Oxidation of polyethylene under irradiation at low temperature and low dose rate. Part II. Low temperature thermal oxidation

This paper deals with the kinetic modelling of unstabilised polyethylene thermal oxidation, particular attention being paid to the domain of low temperatures, typically below 80 °C. Experimental data show that the temperature dependence of the induction time t_i and the steady state rate of oxygen absorption r_S display a discontinuity at 80 °C. A model based on the hypothesis that this discontinuity concerns only the PO₂ bimolecular combination processes and is essentially explained by the competition between terminating and non-terminating PO₂ + PO₂ reactions, was proposed. With pertinent values of the Arrhenius parameters of the elementary reactions under consideration, the model fits well the experimental data (in the 40-200 °C temperature range) and is consistent with previously analysed results of radiochemical ageing. According to this model, 35-40% of the bimolecular PO₂ combinations would not be terminating at 45 °C and this proportion would increase with the temperature. Concerning terminations, the relative fraction of coupling processes, leading to peroxide bridges, would decrease relatively to the disproportionation processes when the temperature increases.     

Polyethylene thermal oxidative stabilisation in carbon nanotubes based nanocomposites

Multiwall carbon nanotubes (MWNT)/linear low density polyethylene (LLDPE) nanocomposites were studied in order to understand the stabilisation mechanism for their thermal and oxidative degradation. Thermogravimetry coupled with infrared evolved gas analysis and pyrolysis gas chromatography-mass spectrometry demonstrate that MWNT presence slightly delays thermal volatilisation (15-20 °C) without modification of thermal degradation mechanism. Whereas thermal oxidative degradation in air is delayed by about 100 °C independently from MWNT concentration in the range used here (0.5-3.0 wt.%). The stabilisation is due to formation of a thin protective film of MWNT/carbon char composite generated on the surface of the nanocomposites is shown by SEM and ATR FTIR of degradation residues. The film formation mechanism is discussed.     

A kinetic evaluation of the thermal oxidation of a phenol stabilised polybutadiene

The thermal oxidation of hydroxy telechelic polybutadiene stabilised with 2,2′-methylene-bis(4-methyl-6-tert-butylphenol) was studied at 100 °C using weight changes and stabiliser quantification by liquid chromatography. The extended induction time relative to the unstabilised sample and the initial rate of stabiliser depletion were found to be proportional to the initial stabiliser concentration.Previously published kinetic models, based on the hypothesis that the stabiliser is only consumed by reaction with peroxy radicals and that the stationary state assumption is appropriate, were examined and found insufficient to explain the observations. An improved model was suggested assuming the contribution of a phenol oxygen reaction that results in a competing oxidation of the stabiliser itself. Experimental and theoretical arguments in favour of this model refinement are proposed.     

Thermal oxidation products and kinetics of polyethylene composites

The thermal oxidation behavior of high-density polyethylene (HDPE) composites was investigated at 60 °C, 90 °C and 110 °C, using Fourier transform infrared (FTIR) spectroscopy and thermogravimetric analysis (TGA). The inorganic fillers do not modify the thermal oxidation mechanisms of HDPE. However, they have great effects on the thermal oxidation kinetics—both the activation energy and the pre-exponential factor increase. That means that although the addition of inorganic fillers retards the onset of thermal oxidation of HDPE, once the oxidation begins, it proceeds much faster than that of HDPE. Stability evaluation of HDPE composites by TGA was not consistent with the result by FTIR. The inorganic fillers influence the oxidation products and their distribution greatly. HDPE/STC and HDPE/mica oxidations were delineated by large amount of carbonyl formation, especially esters and ketones, while HDPE/wollastonite and HDPE/diatomite showed minimum carbonyl formation. In HDPE composites, there is a good relation between the carboxylic formation and the carbonyl index.     

