Nonlinear oscillatory shear tests of pressure-sensitive adhesives (PSAs) designed for transdermal therapeutic systems (TTS)

Transdermal therapeutic systems (TTS) based on pressure-sensitive adhesives (PSAs) allow for the application of pharmaceutical substances via diffusion through the skin. The rheological performance of PSA is largely investigated within small amplitude oscillatory shear (typically up to 1 %), although the skin motions exceed strains beyond 40 %. In this paper, amine-compatible (AC) and non-amine-compatible (NAC) silicone-based PSA compounds differing in the resin content were subjected to strain amplitude sweeps in a twin drive rheometer. Carreau-Yasuda-like fitting of storage and loss moduli curves intercepts the substantial effect of resin content on both compounds; up to four times higher, moduli of AC compounds were determined in SAOS, and their higher molecular mass combined with enhanced interactions contributed to an earlier transition to the nonlinear viscoelastic region. In the nonlinear range, elastic and viscous properties are affected by strains in a different manner with the trend favorable for the PSA application as TTS. The third relative higher harmonic from Fourier transformation I3/1 as well as intra-cycle strain stiffening and shear thickening ratios provide information relevant for an optimization of PSA subjected to large deformations