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.     

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.     

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.     

Polyethylene stabilization against thermal oxidation by a trimethylquinoleine oligomer

The thermal oxidation at 110 and 120 °C of polyethylene (PE) films stabilized by 0.1, 0.2 and 0.5% of a trimethylquinoleine (TMQ) oligomer has been studied by IR spectroscopy (carbonyl build-up) and by DSC (measurement of the oxidation induction time at 200 °C). The induction period increases almost proportionally to the TMQ concentration and the TMQ efficiency (as estimated by the ratio t_ind/[TMQ]₀) increases when lowering the temperature. Some features of stabilizer consumption kinetics and the dependence of maximum oxidation rate with initial stabilizer concentration were compared to experimental results obtained for stabilization by hindered phenols (Irganox 1010) and to literature data for sacrificial (e.g. hindered phenols) and regenerative (Hindered Amine Stabilizers) antioxidants. These comparisons led to classify TMQ in the category of Hindered Amine Stabilizers (HAS), which was confirmed by a kinetic analysis. Only the scheme taking into account the specific features of HAS (role of NO radicals, regeneration from alkoxyamines) was able to correctly simulate the oxidation behaviour of TMQ stabilized PE.     

Physico-chemical aspects of polyethylene processing in an open mixer. Part 19: Mechanisms and kinetics of vinyl and vinylidene group formation

Vinyl and vinylidene group formation is detected in the initial stages of polyethylene processing. In the high temperature range (170-200 °C) the amount formed is small but significant. Formation of these double bonds is usually obscured by their rapid consumption. Bimolecular hydroperoxide decomposition does not seem to be an important source for these products in the early stages of processing. Vinyl and vinylidene group formation can be attributed mainly to intramolecular decomposition of special hydroperoxide groups. The data suggest vinyl groups to arise from secondary hydroperoxide groups formed in α-position to methyl branching. Intramolecular hydroperoxide decomposition involving a primary hydrogen atom from the methyl group yields a vinyl group and an aldehyde. Vinylidene groups seem to arise from secondary hydroperoxide groups formed in α-position to quaternary structures that necessarily include one methyl group. Intramolecular hydrogen abstraction of a primary hydrogen atom from the methyl group yields a vinylidene group and an aldehyde. The calculated rate parameters are in agreement with the thermochemical estimations relative to intramolecular abstraction of primary hydrogen atoms for both reactions. Vinyl groups are also formed on bimolecular hydroperoxide decomposition. The yield of vinylidene groups from the last reaction is negligible.     

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.     

Interfacial interaction in kaolin-filled polyethylene composites

Kaolin-filled polyethylene composites were prepared under different mixing conditions. Viscoelastic properties of specimens were measured by torsion pendulum tests. Fracture behaviour was studied by tensile tests, and fracture surfaces by scanning electron microscopy and ESCA. Fracture properties turned out to be strongly influenced by processing conditions and to correlate with the amount of undissolvable polyethylene bound to the filler surface. This bound layer has an appreciably lower heat of fusion than the unfilled polyethylene.Dedicated Prof. Dr. R. Bonart on the occasion on his 60th birthday.     

Synthesis and kinetics of non-isothermal degradation of amide grafted high density polyethylene

The grafted structures from the reaction of high density polyethylene with maleic anhydride (PEgMA) was reacted further with 1,4-diaminobutane to synthesize amide grafted polyethylenes. Grafted amic-acids were partially converted to imide groups during the reaction. Grafted products were identified by titration, elemental analysis and FTIR. DSC analysis indicated that C_p for amide grafted product increased about 47% with respect to PEgMA, its crystallinity increased 50% with respect to PE and 18% with respect to PEgMA. Analysis of kinetics of degradation was performed by thermogravimetry; degradation kinetic parameters of these amide grafted products have not been evaluated or reported. The degradation of Amide grafted polyethylene is similar to HDPE but with smaller activation energy indicating that it is the most stable product. F_n and A_n kinetic models seemed more suitable than diffusion models.     

Physico-chemical aspects of polyethylene processing in an open mixer. Part 23: Mechanisms and kinetics of trans-vinylene group consumption

There are many potential reactions for trans-vinylene groups in oxidizing polyethylene melts. The main possibilities are reactions with peroxy radicals, molecular oxygen, hydroperoxides and peracids. These different reactions can all contribute to the removal of trans-vinylene groups to some extent. This is especially so, for the reactions with hydroperoxides that have been found to be the dominant reactions with vinylidene and vinyl groups in the low temperature range. The reaction with peroxy radicals is thought to be as important relatively as with vinylidene groups. Therefore, the importance of the reaction is decreasing with increasing temperature. However, the most characteristic reaction for trans-vinylene groups can be detected without any doubt only in the advanced stages of processing. It is mechanical stress induced oxygen addition to the double bond. The discussion shows that the reaction should be important from the beginning of processing. The reaction cannot operate with vinyl and vinylidene groups, which are not part of the polyethylene main chain. After oxygen addition to the trans-vinylene group, the “ene” reaction yields an allylic hydroperoxide so that the double bond is not immediately removed. It is acid catalyzed hydroperoxide decomposition that leads to chain scission with aldehyde formation at the new chain ends.     

