Polymer Degradation and Stability (v.91, #1)

Direct insertion probe pyrolysis mass spectrometry (DIP-MS) analyses of poly(methyl methacrylate) (PMMA), poly(vinyl acetate) (PVAc) and binary PMMA/PVAc guests, coalesced from their inclusion compounds (ICs) formed with host γ-cyclodextrin (γ-CD) through removal of the γ-CD host, have been performed. A slight increase in the thermal stabilities of the coalesced polymers were recorded both by TGA and DIP-MS compared to the corresponding as-received polymers. The DIP-MS observations pointed out that the thermal stability and degradation products of these polymers are affected once they are included inside the IC channels created by the stacked host γ-CDs. DIP-MS observations suggested that the degradation mechanisms for PMMA and PVAc chains in their coalesced blend were significantly altered from those observed in their as-received and solution blended samples. This was attributed to the presence of specific molecular interactions between the intimately mixed PMMA and PVAc chains in their coalesced blend.
Keywords: Cyclodextrin; Inclusion compound; Poly(methyl methacrylate); Poly(vinyl acetate); Blend; Thermal degradation;

Influence of oxygen plasma treatment on poly(ether sulphone) films by Jiachun Feng; Guian Wen; Wei Huang; En-Tang Kang; Koon Gee Neoh (12-20).
Poly(ether sulphone) (PES) is one of the most widely used materials in the micro-electronics industry and a good candidate for the substrates of flexible optoelectronic devices. In this work, the influences of oxygen plasma treatment on the surface chemical composition, surface morphology and optical transparency of PES films were investigated by means of X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM) and UV–visible spectrophotometry. The possible relations between the optical transparency of the substrate and the surface roughness and chemical composition were also studied. The oxygen plasma treatment seriously changed the surface chemical composition and made the surface more rough. Considerable amounts of sulphate species were found on the plasma-treated surface and the surface roughness values (R a) increased monotonically with the increase of the treatment time. The PES films treated by 5 min, 15 min, 30 min and 45 min oxygen plasma demonstrated transmission of approximately 98, 94, 68 and 46%, respectively, in the wavelength range of 400–780 nm. The oxygen plasma induced decline of optical transparency of PES films might be attributed to both the increase of surface roughness and the changes of chemical composition of the film surface.
Keywords: Poly(ether sulphone); Surface; Oxygen; Plasma;

The thermal stability of a novel phosphorus-containing aromatic poly(ester-amide) ODOP-PEA was investigated by thermogravimetric analysis (TGA). The weight of ODOP-PEA fell slightly at the temperature range of 300–400 °C in the TGA analysis, and the major weight loss occurred at 500 °C. The structural identification of the volatile products resulted from the ODOP-PEA pyrolysis at different temperatures was performed by pyrolysis-gas chromatography/mass spectrometry (pyrolysis-GC/MS). The P–C bond linked between the pendant DOPO group and the polymer chain disconnected first at approximately 275 °C, indicating that it is the weakest bond in the ODOP-PEA. The P–O bond in the pendant DOPO group was stable up to 300 °C. The cleavage of the ester linkage within the polymer main chain initiated at 400 °C, and the amide bond scission occurred at greater than 400 °C. The structures of the decomposition products were used to propose the degradation processes happening during the pyrolysis of the polymer.
Keywords: Poly(ester-amide); Thermal degradation; TGA; Pyrolysis-GC/MS;

