Amid rising concerns about the environmental and human threats posed by rubber, Bridgestone and Michelin lead the search for green alternatives

Rubber: ubiquitous and versatile. It’s in everything from tyres to chewing gum; condoms to golf balls. Millions of tonnes of natural and synthetic rubber are produced each year in a market worth billions. But is it sustainable?

The answer is no. Natural rubber prices are volatile, and there are very real climatic and environmental threats to production, along with concerns about the exploitation of workers and destruction of biodiversity.

Tyre makers, the biggest users of rubber, have been much slower than consumer goods companies, which depend on palm oil, to take a close look at their supply chains -  an issue highlighted earlier this summer, when Michelin became the first tyre company to announce plans to source deforestation-free rubber.

Yet as researchers from the University of East Anglia’s School of Environmental Sciences point out, rubber plantations cover about 70% of the land area occupied by oil palm. And almost the entire global supply of natural rubber – some 11 million tonnes in 2015 – comes from Southeast Asia, and is threatened by climate change and vulnerable to disease and environmental hazards.

An international research effort led by scientists at Edinburgh’s Botanic Gardens estimated that more than 50% of plantations in the region might not be sustainable. Land that would have provided food or preserved biodiversity is being destroyed as rubber plantations expand into areas outside their natural tropical habitat. Some plantations have been converted to more lucrative oil palm, pushing rubber production out into areas where water is at a premium and where they might not survive frosts and wind.

Moreover, new plantations are grown in monoculture as opposed to the traditional mixed forest zones, which also threatens biodiversity and livelihoods.

Synthetic rubbers, on the other hand, do not have the same land use issues, but they are petroleum-based and energy-intensive to produce, so the search is on to find additional sources of the flexible polymer on which we all depend.

The new rubbers

There are two routes to the new rubbers: either they can be extracted directly from one of the many plants that can make natural rubber; or created by feeding plant sugars to genetically engineered micro-organisms, which will make the building blocks for rubber manufacture.

Polyisoprene (to give natural rubber its chemical name) is actually made by around 2,500 plants, but there are two really promising candidates that could supply a sustainable source. These wouldn’t replace the hevea natural rubber produced in the tropics, but they would have none of its problems..

Those contenders are the Kazakh dandelion, a relative of the common dandelion, and a bushy plant, guayule. Guayule grows in semi-arid regions of south-west America and Mexico; while the Kazakh dandelion (Taraxacum kok saghyz or TKS) can cope with colder northern regions. Neither would compete with hevea for land resources. Nor would they compete with food crops, as both can grow in less hospitable soils.

But do they offer a truly sustainable alternative? Guayule certainly does, according to Professor Katrina Cornish who leads the research on the two plants at Ohio State University. There are some criteria: can all parts of the plant be used, leaving no waste? "Both need a 100% crop consumption plan. With guayule we pretty much know what that is; with TKS, we’re still working on it." Both plants absorb carbon, and if they displace emissions-intensive synthetic rubbers there is a huge bonus.

To be truly sustainable and resilient, they need to be flexible enough to meet market demands: TKS rubber is found in the roots so the whole plant is harvested annually. It would be part of a crop rotation scheme, or grown on demand. Since it won’t be eaten, there might not be huge concern about the plant absorbing contaminants, suggests Cornish. She thinks it would be worthwhile planting TKS on marginal land such as old mine strips: indeed, it might eventually have the advantage of cleaning up old industrial land.

Because the precious rubber latex is found in guayule’s branches and stems, these could be harvested for a decade or so, before the whole plant is dug up and used.

Another plus is that bees like them: "With guayule we don’t put fungicide or heavy doses of chemicals," says Cornish. "It’s useful for bees to have an uncontaminated source of pollen."

Bees also like the dandelion that the university’s researchers have been breeding. They’ve named this high-rubber yielding variety Buckeye Gold, because they believe it could be Ohio’s next cash crop. However, it’s too early to say whether diseases or pests might yet arise from large scale planting.

Tyre makers key

While big tyre makers have been slow to look at the sustainability of their existing natural rubber supply chains, they are investing in research into the new natural rubbers.

