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Effects of pigments on film-coating systems
Jun 24, 2018

Because of their very diverse nature it can be expected that the effects of pigments on film-coating
systems can be rather complex.
Mechanical effects
In general, the presence of pigments will reduce the tensile strength of a film, increase the elastic modulus and decrease the extension of the film under a tensile load. All of these are, of course, negative effects. However, as pigments consist of discrete individual particles the need for efficient pigment dispersion should be emphasized. Another generalization is that the lower the particle size of the pigment concerned, the smaller will be the deleterious effect on film properties. These effects are of
some importance in the consideration of stress-related film-coating defects. Lehmann & Dreher (1981) describe the property displayed by several of the acrylic film-coating polymers, that of being able to
bind substantially higher quantities of pigment than is possible for example with the cellulosics. The authors point to the advantages of mechanical stability and resistance to attrition achieved.
Aulton et al. (1984) have examined the effect of a wide range of pigments on the mechanical properties of cast films of HPMC (Methocel E5). In addition to confirming the general effects above, they emphasized the need to consider the whole stress-strain diagram and not to merely one feature in isolation. For instance, a pigmented film may well show very little decrease in tensile strength compared with the unpigmented film; however, a consideration of the area under the curve could show significant differences . The term ‘work of rupture’ was coined by the authors for this particular parameter. In comparing the effects of different pigments the authors concluded that there were
pigment-specific effects and that the pigment was not merely occupying space in an inert manner or behaving as an inert diluent. The pigment effect has also been discussed by Rowe (1982) in a study on the effect of pigments on edge splitting of tablet film coats. Talc was seen to be an exception to the general behaviour of pigments. The reason postulated was that as talc exists as flakes it orientates itself parallel to the surface of the substrate in a restraint on volume shrinkage of the film parallel to the plane of coating .
In another study, Okhamafe & York (1985a) have looked at the mechanical properties stated above for free films in combination with PVA or PEG 1000 and loaded with talc or titanium dioxide. Broadly,
the results were in agreement with the findings of Aulton et al. (1984). The results were presented not only in mechanical terms but polymer-pigment interactions were also taken into account in either rein-forcing the mechanical effect or working against it. For example, in the case of high pigment-polymer interaction, the loss of film elongation was greatly potentiated. The same authors, in further work (1985b), have examined the effect of pigmented and unpigmented films on the adhesion of those films to the surfaces of aspirin tablets. They found that pigments
incorporated in an applied film can exert two opposing effects on adhesion: one decreases adhesion by increasing internal stress and the other increases adhesion by strengthening the film-tablet surface interation. From the results obtained, the adhesion of HPMC films was initially increased in the presence of talc because of a stronger film-tablet
interface and a smaller increase in the internal stress of the film, but above 10% by weight of the pigment, the internal stress factor began to dominate and adhesion fell.
In a large comparative study (Gibson et al., 1988), the effect of the iron oxide pigments titanium dioxide, talc, erythrosine lake, and sunset yellow lake were examined upon HPMC (Pharmacoat 606)
films plasticized with PEG 200. The authors concluded that the Young’s modulus of the films is raised by the pigments to an extent that largely depends upon pigment shape and can be predicted by existing
theories. The exceptions are titanium dioxide and the lake pigments which have less of an effect on the modulus than expected due to polymer-pigment interactions or, in the case of the lake pigments, to a
loose particle structure. The ultimate tensile properties of the films depend mainly on the concentration of the particles added. Pigments cause a large decrease in tensile strength except in the cases of yellow
or black iron oxides which are not weakened to such an extent because the shape of the particles allows the growth of flaws to be retarded. If the thermal expansion coefficients of the matrix and filler promote
premature cracking on cooling from the fabrication temperature, then the introduction of filler in any concentration is detrimental to the tensile strength of the system








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