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Glynn Holleran, Vice President, VSS Asphalt Technologies Back to VSS Technology Papers Library 1. INTRODUCTION Basic information on emulsification has been presented in the "Understanding Emulsions"sections of the technology transfer documentation. The purpose of this section is to discuss the important factors in emulsification, stability and aggregate interaction of asphalt emulsions. It is by no means comprehensive but is to give some broad guidelines to understanding and carrying out formulation of asphalt products. 2. Basics Of Emulsifiers and Emulsification: 2.1 Making Choices in formulation. Distributions of one phase into another are used extensively to allow a difficult to handle or even dangerous material to be successfully and easily applied. Mixtures of materials are commonplace. The dispersion of immiscible materials is one such example. Immiscible materials may be suspended, slurried, force mixed. If the particles can be made colloidal (sub micron) then a colloidal solution may be made. This is not a solution in the normal sense where one material is dissolved in another but close to it. Such a system is thermodynamically stable and will not break without being modified (by heat additives, evaporation etc). An emulsion is a dispersion of one immiscible phase in another but does not need to be a colloidal to be useful. In asphalt emulsions the particles are 1-10 micron.
How coarse or how fine the emulsion is dependent on the method of manufacture, the asphalt, the emulsifier and the storage and handling. In considering the formulation of an asphalt emulsion the following questions need to be asked:
Questions are: viscosity range, binder level, life, application conditions, other components, eg aggregates? Eg: Slurry emulsion: must be relatively low viscosity as it must mix well with aggregates, it must have a shelf life of at least a week, it must be stable to pumping through equipment at the conditions of application. You are restricted by the asphalt that you have and the aggregates that you have. Decision: Viscosity: 15-90s 25C, Sieve <0.3% sedimentation <5% in 5 days. Cationic emulsion. b) Emulsifiers Availability and cost are important. However the technical aspects are the most important to consider.
Roadchem® Chemicals. Roadchem® are a wide range of emulsifiers, additives and chemicals. VSS creates a technical solution to suit the bitumen and application required. Products:
As a first cut select the emulsifier for the general application then proceed with recommended levels of emulsifier and follow the trouble shooting guide to achieve the final properties. For most asphalt emulsions mid internal phase ratio particle charge is very important, that is the chemical zeta potential of the system determines stability. Also its interaction with the aggregate is important. The table below shows the zeta potential, approximate treat rates and aggregate surface charge.
Remember that all emulsifiers are mixtures, this is largely based on the source of the raw material. Common bases are:
c) Equipment. In general a very high shear system is preferred. This ensures a controlled and small particle size.
d) Raw Materials. The raw materials are very important to the formulation of the emulsion. i) Asphalt. Selection criteria for asphalt must be based on the final application and performance properties. Emulsifiers as shown above have been shown in durability tests to increase durability but they may also soften the final binder. Select based on rheological requirements. Thus higher penetration for cooler conditions, polymer modified (latex) for cool and hot conditions, harder asphalt for high temperatures. The method of manufacture of the asphalt must be taken into consideration too. Oxidized asphalt has depleted levels of aromatic oils and polar aromatic (resin) fractions, this may make emulsification difficult and the emulsion produced is less stable. PPA based asphalt has very high levels of aromatic resins but relatively low amounts of napthenic and paraffinic oils , this usually results in easy and rapid emulsification but the emulsifier will tend to move out of the asphalt and destabilize the emulsion over several days. In such cases co-emulsifiers or addition of aromatics or other chemicals will need to be considered and VSS should be contacted. Straight run materials are almost always preferred for emulsification. ii) Water The water must be potable. High levels of dissolved salts are not generally a problem in most cationic emulsions, however metal salts such as iron chloride, iron oxide and similar materials may cause precipitation of some fatty diamine type emulsifiers. Citric acid added at 0.1-0.3% will overcome this problem. Otherwise the water must be free from suspended clay or any organic materials. iii) Additives: Additives that may be considered are used to solve specific problems. The source of the problem must be defined to correctly choose. Calcium chloride for example will densify water phases reducing sedimentation, It will compress the double layer thereby reducing viscosity, it will reduce osmotic pressure in salty asphalt thereby controlling viscosity, it will reduce the tendency to inversion. Roadchem 705 will increase stability by co-emulsification, creating a higher charge density on emulsified particles and hence increasing particle repulsion. Non ionic emulsifiers can allow higher levels of packing of emulsifier. Gelling or thickening agents can gel the water phase either completely or partially to increase resistance to sedimentation etc. Aromatic oils can decrease particle size, increase solubility of the emulsifier in the asphalt and increase stability. Etc. 2.2 Emulsions and Emulsion Stability. 2.2.1 How is an emulsion formed? The basis of emulsification is the creation of small (1-5 micron) particles of asphalt that are coated with a chemical that allows the particles to stay apart. The emulsion must, however break back to asphalt films to be able to perform its function of coating, water proofing, adhering etc. Thus most emulsion chemistry is about control. When we extend this to aggregate interactions this becomes more complicated. Almost always mixtures of emulsifies and additives are more effective than single emulsifiers. In most asphalt cases this is already done by the manufacturer, VSS blend to this end. Double Layers Consider an emulsified particle
The particle has emulsifier ions that are dissolved in the asphalt (bitumen) and also in the water phase. Thus two particles when they approach each other are repelled. This is simple in concept, however particles cannot exist in a charged state. This must be balanced.
