R. O. Carter

Determination of ultraviolet light absorber longevity and distribution in automotive paint systems using ultraviolet micro-spectroscopy

Ultraviolet light absorber additives (UVAs) are added to the basecoat and clearcoat layers of modern, multi-layer, automotive paint systems in order to protect the clearcoat and screen the underlying layers from harmful UV light. Unfortunately, these UVAs are subject to photodegradation resulting in diminished UV protection as a paint system weathers. This paper describes an ultraviolet micro-spectroscopic technique that has been developed to measure ultraviolet light absorber content, location, and longevity in the clearcoat layer of complete, intact, automotive, paint systems. Three automotive paint systems containing different UVAs were examined initially and after exposure in Florida. UVA content and distribution were determined in each of the paint systems with weathering and the longevity was determined. This work demonstrates an accurate, unambiguous technique for quantifying UVA loss in coatings.

Photo-stabilization and photo-degradation in organic coatings containing a hindered amine light stabilizer: Part IV—Photo-initiation rates and photo-oxidation rates in unstabilized coatings

Abstract The chemistries and rates of photo-oxidation have been studied in a series of acrylic melamine coatings as a function of exposure condition. Electron spin resonance (ESR) spectroscopy has been used to determine the photo-initiation rate (PIR) of free radicals using a nitroxide doping technique. Infrared spectroscopy has been used to follow changes in the relative concentrations of different functional groups. In particular, the rate of carbonyl group formation has been measured. This rate has been found to be proportional to the square root of the photo-initiation rate, confirming that the formation of carbonyl species in these coatings is a free radical induced photo-oxidation process. Infrared spectroscopic measurements of carbonyl formation can be used to evaluate the effectiveness of free-radical scavengers such as hindered amine light stabilizers in retarding photo-oxidation in polymer coatings.

Photoacoustic infrared spectroscopy for evaluation of an ABS as an automotive interior material

Abstract Photoacoustic Fourier transform infrared spectroscopy (PAS-FTIR) has been demonstrated to be a chemical diagnostic tool to track the degradation of a commercial, fully formulated, molded acrylonitrile-butadiene-styrene material without special sample preparation. Samples obtained from plaques undergoing standard, natural and accelerated exposure as part of an on-going engineering study were examined. Additionally, a series of xenon are exposed samples with exposures according to the SAE J-1885 protocol were also examined to follow the relative rates of reaction for the changes observed above. Five days of xenon are exposure was approximately equal to 12 months exposure in Florida for this material. The decomposition process resulted in the formation of a brittle top layer similar to that seen in exterior exposure of this material. Photochemical oxidation was the cause of the decomposition and not heat. The decomposition was color independent for the three colors examined. A pigmented paint layer was found to protect the material from the photochemical process.

The characterization of polymer surfaces by photoacoustic fourier transform infrared spectroscopy

Abstract Weathered surfaces of intact polymer films on standard test substrates have been characterized with photoacoustic detection Fourier transform infrared spectroscopy with minimum sample preparation. Vibrational spectra were obtained from the surface of fully formulated and pigmented paint samples aged in Florida testing, and in QUV and carbon arc accelerated weathering environments. Single- and two-component polyester urethane resin films on polybutyleneterephthalate-polycarbonate-rubber substrate were examined to a depth of approximately 8–12 μm. The samples were analyzed for differences in chemical changes occurring during natural and accelerated testing. Spectra for these samples were compared with those obtained by attenuated total reflection (ATR) infrared spectroscopy, which requires time-consuming sample preparation. Photoacoustic detection infrared is applicable for heavily pigmented and opaque samples which are difficult to examine by conventional transmission or reflection techniques. The essence of photoacoustic detection in a Fourier transform infrared experiment is discussed.

The Application of Linear PA/FT-IR to Polymer-Related Problems

A simplified method has been developed which allows the computation of linear photoacoustic FT-IR spectra using only one sample and one reference interferogram from a commercial, rapid-scan spectrometer. The application of linear computation to photoacoustic infrared data can produce spectra similar to transmission spectra through the reduction, if not elimination, of the saturation artifacts typical of amplitude computation, the usual mode of producing a PA/FT-IR spectrum or a transmission mode spectrum. Comparison of the resulting linear spectra with amplitude spectra has been used to demonstrate the existence of surface layers on top of substrates. Qualitative identification of the surface layer is easier in this mode than in the case where amplitude spectra are obtainted at the extremes of instrument mirror velocity. Additionally, information concerning a polymer matrix in the presence of up to 25 wt % carbon black is contained in linear photoacoustic spectra. Linear photoacoustic FT-IR spectra of carbon-filled samples are easier to interpret in comparison to spectra obtained by amplitude methods.

