Latex film coalescence

As has already been made clear, interdiffusion is of great importance for the development of the physical properties of latex films [94]. In order to learn how to optimise the performance of a wide variety of coatings formulations, a deeper understanding of the coalescence process is needed. The essential feature that one needs to understand is the role of inter-particle polymer diffusion once the water has evaporated and the nascent film has formed. Although, as reported above, latex film coalescence processes have been studied [90-94], a much better understanding of these processes is needed. In this section, the process of core-shell latex film coalescence and the dynamics of surface structure development of latex films will be discussed in the light of recent MTDSC studies by the authors. 

The removal of direct carbon replicas is dependent upon the polymer. Boiling xylene vapor was used to remove drawn PE from replicas [296] in work on drawn polymer morphology. Hobbs and Pratt [297] described a direct carbon replica method for replication of a PBT impact fracture surface by evaporation of platinum at 20° and PBT removal in hexafluor-oisopropanol (HFIP). Latex film coalescence in poly(vinyl acrylate) homopolymer and vinyl acrylic copolymer latexes was studied using direct replicas [298]. As the latex films have a low glass transition temperature, they were cooled by liquid nitrogen to about -150°C in the vacuum evaporator and shadowed with Pt/ Pd at 45° followed by deposition of a carbon support film at 90° to the specimen surface. The latex films were dissolved in methyl acetate/ methanol. TEM micrographs of the latex films show the difference between films aged for various times (Section 5.5.2). 

Fig. 5.77 Vinyl acetate latex film coalescence is shown by TEM of platinum-palladium-carbon replicas from aged latex films cast on glass. TEM micrographs show the particulate nature of a film aged for 8 h (A and B) compared to the flat film observed after 15 days aging (C).

Modification of mortar and concrete in the presence of a latex occurs by concurrent cement hydration and formation of a polymer film (coalescence of polymer particles and the polymerization of monomers). Cement... 

Formation of solubilized surfactant-latex complexes can influence the properties and performance of vinyl acrylic latexes prepared with NaLS and other penetrating type anionic surfactants. Such complexes seem to affect glass transition temperature and film coalescence process (12). 

This emulsion is not a liquid-liquid system, but is an aqueous dispersion of solid polymer particles. Therefore, if the Tg were above room temperature (at which the emulsion is applied), the polymer segments, lacking segmental mobility, would not diffuse readily from one particle of the emulsion into another after evaporation of the water medium in which the copolymer emulsion is prepared. The result would be a powdery film. Conversely, when the Tg is reduced below room temperature, segmental mobility in the copolymer leads to diffusion and formation of a flexible, strong adhesive film from the latex by coalescence of the emulsified particles during drying at room temperature. 

Use High-boiling solvent for nitrocellulose lacquers, epoxy resins, multicolor lacquers film-coalescing aid for polyvinyl acetate latex. 

Butoxyethanol acetate is primarily used as a high-boiling, retarder solvent (i.e., an active, slow-evaporating solvent which ensures smooth film formation) for nitrocellulose lacquers, acrylic enamels, epoxy resins, and multicolor lacquers (Leaf 1985 as cited in NIOSH 1990 Lewis 1993). 2-Butoxyethanol acetate is also a film-coalescing aid for polyvinyl acetate latex and is used in some ink and spot remover... 

Figure 14.22 Plots of the mean effective diffusian coefiiciem at 36 C as a function ct / for series of FBMA latex films contruning increasing amounts of TPM as a coalescing tud [S9]. The many data points on the lowermost line create a masto- curve obtained by use of the Fujita-Doolittle expression [60]...

Zhao ct al. [61] used ATR-FTIR to analyse flie surface composition of coalesced poly(MMA-butyl acrylate) latex films. 

The Vinnik group [59b] has recently found that nonionic surfactants act like coalescing solvents in promoting interdiffusion in PBMA latex films. Kim et al. [55] have reported that cetyl and steaiyl alcohol act as plasticizers to promote interdiffusion in PS latex films. 

Following a related approach, Castelvetro et al. reported the formation and properties of hybrid latex films resulting from the coalescence of low 7 poly(BA-co-MMA-co-MPTMS) terpolymer latex particles coated by a silica shell [78], The latex was synthesized at neutral pH by semi-continuous emulsion polymerization under starved-feed conditions in order to protect the MPTMS monomer from premature hydrolysis and condensation reactions. A substantial amount of free silanols were therefore available for further reaction with the silica precursor. In order to avoid the formation of a densely crosslinked silica network around the latex core, which may significantly alter film formation, the pH was kept at around 2 (at this pH, hydrolysis is promoted and condensation is significantly retarded). TEM and AFM studies of the nanocomposite film indicated that the silica shell formed a continuous percolating network throughout the polymer matrix. A porous film of interconnected hollow silica spheres was next elaborated by thermo-oxidative decomposition of the organic phase. 

The theory of coalescence required two stages the first was an agglomeration step which was driven by drying of the latex film, allowing the particles to be pushed together into close-packed adhesive contact, but with small adhesion because of the presence of water the second was a sintering step in which the work of adhesion increased as the last water was removed and elastic deformation occurred with shrinkage. 

At points on the bar above the MFFT, an applied latex will coalesce to produce a clear film if no pigment or other opaque materials are present. 

SANS along with other techniques like AFM allows the monitoring of latex particle coalescence and polymer chain inter-diffusion in greater and greater detail. Such fundamental understanding of film formation allows modification to polymerization reactions/ coating recipes and methods of film formation and the development of better quality films. ... 

The resulting latex suspensions were cast into film materials by evaporating the water. Analysis of an ultrathin cross-section of the film by cryo-TEM indicated successful composite latex particle coalescence and film formation, the clay plates forming a three-dimensional honey-like structure as a consequence of their original localization at the external surface of the polymer particles (Fig. 4.20). 

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