Our last article talked about a manufacturer of precision optical equipment that designed a plastic cover for a device with specifications for chemical composition, surface texture, color and optical transmission. However, after switching vendors the product began to fail critical performance tests. As a result, several experiments were conducted to find the root cause of the failed plastic cover. We discussed UV-Visible Spectroscopy and Thermogravimetric Analysis (TGA) experiments, which revealed some clues but did not give us the root cause. In this article we will cover two more techniques and reveal the conclusion.
Infrared spectra of small pieces of the two covers were collected using an integrated diamond Attenuated Total Reflectance (ATR) with an FT-IR spectrometer. The instrument uses a detector that permits the collection of combined mid- and far-IR ATR spectra down to 100 cm-1. Collecting spectra into the far-IR allows easy measurement and identification of inorganic fillers in plastic parts. When combined with an automated beamsplitter exchanger, mid- and far-IR spectra can be automatically collected to provide a single spectrum of a sample from 4000 to 100 cm-1. 2.
Inspection of the infrared spectra of the two plastic pieces showed the polymer composition to be similar, but the original plastic part had an elevated baseline below 800 cm-1 and a sharp peak at 360 cm-1 that were absent or very weak in the spectrum of the replacement part. The peak at 360 cm-1 was below the range of a typical mid-IR spectrometer equipped with a KBr beamsplitter.
In this analysis a solid substrate far-IR beamsplitter made the far-IR range accessible without degrading spectral performance across the entire range.
Additional differences between the spectra were emphasized through a spectral subtraction software routine. The difference spectrum showed small peak shifts in the polymer bands, indicating a small polymer composition difference between the two parts, typical when comparing plastic parts made by different suppliers, but a significant spectral difference was also observed below 800 cm-1.
An FT-IR library search of the difference spectrum against a forensic library of automobile paint pigments and fillers matched rutile, one of the crystalline forms of titanium dioxide, indicating a formulation difference between the two covers.
To confirm the conclusions drawn from the infrared analysis, the two samples were also analyzed using a Raman sample compartment FT-Raman accessory on the spectrometer. The Raman accessory permitted easy collection of Raman spectra with a near-infrared beamsplitter and Indium gallium arsenide (InGaAs) detector mounted inside the spectrometer. The ‘good’ cover was susceptible to burning with the 1064 nm laser, requiring the use of a defocused beam for collection of Raman data. The failed cover did not require this defocusing, again indicative of a difference in a key component.
FT-Raman spectroscopy allowed collection of spectra into the far-IR region, complementing the capability of the FT-IR spectrometer with the built-in ATR and beamsplitter gaining access to this region. Again, the two spectra were very similar, demonstrating similar polymer composition, with small differences in the spectra observable below 800 cm-1, clearly seen in the difference spectrum. A library search of the difference spectrum against a Raman library for minerals identified the difference between the two plastic parts as rutile, confirming the identification from infrared analysis.
So, what was the conclusion of the four experiments conducted because a switch of suppliers for molded plastic covers led to failures of a precision optical measurement device? Why was ambient light leaking into the device causing erroneous measurements for low light level measurements?
Diffuse transmission measurement of the parts by UV-Visible spectroscopy confirmed that the failed cover did not meet the specification for maximum transmittance. Thermogravimetric analysis demonstrated that the composition of the original cover contained approximately 3% more, by weight, of an inorganic filler compared to the replacement cover. ATR infrared analysis over the mid and far-IR spectral regions showed that the original cover had significantly higher rutile (titanium dioxide) content than the replacement cover. The infrared results were confirmed by FT-Raman spectroscopy. The root cause of the failure tracked with the lower rutile content, and corrective action was implemented.
This study clearly showed the importance of having several tools available for Root Cause Analysis.
To get more specifics, including spectra, references, and instruments used, download the application note.