Effect of natural antioxidants on the stability of polypropylene films

A preliminary study on the efficiency of agri-food industry wastes as stabilizers for polypropylene (PP) films is reported. Several analytical techniques were employed to evaluate the stabilization effectiveness of the additives. DSC and CL analysis performed on unaged samples confirmed the antioxidant activity of natural additives, and provided the following order of efficiency: red grape seeds > white grape seeds > tomato extracts. The films were also artificially aged at 70 °C, and FTIR spectroscopy confirmed the stabilization trend obtained from the unaged films. Kinetic analysis of TG data alongside tensile tests indicated that the tomato extract is a good thermal and processing stabilizer, but it is sensitive to oxidation. In contrast, grape seeds provide long-term stabilization to PP under conditions of oxidative degradation. Our results show that tomato and wine processing by-products have good potential to be exploited as a low-cost source of value-added phytochemicals.     

Hydroperoxide build-up in the thermal oxidation of polypropylene - A kinetic study

The thermal oxidation of unstabilised polypropylene (PP) was studied at 80 °C under various oxygen pressures: 0.02, 0.5 and 5.0 MPa, and, under 5.0 MPa oxygen pressure at various temperatures: 60, 80, 100 and 120 °C. Hydroperoxides were titrated using a chemical titration method and modulated DSC (taking an enthalpy of −325 kJ mol⁻¹). Starting from a previous kinetic analysis of carbonyl growth in same exposure conditions, we have tried to simulate experimental results by a model based on the classical mechanistic scheme in which initiation results from POOH (mainly bimolecular) decomposition. The model, which takes into account substrate consumption and does not rely on usual simplifying assumptions (steady state for radicals, long kinetic chains, interrelations between termination rate constants), generates kinetic curves with the same shape as experimental ones and predicts well the effect of O₂ pressure and temperature on hydroperoxide and carbonyl concentrations.     

Synthesis of 3,5-ditert-butyl-4-hydroxybenzoates and their thermal antioxidation behavior for polypropylene

Butyl 3,5-ditert-butyl-4-hydroxybenzoate (BB), octyl 3,5-ditert-butyl-4-hydroxybenzoate (OB), and dodecyl 3,5-ditert-butyl-4-hydroxybenzoate (DB) were synthesized and characterized, and their performance as antioxidant for polypropylene (PP) were also investigated. The thermooxidative stability of PP with different antioxidants were assessed by the measurements of long-term accelerated aging, differential scanning calorimetery, and thermogravimetric analysis. The results showed that 3,5-ditert-butyl-4-hydroxybenzoates could effectively inhibit the oxidation degradation of PP. And it was also found that the antioxidant efficiency of BB, OB, and DB for PP was in the order of DB > OB > BB.     

Effects of halloysite nanotube orientation on crystallization and thermal stability of polypropylene nanocomposites

The polypropylene/halloysite nanotubes (PP/HNTs) nanocomposites were prepared via water-assisted injection molding (WAIM) and compression molding (CM). HNTs were highly oriented in WAIM parts due to the strong shear effect; whereas HNTs were randomly oriented in the CM one. The orientation of HNTs had little influence on their nucleating efficiency for the PP. However, the HNTs selectively induced α-form crystal at high cooling rates; whereas they showed β-nucleating activity at low cooling rates. Thermal analyses revealed that the HNTs delayed thermal degradation onset in the initial degradation stage, whereas they sped up the thermal degradation in the main volatilization stage at the contents of 5 and 8 wt%. The simultaneous thermogravimetric analyses and differential scanning calorimetry measurements revealed that, at a low content, the direct stabilizing effect of HNTs on PP contributed largely to the increased thermal stability of the WAIM PP/HNTs nanocomposites rather than their barrier and entrapment effect on the volatile products.     

Effect of oxygen pressure on the oxidation kinetics of unstabilised polypropylene

The thermal oxidation of unstabilised polypropylene films at 80 °C and various oxygen pressures ranging from 0.02 MPa to 5 MPa has been studied by FTIR spectrophotometry (carbonyl growth). The induction time decreases and the maximum oxidation rate increases quasi-hyperbolically when the oxygen pressure increases. The asymptotic behaviour (corresponding to the regime of oxygen excess) is not reached at the highest pressure under study. A kinetic model derived from a classical mechanistic scheme but free of simplifying hypotheses, has been used to simulate the observed behaviour and to determine the elementary rate constants. It is shown that a good simulation of kinetic curves of carbonyl build-up in the whole pressure interval under study can be obtained with a set of physically reasonable rate constant values. The “inverse problem” cannot be, however, totally solved because certain constants are interdependent so that some rate constant values have to be arbitrarily chosen or taken from the literature.     

