ANALYSIS OF EMULSION STABILITY AND ASPHALT COMPATIBILITY G. Holleran, Vice President, Valley Slurry Seal Company, USA Jeffrey R. Reed, President, Valley Slurry Seal Company, USA Back to VSS Technology Papers Library ABSTRACT The stability of asphalt emulsions plays a key role in their application and the logistics of their manufacture. Through experience with various types of asphalt made in different ways with different crude sources VSS has developed an approach to stability and property enhancement. This paper reviews the instability mechanisms found in asphalt emulsions and their primary causes. It describes the use of visual microscopy and asphalt analysis to look at the stability characteristics of emulsions made with a range of asphalts and emulsifiers. Case studies where this approach has proved successful in Eastern Europe, Asia and the Pacific Rim are discussed. The effect of polymer modification and the compatibility with asphalt is also discussed with respect to the emulsion outcomes. This is discussed with relation especially to asphalt rubber modified emulsions. 1. INTRODUCTION 1.1 Basic Asphalt Chemistry Asphalt is a mixture of many chemical types. It is the chemical composition of asphalt that determines its properties in whatever end use that is chosen. This is because asphalt is made up of polar and non polar compounds in complex association. The interaction of polar compounds determines asphalt structure and mechanical properties. The chemistry of the asphalt produced depends on two main parameters, the crude source and the manufacturing process. The main sources of crude oils are USA, Middle East, Eastern Europe, parts of Africa, the Caribbean (e.g. Venezuela and Mexico), North Sea, parts of Asia and Micronesia (e.g. Indonesia, PNG). All together there are more than 1500 (1) sources of crude oil. They range from heavy black liquids (e.g. Boscan) to straw colored liquids (e.g. Beryl). They have different levels of bituminous materials). The chemistry may be basically paraffinic, napthenic (straight chains or rings) or aromatic. This will effect the asphalt properties and influence the method of manufacture and applications. Crudes are classified as heavy, medium or light, depending on the level of bituminous materials. 1.2 Elemental and Molecular Composition Asphalt is a derivative of organic materials produced in process­es that have spanned millions of years. They are not a single chemical species but a complex mixture of organic molecules that vary widely in composition from non polar saturated hydrocarbons to highly polar , highly condensed ring systems. They also contain trace amounts of metals, mainly vanadium and nickel (2). Although all asphalts are predominantly carbon and hydrogen , most of the molecules contain at least one hetero (S,N,O) atom. Because the hetero atoms often impart functionality and polarity , they have a disproportionate effect on the properties. Elemental analysis is an averaging that gives little information on molecular arrangements and so sheds little light on physical properties. The organic origin also ensures that the exact chemical species present are diverse. Carbon in aromatic ring systems is about 25- 35% of the total carbon. The aromatic carbons are incorporated in condensed ring systems containing 1-10 rings. These ring systems may be associated with napthenic ( saturated) ring systems and they both may have attachments of various branched and linear hydrocarbons. (3) Carbon associated with napthenic ring systems are of the order of 15-30% of total carbon.(3) Non aromatic or napthenic hydrocarbons are present alone or as side branches (3) and account for 35-60% of the carbon content. Other structures are present in different asphalts. The hetero atoms are often present in sufficient amounts such that on average every molecule has one. These may be in the rings, in non ring components or as functional groups attached to compounds. These, together with polarizable aromatic rings, contribute polarity. Correlation of properties with discrete chemical groups are close to impossible due to the huge diversity, for this reason asphalt is usually considered as consisting of fractions. The influence of each fraction becomes the key consideration in emulsability properties. 1. 3 Fractionation 1.3.1 Simple Fractionation Asphalts can be classified most simply as two major fractions : Asphaltenes and Maltenes.