Thermomechanical degradation of macromolecules

Extensional flow techniques are used to investigate thermomechanical scission of polymer solutions from ambient temperatures up to 150°C. We report precise central scission of chains beyond a critical fracture strain-rate. These results can be well accounted for by a Thermally Activated Barrier to Scission (TABS) model. We speculate upon the origin of degradation in simple shear flows and report novel results on degradation in porous media and ultrasonic sound fields, which contain dominant extensional components. Finally, we show how the nature and degree of degradation is affected by concentration and polydispersity. In semi-dilute entangled solutions, the degradation rates increase, are much higher for polydisperse solutions and the scission becomes progressively more random along the chain.Dedicated to Professor H. H. Kausch on the occasion of his 60th birthday     

Post-irradiation degradation of polypropylene

We present the mechanical changes (elongation at break, tensile strength, modulus of elasticity) and electrical changes (volume and surface resistivity, tg, , breakdown voltage) ofisotactic polypropylene (PP) due to -radiation. Ageing of PP was achieved in air by using a¹³⁷Cs -source with 400 Ci activity; the dose rate was of 8·10⁴ rad/h, and the integrated doses ranged from 2 to 160 Mrad. Severe deterioration of mechanical properties was observed. The changes of electrical properties were not as significant as those of mechanical characteristics. These variations are caused by the increase in density of crosslinking chain-scission and by oxidative processes. The rate of extent of degradations has been evaluated by the growth of the hyydroxy and carbonylbands in the IR spectrum By using FTIR mapping, we have also investigated the degradation of PP which had been irradiated in air 15 years ago. We determined the contour lines of FTIR absorption bands of carbonyl groups at different regions of the cross-section of an irradiated sample. Dust particles of irradiated brittle PP were also studied. The FTIR showed that the carbonyl band is broad, indicating a mixtures of many different functional groups (ester, acid, ketone, lactones). We determined also the gel content at different irradiation doses.     

Radiation degradation of polypropylene

Post-irradiation degradation of isotactic polypropylene irradiated by Co⁶⁰-γ-ray has been followed for 12 months. Effects of irradiation doses (10–100 kGy) up on the change of the structure and mechanical properties as well as flowability of this polymer has been studied. Carbonyl index increases with increasing post-irradiation time. The rate of which was much higher for doses above 50 kGy. Tensile strength declines with time and those samples irradiated above 50 kGy become quite brittle, just after irradiation. Results reveal that post-irradiation degradation of polypropylene irradiated by γ-ray occurs via chain scission mechanism.     

Thermomechanical degradation of polyisobutylene/polystyrene blends

The thermomechanical degradation of polyisobutylene/polystyrene blends, carried out at different temperatures and mixing speeds, has been followed by means of rheological measurements. At high rotational speeds all the blends degrade significantly, but the extent of degradation increases the lower the MFI of the virgin sample. The effect of temperature is very complex, depending on the temperature and on the composition.     

Thermoxidative degradation of polypropylene

The thermoxidative behavior of atactic and isotactic polypropylene under dynamical thermoxidative conditions has been studied. It has been established that, with the increase of the heating rate, the development of the oxidative processes are diminished and consequently a modification in the reaction mechanism takes place. One can notice at the same time that the oxidative processes are more intense in the case of the atactic polymer. The 5–15°C/min heating rates determine significant differences between the thermal behavior of the samples, permitting the elaboration of the standard curves useful in fast determination of the atactic content of the industrial products by routine analysis.     

Thermomechanical degradation of PLA‐based nanobiocomposite

Nanobiocomposites are a new class of biodegradable polymer materials with nanometric dispersion of inert particles in a biodegradable polymer matrix that show very interesting properties often very different from those of conventional‐ filled polymers and also biodegradability. An important issue in the applications of the biodegradable polymers is their easy degradability during processing due to the thermomechanical stress or to the presence of humidity. In this work, the thermomechanical degradation behavior of a nanobiocomposite made by a PLA‐based blend and an organomodified montmorillonite has been investigated. The degradation kinetics has been followed by means of rheological, mechanical and morphological characterization. In particular, the influence of temperature and of the presence of humidity have been considered. The presence of the nanoparticles slightly increases the thermomechanical degradation of the pure matrix and in particular with increasing time and temperature processing. In the more severe conditions, indeed, the organomodifier undergoes some slight decomposition of the organomodifier of the clay because of the Hoffmann elimination. The radicals formed through this decomposition enhance the degradation of the matrix. However, this decomposition is at the first stage, and the evolved CO₂ remains entrapped in the clay increasing the level of intercalation and causing also some exfoliation. Then the morphology of the nanobiocomposite changes because of the processing conditions. Moreover, the thermomechanical degradation remarkably increased if the materials are not pre‐dried because of the hydrolytic degradation of the biodegradable polyesters of the matrix. Copyright © 2015 John Wiley & Sons, Ltd.     

