Polymer Degradation and Stability (v.89, #3)

Thermal stability of nanocomposites based on polypropylene and bentonite by Florêncio G. Ramos Filho; Tomás Jeferson A. Mélo; Marcelo S. Rabello; Suédina M.L. Silva (383-392).
A polycationic bentonite clay (PB) was organically modified with a quaternary organic salt and added to isotactic polypropylene (PP). The compounds were prepared by melt intercalation using a twin extruder then characterized by X-ray diffraction (XRD), thermogravimetry (TG) and scanning electron microscopy (SEM) for clarifying the composite nanostructure. Compression moulded films were exposed to a thermal oxidative environment at 110 °C in an attempt to evaluate the thermal stability of PP matrix after chemical modification of bentonite. The carbonyl index results, as obtained by infrared spectroscopy, showed that the modified clay had higher thermal stability in the solid state than the natural clay. This may be connected to a higher dispersion of clay particles, reducing oxygen diffusion through the sample. On the other hand, the degradation of the composites was more intense than the unfilled polymer and this may be due to the presence of acidic sites on the clay surface that act as a catalyst to the polymer oxidation, and/or due to salt decomposition, initiating the free radical degradation of PP.
Keywords: Bentonite; Polypropylene; Nanocomposites; Thermal degradation;

On the use of femtosecond laser ablation to facilitate spectroscopic depth profiling of heterogeneous polymeric coatings by Lionel T. Keene; Thomas Fiero; Clive R. Clayton; Gary P. Halada; David Cardoza; Tom Weinacht (393-409).
Spectroscopic depth profiling of organic coatings is useful for characterizing the nature and spatial distribution of weather-induced aging on a coating system. Due to their highly heterogeneous compositions and general opacity, high-solids organic coatings can be extremely difficult to depth profile using established techniques. This work evaluates the feasibility of using femtosecond time-scale laser pulses as a method for preparing coating samples for the purpose of depth-resolved spectroscopic analysis. Samples were prepared by ablating square regions of various depths in military coating samples. Ablated regions were characterized morphologically using a custom scanning confocal profilometer and spectroscopically using Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy/Energy Dispersive Spectroscopy (SEM/EDS) and dispersive Raman spectroscopy. Depth profiles collected from ablated samples were compared with profiles of the same material collected using a cross-sectional transmission-mode FTIR technique. A threshold level was found, below which a compound-selective ablation process was observed. Additional ether cross-linking was found to occur at moderate/high energy levels. Material in the residual interaction volume showed indications of reconstituted urethane functionality.
Keywords: Multiphoton-absorption photopolymerization; FTIR; Polyurethane; Depth-profile; Photodegradation;

Thermal degradation of chemically modified polysulfones by Gábor Molnár; András Botvay; László Pöppl; Kornél Torkos; József Borossay; Árpád Máthé; Tünde Török (410-417).
The thermal stability of some known (bromomethylated, carboxylated, brominated, nitrated) and novel (tertiary amine) polysulfone (PSU) derivatives has been studied by thermogravimetry (TG). The thermal decomposition products have been identified by various coupled mass spectrometric (MS) methods. From these data, the possible decomposition mechanism was deduced.
Keywords: Polysulfone derivatives; Thermal stability; Thermal degradation; Thermogravimetry; Mass spectrometry;

Polypropylene-graft-maleic anhydride (PPgMA)/montmorillonite nanocomposites have been prepared using direct melt intercalation techniques (extrusion and injection molding). Characterization of the nanocomposites, performed with X-ray diffraction and transmission electronic microscope techniques, suggested an importance of the mode of preparation and the type of organic modification that was used in the montmorillonite on the dispersability of clay in the polymer matrix. Injected mold nanocomposites were generally exfoliated in opposite to extruded samples that presented different structures (exfoliated, immiscible). Thermal degradation and fire behaviour of the corresponding nanocomposites displayed an improvement in comparison to pure PPgMA; this improvement was more important for injected mold specimen than for the extruded ones. It was ascribed to the presence of clay layers which act as diffusion barriers and reduced the mass loss; results that got the support of Limiting Oxygen Index measurements. A deterioration of these properties was observed on UV-irradiated nanocomposites. This was ascribed to the generation of weak points issue from the photo-oxidation reactions that take place mainly on the sample surface as shown in the oxidation profile. The oxygen starvation and reduced radical mobility, due to the presence of clay platelets, are blamed to be responsible for this oxidation profile. On the other hand, pure PPgMA showed an outstanding improvement of the fire behaviour with ageing time. Improvement that was attributed to cross-linking reactions which create a compact structure that is less inclined to volatilize.
Keywords: Polypropylene-graft-maleic anhydride; Nanocomposites; Photo-ageing; Thermal stability; Fire retardancy; Limiting Oxygen Index;

