Polymer Degradation and Stability (v.73, #1)

Aging effect on the reflectivity measurements of polychloroprene matrices containing carbon black and carbonyl-iron powder by Magali Silveira Pinho; Maria Luisa Gregori; Regina Celia Reis Nunes; Bluma Guenther Soares (1-5).
The effect of aging, arising from salt spray exposure, on the microwave reflectivity was studied for polychloroprene (CR) matrices, containing different kinds of active material in particulate form. The materials used as microwave absorbers in different contents were carbon black (CB) and carbonyl-iron powder (CIP). The microwave reflectivities were determined for the frequency range of 8–16 GHz. The formation of a superficial layer of iron oxide on CIP:CR composites was responsible for a reduction and change of microwave absorption towards a higher frequency. The reflectivity curves of CB:CR composites did not display any significant difference before and after the exposure.
Keywords: Microwave absorbing materials; Polychloroprene; Carbon black; Carbonyl-iron powder; Salt spray test; Waveguide;

Effect of dopant ions on thermal stability of conducting poly(o-phenitidine) by R.C. Patil; S. Radhakrishnan; K. Ogura (7-13).
The thermal degradation/stability of poly(o-phenitidine) doped with different sulfonic acids was investigated with the help of thermogravimetric analysis coupled with mass spectrometer. Thermal decomposition behaviour was found to depend on the nature of dopant ion and the thermal stability was in the order of DBSA>CSA>PTSA. The dopant ion was found to dissociate prior to the main chain scission. These findings could be explained on the basis of strong coupling/complexation between the conducting polymer and the dopant as well as the stability of the individual dopant species.
Keywords: Conducting polymers; Thermal stability; Poly(o-phenitidine); Sulfonic acid dopants;

Thermo-oxidative stability of PP waste films studied by imaging chemiluminescence by Gustav Ahlblad; Pieter Gijsman; Björn Terselius; Anna Jansson; Kenneth Möller (15-22).
Imaging chemiluminescence (ICL) was used to study the oxidative stability of PP waste films and of unstabilised PP particles for comparison. It was found that the unstabilised particles oxidised homogeneously with no induction period, whereas stabilised PP waste films showed a considerable distribution of induction times sometimes affected by the presence of threadlike defects. Individual specimens of a population of PP films all showed a sigmoidal increase of the CL intensity after the induction period. A sequence of chemiluminescence images showed a propagating oxidation front and a perfect match of the fraction of oxidising material covered by the front and the increase in chemiluminescence intensity. The role of physical spreading in the oxidation of stabilised PP material was clearly demonstrated. On artificial recycling of these films ICL showed a significant change in the oxidative stability (residual lifetime) much earlier than FTIR (carbonyl index) and DSC (oxidation induction temperature).
Keywords: Chemiluminescence; Imaging; Oxidation; Polypropylene;

The early stages of thermal degradation of poly(styrene sulfone)s were studied with polymers from the copolymerization of styrene (M) with sulfur dioxide (S). The experiments were carried out in the temperature range 190–200°C in the early conversion range of 0–0.4% in nitrogen. The activation energy of thermal degradation was found to be in the range 180–300 kJ mol−1. The activation energy decreased with an increase in the content of sulfur dioxide in the polymer and with an increase in the content of SMS triad sequences. The activation energies of thermal degradation of SMS and MMS sequences were calculated as 175 and 390 kJ mol−1, respectively. It was found that SMS triad monomer sequence is the most sensitive microstructure in thermal degradation of the poly(styrene sulfone)s.
Keywords: Thermal degradation; Poly(syrene sulfone); Microstructure;

Thermal analysis of poly(vinyl alcohol)/graphite oxide intercalated composites by Jiayan Xu; Yuan Hu; Lei Song; Qingan Wang; Weicheng Fan; Guangxuan Liao; Zuyao Chen (29-31).
Poly(vinyl alcohol)/graphite oxide intercalated nanocomposites have been prepared and characterized by X-ray diffraction. Their differential scanning calorimetry and thermogravimetry results suggest that the content of graphite oxide change the glass transition temperature of nanocomposites, and the increase of graphite oxide content can cause the increase of the oxidation temperature of nanocomposites and decrease of their mass loss rate.
Keywords: Graphite oxide (GO); Intercalation; Poly(vinyl alcohol) (PVA);

