Polymer Degradation and Stability (v.76, #1)

Thermal decomposition processes in polybenzoxazine model dimers investigated by TGA–FTIR and GC–MS by Kasinee Hemvichian; Apirat Laobuthee; Suwabun Chirachanchai; Hatsuo Ishida (1-15).
The thermal decomposition processes of a model compound containing Mannich bridge and a series of polybenzoxazine model dimers are investigated using TGA–FTIR and GC–MS. The 2,4-dimethylphenol-based benzoxazine dimers degraded into smaller and highly volatile compounds, leaving no char at the end of degradation. The p-cresol-based benzoxazine dimers degraded into smaller and highly volatile products as well, nevertheless some of which are able to undergo crosslinking and aromatization processes and form char. The major decomposition products for modified p-cresol-based dimers are amines and ester compounds.
Keywords: Benzoxazine; Polybenzoxazine; Thermal degradation; FTIR; GC–MS;

The kinetics of degradation of polysiloxanes with different starting chemistries is reported in this paper. The polymers used in the study were vinyl-terminated polydimethylsiloxane (PDMS) and poly(diphenyl-dimethyl)siloxane (3.5 and 25% diphenyl content, DP-3.5 and DP-25). These polymers were inert pyrolyzed under isothermal conditions at temperatures from 325 to 400 °C for 5 h. Additionally, these polymers were pyrolyzed under dynamic heating (rate 6.25 °C/min) up to 925 °C. The kinetic rate constants were calculated for each of the polymers, from both isothermal TGA and dynamic, non-isothermal TGA. The Van Krevelen and Coats-Redfern methods of analysis were used for calculating the energy of activation from dynamic TGA. The various analysis techniques resulted in consistent trends. The calculated ΔE values decreased with the incorporation of phenyl groups in the polysiloxane. The pre-exponential factors also decreased upon incorporation of phenyl groups, most likely due to the reduced mobility of the polymer backbone upon this chemical change.
Keywords: Pyrolysis; Polysiloxanes; Thermogravimetric analysis; Kinetics; Degradation;

Electron spin resonance imaging (ESRI) was developed in our laboratory as a method for spatial and spectral profiling of radicals formed during polymer degradation. We present the application of this approach to the study of thermal degradation at 393 K of poly(acrylonitrile-butadiene-styrene) (ABS) containing 1 or 2% w/w Tinuvin 770 as the hindered amine stabilizer (HAS). The spatial distribution of the HAS-derived nitroxide radicals, obtained by 1D ESRI, was homogeneous at short treatment times but became heterogeneous after treatment times ⩾ 70 h. The spatial variation of the ESR line shapes with sample depth was visualized by 2D spectral–spatial ESRI. Nondestructive (“virtual”) slicing of the 2D images resulted in a series of ESR spectra along the sample depth, which were used to deduce the relative intensity of nitroxide radicals present in two distinct sites. The two sites represent radicals located in butadiene-rich and SAN-rich domains, respectively. Taken together, 1D and 2D ESRI allowed the determination of the extent of degradation within morphologically-distinct domains as a function of sample depth and treatment time. The conclusions from the ESRI experiments were substantiated by attenuated total reflectance (ATR) FTIR spectroscopy of the outer layer (500 μm thick) of the polymer. Comparison of the two techniques suggested that the advantage of the ESRI method is its ability to provide mechanistic details on the early stages of the ageing process. ESRI and FTIR data indicated that the larger Tinuvin 770 content in the polymer, 2%, leads to less efficient stabilization.
Keywords: Polymer degradation; Hindered amine stabilizers (HAS); Electron spin resonance (ESR); Electron spin resonance imaging (ESRI);

