The thermo-oxidative degradation of metallocene polyethylenes: Part 2: Thermal oxidation in the melt state

The thermo-oxidative melt degradation of different metallocene polyethylenes (mPEs) was investigated in a torque rheometer open to air at 225 °C and 10 rpm. The mPEs differed essentially according to their initial melt index, molar mass distribution, density and ash content, but one characteristic was changed at a time in order to assess the influence of each specific property in the thermo-oxidative degradation of the PEs investigated. Crosslinking was found to dominate at the early stages of degradation during mastication for most polymers where reactions of alkyl radicals to vinyl groups were considered to be the dominant reaction. Furthermore, discolouration was attributed to both excessive levels of catalyst residues and extensive formation of conjugated systems. Finally, it was concluded that the polymer melt viscosity, i.e., molar mass and shape of molar mass distribution, appeared to govern the processing stability of the mPE. These results confirm the importance of shear as the major source for initiation of free radicals formed by homolytic fission caused via mechanical cleavage of polymer chains.     

Thermal stability and degradation products analysis of benzocyclobutene-terminated imide polymers

Benzocyclobutene-terminated imides were prepared and fully characterized with ¹H NMR, MS, and FT-IR. The thermal degradation of polymers was investigated by using thermogravimetric analyzer (TGA) and high-resolution pyrolysis-gas chromatography–mass spectrometry (HR-Py-GC–MS). TGA showed that thermal degradation of the polymer was a single-stage process in N₂, whereas a three-stage degradation in air atmosphere. The major involved products were found to be CO₂, naphthalene and naphthalene derivatives. Degradation mechanism of the polymer was suggested and the relationship between structures of the polymer and degradation products was also discussed.     

Study of the degradation mechanisms of polyethylene during reprocessing

The present work aims at evaluating the behaviour of various grades of high and low density polyethylene subjected to continuous injection moulding operations. It is based on a wide range of experimental techniques that allowed the establishment of relationships between the observed property changes and the structural modifications developed in the polymers. The study confirmed the coexistence of two distinct degradation mechanisms, crosslinking and chain scission, that occur due to thermo-oxidative reactions arising during the different injection moulding stages. The relative importance of these two mechanisms depends on the material structure and on the processing conditions used. It was also possible to confirm that the polyethylenes with lower molecular mass show low sensitivity to degradation during reprocessing. Furthermore, the more branched polymers (as is the case of LDPE) also proved to be less sensitive to thermo-degradation phenomena. Overall, it was concluded that the primary recycling of polyethylene, if performed under well controlled conditions, will lead to only minor material property losses.     

Investigating the role of metal oxidation state on the degradation behaviour of LDPE

Transition metal stearates have been reported to act as effective pro-oxidants for polyethylene, even at trace concentrations. This study is an attempt to investigate the effect of the oxidation state of a metal on its pro-oxidant nature. Three metal stearates, namely manganese, iron and cobalt, in their common oxidation states (+2 and +3), were synthesized and their effect on the photo-oxidative and thermo-oxidative degradation of low-density polyethylene (LDPE) films has been investigated. Films of 70 ± 5 μ were prepared by film blowing technique, exposed to xenon arc weatherometer and air oven at 70 °C for extended time periods. The chemical and physical changes induced by this exposure were followed by monitoring the changes in mechanical properties (tensile strength and elongation at break), carbonyl index (CI), molecular weight (viscometry), MFI, density, and thermal properties. The results were analysed to explain the structural and chemical modifications taking place in the polymeric matrix as a result of aging. The studies reveal that the oxidation state of the metal did not affect its ability to initiate and accelerate degradation. The thermo-oxidative degradation in the presence of metal stearate was found to follow the order: cobalt > manganese > iron. However, iron stearate was capable of initiating photo-oxidative degradation to the same extent as cobalt and manganese, in the concentration range investigated. The results indicate that iron is primarily an effective photo-oxidant, while cobalt and manganese can act both as photo-oxidant as well as thermo-oxidant.     

