(8c) Latex Film Formation: Influence of Coalescent Distribution on Film Formation Behavior

There are three primary stages of latex film formation: 1. Evaporation of water, 2. Deformation and close packing of latex particles, and 3. Interdiffusion of polymer chains.  The final step of this process is essential to creating defect-free, mechanically robust films.  Historically, high performance coatings were made using a formulation containing a high-T_g polymer and a volatile coalescent.  The volatile coalescent enables film formation by softening the polymer latex particles.  This softening enables polymer chain interdiffusion, and upon evaporation of the volatile coalescent, a harder, high-T_g coating is created.  Recent and proposed changes in regulations drive a market need for materials with no/low volatile organic compounds (VOCs), yet provide good performance.  Typically, low VOC coating formulations contain polymers with low T_gs.  Coatings from these low-T_g paints generally have a soft/tacky feel and poor performance.  These shortfalls are particularly prevalent in glossier formulations that contain low pigment loadings.  Alternatively, harder, high-T_g binders can be used in low-VOC formulations by incorporating permanent plasticizers.  However, when permanent plasticizers are used at sufficient levels to enable low temperature coalescence, the resultant coatings also have a soft/ tacky feel that will not improve over time.  

One solution to the inherent tradeoff between film formation and performance in low VOC coatings is to use more effective coalescents.  The efficiency of a coalescent is primarily a function of its solubility within the polymer latex particle and the DT_g between the polymer and coalescent.  Estimates of T_g depression, required to enable film formation, can be calculated using well-known mixture rules such as the Flory-Fox equation.  Beyond these simple considerations, there may be are other factors that influence the efficiency of a coalescent.  One possible such factor is the distribution of the coalescent within the polymer latex particle.  Understanding how this factor may influence performance requires a technique to measure the coalescent distribution within the particle.  In this presentation, the use of small-angle neutron scattering to evaluate coalescent distribution within latex particles will be discussed and evaluated.  The ability of this technique to evaluate small molecule behavior within a latex particle will hopefully aid in the development of more efficient coalescents and reduce VOCs required for high performance coatings.