Polymer Degradation and Stability (v.72, #2)

Photooxidation of dried linseed oil films with various thicknesses is analysed. Distributions of the photoproducts resulting from photooxidation are determined by photoacoustic IR spectrometry and by micro-IR spectrometry. Oxidation profiles are observed and it is shown that oxidation can extend up to 300 μm. This result is confirmed by micro-fluorescence analysis of the photo-oxidized sample. On the other hand, the cross-sectional analysis of the exposed films suggest that crosslinking occurs in the oxidized layers, which could prevent some of the low molecular weight products formed from migrating from the solid film towards the surrounding atmosphere.
Keywords: Linseed oil; Micro-infrared spectrometry; Photoacoustic; Fluorescene;

Mechanisms of photooxidation of polyglycerol by Sandrine Morlat; Nathalie Cezard; Bernard Loubinoux; Jean-Louis Philippart; Jean-Luc Gardette (199-208).
The photooxidation behaviour of polyglycerol (PG) has been studied under irradiation at long wavelengths (λ ⩾300 nm) at 40°C and in the presence of oxygen. Photoproducts have been identified by FTIR analysis coupled to chemical treatments and analysis of the gas phase was performed by mass spectrometry. The oxidation kinetic curves were plotted as a function of irradiation time depending on the O2 pressure. Results of thermooxidation at 100°C of PG samples are also compared to photooxidation results. A mechanism accounting for the main routes of oxidation is proposed.
Keywords: Polyglycerol; Photooxidation; Thermooxidation; Infrared spectrometry; Mass spectrometry;

Testing of organic matrix durability in photochemical ageing using ablation measurements by L Guillot; L Monney; C Dubois; A Chambaudet (209-215).
The ablation of two epoxy matrixes (DGEBA–IPDA and DGEBA–MTHPA) and an orthophthalic polyester matrix by artificial UV-visible irradiation has been studied. The photo-chemical removal of these organic matrixes after photo-ageing has been quantified using a two-dimensional profile measurement technique. The results have made it possible to describe and compare the ablation's evolution in the two organic matrixes under the same artificial conditions. This method has the advantage of being very accurate and fast. Moreover, it also allows for the ability of additives to increase the durability of organic matrixes, whether separate or included in composite materials, to be tested.
Keywords: Photooxidation; Epoxy matrix; Ablation; Ageing;

The recycling of polymer blends has gained increasing attention in the world due to economic and environmental considerations. The exposure of recycled polyethylene blends to low doses of 60Co gamma radiation improves their mechanical properties due to crosslinking. It is agreed that gamma irradiation in air may cause degradation with oxidation occurring near the surfaces. Usually degradation means worsened properties. Then, it is important to estimate the extent of oxidation depth under those conditions. Samples of recycled polyethylene blends after exposure to gamma rays were examined by scanning electron microscopy (SEM). Permanganic reagent, developed for revealing semicrystaline morphologies in polyolefins under the electron microscope, has been used to monitor the superficial degradation. A detailed characterization of the material behavior has been made by mechanical and physico-chemical testing. The data presented in this article for recycled polyethylene blend after exposure to gamma rays in the air provide evidence of that the increase of radiation dose promotes a higher surface degradation. The effect of gamma radiation is clearly seen in the specimens irradiated with different doses and may be related to the changes in the mechanical properties.
Keywords: Recycling; Polyethylene blends; Gamma irradiation; Scanning electron microscopy;