The application of a novel profluorescent nitroxide to monitor thermo-oxidative degradation of polypropylene by Aaron S. Micallef; James P. Blinco; Graeme A. George; Damien A. Reid; Ezio Rizzardo; San H. Thang; Steven E. Bottle (427-435).
The novel profluorescent nitroxide, 1,1,3,3-tetramethyldibenzo[e,g]isoindolin-2-yloxyl (TMDBIO) was investigated as a probe for the formation of polymer alkyl radicals during the thermo-oxidative degradation of unstabilised polypropylene. TMDBIO possesses a very low fluorescence quantum yield due to quenching by the nitroxide group; however, when the free-radical moiety is removed by reaction with alkyl radicals (to give an alkoxyamine), strong fluorescence is observed. Using spectrofluorimetry, the reaction of the nitroxide with polymer alkyl radicals during oxidation has been monitored. Significantly, the trapping of polymer alkyl radicals during the “induction period” at 120 °C is observed, when it is not possible to detect changes in the polymer using either chemiluminescence or infrared spectroscopy. This highlights the sensitivity of this method and represents the direct observation of free-radical generation in polypropylene in the “induction period”. TMDBIO also successfully stabilises polypropylene under thermo-oxidative conditions, which is consistent with its action as a radical trap. At elevated temperatures (150 °C), at the end of the “induction period” when the polymer is extensively degraded, the fluorescence decreases, due to secondary oxidation of the TMDBIO.
Keywords: Profluorescent nitroxide; Radical trap; Alkoxyamine; Polypropylene; Thermal oxidation; Stabilisation; Degradation; Induction period; Fluorescence; Chemiluminescence;

Electron beam induced crosslinking of nylon 6 with and without the presence of TAC by Susan Dadbin; Masoud Frounchi; Davood Goudarzi (436-441).
The influence of electron beam irradiation on neat nylon 6 was manifested by formation of molecular branches deduced from increase in melt flow index and intrinsic viscosity. Crosslinking of nylon 6 was carried out using electron beam irradiation in presence of triallyl cyanurate (TAC). The crosslinked samples showed a high level of gel content. At a given dose, increasing the TAC level from 1% to 3% increased the gel content of the samples slightly and correspondingly decreased the percent of water absorption. The melting temperature decreased with increasing the dose and the TAC level. All samples containing TAC showed some decrease in crystallinity under exposure to electron beam. The crosslinked nylon 6 samples showed higher thermal stability as it was verified by its heat deflection temperature. Elongation at break decreased while the tensile strength and yield stress increased as a function of TAC level and absorbed dose.
Keywords: Nylon 6; Triallyl cyanurate; Electron beam; Crosslinking; Heat deflection temperature;