The ultrasonic degradation of poly(vinyl acetate) was carried out in the presence of an oxidizing agent, benzoyl peroxide. The time evolution of the molecular weight distribution (MWDs) was determined by gel permeation chromatography. A mechanism based on radical reactions is proposed. The rate coefficients for these reactions were determined by analyzing the MWDs by continuous distribution kinetics. A stoichiometric kernel that accounts for preferential mid-point breakage of the polymer chains was used. The degradation rate coefficient of the polymer decreased with increasing benzoyl peroxide concentration, as predicted by the model.
Keywords: Sonochemistry; Degradation; Poly(vinyl acetate); Continuous distribution kinetics; Benzoyl peroxide;

Mathematical analysis of the mechanical behavior of 60Co-irradiated polyolefin blends with and without woodflour by C Albano; J Reyes; J González; M Ichazo; R Poleo; E Davidson (39-45).
The aim of this work was to analyze by means of simple mathematical models the effect of different integral doses of γ irradiation on the mechanical properties of blends of polypropylene (PP) with virgin and recycled high density polyethylene (v-HDPE, r-HDPE), with and without woodflour-type filler. Analysis of the results at an integral irradiation dose between 0 and 50 kGy shows that crosslinking predominates in the PP/v-HDPE blend whereas degradation is predominant in the PP/r-HDPE blend, due to the characteristics of the recycled material. For the 50–70 kGy interval, both blends (PP/v-HDPE, PP/r-HDPE) suggest the possibility of competing processes, chain breaking being the most marked in the PP/r-HDPE blend. For the 0–50 kGy range in the blends with woodflour, the behavior of the PP/v-HDPE/woodflour composite was more complex since the polynomial equations are of third order for both tensile properties analyzed: Young's modulus and elongation at break. The behavior of the PP/r-HDPE/woodflour composite is complex suggesting simultaneous crosslinking and degradation. Both composites show a bimolecular process within the 50–70 kGy range which signifies a second-order dependence.
Keywords: Mechanical properties; Mathematical models; Irradiation; Polyolefin blends; Composites;

Vacuum pyrolysis of commingled plastics containing PVC II. Product analysis by R. Miranda; H. Pakdel; C. Roy; C. Vasile (47-67).
Vacuum pyrolysis of individual plastics including high density polyethylene (HDPE), low density polyethylene (LDPE), polypropylene (PP) and polystyrene (PS) has been performed at a final temperature of 500°C and under a total pressure of 2 kPa. A PVC sample and a commingled plastic sample composed of HDPE/LDPE/PP/PS/PVC have been pyrolyzed in two consecutive steps at 360 and 520°C in order to separately trap HCl. A pyrolysis set-up consisting of a bench scale batch reactor with a condensation train was used to collect the pyrolysis products. The main gaseous and liquid products, including the chlorinated hydrocarbons, were identified by chromatography. Global and chlorine mass balances were performed. The yields and pyrolysis product composition from each single polymer were compared with those from the commingled plastics. At 520°C, 99.55 wt.% of the total chlorine was trapped as HCl corresponding to 4.6 wt.% in yield, on an original feedstock basis, followed by 62.0 wt.% wax, 31.1 wt.% oil, 1.0 wt.% gas and 1.2 wt.% solid char residue (loss was 0.1 wt.%). The pyrolysis oil contained 12 ppm Cl on a pyrolysis oil basis. This value was less than what was expected (153 ppm) from the theoretical values which were calculated when the individual polymers were pyrolysed. The chlorinated hydrocarbons formed during the pyrolysis of commingled plastics were different from those obtained during the thermal decomposition of pure PVC. However, the hydrocarbons produced in the oil fraction were similar to those identified during the pyrolysis of the single plastics.
Keywords: Commingled plastics; PVC; HDPE; LDPE; PP; PS; Thermal degradation; Vacuum; Pyrolysis;

