Often times the distinction given to particle sizes of materials also reflects its ability to perform better in the context of bituminous cold technologies for roads and specifically in applications of neat and polymer modified bitumen emulsions. It is very common in most scientific conversations to mention the importance of particle size. To answer the question posed, this discussion supports the notion and narrative that that size matters. The contestations for why size matters seek to evaluate microemulsions as the length scale prefix to the word emulsion focuses specifically on the micro-scale. Key is the understanding of microemulsion constituents which are macroscopically homogenous mixtures of water, bitumen and surface-active agent. Microscopically, the mixture is made up of water-abundant and bitumen-abundant phases separated by an amphiphilic surface-active film.
This article will focus on understanding the components of microemulsions, and their formulation and behaviour as is related to applications in cold mix asphalt through cold bitumen emulsion slurry dispersions.
Background of Emulsion Science
Historically, the first documented extraordinarily stable emulsion was formulated by Ramsden and Pickering in 1901 and was known as the Pickering emulsion, which used solid particles at the interface of two immiscible liquids. Schulman and coworkers originally proposed the word microemulsion although the first publication had appeared in the early 1940s. Development of surfactant science in the 1960s which focused on bulk phases paved way for microemulsion research in the 1970s and 1980s. Microemulsions bear their name from the diameter of emulsion droplets which are determined by the amount of surfactant. It consists of thermodynamically stable oil droplets of diameters ranging from 100 to 1000A. Figuratively, the interfacial tension at the amphiphilic surface-active film is less than 0.0001 mJ/m relative to just an oil/water interface without the surfactant which is 50 mJ/m. Clearly the role of the surfactant is critical in decreasing interfacial tension which leads to a significant reduction in capillary retention force and allows for oil droplets to deform and move easily through pores allowing for better penetration and binding. Given this background the options of microemulsions can either be oil/water or water/oil both determined by volume fraction of the dispersed and continuous phase. Bitumen or polymer-modified bitumen is represented as the oil for applications unique to the context of the discussion.
Micro vs Macro
The knowledge of microemulsion phase behaviour can lead to a direct correlation with macroemulsion properties. A good example of the differences between microemulsions and macroemulsions is that the former has droplets that are too small to scatter light whereas the latter has large enough droplets to scatter light. Therefore, microemulsions are transparent whereas macroemulsions are opaque. In order to understand the molecular behavior of microemulsions the phenomenon of self-diffusion is key in the transition from an oil in water to a water in oil droplet structure and support for bicontinuous structures at a molecular level. At high temperatures the water in oil structure is dominant. Emulsion breaking which is at a macroscopic level can happen through coalescence driven by a decrease in surface area with a concurrent gain in surface free energy, nucleation as a result of droplet proximity and Ostwald ripening in which few emulsion droplets grow larger at the expense of the majority shrinking and disappearing. They are three types of bitumen emulsions with the classification based on setting on contact with unbound granular material surfaces. Rapid-setting emulsions are very reactive and set quickly when in contact with clean aggregates of low surface area such as chip seal chippings for surface dressings. Medium-setting emulsions set slightly slower than rapid-setting and are commonly used in open-graded mixes. Lastly, slow setting emulsions used with reactive aggregates of high surface area.
Emulsion Formulation and Application
Formulation of conventional bitumen is performed in a colloidal mill, with hot liquid bitumen at 1400C and aqueous surfactant are mixed for a very short period of time and pass through a rotor-stator system with very miniscule openings of less than a millimeter and high shear of up to 3000 revolution per minute. On the other hand, formulation of high internal phase ratio bitumen emulsions which lead to bitumen microemulsions is performed in special reactor. The bitumen is placed in the reactor until reaching a uniform temperature of 900C whilst the aqueous surfactant phase is prepared at the selected concentration and pH set at 2.5 using hydrochloric acid. Once the bitumen has reached the required temperature, the aqueous phase is introduced at low agitation then slowly increased to 680 rpm and is maintained until the desired particle size. Water with an acid pH is used to dilute to 60% bitumen content. Typically for cationic bitumen emulsions the average diameter is about 3 to 6 micrometers.
Are environmental regulations, health and safety concerns or potential profit loss a concern right now?
Slurry or microemulsion is the cold mix asphalt specific application of bitumen emulsion either in their neat or modified form in microsurfacing placed over the cheap seal to eliminate risks associated with loose chips as well as achieving the desired surface texture. Improved coating quality in cold recycling can be achieved with recycled asphalt pavement millings when bitumen emulsion is used since it provides a greater specific surface area. Preference for cold recycling is because majority of the projects are performed in-place rendering it economical as opposed to plant recycling which gives better results with regards to homogeneity. Grave emulsions originally developed in France use bitumen emulsion as a binder at low binder content than other cold mix asphalt. It finds its applications in base courses and wearing courses for light traffic loads.
So it does matter!
As well as their functionality, the benefits of using microemulsions in cold mix asphalt technologies include reduced fuel usage and greenhouse gas emissions which conforms with sustainable development, better working conditions and improved field compaction. Practically speaking, the fact that aggregates do not require heating as mixing and laying can occur at ambient temperature renders the technology to be more environmentally friendly. However, it is important to mention that cold asphalt mixtures contain substantial air-voids that could possibly trap enough water to impair short and long-term bonding between the aggregates and bitumen leading to a weakened asphalt layer. All things being equal, cold mix asphalt and cold bitumen emulsion slurry dispersions would not practically work if size was not of the essence – so we can state that ‘Yes, size does matter’.
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REFERENCES
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