Studies on the photo-oxidative degradation of LDPE films in the presence of oxidised polyethylene

This paper reports the results of photo-oxidative degradation studies of LDPE in the presence of varying amounts of oxidised polyethylene (OPE), which was prepared by heating LDPE films containing 0.1% cobalt stearate in oxygen atmosphere at 100 °C. OPE, with a CI of 12 was used as an additive for LDPE. Varying amounts of OPE (0.5-5%) were blended with polyethylene in an extruder and films of 70 μm thickness were prepared by film blowing process. The physico-chemical properties of the films were evaluated and these were found to be proportional to the amount of OPE. The films thus obtained were subjected to UV-B exposure at 30 °C for extended time periods. The chemical and physical changes induced by UV exposure were followed by monitoring the changes in mechanical properties (tensile strength and elongation at break), carbonyl index (CI), morphology, molecular weight, MFI and DSC crystallinity. Incorporation of OPE was found to be effective in initiating the photo-degradation of LDPE in relatively short span of time and the degradation was found to be proportional to the amount of OPE in the formulation.     

Accelerated aging of LDPE films containing cobalt complexes as prooxidants

This paper deals with the synthesis and characterization of two cobalt complexes, namely cobalt styrene maleate copolymer (CSMA) and cobalt stearate (CS) and studying the effect of these complexes on the degradation behavior of Low Density Polyethylene (LDPE) films. Seventy micron films containing these additives were prepared by sheeting process and were subsequently exposed to two different degradative environments, i.e. UV-B irradiation and heat. The degradation was monitored by measuring the changes in tensile strength, elongation at break, Carbonyl Index (CI), Melt Flow Index (MFI) and density. It was observed that although both the additives contained the same metal and similar type of bonding, CSMA was incapable of initiating thermal/photodegradation of LDPE while the analogous CS at the same concentration range significantly accelerated degradation. Films were also subjected to soil burial tests for a period of 1 year and the degradation was monitored by weight loss measurements. Multiple extrusions of LDPE were performed in the presence of the two additives to investigate their practicability for reprocessing. Overall migration of constituents from these films was determined in food simulants to evaluate the application of these films as food packaging material.     

Effect of benzil and cobalt stearate on the aging of low-density polyethylene films

We report an investigation of the effect of benzil and cobalt stearate on the degradation behaviour of LDPE films. Thin films (70 μ) containing these additives were prepared by sheeting process. The effect of heating, exposure to UV-B and natural weathering of LDPE films in the presence/absence of additives was investigated. Changes in the tensile properties, carbonyl index and density were used to investigate the degradation behaviour. Attempts have been made to correlate the results as a function of mixed additives. In contrast to the activity of typical triplet activators, benzil was found to be incapable of initiating thermal- or photo-degradation of LDPE films. However, an accelerated rate of oxidation was observed, primarily due to cobalt stearate, in the case of compositions containing a combination of benzil and cobalt stearate. All samples were found to be more susceptible to thermo-oxidation than to UV or natural weathering.     

Characterizing blends of linear low-density and low-density polyethylene by DSC

Summary A two-step isothermal annealing (TSIA) procedure is described that enables the endothermic peaks of low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE) and their blends, to be satisfactorily resolved during analysis by differential scanning calorimetry. A modified form of multistep isothermal annealing, the TSIA procedure produces a highly characteristic profile of the blend components by facilitating the segregation of the phases based on branch density. It is proposed that the TSIA procedure may have significant merit in the identification and quantification of the components in an unknown blend as well as increasing the sensitivity in analytical procedures aimed at blend component quantification.     

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.     

