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.     

Formation of polymer-bonded nitroxyl radicals in the UV stabilization of polypropylene by a bifunctional hindered amine light stabilizer

The formation of polymer-bonded stabilizer derivatives during photooxidation of isotactic polypropylene, which contains the hindered amine light stabilizer bis(2,2,6,6-tetramethyl-4-piperidinyl) sebacate and the corresponding bisnitroxyl radical, was investigated. The photooxidized polypropylene films that contain these additives were studied by ESR, IR, and nitrogen analysis before and after exhaustive solvent extraction of the photooxidized films. ESR showed that under the conditions in use a maximum of 20% of the nitroxyl radicals formed from the hindered amine was bonded to the polymer chain. Regeneration of nitroxyl radicals from the polymer-bonded stabilizer derivatives under photooxidative conditions indicated that the stabilizer was bonded to the polymer chain by O-alkyl-substituted hydroxylamine linkages >N? O? PP.     

Interactions of stabilizers during oxidation processes

The antioxidative action of mixtures of phenols, phosphites, HALS, ^a) and some of their transformation products in various compositions has been studied in the thermo- and photo-oxidation of hydrocarbons and polypropylene under different conditions. In the AIBN-initiated oxidation of hydrocarbons at low temperatures (< 80°C), hindered phenols, hindered aryl phosphites and the nitroxyl derivatives of HALS act antioxidatively when used individually in appropriate concentrations. Secondary HALS do not show any induction period, but a certain retardation of the oxidation process after some reaction time. The inhibiting efficiency of nitroxyls observed cannot be explained completely by the currently accepted action mechanisms of HALS, but is also related to the reaction of the nitroxyls with alkylperoxyl radicals. In mixtures with hindered phenols, HALS have almost no influence on the rate of thermooxidation at low temperatures. Their nitroxyl derivatives, however, always exhibit synergism, most pronounced when both stabilizers are used in equimolar ratios. During the photooxidation phenols lower the efficiency of HALS. The influence of mixtures of stabilizers on the oxidative stability of polypropylene is rather different and depends on the oxidation conditions, the structure, the concentration and the ratio of the stabilizers. Synergistic as well as antagonistic effects are observed. Both aliphatic and aromatic phosphites studied act synergistically when used together and with phenols. This demonstrates that for acting as synergist for phenols, the hydrogen peroxide decomposing capability of the phosphites, but not their chain breaking activity is important. HALS-phosphites and phosphonites, containing amine and phosphorus units in one molecule, are highly effective inhibitors of photo- and thermooxidation and exhibit lower critical antioxidant concentrations and longer induction periods than phosphites alone. They even exceed the efficiency of phenols in many cases. Transformation products of phenolic antioxidants investigated act differently and in many cases contrarily under photo- and thermooxidative conditions. Therefore, they influence the efficiency of stabilizer mixtures also in a different way.     

Spatial resolution of degradation in stabilized polystyrene and polypropylene plaques exposed to accelerated photodegradation or heat aging

Spatial resolution of photo and thermooxidation processes in polypropylene (PP) and polystyrene (PS) plaques was studied by electron spin resonance imaging (ESRI) and attenuated total reflectance Fourier-transform infrared spectroscopy (ATR FTIR). Polymer plaques made of PP and PS stabilized with various systems based on hindered amine stabilizers (HAS) were exposed to accelerated photodegradation and to heat aging in air atmosphere. Concentration profiles of nitroxides generated during the degradation process in the plaques were measured by ESRI, carbonyl and hydroxy groups were identified by ATR FTIR, transparency of the plaques in the actinic part of solar radiation, including a part of UV-B (295-315 nm), full range of UV-A (315-400 nm) and visible radiation was measured by optical techniques. A mechanism of the degradation consistent with all experimental data is suggested.     

Mechanisms of antioxidant action: Nitrosamines as heat and light stabilisers for polypropylene

N-Nitrosamines are shown to be very effective melt stabilisers for polypropylene and one of these, derived from the hindered amine light stabiliser, Tinuvin 770, is almost twice as effective as the latter. The mechanism is believed to involve the formation of two kinds of nitroxyl radical, the first derived from the amine and the second formed by the reaction of nitric oxide with alkyl, and possibly alkoxyl, radicals. Nitrosamines which do not dissociate to NO are neither melt stabilisers nor uv stabilisers.     

