil in the raw material, or poor washing in the factory. Ash and dirt often move together. A lot with low dirt but high ash points to fine mineral contamination that the sieve test does not catch.
Volatile matter
Volatile matter is the weight lost on drying, which is almost all water. Rubber that is not fully dried can develop mould in storage, lose weight in transit and give off steam in the mixer. A result near the 0.80% limit is a warning sign for long sea voyages to Asia.
Nitrogen
Nitrogen is a proxy for protein content. Proteins act as natural accelerators, so nitrogen affects cure speed. High or variable nitrogen makes cure times harder to hold. It also increases water absorption.
Po and PRI
Po is the initial Wallace plasticity of the raw rubber. A higher Po means harder rubber with longer polymer chains.
PRI is the percentage of plasticity the rubber keeps after heating at 140 °C for 30 minutes. It shows how well the rubber resists breakdown by oxidation. A low PRI means the rubber softens quickly in the mixer and in service. Rubber made from old or overheated coagulum tends to have low PRI.
PRI is the parameter tyre makers watch most closely. The ISO minimum is 40, but many contracts ask for 50 or 55 for carcass compounds.
Mooney viscosity
Mooney viscosity measures how stiff the rubber is at mixing temperature. It is not part of the TSR20 grade, but it is often the parameter that causes the most trouble in a factory, because mixing cycles and energy use are set around it.
If your process is sensitive, put a Mooney range in the contract, for example 75 to 90, and ask for results per lot. Mooney in natural rubber also rises in storage, a process called storage hardening. A lot tested in Monrovia will usually read higher after a long voyage.
How to write the specification into a contract
- Cite the standard. Name ISO 2000 and the edition, or the national scheme you use.
- Add tighter limits only where they matter. Every limit tighter than the grade narrows supply and may raise the price.
- Name the test methods. Results from different methods are not directly comparable.
- Say where results are final. At loading, by a named surveyor, or at discharge, with a process for disputes. See our guide to inspection and certificates of analysis.
- Agree the sampling plan. How many bales per lot are sampled, and how.
TSR20 or a cleaner grade?
If your product needs lower dirt and higher PRI than TSR20 can guarantee, compare the grades in TSR10 vs TSR20 vs RSS. For most tyre carcass, truck tyre and general rubber goods compounds, a well-specified TSR20 is the right balance of cost and consistency.
When you request a quote, send the full specification you need and we will tell you which limits we can guarantee.