Stable Colloidal Polymer System
Our system may be defined as a STABLE COLLOIDAL POLYMER SYSTEM in which the continuous phase is exclusively or primarily aqueous. The very high molecular weight polymer maintains low product viscosities for the coating. The workable viscosities offer ease of application.
The process of drying goes through a transition state of changing from the liquid to the solid state. This process is commonly referred to as the coalescence. The film formation begins as the wet polymer coating existing as micro droplets, micells, of solid material dispersed in water. Some coalescent solvent will migrate into the polymer. As the level of water decreases during the drying process, the polymer particles become forced together closer and closer. When the repulsive forces cannot keep the micells separated, the polymer solidifies.
Below the glass transition temperature, the polymer particles will not polymerize and the coating will exhibit brittleness, cracking, stiffness, and transparency. The coalescent solvent will insure that the polymer particle changes in properties associated with the virtual cessation of the molecular motion on the local state. Simply, the polymer particles are soft enough at the point of coalescence to form a continuous film. The coalescent slowly evaporates to allow the coating to develop the maximum durability and appearance properties.
The dry film becomes a tough and resistant continuous polymer film. Below the minimum film formation temperature, the polymer cannot form a continuous film. Also, the coalescence of the individual polymer particles helps to minimize the use of surface-active agents. Relative humidity affects the evaporation of both coalescent solvents and water. Both give entirely different results.
During film formation at low to moderate relative humidity, water leaves the coating more rapidly than the coalescent solvent. The density of the polymer particles is reached more quickly. The coating films quickly and partially coalesces. The coalescent solvent must diffuse through the partially formed film before it can volatilize into the air. The time period is a slow process. This process retards the early development of film hardness. Tack free time is increased. The quality of the final film is good.
High relative humidity changes the situation. Water evaporates slowly with the coating staying wet and without forming a film. Coalescent evaporation continues at a fast rate since it does not diffuse through the partially formed film. Even if coalescent solvent has left the coating for the polymer particles to return to their normal hard state. Without enough coalescent to soften the polymer particles, adequate film formation is not possible.
Air circulation on film formation reduces the situation with relative humidity. Circulating the air in heated oven increases evaporation by replacing moist air with dry air that will absorb water more readily. The force drying at elevated temperatures will resolve high relative humidity. The heat air reduces the relative humidity even when the surrounding outside air has a high relative humidity.
The additions of various additives have resolved defects that originally would have been present. Some examples are defoamers eliminating micro-foam caused during manufacturing, defoamers eliminating air-entrapment during filling, preservatives and biocides destroying unwanted micro-organisms, freeze thaw agents for freeze stability, slip agents for slip resistance, and surface tension agents for flow and surface leveling.