Nanotechnology has taken over many discussions on material properties and behaviour. However, in bitumen emulsion studies rarely do conversations revolve around the contribution of emulsion technology at a nanoscale. So we ask; Why Nano? The definition of nano refers to the particle size with dimensions of the ratio of one to the billionth of a metre in metric terms. Bitumen emulsion bulk properties are dependent on what happens at a nanoscale level. We seek to highlight the bitumen emulsion structure to function relationship whilst intricately looking at an interesting class of emulsions known as High Internal Phase Ratio Emulsions (HIPREs). The article seeks to further evaluate the applicability of crumb rubber from end of life tyres in crumb rubber modified high internal phase ratio bitumen emulsions. Firstly HIPREs will be analyzed from particle size and formulation approach. This will be followed by crumb rubber and its sieve analysis from a grading perspective in relation to the contribution in HIPREs. Finally, challenges and realistic deductions will be addressed in terms of sustainability of the use of crumb rubber in HIPREs.

What’s all the HIPRE about?

The formulation of bitumen emulsions using HIPRE technology presents a different approach to traditional colloid mill technology. Viscosity is critical to the application of bitumen emulsions, the former results in high viscosity nano-emulsions whereas the latter produces low viscosity emulsions. The distinction between the two is evident in particle size and geometry of bitumen globules. HIPREs emulsification offers the ability to precisely control the morphology of the nanobitumen emulsions based on the determinant effect on micro and macro properties. As size is of the essence,  nanobitumen emulsions provide significant improvement over the fundamental material properties rendering them superior to other bitumen emulsion technologies. Technically, nanobitumen emulsion formulation yields a concentrate form, with between 75% to 95% by weight of bitumen dispersed phase. The aqueous phase consists of the surfactant in water enabling fine emulsions with little variation in nanobitumen particle size distribution . But how then is the bitumen mixed with the aqueous phase? This happens at the bitumen required temperature through introduction of the aqueous phase at low agitation which increases gradually until the desired nanobitumen size is achieved. However, the drawback to concentrated nano-emulsions lies in difficulty to handle which can be overcome through dilution with water with an acid pH to 60% volume fraction of the in order to match conventional bitumen emulsion viscosity.  

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Crumb rubber properties and behaviours

Crumb rubber is obtained from the end of life tyres either through ambient grinding or cryogenic grinding. Preference in most cases is given to ambient grinding owing to the challenges associated with the sharp edges of crumb rubber obtained from cryogenic grinding. Could we also evaluate crumb rubber from a nano-perspective? Interestingly, yes and we will find out shortly so as to why. The source of the crumb rubber is very important as it determines the reactivity with nanobitumen. Generally, truck tyres have more natural rubber than synthetic rubber components that can be manipulated at a nano-polymer level. The polymeric moieties of rubber differ as a result of different molecular structures which render the bulk properties in the rubber component added to the nano-emulsions. As crumb rubber is not just rubber alone the contribution of other additives such as carbon black at a nanoscale also contribute to the efficacy of crumb rubber in nanobitumen emulsion. Bitumen phase preparation at a macroscopic level is done by heating a quantity of bitumen to a maximum temperature of 1800C. The crumb rubber powder and stabilizing agent such as polyphosphoric acid are added to the preheated bitumen followed by stirring and cooling to a temperature of at most 1100C. Macroscopically, crumb rubber particle size charts from sieve analysis offer different ranges of mesh sizes applicable in crumb rubber with examples ranging from 30 to 200 mesh. The connection is then between reactions of the components of the crumb rubber at a nano-level effecting respective bulk behaviour of crumb rubber in nanobitumen.  

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The challenges associated with use of crumb rubber in nanobitumen emulsions are related to digestion ability limitations based on crumb rubber mesh size. Phenomenon like agglomeration of microscopic particles create regions of particle density which can create zones of dispersion in nanobitumen. High values of sieving have been obtained with crumb rubber modified bitumen emulsions owing to incomplete digestion of crumb rubber in the bitumen which leads to retention in for example the 70-mesh sieve. What that translates to is incomplete zones of interaction at a nanochemistry level as the exposure of the crumb rubber is different for the available nanobitumen. Ultimately, very high viscosity values that are obtained and are almost three-fold to those of conventional bitumen emulsions. Achieving excellent settling capacities is also key to storage stability as particle sizes distribution over time tend to settle at the bottom of the container if not fully digested or are out of phase. From a residual bitumen point of view, post-curing the HIPRE technology enables far superior engineering properties to conventional neat bitumen emulsions owing to benefits derived from the crumb rubber and incorporated into the nanobitumen. Crumb rubber modified bitumen emulsions are used in cold in-situ recycling where mixes are made from milled and reclaimed materials and in half warm mixtures for asphalt which combine with warm mix additives such as Fischer Tropsch wax. Generally, the former and the latter have incentives of working at ambient application temperatures and lowering of manufacturing temperatures associated with use of HIPRE technology respectively. 

So, why Nano?

Rather than just joining the line of people spruiking the nanotechnology buzzword, we can see the understanding of the composite nature of nanobitumen emulsions and crumb rubber is very important. Separately, they are immiscible, but crumb rubber can be dispersed into the bitumen phase prior HIPRE emulsification. The role played by nanochemistry in facilitating the reaction at a nano-scale level entails interaction between bitumen components of varying nanosizes with the nanopolymeric units of the rubber component of the crumb rubber. The contribution of nanocarbon black particles has also been identified as part of the composite crumb rubber-bitumen reaction working to enhance nanochemistry of the system. We reiterate that material properties at the macro or end user scale are highly dependent on nanostructures and nanoparticles, which interact at a molecular level, and resultant engineering properties reflect the synergy in individual and co-interaction effects of the composite system. 

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