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Manufacturers who use plastic parts in their products and processes often need to analyze failed parts to determine the root cause of the failure and appropriate corrective actions.
This type of analysis usually uses infrared and Raman spectroscopy to evaluate the chemical composition of the components, ultraviolet-visible spectroscopy to study their transmittance and color, and thermal analysis to determine their physical properties.
This article outlines a study that used all these tools to determine the cause of failure of plastic parts.
Thermo Scientific Nicolet iS50 FTIR spectrometer. Image source: Thermo Fisher Scientific-Material and Structure Analysis
A company that makes precision optical devices designed a plastic cover for the device. The part is designed to have specific characteristics in terms of its chemical composition, surface texture, color and optical transmission.
The lid is made of a polycarbonate-acrylonitrile butadiene styrene (PC-ABS) mixture. It also contains enough titanium dioxide to provide a subtle off-white color and ensure a light transmittance of less than 0.01% T over a wide spectral range (from ultraviolet to near infrared).
The opacity of this part is necessary to prevent ambient light from entering the optical device and adversely affecting low light level measurements. Initially, it was found that all the supplied parts were within the specification range, and the product performance was satisfactory.
Implemented a redesign project to reduce costs and make the product more competitive. This process involves the commissioning of various components from alternative suppliers-including the lid.
A new supplier’s quotation is lower than the original covered supplier, and it is found that the test part meets all opacity requirements, so the production contract for the part is awarded to the new supplier.
Soon after this change, the product began to fail key performance tests, and the failure could be traced back to the presence of ambient light causing the background to rise, which affected low-level optical measurements.
Visual inspection did not find any obvious difference from the original parts, but many control experiments confirmed that the failure was caused by the new cover.
Therefore, it is necessary to use a series of applicable techniques for root cause analysis to quickly identify and control the problem.
A Thermo Scientific™ Evolution™ 220 UV-Visible spectrophotometer and integrating sphere were used to perform diffuse transmission measurements on the original lid and the failed lid (Figure 1).
Figure 1. Evolution 220 UV-Visible spectrophotometer (left) and the integrating sphere accessory for the sample chamber (right). Image source: Thermo Fisher Scientific-Material and Structure Analysis
The cover deliberately contains a large number of particles, designed to effectively scatter any transmitted light. The transmittance is measured with an integrating sphere.
Cover slips from functional and malfunctioning equipment were placed on the transmission port of the sphere, and the spectrum was from 220 to 800 nm (Figure 2).
Figure 2. Diffuse transmission UV-Vis spectra of the failed protective layer (blue) and the good protective layer (red) collected using the Evolution 220 UV-Vis spectrophotometer and integrating sphere accessory. Image source: Thermo Fisher Scientific-Material and Structure Analysis
Although the transmittance cannot be measured through the functional cover, significant transmittance is measured through the failed cover, including the visible part of the spectrum greater than 7% T.
These findings explain the poor performance of the device due to light leakage when operating under environmental conditions. However, these findings alone are not enough to determine the root cause.
TA Instruments™ Thermogravimetric Analyzer is used to measure the small pieces of the two lids to determine the overall composition (Figure 3). 1
Figure 3. The thermogravimetric analysis weight loss curve of a good covering layer (red) and a failed covering layer (blue), indicating that the inorganic content of the good covering layer is significantly higher than that of the failed covering layer. Image source: Thermo Fisher Scientific-Material and Structure Analysis
Before cooling to 550 °C, the sample was heated from ambient to 650 °C at a rate of 20 °C per minute under a N2 purge. Then they are heated again to 1000°C at a rate of 20°C per minute under air purge.
The application of the initial heating ramp under nitrogen pyrolyzes the organic components of the lid, while the final temperature ramp in the air burns the remaining organic components. This process results in leaving only oxides of inorganic components.
The organic decomposition curves of the two lids are almost the same, confirming that they have the same plastic composition.
It was found that the functional covering contained 5.4% by weight of residual inorganic components, while the failed covering contained 2.2% by weight of inorganic components. This significant difference in the content of inorganic fillers provides a strong clue as to the source of light leakage.
The integrated diamond iS50 ATR on the Thermo Scientific™ Nicolet™ iS50 FTIR spectrometer is used to obtain the infrared spectrum of each cap piece (Figure 4).
Figure 4. Nicolet iS50 FT-IR spectrometer, built-in diamond iS50 ATR, iS50 ABX automatic beam splitter exchanger and sample chamber iS50 Raman accessories. Image source: Thermo Fisher Scientific-Material and Structure Analysis
The built-in iS50 ATR of Nicolet iS50 has a dedicated detector that can collect combined mid-infrared and far-infrared ATR spectra as low as 100 cm-1-these factors can easily measure and identify inorganic fillers in plastic parts.
The powerful combination of these features and the iS50 ABX automatic beam splitter switch on the Nicolet iS50 spectrometer means that mid-infrared and far-infrared spectroscopy programs can be automatically collected before being spliced together with Thermo Scientific™ OMNIC™ Macros\Pro™ Visual Basic.
This provides a single spectrum of samples from 4000 to 100 cm-1.2
The ATR spectrum of the plastic part (Figure 5) was corrected using the advanced ATR correction algorithm of OMNIC software. 3 The algorithm considers and accounts for the relative intensity change caused by the penetration depth of the sample as a function of wavelength.