Thermal oxidation of polybutadiene. Part 1: Effect of temperature, oxygen pressure and sample thickness on the thermal oxidation of hydroxyl-terminated polybutadiene

An additive-free, uncrosslinked, hydroxyl-terminated polybutadiene of predominant trans 1-4 structure was thermally oxidized at temperatures ranging from 60 to 120 °C, under various oxygen pressures (between 0.01 and 3.1 MPa). Samples of thickness ranging from 5 to 1000 μm were studied by gravimetry (mass changes due to oxygen absorption) and infrared spectrophotometry (hydroxyl and carbonyl build-up, double bond consumption). The effects of film thickness, oxygen pressure and temperature on oxidation kinetics are discussed in terms of branched radical chain mechanisms.     

Kinetic modelling of stabilization coupled with stabilizer loss by evaporation. Case of dithioester stabilized polyethylene

The thermal oxidation of medium density polyethylene (PE) films stabilized by various weight ratios of DiLaurylThioPropionate (DLTP) or DiStearylThioPropionate (DSTP) has been studied in air at 110 and 120 °C. DSC was used for the measurement of an induction time at 190 °C (directly linked to the concentration of residual stabilizing species) and FTIR spectrophotometry for the measurement of the total concentration of ester groups originating from unreacted and reacted stabilizer molecules. The growth in PE carbonyl groups is also recorded and the induction periods at the exposure temperatures are determined. The results show that the physical loss of these hydroperoxide decomposers cannot be neglected, and also imply that simple evaporation-reaction models are inadequate to predict the kinetic behaviour of these systems. A new model is tentatively proposed. It assumes the coexistence of two stabilizer phases: the insoluble (dispersed) phase as the stabilizer excess relatively to the solubility threshold, and the soluble phase from which evaporative loss and the chemical stabilization can occur. Both phases are in equilibrium where the insoluble phase acts as a reservoir for the soluble one.     

Modelling of thermal oxidation of phosphite stabilized polyethylene

The thermal oxidation behaviour of polyethylene films stabilized by various weight ratios of organophosphites (Irgafos 168) has been studied at selected temperatures. The duration of the induction period was found to increase proportionally with the stabilizer concentration, even at temperatures as low as 80 °C. Particular attention was paid to the phosphite-phosphate conversion during the induction period. A kinetic model, involving volatile and partially soluble hydroperoxide decomposers, was developed in order to simulate these results. With the use of kinetic parameters that can be at least tentatively justified from theoretical considerations, this model gave simulations in reasonable agreement with the experimental observations for stabilizer depletion and carbonyl formation. Of particular note is the fact that, even for non-trivial results such as the shape of the phosphite versus phosphate concentration plots, or phosphate build-up, there was also a quite good agreement.     

Photochemical stabilization of linear low-density polyethylene/clay nanocomposites: Towards durable nanocomposites

This article reports a study of the chemical modifications of LLDPE/nanoblend nanocomposites exposed to UV light in conditions of artificially accelerated ageing and natural weathering. Analysis by infrared spectroscopy of the chemical modifications produced by photoageing shows that the presence of an organo-clay leads to the decrease of the oxidation induction time of the polymer (LLDPE), which results in lower durability of the nanocomposites. Protection against photooxidation was tested with different kinds of UV stabilizers and with a metal deactivator. It is shown that the metal deactivator is very efficient in stabilizing the nanocomposite since it totally cancels the prodegradant effect of the organo-clay. This confirms the role played by iron impurities in natural clays. The use of a metal deactivator offers a new insight into the stabilization strategy for nanocomposites.     

Physico-chemical aspects of polyethylene processing in an open mixer. Part 30: Formal kinetics of γ-lactone formation at a constant rate