Synthesis and comparative biodegradability studies of three poly(alkylene succinate)s by Dimitrios N. Bikiaris; George Z. Papageorgiou; Dimitris S. Achilias (31-43).
Poly(alkylene succinates) were synthesized from succinic acid and aliphatic diols with 2 to 4 methylene groups by melt polycondensation. DSC, 1H NMR, WAXD and molecular weight measurements were used to characterise the polymers. Biodegradability studies of polyesters with the same average molecular weight, included enzymatic hydrolysis for several days using Rhizopus delemar lipase at pH 7.2 and 30 °C. DSC traces of biodegraded polyesters revealed that hydrolysis affected mainly the amorphous material. For all polyesters an increase in glass transition, melting point and heat of fusion was recorded. In the first days of enzymatic hydrolysis, fast rates of mass loss were observed accompanied by a rapid reduction of intrinsic viscosity and molecular weight, thus indicating a mixed endo- and exo-type hydrolysis mechanism. Afterwards, it turned to an exo-type mechanism, taking place in the crystalline phase, since after 15–25 days of enzymatic hydrolysis molecular weight was stabilized, while mass loss kept on decreasing though in a slower rate. End-group analysis revealed that carboxyl and hydroxyl groups increased due to ester bonds' scission. The biodegradation rates of the polymers decreased following the order PPSu > PESu ≥ PBSu and it was attributed to the lower crystallinity of PPSu compared to other polyesters, rather than to differences in chemical structure. Finally, a simple theoretical kinetic model was developed and Michaelis–Menten parameters were estimated.
Keywords: Enzymatic hydrolysis; Biodegradable polymers; Aliphatic polyesters; Succinic acid;

Calorimetric and thermogravimetric studies of UV-irradiated polypropylene/starch-based materials aged in soil by J.M. Morancho; X. Ramis; X. Fernández; A. Cadenato; J.M. Salla; A. Vallés; L. Contat; A. Ribes (44-51).
We studied the biodegradability in soil of mixtures of polypropylene and a starch-based biodegradable additive. The changes in their properties were studied using calorimetry and thermogravimetric analysis. To observe the effect of UV radiation, the mixtures were photo-oxidized before biodegradation. The results were compared with those obtained from previous studies on non-photo-oxidized samples. Using calorimetry, we observed changes in the crystallinity of the samples and in their crystallization kinetics, which we analyzed using the Avrami equation. Photo-oxidation was found to reduce the crystallinity of the mixtures while degradation in soil increases it. Using thermogravimetry, we observed changes in the thermal stability and in the associated kinetic parameters, which we determined using an isoconversional integral method. Biodegradation tended to increase the thermal stability of the starch units and did not affect the polypropylene. Photo-oxidation tended to decrease the thermal stability of the mixture, although it may make the starch slightly more stable. The thermooxidative degradation of the mixtures was also studied.
Keywords: Biodegradation; Differential scanning calorimetry; Photo-oxidation; Thermal degradation;

Long-term heat stabilisation by (natural) polyols in heavy metal- and zinc-free poly(vinyl chloride) by Johan Steenwijk; Rik Langerock; Daan S. van Es; Jacco van Haveren; John W. Geus; Leonardus W. Jenneskens (52-59).
The long-term heat stabilisation efficiency of (natural) polyol additives in heavy metal- and zinc-free poly(vinyl chloride) (PVC) has been investigated. It is shown that polyols, such as sorbitol and xylitol, markedly reduce the dehydrochlorination rate and improve Congo Red values. Extraction experiments on unprocessed and ground-processed PVC-sorbitol (1.0 phr) mixtures after thermal degradation at 200 °C revealed that especially in the ground-processed PVC-sorbitol system, sorbitol is partly converted into its mono- and dianhydro-derivatives 1,4-sorbitan and isosorbide, respectively. Apparently, the HCl released during thermal degradation acts as the catalyst. Similar intramolecular cyclodehydration reactions also occur with the natural polyols, erythritol and xylitol, under these conditions. Scrutiny of the measured dehydrochlorination rates and the Congo Red values for ground-processed heavy metal- and zinc-free PVC–polyol mixtures show that in particular polyols containing primary hydroxyl groups exert long-term heat stabilisation and that they act as efficient HCl scavengers.
Keywords: PVC; Additives; Thermal degradation; Stabilisers; Renewable resources; Sorbitol; 1,4-Sorbitan; Isosorbide; Intramolecular cyclodehydration;