Investigating the role of anionic surfactant and polymer morphology on the environmental stress cracking (ESC) of high-density polyethylene by Afshin Ghanbari-Siahkali; Peter Kingshott; Dag Werner Breiby; Lise Arleth; Carina Koch Kjellander; Kristoffer Almdal (442-453).
The influence of a commercial grade and a pure anionic surfactant (dodecylbenzenesulfonic acid sodium salt, DBS) in aqueous solutions on the environmental stress cracking (ESC) of high-density polyethylene (HDPE) has been investigated. Injection moulded HDPE samples in “dog-bone” shape were exposed to the aqueous solutions of surfactant (0.14 M) at 80 °C under strain (ɛ  = 1.2%) for 5 days. Both the commercial and pure surfactants caused cracks under the experimental conditions. The change in surface chemistry of the HDPE was followed using attenuated total reflectance Fourier transform infrared (ATR-FTIR) and Raman spectroscopy. The thermogravimetric analysis (TGA) did not show any significant differences in thermal stability of the samples studied. Differential scanning calorimetry (DSC) analysis, however, revealed an increase in crystallinity of the exposed samples. Wide Angle X-ray Scattering (WAXS) indicated that the HDPE sample has a boundary of both preferred and random (isotropic) crystallite orientations close and far away from the injection moulding inlet, respectively. The structure of the surfactants in solution was investigated by light scattering (LS). This showed that the commercial grade surfactant forms large aggregates (e.g., 500 nm) both at room temperature and at the exposure temperature of 80 °C, whereas the pure surfactant has a critical micelle temperature (CMT) of around 45 °C. Below the CMT aggregates are formed, and above the CMT the surfactants are unimeric. Matrix-assisted laser desorption ionisation time-of-flight mass spectrometry (MALDI-TOF-MS) analysis revealed that the commercial surfactant has a wide distribution of hydrocarbon chains from C-10 to C-13, whereas the pure one contains predominantly hydrocarbon dodecyl (chain of C-12).
Keywords: HDPE; ESC; Surfactant; DBS; Crystalline orientation; Degradation; CLSM; WAXS; Light scattering; Raman;

The evolution of the gases evolved during the catalytic pyrolysis of three commercial ethylene-vinyl acetate copolymers has been studied by TGA/FTIR. Three zeolites have been selected as catalysts: HUSY, HY and HZSM-5. The results obtained have been compared with those corresponding to the thermal pyrolysis and to the catalytic pyrolysis in the presence of mesoporous MCM-41. The effect of the catalysts consists mainly an increase in the yield of alkenes and a shortening and/or an increasing of the branching of the hydrocarbons obtained. The extent of such effects depends on the catalytic activity of the acid solids studied.
Keywords: Catalytic pyrolysis; EVA copolymers; TGA; FTIR; MCM-41; HUSY; HZSM-5; HY;

Study of hygrothermal ageing of glass fibre reinforced PET composites by M.P. Foulc; A. Bergeret; L. Ferry; P. Ienny; A. Crespy (461-470).
Hygrothermal ageing has been investigated on glass fibre reinforced polyethylene terephthalate (PET) composites using complementary techniques and a multiscale approach in order to identify the different steps of the material's degradation. For early ageing times (t  < 6 h), DMTA tests give evidence of the plasticisation of the PET matrix. GPC measurements and acid end group titration show that the chemical degradation step of the composites occurs immediately and that the main degradation mechanism is random chain scission. The changes in morphology resulting from hydrolysis, investigated through DSC and SAXS experiments, reveal a decrease in the long period that may result from the diffusion of oligomers out of the spherulites. The water uptake for long ageing times is attributed to an interfacial debonding which induces an osmotic pressure in this area. Photomechanical measurements highlight the development of microcracks within the aged material that induced an increase in the variation of material volume.
Keywords: PET composites; Hygrothermal ageing; Plasticisation; Hydrolysis; Osmotic cracking; Interfacial debonding; Photomechanics;

A model of radiation-induced degradation of the poly(vinylidene fluoride) surface during XPS measurements by I.V. Voinkova; N.N. Ginchitskii; I.V. Gribov; I.I. Klebanov; V.L. Kuznetsov; N.A. Moskvina; L.A. Pesin; S.E. Evsyukov (471-477).
Carbon core-level spectra of poly(vinylidene fluoride) (PVDF) have been studied by X-ray photoelectron spectroscopy (XPS) using a non-monochromatic (Al K α) source. Surface degradation has been observed during prolonged measurements. Monotonic modification of the C1s-line satellite during dehydrofluorination of PVDF suggests the emergence of additional possibilities for inelastic energy losses, which may be due to the formation of some new peculiarities in the combined density of states (DOS) caused by the change in the hybridisation mode of carbon valence electrons. A simple mathematical model has been proposed to describe the radiation-induced carbonisation of PVDF. The calculated data nicely fit experimental results.
Keywords: Poly(vinylidene fluoride); XPS; Radiative degradation; Mathematical model;

