Effect of cobalt carboxylates on the photo-oxidative degradation of low-density polyethylene. Part-I

The effect of three cobalt carboxylates of increasing chain length, namely cobalt laurate, cobalt palmitate and cobalt stearate on the photo-oxidative degradation of low-density polyethylene (LDPE) films has been investigated. LDPE films containing cobalt carboxylates were irradiated with UV-B light at 30 °C for extended time periods. FTIR spectroscopy, mechanical testing, morphological studies, molecular weight, density and MFI measurements were performed to monitor the degradation behaviour. The results of these studies were analysed to explain the structural and chemical modifications taking place in the polyethylene matrix due to UV-B exposure. FTIR studies indicate that the degradation is dominated by formation of carbonyl and vinyl species. The studies on mechanical properties reveal that samples containing cobalt carboxylates, become mechanically fragile after UV exposure for 400 h, while neat LDPE exhibits insignificant changes during this period. The degradation was found to increase proportionally with increasing chain length and follows the order CoSt₃ > CoPal₃ > CoLau₃. Migration studies were performed on food simulant systems to investigate the applicability of these films for food packaging.     

Degradation of pre-aged polymers exposed to simulated recycling: Properties and thermal stability

Accelerated thermal and photo-aging of four homopolymers, low-density polyethylene (LDPE), high-density polyethylene (HDPE), polypropylene (PP) and high-impact polystyrene (HIPS), was performed and the impact of subsequent reprocessing conditions on their properties studied. Polymer samples oven-aged at 100 °C for varying periods of time or UV irradiated in a Weather-o-meter (WOM) at λ = 340 nm were reprocessed in a Brabender plasticorder at 190 °C/60 rpm for 10 min. Chemical changes and the evolution of rheological and mechanical properties accompanying the gradual degradation of the individual polymers were monitored and evaluated (DSC, FTIR, colorimetric method, MFI, tensile impact strength). LDPE and HIPS were found to be more susceptible to thermo-oxidation than HDPE and PP, whereas HDPE and PP were affected to a greater extent by UV exposure; the crucial role here is being played by the stabilization of the studied resins. In HDPE the scission and crosslinking reactions competed both in thermo-and photo-degradation. In the case of LDPE, scission prevailed over branching during thermo-oxidation, whereas photo-oxidation of the same sample led predominantly to crosslinking. Abrupt deterioration of the LDPE rheological properties after one week of thermal exposure was suppressed by re-stabilization. The scission reaction was also predominant for PP during thermo-oxidation, and it took place even faster during UV exposure. In the case of HIPS a slight photo-degradation of PS matrix is accompanied by simultaneous crosslinking of the polybutadiene component.     

A novel zinc chelate complex containing both phosphorus and nitrogen for improving the flame retardancy of low density polyethylene

A novel metal chelate complex containing phosphorus, nitrogen and zinc (II) ion was synthesized and used as the flame retardant of low density polyethylene (LDPE). The zinc chelate complex was synthesized by reacting zinc acetate with the ligand of tetraethyl (1,2-phenylenebis(azanediyl)) bis (2-hydroxylphenylmethylene) diphosphonate (TEPAPM). The chemical structure of the target Zn-TEPAPM was confirmed by FTIR, ¹H NMR, ¹³C NMR and ³¹P NMR spectra. The flame retardancy and thermal behavior of LDPE containing various amount of Zn-TEPAPM were investigated by limiting oxygen index test, thermogravimetric (TG) analysis and cone calorimetry. The results show that Zn-TEPAPM can greatly increase the thermal stability, the char formation and smoke suppression ability of LDPE. The TG curves show that even when the filler content of Zn-TEPAPM is as low as 1 wt% in LDPE, the onset degradation temperature of LDPE is increased from 429 °C to 442 °C, and the maximum degradation temperature is increased from 469 °C to 488 °C. Also, a reduction of 32% for the peak heat release rate (PHRR) is obtained in the cone test. Moreover, Zn-TEPAPM is demonstrated to be a very effective synergist of ammonium polyphosphate (APP). When 1 wt% of Zn-TEPAPM was introduced into the LDPE/APP (mass ratio 80/19) blend, the PHRR value is reduced by 32%, compared with that of LDPE/APP blend without Zn-TEPAPM, and the char layer becomes more compact and intact.     

