Single-step dipping

This possibility is illustrated in Fig. 25. The dip seems considerably too deep to be plausible. So while we cannot rule out this possibility, without firm evidence for the existence of such an intermediate, we will invoke Ockham s razor and return to the single-step model. 

Single step dipping with a mixture of EDT (Baytron M)/polymerizer/organic solvent. Iron(III) p-toluene sulfonate (tradename Baytron C) can be used as the polymerizer. 

Multiple dipping finally increases the maximum allowable thickness of the (calcined) crack-free membrane with respect to that obtainable in a single step [13,14,18]. At each given set of experimental conditions there is a certain critical layer thickness which should not be exceeded if cracking during drying and/or calcination is to be avoided. 

The experimental observation that there exists a critical thickness above which cracking occurs cannot easily be explained. Brinker [1] discusses a theory which explains that very thin layers can bear much larger stresses because critical cracks carmot be formed unless a certain critical thickness is surpassed. This thickness is estimated to be equal to or less than 1 pm and Brinker comes to the conclusion that thicker films will always crack. This is certainly not the case for alumina, titania and zirconia films for which much larger (alumina) to larger (titania) thicknesses are observed. As shown in Table 8.2 critical thicknesses of a few pm in single-step dip-coated films occur and critical flaws are smaller than this thickness and so can be present. 

Table 3. Pexul Polyester Single-Step Adhesive Dip.

One of the first single-step adhesive dips for polyester was developed by ICI. This adhesive was based on a co-resin of resorcinol and... 

As is shown in Fig. 8.4 the film growth rate is much smaller in the second step compared to that in the first one. This is mainly due to the additional resistance offered by the first applied layer. It should be noted that in both single and multiple dipping the driving force for film formation (capillary pull) is always exerted by the dry portion of the support. A multilayer membrane is shown in Fig. 8.5. 

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