Synergistic effect of natural zeolites on flame retardant additives by H. Demir; E. Arkış; D. Balköse; S. Ülkü (478-483).
The thermal degradation and flammability of intumescent fire-retardant polypropylene matrix composites consisting of ammonium polyphosphate (APP) as an acid source and blowing agent, pentaerythritol (PER) as a carbonisation agent and natural zeolite (clinoptilolite rich, Gördes II) as a synergistic agent were examined. APP and PER combinations were examined at different ratios (0.25, 0.33, 0.5, 1, 2, 3, and 4) for optimisation of the formulation for flame retardancy. The zeolite was incorporated into flame retardant formulation at four different concentrations (1, 2, 5, and 10 wt%) to investigate the synergism between the flame retardant materials. The thermal degradation and flammability of composites were characterized by thermogravimetric analysis (TGA) and by limiting oxygen index (LOI) measurement and horizontal burning (HB) tests. A synergistic effect in flame retardancy was observed when natural zeolites were used in combination with APP and PER.
Keywords: Natural zeolite; Intumescence; Flame-retardant polypropylene;

Ethylene-propylene-diene (EPDM) elastomers, containing various concentrations of the third monomer, 5-ethylene-2-norbornene (ENB), were crosslinked using dicumyl peroxide. The influence of the amount of peroxide on the UV stability was investigated. The crosslinked EPDM samples were UV aged using a weather-O-meter (WOM) and by oxygen absorption experiments. It was found that crosslinking of ENB-containing EPDM with dicumyl peroxide dramatically influenced the UV ageing. For WOM ageing, linear relationships were found between relative carbonyl absorbance and third monomer content as well as between relative carbonyl absorbance and peroxide concentration, at a fixed ageing time. The values of the relative carbonyl absorbance as a function of the WOM ageing time showed a trend comparable to that observed in the oxygen absorption experiments. We showed previously [Snijders EA, Boersma A, van Baarle B, Noordermeer J. Effect of third monomer type and content on the UV stability of EPDM. Polym Degrad Stab, in press] the linear trends for the relative carbonyl absorbance and the oxygen absorption rate as a function of the third monomer content for uncompounded, non-crosslinked EPDM elastomers. The microhardness of the various crosslinked EPDMs as a function of the WOM ageing time showed a maximum, although the samples were crosslinked before UV ageing. The decrease in microhardness, after the maximum, did not depend on the peroxide concentration. However, it was linearly dependent on the third monomer content. The crosslink density as a function of the WOM ageing time was determined using equilibrium swelling experiments of the samples in decalin. These results showed a trend comparable to that for microhardness. It was also shown that the relationship between the soluble part of the sample and the microhardness throughout the WOM ageing was linear. This shows indeed that upon UV ageing crosslinking and chain-scission reactions compete.
Keywords: Ethylene-propylene-diene rubber; EPDM; 5-Ethylidene-2-norbornene; Dicumylperoxide; UV; Stability; Degradation;

The composite containing 20 wt% of osteoconductive β-tricalcium phosphate and 80 wt% of bioabsorbable poly-l,d-lactide (ll/dd dimer ratio 96/4) was manufactured using twin-screw extruder and self-reinforcing technique. The self-reinforcing created the porous surface and increased the mechanical properties of the composites. The in vitro degradation behavior of the self-reinforced composite was investigated comparatively with plain self-reinforced matrix polymer. The samples studied were characterized determining the changes in mechanical properties, thermal properties, molecular weight, mass loss and water absorption in phosphate buffered saline at 37 °C for up to 104 weeks. The results showed that the presence of the β-tricalcium phosphate modified the degradation of the matrix polymer. The degradation rate of the self-reinforced composites was slower than the degradation rate of the self-reinforced matrix polymer.
Keywords: Hydrolytic degradation; Composite; Polylactide; Self-reinforced; Tricalcium phosphate;

