Polymer Degradation and Stability (v.91, #7)

Calendar (I).

The physical properties of pressure sensitive rubber composites by M.H. El Eraki; A.M.Y. El Lawindy; H.H. Hassan; W.E. Mahmoud (1417-1423).
Nitrile butadiene rubber, NBR, structural foam of different apparent densities was obtained by using different concentrations of foaming agent, azodicarbonamide, ADC/K. The true stress–strain characteristics, in case of compression, of foamed samples after the application of cyclic stress–strain were measured. The effect of the cyclic stress–strain on strain energy density of ADC/K foaming agent-filled NBR rubber composites was studied. The mechanical parameters were found to depend on the foaming agent concentration and on the pre-cyclic fatigue number. Results also indicated that the strain energy decreased with filler concentration.The effects of the cyclic stress–strain on the conductivity of ADC/K foaming agent-filled NBR rubber composites were studied. The electrical properties were found to depend on the foaming agent concentration, the strain amplitude and the number of stress–strain cycles of pre-strain. This study was assisted by the current–voltage characteristics which were measured under the effect of different compression ratios: 0%, 5%, 10%, 15%, 20%, 25% and 30%. The free current carrier mobility and the equilibrium concentration of charge carriers in the conduction band were produced as functions of compressive strain. Results also indicate that there is a linear variation between pressure and conductivity for all samples, which means that these samples can be used as a pressure sensor.
Keywords: Foam; Hysteresis loss; Pre-strain; Pressure sensor;

Long term degradation of poly(ɛ-caprolactone) films in biologically related fluids by Juan Peña; Teresa Corrales; Isabel Izquierdo-Barba; Antonio L. Doadrio; María Vallet-Regí (1424-1432).
The aim of this work was to study the long term degradation behaviour of poly(ɛ-caprolactone) (PCL) films, potentially useful as substrates for tissue engineering, obtained by two different methods (compression moulding or casting in chloroform) in two biologically related media: phosphate buffered solution (PBS) and Dulbecco's modified Eagle's medium (DMEM). The films were characterized at different degradation times by differential scanning calorimetry (DSC), Scanning Electron Microscopy (SEM) and Fourier Transform Infrared Spectroscopy (FTIR). The molecular weight was determined by Gel Permeation Chromatography (GPC). Chemiluminescence (CL) was used to assess physical or chemical changes from the early stage of the degradation. A different behaviour is observed in samples immersed in PBS when compared with those treated in DMEM. In this medium, the degradation after one year and a half (18 months) shows that although the chemical structure has been modified, the layers become more fragile but maintain their consistency. A higher degradation rate is obtained for membranes obtained by casting with respect to those obtained by compression moulding.
Keywords: Degradation; Chemiluminescence; Biodegradable scaffold; Tissue engineering;

Role of the interphase dynamics in the induction time of the thermo-oxidation of isotactic polypropylene by Mario Hoyos; Pilar Tiemblo; José Manuel Gómez-Elvira; Lyda Rychlá; Jozef Rychlý (1433-1442).
This work completes an earlier study on the influence of molar mass and microstructure on the thermal stability of low molar mass isotactic polypropylenes (iPP). The relative dependence of induction time (t i) on both parameters has been assessed for new metallocene iPP samples (M-PP) with molar masses from 65,000 to 182,000. The new M-PP series includes one metallocene ethylene–propylene copolymer (M-EP) with an ethylene content of 2.6 mol%.When the new t i data are considered together with those previously reported, neither the molar mass nor the microstructure, taken as sole parameters, is able to explain the global t i evolution of the M-PP samples.On the basis of the results corresponding to the M-EP copolymer, it is proposed that local chain dynamics occurring at high free volume regions, associated to propylene segment interruptions, play a main role in the ability of the interphase to initiate the oxidation. The correlation of the characteristic interphase chain dynamics, as measured by DMTA, with t i data supports this suggestion. This hypothesis provides a more unified insight about the actual origin of the iPP thermo-oxidation, as it integrates the influence of parameters which have been found to drive partially the thermal stability of iPP, in particular, molar mass and microstructure.
Keywords: Polypropylene; Interphase dynamics; Thermo-oxidation; Microstructure;

