M. Iring

Study of the thermal oxidation of polyolefines—IX: Some differences in the oxidation of polyethylene and polypropylene

Abstract In the thermal oxidations of polyethylene (PE) and polypropylene (PP), both similarities and differences are found. In the initial stage, the ratio of the hydroperoxide formed and the amount of oxygen absorbed is independent of both the nature of the polymer and the actual reaction conditions. With increased conversion, however, this ratio decreases to a greater extent in the case of PE than for PP. Most of the carbonyl groups formed in the PE chain during oxidation are ketones and carboxyls, but in PP there are also carbonyls with different structure. The amount of carbonyl groups in PE is approximately 1.5 times that observed for PP. The number of scissions involving considerable molecular weight decrease is higher for PE than for PP: for identical conversions, the ratio of scissions is S PE / S PP ∼ 6.

Decomposition of peroxides of oxidised polypropylene studied by the chemiluminescence method

Abstract It was found that the initial increase in light emission from pre-oxidised polypropylene, measured in a nitrogen atmosphere, is due to decomposition of hydroperoxides in the system. Decomposition is a bimolecular reaction with the probable participation of chemisorbed oxygen. A new initiation reaction step is proposed which contributes to the deterioration of the polymer properties in the induction period of oxidation.

The thermo-oxidative degradation of polyolefines—Part 10. Correlation between the formation of carboxyl groups and scission in the oxidation of polyethylene in the melt phase

Abstract Films of low density polyethylene have been degraded under an oxygen atmosphere at temperatures above the semicrystalline melting point. Time, conversion and temperature dependence of carboxyl group formation and chain scission have been studied. After induction periods we found linear dependences both in function of time and conversion. One third of absorbed oxygen forms carboxyl groups and the absorption of 3·57 mmol oxygen per monomer unit is needed for one chain scission. Maximum rates of carboxyl formation and chain scission have Arrhenius temperature dependence with 33·5 kcal/mole activation energy. The number of carboxyl groups and chain scissions are always practically the same; we assume that the isomerisation of secondary alkyl peroxy radicals simultaneously causes chain scission and carboxyl formation.

Thermal oxidation of Linear Low Density Polyethylene

Abstract The oxidative stability of Linear Low Density Polyethylene (LLDPE) melts was studied under static and dynamic conditions. Static oxidation of two types of LLDPEs (n-hexene and 4-methyl-pentene-1 comonomers) containing different amounts of comonomer units was carried out at 150°C. The oxidizability of the samples was characterized by the induction period (t i,O 2 ) and the maximum rate of oxygen uptake (W O 2 ,max ). The induction period was found to be inversely proportional to the vinyl content of the polymer while W O 2 ,max was proportional to the amount of chain branching. Dynamic degradation during moulding in a closed mixing chamber at 200°C was studied on stabilized LLDPE containing a degree of branching of 6,5 n-butyl/1000 C and HDPE containing no short chain branching. The degradation of LLDPE involved chain scission (and oxygen uptake), and that of HDPE, chain scission and crosslinking. The degradation of LLDPE melt resulted in only a slight change in the physical properties of the solidified polymer but, in the case of HDPE, a considerable decrease in structural order was found. As a result of moulding, the remaining stability decreased drastically, while mechanical properties revealed a considerable decrease only after the stabilizer was used up.

Study of the thermal oxidation of polyolefines—VIII: Volatile products of polypropylene thermal oxidation

Abstract In the oxidation of isotactic polypropylene, qualitative and quantitative studies were made of organic volatile products by gas chromatographic methods. Experimental results and further data have been compared. Most of the products formed during degradation are acetone, and other ketones and aldehydes. Propylene, acetic acid and 2.4-dimethyl furane have also been identified. The products are formed in an accelerating kinetic process. There was linear relationship between the volatile products measured and the amount of oxygen consumption: approximately 2% of the oxygen absorbed is built into low molecular weight organic carbonyl compounds.

