Polymer Degradation and Stability (v.89, #2)

Weathering of wood modified with the N-methylol compound 1,3-dimethylol-4,5-dihydroxyethyleneurea by Y. Xie; A. Krause; C. Mai; H. Militz; K. Richter; K. Urban; P.D. Evans (189-199).
N-methylol compounds are used as a wrinkle-resistant finish in the textile industry. They are expected to enhance the resistance of wood to weathering because they can cross-link the cell wall and dimensionally stabilise wood. Scots pine veneers were modified with 1,3-dimethylol-4,5-dihydroxyethyleneurea (DMDHEU) to weight percent gains (WPG) of 10%, 27% or 48% and exposed to artificial weathering. Initially, weight losses of unmodified veneers were significantly greater than those of DMDHEU treated specimens even though DMDHEU was leached from wood at a higher rate than loss of wood substance. The weight losses of all treated veneers after 144 h of weathering, however, were similar to those of the unmodified controls. Therefore we conclude that in the short term DMDHEU treatment can restrict weight losses of wood during weathering, which occur due to degradation of lignin and hemicellulose and loss of degraded wood fragments from wood.Infrared spectroscopy suggested that treatment of wood veneers with DMDHEU to high WPG (48%) stabilised lignin to some extent. Tensile strength losses of DMDHEU treated veneers during weathering were lower than those of untreated veneers. DMDHEU treatment, however, had a deleterious effect on the tensile strength of the veneers, possibly associated with the presence of magnesium chloride catalyst in the treatment solution. Scanning electron microscopy revealed that DMDHEU treatment was highly effective at preventing the degradation of the wood cell wall during weathering. Tracheids in unmodified veneers became distorted within 48 h of weathering exposure, whereas cells in modified veneers, especially those reacted to higher weight percent gains, retained their shape even after 144 h weathering.
Keywords: Artificial weathering; UV degradation; Wood modification; DMDHEU; Electron microscopy;

Effect of third monomer type and content on the UV stability of EPDM by Emile A. Snijders; Arjen Boersma; Ben van Baarle; Jacques Noordermeer (200-207).
Various uncompounded ethylene–propylene–diene (EPDM) elastomers were photo-oxidized using standard Weather-O-Meter (WOM) aging and oxygen absorption measurements during UV irradiation. The influence of the type and amount of diene on the UV stability was investigated. FTIR (ATR) measurements of the carbonyl absorbance after WOM aging were comparable with the outcome of the oxygen absorption tests. Both techniques showed a decrease in UV stability with increasing diene content. In addition, EPDM containing 5-ethylidene-2-norbornene (ENB) as the third monomer showed a higher UV stability compared to EPDM containing dicyclopentadiene (DCPD) as the third monomer. Furthermore, a linear relation was found between the carbonyl absorbance after WOM aging and the third monomer content for DCPD- and ENB-containing EPDM samples with comparable ethylene/propylene (C2/C3) ratio. The microhardness (International Rubber Hardness Degree) and the Young's modulus of the various EPDMs as function of the WOM irradiation time showed a maximum. This maximum is the result of parallel photo-crosslinking and chain-scission reactions, the latter being dominant in the later stages of the oxidation process. Combining the results from the FTIR measurements with the Young's modulus showed that DCPD-containing EPDM has a higher propensity to crosslinking reactions compared to ENB-containing EPDM.
Keywords: Ethylene–propylene–diene rubber; EPDM; 5-Ethylidene-2-norbornene; Dicyclopentadiene; Carbonyl; UV; Stability; Degradation;