"I think there’s potential for the tyre industry to help bring these new natural rubbers to production scale," says Amy Randall, manager of innovation and technology development at Bridgestone Americas. Bridgestone’s over-arching aim is to use only sustainable raw materials in its tyres by 2050.

On the other side of the Atlantic, Italian chemicals group Versalis is also investing in guayule research. This is part of a wider long-term plan to convert its petrochemical refineries into bio-refineries, which will make a range of chemicals as well as its own synthetic rubbers. Versalis is convinced there’s a future for guayule in Europe – which imports 1.2 million tonnes of natural hevea rubber a year, almost one tenth of the global supply.

Both Bridgestone and Versalis are developing technologies to maximise the value of the entire plant. In addition to rubber, guayale contains energy-rich turpene resins, which potentially have a future in the chemicals industry or as a feedstock for biofuels. It is thought that the residual biomass (or bagasse) could be used in the building industry.

Versalis believes guayale could be a route to a bio-based version of butadiene, which is one of the most important building blocks for synthetic rubber. Exploiting the whole plant would not only be a more sustainable option, but a commercially necessary one, because extracting and processing the rubber alone is difficult and expensive.

A whole new industry

The companies exploring guayule and TKS are effectively building a new industry. "You’re creating everything, from the supply chain of biomass to the multiple products developed from the biomass," says Dr Bill Niaura, Bridgestone’s director of business development. Bridgestone has even had to develop its own equipment to harvest the plants on its guayule farm.

The first step to exploiting the biomass is to breed plants that can produce a reliable yield. Niaura says the process of going from a wild to a "true" variety with stable traits will take five to seven years. Versalis is in its fourth year of successful planting, and is expanding its test beds from southern Italy to Sicily.

What will accelerate plant breeding programmes is the complex work under way at the US Department of Agriculture, where scientists are sequencing the guayule genome. "Our job is yield: can we produce enough to compete with hevea?" explains Dr Bill Belknap, a research plant physiologist with the Agricultural Research Service.

Coming up with the genome is important because it takes two years for a guayule plant to grow to maturity; a two-year wait to see if a promising candidate will actually produce improved yields. Belknap’s team has been able to take advantage of extraordinary advances in sequencing technology and will soon publish a reference genome. Scientists will use it to produce a map of the genetic markers, which will tell them which plants have particular traits –  in this case, good latex production.

The challenge then is moving from individual plants to large-scale production, says Jeff Martin, founder of Yulex, another company looking forward to receiving the reference genome. Yulex pioneered guayule as a solution to natural rubber latex allergies and is making big strides towards breeding a high-yield variety. "A decade ago there was nothing; now you’ve got the top tyre companies exploring guayule and TKS," says Martin. "Guayule rubber is of exceptional quality; it has the capability to be commercially viable." He is confident that farmers will add guayule to their crop mix.

Even then, there is a long way to go. Niaura points out that "there is no system in place to create commercial quantities of guayule seed. If SE Asian rubber disappeared –we just can’t replace it today".

Guayule has a very different chemistry from hevea, and needs processing to compensate for the differences. Bridgestone has built a bio-rubber process research centre that, two years on, is producing tyre-grade rubber. It has made prototype tyres where all the hevea natural rubber components were replaced with its guayule rubber.

The challenge of the processing also has its upside: Niaura says what’s exciting is the opportunity to control the many variables and influence the technical parameters of the end product: He predicts manufacturers will have more control and product consistency than with hevea rubber.

Versalis’s experience suggests there may even be some performance gains to be had with guayule rubber, especially in the premium tyre market. It’s been supplying guayule rubber to Italy's Pirelli, which earlier this year announced had successfully tested the rubber in tyres on its top-end Maserati models.

Making guayule pay

But there’s a balancing act: when rubber prices are low (as now) then the bagasse and resins have to earn money. "That’s the 800lb gorilla in the room," says Niaura. "Bridgestone has built a bioprocessing plant, not a rubber plant."  

