Aging behavior and thermal degradation of fluoroelastomer reactive blends with poly-phenol hydroxy EPDM

In this paper, the aging behavior of the reactive blends of fluoroelastomer (FPM) with poly-phenol hydroxy ethylene propylene diene monomer rubber (PHEPDM) in hot air was firstly investigated. The aging mechanism was analyzed by the swelling experiment, attenuated total-reflectance Fourier-transform infrared (FTIR-ATR) spectroscopy and X-ray photoelectron spectroscopy (XPS). The results showed that the aging process increased the crosslinking density and the content of double bond. The O/F or O/C ratios increased and then decreased during aging because of the oxidation reaction of molecular chain and the surface migration of fluoro group. Secondly, thermogravimetric analysis (TGA) was used to study the thermal degradation behavior of the reactive blends. The apparent degradation activation energy (E) of FPM/PHEPDM reactive blends was calculated by the Kissinger and Coats-Redfern methods, respectively. The results showed that the FPM/PHEPDM reactive blends had higher thermal degradation temperature but lower E than FPM. Both the thermal degradation process of FPM/PHEPDM reactive blends and FPM were determined by nucleation and growth mechanism (Am). The general mechanism function was [−ln(1 − α)]^1/m. The optimum value of m was between 1/3 and 1/2 for FPM/PHEPDM reactive blends, but 1/2 for FPM. From the results above, it was deduced that the special structure of PHEPDM made itself surrounded by fluoroelastomer and protected from hot-air aging and thermal degradation.     

Thermal degradation and stability of epoxy nanocomposites: Influence of montmorillonite content and cure temperature

The thermal behaviour and stability of epoxy nanocomposites were studied by thermogravimetric analysis (TGA). The nanocomposites consisted of a trifunctional epoxy resin, a hardener containing reactive primary amine groups and clay nanoparticles (i.e. montmorillonite), previously treated with octadecyl ammonium. Three levels of nanoclay content (0, 5 and 10%) and three temperature levels (120, 150 and 200 °C) were used. The exfoliation of nanoparticles within the material was analyzed by X-ray diffraction (XRD). The cure conversion was determined by Fourier transform infrared (FTIR) spectroscopy by selecting the suitable band for epoxide functional groups. The study demonstrated that the nanoclay greatly accelerates the cure, at the different cure temperatures studied. Finally, the thermal stability of the various nanocomposites was established by calculating various characteristic temperatures from thermograms as well as conversion and conversion derivative at maximum decomposition rate. The collisions between resin molecules, which are trapped within the nanoclay galleries, were less effective because they were protected against thermal degradation by the galleries. However, once the collision was effective, the thermal activation occurred more readily.     

Poly-silafluoroalkyleneoligosiloxanes: a class of fluoroelastomers with low glass transition temperature

This work reports some results about the synthesis of unsaturated poly-silafluoroalkyleneoligosiloxanes—derived from TFE telomers—which, after crosslinking, gave elastomeric materials characterized by good flexibility at low temperature, glass transitions below −45 °C and good thermooxidative stability over 250 °C. They are proposed as alternative materials with respect to polyfluoroolefin elastomers.     

Effect of ZnO and organo-modified montmorillonite on thermal degradation of poly(methyl methacrylate) nanocomposites

