Degradation Of High Density Polyethylene Research Articles

Applicability of non-isothermal model-free predictions for assessment of conversion vs. time curves for complex processes in isothermal and quasi-isothermal conditions

The non-isothermal model-free prediction procedures used for assessment of degradation isotherms of materials and products are presented and critical analyzed. To this end, these methods were applied to the following experimental data: thermo-oxidative degradation of poly (vinyl alcohol) (PVA) and thermal degradation of high density polyethylene (HDPE). The predicted conversion vs. time curves were compared with the experimental ones obtained in quasi-isothermal conditions as well as with those calculated using the degradation mechanisms and corresponding kinetic parameters obtained by Thermokinetics software package. The errors of predictions performed by model-free and model-based procedures were evaluated. Although the used software package considers relative simple reaction schemes containing maximum six steps, the predicted conversion vs. time curves by model-based procedure exhibit lower deviations from the experimental ones than those calculated by model-free procedures. The causes of the prediction errors are discussed.

An iterative model-free method to determine the activation energy of heterogeneous processes under arbitrary temperature programs

A new iterative model-free (isoconversional) method with integration over a given range of conversion for the determination of the activation energy using non-isothermal data recorded at arbitrary temperature programs has been suggested. The advantages of applying this method in comparison with differential linear and nonlinear methods and advanced nonlinear method are put in evidence. The applicability of the suggested method was checked for simulated data for a single-step process, thermal decomposition of calcium carbonate, and thermal degradation of high density polyethylene.

Thermal degradation of rice-bran with high density polyethylene: A kinetic study

Thermal degradation behavior of mixtures of rice bran (RB) and high density polyethylene (HDPE) was investigated by thermo-gravimetric analyses (TGA) under dynamic conditions in nitrogen atmosphere and was compared with that of individual materials. Experiments were carried out in the range of ambient temperature to 900 °C at two heating rates (5 and 20 °C per minute). Kinetic analysis indicated that activation energy for pyrolysis of RB, HDPE and those for RB-HDPE mixtures varied with rate of heating as well as with the three temperature ranges. This variation has been explained on the materials’ decomposition behavior. Maximum difference between experimental and theoretical mass loss (Δm) was 26% at 475 °C and 34% at 489 °C at the heating rates of 5 and 20 °C per minute, respectively. These maxima indicate stronger interactions at corresponding temperature between RB and HDPE during copyrolysis. Reduction in activation energy for pyrolysis, lower temperatures at which rate of decomposition is highest, and negligible quantity of the residue suggest a synergism between thermal degradation of RB and HDPE.

Thermal-catalytic degradation of polyethylene over silicoaluminophosphate molecular sieves—A thermogravimetric study

The present work is aimed at recycling plastic wastes economically and efficiently, for which pure high density polyethylene (HDPE) has been initially selected for the investigations. Thermogravimetric technique has been used to investigate, analyze and compare the thermal and catalytic degradation of HDPE. The catalytic degradation was investigated over the medium pore silicoaluminophosphate, SAPO-11 molecular sieve. The thermogravimetric evaluation was performed using 2–30 wt% catalyst, and the apparent activation energies for the thermal and catalytic polymer degradation were estimated using various iso-conversional methods. The apparent activation energy was found to be lower when SAPO-11 was used compared to the direct thermal degradation of HDPE. The activation energy and coke levels are comparable to the medium pore zeolite ZSM-5 and lower than the values obtained over large pore zeolites reported in literature.

Thermal Degradation of HDPE in a Batch Pressure Reactor: Reaction Time and Mechanical Stirring Effect

The effect of reaction time and mechanical stirring on thermal degradation of high density polyethylene(HDPE) was studied at 350°C under nitrogen atomosphere in a batch pressure reactor. Changes in molecular weight(MW), molecular weight distribution (MWD), and crystalline behaviors of the degraded products were investigated by gel chromatography (GPC) and differential scanning calorimetry (DSC). It was found that MWD curves all shifted toward lower molecular weight with increasing reaction time, with both the extent of the movement and its showing a rapid initial drop and then leveling off. In a short period of reaction time, the MW, MWD and crystalline behaviors of the degraded products were affected notably by the mechanical stirring. The of the degraded products without stirring was lower than that of products with stirring in the same time, which should be related to the large difference of temperature distributions in the reactor. When the reaction time reached 4 h, the of the degraded products had dropped to about 5 × 103g/mol from about 3 × 105g/mol for the original , and the product did not show the melting and crystallization behaviors of high density polyethylene again.

