Modifying asphalt with SBS polymers is not “asphalt + SBS + mixer”: the result depends on the chemistry of the base asphalt. SBS needs a sufficient aromatic fraction to dissolve and swell; if the base is paraffinic, the polymer does not disperse, separates in the tanks and does not improve performance. That is why the first step is a SARA test in the laboratory, not switching on the colloid mill.
This article was originally published in our construction digital magazine, within the editorial line dedicated to civil engineering and pavements.
The analysis is by Eng. Pablo del Águila, international transportation infrastructure consultant and pavement specialist, expert in asphalts, concretes and materials, CEO of Grupo Empresarial Camineros and director of the Escuela de Camineros in Florida, USA.
You can also read the version published in the December 2025 issue of Construcción en Panamá magazine (in Spanish).
Why does the base asphalt determine whether modification succeeds?
In conventional asphalt, asphaltenes are dispersed in a maltene phase (saturates, aromatics and resins). Resins, rich in polar nitrogen, sulfur and oxygen compounds, stabilize the asphaltenes and prevent them from flocculating.
Adding SBS does more than incorporate a polymer: it changes the solubility balance of the maltenes, redistributes aromatics and resins between the asphalt and the polymer, and can break the colloidal equilibrium. If the base was already near its stability limit, the polymer captures part of the aromatics and resins, the asphaltenes flocculate and phase separation, sediment and erratic binder behavior appear.
Starting point: the SARA test
The SARA test determines the asphalt fractions: Saturates, Aromatics, Resins and Asphaltenes. It can be performed, for example, by thin-layer chromatography with flame ionization detection (TLC–FID) using IATROSCAN equipment.
It shows whether the base is predominantly paraffinic (high saturate content) or aromatic (high aromatic and resin content). According to Del Águila, that distinction “is the difference between an asphalt that will be modified easily and one that will only cause problems.”
What does SBS need to work?
Aromatics and, to a large extent, resins act as the polymer’s solvent. If SBS dissolves and swells properly, a continuous and stable polymer–asphalt network forms, providing:
- A real improvement in modulus and elastic behavior.
- Greater rutting resistance.
- Better fatigue performance.
- Stability during tank storage.
Without an adequate aromatic base, SBS becomes an uneconomical solution with poor and unpredictable results.
What happens if the base is paraffinic?
- The polymer has limited solubility and does not fully disperse or swell.
- Phase separation appears during storage, with polymer layers even with agitation.
- Laboratory Brookfield viscosities do not represent actual plant behavior.
- Viscosity rises, but without gains in rutting, fatigue or thermal cracking.
- Operational problems increase: excessive temperatures, poor workability and batch-to-batch variability.
In short: a lot of effort and cost for a “modification” that is not really one.
How to modify a paraffinic base
If only a paraffinic base is available, the technical route is to prepare the base first and then add the polymer:
- Adjust the composition: add aromatic oils or extracts in controlled proportions, checking penetration or PG grade, viscosity at 135–165 °C and, if possible, a new SARA test.
- Use compatibilizers judiciously: small doses of polyphosphoric acid (PPA) or sulfur can stabilize the network, but excess PPA pushes the binder toward plastomeric behavior.
- Optimize SBS type and content: choose linear or radial SBS to suit the adjusted base and start in the lab with moderate contents of 3–4%, verifying rheology (DSR, BBR), storage stability and industrial feasibility.
If the SARA test reveals a negligible asphaltene content, the soundest solution may be to blend it with a more aromatic and asphaltenic binder, or to discard that base.
What should never be done
- Add SBS “by trial and error” to any asphalt until viscosity rises.
- Compensate for poor compatibility by increasing the polymer percentage.
- Use diesel or fuel oil as reducing oils without control or testing.
- Modify without a prior study of the SARA fractions.
Frequently asked questions about modified asphalt
What is SBS-modified asphalt?
A binder to which styrene-butadiene-styrene polymer is added to improve its elasticity and its resistance to rutting and fatigue.
What is the SARA test?
An analysis that separates asphalt into saturates, aromatics, resins and asphaltenes, showing whether the base is compatible with the polymer.
What percentage of SBS is used?
Del Águila recommends starting in the laboratory with 3–4% and scaling up only when the base shows good compatibility.
A system, not a recipe
Modifying with SBS means designing a polymer–asphalt system consistent with the chemistry of the base. The SARA test is not an optional analysis: it determines whether the modification is a smart investment in pavement performance or a cost overrun with no benefit to the client or the project.
