Film-formation temperature

The freeze/thaw (F/T) stability of a polymer emulsion serves as a macroscopic probe for investigating the properties of the average particle in a polymer emulsion. A review of the factors which contribute to this stability is included. A study of styrene-ethyl acrylate-methacrylic acid polymers shows the existence of a minimum in the plot of minimum weight percent acid required for F/T stability vs. the minimum film formation temperature (MFT) of the polymer. This is considered to be a function of both the amount of associated surfactant and the minimum acid content. Thus, both the type of surfactant and the copolymer ratio—i.e., MFT—play major roles. Chain transfer between radicals and polyether surfactant resulting in covalently bonded surfactant-polymer combinations is important in interpreting the results. 

Minimum Film Formation Temperature (MFT). Equipment described by Protzman and Brown (24) was built, calibrated, and used for all measurements. A series of 25% emulsions, adjusted to pH 9.5 with NH3, was used throughout unless otherwise indicated. 

Minimum Film Formation Temperature C >90 Ionic Charge Anionic pH as packed 9.8 Density Ibs/US gal 8.7 Brookfield Viscosity, cP 100 Performance Properties ... 

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... 

The percentage of solids in the dispersion adhesives is 35-55%, in some cases as much as 70%, the rest is water, which after the adhesive application must either be completely evaporated (ventilation time) or diffuse into the joined parts (wood, textiles, concrete, foams, leather). The film formation temperature in types containing plasticizers and flexibilizing additives is lower than in types that are free of plasticizers. Additives, so-called high boilers (ethyl glycol, ethyl glycol acetate, ethylhexanol), are used to bring the film formation temperature down. 

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. 

Density (25°C), approx. Ib/gal Surface tension (25°C), approx, dynes/cm Min. film formation temperature °C T... 

The addition of comonomers during polymerization enables a higher flexibility to be obtained compared to PVAc homopolymers. This causes also a lower glass transition temperature and a lower minimum film formation temperature. Possible comonomers are acrylic acid esters (butylacrylate, 2-ethylhexylacrylate), dialkylfumarates, ethylene, and others. 

The investigations were carried out using 2 polymers (Pol5mier 1 - redispersible powder with a film formation temperature of 5 °C, Polymer 2 - dispersion with a film formation temperature of 30 °C). Both based on styren-acrylic ester copolymer. 

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. 

The following experiment should demonstrate the influence of particle size and minimal film-formation temperature (which is connected with the glass transition temperature and therefore with the chemical structure of the polymers) on the properties of films, prepared from aqueous dispersions. 

---