Effect of hindered piperidine compounds on the thermal reduction of p-benzoquinone in polypropylene and its effect on photo-oxidation

The effect of a hindered piperidine compound and stable nitroxyl radical on the thermal reduction of p-benzoquinone in polypropylene has been examined using ESR, uv-visible and fluorescence spectroscopy. Thermal reduction to hydroquinone was inhibited by both compounds. With the hindered amine the nitroxyl radical was regenerated through the formation of the hydroxylamine whereas, with the nitroxyl radical, there was quantitative conversion to the hydroxylamine and no nitroxyl radical regeneration. Both inhibition processes were found to antagonise the photo-stabilising action of the hindered piperidine compounds due to the regeneration of the quinone.     

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.     

PIXE Spectroscopy for Determination of Volatile Corrosion Inhibitor Concentration in Anticorrosion Polymer Films

Summary. Anti-corrosion polymer films used for 0 to 6 years were investigated to determine the type and content of volatile corrosion inhibitors (VCI) and how long the films emit it to protect the metal surface encased by the film. By FTIR and DSC low density polyethylene (LDPE) was identified as the polymer carrier. Particle induced X-ray emission (PIXE) spectroscopy of anti-corrosion polymer films revealed significant concentration of molybdenum implying that corrosion inhibitor was molybdate based. The films were divided into three groups according to different rate of the VCI concentration decrease with increasing age. The division of films into groups and diffusion of VCI was confirmed by significant decrease of Mo content determined by PIXE spectroscopy after accelerated ageing in vacuum at 50°C. It was concluded that films with initial concentration of Mo above 6000 ppm emit VCI for at least 6 years while the films with less than 4000 ppm of molybdenum do not emit the VCI in the ambient conditions.     

PIXE Spectroscopy for Determination of Volatile Corrosion Inhibitor Concentration in Anticorrosion Polymer Films

Anti-corrosion polymer films used for 0 to 6 years were investigated to determine the type and content of volatile corrosion inhibitors (VCI) and how long the films emit it to protect the metal surface encased by the film. By FTIR and DSC low density polyethylene (LDPE) was identified as the polymer carrier. Particle induced X-ray emission (PIXE) spectroscopy of anti-corrosion polymer films revealed significant concentration of molybdenum implying that corrosion inhibitor was molybdate based. The films were divided into three groups according to different rate of the VCI concentration decrease with increasing age. The division of films into groups and diffusion of VCI was confirmed by significant decrease of Mo content determined by PIXE spectroscopy after accelerated ageing in vacuum at 50°C. It was concluded that films with initial concentration of Mo above 6000 ppm emit VCI for at least 6 years while the films with less than 4000 ppm of molybdenum do not emit the VCI in the ambient conditions.     

Photostabilization of commercial polypropylene by hindered piperidine compounds: An ESR and luminescence study

The photostabilization of commercial polypropylene by a hindered piperidine stabilizer, bis [2,2,6,6-tetramethyl-4-piperidinyl] sebacate (I) and by a model N-oxy radical compound, 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxy (II) is examined using ESR and luminescence spectroscopy. ESR spectroscopy shows that I operates through the formation of a stable nitroxyl radical in the polymer. On the other hand, II disappears rapidly during the early stages of photo-oxidation but continues to act as an effective stabilizer. A low steady-state equilibrium concentration of nitroxyl radicals is believed to be responsible for the high photostabilizing efficiency of I. Both compounds also inhibit the photolysis of the luminescent α,β-unsaturated impurity groups present in the polymer; possible mechanisms are discussed.     

The effect of UV light on the thermooxidative stability of linear low density polyethylene films crosslinked by ionizing radiation

The effect of ultraviolet (UV) light on the thermooxidative stability of Linear Low Density Polyethylene(LLDPE) films was studied. LLDPE was stabilized with phenolic type antioxidant known as Irganox 1010, hindered amine light stabilizer known as Chimmasorb 944 and phenolic type gamma stabilizer. The influence of these additives on the thermooxidative stability of gamma and UV irradiated LLDPE were investigated by isothermal Differential Scanning Calorimeter (DSC). The oxidation induction time (OIT) experiments indicate that antirad free LLDPE films which contains antioxidant and UV stabilizer are more sensitive to gamma and UV radiation. On the other hand, films which contain antirad and irradiated to different doses of γ-radiation demonstrated improved thermooxidative stability.     