Figure 5. Advance ATR corrected infrared ATR spectrum of a good plastic cover (top), a failed plastic cover (middle), and the difference spectrum between the two (bottom). Image source: Thermo Fisher Scientific-Material and Structure Analysis
It also explains the peak shift of the infrared spectrum due to the difference in refractive index between the ATR crystal and the sample.
A review of the infrared spectra of the two plastic parts revealed similar polymer composition, but the original plastic part showed a baseline rise below 800 cm-1 and sharp peaks at 360 cm-1 (Figure 6). These peaks are very Weak or even non-existent in the spectrum of replacement parts.
Figure 6. Overlay of the advanced ATR correction spectra of good coverage (blue) and failed coverage (red) over the spectral region from 940 to 100 cm-1. Please note that the elevated baseline and 360 cm-1 absorption band in the spectrum of a good cover is absent or significantly reduced in the spectrum of a failed cover. Image source: Thermo Fisher Scientific-Material and Structure Analysis
Although the peak at 360 cm-1 is lower than the range of a standard mid-infrared spectrometer equipped with a KBr beam splitter, using iS50 ABX with a solid-substrate far-infrared beam splitter can access the far-infrared range without affecting its performance The high performance of the entire series.
The use of spectral subtraction emphasizes the additional differences between spectra. The difference spectrum (Figure 5, bottom) shows a slight peak shift in the polymer band, indicating a difference in polymer composition between the two parts.
Although this difference appears when comparing plastic parts made by different manufacturers, noticeable spectral differences should also be noted below 800 cm-1.
A library search was performed on the difference spectrum and evaluated according to the forensic library of automotive fillers and paint pigments4 (Figure 7), and the results matched rutile (a crystalline form of titanium dioxide). This shows that there are significant formulation differences between the two covers.
Figure 7. FT-Raman difference spectrum (blue) between good and failed overlays, and the best match for the forensic car paint pigment and filler library (red) in the library search, identified a higher concentration of rutile (titanium dioxide) ) On the very good cover. Image source: Thermo Fisher Scientific-Material and Structure Analysis
In order to verify the results obtained by infrared analysis, the two samples were also analyzed through the iS50 Raman sample chamber FT-Raman accessory on the Nicolet iS50 spectrometer (Figure 4).
The iS50 Raman accessory can be easily inserted into the sample chamber of the Nicolet iS50 FTIR spectrometer without the use of external modules-this is a typical requirement for other FTIR spectrometer systems.
Thanks to the near-infrared beam splitter and InGaAs detector installed inside the spectrometer, the iS50 Raman accessory facilitates the direct collection of Raman spectra.
Figure 8 shows the FT-Raman spectra and their spectral difference spectra obtained from functional coverage and failure coverage.
Figure 8. FT-Raman spectra of the good cover (top), the failed cover (middle), and the subtraction result between the two (bottom). Image source: Thermo Fisher Scientific-Material and Structure Analysis
FT-Raman spectroscopy can also collect the spectrum into the far-infrared region. In accessing this region, it can confidently complement the capabilities of the Nicolet iS50 FTIR spectrometer and the built-in iS50 ATR and ABX.
Using this method, the two spectra were found to be very similar, again highlighting the similar polymer composition. Small differences are again visible in the spectrum below 800 cm-1, as shown by the difference spectrum.
The difference spectrum library search for the mineral Raman library 5 (Figure 9) also identified the difference between the two plastic parts as rutile, verifying the results of infrared analysis.
Figure 9. FT-Raman difference spectrum between good and failed coverings (top), and the top library search results for the mineral Raman library (bottom), identifying higher concentrations of rutile (titanium dioxide) in good coverings . Image source: Thermo Fisher Scientific-Material and Structure Analysis
Incorrect measurements of low light level measurements are caused by ambient light leaking into the device. Diffuse transmission measurement of the part by ultraviolet-visible spectroscopy, it is determined that the failed cover does not meet the maximum transmittance specification.
Further investigation by thermogravimetric analysis showed that the composition of the original cover layer contained approximately 3% inorganic filler (by weight) compared to the replaced cover layer.
Infrared ATR analysis in the mid-to-far infrared spectral region showed that the rutile (titanium dioxide) content of the original cover layer was much higher than that of the replacement cover layer-the result was subsequently confirmed by FT-Raman spectroscopy.
The sample survey provided here clearly illustrates the importance of using the appropriate range of tools in root cause analysis.
Many of the tools used in this study can be used on the Nicolet iS50 FTIR spectrometer. Thanks to its built-in iS50 ATR and iS50 Raman accessories, the instrument can collect multi-range spectra without affecting quality or speed.
The powerful analysis combination provided by Thermo Scientific UV-Vis and FTIR instruments, as well as the addition of thermogravimetric analysis, is of decisive significance for determining the root cause of the failure of the plastic cover in this example.
This information is derived from materials provided by Thermo Fisher Scientific-Materials & Structural Analysis and has been reviewed and adapted.
For more information on this source, please visit Thermo Fisher Scientific-Material and Structural Analysis.
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