Emulsions solubility parameters

When more than routine water resistance is required, a copolymer vinyl acetate emulsion can be used. The plasticizing comonomer in the polymer particles increases their intrinsic coalescing ability thus, they can coalesce more readily than homopolymer particles to a film that has a higher resistance to water. This resistance to water does not extend to the organic solvents, however, which are better resisted by homopolymer films. The soft copolymers have lower solubility parameters than homopolymers and are more readily attacked by solvents of low polarity, eg, hydrocarbons. 

In emulsion polymerization, NBR with acrylonitrile content between 15 and 50% can be obtained. The increase in the acrylonitrile content in the NBR produces an increase in the polar nature and solubility parameter in the copolymer [12]. Furthermore, the increase in acrylonitrile content improves the resistance to oils and also increases the glass transition temperature of the copolymers from -60 to-lO C. 

The copolymers were patented by Wiley, Scott, and Seymour in the early 1940s. A typical formulation for emulsion copolymerization contains vinylidene (85 g), vinyl chloride (15 g). methylhydroxypropylcellulose (0.05 g), lauroyl peroxide (0.3 g) and water (200 g), More than 95 per cent of these monomers are converted to copolymer when this aqueous suspension is agitated in an oxygen-free atmosphere for 40 hrs at 60°C. The glass transition temperature of the homopolymer is — 17°C. It has a specific gravity of 1.875 and a solubility parameter of 9.8. 

Prior to this discovery, in 1954 Silberberg and Kuhn (62) were first to study the polymer-in-polymer emulsion containing ethylcellulose and polystyrene in a nonaqueous solvent, benzene. The mechanisms of polymer emulsification, demixing, and phase reversal were studied. Wetzel and Hocks discovery would then equate the pressure-sensitive adhesive to a polymer-polymer emulsion instead of a polymer-polymer suspension. Since the interface is liquid-liquid, the adhesion then becomes one type of R-R adhesion (35, 36). According to our previous discussion, diffusion is not operative unless both resin and rubber have an identical solubility parameter. The major interfacial interaction is physical adsorption, which, in turn, determines adhesion. Our previous work on the wettability of elastomers (37, 38) can help predict adhesion results. Detailed studies on the function of tackifiers have been made by Wetzel and Alexander (69), and by Hock (20, 21), and therefore the subject requires no further elaboration. 

EflEect of Viscosity and Solubility Parameter of a Nonreactive Liquid Additive on the Emulsion Polymerization of Styrene... 

It is the purpose of this investigation to attempt to modify a conventional styrene emulsion by the addition of a nonreactive oil soluble additive with different combinations of viscosity and Hildebrand solubility parameters. It was anticipated these additives would Induce the same heterogeneous condition as in a monomer system with poor polymer solubility. 

Benzene is termed a good solvent for polystyrene since Its solubility parameter (6=9.2H) Is within a previously established range of 1.8 for polystyrene (6=9.2H). When hexane (6=7.3H) was used at the same concentration, very little polymerization retardation was observed. The intrinsic viscosity and GPC elution times of the polymer resulting from the hexane modified emulsion Indicated it was substantially lower in molecular weight than the control. 

As shown In Figure 4, the rate of polymerization of styrene was retarded by good nonvlscous solvents such as benzene, cyclohexane, and octane whose solubility parameters (6) were within 1.5H of that of polystyrene at styrene to additive ratios of 3 to 1. The absolute rates were slightly Increased In poorer nonvlscous solvents such as heptane and hexane and were fastest In viscous nonsolvents such as dllsoctyl phthalate and Nujol. Rate studies Indicated a Rp dependency on [E] substantially greater than unity for the styrene emulsion systems modified with viscous poor solvents. 

For nonionic surfactants, an optimization of the process was achieved by using a similar approach to the so-called Cohesive Energy Ratio (CER) concept developed by Beerbower and Hill for the stability of classical emulsions (H). Its basic assumption is that the partial solubility parameters of oil and emulsifier lipophilic tail and of water and hydrophilic head are perfectly matched. Thus, the Vinsor cohesive energy ratio Ro, which determines the nature and the stability of an emulsion, is directly related to the emulsifier HliB (hydrophile-lipophile balance) by... 

Mandal S and Pangarkar VG. Pervaporative dehydration of l-methoxy-2-propanol with acrylonitrile based copolymer membranes prepared through emulsion polymerization A solubility parameter approach and study of structural impact. J. Memb. Sci. 2002 209(l) 53-66. 

There have been numerous attempts to determine HLB numbers from other fundamental properties of surfactants, e.g., from cloud points of nonionics (Schott, 1969), from CMCs (Lin, 1973), from gas chromatography retention times (Becher, 1964 Petrowski, 1973), from NMR spectra of nonionics (Ben-et, 1972), from partial molal volumes (Marszall, 1973), and from solubility parameters (Hayashi, 1967 McDonald, 1970 Beerbower, 1971). Although relations have been developed between many of these quantities and HLB values calculated from structural groups in the molecule, particularly in the case of nonionic surfactants, there are few or no data showing that the HLB values calculated in these fashions are indicative of actual emulsion behavior. 