Thermal oxidation and molding feasibility of cycloolefin copolymers (COCs) with high glass transition temperature by Mark O. Liu; Hong-Fan Lin; Ming-Ching Yang; Mei-Jyun Lai; Chih-Chieh Chang; Ming-Cheng Feng; Po-Ling Shiao; In-Mau Chen (1443-1447).
Cycloolefin copolymers (COCs) with high glass transition temperature (T g  = 203 °C) have been synthesized and pelletized by extrusion molding. However, their colors change from transparent to yellow during extrusion molding because of thermal oxidation and generation of alkene groups. We have successfully blended several antioxidants (Irganox 1010, Irgafos 168, Irganox HP2225 and Irganox HP2921) into lab-made COCs to avoid the discoloration. The experimental results show that Irganox HP2921 is the best antioxidant among the antioxidants used and can effectively not only suppress thermal oxidation but also eliminate the color stain.
Keywords: Cycloolefin copolymer; Extrusion molding; Discoloration; Color stain; Antioxidant;

Loss and recovery in hydrophobicity of silicone rubber exposed to corona discharge by Yong Zhu; Masahisa Otsubo; Chikahisa Honda; Shou Tanaka (1448-1454).
This paper describes the loss and recovery in the hydrophobicity of unfilled high temperature vulcanized silicone rubber (HTV-SR) resulting from corona discharge. In this study, HTV-SR specimens were exposed to corona stress generated by a parallel needle–plane electrode system, and physicochemical analyses on the surface layer of SR before and after corona discharge treatment were carried out by using Fourier transform infra-red (FTIR) spectroscopy, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). It is demonstrated that hydrophilic OH groups that are by-products of ageing can be formed instead of hydrophobic CH3 groups on the surface of aged SR and the corona discharge plays an important role on the temporary loss of hydrophobicity. After a substantial period of ‘rest’, the hydrophobic recovery of SR results from the diffusion of low-molecular weight (LMW) silicone fluid from the bulk onto the aged layer.
Keywords: Silicone rubber; Hydrophobicity; Corona discharge; SEM; FTIR; XPS;

Poly(methyl methacrylate)/silica/titania ternary nanocomposites with greatly improved thermal and ultraviolet-shielding properties by Haitao Wang; Peng Xu; Sheng Meng; Wei Zhong; Weichuan Du; Qiangguo Du (1455-1461).
Poly(methyl methacrylate) (PMMA)/silica/titania ternary nanocomposites with covalent bonding interaction between polymer and inorganic phases have been prepared using a novel non-hydrolytic sol–gel method. Transmission electron microscope (TEM) image of silica/titania binary inorganic component indicates a core-shell-like structure. Scanning electron microscope (SEM) images suggest that the well dispersed silica/titania particles in the hybrid are on the nanometer-scale. The transparencies of nanocomposites are maintained in visible region while the absorption band in ultraviolet (UV) region is red shifted with increasing inorganic content. The thermogravimetric analysis (TGA) results show that the thermal stability of PMMA copolymer increases dramatically with the addition of silica/titania moieties both in nitrogen and in air.
Keywords: Nanocomposites; Non-hydrolytic sol–gel method; Thermal properties; Thermogravimetric analysis (TGA); Transmission electron microscopy (TEM);

Electron spin resonance imaging (ESRI) was applied to the study of thermal degradation at 393 K of poly(acrylonitrile-butadiene-styrene) (ABS) prepared by emulsion polymerization and containing 25% wt butadiene (ABS-25B). The polymer was doped with 1 or 2% wt Tinuvin 770 as the hindered amine stabilizer (HAS). The spatial distribution of the HAS-derived nitroxide radicals, obtained by 1D ESRI, was initially homogeneous, but became heterogeneous through sample depth with increasing treatment time, t. The spatial variation of ESR line shaping with sample depth was visualized by 2D spectral-spatial ESRI. ESR spectra along the sample depth, obtained by nondestructive (“virtual”) slicing of the 2D images, were used to deduce the relative intensity of nitroxide radicals present in two dynamically distinct sites; the sites were assigned to butadiene-rich (fast component) and SAN-rich domains (slow component), respectively. 1D and 2D ESRI allowed the determination of the extent of degradation within morphologically-distinct domains as a function of sample depth and treatment time. The results from the ESRI experiments were substantiated by attenuated total reflectance (ATR)-FTIR spectroscopy of the outer layer (500 μm thick) of the polymer. Both techniques indicated faster degradation of polymer samples that contained the higher HAS content, 2% wt. Comparison with the results obtained for a parallel study of ABS prepared by mass polymerization and containing 10% wt butadiene (ABS-10B) indicated clearly that the rate of degradation of the polymer prepared by emulsion polymerization (ABS-25B) is significantly reduced. This result can be explained by the formation of cross-linked “composite” networks during emulsion polymerization, which leads to greater thermal stability.
Keywords: Polymer degradation; Poly(acrylonitrile-butadiene-styrene) (ABS); Hindered amine stabilizer (HAS); Electron spin resonance (ESR); Electron spin resonance imaging (ESRI);

