Permeability barrier coatings

In suppositories cetyl alcohol is used to raise the melting point of the base, and in modified-release dosage forms it may be used to form a permeable barrier coating. In lotions, creams, and ointments cetyl alcohol is used because of its emollient, water-absorptive, and emulsifying properties. It enhances stability, improves texture, and increases consistency. The emollient properties are due to absorption and retention of cetyl alcohol in the epidermis, where it lubricates and softens the skin while imparting a characteristic velvety texture. 

In order to avoid diffusion in the plgment/binder interface, in-terfaclal bonds between lx>th phases should be as water-resistant as possible. If permeability is taken as a measure, properly formulated barrier coatings may coiipare in corrosion protective efficiency with normal coatings, the thickness of vhlch is tvo or three times as high. 

Polyolefins can be coated with other materials, such as poly(vinylidene chloride), in order to reduce the permeability by gases of the walls of blow mouldings or film (thus extending the shelf-lives of products contained in them) however, barrier coatings of this nature cannot be applied satisfactorily to the untreated surfaces. 

For barrier coatings to be effective it is necessary to have (i) good adhesion of the barrier film on the metal surface (this is generally accomplished by multicoat procedure to a certain extent) (ii) proper choice of the polymers to control the permeability of oxygen and (iii) optimum film thickness achieved by multicoats or thin-film system, depending upon the envisaged application. 

The surface tissue of the mouth is squamous epithelium, and the cells lining the cheek are dead and enucleated. The effective permeability barrier is quoted as between 10 9 and 10 5 cm/s due in part to the activity of membrane coating granules... 

The fact that silicone coatings are able to prevent silver migration has, of course, been known for over ten years. But the mechanism for this has not been understood. Silicones, as a general polymer class, have a very high permeability to water vapor. Hence, silicones cannot be considered as "barrier coatings" which function by excluding water from the region between the conductors. 

Pull test results for all three substrates showed that the films enriched with tin have higher adhesive and cohesive strengths than those of higher carbon content. The metallic films acted as good water vapor permeability barriers in accord with what is expected for metal coatings. 

Clay nanocomposites are also being developed as barrier coatings for film and for containers. The nanocomposite is deposited on the film from a solution of PVOH/ EVOH copolymer in a mix of water and isopropyl alcohol which has been used in a supersonic dispersion system to nano-disperse 7 nm diameter silica and titanium dioxide particles. The ratio of polymer to silica depends on the barrier properties required. Typical microgravure equipment can be used to coat the solution onto a plastic substrate. The result reportedly is a transparent barrier coating which is superior to silica- and alumina-coated films, and is comparable to aluminum-coated materials. Oxygen permeability at a coating thickness of 2 pm is less than 1 cc/m d atm, and moisture permeation less than f g/m d. Costs are reported to be competitive with ceramic coatings [4]. 

Ethylene Copolymers. Ethylene-vinyl acetate copolymers differ according to their vinyl acetate content and molecular mass. With increasing vinyl acetate content, compatibility with paraffin waxes decreases, but that with other binders increases. Low molecular mass types containing 25-40% vinyl acetate are readily or sufficiently soluble in solvents. With a 40% vinyl acetate content, they can be combined with polar resins and nitrocellulose. Terpolymers with free carboxyl groups exhibit improved adhesion. Ethylene-vinyl acetate copolymers are primarily added to waxes to improve their properties, but are also used to increase flexibility and adhesion in paints, printing inks and adhesives, and for hot melt coatings. Ethylene-vinyl acetate copolymers have low water vapor and gas permeabilities (barrier effect). 

Paint the cathodic material. This provides a barrier against electrolyte contact with the cathode however, this is not simply accomplished in many cases due to the permeability of coatings by moisture combined with the generation of hydrogen and hydroxyl ion on the cathode surface. The combined effect is to strip all but the most alkali-resistant coatings. 

Modified NR latex can be also blended with lignin to produce a paperboard barrier coating in order to replace unrecyclable wax coating materials. The blends containing lignin showed a reduction of water vapour permeability in coated paper when compared with coatings without lignin. ... 

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