A comparative study on thermal and catalytic degradation of polybutylene terephthalate

A comparative study on the thermal and catalytic degradation of polybutylene terephthalate (PBT) at atmospheric pressure was conducted. The weight loss of PBT under thermal degradation was significantly influenced by the temperature between 360 °C and 380 °C, but little affected by the PBT particle size. Four groups of catalysts include metal chloride, metal oxide, metal acetate, and metal copper powder were used to test PBT degradation activity. Copper (II) chloride is the most active one for increasing the percentage PBT weight loss more than 100% in comparison with the result of thermal degradation at a temperature of 360 °C for 30 min. PBT and catalyst mixtures can be prepared by impregnation and physical method, the former resulted in a better PBT degradation. The percentage PBT weight loss in the presence of CuCl₂ increased steadily between 320 °C and 380 °C which was different from the results of thermal degradation. The time for obtaining a same percentage PBT weight loss reduced effectively when compared to the catalytic to thermal degradation. The weight ratio of CuCl₂/PBT was tested between 0 and 0.2 and the optimal ratio was 0.1. The gaseous product distribution analyzed by GC/MS for PBT thermal and catalytic degradation revealed almost the same and the major products were ethane, carbon monoxide, carbon dioxide, 1-butene, 2-butene, 1,3-butadiene, and butadiene dimmer. But the relative abundance of major products was changed, especially for 1,3-butadiene increased dramatically, and a new chlorocompound was produced in catalytic degradation. In condensed liquid product, both the number and the molar mass of components were more and greater than that of in gaseous product and 4-heptylacetophenone was the most abundance product. In PBT catalytic degradation, 4-heptylacetophenone and some products were decreased and some even disappeared completely while the abundance of benzoic acid increased and three new products were generated.     

Effect of dissolution-based recycling on the degradation and the mechanical properties of acrylonitrile-butadiene-styrene copolymer

A dissolution-based recycling technique for acrylonitrile-butadiene-styrene copolymer (ABS) is proposed, and the effects of repeated recycling cycles are studied measuring changes in chemical structure, melt viscosity, and tensile and impact properties. Acetone as solvent, 0.25 g/ml concentration, room temperature and 40 min for dissolution have been found to be the most reliable recycling parameters. FTIR, DSC and MFI results have shown that the dissolution-based recycling itself does not degrade the ABS. However, TGA analysis suggests that during the dissolution some stabilizers are probably eliminated, and consequently degradation takes place in the following injection moulding step. Darkening of recycled ABS is attributed to the butadiene degradation, pointed out by FTIR results. Otherwise, the chemical structure of the SAN matrix has not been modified, but its molecular weight has been reduced. The modulus of elasticity is not affected even after four recycling cycles. However, yield stress and impact strength decrease after the first recycling cycle, and remain constant in the following steps.     

Compatibilized polyamide 6/polyethylene blend-clay nanocomposites: Effect of the degradation and stabilization of the clay modifier

Blends of polyamide 6 (PA6) and high-density polyethylene (HDPE) were compatibilized using an already investigated method and a sample of Cloisite 15A, a montmorillonite modified with ammonium quaternary salts was added. The blends were prepared in a twin screw extruder and characterized from a morphological, rheological and mechanical point of view. The results indicated that, despite a good morphology achieved in the filled blends and a moderate intercalation of the clay, the mechanical properties are far from being good, especially the ultimate properties.In order to investigate the possible influence of the inhibition of the crystallization and of the degradation of the organic modifier of the clay, DSC measurements and FTIR-ATR were carried out. The results confirm that the clay causes a slight decrease of the crystallization, particularly in the HDPE phase. In addition, in the preparation conditions, the clay modifier is sensitive to thermo-oxidation. Both features can, therefore, explain the bad mechanical performance, even if the degradation effects seem to be more important. In order to prevent, or at least to reduce, the thermo-oxidation, a stabilizing system was added to the filled blends. In this case, the mechanical properties are improved for the entire compatibilized blend set.     