A low density polyethylene (LDPE) film used as an air and water vapour barrier for 15 years in an exterior wooden wall construction has been examined regarding ageing properties. The wall in question was part of a small house used for building material testing purposes only. During its use, the major part of the film was firmly pressed between plywood boards made of spruce and plasterboards. By pure chance, however, parts of the film had been hanging loose over the plasterboard into the room and thus exposed to room temperature air as opposed to the rest of the film. Different parts of the same LDPE film had thus been exposed to natural ageing in different environments allowing for interesting comparisons to be made. Tensile testing, size exclusion chromatography (SEC), oxygen induction temperature measurements, UV and FTIR spectroscopy, and MALDI mass spectrometry have been used to examine the differently aged parts of the LDPE film. It was found that the film contained the antioxidant β-(3,5-di-tert-butyl-4-hydroxyphenyl)-propionic octadecyl ester (trade name Irganox 1076). Despite the fact that the part of the film found inside the wall had lost about 75% of its initial antioxidant concentration as compared to the part exposed to the room's environment, the latter had a substantially lower stabilising power as indicated by a considerably lower oxygen induction temperature. Most likely,the antioxidant has been “deactivated” through its protective action in preventing oxidative degradation of the polymer of the “room” film. The loss of antioxidant in the “wall” film can most likely be attributed to migration and loss to the surrounding wall construction materials. Despite substantial losses of active antioxidant for both parts of the film, through different mechanisms, both parts have retained their tensile properties as indicated by normal values of elongation- and stress-at-break. Furthermore, the SEC chromatograms indicate no degradation of the polymer in neither parts of the film. Accordingly, the polymer itself seems not yet to have been affected by ageing.
Keywords: LDPE; Ageing; Polymer degradation; Antioxidant; Migration; Diffusion;

Experimental study of polystyrene thermolysis to moderate conversion by W.Jerome Sterling; Katherine S. Walline; Benjamin J. McCoy (75-82).
A general model for polymer degradation by concurrent random and chain-end processes was developed using continuous distribution kinetics. Population balance equations based on fundamental, mechanistic free radical reactions were solved analytically by the moment method. The model, applicable to any molecular weight distribution (MWD), reduces to the cases of independent random or chain-end scission. Polystyrene degradation experiments in mineral oil solution at 275–350°C supported the model and determined reaction rate parameters. The degradation proceeded to moderate extents requiring a MW-dependent random scission rate coefficient. Throughout the experiments, the polymer MWD, which was measured by GPC, maintained the exponential form, so that the logarithm of MWD varies linearly with MW, with slope directly related to the number-average MW. Polystyrene random scission activation energy was 7.0 kcal/mol, which agrees well with other thermolysis investigations but is lower than that found by pyrolysis due to fundamental differences between the processes. Magnesium oxide, added as a heterogeneous catalyst in solution, was found to have no effect on polystyrene degradation rate.
Keywords: Distribution kinetics; Random scission; Pyrolysis; Molecular weight distribution; Polystyrene degradation;

The thermal degradation of poly(vinyl acetate) (PVAc) was carried out in the presence of various concentrations of aluminum chloride (AlCl3) at different temperatures. The time evolution of molecular weight distributions (MWDs) was determined by gel permeation chromatography. A continuous distribution kinetic model was used to evaluate the degradation rate coefficient assuming the reaction is first order in polymer concentration and that the decomposition of AlCl3 is first order. Experimental data indicated that the presence of AlCl3 greatly enhanced the rate of degradation of PVAc. The activation energy of the degradation rate coefficient for PVAc in presence of aluminium chloride was 14.2 kcal/mol.
Keywords: Thermal degradation; Poly(vinyl acetate); Aluminum chloride; Catalytic degradation; Continuous distribution kinetics;

The polymer precursor technique is a simple and inexpensive method for the preparation of high temperature superconductor (HTSC) powders and films. This paper presents the results of thermoanalytical (TA) and FTIR investigations of poly(methacrylic acid) (PMAA) and Y–Ba–Cu poly(methacrylic acid) precursors. There are significant differences in the thermal degradation in air and in inert atmosphere. This knowledge is used to explain the specific two-step preparation procedure of the HTSC thin film. Superconducting transition temperatures of resulting thin film samples are reported. Furthermore, the preparation of Ca and K doped YBCO from analogous PMAA precursors was investigated. The influence of these dopants on the superconducting transition temperature is demonstrated.
Keywords: Polymer degradation; Precursor; High temperature superconductor preparation;

