1. Introduction

The importance of particle size in emulsions has been discussed in many papers (1,2,3). It is a determinant of emulsion stability, coating, break rate and cure rate. This is, of course, not the whole story, as formulation, raw materials and aggregates are also critical. However, particle size and particle size distribution are important variables and are controllable with formulation, raw materials and the equipment used to manufacture the emulsion. Many of the processes of breaking and curing are directly dependant on it (3).

This paper discusses particle size effects on laboratory and field performance. The main conclusions that could be drawn are that particle size (PS) and particle size distribution (PSD) have an effect on microsurfacing and quick set slurry break, coating of aggregate, cure time and traffic time. This finding was applied to adjustment of the mix designs and later to equipment technology for other customers. The variables that affect emulsion and end use performance are many, but an optimized balance can be achieved and even shortcomings of some systems overcome e.g asphalt quality (4). This information is useful in scaling to a full-scale production with a colloid mill that is unfamiliar.

2. Potential Effects of Particle Size and Distribution

The work of Durand and others (1) shows some correlations between emulsion properties and particle size distribution and size. It needs to be pointed out that emulsion instability and controlled break are different effects. There is no doubt that coarse emulsion will break faster in the silica flour test or in straight aggregate mixing tests (coagulation), but what needs to be considered is break after adequate coating of aggregate particles and the formation of a cohesive mixture. There are three main steps in achieving a trafficable Microsurfacing (3):

• Break
• Coating and film formation
• Cure

2.1 Break

This is the flocculation and coalescence within the emulsion and the reaction with the aggregate surface. The PS and PSD could have several effects here. Firstly, on coalescence and flocculation, if the emulsion is already coarse, the floccs will be large and create large coalesced particles. Such particles will have the charge spread over a larger particle and the charge density at the point of contact with the particle will be reduced; hence, as the contact angle is reduced and the rate of reaction is reduced, the thermodynamic break rate will be reduced. The kinetic effect of access to the aggregate surface could be significant. That is, larger particles interfering with each other and flocculating and coalescing within the emulsion rather than reacting with the surface. This may give the appearance of a fast set but be more likely to create false slurries and water entrapment.

Inversion effects too can occur in high binder content emulsions such as are used in chip seals. Coarser emulsions are more likely to invert.

2.2 Coating and Film Formation

This refers to the binder evenly coating all particles and forming a coherent film. The above kinetic effects will affect the coating of the particles and, as we are dealing with graded fine aggregates in slurry surfacing aggregates, will have a tendency to break on the larger surface area fines and not coating large particles at all. Coherent films require even coalescence on the particle surface, and entrapped water will interfere with this, leading to a less cohesive mix. PSD is an important factor in film formation and a range of sizes that fit together will assist this process.

2.3 Cure

This is the steady loss of water from the system and the stiffening of the total mix as cohesion increases. Entrapped water will clearly inhibit cure. Rejection of water from the aggregate surface is both a thermodynamic and kinetic effect. The thermodynamics relate to the energy differences between the emulsifier and the aggregate charge, the kinetic to the diffusion controlled loss of water through the coalescing binder. So the potential effects on performance are that coarse emulsions with wide PSD will be more likely to give poor coating, false slurries, and poor cure rate expressed as cohesion build up or traffic time.

3. Controlling Particle Size

Several papers have been written describing methods to improve particle sizing of emulsions by methods of formulation and adjustment of asphalt chemistry (3,4). The methods usually involve improvement of chemical systems, doping of asphalt with surface active agents, tailoring asphalt composition and optimization of manufacturing conditions. The approach is basically to improve dispersion of the asphalt in the mill, and to stabilize the emulsion against early flocculation and coalescence. However the physical act of milling is the main determinant of initial particle size for a given asphalt and emulsifier system. The particle size is determined by the shear in the mill and the residence time (1,5).