Kinetic study of the thermal degradation of high density polyethylene

The thermal degradation of high density polyethylene has been modelled by the random breakage of polymer bonds, using a set of population balance equations. A model was proposed in which the population balances were lumped into representative sizes so that the experimentally determined molecular weight distribution of the original polymer could be used as the initial condition. This model was then compared to two different cases of the unlumped population balance which assumed unimolecular initial distributions of 100 and 500 monomer units, respectively. The model that utilised the experimentally determined molecular weight distribution was found to best describe the experimental data. The model fits suggested a second mechanism in addition to random breakage at slow reaction rates.     

Effect of residual contaminants and of different types of extrusion processes on the rheological properties of the post-consumer polypropylene

Rheological measurements were conducted to verify the influence of different mechanical recycling processes and the presence of contaminants on the degradation of post-consumer polypropylene. Firstly, polypropylene (PP) was contaminated to simulate a post-consumer material, following the protocol recommended by the FDA. PP was subsequently recovered (washed and dried) and the samples were submitted to different extrusion processes. The rheological data demonstrated that the different types of processing applied and the presence of contaminants altered the molecular structure of the samples. The contaminants acted as agents that accelerated the polymer degradation. The contaminated samples submitted to higher shear rates exhibited greater decrease in their molar mass and a slight narrowing in the molar mass distribution. Also, it was observed that the most degraded samples showed decrease in their molar mass, in the viscosity and in the level of their molecular entanglements. These samples also exhibited a more Newtonian behavior and their molar mass distribution showed a slight narrowing. By calculating the ratio of the molar mass it was possible to quantify the degree of degradation of PP samples, confirming the results obtained.     

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.     

The thermo-oxidative degradation of metallocene polyethylenes. Part 1: Long-term thermal oxidation in the solid state

The long-term thermo-oxidative degradation of different metallocene polyethylenes (mPEs) was investigated in the solid state (oven ageing in air at 90 °C). The mPEs differed essentially according to their initial melt index, molar mass distribution, density and ash content. Two Phillips-type HDPEs were also studied for comparison. The rate of oxidation was assessed by carbonyl index measurements. Initial vinyl unsaturation and short chain branch contents were correlated with induction times. The hydroperoxide concentrations of the aged materials and the levels of metal catalyst residues were also evaluated. Outstanding oxidative stability was exhibited for all the mPEs in the solid state relative to the Phillips HDPEs. This was believed to be due to the low concentration of both adventitious metal catalyst residues and initial vinyl unsaturation. Degree of short chain branching also appeared to influence the oxidative stability of the mPEs. Besides inherent properties, the density/crystallinity appeared to be the main factor influencing their oxidative stability. In the case of the Phillips HDPEs, the high level of vinyl unsaturation together with chromium catalyst residues may contribute to their poor oxidative stability.     

Processing stability of polypropylene impact-copolymer during multiple extrusion - Effect of polymerization technology

Three commercially available polypropylene impact-copolymers (ICPP) produced by Innovene (INN), Spheripol (SPH) and Unipol (UNI) technologies were subjected to multiple extrusion using a twin-screw extruder W&P ZSK25 at 220 °C. Processing stability and changes in properties induced by extrusion were investigated. The materials were of similar MFR ∼6 dg/min, similar ethylene contents ∼7.5 wt. % and the same type and level 1200 ppm of phenol/phosphite stabilizer system was used. Ranking INN < UNI < SPH in processing and long-term (LTHA) stabilities observed was primarily related to the reactivity of catalyst residues rather than to other factors, such as contents of ethylene, quantity of extractables, EPR phase composition, levels of ash or individual elements in it. The mechanically demanding multiple extrusion conditions and consequently different extent of processing degradation, however, induced only minimum changes in morphology and impact strength of the solid ICPP matrix. Thus, regardless of changes in melt-flow properties induced by extrusions, all the three grades even after 5th extrusion at 220 °C exhibited Charpy notched impact strength at 23 °C only minimally changed.     

