53P
`
`MOISTURE PERMEATION MECHANISM OF SOME AQUEOUS-BASED F I L M COATS
`A.O. Qkhamafe, P. York, Postgraduate School of Studies in Pharmacy, University of
`Bradford, Bradford, West Yorkshire, BD7 lDP, U.K.
`The sorption-desorption technique (Sacher & Susko 1981) has been used to evaluate
`the permeation characterisitics of water-based free films of hydroxypropyl methyl-
`cellulose (HPMC) containing either polyethylene glycol (PEG) 400, PEG 1000 or
`polyvinyl alcohol (PVA) - degree of hydrolysis 88%.
`This method provides a deep-
`er insight into permeability mechanisms than the transmission cup method as well
`as affording practical advantages in avoiding sealing problems and eliminating
`pore effects.
`Films, cast by a pouring technique on the inside of a rotating perspex cylinder
`maintained at 40 - 42OC, were conditioned for 7 days at ZOOC, 58% R.H. before
`The measuring apparatus incorporates an automatic recording microbalance
`test.
`is hung, and humidity and vacuum lines, placed in a
`from which the film specimen
`controlled temperature room (20 -+ l0C).
`The system plus the test film was evac-
`uated at l0-4mbar overnight, then a steady stream of moist air (75% R.H.) was
`Desorption was carr-
`passed over the film until equilibrium weight was attained.
`The integral diffusion
`ied out under vacuum until a constant weight was reached.
`coefficient, D, was calculated employing Boltzmann's solution of Fick's law put
`forward by Long & Thompson (1955) for concentration dependent diffusion, using
`the initial linear slopes of plots of Me/Mt against tz/l for sorption and desorpt-
`Mt is the amount sorbed or desorbed at time, t, Me the equilibrium moisture
`ion.
`concentration and 1 the film thickness. The permeability coefficient, P, is
`given by (Crank 1975): P = DS where S is obtained from Henry's law using the
`expression S = c/p where c is the sorption equilibrium concentration and p is the
`vapour pressure of the moist air.
`
`(b)
`
`D
`
`102 :::I-..-
`
`X
`
`D x
`
`50 :
`
`2 0 4 0 6 0 8 O x K )
`C onc (%Ww)
`Conc(%Yw)
`Conc(%Vw)
`2 -1
`-3
`3
`Fig. 1: Showing variation of D(cm2s-'),
`S[cm cm (cmHg)-l] and P[cm s (~mHg)-~]
`with concentration of PEG 400 (D), PEG 1000 (A) and PVA (0) at 2OoC, 75% R.H.
`The enhanced segmental mobility of HPMC brought about by the plasticking activ-
`ity of the PEG'S is believed to be responsible for the increase in diffusion
`PVA, however, causes a
`coefficient with increase in concentration (Fig l(a)).
`decrease in diffusion coefficient, suggesting that it has an anti-plasticising
`effect. The longer ethylene backbone (and hence lower hydrophilicity) of PEG
`1000 makes it a weaker plasticiser than PEG 400. PEG 400 and 1000 (glycols) have
`twice the number of -OH groups of PVA per molecule forming more hydrogen bonds
`with HPMC and leaving the film former with fewer bonding sites for permeating
`moisture. This accounts for the lower value of solubility coefficient (Fig.l(b)).
`The striking similarity of Figs. l(a) and (c) indicates that in the film systems
`evaluated, moisture permeability is influenced more by change in diffusion coeffi-
`cient than in solubility coefficient.
`Crank, J. (1975). The Mathematics of Diffusion, pp. 45 - 4 6 , Oxford University
`Press
`
`Long, F . A . , Thompson, L.J. (1955) J.Polymer Sci. 15: 413-426
`Sacher, E., Susko,J.R.(1981) J.App1. Polymer Sci. 26: 679-686
`0022-3573/82/120053 P-O1$02.50jo
`@ 1982 J. Pharrn. Pharmacol ,
`
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