Polyethylene photooxidation

F. Gugumus, Findings pertaining to polyethylene photooxidation, Makromol. Chem. Macromol. Symp. 1989, 25, 1-22. 

Bocchini, S., Di Blasio, A., Frache, A. Influence of MWNT on polypropylene and polyethylene photooxidation. Macromol. Symp. 301, 16-22 (2011)... 

Various compounds such as thiobisphenol were reported to exhibit a pro-oxidant effect in the early period of polyethylene photooxidation, which is accelerated as their concentration is enhanced, while on the later irradiation period the carbonyl build up was retarded [72]. The retardation of the carbonyl accumulation was obtained with a compound based on dodecyl-3,3 -thiodipropionate over the whole period of ultraviolet exposure. Kinetic data showed [72] that the concentration of 0.1 w/w% of thiobisphenols determines the most effective retardation for flie accumulation of carbonyl group in LDPE films. 

In semi-cristalline polymers, rate-enhancement under stress has been frequently observed, e.g. in UV-photooxidation of Kapron, natural silk [80], polycaprolactam and polyethylene terephthalate [81]. Quantitative interpretation is, however, difficult in these systems although the overall rate is determined by the level of applied stress, other stress-dependent factors like the rate of oxygen diffusion or change in polymer morphology could occur concurrently and supersede the elementary molecular steps [82, 83], Similar experiments in the fluid state showed unequivocally that flow-induced stresses can accelerate several types of reactions, the best studied being the hydrolysis of DNA [84] and of polyacrylamide [85]. In these examples, hydrolysis involves breaking of the ester O —PO and the amide N —CO bonds. The tensile stress stretches the chain, and therefore, facilitates the... 

In the present paper, we describe how photodegradation of low density polyethylene films was enhanced by uniaxial elongation. An explanation of the enhancement process is given based on the photooxidation and deformation mechanisms, and the photodegradation products. 

Photooxidation Diffusion Chemical changes due to photochemical reactions Introduction of contaminants from man-made materials, such as solvents from polyvinyl chloride (PVC) materials and PVC cement, plasticizers, and phthalates from polyethylene and polypropylene materials Protection from exposure to light, use of amber glass bottles Use of inert materials (PTFE, fiberglass-reinforced epoxy materials) steam-cleaning of groundwater well components prior to installation... 

Despite the numerous papers devoted to photooxidation of hydrocarbon polymers [21], the initiation step has not been clearly established yet even for polyethylene or polystyrene which were the most studied [22,23]. Difficulties which follow from solution of this problem consist in the necessity of analysis of small amounts of decomposing unstable structures and products which are thereby formed. Moreover, photoinitiation does not include one reaction only but the overall complex of many chemical and physical processes, which importance depends on experimental conditions. 

The photooxidation mechanisms of polyolefins continues to attract much interest and controversy. A comparison of the photooxidation rates of linear low density polyethylene with low density polyethylene indicates that catalyst levels in the former have little, if... 

Conjugated chromophores develop from the PBT and these will screen the photoinduced decomposition of the hydroperoxides in PP. This effect results in an accumulation of hydroperoxides from the PP. Iron diethyldithio-carbamate has been shown to exhibit an initial stabilisation effect on polyethylene followed by sensitisation as does anthraquinone . Trace volatiles have been measured during the photooxidation of polyethylenes while ferric stearate/cerium(III) mixtures and starch materials are good degradants . ... 

The photooxidation kinetics of branched polyethylenes has been found to be strongly influenced by processing history and the incorporation of poly(2,6-dimethyl-l,4-phenylene... 

Starch and cellulosic materials are frequently used as fillers in degradable materials. The addition of starch to LDPE in combination with a pro-oxidant increases the photooxidation rate and the formation of hydroperoxides and carbonyl groups. Starch alone does not increase the photooxidation rate. The addition of starch to LDPE increases its stability in 80°C water. Slower degradation in water is due to leaching out of the pro-oxidant. The addition of starch causes biodegradation process under soil burial conditions. Further increase in the degradation rate can be achieved by preheating polyethylene filled with starch. ... 

Figure 10. W absorbance of polyethylene before and after photooxidation...

However, a thiobischenol reduced the rate of hydroperoxide buildup in both clear and black films and in combination with carbon black showed synergistic behavior in inhibiting the photooxidation of polyethylene (17). 

Figure 11. Photooxidation rate of branched polyethylene films containing 0.1% Neotex ISO carbon black (9) and 0.1% substituted o-hydroxybenzophenone (H). Also shown are the rates of unprotected films exposed to direct radiation (A), to radiation transmitted by the black screen (O), and by the film cotaaining the...

The mechanisms of the photooxidation of polyethylene and polypropylene have been discussed in depth with particular emphasis on the importance of hydroperoxides as the precursor to free radical formation . Both the kinetics and nature of the photooxidation products of the polymers are markedly controlled by these species especially polypropylene. On the other hand the density of polyethylene has been found to play an important role on the photooxidation rate of the polymer . Here the photostability of the polymer decreased with decreasing film density indicating that oxygen diffusion is impaired by the crystallites and therefore improves stability. In fact, other workers have found that the crystalline regions of polyethylene are unaffected by irradiation in air . These workers also found new crystalline regions are formed on irradiation due to the smaller polymer fragments... 

A computer model has been developed which can generate realistic concentration versus time profiles of the chemical species formed during photooxidation of hydrocarbon polymers using as input data a set of elementary reactions with corresponding rate constants and initial conditions. Simulation of different mechanisms for stabilization of clear, amorphous linear polyethylene as a prototype suggests that the optimum stabilizer would be a molecularly dispersed additive in very low concentration which can trap peroxy radicals and also decompose hydroperoxides. 

As a starting point for this computational approach to the photooxida-tive process in polymeric materials, we have examined the simplest prototype neat, amorphous, linear polyethylene above its glass transition temperature. In practice, polyethylene is partially crystalline, and contains truly linear olefins, vinylidene groups, ketones and hydroperoxides in addition to the short side chains. Much insight has already been gained into the photooxidation process by conventional experimentation on such polymers (12,13), yet several important questions still remain. Several good reviews have appeared recently (14-16). 

In principle, the computational approach to the kinetics of the complex photooxidation process can give meaningful insight into the effects of outdoor weathering of hydrocarbon polymers. For clear amorphous linear polyethylene, the model suggests that the optimum stabilizer would be a molecularly dispersed additive in very low concentration which could trap peroxy radicals. An additive which decomposes hydroperoxides would also be effective but would require higher concentrations. The useful lifetime of unstabilized polyethylene is predicted to vary from a few months in hot weather (100°F) to almost two years in cool weather (45°F), which correlates well with experimental results and general experience. 

---