PMB Storage Stability as a Property of the Bitumen × SBS Pair

A polymer-modified binder is a two-phase system, and its behavior during hot storage without stirring describes neither the rubber grade alone nor the bitumen alone, but the specific pair together with the preparation mode. The SARA composition of the base bitumen correlates with this behavior, yet in published data different research groups find different governing fractions — and no single compositional parameter gives an unambiguous prediction. Changing the base bitumen therefore means changing the system, and the stability of that system is established by testing the pair, not by recalculating the dosage.

What exactly separates in the tank

Polymer and bitumen do not form a true solution. The rubber selectively absorbs those bitumen molecules that have the highest affinity to it, and the system splits into two phases of different composition — polymer-rich and asphaltene-rich. The review by Polacco and co-authors calls this process an internal distillation: the polymer "usually is selectively swelled by the most affine asphalt molecules and exerts a sort of internal distillation, driving the formation of a polymeric phase and a residual asphalt phase of different compositions."

This morphology is thermodynamically unstable. What happens next is a matter of kinetics: "Whether they separate or not is a mere kinetic condition that has critical importance during prolonged storage at high temperatures and absence of stirring." After the mill, the binder sits in a metastable state, and the lifetime of that state remains a technological characteristic of the specific system.

The direction of migration is set by the density difference. The polymer-rich phase is lighter, so in a vertical tube it moves upward while the asphaltene-rich phase settles: "The lighter PRP phase will migrate to the top part of the tube, while ARP will settle at the bottom." That is why the laboratory storage stability test compares the properties of the top and bottom parts of the sample after storage.

SARA composition of bitumen: what it correlates with

The classical tool for describing bitumen is the split into saturates, aromatics, resins, and asphaltenes. Hence the Gaestel colloidal instability index: the ratio of the sum of saturates and asphaltenes to the sum of aromatics and resins.

The classical compatibility framework is formulated through aromatics: according to the Polacco review, "a high aromaticity and low asphaltene content favors the compatibility," and Lu and Isacsson, cited there, state the requirement directly — "to produce a polymer modified binder with good compatibility, the content of aromatics of the base bitumen should be sufficiently high for the polymer to compete with asphaltenes." The same review also gives a target distribution of components: a colloidal instability index of approximately 0.3, light-component content above 55%, and a saturates-to-aromatics ratio of approximately 0.55.

Further on, the same review checks these targets on two bitumens — and they diverge. For a pair of bitumens with aromatics contents of 16 and 27% and asphaltene contents of 16 and 20%, the instability indices came to 0.37 and 0.41; a result consistent with all three parameters at once proved unattainable, because the bitumen with the higher aromatics content also had the higher index. The reviewers state their conclusion without softening: "the use of 'compositional' parameters cannot lead to an unambiguous prediction of compatibility and stability" of PMB.

A 2021 correlation study from the University of Innsbruck tested the same dependence on six bitumens of different origins and penetration grades, blended with one branched SBS at a dosage of 5 wt.%. The SARA composition was determined by thin-layer chromatography with flame ionization detection.

The state of the system these numbers belong to: 5 wt.% branched SBS, polymer added at 180 °C, dispersed with a rotor-stator homogenizer at 190 °C for 60–90 min. Phase separation was assessed by fluorescence microscopy of samples prepared by freeze-fracturing per EN 13632; storage stability — by the tube test per ASTM D7173.

The result of this study diverged from the classical framework. The degree of swelling correlated with the saturates content, not with aromatics: "primarily, saturates migrate into the polymer-rich phase, causing swelling." Phase inversion was observed in bitumens with saturates content slightly over 5%. The softening point difference between the top and bottom parts of the tube turned out to be higher for bitumens with lower saturates content, and for the remaining fractions the authors recorded no significant correlations — and named the divergence from the literature directly: "No correlation was found with the aromatics fraction, contrary to the literature, where the aromatics fraction is reported to be important for compatibility."

The two works do not refute each other: the bitumen sets differ, the polymer differs, the tube test method differs, and the Innsbruck authors themselves note that aromatics and resins correlated with each other in their set, which complicates separating the effects. For the engineer preparing the binder, a practical boundary follows: the compositional parameter of the base bitumen helps predict the direction of risk and plan the tests, but does not replace testing the specific pair.