(4) a) Asphaltenes The asphaltenes consist of highly condensed planar and hetero atom polar groups , polarizable aromatic ring systems and large amounts of hetero atom polar functional groups. The fraction is defined as the proportion of material precipitated when a straight chain alkane is added to the asphalt. The portion of the asphalt that is precipitated varies with the alkane used. Normally n hexane (ASTM) or n Pentane (IP) are preferred. Asphaltenes are a product of resin development by geological or processing effect. They need not be high molecular weight but are the most polar of the fractions. This explains why the fraction level varies with solvent used. The solvents are all non polar, it follows that smaller molecules will disrupt the dispersion of polar molecules most of all. For larger molecules such as hep­tane the disruption is less and hence only the most polar molecules are precipitated out. It has been found that the asphaltenes are agglomerations of the most polar molecules in the asphalt and as such are only able to be dissociated from one another by dilution or some energy source such as heat or ion emission. When molecular weight is determined by field ionization mass spectroscopy of this fraction the levels found are much lower than those traditionally recorded by vapour pressure osmometry.(1000 c/f 20,000). This indicates that the asphaltenes are not large molecules but highly interactive polar molecules.The polarity of the asphaltene component is derived by the presence of hetero atoms (S,O,N). These are functionalized and the functional groups are polar groups. In recent time it has been established that the alkane extraction process also may precipitate higher molecular weight , straight chain hydrocarbons , and straight chain hydrocarbons attached to aromatic rings. These materials have a wax like behavior that may cause low temperature cracking due to a free volume collapse in cooling (c/f the hole in the fat that appears on cooling). (5). These are not simple waxes as they have much greater molecular weights. b) Maltenes (Petrolenes) This is the remaining portion of the asphalt. It consists of two fractions, OILS and RESINS. Separation to these fractions can be easily carried out by Clay/gel separation.(6) i) Oils These oils are the liquid part of the asphalt and consist of n, iso and cyclo paraffins and condensed napthenes with some alkyl aromatics. The aromatic portion is mostly napthoaromatic hydrocarbons with three or four napthenic rings per molecule. The fraction is non polar. The oils have a key feature of dispersing polar agglomerations. This is especially true of the aromatic and napthenic oils. Thus this component is important in emulsification. ii) Resins The resins are chemically very similar to the asphaltenes. That is, they are a transition from oils to asphaltenes. The resins consist of mainly polycylic molecules containing saturated, aromatic and heteroaromatic rings and hetero atoms in various functional groups. The resins are not as polar as the asphal­tenes and hence are not as interactive. There are significant differences between the chemical compositions of oils, resins, and asphaltenes from different crude sources and the percentages of these fractions will also vary. The resins are often aromatic and emulsifiers have high solubility in such species. The asphaltenes are the thickening agent or structuring agent, the polar aromatics also contribute structure and ductility. The fluidity is imparted by the saturate and napthene aromatic fractions, which , in combination with the asphaltenes produce complex flow behavior. The chemical interaction of these species with emulsifiers is a key determinant of emulsion performance. 1.3.2 Complex Fractionations Many attempts have been made to fractionate asphalt to its active components, as they exist in nature. The techniques below illustrate the main methods used in the last 50 years.(4). For example: Corbett Selective Adsorption/ Desorption Roestler/ Sternberg Chemical Precipitation Traxler Partitioning with Partial Solvents Boduszynski Chemical Reactivity Jennings HPLC Brule GPC Clay/ Gel All of the fractionation techniques separate out the influential fractions in asphalt structuring and the compatibility of solvent type effects of the oil or neutral phases. It is beyond the scope of this presentation to consider these methods in detail. However, it is worth discussing the Fractionation developed by the SHRP program as this is the latest and , with the Clay- gel, the most useful for the purposes of asphalt selection and modification for performance modification. 1.3.3 SHRP Fractionation The fractionation technique used is aimed at extracting species from the asphalt that actually exist in nature and examine their interaction. It is a refinement of the work of the above workers. a) Size Exclusion Chromatography (7) This may be carried out in simple glassware columns packed with swollen beads. The pore structure of the gel beads determines the rate of flow through the col­umn. The solvent phase flows straight through and the associated phase is slowed. The two fractions derived from this method are termed SEC I and SEC II. The former is the associated phase and the latter the dispersing or solvent phase. The SEC I phase give asphalt its viscoelastic properties. This can be observed by measurement. In field terms a high SEC I fraction means greater structure, less thermal susceptibility and better performance at high temperatures. Associations are confirmed by fluores­cence experiments. Associations quench fluorescence. The SEC II or solvent phase is the oil fraction that the