Effects of repeated extrusion on the properties and durability of rigid PVC scrap by Nazdaneh Yarahmadi; Ignacy Jakubowicz; Thomas Gevert (93-99).
Determining retained properties and durability are among the most important tasks when evaluating the possibility of mechanical recycling of plastic waste. In this investigation, profiles made of rigid PVC have been studied after various numbers of repeated extrusions to gain knowledge about the changes of properties and durability after conventional processing. New indoor profiles of PVC were re-extruded, from one to five times without adding new additives. The material was characterised after each extrusion using measurements of colour, degree of gelation, stress–strain at break and UV-Vis-NIR spectroscopy. The durability of the material was evaluated by accelerated ageing in ventilated heat ovens with low laminar airflow, at various temperatures. Changes in elongation at break, colour and chemical characteristics, depending on degradation, were measured and compared for the material after each extraction and various periods of ageing time. The material showed an improvement of mechanical properties after the second extrusion. This is explained by an increase in the degree of gelation of the PVC material. The main colour change due to repeated extrusion was yellowing (Δb). In accelerated heat ageing, the main colour change was towards red (Δa) and a change in greyness (ΔL) as a result of thermal dehydrochlorination. The activation energies of the degradation were also determined for the material after each re-extrusion and were found to decrease with an increasing number of extrusions. The estimated lifetime for indoor use at room temperature for the material after five extrusions was one-third of the lifetime of the material extruded only once. Despite this finding, a long lifetime can be expected even for the material subjected to five consecutive extrusions, making the material suitable for mechanical recycling.
Keywords: Rigid PVC; Repeated extrusion; Recycling; Accelerated ageing; Lifetime; Activation energy;

Detection of toxicity released by biodegradable plastics after composting in activated vermiculite by Francesco Degli-Innocenti; Gaetano Bellia; Maurizio Tosin; Anu Kapanen; Merja Itävaara (101-106).
The composting test method based on activated vermiculite is a comprehensive system for the assessment of the environmental impact of biodegradable plastics. It allows, in a single test, (i) the measurement of the mineralization of the polymer under study; (ii) the retrieval of the final polymeric residues and (iii) determination of the biomass (to make a final mass balance); (iv) detection of breakdown products of the original polymer. In this study it is shown that the vermiculite test method is also suitable to perform ecotoxicological studies. The Flash test is a method based on kinetic measurement of bioluminescence by Vibrio fischeri, and was applied to evaluate the toxicity of compost samples and vermiculite samples after the biodegradation of a polyurethane (PU) based plastic material. Toxicity was detected in vermiculite samples contaminated by 4,4′ diamino diphenyl methane (MDA), a toxic breakdown product released by the PU moiety, as shown by HPLC. On the other hand, neither toxicity nor the presence of MDA was detected in mature compost. A recovery experiment previously performed had shown a 10% MDA recovery yield from mature compost. The possibility of testing the ecotoxicity of extracts obtained from mineral matrix after biodegradation makes the vermiculite test system particularly interesting for the overall assessment of the environmental impact of biodegradable plastics.
Keywords: Biodegradation; Composting; 4,4′ Diamino diphenyl methane; Ecotoxicity; Flash test; Polyurethane; Vermiculite; Vibrio fischeri;

The photochemical stability of collagen type I from young albino rat tail tendon in the presence of glutathione (GSH) was investigated using viscosimetry, UV–vis spectrophotometry and spectrofluorimetry. The results suggest that glutathione makes collagen less resistant to the photolytic action of UV radiation. However, this antioxidant can protect the protein from UV photo-oxidation and the protection effect depends on its concentration in the protein solution.
Keywords: Collagen; Rat tail tendon; UV radiation; Glutathione;

The compatibility of ethylene–propylene–diene terpolymer with polypropylene is assessed by differential scanning calorimetry using the O'Neill method over the temperature range from 335 to 410 K. Molar heat capacities of various blends of these two polyolefins were determined for four compositions (20/80, 40/60, 60/40 and 80/20% w/w) and for individual components. Linear relationships between the contributions of components to overall C p values were evaluated for unprocessed and γ-irradiated mixtures. The composition of polymer specimens does not influence the relative contribution of constituents to the calculated C p values for tested blends exposed to the same dose. The effect of dose is pointed out for the characterisation of radiation compatibility.
Keywords: Ethylene–propylene terpolymer; Polypropylene; Blends; Compatibility; DSC; Molar heat capacity;

