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11 Table 2-2. Summary of conditions investi- 1. Condensation of volatile compounds on the lid of the SAFT gated during the selection study. before they entered the air-cooled condenser, Test Conditions Investigated 2. Inefficiency of the air-cooled condenser allowing volatiles Modified German Rotating Morton versus smooth flask to pass completely through the VCS, Flask Rotational speed 3. Production of fewer volatile compounds in the SAFT com- Mixing enhancers Scrapers pared to the RTFOT due to the shorter duration of the Stirred Air Flow Test Impeller type SAFT conditioning procedure and the lower airflow rate Position of air supply Rotational speed of impeller used in the SAFT, 4. Rapid saturation of the small air bubbles produced in the SAFT with volatiles so that they are not able to absorb addi- · Visual assessment of the degree of mixing during the test, tional volatiles as they move upward through the binder, and · Visual assessment of separation for the two polymer modi- 5. Suppression of volatilization caused by the build up of air pressure in the SAFT. fied binders, and · Potential for implementation as a specification test. The VCS study consisted of a series of small experiments to evaluate these potential causes and to design a more effective The second part of the selection study was a formal exper- iment designed to address whether the degree of aging in the VCS for the SAFT. The product of the VCS study was an prototype long-term versions of the tests was affected by the improved VCS system employing reusable adsorbents that large differences in viscosities for neat and modified binders are commonly employed for chromatographic analyses. at the selected aging temperature of 100°C. To emphasize the Findings from the VCS study are presented in Chapter 3. The significance of the "viscosity effect," in the PAV condition, VCS study is documented in Appendix C (available on the the unmodified binder has the consistency of light cream TRB website). whereas the modified binder has the consistency of molasses. Versions of the long-term tests judged successful based on the 2.5 SAFT Optimization Study first part of the selection study were subjected to this formal experiment. In this experiment, the PG 58-28 and the SBS- During NCHRP Project 9-36, the Texas Department of modified PG 82-22 were aged in the prototype long-term ver- Transportation contracted with James Cox and Sons, Inc., to sion of the test and in the PAV. Rheological measurements at produce a commercial version of the SAFT. Figure 2-4 shows high, intermediate, and low pavement temperatures were photographs of the prototype and commercial versions of the used to compare the level of aging to that produced by the PAV. SAFT. The major difference between the prototype and com- Replication was included in this experiment to permit statisti- mercial versions is how the binder in the aging vessel is heated. cal analysis of the differences in aging that were observed. In the prototype device, the aging vessel was heated by direct Based on the results of the selection study, the SAFT was contact with a heating mantle, while the commercial version chosen for further development as an improved procedure for used an oven to heat the aging vessel. Because the aging vessel short-term aging of binders. Relevant findings from the selec- was in direct contact with the heating mantle in the prototype, tion study are presented in Chapter 3 of this report. The selec- the aging vessel and the binder in contact with it were exposed tion study is documented in detail in Appendix B (available to very high temperatures during the heat-up portion of the on the TRB website). test. In the commercial version, the oven was limited to a tem- perature approximately 13°C above the test temperature, so the temperature of the aging vessel and the binder in contact 2.4 Volatile Collection with it were much lower. This resulted in less aging in the com- System Study mercial version compared to the prototype for the same oper- The prototype SAFT included a volatile collection system ating parameters. The difference in aging between the proto- (VCS), which consisted of a copper coil condenser operated type and commercial versions of the SAFT necessitated a study at ambient temperature. Data published for several binders to optimize the operating parameters of the commercial version during the development of the SAFT showed that the mass of of the SAFT to reproduce the aging from the RTFOT for neat volatiles collected was a factor of 10 lower than the mass change binders. This study was called the SAFT optimization study. in the RTFOT (1). Since mass change during the RTFOT The SAFT optimization study included two parts. The first includes mass loss due to volatilization and mass gain due to part was a series of tests to verify that the heat-up phase of the oxidation, the RTFOT mass change was expected to be less test does not result in significant aging of the binder. During than the mass of the volatiles collected with the SAFT. Several the heat-up phase, the temperature of the binder is increased possible causes for this discrepancy were identified including from approximately 100°C to the testing temperature of 163°C