For Randall the solution is to exploit guayule’s non-rubber potential. "How to effectively utilise the co-products ­– that’s where we are really stretching ourselves now." She says Bridgestone is working with vendor partners on uses for the co-products. It is finding interested parties but is tight-lipped about who they are. The resins have many chemical constituents: to fully exploit them means building a resin refinery on top of the bio-refinery.

"Many people think that when guayule is fully developed, the turpene resins will be more valuable than the rubber itself," suggests Martin. And that would mean the rubber would be commercially viable.

"We’re pushing; we need people to pull," says Randall: "That’s when we’ll have made progress."

The bagasse also has potential as a biofuel – putting it on a more sustainable footing than crops which are grown solely for energy.

Bridgestone is also exploring TKS, and is part of a project led by Ohio State University’s PENRA programme. The US tyre company is putting the rubber extracted from TKS through its paces at its research centres in the US and Japan. Randall says once Bridgestone's guayule processing capability is optimal, the company will be ready for TKS.

TKS has a much more similar chemistry to hevea rubber than guayule, says Professor Cornish. "The big companies are interested because it’s almost a drop-in [replacement]."

In Europe, Continental has pioneered its use, producing prototype tyres where the tread is made from the dandelion rubber. It says the performance of these prototypes on snow and ice and in wet and dry conditions was comparable to the same tires built from hevea rubber. Continental is working on a five- to 10-year timeframe, by which time it hopes to produce a range of tires using TKS.

Versalis sees another route to rubber: using the exhausted guayule bagasse to produce bio-butadiene. This would create a sustainable alternative to one of the most important raw materials for the chemicals industry: butadiene.

Butadiene is produced on a massive scale (some 11 million tonnes a year, equal to the supply of natural rubber) and is the starting point for many synthetic rubbers, as well as being used in plastics and lubricants.

It is usually produced when oil is broken down into its constituent parts in refineries, but there have been concerns about potential shortages, as the US has moved towards shale gas. Shale gas yields smaller quantities of butadiene than does crude oil. But Versalis says any future shortage of fossil-fuel derived butadiene is of secondary importance to growing demand for products with a “higher renewable and sustainable footprint".

Using GMOs to make rubber

For the past three years Versalis has also been working with US biotech firm, Genomatica, to produce bio-butadiene from non-food biomass, using genetically engineered micro-organisms. It’s operating in a crowded field: at least seven consortia worldwide are pursuing the same goal. 

In February, Versalis announced what it described as "a remarkable milestone" for the rubber industry: it had not only produced significant quantities of bio-butadiene monomer from a fully renewable feedstock, but had polymerised it to make bio-polybutadiene. According to the Italian firm, this is the first time the rubber polymer has been made from a non-fossil fuel source.

Work is continuing to improve each stage of the process, and it has been successfully testing the bio-polybutadiene rubber both alone and as part of the preparation of other synthetic rubbers in its elastomers portfolio. Once this R&D phase is complete and all the results assessed, it expects to be ready to commercialise the process.

According to Martin: "Versalis is one of few companies in the world with the breadth of expertise to develop the concept of the bio refinery."

Michelin

French tyre maker Michelin also has ambitions to develop bio-based butadiene. It is a partner in a €52m (£45m) public research project called BioButterfly

Its goal is to lay the groundwork for a bio-based rubber industry in France. Not only do the partners want to produce economically competitive bio-butadiene for rubber manufacture, they want to adapt the process for all uses of bio-butadiene. Together with its partners, public sector research group IFP Energies Nouvelles and its spin-out Axens, Michelin aims to have an industrial demonstrator by the end of the eight-year project.

According to Philippe Esnault, director of strategic partnerships for materials and components, the project’s exploration phase is over and the next important decision is about building a process demonstrator to validate all the steps in the complete process chain.

Esnault says the challenges are "numerous and exciting". The partners have to design an industrial production pathway that "combines technical performance, respect for the environment and that is also economically competitive" with butadiene rubber.

There’s no decision yet on biomass feedstock, but Michelin says it has plenty of options, and will choose the source with the best sustainability characteristics.

