Imaging Analysis of Scratch Reduction in Poly (Methyl Methacrylate) (PMMA) Plastic with NOVUS Polishing Products

Remediation of disfiguring scratches and abrasions on aging plastics challenges even the most experienced conservator. This study assesses the effect of three NOVUS polishing products on remediation of poly (methyl methacrylate) (PMMA). Samples were prepared to mimic the depth and type of scratches observed during conservation treatment performed on vacuum-formed reliefs from the 1960s. The study compares samples of new/unaged PMMA to naturally aged PMMA samples. Two different methods were successfully used to assess the polishing efficacy, which is notoriously challenging to document in transparent and reflective media: (1) image segmentation analysis of photographs to yield scratched area fractions, using three approaches (Li-thresholding, random forest, and convolution neural network); and (2) optical profilometry measurements, which provide surface root-mean-square roughness (R-RMS) values. Both methods support the qualitative observations that polishing with the abrasive NOVUS products significantly reduced the surface damage; scratch area fractions decreased from 50-60% to 1-5% across the aged PMMA types after treatment, corresponding to an similar to 85% decrease in surface roughness. Critically, the two methods are complementary: the image segmentation results demonstrated a greater visible difference between two degrees of polishing treatments on the new material, while profilometry results were able to parse differences in the degree of scratch reduction between the aged and new materials.