Minimum film-forming temperature MFFT

Minimum boiling azeotropes, 3 802-807 Minimum film-forming temperature (MFFT), 16 291 20 378-379 Minimum film thickness equation, 15 211 Minimum fluidization velocity, 11 793, 797-798... 

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). 

Copolymer composition has a direct effect on the Tg of the polymer, which determines the minimum film forming temperature (MFFT) of the latex and the application. Thus, a 95/5 wt/wt butyl acrylate/methyl methacrylate is an adhesive, whereas a 50/50 copolymer of the same monomers is a binder for paints. Copolymer composition affects properties such as resistance to hydrolysis [4] and weatherability. In situ formed blends of random copolymers of different compositions may be beneficial for application properties [5]. Conventional free-radical polymerization, which is the process used to manufacture almost all commercial emulsion polymers, does not allow the production of block and gradient copolymers (accessible by means of controlled radical polymerization [6], Section 3.3). Nevertheless, graft copolymers are frequently formed, and the extension of grafting largely determines the application properties. Thus, grafting determines the size of the rubber domains in ABS polymers, and the toughness of these polymers increases with rubber size. 

Another interesting parameter is the minimum film-forming temperature (MFFT), which is the minimum temperature at which a polymeric material is able to coalesce to form a film. At temperatures below MFFT, a white opaque or powdery material is formed, whereas a clear, transparent film is formed at temperatures equal to or greater than MFFT. The MFFT has implications in coating processes. The temperature in the mass during coating must be above the MFFT in order to ensure film formation. 

Low ambient temperature is quite detrimental to water-based emulsion adhesives. When the temperature is too low, the dispersed polymer particles will not coalesce or fuse together, and it will prevent the formation of a strong continuous film of adhesive. Instead, a powdery, cracked film will occur if the temperature is lower than a required minimum. There is a minimum film-forming temperature (MFFT) below which there will be no film formation and consequently no bonding. This temperature may be measured with several test methods ASTM D 2354 and ISO 2115, by using specific equipments such as the Sheen Instrument minimum film temperature bar (Fig. 19) a naicroprocessor controlled stainless steel plate is cooled at one end and heated at the other. The sample to be tested is laid down at 75 JLm film thickness and 25 mm width. After 45 -90 min, a clearly defined coalescence zone will be visually obvious, and the temperature at this point will be recorded as MFFT. 

Emulsion latices film form by evaporation of the water phase followed by coalescence of the polymer molecules. For the polymer to form a coherent film it must be of sufficiently high molecular weight and also the temperature must be above the minimum film forming temperature (MFFT) of the system. The MFFT is governed by the T of the polymer, and is normally somewhat lower than the T due to the plasticising effect of the surfactant and any additives present in the film. 

Each emulsion polymer or co-polymer is characterised by its minimum film forming temperature (MFFT), which, if found to be too high for a given application, can be lowered by the addition of plasticiser or high boiling point coalescing solvent... 

The efficiency of the AB group of coalescents towards almost all polymer types has aheady been mentioned. For styrene-acrylics and vinyl acetate — Veova copolymers, the diesters have a significant edge in performance compared with other members of that group. This advantage is shown in the reduction in amount required to attain a particular minimum film forming temperature, MFFT or the actual MFFT for a given addition level. 

Note MFFT, minimum film-forming temperature. Source Arkema, Inc., 2008 (With permission). 

Film formation requires deformation of polymer particles and reptation of polymer chains, and is strongly temperature dependent. The temperature at which a film will form is commonly measured on a minimum film formation temperature (MFFT) bar. Latex is applied to a metal bar with a pre-assigned temperature gradient. The coating is allowed to dry and a number of transitions are noted. Below a certain temperature the film displays cracks. This is called the crack point MFFT. At a lower temperature there is a transition from cloudy to clear, as the pores between particles become much smaller than the wavelength of light. This is called the cloudy-clear MFFT. A further transition is the temperature at which the film is able... 

Dispersions of insoluble polymer particles form films by coalescence of the particles. The largest volume of such coatings use latexes as a binder. The lowest temperature at which coalescence occurs to form a continuous film is called its minimum film-formation temperature (MFFT). A major factor controlling MFFT is the Tg of the polymer particles. The MFFT of latex particles can be affected by water, which can act as a plasticizer (5). Most latex paints contain volatile plasticizers, coalescing solvents, to reduce MFFT. The mechanism of film formation from latexes has been extensively studied the papers in References 6-9 review various theories associated with it. Film formation occurs by three overlapping steps evaporation of water and water-soluble solvents that leads to a close packed... 

Minimum film formation temperature (MFFT) is the minimum temperature needed for a binder to form a coherent film. This measurement is based on, although not identical to, the glass transition temperature (Tg) of the polymer. 

Latex adhesives are applied to the substrate and the water is allowed to dissipate this can be by evaporation, or in the case of porous substrates such as paper and wood, by passage into the adherend. To obtain a cohesive adhesive it is necessary for the particles to coalesce, and this can only occur above a critical temperature (which is different for each latex) known as the minimum film-formation temperature (MFFT) this is the lowest temperature at which the latex can form a film. 

If the two bath temperatures have been chosen correctly, the emulsion will dry to a continuous film at the hot end of the bridge and will be cracked and crazed at the cold end. The boundary between the cracked and intact films occurs at the minimum film forming temperature. The MFFT is generally slightly lower than the Tg because of the plasticising effect of water on the polymer film. 

Most printing ink emulsions are resin supported . Printing ink emulsion polymers contain a support resin to reduce MFFT (minimum film forming temperature), and insure film coalescence. A support resin also decreases the need for surfactants. A support resin provides ink re-wettability, improves compatibility with pigment dispersions, and improves ink transfer and printability. Support resins are typically styrene acrylic polymers with acid functionality that are amine neutrahzed. 

The MFFT can be measured in the laboratory as the minimum temperature at which a cast latex film becomes clear. This is simply because if the coating has not formed a coherent film, it will contain many voids between polymer particles. These voids create internal surfaces within the film, which cause the opacity. 

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