From component composition to the binder after storage

The evidence base for storage stability is built in levels, and each level answers its own question.

What has arrived. The bitumen's penetration grade, its certificate values and, if available, the SARA composition; for the rubber — the macromolecule architecture, bound styrene content, diblock content, melt flow rate. This level establishes the identity of the components and gives grounds to expect one direction of behavior or another.

How the pair behaved in our process. Morphology after dispersion — the size and shape of the polymer phase, the presence of undissolved granules, signs of phase inversion. Fluorescence microscopy and the glass-plate check work here, and the process parameters are the polymer addition temperature, the dispersion temperature and duration, and the mill mode. This level proves that the rubber has actually entered the system rather than being present in it as a separate phase.

What the binder remains after storage. The tube test per EN 13399 and the class properties taken after storage, not only on a fresh sample. EN 13399 in its current 2017 edition states its own boundary directly: "This European Standard specifies a method for measuring the storage stability at high temperatures," with a note that modified bitumens "display phase separation under certain conditions." The method answers the question of how the system behaves under the storage conditions set by the method — and does not describe the behavior of the binder in your tank, with different geometry, different time and possible stirring.

None of these levels replaces the next: component identity does not describe the morphology, and the morphology after the mill does not describe the state of the binder after a day in the tank. The boundary of the laboratory level runs further still: in Material Wizard's experience, divergences between the laboratory result and the binder's behavior in production have occurred, and the causes lay in factors the sample does not cover — above all the temperature regimes at the preparation, storage and laying stages. A compositional parameter does not become critical merely because it is convenient to measure. Criticality is established by correlation: once the chain "base bitumen composition → morphology after the mill → difference in properties after storage" has been accumulated for a specific pair, the compositional parameter becomes a working acceptance criterion. Until then it remains a screening indicator that sets the scope of checking for a new bitumen batch.

Risk coordinates when changing the base bitumen

The scope of checking a new bitumen batch is best defined not by a list of ready-made scenarios but by several independent axes that overlay one another.

History of the pair. How many industrial batches have already been made on this exact "bitumen supplier × rubber grade" pair, what spread of tube test results has been accumulated, and whether any of them sit at the edge of the tolerance.

Status of the change. What exactly changed: a different supplier, a different feedstock source of the same supplier, a different penetration grade, a different rubber batch, a modified mill mode. A change in the base bitumen composition and a change in the dispersion mode give a similar result in the protocol but require different actions. In Material Wizard's practice, switching to a new base bitumen required revising the formulation settings — transferring the previous settings to the new pair did not produce a working result.

Thermal and logistic state of the binder. Temperature and duration in the tank before shipment, presence and frequency of stirring, number of transfers, reheating after idle time. These conditions determine how far phase migration manages to advance, and they are what makes the laboratory result transferable or non-transferable to production.

Sensitivity of the application. Binder class and criticality of the object. The requirements for a bridge-deck binder and for a lower-category road differ not only in the tolerances but also in the cost of a batch pulled from shipment.

One bitumen batch sits on all four axes at once, and it is the combination that determines whether checking the certificate is enough or a full tube test with class properties after storage is needed.

Stabilization: what it adds to the system

When a pair shows separation, stabilization changes the nature of the bond between the phases. The first route works with bond chemistry; its most common tool is sulfur: it is assumed that "sulfur chemically crosslinks the polymer molecules and chemically couples polymer and asphalt through sulfide and/or polysulfide bonds." The Polacco review presents this mechanism with an honest caveat: the assumption rests on the well-studied vulcanization of unsaturated rubbers, yet "clear evidence for the occurrence of such chemical reactions in the case of PMAs has not yet been reported."

The technological limits of the method are described in the same source and concern the sequence of operations rather than the formulation itself. The network must form after the polymer is already well dispersed and swollen, since introduced earlier it would "freeze the morphology at an earlier stage"; the degree of crosslinking must stay far enough from the gel point; premature sulfur addition can "lead to an unusable heterogeneous bitumen-polymer." One of the patent regimes cited in the review prepares a homogeneous PMB first, then adds 0.1 to 3 wt.% sulfur with further stirring for 20–60 min. The review separately names the risk of hydrogen sulfide release during preparation.