Thermal degradation behaviors of some branched and linear polysiloxanes by Wenjun Zhou; Hui Yang; Xingzhong Guo; Jingjuan Lu (1471-1475).
The thermal degradation behaviors of some branched and linear polysiloxanes were studied by thermogravimetric (TG) analysis and infrared (IR) spectroscopy, and the effect of phenyl content on the thermal stability of the tested branched polysiloxanes was investigated. The branched polysiloxanes with phenyl obtain higher thermal stability at lower temperature. The degradation residues of the examined branched polysiloxanes at 800 °C are rather high, 65.1–77.6% in N2 and 40.5–66.5% in air, respectively. However, the amount of solid residue increases with decrease in phenyl content in both N2 and air atmospheres. It was found that the thermal stability of the branched polysiloxanes is higher than that of the linear one, which indicated that the branched structure benefits the formation of a crosslink in the solid residue.
Keywords: Polysiloxane; Thermal degradation; Thermal stability; Phenyl content; Branched structure;

Kinetic study of the thermal degradation of high density polyethylene by Michael Wallis; Suresh K. Bhatia (1476-1483).
The thermal degradation of high density polyethylene has been modelled by the random breakage of polymer bonds, using a set of population balance equations. A model was proposed in which the population balances were lumped into representative sizes so that the experimentally determined molecular weight distribution of the original polymer could be used as the initial condition. This model was then compared to two different cases of the unlumped population balance which assumed unimolecular initial distributions of 100 and 500 monomer units, respectively. The model that utilised the experimentally determined molecular weight distribution was found to best describe the experimental data. The model fits suggested a second mechanism in addition to random breakage at slow reaction rates.
Keywords: Degradation; Thermogravimetric analysis; Polyethylene; Population balance;

Low density polyethylene (LDPE) was modified to introduce biodegradability by grafting highly hydrophilic monomers (which can act as nutrients for microorganisms) such as glucose by a novel melt phase reaction using Brabender plasti-corder in the presence of ceric ammonium nitrate (CAN) to obtain 4-O-hydroxymethyl d-arabinose (sugar) end-capped LDPE (Su-g-LDPE) at a maximum grafting of 16%. The grafted polymers were characterized by FTIR, thermal analysis, WAXD and mechanical property measurements. The biodegradability of Su-g-LDPE was carried out by soil-burial test and by optical density measurements in presence of an aerobic bacterium Pseudomonas sp. The degraded polymer shows changes in weight, crystallinity and inherent viscosity. Optical density of the medium registered an increase with degradation. FTIR spectra of the degraded samples showed 70% decrease in the ketone carbonyl index (ν 1719/ν 1465) of Su-g-LDPE indicating microbial degradation of LDPE matrix, which was further confirmed by SEM micrographs. The present data support a microbial oxidation process involving β-oxidation whereby the carbonyl is further oxidized to carboxylic acid and affects cleavage of the LDPE chain at the ends.
Keywords: Low density polyethylene; Glucose; Melt phase grafting; Biodegradation;

Acquired biodegradability of polyethylenes containing pro-oxidant additives by Marek Koutny; Martine Sancelme; Catherine Dabin; Nicolas Pichon; Anne-Marie Delort; Jacques Lemaire (1495-1503).
Biodegrability of high density polyethylene film (HDPE) and low density polyethylene film (LDPE) both containing a balance of antioxidants and pro-oxidants was studied with defined microbial strains particularly with Rhodococcus rhodochrous and Nocardia asteroides in mineral medium. After an abiotic pre-treatment consisting of photooxidation and thermo-oxidation corresponding to about 3 years of outdoor weathering the samples were inoculated, incubated up to 200 days and during the period their metabolic activities were followed by measuring adenosine triphosphate content. Simultaneously the cultures were also monitored by optical microscopy and FTIR spectroscopy. The first initial phase of fast growth caused by the presence of low molecular extractable compounds was followed by a long period of stabilized metabolic activity suggesting that microorganisms continued to gain energy from the substrate but evidently at a much slower rate. Complementary analysis performed at the end of incubation revealed that during the experiment time biodegradation processes probably affected surface layer of materials only.
Keywords: Adenosine triphosphate; Polyethylene; Biodegradation; Pro-oxidant; Photooxidation; Thermo-oxidation;