Effect of photoirradiation on potassium persulfate-surface oxidation of low-density polyethylene film by Hitoshi Kubota; Yayoi Hariya; Shin-ichi Kuroda; Takashi Kondo (223-227).
Surface oxidation of low-density polyethylene (PE) film (thickness=30 μm) with potassium persulfate (KPS) was investigated at 50°C in water. The surface oxidation was followed by ATR-IR measurement of the oxidized PE film, where the absorbance ratio of the carbonyl group at 1716 cm−1 to the methylene group at 1460 cm−1 was used as a measure of the oxidation. The absorbance ratio increased with increasing the reaction time (∼12 h) and the concentration of KPS (0.01–0.30 mol/l). The decomposition of KPS was largely accelerated by photoirradiation (λ>290 nm), resulting in the promoted surface oxidation of PE film, by which hydroxyl groups were introduced on the oxidized surface. Vinyl monomers such as acrylamide, N-isopropylacrylamide and glycidyl methacrylate could be grafted onto the oxidized surface using ceric salt initiator, which was confirmed by ATR-IR measurement of the grafted films.
Keywords: Surface oxidation; Low-density polyethylene film; Potassium persulfate; Photoirradiation; Ceric salt-initiated grafting;

Caligonum monogoliacum and Tamarix spp. were pre-treated with ethanol–H2O (60/40,v/v) under acid catalyst (0.2 N HCl) at 70°C for 4 h and then post-treatment with 2% H2O2 at pH 11.5 for 16 h at 45°C, respectively. Alkaline peroxide post-treatment released a higher amount of lignin (10.6% from Caligonum monogoliacum and 12.1% from Tamarix spp.) as compared to acidic organosolv pre-treatment (5.5% from Caligonum monogoliacum and 6.4% from Tamarix spp.). The two-stage treatments together solubilized or degraded 84.3 and 90.2% of the original lignins from Caligonum monogoliacum and Tamarix spp., respectively. Analyses of the acid-insoluble lignin fractions revealed that alkaline peroxide post-treatment resulted in a substantial oxidation of the isolated lignins as they are enriched in carbonyl and carboxyl groups. In comparison, the isolated four acid-insoluble lignin samples were further characterized by both degraded methods such as alkaline nitrobenzene oxidation and thermal analysis, and non-degradation techniques such as ultraviolet (UV), Fourier transform infrared (FT-IR), and carbon-13 magnetic resonance spectroscopy (13C-NMR) as well as gel permeation chromatography (GPC), and their results are reported.
Keywords: Caligonum monogoliacum; Tamarix spp.; Lignin; Degradation; Thermal stability;

Mechanical and transport properties of irradiated linear low density polyethylene (LLDPE) by Carlo Naddeo; Liberata Guadagno; Simonetta De Luca; Vittoria Vittoria; Giovanni Camino (239-247).
Films of a new linear low density polyethylene (LLDPE) were exposed to accelerated weathering in a UV chamber at 60°C for increasing times. The changes in the mechanical properties, due to the degradation were followed by determining the elastic modulus (E), the stress at the yield point (σ y), the post yield stress drop (PYSD), the stress at the breaking point (σ b), the strain at the breaking point (ϵ b), and the toughness (t). They were correlated to the exposure time and to the carbonyl index, previously determined on the same samples. All the mechanical parameters were found to be very sensitive to the chemical degradation and to the morphological changes. Furthermore the curves of the mechanical parameters as a function of the exposure time allowed the determination of the point beyond which the material becomes useless. This point corresponds to 150 h of irradiation and to a carbonyl index of 0.3. The transport properties, diffusion and sorption, of a non polar molecule, n-pentane and a more polar molecule, dichloromethane were also correlated to the changes in the molecular structure.
Keywords: LLDPE; Spherilene; Accelerated photodegradation; Mechanical properties; Transport properties;

Chemiluminescence accompanying oxidation of lactam-based polyamides stabilized with phenols and secondary aromatic amines as well as with a hindered nitroxy radical was measured. The yields of chemiluminescence per mol of oxygen absorbed were found to be higher for oxidation of polyamide in the presence of antioxidants than in their absence depending on the content of amino and carboxylic end-groups. Because antioxidants react with peroxy radicals by several consecutive inhibition reactions with different yields of chemiluminescence, the intensity of chemiluminiscence also depends on the rate of initiation of the oxidation. The relation between the intensity of chemiluminescence and the rate of oxygen consumption during oxidation of stabilized polyamides differed profoundly for various polyamides depending on their end-group content, on the concentration of antioxidant and also on the conditions of oxidation, especially on the rate of initiation determining the rate of antioxidant consumption. Redox inhibition reactions were found to be the predominant source of chemiluminescence accompanying the oxidation of N-alkylamides in the presence of antioxidants.
Keywords: Lactam-based polyamides; Antioxidants; Oxidation of stabilized N-alkylamides; Chemiluminescence of inhibition reactions;

