If one accepts the old adage “The more things change, the more they stay the same…” is true, does it then follow that “the more things stay the same, the more they (can) change…” Any truthful response to this question will of course depend on one’s perspective. For instance, take the tire. Since the invention of the wheel sometime between 5000 and 4000 BC, it has remained, for all intents and purposes, round; no doubt a critical feature to its performance. Historical adoption of the tire as a vehicle of utility has, from its inception, been mostly dictated by the evolution of the prevailing supporting infrastructure, i.e. the design and construction of roadways. However, since the wheel’s inception its material of construction (MoC) has only gradually evolved from readily available, but brittle stone, to solid, grainy wood sometime after 3000 BC, to the light-weight spoke-reinforced wood wheel of the Bronze Age around 2000 BC, and then only recently to the rubber-based, pneumatic tire introduced in the late 1870s. The single most important technological event that enabled the pneumatic rubber tire came from Charles Goodyear who, in 1839, invented vulcanized rubber. As a point of reference, the world’s most recognizable tire maker Goodyear, of no relation to the inventor of vulcanized rubber, was only founded in 1898.
To this day, the vulcanized rubber tire itself continues to undergo significant proliferation and technological evolution. Engineering and formulation performance enhancements have led to the tire impacting almost, if not all facets of modern society from transportation, to manufacturing, to international commerce, among many other things. World demand for tires is forecast to rise 4.7% per year through 2015 to 3.3 billion units with a corresponding aggregate value over the same span to $220 billion, compounding at 6.5% annually.1 Motor vehicle tires account for roughly 60% of that aggregate demand, and the Asia-Pacific region accounts for over half of global tire demand and therefore is the largest progenitor of used tires.1 In 2003, the US alone generated nearly 290 million used tires of which 130 million were incinerated for their fuel value, 100 million were recycled via shredded crumb after removal of the supporting steel belts into new products such as playground mulch, running tracks, and roadbeds.2,3 At the Federal level, the EPA established a waste management hierarchy, in order of preference of reduce, reuse, recycle, waste-to-energy, and appropriate disposal. Like many US states, Georgia fully embraces practical, environmentally-conscious means of recycling scrap tires and recently amended road construction specifications to include recycled tire rubber crumb as an alternative to conventional oil-based polymers for road asphalt production.4 Rubberized road asphalt provides the unique benefits of reduced pavement thickness for a given traffic flow, replaces higher cost oil-based materials, requires less maintenance during the service lifetime of the road, and reduces noise to increase the quality of life for nearby residents. Clearly, used tire crumb is a useful store of an engineering solution.
In a very compelling development, Plenio et al. recently demonstrated the feasibility of an alternative means of extracting significantly greater value from spent vulcanized tires5 than that offered by current pyrolysis and engineered crumb recycling methods. In an elegant “proof-of-concept”, the authors employed two metathesis-active species, the Grubbs-Hoveyda complex 1 and a new, electron-deficient NHC containing complex 2 shown in Scheme I under an ethylene atmosphere in a process called ethenolysis to catalytically breakdown the highly cross-linked structure of spent End of Life Tire (ELT) crumb as depicted in Scheme II. Notably, the steel-free ELT crumb is the same as that used for road asphalt applications described above. The authors were able to recover up to 50 wt% of granulate crumb as a toluene soluble oligomer solution. On a weight basis, a typical pneumatic tire contains roughly 14.5% steel, 28% carbon black, 16.5% of various fillers, and 14% natural and 27% synthetic rubbers, respectively. Therefore, assuming all other formulation components are still present, ethenolysis of ELT crumb using either 1 or 2 gives rise to > 90% organic polymer mass recovery.The authors selected the two catalysts based on a variety of prior screening reactions using squalene and pristine natural rubber as models with nearly identical oligomeric distributions compared to the ELT ethenolysis results.6 The ethenolysis process and its variant alkenolysis have found utility in a variety of controlled molecular weight reductions and transformations such as main chain-polyolefin depolymerization7 and the cleavage of internal double bonds for a variety of specialty chemicals such as 9-decenoate derived from renewable plant oils.8
Most notable though in Plenio’s work is the significant reactivity and selectivity of each catalyst in breaking down the sterically congested tri-substituted olefin bond of the natural rubber’s polyisoprene component. The authors observed that at 0.0074 mmol, or about 0.5 wt%, of either catalyst 1 or 2 per gram of ELT crumb charged gave rise to 0.5 grams of organic solubles primarily composed of valuable oligomeric 1,4-cis isoprenes. The preferred conditions employed 6.0 grams of crumb, or granulate of a respective particle size suspended in 150 mL of toluene at 80°C under an ethylene pressure of 7 bar for 4 to 20 hours. Therefore, ethenolysis of vulcanized ELT rubber offers significantly higher value than the simple BTU content of thermal recycling described above. MS analysis of the soluble fraction showed very similar oligomeric 1,4-cis isoprene species to that obtained from natural rubber suggesting cross-metathesis, even in solution, was not competitive. The RHC=CHR fragments observed are attributed solely to competitive synthetic polyisobutylene self-metathesis processes. Work by Wagener9 and Coughlin10 predating that of Plenio’s described here may further contribute to an optimized, solvent-free process devoid of terminating side reactions, respectively, that is readily amenable to scale-up and real-life application.