There are only few mechanisms susceptible to explain γ-lactone formation at a constant rate. The formal kinetics based on these mechanisms proves to be a useful tool in the attempt to estimate the likeliness and possible relative amount of their contribution. The α,γ-keto-hydroperoxides formed in 4-position to hydroxyl groups are decomposed very rapidly at the temperatures of the experiments. The decomposition yields a carboxylic acid group in 4-position to the alcohol group and is first choice for explaining γ-lactone formation at a constant rate. However, the activation energy deduced from the formal kinetics developed for this mechanism is rather small with about 3.6 kcal/mol and hardly in agreement with the experimental value of 29.8 kcal/mol. This leads to the re-examination of the experimental data. Separate fitting of the data for the low temperature range yields the value of 4.1 kcal/mol. This value is sufficiently close to the value deduced from the formal kinetics to be compatible with it. The formal kinetics indicates also that on passing from air to pure oxygen the rate should increase by a factor of about 1.7. This is sufficiently close to the experimental value of about 2 for agreement. It is concluded that the mechanism examined can account for the bulk of the γ-lactone formed at a constant rate.The calculations for 1-peroxy-2,5-di-hydroperoxides and 1,4-keto-hydroperoxides do not yield conclusions that are as straightforward as those for the α,γ-keto-hydroperoxides in 4-position to hydroxyl groups. Although the estimated activation energies are roughly compatible with the experimental value for the low temperature range, the increase with the oxygen concentration is significantly larger than that observed experimentally. Hence, the contribution of these intermediates to the constant rate of γ-lactone formation can only be minor.     

Thermal oxidation of polybutadiene. Part 2: Mechanistic and kinetic schemes for additive-free non-crosslinked polybutadiene

The thermal oxidation of an additive-free, uncrosslinked, hydroxy-terminated polybutadiene has been studied at temperatures ranging from 60 to 120 °C and oxygen pressures from 0.01 to 3.1 MPa by gravimetry, IR spectrophotometry and chemical titration of epoxides and hydroperoxides for samples of 20 and 265 μm thicknesses. A mechanistic scheme with 17 elementary steps among which radical additions to double bonds, decomposition of the resulting alkyl radicals and the corresponding terminations was proposed on the basis of qualitative observations and literature data. A kinetic scheme, composed of 8 differential equations for the 8 reactive species plus 4 equations for the measured quantities, has been derived from the mechanistic scheme. The kinetic parameters, essentially elementary rate constants, have been determined using an inverse approach. A set of physically reasonable parameter values has been obtained. With these parameters, the kinetic model is able to generate kinetic curves of the mass gain, carbonyl build-up, hydroxyl build-up, double bond consumption, epoxide build-up and hydroperoxide build-up, reasonably close to experimental ones, in the full domain of temperatures, oxygen pressures and sample thickness under consideration.     

Effective thermal oxidation of isopropanol by an NAD model

The reaction of 10-methylacridinium cation (MA⁺) with isopropanol in the parent alcohol medium under dark, oxygen-free, and refluxing conditions gave hydride transfer product 10-methyl-9,10-dihydroacridine (MAH). The kinetics of the alcoholic oxidation reaction, including the kinetic isotope effect and the kinetic temperature effect, were determined. Hydride transfer is involved in the rate-determining step.     

Structure evolution mechanism of poly(acrylonitrile/itaconic acid/acrylamide) during thermal oxidative stabilization process

Polyacrylonitrile (PAN) polymers with different compositions were prepared by an efficient aqueous free-radical polymerization technique.Thermal properties of polyacrylonitrile homopolymer (PAN),poly(acrylonitrile/itaconic acid) [P(AN/IA)] and poly(acrylonitrile/itaconic acid/acrylamide) [P(AN/IA/AM)] were studied by Fourier transform infrared spectroscopy,X-ray diffraction,differential scanning calorimetry and thermogravimetry in detail.It was found that AM had the ability to initiate and accelerate thermal oxidative stabilization process,which was confirmed by the lower initiation temperature and broader exothermic peak in P(AN/IA/AM) as compared with that in P(AN/IA) and PAN.The intensity of heat releasing during the thermal treatment was relaxed due to the presence of two separated exothermic peaks.Accompanied by DSC analysis and calculation of the apparent activation energy of cyclization reaction,two peaks were assigned to the ionic and free radical induction mechanisms,respectively.The higher rate constant in P(AN/IA/AM) indicated that the ionic mechanism actually had a kinetic advantage at promoting thermal stability over the free radical mechanism.This study clearly show that the synthesized P(AN/IA/AM) terpolymers possess larger room to adjust manufacture parameters to fabricate high performance of PAN-based carbon fibers.     

