Compressive deformation of a translucent material containing an opaque contrasting filler as a pressure indicator mechanism*

Many strategies have been developed for pressure-indicating materials due to their importance in areas such as monitoring of structural integrity, impact analysis, and food processing. The prevailing technology for pressure-indicating films has its origins in the development of microcapsule-based carbonless copy paper in the mid-20th century, but this approach suffers from material and chemical complexity, non-continuous response, and limited sustainability. In this work, an overlooked mechanism for pressure-responsive color change is demonstrated using cellulose acetate membranes prepared by direct immersion annealing with different loadings of activated charcoal. Compressive plastic deformation of the translucent cellulose acetate leads to a decrease in optical path length and a concomitant increase in the visibility of the opaque contrasting filler. Membranes were characterized by ATR-FTIR, TGA, cross-sectional SEM, AFM, optical microscopy, and gas adsorption properties. A linear relationship between applied pressure and resultant pressure mark brightness in the range of 12–56 MPa was observed for membranes bearing 1–7 wt% activated charcoal. Comparison of pressure mark patterns with cross-sectional SEM supports the importance of the direct-immersion-annealing-based porous morphology for the tuning of indicator sensitivity and dynamic range. A simple drop test was used to illustrate the robustness and utility of these indicators in optical impact assessment.