The combined effect of photodegradation and stress cracking in polystyrene by A.R. Sousa; K.L.E. Amorim; E.S. Medeiros; T.J.A. Mélo; M.S. Rabello (1504-1512).
This work aims to analyze the effects of photodegradation on the stress cracking resistance of polystyrene. Injection moulded samples were exposed to the ultraviolet light for various times in the laboratory prior to solvent contact. The bars were then stressed in a tensile testing machine under the presence of butanol. During this period the stress relaxation was monitored and the ultimate properties were evaluated after selected periods of stress cracking. Complementary tests were done by size exclusion chromatography and by scanning electron microscopy. The results indicated that butanol causes significant modification in polystyrene, with extensive surface crazing as well as reduction in mechanical properties. This is intensified under higher mechanical stress. The previous degraded samples showed a higher level of stress relaxation and a greater loss in tensile strength in comparison to the undegraded ones. The synergist action of photodegradation and stress cracking in polystyrene may be a consequence of the chemical changes caused by oxidation like the formation of polar chemical groups and the reduction in molecular weight.
Keywords: Polystyrene; Photodegradation; Stress cracking; Mechanical properties;

Towards the development of thermally latent novolac-based char formers for ABS resins by Kangjun Seo; Jinhwan Kim; Jin-Young Bae (1513-1521).
Novel novolac-based char formers for ABS resins, [PN–PI], [PN–BPI], [CN–PI] and [CN–BPI], were prepared from phenol formaldehyde novolac (PN), cresol formaldehyde novolac (CN), phenyl isocyanate (PI), and 4-biphenyl isocyanate (BPI) via a simple urethane-forming reaction. The four compounds were used as thermally latent char formers for this study. Thus, a two component system employing novolac–isocyanate adduct as a char former, and tetra-2,6-dimethyl phenyl-resorcinol diphosphate (DMP-RDP) as a phosphorous-based flame retardant was blended with ABS, and the thermal degradation behaviour and flame retardancy were determined by thermogravimetric analysis (TGA) and LOI test. The mixtures show a synergistic effect between DMP-RDP and novolac–isocyanate adduct on the flame retardance enhancement of ABS. Those containing higher molar mass CN–BPI adduct are found to be most flame retardant, and a LOI value as high as 35 is obtained.
Keywords: Novolac; ABS; Char former; Flame retardancy;

Two-dimensional infrared (2D IR) correlation spectroscopy was applied to study the structural changes occurring in the decomposition of PHEA-co-MMA/SiO2. Complicated absorption spectral changes were observed in the heating process. 2D IR analysis indicates that during heating, covalent bonds, (Si–O–C), between the polymer and the inorganic moiety were formed, which was the main factor in the improvement in thermal properties of the hybrids such as the decomposition temperatures (T d). The thermal stability of the hybrids was also studied by solid-state 29Si MAS NMR spectroscopy and TGA tests. Their results complemented each other well.
Keywords: Two-dimensional infrared; Correlation spectroscopy; Hybrid; Thermal degradation;