Studies of synergism between carbon black and stabilisers in LDPE photodegradation by J.M. Peña; N.S. Allen; M. Edge; C.M. Liauw; B. Valange (259-270).
The interaction between two commercial hindered piperidine compounds, three commercial antioxidants, a secondary antioxidant and two types of furnace carbon black (CB) (with a different surface area and surface chemistry) in the photo-oxidation of LDPE film have been studied using FTIR infrared spectroscopy. The nature of stabiliser-CB interactions has also been analysed through the use of flow microcalorimetry. The stabilisers used are, a commercial polymeric HALS, namely Chimassorb 944®, and Tinuvin 622® LD®, primary and secondary antioxidants, namely Irganox 1010® and Irganox 1076®, Cyanox 1790® and Irgafos 168®. For the fillers, two carbon blacks were chosen with different features as the specific surface particle size and chemistry. During photo-oxidation the interactions are variable, being both antagonistic and synergistic. Generally for the primary antioxidants alone their photostabilising effects on the polymer film are low or negligible and can be related to the stability of the additives. However, in the presence of a hindered piperidine light stabiliser no such correlations between polymer and antioxidant stability exist. In most of the cases, antagonistic interactions were found between HALS and the antioxidants. Differences in the performance of the polymeric type of HALS were evidenced, as well as between the CB grades. Less antagonism was found between primary and secondary antioxidants with CB for two-additive formulations. This effect is overcome by a powerful synergistic interaction in three additive formulations. Antagonism was found between one HALS and CB. It is concluded that the nature of the CB plays an important role in controlling their performances as a stabilising agent alone, as well as their interactions with light stabilisers and antioxidants.
Keywords: Stabilisation; Carbon black; Polyethylene; Photodegradation; Synergism;

Double stage oxidation of polyethylene as measured by chemiluminescence by Rastislav Broska; Josef Rychlý (271-278).
The double stage oxidation of LDPE has been investigated by chemiluminescence (CL) with the aim of determining the processes corresponding to each oxidation stage. The nonisothermal chemiluminescence exhibiting two peaks used for monitoring evolution of hydroperoxides in thermo-oxidatively aged samples confirms two stages of oxidation and reveals a gradual reduction in stability of formed peroxides and in crystalline content of recrystallised sample after ageing. The effect of oxygen concentration on CL of PE oxidation has been examined indicating that the double stage oxidation is related to the transfer of the system to a strictly oxygen diffusion controlled process. The results are supported by FTIR measurements and determination of gel content which reveal that crosslinking takes place only in the second oxidation step.
Keywords: Chemiluminescence; Polyethylene; Oxidation;

Scission and crosslink concentrations have been determined in polypropylene samples using gel permeation chromatography molecular weight distributions and computer simulations. Measurements have been made at different depths within injection moulded bars 3 mm thick after 4 weeks ultraviolet (UV) exposure. The scission and crosslink concentrations were found to be 0.025 and 0.006 mol/kg respectively at the surface and 0.004 and 0.001 mol/kg respectively at the centre. The values at the unexposed face were similar to those at the exposed face. Samples exposed to UV while loaded in uniaxial tension (10 MN/m2) showed markedly increased chain scission but little change in the crosslinking rate.
Keywords: Photo-oxidation; Polypropylene; Scission/crosslinking ratio; Depth profiling; Stress-aided oxidation;

