Microporous coatings

Zentner and coworkers [24,26] utilized this information in their development of a system that releases this drug over a 24 hr period. The use of NaCl to modulate the release of diltiazem presents an interesting problem in that the concentration of the solubility modifier must be maintained within certain limits and below its saturation solubility within the device. To solve this problem, core formulations were developed that contained both free and encapsulated NaCl. The encapsulated NaCl was prepared by placing a microporous coating of cellulose acetate butyrate containing 20 wt% sorbitol onto sieved NaCl crystals. The coated granules released NaCl over 12-14 hr period via an osmotic mechanism into either water or the core tablet formulation. The in vitro release profile for tablets (core I devices) containing 360 mg of diltiazem HC1 and 100 mg of NaCl equally divided between the immediate release and controlled release fractions... 

The major advantage of the microporous coatings is the speed with which they absorb inks as a result of their porosity. This instantaneous absorption provides, in effect, instant drying. Additional characteristics of the microporous layer are the high water fastness and its compatibility with both dye and pigment inks. 

There are different ways that low energy surfaces can be applied to textiles. The first way is mechanical incorporation of the water-repellent prodncts in or on the fibre and fabric surface, in the fibre pores and in the spacing between the fibres and the yams. Examples of these are paraffin emulsions. Another approach is the chemical reaction of the repellent material with the fibre snrface. Examples of these are fatty acid resins. Yet another method is the formation of a repellent fihn on the fibre surface. Examples of these are silicone and flnorocarbon prodncts. The final approach is to use special fabric constructions like stretched polytetrafluoroethylene films (Goretex), films of hydrophilic polyester (Sympatex) and microporous coatings (hydrophilic modified polynrethanes). 

The drying and sintering of sol-gel films (mesoporous and microporous coatings) is discussed in Chapter 8. The drying of sol-gel films during dip-coating is discussed by Hurd and Brinker [43]. 

Fig. 3.10 Scanning electron micrograph showing microporous coating on needlefelt substrate.

Improved membranes have been the key to recent advances in ultrafiltration. The finest niter papers have pore diameters of as small as 1000 nm (1 micron) whereas ultrafilter membranes can be made with pore diameters from 1000 nm to as small as 2-3 nm. For many years cellophane or freshly formed films from collodion (nitrocellulose) were used, but now a number of manufacturers supply strong, flexible, and durable membranes of remarkably uniform pore size yet with high porosity, permitting rapid flow of water. Porous glass membranes have also been developed as well as porous carbon. Po rous ceramic with a microporous coating provides an ultrafilter highly resistant to high temperature and chemical attack. 

The microporous barrier layer breathes primarily through a permanent air permeable pore structure. Diverse techniques have been used to manufacture microporous coatings and films. The most important methods are listed below. 

The past decade has seen significant advances in the ability to synthesize different types of microporous coatings with ordered structures from a wide range of different precursors. Sol-gel hydrothermal synthesis is one of the most promising methods for obtaining zeolitic coatings (films and membranes) on the internal surface of channels of catalytic microstructured reactors. Zeolite[Pg.277]

A.V. Patsis and E.H. Henriqnes, Interdiffusion in complex polymer systems used in the formation of microporous coatings. Journal of Polymer Science Part B Polymer Physics 28 (1990) 2681-2689. 

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