A new indicator of natural rubber quality

To strengthen the position of natural rubber in relation to its synthetic rival, a CIRAD team has just used an innovative technique to identify a new indicator of the quality of this noble material.

Natural rubber has properties that are still unsurpassed, and many specific uses: aircraft and truck tyres, vehicle engine supports, high-speed train suspension parts, industrial glues and adhesive tapes, elastic yarns, gloves, condoms, etc. However, it is a product of biological origin, which makes its properties more variable than those of its synthetic competitors. The challenge is thus to find relevant indicators of its quality, and to this end, it has proved necessary to look into its very structure.

Natural rubber differs from its synthetic counterparts through its more complex structure. When it is dissolved in a conventional solvent, the structure is gradually and partially destroyed. A certain proportion of the natural rubber remains insoluble. This fraction is commonly referred to as the gel phase (a network of reticulated polymers swollen with liquid), or as a macrogel. The soluble fraction contains rubber macromolecules and a variable quantity of microaggregates that make up a microgel. Gel has a major impact on the rheological properties of the material, and thus very probably on blending performance (blending being one of the natural rubber processing stages). CIRAD has thus studied various aspects of its macromolecular structure (links with rheological properties, gel formation mechanisms, etc).

To date, only macrogel has been quantifiable, but researchers are now also studying microgel. To this end, they have developed a methodology based on the size exclusion chromatography (SEC) technique. The solution containing the microgel is filtered and then centrifuged. This makes it possible to analyse macromolecular structure of and quantify the microgel. By providing a more comprehensive characterization of natural rubber structure, this research has opened the way for a clearer understanding of the links between that structure and the properties of the end product. This should result in the emergence of new, more relevant quality indicators than the criteria currently used.

The natural rubber currently used in industry comes from a singler species originating from South America, Hevea brasiliensis. Rubber trees are a veritable “green factory”, producing an elastomer from a renewable energy source. It takes 0.4 TOE (tonnes of oil equivalent, or the energy produced by one tonne of oil) to produce one tonne of natural rubber, but 3.7 to 5 TOE to produce one tonne of synthetic rubber. At current consumption rates, using natural rubber thus saves around 20 million TOE, ie around 40% of annual consumption for transport purposes in France, or 3% of total annual oil consumption in the European Union.