Unplasticised poly(vinyl chloride) (uPVC) subjected to thermophysical aging and cyclic stress (fatigue aging) was examined by differential scanning calorimetry. The case of thermophysical aging alone was also investigated in order to provide a control. Interestingly, an initial increase in mobility was found at the early stages of aging prior to the usual reduction in mobility for uPVC thermophysically aged without a stress input. It was found in aged uPVC that cyclic stress (at a stress maximum of 40 MPa) caused an initial decrease in the peak temperature for excess enthalpy (T P) followed by an increase, and simultaneously an initial increase in the equilibrium inflection temperature (T EI) followed by a decrease, as a function of number of stress cycles. These results can be attributed to the competing effects of de-aging (rejuvenation) and aging enhanced by the application of cyclic stress. It is concluded that fatigue aging is a combination of two dissimilar structural processes: one permanent and one non-permanent.
Keywords: Cyclic stress; Enthalpy relaxation; Thermophysically aged polyvinyl chloride; Aging;

Many reactions can yield ketones on polyethylene oxidation. Separation of the experimental data into parts arising from contributions by different mechanisms is one of the main problems in attributing ketone formation to specific reactions. This is achieved in polyethylene melts by fitting separately the data pertaining to the initial and those relative to the advanced stages of processing to an equation formed by a linear and a quadratic term in processing time. It shows that ketone formation in the high temperature range (170–200 °C) can be separated into two main parts that can be attributed to two different mechanisms. The first part corresponds to ketone formation without an induction period, i.e. at a constant rate at a given temperature. It is based on a cage reaction that is part of the chain propagation reaction. It involves a peroxy radical and the tertiary hydrogen atom of the secondary hydroperoxide whose formation on hydrogen abstraction by a peroxy radical is at the origin of the new peroxy radical. Monomolecular decomposition of the α-hydroperoxy alkyl radical formed in the reaction yields a ketone and a hydroxyl radical, which is continuing chain propagation. This part of ketone formation can be accounted for by solution-type homogeneous kinetics.The second part of the ketone formed shows a significant induction period which increases with decreasing temperature. It corresponds to ketone formation resulting from various hydroperoxide decomposition reactions. This mechanism, overall the same as that for alcohol formation, is well accounted for by the heterogeneous kinetics already used for explaining alcohol formation.
Keywords: Polyethylene; Thermal oxidation; Processing; Hydroperoxides; Oxidation products; Ketones; Thermolysis; Mechanisms; Kinetics;

The thermal degradation of a series of poly-n-alkyl acrylates, from methyl to dodecyl, and poly-n-alkyl methacrylates, from butyl to nonyl, has been studied in isothermal conditions at 400–650 °C using pyrolysis-gas chromatography/mass spectrometry. The type and composition of the pyrolysis products gave useful information about the mechanism of thermal degradation. It was shown that the main thermal degradation processes for poly-n-alkyl acrylates are random main-chain scission with the formation of monomer, dimer, saturated diester, trimer, corresponding acetate and methacrylate, and non-radical side-chain reaction through six-member ring transition state. The most abundant degradation products coming from the alkyl ester decomposition are the corresponding olefin, aldehyde, and alcohol. Poly-n-alkyl methacrylates yield monomer as the predominant degradation product in all investigated pyrolysis conditions. Unlike poly(methyl methacrylate) which gives quantitative yields of monomer, however, the poly-n-alkyl methacrylates with longer alkyl chain produce also significant amounts of olefin and methacrylic acid.
Keywords: Poly-n-alkyl acrylates; Poly-n-alkyl methacrylates; Thermal degradation mechanism; Pyrolysis-gas chromatography/mass spectrometry; Pyrolysis products;