One of the goals of BioButterfly is to reduce greenhouse gas emissions as well as the environmental impact of the entire production chain.

Race to develop bio-isoprene

The other area of intense research interest is bio-isoprene. The rubber industry makes its own synthetic version of nature’s polyisoprene (the primary chemical constituent of natural rubber) from petrochemical sources. Again there are several international consortia grappling with the task.

It’s not just a technical challenge: when prices for both natural rubber and oil are as low as they are now –making raw materials cheap –it’s difficult to justify the capital investment needed to produce a greener alternative.

One such casualty seems to be the partnership between Chemicals giant DuPont and tyre maker Goodyear. The two were far enough ahead in producing bio-polyisoprene to make a tyre that was showcased at the UN climate change conference in Copenhagen in 2009. Goodyear’s experience suggested that the bio-isoprene polymer was virtually identical in performance to the petroleum-based polymer. It was made by genetically engineered micro-organisms that could feed on plant products (such as glucose) to produce isoprene gas; this in turn was collected and purified. Its developer, US biotech firm Genencor was bought by DuPont in 2011. DuPont said in a statement: "Due to market forces, the two companies have chosen to suspend their partnership on the bio-based isoprene research."

However, others are ploughing ahead. The US biotech firm Amyris also makes bio-isoprene, using a genetically engineered yeast, so instead of producing ethanol, the yeast produces hydrocarbons. It feeds on a variety of plant sugars to produce renewable isoprene. This technology is showing "great promise", the company says.

Amyris has been working on developing a source of bio-isoprene with Michelin since 2011. In 2014, Brazilian Chemicals group Braskem brought its process and chemical engineering expertise to bear on the project. Michelin is tight-lipped about progress, saying only that it is "well on track". But it has begun a third phase, which will allow the partners to assess the competitiveness of the project on social, economic and environmental fronts.

The potential benefit is "improved ease of manufacturing and a more secure supply source in the face of volatile isoprene supply and pricing," suggests Amyris. Esnault says: "We are working today to integrate the lifecycle analysis so we can develop the bio-materials, not only to positively impact on carbon emissions from production phases but also to those relating to the tyre’s usage – including its end of life."

Amyris has another bio-based route to rubber: biomass converted to a molecule called farnesene, again using genetically engineered yeast. Japanese firm Kuraray has developed liquid rubber using farnesene, which has been tested by tyre makers. An Amyris spokesman said: "We’ve seen some indication that tyres made from it [liquid farnesene rubber] may last longer, with better wet-grip road performance."  Earlier this year Amyris announced that such is the demand for its farnesene, its plant capacity is sold out for the next four years . 

Renewable doesn't equal biodegradeable

These technologically complex and capital-intensive efforts to produce natural and synthetic rubbers from renewable sources will have major advantages in terms of carbon footprint, diversity of supply and logistics. But no one’s sure about further down the line.

Will they have a longer lifespan or be easier to recycle? Renewable doesn’t equal biodegradable. The tyre makers have little to say about those aspects of sustainability, although performance and lifespan of the new materials has to be at least as good as existing ones.

"There may be performance or lifetime benefits in other products but those discoveries could never be made unless a large/influential industry like the tyre industry helps to drive this technology forward," says Randall. 

Cornish is trying hard to encourage the use of guayule and TKS in non-tyre applications, which would present real opportunities for recycling, especially where the rubber doesn’t have to be as long lasting: surgical gloves would be one example. Nike has made a patent application for a guayule and synthetic rubber combination for footwear outsole, but the company says it is not using guayule in any commercial products.

In terms of its recycling potential, Randall expects guayule to be equivalent to hevea rubber. "But you can’t be satisfied with it being equal," she says. "That’s not a long-term feasible strategy."

For that, much more knowledge of the chemistry is needed, and who knows what the future holds? What is certain is that the big corporations with deep pockets will have a major influence on the direction of travel.

sustainable rubber  sustainability  Environment  climate change  Guayule  emissions  supply chains  technology  Chemicals  GMOs  bio-polybutadiene  BioButterfly