Making Blown Bitumen
Blown Bitumen begins its life much like any other asphalt. It is taken from the bottom of a vacuum distillation unit in a refinery, however blown asphalt is then taken through an additional oxidation processing step.
The temperature of the vacuum tower bottoms is maintained in the blowstill between 450°F and 500°F while air is blown into it. While this process is often called oxidation (resulting in oxidized bitumen) there are more reactions going on during the air blowing process. Some of these reactions include dehydrogenation, condensation, polymerization and many others.
Once the desired characteristics for that batch are met, the process ends and the asphalt is then sent to storage for shipment by truck or rail. It may also be used on site for the production of roofing products. Typical end point parameters include a softening point 215°F and a penetration of 20 for roofing grade asphalts. Common grades of paving grade asphalts are 95-25, 85-25, 90-40 and 115-15. Some paving grade asphalts are modified by adding polyphosphoric acid to blowstill prior to starting the air flow.
Properties of Blown Bitumen
There are several advantages to blown asphalt. One is that it is much more chemically stable than regular asphalt. This stability also translates into added durability.
One of the more interesting features that blown asphalt has is a higher softening point. This means that it can remain in place longer than alternatives. It retains the additional durability during the elevated temperatures, as well.
Blown Bitumen is completely water resistant, making it well suited to a variety of sealant and joint filling compounds. It is also more flexible than regular asphalt, which is why it is often used to make roofing products such shingles.
Uses of Blown Asphalt
Blown asphalt has a wider range of uses due to its water resistance, added durability and chemical stability. Approximately 90% of the asphalt produced in the US goes into paving and 10% goes into roofing products such as shingles and rolled roofing products.
There are many other special applications that include:
- Pipe coating enamels
- Concrete joints
- Waterproof lining for reservoirs and canals
- Sound dampening felts
- Anti-slip coatings