The views from solution-type chemistry cannot be transposed to polymer melts. The bimolecular reaction between hydroperoxide and alcohol groups is a typical example in this respect. It is the dominant hydroperoxide decomposition reaction in the advanced stages of polyethylene processing in the high temperature range (170–200 °C). In part, the alkoxy/alkoxy radical pair resulting from the decomposition reacts directly in the cage by disproportionation and combination. In part, it is transformed into a caged alkoxy/alkyl radical pair following hydrogen abstraction by one of the alkoxy radicals. The alkoxy/alkyl radical pair reacts by combination or disproportionation in the cage or is transformed into an alkyl/alkyl radical pair on hydrogen abstraction by the remaining alkoxy radical. Combination or disproportionation of the alkyl/alkyl radical pair is considerably faster than oxygen addition and/or cage disruption. The bimolecular reactions of the three caged radical pairs remove the free radicals from the polymer melt. There is no chain reaction initiated by those radicals. This explains previous findings concerning the absence of initiation by the hydroperoxides formed and decomposed in polyethylene melts. The relative quantities of oxidation products calculated from the model are compatible with the experimental results. The comparison yields quantitative values for the probabilities of the different cage reactions. The rate constants for the reactions between the alkoxy/alkoxy and alkoxy/alkyl radical pairs can be deduced from the probabilities, taking intramolecular hydrogen abstraction by the alkoxy radical as a reference. They are comparable to the corresponding rate constants for low molecular mass compounds in solution. The knowledge of these values allows calculation of the oxidation product yields at any temperature.
Keywords: Polyethylene; Thermal oxidation; Hydroperoxides; Oxidation products; Alcohols; Ketones; trans-Vinylene; Ethers; Thermolysis; Mechanisms;

UV-laser-assisted degradation of poly(methyl methacrylate) by C. Wochnowski; M.A. Shams Eldin; S. Metev (252-264).
Poly(methyl methacrylate) (PMMA) has been irradiated by UV-laser light with different wavelengths (193 nm, 248 nm and 308 nm) in order to investigate the photolytic degradation of the physical–chemical molecular structure. The photoinduced chemical reactions at the polymer surface have been investigated by QMS, XPS, FTIR and NMR in order to clarify the degradation mechanism. It was also shown that in the UV-illuminated area a modification of the refractive index can be achieved which strongly depends on the irradiation conditions.
Keywords: UV-excimer laser; Poly(methyl methacrylate); X-ray photoelectron spectroscopy (XPS); Norrish reaction; Refractive index modification;

The procedures used in some very recent works for evaluation of the kinetic parameters from non-isothermal data corresponding to thermal and thermo-oxidative degradation of polymers and polymeric materials are critically analysed. It is pointed out that only the use of a set of conversion degree vs. temperature curves recorded at different heating rates can give reliable information on the degradation mechanism and in some suitable cases, can lead to the true kinetic parameters. A general algorithm for evaluation of the kinetic parameters from the non-isothermal data is suggested. The application of this algorithm for a set of simulated non-isothermal data has been shown that when the activation energy is independent of the conversion, the invariant kinetic parameters method associated with the criterion of independence of kinetic parameters on the heating rate is recommended for evaluation of the kinetic triplet (activation energy, pre-exponential factor, conversion function).
Keywords: Non-isothermal kinetics; Polymers; Polymeric materials; Thermal and thermo-oxidative degradation;

Ozone degradation of ribonucleic acid (RNA) by Franco Cataldo (274-281).
RNA (ribonucleic acid) from Torula utilis was subjected to ozonization both in aqueous solution and in the solid state. The ozonization in aqueous solution was followed by spectrophotometry and polarimetry. The latter technique was extremely useful in following the RNA degradation as function of the amount of O3 added to the solution. The RNA specific optical rotation drops to zero following a linear relationship with the amount of O3 added to the solution. The loss in the chirooptical properties of RNA and the reduction of the specific viscosity of the solution were interpreted in terms of denaturation of the RNA macromolecule and in terms of oxidation of the purine and pyrimidine bases which are the pendent groups of RNA macromolecule. The polarimetric measurement of the reaction between O3 and RNA has also allowed the measurement of the stoichiometric molar ratio at which the RNA optical rotation vanishes completely. This RNA/O3 molar ratio was found to lie in the range of 2.0–1.5, suggesting that the RNA denaturation is complete when at least 50% of the RNA monomeric units have reacted with O3. The RNA/O3 molar ratio was also checked iodometrically and found about 1.3. The differences between the polarimetric and iodometric stoichiometry are discussed.RNA ozonized in powder form in the solid state was found to be significantly less reactive than in the case of the aqueous solution. After 24 h ozonization in the solid state, RNA shows a certain reduction in the specific optical rotation (much less compared to RNA ozonized in aqueous solution at the equivalent nominal molar ratio) suggesting the occurrence of a certain degree of denaturation. The FT-IR spectra of the ozonized RNA are discussed in comparison to the reference pristine RNA spectrum.
Keywords: RNA; Ribonucleic acid; Ozone; Ozonization; Polarimetry; Iodometry; Viscometry; Denaturation; Stoichiometry;

