James Edward Pickett

Photodegradation of UV screeners

Abstract The UV screeners that are used to help photostabilize polymers and coatings are themselves subject to photodegradation over the course of a few years of exposure. A simple computer program has been devised to model this process. All of the chromophores of the commercially available screeners degrade, but at varying rates. The rates are highly dependent on the matrix since hydrogen bonding effects and free radicals can lead to decomposition of the screeners. Concentration seems to play no role, and hindered amine light stabilizers (HALS) stabilize the screeners only to the extent that they reduce free radical degradation of the matrix.

Degradation kinetics of CIGS solar cells

The moisture-induced degradation rate of CIGS solar cells at 22–85oC and 0#x2013;100% RH has been measured and fit to a kinetic rate expression. This expression is coupled to a model of moisture diffusion into a package and typical meteorological input data to create a cumulative damage model to predict lifetime of packaged cells. Analytic solutions are also derived. In the diffusion-controlled regime, the life depends on the square root of package permeability and cell degradation rate. Early results indicate that a barrier of ≪10−4 g/m2/day or better will be required to assure 20 year lifetime in Miami for the Global Solar cells used here. Other less stable cells will require 10–100X better moisture barrier. Estimated acceleration factors for damp heat (85C/85%RH) vs. Miami range from 15X to 50X, depending on the package, since diffusion through the package is accelerated differently than the cell degradation kinetics. These low accelerations are a direct result of the square root dependence in this diffusion-controlled process.

Highly Predictive Accelerated Weathering of Engineering Thermoplastics

Current accelerated weathering protocols such as SAE J1960 or ASTM G26 do not provide reliable, predictive results for engineering thermoplastics. Correlation factors among resin types and even different colors of a single resin have variations that are 60-100% of the mean at the 95% confidence level, making these tests useless for lifetime prediction or even reliable ranking of materials. We have developed improved conditions using CIRA/ sodalime-filtered xenon arc; a more rain-like water spray, and occasional sponge-wiping of the samples. The data for gloss loss and color shift agree very well with Florida data giving a correlation factor of 3100′680 kJ/m 2 (at 340 nm) per Florida year at the 95% confidence level. The acceleration factor is 7.6x.

Poly(3,4-dibutoxythiophene vinylene): A new processable conducting polymer with unusual optical properties. Synthesis, characterization and stability

Abstract Poly(3,4-dibutoxythiophene vinylene) was prepared and investigated as a new, processable conducting polymer. Having the vinylene unit to reduce steric interactions, substitution of the thiopene in both the 3- and 4- position is possible without loss of conductivity. The dibutoxy substitution yields advantages in processing, as the high molecular weight polymer dissolves easily in common organic solvents. The polymer can be cast into tough, flexible, free-standing films. The FeCl 3 doped polymer films are nearly optically transparent and have conductivities of 1 Scm −1 . The stability of the undoped polymer to light, oxygen, ozone and water is reported.

An efficient synthesis of dihydrocoumarins

Abstract Dihydrocoumarins are obtained in 40 to 60% yields in one step by heating acrylic esters with an excess of a phenol in the presence of base catalysts.

Thermal oxidation of extruded poly(2,6-dimethyl-1,4-phenylene oxide)

Abstract The oxidation of extruded polyphenylene ether resin at moderate temperatures has been shown to obey Arrhenius kinetics with first-order dependence on the hydroxyl endgroups and half-order dependence on oxygen. In blends, the oxidation is overall second order in uncapped resin. The phenolic endgroup catalyzes the oxidation of the benzylic methyl groups, so capping the resin leads to a much reduced oxidation rate. The rate of oxidation of the capped commercial resin is approximately the same as that of pure capped model compounds, showing that other factors such as residual catalyst or structural irregularities play only a minor role in promoting the oxidation. A mechanism involving attack of the endgroup phenoxyl radical on the benzylic methyl group is suggested as the first step in the oxidation of the uncapped polymer.

Lifetime Predictions for Hardcoated Polycarbonate

Polycarbonate (PC) is an attractive material for many glazing applications because of its low weight compared with glass, and high impact resistance compared with other transparent polymers. Polycarbonate glazing usually is coated to impart weatherability and scratch resistance. Early abrasion-resistant coatings lacked UV absorbers and failed by delamination after about 12 months of 45° south exposure in Florida. The highest performing wet coatings consist of UV absorbers co-polymerized with a silicone/colloidal silica nanocomposite. Such first generation silicone hardcoats (SHC) typically would weather in Florida for three to five years before the underlying PC began to turn visibly yellow and the coating would delaminate. Some coating formulations also were subject to Chapter 10

Effect of stratospheric ozone depletion on terrestrial ultraviolet radiation : a review and analysis in relation to polymer photodegradation

Abstract Current satellite and ground-based data indicate a long-term decrease in global ozone levels of 2·7 ± 1·4% per decade, although recently the rate may have increased to about 4% per decade outside of the tropics. There is yet little evidence of an accompanying wide-spread increase in terrestrial UV levels, but calculations predict increases of a similar magnitude in coming years. The manifestation of increased UV will be as a broad band centered about 310 nm with only very little absolute increase in wavelengths shorter than 295 nm. Unless sustained ozone losses exceed 15% during the summer months, the effect should be inconsequential for most polymers. The effects of sun position on daily and seasonal variations in UVB are discussed.

Hydrolysis Kinetics and Lifetime Prediction for Polycarbonate and Polyesters in Solar Energy Applications

The hydrolysis kinetics of polyesters and BPA polycarbonate appear to be second order in water, that is, second order in relative humidity (RH). This finding, combined with activation energies for hydrolysis, was used in a service life prediction model for a photovoltaic (PV) module front sheet application. The modeling process involves: (1) finding finely time-parsed climatic data for a benchmark location, (2) calculating module temperature and RH from the climatic data, and (3) applying the kinetics to determine the amount of degradation that occurs in 1 year relative to accelerated laboratory conditions, 85 °C and 85 % RH in this case. Acceleration factors under laboratory conditions are very high, especially for poly(ethylene terephthalate) (PET), leading to predicted lifetimes of several centuries if hydrolysis is the only degradation mode. The model was tested for sensitivity to assumptions and experimental uncertainties, and hydrolysis can be considered unimportant in this solar energy application. Concerns about PET suitability on the basis of its relatively short embrittlement times in the 85 °C/85 % RH damp heat test are unfounded.

Service Life Prediction: Why Is This so Hard?

Service life prediction, in general, and weathering lifetime, in particular, have been persistent problems despite nearly a century of work. Part of the problem has been a gap between the work of scientists studying material degradation and the practices, needs, and desires of engineers who make the specifications and standards that drive most testing. In this chapter, we describe why this gap exists and propose a way to close it using the methodology of service life prediction to develop predictive test conditions for specific classes of materials.