Autoignition temperature
Autoignition temperature is defined as the temperature at which a substance will ignite in the absence of an external ignition source such as a flame or spark. It is subjective and is highly dependent on the test method used. Probably the best known test method is ASTM E659 in which a small sample is introduced into a preheated glass flask. The flask is observed in darkness for ten minutes and ignition is deemed to have occurred if a significant temperature rise or a flame is observed. The test conditions take into account the ‘induction period’ observed in many substances before auto ignition occurs.
Although there are few published data, the auto ignition temperature of bitumen is around 400C. The temperature is largely independent of bitumen grade although the softer grade temperatures are slightly lower. It has been known for autoignition temperatures to be exeeded in bitumen boilers during roofing operations: this practice is highly dangerous, as well as having a deleterious effect on the properties of the bitumen.
| Grade | Temperature (°C) at viscosity in Pa.s (0.1 P) | |||||
| Penetration grades | 20 | 5 | 2 | 1 | 0.2 | 0.1 |
| 200 pen | 70 | 85 | 97 | 108 | 138 | 156 |
| 100 pen | 78 | 94 | 106 | 117 | 149 | 166 |
| 70 pen | 85 | 101 | 113 | 123 | 155 | 172 |
| 50 pen | 90 | 105 | 118 | 128 | 160 | 177 |
| 35 pen | 95 | 111 | 124 | 135 | 168 | 185 |
| 25 pen | 101 | 117 | 130 | 142 | 175 | 193 |
| 15 pen | 111 | 128 | 141 | 153 | 186 | 205 |
| Hard grades | ||||||
| H80/90 | 133 | 150 | 163 | 172 | 206 | 224 |
| H100/120 | 154 | 170 | 184 | 195 | 227 | 245 |
| Oxidized grades | ||||||
| R85/25 | 126 | 144 | 157 | 169 | 205 | 225 |
| R85/40 | 127 | 144 | 157 | 168 | 201 | 220 |
| R95/25 | 143 | 161 | 176 | 188 | 225 | 245 |
| R115/15 | 166 | 186 | 202 | 216 | (256) | – |
| R135/10 | 190 | 210 | 225 | 240 | – | – |
