Photo-oxidative stability

Now, consider stability. If a satisfactory initial system or component performance and cost are assumed, then in many cases the critical issue is to maintain the physical behavior of materials adjoining an interface for up to 30 years. The physical behavior may include properties that directly influence solar device performance, such as reflectance, transmittance, absorptance, emittance, and photovoltaic efficiency or solar device performance may be indirectly affected by properties such as adhesion, permeability, photo-oxidative stability, or interdiffusion. The required stability of interfaces in SECS components is counter to basic physics and chemistry, because atoms at interfaces must be more reactive and thermodynamically less stable than when in the bulk of materials (2). Yet, the density of solar energy requires deploying systems with large interfacial... 

Miscellaneoiis Polymers.—Allen et al. have studied the post-cured photo-oxidative stability of di- and tri-acrylate resins having structures (9) or (10)... 

Since UV absorbers cannot completely prevent the initiation reactions above, we felt that better antioxidants were necessary to obtain improved long-term photo-oxidative stability of polyolefins. We reasoned that stable radicals which scavenge radicals formed in the oxidative process might compensate for the imperfections of the absorbers. On the basis of the assumption that the diaryl nitroxyl radicals, rather than their parent diarylamines, are the active antifatigue species in rubber protection, we first attempted to obtain new stable nitroxyl radicals. 

In the 1950s, Feller (1972) showed that fluorinated acrylics had even higher stability than those currently available. These have been extensively researched for use on external stone surfaces (see Table 8.2 for typical examples). They cannot be used as homopolymers, because they have relatively high Tg values and dissolve in few solvents. As has been similarly shown with the parent polymers, the distribution of the fluorine substitutents appears to be important in determining the photo-oxidative stability of the polymer (Signori et al., 2006). However, suitable polymers have yet to be developed that meet the other requirements of this usage (Toniolo et al., 2002). 

Chiantore O, Lazzari M (2001) Photo-oxidative stability of pataloid acrylic protective polymers. Polymer 42 17-27... 

High chemical purity is required of additives and fillers. They should contain no or only very few heavy metal ions, which can catalyze the aging process in plastics. Copper, manganese, and cobalt ions (see Section 1.4.2.1.2) influence thermal-oxidative and/or photo-oxidative stability of plastics negatively [86]. 

Traces of copper or iron scarcely influence the photo-oxidative stability of PPE. However, if PPE containing copper or iron compounds is mixed with impact modified polystyrene (SB), the result is a significant reduction in photo-oxidative stability. Iron(III)chloride causes strong degradation, followed by copper(II)chloride and acetate the least damaging is iron(III)acetate [86]. 

Geuskens, G., Bastin, P. Photo-oxidation of polymers Part VI - Influence of thermal oxidation on the photo-oxidative stability of a styrene-acrylonitrile copolymer. Polymer Degradation and Stability, 4 (1982), p. 111-121... 

Gorrasi, G., Sorrentino, A. Photo-oxidative stabilization of carbon nanotubes on polylactic acid. Pol5nn. Degrad. Stab. 98, 963-971 (2013)... 

Protection of polymers against thermal and photo-oxidative degradation is achieved with appropriate stabilizers that ensure the desirable polymer properties throughout the entire service life of the polymer. Compatible and polymeric stabilizers usually give the best protection. In order to avoid migration and evaporation, polymeric stabilizers are used. 

An alternative mechanism by which additives may protect polymers from photo-oxidation is radical trapping. Additives which operate by this mechanism are strictly light stabilizers rather than antioxidants. The most common materials in this class are the hindered amines, which are the usual additives for the protection of poly (ethylene) and poly (propylene). The action of these stabilisers is outlined in Reactions 8.3-8.5. 

Ranby, B. and Rabek, J., Photo Degradation, Photo Oxidation and Photo Stabilization of Polymers, Wiley, New York, 1975, 75. 

It is interesting to note that the most critical spectral changes occur at the initial stages of photo-oxidation. On further exposure of the samples (as a thin film on KBr) to UV light, a certain stabilization of chemical changes in the structure of the oligomers studied takes place along with a decrease in the intensity of the major absorption bands. 

Consequently conventional antioxidant mechanisms must be expected to protect against photo-oxidation. Thus hydroperoxide decomposition to inert molecular products will reduce the rate of photoinitiation and scavenging of any of the free radical species will be beneficial, although the effectiveness of conventional antioxidants in photo-oxidations is limited by their own stability and the photo-sensitizing propensity of their products (3,). 

Most kinetic treatments of the photo-oxidation of solid polymers and their stabilization are based on the tacit assumption that the system behaves in the same way as a fluid liquid. Inherent in this approach is the assumption of a completely random distribution of all species such as free radicals, additives and oxidation products. In all cases this assumption may be erroneous and has important consequences which can explain inhibition by the relatively slow radical scavenging processes (reactions 7 and 9) discussed in the previous section. 

Substituted tetramethylpiperidines and their oxidation products appear to act as UV stabilizers for polypropylene by weakly scavenging PP, PPO2 and possibly other radicals involved in the photo-oxidation. Their weak scavenging ability is off-set by the... 

Again, polyesters based on naphthalene-2,6-dicarboxylic acid can meet the desired properties, plus have other additional advantages. Food components containing double bonds or conjugated double bonds are extremely sensitive to light-induced reactions, such as photo-oxidation. The addition of UV stabilizers is therefore necessary to protect the content of PET bottles against UV irradiation. PET, in contrast to PEN, does not show intrinsic UV protection properties. 

The thermal stability and lightfastness of polyesters is particularly necessary for technical and high-performance applications. The modification of the polymer causes disorder and affects the stability as well as some other properties. PET modified by DEG suffers particularly from photo-oxidative reactions due to the presence of the sensitive ether bonds. These copolymers need special stabilization depending on the kind and degree of modification. The UV stability can also be influenced by the technology of the process, whereby slight improvements of DMT-based polymer are observed [29],... 

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