The dynamic thermal degradation of poly(propylene carbonate)s (PPC)s with alternating molecular structure and varying molecular weights was studied using TG–DTA. Because of the thermal hysteresis and the dissolved air for bulk samples, it is beneficial to employ film samples for kinetic analysis. It was found that the DTA curve for low molecular weight PPC had both an endothermic peak and an exothermic peak, whilst the DTA curve for high molecular weight PPC exhibited only the exothermic peak. PPC with high molecular weight showed better thermal stability than that with low molecular weight. Kinetic analysis of the dynamic TG curves for PPCs with varying molecular weights was carried out using the Ozawa method. The decomposition activation energy of PPC was calculated to range from 105 to 118 kJ/mol. The influence of nitrogen flow rate on the decomposition of PPC was also investigated and is discussed.
Keywords: Polycarbonate; Carbon dioxide; Thermal decomposition; Ozawa method;

Abiotic hydrolysis of poly[(R)-3-hydroxybutyrate] (PHB) was investigated in acid and base media by monitoring the formation of two monomeric hydrolytic products, 3-hydroxybutyric acid (3HB) and crotonic acid (CA) from three types of PHB matrix, native amorphous granules, precipitates and solvent-cast films. Control experiments were also performed with five esters of 3-hydroxybutyric acid. The monomeric products were not released from the PHB specimens in acidic solutions (0.1–4 N H+), but were measured as the major hydrolytic products from alkaline hydrolysis (0.1–4 N OH). The amorphous PHB granules decomposed 30 times faster than the crystallized PHB precipitates and solvent-cast films. Formation of both CA and 3HB followed the quasi-0th-order kinetics and their mass ratio (CA/3HB = 0.42–0.68) increased with temperature because of the different activation energies, 88.4 kg/mol for CA formation and 78.9 kJ/mol for 3HB formation. Kinetic analysis and hydrolysis results revealed that dehydration of the 3-hydroxyl group is not a favorable reaction, compared to ester bond hydrolysis, under the chosen experimental conditions. A transient six-membered ring structure involving two neighboring monomers is most likely needed for crotonate formation.
Keywords: PHB; Polyhydroxyalkanoates; Biopolyester; Abiotic hydrolysis; 3-Hydroxybutyric acid; Crotonic acid;

The change in polymer structure induced by irradiation with high-energy electrons (6 MeV) was studied for a polymer film of poly(ether sulphone) (PES). This kind of irradiation has been chosen as a method of generating crosslinking and chain-scission. It was found from the measurements of gel fraction that the probabilities of crosslinking and chain-scission depend on the irradiation dose. Sol–gel measurements gave G (crosslinking)  = 0.052 and G (chain-scission)  = 0.012. Consequently, increasing irradiation dose resulted in higher degree of crosslinking.The correlation between the degree of crosslinking and fluorescence lifetimes has been obtained. Thus, an increase in the degree of crosslinking shifts the distribution of fluorescence lifetimes towards shorter values.
Keywords: Poly(ether sulphone); Sol–gel studies; Fluorescence lifetimes distributions; High-energy electrons irradition;