Effects of microparticles on isotactic polypropylene: Thermomechanical and thermal properties

Five types of melamine-formaldehyde microcapsules, that is, with shells of different compositions [melamine formaldehyde shell or a melamine formaldehyde-poly(hexamethylene adipate glycol) shell] and containing or not a flame retardant, diammonium hydrogen phosphate (DAHP), have been prepared by an in situ polymerization method and have been added to an isotactic polypropylene matrix (iPP) by melt blending at 5 wt %. Wide-angle X-ray diffraction and differential scanning calorimetry were employed to investigate the crystallization behavior of the prepared iPP/microparticles composites. The tensile properties and the thermal stability were also evaluated. It was stated that the morphology and the shape of the microparticles not only influence the crystallization behavior but also the thermomechanical properties of these composites. Thus, rougher microparticles act as a nucleating agent for the iPP, and in the presence of microparticles containing DAHP, the α- and the β-crystals are formed. Moreover, the presence of microparticles improves the thermal stability of the iPP. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2566–2576, 2008     

Atactic polypropylene thermal degradation test

Atactic polypropylene was subject to thermal treatment the temperature range 200–260°C for 2–4 h. Polymer structure changes assessment was made by derivatograph.     

Natural photo-aging degradation of polypropylene nanocomposites

The natural photo-aging degradation of polypropylene (PP), PP/CaCO₃ and PP/SiO₂ nanocomposites were studied outdoor for up to 88 days. The chemical structure characterized by Fourier transform infrared spectroscopy (FTIR) and pyrolysis gas chromatography-mass spectroscopy (PGC-MS) showed that PP nanocomposites are much more susceptible to photo-degradation than unfilled PP. And the oxidation rate is faster with more filler amount. There are lots of chain scissions happened in PP nanocomposites, accompanied with the formation of ketone, alcohol, ester and unsaturated double bond. This severe chain scission led to great decrease of M_n and M_w, and the consequent small fragments would re-crystallize and increase the crystallinity of the nanocomposites. However, these effects do not relate to the ultraviolet character of the two nano fillers.     

Gradient structure of polypropylene composites filled with carbon black

Russian Chemical Bulletin - Gradient polymer composites were prepared by melt-mixing of carbon black and polypropylene in a compounder and subsequent hot pressing of pellets. The thermally treated...     

Impact behaviour of polypropylene filled with multi-walled carbon nanotubes

Multi-walled carbon nanotubes/polypropylene composites were compounded using a twin-screw extruder. Here, nanotubes with different lengths, i.e. 1-2 μm and 5-15 μm, respectively, were applied at a constant volume content of 1%. Notched Charpy impact tests showed that toughening effects of nanotubes depended highly on testing temperatures. The impact resistance was notably enhanced at a temperature above the glass transition temperature of matrix. Longer nanotubes performed more effective in toughening compared to the shorter ones. The increment of impact resistance of nanotube-filled polypropylene was considered due to enhanced load-carrying capability and much-increased deformation of matrix. SEM fractography further revealed the toughening mechanisms in a micro-scale. The impact energy was improved via nanotube breakage and pullout, which likely led to a series of energy consuming actions. In addition, the smaller spherulite size induced by nanotubes would be favourable to the impact resistance partially.     

Nucleation and crystallization of polypropylene filled with BaSO4

The influence of high concentration of BaSO₄ as nucleating agent on crystallization of fiber-forming polypropylene was studied by DSC. The work presents experimental and calculated values of melting and crystallization enthalpies of filled polypropylene and the influence on the formation of interface interactions between filler and polymers. These results show minimal interactions of components (BaSO₄ and polypropylene) under experimental conditions.     

Rheology of highly filled polypropylene compounds: Measurement and applications

It is common knowledge among rheologists as well as polymer engineers that compounds of thermoplastics with a high volume content of mineral fillers show a viscoelastic behaviour which is significantly different from homogeneous systems. A strong nonlinearity is combined with the development of a second plateau region of the dynamic moduli at lower frequencies. This for one makes the rheological characterization of these materials more complicated, as could be shown in a round robin test, including a PP-compound with talc. The linearity criterion has to be handled carefully to achieve comparable and representative results. However, the viscoelasticity gives access to a quick characterization method regarding dispersion quality in compounds. This can be shown for the case of PP-talc-compounds, where correlations between relaxation spectrum and impact strength could be developed.     