Thermal and photo-oxidative behaviour of hindered piperidine compounds in polypropylene: Importance of hydroxylamine in stabilisation

The thermal oxidation behaviour of three hindered piperidine compounds has been examined in polypropylene film by oven ageing in air at 140°C together with its subsequent effect on rate of photo-oxidation. The amine, bis[2,2,6,6-tetramethyl-4-piperidinyl] sebacate, shows a rapid growth in nitroxyl radical concentration due to reaction with hydroperoxides whereas the bis-nitroxyl derivative is relatively stable and decreases in concentration very slowly, giving the hydroxylamine. The ageing process results in an improvement in photo-stability and this is associated with the reaction of the nitroxyl radical with a tertiary radical to give the hydroxylamine and ethylenic unsaturation. The mono nitroxyl compound behaves differently during the early stages of ageing in that there is a rapid decrease in concentration and this is followed by a rapid fall in photo-stability. Only during the latter stages of ageing does hydroxylamine production appear to control photo-stability.     

Synthesis,Characterization, and Performance Evaluation of Polymeric HALS in Ethylene‐Propylene‐Diene Terpolymer (EPDM)

A polymeric hindered amine light stabilizer (HALS), wherein the hindered amine functionality was attached to the maleic anhydride graft ethylene‐propylene‐diene terpolymer (EPDM) was synthesized. This involves photoinduced grafting of maleic anhydride groups on unsaturated sites of EPDM, followed by incorporation of amino terminated HALS. The grafting and functionalization reactions were characterized by FTIR and ¹³C NMR spectroscopy. The surface changes upon degradation are studied by SEM. The photostabilizing efficiency of this polymeric HALS was studied and compared with conventional HALS under accelerated weathering conditions. The HALS grafted EPDM showed significantly improved photostabilizing efficiency.     

Photo-stabilization of polypropylene film by surface photo-grafting

The photo-stability of polypropylene film photo-grafted on the suface with a hindered amine light stabilizer, 2,2,6,6-tetramethyl-4-piperidinyl-meth-acrylate (MTMP), was compared with films stabilized by addition of MTMP and the homopolymer of MTMP, respectively. It is shown that the stabilizing effectiveness of surface photo-grafting is much higher than that of the other methods. The high effectiveness of surface photo-grafting, and its possible reasons, are discussed.     

Mechanisms of antioxidant action: Behaviour of a hindered piperidine and related oxidation products during processing and photo-oxidation of polypropylene

Evidence is presented to show that a commercial hindered piperidine (I(A)) is not an effective melt stabiliser for polypropylene whereas related nitroxyl radicals (II(A)) and (II(B)) and hydroxylamine (III(B)) are highly effective. These results are explained on the basis of an oxidative transformation of the piperidine to the nitroxyl during processing and the involvement of the latter in a cyclical regenerative process in which the nitroxyl acts as a chain-breaking acceptor (CB-A) antioxidant and the derived hydroxylamine as a chain-breaking donor (CB-D) antioxidant.The same CB-A/CB-D cycle operates during photo-oxidation of polymer films containing each of the additives. The nitroxyl radical concentration reaches a stationary concentration in the polymer, irrespective of whether it is added as nitroxyl or as parent amine. The derived hydroxylamines are substantially more effective than the nitroxyl radicals as ultraviolet stabilisers.     

Stabilization of low density polyethylene with sterically hindered derivatives of 4-hydroxybenzoic acid

In comparison with other commercial light stabilizers, sterically hindered 4-hydroxybenzoates are found to possess only a weak power, if they are used for low density polyethylene (LDPE). It is known that these benzoates exhibit significant synergistic effects together with 2-hydroxybenzophenones and hindered amine light stabilizers in light stabilization of high-density and linear low-density polyethylene. The light stabilizing efficiencies of the above mentioned stabilizers and mixtures of them in different weight ratios are determined by weathering in a Xenotest 150 unit. Only small synergistic effects of about 15 −30% are found for light stabilizing LDPE. The effects can be explained by the stabilizing efficiency of sterically hindered 4-hydroxybenzoates during processing. Combining of light stabilizing structure components like 2-hydroxy-benzophenone resp. 2, 2, 6, 6-tetramethylpiperidine with sterically hindered 4-hydroxybenzoates in one stabilizer molecule only, from the first mentioned combination, results a powerful light stabilizer. Its efficiency is the sum of the powers of the single structure components.     

