Minimum film-forming temperature MFT

At the phenomenological level it may be known that Solvent A works well with acrylic dispersions while Solvent B is better for vinyl dispersions. Experimentally the important question of how much to add can be determined by systematic experimentation. However, for fine tuning some theoretical knowledge about the influence of transient solvent on the glass transition temperature (Tg), and tihe influence this will have on the minimum film forming temperature (MFT) is usefiil. This would undoubtedly lead to some consideration of the distribution (partitioning) of the solvent between the aqueous and polymer phases, which can be... 

Ethyl acrylate-methyl methacrylate copolymers were provided by the Rohm and Haas Co. and films were formed on the impedometer at temperatures which bracketed their minimum film-forming temperatures (MFT). The value of G above w jch a coherent film could not be obtained was ca. 10" dynes/cm which constituted the practical limit of operation of the device. 

Films were prepared from the two sets of samples described in Table 2. One immediately apparent difference is that the PBMA/PEO latex formed transparent films at 20°C, whereas the PBMA dispersion required higher temperature ( 30 C). The surface PEO chains appear to lower the minimum film forming temperature [MFT] of the latex. Interdiffusion measurements were carried out on these two films, and f values were calculated, as is shown in Figure 4. These results are quite surprising. One notices first that the surface PEO chains promote interdiffusion at early times. This result is consistent with the PEO also lowering the MFT of the system, and suggests that PEO acts as a plasticizer for the particle surface. 

This effect has a major impact on the use of polymers in aqueous systems. In the case of a polymer in solution, the presence of the solvent plasticizes the polymer during film formation. A polymer with a high Tg, i.e., one greater than ambient temperature, can, when in solution be applied at temperatures below its Tg. In the case of an emulsion polymer, water is a non-solvent in the system and film formation below the polymer Tg is unlikely. The temperature at which a coherent film may be formed from a solution or emulsion based system is known as the Minimum Film Forming Temperature (MFFT or MFT). 

The essential property of any polymer used as an adhesive is that it should have good cohesion and stick to the surfaces it joins. This can occur only if the polymer forms a film on application or at some stage during the joining process. In the case of a water suspension such as a PVA or an EVA adhesive, this involves a smooth coalescence of the polymer particles as the water evaporates. For this to occur, the polymer must be above its minimum filming temperature (MFT). Some applications lend themselves to the application of heat. However, most rely only on the evaporation of water at ambient temperature, coupled with absorbence (or wicking ) into the substrate, if porous. 

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