Since a few years ago, a topic of interest consists in developing composites filled with nanofillers to improve thermal degradation and flammability property of poly(methyl methacrylate) (PMMA). In the present work, the effects of ZnO nanoparticles and organo-modified montmorillonite (OMMT) on the thermal degradation of PMMA were investigated by thermogravimetric analysis (TGA). PMMA-ZnO and PMMA-OMMT nanocomposites were prepared by melt blending with different (2, 5, and 10 wt%) loadings. SEM and TEM analyses of nanocomposites were performed in order to investigate the dispersion of nanofillers in the matrix. According to TGA results, the addition of ZnO nanoparticles does not affect the thermal degradation of PMMA under an inert atmosphere. However, in an oxidative atmosphere, two contrary effects were observed, a catalytic effect at lower concentration of ZnO in the PMMA matrix and a stabilizing effect when the ZnO concentration is higher (10 wt%). In contrast, the presence of OMMT stabilizes the thermal degradation of PMMA whatever be the atmosphere. Differential thermal analysis (DTA) curves showed surprising results, because a dramatic change of exothermic reaction of the PMMA degradation process to an endothermic reaction was observed only in the case of OMMT. During the degradation of PMMA-ZnO nanocomposites, pyrolysis-gas chromatography coupled to mass spectrometer (Py-GC/MS) showed an increase in the formation of methanol and methacrylic acid while a decrease in the formation of propanoic acid methyl ester occurred. In the case of PMMA-OMMT systems, a very significant reduction in the quantity of all these degradation products of PMMA was observed with increasing OMMT concentration. It is also noted that during PMMA-OMMT degradation less energy was released as the decomposition is an endothermic reaction and the material was cooled.     

A thermal degradation mechanism of polyvinyl alcohol/silica nanocomposites

The thermal degradation mechanism of a novel polyvinyl alcohol/silica (PVA/SiO₂) nanocomposite prepared with self-assembly and solution-compounding techniques is presented. Due to the presence of SiO₂ nanoparticles, the thermal degradation of the nanocomposite, compared to that of pure PVA, occurs at higher temperatures, requires more reaction activation energy (E), and possesses higher reaction order (n). The PVA/SiO₂ nanocomposite, similar to the pure PVA, thermally degrades as a two-step-degradation in the temperature ranges of 300-450 °C and 450-550 °C, respectively. However, the introduction of SiO₂ nanoparticles leads to a remarkable change in the degradation mechanism. The degradation products identified by Fourier transform infrared/thermogravimetric analysis (FTIR/TGA) and pyrolysis-gas chromatography/mass spectrometric analysis (Py-GC/MS) suggests that the first degradation step of the nanocomposite mainly involves the elimination reactions of H₂O and residual acetate groups as well as quite a few chain-scission reactions. The second degradation step is dominated by chain-scission reactions and cyclization reactions, and continual elimination of residual acetate groups is also found in this step.     

The thermal degradation behaviour of polydimethylsiloxane/montmorillonite nanocomposites

A series of novel polydimethylsiloxane/montmorillonite (PDMS/MMT) nanocomposites was prepared. The thermal degradation behaviour of these nanocomposites was studied by means of Thermal Volatilization Analysis (TVA) and Thermogravimetric Analysis (TGA). The major degradation products were identified as cyclic oligomeric siloxanes from D₃ to D_7, and higher oligomeric siloxane residues. Other minor degradation products include methane, bis-pentamethylcyclotrisiloxane, propene, propanal, benzene and dimethylsilanone. The results demonstrate that the nanoclay significantly alters the degradation behaviour of the PDMS network, modifying the profile of the thermal degradation and reducing the overall rate of volatiles evolution. The results also indicate that the nanoclay promotes the formation of dimethylsilanone and benzene by inducing low levels of radical chain scission.     

A model for thermal degradation of hybrid nanocomposites

A model is developed for thermal degradation of polymer nanocomposites. A composite is thought of as an equivalent network of linear chains with attached side-groups. Thermal degradation is treated as combination of (i) binary scission (fragmentation) of backbone chains, and (ii) detachment of side-groups and their subsequent annihilation (diffusion to the surface of a sample and desorption). An explicit solution is derived for the kinetic equation. This solution involves three adjustable parameters that are found by fitting observations on isotactic polypropylene reinforced with carbon nanofibres. Good agreement is demonstrated between the experimental data and the results of numerical simulation.     

