VSS ASPHALT (BITUMEN) TECHNOLOGIES


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ROADCHEM® ARE™

For many years the benefits of asphalt rubber have been used in roads in hotmix and in hot -sprayed chip seals. Now VSS Technology has developed a new use.

Asphalt Rubber emulsions for chip seal and slurry surfacing.

Roadchem® ARE™ HBCE™ is a high binder chip seal emulsion with 15% modification. This product can be used for SAM and SAMI seals.

Roadchem® ARE™ MSE is a microsurfacing emulsion used for fast traffic systems.

VSS Asphalt Technologies reported in Asphalt Rubber 2000 Portugal meeting. ISAET 1996 and REAAA in 1998 the use of crumb rubber in various forms in emulsions and the possibilities for direct emulsification.

The development of direct emulsification techniques relies on the manufacture of fine rubber dispersion. VSS technology is used to make this material.

This is then emulsified with Roadchem® emulsifiers to a stable emulsion.

VSS will be marketing these emulsions soon.

Contact VSS Asphalt Emulsion Technologies for more information.

1. Introduction.

The purpose of a road surfacing is Roads are in essence mechanisms by which stress applied from moving wheel loads are transferred to the earth. To operate effectively they must not crack, rut or wash away. The distress of road surfaces generally take one of these forms. However in instances where the road surface or structure has failed then the rehabilitation method must address and correct the failure mode.

Asphalt (bitumen) is a naturally occurring material and has limitations based on its chemistry, the method that was used for its refining and the materials with which it is blended, for example cutter or other diluents . Addition of polymeric materials has been widely used to extend the application of asphalt and improve its properties, particularly in the area of thermal susceptibility and flexibility.

A significant problem in western countries has been the development of a vast used tyre stockpile. Tire rubber is a mixture of many materials including some polymers. This material has been used extensively in asphalt rubber modified chip seals gap graded hot mix and dense graded hot mix. However to meet many environmental emission standards a cold method would be better suited.

2. To make an emulsion requires a compatible asphalt rubber binder:

The compatibility of asphalt / polymer systems is the structural arrangement of the polymer particles , chains or groups within the asphalt matrix.

In asphalt rubber compatibility is an issue that has received little attention mostly as the material is used hot and soon after manufacture. A reaction is claimed to occur in which the asphalt and the rubber particle interact to form a gel coated particle. This is analogous to the process of swelling that occurs in polymer asphalt systems. See figure 1.

How well this model reflects the actual situation and the relative effect of particle sizing is not sure but , based on polymer and asphalt chemistry it seems adequate. It also explains why there is a significant change in properties over time as such a system is not thermodynamically stable. That is the large increase in viscosity over its early life is due to the continuation of this solvation process.(15).

This can be shown in examination of micrographs of asphalt rubber digested with and without extender oil and in low shear mixing compared to material put through a colloid mill. As shown in figure 2 and figure 3 . ( relative sizing is important ;all are to same scale).

LHS Low shear 200C 1 hr	RHS Low Shear 200C 1 hr.
Figure 2

LHS no extender oil, High Shear	RHS with extender Oil, High Shear
Figure 3

However it is claimed that materials that have been digested for longer periods or those which have finer morphologies exhibit poorer fatigue properties . This is explained by a mechanism similar to that which is employed in materials such as high impact polystyrene to make them more impact and crack resistant. In such materials a rubber in this case butyl rubber or butadiene rubber) is dispersed in relatively large (above micron) sizing particles. The fatigue fracture begins as a crazing in the dispersing asphalt) phase and propagates by the increase of strain at the root of the craze ; this leads to cracking and the crack similarly propagates. If this crack encounters a large mass of flexible material that can dissipate the strain, then the crack will be blunted. It cannot simply go around the mass because of the magnitude of its size.

Morphology has a dramatic effect on rheology This has been demonstrated by rheological studies of modified binders and the subsequent mix and product properties. Compatible systems perform better.

In emulsification the presence of large particles would make for a coarse unstable emulsion.

3. Emulsification

Polymer emulsions have been used to a great extent in chip sealing and slurry/microsurfacing The polymers generally are pre-blended with the asphalt, co-milled as latex either by direct injection into the soap or pre mixed with the soap, or post added .