Thermal stability and degradation behavior of novel wholly aromatic azopolyamide-hydrazides by Refah Farhan Al-Ghamdi; Mona Mohamed Fahmi; Nadia Ahmed Mohamed (1530-1544).
Thermal stability and degradation behavior of a series of novel wholly aromatic polyamide-hydrazides containing azo groups in their main chains have been investigated in nitrogen and in air atmospheres using differential scanning calorimetry (DSC), thermogravimetry (TG), infrared spectroscopy (IR) and elemental analysis. The influences of controlled structural variations and molecular weight on the thermal stability and degradation behavior of this series of polymers have also been studied. The structural differences were achieved by varying the content of para- and meta-substituted phenylene rings incorporated within this series. Azopolyamide-hydrazides having different molecular weights of all para-substituted phenylene type units were also examined. The polymers were prepared by a low temperature solution polycondensation reaction of p-aminosalicylic acid hydrazide [PASH] and an equimolar amount of 4,4′-azodibenzoyl chloride [4,4′ADBC] or 3,3′-azodibenzoyl chloride [3,3′ADBC] or mixtures of various molar ratios of 4,4′ADBC and 3,3′ADBC in anhydrous N,N-dimethyl acetamide [DMAc] containing lithium chloride as a solvent at −10 °C. All the polymers have the same structural formula except the mode of linking phenylene units in the polymer chain. The results clearly reveal that these polymers are characterized by high thermal stability. Their weight loss occurred in three distinctive steps. The first was small and assigned to the evaporation of absorbed moisture. The second was appreciable and was attributed to the cyclodehydration reaction of the hydrazide groups into 1,3,4-oxadiazole rings by losing water, combined with elimination of azo groups by losing molecular nitrogen. This is not a true degradation but rather a thermo-chemical transformation reaction of the azopolyamide-hydrazides into the corresponding polyamide-1,3,4-oxadiazoles. The third was relatively severe and sharp, particularly in air, and corresponded to the decomposition of the resulting polyamide-1,3,4-oxadizoles. In both degradation atmospheres, the improved resistance to high temperatures was always associated with increased content of para-phenylene moieties of the investigated polymer. The better thermal stability of the wholly para-oriented type of polymer relative to the other polymers is attributed to its greater chain symmetry which is responsible for its greater close packing, rod-like structure and consequently stronger intermolecular bonds which would be more difficult to break and therefore more resistance to high temperatures. Further, with exception of 160–200 °C temperature range, where the lower molecular weight samples showed considerable weight losses which were most probably due to hydrogen bonded DMAc, all the wholly para-oriented phenylene type of polymer samples behaved similarly regardless of their respective molecular weight. This seems to indicate that the structural building units responsible for high thermal stability of the polymers are their characteristic groups, such as aromatic moieties, amide and hydrazide linkages in case of azopolyamide-hydrazides, and 1,3,4-oxadiazole rings, aromatic nuclei and amide linking bonds in case of polyamide-1,3,4-oxadiazoles, rather than the longer chain segments.
Keywords: Azopolyamide-hydrazides; Thermal and thermo-oxidative stability; Differential scanning calorimetry; Thermogravimetric analysis; Polyamide-1,3,4-oxadiazoles;

Mechanism of glycolysis of nylon 6,6 and its model compound by ethylene glycol by Kap Jin Kim; D. Manjula Dhevi; Jong Soon Lee; Young Dal Cho; Eun Kyung Choe (1545-1555).
The degradation mechanism of nylon 6,6 and its model compound hexamethylenebis(hexamide) (HMHA) by ethylene glycol (EG) was studied in detail. Glycolysis of both model compound and nylon 6,6 was carried out with stoichiometric excess of EG at high temperature (250 and 275 °C, respectively) in an autoclave reactor as a closed system. Samples were collected at predetermined intervals and characterized by FT-IR, GC–MS, and GPC. FT-IR studies of model compound glycolysis showed no significant increase in the ester band after 11 h indicating that the glycolysis reaction may have reached equilibrium. GC–MS data revealed the presence of ester as well as unreacted model compound. GC–MS data for nylon 6,6 revealed the presence of compounds having β-hydroxyethylester group, bis(β-hydroxyethyl)adipate, and δ-valerolactone as the degradation products. From GPC data, as the glycolysis time of nylon 6,6 increased, both the number average and weight average molecular weights decreased indicating that the main chain of nylon 6,6 was broken into low molecular weight compounds during glycolysis.
Keywords: Nylon 6,6; N,N-hexamethylenebis(hexamide); Ethylene glycol; Glycolysis mechanism;

Evaluation of the rate of abiotic degradation of biodegradable polyethylene in various environments by Ignacy Jakubowicz; Nazdaneh Yarahmadi; Henrik Petersen (1556-1562).
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.
Keywords: Degradable polyethylene; Thermo-oxidation; Pro-degradant; Molecular weight; Biodegradation; Mineralization;