Thermal analysis applied to the characterization of degradation in soil of polylactide: II. On the thermal stability and thermal decomposition kinetics

The disposal stage of polylactide (PLA) was assessed by burying it in active soil following an international standard. Degradation in soil promotes physical and chemical changes in the polylactide properties. The characterization of the extent of degradation underwent by PLA was carried out by using Thermal Analysis techniques. In this paper, studies on the thermal stability and the thermal decomposition kinetics were performed in order to assess the degradation process of a commercial PLA submitted to an accelerated soil burial test by means of multi-linear-non-isothermal thermogravimetric analyses. Results have been correlated to changes in molecular weight, showing the same evolution as that described by the parameters of thermal stability temperatures and apparent activation energies. The decomposition reactions can be described by two competitive different mechanisms: Nucleation model (A2) and Reaction Contracting Volume model (R3). The changes in the kinetic parameters and kinetic models are in agreement with the calorimetric and dynamic-mechanical-thermal results, presented in the Part I of the study [1].     

Thermal stability and degradation of starch derivatives

Thermal behaviour of different starch derivatives, i.e. starch esters and ethers having degree of substitution (DS) in the range of 0.02–0.18 were studied. Potato, maize and wheat starches were used. Measurements were carried out by coupled thermal analysis/ mass spectrometry method (STA-MS) in air atmosphere. The major DTG peak during the investigation for starch derivatives is observed below 300°C. The mass loss up to a temperature of 300°C is about 50%. The most abundant ions found areH₂O⁺ and CO₂ ⁺. For the studied starch derivatives with a low degree of substitution (DS<0.18) no correlation was found between thermal stability and the level of substitution regardless of the nature of substitution.     

Thermal degradation behaviors of polybenzoxazine and silicon-containing polyimide blends

Thermal behaviors of polymer blends between common-type polybenzoxazine (PBA-a) and polysiloxane-block-polyimide (SPI) were studied using Dynamic Mechanical Analysis (DMA) and Thermogravimetric Analysis (TGA). The polymer blends showed only one glass-transition temperature (T_g) that increased as the content of SPI increased. Synergistic behavior in the char formation of the alloys was clearly observed. The DTG curves showed three stages and two stages of decomposition reaction in neat PBA-a and SPI, respectively. For the blending systems with 25 wt%, 50 wt%, and 75 wt% of SPI, the DTG thermograms of the blends exhibited four stages of thermal decomposition reaction. The apparent activation energies (E_a) of each step were determined using Kissinger method, Flynn-Wall-Ozawa method and Coats-Redfern method. The type of solid state mechanism was determined by Criado method. From the calculation, the solid state thermal degradation mechanism is proposed to be F1 (random nucleation with one nucleus on the individual particle) type for PBA-a, SPI, and their blends.     

Influence of the repeated extrusion on the degradation of polyethylene. Structural changes in low density polyethylene

The influence of the repeated extrusion on the molecular parameters of low density polyethylene (LDPE) Bralen NA 7-25 was studied. Virgin polyethylene was submitted up to 20 extrusion cycles and the processed samples were fractioned using precipitation fractionation. Non-fractionated samples and the individual polymer fractions were characterized by their weight average molar masses M_w (static light scattering), number average molar masses M_n (osmometry) and limiting viscosity numbers [η] (viscometry). Rheological properties in terms of shear viscosity curve, zero shear viscosity and flow activation energy were also determined by using high pressure capillary rheometer. The course of the changes in molecular parameters of LDPE is influenced both by the initial polymer structure and by the changes induced by the mechano-chemical degradation. The suggested degradation mechanisms during multiple extrusion of Bralen are chain scission predominating in the early stage of processing followed by recombination of macromolecules resulting in crosslinking and formation of microgel, which is clearly notable for the samples extruded 3-20 times.     

