Structure of Waterborne Organic Composite Coatings

Effect of internal structure and resin deformability on drying rate and stress in convective drying of silica-latex coatings

Latex paint is an aqueous dispersion of nano-sized polymer particles that can form a thin film by itself or mixed with rigid particles. We have developed an apparatus that can simultaneously measure drying rate and stress generation and have investigated the film formation process of a latex-only coating layer under convection drying. In the present study, we adopted the same method to investigate the film formation process of the silica-latex coating layer. As a result, we were able to systematically correlate the drying rate change by the equivalent thickness of latex particles accumulated with silica particles at the drying surface. Furthermore, it is unveiled that the drying rate in the former stage depends on drying temperature, while the drying rate changed to be dominated by silica content after the particle-packing layer was formed over the entire coating layer. On the other hand, the model we proposed for stress generation, considering the temperature effect on latex deformability, was found to be applicable to the present experimental system by replacing a portion of deformable particles with rigid particles.

How latex film formation and adhesion at the nanoscale correlate to performance of pressure sensitive adhesives with cellulose nanocrystals

Emulsion polymerized latex-based pressure-sensitive adhesives (PSAs) are more environmentally benign because they are synthesized in water but often underperform compared to their solution polymerized counterparts. Studies have shown a simultaneous improvement in the tack, and peel and shear strength of various acrylic PSAs upon the addition of cellulose nanocrystals (CNCs). This work uses atomic force microscopy (AFM) to examine the role of CNCs in (i) the coalescence of hydrophobic 2-ethyl hexyl acrylate/n-butyl acrylate/methyl methacrylate (EHA/BA/MMA) latex films and (ii) as adhesion modifiers over multiple length scales. Thin films with varying solids content and CNC loading were prepared by spin coating. AFM revealed that CNCs lowered the solids content threshold for latex particle coalescence during film formation. This improved the cohesive strength of the films, which was directly reflected in the increased shear strength of the EHA/BA/MMA PSAs with increasing CNC loading. Colloidal probe AFM indicated that the nano-adhesion of thicker continuous latex films increased with CNC loading when measured over small contact areas where the effect of surface roughness was negligible. Conversely, the beneficial effects of the CNCs on macroscopic PSA tack and peel strength were outweighed by the effects of increased surface roughness with increasing CNC loading over larger surface areas. This highlights that CNCs can improve both cohesive and adhesive PSA properties; however, the effects are most pronounced when the CNCs interact favourably with the latex polymer and are uniformly dispersed throughout the adhesive film. This article is part of the theme issue 'Bio-derived and bioinspired sustainable advanced materials for emerging technologies (part 1)'.

Water-based acrylic coatings reinforced by PISA-derived fibers

Polymerization-induced self-assembly (PISA) provides nanofibers that may be used as reinforcing fillers for all-organic aqueous coatings.

Quantifying the synergistic effect of dispersion state and interfacial adhesion contributions on impact strength of core shell rubber-toughened glassy polymers

Synergistically improved impact strength of a core shell rubber-toughened glassy polymer was rationalized with disparities ratio of |ΔT/ΔD|.

Switching off the tackiness of a nanocomposite adhesive in 30 s via infrared sintering

Soft adhesives require an optimum balance of viscous and elastic properties. Adhesion is poor when the material is either too solidlike or too liquidlike. The ability to switch tack adhesion off at a desired time has many applications, such as in recycling, disassembly of electronics, and painless removal of wound dressings. Here, we describe a new strategy to switch off the tack adhesion in a model nanocomposite adhesive in which temperature is the trigger. The nanocomposite comprises hard methacrylic nanoparticles blended with a colloidal dispersion of soft copolymer particles. At relatively low volume fractions, the nanoparticles (50 nm diameter) accumulate near the film surface, where they pack around the larger soft particles (270 nm). The viscoelasticity of the nanocomposite is adjusted via the nanoparticle concentration. When the nanocomposite is heated above the glass transition temperature of the nanoparticles (T(g) = 130 °C), they sinter together to create a rigid network that raises the elastic modulus at room temperature. The tackiness is switched off. Intense infrared radiation is used to heat the nanocomposites, leading to a fast temperature rise. Tack adhesion is switched off within 30 s in optimized compositions. These one-way switchable adhesives have the potential to be patterned through localized heating.

Structure of interacting aggregates of silica nanoparticles in a polymer matrix: small-angle scattering and reverse Monte Carlo simulations

Reinforcement of elastomers by colloidal nanoparticles is an important application where microstructure needs to be understood-and, if possible, controlled-if one wishes to tune macroscopic mechanical properties. Here, the three-dimensional structure of large aggregates of nanometric silica particles embedded in a soft polymeric matrix is determined by small angle neutron scattering. Experimentally, the crowded environment leading to strong reinforcement induces a strong interaction between aggregates, which generates a prominent interaction peak in the scattering. We propose to analyze the total signal by means of a decomposition in a classical colloidal structure factor describing aggregate interaction and an aggregate form factor determined by a reverse Monte Carlo technique. The result gives new insight to the shape of aggregates and their complex interaction in elastomers. For comparison, fractal models for aggregate scattering are also discussed.

Small-angle neutron scattering study of particle coalescence and SDS desorption during film formation from carboxylated acrylic latices

Four monodisperse core-shell latices were synthesized for small-angle neutron scattering (SANS) studies, differing by the acrylic acid content in the particle shell (1 or 4 wt%) and the T(g) of the acrylic core (around -40 or 10 degrees C). In a first part, the coalescence kinetics of the surfactant-free latices were studied. It was shown that coalescence was hindered by an increase in the acrylic acid content of the shell, pH of the latex, and Tg of the core. These results could be interpreted in terms of chain mobility in the shell and in the core. Upon coalescence, the hydrophilic phase was segregated in spherical, polydisperse domains with an average diameter of 110 nm. In a second part, labeled SDS was used to follow desorption of the surfactant during film formation. It was shown that desorption occurred early in the film formation process when the latex still contained around 20% of water. A small fraction of the surfactant remained irreversibly adsorbed at the particle surface.