Photoinduced chemical crosslinking activity and photo-oxidative stability of amine acrylates: photochemical and spectroscopic study by Norman S Allen; Michaela C Marin; Michele Edge; Derek W Davies; John Garrett; Frank Jones (119-139).
A number of dialkyl and heterocyclic amine-terminated diacrylate resins were prepared by a Michael addition reaction of the appropriate secondary amine with a triacrylate diluent monomer. Photocuring rates are found to be dependent upon the extent of UV absorption of the photoinitiator used with ITX in this case exhibiting the most rapid cure. There is no consistency in cure rate with amine structure. Transient absorption spectra on conventional microsecond flash photolysis are assigned to the formation of free radical intermediates by electron transfer. Photoinduced polymerisation activities of the resins measured by real-time infrared spectroscopy appear to be closely related to the formation of such transient species. The initiator structure is the more determining factor. Their photo-oxidative stability and photoyellowing (by UV derivative absorption) are studied after UV and electron-beam curing through Fourier transform infra-red (FTIR) and second derivative UV spectroscopic methods and hydroperoxide analysis. On photo-oxidation UV cured diluent monomer undergoes oxidation at a higher rate than EB cured systems. Initial photo-oxidation of the coatings as measured by hydroperoxide analysis and hydroxyl index via FTIR analysis exhibited similar changes. Again the oxidation profiles are dependent upon the terminal amine structure with 1-methylpiperazine exhibiting the strongest oxidation. Hydroxyl index showed generally an initial rise followed by a sharp decline and then a slow increase over much longer irradiation periods. The presence of the amine functionality is found to be an effective scavenger of oxygen and hydroperoxide formation in EB cured coatings. In the UV cured coatings the hydroperoxide levels are found generally to be significantly higher than in EB systems due to the photosensitising effect of the residual photoinitiator. Diluent monomer terminated with dialkylamine groups are found to be more prone to oxidation and UV absorption increase than alkanolamines, cycloaliphatic amines and heterocyclic amines. UV cured resins exhibit a more facile transient photoyellowing than the same electron-beam cured systems, and this is associated with hydrogen-atom abstraction and oxidation of the alkylamine group by the residual photoinitiator enhancing the rate of hydroperoxidation of the amine group.
Keywords: Photochemistry; Photopolymerisation; Photocuring; Photoinitiators; Amine acrylates; Hydroperoxides; Yellowing; Photooxidation;

Effects of microtoming on oxidation of ultra high molecular weight polyethylene (UHMWPE) by K Jacobson; L Costa; P Bracco; N Augustsson; B Stenberg (141-150).
It has been found that the rate of oxidation, as measured by the growth of the carbonyl band of oven aged microtomed UHMWPE films, is dependent on the velocity at which the films are microtomed. The same observation was made when real time chemiluminescence measurements were performed in air at 120°C. The lower the cutting velocity, the faster the oxidation. By using NO derivatisation of unaged films, it was discovered that the hydroperoxide contents were also higher the lower the cutting velocity was. The knife used was quite blunt and left scratch marks on the films. When a sharper knife was used, no clear correlation could be found between the cutting conditions and the rate of oxidation. The films cut with the sharp knife did, however, also contain hydroperoxides. The most probable explanation of the observation that additional hydroperoxides were formed during cutting with the blunt knife is oxidation induced by the cobalt present in the tungsten carbide knife. It was found that the oxidation of the films was accelerated by just placing the knife on top of them during oven ageing.
Keywords: UHMWPE; Ultra high molecular weight polyethylene; Microtomy; Oxidation; Chemiluminescence; Hydroperoxides;

The influence of dicumyl peroxide (DCP) (5, 10 and 15% by mass) on the structure and thermal properties of two different paraffin waxes was investigated. For both waxes an increase of gel content was observed for increasing DCP concentration. Gel permeation chromatography, however, showed a decrease in intensity of the main peak, and the development of a lower molar mass peak. Differences were observed between the DSC curves for the untreated and treated samples, but these could not, with certainty, be linked to either cross-linking or degradation. TGA curves showed two well-developed decomposition steps for treated samples, which is in line with the development of a second peak in the GPC chromatogram.
Keywords: Paraffin wax; Dicumyl peroxide; Degradation; Cross-linking; DSC; TGA; GPC;

The thermal and mechanical properties of linear low-density polyethylene (LLDPE)/wax blends, prepared in an extruder, were investigated, using differential scanning calorimetry (DSC) and tensile testing. DSC measurements indicated that blends consisting of 10 and 20% of wax are probably miscible in the crystalline phase. For 30% and more wax, phase separation of the two components were observed. The DSC curves of both pure LLDPE and blends show, beside the main exothermic peak, another small peak at about 70°C. Since this peak was observed for both pure LLDPE and the blends, it is probable that its origin is partly in the LLDPE structure, but it is also influenced by wax crystallization. An increase in Young’s modulus with an increase in wax content was observed. An increase in wax content causes a decrease in elongation at yield. A small increase in yield stress was observed for blends consisting of 10 and 20% of wax. For blends consisting of 30% and more wax, no yield point, but brittle rupture, was observed.
Keywords: LLDPE/wax blends; Thermal analysis; Tensile properties;

