Testing and analysis of footwear insole copolymers by FTIR-ATR, non-sampling Raman probe spectroscopy, and thermal gravimetric analysis

Abstract

The footwear industry generates hundreds of billions of dollars in revenue each year. However, e-commerce, online retailers' markets, and social media platforms have promoted footwear cross-border counterfeiting along the footwear supply chain with economic implications for shoe producers and marketers, lowering the company's sales and profit margins. This study reports the first combined use of FTIR-ATR, non-sampling Raman probe spectroscopy and Thermal Gravimetric Analysis (TGA) for copolymer analyses of different classes of footwear. Specifically, one hundred and ninety-nine (199) shoe samples of various brand names—including high-end shoes, low-end shoes, heels, boots, men's dress shoes, flats, and sneakers—were collected via donation or purchasing from local stores and subjected to FTIR and Raman spectra measurement. The FTIR-ATR and Raman spectra of poly (ethylene vinyl acetate) ATEVA copolymers reference standards were collected and utilized to construct calibration curves. ATEVA copolymers reference standards and the insole footwear were further subjected to TGA to evaluate the polymers' thermal decomposition and stability pattern. The figure-of-merit (R² ≥ 0.9542) demonstrates the linearity of the calibration curve. The overall results (15 ± 5.2 %, 23 ± 9.0 %, 21 ± 9.0 %, 25 ± 11 %, 19 ± 8.2 %, and 23 ± 10 %) of % copolymer was determined in the heels, sandals, boots, men's dress shoes, flats, and sneakers respectively. The FTIR and Raman spectra and the determined % copolymers content may provide insight into footwear insole thermal degradation, stability, and insole durability. The thermogram decomposition energy of footwear polymers vary, but footwear insole polymers degrade around 600 °C, and weight loss continues gradually with an increase in temperature. This study finds practical applications by U.S. Customs and Border Protection officers, forensic scientists, law enforcement, and quality control officers for forensic footwear, fabric, and automobile polymer analysis for quality checks for consumer goods standard compliance.