Kinetics of thermal degradation and estimation of lifetime for polypropylene particles: Effects of particle size

The thermal stability and degradation behavior of polypropylene (PP) particles having diameter varying from few micrometers to nanometers were studied by thermogravimetric analysis (TGA). The PP particles of average diameter ∼20 μm, ∼10 μm, ∼5 μm, ∼1 μm and <500 nm were studied over a range of temperature from 25 to 600 °C in N₂ atmosphere and heating rates of 5, 10 and 15 °C/min. Thermal stability of PP particles initially decreases and then increases as particle size further decreases to nanometer scale. The five single heating rate techniques such as Friedman, Freeman-Carroll, Chang, Coats-Redfern and second Kissinger; and three multiple heating rate techniques such as the first Kissinger, Kim-Park and Flynn-Wall were used to compute the kinetic parameters of degradation reaction, e.g., activation energy (E_a), order of reaction (n) and frequency factor [ln(Z)]. The lifetime of macro-, micro- and nanosized PP particles was also estimated by a method proposed by Toop. It was found that the activation energy and lifetime of nanosized PP particles are moderately high compared to the microsized PP particles. Moreover, the decomposition temperature, order of reaction (n), frequency factor [ln(Z)] not only depend on the heating rate and calculation technique but also on the particle size of polymer. The results are compared with macrosized PP.     

Profluorescent nitroxides: Thermo-oxidation sensors for stabilised polypropylene

The profluorescent nitroxide, 1,1,3,3-tetramethyldibenzo[e,g]isoindolin-2-yloxyl (TMDBIO) was investigated as a probe for the radical-mediated degradation of stabilised polypropylene. TMDBIO has been previously shown to be a sensitive probe for free-radical degradation during the thermo-oxidation of unstabilised polypropylene. Here we report on the effect that adding hindered phenol or phosphite stabilisers to polypropylene has on the free-radical sensing ability of TMDBIO during thermo-oxidation. In addition, novel dual-functional, hindered phenol containing profluorescent nitroxides, 5-[2-(4-hydroxy-3,5-di-tert-butylphenyl)ethenyl]-1,1,3,3-tetramethylisoindolin-2-yloxyl (HSTMIO) and its derivatives were investigated as probes for the radical-mediated degradation of polypropylene. These dual-functional probes were shown to be efficient stabilisers for polypropylene during thermo-oxidation at 150 °C in oxygen and sensors of thermo-oxidation during its early stages, in the so-called “induction period”.     

Gas permeation and mechanical properties of polypropylene nanocomposites with thermally-stable imidazolium modified clay

Dialkyl imidazolium salt with better thermal stability than the commonly used dimethyldioctadecyl ammonium salt was synthesized and ion exchanged on the montmorillonite surface. Polypropylene nanocomposites with different volume fractions of the obtained organo-montmorillonite (OMMT) were prepared and the effect of the modified clay on the gas barrier and mechanical properties was studied. Wide angle X-ray diffraction (WAXRD) and transmission electron microscopy (TEM) were used to investigate the microstructure obtained. Thermal behavior of the composites analyzed by thermogravimetric analysis was observed to enhance significantly with the filler volume fraction. The gas permeation through the nanocomposite films markedly decreased with augmenting the filler volume fraction. The decrease in the gas permeation was even more significant than through the composites with ammonium treated montmorillonite. Better thermal behavior of the organic modification owing to the delayed onset of degradation hindered the interface degradation along with detrimental side reactions with polymer itself. Transmission electron microscopic studies indicated the presence of mixed morphology i.e., single layers and the tactoids of varying thicknesses in the composites. The crystallization behavior of polypropylene remained unaffected with OMMT addition. A linear increase in the tensile modulus was observed with filler volume fraction owing to partial exfoliation of the clay.     

Thermal and crystallization studies of short flax fibre reinforced polypropylene matrix composites: Effect of treatments

The effect of fibre treatments on thermal stability of flax fibre and crystallization of flax fibre/polypropylene composites was investigated. For thermal stability study, flax fibres have been treated using maleic anhydride, maleic anhydride polypropylene copolymer, vinyltrimethoxy silane and alkalization. In order to compare thermal stability of flax fibres thermogravimetry (TG) analysis has been used. Kinetic parameters have been determined by Kissinger method. Results showed that all treatments improved thermal stability of flax fibres. For crystallinity analysis, three different techniques have been used, differential scanning calorimetry analysis (DSC), pressure–volume–temperature (PVT) measurements for analysis of volume shrinkage and polarized optical microscopy (POM). All techniques results showed that addition of flax fibre increased crystallization rate. Besides, depending on fibre surface treatment and crystallization temperature, flax fibre/PP composites can show transcrystallinity.     

