High Internal Phase Emulsion (HIPE): Assignment

Added on - Oct 2019

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As defined by Lissant (1974), high internal-phase ratio emulsions are those with more than 74%internal phase volume,Φ. TheΦvalue of 74% represents the maximum volume that can beoccupied by uniform non-deformable spheres when packed in the most efficient manner. Thesedays, also known as high internal phase emulsion (HIPE), the value ofΦcan be as high as 99%.At this high value ofΦ, closely packed monodispersed spheres is no longer physically possible ininternal or dispersed phase. Thus, at this high internal phase volume, the shape is deformed intonon-spherical polyhedral droplets which appeared to be monodispersed in size, as quoted byCameron and Sherrington (1996) on the work done by Lissant. The droplets have relatively largecontact area, are surrounded by continuous phase and stabilized by thin surfactant films. Thecontinuous phase, which generally constitutes less than 26% of the final volume of HIPE,normally contains monomer, cross-linking agent, surfactant and oil-phase initiator. Due to HIPEunique characteristics, HIPE have been used for many years in many applications such as foodpreparation, cosmetics, oil recovery and many others. One of the most important applications ofHIPE is the ability to be used as template systems for the synthesis of a range of polymericmaterials.The HIPE processing can be divided into two stages as discussed by Akay et al. (2005). During thefirst stage of the processing, the dispersed (aqueous) phase is continuously dosed into a mixingvessel containing the continuous phase (oil phase). Care is taken in minimizing the jet mixing ofthe two phases since addition of aqueous phase alone creates mixing. There is a reduction in thedroplet size of the aqueous phase due to the rotation of the impeller during dosing. In thesecond stage of processing, further mixing is carried out upon completion of dosing in order toreduce aqueous phase droplet size (i.e. size of pores after polymerization) and to obtain HIPE ofnarrow droplet size distribution. No additional mixing (homogenization) stage is needed for thecase of a very low dosing rate.In the case of very large relative dosing rate and small mixing rate, instead of HIPE formation,dilute (low) internal oil-in-water (O\W) emulsion is formed. When HIPE is stable, polymerizationwithout phase separation will take place.The monomer-based HIPE can be polymerised to obtain micro-porous polyHIPE polymers (PHP).Barby and Haq (1982) discovered that open-cell HIPE-based polymer can be polymerised byusing relatively simple low HLB (Hydrophile-Lipophile Balance) surfactant and HIPEs composingof styrene-divinylbenzene (DVB) as shown schematically in Figure 2.1.The internal (aqueous) phase used in preparing the HIPE can be easily and rapidly removed fromthe PHP to produce a highly porous material with very low density. Another importantcharacteristic of PHP is that it can be specifically tailor-made according to its application. Forexample, PHP can be produced with specific interconnect size, d, e.g. as d of 0<d/D<0.5, D ispore size. Moreover, a highly porous interconnected monolithic material of PHP with a well-defined and uniform microstructure of very low dry density can also be produced. The structureof PHP is shown in Figure 2.2 (adapted from (Akay et al., 2005b)). The materials can be producedover a wide range of pore size, D, (0.5 μm<D<5000 μm), based on the conditions of the startingemulsions. PHP having pore size greater than 200 μm can be produced through a coalescencepolymerisation technique (Akay et al., 2005a; Akay et al., 2002). Furthermore, the porosity ofPHP surface can be controlled by varying the surface chemistry of materials against which theHIPEs are polymerised. This allows the production of asymmetric materials.Due to PHPs unique structures and properties, PHPs have made ways in diverse fields ofintensified processes, especially in biology, where their applications include the discovery of a
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