Thermal properties of high density polyethylene composites with natural fibres: Coupling agent effect

High density polyethylene composites with curaua fibres were prepared using an intermeshing co-rotating extruder and two different coupling agents. The thermal stability of the components was studied by thermogravimetric and differential scanning analysis, as well as by the oxidation induction time. Maleic anhydride grafted polyethylene, used as coupling agent, affected the composite stability more markedly than did poly(ethylene-co-vinyl acetate). However, oxidation induction times were analogous for composites with and without coupling agents. Results also indicated that a higher fibre-matrix interaction precludes the crystallinity enhancement caused by the fibre.     

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.     

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.     

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.     

Comparison of the biodegradability of various polyethylene films containing pro-oxidant additives

The biodegradability of high density polyethylene films (HDPE), low density polyethylene films (LDPE) and linear low density polyethylene films (LLDPE) with a balanced content of antioxidants and pro-oxidants (manganese + iron or manganese + iron + cobalt) was studied. Abiotic pre-treatment consisting of photooxidation and thermal oxidation corresponding to about three years of outdoor weathering (including 3-4 months of exposure to daylight) was monitored by FTIR and SEC measurements. The oxidized samples were then inoculated with the strain Rhodococcus rhodochrous in mineral medium, and incubated up to 180 days. The metabolic activity of the bacteria was assessed by measuring adenosine triphosphate content (ATP) and the viability of the cells. Complementary experiments were performed by ¹H NMR spectroscopy to monitor the biodegradation of soluble molecules excreted from the polymer in the incubation medium. Finally SEM was used to visualize the formation of a biofilm at the surface of the polymer. Three samples among the 12 tested were investigated in compost and soil environments. The results show that the main factor controlling the biodegradability of the polyethylene films is the nature of the pro-oxidant additive and to a lesser extent that of the matrix. Except for the samples containing very high content of cobalt additive, the various polymer films were used as substrates by the bacteria.     

Thermolysis of polyethylene hydroperoxides in the melt, Part 12: Mechanisms and kinetics of thermolysis

The thermolysis of polyethylene hydroperoxides is attributed to the reaction of two hydroperoxide groups. This bimolecular reaction appears as a first-order reaction with the mean values of the hydroperoxide concentrations that can be used for the experimental verification of the kinetics. In low molecular mass liquids and solutions these findings would be irreconcilable. However, in polymer melts, this contradiction is more apparent than real. It is a consequence of the heterogeneous kinetics valid in polymer melts. The bimolecular reaction involves the decomposition of pairs of hydroperoxide groups that are relatively close in the elementary oxidation volumes. By diffusion these hydroperoxide groups can come close enough for reaction. From the chemical point of view the decomposition is a bimolecular reaction. However, from the kinetic point of view it is a first-order reaction of the hydroperoxide pairs. The dependency of the first-order rate on the initial hydroperoxide concentration is explained by the heterogeneous kinetics. The activation energy of the overall process can be related to the sum of the activation energies pertaining to the chemical reaction and to the diffusion process.     

Thermolysis of polyethylene hydroperoxides in the melt. Part 9: Re-examination of the experimental kinetics of hydroperoxide decomposition

The experimental kinetics of decomposition of polyethylene hydroperoxides in the melt is re-examined. It is found that the rates determined are more accurate if only the “free” hydroperoxides are taken into account instead of the total hydroperoxides that include also the “associated” hydroperoxides. Then, decomposition of polyethylene hydroperoxides in the melt can be attributed unambiguously to a first-order reaction that is valid in the whole time range of the thermolysis experiments. Nevertheless, the first-order rate constant determined this way increases with the initial hydroperoxide concentration. This constitutes a significant difference with the first-order rate constants that are valid in low molecular mass chemistry and are independent of the initial concentration of the reacting species. It has already been concluded previously that this experimental first-order rate cannot be attributed to true monomolecular hydroperoxide decomposition. Hence, another or other reactions must be envisaged for the interpretation of the specific first-order decomposition of the hydroperoxides in polyethylene melts.     

Physico-chemical aspects of polyethylene processing in an open mixer. Part 18: Mechanisms and kinetics of trans-vinylene group formation

Numerous reactions can yield trans-vinylene groups on polyethylene oxidation. The first problem on data interpretation consists in the separation of the experimental data into components corresponding to well-defined mechanisms. This is achieved by fitting the experimental data into an equation comprising a linear and a parabolic term. The linear term corresponds to trans-vinylene formation at a constant rate from the beginning of the experiments. It can be attributed to trans-vinylene formation on direct decomposition of polyethylene peroxy radicals. The second term accounts for trans-vinylene group formation on cage reactions of various free radicals resulting from hydroperoxide decomposition.The first mechanism can be interpreted by formal homogeneous kinetics. Formation of trans-vinylene groups according to the second mechanism can be accounted for by the heterogeneous kinetics. It proceeds in parallel with the formation of alcohols and ketones. However, reaction of the double bonds with various reactive species in the oxidizing polymer melt does not only lead to a limiting value of the concentration in the advanced stages of polyethylene processing, but also affects the accuracy of the calculations already in the early stages.     

Electrically conductive composites of polyethylene filled with polyamide particles coated with silver

New types of electrically conductive polymeric composites were prepared on a base of high-density polyethylene (HDPE) matrix filled with silver-coated polyamide (PA) particles. The electrical, mechanical and adhesive properties of those composites are reported in this paper. The percolation concentration of the filler within a matrix was found to be 4 vol.%. Composites filled with high filler content were highly electrically conductive; their electrical conductivity reached the value of 6.8 × 10² S cm⁻¹. Mechanical properties and rheology of these composites were discussed. The adhesive properties of the composites to metal sharply increased with an increase in the filler content.