Film formation MFFT

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... 

A series of latex copolymers were prepared using a typical emulsion polymerization recipe and procedure only the monomer composition was varied. The control composition (80/20 vinyl acetate/butyl acrylate) is similar to that used for interior latex paint. Table V lists the compositions and properties of the latexes. Percent solids, pH, and particle size are similar for all the latexes. Viscosity varies somewhat, but is within limits for this type of latex. The only unreacted monomer detected was the vinyl acetate. Thus, the incorporation of VEC into the emulsion polymerization via the monomer mixture did not affect the latex synthesis. The Tg and minimum film formation temperature (MFFT) of the latexes increase with increasing VEC content, which is expected based on the previous results. 

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). 

Mechanical stability of dispersions was determined using a laboratory centrifuge with rotational speed of 3700 rpm. Particle size, particle size distribution and zeta potential of the dispersions was determined using Malvern Zeta Sizer 4 equipment. Minimum film formation temperature (MFFT) was measured using a Coesfeld apparatus. 

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. 

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... 

FIGURE 2.16 The effect of minimum film-formation temperature (MFFT) of a dispersion on its drying behaviour. Primal (Rohm Haas) dispersions were allowed to dry at room temperature, =21 °C, in a silicone rubber mould. An MFFT above room temperature results in incomplete coalescence of particles and poor film formation. The cracks and distortion that occur in the films result from the movement of water and shrinkage during drying, (a) AC-34 MFFT, 12 °C. (b) AC-73 MFFT, 37 °C. (c) B-85 MFFT, 90 °C. 

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. 

Two MFFTs appear to exist wet MFFT and dry MFFT. The normal, or wet, MFFT is that which is seen under normal circumstances — wherein a latex is applied at an ambient temperature above the polymer s T, and film formation follows the three stages described in Section 3.3. This wet MFFT is associated with particles deforming due to a receding air-water interface. 

The higher temperature at which a previously uncoalesced latex deforms is the dry MFFT. This is associated with much smaller quantities of water between particles. The role of the water at this higher dry MFFT is not well understood. It may be that the smaller amounts are able to deform the particles because a different deformation mechanism is possible at the elevated temperature. Or, it may be that the polymer particle is softer under these circumstances. The phenomenon is interesting and may be helpful in improving models of latex film formation [21-24]. 

Devon et al. [33] prepared a series of acrylic latexes with core-shell particle morphologies. The minimum film formation temperatures (MFFT) of the latexes are expected to change with the core-shell characteristics in the following order ... 

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. 

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. 

It is critical that the MFFT is below the temperature prevailing during the drying of the film, otherwise a discontinuous, hazy film is formed because the polymer did not film form at that temperature. The formation of a polymer film form or latex film is different to all other types of film formation. Not only has the water to disappear by evaporation or absorption into substrate, but during this process the polymer particles must fuse together. 

Obviously, the particle size is important because it affects the capillary and other film formation forces. However, varying particle size has minimal effect on MFFT. The T of the comonomers and the presence of external plasticisers are most important for film formation. 

Hydrophobic solvents decrease the MFFT most efficiently and increase the hardness of the coating due to improvements in film formation. They also penetrate into the shell and swell the polymer, increasing the viscosity dramatically. Coalescing solvents distribute throughout the three phases, water, polymer shell and polymer core, in differing volumes. Optimum film formation takes place when the coalescent is present in the shell of the polymer. 

Abrasion and impact resistance of acrylic coatings can be improved with the addition of up to 30% of a water borne polyurethane. The addition of a polyurethane dispersion as a physical blend improves the inherent film strength and film formation of the aciylic latices. Polyurethanes generally have better film forming properties because of their lower MFFT at identical surface hardness. 

NeoCiyl A-1049-U is an anionic aciylic dispersion, 40 %nvc with a MFFT of 40 C. NeoCiyl BT-21 is an acrylic dispersion, 40 %nvc. Sudranol 100 is a wax emulsion to improve water repellency. Steinapol SBDO-70 (Rewo) can be used as the wetting ad tive. For good film formation, a combination of coalescents and plasticisers are added. 

Disperse the millbase to Hegman 7 and add the let down. Viscosity and pH are adjusted with ammonia. Rhoplex WL-91 (Rohm and Haas) is a hard polymer for good block resistance, 41.5% nvc with a MFFT of 52"C. Plasticisers and coalescents are required for good film formation. 

As an aqueous dispersion can only dry above 0 °C, the MFFT and white-point temperature are only defined above this value. The control of the polymer layer thickness is crucial for the measurements. Mechanical stress may develop during film formation (particularly when crosslinking is involved) which leads to crack formation above a certain layer thickness. A further point which should be considered is that very short drying times are often used in dispersion processing, for example on coating machines. In this case, the MFFT may well he above the value determined according to ISO 2115. The discrepancy is caused by kinetic limitations in water evaporation and polymer interdiffusion [24]. 

Plasticizers (i.e. propylene glycol) and coalescing solvents (i.e. glycol ether) are added to maintain satisfactory printability and satisfactory MFFT (minimum film formation temperature). 

The theory that a reduction in particle size to the nanorange could yield innumerable benefits has been discussed by Takeuchi et al. [76], They stated that whereas the MFFT was key in the formation of a clear, constant film, this temperature was subject to factors such as the glass transition temperature of the polymer, the particle morphology, water content, excipients such as plasticizer, and importantly particle size. By using nanosuspension of enteric polymers, an enhanced coalescence should occur partly due to the large increase in surface area and consequent increase in surface energy. 

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