By Penny Carmichael, on 5 November 2013
– Article by Jack Humphrey
Bitumen has been used by the Egyptians for mummification, by the French to cure vine rot, by the newspaper industry for ink and by BMW for soundproofing. It has a wide range of uses thanks to its curious balance of properties. This week our talk was given by Dr Ian Lancaster, the technical manager of the UK arm of the Swedish oil products firm Nynas.
Bitumen (or Asphalt in the US) is the residue left after crude oil has been processed. It it very difficult to define chemically but a rule of thumb is that it can contain any hydrocarbon with a length greater than 25 carbons – a possible 36 million molecules before considering stereochemistry. The classical definition describes bitumen as a colloid, a substance dispersed throughout another substance – milk being an example. The colloid model has large bulky “asphaltenes” dispersed throughout “maltenes”, which may be oils, aromatics and resins. The difficulty in testing this model is that bitumen is black and so light microscopy cannot probe it. Nynas prefer to use solubility parameters to classify its bitumen. By titrating in three different solvents a three-dimensional “solubility sphere” can be constructed. In this way it is possible to discern the asphaltenes from the maltenes. Another way to observe the components of bitumen is to fractionate it in different solvents. Different mixtures can give different properties. The rigidity of bitumen is given by the asphaltenes. The resin component acts as an emulsifier. The aromatic compounds with their polar groups give the adhesive properties. The saturated oils give bitumen its fluidity. As well as hydrocarbons there are trace amounts of metals such as nickel and vanadium. It is possible to fingerprint a source of bitumen by the specific concentrations of these metals.
Bitumen ages over time and this is what makes potholes a problem. Heat, oxygen and UV light all affect bitumen, making it more brittle. This can be due to evaporation of the saturated oils, through polymerisation of the asphaltenes and through other chemical changes.
Bitumen’s most useful property is its adhesion. By combining with rock aggregate the bitumen acts as the glue to form the strong material used in roads and in roofing. This is due to hydrogen bonding between carboxyl groups in the bitumen with hydroxyl groups in the aggregate. If water can get in between the bitumen and the aggregate then it can displace the bitumen. This is called delamination and can be observed in potholes. The inclusion of additives helps to reduce this process.