Thermooxidative results

Thermal, Thermooxidative, and Photooxidative Degradation. LLDPE is relatively stable to heat. Thermal degradation starts at temperatures above 250°C and results in a gradual decrease of molecular weight and the formation of double bonds in polymer chains. At temperatures above 450°C, LLDPE is pyrolyzed with the formation of isoalkanes and olefins. 

Thermal, Thermooxidative, and Photooxidative Degradation. Polymers of a-olefins have at least one tertiary C-H bond in each monomer unit of polymer chains. As a result, these polymers are susceptible to both thermal and thermooxidative degradation. Reactivity in degradation reactions is especially significant in the case of polyolefins with branched alkyl side groups. For example, thermal decomposition of... 

Methods for recycling used plastic materials are reviewed. Emphasis is placed on the research projects into chemical recycling methods for used plastics at the Leuna location. These include development of a process for the thermaL thermooxidative pretreatment of used plastic materials, utilisation of pretreated used plastic materials in the visbreaker by gasification and by hydrogenation and the production of wax oxidates from pretreated used plastics. The results are discussed. 

Replacing the hydrogen in 68 with a phenyl group leads to the secondary acetylenic monomer 70. It was believed that this disubstituted acetylene would suppress the reaction of acetylene with itself and insure that there was an acetylene functionality available for reaction with the o-quinodimethane at 200 °G The DSC of 68 showed the presence of a single exothermic peak at 263 °C which the authors felt was adequate evidence for the occurrence of a Diels-Alder reaction between the acetylene and benzocyclobutene. Unfortunately they did not report on any control experiments such as that between diphenylacetylene and simple benzocyclobutene hydrocarbon or a monofunctional benzocyclobutene in order to isolate the low molecular weight cycloaddition product for subsequent characterization. The resulting homopolymer of 68 had a Tg of 274 °C and also had the best thermooxidative stability of all of the acetylenic benzocyclobutenes studied (84% weight retention after 200 h at 343 °C in air). 

Somewhat greater improvements were found in the thermooxidative stability of the fluorinated materials, though the effect for most part was still only moderate with the greatest difference found to be about 40°C. These stability improvements were most notable when comparisons were made at the 2% index, and (in contrast to the anaerobic results) often became less pronounced at the 5 and 10% weight loss indexes. 

The technological thermal stability—i.e., the proceeding of color development in thermooxidative surroundings—was evaluated in an air circulating oven at 190 °C, removing the samples at 5-minute intervals. The results obtained were essentially the same for PVC/EPR and for PVC. 

Because PBI is expensive, other thermostable polymers were explored and tested as catalysts (246). A cross-linked version of a polyimide (PI) support with incorporated triazole rings (12b) gave better results than PBI for the epoxidation of cyclohexene. Moreover, it can be reused in the cyclohexene epoxidation at least 10 times without any loss of activity (247). Even less expensive, but thermooxidatively stable materials include polysiloxane-based resins, which have also been used for incorporation of Ti (see Section II,A). In this case, the synthesis comprises the polymerization of TEOS and an oligomeric dimethyl silanol with the addition of functional trialkoxysilanes such as trimethoxysilyl-2-ethylpyridine instead of Ti(OiPr)4 (248). Preliminary results show that the activity per Mo atom is higher than that of PBI-Mo. Furthermore, the degree of leaching of Mo is very low. Thus, it is expected that the polysiloxane-based systems may soon find wide application in oxidation chemistry. 

If the carboxylic acids on the cellulosic chain are not the major cause of the thermooxidative decay of old cellulosic textiles, one must consider the carbonyl species, particularly the aldehydes on the C2 and Q) carbons. Nikitin (14) noted that the primary autoxidation process is a reaction of molecular oxygen with aldehyde groups, which initiates a chain reaction resulting in more profound changes and decomposition of the molecule . Thus, reduction of the aldehyde groups should lead to improved stability of degraded cellulose. 

Examination of Figure 5 shows that degradation in the early stages is linear with time but occurs at a rate somewhat higher than the rate of degradation found at later times. This phenomenon is observed at temperatures down to 120 °C. Apparently we are observing at least two phenomena, one in which the cellulosic chains are readily broken, and one in which the chains are more resistant to thermooxidative attack. It is not necessarily true that these results indicate different chemical mechanisms at work. Consider the model described by Rowland et al. (18) in which cellulose is a somewhat defective crystal composed of... 

Production and use of PVC occur in the presence of air, i. e. in the presence of oxygen. Therefore, it is surprising that the mechanistic details of thermooxidative degradation of PVC are still not fiilly revealed. The major reactions of this process are shown in Scheme 1. As indicated in this Scheme, thermal dehydrochlorination yields HCl and simultaneously sequences of conjugated double bonds (polyenes) in the chain. The reactive polyenes lead to peroxides in a reaction with oxygen followed by the formation of radicals. Subsequent chain reactions result in additional initiation of HCl loss and further oxidative processes (/, 8). 

As one might expect, a different picture emerges for the thermooxidative stability as reflected in the results of TO and ITG tests performed in air. The participation of oxygen in the overall degradation process leads to the intermediacy of imidazole free radicals and oxygenated free radicals with concomitant chain scission and the net result is a rather catastrophic weight loss observed as the test temperature approaches the 450-500 C level. The representative TG thermograms of Hg. 7, obtained in air, Dlustrate this trend... 

Results of experiments conducted so far indicate that pyrolysis offers a number of basic advantages when compared with thermooxidative waste treatment. Examples of said advantages are ... 

From the chemist s point of view, the keto defect sites can be formed during polymer synthesis as a consequence of incomplete monomer alkylation, as well as a result of photo-, electro-, or thermooxidative degradation processes occurring after polymer synthesis. Acting as low-energy trapping sites... 

The abrasive wear of plastics occurs as a result of strong adhesive interaction, fatigue, macroshearing, abrasive action, thermal and thermooxidative interaction, corrosion, cavitation, etc. Fillers are involved in these processes because mineral... 

Table 4 (reproduced from [38]) summarizes the adsorption characteristics of the initial and polymethylhydrosiloxane-modified macroporous silica with a pore radius r = 57.0 nm. Presented also are the adsorption characteristics of surface-porous adsorbent (see Section 4.2) prepared by the thermooxidation of modified silica, in a narrow temperature range. The values of the water molecular area WH2O and the heat of wetting q show that the modification results in the hydrophobization of the surface, while subsequent thermooxidation increases the adsorbent specific surface area. 

We believe that a consistent interpretation of the results presented can be obtained based on the assumption that the microscopic structure of the adsorbent surfaces studied may be associated with two types or scales of surface porosity (i) inhomogeneities with a characteristic size substantially smaller than that of the adsorbate molecule (surface inhomogeneity due to the atomic structure of the adsorbent), and (ii) inhomogeneities possessing a characteristic size comparable to that of the adsorbed molecule (surface microporosity arising from removal of fragments of the modifier layer during thermooxidation). 

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