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SURFACE FREE ENERGY VALUES AS PARAMETERS IN THE PREDICTION OF ADHESIVE BOND STRENGTHS DEVELOPED BY TABLET FILM COATINGS

Date

1971

Journal Title

Journal ISSN

Volume Title

Publisher

ORCID

Type

Degree Level

Doctoral

Abstract

The object of the investigation was to find a set of parameters that could be used in predicting the adhesive bond strength developed by a film coating on a tablet surface. To predict the strength of an adhesive bond it was necessary to find measures of the cohesional energy densities (CED) of the tablets and the coating solutions. In areas other than pharmacy various means had been found of estimating the CED of a variety of substances. These CED values have been used to explain and predict adhesion of adhesive materials to a wide range of substrates. Estimates of the CED of liquids can be obtained by surface tension (yL) measurements. Critical surface tension (yc) values which are estimates of the surface free energy of solids, both homogeneous and heterogeneous, have been obtained by the use of the Zisman technique. The yc values of different acetylsalicylic acid (ASA) tablets were determined using a variety of liquids and a series of methanol-water mixtures. The ASA tablets were coated with different methyl-cellulose coating solutions and the dried films peeled from the tablet surfaces. The forces required to cause the peeling of the film coat-ings were compared with the yc values of the tablets and a correlation was seen to exist. This prompted further investigation into the use of yc values as a means of explaining and predicting the adhesive bond strength developed by a film coating. A study was undertaken to determine the variation of Yc values when different series of test liquids were employed. It was found that yc varied depending on the type of test liquids used. The slopes of these plots were found to vary considerably and as 0, the contact angle, became increasingly large the plots ceased to be linear. It was con-cluded that although the determination of yc values represented an important advance, by themselves they were not completely adequate in characterizing a tablet surface. Young's equation, which is the basis of the yc determinations did not adequately describe the nature and the extent of interaction between a liquid adhesive and a solid surface. A modified Good-Girifalco-Fowkes-Young equation was employed to determine the operative dispersion (D) and polar (P) force compon-ents ,of the surface free energies of liquids (yL) and solids (ys). The values determined for the ASA tablets did not coincide with the yc values. The difference between yc and depended on the degree to which and yL of the liquids matched and balanced the ys and ys values of the tablets. Determination of yLD , yL P , ysD and and the extent of their interaction explained the variations in yc values, differences in the slopes of the Zisman plots and their departure from a linear relationship. ASA tablets whose and ys values had been determined, were coated with six different coating solutions. The surface free energy components of the solvents, solvent blends, final solution and dried films were determined. The quality of the film coatings was noted and the force required to peel the coatings was measured with a transducer linked to a recorder. It was found that a knowledge of the relative presence of the various surface free energy components when considered with other relevant factors made it possible to explain the strengths of the adhesive bonds and the qualities of the film coatings produced. The ability to explain these observed phenomena makes it possible to optimize conditions for a good film coating and to predict the type of coating and the extent of bonding that may be expected.

Description

Keywords

adhesive bond strength, cohesional energy densities

Citation

Degree

Doctor of Philosophy (Ph.D.)

Department

Pharmacy

Program

Advisor

Committee

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DOI

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