Effect of catalyst loading on kinetics of catalytic degradation of high density polyethylene: Experiment and modelling

Catalytic degradation of high density polyethylene (HDPE) using silica-alumina has been investigated in a thermogravimetric analyser, and the degradation kinetics determined using a population balance model recently developed in our laboratory. The incorporation of multisite adsorption into the model greatly improved the fit to experimental data. It is proposed that both thermal and catalytic cracking occur simultaneously, effectively through a two-step process: cracking of the large initial polymer molecules dominated by the catalyst with an activation energy of approximately 174 kJ/mol, followed by further breakage strongly influenced by thermal cracking with an activation energy of approximately 256 kJ/mol, so that it is the thermal degradation that is especially responsible for over-cracking and formation of gaseous products. In addition, it is found that the pre-exponential factor has a linear dependence on the catalyst loading. The breakage kernel used in the model allows for random scission, with mid-point being the most probable, so that the product distribution does not comprise a single peak moving smoothly through time—but peaks form at several discrete sizes. The model can predict product distributions at various conditions; however, as the model does not incorporate any specific mechanisms for adsorption and reaction, more direct investigation of the product distributions is also needed. This is of industrial importance as these products are economically attractive for the production of liquid fuels. The required reaction time can be predicted for a specific product distribution.

Thermo‐oxidative degradation of HDPE as a function of its crystalline content

AbstractThe composition, the thermal properties, and the kinetics of the thermo‐oxidative degradation of high‐density polyethylene (HDPE) were studied as a function of the increasing crystalline fraction, which resulted from the selective extraction of the amorphous part, through digestion by immersion in fuming nitric acid (HNO3) for different periods of time. The chemical and thermodynamic changes in HDPE, brought about by digestion in nitric acid for different periods of time, are discussed. Changes in the chemistry and microstructure of the HDPE, as a function of acid treatment for different periods of time, were studied using infra‐red spectroscopy (FTIR), gel permeation chromatography (GPC), and thermal analysis (DSC and TGA), as well as scanning electron microscopy (SEM). These studies were carried out as a function of the extracted amorphous fraction of HDPE samples via digestion in HNO3. These studies showed that in the first stages of the acid chemical attack, the amorphous part first undergoes a chemical modification and then dissolves into the strong acid medium. The total crystalline fraction apparently decreases during the first stages of the chemical attack and then increases as the amorphous part is extracted. TGA results show that as the selective extraction of the amorphous part occurs, there is a displacement of the thermo‐oxidative degradation toward higher temperatures. The kinetics of the thermo‐oxidative degradation as a function of the extraction of the amorphous part was followed according to the Horowitz‐Metzger method, and it was found that as the concentration of the crystalline fraction increases, the activation energy for the thermo‐oxidative degradation increases. SEM studies show that the extraction of the amorphous part does not affect the size of the crystalline lamellar thickness of HDPE. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1906–1915, 2009

Comparative study of the effect of different nanoparticles on the mechanical properties, permeability, and thermal degradation mechanism of HDPE