Ground tyre rubber usually can not be added so simply.

Pre-blended crumb, must be very finely dispersed , free of any metal ( to avoid damage to the emulsion mill) to be able to be emulsified .

Other methods by which tyre rubber may be incorporated in emulsion are post addition approaches.

Ground solid crumb rubber has been added into slurry mixes as a dry ingredient, similar to the method mentioned above. In such a case ,the rubber becomes a part of the aggregate phase and acts mainly as a filler. Such processes are in general use in USA .

However, to create much of a change in elasticity, increase in cohesion, and other desirable properties ,the rubber needs to be fully or partially digested so that it may coat particles. This is the basis of the process to be discussed.

The increase in cohesion should improve properties such as deformation resistance ( in rut filling) ,surface abrasion resistance, crack resistance and allow increased binder films without flushing.

4. Dispersion and Post Addition.

The process referred to as the wet process may be adapted for use with emulsions. If a suitable dispersing agent can be used that allows dispersion in the emulsion water phase, then a material capable of being post added may be produced. The type is of obvious importance as it must not create an environmental hazard , nor degrade either the asphalt properties nor those of the rubber. On the other hand, if the solvent is able to swell or soften the rubber, then it may improve wetting and adhesion.

A range of agents has been used to optimize the dispersion and other additives such as wetting agents and carbon black included. In general terms , an oil additive is preferable with a high aliphatic content and a boiling range that meets emission requirements but also allows swelling of the rubber. The material will be referred to by its designation of RG-1. RG-1 is a semi swelled dispersion of crumb rubber (40-50%) in a petroleum solvent. It is supplied as a free running high viscosity material that can be readily poured and pumped.

RG-1 is used by post addition into the emulsion with simple mixing.(figure 5 shows some properties). As may be seen the emulsion is not greatly affected by addition of RG-1 except that the sedimentation rate is high. This is not surprising as the RG-1 is a separate phase and so the emulsion must be thoroughly mixed before use. There is no obvious breaking caused by the presence of the RG-1 and this is true to concentrations in excess of 20%.

Figure 4 Emulsion Properties

5. Direct Emulsification

Direct emulsification of asphalt rubber has been reported in a number of patents .

These patents are not very specific on the composition of the asphalt rubber but do have in common that the asphalt and rubber are preblended and gelled.

Although high levels of addition are claimed they seem very difficult to achieve.

The work carried out here uses the compatibility approach indicated in section 2 and has been aimed at developing as high a concentration of asphalt rubber as possible.

Analysis of the asphalt rubber used indicates that high SBR content materials produce the best results.

As fine a dispersion of asphalt rubber as possible is attained by use of additives , asphalt composition control and high shear blending. Particle size of the rubber must be of an order of magnitude lower than the desired emulsion particle size.

The principles of emulsification and the effect of various parameters was discussed by the authors previously .  

6. Emulsification of Asphalt Rubber Blends

Blends of asphalt rubber were prepared at a set percentage of 15%. The rubber used was from a single source and represented standard car radial tyre rubber predominantly synthetic rubber. The asphalt was compatibilised with the rubber by use of aromatic oil and added carboxylic acid.

Figures 5, 6 and 7 show the morphologies of three types. One where low shear was used. One where the system was compatible to a very fine distribution of 1-10 microns and an intermediate with some larger rubber particles.

This was emulsified with a low zeta potential emulsifier with a stabilizing additive to optimize double layer thickness and create the most stable emulsion . A modified asphalt was produced to give a balance of components with a high aromatic oil content.

Figures 8, 9 and 10show the resulting three emulsions . As may be seen the coarser blends gave coarser emulsions.

Figure 4 gives the emulsion properties. Again the best emulsions were produced with the finest dispersions.

It is clear that the emulsion can be formed and used. However the morphology is a prime determinant of the physical properties and this would seem to be a compromise from traditional asphalt rubber materials. In short this is a different material.

7. Application:

Slurry and Microsurfacing:

It has been shown in slurry and microsurfacing applications that properties of the mixes are improved significantly in cohesion, abrasion resistance and flexibility.

Chip Seal:

It has been shown that chip seals with such emulsions have significantly improved resistance to stone loss and improved flexibility.