Recent Improvements in Hot Applied Crack and Joint Sealant Materials

Abstract: Most hot applied crack and joint sealant materials are formulated from asphalt as the base resin, and rely primarily on empirical tests to target the necessary mechanical and thermal properties needed for performance in the roadway. Examples of commonly used empirical tests are the ring and ball softening point for estimating resistance to flow at high temperatures, and cone penetration to assess flexibility at intermediate and low temperatures.

More recently, innovators have been targeting improvements in hot applied crack and joint sealant materials by taking advantage of readily available raw materials not based on asphalt, but rather on renewable raw materials, while at the same time, exploring non-empirical, rheology-based performance tests that better predict mechanical and thermal performance in the field. For example, pine rosin ester base resins derived from the pulping of softwood trees in the paper making industry are a readily available, renewable, raw material that can provide unique performance attributes in hot applied crack and joint sealants. Sealants formulated from pine rosin esters as the primary base resin, along with suitable plasticizing oils to improve flexibility at low and intermediate temperatures, have been found to be surprisingly compatible with styrene-butadiene block copolymers. Crack and joint sealants formulated from pine rosin esters suitably modified with plasticizers and polymers give unique rheological performance, both at high and low temperatures, along with inherent resistance to oxidative aging that may promote longer term performance in the field compared to asphalt based crack and joint sealants.

Properly formulated pine rosin ester-based crack and joint sealants have been found to have an unusually enhanced ability to relax stress at low and intermediate temperatures by undergoing viscous deformation or flow that is constrained by a properly developed polymer network. This behavior is easily seen in “Black Space” Diagrams where complex stiffness (G*) is plotted versus the phase angle collected using the dynamic shear rheometer (DSR). Data indicates very high phase angles, in excess of 70° or even 80° at stiffness values corresponding to low and intermediate temperatures at which the crack and joint sealant will sustain most of its exposure in the field. Meanwhile, the level of permanent, unrecovered strain at high temperatures, as measured using the DSR Multiple Stress Creep and Recovery (MSCR) test at 80°C shows very low levels of unrecovered strain. This performance is believed to be due to the development of a strong polymer network from the high degree of chemical compatibility between the polymer and the base resin. In summary, the pine rosin ester formulated sealants show excellent visco-elastic behavior.
Finally, innovators have found that properly formulated sealants demonstrate a unique ability to resist the effects of oxidative aging when compared to standard, asphalt-based sealants. Following aging in the Pressure Aging Vessel (PAV), these pine rosin ester sealants show Glover-Rowe aging parameters that are typically less than half those demonstrated in asphalt based sealants. Furthermore, retained ductility are typically 75% or greater for pine rosin ester based sealants, and 25% or less for asphalt based sealants.