Polymer Degradation and Stability (v.69, #3)

Flame retardancy in thermoplastic polyurethane elastomers (TPU) with mica and aluminum trihydrate (ATH) by Ubirajara Almeida Pinto; Leila Léa Yuan Visconte; Jorge Gallo; Regina Célia Reis Nunes (257-260).
Two types of aluminum trihydrate (ATH), with or without surface treatment, and mica were incorporated into TPU composites. The quantity of mica that should be used has been previously evaluated from mechanical properties. In the present work, different mixtures of TPU/ATH/mica were prepared and those compositions presenting fire retardancy were identified through the two principal tests in this area: vertical burning (UL94) and oxygen index (ASTM D2863). The influence of mica on fire resistance was also evaluated. The objective of this work was to obtain good flame retardancy in TPU–mica composites and an optimum cost-performance balance as a consequence of mica addition.
Keywords: Thermoplastic polyurethane; ATH; Fire retardancy; Mica;

To effect useful changes in jute fibre, we graft copolymerized with methyl methacrylate (MMA) initiated by ceric ions and studied optimization of grafting parameters as a function of various reaction conditions. Jute and its graft copolymer thermally degrade in one and two stages, respectively, but follow the same degradation mechanism. The degradation mechanism and thermal kinetics were evaluated by applying seven kinetic degradation models. Both fibres decompose following the R-2 (PBR-cylindrical symmetry) kinetic equation. Initially, the thermal stability of the graft copolymer is better, as evident from higher energy of activation (E act) and higher initial decomposition temperature (IDT). Swelling (Ps) of graft copolymers increases as a function of grafting and at a particular graft level follows the order: DMF>H20>iso-propanol.
Keywords: Jute fibre; Thermal degradation; Swelling behaviour;

Styrene (ST) and methyl methacrylate (MMA) have been copolymerized with a variety of comonomers containing covalently-bound phosphorus-containing groups, including vinyl phosphonic acid, several dialkyl vinyl phosphonates, and various vinyl and allyl phosphine oxides. The flame retardance of these polymers has been preliminarily assessed through thermogravimetric analysis and measurements of limiting oxygen index (LOI) and char yields. All the phosphorus-containing polymers produce char on burning (and also on heating in air or nitrogen) and have LOIs higher than those of the parent homopolymers, indicating significant flame retardance involving condensed-phase mechanisms. However, despite there being general correlations between LOI, char yield and phosphorus-content, some copolymers have higher than expected LOI and/or char yield, whilst others have lower, indicating that phosphorus environment is important. In order to explore mechanisms of flame retardance in more detail, laser pyrolysis/time-of-flight mass spectrometry and mass spectrometric thermal analysis have been applied to study the decomposition behaviour of three of the MMA copolymers: those containing pyrocatecholvinylphosphonate (MMA-PCVP), diethyl-p-vinylbenzylphosphonate (MMA-DEpVBP) and di(2-phenylethyl)vinylphosphonate (MMA-PEVP) as comonomers. The laser pyrolysis experiments provide an insight into probable polymer behaviour behind the flame front in a polymer fire and show that copolymerization of MMA with the comonomers does not greatly change the pyrolysis mechanism compared with that of poly(methyl methacrylate) (PMMA). However, the amounts of MMA monomer evolved during pyrolysis are much reduced for the copolymerized samples. Since MMA is the major fuel evolved during the combustion of PMMA and its copolymers, this effect must be a major contributing factor to the reduced flammability shown by the copolymers. MMA-DEpVBP underwent the most extensive decomposition following laser pyrolysis. In particular, significant amounts of highly flammable methane and ethene were detected. Such increased amounts would occur also if the copolymer were to be exposed to high temperature conditions when burnt. Hence, its seems reasonable that the MMA-DEpVBP has a lower LOI value than expected, despite it giving a relatively high yield of char. Mass spectrometric thermal analysis studies of the MMA-PEVP provide evidence that the PEVP unit decomposes around 200°C, eliminating styrene, with evolution of MMA reaching a maximum some 50°C higher. Possible mechanisms for these processes are suggested.
Keywords: Flame retardance; Acrylic; Styrenic; Phosphorus; Mechanism;