Mn2Al-LDH- and Co2Al-LDH-stearate as photodegradants for LDPE film

The layered double hydroxides (LDH) Mn₂Al-LDH-stearate and Co₂Al-LDH-stearate were prepared by a surfactant-assisted intercalation of the corresponding precursor LDH-CO₃ forms. These compounds were evaluated as potential photodegradant additives in low density polyethylene films with a thickness of ca. 40 μm. They were incorporated into blown polyethylene films via a 10% masterbatch. The films were subjected to accelerated ageing in a QUV weatherometer. The machine was fitted with A320 lamps and operated on a dry cycle at 63 °C and an irradiance of 0.67 W/m². It was found that 100 h of QUV exposure was sufficient to cause mechanical embrittlement of films containing as little as 0.1% of either active additive.     

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.     

Natural weathering test for films of various formulations of low density polyethylene (LDPE) and linear low density polyethylene (LLDPE)

Natural (outdoor) weathering test was performed to investigate the UV stability of thin films (0.06 mm) of linear low density polyethylene (LLDPE) and low density polyethylene (LDPE). The PE films were prepared from various formulations of LLDPE and LDPE resins. Some of these films contained a single high molecular mass HALS only, along with a primary antioxidant (i.e. Irganox 1010) and a secondary antioxidant (i.e. Irgafos 168 or Alkanox TNPP), while others contained HALS and UVA (i.e. Chimassorb 81 or Tinuvin P or Tinuvin 326) along with these antioxidants. The HALS used was either an oligomeric or a synergistic mixture of a high molecular mass (HMM) hindered amine stabilizer and co-additives. The UV stability was investigated by exposing the prepared films at 45° towards south in the direct sunshine up to 365 days. Fifty percent of tensile strength retention was determined for all these exposed films and it was found that the films containing a single HALS gained improved UV stability by about two to 12 fold over the pure films. On the other hand, films that contained a combination of HALS and UVA obtained further improved UV stability over the films containing a single HALS (both have antioxidants). Films containing a single HALS reached 50% TS retention within 205 days, whereas, films containing a combination of HALS and UVA reached 50% TS retention within 590 days, which is about three times further improvement in UV stability.     

Synthesis of TRITON™ X-based phosphate ester surfactants and their self-charring behavior

This paper describes the synthesis and characterization of a series of TRITON™ X-based surfactants with a predominantly alkyl phenol ethoxylate (APE) backbone and a phosphate ester chain end. Four phosphate-terminated TRITON™ X (or APE) derivatives (OPE2-OPO(OH)₂, OPE5-OPO(OH)₂, OPE10-OPO(OH)₂, and NPE10-OPO(OH)₂) were prepared from commercially available octyl phenol ethoxylate (OPE) of different oxyethylene units (n = 2, 5 and 10), nonyl phenol ethoxylate (NPE) of 10 oxyethylene units and phosphorous pentoxide via a simple condensation reaction. Depending on their composition and chain length of oxyethylene units used in the reaction, the surfactants show different self-charring behaviors. The phosphate-terminated TRITON™ X of the lowest number of oxyethylene units (i.e. OPE2-OPO(OH)₂) gives the largest amount of char (up to 23 wt%) at 600 °C under air condition. The carboxylic acid-terminated TRITON™ X derivatives (i.e. OPE-COOH) were also tested for comparison.     