Synergism of hindered amine light stabilizers and UV-absorbers

The synergism of hindered amine light stabilizers (HALS) and UV-absorbers is very important in the practical photo-stabilization of polymers. As very little is known, however, the relationship between the composition of the mixture of the two kinds of light stabilizer and its photo-stabilizing efficiency was investigated in four common polymers. A high level of synergism was observed in polypropylene, high density polyethylene and ABS resin. A moderate synergistic effect was obtained in polystyrene. The maximum efficiency was observed at mixing ratios of hindered amine light stabilizer to UV-absorber of about 75:25 in polypropylene and high density polyethylene, 90:10 in ABS resin and 80:20 in crystal polystyrene.The synergism is due to different stabilizing mechanisms of hindered amine light stabilizer and UV-absorber, and is explained by the diffusion of very effective hindered amine light stabilizer from the polymer bulk, which is protected by the UV-absorber, towards the surface layer where photo-oxidation proceeds.     

Polyethylene stabilization against thermal oxidation by a trimethylquinoleine oligomer

The thermal oxidation at 110 and 120 °C of polyethylene (PE) films stabilized by 0.1, 0.2 and 0.5% of a trimethylquinoleine (TMQ) oligomer has been studied by IR spectroscopy (carbonyl build-up) and by DSC (measurement of the oxidation induction time at 200 °C). The induction period increases almost proportionally to the TMQ concentration and the TMQ efficiency (as estimated by the ratio t_ind/[TMQ]₀) increases when lowering the temperature. Some features of stabilizer consumption kinetics and the dependence of maximum oxidation rate with initial stabilizer concentration were compared to experimental results obtained for stabilization by hindered phenols (Irganox 1010) and to literature data for sacrificial (e.g. hindered phenols) and regenerative (Hindered Amine Stabilizers) antioxidants. These comparisons led to classify TMQ in the category of Hindered Amine Stabilizers (HAS), which was confirmed by a kinetic analysis. Only the scheme taking into account the specific features of HAS (role of NO radicals, regeneration from alkoxyamines) was able to correctly simulate the oxidation behaviour of TMQ stabilized PE.     

光交联聚乙烯材料的光稳定性研究

本文中用紫外光敏化的方法对低密度聚乙烯进行了交联,并利用ESCA和ATR-IR等手段研究了交联后材抖的光稳定性及其改进办法。     

Role of the UV absorber as a matrix in matrix‐assisted laser desorption/ionization mass spectrometric analysis of a mixture of a UV absorber and a stabilizer

A mixture of a UV absorber (Tinuvin 234 or Tinuvin 329) and a UV stabilizer (Tinuvin 770) was analyzed using matrix‐assisted laser desorption/ionization mass spectrometry (MALDI‐MS) without any matrix. Fragmentation patterns of the UV absorbers and stabilizer were also investigated. The mass spectra showed the [M+H]⁺ ions and some fragment ions. Tinuvin 234, Tinuvin 329, and Tinuvin 770 generated three (m/z 119, 370, 432), one (m/z 252), and two (m/z 124 and 140) fragment ions, repectively. These fragment ions can be used to identify the chemical structures of the UV absorbers and stabilizer. Since the UV absorber performed a role as the matrix, the ion abundance of the UV stabilizer was enhanced by mixing with the UV absorber. When organic materials extracted from polypropylene (PP) containing the UV absorber and stabilizer were directly analyzed using MALDI‐MS without any matrix, the protonated molecule of the UV stabilizer was detected in abundance but the product ions of the UV absorber were not observed. When 2,5‐dihydroxybenzoic acid was used as a matrix, the protonated molecule of the UV absorber was observed. Copyright © 2010 John Wiley & Sons, Ltd.