At the interface the charge density, as measured by the zeta potential determines the stability of the emulsion and the thickness of the double layer are determined thus by the molecular size of the emulsifier, its ability to dissolve in the asphalt and its ability to dissolve in the water phase, this is its HLB (Hydrophile/ Lipophile Balance). The higher the number the more the emulsifier will be in the water phase the lower the more in the asphalt. HLB shows the ability of the emulsifier to maximize its presence at the interface. Care must be taken in the use of HLB however as it itself does not measure stabilizing power, it must be used in conjunction with the zeta potential. Emulsifier choice is based on this and the aggregate interaction. The double layer may be manipulated and the surface charge may also be manipulated. All such emulsifier systems are equilibriums. That is the double layer thickness and surface charge density are functions of time. In cationic rapid set systems only about 30% of the added surfactant is available at the interface within several hours of emulsification. In medium set systems it is about 45% and in slow set systems about 60% That is the system may be unstable. If the surface charge is insufficient the emulsion might flocculate and coalesce. Methods such as asphalt doping might have to be considered. Methods to reduce double layer thickness by compression are possible (calcium chloride), or use of nonionic surfactants (polyethylene glycol C17 type). Some emulsifiers form Miscelle structures with time or at particular pH ranges, this is true especially where slow setting quaternary ammonium compounds are used.
Miscelles reduce the effective emulsifier by forming large agglomerations that are too large to approach the interface and which have the lipophile ends protected from the asphalt. 2.2.2 Emulsion Stability and Instability. In asphalt emulsions the oil droplets are relatively large (order of microns). Because of the large surface area per drop the excess Gibb's energy per drop is high and cannot be compensated for by entropy contributions. The stability of such a disperse system are thus characterized by the time dependency of its its basic parameters, namely how ell is it dispersed, what is the surface charge and how small the particles. The stability mechanisms most of interest for asphalt emulsions are:
a) Sedimentation: The sedimentation of an emulsion determines how long it may be stored. It is tested by a simple separation test. There are several mechanisms that effect sedimentation. Spheres in a dispersed medium settle at a rate described by Stokes Law. Rate of sedimentation is proportional to p r2 Where p is the density of the dispersed phase, r is the radius of the particles and k is a constant. That is the rate of sedimentation increases with density and decreases with reducing particle size as a squared function. That is halve the particle size and reduce sedimentation by a factor of 4. Other mechanisms that are present are Buoyancy: This is dependent on density. Increased density lower buoyancy as the particle must displace more liquid to float. This factor can be changed by densifying the water phase with calcium chloride. Brownian Motion: This occurs only in very fine (2-3 micron and sub micron particles). This is a sort of internal mixing, however this can accelerate flocculation- coarsening because it allows collisions of particles. For stable highly dispersed systems they may start off as sedimentation stable but other mechanisms that coarsen the emulsion will alter this.
b) Flocculation. When two particles approach each other several types of interactions may occur. There are two main ways that the colloidal interactions influence flocculation. The first is collision effciency which is the probability that a pair of colliding particles will form an aggregation and the second is the strength of such aggregations.
There are two steps, transport and attachment therefore. Almost all colloidal interactions are of short range. Van Der Waals forces are primary attractive forced that occur over distances of several hundred angstroms. Double layers are usually larger than this and so the thickness of the double layer is critical. Also gelled dispersing phases will slow down collisions and thus make flocculation more difficult. Transport variables include pumping and shearing, temperature reduction, increase in temperature. Attachment variables include zeta potential, double layer thickness, emulsifier type. The kinetics of flocculation will be determined by the balance of these factors. Hence warming an emulsion can reduce shearing hence flocculation. The rate of flocculation relative to final coalescence is also critical as this will determine the entrapment of water in the final particles. Water in the particles will dramatically increase viscosity and, while this may reduce sedimentation it can make the final emulsion out of specification and retard its cure rate. Flocculated particles act as if they are larger particles and settle faster as per Stokes law. c) Coalescence. The coalescence of flocculated particles is a function of the surface charge and factors such as shearing (orthokinetic) and temperature. It is the next step from flocculation and occurs as a spherical shape is energetically lower. This is a second order reaction. The effect of shear can be to break up aggregates (disruption) or cause coalescence. This is described in the Bernoulli equation: Rate of coalescence = da/dt =k1cn - k1 c (dj 3/D + dj 3/D ) 3 n 2 kd is the disruption rate constant, kc is the is the coalescence rate constant independent of droplet sizes di and dj , D is the fractal dimensionality. n is the number of aggregated particles per volume. This is the reason that low shear mixing often breaks up floccs in electrically stable emulsions.