Thermal degradation kinetics of the biodegradable aliphatic polyester, poly(propylene succinate) by K. Chrissafis; K.M. Paraskevopoulos; D.N. Bikiaris (60-68).
The preparation of the biodegradable aliphatic polyester poly(propylene succinate) (PPSu) using 1,3-propanediol and succinic acid is presented. Its synthesis was performed by two-stage melt polycondensation in a glass batch reactor. The polyester was characterized by gel permeation chromatography, 1H NMR spectroscopy and differential scanning calorimetry (DSC). It has a number average molecular weight 6880 g/mol, peak temperature of melting at 44 °C for heating rate 20 °C/min and glass transition temperature at −36 °C. After melt quenching it can be made completely amorphous due to its low crystallization rate. According to thermogravimetric measurements, PPSu shows a very high thermal stability as its major decomposition rate is at 404 °C (heating rate 10 °C/min). This is very high compared with aliphatic polyesters and can be compared to the decomposition temperature of aromatic polyesters. TG and Differential TG (DTG) thermograms revealed that PPSu degradation takes place in two stages, the first being at low temperatures that corresponds to a very small mass loss of about 7%, the second at elevated temperatures being the main degradation stage. Both stages are attributed to different decomposition mechanisms as is verified from activation energy determined with isoconversional methods of Ozawa, Flyn, Wall and Friedman. The first mechanism that takes place at low temperatures is auto-catalysis with activation energy E  = 157 kJ/mol while the second mechanism is a first-order reaction with E  = 221 kJ/mol, as calculated by the fitting of experimental measurements.
Keywords: Poly(propylene succinate); Aliphatic polyester; Biodegradable polymer; Thermal degradation; Thermogravimetry;

Chemical degradation of crosslinked ethylene-propylene-diene rubber in an acidic environment. Part I. Effect on accelerated sulphur crosslinks by Susanta Mitra; Afshin Ghanbari-Siahkali; Peter Kingshott; Helle Kem Rehmeier; Hans Abildgaard; Kristoffer Almdal (69-80).
The time-dependent chemical degradation of accelerated sulphur cured ethylene propylene diene rubber containing 5-ethylidene-2-norbornene as diene in an acidic environment (20% Cr/H2SO4) was investigated. Two different rubbers with a similar ethylene to propylene ratio and diene content but with a significant difference in molar mass and level of long chain branching were used in the study. The molecular mechanisms of the chemical degradation occurring at the surface were determined using surface analysis (X-ray photoelectron spectroscopy and attenuated total reflectance Fourier transform infrared spectroscopy). The results reveal formation of several oxygenated species on the surface as a consequence of the acid attack. Furthermore, the crosslink sites of the exposed rubber samples are also found vulnerable to hydrolytic attack as evidenced by the decrease in crosslink density. The extent of surface degradation was strong enough to affect the bulk mechanical properties. Changes in mechanical properties were also monitored through determining retention in tensile strength, (%) elongation at break, modulus at 50% elongation, and change in micro-hardness. A negative correlation is also established between retention in modulus at 50% elongation and decrease in crosslink density. Scanning electron microscopy reveals the topographical damage at the surface due to the aqueous acid induced chemical degradation. The results indicate that the chemical degradation proceeds mainly via hydrolysis of crosslinks but upon prolonged exposure, the oxygenated species tend to combine with each other. The effect of molar mass and level of long chain branching also influences the chemical degradation.
Keywords: Chemical degradation; ENB-EPDM; Rubber; XPS; ATR-FTIR; Crosslink density; Mechanical properties; SEM;