Degradation and stabilisation of poly(ethylene-stat-vinyl acetate): 1 - Spectroscopic and rheological examination of thermal and thermo-oxidative degradation mechanisms

The degradation of ethylene vinyl acetate (EVA) copolymers was compared with low density polyethylene (LDPE), poly(vinyl acetate) (PVAc) and poly(vinyl chloride) (PVC) using FTIR, UV-visible and fluorescence spectroscopy as well as thermal and rheological analyses. Thermal, thermo-oxidative and photo-oxidative studies were conducted. Thermo-oxidation below 180 °C shows more similarities between EVA and LDPE. The luminescence spectra of degraded EVA and LDPE were almost identical but very different to that of PVAc. UV-vis analysis showed that the polyenes present in aged PVC were unlikely to be the same species responsible for the observed colour formation in aged EVA. It is suggested that they are polyconjugated carbonyl products. Rheological analysis also showed the evolution of crosslinking reactions during thermo-oxidation. FTIR studies after thermal degradation in inert conditions 290 °C showed complete loss of the ester functionality and associated lactone formation along with some evidence for ketonic and unsaturated carbonyl groups. Degradation in air at 180 °C, however, revealed that loss of the ester group was not so marked, with PVAc exhibiting the greatest stability. This was in line with the induction time to onset of autocatalytic carbonyl growth at 180 °C; the latter showed an apparent exponential decrease with increasing vinyl acetate content up to 28% w/w. Fluorescence analysis produced trends that complemented those of carbonyl index; the time to decomposition of initial fluorescent α,β-unsaturated carbonyl species coincided with the time to onset of carbonyl growth. Furthermore, the rate of formation of the new fluorescent species produced in EVA, and LDPE was similar to that of carbonyl growth. These new fluorescent species are therefore likely to be di- or tri-carbonyl products.     

Evaluation of the rate of abiotic degradation of biodegradable polyethylene in various environments

The rate of abiotic degradation of polyethylene (PE) films containing a manganese pro-degradant has been studied in various environments at 60 and 70 °C. The degradation was monitored from the change in molecular weight and the elongation at break after exposure to dry and humid air. It was observed that moisture had a strong accelerating effect on the rate of thermo-oxidation of PE films. However, despite the humidity level in the compost environment being similar to that in humid air, the rate of degradation in compost was much slower. It is proposed that ammonia and/or hydrogen peroxide generated by microorganisms in the compost can be responsible for the deactivating effect, as aqueous solutions of these compounds significantly retard the rate of degradation.     

Degradation of abiotically aged LDPE films containing pro-oxidant by bacterial consortium

The degradation of abiotically aged low density polyethylene (LDPE) films containing trace quantities of a representative pro-oxidant (cobalt stearate) was investigated in the presence of well defined enriched microbial strains namely, Bacillus pumilus, Bacillus halodenitrificans and Bacillus cereus in Basal salt medium. The films were initially subjected to an abiotic treatment comprising UV-B irradiation, and subsequently inoculated with the bacterial strains. The degradation in the polymeric chain was monitored by changes in the mechanical, morphological, structural and thermal properties. The abiotic treatment led to the formation of extractable oxygenated compounds as well as unoxidised low molecular weight hydrocarbons, which was confirmed by GC-MS studies. These were utilized by the bacterial consortium in the subsequent biotic phase and led to a mass loss of the polymer (8.4 ± 1.37%), which was also accompanied by an increase in the bacterial count. A decrease in the surface tension of the cell free medium was observed, which indicates that the bacterial consortium produced extracellular surface active molecules in order to enhance the bioavailability of the polymeric fixed carbon. The spectroscopic investigations reveal that the bacteria preferentially consume the oxygenated products leading to a decrease in the Carbonyl Index (CI), which in turn leads to an increase in the initial decomposition temperature as observed in the TGA traces. The morphological investigations reveal a biofilm formation on the surface, which was found to be scattered in certain regions and not uniform on the polymeric surface.     

