Lignin, a plant molecule that makes trees sturdy and vegetables crunchy, could help make our roads and roofs more sustainable. Researchers at the Netherlands Organisation for Applied Scientific Research (TNO) have integrated lignin into the mixture currently used to create asphalt. Their findings were presented at the 249th National Meeting & Exposition of the American Chemical Society held last month.
Bitumen, the main ingredient in asphalt and roof sealants, is a by-product of crude oil production. But oil is a non-renewable source, and fluctuations in the oil market have made it difficult to get high-quality bitumen. So a team at TNO, led by senior scientist Ted Slaghek, sought out ways to alleviate the problem.
[L]ignin 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 pollutant, he says.
Lignin shares a lot of similar traits with bitumen, and so could be considered an environmentally-friendly option for maintaining – and even improving – the quality of our roads for relatively cheap.
And the asphalt can be tailored to suit regional needs, depending on the climate conditions that affect the roads.
In order for any of the lignin-bitumen mixtures to work, lignin must be incorporated into the bitumen at the molecular level. The type of lignin used doesn’t factor in – whether it is from pine, straw or wood pulp, lignin’s usefulness comes from how it is extracted. He and his team are currently patenting their extraction method, which works for roads, roofs and sealant, as the bitumen used in all three is the same.
He and his team are currently working with local government in the Netherlands to test the lignin-based asphalt on a wider scale, with plans to build a stretch of bicycle path, parking spot or section of road with one of the mixtures.
Tweaking lignin also plays a role in our quest for sustainable alternatives to petrol – although its sticky rigidity is actually seen as a bit of a problem here. Scientists are using cellulose from plant waste to create ethanol, but cellulose is fused together by lignin. So they are genetically engineering plants with lignin that breaks down easily in order to isolate the cellulose.
Megan Toomey is a freelance journalist based in Melbourne.
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