Solar radiation, polymers

The ordinary monomer units of PVC are not expected to absorb any terrestrial solar radiation (1>.2>.3 A) Thus, under the usual ambient conditions, photodegradation of the polymer must be initiated by chromophoric impurities. These impurities may simply be structural defects in the PVC itself, or they may be extraneous substances that have been incorporated into the polymer. Several of these potential photosensitizers are discussed in the following sections. 

Photolysis of atmospheric pollutants by solar radiation results in an increase of ozone concentration in certain urban areas and is the cause of a sequence of oxidation reactions with polymers. Ozone reacts with practically all organic materials especially with alkenes. The rate of its reaction with alkene is several orders of magnitude higher than that with alkane. The ratio of the rate constants of ozone with ethene/ethane is 1.5 x 105, with propene/propane 1.6 x 106, and with butene- 1/butane 1.1 x 106, at room temperature [5],... 

Chromophores (Ch) are transformed after absorption of the actinic solar radiation in excited singlet ( Ch ) and triplet (3Ch ) states (Rabek, 1996) (Eq. 3-11). Excited chromophores sensitize the formation of macroalkyls from the matrix polymer (Eq. 3-12a) and singlet oxygen from the ground state oxygen (Eq. 3-12b) and accelerate homolysis of POOH via an exciplex (Eq. 3-12c), Reaction scheme 3-3. 

The prime factors causing degradation of polymers are UV light and the heating effects of solar radiation. Additional relevant factors include ozone, atmospheric contaminants and induced radiation. It is the impurities present in the commercial polymers that initiate photochemical reactions because pure saturated polyolefins... 

Copolymers of vinyl chloride and methyl vinyl ketone undergo chain scission with concomitant rapid decreases in tensile strength and elongation when exposed to near ultraviolet li t and solar radiation. Free radicals formed by the homol3rtic scission of the acyl group apparently deplete the stabilizers used and lead to rapid discoloration of the polymer, presumably by the usual radical chain reaction involving the production of HCl and conjugated double bonds. 

The physical behavior of an antioxidant is a major factor affecting its permanency, efficiency, and acceptability, especially when the polymer (e.g., fibers, films) artifact is placed directly in contact with aggressive environments, e.g., organic solvents including dry cleaning solvents, hot water and detergents, hot oils, acids, fatty food, hot air, and intense solar radiation. Physical factors, which control the permanence and efficacy of an antioxidant include distribution, solubility, diffusivity, volatility, and leachability. 

The growth law for a polydisperse aerosol can be determined by measuring the change in the size distribution function with lime. In experiments by Heksler and Friedlander (1977), small quantities of organic vapors that served as aerosol precursors were added to a sample of the normal atmospheric aerosol contained in an 80-m bag exposed to solar radiation. The bag was made of a polymer film almost transparent to. solar radiation in the UV range and relatively unreactive with ozone and other species. Chemical reaction led to the formation... 

Aliphatic polyamides (PA) and products on their base are polymers with low stability to ultra-violet irradiation and under the effect of solar radiation they destruct less than in a year [1]. In natural conditions ultra-violet part of solar spectrum with wave-length 290-350 nm is more dangerous for these polymers, though aliphatic PAs in this field have very weak absorption of chromophore amide group [2, 3], That is why while examining the problem of PA phototransformations, unlike other polymers, both possibility of light absorption by impurities and self-absorption should be taken into consideration [4],... 

Oxygen atoms might react with polymers either by a direct reaction or by an energy-transfer process. Since solar radiation on Earth has wavelengths longer than about 300 nm, this process does not seem important as an initiation step of polymer photooxidation by solar radiation. 

A polymethylmethacrylate film is transparent and does not absorb solar radiation. Therefore photodegradation of this polymer is relatively small. ESR studies on photo-induced radical formation have been reported by a few workers (90-94). 

Although polymers vary widely in their resistance to photodegradation, almost every polymer deteriorates on continued exposure to solar radiation during outdoor weathering. 

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