Analytical protocol to study the food safety of (multiple-)recycled high-density polyethylene (HDPE) and polypropylene (PP) crates: Influence of recycling on the migration and formation of degradation products

An analytical protocol was set up and successfully applied to study the food safety of recycled HDPE and PP crates. A worst-case scenario was applied that focused not only on overall migration and specific migration of accepted starting materials but also on migratable degradation products of polymers and additives that may be formed during mechanical recycling.The analytical protocol was set up to cover a wide variety of possible migrants. Identification and semi-quantification were possible for almost all migrants that increased significantly with increasing mechanical recycling steps for both the HDPE and PP crates.It was concluded that the analytical protocol was suitable to study the influence of (multiple) recycling on the food safety of plastic materials. The protocol can be applied to other plastic food contact materials and provides valuable information on the food safety of the recycling process and the resulting recycled food contact materials in addition to challenge testing.     

Thermal degradation studies of alkyl-imidazolium salts and their application in nanocomposites

Increasing the thermal stability of organically-modified layered silicates is one of the key points in the successful technical application of polymer-layered silicate nanocomposites on the industrial scale. To circumvent the detrimental effect of the lower thermal stability of alkyl ammonium-treated montmorillonite, a series of alkyl-imidazolium molten salts were prepared and characterized by elemental analysis, thermogravimetry (TGA) and thermal desorption mass spectroscopy (TDMS). The effect of counter ion, alkyl chain length and structural isomerism on the thermal stability of the imidazolium salts was investigated. Alkyl-imidazolium-treated montmorillonite clays were prepared by ion exchange of the imidazolium salts with Na-montmorillonite. These organically-modified clays were characterized by X-ray diffraction (XRD), TDMS and thermogravimetry coupled with Fourier transform infrared spectroscopy (TGA-FTIR), and compared to the conventional quaternary alkyl ammonium montmorillonite. Results indicate that the counter ion has an effect on the thermal stability of the imidazolium salts, and that imidazolium salts with PF₆⁻, N(SO₂CF₃)₂⁻ and BF₄⁻ anions are thermally more stable than the halide salts. A relationship was observed between the chain length of the alkyl group and the thermo-oxidative stability; as the chain length increased from propyl, butyl, decyl, hexadecyl, octadecyl to eicosyl, the stability decreased. The results also show that the imidazolium-treated montmorillonite has greater thermal stability compared to the imidazolium halide. Analysis of the decomposition products by FTIR provides an insight about the decomposition products which are water, carbon dioxide and hydrocarbons.     

Thermal stability of the copolymers of silicon-containing arylacetylene resin and acetylene-functional benzoxazine

Acetylene-functional benzoxazine resin (AFBEN) was used to modify silicon-containing aryacetylene resin (SCAAR). The thermal stability, degradation and ablation mechanisms were investigated. Based on the results of Py/GC/MS methods, we concluded that the cross-linking reactions of poly(dimethylsilyleneethynylene-phenyleneethynylene)(DMSEPE, a class of SCAAR) were due to (1) the Diels-Alder reaction between Ph-CC and CC, (2) the ring trimerization of CC and (3) radical polymerization of CC to form polyene structure. During the curing of AFBEN, polyene was the main product of the polymerization of ethynyl in AFBEN and the activation of the para positions of the aniline ring increased due to the introduction of ethynyl. XRD and XPS results showed that SiO₂ and SiC formed on the surface of the composites in the process of ablation.     

Thermal degradation of polystyrene in the presence of hydrogen by catalyst in solution

For the first time, low temperature degradation (170-240 °C) of polystyrene in benzene is carried out in the presence of hydrogen using iron(III) oxide catalyst. The effect of temperature, catalyst loading and polymer loading on degradation are studied in hydrogen atmosphere. Degradation is also carried out at different initial hydrogen partial pressure. The time dependent molecular weight is calculated using viscosity average method. It is found that the degradation is enhanced considerably in the presence of hydrogen and followed random degradation chain scission. A random degradation kinetic model of Kelen [Kelen T. Polymer degradation. New York: Van Nostrand Reinhold Company; 1983.] is used to estimate the degradation rate constants. Empirical correlations are proposed to account for the effect of catalyst loading and initial hydrogen partial pressure on degradation. The true thermal degradation rate constants are calculated using these proposed correlations at given catalyst loading and initial hydrogen partial pressure with varying temperature. The frequency factor and activation energy are also determined using Arrhenius equation considering the true thermal degradation rate constants.