It seems like everything these days is starting to be bio-based. And that’s good because resources like fossil fuels are finite and may one day no longer be an option to produce the materials we use every day. Materials like asphalt.
This is why researchers all over the world are looking at ways to produce a more sustainable asphalt product through different materials. The latest research from The Netherlands is showing that asphalt can actually be produced from plants. That’s a far cry from oil-based bitumen.
At the annual American Chemical Society meeting, researcher Ted Slaghek presented his lignin-based road concept.
A lignin is a common-but-complex material that makes trees sturdy and cornstalks stand.
Microalgae lingins have long been known for their use in applications like cosmetic dyes and food supplements. They reportedly account for nearly a third of all the organic carbon in the biosphere, and yet we really haven’t had much of a use for the material. It’s often burned to generate electricity at paper mills, but burning this material is not only wasteful, but releases soot and greenhouse gasses into the atmosphere.
That's why Slaghek and his colleagues are thrilled about their success in creating a lignin-based seal for asphalt mix.
Bio not Bitumen
Traditionally, a by-product of crude oil production called bitumen is the main sticky ingredient in asphalt and roof sealants. However, it's no secret that oil is a limited commodity, and even the by-product of its refinement may soon disappear.
"In the long term, we have to move to renewable products that we can harvest every year," Slaghek said. "It should be logical to use natural organic raw materials instead of crude oil."
Slaghek and his team used a hydrothermal liquefaction process, i.e. pressurized water (in a subcritical state), to transform this microalgae waste into a black, viscous, hydrophobic substance (bioasphalt) that closely resembles petroleum-derived asphalt. The process currently achieves a conversion efficiency of 55%.
Even though the chemical composition of bioasphalt is completely different from its petroleum-derived counterpart, they have similarities, including their black color. A liquid at temperatures exceeding 212°F, bioasphalt can be used to coat mineral aggregates; viscoelastic at -4°F to 140°F, it ensures the cohesion of the granular structure while supporting mechanical loads and relaxing stress.
This innovation offers a new possible option for the road building industry, which is entirely dependent on petroleum today. The types of bioasphalt developed so far relied on oils of agricultural origin (which could be needed for human nutrition) or from the paper industry, mixed with resins to improve their viscoelastic properties. Microalgae cultivation does not require the use of arable land so it offers an attractive solution.
Still, Slaghek and his team have some things to work out. As it stand, the team has developed lignin mixtures that can extend the lifespan of a road in warm temperatures, but in cold areas, these mixtures become exceptionally brittle, increasing the chances that the road will chunk and crack, potentially damaging cars. Not something we’d want in almost two-thirds of the US.
So the researchers are developing more than one lignin cocktail - a variety of asphalts for different climates.
The team also plans to build a 100-meter stretch of bicycle path this year using one of the lignin mixtures - a proof-of-concept that a sustainable "green road" can truly work out.