Radiation-induced degradation of sodium alginate by Naotsugu Nagasawa; Hiroshi Mitomo; Fumio Yoshii; Tamikazu Kume (279-285).
Alginates were irradiated as solids or in aqueous solution with Co60 gamma rays in the dose range of 20 to 500 kGy to investigate the effect of radiation on alginates. Degradation was observed both in the solid state and solution. The degradation in solution was remarkably greater than that in the solid. For example, the molecular weight of alginate in 1% (w/v) solution decreased from 6×10−5 for 0 kGy to 8×10−3 for 20 kGy irradiation while the equivalent degradation by solid irradiation required 500 kGy. Degradation G-values were 1.9 for solid and 55 for solution, respectively. The free radicals from irradiated water must be responsible for the degradation in solution. The degradation was also accompanied by a color change to deep brown for highly degraded alginate. Little color change was observed on irradiation in the presence of oxygen. UV spectra showed a distinct absorption peak at 265 nm for colored alginates, increasing with dose. The fact that discoloration of colored alginate was caused on exposure to ozone suggests a formation of double bond in the pyranose-ring.
Keywords: Radiation; Degradation; Sodium alginate; Molecular weight; G-value; Discoloration;

Polyisoprene can be cross-linked by an initial lithiation followed by reaction with both monochloro compounds and dichloro compounds. The monochloro compounds effect crosslinking through a lithium-chlorine exchange route while the use of dichloro compounds links the PIP chains with the spacer between the two chlorine atoms. A significant amount of char is produced from compounds which have been cross-linked with aromatic dihalides while aliphatic dihalides do not produce significant char.
Keywords: Polyisoprene; Cross-linking; Thermogravimetric analysis; Char; Lithiation;

Successful prediction of long-term weatherability performance of organic coatings in different markets requires an understanding of the relationship between the rate of weathering in specific outdoor and accelerated tests with those observed in the field within that market. Since the rate of weathering is, to a large extent, controlled by the rate of coating photo-oxidation, the development of such relationships requires the following information: (1) identification of the critical environmental variables that control photo-oxidation rate; (2) determination of the dependence of the rate of photo-oxidation for coatings of interest on those variables; (3) determination of the distribution of the values of the critical variables in the given market relative to those for the controlled exposure. For general polymer photo-oxidation, the critical environmental variables are well known: light intensity, wavelength distribution, and temperature. For some polymeric materials, including those widely used in automotive coatings, humidity and moisture also play an important role in weathering. In this paper, three specific models for the dependences of coating photo-oxidation on these variables are developed. Using these models and measured or calculated distributions of light intensity, wavelength, temperature and humidity in specific automotive markets, predictions are made of the length of time required for standard outdoor exposures to insure performance in those markets.
Keywords: Weathering; Photooxidation; Paints; Coatings; Service life prediction;

A number of exposure models have been developed to account for the variation in photo-oxidation rate with light intensity, wavelength distribution, temperature and humidity. These models have proven useful in accounting for the different observed rates of photo-oxidation and other weathering phenomena in different outdoor locations. In this paper, the same models are applied to calculate acceleration factors of various accelerated weathering tests relative to standard outdoor Florida weathering. The predicted acceleration factors are in reasonable agreement with the ranges commonly observed for various accelerated tests. The models also can be used to assess the sensitivity of the acceleration factors to material differences and to variations in environmental conditions within a given chamber and from chamber-to-chamber. The critical importance of variability to the reproducibility and reliability of accelerated test results are discussed. Difficulties of comparing measurements made outdoors with values for accelerated tests are also discussed. Of particular note is the need for all exposures to measure and report actual sample temperatures rather than air or black panel temperatures.
Keywords: Weathering; Photooxidation; Paints; Coatings; Accelerated tests; Accelerated factors;

Polyvinylimidazole–silica hybrid materials (PVI–SiO2) containing various proportions of covalently bonded silica were prepared from [3-(methacryloxy)propyl]trimethoxysilane modified polyvinylimidazole (PVI), water and tetramethoxysilane (TMOS) via a sol–gel technique. Infrared (IR), 29Si and 13C nuclear magnetic resonance (NMR) spectroscopy, differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were used to analyse the structures and properties of hybrids with various proportions of TMOS. The apparent activation energies E a for desorption of water and decomposition of PVI chains in hydroxylated hybrids were respectively around 55 and 35 kJ/mol, while the values were increased by about 10 kJ/mol in dehydroxylated hybrids.
Keywords: Polyvinylimidazole; Silica; Hybrid; Sol–gel; Stability;