Physico-chemical aspects of polyethylene processing in an open mixer. Part 32: Formal kinetics of γ-lactone formation from secondary products. Heterogeneous kinetics

Oxidation of aldehydes and γ-hydroxy-trans-vinylene groups can yield γ-lactones. These intermediates account for γ-lactone formation in the advanced stages of polyethylene processing in air. The acyl-peroxy radical formed on free radical induced oxidation of aldehydes can abstract intramolecularly a δ-hydrogen atom to yield a peracid. Reaction of the alkyl radical formed in this reaction with the hydroperoxide group of the peracid gives a γ-lactone with simultaneous release of a hydroxyl radical. The calculated rate of γ-lactone formation according to the mechanism envisaged decreases slightly with increasing temperature (activation energy of about −5 kcal/mol). It is in agreement with the experiments that do not show significant activation energy in the high temperature range for the advanced stages of polyethylene processing. The calculated rate of γ-lactone formation is found to increase by a factor of about 2.7 if the processing experiments are performed in pure oxygen instead of in air. This is close to the experimental factor of about 2.Peroxidation of γ-hydroxy-trans-vinylene groups can also yield γ-lactones. The first possibility involves addition of a peroxy radical to the double bond followed by oxygen addition to the alkyl radical. This reaction possibly yields an α-peroxy-hydroperoxide. Intramolecular decomposition involving the two reactive groups of the α-peroxy-hydroperoxide can give an ozonide that on thermal decomposition yields among others an acid group in 4-position to the alcohol. The activation energy calculated is strongly negative so that the rate should decrease strongly with increasing temperature. Hence, the mechanism cannot contribute significantly to γ-lactone formation in the whole temperature range of the experiments. This is so in spite of the fact that the rate is estimated to increase by a factor of about 1.7 on passing from air to pure oxygen, which is close to the experimental value of approximately 2. The second possibility of transformation of γ-hydroxy-trans-vinylene groups is based on stress-induced oxygen addition to the double bond. Acid catalyzed decomposition of the allylic hydroperoxide that is formed in the reaction yields a pair of aldehydes with one of the aldehyde groups in 4-position to the alcohol group. Peroxidation of the aldehyde pair can give an acid group in 4-position to the hydroxyl group so that a γ-lactone can be formed. The activation energy calculated for the process is very small and the effect of the oxygen concentration corresponds to an increase by a factor of approximately 4.5 on passing from air to pure oxygen. It is postulated that simultaneous contribution by different mechanisms might well account for the experimental value of about 2.The heterogeneous kinetics discussed in detail allows for complementary data interpretation. It is especially suited for the understanding of the advanced stages of polyethylene processing, after some induction time.     

Physico-chemical aspects of polyethylene processing in an open mixer. Part 31: Formal kinetics of γ-lactone formation from additional primary products

The potential contribution of primary oxidation products to γ-lactone formation in polyethylene is discussed. The 1,4,6-hydroperoxy-keto-hydroperoxides and the 1,2,5-tris-hydroperoxides are investigated in this work. Their formation and decomposition is examined with respect to γ-lactone formation at increasing rates in the initial stages and possibly at constant rates in the advanced stages of polyethylene processing. The formal kinetics based on the mechanisms is used to check the effect of the temperature and of the oxygen concentration. It is found that the activation energy calculated for the two mechanisms envisaged can account for different experimental values valid in the initial or advanced stages of polyethylene processing. However, the calculated increase of the rate on passing from air to pure oxygen is always much larger than the experimental value. Hence, the mechanisms examined can contribute to part only of the γ-lactone found experimentally. They should necessarily be complemented by additional mechanisms that show smaller increase with the oxygen concentration than that found experimentally.     

Epoxides in the thermal oxidation of polybutadiene

Polybutadiene films were aged under air or high oxygen pressure (3.1 MPa). In both cases, the amount of epoxide formed was titrated. The results show that the epoxide formation rate is a decreasing function of oxygen concentration that validates the mechanism proposed by Mayo. According to this mechanism, epoxides are generated from the decomposition of β-peroxy alkyl radicals resulting from the addition of peroxy radicals on double bonds.     