This can be expressed by :

Shear Rate = (2 p R V / 60 E) ………(1)

where R = Colloid Mill Radius
V = Rotation speed (rpm)
E = gap dimension (microns)

That is, the particle size is a function of mill diameter, gap and peripheral speed(1). A correlation between this shearing and the d50 value and , a correlation between PSD and the initial particle sizes in the mill has been reported (6).However, mill configurations internally are quite different, with different tooling and effective gap sizes created by this.

For this reason, the relationship is changed to:

Shear Rate = (2 p R V / 60 e) M f ……………… (2)

Where M f is the mill factor.

The mill factor is the increase or decrease in shear created by the mill configuration and tooling. Various mills with different M f values were used in this study to achieve different shear rates.(8) as shown in Table 1.

Manufacturing conditions are also very important; the main manufacturing parameters are pH, exit temperature and the delta between the bitumen and the soap temperature. This is shown in figure 1a, b and c. It is clear that there is an optimum for temperature and pH conditions, and this needs to be determined for every system used!

Figure 1a Effect of Exit temperature on Particle size

Figure 1 b Effect of The difference between Soap and Bitumen temperature

Figure 1c Effect Of pH ( from reference 9)

The addition method of additives such as polymer too can be important. Co milling of latex has been found to give better results in particle size than post adding or adding in soap. ( 9,10,11).

4. Experimental Studies

4.1 Surfacing Effects

Several emulsions were made on several mill types as shown in table 1 and using Arabian crude based bitumen.

4.2 Mix Results

All mixtures were formulated with 16% emulsion, 1% cement and 8% water. Table 3 shows the results.

It is clear that most properties are dependant on coating, and this is highly influenced by particle sizing and distribution. Finer, narrower distributions produce superior results.

4.3 Chip Seal Emulsion Effects

The inversion temperature was measured using a Brookfield viscometer and hotplate, and measuring where viscosity took a sudden increase as the material cooled. This showed that the emulsion- a CRS-2p inverted at much lower temperatures for finer emulsions. For chip seals this would mean a longer time for aggregate to be wetted and adhere.

Figure 2 Particle Size Effects on Inversion of High Binder Emulsions

Other properties can be seen in figures 3,4 and 5.

Figure 3 Effect of particle size on film formation

This test showed that finer emulsions coated better and formed films faster,thereby increasing water resistance.

Figure 4 Effect of Particle Size on Stone retention

This shows that stone retention is improved, probably due to better coating, and faster break with less water retention in film.

Figure 5 Effect of Humidity and temperature of Cure

5. Field Results

It has been shown that particle sizing is important to field performance (8). The results show that finer particle sizes, and narrower distributions, set and cure faster at a given temperature. This appears to be related to improvement of the coating ,and breaking characteristics of the emulsions.

6. Conclusions

• Particle size and distribution affects emulsion physical properties
• Control of particle size may be carried out by control of mill configuration (Mf)
• Mixture properties that are controlled by coating and reaction rate between aggregate and emulsion are improved by reducing d50 and the narrowing distribution.

7. References

1. Durand,G ,Piorier,J.E (1996) AEMA International Symposium On Asphalt Emulsions Washington D.C.
2. Booth,E.H, Gaughan R, G, Holleran, G (1994) Australian Road Research Board International Conf Perth.
3. Holleran, G (1999) AEMA International Symposium on Asphalt Emulsions Washington D.C
4. Holleran, G (1999) International Slurry Surfacing Meeting Puerta Villarta Mexico.
5. Province, R (1986) Workshop on Bitumen Emulsions Melbourne Australia
6. Holleran US Patent 5,518,538
7. Holleran, G (1999) ISSA/AEMA Meeting Amelia Island Florida.
8. Holleran G, Reed, J,R (2002) Effect Of Emulsion Particle Size and Distribution On Microsurfacing Applications, ISSA International Congress March 2002 Berlin
9. The Size and Charge Of Concentrated Bitumen Emulsions ( 2000) Colloidal Dynamics
10. Holleran G ,(2003) Benefits and Uses Of Polymers In Slurry Systems ISSA Workshop Las Vegas
11. Holleran, G Reed J The Effect Of Particle Sizing On the Performance Of Slurry Surfacing Lyon 2002