Synthesis and selective IR absorption properties of iminodiacetic-acid intercalated MgAl-layered double hydroxide

An MgAl-NO₃-layered double hydroxide (LDH) precursor has been prepared by a method involving separate nucleation and aging steps (SNAS). Reaction with iminodiacetic acid (IDA) under weakly acidic conditions led to the replacement of the interlayer nitrate anions by iminodiacetic acid anions. The product was characterized by XRD, FT-IR, TG-DTA, ICP, elemental analysis and SEM. The results show that the original interlayer nitrate anions of LDHs precursor were replaced by iminodiacetic acid anions and that the resulting intercalation product MgAl-IDA-LDH has an ordered crystalline structure. MgAl-IDA-LDH was mixed with low density polyethylene (LDPE) using a masterbatch method. LDPE films filled with MgAl-IDA-LDH showed a higher mid to far infrared absorption than films filled with MgAl-CO₃-LDH in the 7-25 μm range, particularly in the key 9-11 μm range required for application in agricultural plastic films.     

Ageing and thermal degradation of plasma polymerised thin films derived from Lavandula angustifolia essential oil

The ageing and thermal degradation of polymer thin films derived from the essential oil of Lavandula angustifolia (LA) fabricated using plasma polymerisation were investigated. Spectroscopic ellipsometry and Fourier transform infrared (FTIR) spectroscopy were employed to monitor the optical parameters, thickness and chemical structure of the polyLA films fabricated at various RF powers over a period of 1400 h. The bulk of the degradation under ambient conditions was found to occur within the first 100 h after fabrication. The thermal degradation of the polyLA films was also investigated using the ellipsometry and FTIR. An increase in thermal stability was found for films fabricated at increased RF power levels. Between 200 and 300 °C, the properties indicate that a phase change occurs in the material. Samples annealed up to 405 °C demonstrated minimal residue, with retention ranging between 0.47 and 2.2%. A tuneable degradation onset temperature and minimal residue post-anneal demonstrate that the polyLA films are excellent candidates for sacrificial material in air gap fabrication.     

Oxo-biodegradable polyolefins show continued and increased thermal oxidative degradation after exposure to light

Samples of polyethylene (PE) and polypropylene (PP) with 2% Renatura™ pro-oxidant additive were subjected to varying times of weathering (UVCON; ISO 4892-3), transferred to dark thermal exposure (at 70 °C) and the subsequent degradation monitored by measuring elongation at break and tensile strength during intervals up to 45 days. The results demonstrate that the oxidative degradation, after initial light exposure, continues more rapidly in the dark thermal conditions and that increased exposure to light increases thermal oxidative degradation.     

Effect of crosslinking on the properties of composites based on LDPE and conducting organic filler

New types of composites were prepared using low-density polyethylene (LDPE) filled with modified organic filler, Canadian switch grass coated with polypyrrole (PPy). The grass surface was entirely covered when 10 wt.% of pyrrole was used for the modification, as confirmed by scanning electron microscopy and infrared spectroscopy. LDPE composites filled with modified grass were prepared by melt mixing and their properties were compared with the properties of the composites filled with unmodified grass. The influence of crosslinking, induced by 1 wt.% of peroxide, on mechanical, thermal and electrical properties of the composites was investigated. Crosslinking enhanced the tensile strength of the prepared composites in the entire range of the filler content. The Young’s modulus of the composites prepared by crosslinking is slightly lowered when compared with the uncrosslinked composites if the filler content is less than 60 wt.%, for higher filler content it is increased. The conductivity of the uncrosslinked composites containing 40 wt.% of grass modified by PPy was in the range 1 × 10⁻⁶ S cm⁻¹, which is a value by 5 orders of magnitude higher than the conductivity of the crosslinked materials. The presence of PPy on grass surface leads to a reduction of crosslinking of the LDPE matrix.     

Nanostructured ultraviolet resistant polymer coatings

Nanoparticle-embedded acrylic coatings that can absorb copious amounts of UV radiation yet scatter little were developed to protect base fabrics from sun-induced degradation. Zinc oxide and titanium dioxide nanoparticles with diameters ranging from 15 to 70 nm were used. Nanoparticles (5 wt%) were dispersed in acrylic emulsions. Nanoparticle-embedded acrylic films of 10 μm and 20 μm thick were prepared and bonded to Kevlar fabric. Mechanical tests as well as infrared, visible and UV spectroscopy were used to characterize nanoparticle-embedded acrylic emulsions and coated Kevlar fabric.The changes in mechanical and chemical properties of Kevlar fabric after a day and week of intense UV exposure were assessed using tear and strip tensile testing, UV, visible and infrared spectroscopy, and wide and small angle X-ray analysis. Tear and tensile data, with support from UV results, showed that 20 μm nanoparticle-embedded coatings largely prevented degradation of Kevlar fabric, allowing only 5% of the degradation that occurred in the unprotected fabric after a week of UV exposure.     