AbstractIn the present study, different series of high‐density polyethylene (HDPE) nanocomposites were prepared by melt mixing on a Haake‐Buchler Reomixer, containing 2.5 wt % of multiwall carbon nanotubes, pristine and modified montmorillonite, surface‐treated and ‐untreated SiO2 nanoparticles. From transmission electron micrographs, it was found that beyond a fine dispersion of nanoparticles into HDPE matrix, there are also some aggregates easily discriminated. As a result, there was a decrease in the tensile and impact strength of most of nanocomposites except Young's modulus, which was increased. Storage modulus as recorded from dynamic mechanical analysis was also increased in all nanocomposites, because HDPE becomes stiffer due to the incorporation of nanoparticles. The nucleation behavior of nanoparticles during crystallization was found to have no obvious effect on melting and crystallization temperature of HDPE. However, a small decrease in the heat of fusion in all nanocomposites was evidenced. Gas permeability of HDPE matrix in O2, N2, and CO2 was reduced in all nanocomposites compared with neat polymer. Thermal stability of HDPE was also enhanced due to the incorporation of different nanoparticles. From the kinetic analysis of thermal decomposition of HDPE, it was concluded that to describe the thermal degradation of HDPE and the studied nanocomposites, two consecutive mechanisms of nth‐order autocatalysis have to be considered. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Following the First Steps of UV Degradation of High Density Polyethylene by Fluorescence Spectroscopy

Ultraviolet radiation is one of the major factors that affect weathering of polymers. Indeed, diffusion of oxygen containing groups [-C-O, C=O and C(=O)O-] into the molecular chains of polymers is particularly enhanced by UV irradiation in air, thus entailing structural and chemical changes such as polymeric chain breaking, cross-linking and oxidation. The goal of this study is to predict the photo-chemical ageing of polymers upon UV radiation through a non destructive method: fluorescence spectroscopy. In the case of high density polyethylene (HDPE) the addition of fluorophore is required to make it fluorescent. The spectral features of fluorescence are very sensitive to the interactions between the excited molecules and the matrix. In this way, Fourier-Transform InfraRed spectrophotometry (FTIR) and Differential Scanning calorimetry (DSC) have been employed to correlate the fluorescence spectra with the degradation phenomena.

Influence of cleaning conditions on the degradation of recycled HDPE

AbstractOne of the materials most commonly found in municipal plastic waste (MPW) is High Density Polyethylene (HDPE), which is used particularly for food packaging and cleaning product containers. Postconsumer HDPE is generally contaminated with other impurities and, therefore, a cleaning process is necessary before it can be reprocessed. The aim of this study was to investigate the influence of the cleaning conditions on the oxidative degradation of postconsumer HDPE. These plastic materials were washed in NaOH solution, liquid detergent (D), and a mixture of NaOH/D (1 : 1). Results for the melt flow rate (MFR) and capillary rheometry showed that NaOH has a great influence on the oxidative degradation of HDPE. The HDPE treated with NaOH solution had a lower decomposition temperature, observed through Thermal Gravimetric (TG) analysis, and a lower the Oxidative Induction Time (OIT), obtained via Differential Scanning Calorimeter (DSC). Finally, the oxidative degradation of HDPE was verified through a higher carbonyl index. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Preparation and Characterization of Partially Degraded High-Density Polyethylene in an Internal Mixer

The partial degradation of high-density polyethylene was done in an internal mixer for different residence time. The oxidative thermomechanical degradation resulted in the formation of carbonyl groups in the polymer. The carbonyl index values estimated from the FTIR studies indicate the increase of carbonyl groups without any induction time. The common phenomenon during oxidative degradation of polyethylene is the increase in trans-vinylene groups and decrease in vinyl groups and these phenomena are observed in the current investigation. Owing to the chain scission occurring during the mixing at processing temperature, the change in morphology was studied by the amount of crystallinity estimated through DSC. The variation in the molar mass of the material was followed by SEC method.

Thermal degradation of polyethylene modeled on tetracontane

The thermal degradation of tetracontane as an idealized monodisperse “polymer” of 40 carbon atoms was modeled based on a detailed reaction mechanism consisting of simultaneous or subsequent H-abstraction, β-scission and backbiting reactions (intramolecular H-shifts). A stiff differential equation solver with a postprocessor program to compute the concentrations of each species and the contribution of each reaction to their production was used. The model correctly predicts degradation at low conversion and indicates objectives for further research to improve accuracy at higher conversions. Rate of production analysis quantifies reaction pathways contributing to the formation and consumption of species and reveals specific H-abstraction and β-scission reactions, the complex impact of backbiting reactions, interdependence between production rates of alkyl and alkenyl radicals, dominant role of small alkyl radicals. The data obtained by the model are compared to the experimental product distribution of the degradation of high density polyethylene measured at 500 °C and 20 s reaction time in a micropyrolizer reactor.