d) Inversion: Water Entrapment. In inversion the particles of the emulsion come together so quickly that the the water is trapped. This occurs in only rapidly coalescing systems, especially very high binder emulsions.
e) Ostwald Ripening. An emulsion might be stable against flocculation and coalescence with time but may still coarsen over time. This will cause increased sedimentation. This phenomenon is related to the dependence of solubility on droplet radius. (Kelvin effect). The Ostwald ripening rate is dependent on the asphalt chemistry and may be adjusted by addition of parrafinic solvents to the asphalt. The rate is a constant that depends on the interfacial tension. One of the effects is to create a bimodal distribution. 2.3 Emulsion Breaking and Curing. 2.3.1 Film Formation. The application of emulsions relies on their ability to form films on aggregates. This section will not go into this in great detail but film formation concerns coalescence without entrapment of water into a continuous film. It can be described in terms of Young's equation. Wetting (s/l) = interfacial energy a/l - interfacial energy s/l X cos(contact angle). Film formation is function of kinetic factors such as temperature, viscosity and internal stability and thermodynamic factors such as are described in the Young equation. Films will form more slowly at low temperatures, with larger particles and with higher viscosity. Addition of solvents or coalescing agents will assist, especially at low temperature. Adhesion can be enhanced by adhesion agents doped into the asphalt. 2.3.2 Breaking Emulsions break by destruction of the double layer. The main mechanisms thus are:
The second is of great interest for road emulsions as it describes the chemical interaction between the emulsion and the aggregate.
The mechanism is not wholly discerned but appears to be as follows. The stable emulsion particles are attracted to the surface of the aggregate where emulsifier from the bulk solution interacts with the negative charges. This changes the equilibrium and the emulsion is destabilized. This in turn leads to deposition of asphalt on to the aggregate surface. This is where the wetting and film formation become critical. The rate of film formation will determine the adhesion and the emulsifier/formulation will determine the breaking rate. The internal flocculation and coalescence will also have an effect. Within the forming film the particles are in closer proximity and these mechanisms will accelerate. However if too fast water will be trapped and the binder film forming capability will be diminished. The bulk part of the emulsion remote from the aggregate surface will break by flocculation and coalescence In slurry and microsurfacing the rate will be controlled by additives. 2.3.3 Cure. The cure of an emulsion film is often confused with break. Cure is simply the loss of water from the film and bulk emulsion. Cure Rates are thus dependent on water content, rate of evaporation and the diffusion of water through the curing binder. In systems with strong energy differences between the aggregate surface and the emulsified binder an extra driving force is present to push water away from the aggregate/ binder interface. This is especially true in microsurfacing. Emulsifier choice relative to aggregate is thus a very key issue. Cement is often used to enhance this. 3.0 Equipment Effects The effect of equipment that is used to handle and apply the emulsion is significant. This has been extensively discussed in handling manuals. In short, sources of shear will coarsen and break the emulsion, depending on the level of shear (and this is temperature and equipment dependent). In spraying applications the nozzles used for spraying will also exert a shear. Because this is for a short time a stable emulsion will not usually break. Its rheology will change, however as asphalt emulsions are shear thinning, that is they reduce in viscosity with shear. Depending on the asphalt, the emulsifier and additives and their levels the material may also be thixotropic, that is, its viscosity will recover rapidly after spraying. As spraying is the last action in application no long term effects are relevant. In cold mix, slurry or other pugmill mixing applications the shear stability is of much greater importance. The pugmill creates a high level of shear. In such operations the pugmill should coat the aggregate with emulsion NOT break the emulsion. If the emulsion is not sufficiently stable there will be a tendency for the emulsion to have an accelerated break. This leads to poor coating, especially of course particles and leads to a mix that has uncoated particles. This decreases strength, durability and resistance to water in the final mix. Freeze thaw resistance is also greatly reduce fatigue and pothole due to water damage and freeze thaw.
4.0 Conclusions Asphalt is a chemically complex material and emulsification increases this complexity. It is important in all applications that the chemistry of the system is considered to optimize the performance of emulsion products. Back to VSS Technology Papers Library Copyright © 2000 Valley Slurry Seal Co All Rights Reserved. |