Thermal and thermo-oxidative behaviour of butadiene–styrene copolymers with different architectures by Silvestru Bogdanel Munteanu; Mihai Brebu; Cornelia Vasile (501-512).
The thermal and thermo-oxidative behaviour of three types of styrene–butadiene (BS) copolymers namely (poly(styrene-b-butadiene-b-styrene) linear triblock copolymer, poly(butadiene-b-styrene)4 star block copolymer and a poly(butadiene-r-styrene) random copolymer) have been studied. It was shown by FT-IR, DSC and DMTA experiments that during the heating up to 200 °C, chain-scission and crosslinking reactions occur in all copolymers. The temperature interval over which this process occurs is larger and at higher temperatures for the random copolymer compared with the two block copolymers. The random copolymer proved to have the highest thermal stability due to the stronger interactions between the styrene and butadiene units. Concerning the thermal degradation/decomposition products, the three BS copolymers produce similar amounts of gases but different quantities of oils. The distribution of the compounds in BS degradation oil depends on the copolymer structure, while the residue structures are similar for all studied copolymers.
Keywords: Styrene–butadiene copolymers; Thermal; Thermo-oxidative degradation; Stability;

Simple observation of surface wetting phenomena, i.e. the behaviour of droplets and applied films on polymer surfaces is shown to be a promising approach to predict whether ESC will occur in given liquid/polymer systems. The present study included polycarbonate (PC), cyclic olefinic copolymer (COC), and acrylonitrile/butadiene/styrene (ABS) polymers. The most certain indication of a potential ESC problem is when a droplet of challenge liquid spontaneously spreads on the polymer surface. The ability to predict whether ESC will occur based on the behaviour of applied films appears to be possible but is less certain. Films that do not retract are a good indication of potential ESC phenomena. Films that do retract may just be an indication of a contaminated surface. This leads to a false sense of security. Therefore the film wetting/dewetting phenomena must be used with special care if potential ESC problems are to be predicted by this method.
Keywords: Environmental stress cracking (ESC); Surface wetting; Droplet spreading test; Wetting tension; Surface resistance;

The product yields are important characteristics of the various hydroperoxide decomposition reactions. The true bimolecular hydroperoxide decomposition is thought to be the dominant reaction in the early stages of polyethylene processing. The reaction probabilities and theoretical yields calculated for the reaction account for the ratio of alcohols to ketones in these early stages. The calculations yield a rate constant for the disproportionation between the alkoxy and peroxy radical that is of the same order as published data for corresponding very low molecular mass radicals. The other reactions of the caged radical pairs are the same as those discussed for the reaction between a hydroperoxide and an alcohol group. They easily account for the experimental data.Pseudo-monomolecular hydroperoxide decomposition involving a hydroperoxide group and a segment of the polymer requires only parameters already envisaged previously. It does not seem to be important for polyethylene processing. However, it has been envisaged for thermolysis of polyethylene hydroperoxides in a press. The product yields calculated in this work for this reaction do not agree with the data from thermolysis of polyethylene hydroperoxides. The calculations allow concluding that pseudo-monomolecular hydroperoxide decomposition cannot be the only reaction on thermolysis of polyethylene hydroperoxides.
Keywords: Polyethylene; Thermal oxidation; Processing; Hydroperoxides; Oxidation products; Alcohols; Ketones; trans-vinylene; Thermolysis; Mechanisms;

On the action of ozone on whole bovine blood by Franco Cataldo; Luigi Gentilini (527-533).
The reaction between whole bovine blood and ozone has been studied. The ozonization reaction was monitored spectrophotometrically by using the B-band and the Q-bands of hemoglobin contained in the red blood cells. Ozone causes the destruction of the hemin prosthetic groups present in the hemoglobin of the red cells and the toxicological and medical implications of this fact are briefly discussed.Although hemoglobin of the red cells appears to be the preferred target of ozone attack, there are other substrates in blood which are susceptible to ozonolysis reactions: fatty acids and cholesterol. The reaction between cholesterol and ozone was explored both polarimetrically and spectrophotometrically and it has been shown that this molecule also can be degraded by ozone in blood.
Keywords: Ozonolysis; Bovine blood; Hemoglobin; Methemoglobin; Cholesterol; Decomposition toxicology; Ozono-therapy;