Reaction mechanism of styrene monomer recovery from waste polystyrene by supercritical solvents by Huang Ke; Tang Li-hua; Zhu Zi-bin; Zhang Cheng-fang (312-316).
The degradation of polystyrene in supercritical benzene, toluene, ethylbenzene and p-xylene was studied at 310–370 °C, 4.0–6.0 MPa using a novel fast process. Supercritical degradation has unique properties such as excellent heat and mass transfer. By using this technique, polystyrene has been successfully depolymerised into monomer, dimer and other products in a very short reaction time with high conversion. Toluene used as supercritical solvent was more effective than other solvents such as benzene, ethylbenzene and p-xylene for the recovery of styrene from polystyrene, though the conversions of polystyrene were similar in all the above solvents. The highest yield of styrene obtained from polystyrene in supercritical toluene at 360 °C for 20 min reached 77 wt.%. The reaction mechanism consists of depolymerisation of polystyrene and secondary reactions due to closed system. This study indicated that different supercritical solvents affected these two processes differently.
Keywords: Supercritical solvents; Degradation; Polystyrene;

In the liquid-phase cracking of bisphenol-A types of epoxy resin and polycarbonate resin at 440 °C in the presence of solvents such as tetralin, decalin or cyclohexanol, we recovered more than 40 wt% of phenol and isopropylphenol as monomers. On the other hand, bisphenol-A, which was common monomer from epoxy resin and polycarbonate resin, could also be recovered with high yield, when reaction was carried out at lower temperature of 300–350 °C. However, degradation of polycarbonate resin did not proceed in the thermal conditions without catalyst. Mechanistic difference between epoxy resin and polycarbonate resin in the liquid-phase cracking is discussed on the basis of the product distribution from resins and their appropriate model compounds.
Keywords: Liquid-phase cracking; Monomer recycling; Bisphenol-A; Epoxy resin; Polycarbonate resin;

The thermal degradation behaviour of hemp (Cannabis sativa L.) fibres under a nitrogen atmosphere was investigated by using thermogravimetry (TGA). The kinetic activation energy of treated fibres was calculated from TGA data by using a varied heating rate from 2.5 to 30 °C/min. The greater activation energy of treated hemp fibre compared with untreated fibre represented an increase of purity and improvement in structural order. A hydrophobic solvent affected the degree of non-cellulosic removal. Mercerisation and enzyme scouring removed non-cellulosic components from the fibre; however, structural disruption was observed after higher alkaline concentration, 20 %wt/v and longer scouring time, respectively. Structural disruption was observed by X-ray measurement. The FTIR results indicated an elimination of the non-cellulosic components by the mercerisation treatment and a specific removal of low methoxy pectin by use of pectate lyase enzyme (EC 4.2.2.2). An increase of temperature at the maximum rate of degradation and the rate of weight loss was characteristic of the purity and structure of treated hemp fibre.
Keywords: Cellulose fibre; Thermal degradation; Kinetics; Scouring; Crystallinity;

Hydrolytic degradation of poly(ε-caprolactone) in the melt by Hideto Tsuji; Tomoyuki Ono; Takashi Saeki; Hiroyuki Daimon; Koichi Fujie (336-343).
Rubbery biodegradable polyester poly(ε-caprolactone) (PCL) having a relatively low melting temperature (ca. 60 °C) was hydrolysed in the melt in high-temperature and high-pressure water at a wide temperature range of 175–370 °C for periods up to 30 min and formation and decomposition of 6-hydroxycaproic acid (6-HCA) and molecular weight change of PCL were investigated. More than 90% of PCL was hydrolysed to water-soluble oligomers and monomer within 30 min when hydrolysed in the temperature range of 300–370 °C. Within 30 min, 6-HCA was obtained at the maximum yields exceeding 80% as far as hydrolysis was carried out in the temperature range of 300–370 °C, whereas the maximum yield decreased to 15% due to incomplete hydrolysis when hydrolysis temperature was lowered to 250 °C. The optimal temperatures for PCL (300–370 °C) were higher than those for poly(l-lactide) (PLLA) (≤270 °C) and poly[(R)-3-hydroxybutyrate] [R-P(3HB)] (≤200 °C), meaning the high stability of 6-HCA at high temperatures. Too long hydrolysis of PCL at 300–370 °C caused the decreased yield of 6-HCA due to its decomposition. The hydrolysis of PCL proceeds homogeneously and randomly via a bulk erosion mechanism. The molecular weight of PCL decreased exponentially without formation of specific low-molecular-weight chains originating from crystalline residues. The activation energy for the hydrolysis in the melt (ΔE h) obtained for PCL, 72.3 kJ mol−1 is higher than the 51.9 kJ mol−1 for PLLA, lower than 126 kJ mol−1 for R-P(3HB), and much lower than 233 kJ mol−1 for poly(ethylene terephthalate). This study revealed that the hydrolysis of PCL in the melt is an effective and simple method to obtain its monomer 6-HCA and to prepare PCL having different molecular weights without containing the specific low-molecular-weight chains, because of the removal of the effects caused by crystalline residues.
Keywords: Hydrolytic degradation; Hydrolysis; Recycling; Biodegradable; Biomaterials; Polyesters;

