Infrared Spectroscopy Example Analyses (FTIR)
- The case studies collected below show some of the very broad capabilities of FTIR.
- Identification of Defects in Injection Molded Plastic Components
- Observation of Epoxy Curing Kinetics
- Identification of Flooring Off-gas Irritant
- Identification of Oil Defects in Thermoset Polymer
- Germanium ATR Crystal for Evaluation of Carbon Black filled Rubber
- Component Migration in PEBAX Catheter Tubing
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FTIR Identification Of Defects In Injection Molded Aerospace Components
Several polyamide injection molded components were submitted for analysis. The samples contained three types of defects:
- Brown Fibrous Inclusions (Foreign Object Debris)
- Light Brown Discoloration
- White Opaque Regions
1. Brown Fibrous Inclusions. The infrared absorption spectrum of the surrounding polyamide is shown in blue. This spectrum shows the typical two large amide bands in the 1600cmˉ¹ region and sharp N-H stretch peak at ~3300cmˉ¹. Spectra from two defects are shown in green and red (click image to expand). These spectra show additional broad O-H stretch absorbance in the 3000-3500cmˉ¹ region and a broad glycosidic linkage absorption peak in the 1000cmˉ¹ region. These IR observations combined with the fibrous appearance of the defects indicate the material is cellulose, likely cardboard debris contaminant within the hopper. A reference spectrum for cardboard (cellulose) is shown in yellow.
2. Light Brown Discoloration. The sample shown above has been milled thin enough to be viewed in transmission. Viewed this way we can see the surrounding glass-filled polyamide and a the central brownish defect with no glass fill (click image to expand). In the spectra stack to the right, the infrared absorption spectrum of the surrounding polyamide is shown in blue. An absorbance spectrum from the contaminated region is shown in red (click image to expand). This spectrum shows some additional peaks in the lower wave-numbers in addition to some changes in the 1700cmˉ¹ region. The green spectrum highlights the differences between these spectra (it was produced by subtracting the polyamide out of the red spectrum). After performing this subtraction it straightforward to match the contamination spectrum to a polyetherimide reference spectrum (purple). These results indicate that residual polyetherimide was in the injection hopper from a previous manufacturing run.
3. White Opaque Regions. In the spectra stack to the right, the infrared absorption spectrum of the surrounding polyamide is shown in blue. The absorbance spectrum from the defective region is shown in red (click image to expand). This spectrum shows addtional peaks in the low wave-number which are characteristic of glass Si-O bonds. The reference glass fiber spectrum shown in purple matches closely with the the IR spectrum of defective region. Additional optical microscopy confirmed that these white opaque regions contained voids with a large quantity of un-coated glass fibers.
Observation of Epoxy Curing Kinetics
The necessary cure time for epoxies can be determined by Fourier Transform Infrared Spectroscopy (FTIR). Different molecular bonds absorb different wavelengths of infrared light. The curing of epoxy is monitored by measuring the area of the epoxide ring absorbance peak at 915cmˉ¹ (10.9µm wavelength).
The epoxide group is consumed in the reaction with the epoxy curing agent. Measurements of peak over time compared with its initial size allows for determination of the percent conversion of the epoxide reactants. The effects of many variables can be analyzed including temperature, humidity, filler content, bond line thickness. Potential benefits include: improved process throughput efficiency, increased bond strength, increased product consistency, and increased product reliability. |
Similar FTIR techniques can be used to determine ideal usage times, cure times, and cure conditions for polyurethanes, acrylates, silicones, sol-gels, polyimides, and more. Applications include adhesives, rubbers, inks, primers, paints, adhesion promoters, and more.
For assistance with your process design please contact us at: info@maplaboratory.net
Flooring Off-gas Identification
Many months after installation, a polyurethane flooring still exhibited a strong odor and contained large cloudy opaque regions. In order to address air quality concerns a section of the flooring was scraped up and submitted for identification of the off-gassing odor.
The gas from the sample chamber was pumped into an FTIR gas cell and analyzed in transmission. The resulting infrared transmission spectrum contained peak structures characteristic of carbon-flourine bonds with additional peak structures matching short chain aldehydes. The majority of the gas was identified as parachlorobenzotrifluoride (PCBTF), a common solvent used as a low VOC replacement for xylene and toluene (match to reference PCBTF spectrum is shown below). The additional light aldehyde emission is common for polyurethane lacquers. The solid coating materials were analyzed by FTIR-ATR. Comparison of the cloudy regions of the flooring coating with the clear regions confirmed the presence of trapped PCBTF solvent. The findings were consistent with over-application and insufficient cure time between coatings. |
Oil Defects in Thermoset Polymer
The thermoset polymer and PTFE cover sheet shown below were submitted for identification of the oil which was pressed out around the outside of the polymer and present in three bubble-like defects. FTIR results indicated the oils were a mixture of plasticizer and flame retardant present within the material. Separation of the oils was prevented by additional degassing of the polymer mix and reducing the maximum cure temperature.
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Use of Germanium ATR for Evaluation of Carbon Black filled Rubber
Attenuated Total Reflection (ATR) is a convenient way to quickly capture infrared absorption spectra of materials. The test specimens are placed directly on an ATR crystal requiring little/no sample preparation. The ease of analysis has some trade-offs:
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For highly absorbing materials and materials with high refractive index like carbon black this leads to baseline drift and sampling artifacts.
   Note: Longer wavelengths (λ) correspond to smaller wave numbers (cm−1)
              Wavenumber is equal to how many wavelengths fit in 1 cm (1/λ).
These sampling artifacts can complicate identification of carbon black filled rubbers. It is possible to capture more discernable spectra by changing the ATR crystal to a higher refractive index material such as germanium. A higher refractive index ATR crystal has the effect of decreasing the IR penetration depth. Germanium has a refractive index of 4.0 while the standard ATR crystal (diamond) has a refractive index of 2.4.
See the figure below of the infrared absorption spectra of the same specimen captured with a diamond and germanium ATR. Note how much easier it is to see the peaks revealing the nitrile butadiene rubber (NBR) composition of the material when using the germanium ATR.
See the figure below of the infrared absorption spectra of the same specimen captured with a diamond and germanium ATR. Note how much easier it is to see the peaks revealing the nitrile butadiene rubber (NBR) composition of the material when using the germanium ATR.
Component Migration in PEBAX Catheter Tubing
Background:
Surface residues were causing difficulties forming good bonds to polyether block amide (PEBAX) catheter tubing. Even after cleaning surface residues were observed to return after reflowing (heating) the tubing to form butt-welds. FTIR was implemented to determine the composition of the residues and suggest an appropriate cleaning solution. A fresh batch of tubing was cleaned with a DI water wipe to remove preexisting water soluble surface residues. A fresh butt-weld was formed and the surface of the parts were analyzed by FTIR. FTIR showed an abundance of polyether on the surface of the parts after heating. This surface polyether was observable as a residue on the FTIR-ATR crystal after removing the tubing from the ATR crystal. The shallow sampling depth of the FTIR-ATR analysis (~5µm) also allowed observation of a corresponding decrease in the amide component of the polyether block amide as the amides were occluded by the surface film of polyether. FTIR analysis was repeated at several time points after the initial heating. The time series FTIR analysis allowed observation of the diffusion of polyether oligomer as it re-incorporated with the PEBAX. The rate of this diffusion varied for the different durometer PEBAX materials. Adhesion to the catheter tubing was improved by adding additional DI water cleaning steps to the manufacturing process. |
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