Coating materials cellulose

Cellulose nitrate also has widespread use as an adhesive and coating material. Whereas stabilizers are added to products, eg, sodium carbonate as a neutralizer, many conservators are hesitant to use cellulose nitrate materials because of the inherent instabiUty and the dangers to the object from nitric acid, formed when the nitric oxide combines with moisture. 

Technique of thin-layer chromatography. Preparation of the plate. In thin-layer chromatography a variety of coating materials is available, but silica gel is most frequently used. A slurry of the adsorbent (silica gel, cellulose powder, etc.) is spread uniformly over the plate by means of one of the commercial forms of spreader, the recommended thickness of adsorbent layer being 150-250 m. After air-drying overnight, or oven-drying at 80-90 °C for about 30 minutes, it is ready for use. 

Shellac is the oldest known material that has been used as enteric coating material. However, as a natural material, it lacks a crucial quality criterion of more modern polymers (i.e., batch-to-batch reproducibility). Hence, the most commonly used polymers today are the synthetic methacrylate copolymers or semisynthetic derivatives of cellulose. The main structural element of these polymers is an acidic function (either phthalate or methacrylic acid), which is responsible for the pH-dependent dissolution. 

A survey of the German market showed that more than 50% of enteric formulations were coated with methacrylate copolymers, about 40% with cellulose derivatives, 5% with shellac, and 3% with other materials [1], Enteric coating materials (Table 1) are described in various publications [21, 22], In addition to polymers mentioned in Table 1, others are being studied (e.g., to obtain release at lower pH) [23], Polymers with a dissolution at lower pH are intended for the protection of drugs in acidic medium and not for the protection of the gastric mucosa. 

Naproxen sodium tablets were used in an in vivo-in vitro evaluation with four different polymeric dispersions, cellulose acetate phthalate (CAP), cellulose acetate trimellitate (CAT), 50 50 CAP/CAT, and methacrylic acid copolymer [64], The study indicated that coating material that dissolves at a more acidic pH in vitro (such as CAT at pH 4.5) will also dissolve at a more acidic pH in vivo (i.e., the coating dissolves higher up in the GI tract). In addition, it was found that aging did not markedly affect dissolution characteristics of CAT or methacrylic acid copolymer-coated tablets. 

Frohoff-Hulsmann, M.A. Schmitz, A. Lippold, B.C. Aqueous ethyl cellulose dispersions containing plasticizers of different water solubility and hydroxypropyl methyl-cellulose as coating material for diffusion pellets. I. Drug release rates from coated pellets. Int. J. Pharm. 1999, 177, 69-82. 

Cellulose acetate has offered little in fulfilling the requirements of a protective coating material. Although stable to light, it is restricted in use by its limited solubility in solvents, and its poor compatibility with resins. It has been used only for special purposes as a lacquer material, and to a limited extent for surface coatings on paper and cloth. Cellulose mixed esters appear more promising for th se uses. Cellulose acetate... 

Cellulose acetate phthalate (CAP) is used as an enteric film coating material, or as a matrix binder for tablets and capsules.Such coatings resist prolonged contact with the strongly acidic gastric fluid, but dissolve in the mildly acidic or neutral intestinal environment. 

A reconstituted colloidal dispersion of latex particles rather than solvent solution coating material of cellulose acetate phthalate is also available. This white, water-insoluble powder is composed of solid or semisolid submicrometer-sized polymer spheres with an average particle size of 0.2 pm. A typical coating system made from this latex powder is a 10-30% solid-content aqueous dispersion with a viscosity in the 50-100 mPa s range. 

Lin SY, Kawashima Y. Drug release from tablets containing cellulose acetate phthalate as an additive or enteric-coating material. Pharm Res 1987 4 70-74. 

As biopolymers, cellulosics soluble in organic solvents are essentially used as semipermeable membranes (e.g. reverse osmosis separation), and for drug delivery in coating materials for reservoir or osmotic systems and excipients for matrix systems (e.g. films, compressed tablets). Recent advances in the latter fields will be reviewed. 

Physicochemical studies of the solution properties of amylose benzoate by light-scattering, osmometric, and viscosimetric techniques showed that the molecules behave as coils. Factors affecting the rate of dissolution of cellulose acetate phthalate in aqueous solution are important from the point of view of use as an enteric-coating material. The removal of the benzoyl group during methanolysis is slow and concurrent with the methanolysis, " whereas deacetylation precedes methanolysis. 

In the case of natural polymers, polysaccharides like cellulose and chitosan are widely studied as antimicrobial films and barrier/coating materials. The following section reviews the recent studies on cellulose and chitosan having current and potential use as antimicrobial material for active food packaging systems. 

In liquid chromatography greater efficiencies and faster analysis times have resulted from the use of smaller particle diameters, within the pressure limitations imposed by instrumentation and support materials. While HPLC has demonstrate improved column efficiencies due to the use of superior support properties, the low-pressure modihed-cellulose and liquid-coated materials have often shown superior selcctivities with regard to nu cleic acid compounds. 

Products and Uses Cellulose esters or ethers in coating materials used on metal, protective, or decorative coatings also in paper products, textiles, plastics, furniture polish, and nail polish. 

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