associated phases are dispersed in. The better this fraction is as a solvent the less brittle the asphalt will be and the viscoelasticity is suppressed. The SEC II fraction is a viscous oil and the SEC I fraction is a black solid. In practical terms the fractions here could be considered as SEC I fraction equals asphaltenes and some of the resins (polar). The SEC II fraction consists of the oils and some resins. Thus the technique gives a more accurate measure of the viscoelastic proper­ties based on composition than just measuring asphaltenes. The fractions may be further extracted, however for emulsion ,a knowledge of the associated and dispersing phase levels can give an insight to any adjustments that can be made to improve emulsability. b) Ion Exchange Chromatography (8) The SEC I fraction is the structuring portion, that is the fraction that gives asphalt its unique rheology. However this too is made up of a mixture of materials. The technique to separate out the fractions developed also uses simple preparative chemistry and takes advantage of the difference in polarity and charge of materials. The process is Ion Exchange chromatography. In this process either the entire asphalt or the SEC I fraction may be separated. The primary fractions produced from this separation are acid fraction, basic fraction, and amphoterics, these constitute the SEC I phase. The amphoteric fractions are black solids, the basic fractions are intractable tacky semi solids and the acid fractions are viscous liquids. The neutral fractions are liquids. The microstructural model of asphalt would predict that the fractions separat­ed will be fundamentally different in chemical composition. It would also predict that the greatest degree of interaction will be between the polyfunctional material, ie the amphoterics. The amphoteric concentration is the main building block of asphalt viscoelasticity. c) Supercritical fluid Chromatography (9) The SEC I fraction corresponds to the neutrals. That is, the non polar fraction. The composition of this fraction determines the manner in which associations can form and hence the asphalt properties. Put simply, the better the solvent the more disperse the material. There are other materials present too. These include some wax like materials (although much higher molecular weight). In some asphalts a precipitate has been observed on standing. This has been linked to low temperature cracking. That is high molecular weight fractions in the neutrals phase can cause embrittlement at low temperatures by causing a free volume collapse. The method of separation is called supercritical fluid chromatography. This precipitates out different fractions of different solubility at varying conditions of temperature and pressure. It was found that the materi­als range in carbon number from C40 to C110. The fractionation of sol type asphalts shows higher carbon number and more aromatic nature than gel types. Higher aromaticity will give better dispersion and fewer associations. High molecular weight materials may come down in heptane , creating an illusion of high asphaltenes and hence a false correlation between low temperature crack­ing and polar content. 1.4 Models Models are useful in conceptualizing what may happen and lead to valuable insights to likely trends in behavior. 1.4.1 Colloidal Model (10) This is the traditional model where solid asphaltene particles are dispersed in the maltenes fraction. The asphaltene is the centre of a miscelle and these are peptized by the polar aromatic fractions absorbed from the Maltenes. A sol type asphalt has the asphaltenes fully dispersed.In a gel type, the miscelles are not fully dispersed. Sol asphalts are Newtonian, gel asphalts are non Newtonian. Real asphalts exhibit some character of both sol and gel. Generally most asphalts are Newtonian at temperatures higher than 60C, at lower temperatures they exhibit non Newtonian behavior. This fact makes the asphalt set as it cools. This setting appears to be related to structuring of the asphalt. This structure is due to molecular orientation at aggregate surfaces and ordering in the bulk asphalt due to interactions of polar species. This model is useful to conceptualize emulsion effects. (11). Gelled particles are difficult to disperse and emulsify. 