A mathematical model for the random scission of linear polymers is presented. The model takes the form of a set of ordinary differential equations which describe the evolution of the MW distribution as a function of the fraction of bonds broken. An exact solution, valid for large initial MW, is derived from the equations and compared with results from a Monte-Carlo type simulation. Isothermal thermogravimetric experiments using polyethylene are used to suggest a relationship between the rate of bond breaking and temperature and this is then used to compare model predictions for the rate of degradation with constant heating rate thermogravimetric experiments. Excellent agreement is found between theoretical predictions and experimental results for the case of a standard sample of polyethylene with number-average MW 2015.
Keywords: Random scission; Linear polymer; Polyethylene; Thermal degradation; Mathematical model;

The effect of exposing polypropylene (PP) to γ-irradiation prior to the catalytic pyrolysis over a HY-zeolite was investigated using a thermobalance and a semi-batch reactor. A significant increase in the rate of the catalytic pyrolysis was realized when PP was exposed to a small irradiation dose of 10 kGy. The high reactivity of the irradiated PP was conjugated with low yields of residue and coke in addition to enhanced selectivity for light distillate (C7–C10). Examining the effect of pyrolysis temperature revealed that the catalytic pyrolysis preferred high temperature among the investigated temperature range of 325–375 °C. A possible reaction mechanism was discussed based on the observed results and the reaction mechanisms reported in the literature. The results suggested the applicability of the proposed pyrolysis method for enhancing the catalytic conversion of plastic waste into useful hydrocarbons.
Keywords: Irradiation; Catalytic pyrolysis; Polypropylene; Chemical recycling;

Thermal degradation of poly[(R)-3-hydroxybutyrate], poly[ε-caprolactone], and poly[(S)-lactide] by Yoshihiro Aoyagi; Koichi Yamashita; Yoshiharu Doi (53-59).
Thermal degradations of poly[(R)-3-hydroxybutyrate] (PHB), poly[ε-caprolactone] (PCL), and poly[(S)-lactide] (PLA) were investigated under both isothermal and non-isothermal conditions. In isothermal degradation experiments, the three polyesters showed quite different time-dependent profiles of weight loss and the number–average degree of polymerization, reflecting their different degradation mechanisms. Thermogravimetric analysis and pyrolysis coupled with GC/MS suggest that PHB is degraded by a random chain scission (cis-elimination), while PCL is degraded by an unzipping depolymerization from the hydroxyl end of the polymer chains. In contrast, the thermal degradation behavior of PLA was very complex because various reactions occurred concurrently.
Keywords: Thermal degradation; Thermoanalysis; Pyrolysis; Poly(3-hydroxybutyrate); Poly(ε-caprolactone); Poly(lactide); Biodegradable polymer;

Catalytic recycling of the mixture of polypropylene and polystyrene by Jong-Ryeol Kim; Jik-Hyun Yoon; Dae-Won Park (61-67).
Catalytic degradation of the mixture of polypropylene (PP) and polystyrene (PS) was investigated in a semi-batch reactor. For all the catalysts (HNZ, SA, HZSM-5) tested, the main degradation products were liquid oils with gasoline range carbon number distributions. A natural clinoptilolite zeolite (HNZ) showed as good catalytic performance as silica-alumina (SA). HZSM-5 catalyst produced the highest amount of gaseous products. Increase of degradation temperature resulted in a decrease of ethylbenzene and propylbenzene, but an increase of styrene. Thermogravimetric analysis revealed the interaction of PP and PS in the catalytic degradation.
Keywords: Catalytic degradation; Mixed plastics; Polypropylene; Polystyrene; TGA;

Poly(olefin sulfone)s obtained by the free radical copolymerization of different olefins, vinyl chloride, acrylamide, and styrene, with sulfur dioxide, denoted PVCS, PAaS and PStS, have been investigated by thermogravimetric analysis (TGA). The activation energy, pre-exponential factor, and decomposition order of thermal degradation of these poly(olefin sulfone)s with similar SO2 mole fractions were obtained from TGA measurements in nitrogen. Comparison of the values of activation energy at the early stage, shows that the thermal stability decreases in the order: PStS>PVCS>PAaS; this can be explained by the electronegative effect of the substituent in the side chain of the olefin. Moreover, in the later stage, the thermal stability decreases in the order: PStS<PVCS<PAaS; this phenomena can be explained by the effect of the polyolefin structure on the thermal stability.
Keywords: Thermal degradation; Poly(styrene sulfone); Poly(acrylamide sulfone); Poly(vinyl chloride sulfone);

