Asphalt paving compositions, which include a mixture of asphalt binder and aggregate, have long been employed in the manufacture of pavements. The asphalt binder compositions often included polymeric additives and are therefore often referred to as modified or polymer-modified asphalt binder compositions. The presence of polymeric materials is believed to improve the behavior of the pavements, particularly at extreme temperatures.
While elastomeric polymers having low crystallinity are often advantageously employed, the presence of high Tg polymers also produces benefit. Therefore, particularly useful polymers include block copolymers including low Tg amorphous segments or blocks and high Tg amorphous segments or blocks. For example, block copolymers including a block of styrene and a block of butadiene may be used including polymers such as block styrene-butadiene-styrene (SBS).
In manufacturing the modified asphalt binder compositions, polymer is introduced with molten asphalt. Under sufficient conditions of mixing and/or shear, the polymer is dissolved and/or dispersed into the molten-asphalt composition. It is desirable to uniformly disperse the polymer throughout the composition.
It has been discovered that certain polymers, such as styrene-butadiene-styrene block polymers do not disperse as efficiently into the molten asphalt. This is especially true with high molecular weight polymers. As a result, greater energy requirements and time are required to achieve a desirable dispersion of the polymer within the asphalt.
Conventional approaches to improving the dispersibility of the polymers into the molten asphalt includes the use of lower molecular weight polymers, which generally disperse more readily into the asphalt composition. Unfortunately, by using lower molecular weight polymers, the advantages sought by the use of a higher molecular weight polymers cannot be fully achieved. For example, certain binder and/or pavement performance ratings cannot be achieved with low molecular weight polymers.
Thus, there is a need to improve the dispersibility of polymers, particularly polymers including high Tg blocks, without deleteriously sacrificing performance attributes that are advantageously achieved with higher molecular weight polymers.