Beerbower (36) has correlated solubility parameter with emulsifier selection with some success. Following Winsor (37), he calculates a ratio of the lyophobic to hydrophilic portions of emulsifiers using Hansen s three-component solubility parameter values. In the one test reported, there seems to be excellent correlation of the optimum ratio with stability of the emulsion. 

The choice of emulsifier is critical since it controls the stability of the emulsions prior to and after polymerization. Moreover, polymerization conditions typically represent destabilizing factors vigorous stirring, temperature rise and evolution of acrylamide content in the aqueous phase. In the case of inverse emulsions, the HLB values mostly used by the formulators range between 4 and 6. Some attempts were made to predict quantitatively the optimal HLB value corresponding to the most stable dispersions [18,19]. The treatment was based on the so-called cohesive energy ratio (CER) concept devekq)ed by Beer-bower and Hill for conventional emulsions [20]. Tins approach is based on a perfect chemical match between the partial solubility parameters of oil (ig)... 

I summarize briefly below the basic concepts of this approach, which is derived from that developed by Beerbower and Hill [31] for the stability of classical nonionic emulsions, which is referred to as the cohesive energy ratio (CER) concept. The treatment lies in a perfect chemical match between the partial solubility parameters of oil ( ) and surfactant lipophilic tail 6]) and of water and hydrophilic head. Under these conditions, one obtains for the optimum HLB (hydrophile-lipophile balance) of the surfactant the relation... 

FIGURE 11.10. The optimum functioning of a surfactant as an emulsion stabilizer depends on the correct balance of its solubility parameter with those of the two phases. The proper balance gives optimum adsorption at the interface (a). If its value is too close to that of the external phase, the surfactant will be too soluble in that phase and will not adequately adsorb (b). If it is too close to that of the internal phase, the same will occur, but in the internal phase (c). 

Since the cloud point of a surfactant is a structure related phenomenon, it should also be related to HLB, solubility parameter, cmc, and other parameters, as is found to be the case. Clearly, temperature can play an important role in determining surfactant effectiveness where hydration (or hydrogen bonding) is the principal mechanism of solubilization. Because of the temperature sensitivity of such materials, their activity as emulsifiers and stabilizers also becomes temperature sensitive. In particular, their ability to form and stabilize o/w and w/o emulsions may change dramatically over a very narrow temperature range. In fact, an emulsion may invert to produce the opposite emulsion type as a result of temperature changes. Such a process is termed phase inversion, and the temperature at which it occurs for a given system is its phase inversion temperature (PIT). 

Clearly, the process of selecting the best surfactant or surfactants for the preparation of an emulsion has been greatly simplified by the development of the more or less empirical but theoretically based approaches exemplified by the HLB, solubility parameter, and PIT methods. Unfortunately, each method has its significant limitations and cannot eliminate the need for some amount of trial-and-error experimentation. As our fundamental understanding of the complex phenomena occurring at oil-water interfaces, and of the effects of additives and environmental factors on those phenomena, improves it may become possible for a single, comprehensive theory of emulsion formation and stabilization to lead to a single, quantitative scheme for the selection of the proper surfactant system. 

Foaming ability of surfactants can also be correlated with the respective solubility parameter, as discussed for emulsions in Chapter 11. In this case, the solubility of the surfactant must be properly balanced—that is, be soluble enough to attain a significant concentration in solution, but not so soluble that significant adsorption does not occur. 

The next series of Figs. 5-7 illustrate the use of a relatively new aromatic modified terpene resin as a tackifier for natural rubber latex. Again, the properties of probe tack, quick stick, peel and shear adhesion were measured for various rubber/resin ratios. We observe that this resin tackifies the natural rubber latex quite well, yet the curves are substantially different from those generated with the beta-pinene resin emulsion. These differences can be attributed to differences in solubility parameter (caused by compositional differences) and molecular weight and distribution of the two resins. 

Research into optimal formulations is based on the idea of cohesive energy ratio (CER). This was originally developed to stabilise classic non-ionic surfactant emulsions [6.13]. Despite its limitations, the CER concept unifies the ideas of solubility parameters and HUB. Recall that the HLB is a measure of the emulsifying power of surfactants and is based on their hydrophile-lipophile balance [6.3]. It can be calculated from a simple formula involving only relative weights of sequences HLB = 20 x Mh/Mt, where Mh is the molecular weight... 

Equivalence between the lipophilic part of the surfactant and the oil means that (6.1) can be used to calculate the required HLB for an oil whose solubility parameters are known, depending on the nature of the emulsion desired. 

HLB (hydrophilic-lipophilic balance) Yes (group contribution methods, solubility parameters) Design of emulsions including stability of emulsions and determining the emulsion type... 

---