A new methodology to evaluate the mechanical properties of elastomeric closures

A quantitative method was developed to evaluate the mechanical properties of elastomeric closures commonly used in liquid parenteral dosage forms. The new methodology was based on the compression of individual stoppers facing upwards, with a stainless steel probe attached to an Instron testing machine. The Instron recorded the maximum force exerted by the closure at a preset rate and displacement value. The mechanical properties were also evaluated with the standard needle penetration force test. The compression test was found to be more sensitive in ascertaining differences in the mechanical properties of stoppers compared to the needle penetration test. Changes in the mechanical properties of closures exposed to various concentrations of acetate buffer, pH and buffer species were also evaluated in this study. The mechanical properties of all stoppers tested were affected by the acetate buffer concentration, even though their physical appearance did not change. The compression force was significantly decreased with an increase in buffer concentration, whereas the needle penetration force values were not significantly different. Since there was no significant effect of pH or citrate buffer species on the mechanical properties of closures, the changes in compression force observed were attributed to the acetate buffer species. In addition, an increase in the duration of the autoclaving cycle significantly affected the mechanical properties of polyisoprene stoppers, whereas the properties of bromo and chlorobutyl stoppers remained unchanged.