Hydrophobic tablet coatings

Sustained release wax matrix tablets are generally considered difficult to coat with aqueous polymeric dispersions due to the poor wettability of the hydrophobic tablet surface. "" The physicochemical... 

Felton LA, McGinity JW. Influence of plasticisers on the adhesive properties of an acrylic resin copolymer to hydrophilic and hydrophobic tablet compacts. Int J Pharm 1997 154(2) 167—178. Okarter TU, Singla K. The effects of plasticisers on the release of metoprolol tartrate from granules coated with a polymethacrylate film. Drug Dev Ind Pharm 2000 26(3) 323—329. 

The affinity to hydrophilic and hydrophobic surfaces is particularly useful in the hydrophilization of a wide range of substances, ranging from hydrophobic tablet cores - to permit sugar or film-coating, to medical plastics. 

Affinity to hydrophobic surfaces Adhesion of the tablet coating to cores with hydrophobic substances... 

Sugar coatings are particularly susceptible to cracking when they are applied to large batches of tablet cores that are dried rapidly. As most active substances are hydrophobic, copovidone is useful as an additive to prevent the tablet coating cracking away from the tablet core during manufacture. 

In extmded products, plasticizers play the usual role of process additive and component which adds elasticity to the product. In addition to these typical roles, plasticizers were found to perform some roles typical of the product into which they were incorporated. In pharmaceutical products, dmg controlled-release properties were regulated with the amount of the plasticizer present in tablet coating. In addition, the type of plasticizer could also change properties of coating by affecting the balance between hydrophobic polymer component and hydrophilic excipients. ... 

Erodings of Slow-Releasing Core Tablets. The sustained-dose portion of a dmg is granulated with hydrophobic materials such as waxes, fatty acids, or fatty alcohols and compressed into a core. The initial dose is added to the core by a modified sugar coating process or by compression coating. Thus, a tablet within a tablet is created. The core erodes slowly to release the active ingredient. 

The implant consists of a tablet-shaped ganciclovir reservoir. The drag is initially completely coated with poly(vinyl alcohol) (PVA) and then coated with a discontinuous film of hydrophobic, dense poly (ethylene-co-vinyl acetate) (EVA). Both polymers are nonerodible and hydrophobic (the PVA used in the implant is cross-linked and/or high molecular weight, to ensure it does not dissolve when exposed to water). The entire assembly is coated again with PVA to which a suture tab made of PVA is attached (Figure 4.5). 

Another system was based on a swelling core tablet and a surrounding coating consisting of a combination of hydrophobic and hydrophilic polymers. The insoluble hydrophilic polymer, such as calcium pectinate or calcium alginate, was dispersed in the coating and served as a channel-former in order to control the water penetration. The core contained a swelling, but... 

The main use of ethylcellulose in oral formulations is as a hydrophobic coating agent for tablets and granules. Ethylcellulose coatings are used to modify the release of a drug, to mask an unpleasant taste, or to improve the stability of a formulation for example, where granules are coated with ethylcellulose to inhibit oxidation. Modified-release tablet formulations mav also be produced using ethylcellulose as a matrix former. " ... 

Hydrophobic surface of the tablet core Improvement in adhesion of subsequent coatings by hydrophilization of the surface... 

It is widely accepted that hydrophobic lubricants show no toxic effects since they have a poor dissolution and absorption in the gastrointestinal tract [5], They are commonly used in the range of 0.25-5.0% (w/w) for tablet compression (Table 1). However, at very low levels could function as a glidant (flow enhancer). The use of an excessive amount of lubricant could impede flow because the interparticulate adhesion forces would surpass the interparticulate friction forces between the host particles. Moreover, lubricants are always added last after all other components have been thoroughly blended to minimize their deleterious effect. The optimal blending time (usually from 2 to 15 min) depends on their chemistry and amount since it determines the degree particle coating [6]. 

Adhesion between the polymeric film and substrate is a major concern. Poor adhesion could result in flaking or peeling of the coating from the substrate core. Moisture could accmnulate at the film-substrate interface and compromise the mechanical protection provided by the coating. Polymer adhesion is related to both film-substrate interfacial interactions and internal stresses within the film. Polymer adhesion can be evaluated by peel tests or butt joint tests. Apart from the specific properties of the polymers, excipients used in tablet formulations can influence film-tablet adhesion. Since adhesion between a polymer and the tablet smface is due primarily to hydrogen bond formation, hydrophobic agents may decrease adhesion by... 

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