J.L. Gerlock

The effects of weathering on the mechanical performance of automotive paint systems

The durability of automotive paint systems continues to be a great concern to both auto companies and their coating suppliers. Recent advances in assessing the durability of coatings by measuring weathering-induced chemical composition changes have greatly increased our ability to discern superior from inferior coatings. However, different coatings will likely tolerate different amounts of weathering-induced chemical composition changes while still maintaining their mechanical integrity. Thus, a means of linking chemical composition changes to changes in relevant mechanical properties would be highly desirable. The fracture energy, the amount of mechanical energy required to propagate a crack in a material, is a sensitive measure of the brittleness of a material and is relevant to a number of potential failure mechanisms in automotive paint systems. The fracture energy of clearcoats can vary widely depending on the formulation of the clearcoat (initial chemical composition and additive package) and on the amount of weathering. Weathering embrittles most coatings. Weathering-induced changes in the fracture energy are related to chemical composition changes occurring in the clearcoat. Because the brittlest materials will not crack without an applied stress, the stress distribution in complete paint systems as a function of weathering must also be known to accurately anticipate mechanical failures. Measuring thermoelastic constants of individual layers allows for computation of the stresses in complete paint systems. Stresses tend to increase with weathering. The presence of flaws in the clearcoat changes the stress distribution dramatically. Coupled with fracture energy measurements, the stress measurements provide additional insight into paint system failure mechanisms.

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.

Assessment of photooxidation in multi-layer coating systems by time-of-flight secondary ion mass spectrometry

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) has been used to assess the formation of 18O-labeled photooxidation products in two multi-layer coating systems held in 20% 18O2/80% nitrogen atmosphere during brief Xenon arc weatherometer exposure. A four-layer model coating system without hindered amine light stabilizer or ultraviolet light absorber additives was examined first. The model was exposed for 8 days in the weatherometer, and then its TOF-SIMS-18O− response was recorded. This response tracks the known photooxidation resistance of the coating layers used to prepare the model system. Next, the technique was extended to a fully formulated six-layer ‘repair’ paint panel that had been weathered for 4 years in Florida prior to 10 additional days of weatherometer exposure in 20% 18O2/80% nitrogen atmosphere. The TOF-SIMS-18O− response observed clearly suggests that the TOF-SIMS-18O− technique can be used to assess the relative photooxidation rates of individual coating layers in fully formulated, multi-layer coating systems.

Rates of photooxidation induced crosslinking and chain scission in thermoset polymer coatings-I

Chemical stress relaxation was used to measure the rates of crosslinking and chain scission during accelerated weathering in two acrylic/melamine clearcoats that photooxidize at different rates. The more photooxidation-resistant clearcoat undergoes an initial increase in crosslink density followed by a steady decrease, while the more photooxidation-prone clearcoat undergoes primarily chain scission until approximately 800 h of accelerated weathering, after which the rate of crosslink formation begins to exceed the rate of chain scission. The net changes in crosslink density mirror those found using dynamic mechanical analysis. Clearcoat films weathered under no stress displayed the same amount and type of chemical composition change as the films weathered under stress. Chemical stress relaxation appears to be a ready means of assessing the influence of the fate of free radicals on the physical properties of the clearcoats.

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.

On the combined use of UVA, HALS, photooxidation, and fracture energy measurements to anticipate the long-term weathering performance of clearcoat/basecoat automotive paint systems

Transmission UV spectroscopy measurements of clearcoat ul traviol et light absorber (UVA) disposition, electronspin resonance (ESR) spectroscopy measurements of clearcoat and basecoat Active hindered amine light stabilizer (HALS) disposition, and transmission Fourier transform infrared (FTIR) measurements of photooxidation have been carried out on 5 µm thickslioes of clearcoat/basecoat/primer/e-coatpaint systems on steel panels as a function of outdoor exposure. These analysis results are combined with clearcoat fracture energy measurements to assess the possibility that a clearcoat/basecoat paint system will resistcatastrophic cracking/peeling failure at long times. Taken together, all results indicate that these nontraditional paint weathering performance metrics should be added to the existing repertoire of paint weathering performancemetrics to ensure that inferior clearcoat/basecoat automotive paint systems are not introduced into service.

In situ e.s.r. monitoring of the coordination and oxidation states of copper in Cu-ZSM-5 up to 500°C in flowing gas mixtures: 1. Interaction with He, O2, NO, NO2, and H2O

Abstract A flow cell was used for the monitoring of the state of copper ions in Cu-ZSM-5 in situ at high temperatures by e.s.r. Reduction of the most reactive square-planar coordinated Cu 2+ ions to Cu + occurs at 500°C in methane flow. The reoxidation of cuprous to cupric ions at 500°C by NO or O 2 is very fast; and, at 500°C, in mixtures of CH 4 with excess oxygen, most of the copper in CuH-ZSM-5 retains the cupric state. The formation of weak adsorption complexes between isolated Cu 2+ cations and CO molecules at 200°C leads to a measurable change in coordination. An activated formation of strong adsorption complexes, with charge transfer, takes place on treatment of the CuH-ZSM-5 by CO at 100–300°C. Subsequent heating in He flow removes all ligands at T > 400°C. At 500°C, in CO flow a slow reduction of Cu 2+ takes place. Cooling of the CuH-ZSM-5 in a mixture [CO + NO] is accompanied by a change in Cu 2+ coordination which is indicative of a mixed adsorption complex.

Photo-stabilisation and photo-degradation in organic coatings containing a hindered amine light stabiliser. Part I—ESR measurements of nitroxide concentration

Abstract Quantitative measurements of the variation in nitroxide free radical concentration on exposure to ultraviolet (uv) light are reported for a melamine crosslinked acrylic copolymer film doped with a hindered amine light stabiliser (bis (2,2,6,6-tetramethyl-4-piperidinyl) sebacate) as a function of hindered amine concentration, light intensity and humidity. The concentration of nitroxide is found to rise to a maximum and then slowly decrease. The following parameters can be derived from the nitroxide data: the net rate of formation of nitroxide during the early stages of exposure, the maximum nitroxide concentration, the time to maximum and the net decay rate at long exposure times. The observed variations in these parameters with exposure conditions are discussed. It is found that the humidity of the exposure greatly influences the degradation and stabilisation chemistry of acrylic-melamine coatings.

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.

Determination of active HALS in automotive paint systems II: HALS distribution in weathered clearcoat/basecoat paint systems

ESR is used to quantify the steady-state concentration of HALS-based nitroxyl radicals and the concentration of nitroxyl radicals produced when HALS and its inhibition cycle products are oxidized with peracid for four clearcoat/basecoat paint systems differing in HALS type, HALS concentration, clearcoat chemistry, and outdoor exposure conditions. Clearcoat bulk and surface analysis results are compared. Step-wise analysis is used to determine the distribution of HALS and its inhibition cycle products in two paint systems as a function of exposure. Micro-UV spectroscopy is used to determine the distribution of ultraviolet light absorber additive in the same paint systems. Taken together, the results indicate that the concentration behavior of HALS and UVA additives during the early stages of weather exposure can provide direct insight into the long-term weathering performance of clearcoat/basecoat paint systems.