Microstructure to property relations in a family of millable polyurethane fluoroelastomers

A series of segmented polyurethane fluoroelastomers based on perfluoropolyether macromers and functionalized with allyl groups is presented. Their peroxidic vulcanization behaviour was studied on both unfilled and carbon black filled compounds by dynamic rheometry. The optimal amount of allyl groups and peroxy content was determined. Formation of polyphasic structure with phase segregation of a low T_g fluorinated moiety was shown by calorimetric analysis. The low T_g value is not affected by vulcanization which, on the other hand, is effective in the elimination of any residual crystalline order of the urethane phase. The vulcanized compounds were characterized by tensile tests, dynamic-mechanical analysis, solvent swelling, compression tests and non-isothermal stress relaxation measurements. Mastercurves were successfully built based on time-temperature superposition principle. It resulted that polyurethanes based on symmetric diisocyanates show better chemical resistance and low temperature viscoelastic behaviour, and are promising candidates for the development of high performance low temperature sealing materials.     

Silver sulfide/poly(3-hydroxybutyrate) nanocomposites: Thermal stability and kinetic analysis of thermal degradation

The non-isothermal degradation of poly(3-hydroxybutyrate) (PHB) and silver sulfide/poly(3-hydroxybutyrate) (Ag₂S/PHB) nanocomposites was investigated using thermogravimetric (TG) analysis. In the composite materials, Ag₂S caused the degradation of PHB at a lower temperature as opposed to that of neat PHB. Moreover, an increase Ag₂S loading in the PHB decreased the onset temperature (T_onset) of thermal degradation, whereas it was raised upon augmenting the heating rate. From Kissinger plots, the observed trend of the degradation activation energy, E_d, was attributed to polymer-particle surface interactions and the agglomeration of Ag₂S. The thermal degradation rate constant, k, was linearly related to the Ag₂S loading in PHB. Thus, the Ag₂S nanoparticles effectively catalyzed the thermal degradation of PHB in the Ag₂S/PHB nanocomposites. Differential scanning calorimetry (DSC) data also supported the catalytic property of Ag₂S.     

Thermal and thermo-oxidative degradation of high density polyethylene/fullerene composites

Fullerene (C₆₀)/high density polyethylene (HDPE) composites were studied in order to understand for their behaviors on thermal and thermo-oxidative degradation. Under different atmosphere, the influences of C₆₀ on the thermal stability of HDPE are different. Thermogravimetric analysis coupled to Fourier transform infrared spectroscopy (TG-FTIR) and pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS) demonstrate that in N₂ the addition of C₆₀ increases the onset decomposition temperature by about 10 °C with more heavy compounds (more than 34 carbon). Also the thermal stability of HDPE in air is remarkably improved with the addition of C₆₀. When the content of C₆₀ is 2.5 wt% the onset decomposition temperature increases by about 91 °C. The results of viscoelastic behavior and gel content reveal that C₆₀ can trap the alkyl radicals and alkyl peroxide radicals to inhibit hydrogen abstraction to suppress the chain scission and preserve the long chain structure. However, in the absence of C₆₀ or with low C₆₀ concentration, hydrogen abstraction occurs, resulting in the formation of a series of alkyl radicals and alkyl peroxide radicals, which accelerates the chain scission and plays a leading role in the thermal oxidative degradation.     

Comparing the effect of nanofillers as thermal stabilizers in low density polyethylene

A study of the thermal degradation under inert and oxidative conditions of LDPE and three 5-wt% nanocomposites has been performed. The bases of comparison were the geometries of the nanofillers (spherical, fibrous and laminar) and the sample thickness. Homogeneous and well-dispersed materials were obtained with the three nanoparticles, ensuring a relevant comparative analysis.The thermal degradation curves obtained from TGA under nitrogen flow did not show significant differences in behaviour within the nanocomposites and the reference LDPE. However, the results for the thermo-oxidation study showed a strong stabilization effect for both fibrous and laminar silicates, but not for the spherical silica nanoparticles. A kinetic study of the degradation under isothermal conditions showed that the nanocomposites made from fibrous and laminar silicates degraded following the mechanisms observed for thin films independent of the sample thickness. These results suggested the occurrence of a protective layer against thermo-oxidation on the film surface. Chemical analysis of the degraded surfaces by IR and EDX measurements gave data to explain these differences in behaviour.     

