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POLYMER CHARACTERISTICS
Jun 20, 2018

POLYMER CHARACTERISTICS

1.Solubility

Inspection of the solubility characteristics of the film-coating polymers show that the following have a good solubility in water: HPMC, HPC, MC, PVP, PEG plus gastrointestinal fluids and the common organic solvents used in coating.
Acrylic polymers used for conventional film coating include methacrylate amino ester copolymers. These bcome water soluble by swelling, increasing permeability in aqueous media. The polymer in its unmodified form is however soluble only in organic solvents. Where it is proposed to use an aqueous solvent for film coating it is necessary to consider, first, the need to minimize contact between the tablet core and water and, secondly, the need to achieve a reasonable process time. Both can be achieved by using the highest possible polymer concentration (i.e. the lowest possible water content). The limiting factor here is one of coating suspension viscosity.

2. Viscosity

HPMC coating polymers, for example, are available in a number of viscosity designations defined as the nominal viscosity of a 2%w/w aqueous solution at 20°C. Thus a 5mPa s grade will have a nominal viscosity of 5 mPa s in 2% aqueous solution in water at 20°C and similarly with 6 mPa s, 15 mPa s and
50 mPa s grades. Commercial nomenclature for these grades may still describe them as ‘5 cP’ etc. Commercial designations such as E5 (Methocel) or 606 (Pharmacoat) also correspond with the viscosity designation, such that for example Methocel E5 has a nominal viscosity of 5mPa s under the previously
described standard conditions. While Pharmacoat 606 would have a nominal viscosity of 6 mPa s under the same conditions.
Considering the final polymer solution to be sprayed, a normal HPMC-based system would have a viscosity of approximately 500 mPa s. Inspection of Fig. 2.3 shows that if, for instance, a 5 mPa s grade is used (E5) a solids concentration of about 15%w/w can be achieved. This has the advantage over, for example, a coating solution prepared from a 50 mPa s grade (E50) where only a 5%w/w solids concentration could be achieved. The lower viscosity grade polymer permits a higher solids concentration to be used, with consequent reduction in solvent content of the solution. The practical
advantage to be gained is that the lower the solvent content of the solution, the shorter will be the processing time as less solvent has to be removed during the coating procedure. This beneficial interaction between polymer viscosity and possible coating solids is self-limiting in that very low viscosity polymers will suffer from poor film strength due to low
molecular weight composition. Delporte (1980) has examined polymer solution viscosities in the 250–300 mPa s range and has concluded that 5 mPa s HPMC is preferable to the use of 15 mPa s material. Furthermore, Delporte advocated the use of elevated temperature coating media in order to additionally
increase solids loadings via a decrease in viscosity.

3. Permeability

One of the reasons for coating tablets is to provide a protection from the elements of the atmosphere such that a shelf-life advantage for the product may be gained.With the continuing change from sugar- to film-based coating has come associated problems of stability due to sugar-coating techniques providing a better moisture barrier than that offered by simple
non-functional cellulosics or acrylics. Usually the moisture permeability of a simple film may be decreased by the incorporation of water-insoluble polymers, however disintegration and dissolution characteristics of the dosage form must be carefully checked. Permeability effects can be assessed practically by a technique of sealing a sample of cast film over a small container of desiccant or saturated salt solution, the permeability to water vapour being followed
by successive weighings to determine respectively weight gain or weight loss (Hawes, 1978). In addition to being tedious to perform, the results are only comparable when performed under identical conditions. Using similar techniques Higuchi & Aguiar (1959) demonstrated that water vapour permeability of a polymer is dependent on the relative polarity of the polymer. Both Hawes (1978) and Delporte (1980) have seen little difference in water vapour permeability between two commercial grades of HPMC (E5 and E15) which differ only in molecular weight. Okhamafe & York (1983) have used an alternative method of assessing water vapour permeability, and that is a sorption-desorption technique to evaluate the performance of two film-forming polymers, HPMC (606) and polyvinyl alcohol (PVA). Addition of PVA to the HPMC was seen to enhance very effectively the moisture barrier effect of the HPMC. The authors ascribe this behaviour to the possible potentiation of the crystallinity of the HPMC by the PVA.
Sometimes permeability of other atmospheric gases is of concern, particularly that of oxygen. This area has been studied by Prater et al. (1982) who examined the permeability of oxygen through films of
HPMC. These workers used a specially constructed cell which held a 21 mm diameter sample of the film. The passage of gas into the acceptor portion of the cell was monitored by using a mass spectrometer detection system. Earlier, Munden et al. (1964) had also determined oxygen permeability through free films of HPMC. They concluded that there was an inverse relationship between oxygen permeation and water vapour transmission. These results were obtained using a technique of sealing the films across a container of alkaline pyrogallol and measuring the consequent solution darkening. As
Prater et al. (1982) point out, this method is not only tedious but water vapour from the pyrogallol is capable of plasticizing the film and modifying the result.

4. Mechanical properties

Some of the film mechanical properties of concern are:
• tensile strength
• modulus of elasticity
To perform any function a film coat must be mechanically adequate so that in use it does not crack, split or generally fail. Also, during the rigours of the coating process itself the film is often relied upon for the provision of some mechanical strength to protect the tablet core from undue attrition.
These attributes may be conveniently measured by tensile tests on isolated films although other techniques such as indentation tests have a part to play. Much discussion has also taken place in the literature on the merits and validity of examining isolated films as opposed to examination of a film
produced under the actual conditions of coating. Both arguments have been reviewed by Aulton (1982).
Suffice it to say that much useful data can be obtained relatively easily from isolated films which, in practice, has demonstrated the validity of such techniques.

• work of failure
• strain.
• Tensile strength: The most important parameter here is the ultimate tensile strength, which is the maximum stress applied at the point at which the film breaks.
• Tensile strain at break: A measure of how far the sample elongates prior to break.
• Modulus (elastic modulus): This is applied stress divided by the corresponding strain in the region of linear elastic deformation. It can be regarded as an index of stiffness and rigidity of a film.
• Work of failure: This is numerically equivalent to the area under the curve and equates to the work done in breaking the film. It is an index of the toughness of a film and is a better measure of the film’s ability to withstand a mechanical challenge than is a simple consideration of tensile
strength All these properties of a polymer film are related to its molecular weight which, in turn, affects the viscosity of the polymer in solution. In general, apart from the acrylics, the different types of individual polymers are available in various commercial viscosity designations. These designations rely on the description of a standard solution in a specified solvent, as previously indicated.2.6.5 Tackiness In a film-coating sense, tack is a property of a polymer solution related to the forces necessary to separate two parallel surfaces joined by a thin film of the solution. It is a property responsible for processing difficulties and is a limitation on the use of some polymers, e.g. hydroxypropyl cellulose







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