Put a chemist and road-builder together and you have a powerful mix. By joining forces in the laboratory, Dr Ted Slaghek and Dave van Vliet of TNO created a type of bitumen, with improved properties, based on lignin.
A study by TNO, the Netherlands Organisation for Applied Scientific Research, shows that the plant molecule lignin in asphalt and sealant mixtures helps roads and roofs hold up better under various weather conditions and make them more environmentally friendly.
Slaghek says that in the long term, industry will have to move to renewable products that it can harvest every year and it should be logical to use natural organic raw materials instead of crude oil.
Slaghek explains that lignin is a renewable resource that makes up as much as a third of the dry material in trees, where it keeps out water and binds together other components of plant biomatter, like cellulose. Lignin is also plentiful and therefore, inexpensive because it is removed as a waste product during the paper-making process. More than 50 million tons are produced globally as waste each year. Most of this is burned to generate electricity to run the paper mills. Burning lignin is not only wasteful, it releases soot and other pollutants, he says.
Because lignin shares many characteristics with bitumen, it could become an environmentally friendly additive to help reduce the amount of bitumen needed for construction activities. Like bitumen, lignin is a large molecule with a number of carbon rings.
The researchers first thought that this similarity meant that they could just add lignin to the bitumen, like other polymer additives that are already used to improve sealants, but Slaghek says that for the mixtures to work, lignin must be integrated into the bitumen on the molecular level, not just mixed into it. By integrating the lignin, he can reduce the amount of bitumen needed by as much as half, although he found that the best-performing mixtures required less.
As with other additives, lignin makes sealants perform even better-but those polymer additives come from petroleum sources, making them just as problematic as bitumen.
Slaghek’s team has developed a number of lignin-bitumen mixtures that make the asphalt harder in warm weather, preventing rutting and adding a few years to a road’s lifespan.