ADVANCED MATERIALS & PROCESSES | NOVEMBER/DECEMBER 2023 36 The following case study illustrates the effectiveness of FTIR in assessing molecular degradation. CASE STUDY: BRITTLE PERFORMANCE OF POLYSTYRENE HOUSINGS Housings from two different production lots exhibited significant variation in performance during quality testing. Specifically, the difference was detected during impact testing per- formed on the molded parts. One of the lots exhibited superior impact properties and ductile behavior, similar to that of a previous production run. The other lot demonstrated a marked reduction in impact properties, as evidenced by lower impact strength along with relatively brittle behavior (Figs. 2 and 3). The housings were specified to be produced by injection molding using a high-impact poly- styrene (HIPS) resin. Testing and results: Visual exam- ination of the impacted housing specimens confirmed the reported property variation. Specimens representing the retained reference parts from an earlier production lot showed significant ductility in the form of stress whitening and deformation. Conversely, the new production lot samples exhibited characteristics of brittle fracture. The two lot samples were analyzed using FTIR in the attenuated total reflectance mode. A comparison of the results showed a general match. Both sets of spectra exhibited absorption bands characteristic of styrene and butadiene functionalities (Fig. 2). However, variation in the relative intensities of the styrene and butadiene bands was observed (Fig. 3). Band intensity measurements were taken for the butadiene and styrene functionalities at 964 and 751 cm−1, respectively, for the two lot samples. The results, as shown in Table 1, clearly illustrate that the samples that performed in a brittle manner had a relatively lower level of butadiene content. Conclusions: At the conclusion of this testing, the variation in housing performance was attributed to the use of two different resins to mold the parts. Although both production lots were molded from HIPS, the relative level of butadiene functionality was significantly lower in the brittle parts. Butadiene is added as an impact modifier. As such, it enhances the ductility of the resin as increasing amounts are added to the resin mix. ~AM&P For more information: Jeffrey A. Jansen, engineering manager, The Madison Group, 2615 Research Park Dr., Madison, WI 53711, 608.231.1907, info@madisongroup.com. Fig. 2 – FTIR spectral comparison showing generally good agreement between brittle lot and ductile lot materials. Both spectra contained absorbances that are associated with styrene and butadiene functionalities. Fig. 3 – FTIR spectral comparison illustrating a relatively lower level of absorption associated with butadiene functionality in the brittle lot material compared with the ductile lot material.
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