ABSTRACT
Because of the hydrophobic nature of polydiorganosiloxane, the cross-linked hydroxy end-blocked PDMS elastomer coatings are relatively impermeable to hydrophilic and ionic compounds [81]. The permeability to such compounds, however, can be increased by adding a water-soluble or pore-forming component to the dispersion [81,82]. Li and Peck [77,78] and Woodward et al. [75] used water dispersible/soluble or poreforming agents (e.g., PEG 1450, 3350, or 8000) as additives to increase the permeability of the coatings to a variety of agents. Fig. 21 shows the effects of the molecular weight of polyethylene glycols and the solid content on the apparent viscosity of cross-linked hydroxy end-blocked PDMS-PEG dispersions. As is evident, with the same solid content, dispersions containing the higher molecular weight PEG are comparatively more viscous. As the solid content increases, the viscosity of the dispersion also
Fig. 21 The effect of total solids content on the apparent viscosity of dispersions with total solids consisting of silicone and silica in a ratio of 2 to 1 and 30% polyethylene glycol [key: (•) PEG 8000; (A) PEG 3350; and (•) PEG 1450]. (From Ref. 77.)
increases. A more pronounced increase in viscosity occurred in the system that contained the highest molecular weight PEG. At a total of 30% PEG solid content, the viscosity of the dispersion was less than 25 cps. Other water-soluble substances such as low molecular weight PEGs, glycerin, propylene glycols, sucrose, and glucose, were also evaluated. These provided sticky and soft films, especially at higher concentration (> 10%) and, therefore, were not regarded as useful.