Effect of clay organomodifiers on degradation of polyhydroxyalkanoates

Ammonium surfactants are commonly used as clay organomodifiers in nanocomposites. Their effect on the thermal- and thermo-mechanical degradation of polyhydroxyalkanoates (PHAs) is reported. Two poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV) grades were tested and compared to polyhydroxybutyrate (PHB). Thermal stabilities were determined from thermogravimetric data and determination of molecular weight changes after processing was performed. The data revealed that all surfactants enhance the PHBV degradation. The results also highlight the preponderant effect of the initial M_w rather than the HV content on the thermal stability. The thermo-mechanical study confirmed the role of surfactants and their different behaviour towards PHBV degradation. This study demonstrates that all surfactants enhance the PHA degradation since their decomposition products most likely act as catalytic agents.     

Thermal degradation of high density polyethylene in a reactive extruder

The thermal degradation of high density polyethylene was conducted in a reactive extruder at various screw speeds with reaction temperatures of 400 °C and 425 °C. The residence time of the extruder was estimated and the molecular weight distribution of the fed plastic and reaction products was analysed using gel permeation chromatography. A continuous kinetic model was used to describe the degradation of the high density polyethylene in the reactive extruder. The breakage kernel and the scission rate model parameters were estimated from the experimental data for a variety of cases. It was found that purely random breakage and a scission rate which had a power law dependence on molecular size of 0.474 best described the experimental data.     

Effect of manganese nanoparticles on the mechanical, thermal and fire properties of polypropylene multifilament yarn

Polypropylene filled with 10 wt% of inorganic nanoparticles has been prepared by melt blending. The fillers investigated were manganese oxides (MnO and Mn₂O₃) and manganese oxalate (MnC₂O₄). The morphology and thermal stability of these nanocomposites have been studied by transmission electron microscopy (TEM) and thermogravimetric analysis (TGA). The experimental results reveal that the addition of 10 wt% manganese oxides improves the thermal stability in air of polypropylene by about 70-80 °C. In a second step, these nanocomposites have been processed by melt spinning in order to produce multifilament yarn. The mechanical properties of these filaments have then been characterized. It is shown that just the addition of Mn₂O₃ improves the mechanical properties of polypropylene filaments. The flammability of these nanocomposites used as knitted fabrics has finally been evaluated with a mass loss calorimeter at 35 kW/m². This kind of experiment has not revealed a real improvement of fire properties.     

Nanoclay and carbon nanotubes as potential synergists of an organophosphorus flame-retardant in poly(methyl methacrylate)

This study explores whether nanoparticles incorporated in polymers always act as synergists of conventional flame-retardant additives. For this purpose, two different filler nanoparticles, namely organically modified layered-silicate clay minerals or nanoclays and multi-walled carbon nanotubes, were incorporated in poly(methyl methacrylate) filled with an organophosphorus flame-retardant that acts through intumescence. Effective dispersion techniques specific to each nanoparticle were utilized and prepared samples were thoroughly characterized for their nanocomposite morphologies. Nanoclays were shown to outperform carbon nanotubes in respect of improving the fire properties of intumescent formulations assessed by cone calorimeter analysis. An intriguing explanation for the observed behaviour was the restriction of intumescence by strong carbon nanotube networks formed on the flaming surfaces during combustion contrary to enhanced intumescent chars by nanoclays. Carbon nanotubes surpassed nanoclays considering the thermal stability of intumescent formulations in thermogravimetry whereas mechanical properties were significantly superior with nanoclays to those with carbon nanotubes.     

Thermal stability of polypropylene-clay nanocomposites subjected to laser pulse heating

Polypropylene based nanocomposites filled with montmorillonite nanoclay prepared by twin screw extrusion have been studied for thermal stability at high heating rates. In contrast to traditional thermal stability and flammability studies of polymer nanocomposites using heating rates on the order of tens of degrees per minute, this study achieves heating rates that are six orders of magnitude higher. This was accomplished using laser pulse heating. The results show that the nanoclay increases thermal stability of the polymer, as measured by a decrease in the mass loss for a laser pulse at a given energy and intensity. Electron microscopy and various spectroscopic techniques show that a silicate-rich char layer may provide the mechanism for protection of the polymer and decreased degradation rates. The results of the study are compared to the typical results found in traditional thermal stability testing.     

