One of the questions that arises from the use of modified asphalts is to what extent it is possible to increase the level of polymer in the formula or degree of modification, to further increase the advantages that the modified ones present over the conventional ones.

In standard modified asphalts, the increase in the amount of polymer in the solution leads to an increase in the viscosity of the asphalt. The high viscosities make its handling in the industry impractical, since the asphalt must be heated to high temperatures so that it can be pumped and adhered to the stone aggregates when manufacturing asphalt mix. In a context where the price of fossil fuels and environmental responsibility are increasing, this does not seem to be a viable option.

There are SBS polymers with a high vinyl content (1) that have better compatibility with asphalt. This is due to the fact that it is made up of shorter chains compared to those of the SBS polymers present in the standard modified ones, which also translates into a lower viscosity of the solution (1). Short chains also make the reaction rate faster, further increasing compatibility. All this makes it possible to increase the amount of modifying agent in the asphalt binder without the aforementioned problems occurring. Also, the manufacturing methods are essentially the same with some additions that we will see in this section.

Intermolecular structure

Highly modified asphalts have some key differences that set them apart from standard asphalts, such as the aforementioned amount of polymer. Highly Modified Asphalt, HiMA, is made with 7.5% polymer by weight. From this value, a change in the intermolecular structure of the ligand is observed, which is essential to classify it as highly modified. The continuous matrix becomes formed by the SBS chains and the asphalt molecules become the dispersed phase, creating colloidal groups, as shown in Figure 1. This means that the mechanical properties of the binder are largely determined by the polymeric network, being able to show improvements with respect to standard modified asphalts, as shown by various studies (1,2). The continuous matrix change phenomenon is due to the strong interaction of the SBS polymers with the asphalt, which allows them to increase up to ten times their own volume when associated with maltenes.

Figure 1. Scheme of the transformation of the molecular structure of the binders with the increase of a modifying agent. Adapted from Field and Laboratory Study of High-Polymer Mixtures at the NCAT Test Track (2).

Bibliography

1. Scholten, Erik J., Vonk, Willem y Korenstra, Jan. Towards green pavements with novel class of SBS polymers for enhanced effectiveness in bitumen and pavement performance . Varsovia : 2nd International Conference on Environmentally Friendly Roads, 2009.
2. Timm, David H., and others. Field and Laboratory Study of High-Polymer Mixtures at the NCAT Test Track. Auburn : National Center for Asphalt Technology, 2012.