New stabilizers for poly(vinyl chloride) based on 2′-hydroxy-bezalacetophenone derivatives have been obtained and their photostabilisation effectiveness in PVC investigated. The photostabilizers were incorporated into the PVC when the polymer was thermally stabilized (at a temperature of about 170 °C). Artificial aging exposure has shown that 2′-hydroxy-3′,5′-dichlorobenzalacetophenone (2′H3′,5′DClBAP) behaves well as photostabilizer in a semirigid PVC plaque or a transparent extruded film (200 μm thick) at 0.2% concentration. A study of the IR and electronic absorption spectra in a large variety of acidic or basic solvents leads us to conclude that 2′H3′,5DClBAP resists the action of chemical agents with which it may come into contact when used in a plastic material.
Keywords: Photostabilizer; Absorption spectra; Medium;

The abnormal structures formed in a liquid crystalline aromatic polyester (LCP), 4-hydroxybenzoic acid (HBA)/2-hydroxy-6-naphthoic acid (HNA) copolymer, thermally treated up to 500 °C were characterized in detail by pyrolysis-gas chromatography in the presence of an organic alkali, tetramethylammonium hydroxide (TMAH). On the pyrograms of the thermally treated LCP samples observed at the reactive pyrolysis temperature of 300 °C, the peaks reflecting the abnormal structures such as the branching and condensed structures were observed together with those of the dimethyl derivatives of HBA and HNA reflecting the main chain and those originated from the end groups. Generally, the yields of all the characteristic products reflecting the abnormal structures increased with the rise of the thermal treatment temperature. Furthermore, it was confirmed that the condensation reactions might occur in preference to the branching formations in the LCP sample at higher treatment temperature around 500 °C. Finally, it was suggested from the average molar composition between HBA and HNA units observed in the specific products that the preferential elimination of the HBA unit in the polymer chain might take place during the thermal treatment of the LCP sample, while the abnormal structures such as the branching and the condensed structures were also confirmed to occur preferentially at the HNA units rather than the HBA ones.

Thermolysis of polyethylene hydroperoxides restricted to the initial stages can be readily understood. The same is true for the oxidation products formed on thermolysis. These products are essentially the same as those formed on processing in mixers open to air. They are mainly alcohols and ketones, in-the chain ketones and methyl-ketones. Carboxylic acids and aldehydes are also formed in a small amount. The same is valid for trans-vinylene and γ-lactone groups. The kinetics of oxidation product formation in the initial stages are clearly related to the corresponding kinetics for hydroperoxide decomposition. However, the various products show significant differences in this respect. Thus, thermolysis of hydroperoxides in the initial stages consists of direct hydroperoxide decomposition and free radical induced hydroperoxide decomposition. Formation of carbonyl groups corresponds also to such a process. This means that carbonyl groups result from both direct and induced hydroperoxide decomposition. However, alcohol groups are formed essentially in reactions following direct hydroperoxide decomposition according to the pseudo-monomolecular reaction. Formation of γ-lactones does not proceed according to the same general scheme. Its rate is increasing considerably with the initial oxidation of the samples. Hence, it looks as if it were formed out of products accumulating in the oxidizing polyethylene melt.
Keywords: Polyethylene; Hydroperoxides; Melt; Thermolysis; Kinetics; Oxidation products; Alcohol; Carbonyl; Trans-vinylene; γ-Lactone;

