MR PROFILING OF DRYING IN ALKYD EMULSIONS: ORIGINS OF SKIN FORMATION

摘要

For the first time, NMR profiling has been successfully used to study vertical profiles (~1H density and T_2 relaxation time) during alkyd emulsion film formation with better than 10 ****m pixel resolution. From a basic mass loss experiment, we have shown that in the later stage of the film formation, evaporation of a few percent of either solvent (initially present in the alkyd resin) or water trapped below a skin layer occurs. MR profiling of alkyd emulsions found that a skin layer having a lower T_2 developed during the later stages of drying. It grew in thickness over time. Our first hypothesis was that these developing profile shapes were related to the evaporation of xylene from successively deeper regions of the film. Xylene is known to plasticise the alkyd, therefore it could lead to more mobile polymer chains and so to higher T_2 values. Experiments using a NMR benchtop spectrometer indicated that the large difference in T_2 between the skin and the rest of the film would require an unrealistically high xylene concentration (4.8 wt%). An experiment on an emulsion enriched with xylene found that xylene evaporates from the film quicker than first expected. Our second hypothesis was that a small amount of water was trapped below a skin layer. This hypothesis is supported by the predictions of the process model of Routh and Russel From the mass loss experiment we conclude that up to 5 wt% of water may be trapped in the film once a skin is formed. The water has to diffuse through this skin layer, the thickness of which was found to increase linearly with the square root of time. The data are consistent with a diffusion coefficient of water in alkyd of 1.5x10~(-9) cm~2 s~(-1). This interpretation is more probable than the xylene hypothesis. These encouraging results suggest ideas for further investigation. GARField experiments could be performed in order to gain additional information on the initial stages of alkyd film formation. Alkyd emulsions prepared using D_2O could strongly reinforce the hypothesis of water diffusion through an increasingly thick skin layer. Indeed, profiles obtained from such alkyd films drying in controlled atmosphere should be free of any non-uniformity. On the other hand, if such films were cast and dried in the normal atmosphere, the uptake of protons from the water vapour of the air could help to estimate the time scale and the amount of water trapped in the alkyd film. Infrared ellipsometry could also be used to confirm the presence of trapped water. Finally, by varying experimental parameters such as temperature, relative humidity, air flow, solid content and initial film thickness, we could use GARField profiles to test the theoretical models of film formation from colloidal dispersions.