Jacky Mallegol

Influence of drier combination on through-drying in waterborne alkyd emulsion coatings observed with magnetic resonance profiling

Achieving fast and uniform crosslinking in alkyd coatings poses a challenge to formulators that demands a fundamental understanding of drier efficiency. In recognition of this, we have examined the physical changes that accompany autooxidative crosslinking in alkyd films (cast from waterborne emulsions) in the presence of various combinations of metal carboxylate driers. A newly developed type of magnetic resonance (MR) profiling was used in conjunction with conventional techniques: Beck-Koller drying tests, pendulum hardness, and mass uptake. MR profiling noninvasively probes the molecular mobility of the alkyd as a function of depth (with a pixel resolution of about 9 µm), over drying times ranging from minutes to weeks. It thereby indicates drier efficiency via its sensitivity to viscosity build-up during drying and to subsequent film hardening. We show unequivocally that more uniform crosslinking is achieved using a combination of a primary (Co) and a secondary (Ca) drier, in support of conventional belief. Furthermore, these results yield new insight into the chemical mechanisms induced by the driers and are thus of clear benefit to coatings researchers and formulators. Notably, the secondary driers improve the efficiency of the hydroperoxide decomposition reactions, but they are only active during an initial period, after which crosslinking nonuniformity develops.

Skin Development during the Film Formation of Waterborne Acrylic Pressure-Sensitive Adhesives Containing Tackifying Resin

Tackifying resins (TR) are often used to improve the adhesive properties of waterborne pressure-sensitive adhesives (PSAs) derived from latex dispersions. There is a large gap in the understanding of how, and to what extent, the film formation mechanism of PSAs is altered by the addition of TR. Herein, magnetic resonance profiling experiments show that the addition of TR to an acrylic latex creates a coalesced surface layer or “skin” that traps water beneath it. Atomic force microscopy of the PSA surfaces supports this conclusion. In the absence of the TR, particles at the surface do not coalesce but are separated by a second phase composed of surfactant and other species with low molecular weight. The function of the TR is complex. According to dynamic mechanical analysis, the TR increases the glass transition temperature of the polymer and decreases its molecular mobility at high frequencies. On the other hand, the TR increases the molecular mobility at lower frequencies and thereby promotes the interdiffusion of latex particles to create a skin layer. In turn, the skin layer is a barrier that prevents the exudation of surfactant to the surface. The TR probably enhances the coalescence of the latex particles by increasing the compatibility between the acrylic copolymer and the solids in the serum phase.

Morphology and elasticity of waterborne acrylic pressure-sensitive adhesives investigated, with atomic force microscopy

The morphology of pressure sensitive adhesives (PSAs), especially at the surface in contact with a release liner, is expected to have a dominant influence on the tack strength and energy in an application. We have used tapping-mode atomic force microscopy to determine the morphology at the surfaces of freshly-cast waterborne acrylic PSAs over lateral length scales of a few μm. We demonstrate that topographical features on silicone release liners can be used to pattern the PSA surface in contact with it. Control of the texture of a PSA surface can potentially be exploited to tailor its properties. Latex particle boundaries are much better defined at the air surface of the PSA in comparison to its back face. A series of experiments suggests that this difference results from the distribution of water-soluble species within the dry film. The pressures and processes involved in the transfer lamination process do not alter the PSA morphology. The first reported AFM images of the response of these materials to pressure and shear provide insight into the deformation mechanisms. Amplitude-distance curves on PSA surfaces show that there is a small decrease in tack and an increase in stiffness after ageing for 13 months.

Diffusion behavior of isobornyl acrylate into photopolymerized urethane acrylate films: influence of surface oxidation during curing.

The diffusion of liquid isobornyl acrylate (iBoA) into photopolymerized films of urethane acrylate was shown to be strongly dependent on curing conditions. Fourier transform infrared (FTIR) monitoring of iBoA sorption into films photopolymerized in oxygen-free conditions reveals that diffusion proceeds according to Ficks laws. The diffusion coefficient and the total amount of absorbed monomer are dependent on the degree of cure of the polymer network. When polymerized in air, the films exhibit an anomalous behavior, with a lag time increasing with the dose of applied radiation. At a comparable degree of cure, films photopolymerized in air showed a retarded diffusion with a characteristic diffusion constant close or equal to the Fickian diffusion coefficient observed with the film polymerized under nitrogen, suggesting that the retardation phenomenon was due to differences in surface interactions with liquid the monomer. X-ray photoelectron spectroscopy (XPS) measurements clearly revealed the occurrence of surface oxidation attributed to direct photolysis of urethane functions by the short wavelength component of the incident UV light. The lag time observed in samples polymerized in aerobic conditions is interpreted by the existence of H-bonded hydroperoxides forming a temporary barrier retarding monomer diffusion.