Polystyrene/graphite nanocomposites: effect on thermal stability by Fawn M Uhl; Charles A Wilkie (111-122).
Nanocomposites consisting of polymer and clay have been shown to exhibit a significant reduction in flammability and an increase in mechanical properties. This work examines the effect of thermal stability and mechanical properties of nanocomposites prepared from potassium graphite and styrene. Synthesis of nanocomposites was accomplished by using potassium graphite (KC8) as the initiator in the polymerization of styrene. A slight increase in thermal stability is observed but mechanical properties are decreased.
Keywords: Nanocomposites; Polystyrene; Graphite; Thermal stability; Cone calorimetry; Potassium graphite;

A linear low density polyethylene (LLDPE) grafted with dibutyl maleate (DBM) (PE-g-DBM) was prepared in the presence of dicumyl peroxide (DCP) and characterized by FTIR, melt index, and contact angle. The effects of PE-g-DBM as a compatiblizer on the mechanical properties, morphological structure and crystallization behaviors of magnesium hydroxide (MH)-based halogen-free flame retardant (HFFR) LLDPE blends have been investigated using scanning electron microscope (SEM), differential scanning calorimetry, and mechanical properties measurements. The results show that a suitable amount of PE-g-DBM compatiblizer can increase the adhesion between MH and polyethylene and thus improve the mechanical properties of LLDPE/MH/PE-g-DBM blends. The crystallinity degree of the blends decreases with increasing PE-g-DBM content.
Keywords: Dibutyl maleate-grafted LLDPE; Magnesium hydroxide; LLDPE; Mechanical properties; Morphology; Crystallization;

Experimental analysis and numerical simulation for biodegradability of polyethylene by Fusako Kawai; Masaji Watanabe; Masaru Shibata; Shigeo Yokoyama; Yasuhiro Sudate (129-135).
Biodegradability of polyethylene wax (PEwax) was studied both experimentally and analytically. The weight loss of PEwax (0.5%) by a microbial consortium in approximately 3 weeks was 31.5% and the top molecular weights shifted to higher values. One of the features observed experimentally was fast consumption of low molecular weight-PEwax. On the other hand, one may speculate, theoretically, that hydrocarbons such as PE should be subject to an initial oxidation to yield carboxylated compounds at their terminals, which are then depolymerized via β-oxidation process. A mathematical model based on these theoretical and experimental aspects is proposed and a method of analysis is illustrated. β-Oxidation rates and consumption rates of low molecular weight-PEwax are determined numerically, and the temporal change of weight distribution is simulated. Numerical results are compared with experimental results, and the governing mechanism of biodegradation is interpreted. We conclude that the primary factors in biodegradation of polyethylene are an initial oxidation at the terminals of molecules followed by β-oxidation and consumption of low molecular weight-PEwax. The most reliable biodegradation limit was approx. 2000.
Keywords: Biodegradability; Polyethylene; Numerical simulation; β-Oxidation; Terminal oxidation;

Attempts were made to use a ground tire rubber (GTR) fraction (particle size: 0.4–0.7 mm) to produce thermoplastic elastomers (TPEs) composed of low-density polyethylene (LDPE), fresh rubber and GTR with and without dynamic curing. GTR has been thermomechanically decomposed in the presence (GTRPM) and absence of processing oil (GTRM) and its decomposition was followed by the increase in the acetone-soluble fraction. The composition of the TPEs was constant, viz. LDPE: rubber:GTR=50:25:25 wt.%. As fresh gums styrene/butadiene (SBR), natural (NR) and ethylene/propylene/diene (EPDM) rubbers were used in blends with GTRR (reference, not treated), GTRM and GTRPM at 1:1 ratio. Dynamic vulcanisation occurred either by sulfur or by peroxide curatives. Based on the thermomechanical and mechanical properties of the resulting TPEs, the best performance was achieved by recipes containing GTRM and EPDM after dynamic vulcanisation with sulfur. This was traced to the dual compatibilizer effect of the EPDM. The ethylene segments of the EPDM are compatible with the LDPE, whereas the surface of the partially decomposed GTR may be involved in both molecular entangling and interphase sulfur crosslinking mechanisms which all yield a better interfacial bonding between the GTR particle and the matrix. The phase structure of the TPEs were assessed by dynamic-mechanical thermal analysis (DMTA) and fractography (etched cryogenic fracture surface) and discussed.
Keywords: Low-density polyethylene (LDPE); Ground tire rubber (GTR); Thermoplastic elastomer; Thermomechanical degradation; Dynamic vulcanisation;

