Interactions and Network Formation Theories

The modeling of latex systems is not trivial as a result of the many possible variations that can be present. For stability purposes, most latex resins are synthesized with oligomeric acids or polymer fragments grafted to their surfaces. These surface modifications along with latex particle size dictate the amount of surfactant and associative thickener that can be adsorbed. Another issue for studies on latex is that they are synthesized in various median particle sizes and particle size distributions, and are commonly blended to make bi- and multimodal particle size distributions (Chen, Wetzel, Ma, & Glass, 1997). All of these issues must be accounted for when attempting to define the interactions between the latex, surfactant, and thickener. The complications are increased with pigmented systems, which create another variable in the interaction scheme for a given formulation. One traditionally accepted viscosifying mechanism for a fully formulated system is show in Figure 5 (Münzenberg, 2011).

Figure 5. Schematic showing the traditional theory for the formation of a transient network of a fully formulated HEUR thickened system (Münzenberg, 2011).

The adsorption of HEUR polymers onto latex surfaces through hydrophobic end groups, with the PEO portion forming a thin shell is understood and recognized; however, the effects of component contributions of core-shell hydrodynamic volume, particle aggregation, viscous drag on aggregated particles, and HEUR transient network on a given system’s rheology is unclear (Beshah, Izmitli, Van Dyk, Rabasco, & Bohling, 2013). Early research proposed a transient network, shown in Figure 5, in which HEUR polymers form flower-like micelles which coexists with latex. In this model, some of the

shown centrifugation to disrupt the weakly bound hydrophobes resulting in inaccurate detection of free HEUR in the aqueous phase (Beshah, Izmitli, Van Dyk, Rabasco, &

Bohling, 2013). A newer technique developed by Beshah et al. utilized an in situ method to detect the free HEUR in the aqueous phase as a way to circumvent the perturbation of whichever networks are present in the system. This test included a PFGNMR of a neat system of HEUR, latex, and water which counteracted the idea of the formation of flower-like micelle formation of HEUR. A simple cartoon representation of this referenced work is shown in Figure 6 (Beshah, Izmitli, Van Dyk, Rabasco, & Bohling, 2013).

Figure 6. Simple schematic of the old (left) and current model (right) based on PFGNMR results for 1% HEUR and 30% latex composite. Small black dots represent hydrophobic end groups of HEUR molecules (Beshah, Izmitli, Van Dyk, Rabasco, & Bohling, 2013).

This test was conducted in a 1% HEUR and 30% latex composite which is a typical representation of commercial paints and coatings. The study concluded that at this level of HEUR and latex there was no flower-like micelles typical of the formation of a transient network shown on the left side of Figure 6. The study does mention that at higher levels of HEUR that flower-like and transient networks may arise, but the scope of

mechanism was proven for a simple system of latex, thickener, and water so the application to a fully formulated system may be different. The formation of a transient network with interactions of HEUR to HEUR via the hydrophobic ends has been shown not to exist in an aqueous system; however, this has not yet been proved for a fully formulated system.

Both models of the formation of networks for a HEUR thickened system are an attempt to define the viscosifying mechanisms present. Each model has limitations, and for this project report the latter model presented by Beshah et al. in 2013 will be considered more accurate.

The nature of the weak van der Waals type hydrophobe-hydrophobe interactions creates an adsorption-desorption exchange of the hydrophobe groups of the HEUR molecules and latex particles. As a result, the stronger the interaction between the hydrophobes of HEUR and hydrophobic latex surfaces, the lower the exchange rate and thus the lifetime of the hydrophobe in the aqueous phase. This means that exchanges in this given system are mainly between loop to loop, loop to direct bridge, and perhaps direct bridge to direct bridge (Beshah, Izmitli, Van Dyk, Rabasco, & Bohling, 2013). Formation of bridges loops, and chain transport between surfaces have been studied and are dependent on HEUR hydrophobes, concentrations, hydrophobicity, particle size, and others (Dewalt, Gao, & Ou-Yang, 1996).

Another interaction that must be considered is hydrophobic interactions introduced via

the viscosity increase will reach a maximum and the show a decrease in viscosity. The decrease in viscosity with an increase in surfactant concentration is typically observed above the CMC of the surfactant (Glass, 2001). The use of colorants in paints, which contain high levels of surfactants, has been an issue resulting in viscosity loss. Much research has been dedicated to develop a rheology modifier with improved viscosity stability with the additions of surfactant. One newer rheology modifier technology has been developed which reduces the viscosity loss upon tinting. The improved viscosity stability is achieved essentially through a new viscosity-building mechanism (Saucy, 2008). Two classifications that are commonly used for associative thickeners are high-shear viscosity builder (also called an ICI builder) and mid-high-shear viscosity builder (also called KU builder). KU, which is called a Krebs Unit, is a viscosity measurement most widely used in architectural paints with typical KU values ranging from 90-100 (Wicks, Jones, Pappas, & Wicks, 2007). The ICI builder is typically used in higher amounts than the KU builder, and has a higher molecular weight than the KU builder as well. The difference in molecular weights, shown in Figure 7, leads to the two thickeners occupying varying volumes of space when adsorbed to the latex. This difference in occupied space hinders access to the lower-molecular weight KU builder, and essentially inactivates a fraction of the thickening polymers of the system. When surfactants are added, displacement of thickener will occur for both the high- and shear builders, but some compensation will be provided through the now activated mid-shear thickeners that were not previously participating in network building. In addition, the displaced polymer chains can participate in building an extended linked network that was not accessible before addition of surfactant (Saucy, 2008).

Figure 7. Schematic representation of the hydrodynamic volumes of low and high molecular weight thickeners that are adsorbed onto the latex surface (Saucy, 2008).