Medicated Pressure Sensitive Adhesive Patches from STR-5L Block Rubber: Effect of Preparation Process

STR-5L is a high quality block rubber in Thailand that is interesting to apply in medical and pharmaceutical products because it has very low impurity but high uniformity. In this study, medicated pressure sensitive adhesive (PSA) patches were developed by melt blending technique using hydroxyethyl cellulose (HEC) as tackifier and paraffinic oil as softener. Two rolls mill was used to blend all ingredients and the thin PSA patches were rolled out. Various preparation parameters were studied such as initial viscosity of rubber (60 or 80 Mooney viscosity; MV), mastication time (5-20 minutes), step of mixing, mixing time (35-80 minutes) and gap between rollers (0.1-0.4 mm). The suitable processing conditions were optimized. It was found that the rubber having initial viscosity of 80 MV provided better physical properties, for example, higher force T-peel, lap shear strength and shear holding time. Longer mastication time increased the shear holding time of patches. At 70 minutes of mixing time, the patches showed the highest shear holding time and did not leave any residue on the testing equipment surface. Moreover, the shear holding time decreased when the gap between rollers was expanded. Mixing method with the sequential addition of STR-5L, HEC and then paraffinic oil, provided good PSA patches. However, preparation process did not affect the plasticity retention index of PSA patches due to there were not significant change of values when those all parameters were varied. Next, lidocaine or its hydrochloride salt in powder form could be blended into this PSA to be the homogeneous patches. This lidocaine PSA patches for local anesthetics application on the skin would be evaluated in further study.