Photo-degradation and photo-oxidation of polyolefins: Importance of oxygen-polymer charge transfer complexes

The effect of prior irradiation (λ's > 300 nm) in an inert atmosphere on the subsequent photo-oxidative stabilities of polypropylene and low density polyethylene films has been examined using luminescence, infra-red and ultraviolet absorption techniques. Prior prolonged irradiation in an inert atmosphere of nitrogen was found to have no significant effect on the subsequent rates of photo-oxidation of the polymer films. The importance of oxygen-polymer charge transfer complexes in initiating photo-oxidation of the polymers is discussed in relation to the behaviour of other major photo-initiators such as carbonyl/unsaturated carbonyl and hydroperoxide groups.     

Rapid measurement of polymer photo-degradation by FTIR spectrometry of evolved carbon dioxide

A novel method, allowing simultaneous UV exposure of a polymer sample and IR interrogation of the vapour in a specially constructed cell, has been applied to polyethylene (PE) samples containing TiO₂ pigments with different photoactivities. Measurements of the CO₂ generated by films exposed to ultraviolet irradiation (UV) were completed in 5 h - very much less than conventional accelerated tests. The TiO₂ pigments used included anatase and rutiles with different surface treatments. Anatase pigmented material gave significantly higher CO₂ emission than unpigmented PE whilst rutile-pigmented PEs either gave reduced CO₂ emission or enhanced emission, according to the surface treatment. The ranking of the pigments as protectants or pro-degradants correlated well with the carbonyl index measured after more than 300 h exposure to UVA fluorescent tubes in a QUV machine.The method was then used to probe mechanistic aspects of the photo-oxidation of pigmented polyethylene (PE) film. For unpigmented polymer the photo-degradation was sensitive to changes in the small fraction of incident UV below 300 nm, but for pigmented films this was much less important. This is because unpigmented film degrades by direct photochemical attack whereas, for pigmented film, photocatalysis by TiO₂, which absorbs in the 300-400 nm region, is important. For films whose photo-oxidation was dominated by photocatalysis by the TiO₂ the rate of oxidation was shown to vary as the square root of the UV intensity. By contrast, for unpigmented films the rate of direct photochemical oxidation was linearly proportional to UV intensity. The difference is a consequence of the controlling role of electron-hole recombination in photocatalytic processes. For both unpigmented and pigmented films the rate of oxidation was shown to increase with increasing humidity and oxygen content of the atmosphere.     

Degradation products formed during UV exposure of polyethylene-ZnO nano-composites

Photodegradation of low density polyethylene (LDPE) containing nano-particulate ZnO has been studied using FTIR to follow the development of oxidation products in the polymer film and to monitor carbon dioxide evolved as a principal product of oxidation. The degradation behaviour of ZnO-free LDPE has been compared with that of compounds containing 0.25% and 0.75% ZnO and these results are compared with those obtained using similar films containing nano-particulate TiO₂. Under UV exposure, the presence of ZnO accelerated the development of carbonyl groups and CO₂ production. The carbonyl group development was more rapid when TiO₂ was used whereas ZnO caused greater CO₂ generation. Carbonyl group development seemed to correlate better with the reduction in mechanical properties whereas CO₂ generation correlated better with weight change measurements. The influence of ZnO on the oxidation pathways in LDPE is discussed; it is proposed that photo-oxidation is relatively much more likely to occur at terminal sites (rather than at pendent sites) when ZnO is present.     

Cytocompatibility of surface ground PE doped with calcium salt of 6-carboxycellulose

We studied the physical–chemical properties of polyethylene (PE) films doped with calcium salt of 6-carboxycellulose (Ca-oxy) in concentrations 0–20 wt.% and their interactions with cells. The interaction between the PE/Ca-oxy composite film and 3T3 cells has not been described so far. Modification by surface grinding to enhance material cytocompatibility was studied. The surface wettability of doped films was tested by contact angle measurements and the surface morphology by SEM microscopy and profilometry. Adhesion and proliferation of tissue cells (3T3 mouse fibroblasts) were studied in vitro on pristine and ground doped PE films. The ground samples have a higher contact angle values and the number of proliferated cells is greater on the ground samples than on pristine ones. The presence of Ca-oxy in PE film increases the adhesion of 3T3 cells on the doped substrate and on ground samples adhere more cells than on pristine ones.