Multi-component analysis of low-density polyethylene oxidative degradation

The oxidative degradation of polyethylene in various conditions has been studied. In order to gain insight into the oxidation process, a method for the curve-fitting analysis of the IR carbonyl band between 1800 cm⁻¹ and 1600 cm⁻¹ in oxidized low-density polyethylene (LDPE) has been developed. Up to 10 components were needed to fit the band envelope, whose assignments and peak positions were based on the literature and on the synthesis of an appropriate model compound. The determination of the other band parameters, such as peak width and peak shape, necessary for reliable best fitting of the absorbance envelopes, was obtained by overall fitting optimization process. By using the available extinction coefficients, the quantitative determinations of the main oxidized species, i.e. ketones, carboxylic acids, esters, γ-lactones and ketoacids, were obtained with a reasonable confidence by rigorous parameter setting. The method was applied to the IR analysis of LDPE samples oxidized in different conditions (under thermal and irradiation stimulation), either as beads or films, as a function of time. Total hydroperoxide concentrations were also quantitatively estimated by a modified iodometric titration procedure. A good linear correlation between concentrations estimated by chemical titration and by intensity analysis of the free hydroperoxide IR band was observed.     

Stability of ZSM-11 and BETA zeolites during the catalytic cracking of low-density polyethylene

The stability of H-ZSM-11 (H-Z) and H-BETA (H-B) zeolites during the catalytic degradation of low-density polyethylene (LDPE) was studied using the same sample of catalyst in eleven consecutive cycles. The gaseous hydrocarbons, liquid hydrocarbons and waxes generated in each cycle were analyzed as well as the used catalyst. The zeolites were characterized by XRD, FTIR of adsorbed pyridine and N₂ adsorption, while the physical mixtures of LDPE/zeolites were subjected to TG-DTG analysis.The H-Z zeolite exhibited an important stability during the successive cycles of LDPE conversion. On the contrary, the behavior H-B zeolite was completely different; from the sixth cycle the yields of products changed progressively, approaching to that obtained in a purely thermal process.The yields of accumulated coke increased steadily throughout the cycles up to maximum values in the eleventh cycle of ∼6 and ∼15 wt% for H-Z and H-B, respectively. These results were confirmed by TG under air flow.     

Catalytic cracking of low-density polyethylene over H-Beta and HZSM-5 zeolites: Influence of the external surface. Kinetic model

The effect of the addition of H-Beta and HZSM-5 zeolites on the pyrolysis of commercial LDPE has been studied using dynamic experiments in a thermobalance. The H-Beta zeolite shows an important influence on the decomposition of the polymer even at a very low zeolite concentration. A remarkable effect in the first steps of LDPE decomposition on increasing the catalyst loading has been observed and a two-step process occurs in the presence of H-Beta zeolite. A pseudo-kinetic model has been proposed in order to adequately reproduce this behaviour. The model developed has been applied to simultaneously correlate the TG and DTG curves obtained under dynamic conditions at heating rates of 5, 10 and 35 °C/min and at several catalyst concentrations. The model is capable of simultaneously correlating all the experiments, thus demonstrating the validity of the hypothesis suggested and revealing a notable ability as a predictive model.     

Effect of sol-gel derived nano-silica and organic peroxide on the thermal and mechanical properties of low-density polyethylene/wood flour composites

The influence of nano-silica, synthesized and mixed with low-density polyethylene (LDPE) through a sol-gel process, on the thermal and mechanical properties of LDPE and LDPE/wood flour (WF) composites, prepared in the absence and presence of dicumyl peroxide, was investigated. Scanning electron microscopic (SEM) analyses show a uniform dispersion of silica nano-particles of size 10-50 nm in the matrix, and Fourier-transform infrared (FTIR) spectroscopic results indicated interaction between the nano-silica and the LDPE matrix, which seems to improve for samples prepared in the presence of dicumyl peroxide (DCP). WF and nano-silica, as well as the presence of DCP during sample preparation, substantially improve the thermal stability of the LDPE matrix. The tensile strength of the samples decreased with increasing wood flour content, while the tensile modulus substantially increased. The presence of nano-silica gave rise to lower values for both tensile strength and tensile modulus, while higher tensile strength (and an increase in tensile strength with WF content) is observed for samples prepared in the presence of DCP. The tensile modulus increases with increasing WF content, but is not substantially influenced by the presence of nano-silica or by sample preparation in the presence of DCP. The DMA results were in line with the tensile results.     