Chemical degradation of crosslinked ethylene-propylene-diene rubber in an acidic environment. Part II. Effect of peroxide crosslinking in the presence of a coagent by Susanta Mitra; Afshin Ghanbari-Siahkali; Peter Kingshott; Helle Kem Rehmeier; Hans Abildgaard; Kristoffer Almdal (81-93).
An investigation on the time-dependent chemical degradation of ethylene-propylene diene rubber containing 5-ethylidene-2-norbornene as diene cured by peroxide crosslinking in the presence of a coagent in an acidic environment (20% Cr/H2SO4) has been made. Two types of rubber, with comparable monomer composition, but having significant differences in molar mass and levels of long chain branching were tested. Dicumyl peroxide and triallylcyanurate under similar conditions were used for curing the rubbers. The molecular mechanisms of chemical degradation at the surface were studied using X-ray photoelectron spectroscopy and attenuated total reflectance Fourier transform infrared spectroscopy, which demonstrate that several oxygenated species evolve during exposure. The primary process of degradation is hydrolytic attack on the crosslink sites, which is manifested by a decrease in crosslink density. The surface degradation is found to be strong enough to alter the bulk mechanical properties as observed by the change in retention in tensile strength, elongation at break, modulus at 50% elongation and, the change in micro-hardness. Retention in modulus at 50% elongation is found to follow a negative linear correlation with decrease in crosslink density. With higher molar mass and level of long chain branching more crosslinking occurs and thus comparatively more hydrolytic attack ensues. Scanning electron microscopy shows that the surface topography is significantly altered upon exposure and supports the notion of the dependence of degradation on the crosslinking density of the samples. Importantly, the coagent used in this study is shown to enhance the chemical degradation through formation of weaker sites for hydrolysis. The results also show that upon prolonged exposure the resulting oxygenated species tend to combine with each other.
Keywords: Chemical degradation; ENB–EPDM; Rubber; XPS; ATR-FTIR; Crosslink density; Mechanical properties; SEM;

Degradation of poly(ester-urethanes) and poly(acrylic-urethanes), as a base for automotive paintings in interior applications, has been studied by chemiluminescence. The samples were clearcoat and black-pigmented paints, unstabilised and stabilized with HALS Tinuvin 292 and UV absorber Tinuvin 1130, exposed to various doses of artificial weathering in Xenotest and Solisi equipment. Chemiluminescence has appeared a powerful tool to evaluate the oxidation stability of various polyurethane systems. From the dependences of oxidation onset temperature on heating rate, the kinetic parameters describing the dependence of induction periods on temperature have been obtained. The kinetic parameters enabled us to calculate the length of the induction period for a chosen temperature, the protection factors of various additives and the residual stability of the polymer after an artificial ageing stress. It has been found that the loss of residual stability with ageing dose obeys a first-order relationship. Equivalence between the two methods of artificial ageing has been determined. The results indicate that the equivalence depends on the polymer composition. The procedure presented here can also be applied for the determination of equivalence of accelerated and field tests so contributing to establishing a reliable correlation between them.
Keywords: Chemiluminescence; Polyurethane; Accelerated ageing; Thermo-oxidation; Residual stability; Automotive coating;

Two series of poly(ethylene terephthalate-co-oxalate-co-sebacate) (PETOXS) have been synthesized by melt polycondensation using diethylene oxalate (DEOX) as a starting material. NMR quantified the composition, structure, and average sequence length of the copolyesters. Melting and crystallization properties differ from each other on the basis of PETOXS feature. Analysis of TG traces determined that initial degradation temperatures were affected by the content of PET. It was observed that the Young's modulus and the maximum tensile stress increased with increasing content of poly(ethylene oxalate) (PEOX) in aliphatic units, whereas the elongation at break considerably decreased. Obvious weight loss was observed in alkali hydrolysis experiments, and the degradation rates are subject to distinct factors when the ratio of two aliphatic polyester units is varied. DSC was performed to degraded copolyester samples, and the variation of melting temperature and crystallinity were investigated.
Keywords: Poly(ethylene oxalate); Copolyester; Degradation; Synthesis;