The mechanism and kinetic model for thermal decomposition behavior of poly (phenylene sulfide) (PPS) was studied by stepwise pyrolysis/gas chromatography/mass spectrometry (stepwise Py-GC/MS) and thermogravimetry analysis/mass spectrometry (TG/MS) techniques. The composition of evolved gases was determined by Py-GC/MS analysis. The mechanism and kinetic model of the pyrolytic behavior of PPS was investigated by TG/MS from room temperature to 900°C. The mechanism indicated apparently one-stage pyrolysis, mainly by depolymerization, main chain random scission and carbonization. The initial scission of PPS was depolymerization and main chain random scission to evolve benzenethiol and hydrogen sulfide, respectively, as major products; while depolymerization dominated in lower temperature pyrolysis and main chain random scission dominated in higher temperature pyrolysis. The chain transfer of carbonization also produced in initial pyrolysis and gradually dominated at the higher pyrolysis temperature to form the high char yield of solid residue. Based on the analytical result, the kinetic model of pyrolysis was proposed and calculated. The calculated kinetics was compared with the experimental result, which was quite good consistence.
Keywords: PPS pyrolysis; Mechanism and kinetic model; Stepwise Py- GC/MS and TG/MS;

Evolution with temperature of the ESR signal of bromine doped poly(N-vinylcarbazole) The doping temperature used as parameter by S Touihri; P Molinié; S Ouro Djobo; K Napo; G Safoula; J.C Bernéde (333-340).
Poly(N-vinylcarbazole) powder (PVK) has been doped with bromine at 293, 303, 323, 363, 383 and 423 K for 24 h. These powders have been systematically studied by electron spin resonance (ESR) at temperatures varying from 300 to 523 K and reverse. It is shown that the stability of the ESR signal after “temperature cycling measurements” depends on the doping temperature. The modifications of the signals during and after measurements are explained by bromine reaction with PVK. The PVK is partly decomposed with formation of carbazole monomers embedded in a disordered polymer matrix. The degree of degradation of PVK depends on the doping temperature. It has been checked by X-ray diffraction and X-ray photoelectron spectroscopy (XPS). Moreover a correlation between room temperature conductivity and PVK modification is established.
Keywords: Poly(N-vinylcarbazole; Bromine doping; Electron spin resonance; Conductivity;

An exponential decay function for polymer degradation in turbulent drag reduction by Hyoung J. Choi; Chul A. Kim; Jeong-In Sohn; Myung S. Jhon (341-346).
The mechanical degradation of high molecular weight polymers under turbulent flow conditions was investigated using a rotating disk apparatus. The validity of the empirical exponential decay function, which has been used to represent the degradation phenomenon in polymer induced turbulent drag reduction in a pipe flow, has been investigated. Results show that the single exponential decay model is not universally suitable for all polymeric drag reducers, but it effectively represents shear resistant, drag reducing agents, including the various polysaccharides, and is also applicable in describing short time degradation behavior.
Keywords: Drag reduction; Turbulence; Polymer degradation; PEO; Mechanical degradation; Polysaccharide; Xanthangum;

The effects of ultra-violet irradiation on the microstructure of an oxygen free polymer viz., acrylonitrile-butadiene-styrene and a polymer containing oxygen viz., polycarbonate have been investigated using positron annihilation lifetime measurement. Lifetime results in both polymers in the early stages of irradiation indicate that chain-scission leading to free radical formation is the predominant process. Radiation induced cross-linking becomes dominant in the later stages of irradiation. An interesting feature observed in the present studies is that an oxygen containing polymer attains free volume stability which is attributed as due to the formation of photostabilisers whereas no such stability is observed in the case of an oxygen free polymer. UV absorption spectroscopy results supplement the positron lifetime results that oxygen containing polymer becomes photostabilised.
Keywords: Positron lifetime; Polymer; Free volume; UV degradation; Conformation;