The study of regenerated cellulose films toughened with thermoplastic polyurethane elastomers

Regenerated cellulose blend film with thermoplastic polyurethane (TPU) was successfully prepared by coagulating cellulose/TPU solution with water in the presence of a thermoplastic polyurethane elastomer (TPU). Compared with pristine regenerated cellulose film, the toughness and thermal stability of the blend film was significantly improved. For example, the elongation at break was increased from 11% of pristine cellulose film to 51% of blend film with 20 wt. % TPU. The 50% weight loss temperature of this blend film was increased by 33 °C compared to neat cellulose. The relaxation transition temperature of cellulose was decreased with the addition of TPU through dynamic mechanical thermal analysis. The oxygen permeability was decreased from 2.3 × 10⁻¹⁰ cm³ cm/cm² s Pa of pristine cellulose film to 0.08 × 10⁻¹⁰ cm³ cm/cm² s Pa of the blend film with 20 wt.%. TPU The X-ray diffraction spectra showed that the crystallinity of cellulose decreased with incorporation of TPU. The images of scanning electron microscope discovered that there was good compatibility between cellulose and TPU. TPU was nano-dispersed in cellulose matrix. The blend film still maintained quite good transparency.     

The investigation of molecular affinity involved in poly(ethylene glycol)-based polymer-dispersed liquid crystal display

Acrylic polyethylene glycol(PEG)-based polymer-dispersed liquid crystal (PDLC) films have been fabricated to investigate the effect of intermolecular interactions on PDLC performance. For this purpose, the amphiphilic liquid crystal and polymers are selected as PDLC composite materials. The acrylic PEG contents are varied from 0 to 66.66 mol wt.% in order to understand the effects of different levels of additions on the microstructure and electro-optical properties of the PDLC films. For this intention, polarized optical microscopy and UV–vis spectroscopy are used. The extent of phase separation and anchoring energy are also examined using Fourier transform infrared (FTIR) spectroscopy and contact angle measurements in consequence of acrylic PEG addition. The contrast ratio, threshold voltage, as well as saturation voltage, tended to increase with the addition of acrylic PEG. The molecular affinity involved in the polymer matrix and LC molecules affected the phase separation which is responsible for the formation of domain size; this accordingly changed the electro-optical properties of PDLC film.     

Influence of thiobisphenols on the photo-oxidation of low density polyethylene

The influence of thiobisphenols, some products of their thermo-oxidative transformation and model compounds on the photo-oxidation of low density polyethylene (Ld PE) has been studied. Thiobisphenols are found to have a pro-oxidant effect in the early period of exposure, which increases with their concentration. On the other hand, the carbonyl build up was retarded in a later irradiation period. Similar effects have been observed with some thiobisphenol transformation products, bis(3,5-di-tert. butyl-4-hydroxybenzyl) sulphide and its oxidation products, sulphoxide and sulphone, respectively, and model 4,4′-methylenebis(2-methyl-6-tert. butylphenol). Dodecyl 3,3′-thiodipropionate retarded the carbonyl build up during the whole period of ultraviolet irradiation. Kinetic data show that a concentration of 0·1% by weight of thiobisphenols is the most effective in retarding the carbonyl build up in Ld PE films. The formation of non-alcoholic oxygenated structures seems to be a specific effect of the sulphur containing compounds under study.     

Influence of Natural and Accelerated Weathering on the Mechanical Properties of Low-Density Polyethylene Films

Natural and accelerated weathering tests were performed to inspect the effect of antioxidants on low-density polyethylene (LDPE) films used as greenhouse covering materials. The LDPE pellets were extruded and blown into a film using a twin-screw extruder and film blowing machine, respectively. The film with 0.2 wt.% Alkanox-240 (AN-0.2) stabilizer showed the highest tensile strength (11 MPa) among all samples during 90 days of natural as well as accelerated weathering. The elastic modulus of the film with 0.5 wt.% of Good-rite (GR-0.5) increased after weathering from approximately 91.8 to 138.9 MPa, and showed the best performance. Morphological images of the neat LDPE film during weathering showed some cracks and grooves, while those of stabilized films showed fewer cracks. Moreover, the estimation of the rapidity of the accelerated method compared to the natural one was approximately nine times faster in Riyadh during the summer season (June–August). The present study suggests that the addition of antioxidants can improve the tensile strength, stability, and, hence, the effectiveness of these films. The best antioxidants were found to be 0.2 wt.% Alkanox and 0.5 wt.% Good-rite antioxidants.     