1.4.2 Microstructural (12) A variant on the colloidal model that extends the understanding is the micros­tructural. Asphalt is a continuum of polar and non polar material ie a homogeneous, self compatible mixture consisting of a variety of molecular species that are mutually dissolved or dispersed. This creates areas of order or structure depending on the concentrations of polar material. The only differential between asphaltenes and resins is in polarity and thus the degree of potential associations. Thus structuring depends as much on asphaltene chemistry as concentration. The resinous proportion is less interactive and the oils are not interactive at all. 1.5 Representation Of Composition What the fractionation and the model theories give us is the capability to conceptualize the way asphalt flows and reacts to stress. It can thus allow a rational approach to crude selection and manufacturing methods. The composition of asphalt as shown by the different fractionation techniques and conceptualized in the microstructural model, is good evidence that the basic approach of measurement of polarity differences by asphaltene/ resin and oil measurement is sound. These components may be easily and directly measured using either clay- gel analysis or using the Iatroscan instrumented chromatog­raphy method (MARK V). They can be used directly to plot a triangular diagram of composition. Then, by either empirical or theoretical means a compositional region may be established for performance optimization. See figure 1. Figure 1. (6) Compositional Assessment This performance / compositional area will be crude dependent to the extent that the exact chemistry of the fractions is not considered. However, for chemically manipulated systems where the asphalt chemistry is, to some extent controlled, it will be practically independent. Thus the system would be ex­pected to operate best on low asphaltene crudes that have been modified to higher asphaltenes asphalts. Put plainly, once a suitable correlation has been found for a crude source then the composition can be used to check performance. The emulsification of asphalt is closely related to the dispersion of asphaltene – polar type materials (11) In the VSS method the A960 cut is analyzed together with clay gel and or Iatroscan analysis to assess the emulsability of the asphalt and potential problems. This is not a completely codifiable methodology and gives directions rather than blend ratios. It is about re balancing the composition. To reduce potential problems. This has been successfully used for slurry surfacing emulsions (6). 2. STABILITY MECHANISMS IN EMULSIONS 2.1 The Colloidal State The subject of colloid science covers a very wide range of materials from beer to road building materials. What these systems have in common is particles of solid or liquid (discontinuous phase) distributed in a liquid or gas (continuos phase).Systems are colloidal if the particle sizing falls within the range 0.001 – 1 micron. When particles are > 1 micron the system is a dispersion and the bulk properties are more important than the interfacial ones. Thus an asphalt emulsion is partly colloidal, partly dispersion.- a colloidal dispersion whose properties are a mixture. The following discusses some of these issues. Asphalt emulsions are distributions of a semi solid in a liquid phase. 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. 2.2.2 Double Layers Consider an emulsified particle Figure 2 Representation Of 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. Figure 3 Representation Of a Double layer. 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. Figure 4 Reduction of Charge over distance (13) All such emulsifier systems are equilibriums , but asphalt emulsions, which are thermodynamically unstable, do not achieve this equilibrium. 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 basic parameters, namely how well is it dispersed, what is the surface charge and how small the particles are. The stability mechanisms most of interest for asphalt emulsions are: Sedimentation Flocculation Coalescence Inversion (water entrapment) Ostwald Ripening Attention to these aspects can be useful in solving emulsion problems. 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. Figure— Figure 5 (13) Settling Particle Spheres in a dispersed medium settle at a rate described by Stokes Law. Rate of sedimentation is proportional to rr 2 Where r is the density of the dispersed phase and r is the radius of the particle. 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 mostly in very fine ( 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 efficiency which is the probability that a pair of colliding particles will form an aggregation and the second is the strength of such aggregations. Figure 6 Flocculation. There are two steps therefore, transport and attachment. Almost all colloidal interactions are of short range. Van Der Waals forces are primary attractive forces 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 than the irregular shape of a flocculated particle. 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 =k1dn - k1 c (di 3/D + dj 3/D ) 3 n 2 Where k d is the disruption rate constant , k c is the coalescence rate constant independent of droplet sizes d i and d j , 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 flocculated particles in electrically stable emulsions. Figure 7 Coalescence d) Inversion: Water Entrapment In inversion the particles of the emulsion come together so quickly that the water is trapped. This occurs in only rapidly coalescing systems , especially very high binder emulsions. Figure 8 Inversion 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 a 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: Flocculation /coalescence: this is accelerated by evaporation ( reduced dispersing phase), shearing, etc. as indicated above. It does not involve film formation. Aggregate Interaction The second is of great interest for road emulsions as it describes the chemical interaction between the emulsion and the aggregate. Figure 8 Reaction with stone 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 this is 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. In chip sealing high binder content emulsions will accelerate the break in the bulk emulsion. 