Thermal degradation of high density polyethylene in a reactive extruder

The thermal degradation of high density polyethylene was conducted in a reactive extruder at various screw speeds with reaction temperatures of 400 °C and 425 °C. The residence time of the extruder was estimated and the molecular weight distribution of the fed plastic and reaction products was analysed using gel permeation chromatography. A continuous kinetic model was used to describe the degradation of the high density polyethylene in the reactive extruder. The breakage kernel and the scission rate model parameters were estimated from the experimental data for a variety of cases. It was found that purely random breakage and a scission rate which had a power law dependence on molecular size of 0.474 best described the experimental data.

Feedstock recycling of polyethylene over AlTUD-1 mesoporous catalyst

The catalytic degradation of high density polyethylene (HDPE) was investigated using AlTUD-1 as catalyst, a recently discovered mesoporous aluminosilicate. The catalytic activity of AlTUD-1 was evaluated by TGA measurements, using a polymer/catalyst ratio of 9:1. AlTUD-1 has a Brønsted acidic behaviour, three-dimensional (3D) connectivities and a pore diameters between 2 and 50 nm. Compared to HY zeolite, the large pore size of AlTUD-1 enhances a selective catalytic degradation of the polymer and prevents rapid deactivation. Moreover, the apparent activation energy of polymer cracking is much lower than with HY zeolite. For these reasons, AlTUD-1 is a potentially interesting catalyst for the catalytic cracking of plastic waste into liquid fuels.

Waste catalysts for waste polymer

Catalytic cracking of high-density polyethylene (HDPE) over fluid catalytic cracking (FCC) catalysts (1:6 ratio) was carried out using a laboratory fluidized bed reactor operating at 450 °C. Two fresh and two steam deactivated commercial FCC catalysts with different levels of rare earth oxide (REO) were compared as well as two used FCC catalysts (E-Cats) with different levels of metal poisoning. Also, inert microspheres (MS3) were used as a fluidizing agent to compare with thermal cracking process at BP pilot plant at Grangemouth, Scotland, which used sand as its fluidizing agent. The results of HDPE degradation in terms of yield of volatile hydrocarbon product are fresh FCC catalysts ≫ steamed FCC catalysts ≈ used FCC catalysts. The thermal cracking process using MS3 showed that at 450 °C, the product distribution gave 46 wt% wax, 14% hydrocarbon gases, 8% gasoline, 0.1% coke and 32% nonvolatile product. In general, the product yields from HDPE cracking showed that the level of metal contamination (nickel and vanadium) did not affect the product stream generated from polymer cracking. This study gives promising results as an alternative technique for the cracking and recycling of polymer waste.

Recycling of Polymer Waste with Fluid Catalytic Cracking Catalysts

Feedstock recycling of high-density polyethylene (HDPE) over fluid catalytic cracking (FCC) catalysts (1:6 ratio) was carried out using a laboratory fluidized bed reactor operating at 450°C. Fresh and steam deactivated commercial FCC catalysts with different levels of rare earth oxide (REO) were compared as well as used FCC catalysts (E-Cats) with different levels of metal poisoning. Fresh FCC catalysts gave the highest results of HDPE degradation in terms of yield of volatile hydrocarbon product. Meanwhile, steamed FCC catalysts and used FCC catalysts showed similar but lower yields. Overall, the product yields from HDPE cracking showed that the level of metal contamination (nickel and vanadium) did not affect the product stream generated from polymer cracking. This study gives promising results as an alternative technique for the cracking and recycling of polymer waste.

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.