Effect of different nanoparticles on HDPE UV stability

In the present study different series of HDPE nanocomposites were prepared by melt mixing on a Haake-Buchler Reomixer, containing 2.5 wt% of multiwall carbon nanotubes, pristine and modified montmorillonite, and SiO₂ nanoparticles. Nanocomposites in the form of thin films were exposed to UV irradiation at 280 nm at constant temperature (25 °C) and constant relative humidity (50%) for several times. From tensile strength and Young’s Modulus measurements it was verified a high increase with initial UV irradiation times (till 100 h) and a slight reduction thereafter. The increase was higher in nanocomposites compared with neat HDPE, except these containing MWCNTs, and was attributed to the crystallinity increase in the particular samples. The mechanical properties reduction at higher UV irradiation times was attributed to the extensive macromolecular chain scission causing irregularities and holes in film surfaces. However, from FTIR study it was found that SiO₂ and organically modified montmorillonite cause a serious effect on HDPE during UV degradation. New chemical compounds containing carbonyl, vinyl and hydroxyl groups were formed. It seems that these nanoparticles have an accelerating effect acting as catalysts to HDPE photo-oxidation. This was also verified from micro-Raman analysis. Untreated montmorillonite has also a small influencing effect while neat HDPE and nanocomposites containing multiwall carbon nanotubes have the highest UV stability.     

Effects of melt processing conditions on photo-oxidation of PP/PPgMA/OMMT composites

This article reports the studies of photo-oxidative behaviour of polypropylene/maleic anhydride-grafted polypropylene/organic modified montmorillonite (PP/PPgMA/OMMT) composites prepared by two different melt processing methods. Samples of pristine polypropylene (PP) and PP/PPgMA/OMMT composites were prepared in an internal mixer and in a twin screw extruder. The samples were exposed to long wavelength radiations (λ > 300 nm) for the photo-oxidation. The samples were examined by FTIR, X-ray diffraction and microscopy. Similar to the pristine (PP), it is found that the photo-oxidation process in the composites depends on the melt processing conditions, which could cause the deterioration of organic modifier of the clay and the polymer matrix. The new radicals formed in addition to the iron impurities in the montmorillonite accelerate the photo-oxidation.     

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.     

The role of organoclay and matrix type in photo-oxidation of polyolefin/clay nanocomposite films

In this paper the photo-oxidation behaviour of polyolefin/clay nanocomposite films was studied; in particular, the effect of the amount of organo-modifier and the matrix polarity on the photo-oxidation was investigated. Two different organo-modified clays and compositions of LDPE/EVA blend films were used and the photo-oxidation was followed by mechanical and spectroscopic analyses.The organoclay and matrix type strongly influence the photo-oxidative behaviour of nanocomposite films. The films filled with CL15A show a faster loss of mechanical performance and higher carbonyl formation with respect to the films filled with the CL20A. Additionally, the LDPE based nanocomposite undergoes photo-oxidation more rapidly than the EVA based one.     

Low density polyethylene-montmorillonite nanocomposites for film blowing

Low density polyethylene nanocomposites were prepared using differently modified montmorillonite (MMT) and different compatibilizers. The best results were obtained for MMT with largest gallery distance. The most exfoliated system was further optimized for superior mechanical properties by varying the compounding condition. The criteria were mechanical properties of nanocomposites and X-ray proofs of exfoliation. The optimized nanocomposites were used for film blowing. The effect of blow ratio on mechanical properties and oxygen permeation of films was evaluated for two best nanocomposites and two films blown from pristine polyethylene. The texture of crystalline phase of blown films was analyzed by X-ray pole figure technique, SAXS and AFM. Two components of texture were detected, the first component related to the molecular orientation of polyethylene by film blowing and take-up and the second connected with the formation of free surfaces of the film. The crystallinity degree from DSC and long period determined from SAXS of polyethylene component were nearly independent of the additives. It indicated that the compatibilizer was preferentially located around clay platelets and did not enter the amorphous layers of polyethylene. Also the orientation of clay platelets was determined by FTIR using 1080 cm⁻¹ band characteristic for Si-O bonds. A clear correlation of oxygen permeativity of blown films with clay platelets orientation and degree of exfoliation was evidenced.     

Determination of radiolysis products in gamma-irradiated multilayer barrier food packaging films containing a middle layer of recycled LDPE

Volatile and non-volatile radiolysis products and sensory changes of five-layer food packaging films have been determined after gamma irradiation (5–60 kGy). Barrier films were based on polyamide (PA) and low-density polyethylene (LDPE). Each film contained a middle buried layer of recycled LDPE or 100% virgin LDPE (control samples). Data showed that a large number of radiolysis products were produced such as hydrocarbons, alcohols, carbonyl compounds, carboxylic acid. These compounds were detected in the food simulant after contact with all films even at the lower absorbed doses of 5 and 10 kGy. The type and concentration of radiolysis products increased progressively with radiation dose, while no new compounds were detected as a result of the presence of recycled LDPE. In addition, irradiation dose appears to influence the sensory properties of table water in contact with films.     