Evolution of products during the degradation of polyethylene in a batch reactor

The thermal degradation of two polyethylene samples (LDPE and HDPE) has been carried out in a batch reactor under dynamic conditions. The evolution of products generated after regular intervals of 5 min (temperature increments of approximately 25 °C) has been analyzed. The behaviour of LDPE and HDPE has been compared, and no differences in the quantity and weight fraction of the gaseous products obtained have been found. For both polymers, n-paraffins are the major products at the very beginning of the process, while as the decomposition proceeds 1-olefins are more abundant. The condensed fraction is much larger than the gaseous fraction and its analysis reveals some differences between the behaviour of LDPE and HDPE at the beginning of the degradation process. These differences disappear at higher temperatures where more similar trends are observed. 1-Olefins, n-paraffins, dienes and olefins with wide carbon number distributions are the most important condensed compounds obtained in the thermal degradation of both polyethylenes. The formation of 1-olefins and n-paraffins begins at slightly lower temperatures than for dienes and olefins. On the other hand, as the temperature increases, the amount of low and high molecular weight compounds increases at the expense of intermediate molecular weight products and the former become the most important by the end of the degradation process. This behaviour could be related to the thermal cracking of waxes through secondary reactions.     

Study on rheological properties and relaxational behavior of poly(dianilinephosphazene)/low-density polyethylene blends

Rheological properties and relaxational behavior of blends of low-density polyethylene (LDPE) and poly(dianilinephosphazene) (PDAP) have been investigated to check miscibility and molecular relaxations in the crystalline and amorphous phases. In the studied shear rate range, all PDAP/LDPE blends exhibited a shear thinning behavior. The experimental data were fitted using the logarithmic rule and serial model to investigate the miscibility of blends. It was found that LDPE and PDAP can achieve a certain degree of miscibility in the molten state. The dynamic mechanical α-, β- and γ-relaxation behavior obtained from dynamic mechanical thermal analysis imply that the two components in the amorphous phase were miscible. The wide-angle X-ray diffraction result showed that these two components interact with each other.     

Extraction and quantification of antioxidants from low-density polyethylene by microwave energy and liquid chromatography

In this paper a experimental design is applied to optimize the quantification of hindered phenol Irganox 1076, phosphite antioxidant Irgafos 168 and their oxidized product tri[2,4-di-tert-butylphenyl]phosphate from low-density polyethylene (LDPE). The developed analytical method consists of two steps: microwave-assisted extraction and reversed-phase liquid chromatography (LC) coupled with ultraviolet diode-array detector. A Plackett-Burman design was carried out in order to find the significant experimental parameters affecting the antioxidants extraction by microwave energy. These parameters were subsequently optimized by a central composite design. The performed method allows extracting the studied antioxidants at low temperature in a short time without degradation of phosphite antioxidant Irgafos 168.     

Blends of low-density polyethylene with nylon compatibilized with a sodium-neutralized carboxylate ionomer

An ethylene-methacrylic acid copolymer partially neutralized with sodium (Na-EMAA) was successfully used to compatibilize Nylon 6 (Ny6) and low-density polyethylene (LDPE) blends. The phase morphology and thermal behavior of these blends were investigated over a range of compositions using a variety of analytical techniques. The addition of small amounts (0.5 phr) of Na-EMAA improved the compatibility of Ny6/LDPE blends as evidenced by a significant reduction in dispersed phase sizes. TGA measurements demonstrated an improvement in thermal stability when Na-EMAA was added to either LDPE or Ny6. DSC results of Ny6/Na-EMAA binary blends showed that with increasing Na-EMAA content, the crystallization temperature of Ny6 phase decreased indicating that Na-EMAA retarded crystallization of Ny6. TGA and DSC results indicate that chemical reactions might have taken place between Ny6 and Na-EMAA, a hypothesis confirmed by the Molau test.     

COMFA of the acute toxicity of phenylsulfonyl carboxylates to Vibrio fischeri

From the Comparative Molecular Field Analysis (CoMFA) method, the paper describes a three-dimentional quantitative structure–activity relationship (3D-QSAR) model for the acute toxicity log EC₅₀ (15min-EC₅₀ in μmoll^-l) of 56 phenylsulfonyl carboxylates on Vibrio fischeri. The achievement of a high leave-one-out (LOO) cross-validated correlation coefficient q² of 0.790 with four optimum components indicates the significance of the correlation of the steric and electrostatic fields with the biological activities. The key features in the CoMFA contour maps are critical to trace the important properties and gain insight into the toxic mechanism of the tested phenylsulfonyl carboxylates.     

