EFFECT OF FORMALDEHYDE ON THE PHYSICOCHEMICAL PROPERTIES OF SOFT GELATIN CAPSULE SHELLS

The effect of formaldehyde (FA) on three physicochemical properties of soft gelatin capsule shells was studied to interpret a change in the internal structure of those capsule shells whose disintegration time w as remarkably prolonged by long-term storage at 40 degrees C or more. Shells with different degrees of crosslinking were prepared by immersi ng the shells in 0-3% FA solutions for 10 min. These shells were evalu ated by measuring the following three physicochemical properties: equi librium swelling ratio (S-eq), the gel strength of the swollen shells, and the percent of gelatin dissolved after 10 min (D-10) from the she lls. When the soft gelatin capsule shells were crosslinked by the trea tment with FA, S-eq determined by second-order kinetics in the swellin g of the shells decreased linearly with an increase in the amount of F A. The gel strength of the swollen shells increased with an increasing amount of FA, whereas the D-10 of gelatin from the shells was diminis hed drastically at a given small amount of FA. From these findings, we confirmed that a three-dimensional network was formed tightly by inte rchain covalent crosslinking in the shells by treatment with FA. These results were, on the whole, closely similar to the phenomenon observe d in the disintegration test for the soft gelatin capsules stored long -term at 4O degrees C or more. Therefore, it was suggested that the ev aluations of these physicochemical properties were available also for capsule shells treated by heating to clarify a mechanism for the prolo ngation of the disintegration time of the soft gelatin capsules.