Wood coated with urethane-modified acrylic coatings was susceptible to yellowing induced by light. To find out the mechanism for inhibiting its photodiscoloration, wood specimens covered with separate acrylic clear free films containing stabilizers were used to simulate coated wood. Ultraviolet light absorbers tested in this study reduced not only the discoloration of coating films but also those of the underlying wood. HALS was not helpful in reducing the photodiscoloration of wood coated with aromatic urethane-modified acrylate containing light absorbers, but it showed a significant synergistic effect on the inhibition of photodiscoloration of wood coated with aliphatic urethane-modified acrylate containing light absorbers. Comparisons of the UV transmission spectra of coating films revealed that HALS preserved the absorption capability of the light absorber in aliphatic urethane-modified acrylate after the lightfastness test, therefore elongating the service life of light absorber and, consequently, that of the coating.
Keywords: Photodiscoloration; Wood; Aromatic urethane-modified acrylate; Aliphatic urethane-modified acrylate; Ultraviolet light absorber; HALS; Synergism;

Thermal decomposition kinetics of poly(trimethylene terephthalate) by Xue-Song Wang; Xin-Gui Li; Deyue Yan (361-372).
Poly(trimethylene terephthalate) samples with various molecular weights have been prepared by changing reaction conditions via trans-esterification method. The dynamic thermogravimetric kinetics of these PTT samples under argon, air, and nitrogen were analyzed by Freeman–Carroll, Friedman, and Chang techniques. The effects of molecular weight on the thermostability and the decomposition kinetic parameters of PTT under argon and air were discussed systematically. The effects of atmosphere (air, argon, and nitrogen) were also investigated. The experimental data showed that under argon atmosphere PTT exhibited mainly one decomposition step, and all of the initial degradation temperature, the temperature at the maximum weight-loss rate, the activation energy E, order n and frequency factor Z for decomposition increased with molecular weights. However, under air atmosphere PTT exhibited two main degradation stages. Characteristic temperatures increased but decomposition kinetic parameters decreased with molecular weights for the first decomposition stage. Nevertheless little variation of characteristic temperatures and degradation kinetic parameters has been observed for the second degradation stage. In addition, the decomposition process of PTT (or PET) in argon is similar to that in nitrogen, while much different degradation mechanism can be found in air. In brief, PTT and PET show much lower initial decomposition temperature and degradation-activation energy in air than in nitrogen and argon.
Keywords: Poly(trimethylene terephthalate); Molecular weight; Thermogravimetry; Thermostability; Degradation kinetics;

Thermal degradation of cyclodextrins by Francesco Trotta; Marco Zanetti; Giovanni Camino (373-379).
The thermal degradation of cyclodextrins and substituted β-cyclodextrins has been studied. It is found that in inert atmosphere they all decompose in one major step (252–400°C) leaving a residue (Char) which is thermally quite stable, decomposing at a low rate at higher temperature. Temperature of decomposition, char yield and thermal stability depend on the type of substituent of the cyclodextrin. On heating in air, the first degradation step is not affected whereas the char is oxidised to volatile products below 600°C apart from the cases of amino and phosphate substituted cyclodextrins, which give a ceramic-like residue stable to above 800°C. The charring process involves opening of the cyclodextrin rings followed by a chemical evolution similar to that of cellulose with loss of the glucosidic structure and hydroxyl groups and build up of unsaturation, carbonyl groups and aromatic structures. The kinetics and yield of the various processes, as well as the final structure of the char at high temperature, depend on the substituent of the cyclodextrin. Carbon dioxide, water, levoglucosan and furans are the major volatiles evolved from cyclodextrin degradation as from cellulose. Products deriving from substituents show that they do not simply behave as leaving groups but rather take part in the charring. Moreover, the relative amount of CO2 and levoglucosan which are respectively related to competing charring and depolymerisation in cellulose, do not relate to the char yield of substituted cyclodextrins.
Keywords: Cyclodextrins; Thermal degradation; Charring;

The thermal stability of blends of polyurethane with three different elastomers having reactive functional groups has been studied. The blends have been prepared by three different blending techniques, keeping the blending ratio constant. It was found that the thermal stability of the blends was dictated by the blending technique. Blends prepared by the masterbatch technique degraded at a lower temperature. The degradation temperatures were increased when preblending and preheating/preblending techniques were adopted. This may be due to the formation of interchain crosslink bonds between the two elastomer phases, the extent of which was more in the preheated/preblended technique.
Keywords: Polyurethane; Masterbatch; Preblending; Preheating preblending; Interchain crosslinking; Vulcanization;