At their most basic level, surface coatings are used to protect the surface of a material from the environment, but you probably already knew that. Film formation, a term typically used for water-based coatings (latex-type polymers), is critical to the protective properties of a coating. Film formation happens once the water-based coating is applied and the water evaporates during drying, causing the particles to “smush” together (coalesce) to create a uniform film. The coalescence of the indiscreet particles within a water-based polymer is what creates a good protective paint. Water-based coatings are different from solvent-based coatings where the polymers are dissolved, which means there is no coalescence step.
Film formation would not happen if the particles simply dispersed on the surface without coalescing. It would be like dumping a bucket of ball bearings on a table. They would move freely, acting as separate entities without forming a barrier. If those same ball bearings were glued together, they would create a solid, unified barrier. To create a dependable protective coating, the polymer particles in a good system will transform from being discrete entities to a uniform film.
Factors to consider that will affect the film formation process:
- Coating formulation and environmental conditions
- Glass transition temperature of the polymer (Tg)
Each of these factors needs to be considered during the paint formulation process to maximize film formation to create the best coating. Following are some ways to optimize the coating to have the broadest application window by taking into account these three factors.
There are two ways that can help coalesce the particles:
- Increase the temperature during application to soften the polymer through a melting process.
- Use a cosolvent (glycol ether type cosolvents are most commonly used) to slightly swell the polymer, allowing it to merge together more efficiently.
Choosing the proper technique depends on the application. For example, a part made during an industrial manufacturing process might go through a baking process (an oven), so the increased temperature technique is a natural fit because there is already a heating step, which means less cosolvent is needed.
When using a cosolvent, it’s important to choose the correct type and amount for your polymer. Most latex polymers require a cosolvent to aid film formation, which softens the copolymer and enables easier particle coalescence. Some polymers need a lot more help from these two mechanisms to form a good film. Others are innately softer and do it more readily on their own. The harder a polymer is, the more help it will need for film formation.
Plasticizers can also be used (phthalates are the most common), which behave similarly to cosolvents, but are classified differently. Plasticizers don’t evaporate, so they don’t exit your film the way a cosolvent would. They also often reduce the overall hardness of a polymer system.
The lower the temperature at the application point, the more difficult it is to realize a good film formation. To apply a coating in low temperatures, the formulation must be optimized. Humidity is another factor. If it’s too low, paint will sometimes dry very fast, which can lead to more porous films. High humidity, on the other hand, will reduce the speed of water evaporation from a paint film, which can allow more time for effective coalescence.
Glass Transition Temperature
Glass transition temperature (Tg) and minimum film formation temperature (MFFT) are related. Tg is the temperature at which the polymer goes from a hard, glassy state to an amorphous, rubbery state. A low Tg polymer will be soft at room temperature and form a film more easily. The higher the glass transition temperature, the polymer will be harder and require more effort to get the film to form, which means higher cosolvent levels or higher temperatures if baking or the use of plasticizer.
These factors are very application driven. There are also government regulations to consider, with each coating/application having rules that govern how much VOC can be in the formulation.
As an example, coating a piece of farm equipment requires a hard polymer to be scratch resistant and extremely durable. Higher cosolvents can be used in this situation, and government regulations allow it. A roof coating is more elastic, and those types of applications allow only a low level of cosolvent because they don’t require it.
There are many factors and techniques that must come together to create a functional, quality coating. Contact your Lubrizol account manager to tap into our film formation expertise.