Polyvinyl alcohol (PVA) is a copolymer of vinyl alcohol and vinyl acetic acid. Many modified PVAs with various functional groups and numerous properties have been developed, and their basic characteristics determined by polymerization and saponification.¹

Hoshi et al. synthesized PVA copolymer by the copolymerization of acrylic acid (AA) and methyl methacrylate (MMA) with PVA and developed hard capsules that could be filled with liquids, such as Macrogol 400.²

As PVA copolymer film has extremely low oxygen permeability, its use as a film coating was evaluated. It was found that PVA copolymer could be used to coat tablets and/or granules if it was processed into a low-molecular-weight solution that could be sprayed.^3,4

This article primarily focuses on the properties of PVA copolymer, and its use as a film coating for tablets and microparticles. It also includes the results of scale-up experiments.

Development

The PVA copolymer developed for encapsulation copolymerizes PVA, AA and MMA at high degrees of polymerization to provide capsules with the desired hardness and solubility.

Figure 1

When a solution of the polymer is sprayed by the two-fluid method, which is usually used for film coating, the liquid drops are not micronized and become thread-shaped dust (Figure 1) that cannot be sprayed. This thread-forming property is the result of the formation of an extensive elastic network in the polymer solution. To contend with this problem, the development of a low-viscosity polymer suitable for spraying was attempted by decreasing PVA's degree of polymerization.⁵

PVA with a degree of polymerization of 500 and an 88% degree of saponification was copolymerized with AA and MMA at the same copolymerization ratio used for encapsulation. The PVA copolymer solution obtained was sprayed and the droplet size was measured with a laser diffraction droplet size analyser (LDSA1400A; Tohnichi Computer Applications Co. Ltd, Japan).

Table I Droplet size at spraying of water solution.

As shown in Table 1, the mean particle size and size distribution of the liquid drops approximated those of hypromellose solution (HPMC [TC-5; Shin-Etsu Chemical Co. Ltd, Japan]) for film coating, which was used as a control, indicating that this copolymer makes spray coating possible.

Tablet film coating

PVA copolymer can be dissolved in water at room temperature, but dissolves more easily with heating and yields a slightly emulsified turbid solution.

The solution has low viscosity, even at high concentration, and adjustments for high-concentration solutions are possible as viscosity can be lowered by heating. The practical elongation of the film obtained from the polymer is approximately 34 times greater than that of HPMC.² The addition of plasticizer is not required, which eliminates the the risk of drug–plasticizer interaction, as well as any appearance change that could be caused by plasticizer migration.

The film obtained is clear and transparent, and can be used for white tablets as there is no discolouration caused by the remaining functional groups, as is seen in a cellulose polymer system. It also has the advantage of further increasing the filling rate pigment volume concentration as it has a high-adhesive strength with other film coating materials, such as pigments, opacifiers and lubricants. The results of this evaluation are described hereafter.

Strength of film adhesion to tablet core. A total of 5 mg of PVA copolymer or HPMC was coated on 150 mg tablet cores containing 10% wax for sustained release with embossing.