Reducing end-group of cellulose as a reactive site for thermal discoloration

Thermal discoloration of cellulose (Avicel PH-101 and Whatman No. 42 filter paper) was studied in N₂ at 160-280 °C with glycerol-treated and NaBH₄-reduced samples, to understand the role of the reducing end. Thermal discoloration of glycerol-treated Avicel PH-101, in which some of the reducing ends were converted into glycosides (non-reducing ends), was suppressed compared with the original cellulose, and the level of suppression was directly related to the extent of glycosylation of the reducing ends. The stabilization efficiency of glycerol-treated Whatman No. 42 filter paper suggested that the reducing ends newly formed by reduction of the degree of polymerization (DP) (to about 200) during heat treatment contributed to the discoloration. The important role of the reducing ends in thermal discoloration was supported by the stabilization of Avicel PH-101 by reduction with NaBH₄ (giving a reducing end content that was 2% of that of the original cellulose). Thermally induced discoloration was also inhibited by heating cellulose in suspension in the polyether tetraethyleneglycol dimethylether, which has been reported to inhibit the thermal degradation of reducing sugars.     

Anisotropic and heterogeneous thermal expansion of polyethylene foam blocks: Effect of thermal treatments

Crosslinked closed cell polyethylene foams produced in blocks by compression moulding present an anisotropic and heterogeneous thermal expansion behaviour when the temperature is increased. This paper analyses the main reason for this particular behaviour and presents a way to reduce it by using thermal treatments.In order to perform this analysis, an experimental study on the cellular structure, lamellar distribution and thermal expansion is presented as a function of two kinds of thermal treatments. The experimental results have showed that the main factor controlling the foams thermal expansion is an anisotropic and heterogeneous cellular structure of the original foams. It has been also proved that an adequate thermal treatment allows homogenising the foams thermal expansion.     

Hydroperoxide build-up in the thermal oxidation of polypropylene - A kinetic study

The thermal oxidation of unstabilised polypropylene (PP) was studied at 80 °C under various oxygen pressures: 0.02, 0.5 and 5.0 MPa, and, under 5.0 MPa oxygen pressure at various temperatures: 60, 80, 100 and 120 °C. Hydroperoxides were titrated using a chemical titration method and modulated DSC (taking an enthalpy of −325 kJ mol⁻¹). Starting from a previous kinetic analysis of carbonyl growth in same exposure conditions, we have tried to simulate experimental results by a model based on the classical mechanistic scheme in which initiation results from POOH (mainly bimolecular) decomposition. The model, which takes into account substrate consumption and does not rely on usual simplifying assumptions (steady state for radicals, long kinetic chains, interrelations between termination rate constants), generates kinetic curves with the same shape as experimental ones and predicts well the effect of O₂ pressure and temperature on hydroperoxide and carbonyl concentrations.     

Thermal oxidation of metallocene ethylene-1-olefin copolymer films during one year oven aging

Metallocene ethylene homopolymer, ethylene-1-hexene and ethylene-1-octadecene copolymer films were characterized and analysed during their aging in a forced draft oven at 60 °C and compared to commercial polymers (HDPE and LDPE). Each polymer is essentially different in its structure, and the variables used in the present study are comonomer size and content in the main linear backbone polymer. The polymers were characterized initially using GPC, DSC, FTIR, and tensile tests. Later, at different time intervals, samples were removed from the oven and characterized using FTIR and GPC in order to detect changes in chemical structure, size, and molecular weight distribution due to thermo-oxidative aging. At the end of aging, films were subjected to tensile tests to quantify the effect of time on their useful life. As a qualitative reference parameter, the carbonyl index, the ratio of the infrared absorbance of the CO stretching band at 1715 cm⁻¹ and the absorbance of a reference band at 718 cm⁻¹, was determined. Kinetic thermo-oxidation is related to comonomer size and content in the main backbone polymer. As comonomer size decreases or comonomer content increases, degradation rate increases. The scission factor (S) and carbonyl index (CI) graphs of each material show a slope increase which is related to the autocatalytic rate of oxidation.