Rubber devulcanised by bacteria
A UK biotech company is launching a new technology that uses bacteria to remove sulphur from vulcanised rubber. Simon Frost reports.
When Thomas Hancock and Charles Goodyear were developing vulcanisation, undoing the process was not their priority. One of materials science’s most significant discoveries, vulcanisation is what allows rubber to be used in truck tyres, machine belts and shoe soles – but cross-linked sulphidic bonds remain difficult to reverse, widely limiting rubber’s end-of-life use to downcycling models such as crumbing for re-use in heat recovery or as a filler material in floorings. Now, a UK-based biotech company is launching a devulcanisation technology that uses a species of bacteria to selectively digest sulphur from rubber at room temperature.
‘Our scientists were busy looking at environmental remediation in a number of applications, and through this process they discovered an organism that lives in coalmines that has the ability to degrade certain kinds of hydrocarbon bonds,’ Barry van Bergen, CEO of Recircle Ltd, UK, told Materials World. The research was led by Professor Nick Christofi at Edinburgh Napier University, UK. ‘With a little bit of coaxing, in a very natural environment, they were able to get the organism to selectively digest the sulphur of rubber compounds,’ van Bergen explained.
While common rubber crumbing processes use combinations of extrusion, chemical processing, cryogenic grinding and heat to create a reusable product, Recircle uses ambient mechanical crumbing, before carrying out its bacterial process at 30°C without the use of chemicals. ‘Many of the processes that are out there create products that are quite difficult to incorporate in any degree of quantity, and typically they degrade the properties of the material, so you can really only use them as a filler at an incorporation rate that can be limited to as low as 5% in some cases,’ van Bergen said, although other processes allow incorporation at substantially higher ratios.
Recycled crumb has to be compounded with virgin rubber, but van Bergen claims that in addition to low incorporation rates, current recycled rubber crumb materials do not adhere well to virgin rubber, creating poor structural properties that limit their reuse to applications such as roadbeds and flooring for children’s playgrounds. The other dominant re-use of rubber is in heat recovery, which van Bergen describes as ‘a tragedy’, as less than 30% of the energy put into the material’s production is recovered.
In contrast, van Bergen said, the mild conditions used for Recircle’s process mean that it can conserve the rubber’s desirable properties such as elasticity, elongation and abrasion resistance, while achieving incorporation rates of up to 70% in high-quality rubber composites – other technologies claim similar incorporation rates, but for less demanding products such as replacement car mats, where a high specification is not required.
‘It’s a very gentle process, and the outer texturised surface creates a huge amount of surface area and fresh sites for re-bonding chemically with new rubber composite – that’s why we’re getting such promising structural properties,’ van Bergen said – although one rubber industry figure, who requested not to be named, played down this claim, noting that only around 1% of reactive sites in natural rubber are consumed during vulcanisation. Van Bergen responded, ‘There is some nuance here of course. If you continued and allowed more to be reacted, you get very hard rubber with unsuitable characteristics. So if you took this rubber, all ground up and not treated, you would simply drive it towards further curing, thereby altering the desired properties. Take the sulphur off those original bonding sites and you can form the appropriate level of cross-linking, rather than adding more.’
Martyn Bennett, Chief Scientist at ARTIS, UK, told Materials World, ‘Incorporation of crumb into a matrix has been challenging people for years – if you get it small enough it works well, if you do it quickly enough it bonds well, but both options require additional cost to the basic crumb. Strictly speaking, you don’t need or even want to fully devulcanise the rubber – all you need is active sites on the surface to form a good bond to the matrix. Doing that effectively is the key. I would be very keen to see the results of 70% fill, by weight or volume.’
Bennett summarised that, ‘As a scientist, I remain sceptical until I have seen the evidence and I still have several questions unanswered, but that doesn’t mean I don’t think it’s a great idea – if it works commercially, it will be fantastic.’ Bennett is eager to see data on the treated crumb alongside control information against untreated crumb of the same size. ‘If the bugs are efficient, a continuous process could be developed with a suitable throughput and appropriate pre-processing such as ultra-high-pressure water crumbing. What about the energy balance – it’s a wet process and the material has to be dried before use […] in short, it’s a fantastic idea on paper – let’s see the data.’
Recircle’s process requires a strictly controlled feedstock – specific rubber formulations will be processed separately to allow for rubber composites to be reproduced with their particular properties intact, ‘For example, taking material from part of a mining truck tyre and then putting the reclaimed material back into the same part of a new mining truck tyre – rubber from a car tyre wouldn’t provide the suitable formulation,’ van Bergen said.
Bob Kind, Technical Director of Polymer Recyclers Ltd, UK, told Materials World that his company patented ReTern, a process for converting rubber crumb into a masterbatch for use as a substitute for rubber hydrocarbon in rubber compounds in 2007, but market economics and a lack of investment and customer interest saw it fall by the wayside. Although ReTern was not a devulcanising process, Kind notes that similar claims of quality were made for ReTern as those being made for Recircle.
In addition to a substantial and strictly controlled feedstock, Kind says that for Recircle’s technology to be a success, it must create very similar properties to the original material, ‘If there is a deficiency, then this may be compensated by judicious compounding,’ he noted. Kind also said that the product must be uniform and comply with material specification, and that the overall cost must constitute a significant saving.
Perhaps today the market is in a better place to receive a new recycled rubber technology. With the advent of electric and autonomous vehicles, demand for natural rubber in the tyre industry is likely to accelerate, although forecasting rubber consumption is notoriously difficult, particularly in terms of synthetic versus natural.
The International Rubber Study Group (IRSG) reported in March 2017 that global natural rubber production would reach 12.9 million tonnes in 2017, compared with the 12.4 million tonnes produced in 2016. Synthetic rubber accounts for even more, bringing the total global production of rubber up to more than 29 million tonnes. In its 2016 World Rubber Industry Outlook report, the IRSG forecast that while total rubber consumption will increase by an average of 2.7% annually to 2025, the outlook for natural rubber supply will be sufficient to meet the increased demand, predicting a surplus of a million tonnes by 2020.
A study by the University of East Anglia and University of Sheffield, UK, published in Conservation Letters in 2015, however, estimated that 4.3–8.5 million hectares of additional rubber plantations would be required to meet projected demand for rubber by 2024. In equivalent terms, van Bergen explained, ‘We might need to plant a country roughly the size of Ireland to meet the demand in 2024. The scale is just huge, and the use of land for rubber plantations is in competition with food production. We have a technology that could properly put the product back into use and reduce the need for primary raw material.’
Recircle is already working with customers spanning multiple sectors, rubber types and applications. ‘Feedback from customers on testing and incorporated product quality has been great,’ says van Bergen, ‘and we think there’s a bright future ahead.’
Recircle’s website dedicated to this new technology goes live this month at www.recirclerubber.com