Optimisation of the effect of colemanite as a new synergistic agent in an intumescent system by U. Atikler; H. Demir; F. Tokatlı; F. Tıhmınlıoğlu; D. Balköse; S. Ülkü (1563-1570).
An intumescent system including ammonium polyphosphate (APP) as an acid source and blowing agent, pentaerythritol (PER) as a carbonific agent and colemanite as a synergistic agent is used to enhance flame retardancy of polypropylene (FR-PP). In order to investigate the synergism between colemanite and the flame retardant materials (APP and PER), D-optimal mixture design was employed. The limiting oxygen index (LOI) and amount of residue (AoR) were accepted as response 1 and response 2, respectively. Applying D-optimal strategy, 18 experiments were performed. Filler content was fixed at 30 wt% of total amounts of flame retardant PP composites. Constraints were determined according to the ratio of APP/PER ranging between 1 and 3. Statistical analysis of the cubic model revealed that lack of fit (LoF) was not significant for the cubic and linear model for both responses. The model suggested an optimum composite formulation with concentration levels 65% of APP, 28% of PER and 7% of colemanite that gives an LOI of 40.3. The experimental LOI and AoR of optimum formulation were achieved as 39.3 and 21.4 with 2.5% and 2.2% errors, respectively.
Keywords: Colemanite; Intumescent system; D-optimal mixture design; Polypropylene;

Migration of phenolic antioxidants from linear and branched polyethylene by M. Lundbäck; M.S. Hedenqvist; A. Mattozzi; U.W. Gedde (1571-1580).
Plaques of linear polyethylene (LPE) and branched polyethylene (BPE) were exposed to oxygen-free media (nitrogen or water) at 75, 90 and 95 °C. The polymers were stabilized with one of the following three bifunctional phenolic antioxidants: Santonox R, Irganox 1081 or Lowinox 22M46. The initial concentration of antioxidant in the plaques was ∼0.09 wt.%. After ageing, the oxidation induction time profiles obtained by differential scanning calorimetry often became very flat, which indicated that migration was controlled by the boundary loss process. The unexpected higher migration rate from LPE than from BPE was due to the dominance of the boundary loss process. It is proposed that the low boundary loss rate in BPE was due to the presence of a thin liquid-like (oligomeric) surface layer which developed during ageing of this polymer. A qualitative relationship was found between the boundary loss rate to water and the polarity of the antioxidant. The antioxidant diffusivities in LPE and BPE were approximately equal, a finding which, in view of the morphological analysis estimating the geometrical impedance factor, indicated that the constraining effect of the crystals on the non-crystalline fraction was not sensed by the antioxidant molecules. It is suggested that the large molecular size and the low segmental flexibility of the antioxidant molecules inhibited their ability to penetrate the interfacial component.
Keywords: Polyethylene; Migration; Phenolic antioxidants; Morphology;

Polyolefins with controlled environmental degradability by David M. Wiles; Gerald Scott (1581-1592).
Antioxidants and stabilisers, developed to increase the durability of polyolefins, in combination with prooxidant transition metal complexes provide industrial products with widely variable but controlled lifetimes. The low molar mass oxidation products formed during photo-oxidation and thermal oxidation are biodegradable and oxo-biodegradable polyolefins are now widely used in agricultural applications and in degradable packaging as examples. The scientific basis for the performance of oxo-biodegradable materials is explained with reference to naturally occurring macromolecules. Comparison with hydro-biodegradable materials is made and the need is demonstrated for performance standards to be developed that mimic nature's resource recovery mechanism, that of oxo-biodegradation.
Keywords: Oxo-biodegradable polyolefins; Peroxidation; Prodegradants; Antioxidants; Mulching films; Compostable packaging;

As part of a study on the kinetic modelling of polyethylene oxidation under irradiation at low temperature and low dose rate, this first part deals with the kinetic regime in which thermal initiation, linked to hydroperoxide decomposition, is negligible compared to radiochemical initiation due to polymer radiolysis. The kinetic analysis is based on results published 30 years ago by Decker, Mayo and Richardson. A small modification of their mechanistic scheme, consisting in the introduction of a non-terminating bimolecular combination of PO2 radicals, leads to a more consistent set of radiochemical yield values. The most significant change is a decrease in the radiochemical yield of radicals Gi from 10 to 8. At 45 °C, termination of PO2 radicals is not very efficient: 35–40% of the PO2  + PO2 encounters are non-terminating, 75% of the termination events lead to peroxide bridges, the rest is a disproportionation according to the Russell mechanism.
Keywords: Polyethylene; Irradiation; Oxidation; Kinetics; Radiochemical yields; Termination;