Immobilization of antioxidants via ADMET polymerization for enhanced long-term stabilization of polyolefins

Novel macromolecular antioxidants with multiple hindered phenolic antioxidant moieties along a linear, unsaturated olefinic backbone are prepared and their antioxidative ability in polypropylene (PP) blends is investigated. Firstly, α,ω-diene monomers bearing one alcohol functionality are prepared from 10-undecenoic acid, sourced from the renewable resource castor oil, and are subsequently coupled to hindered phenolic antioxidants to prepare antioxidant bearing α,ω-diene monomers. The preparation of macromolecular antioxidants is then described, via ADMET polymerization of the hindered phenol bearing monomers. Upon blending with polypropylene, the resultant blends show excellent antioxidative stabilization in comparison to commercially available antioxidants and thus represent promising additives for the long-term stabilization of polyolefins in extreme environments, for example in the construction of solar water heating systems.     

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.     

Extraction of polypropylene additives and their analysis by HPLC

Summary The polypropylene additives were extracted by dissolution-precipitation and Soxhlet. The Soxhlet method was adapted for the extraction of phosphorous antioxidants. The RP HPLC method with quaternary gradient elution separated five chemical groups of additives: lower molecular mass di-tert-butyl phenol (D.T.B.P.), hindered amine light stabilizers (Tinuvin 326), hindered phenolic antioxidants (Irganox 1010) and phosphorous antioxidants (Irgafos 168 and Ultranox 626) with their degradation products.     

Effect of acid treated multi-walled carbon nanotubes on the mechanical, permeability, thermal properties and thermo-oxidative stability of isotactic polypropylene

The effect of acid treatment of multi-walled carbon nanotubes (MWCNTs) on the mechanical, thermal and mainly thermo-oxidative stability of isotactic polypropylene (iPP) was evaluated. From the acid treatment surface carboxylic groups were mainly formed, while the nanotubes' length was gradually reduced by increasing the treatment time. Young's modulus, tensile strength and storage modulus of the iPP/MWCNT nanocomposites were increased by increasing the treatment time of the MWCNTs, due to finer dispersion inside the polymer matrix, as revealed by TEM and micro-Raman spectroscopy. Furthermore, the nanotubes acted as nucleating agents, an effect more pronounced with finer filler dispersion. Thermal stability in an inert atmosphere also increased. Thermo-oxidative stability tests in air and O₂ revealed that oxidative degradation took place in two stages. In the first stage, corresponding to temperatures up to 230 °C, the MWCNTs accelerated the oxidation of iPP, while at higher than 300 °C temperatures the trend was reversed. Incubation studies proved that, at the first stages, oxidation was due to random chain scission of iPP and oxygen uptake. This behaviour was accelerated by the MWCNTs' surface carboxylic groups and, as found by O₂ permeability studies, was mainly a surface process. In the second stage, due to the shielding effect of MWCNTs, the removal of the gases produced during decomposition was hindered. At this stage the presence of MWCNTs resulted in more thermo-oxidatively stable nanocomposites.     

Thermal degradation analysis and XRD characterisation of fibre-forming synthetic polypropylene containing nanoclay

Flammability of synthetic fibres is significantly worse than that of bulk polymers because of the high surface area to volume ratio and the low tolerance to high filler loadings in the fibre production process. Introducing nanocomposite structures has the potential to enhance the char formation at relatively low loadings of nanoparticulate fillers and hence can reduce the flammability of synthetic polymers and fibres.This paper reports thermal degradation analysis results in conjunction with TG analysis under different atmospheres and further studies of X-ray diffraction characterisation of fibre-forming polypropylene containing selected dispersed nanoclays.The concentrations of hydrocarbons, carbon monoxide and carbon dioxide released during the TG analysis have been monitored and analysed by using a combined electrochemical infrared analyser. The intensity changes of the crystallinity peaks and nanoclay peaks in the polymer and composites are discussed.     

Development of a novel potentiometric titration method for determination of polypropylene degradation

In this study, a novel potentiometric titration of hydroperoxide in degraded polypropylene (PP) is proposed. This titration is quite sensitive compared with the conventional ones such as UV and manual titrations, and its detection limit was about 2 meq/kg. The sensitivity was equal to that of molecular weight measurement by GPC for the degraded PP and, in addition, the volatilization behavior of the hydroperoxide could be detected. This titration was found to be very effective for the determination of PP degradation.     

