Bitumen Modification

Interaction

Modification

Kraton™ polymers can give bitumen the high-performance characteristics it needs for long term survival of even the toughest climates.

The unique molecular structure of Kraton Corporation interacts with the bitumen to add flexibility, elasticity and toughness over a wide temperature range. This will reduce bitumen's sensitivity to extreme temperatures and significantly enhance its performance properties compared to unmodified bitumen.

Interaction

At lower temperatures, polystyrene and polybutadiene do not mix. Due to the composition of Kraton D polymers, the polystyrene blocks combine into a hard solid islands in a sea of polybutadiene chains.

The rubbery polybutadiene connects the different polystyrene islands to form a network. Polybutadiene remains rubbery even at temperatures as low as -90°C. Polystyrene remains solid up to the polystyrene glass transition temperature at around 100°C. The resulting Kraton D polymer has rubbery properties over a large temperature range. This network structure is strong at temperatures up to 100°C. Since the structure is based on the physical reversible bonds in the polystyrene domains, at higher temperatures the network breaks up into its individual SB and SBS molecules. Re-establishment of the network occurs on cooling. This reversible network is one of the main contributors to the versatility of Kraton polymers.

Absorption

Kraton D SBS polymer has almost the same solution parameters as the maltenes phase of the bitumen. It can absorb up to nine times its own weight of bituminous components, and the extended polymer can occupy a substantial volume fraction of the blend even at low concentrations. There is evidence that at a concentration of 3% in a highly compatible bitumen, the polymer rich phase may have achieved co-continuity with the other (asphaltene rich) phase.

Bitumen composition is important with respect to the final properties of the blend and the level of compatibility. Certain carefully selected bitumens can have, for instance, a single phase structure at temperatures higher than 130°C. The aromaticity and the average molecular weight of the maltenes phase determine the weakening of the styrene domains, and hence empirical properties like softening point.

Interaction

The compatibility of bitumen and polymer is key to the success of the modification. Kraton polymers are ideally suited to be blended with bitumen because of their similar average solubility parameters and their ability to absorb bitumen's oily components, while maintaining the network structure.

This unique combination where a small amount of polymer creates a network structure in the bitumen by absorbing its components and increasing its effective volume, has been integral to the success  of Kraton D polymer modified bitumen

Bitumen is turned from a highly viscous material into a bituminous elastomer, with the key advantages of elastic behaviour over a larger temperature range. If necessary, by the right choice of bitumen, polymer concentration and polymer structure, the temperature range can be extended to cover virtually any climate.

Compatibility

As indicated in the section on bitumen composition, bitumen consists of a mixture of larger and smaller molecules, some of which are oil-like. They are absorbed by the polymer. However, these oil-like components also act to dissolve the larger molecules ('Asphaltenes'). When the polymer absorbs too much of these oily 'solvents', a phase separation can occur, whereby an 'asphaltene' rich layer forms at the bottom and a polymer rich layer forms at the top of the mixture.

The interaction of polymer and different components in the bitumen can also have other effects. A summary of these effects are shown in the compatibility chart on the right.

The S Curve

The S curve clearly shows the dramatic impact of a small amount of polymer on empirical properties such as Softening Point.

Although the S-curve does not necessarily reflect the level of performance that can be achieved at the various polymer contents, it surely illustrates that securing complete dissolving and avoiding degeneration due to too severe processing conditions, are important to obtain a certain performance level.

Elastomeric

Addition of Kraton D polymer fundamentally changes the behaviour of bitumen from a highly viscous liquid to an elastomeric product at higher concentrations (+/-12%) These changes can be observed in the table:

 

Mechanical properties of blown bitumen compared to bitumen modified with high concentrations of Kraton D polymer

Measurement

Unit

Blown bitumen

Bitumen
10% Kraton D

Bitumen
12% Kraton D

Elongation

%

140

1500

2000

Permanent set

%

>100

<10

<10

Breaking stress

N

0.5

10

15

Cold Bend pass

°C

- 5

- 20

- 35

Findings:

  • Elongation shows that highly modified (+/-12%) test samples can be extended 20 times their original length before failure
  • Permanent set shows that a less than 10% permanent deformation resulted from that extension
  • The breaking stress had increased by a factor 20 to 30
  • Cold bend indicates its flexibility and non brittle behaviour at very low temperatures

 

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