Catalytic degradation of high density polyethylene over microporous and mesoporous materials

High-density polyethylene (HDPE) was converted to a mixed of lower and higher hydrocarbons over micro (ZSM-11 zeolite with MEL structure) and mesoporous (MCM-41) modified materials. The main liquid hydrocarbons (LHC) were: benzene, toluene, xylenes (BTX), using Zn-, Mo-, and H-containing MEL-zeolites as catalysts. The Zn-zeolite exhibited the highest levels of LHC (>57 wt%) with BTX selectivity of 47 wt%. H-zeolite gave a higher fraction of gaseous hydrocarbons (C1–C4) than Mo- and Zn-zeolites. Zn–MCM-41 produced mainly C5–C16 products. LPG (C3–C4) levels were interesting for all the samples. Thermo gravimetric analyses studies of mixtures catalyst/polymer, have been used to investigate the performance of different catalysts on degradation reactions of the polymer. The catalytic transformation of the polyolefin occurs at lower temperature with respect to thermodecomposition of the pure HDPE (500 °C). A one-pass reactor in the temperature range of 410–500 °C and atmospheric pressure was employed. We observed the complete conversion of HDPE, in the range of temperatures studied (410–500 °C). The synthesized and modified samples after cation incorporation were characterized by TG–DSC, XRD, TPD-FID, FTIR and surface area by BET, in order to verify and to confirm the crystallinity (for microporous materials) and regularity (for mesoporous materials) of the structures, stability and the type of active sites. The reactant was characterized by FTIR, XRD and TG–DSC. Reaction products were analyzed by gas chromatography. By means of development of a competitive catalytic system, we determine the feasibility of the process of degradation of HDPE remainders towards the synthesis of gaseous and liquid hydrocarbons.

The role of chain scission and chain branching in high density polyethylene during thermo-mechanical degradation

The mechanical and thermo-oxidative degradation of high density polyethylene (HDPE) was measured in a twin-screw extruder using various processing conditions. Two types of HDPE, Phillips and Ziegler-Natta, having different levels of terminal vinyl unsaturation were analysed. Mild screw profiles, having mainly conveying elements, have short mean residence times then profiles with kneading discs and left hand elements. Carbonyl and trans-vinylene group concentrations increased, whereas vinyl group concentration decreased with number of extrusions. Higher temperature profiles intensified these effects. The thermo-mechanical degradation mechanism begins with chain scission in the longer chains due to their higher probability of entanglements. These macroradicals then react with the vinyl terminal unsaturations of other chains producing chain branching. Shorter chains are more mobile, not suffering scission but instead are used for grafting the macroradicals, increasing the molecular weight. Increase in the levels of extrusion temperature, shear and vinyl end groups content facilitates the thermo-mechanical degradation reducing the amount of both, longer chains via chain scission and shorter chains via chain branching, narrowing the polydispersity. Phillips HDPE produces a higher level of chain branching than does the Ziegler-Natta type.

Catalytic degradation of high density polyethylene over mesoporous and microporous catalysts in a fluidised-bed reactor

High density polyethylene (HDPE) was pyrolysed over various catalysts using a laboratory fluidised-bed reactor operating isothermally at ambient pressure. HZSM-5 catalysed degradation resulted in much larger amounts of volatile hydrocarbons compared with degradation over non-zeolitic catalysts (MCM-41 and SAHA). When an HZSM-5 was used as a cracking additive in combination with a non-zeolitic catalyst (MCM-41 and SiO 2–Al 2O 3), the solid mixed catalysts produced less gas with a lower loss of gasoline than HZSM-5. MCM-41 with large mesopores and SAHA with weaker acid sites resulted in a highly olefinic product and gave rise to the broadest carbon range of C 3–C 7. Both SAHA and MCM-41 materials allow bulky reactions to occur leading to the generation of coke and subsequently deactivation of the catalyst. This paper presents the conversion of polymers to useful hydrocarbons using various cracking catalysts and attempts to provide a basis for optimising the potential benefit of catalytic polymer recycling.