Thermal and photothermal degradation

Figure 2. Weight loss of PP after irradiation for 15 hr as a function of temperature. Points represent the difference between extents of thermal and photothermal degradations at these temperatures.

Polypropylene. In the thermal and photothermal degradations of PP as prepared as well as In the thermal degradation of the pre-irradiated polymer, the general patterns of volatile products are similar and are qualitatively accounted for In terms of the mechanism of Tsuchlya and Suml ( ), which proposed that the complex mixture of saturated and unsaturated hydrocarbons Is formed in a free radical process in which Inter and Intramolecular transfer play a predominating role. In the photothermal reaction, however, in addition to those products, a very much higher relative concentration of methane and an appreciable amount of... 

The thermal degradation of polypropylene is accelerated by 253.7 nm radiation (photothermal degradation), and methane and ethane appear as additional products [822] these are associated with photoinitiation of the radical process involved, as the mechanisms of the thermal and photothermal reactions are almost identical. The different overall activation energies of the two reactions, 50.1 and 11.7kcal/mol, respectively, may be associated with the different initiation steps. 

The volatile products of thermal degradation of polypropylene (PP) under vacuum in the temperature range 300-360°C comprise a complex mixture of saturated and unsaturated hydrocarbons. Under u.v. radiation at these temperatures (photothermal degradation), the general pattern of products is similar but the rate is Increased, ehtylene appears as a minor product and the relative amount of methane is very much greater, especially as the temperature Is decreased below 300°C. Energies of activation of the thermal, photothermal and photoreactions are 50.1, 33.9 and 11.7 k cal mole" respectively. 

Mass spectra of volatile products from photothermal and thermal degradations at 354°C and of photothermal degradation at 200 and 250 C are illustrated in figure 1. They are all typical of a mixture of hydrocarbons with the main peaks separated by 14 units (CH2). They are generally similar, except that as the degradation temperature is decreased, the lines at mass numbers 15 and 16 become relatively much more intense. These two lines, as well as forming part of the breakdown pattern of higher hydrocarbons, are characteristic of methane. Thus it may be concluded that the relative proportions of methane in the volatile products... 

At high temperatures the image of photodegradation changes completely because additional reactions are involved in the thermal degradation of a polymer [1319]. Many papers have been devoted to the study of photothermal degradation of polyethylene [148,1289], polypropylene [822], poly(ethylene-co-carbon monoxide) [871, 917], polyacrylates and polymethacrylates [821, 829], poly(acrylate-co-methacrylate) [828, 831], poly(vinyl chloride) [311, 702, 768, 891,1097,1319,1792] polystyrene [1491], natural rubber [14] and poly(ether sulphone) [1265]. 

The experiments described in the first part of this paper thus seek to extend our knowledge of the degradation processes which occur in PP, first, by linking photo and thermal processes through photothermal studies and second, by observing the effect of pre-Irradiation on subsequent thermal degradation. 

Figure 1. Mass spectra of volatile products from photothermal and thermal degradations of PP. A, thermal at 354°C B, photothermal at 354°C C, photothermal at 200°C D, photothermal at 250°C.

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