Accelerated degradation of polypropylene by amosite asbestos

The use of differential scanning calorimetry and themogravimetric analysis showed that the accelerated oxidative degradation of polypropylene in the presence of amosite asbestos can be attributed to the catalytic decomposition of peroxides by iron and to air trapped in the fibre bundle.     

The mechanical and thermal behaviors of glass bead filled polypropylene

Notch Izod impact strength of poly(propylene) (PP)/glass bead blends was studied as a function of temperature. The results indicated that the toughness for various blends could undergo a brittle‐ductile transition (BDT) with increasing temperature. The BDT temperature (T_BD) decreased with increasing glass bead content. Introducing the interparticle distance (ID) concept into the study, it was found that the critical interparticle distance (ID_c) reduced with increasing test temperature correspondingly. The static tensile tests showed that the Young's modulus of the blends decreased slightly first and thereafter increased with increasing glass bead content. However, the yield stress decreased considerably with the increase in glass bead content. Dynamic mechanical analysis (DMA) measurements revealed that the heat‐deflection temperature of the PP could be much improved by the incorporation of glass beads. Moreover, the glass transition temperature (T_g) increased obviously with increasing glass beads content. Differential scanning calorimetry (DSC) results implied that the addition of glass beads could change the crystallinity as well as the melting temperature of the PP slightly. Thermogravimetric analysis (TGA) measurements implied that the decomposition temperature of the blend could be much improved by the incorporation of glass beads. Copyright © 2004 John Wiley & Sons, Ltd.     

Thermal degradation of thin films of isotactic polypropylene and polypropylene with ketonic additives

The rate of oxidation of 0.3–0.7 mil films of pure polypropylene is much more rapid than with thicker films. The rate of oxidation increases with the increase in the partial pressure of oxygen and with temperature. The apparent activation energy in oxygen is 22.5 keal/mole. 1,3-Diphenyl-2-propanone added to the polymer acts as an oxidation initiator while p-phenylacetophenone and 4-phenylbenzophenone slightly retard the oxidation. The effects of the additives are more pronounced when the oxidation was carried out in air or at the lower temperatures (90°C) when the oxidation was conducted in pure oxygen. The degree of crystallinity based on the infrared data was found to increase with the degree of oxidation of the polymer.     

Crystallization properties and thermal stability of polypropylene composites filled with wollastonite

The influence of wollastonite (CaSiO₃) content on the crystallization properties and thermal stability of polypropylene (PP) composites was investigated. The results showed that the crystallization temperature, crystallization end temperature and crystallization temperature interval, as well as the degree of crystallinity of the composites, were higher than those of the unfilled PP resin, while the crystallization onset temperature was little changed from that of the unfilled PP resin. The increase of degree of crystallinity for the composites could be attributed to the heterogeneous nucleation of the CaSiO₃ in the PP matrix. The thermal stability increased with increasing filler weight fraction (ϕ_f); the thermal decomposition rate decreased nonlinearly with increasingϕ_f. Finally, the dispersion of the filler particles in the matrix was observed, and the mechanisms of thermal stability and crystallizing behavior were discussed.     

Mechanical properties and morphology of intumescent flame retardant filled polypropylene composites

The intumescent flame retardant (IFR) filled polypropylene (PP) composites were prepared using a twin‐screw extruder. The tensile and impact fracture behavior of the composites were measured at room temperature. It was found that the Young's modulus increased roughly, while the tensile strength decreased slightly with increasing the IFR weight fraction; the toughening effect of the filler on the PP resin was significant. Both the V‐notched Izod impact strength and the V‐notched Charpy impact strength of the PP/IFR composites showed a nonlinear increase with increasing the filler weight fraction (φ_f) as φ_f was less than 20%, then it decreased. The limited oxygen index of the composites increases nonlinearly with increasing φ_f. The relationship between them obeyed a quadratic equation. The impact fracture surface was observed by means of a scanning electronic microscope to understand the toughening mechanisms for the composite systems. Copyright © 2014 John Wiley & Sons, Ltd.     

Pyrolysis of polypropylene: II. Kinetics of degradation

The kinetic law dα/dr=k(1-α)[1-(1-α)^2b]^1,2 proposed for the pyrolysis of polystyrene is shown to be valid for the pyrolysis of polypropylene taking into account only the percentage of isotactic polymer.As the experimental activation energy of 265 kJ mol^?1 is of the same order of magnitude the as the theoretical energy calculated by the equation E = 1 2 (E_ir-E_r)-E_r it can be concluded that the decomposition mechanism is governed by a depolymerization reaction as the principal products obtained are compounds with 3n carbon atoms.