The minimum film-forming temperature (MFFT) is a critical parameter for polymeric dispersions. It determines the lowest temperature at which the paint can coalesce into a continuous film upon drying. All paint formulations include the three major components i.e., polymeric binders, pigments, additives, and solvents.
The binder particles exist as dispersed spheres in water. They provide the crucial film-forming functionality once the water evaporates. However, the polymer chains that make up these particles cannot freely flow at lower temperatures due to restrictions.
- In cold environments, the painted surface becomes thick and viscous resulting in a bumpy surface.
- In extreme cases, the paint may not even form a film at all, and will simply dry as a thick, tacky mess. This can lead to peeling, cracking, and other problems with the coating.
To avoid these problems, paint manufacturers should control the MFFT of their products. They do this by selecting the appropriate polymer, solvent, and additives.
Heating the paint increases chain mobility. This enables the deformation and interdiffusion between binder particles. It is required to transform discrete spheres into a uniform, cohesive film.
Let's have a deeper understanding about this topic:
- What is the minimum film-forming temperature (MFFT) and its mechanisms?
- Why is the importance of MFFT in coating film formation?
- What are the applications of MFFT in coating applications?
- How is MFFT evaluated?
Minimum Film Forming Temperature (MFFT) — Definition and Mechanisms
MFFT is a temperature-related property like glass transition temperature, governing the film formation in coatings. It is a characteristic temperature of polymer below which it can no longer form a film*.
The minimum film-forming temperature is the lowest temperature required to coalesce** an aqueous polymer dispersion (latex or emulsion) when applied to a substrate into a thin film. It is an important property to avoid a coating failure.
- At temperatures at and above the MFFT of the polymer, a smooth film is formed.
- At temperatures below the MFFT of the polymer, continuous film is not formed. This is because the polymer cannot coalesce and thus leads to a white powdery film with cracks.
The formulation factors influencing MFFT enable paint chemists to develop weather-resistant coatings. Insufficient film formation makes the paint unusable for practical purposes. This is regardless of how well other qualities like scrub resistance or color uniformity are optimized.
NOTE:
*Film can be defined as a thin layer of coherent solid matter adhered to a substrate.
**Coalescence is the fusing of molten particles to form a continuous film.
Main steps in the film formation mechanism
Water-based or aqueous systems are either solutions or dispersions. This depends on the water solubility of the polymers. These polymer dispersions differ greatly in their structure from other binders. Film formation in such systems happens once the coating is applied. The water evaporates during drying, causing the particles to coalesce with the help of
film formers. This creates a uniform film. The main steps in the process involve:
- Formation of bond due to cohesion forces between the coating polymer molecules. This happens when the polymer solution is applied to the surface.
- Coalescence of adjacent polymer molecular layers or surfaces occurs through diffusion.
- When most of the water evaporates, the viscosity of the solution increases. This leaves the polymer chains near each other and deposits over a previous polymer layer.
If there is an adequate cohesive attraction between the molecules and sufficient diffusion and coalescence upon the more complete evaporation of the residual water, the individual polymer chains align themselves to form a cohesive film.
Film Formation from Aqueous Polymeric Dispersion
The mechanism of film formation is different in solvent-based systems and water-based systems.
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Solvent-based coatings — The organic solvent evaporates and the polymer chains approach each other. Finally, they form a continuous homogeneous film upon application.
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Water-based systems — The water evaporates and the polymer particles approach each other. Under appropriate conditions, they coalesce to form a homogeneous polymeric film. These conditions include a particular temperature and the presence of sufficient amounts of water and/or other
plasticizers).
MFFT and its relation to glass transition temperature
Complete film formation can only occur above the minimum film-formation temperature. Below this temperature, the polymer particles are very hard. They are not sufficiently deformed during the drying process. Inter-diffusion of polymer chains is then hindered or no longer possible. Thus, resulting in a poorly adhering powdery layer on the substrate.