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 POLYMER ADDITION EFFECTS The effect of adding polymer into an emulsion system is complex. The effects on the rheological properties and morphology of the binders have been discussed in detail elsewhere (14,15). The polymer also effects the emulsion and emulsion stability. Compatibility issues with the asphalt are also important , especially for polymers premixed with the asphalt before application. Figure 9 shows a micrograph of a latex modified emulsion ( comilled) compared to one where a compatible asphalt has been blended with 3% SBS and emulsified. It is clear from this micrograph that the emulsions are similar. However figure 10 shows a micrograph of a poorly dispersed latex system ( post added) together with anv incompatible SBS system.    Figure 9. LHS latex co-milled (3um largest) RHS SBS3% (largest 5 um).    Figure10 LHS post added latex 20um largest RHS SBS 3% largest size15um. The polymer can thus have little effect on PSD or a lot. For this reason comilling of latex is favored. Figure 12 shows that there are some benefits on curing and stone retention (Vialit test). Figure 11 Vialit results: different addition types of latex. An even dispersion as produced in a crumb rubber that has been milled (16) gave an emulsion that had a fine particle size distribution whereas one made with conventionally reacted material had a very coarse distribution and the emulsion was not stable. Dispersion of RG-1 materials showed a multiphase coarse system. This is useful only if used within 24 hours. Dry rubber in a slurry mix showed poor results in comparison. 4.0 MANUFACTURING AND APPLICATION EQUIPMENT EFFECTS 4.1 Manufacturing The effect of manufacturing equipment on emulsion properties has been discussed in the literature (18,19). Particle sizing from the above clearly is a significant effect.The mill system used to create the initial emulsion is critical to long term stability and performance. 4.2 Application The effect of equipment that is used to handle and apply the emulsion is significant. 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 reduced. 5. EXPERIMENTAL WORK AND APPLICATIONS 5.1 Experimental Methods Standard ASTM, ASA and AASHTO methods are used for characterization together with ISSA methods where appropriate. A method was developed for emulsion assessment (19) using visible microscopy. This involved an optical transmission microscope with a camera attached and an ability to magnify at 100-500 times. The emulsion is diluted with distilled water at a rate of 10 parts emulsion to 1 part water. The emulsion is dropped on to a reservoir type microscope slide, and the operator rapidly focuses through 3 fields. A photograph is taken at each focus point. and the particles are counted for sizing based on the known magnification. The particle size distribution is then plotted. Alternatively, this may be done manually with a graticule in the eye piece. The relative reduction in particle sizing may also be observed ,as well as flocculation and coalescence effects. Measurements may also be taken over time to observe the flocculation and coalescence behavior. A method was developed (20) for examining shear susceptibility of coal tar emulsions. It consisted of a Brookfield LVT with spindle 1 (high surface area) rotating at 60 rpm. The viscosity drop is monitored. It has been found that this closely correlates with the increase in particle size. Thixotropic effects are taken into account by allowing the emulsion to recover and re-measuring the viscosity curve. Shear susceptible materials do not exhibit hysteresis. a) Eastern Europe In this project the blown pitch asphalt made an initially excellent emulsion of fine particle size but this reduced in storage stability and particle size coarsened dramatically with time.