Deborah F. Mielewski

Photooxidation kinetics in crosslinked polymer coatings

Abstract Quantitative relationships have been developed between photooxidation rates and hydroperoxide concentrations in acrylic urethane and acrylic melamine coatings. The rate of photooxidation in both urethane and melamine crosslinked coatings is found to be linear in hydroperoxide concentration. In urethane coatings, the photooxidation rate goes to zero as the hydroperoxide concentration goes to zero, while in melamine coatings there is a residual photooxidation rate at zero hydroperoxide concentration. This residual photooxidation appears to be related to melamine photochemistry. The simple relationships derived suggest that hydroperoxide measurements can be used to screen coatings for photostability using outdoor-like exposure conditions.

The role of hydroperoxides in the photo-oxidation of crosslinked polymer coatings

Abstract Iodometric titration has been used to measure hydroperoxide concentrations in a series of unstabilized acrylic melamine and acrylic urethane coatings as a function of laboratory weathering time. Hydroperoxide concentrations are found to be a function of the initial free radical formation rates, the exposure conditions used, and the crosslinker type. For coatings with high initial free radical formation rates, the hydroperoxide level rises rapidly to a maximum and then decreases. For a given copolymer, the hydroperoxide level in the melamine crosslinked coating is always significantly lower than that in the urethane crosslinked coating. For coatings with low initial radical formation rates, the initial rise in hydroperoxide concentration is small. The hydroperoxide concentration in the melamine coating remains small, while that in the urethane coating slowly increases suggesting autocatalytic oxidation. Hydroperoxides contribute significantly to free radical formation at long exposure times, especially in the urethane coatings. The lower level of hydroperoxide in the melamine coatings is attributed to the ability of the melamine crosslinker to decompose hydroperoxides.

Photostabilization and photodegradation in organic coatings containing a hindered amine light stabilizer. Part VII. HALS effectiveness in acrylic melamine coatings having different free radical formation rates

Abstract The ability of hindered amine light stabilizer (HALS) bis-(2,2,6,6-tetramethyl-4-piperidinyl) sebacate to reduce the photooxidation rate in two different acrylic melamine polymer coatings has been studied using both infrared and electron spin resonance spectroscopies. These coatings differ primarily in the rate at which free radicals are formed during photolysis. Under controlled photolysis conditions similar to sunlight (near-ambient), the photoinitiation rate (PIR) of free radicals in one coating is some five times higher than that of the other coating, resulting in a photooxidation rate that is some 2·5 times greater than that for the coating with the lower PIR value. When harsh, short wavelength ultraviolet light is used, the PIR values and photooxidation rates of both coatings are higher. The addition of hindered amine (2% by weight) to the coating with the higher PIR value reduces its photooxidation rate by roughly a factor of 2·6 for both exposures. The addition of the hindered amine to the coating with the lower PIR value reduces its photooxidation rate by a factor of 1·9 for the harsh exposure, but does not reduce its photooxidation rate under near-ambient exposure conditions. These results point out the risk of using harsh, accelerated tests to evaluate stabilizer performance and also suggest that there is a limit to HALS inhibition in acrylic melamine coatings.

Determination of hydroperoxide concentrations in cross-linked polymer coatings containing hindered amine light stabilizers

A procedure for measuring hydroperoxide concentrations in unstabilized, cross-linked polymer coatings as a function of weathering time has been extended for use on coatings containing hindered amine light stabilizers (HALS). The modification involves electron spin resonance spectroscopic measurements of the nitroxide concentration in HALS stabilized coatings. Hydroperoxide concentrations are determined by correcting iodometric titration values for the contribution from nitroxide reduction by I−. The correction is usually small. This procedure has been used to compare hydroperoxide concentrations formed during weathering of unstabilized and HALS stabilized acrylic melamine and urethane cross-linked coatings. The reduction in hydroperoxide level with the addition of HALS is consistent with longer-term infrared spectroscopic measurements of carbonyl growth in these coatings. The results strongly suggest that measurements of hydroperoxides formed during controlled ‘outdoor-like’ exposures can be used for the rapid evaluation of stabilized coating weatherability and the selection of effective HALS additives.

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.

Photo-initiation rate behavior of weathered coatings: ESR-nitroxide decay assay

Abstract A procedure is described to extend the nitroxide decay assay measurement of coating photo-initiation rates to weathered coatings. The procedure involves post-cure doping of coatings with 4-hydroxy-2,2,6,6-tetramethyl-piperidinyloxy free radical in the presence of CH 2 Cl 2 /CH 3 OH vapor at 60°C. The photo-initiation rates of three acrylic-melamine coatings have been followed as a function of ‘QUV-light only’ accelerated exposure. For each of these coatings, the photo-initiation rate drops during the first 10–20 h of exposure and then remains constant at a level indicative of their weatherability for the duration of the exposure. No evidence is found for autoxidation in these coatings.