Fabrication of tri-polymers composite film with high cyclic stability and rapid degradation for cardiac tissue engineering

In this communication, biodegradable and highly elastic silk fibroin/poly(lactide-co-ε-caprolactone)/polyethylene oxide (SF/PLCL/PEO) tri-polymers composite film was fabricated by sol–gel casting technology. The tri-polymers composite film exhibited a high cycle performance and rapid degradation rate by regulating the content of blending of the three polymer contents. The viability of cardiomyocyte cells was demonstrated for both SF/PLCL and SF/PLCL/PEO composite films after 1 day of culture, although the tri-polymers composite film demonstrated superior cell growth, attachment and spreading after culturing for 7 days. Study findings support the potential application of this biocompatible tri-polymers composite film as a heart patch substitute with multi-functionalities.     

Influence of accelerated ageing on the physico-mechanical properties of alkali-treated industrial hemp fibre reinforced poly(lactic acid) (PLA) composites

30 wt% aligned untreated long hemp fibre/PLA (AUL) and aligned alkali treated long hemp fibre/PLA (AAL) composites were produced by film stacking and subjected to accelerated ageing. Accelerated ageing was carried out using UV irradiation and water spray at 50 °C for four different time intervals (250, 500, 750 and 1000 h). After accelerated ageing, tensile strength (TS), flexural strength, Young's modulus (YM), flexural modulus and mode I fracture toughness (K_Ic) were found to decrease and impact strength (IS) was found to increase for both AUL and AAL composites. AUL composites had greatest overall reduction in mechanical properties than that for AAL composites upon exposure to accelerated ageing environment. FTIR analysis and crystallinity contents of the accelerated aged composites support the results of the deterioration of mechanical properties upon exposure to accelerated ageing environment.     

正性远紫外抗蚀剂 Ⅱ.聚苯乙烯砜的荧光光谱研究

利用荧光光谱研究了聚苯乙烯砜正性远紫外抗蚀剂薄膜和溶液的光氧化反应。发现聚苯乙烯砜的荧光随光照时间的延长而逐步减少。这一现象和在未辐照的聚苯乙烯砜中加入微量的芳香氢过氧化物或羰基化合物时的情况相同。这表明:聚苯乙烯砜经光照后荧光的淬灭和体系光氧化过程中产生了氢过氧化物或羰基化合物有关。这一方法适宜于对高聚物光氧化初始阶段的研究。     

Investigating the effect of accelerated weathering on the mechanical and physical properties of high content plastic solid waste (PSW) blends with virgin linear low density polyethylene (LLDPE)

In developing countries, plastic solid waste (PSW) poses a serious threat due to the increase in the dependency on landfilling as well as other environmental issues. Hence, valorising the accumulated waste is essential to promoting more environmentally friendly practices. In this work, the mechanical and physical properties of virgin linear low density (LLDPE) and LLDPE blends with PSW are reported. The formulations studied contained the following virgin to waste ratios (wt.%/wt.%): 100/0, 75/25, 50/50, 25/75 and 10/90. The effect of photo-degradation on the studied specimens was investigated using accelerated weathering tests in a UV chamber. Young's modulus showed an increase with exposure duration due to change in the samples' crystallinity. The loss of mechanical integrity (i.e. stress and strain at rupture) of the samples studied was related to the photo-degradation mechanisms, namely cross-linking and photo-oxidation. Haze and light transmission measured indicated that there was a loss of the amorphous regions in the samples studied after about 200 h of continuous exposure. The total change in colour (ΔE) was estimated but did not show a clear trend, indicating a clear dependency on wash mechanism and continuous loss of polymer colour and degradation.     

Identification of carbonyl species of weathered LDPE films by curve fitting and derivative analysis of IR spectra

We describe the examination of the weathering degradation of LDPE (low density polyethylene - locally produced B24/2 and imported LDPE 2100T), supplied by two different manufacturers and processed into films for greenhouse coverings, over several months in a sub-Saharan region of Algeria. The three IR regions most affected by weathering degradation are 800–1100 cm⁻¹, 1680–1800 cm⁻¹ and 3300–3600 cm⁻¹. The IR spectral region most affected by the aging process is the carbonyl region. Curve fitting combined with derivative spectroscopy revealed that the composite carbonyl band encompasses more than 10 different oxidation products. The most significant among these in terms of absorbance are carboxylic acids, ketones, aldehydes and esters. The oxidation kinetics with respect to the type of LDPE film shows that B24/2 LDPE undergoes less oxidization than LDPE 2100 T. Calculating the concentrations of different carbonyl species compared to their respective absorbances indicates that the aldehydes are the predominant component of the final compound, rather than the carboxylic acids.