The second route shifts the composition of the bitumen phase itself. The review gives an example where, for blends containing 3.5 wt.% SBS, adding 2 to 4% of oil reduced the softening point difference in the tube test from the 15–26 °C level down to 6–8 °C. The third route works with densities: stability is achieved by a composition at which the densities of the two phases converge, and the density difference ceases to be the driving force.

What all three routes share is that they change the system itself. After a stabilizer is introduced, the rheology, the behavior on reheating and the requirements for the sequence of operations change, which is why stabilization is agreed together with the preparation mode.

What to agree before the first batch on a new bitumen

• The base bitumen certificate for this batch: penetration grade, softening point, the values the supplier actually reports.
• The SARA composition of the base bitumen, if the supplier determines it, or an agreement on how often it will be determined.
• The rubber batch document: macromolecule architecture, bound styrene content, diblock content.
• The preparation mode fixed in numbers: polymer addition temperature, dispersion temperature and duration, mill mode.
• Dispersion control before the stability test: glass-plate check or fluorescence microscopy.
• The tube test per EN 13399 with the actual storage conditions recorded, plus the class properties taken after storage.
• Storage conditions of the finished binder in your cycle: temperature and duration in the tank, stirring, number of transfers, reheating.
• The stabilization decision: whether it is introduced, at what stage, and what changes in the sequence of operations.

Five questions about PMB storage stability

1. Can the base bitumen certificate predict whether the binder will separate? The certificate sets the direction of expectations and the scope of checking, but gives no prediction. The Polacco review formulates this as a conclusion from a large body of literature: compositional parameters do not lead to an unambiguous prediction of compatibility and stability. The certificate defines the scope of checking for a new pair; the answer about stability comes from the tube test on that pair.

2. Why does the same rubber grade give different results on different bitumens? Because the measured quantity is a property of the pair. The polymer selectively swells with those bitumen molecules that have the highest affinity to it, and the composition of the two resulting phases, their density difference and the separation kinetics depend on which molecules are available. The Polacco review records this at the level of observations too: for two different base bitumens, the most compatible polymer out of the same pair of candidates turned out to be different.

3. Which bitumen fraction governs rubber swelling — aromatics or saturates? Published data diverge. The classical framework links compatibility to high aromaticity and low asphaltene content. The correlation study of six bitumens with one branched SBS gave a different picture: the degree of swelling correlated with the saturates content, and no correlation with aromatics was found in that set. The two results were obtained on different bitumen sets and different polymers, so for a specific pair the question remains experimental.

4. Does a good tube test result mean the binder will survive storage in our tank? The test describes the system's behavior under the conditions set by the method. The storage time, temperature, vessel geometry and absence of stirring in your tank differ, so the result transfers together with its conditions. Results taken under the same conditions are comparable; when the storage conditions change, they are recorded in the protocol. In Material Wizard's experience, divergences of the kind "sample within tolerance — binder behaves differently in production" have occurred, and the factor turned out to be precisely the temperature regimes of the production cycle, which the laboratory sample does not reproduce.

5. What does sulfur addition do, and what does it not do? Sulfur creates chemical bonds between the polymer chains and the bitumen components, so the phases stop separating under the density difference. It does not compensate for poor dispersion: the network must form after the polymer is already distributed and swollen. Introducing a stabilizer changes the sequence of operations, the binder's rheology and its behavior on reheating, so the decision is made together with the preparation mode.

Products and support

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Sources. G. Polacco, S. Filippi, F. Merusi, G. Stastna. A review of the fundamentals of polymer-modified asphalts: Asphalt/polymer interactions and principles of compatibility. Advances in Colloid and Interface Science 224:72–112 (2015). DOI: 10.1016/j.cis.2015.07.010 · M. Wieser, A. Schaur, S. H. Unterberger. Polymer-Bitumen Interaction: A Correlation Study with Six Different Bitumens to Investigate the Influence of SARA Fractions on the Phase Stability, Swelling, and Thermo-Rheological Properties of SBS-PmB. Materials 14(5):1273 (2021). DOI: 10.3390/ma14051273 · EN 13399:2017 · ASTM D7173 · EN 13632.

This material was prepared with the assistance of artificial intelligence and reviewed by the Material Wizard editorial team. If anything here informs a decision of yours, check it against the primary source — or ask us.