(figures 12,13) The emulsion was tested for shear susceptibility and this was found also to be a problem. (figure 14). In all other aspects the asphalt gave good results- i.e. adhesion, slurry properties, set and cure were acceptable. The initial particle size distribution showed 98% of particles less than 3 micron and a high percentage 1 micron or less. This would indicate that looking at interfacial properties and using a coemulsifier may give improvements. A lignoamine blended with a small amount of tetramine was tried and the results are shown in the figures. IN the field the effect of the modification on the slurry was to improve traffic times as cohesion build was faster. Figure 12 Stability deterioration (6) Figure 13 change in PSD. Figure 14 Shear Susceptibility (6) b) Pacific i) Blown Asphalt. This was a chip sealing application. In this problem ,the high bitumen content emulsion lost viscosity and apparently binder content in storage. Particle sizing showed that the material was being made to a dispersed/colloidal state, that was acceptable,but shearing was causing flocculation and coalescence leading to fall out. This was felt to be due to the blowing process reducing aromatic oil content in the asphalt and, reducing charge density of the emulsifier at the interface. Or at least a disruption of the equilibrium of the emulsifier distribution between the phases. The asphalt was doped with aromatic oil ( Mobilsol 30 type). This appeared to overcome the problem. ii) PD Tar type asphalt. This material was used for general emulsions and exhibited a reduction in stability with time. PD tar materials are high in resins but low in aromatic oils. This allowed a good emulsion initially but it was felt that the emulsifier migrated into the asphalt over time creating an instability. ( see figure15). Blending with vacuum tower bottoms ( straight run materials) and BSFE ( bright stock furfural extract- a highly aromatic oil overcame the problems. ( figure 15) Figure 15 PDA base (6) b) Asia i) Heavy Crude-Slurry/ Microsurfacing emulsions: This used a straight run but heavy crude base asphalt with high asphaltenes. This was not a problem for standard emulsion, but for emulsions made for slurry and microsurfacing applications, the results showed poor emulsification, coarse emulsions with low stability and poor coating. A check of equipment showed that it was lower shear in the laboratory mill compared to the production unit. This gave an increase in particle sizing and hence poor stability. As there was not an apparent problem for standard emulsions for emulsions produced in either the laboratory or plant mill the microsurfacing/Slurry emulsifier compatibility emulsifier compatibility was suspected as the cause of the poor stability. As this is a heavy crude with high asphaltene (>25%) content it was suspected that dispersion of asphaltenes might be might be a problem. Emulsifier was blended with the asphalt prior to milling and this produced satisfactory results . Also the slurry and microsufacing properties were acceptable. The effect of the emulsifier in the asphalt appears to be to give better dispersion by surfactant effect and lower particle size in either mill. ii) Waxy Crude material. The composition and experiment indicated no problems with the emulsion. However, problems were experienced with some of the emulsion setting, cohesion build up and finished properties in slurry and chip seal. Addition of polymer largely addressed these issues. 6 CONCLUSIONS Asphalt is a variable material and this can have a significant effect on the emulsion properties. These must be largely solved on a case by case basis ,but a knowledge of asphalt composition and its effect on emulsions can point into the right directions. Emulsion properties and stability are largely bulk properties ,but the emulsion can be improved by creating finer distributions. This makes interfacial properties more important and provides a chemical solution to stability problems. Polymer addition improves rheological properties but processing and compatibility play important roles in emulsion morphology and performance. 7 REFERENCES Holleran, G (1992) "Seminar on Asphalt Chemistry Madras, New Delhi, Bombay Plancher,H et al (1976) Proceedings of Association Of Asphalt Paving Technologists. Vol 45. Ramsay, J et al (1967) Ind. Chem.Eng Chemical products R&D Vol 6 p231. Kirk Othmer Encyclopedia of Chemical Technology. Branthaver, J (1992) WRI personal communication. Holleran, G, Reed J,R (1999) ISSA 35th Conference Mexico Branthaver, J et al (1992) Fuel Sci and Tech Int Journal Vol 10(4-6).p1003 Branthaver ,J et al (1992) Fuel Sci and Tech Int Journal Vol 10(4-6) p885. Barbour,FA, Branthaver J (1992) Fuel Sci and Tech Int Journal Vol 10(4-6)p 979. Whiteoak ,D (1990) Shell Bitumen Handbook London. Dybalski, J (1986) Bitumen Emulsions Workshop Australian Road Research Board proceedings. Christensen, D Anderson, D.A (1991) Rome Conference on The Chemistry Of Bitumen. Hunter, R,J (1993) "Introduction to Modern Colloid Science" Oxford Science Publications. Holleran,G (1998) " Polymer Modification of Emulsions" California Chip Seal Association Conference Sacramento. Holleran, G (1996) International Slurry Seal Association Workshop Ohio. Holleran, G (1998) Bitumen Asia 98 Singapore. Durand,G Piorier,J E (1996) AEMA International Symposium On Asphalt Emulsions Washington D.C. Holleran, G (1997) Russian Road Federation Conference Tambov. Booth,EH, Gaughan, G, Holleran, G (1988) Australian Road Research Board International Conf Canberra. Johnston, M (1987) ICI Australia Personal communication. Back to VSS Technology Papers Library Copyright © 2000 Valley Slurry Seal Co All Rights Reserved.