Radiation resistant polypropylene blended with mobilizer,: antioxidants and nucleating agent

Post-irradiation storage of medical disposables prepared from isotactic polypropylene renders them brittle due to degradation. To avoid this, isotactic polypropylene [(is)PP] was blended with a mobilizer, dioctyl pthallate (DOP), three antioxidants (hindered amines and a secondary antioxidant) and benzoic acid to obtain radiation-resistant, thermally-stable and transparent material. Different formulations prepared were subjected to gamma radiation to doses of 25 and 50 kGy. Tests of breakage on bending after ageing in an oven at 70°C up to 12 months have shown that the addition of DOP and the antioxidants imparts improved radiation and thermal stability as compared to (is)PP alone or its blend with DOP. All the formulations irradiated or otherwise demonstrated excellent colour stability even after accelerated ageing at 70°C for prolonged periods.     

Development and characterisation of flame-retardant fibres from isotactic polypropylene melt-compounded with melamine-formaldehyde microcapsules

Intumescent flame-retardant textiles have been developed from flame-retardant microcapsules. The work is based on the synthesis of different melamine-formaldehyde microcapsules containing di-ammonium hydrogen phosphate and/or poly(1,6-hexamethylene adipate) by in-situ polymerisation. Two types of shell have been produced, composed of melamine formaldehyde or melamine formaldehyde-poly(hexamethylene adipate glycol). The microcapsules obtained were melt-compounded at 5%-wt with an isotactic polypropylene matrix using a twin-screw extruder, and multi-filaments have afterwards been spun from the various extrudates. The manufactured fibres were mechanically characterized by measuring their tensile properties, and their thermal properties were investigated by DSC and TGA. Finally, knitted fabrics were processed from the multi-filaments: their flame-retardant properties were evaluated by performing a fire test with a cone calorimeter, and their thermal conductivity measured with a Hot Disk. The different thermal behaviours are discussed in terms of the influence of system formulation on the overall thermal degradation, due to interactions between the different components of the flame-retardant microcapsules. The results showed that for one of the structures, an intrinsic intumescent flame-retardant system has been achieved.     

Determination of low levels of polymer additives migrating from polypropylene to food simulated liquids by capillary SFC and solvent venting injection

Summary Methods for the determination of specific migration of polymer additives from polypropylene to aqueous acetic acid and isooctane have been studied. The migrants were extracted from aqueous acetic acid and characterized by supercritical fluid chromatography (SFC) with flame ionization detection (FID) and mass spectrometry (MS). The isooctane simulant was concentrated and injected directly into the SFC. Injection of extracts after incubation, using the solvent venting injection technique in the SFC, enabled additive concentrations as low as 10 ppb in 150 ml simulant to be determined, corresponding to a specific migration level of approximately 1 g additive/dm² polymer.     

Thermal behaviour of compatibilised polypropylene nanocomposite: Effect of processing conditions

The thermal properties and fire behaviour of polypropylene (PP) nanocomposites were investigated using differential scanning calorimetry, dynamic-mechanical analysis, thermogravimetric analysis and glow wire test. In order to study the morphological structure of the materials obtained, TEM and XRD analyses were also carried out. The nanocomposites were prepared using the melt intercalation technique in a co-rotating intermeshing twin screw extruder. Particular attention was given to studying the influence of different processing conditions (barrel temperature profile and screw rate) and compositions of PP-nanoclay blends (clay content, use of compatibiliser) on the thermal properties of the nanocomposites.The results show that all the properties analysed were strongly influenced by the nanocomposite composition; instead, the processing conditions greatly affect only the dynamic-mechanical properties. DSC curves show that the crystallinity is deeply influenced by the presence of the clay in the matrix, owing to the fact that the filler acts as nucleating agent. DMA curves show that materials processed at low temperature profile and high shear stress, i.e. when a good clay dispersion is achieved, are characterised by an enhanced modulus, thus indicating that the incorporation of clay into the PP matrix remarkably enhances its stiffness and has good reinforcing effects. TGA traces in oxidizing atmosphere show a drastic shift of the weight loss curve towards higher temperature and no variation of the onset temperature (i.e. the temperature at which degradation begins). The TGA analyses in inert atmosphere show instead marked increase of this parameter (about 200 °C) and no shift of weight loss curves. Glow wire results highlight that polymer nanocomposites are characterised by enhanced fire behaviour.     

Thermo-Mechanical Performance of Natural Rubber/Recycled Ethylene-Propylene-Diene Rubber Blends in the Presence of ZnO Nanoparticles

In the present study, recycled ethylene-propylene-diene rubber (R-EPDM) was used in an attempt to create a value-added natural rubber (NR) material based on NR/R-EPDM blends. ZnO nanoparticles were also incorporated in a way to replace the conventional ZnO together with special attention to improve the thermo-mechanical performance of the blends. Polyhedral and spherical shapes of ZnO nanoparticles were synthesized, and they were found to have no impurities, with a dimension ranging from 10 to 40 nm. ZnO nanoparticles not only act as a curing activator but also improve the thermal stability and dynamic mechanical properties of the blends.