Defect free skin

An ideal pervaporation membrane should consist of an ultra thin defect free skin layer (dense layer) supported by a porous support. The skin layer is perm-selective and hence responsible for the selectivity of the membrane. However, the porous support also plays an important role in overall performance of the membrane. The effect of the porous support, of a composite membrane, on the permeation properties of the membrane is discussed in details in the composite membranes... 

Asymmetric polyimide membranes with an ultrathin defect-free skin layer were fabricated by the dry-wet process [15]. Composition of casting solution used for the preparation of asymmetric membranes was 12 wt.% polyimide, 55 wt.% methylene chloride, 23 wt.% 1,1,2-trichloroethene, and 10 wt.% butanol. In the dry process (solvent evaporation) the evaporation period was changed from 15 to 600 s, while in the wet process (coagulation process) the coagulation media was methanol. It was possible to control the thickness of the skin layer by controlling the evaporation period. From this AFM study, it was observed that the nodule formation was controlled by evaporation time, while the coagulation media controlled the roughness parameter. 

The gas permeation stability of an asymmetric polyimide membrane with a thin and defect-free skin layer was investigated by H. Kawakami and co-workers [176,177]. They studied both the effect of molecular weight and the structure of polyimide using a common fluorinated dianhydride, 6FDA (Scheme 3.13). They observed that for 2,2 -fciB(4-aminophenyl) hexafluoropropane (6FDA-6FAP)-based polyimide, despite of different molecular weights, the permeability of N2, O2, and CH4 had almost constant values whereas the CO2 permeability of the asymmetric... 

Most polymers that have been of interest as membrane materials for gas or vapor separations are amorphous and have a single phase structure. Such polymers are converted into membranes that have a very thin dense layer or skin since pores or defects severely compromise selectivity. Permeation through this dense layer, which ideally is defect free, occurs by a solution-diffusion mechanism, which can lead to useful levels of selectivity. Each component in the gas or vapor feed dissolves in the membrane polymer at its upstream surface, much like gases dissolve in liquids, then diffuse through the polymer layer along a concentration gradient to the opposite surface where they evaporate into the downstream gas phase. In ideal cases, the sorption and diffusion process of one gas component does not alter that of another component, that is, the species permeate independently. 

Effect of Evaporation Condition Previous studies on more traditional applications have investigated the effect of increased air velocity, that is, forced-convection conditions for a combination of dry/wet phase inversion techniques to produce defect-free, ultrahigh flux asymmetric membranes with ultrathin skin layers [115-117]. To investigate the effect of evaporation condition on the release rate of drug, tablets were dip coated with CA solution containing 10% CA, 80% acetone, and 10% water and allowed to dry by blowing air across the surface with a blower (forced convection). As a comparison, tablets coated with the same solution were air dried under natural free-convection conditions. 

Because it is difficult to make a selective skin layer perfectly defect-free, a method was proposed by Henis and Tripodi to seal defective pores. Their method was applied to asymmetric PS membranes, which led to the production of commercial prism membranes. 

With these thoughts in mind, it is time to restrict RO performance predictions using the solution-diffusion model to some specially prepared, defect-free, dense films. Such films are unrelated to any functional membrane of either the integrally-skinned or the thin film composite type. 

An automated version of the coin tap technique allowing the production of C-scan images has been developed by Cawley and Adams [56]. Their system compares the time history, or frequency spectrum, of the measured pulse with a reference standard obtained from a defect-free structure. Detection of lOmm diameter defects in I mm thick CFRP skin has been achieved with this technique, while manual coin tapping detected only 20 mm diameter defects. Still, defect detection with the automated system is greatly dependent upon the threshold value. selected. 

The seminal discovery that transformed membrane separation fi-om a laboratory to an industrial process was the development in the 1960s of the Loeb-Sourirajan process to make defect-free ultrathin cellulose acetate membranes [1]. Loeb and Sourirajan were trying to use membranes to desalt water by reverse osmosis (RO). The concept of using a membrane permeable to water and impermeable to salt to remove salt from water had been known for a long time, but the fluxes of aU the membranes then available were far too low for a practical process. The Loeb-Sourirajan breakthrough was the development of an anisotropic membrane. The membrane consisted of a thin, dense polymer skin 0.2-0.5 pm thick sup-... 

A great deal of work has been devoted to rationalizing the factors affecting the properties of asymmetric membrane made by this technique and, in particular, understanding those factors that determine the thickness of the membrane skin that performs the separation. The goal is to make this skin as thin as possible, but still defect free. The skin layer can be dense, as in reverse osmosis or gas... 

If the bond defect were caused by a trapped air bubble in the middle of the overlap, as in Fig. 30, there would be no load transfer to redistribute because no load would have been transferred there, even if the bond were defect-free. In this case, the requirement would be that such defects were separated by sufficient undamaged bonding to still provide the necessary anchor to protect the joint against creep. Also, if the disbond were too large, one would need to check that the skin over the flaw would not buckle under in-plane compression loads. 

In this regard, there is an excellent review article on MMMs for gas separation, with a detailed discussion on the morphology of the interface between the inorganic particles and the polymer matrix (Chung et al. 2007). Unlike many other articles, this deals with asymmetric membranes for both flat sheets and hollow fibers aimed at the formation of an ultrathin defect-free mixed-matrix skin layer. 

The full description of the deep chemical peel procedure is found in Chapter 8. Before the peeling, the subcision (subcutaneous incision) technique is used to free the fibrous bands from the base of the scars. For this purpose we use an i8-gauge 1.5-inch NoKor Admix needle (Becton Dickinson and Co). This needle has a triangular tip similar to No. 11 blade (Fig. 9.5). it allows smooth separation of fibrous cords. The needle is inserted through a skin surface, and its sharp edges are maneuvered under the defect to make subcutaneous cuts or incisions. The depression... 

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