Water-borne paints based on dispersions of styrene-acrylic copolymers, pure acrylic copolymers, copolymers of vinyl acetate-ethylene (VAE) or with VeoVa, versatic and other monomers used in one-component architectural paints, direct-to-metal or wood, dry by evaporation of water and other volatile components and the coalescence of polymer particles , i.e. their sticking together. This process takes place until the mobility of the particles is maintained and it is possible for them to stick together due to the increasing concentration of the polymer in the volume of the coating. This limiting moment is the temperature we call the minimum film forming temperature (MFFT).
As a result of the formation of the coating, the copolymer particles are plastic to the glass transition temperature, which together with MFFT can be lowered to a level even slightly below 0 °C by means of film forming aids – coalescing agents. Otherwise, by preventing the formation of a continuous coating below its forming temperature limit, a cracked coating [Figure 2] will be formed which will have no decorative, and even less protective, properties. Cracks may be invisible at the boundary points of coating formation, but the strength of the coating will still be insufficient due to the lack of adequate cohesive strength between the polymer particles and fillers and other coating ingredients.
As a result of the evaporation of water, coalescing additives and other volatile components, a process of transformation of aqueous dispersions into cohesive film takes place. The transparency of this film without cracks means the formation of the film in relation to the MFFT test standard, however, the strength of this film in the paint requires further tests, especially in terms of the selection of the coalescent dose that is to form the film on absorbent substrates, ensure the appropriate open time, wet edge, etc. Here I recommend my PCI Magazine article on the importance of water miscibility of coalescing agents in relation to film formation on substrates with different water absorbency – link to article in PCI e-magazine.
Determininig of MFFT
To determine the minimum temperature at which the film forming process of the polymer dispersion binder for latex paints takes place, the minimum film forming temperature is determined in accordance with ASTM D2354. The test is carried out using the MFFT-Bar apparatus [Figure 3], which is a block with an electronically controlled plate ensuring a smooth temperature transition, allowing the temperature to be read at the point where the non-cracked film is formed.
In the Spektrochem Paint Technical Center we use the Rhopoint MFFT Bar II, which allows for automatic performance of the MFFT determination of polymer dispersion and for mixtures of polymer dispersions with film forming aids. The device, using the adjustable temperature of the working plate and the supply of compressed air flowing 4 L/min, allows for accurate determination of MFFT in the range from -10 to 60 °C,
A homogeneously mixed emulsion or polymer dispersion or a mixture with coalescing agents is applied to the plate of the apparatus with a 1″ cube applicator. This applicator, through a 75 μm gap, allows the application of a series of strips, which then dry on the working plate of the apparatus in the atmosphere under cover with a standardized air flow. Due to the temperature griadent on the apparatus plate, in the place where the tested sample can no longer form a transparent film, cracks begin to appear, which can take various shapes (from single short cracks, through long cracks, checking along the entire width of the film, to after complete flaking of the film). Examples of cracks are shown in Figure 4.
Determination of the coalescence characteristic curve
The MFFT Bar apparatus is used not only to determine the MFFT of dispersions and emulsions of polymers without additives, but also to determine the characteristics of reducing MFFT by film forming aids. The coalescence of polymer particles can take place at lower temperatures thanks to the addition of coalescing agents which, due to their physicochemical nature, are able to reduce the MFFT to 0 °C or even slightly below.
In order to analyze the ability to reduce MFFT to various levels, ladder studies are performed with coalescents added in various doses to the polymer dispersion, which are usually converted into the total polymer latex or non-volatile content of the polymer emulsion. An example of a curve for characterizing the MFFT determined with different coalescents at different doses is shown in Figure 5.
The determination of the coalescence curve using the MFFT Bar, however, cannot always be the basis for taking the coalescent dose, despite the fact that the MFFT determination indicated a reduction to a certain desired level, especially in the low temperature range of 0-6 °C.
The selection of the film forming aid dose must be supported by tests on the paint in a specific PVC and on the basis of established raw materials. The selection then consists in determining whether the coalescent dose is sufficient to lower the MFFT in the latex paint in the desired temperature range, always taking into account film formation on an absorbent substrate (see the article on this topic published in PCI Magazine), determining the influence of the coalescent on the properties of the coating ( influence on gloss, blocking, resistance to dirt retention, etc.) The MFFT test is a valuable tool to determine the course of MFFT reduction and the starting dose from which further case studies and ladder studies on a specific paint formulation should be carried out.
Differences in ASTM D2354 and ISO 2115 standards
The MFFT determination can also be performed with ISO 2115, which is essentially the same, but in the details of the optional test implementation, the obtained results differ, and these differences depend on the type of polymer dispersion. Basically the MFFT test is performed directly on the MFFT Bar plate, however to keep the instrument clean, the ASTM D2354 standard suggests that a drawdown should be performed on a polyester film which is placed on the build plate and electrostatically equalized with a cloth. In ISO 2115 such a suggestion also occurs, however here the use of aluminum foil is recommended, and thinly dispersed glycerin is used as the temperature conducting layer from the working plate to the foil. Unfortunately, in many cases the comparative results obtained in ASTM D2354 and ISO 2115 tests show even 3-4 °C difference (for dispersion with MFFT in the range of 8-12 °C), which is a big difference – the worse reproducible results are obtained by using ISO 2115 and the recommended aluminum foil with glycerin. Thus, the repeatability of the ASTM D2354 standard is greater, hence in the Spektrochem Paint Technical Center it is used before ISO 2115.
The MFFT test performed in accordance with ASTM D2354 is an extremely useful test for formulating paint formulas based on water-based polymer dispersions and emulsions. It is an assay used for the laboratory determination of MFFT and the starting dose for ladder studies to determine what dose of a given coalescent should be used to lower the MFFT to a desired level, usually close to 0 °C. This is a simple test method that requires the technician to have experience in reading the place where the film is already formed, especially since it is not always one clearly discernible border of the film’s transition from a cracked state to a transparent state. This test is so important when determining recommendations for producers of raw materials – film forming aids, that it is always the first stage of studies for coalescents. It should also not be forgotten that this test can also be used to check that other raw materials do not adversely affect the MFFT and do not increase the temperature value at which the film is formed, e.g. defoamers, which often increase the MFFT of polymer dispersion.