Polystyrene nanocomposites based on an oligomerically-modified clay containing maleic anhydride by Xiaoxia Zheng; David D. Jiang; Charles A. Wilkie (108-113).
An oligomerically-modified clay containing maleic anhydride was used to prepare polystyrene-clay nanocomposites by melt blending and the effect of this modified clay on the thermal stability and fire performance was studied. These nanocomposites were characterized by X-ray diffraction, transmission electron microscopy, thermogravimetric analysis and cone calorimetry. The results show a mixed immiscible/intercalated/delaminated morphology. The maleic anhydride modified clay improved the compatibility between the clay and the polystyrene.
Keywords: Nanocomposites; Polystyrene; Fire retardancy; Maleic anhydride; Clays;

Structural and conductivity changes during the pyrolysis of polyaniline base by Miroslava Trchová; Pavel Matějka; Jitka Brodinová; Andrea Kalendová; Jan Prokeš; Jaroslav Stejskal (114-121).
Polyaniline base has been exposed to various temperatures between 100 °C and 1000 °C for 2 h in air. The mass loss has increased with increasing temperature. FTIR and Raman spectroscopies show the gradual destruction of the PANI structure, the possible formation of intermediate oxime and nitrile groups, and the final conversion to graphitic material. The elemental analysis confirmed the dehydrogenation while the content of nitrogen was nearly constant even after treatment at 800 °C. The conductivity of PANI base, 10−8  S cm−1, increased to ∼10−4  S cm−1 after treatment at 1000 °C; most of the products, however, were non-conducting. Another series of experiments involved the polyaniline base heated at 500 °C for 1–8 h. The studies were performed in connection with the potential flame-retardant application of polyaniline.
Keywords: Polyaniline; Conducting polymer; Conductivity; Infrared spectroscopy; Raman spectroscopy; Carbon;

Thermal degradation of epoxy–silica organic–inorganic hybrid materials by Jelena Macan; Ivan Brnardić; Sebastijan Orlić; Hrvoje Ivanković; Marica Ivanković (122-127).
Degradation kinetics of organic–inorganic hybrid materials based on epoxy resin were investigated by thermogravimetric analysis (TGA). The hybrid materials were prepared from diglycidyl ether of bisphenol A (DGEBA) and 3-glycidyloxypropyltrimethoxysilane (GLYMO) polymerised simultaneously by poly(oxypropylene)diamine (Jeffamine D230). Nanometric level of homogeneity in the hybrids was verified by electron microscopy. Energy of activation of degradation for the hybrids with varying inorganic content, as well as for the unmodified epoxy–amine system, was determined by the isoconversional Kissinger–Akahira–Sunose method, and was found to be significantly higher for the hybrid materials than for the unmodified epoxy–amine system. The degradation process was described by empirical kinetic models. The results indicated that presence of the inorganic network influences the mechanism of degradation of organic phase. Greater thermal stability of hybrid materials was confirmed by other parameters obtained from TGA curves.
Keywords: Degradation kinetics; Hybrid materials; Sol–gel process; Thremogravimetric analysis;

The synthesis of new stabilizer compounds (a combination between 2,2,6,6-tetramethylpiperidine and 2-hydroxyphenylbenzotriazole in one molecule) is reported. Four polymerisable combined stabilizers as well as two unsaturated triazinyl-2,2,6,6-tetramethylpiperidines and two unsaturated triazinyl-2-hydroxyphenylbenzotriazoles as individual stabilizers were synthesized. Their copolymers and terpolymers of the individual stabilizers with acrylonitrile were obtained. Chemical bonding of the stabilizers in the polymer was confirmed spectrophotometrically. The influence of these additives on the photo-stability of the copolymers was studied. The participation of the combined stabilizers in the polymerisation did not significantly affect the molecular weight and polydispersity of the copolymers. A significant stabilizing effect against photodegradation was found.
Keywords: Bifunctional polymerisable stabilizers; 2,2,6,6-Tetramethylpiperidines; 2-(2-Hydroxyphenyl)-benzotriazoles; Phase transfer catalysis; Copolymerisation; Photo-stability;