Study of the thermal oxidation of polyolefines—II: Effect of layer thickness on the rate of oxidation in the melt phase

Abstract A study was made of the kinetics of oxygen absorption and isothermal weight change as a function of layer thickness for molten polyethylene at 157° under oxygen at 650 torr pressure. The effect of the rate of transport processes on the rate of thermal oxidation was found to be conversion dependent. Comparison of the experimental data on the effect of layer thickness on the maximum rate of oxygen absorption with the results of calculations (carried out assuming various overall reaction orders and applying literature data on permeability coefficient) suggests that, during degradation, the reaction rate is affected by the opposed transport processes of oxygen and the volatile decomposition products. The weight change, closely related to the transport process of volatile products, is diffusion-controlled over the whole range of thickness studied. At the same time, below a critical thickness (approx. 0·05 mm), even the maximum rate of oxygen consumption may be considered a function of only the kinetic parameters.

Correlation between Oxygen Uptake and Carbonyl Formation in Polyethylene Oxidation

Abstract The ratio of absorbed O2 and carboxyl groups formed was studied in LDPE oxidation. It was found that this ratio is independent of the reaction conditions in a wide range (20–100 kPa O2 pressure, 90–160°C, and thickness of 20–1170 μm). If the pressure of O2 was 2.7 kPa, less - COOH formed. A semiempirical mathematic equation was constructed to describe the relation between the acid content and oxygen consumption.

Study of the thermal oxidation of polyolefins—III: Pressure and temperature dependence of the rate of oxygen absorption☆

Abstract The kinetics of oxygen absorption by high pressure polyethylene (PE) have been studied between 8 and 650 torr oxygen pressure and in the temperature range 140 to 180°. The mechanism of thermal oxidation has been described and the equation for the rate of oxygen absorption has been derived. The rate equation has been studied in the initial stage and at points of the maximum rate of oxygen absorption. Some rate constant combinations (characteristic of the process) and their temperature dependence have been determined from experimental data. The pressure dependence of the initial and maximum rates of oxygen absorption, in the pressure range investigated, is well described by the proposed mechanism.

Study of the thermal oxidation of polyolefins: Part XII—Thermal oxidation of isotactic and atactic polypropylene in the condensed phase and in solution

Abstract A study has been made of the oxidation of APP and IPP in the condensed phase and in 1,2,4-trichlorobenzene solution. The differences in the oxidation rates and hydroperoxide concentrations between the amorphous APP melt and the semi-crystalline IPP were attributed to the dissimilarities in the supermolecular structure, since those differences did not appear in solution. It was found that in the accelerating stage of the process, the oxidation in solution follows the kinetic rules of degenerate radical chain reactions under steady-state conditions, while in the condensed phase the condition of stationarity is not fulfilled. Solutions of IPP and APP behave similarly in many respects; significant differences were found, however, in the induction period of the process, as well as in the conversion dependence of the carboxyl group concentration and of the number of chain scissions.

Changes in molecular mass characteristics in the autooxidation of isotropic and oriented isotactic polypropylene films: Primary initiation of the kinetic chain

Autooxidation of polymers is accompanied by the destruction of the macromolecule, which leads finally to the deterioration of the polymer. In several publications1 − 7 it was pointed out that the autooxidation kinetics of isotropic isotactic polypropylene (isotropic IPP) differ considerably from those of the oriented material, the greatest difference being observed at the beginning of oxidation, the induction period of the latter being much longer than that of the former. With increasing degree of oxidation, the kinetic differences decrease or disappear. It was also pointed out2,3 that, at identical conversion, a greater proportion of initial strength remains in the oriented, than in the isotropic, samples and this proportion increases with the degree of stretching (λ). These data led to the conclusion that stretched macromolecules ‘resist’ destruction and oxidize more slowly than macromolecules having the compact conformation of the amorphous phase. These observations inspired the present work in which the oxidative destruction of macromolecules was directly followed, using the GPC method to investigate the change of the molecular mass distribution (MMD). The method was expected to yield information about the processes which take place during the induction period of IPP oxidation, during which the rate of oxygen uptake and the accumulation of product are too small to be followed by the usual methods. That is why the reason for the existence of the induction period has not been clarified so far in the uninhibited oxidation of the polyolefins. Any supplementary knowledge about the so far unexplored processes which take place during the induction period is essential in order to elucidate the nature of the phenomenon.