Thermal degradation of organophosphorus flame-retardant poly(methyl methacrylate) nanocomposites containing nanoclay and carbon nanotubes

Filler nanoparticles pave the way for the development of novel halogen-free flame-retardant polymers. The aim of this study was to investigate the thermal degradability, and in particular, the thermal degradation mechanism of organophosphorus flame-retardant poly(methyl methacrylate) (PMMA) nanocomposites containing nanoclay (NC) and multi-walled carbon nanotubes (CNT). For this purpose, thermogravimetry and direct pyrolysis mass spectrometry analysis were utilized. The onset of degradation was delayed through increased maximum degradation temperature and suppressed mass loss corresponding to initial degradation stage with carbon nanotubes and nanoclays, respectively. Possibility of reactions of melamine and/or melamine derivatives and interactions between carbonyl groups of PMMA and phosphinic acid leading to thermally more stable products was increased owing to the barrier effect of filler nanoparticles. In the presence of NC better flame retarding characteristics was detected as anhydride formation, leading to charring being more effective.     

Effect of thermal degradation on rheological properties for poly(3-hydroxybutyrate)

Thermal degradation at processing temperature and the effect on the rheological properties for poly(3-hydroxybutyrate) have been studied by means of oscillatory shear modulus and capillary extrusion properties, with the aid of molecular weight measurements. Thermal history at processing temperature depresses the viscosity because of random chain scission. As a result, gross melt fracture hardly takes place with increasing the residence time in a capillary rheometer. Moreover, it was also found that the molecular weight distribution is independent of the residence time, whereas the inverse of the average molecular weight is proportional to the residence time. Prediction of average molecular weight with a constant molecular weight distribution makes it possible to calculate the flow curve following generalized Newtonian fluid equation proposed by Carreau as a function of temperature as well as the residence time.     

Mechanistic implications of plastic degradation

Plastics have become an indispensable ingredient of human life. Their enormous use is a matter of great environmental and economic concern, which has motivated the researchers and the technologists to induce different degrees of degradations in the plastic. These degradations can be induced in a better way if their mechanistic implications are properly understood. A better understanding of the mechanism for these degradations is also advocated in order to facilitate the proper use of the alternative waste disposal strategies. In view of the facts concerning the plastic degradation, in this review article, we have discussed various types of polymeric degradations along with their mechanisms, which include photo-oxidative degradation, thermal degradation, ozone-induced degradation, mechanochemical degradation, catalytic degradation and biodegradation. This article also discusses the different methods used to study these degradations and the factors that affect these degradations.     

Nanosize and microsize clay effects on the kinetics of the thermal degradation of polylactides

Polylactide (PLA)-montmorillonite (MMT) micro- and nanocomposites based on semicrystalline and amorphous polymers and unmodified or organomodified clays at 5 wt% content were produced by melt mixing. Based on the three different test methods that were used to follow thermal degradation, different conclusions were obtained. During melt processing, thermomechanical degradation was more pronounced in the presence of all fillers, which apparently acted catalytically, but to different degrees. During isothermal degradation in air from 180 °C to 200 °C, degradation rate constants were calculated from novel equations incorporating changes in intrinsic viscosity (IV). Results show that the thermal degradation rate constants of the amorphous PLA and its composites are lower than those of the semicrystalline PLA and its composites. Due to better filler dispersion in the polymer matrix, the thermal degradation rate constants of the nanocomposites are significantly lower than those of the unfilled polymers and their microcomposites under air. As per dynamic TGA data and thermal kinetic analysis from weight losses and activation energy calculations, organomodified nanofillers have a complex effect on the polymer thermal stability; the unmodified fillers, however, reduce polymer thermal stability. These TGA data and kinetic analysis results also support the findings that the thermal stability of the amorphous PLA and its composites is higher than that of the semicrystalline polymer and its composites and the thermal stability of the nanocomposites is higher than that of the microcomposites. In general, mathematical modeling based on random thermal scission equations was satisfactory for fitting the TGA experimental data.