In the rubber industry, ultimate tensile strength is a fundamental mechanical property. This experimental parameter measures the ultimate strength of a vulcanised rubber compound. Even if a rubber product is never pulled close to its ultimate tensile strength, many users of rubber products still regard it as an important indicator of the overall quality of the compound. Tensile strength is therefore a very general specification, and although the end use of a specific product has little to do with it, formulators often have to go out of their way to meet it.

1. General principles

In order to obtain the highest tensile strength, one should usually start with elastomers where strain-induced crystallisation can occur, e.g. NR, CR, IR, HNBR.

2. Natural rubber NR

Adhesives based on natural rubber usually have a higher tensile strength than neoprene adhesives. Of the various grades of natural rubber, No. 1 fume film has the highest tensile strength. It has been reported that, at least in the case of carbon black filled compounds, No. 3 fume film gives better tensile strength than No. 1 fume film. For natural rubber compounds, chemical plasticisers (plastisol) such as biphenyl amidothiophenol or pentachlorothiophenol (PCTP) are to be avoided, as they reduce the tensile strength of the compound.

3. Chloroprene CR

Chloroprene (CR) is a strain-induced crystalline rubber that gives a high tensile strength in the absence of fillers. In fact, the tensile strength can sometimes be increased by reducing the amount of filler. Higher molecular weights of CR give higher tensile strengths.

4. Nitrile rubber NBR

NBR with a high content of acrylonitrile (ACN) gives a higher tensile strength. NBR with a narrow molecular weight distribution gives a higher tensile strength.

5. Influence of molecular weight

By optimisation, the use of NBRs with high meniscus viscosity and high molecular weight gives higher tensile strengths.

6. Carboxylated elastomers

Consider replacing uncarboxylated NBR with carboxylated XNBR and uncarboxylated HNBR with carboxylated XHNBR to improve the tensile strength of the compound.

Carboxylated NBR with a suitable amount of zinc oxide gives a higher tensile strength than conventional NBR.

7. EPDM

The use of semi-crystalline EPDM (high ethylene content) gives higher tensile strengths.

8. Reactive EPDM

Replacing unmodified EPDM with 2% (mass fraction) maleic anhydride modified EPDM in blends with NR increases the tensile strength of NR/EPDM compounds.

9. Gels

Synthetic gels such as SBR generally contain stabilisers. However, when mixing SBR compounds at temperatures above 163°C, both loose gels (which can be blended away) and tight gels (which cannot be blended away and are insoluble in certain solvents) can be produced. Both types of gel reduce the tensile strength of the compound. Therefore, the mixing temperature of SBR must be treated with care.

10. Vulcanisation

An important way to obtain high tensile strength is to optimise the crosslink density, avoid under-sulphurisation, post-vulcanisation and avoid blistering of the rubber during vulcanisation due to insufficient pressure or the use of volatile components.

11. Pressure-drop vulcanisation

For products vulcanised in autoclaves, the formation of blisters and the resulting reduction in tensile strength can be avoided by gradually reducing the pressure until the end of the vulcanisation, this is known as 'pressure drop vulcanisation'.

12. Vulcanisation time and temperature

Longer vulcanisation times at lower temperatures result in the formation of multi-sulphur bond networks, higher sulphur crosslink density and consequently higher tensile strength.

13. Tensile strength can be improved by better blending techniques to improve the dispersion of reinforcing fillers such as carbon black, while avoiding the mixing of impurities or large undispersed components.

14. Fillers

For fillers such as carbon black or silica, the choice of a small particle size with a large specific surface area can be effective in improving tensile strength. Non-reinforcing or filling fillers such as clay, calcium carbonate, talc, quartz sand, etc. should be avoided.

15. Carbon black

To ensure that carbon black is well dispersed, its filling should be increased to the optimum level to improve tensile strength. Carbon black with a small particle size will have a low optimum filling amount. Increasing the specific surface area of the carbon black and improving the dispersion of the carbon black by extending the mixing cycle can improve the tensile strength of the rubber.

16. White carbon black

The use of precipitated silica with a high specific surface area can effectively improve the tensile strength of the compound.

17. Plasticisers

Plasticisers should be avoided if high tensile strength is desired.

18. When vulcanising NBR compounds, conventional vulcanisation is more difficult to disperse evenly, therefore, sulphur treated with magnesium carbonate will disperse better in polar compounds like NBR. If the vulcanising agent is not well dispersed, the tensile strength can be seriously affected.

19. Multi-sulphur bonded crosslinking network

With conventional vulcanisation systems, the crosslinking network is dominated by polysulphide bonds; with EV, the crosslinking network is dominated by single and double sulphide bonds, the former resulting in a higher tensile strength.

20. Ionic crosslinking networks

Ionic cross-linked compounds have a higher tensile strength because the cross-linked points can slip and therefore move without being torn.

21. Stress crystallisation

The combination of natural rubber and neoprene containing stress crystals in the adhesive will help to increase the tensile strength.