The effect of Cu(I) and Cu(II) salts, halides and acetates on the kinetics and mechanism of the branched chain oxidation of hexano-6-lactam was studied. The effect of Cu salts on the oxidation is independent of the oxidation state of Cu ions and is similar to the effect of alkali metal salts; Cu cations do not seem to be oxidized or reduced in the course of the process. Similarly to alkali metal salts, Cu salts also affect the rates of the initiation and termination reactions of the chain oxidation, namely by their ability to coordinate with hydroperoxides and their peroxy radicals. Thus the Cu salts are able to induce homolysis of 6-hydroperoxyhexano-6-lactam, the branching reaction of the chain oxidation, the bimolecular termination of 6-peroxyhexano-6-lactam radicals leading to adipimide and peroxyadipamic acid, and most probably also the redox reaction of 6-hydroperoxyhexano-6-lactam with the aldehyde O=CH(CH2)4CONH2 formed from alkoxy radicals resulting from 6-hydroperoxyhexano-6-lactam homolysis via unstable hydroxy compound. All the above reactions can participate in the final effect of Cu salts on the kinetics and mechanism of N-alkyl amide oxidation.
Keywords: Oxidation of N-alkyl amides; Cu and Ag salts; Influence of salts on the oxidation mechanism; Kinetics and chemiluminescence;

A kinetic analysis based on a set of non-linear heating functions was developed to evaluate the thermal degradation of poly(vinyl butyral)/glass ceramic composites. The non-linear Arrhenius equation with respect to temperature can be exactly solved by introducing this non-linear heating function (integrating factor). The integral formula can then be used to determine kinetic parameters of solid-state reactions applying the non-linear heating strategies. Thermal decomposition of calcium carbonate in nitrogen and air was utilized to evaluate the reliability of the proposed algorithm using TGA. Results show that the kinetic analysis of this case was quite reasonable compared with the data reported in the literature. This analytical approach was applied on the kinetic analysis of PVB/ceramic thermal degradation. The kinetics of the PVB/ceramic degradation was discussed based on the analytical results. The results can be applied to the design and operation of the ceramic thermal processing.
Keywords: Non-linear heating function; Kinetic analysis; Thermal degradation; Poly(vinyl)butyral; Glass ceramic;

Stabilization effect of lignin in polypropylene and recycled polypropylene by A. Gregorová; Z. Cibulková; B. Košíková; P. Šimon (553-558).
The influence of lignin on the oxidative stability of polypropylene (PP) and recycled polypropylene (recPP) plaques has been examined by differential scanning calorimetry under non-isothermal conditions. Lignin was used as a dry powder in the concentration range from 0.5 to 10 wt%. Further, the mixture of lignin with Irganox 1010 was tested. Onset temperatures of oxidation, protection factors and antioxidant effectiveness were evaluated. The results show that lignin exerts a stabilizing effect in both virgin and recycled polypropylene. The protection factor increases with lignin content in the PP matrix. The mixture of lignin with Irganox 1010 exhibits a synergistic influence in the recycled polypropylene. Moreover, the influence of the lignin content on Vicat softening temperature (VST) was determined for the evaluation of polypropylene heat resistance. VST showed that the presence of lignin improves the heat resistance of PP and recPP plaques.
Keywords: Lignin; Irganox 1010; Antioxidant; Polypropylene; Recycled polypropylene; DSC; Vicat softening temperature;

A further investigation of the roles of multi-walled carbon nanotubes and clay in the fire retardancy of ethylene vinyl acetate copolymer (EVA) nanocomposites has been carried out. It was found that the nanotubes played an important role in the reduction of the peak of heat release rate by forming low permeability char containing graphitic carbon. The oxidation resistance of the char is a function of the degree of graphitisation. Adding clay into the nanotube/EVA composite tends to enhance the formation of graphitic carbon. The nanotubes also have the function of reducing surface cracks of chars to increase barrier resistance to the evolution of flammable volatiles and the oxygen ingress to the condensed phase.
Keywords: Carbon nanotubes; Clay; Fire retardant; Nanocomposites; Ethylene vinyl acetate;