ADVANCED MATERIALS & PROCESSES | NOVEMBER/DECEMBER 2023 35 Fig. 1 – FTIR spectral comparison showing distinct differences between the results obtained on various plastic materials. polymeric materials, the most common sampling techniques are transmittance, reflectance, and attenuated total reflec- tance. Additionally, a microscope can be interfaced with the spectrometer to focus the infrared beam and allow the analysis of samples down to 10 µm. Regardless of the sampling technique, the beam of infrared energy is passed through or reflected off the sample and directed to a detector. The obtained spectrum shows frequencies that the material has absorbed and frequencies that have been transmitted. The results generated through FTIR analysis are referred to as an infrared spectrum. The spectrum graphically illustrates the relative intensity of the energy absorbed on the y-axis versus the frequency of the energy on the x-axis. The frequency of the energy can be represented directly in microns (µm) or as reciprocal centimeters (cm−1) referred to as wave numbers. The spectrum can be interpreted manually or, more commonly, compared to voluminous library references with the aid of a computer. The discrete spectral features present in an FTIR spectrum are known as absorption bands, which correlate to functional groups within the molecular structure of the sample. FTIR IN FAILURE ANALYSIS Material identification: Possibly the most important use of FTIR in failure analysis is the identification of the base polymer that comprises the sample. Determining the composition of the failed component is an essential part of the investigation. Because different polymers have a wide variation in their physical, mechanical, chemical resistance, and aging properties, the use of the wrong resin can yield detrimental results in many applications. Fourier transform infrared spectroscopy is well suited for the identification of polymers that have different molecular structures (Fig. 1). Confirming that the failed article was produced from the specified material is a primary consideration of the failure analyst in assessing the cause of the failure. The use of FTIR in characterizing the composition of the plastic resin base polymer is illustrated in the case study included in this article. However, one area in which FTIR is inadequate is differentiating polymers that have similar molecular structures, such as the members of the nylon family, between poly(ethylene terephthalate) and poly(butylene terephthalate), and between polyacetal homopolymers and copolymers. Aside from determination of the base polymer, FTIR can be helpful in characterizing other formulation constituents. FTIR analysis can provide information regarding the presence of additives and filler materials. Due to the nonlinearity of infrared absorptivity of different molecular bonds, it is not possible to accurately state minimum concentration detection limits. However, it is generally considered that materials present within a compounded plastic resin at concentrations below 1% may be below the detection limits of the spectrometer. Given that FTIR is principally used for the analysis of organic materials, its use in the evaluation of inorganic filler materials is somewhat limited. However, some commonly used fillers—such as calcium carbonate, barium sulfate, and talc— produce unique and identifiable absorption spectra. Degradation: FTIR is a valuable tool in assessing a failed component material for degradation such as oxidation and hydrolysis. Molecular degradation, often involving molecular weight reduction, has a significant detrimental impact on the mechanical and physical properties of a plastic material. This degradation can result from several stages in the material’s life, including resin compounding, molding, and service. As a polymeric material is degraded on a molecular level, the bonds comprising the material are altered. Fourier transform infrared spectroscopy detects these changes in the molecular structure. While FTIR cannot readily quantify the level of degradation, it is useful in assessing whether the material has been degraded and in determining the mechanism of degradation. Specifically, several spectral bands and their corresponding molecular structures can be ascertained. These include: the carbonyl band, particularly carboxylic acid, with formation representing oxidation; vinylidene group formation as an indication of thermal oxidation; the abstraction of bands associated with unsaturation resulting from thermal degradation; vinylene functionality for photooxidation; and hydroxyl group formation indicating hydrolysis[4].
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