The current technology in ozone protection of rubber compounds is reviewed. The topics covered are both static ozone protection involving the use of paraffin wax and the dynamic ozone protection which involves the use of disubstituted p-phenylendiamines. The review is integrated with the experimental evaluation in a standard rubber compound of the most common substituted p-phenylenediamines (included a non-staining type). The ozone resistance under dynamic conditions and the flex-fatigue life of the rubber compounds are discussed.
Keywords: Rubber; Ozone; Protection; Paraffin wax; p-Phenylenediamine; PPD;

Chain dynamics and thermal stability of trifluoroacetylated ethyl cellulose by C.W. Yang; K.H. Wu; T.C. Chang; Y.S. Hong; Y.S. Chiu (297-302).
Trifluoroacetyl ethyl cellulose (TFAEC) was prepared by reaction of ethyl cellulose (EC) with trifluoroacetic anhydride. The effect of the trifluoroacetylation on the chain dynamics and thermal stability of the ethyl cellulose was investigated. Chain dynamics of TFAEC were investigated using the 1H spin-lattice relaxation time in the rotating frame (TH ). The trifluoroacetylation reduced the mobility of the polymeric glucose chain. The apparent activation energies (E a) of degradation under nitrogen and air were evaluated by differential and integral methods. The value of E a of TFAEC under nitrogen was affected by the equivalent ratio of trifluoroacetic anhydride to EC, while that under air was less affected.
Keywords: Trifluoroacetylation; Ethyl cellulose; Relaxation; Degradation; Activation energy;

Antioxidant activities of tocopherols by Yasukazu Ohkatsu; Tetsuto Kajiyama; Yuji Arai (303-311).
Tocopherols are known to show high antioxidant activities. Their action mechanism, however, has not yet been discussed sufficiently. In this study tocopherols were investigated kinetically, together with p-methoxyphenol. The rate equation shows that a tocopherol can trap more mol of radicals than two, namely 2.3–2.9 mol, per mol of tocopherol, and can trap alkyl radicals as well as peroxy radicals. The rate equation for p-methoxyphenol, on the other hand, suggested that it behaves similarly to a tocopherol in antioxidant action. The action mechanism of a tocopherol, thus, is estimated from detailed study on the antioxidant action of the smaller and simpler p-methoxyphenol, namely study on the kinetic evaluation and qualitative and quantitative analyses of products obtained after the antioxidant action of p-methoxyphenol.
Keywords: Tocopherols; p-Methoxyphenol; Alkyl radical; Action mechanism;

The dynamic thermo-oxidative degradation of expandable graphite (EG)-based intumescent halogen-free flame retardant (HFFR) LLDPE blends in the condensed phase at 300 or 400°C has been studied in situ by real time Fourier transform infrared spectroscopy. The kinetic characteristics and dynamic changes of various kinds of pyrolysis products during the thermo-oxidative degradation were examined extensively for several LLDPE/EG blends with different HFFR additives, such as the phosphorus–nitrogen compound NP28, ammonium polyphosphate (APP), red phosphorus (RP), and zinc borate (ZB). It has been found (i) that the dynamic monitoring of the LLDPE/EG/HFFR blends during the pyrolysis shows that the breakdown of LLDPE main chains and formation of various kinds of carbonyl products increase with increasing thermo-oxidation time and temperature; the latter have been identified as carboxylic acids, ketone, lactone and cyclic anhydrides; (ii) that the fast formation rate and high initial concentrations of P–O–P and P–O–C products in the phosphorus-containing HFFR systems have a crucial importance for decreasing the thermo-oxidative degradation rate of LLDPE efficiently (the NP28 system is the most efficient in the present study); and (iii) that the pyrolysis temperature has a significant effect on the efficiency of flame retardants, which requires the LLDPE/EG/HFFR formulation should be optimized at certain temperature range in order to form compact intumescent charred layers immediately at the beginning of the thermo-oxidative degradation of the LLDPE blends.
Keywords: Dynamic thermo-oxidative degradation; LLDPE; Expandable graphite; Halogen-free flame retardant; Real time FTIR; Flame retardant mechanism;