The authors also noted that both the molecular weight and amount of the solvent soluble fraction were inversely related to the ELT crumb granulate size. Specifically, for <0.5 mm and >1.5 mm granulate sizes, the yield of organic soluble fraction ranges from 54% to 38%, and the DP from 15 to 8.4, respectively, thus reflecting competitive heterogeneous and homogeneous ethenolysis processes for varying granulate surface area to volume ratios. At catalyst charges greater than 0.0074 mmol/g ELT, solution ethenolysis processes become predominant for a given granulate size suggesting either “active site” saturation, diffusion limitations, Michaelis-Menten kinetics, mass transfer limitations, or passivation layer buildup. At the end of the ethenolysis, the catalyst and insolubles are simply removed via silica plug filtration. Isolation and characterization of the ethenolysis products by preparative HPLC and 13C NMR followed by elemental analysis gave a composition of C5H7.8 close to the C5H8 of the isoprene subunit. The sulfur content in the ELT, the ethenolysis product, and the soluble component were determined by ICP-OES to all be around 2.2%. Along with nitrogen at <0.1%, total C, H, N, and S sum to 96.5%, the balance being attributed to oxygen derived from adventitious oxidation over many years of service on the road.
If past history is any indicator of future trends, one can expect the rubber-based pneumatic tire to be with us for quite some time to come. Clearly, if left unchecked, issues associated with spent rubber tire will only become exacerbated with continued population growth, changing demographics, and increased applications and performance demands by various markets, end-users, and consumers. In an effort to enhance a Green Culture, these same authors among many others are working next to shuttle or bus the recovered terpene subunits to make useful and value-added flavor and fragrance additives, pheromones, or even active pharmaceutical ingredients. Ruthenium-based metathesis may just be the vehicle that allows for the elusive, efficient two-way street to recycle, or bus synthetic and natural rubber building blocks back to their valuable, basic units such as 1,4-cis isoprene so they may go around and around and be used over and over again…to meet society’s ever-growing variety of needs!
1 Freedonia Group in Rubber World February 29th, 2012.
4 Rubber World February 21st, 2013.
5 Wolf, S. and Plenio, H. Green Chemistry 2013, 15, 315-319.
6 Wolf, S.; Plenio, H. Green Chem. 2011, 13, 2008-2012.
7 A). Grubbs, R.H.; Nguyen, S.T. US5,728,917, March 17, 1998. B). Pawlow, J.; Hall, J.; Poulton, J. US8,058,351 B2, November 15, 2011. C). Ong, C.M.; Guo, S.X.; Guerin, F. US20040132891 A1 July 8, 2004.
8 A). Thomas, R.M.; Keitz, B.K.; Champagne, T.M.; Grubbs, R.H. J. Am. Chem. Soc. 2011, 133, 7490-7495. B). Burdett, K.A.; Harris, L.D.; Margl, P.; Maughon, B.R.; Mokhtar-Zadeh, T. Saucier, P.C.; Wasserman, E.P. Organometallics 2004, 23, 2027-2047. C). Grubbs, R.H. Nguyen, S.T.; Johnson, L.K.; Hillmyer, M.S.; Fu, G.C. WO9604289, 1996. B0.
9 Watson, M.D.; Wagener, K.B. Macromolecules 2000, 33, 1494-1496 and references cited therein.
10 Craig, S.W.; Manzer, J.A.; Coughlin, E.B. Macromolecules 2001, 34, 7929-7931 and references cited therein.