Asphalt is commonly derived from crude oil near the end of the refining process. Although used for many products, its two main uses are for roofing and paving roads. Depending on how it is used, asphalt is mixed with varying amounts and types of volatile and flammable solvents. A particular type of asphalt will therefore have specific fire and explosion hazards. The fire and explosion hazards of asphalt products are a concern during manufacturing, storage, and transportation.
The flash point of an asphalt product provides a measure of its flammable and explosive potential. The typical flash point test consists of heating an asphalt sample in a test cup at specified rates of temperature increases and passing an ignition source across the cup at specified intervals. The flash point is the lowest temperature of the liquid sample corrected to a barometric pressure of 760 mm at which an ignition source causes the vapors immediately above the sample to ignite.
PMA 5 Flash Point Tester
Flash point testing for asphalt products has had problems with repeatability. Of particular concern is the flash point testing of asphalt used for roofing shingles. Asphalt used for roofing has high softening points, ranging from 104 to 230 °F. These asphalts are produced by oxidation, or air blowing, which is a highly temperature-dependent process—the oxidation rate increases with increasing temperature. The process requires blowing air through tanks of hot asphalt (around 500 °F) for 30 min to 12 hr. Since the reaction is exothermic, it also requires cooling. The maximum temperature of asphalt during oxidation is usually maintained at least 50 °F below the flash point. Therefore, an accurate flash point measurement is an extremely important safety concern for the air blowing process.
Flash point testing was typically performed using open-cup testing that followed the ASTM International Standard Test Method D 92 (ASTM D 92). Product specialists at Anton Paar have been using an automatic Pensky-Martens closed-cup tester and modifications of the ASTM D 93 testing standards to optimize the flash point testing of asphalt.
Advantages of a Closed-Cup Method
- Provides a more accurate flash point because the volatiles do not escape as easily; therefore, lower flash points are obtained, which leads to better, safer control of asphalt handling
- More closely resembles asphalt storage conditions
- Decreases risk of exposure to hazardous airborne particles produced during heating of the sample
Advantages of an Automated Closed-Cup Tester
- Improved repeatability—operator error is removed
- More efficient—with a manual unit a well-trained operator is required to run the entire test, meaning he or she is exclusively dedicated to only that instrument
- Automatic barometric pressure correction
- Real-time measurements displayed graphically
Here are additional tips on how to optimize flash point testing using a closed-cup tester and the ASTM D 93 standard:
Flash Point Testing is Dynamic
- According to ASTM D 93: the “…flash point test is a dynamic test method that depends on definite rates of temperature increases to control the precision of the test method.” Test results are also affected by sample size, stirrer speed, and rates of applying the ignition source
- Follow the ASTM D 93, Procedure B standards, but adjust the following parameters: try varying the rates of heating gradient (higher rate ≤300 °F and lower rate ≥300 °F) and stirring (115–240 rpm), and start to apply the ignition source at 30–40 °F below the expected flash point
- Standardize your test procedure to provide reliable results
Proper Handling of the Test Sample
- Don’t overfill the test cup. This is tricky because of the need to wear thick protective gloves when handling very hot liquid asphalt
- Remember to preheat the test cup on a hot plate at 140 ºF for 10 min before adding the hot sample. Some types of asphalts will contract during filling of the measuring cup at ambient temperature. This can affect the repeatability of test results between operators
- Asphalt containers to be sampled for testing should be tightly sealed and stored at temperatures and filled to levels specified by ASTM D 93
Inconsistency is the Enemy of Repeatability
The flash point test for asphalt using an automated closed-cup tester and modifications of ASTM D 93 should be standardized for your company and then adhered to rigorously. In the end, the goal is for your company to have a universal method that you can use to compare flash point test results between laboratories and operators so that you can safely store and handle your asphalt products.
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