Degradation of polyethylene by a fungus, Penicillium simplicissimum YK by Keiko Yamada-Onodera; Hiroshi Mukumoto; Yuhji Katsuyaya; Atsushi Saiganji; Yoshiki Tani (323-327).
We isolated a strain of Penicillium simplicissimum, YK, for use in the biodegradation of polyethylene, characterizing the fungus and examining how to treat the polyethylene before cultivation to make degradation more complete. Degradation was monitored by high-temperature gel-permeation chromatography of the molecular weight distribution of polyethylene before and after the fungus was cultivated with it. Polyethylene with starting molecular weights of 4000 to 28,000 had lower molecular weights after 3 months of liquid cultivation with hyphae of the fungus. UV irradiation of polyethylene or its incubation with nitric acid at 80°C for 6 days before cultivation caused functional groups to be inserted into the polyethylene. The strain grew better on a solid medium with 0.5% polyethylene when it was irradiated for 500 h than when it was not irradiated. Polyethylene with a molecular weight of 100,000 or higher after nitric acid treatment had lower molecular weight after 3 months of liquid cultivation with hyphae of the fungus. The efficiency of polyethylene degradation depended on the growth phase in pure cultivation of the fungus. Functional groups inserted into polyethylene aided biodegradation. Bioremediation of polyethylene may become possible.
Keywords: Polyethylene degradation; Bioremediation; Penicillium simplicissimum;

The effect of silica-modification on MFI, MOR and BEA zeolite catalysts was studied in the liquid-phase catalytic degradation of polyethylene wax. Although their acidities and surface areas were not significantly altered with silica-modification, the degradation activities of poor catalysts were greatly enhanced due to the suppression of carbon deposit on the external surface. The catalytic activity and product composition on silica-modified zeolite catalysts supposed the selective inactivation of the external surface.
Keywords: Polyethylene wax; Silica-modified zeolite; Liquid-phase degradation; External surface;

Degradation of poly(l-lactic acid) by γ-irradiation by Pramono Nugroho; Hiroshi Mitomo; Fumio Yoshii; Tamikazu Kume (337-343).
Poly(l-lactic acid) (PLA), was irradiated with γ-rays in air and in vacuum at 25°C. Melting point T m, glass transition temperature T g and number-average molecular weight M n decreased with increasing irradiation dose. At low doses, T m, T g and M n decreased sharply and then the rate became slower. Up to 200 kGy, PLA was predominantly degraded by random chain-scission. Above 200 kGy, recombination reactions or partial crosslinking may occur in addition to chain-scission. The degree of chain-scission G s of irradiated PLA was calculated to be 1.97 in air and 0.83 in vacuum. The decrease in M n , T m and T g of the sample in air was faster than in vacuum because of oxidative chain-scission. Biodegradation of PLA was retarded with increasing dose due to the introduction of crosslinking during irradiation. ESR spectroscopy showed five kinds of radicals assigned to three kinds of ESR spectra.
Keywords: Poly(lactic acid); Biodegradation; γ-Irradiation; Enzymatic degradation;

Thermal degradation of monodisperse polystyrene in bean oil by Dewen Dong; Shigeru Tasaka; Norihiro Inagaki (345-351).
Thermal degradation of monodisperse polystyrene (PS) in bean oil was investigated in the temperature range of 305–350°C. The degradation products were identified by means of gel permeation chromatography (GPC) and Fourier transform infrared spectrometry (FTIR). A significant feature for the investigated system was that PS in the presence of bean oil thermally degraded into larger fragments of the polymer chain instead of monomer or oligomers which are usually generated in the direct thermal degradation. The average molecular weights of degradation products of PS decreased and the polydispersity indexes (M w/M n) increased but did not exceed 2 as the reaction time increased. The kinetics of the degradation reaction was also examined based on the GPC measurements. The results revealed that the degradation of PS in bean oil was a first order reaction and the energy of activation for chain scission was 215 kJ mol−1. The thermal degradation of PS in bean oil is believed to be a random chain scission process involving radical intermolecular transfer between bean oil and intermediate macroradicals of PS.
Keywords: Bean oil; Gel permeation chromatography (GPC); Polystyrene; Thermal degradation;

