The properties of asphalt can be modified by air blowing in batch and continuous processes. Blown asphalt has more and stronger molecular interactions than the original bitumen and is, therefore, more cohesive. Blowing causes the softening point to increase and the penetration to decrease.
However, the increase in softening point is usually more than the decrease in penetration. This means that blowing reduces the temperature susceptibility of bitumen. The effectiveness of blowing depends largely on the original bitumen (i.e. the original mixture of molecular structures).
The regular distillation process is discontinued at some point while the topped crude is still liquid. The heavy fraction is then put into a converter and air is blown through it while it is maintained at a high temperature.
This process is continued until the asphalt has attained desired properties. Often, such asphalts are called Blown asphalt or Oxidized bitumen. However, this is not strictly a true term because not only oxidation but also vaporization, Dehydrogenation, condensation, polymerization and other reactions occur during the air blowing process.
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Because bitumen is used in a number of roofing products and systems calling for different physical properties, both straight-run bitumens, and blown asphalt processed to a range of viscosities, are used in roofing. Historically, both straight run and oxidized bitumens have been manufactured by petroleum refineries, and this remains largely true in Europe today.
A separate description of the global bitumen manufacturing industry describes North American and European bitumen manufacturing processes, and the chemistry and physical properties of straight run and oxidized bitumens, from the refiner’s perspective.
In North America, most oxidized bitumen is produced downstream of refineries, by roofing product manufacturers and other bitumen processors who acquire straight-run bitumen from refineries for use as feedstocks for oxidation operations.
Chemistry of Air Blowing Oxidation, or air blowing, is the process of passing air through heated bitumen to raise the softening point/viscosity of the bitumen while maintaining much of its flexibility at lower temperatures. The use of this process dates from the late 19th century and it is used to make a wide variety of products with markedly different softening points depending on the extent of oxidation used.
On one end of this continuum are very slightly blown asphalt used for felt saturant and modified bitumen membranes, which are oxidized to softening points below 50°C, in the range of air rectified bitumens.
The hardest bitumens typically used in roofing products BUR products meeting North American specifications for Types III and IV, and shingle coatings are oxidized to softening points of roughly 100°C.These products, however, actually lie at an intermediate point on the continuum of bitumen oxidation.
At the upper end of the range are tired processing aids and drilling mud additives oxidized to softening points around 150°C. It is difficult to achieve softening points higher than this with commercial oxidation equipment operated at standard temperatures.
Blown Asphalt Production Process