There are many variants of emulsion polymerisation and many of them are complex, with algorithms that would be useful if one had any hope of knowing the inputs, which, in general, we don't. I had assumed that by now there would be a significant section on the topic. But so far its complexities have defeated me from producing anything appable. If you have any good ideas, please let me know.
This is a very preliminary version of what is hoped to be a more extended discussion of this important topic.
The perfect emulsion polymerisation takes place with perfectly controlled drops of monomer and with perfect control of the initiating species which might be in the water phase or within a micelle, depending on the precise mechanism. The literature reveals a lot of confusion about how to attain this state of perfection. If a favourite surfactant doesn't happen to work "co-surfactants" (usually alcohols) are added for reasons that aren't at all clear to those doing the addition. Droplet size often depends distressingly on system features such a mixing mechanism, so scaling the polymerisation to a larger vessel is challenging. And of course the heat generated by the reaction is a further confusing factor, especially if ethoxylate surfactants are used, with their large temperature coefficient.
HLD-NAC brings a lot of order to this chaos. By ensuring that the HLD is slightly lower than 0, the surfactants are maximally present at the droplet interface, giving maximum efficiency for minimum surfactant. This is important as the residual surfactant is often a nuisance to the downstream processes. And by taking into account the thermal effects on HLD, the process can be engineered appropriately. If, for example, ethoxylates are used then very careful thermal control is required. Using APGs or equivalent, there is much more freedom in terms of temperature, at least as far as the emulsion is concerned, though good control might be required for the polymerisation kinetics themselves.
Use of fishtail diagrams
Because the monomer "oil" is generally not at a 50% level it is important to use the Cc fishtail diagram to find the % surfactant required to fully solubilise the oil and make sure, therefore, that the level of surfactant is below that point and that the HLD is in the "just less than 0" range so that the surfactant is used most efficiently to create the smallest drops with the least effort. If it is hard to get the oil into small drops then the PIF approach is valid - formulate to get HLD=0 at the start in order to disperse the oil into small drops, then shift the system via one of the HLD variables to place it safely in the Type I domain.
Micelles or not micelles?
Emulsion polymerisation can take place in microemulsions (i.e. nano-sized drops) and so-called "miniemulsions" which are produced via energetic dispersion with sub-CMC concentrations of surfactant and tricks (such as addition of hexadecane, see the Ostwald page) are used to ensure that the drops remain small. In both cases there are no micelles around so the whole process takes place within the oil drops. The more classical emulsion polymerisation is controlled via initiators within micelles. It is, therefore, vital that the formulator knows which process is being used and controls the process via the appropriate HLD manipulation.