Thermal decomposition of poly[3-phthalimido-2-hydroxypropyl methacrylate] by M.Fatih Coşkun; Kadir Demirelli; Mehmet Coşkun; Mahmut Doǧru (145-154).
The thermal decomposition behaviour of poly(3-phthalimido-2-hydroxypropyl methacrylate) poly[PHPMA] was investigated by thermogravimetric analysis (TG) and by programmed heating of the polymer from ambient temperature to 500 °C under vacuum followed by product collection, and using IR spectra of partially degraded polymer. The cold ring fraction (CRF) of products volatile at degradation temperature but not at ambient temperature and the solid fraction trapped at −196 °C were identified by GC–MS, 1H and 13C NMR, FT–IR. The liquid fraction trapped at −196 °C was examined by 1H NMR and FT–IR. Its gas phase was identified by FT–IR. On the degradation, the major products of CRF are the monomer (22.8%), N-(2,hydroxyallyl)phthalimide (21.01% by wt.) and N-(2,3-epoxypropyl)phthalimide (16.95% by wt.). The mechanism of thermal degradation, including formation of the major products, is discussed. Whether the polymer was affected by γ- and β-radiation was also tested.
Keywords: Thermal degradation; Phthalimide;

Glucose effect on the biodegradation of plastics by compost from food garbage by Ji-Chul Jang; Pyong-Kyun Shin; Jin-San Yoon; Ik-Mo Lee; Han-Sup Lee; Mal-Nam Kim (155-159).
The variation of physicochemical properties and microbial population were examined in the course of a food garbage composting, and the biodegradation of several plastics in the compost was investigated. Moisture content was controlled in the range of 64±4%, and the thermophilic stage lasted about 2 weeks. In the initial stage of the composting, mesophilic strains were more numerous than thermophilic ones. As the thermophilic stage set in, thermophilic bacteria and actinomycetes outnumbered mesophilic correspondents while fungi were not detected at all. In the cooling and maturing phases, lots of actinomycetes were still found. However bacteria decreased significantly in number, and only a small number of mesophilic fungi reappeared. When glucose was added to the compost, the so-called “priming effect” was observed, in that the amount of CO2 evolved was larger than that predicted by assuming that all the added glucose was mineralized into CO2. However the priming effect decreased as the quantity of the glucose in the compost increased. Addition of 5 wt.% of glucose to the compost increased the number of micro-organisms by 10-100 times. Addition of some biodegradable plastics also increased the number of micro-organisms. However the increased number of micro-organisms was not dependent on the biodegradability of the added plastics.
Keywords: Priming effect; Glucose; Compost; Biodegradation;

Kinetics of thermal degradation of vinyl polyperoxides in solution by Priyadarsi De; Sujay Chattopadhyay; Giridhar Madras; D.N Sathyanarayana (161-170).
The thermal degradation of vinyl polyperoxides, poly(styrene peroxide) (PSP), poly(α-methylstyrene peroxide) (PAMSP) and poly(α-phenylstyrene peroxide) (PAPSP), was carried out at different temperatures in toluene. The time evolution of molecular weight distributions (MWDs) was determined by gel permeation chromatography (GPC). A continuous distribution model was used to evaluate the random chain degradation rate coefficients. The activation energies, determined from the temperature dependence of the rate coefficients, suggest that thermal degradation of polyperoxides is controlled by the dissociation of the OO bonds in the backbone of the polymer chain. Among the three polyperoxides investigated, the thermal stability is the highest for PAPSP and the lowest for PAMSP.
Keywords: Polyperoxides; Thermal degradation; Continuous distribution kinetics; Solution degradation;