High strain-rate behavior of natural fiber-reinforced polymer composites:

The high strain-rate constitutive behavior of polymer composites with various natural fibers is studied. Hemp, hemp/glass hybrid, cellulose, and wheat straw-reinforced polymeric composites have been manufactured, and a split-Hopkinson pressure bar apparatus has been designed to measure the dynamic stress–strain response of the materials. Using the apparatus, compressive stress–strain curves have been obtained that reveal the materials’ constitutive characteristics at strain rates between 600 and 2400 strain/s. Primary findings indicate that natural fibers in thermoset composites dissipate energy at lower levels of stress and higher strain than glass-reinforced composites. In the case of thermoplastic matrices, the effect on energy dissipation of natural fibers vs. conventional talc reinforcements is highly dependent on resin properties. Natural fibers in polypropylene homopolymer show improved reinforcement but have degraded energy dissipation compared to talc. Whereas in polypropylene copolymer, natural fibers result in improved energy dissipation compared to talc. These data are useful for proper design, analysis, and simulation of lightweight biocomposites.

Photo-degradation and photo-stabilization in organic coatings containing a hindered amine light stabilizer: Part VI—ESR measurements of nitroxide kinetics and mechanism of stabilization

Abstract The concentration of nitroxide has been measured in an acrylic melamine coating containing different hindered amine light stabilizers as a function of exposure harshness, time and stabilizer concentration using electron spin resonance spectroscopy. The four hindered amines are based on 2,2,6,6-tetramethylpiperidine and have the following substituents on the piperidine nitrogen: H, CH 3 , CH 2 CH 2 -polymer, and COCH 5 . Comparisons of nitroxide formation and decay kinetics with infrared spectroscopic measurements of hindered amine effectiveness reveal that a major factor in the ability of hindered amine to inhibit photo-oxidation in acrylic melamine coatings is the rate of conversion of the amine functionality to nitroxide. Conversion is fastest for the z.lbond2; group. The conversion of z.lbond2; to nitroxide is also fast and proceeds through the formation of z.lbond2; groups by at least two mechanisms. By contrast, the conversion of z.lbond2;-polymer and z.lbond2; to nitroxide is roughly an order of magnitude slower than that for z.lbond2; accounting for the decreased effectiveness of these stabilizers.

The role of humidity in the photooxidation of acrylic melamine coatings

Abstract Predicting the weatherability of acrylic melamine coatings commonly used as enamel clearcoats requires a detailed understanding of each of the factors that influence photooxidation kinetics. Previous work 1 has shown that the photooxidation rate in coatings can be written as the following function of hydroperoxide concentration: photooxidation rate = K [YOOH] + M . The existence of a measurable photooxidation rate in the absence of hydroperoxide (i.e. a non-zero value of the intercept, M ) has been observed only in melamine crosslinked coatings. It has also been observed that the photooxidation rate in acrylic melamine coatings increases with increasing humidity. In contrast, for urethane crosslinked coatings the value of M is zero, and the photooxidation rate is independent of humidity. In this paper, infrared spectroscopic measurements of functional group changes (e.g. carbonyl growth and crosslink scission) are used to measure photooxidation rates in acrylic melamine coatings during UV exposures at different humidities. Comparisons of these rates to measured hydroperoxide concentrations for the same coatings and exposures reveal that the increase in photooxidation rate with humidity is due to the fact that the intercept M increases with increasing humidity. Since the intercept is zero under dry conditions, the chemical reactions responsible for the intercept in melamine crosslinked coatings must involve both UV light and moisture. These results confirm the importance of accurately controlling the humidity during UV exposure for predicting the weatherability of melamine crosslinked coatings.

Magic angle 13C NMR determination of the fate of ketone end groups during photolysis of crosslinked acrylic copolymers

Abstract Ketone end groups formed during acrylate copolymerization are a main source of free radical formation during the initial stages of weathering in crosslinked coatings formulated with these copolymers. During weathering, the rate of formation of radicals changes. Other chromophores such as hydroperoxides are also observed. To better understand the photooxidation process, magic angle spinning C-13 NMR spectroscopy has been used to measure the change in concentration of ketone end groups in crosslinked acrylic copolymer coatings as a function of weathering time. A clear correlation has been established between ketone concentration, hydroperoxide concentration, and free radical formation rates in weathered coatings.