Thermal degradation of keratin waste

Thermal degradation of sheep wool, human hair and chicken feathers was studied by TG-MSD/FTIR and by pyrolysis followed by GC-MSD analysis in order to identify the degradation compounds and the temperature range in which they are formed. Only small differences were found between the studied keratin samples. They consist mainly in shift of characteristic temperatures of degradation and in relative amounts of compounds in degradation products, especially in aqueous phase. Degradation started with formation of ammonia and CO₂ (from 167 and 197 °C respectively and with maximum evolution at 273 and 287 °C respectively), continues with formation of sulphur-containing inorganic compounds (SCS, SCO, H₂S and SO₂ at 240, 248, 255 and 253-260 °C respectively) and of water (255 °C). Thiols are formed in two stages (257 and 320 °C) while the evolution of nitriles is maximum around 340 °C and continues up to about 480 °C. Phenol and 4-methylphenol are the most important degradation compounds, formed at 370 and 400 °C respectively. Nitrogen was present mainly in aliphatic/aromatic nitriles, pyrroles, pyridines and amides while sulphur was found mainly as sulphides, thiols, thiazoles and thiophenes.     

Studies on copolymerization of N-isopropylacrylamide with poly(ethylene glycol) methacrylate

The paper describes the preparation and characterization of cross-linked homopolymers and copolymers of N-isopropyl acrylamide (NIPAAm) with poly(ethylene glycol) methacrylate (PEGMA, M_n = 526 g/mol). Several copolymer samples were prepared by taking varying amounts of monomers i.e. NIPAAm and PEGMA in the initial feed using hydrophilic (IRGACURE-2959) and hydrophobic (DURACURE-1173) photoinitiator. In order to investigate the effect of reaction conditions, copolymers were prepared below or above the lower critical solution temperature (LCST) using water or water:ethanol (50:50) as solvent and by varying the amounts of cross-linker. Hydrogels prepared under varying reaction conditions were characterized for its swelling behaviour (using optical microscope), phase transition temperature (using DSC) and morphology (using SEM). As expected LCST increased from 35 to 39 °C as PEGMA content in copolymers increased from 1 to 20% (w/w). However, the morphology of hydrogels was found to be independent on the reaction conditions.Copolymer films having an optimum combination of swelling and performance properties were evaluated as switchable cell culture membranes. Hepatic cancer cell lines (Hep G-2) was used to study the cell growth and detachment. Cell growth and detachment were found to be dependent on the copolymer composition. Cell viability was found comparable to trypsin which also supports application of these films as cell culture membrane.     

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.     

Mechanism of the temperature-dependent degradation of polyamide 66 films exposed to water

Experiments were performed to elucidate the degradation mechanism of hot-pressed polyamide 66 upon exposure to water. For films exposed to water over the temperature range 25 °C-90 °C, degradation was monitored using FTIR and solid-state ¹³C NMR spectroscopies. The data are consistent with a mechanism in which (1) a radical is formed on the methylene carbon adjacent to the amide nitrogen, (2) this radical reacts with oxygen to form a hydroperoxide, and (3) the hydroperoxide decomposes to form an imide or a hydroxylated amide, both of which may cleave leading to chain scission. Water appears to facilitate degradation by increasing the flexibility of the polymer matrix through swelling rather than acting as a reactive species, at least at the early stages of the process. An apparent activation energy of 15 ± 2 kJ/mol is observed for the early stages of degradation, suggesting that segmental motions in the polymer associated with water and oxygen sorption or inter-chain radical reactions are indeed key components of the degradation process.     

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.