The degradation of poly(ethylene oxide) (PEO) by electrolysis was investigated in an aqueous chloride or bromide medium adjusted to pH 6.5 at 25°C. The degradation was monitored by the reduction in viscosity of the reaction system. The PEO was degraded more readily by electrolysis in the presence of halide ion than in their absence. The degradability of PEO increased with an increase in halide ion concentration under a constant current and the degradation in a chloride solution is easier than that in a bromide solution. The initiation mechanism of PEO degradation is discussed; a hypohalide ion is generated during electrolysis in the presence of a halide ion, and the hydroxyl radical may be generated from this ion. This radical abstracts a hydrogen atom from the polymer to form polymer alkyl radical, and further cause chain scission.
Keywords: Poly(ethylene oxide); Oxidative degradation; Hydroxyl radical; Electrolysis;

The correlation between accelerated lightfastness testing and indoor exposure in softwood discoloration was evaluated. The discoloration of softwood exposed to indoor environments was simulated by using an accelerated lightfastness tester equipped with UVA-351 lamps. The trends of the discoloration of three species of softwood (western hemlock, China fir, Japanese cedar) in the accelerated lightfastness testing were similar to those in the indoor exposure test. Bathochromic effects occurred in the spectral reflectance curves of wood with increase in exposure time, and consequently, its color turned yellowish-red. Using the acceleration shift factor (ASF), we could obtain a correlation between accelerated lightfastness testing and indoor exposure. Results from the ASFs of three softwoods examined indicate that discoloration in the accelerated lightfastness test is 250 times more severe than that in the indoor exposure test.
Keywords: Softwood; Discoloration; Indoor exposure; Accelerated lightfastness test; Acceleration shift factor;

Influence of processing aids on the thermal and photostabilisation of HDPE with antioxidant blends by Norman S Allen; Eric Hoang; Chris M Liauw; Michele Edge; Eusebio Fontan (367-376).
The thermal and photostabilising performances of two commercial phenol/phosphite (Anox-Great Lakes) and three commercial phenol/phosphite/lactone (Ciba Specialty Chemicals) packages have been determined in HDPE in the absence and presence of two processing aids, calcium stearate and oleamide. Stabilities were assessed using FTIR, thermal analysis and mechanical property changes (tensile, elongation and impact). In terms of thermal stabilisation the phenol/phosphite blends are found to be superior to the Tris–phenol/phosphite/lactone blends. Solid-state (oven ageing) and melt (OIT) stabilities were found to follow a similar trend as does the yellowness index. The Anox blends were found to offer superior stability in terms of oxidative and mechanical performance. Colour inhibition was strong except in the case of calcium stearate. However, in the presence of calcium stearate strong synergism was observed with the Irganox blends and this may be associated with an exchange interaction/complexation between the calcium and the lactone/phosphite derivatives. These complexes appear to have high performance in thermal stabilization possibly through hydroperoxide decomposition and inhibition of carbonyl formation. In contrast, the effects of photostabilisation were significantly different with strong antagonistic effects in the presence of the calcium stearate due to the instabilities of any calcium complexes. Overall, the chemical and mechanical property changes followed a similar pattern.
Keywords: Processing aids; Antioxidants; Synergism; Antagonism; Slip agent;

Thermal behaviour of liquid crystal–polymer blends by D. Filip; C.I. Simionescu; D. Macocinschi (377-380).
Blends of isotropic amorphous polymers, i.e. poly(ether sulfone)s with cholesteryl palmitate as a low molecular weight liquid crystal were characterized by thermogravimetric analysis under dynamic conditions. The resulting data, together with the analysis of activation energies, demonstrated that adding the low molecular weight component caused thermal destabilization of these blends.
Keywords: Liquid crystal; Blend; Poly(ether sulfone); Thermal analysis;