The thermal properties of porous polystyrene-type beads with different degrees of crosslinking by divinylbenzene were studied by thermogravimetry, differential scanning calorimetry, infrared spectroscopy and Raman spectroscopy. The results showed that thermal decomposition temperature of the beads was raised with increased crosslinking in the porous beads, suggesting that their thermal stability could be greatly improved by crosslinking as expected. DSC analysis revealed that porous beads with crosslinking degrees less than 30% (in weight) had a glass transition and that its temperature (T g) was gradually changed from about 100°C (T g of linear polystyrene) to 160°C as the crosslinking was increased. However, T g disappeared and an exothermic peak around 165°C was found for highly crosslinked beads with more than 30% of crosslinking. The exothermic peak might be attributed to the coupling reaction and/or thermal polymerization of pendant double bonds at high temperature as proved by IR and Raman spectra. When crosslinking was less than 10%, a melting process could be observed as an endotherm at about 270°C. Otherwise, if the crosslinking was above 15%, no melting was displayed and instead of it an exothermic phenomenon took place between 250–350°C. The exotherm might be due to the decomposition of some groups formed in the preparation and storage of the beads.
Keywords: Thermal property; Thermal stability; Crosslinked polystyrene; Porous materials; Thermogravimetry; Differential scanning calorimetry;

The thermal stability of lignin-hydroxy terminated polybutadiene (HTPB) copolyurethane and HTPB polyurethane has been studied by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and isothermal aging in air at various temperatures. All the results show better thermal stability of lignin–HTPB copolyurethane than that of HTPB polyurethane. The structural changes in the polyurethanes during isothermal heating have also been studied by IR spectroscopy.
Keywords: Thermal stability; Lignin; Copolyurethane; Hydroxy-terminated polybutadiene; Isothermal aging;

Oxidation fronts in polypropylene as studied by imaging chemiluminescence by Petter Eriksson; Torbjörn Reitberger; Gustav Ahlblad; Bengt Stenberg (177-183).
The oxidation of isotactic polypropylene (PP) films containing three different commercial stabilisers has been studied at 150°C in air using imaging chemiluminescence (ICL). The oxidation was initiated using unstabilised PP powder, UV-irradiation or copper. It was shown that the initiated local oxidation spread as a propagating oxidation front in the stabilised PP films. By comparing the speed with which the propagating oxidation front spread, the performance of different stabilisers could be assessed. Furthermore, the influences of temperature, antioxidant concentration and sample thickness on the front speed were compared. The advantage of this technique is a controlled initiation of the oxidation that minimises the influence of weak points in the films. Furthermore, the experimental time is considerably shortened. The position of the oxidation front has been monitored using both ICL and infrared-microscopy. It was concluded that the technique is effective when evaluating stabiliser performance. Furthermore, it allows the study of oxidation phenomena such as oxidative spreading. The technique was also applied to polyamide 6, however, in this polymer the oxidation did not show the same spreading behaviour.
Keywords: Imaging chemiluminescence; Chemiluminescence; CL; ICL; Oxidation; Polypropylene; PP; Polyamide; Antioxidants; Heterogeneous oxidation; Oxidative spreading;

The formation of gaseous products resulting from thermo-oxidation of isotactic polypropylene (iPP) at 95 and 140°C in an atmosphere composed of a blend of 32O2 and 36O2 has been investigated using mass spectroscopy (MS). In contrast to the results obtained previously in the case of photochemical oxidation, MS analysis shows that thermo-oxidation does not generate 34O2. A possible interpretation is that, in thermo-oxidation, the mechanism involving the recombination of peroxy radicals POO occurs only with a very low probability. As a consequence, it is suggested that, in thermo-oxidation conditions, the main pathway of oxidation involves the formation of hydroperoxides and not recombination of peroxy radicals as proposed in photo-oxidative conditions.
Keywords: Polypropylene; Thermo-oxidation; Mass spectroscopy; Blends 32O236O2;

Inhibition of Chain Reactions by F Gugumus (189).