The relationship between film formation and MFFT of waterborne coatings

The relationship between film formation and MFFT of waterborne coatings

2020-02-22 17:44:24
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The relationship between film formation and MFFT of waterborne coatings
This week, Mr. Li asked us three questions through our questioning water-based paint expert column. This is the second problem:

Problem:

Film formation above MFFT, no film formation below MFFT, what happens during above or below the MFFT, resulting in film formation?

Above MFFT, the emulsion particles are soft and hard below. This phenomenon is associated with glass transition.

response:

On this issue, we published a panel discussion on the blog last year. In the article entitled "The relationship between the glass transition temperature (Tg) of aqueous resin and the minimum film formation temperature (MFFT)", our explanation is:

In general, the higher the glass transition temperature of an aqueous resin such as an aqueous acrylic emulsion or an aqueous polyurethane dispersion, the higher the minimum film forming temperature, the poorer the film forming property at room temperature, and the amount of film forming aid required. The bigger. The reason for this is well known, and the necessary condition for film formation is the mutual fusion of the aqueous resin particles. This requires that the aqueous resin particles have sufficient fluidity at the film forming temperature, that is, the film forming temperature should be higher than the glass transition temperature of the aqueous resin particle resin.
However, the Tg of many aqueous resin products after film formation is higher than the minimum film formation temperature, contrary to the common rules described above. There are several reasons for this.First, the general theory described above is based on several assumptions. It is assumed that the glass transition temperature or softening temperature of the resin in the aqueous resin particles is the same as that of the resin after film formation. Hypothesis 2 is that there is no difference between the aqueous resin particles. Both assumptions can be broken in the actual aqueous resin synthesis process.
First of all, in the past ten years, people have learned from nanomaterials that when the size of the particles reaches the nanometer level (less than 50 nm), the particle surface is increased due to the large increase in the surface area of the particles. The effect on the bulk properties of the particles begins to occur, and the melting point and softening point of the particles are greatly reduced. That is, when the content of the nano-sized particles in the aqueous resin is increased, the glass transition temperature of the particles is lower than the glass transition temperature after film formation.In addition, modern aqueous resin synthesis techniques generally use a multi-step polymerization process to form a multi-layer or multi-phase structure of the aqueous resin particles, and also cause a difference between the aqueous resin particles, that is, the glass transition temperature of the aqueous resin particles. Will be different from each other. The aqueous resin can be formed into a film as long as there are enough soft particles in the system.

Another reason why the aqueous resin film forming temperature is lower than the glass transition temperature after film formation is the reactivity of the film. Some self-crosslinking systems initiate a further crosslinking reaction after film formation, causing the molecular weight of the bulk of the material to increase, resulting in an increase in the glass transition temperature of the film. However, usually this type of effect is relatively small because the ability of the glass transition temperature to increase with molecular weight is limited.

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