The
Tg of the polymer particles in a polymer dispersion corresponds to the MFFT within a few degrees Celsius. The paint can become sticky or tacky as temperature increases before forming a compact film after drying.
Coalescing agents are thus added to polymer dispersions to obtain paints that exhibit a low MFFT. But they do not become sticky at higher temperatures.
Besides the Tg, there are other dispersion and polymer properties that can influence the film formation process. They are particle size, particle size distribution, molecular weight, degree of cross-linking,
emulsifiers, etc.
Importance of MFFT in Coating Film Formation
MFFT must not be above the processing temperature. Like Tg, MFFT determines, the
hardness of the coating film. Coating formulators are often required to check the MFFT of the polymer. This enables them to understand how the polymer will affect the following parameters of coatings:
Knowing the MFFT enables formulators to create a product that cures under the required application conditions. For effective use, paints and coatings based on emulsions should be applied only to surfaces with a temperature above their MFFT. This is usually derived by the manufacturer and printed on the side of the tin.
Efficiency of coalescing agents in film formation process
Coalescing agents are are also known as film-forming agents, aids, or coalescents. They are often added in the coating systems that contain polymer dispersions as the binder. This aids in the
film formation process.
When a coalescing agent is added in a dispersion, it diffuses into the polymer particles and reduces their hardness. It thus acts as a temporary plasticizer. This
optimizes the film formation of polymer dispersions under given conditions of application. It reduces the Tg of water-based coatings below the MFFT.
The CA-lowered T of the polymer gives the reduced MFFT of the dispersion. This allows film formation at lower temperatures. Since it is a “temporary plasticizer”, it then slowly evaporates from the film during or after application. The applied film achieves its original Tg (hardness) and film properties. The requirements of a coalescing agent for a polymer dispersion are as follows:
- It must act as a plasticizer for the polymer used in the polymer dispersion
- It must not dissolve in the solvent used for the polymer dispersion (for example, water)
- It must evaporate much more slowly than the solvent used
Coalescents are less volatile than water and remain in the film for an extended period before completely evaporating. This facilitates the fulfillment of optimum coating properties. They can be used individually or as a blend of two or more coalescent agents.
Learn in detail about the coalescent agents and get better at selecting them for improved film formation.
Practical Applications of MFFT in Coating Formulations
Product development
Formulators use MFFT for the development of latex binders and paint formulations. These can form continuous films under the expected drying conditions. Adjusting parameters like Tg and plasticizers allows tailoring MFFT.
Quality control
MFFT serves as a key quality control test. It ensures freshly produced batches of
paint or coating will form defect-free films without cracks or poor adhesion. Out-of-spec MFFT indicates issues with storage, handling, or ingredients.
Application conditions
Paint applicators rely on the quoted MFFT range of a coating product to set process conditions. For example,
drying/curing temperatures. This ensures that applied paint flows out and coalesces properly before drying. Higher MFFT products may need heated environments.
Troubleshooting
It enables the analysis of coating failures as the measured MFFT provides clues about the stemming of poor film formation. The failure could be due to product issues or inadequate application temperatures being below the polymer's MFFT.
Weatherability prediction
The close normal application temperature to MFFT indicates the stability margin and the potential for film defects to emerge under cooler weather conditions over the coating lifetime.
Standard Test Methods to Evaluate MFFT
The MFFT is usually closely related to the Tg but not synonymous with it. The Tg may be determined by predicted calculation. MFFT is best determined using an MFFT bar, the basic principles of which are described in ASTM D2354.
ASTM D2354
It determines the minimum film formation temperature (MFFT) of emulsion vehicles. This test method evaluates the minimum temperature at which emulsion vehicles coalesce to form continuous films. The term “emulsion” in this test method includes latex vehicles. This test is limited to emulsions having MFFT below 90 °C.
Another related standard is ISO 2115. This method determines white point temperature and MFFT. Alternatively, the influence of a coalescing agent on the Tg can be quickly determined by
DSC measurements.
Testing Standards for Minimum Film Forming Temperature
(Source: Spektrochem Laboratory)
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