The preceding papers of this series were devoted to the identification and quantification of the main chemical changes resulting from the radiochemical ageing of EPDM (77.9% ethylene, 21.4% propylene, 0.7% diene) and EPR (76.6% ethylene, 23.4% propylene) films irradiated under oxygen atmosphere using 60Co γ-rays. It was shown that two processes are involved in the EPDM radio-oxidation. The random γ-radiolysis of the polymer provides a constant source of macro-alkyl radicals that are likely to initiate a selective oxidation of the polymer through free-radicals reactions involving the abstraction of labile hydrogen atoms. In the present paper, infrared spectroscopy has been used to study the γ-degradation of EPDM cross-linked with dicumyl peroxide and/or stabilised with two types of anti-oxidants (hindered phenol or amine-type). The results show that the anti-oxidants are not efficient in preventing oxidation. To understand the lack of efficiency of the stabilisers, the impacts of the various formulations on the rate of degradation of EPDM against chain oxidation involved in thermal and UV ageing were also studied.
Keywords: Radiochemical ageing; EPDM; Stabilisation; Radio-oxidation; Infrared spectroscopy;

The kinetics involved in the thermal decomposition of Kapton® polyimide 100HN under nitrogen atmosphere were studied by applying various fitting techniques to the isothermal and non-isothermal gravimetric data. The correlation of the reaction mechanism fitting, the analytical model fitting and the isoconversional method to these data was examined in relation to the kinetic parameters and the kinetic predictions. The mechanisms for solid-state reactions fit the isothermal data very well but result in highly uncertain values for the kinetic parameters when applied to the non-isothermal data. Isoconversional methods show that the apparent activation energy depends on the extent of conversion but do not provide information for the reaction order and the pre-exponential factor. Three single heating-rate analytical models by Coats–Redfern, MacCallum–Tanner and van Krevelen were analysed using the non-isothermal data. A multi-heating rate model is proposed and its validity is compared to the single-heating rate models on the basis of kinetic predictions.
Keywords: Polyimide; Thermogravimetric analysis; Kinetics of pyrolysis; Arrhenius parameters;

Degradation and stabilisation of poly(ethylene-stat-vinyl acetate): 1 – Spectroscopic and rheological examination of thermal and thermo-oxidative degradation mechanisms by Miguel Rodríguez-Vázquez; Christopher Mark Liauw; Norman Sidney Allen; Michele Edge; Eusebio Fontan (154-164).
The degradation of ethylene vinyl acetate (EVA) copolymers was compared with low density polyethylene (LDPE), poly(vinyl acetate) (PVAc) and poly(vinyl chloride) (PVC) using FTIR, UV–visible and fluorescence spectroscopy as well as thermal and rheological analyses. Thermal, thermo-oxidative and photo-oxidative studies were conducted. Thermo-oxidation below 180 °C shows more similarities between EVA and LDPE. The luminescence spectra of degraded EVA and LDPE were almost identical but very different to that of PVAc. UV–vis analysis showed that the polyenes present in aged PVC were unlikely to be the same species responsible for the observed colour formation in aged EVA. It is suggested that they are polyconjugated carbonyl products. Rheological analysis also showed the evolution of crosslinking reactions during thermo-oxidation. FTIR studies after thermal degradation in inert conditions 290 °C showed complete loss of the ester functionality and associated lactone formation along with some evidence for ketonic and unsaturated carbonyl groups. Degradation in air at 180 °C, however, revealed that loss of the ester group was not so marked, with PVAc exhibiting the greatest stability. This was in line with the induction time to onset of autocatalytic carbonyl growth at 180 °C; the latter showed an apparent exponential decrease with increasing vinyl acetate content up to 28% w/w. Fluorescence analysis produced trends that complemented those of carbonyl index; the time to decomposition of initial fluorescent α,β-unsaturated carbonyl species coincided with the time to onset of carbonyl growth. Furthermore, the rate of formation of the new fluorescent species produced in EVA, and LDPE was similar to that of carbonyl growth. These new fluorescent species are therefore likely to be di- or tri-carbonyl products.
Keywords: Ethylene vinyl acetate; Poly(vinyl acetate); Degradation; Oxidation; Luminescence; Spectroscopic analysis; Discolouration; PVC; Photo-oxidation; Luminescence; UV–vis; Rheometry; Polyenes;