Effect of lubricants on the photo-oxidative degradation of solid poly(vinyl chloride)

Various commercially employed lubricants, such as low molecular weight polyethylene (Type PA-520 Wax), hydroxystearic acid (Loxiol G-21), butyl stearate (Loxiol G-32) and di-stearyl phthalate (Loxiol G-60), have been examined for their active rôle in the photo-oxidative degradation of poly(vinyl chloride) (PVC). The results show that low molecular weight polyethylene has an obvious accelerating effect on the photo-oxidative degradation of PVC, whereas other lubricants reduce the effect of photo-oxidation. None of the lubricants used has an obvious rôle in the chain scission and crosslinking of PVC.     

Thermooxidative degradation of polyethylene. I and II. Structural changes occurring in low-density polyethylene,high-density polyethylene,and tetratetracontane heated in air

Samples of low-density polyethylene (LDPE), high-density polyethylene (HDPE), and tetratetracontane (n-C₄₄H₉₀) free from additives were heated in air at temperatures between 120 and 180°C. As a comparison, “as received” HDPE containing unspecified additives has also been included. The structural changes have been studied with gel chromatography, viscometry, infrared spectroscopy, differential scanning calorimetry, and gravimetric measurements. LDPE, HDPE, and n-C₄₄H₉₀ follow the same course of thermooxidative degradation when they are free from additives and present in the molten state. Both molecular-diminishing and enlargement reactions occur. At temperatures below 150°C molecular enlargement is not observed until after rather long exposure times, whereas at higher temperatures enlargement occurs immediately. The difference is because “peroxide curing” becomes increasingly important above 150°C, whereas ester formation is operating at all temperature levels. Degradation below T_m is restricted to the amorphous phase that results in a different degradation pattern. In accelerated testing work extrapolations of the Arrhenius type in the prediction of structural change are thus not justified, even within the actual narrow temperature range. Neither are changes in commonly used standards like carbonyl content justified as a measure of the changes; for example, in mechanical properties. The stabilizer in the unpurified HDPE not only influences the induction period but also the course of the thermooxidative degradation.     

Structural chemistry of organotin carboxylates: a review of the crystallographic literature

This review describes the structural chemistry of organotin carboxylates, covering data acquired for mono-, di- and tri-organotin compounds and complexes. A brief discussion is given for organotin amino-acid derivatives.     

The action of anthraquinone sensitizers in the photo-oxidative degradation of low density polyethylene: Mechanical evidence of dark processes

Low density polyethylene (LDPE) films containing three quinoid sensitizers, anthraquinone (I), 2-butylanthraquinone (II) and 2-octanoyloxyanthraquinone (III), have been exposed to the uv light of a fluorescent tube. The kinetics of the photo-oxidative degradation have been monitored by measuring infra-red spectra and three mechanical characteristics derived from stress-strain traces, viz. strain at break, Young's modulus and yield stress. During the first stage of the process, the greater solubility of (II) and (III) in LDPE as compared to (I) was clearly manifested in both the mechanical and spectral data. The sensitizers are consumed at the very beginning of the exposure, but once the degradation starts it continues at a higher rate around introduced or intrinsic centers. The increase of yield stress values after a long-term dark storage which followed the irradiation period is ascribed to a slow continuous crystallization of the degraded polymer. No dark storage effect has been found with samples not previously exposed to uv light.     

Adhesive effect of low-density polyethylene gels on polyethylene moldings

Adhesive effect of low density polyethylene (LDPE) gels in organic solvents such as decalin, tetralin, ando-dichlorobenzene on high density polyethylene (HDPE) moldings has been investigated by shearing tests, electron microscopy, and DSC measurements. When heated at 110°C for 2 h, all of the gels showed strong adhesive strengths around 30 kg/cm², which is sufficiently strong for practical uses. It has been found that the adhesive strength increases with the heating temperature and that the temperature at which the heated gel begins to exhibit the adhesive effect depends upon solvents and is about 30° lower than that of the HDPE gels.