Oxidation of polyethylene under irradiation at low temperature and low dose rate. Part II. Low temperature thermal oxidation by N. Khelidj; X. Colin; L. Audouin; J. Verdu; C. Monchy-Leroy; V. Prunier (1598-1605).
This paper deals with the kinetic modelling of unstabilised polyethylene thermal oxidation, particular attention being paid to the domain of low temperatures, typically below 80 °C. Experimental data show that the temperature dependence of the induction time t i and the steady state rate of oxygen absorption r S display a discontinuity at 80 °C. A model based on the hypothesis that this discontinuity concerns only the PO2 bimolecular combination processes and is essentially explained by the competition between terminating and non-terminating PO2  + PO2 reactions, was proposed. With pertinent values of the Arrhenius parameters of the elementary reactions under consideration, the model fits well the experimental data (in the 40–200 °C temperature range) and is consistent with previously analysed results of radiochemical ageing. According to this model, 35–40% of the bimolecular PO2 combinations would not be terminating at 45 °C and this proportion would increase with the temperature. Concerning terminations, the relative fraction of coupling processes, leading to peroxide bridges, would decrease relatively to the disproportionation processes when the temperature increases.
Keywords: Polyethylene; Thermal oxidation; Kinetics; Modelling; Rate constants;

The product yields from the reaction between two hydroperoxide groups have been re-calculated. This is a consequence of the fact that β-scission of secondary alkoxy radicals cannot be neglected in the high temperature range of the polyethylene processing experiments (170–200 °C). It must be taken into account in addition to disproportionation/combination and hydrogen abstraction by alkoxy radicals. The increased complexity caused by the additional reaction results mainly from the larger number of caged radical pairs involved in the reactions and also in the calculations. Among other products it becomes possible to calculate the yields of aldehyde and vinyl groups that would not result from hydroperoxide decomposition in the absence of β-scission. The yields of the main oxidation products such as alcohols, ketones and trans-vinylene groups are reduced to some extent in comparison with the values calculated if β-scission is neglected. The vinyl group yield corresponds to slightly more than 10% of the yield of trans-vinylene groups in the temperature range of the experiments. The aldehyde yield is significantly larger than the vinyl group yield and is important in the whole temperature range examined. Main-chain scissions are important at the temperatures of the experiments. They become more important than the sum of the different combination reactions from a temperature of 200 °C on.
Keywords: Polyethylene; Hydroperoxides; Oxidation products; Alcohols; Ketones; Aldehydes;

ESR studies on poly(vinyl chloride) irradiated at medium and high doses by Juliette Colombani; Jacques Raffi; Thierry Gilardi; Michele Troulay; Bernard Catoire; Jacky Kister (1619-1628).
Room temperature gamma irradiation at medium (1–100 kGy) or high (0.9–25 MGy) doses induces different radicals into pure or industrial poly(vinyl chloride) (PVC); the various ESR signals differ by their shapes and/or values of their g-factors and/or peak-to-peak widths but do not give precise information on the chemical structures of the different radicals as the spectra are only poorly resolved. However, looking at the ESR parameters of the different signals and their evolution during natural ageing of the samples, allows us to make correlations between the radio-induced lines in industrial PVC and the ones induced in pure PVC or in PVC additives.
Keywords: ESR; PVC; Gamma irradiation; Ageing;

Low-density polyethylene (LDPE) and linear low-density polyethylene (LLDPE) with different copper contents were prepared by melt mixing. The copper powder particle distributions were found to be relatively uniform at both low and high copper contents. There was cluster formation of copper particles at higher Cu contents, as well as the formation of percolation paths of copper in the PE matrices. The DSC results show that Cu content has little influence on the melting temperatures of LDPE and LLDPE in these composites. From melting enthalpy results it seems as if copper particles act as nucleating agents, giving rise to increased crystallinities of the polyethylene. The thermal stability of the LDPE filled with Cu powder is better than that for the unfilled polymer. The LLDPE composites show better stability only at lower Cu contents. Generally, the composites show poorer mechanical properties (except Young's modulus) compared to the unfilled polymers. The thermal and electrical conductivities of the composites were higher than that of the pure polyethylene matrix for both the LDPE and LLDPE. From these results the percolation concentration was determined as 18.7 vol.% copper for both polymers.
Keywords: PE–Cu composites; Tensile properties; Thermal conductivity; Electrical conductivity; Differential scanning calorimetry; Thermogravimetric analysis;