Photoacoustic Detection of Rapid-Scan Fourier Transform Infrared Spectra from Low-Surface-Area Solid Samples

We have demonstrated that the PAS cell volume, and the state of the background material as well as the interferometer mirror velocity and the cell gas composition, must be controlled when one is recording spectra of solid samples. An optically thick, totally absorbing material with a volume matching that of the sample is needed in order to properly normalize spectra of solid samples. With the proper detection bandwidth, mirror speeds of up to 0.181 cm/s can be used with helium as a transfer medium, for the cell specified. The improvement attained at high frequency by using helium can be as much as 10-fold over that obtained with air. The Helmholtz design of the cell produces a resonance at 1.4 kHz with air and 2.65 kHz with helium. These resonances are also affected by the volume and composition of the gas in the sample chamber. Thus, it is essential to select an appropriate background sample for the purpose of normalizing spectra.

Infrared Photoacoustic Spectroscopy of Carbon Black Filled Rubber: Concentration Limits for Samples and Background

Infrared spectroscopy studies of the cure chemistry, state of cure, and surface bloom on rubber materials have always been limited by the presence of carbon black in samples. One of the modern methods for recording infrared spectra of solid samples is photoacoustic detection Fourier transform spectroscopy. It has been demonstrated in the past that surface-segregated species can be identified with this technique, but the results are complicated by the presence of carbon black, which limits the optical depth. As for the study of bulk chemistry, photoacoustic detection does not require that the sample be infrared transparent, and the method can be used with samples containing as much as a 15 wt % carbon. At loadings higher than 30 wt %, the material becomes a total absorber and can be used to record an instrument background spectrum.

Evaluation of the Appropriate Sample Position in a PAS/FT-IR Experiment

The experimental selection of the distance between the sample and the window in a photoacoustic FT-IR experiment is shown to be an important parameter when one is optimizing the interferogram intensity. We have experimentally demonstrated the advantages of selecting a small distance rather than the full boundary layer thickness required to ensure complete conversion of thermal to pressure signal at the sample surface. The minimum volume of the sample chamber is a controlling factor in the optimization of the experiment. In this report we demonstrate the value of minimizing sample chamber volume through controlling sample thickness while pointing out the need for care when doing so.

Characterization of fluoroelastomer networks: II. SEC, FTIR, and ODR analysis

The role of an accelerator in the nucleophilic cure of fluorocarbon compounds was further examined by preparing materials without a crosslinker. Under the previously used curing conditions, ODR reveals the formation of a secondary, accelerator-induced network that is unstable at slightly higher temperature, i.e., ≥190°C. Increased crosslink density and color are observed with the cure time, indicating slow curing reactions and likely formation of conjugated unsaturation. SEC of selective solvent-soluble fractions derived from specimens obtained from the key cure stages shows initially a falling refractive-index response. However, this change is followed by a shift in MWDs toward lower average molecular weights. On the other hand, infrared examination shows a continuous increase in the 1718 cm−1 absorption, but no major presence for the 1680 cm−1 absorption seen in the previous study in the presence of a crosslinker. The concurrent increase in the 3114 cm−1 peak confirms the presence of a CFCH double bond. The solvent-soluble portions and bulk specimens have very similar infrared features.

The Examination of Diffusing Materials on Rubber Surfaces by Photoacoustic Infrared Spectroscopy

Abstract Minor amounts of diffusing materials can be beneficial or detrimental to the performance of elastomeric components. Methods to study these materials have been nonexistent or extremely time-consuming and insensitive for routine use. This report shows that photoacoustic detection Fourier-transform infrared spectroscopy (PAS-FTIR) is a simple method for in situ characterization of diffusing materials in elastomers. In this study, PAS-FTIR was used to monitor the appearance and diffusion of two vulcanization by-products to the surface of a carbon-black-filled natural rubber matrix. The compounds were identified from the PAS-FTIR spectra as a salt of stearic acid (most likely zinc stearate) and dimorpholinyl thione. Three rubber formulations, differing only in the particle size and surface activity of the carbon blacks used, were monitored by PAS-FTIR for a 135-day period to investigate the effect of carbon black on the diffusion of the compounds cited above. Diffusion of the dimorpholinyl thione was ...