Invert emulsification

Resins are incorporated in Neoprene latex as solvent-cut emulsions, solventless pebble-milled dispersions, or sometimes as solvent-free emulsions prepared using invert emulsification techniques. In the latter case a resin with a melting point of 80°C (176 F) or lower is melted. Water and surfactants are added to the molten resin and the temperature of the mixture is decreased. Upon reaching a certain temperature, known as the phase inversion temperature, the water in molten resin emulsion spontaneously inverts to form a resin in water emulsion suitable for use in latex adhesives. A resin dispersion which can be prepared in this manner is shown in Table 16. This particular resin dispersion can be used to produce adhesives with moderate hot strength and good open time using the following recipe ... 

Inversion ofMon cjueous Polymers. Many polymers such as polyurethanes, polyesters, polypropylene, epoxy resins (qv), and siHcones that cannot be made via emulsion polymerization are converted into latices. Such polymers are dissolved in solvent and inverted via emulsification, foUowed by solvent stripping (80). SoHd polymers are milled with long-chain fatty acids and diluted in weak alkaH solutions until dispersion occurs (81). Such latices usually have lower polymer concentrations after the solvent has been removed. For commercial uses the latex soHds are increased by techniques such as creaming. 

There is some evidence to suggest that, depending upon the phase volume ratios employed, the emulsification technique used can be of greater importance in determining the final emulsion type than the H LB values of the surfactants themselves [434], As an empirical scale the HLB values are determined by a standardized test procedure. However, the HLB classification for oil phases in terms of the required HLB values is apparently greatly dependent on the emulsification conditions and process for some phase-volume ratios. When an emulsification procedure involves high shear, or when a 50/50 phase volume ratio is used, interpretations based on the classical HLB system appear to remain valid. However, at other phase-volume ratios and especially under low shear emulsification conditions, inverted, concentrated emulsions may form at unexpected HLB values [434]. This is illustrated in Figures 7.4 and 7.5. 

Isothermal emulsification of viscous oils to obtain concentrated oil-in-water emulsions is carried out by inverting a water-in-oil emulsion under flow conditions using an MFCS mixer. Initial water-in-oil emulsion is prepared using a batch mixer (typical shear rate of 100 sec ), which is subsequently inverted to oil-inwater emulsion at extension rates approaching 10 sec It is found that the FIPI starts at a critical extension rate and the extension rate has to be increased to achieve full inversion resulting in submicrometer size emulsion droplets with a very narrow size distribution (size span < 1). ... 

At the oil-rich side, the phase behaviour is inverted temperature-wise as can be seen in the T( wA)-section provided in Fig. 1.7(c). Thus, the near-critical phase boundary 2 —1 starts at low temperatures from the lower n-octane-QoEs miscibility gap (below <0°C) and ascends steeply upon the addition of water. With increasing wA, this boundary runs through a maximum and then decreases down to the upper critical endpoint temperature Tu. The emulsification failure boundary 1 —r 2 starts at high temperatures and low values of wA, which means that only small amounts of water can be solubilised in a water-in-oil (w/o) microemulsion at temperatures far above the phase inversion. Increasing amounts of water can be solubilised by decreasing the temperature, i.e. by approaching the phase inversion. At Tu the efb intersects the near-critical phase boundary and the funnel-shaped one-phase region closes. 

The newly inverted emulsion properties often match the properties of standard emulsions made in the same (new) location, sometimes with extra features such as extremely small drop size formed during the inversion. In fact, industrial plants that produce extremely fine emulsions use a dynamic inversion process even more complex than the one described here, which promotes the surfactant mass transfer from one of the phases to the other in order to trigger a spontaneous emulsification (209, 210). 

Lin et al. [55] showed that the so-called agent-in-oil method can lead to finer and more stable emulsions. The agent is the emulsifier. For example, if a hydrophilic surfactant is added to the oily phase and then if this premix is subsequently diluted with the aqueous phase, a part of the aqueous phase is first solubilized or emulsified in the oily phase to form a W/O primary emulsion. This emulsion can invert to form an 0/W finer emulsion. Refer also to the low-energy emulsification (LEE) process. 

Figure 11.13a shows a microemulsion of the 0.1 M NaCI aqueous solution/Ci2E4/ decane system with 90% aqueous solution and an oil/surfactant ratio of 2.33, at 7°C. The HLB temperature of this system is approximately 18 C. When the sample is rapidly brought to a higher temperature, 40 C in the experiment shown in Figure 11.13b, the sample becomes milky in less than 40 s. If the test tube is inverted, no flow is observed, an indication that complete emulsification has been achieved. The emulsions produced by this emulsification method have finer and narrower droplet size distributions (Figure 11.14) than those obtained by the usual methods. 

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