Small-Angle Neutron Scattering Studies of Composite Latex Film Structure

Abstract

The structure of composite films made of high T g polystyrene (PS) nodules dispersed in a low T g polybutylacrylate (PBuA) matrix was studied by means of small angle neutron scattering. For films cast from mixtures of PS and PBuA latexes, segregation of PS particles leads to dense clusters of PS particles in a PBuA continuous medium. This segregation has the main features of a phase separation. For films cast from two-stage (core-shell) particles, this segregation phenomenon is prevented, depending on the coverage of the PS core by the PBuA shell. Upon annealing the films above the T g of PS, extensive coalescence of PS particles occurred when large contacts were already present in the dry film at room temperature, whereas coalescence could be prevented when PS particles were taken apart by the presence of a PBuA shell. The extent of coalescence had a strong influence on the films mechanical properties.In waterborne coatings applications, polymer films can be obtained by drying polymer particles colloidal suspensions. During the drying process, particles have to approach each other, stick, and finally get deformed from spherical to polyhedral as water evaporates (J-3). Particles made of polymers of low glass transition temperature (T g ) are thus required in such a film formation process but the final (dry) films mechanical properties are poor. A mechanical properties reinforcement can be obtained when solid particles are mixed with the low T % polymer ones. Those solid particles can be either inorganic or organic when a high T g polymer is used. In this latter case, a further structural transformation of the dry films can be obtained by heating the films above the glass transition temperature of the high T g polymer. The weak bonds between solid particles at their points of contact are then strengthened by polymer 4

Drying and film formation of industrial waterborne latices

“…The extent of particle deformation dependent on T mff has been studied using turbidity measurements 9. According to some authors2–5 the rupture of the surfactant layer, separating the deformed latex particles, is prerequisite to encouraging polymer interdiffusion and developing mechanical strength 26. For drying well above T mff particle deformation and polymer diffusion will be sufficiently strong to destroy the network of hydrophilic surfactant material to form a nonporous film.…”

“…A large number of experimental techniques have been used to investigate different aspects of spacing and deformation of the latex particles: Structural changes in films cast from soft particles protected by hydrophilic membranes have been extensively studied by small‐angle neutron scattering (SANS) 4–7. The same technique has also been applied to study particle coalescence and surfactant desorption during film formation from carboxylated acrylic latices 8.…”

Soft core–hard shell latex particles for mechanically strong VOC‐free polymer films

“…Multiphase latex particles have the potential to overcome the stratification issues that arise when using latex blends because they combine two or more polymers of different compositions within the same latex particle. In this case, unlike latex blends, a more homogeneous distribution of the two phases throughout the polymer film is acheived . One common approach is to use particles with a core–shell morphology consisting of a hard core, which reinforces the film, and a soft shell, which allows for film formation at acceptable temperatures .…”

Preparation of core‐shell latex from co‐polymer of styrene‐butyl acrylate‐methyl methacrylate and their paint properties

“…The hard phase while acts as transparent filler providing block resistance, should not be separated from the surface of soft polymer surface. The hard phase has been grafted onto the soft phase during polymerisation (Chevalier et al , 1996) and such systems are called core‐shell latex. Stepwise emulsion polymerisation allows not only the synthesis of core‐shell particles, but also allows the formation of different morphology of particles like‐half moon, strawberry, octopus, mushroom, inverse core‐shell and inclusion structures (Okubo and Lu, 1996; Cho and Lee, 1985; Zhao et al , 1999; Lee and Rudin, 1992).…”