Use of oxide nanoparticles and organoclays to improve thermal stability and fire retardancy of poly(methyl methacrylate) by A. Laachachi; E. Leroy; M. Cochez; M. Ferriol; J.M. Lopez Cuesta (344-352).
Nanocomposites of PMMA–oxide (oxide: nanoparticles of TiO2 or Fe2O3), PMMA–organoclays (organomodified montmorillonite: OMMT) and PMMA–oxide–OMMT were prepared by melt blending with different additive contents. These nanocomposites were studied by thermogravimetric analysis (TGA) and cone calorimetry. The experimental results obtained by TGA show that TiO2 and Fe2O3 nanoparticles improve the thermal stability of PMMA by about 50 °C from 5 wt% of fillers. Cone calorimeter measurements show that the peak of heat release rate is lowered in the presence of oxide nanoparticles in comparison to pure PMMA and that this decrease is higher when the filler content increases. The time to ignition increases in the case of TiO2, but remains constant for Fe2O3. A synergistic effect was also found by the combination of TiO2 and organoclays resulting mainly in an increase of the ignition time and the reinforcement of the barrier effect of the organoclays. A possible mechanism of improvement of the thermal stability and fire retardant properties of PMMA mixed with TiO2, Fe2O3, OMMT and oxide–OMMT is discussed.
Keywords: Thermal degradation; Poly(methyl methacrylate); PMMA; Flame retardant; Fire retardancy; Nanocomposite; TiO2; Fe2O3; Organoclay; Nanoparticles;

A decomposition chemistry and heat transfer model to predict the response of removable epoxy foam (REF) exposed to fire-like heat fluxes is described. The epoxy foam was created using a perfluorohexane blowing agent with a surfactant. The model includes desorption of the blowing agent and surfactant, thermal degradation of the epoxy polymer, polymer fragment transport, and vapor–liquid equilibrium. An effective thermal conductivity model describes changes in thermal conductivity with reaction extent. Pressurization is modeled assuming: (1) no strain in the condensed-phase, (2) no resistance to gas-phase transport, (3) spatially uniform stress fields, and (4) no mass loss from the system due to venting. The model has been used to predict mass loss, pressure rise, and decomposition front locations for various small-scale and large-scale experiments performed by others. The framework of the model is suitable for polymeric foams with absorbed gases.
Keywords: Removable epoxy foam; Decomposition; Confinement; Pressurization; Adsorbed gases;

Modifications induced by gamma-irradiation in ethylene–propylene–diene terpolymer surrounded by methylcyclopentane vapour were studied. Investigations of gel content, mechanical strength (tensile strength and elongation at break), thermal oxidation resistance (oxygen uptake), and the absorbances in IR spectra were performed and showed improvement in the material stability. Several exposure doses were selected (10, 20, 30, 50, and 80 kGy). For comparison, irradiation in the presence of oxygen (mixture of methylcyclopentane vapours and air) was also carried out.
Keywords: Ethylene–propylene terpolymer; Irradiation methylcyclopentane; Mechanical properties; Gel content;