## 1 Introduction  
Polymer materials are widely used in a variety of industries such as construction, aerospace, electronics, automotive, additive manufacturing, and others. In the automotive industry, the number of parts composed of polymer materials has increased as polymeric materials exhibit advantages in minimal corrosion, design freedom, and weight reduction versus conventional materials [1]. The low weight of polymeric materials also provides greater fuel efficiencies, making these materials crucial in the automotive industry. Polymer materials have, moreover, been commonly used as inks for 3D printing in the additive manufacturing industry, in which more than 50% of parts are currently made with polymers [2]. As opposed to metal, polymers degrade under harsh environmental conditions such as exposure to sunlight, high-temperatures, water, soil, and bacteria [3]. Degradation changes various properties of the polymers and is classified into five categories: thermal degradation, photo degradation, chemical degradation, biological degradation, and mechanical degradation. Thermal degradation is caused by prolonged exposure to high temperatures and is a typical route for degradation across all polymer products. Thermal degradation is further classified into two cases, thermal deposition without oxygen and thermal oxidation with oxygen. Photo degradation occurs upon exposure to any wavelength of ultraviolet light. Polymer materials chemically degrade when exposed to rain, sunlight, high-temperatures, and bacteria due to oxidation and hydrolysis of the polymer. Biological degradation is caused by environmental bacteria and only occurs when the polymers are biodegradable. Mechanical degradation refers to the fatigue and fracture of polymeric materials, which is a similar degradation mechanism to that of metals. Many researchers have studied the various routes by which polymers degrade by using several different experimental methods, which are listed in Table 1.  
The mechanical test in the table means a general terminology to characterize the mechanical properties such as elastic modulus, yield strength, and so on. Among these methods, Fourier-transform infrared spectroscopy (FTIR) is a representative technique to analyze polymer degradation and involves obtaining an infrared spectrum of the polymer. Researchers should perform preprocessing and feature selection to extract meaningful information from FTIR spectra, because spectra can include artifacts due to the acquisition environment and operation of the instrument [18]. Recently researchers employed deep learning techniques to analyze vibrational spectra including those obtained by \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Degradation** & **Polymer** & **Testing Methods** & **References** \\ \hline Thermal degradation & Acrylonitrile-butadiene-styrene (ABS) & Mechanical test & [4] \\  & & FTIR & \\  & & Chemiluminescence & \\  & Polyamides & TGA & [5] \\  & & IR & \\  & & Mechanical test & \\  & & FTIR & [6] \\  & & PA, PVC, PS & \\  & Poly(methyl methacrylate) (PMMA) & TA-FTIR & [7] \\  & & TG & \\  & Polyacrylonitrile & TGA & [8] \\  & & FTIR & \\  & & Molecular weight & \\  & & Viscosity & \\ Photodegradation & Polyamide 6,6 (PA66) & DSC & [9] \\  & & WAXD & \\  & & FTIR & \\  & & Mechanical test & \\  & & SEM & \\  & & Synthetic polypropylene (sPP) & FTIR & [10] \\  & & DSC & \\  & & Mechanical test & \\  & & SEM & \\  & & Total & \\  & & Total & \\  & & & \\  & & & \\  & Poly(vinyl chloride) & TGA & [11] \\  & & Chromatographic & \\  & & analysis & \\  & & UV-spectrophotometry & \\  & & IR & \\ Chemical & Polyesters & FTIR & [12] \\ degradation & & Viscosity & \\  & Polyamide 6 & Viscosity Rheological & [13] \\  & & measurements & \\  & Poly(vinyl chloride) (PVC) & FTIR & [14] \\  & & SEM & \\  & & NMR & \\ Biological & Polyethylene & TGA & [15] \\ degradation & & DSC & \\  & & FTIR & \\  & & & \\  & & & \\  & & & \\  & & & \\  & & & \\  & & & \\  & & & \\  & & & \\  & & & \\  & & & \\  & & & \\ \end{tabular}
\end{table}