The first ever elastomeric adhesive was prepared at the end of the 18th Century, consisting of naphtha solutions of natural rubber.¹ While the original rubber probably came from South America, in recent years that region's production has been blighted by a fungus called Microcyclus ulei that attacks the leaves of rubber trees. Today, adhesive manufacturers who still employ natural rubber as their elastomer get it only from Southeast Asia, Latin America and Africa,² with 95 percent of the world's rubber coming from Asia. Now, reports of fungal leaf blight are emerging from these countries, placing rubber supplies under threat once more.³

Figure 1: Localization of the global production of natural rubber. The amount produced in each continent is shown in units of thousands of tonnes.

"If the fungus disease was to reach epidemic proportions, chemical crop protection would be rendered useless," a recent statement from the Fraunhofer Institute for Molecular Biology and Applied Ecology IME in Aachen said. "The natural latex industry could collapse if that were to happen."

Estimates from the US Environmental Protection Agency and Census Bureau suggest that annual sales of rubber-latex based adhesives in the US alone neared $100 million in 2008. Adhesives made from natural rubber are economical and offer excellent tack even when unmodified to a wide range of substrates. They show very good water resistance, but poor resistance to oils and organic solvents unless vulcanized. Natural-rubber based adhesives are highly flexible, resilient, and resistant to fatigue, although they can become brittle with age due to oxidation.

Switching to synthetic rubbers is an obvious course of action for adhesive makers if natural rubber stocks were threatened, but the substitution process will pose challenges, and run counter to industry-wide sustainability efforts. Some adhesives can also be made using rubber recycled from other applications, but this is unlikely to be a long-term solution if a widespread Microcyclus ulei problem emerges.

Fraunhofer scientists have already turned to an unlikely-sounding alternative source of rubber - the Russian dandelion. Widely used to provide rubber during the Second World War, the latex from the flower is difficult to use as it polymerizes immediately upon seeping out. However, the German scientists have identified the enzyme responsible for this and genetically engineered the plant to switch it off. "We obtain four to five times the amount [of latex] we would normally," said Dirk Prfer, head of department at Fraunhofer IME. "If the plants were to be cultivated on a large scale, every hectare would produce 500 to 1000 kilograms of latex per growing season."

Figure 2: Fraunhofer's genetically-modified, natural rubber producing Russian dandelions

In Brazil, Group Michelin is attacking the blight by is breeding fungus-resistant strains of trees, the first of which emerged in 1992.⁴ The pollen-containing stamens of male plants with resistant qualities were used to fertilise the female parts of high-yielding Asian trees by hand. Hundreds of different varieties were created, and screened for resistance. The best performers were then grafted onto the roots of local rubber tree varieties, in an attempt to fuse together the tissues of two plants. 13 of the highest-yielding fungus-resistant varieties created by this approach entered even more detailed testing in 2005. Now Michelin hopes the plantation will produce 5,000 tons of rubber annually by 2015.

For adhesive producers and users alike, supply constraints would pose dilemmas surrounding some very familiar bonding applications. Today, natural rubber is used in pressure-sensitive adhesives for commercial tapes and surgical plasters. Natural rubber is still used in solvent-borne adhesives to produce leather footwear, and in rubber footwear as curing laminating adhesives.

Perhaps the application most likely to be missed if natural rubber ceases to be available will be in self-sealing envelopes. When the latex dries, some soluble non-rubber compounds migrate to the surface by water transport, creating a thin, non-tacky protective film. When pressed against a similar film the protective layers are displaced, allowing the two rubber surfaces to create a bond.

There will always be potential alternative products that can be used for bonding in these applications. However adhesive makers should ensure that they are in a position to begin offering those alternatives whenever it becomes economically appropriate to do so.

Are you involved in the natural rubber supply chain, or do you have experience formulating to substitute natural rubber? If so, please share your experiences using the tools below.

References

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Andy Extance