Stress development and film formation in multiphase composite latexes

Abstract

Designed appropriately, multiphase softcore/hard-shell latex particles can achieve film formation without the addition of a coalescing aid, while preserving sufficient film hardness. Achieving optimal performance in these materials requires an understanding of how particle morphology affects film formation and stress development in the film. In this study, soft-core/hard-shell latex particles with different shell ratios, core and shell glass transition temperatures (T g s), and particle sizes (63-177 nm) were synthesized using a two-stage emulsion polymerization. The film formation behavior of the composite particles was investigated with cryogenic scanning electron microscopy, atomic force microscopy, and measurements of the minimum film formation temperature (MFFT). Results show that film formation was enhanced for particles with thinner hard shells, smaller particle size, and a smaller difference in T g between the core and shell polymers. For example, the MFFT decreased and the particle deformation increased for particles with thinner shells and smaller particle sizes. Stress development during drying was characterized using a cantilever beam bending technique. A walled cantilever design was used to monitor stress development without the complication of a lateral drying front. The film formation behavior and stress development correlated well with practical paint properties like scrub resistance and gloss.

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

“…The increase in Young's modulus was particularly notable in those samples containing 40% of the hard second‐stage polymer as a result of the highly nonspherical hard segments present in the latex films. It should be noted that the yield at break (up to 10 MPa) was substantially higher than what has been obtained previously (2–3 MPa) while having quite ductile films that resisted deformations of at least 300%.…”

“…who showed that similar hard shell type latexes maintained their shape during film formation and led to films with poor cohesion, which often cracked extensively during drying. Although the particle morphology was not extensively characterized in their work the differences with what is observed here is likely as a result of having a complete shell of the hard phase in their system . This highlights the importance of the structure of the latex itself in the final properties of the film.…”

“…Although the films were ductile, resisting deformations up to 150%, the yield stress was limited to 2 MPa perhaps due to use of the very soft core. Price et al . used a T g ≈ 3 °C core and a T g ≈ 60 °C shell and found that MFFT increased from <0 °C for 75/25 w/w core/shell particles to 20 °C for 65/35 and to 35 °C for 50/50 particles.…”

Stress Development in Hard Particle Coatings in the Absence of Lateral Drying

“…The border or wall promotes uniform drying by eliminating thin edges, and allowing the microstructure to develop uniformly across the substrate. In our previous work, cryogenic scanning electron microscopy (cryoSEM) was used to show that the use of walled substrates leads to greater drying uniformity in latex coatings and improves stress measurements . In this study, walled cantilevers are used to measure stress development in ceramic particle coatings using the cantilever deflection technique.…”

Stress Development of Li-Ion Battery Anode Slurries during the Drying Process

“…On the other hand, if the T g of latex was lower than the drying temperature, the drying stress slightly decayed after the maximum but the residual stress still remained. Price et al 33 reported the drying behavior with respect to the shell ratio in suspensions containing soft-core/hard-shell particles. The drying stress in the suspension with a thin hard-shell developed and reached to the maximum stress, while, in the suspension with thick hard-shell, the stress steeply decayed after the maximum and a crack was generated in the film.…”