The effects of transition metal dopants (V(IV), V(V), Mn(II), Cr(III), Mo(V), and W(V)), introduced into TiO2, upon the rate of photodegradation of poly(vinyl chloride) (PVC) films containing TiO2 have been measured. The rates were determined mainly by monitoring carbonyl group formation. In another set of experiments, the rates of chloride ion release from irradiated PVC particles suspended in water undergoing agitation with air or O2 in the presence of particles of doped TiO2 were measured electrochemically. The doping of TiO2 (rutile) with Cr(III), V(V) or Mn(II) reduces the photoactivity of the pigment, while doping by Mo(V) or W(V) enhances its photoactivity; the results obtained from carbonyl index measurements are paralleled closely by those from chloride ion release. Even the most aggressive doped pigments were less reactive than Degussa P25 pigment, while the greatest protection to PVC film was offered by TiO2 particles coated with Al2O3 or SiO2. Overall, the photoactivity of doped TiO2 is a complex function of dopant concentration, the energy levels of the dopants in the TiO2 lattice, their d electronic configuration and their local distribution. Photoactivity is also linked to other factors such as crystal type, particle size distribution and surface area. There is a clear relationship between the tendency of the dopant to induce the rutile-to-anatase transition and its effect in enhancing the photoactivity of the pigment. The characterisation of the doped pigments was achieved using X-ray powder diffraction, EPR spectroscopy, diffuse reflectance UV–vis spectrophotometry, scanning optical and electron microscopy and particle size analysis using LALLS.
Keywords: Anatase; Chromium; EPR; Manganese; Molybdenum; Poly(vinyl chloride); Photodegradation; Rutile; Titanium dioxide; Tungsten; Vanadium; X-ray powder diffraction;

Hydrolytic stability and hydrolysis reaction mechanism of bis(2,4-di-tert-butyl)pentaerythritol diphosphite (Alkanox P-24) by Nerea Ortuoste; Norman S. Allen; M. Papanastasiou; A. McMahon; Michele Edge; Brian Johnson; Klaus Keck-Antoine (195-211).
The excellent processing stability afforded by the commercial phosphate antioxidant, Alkanox P-24 is well known in the literature. However, it is known that Alkanox P-24 is hydrolytically unstable. Enhancement of its hydrolytic stability is therefore a key objective in this work and some binary and ternary blends were developed using other additives that are often used for polymer stabilisation, including the primary antioxidant tetrakis[methylene-3-(3′,5′-di-tert-butyl-4-hydroxyhyphenyl)propionate]methane (Anox 20), acid scavengers calcium stearate (DW) and the hydrotalcite like compound (DHT-4A). An improvement in the hydrolytic stability of Alkanox P-24 was found when it was blended with these additives. A comparison with different physical forms of blends (traditional powders versus recently introduced Non-Dust Blends) was undertaken. Better performance was observed with NDB relative to the free flowing mixed powders. Spectroscopic studies (FTIR, and mass spectrometry) were also undertaken to elucidate the hydrolysis mechanism of the phosphite antioxidant Alkanox P-24. Mechanistic schemes were devised and interpreted. Hydrolysis products of Alkanox P-24 are believed to be involved in the mechanism of stabilisation. In this programme of work, the role of the hydrolysis products was investigated by controlled thermomechanical degradation in an extruder and stabilisation activity evaluated by following the yellowness index and the melt flow rate. The influence of partially hydrolysed Alkanox P-24 on polymer processing was studied. It was found that some active hydrolysis products showed significant antioxidant activity and retarded polymer degradation during processing. Mechanisms for their formation and identity are elucidated.
Keywords: Phosphite; Hydrolysis; Alkanox P-24; Non-dust blends; Phenolic antioxidant; Hydrotalcite; CaSt;