Natural antioxidants for polypropylene stabilization

A study on the efficiency of bio-based compounds as stabilizers for polypropylene (PP) is reported. A water extract from French maritime pine bark (Pycnogenol^®), a by-product containing polyphenols obtained from wine production, and a carotenoid-containing oleoresin from processing of tomatoes were used. Their stabilizing activity was compared with that of a commercial phenolic antioxidant. Thermogravimetric analysis and Oxidative Induction Time measurements performed on unaged samples, as well as infrared spectroscopy on samples aged at 70 °C, provided evidence for the effectiveness of the natural stabilizers. Mechanical characterization was carried out on aged films and injection moulded samples. Experimental results indicated that particularly grape extract could provide long-term stabilization to PP under conditions of oxidative degradation. Therefore, it could be used as efficient and high value-added additive for polypropylene. Pycnogenol^® also showed antioxidant activity, however the achievement of a more homogeneous dispersion in the polymer matrix could improve the mechanical performance of aged samples.     

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.     

Effect of ageing of sisal fibres on properties of sisal - Polypropylene composites

A composite laminate based on natural sisal fibre and polypropylene was prepared by compression moulding. The mechanical properties of the composite were assessed under tensile, flexural and impact loading. Changes in the stress-strain characteristics, yield stress, tensile strength, and tensile (Young's) modulus, due to ageing have been analysed. Important findings with the fresh and aged fibres and their behaviour in composites have been reported and analysed.     

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.     

Irreversible deformation of isotactic polypropylene in the pre-yield regime

In the modeling of the mechanical response of a polymer over a large strain range, the nonlinear viscoelastic and viscoplastic behavior must be considered. For many polymers, nonlinear behavior is observed at low loads, e.g. by a stress-dependence of the creep compliance for stresses above 2 MPa in case of the polypropylene used in this study. Additionally, plastic deformation has been observed at strains below the yield point for several polymers. In this study, the irreversible deformation by cavitation and shear yielding of polypropylene are characterized in the pre-yield regime in uniaxial tensile tests using digital image correlation. The recovery of strain after unloading at a prescribed strain level is measured and used to identify the evolution of the plastic strain during uniaxial tension. An experimental technique for simultaneous determination of the true stress–true strain curve and the degree of stress whitening, which relates to the amount of cavitation, is introduced and the initiation of cavitation is compared to the plastic deformation detected in strain recovery at various temperatures.     

Effect of natural weather on the structure and properties of polylactide/Cloisite 30B nanocomposites

The degradation of polylactide (PLA)/Cloisite 30B nanocomposites under natural weathering was investigated as a function of clay loadings (1, 3 and 5 wt.%) for up to 130 days using Fourier transform infrared (FT-IR) spectroscopy, size exclusion chromatography (SEC), nanoindentation measurements, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and scanning electron microscopy (SEM). For comparative purposes, the neat PLA was also considered. The FT-IR results showed that the photo-oxidation mechanism of PLA was not modified in the presence of Cloisite 30B, but only the degradation rates were accelerated. Moreover, the photo-oxidative degradation of PLA nanocomposite samples led to the formation of vinyl unsaturation, carbonyls, anhydrides and hydroperoxides groups as a result of the occurrence of several chemical mechanisms simultaneously. The decrease of the weight-average molecular weight, and the number-average molecular weight associated with an enhanced polydispersity of the nanocomposite samples indicated that chain scission was the most prominent phenomenon in natural weathering. The thermal degradation of the PLA was faster in the presence of clay. Modulus and hardness measured by nanoindentation increased slightly with exposure time for both neat PLA and PLA nanocomposite samples; the increase is also a function of the clay content. Finally, the weathering effect on the morphology of exposed samples observed by SEM revealed that the fractured surfaces exhibited many voids and cracks. These defects were much more pronounced for the PLA nanocomposites.     

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.     

Photo-oxidation of thick isotactic polypropylene films I. Characterisation of the heterogeneous degradation kinetics

Photo-oxidation kinetics of thick isotactic polypropylene films have been compared to thermal-oxidation kinetics of thin films, and noticeable differences have been found. The non-classical kinetic trend of the former can be described as a three step evolution: a typical induction/autoacceleration POOH build-up at the beginning, an intermediate slower POOH content increase and, finally, a gentler POOH increase, which can be better described by quadratic functions of the oxidation time than by a linear dependence. In addition, a series of oscillations appearing from the beginning of the photo-oxidation are found. This kinetic heterogeneity is suggested to be related to the progression of the oxidation to the inside of the strips. However, the FTIR analysis of the evolution of the POOH band position in both photo and thermal oxidation, enables the observed shape changes to be associated to kinetic stages.     

Processing stability of polypropylene impact-copolymer during multiple extrusion - Effect of polymerization technology

Three commercially available polypropylene impact-copolymers (ICPP) produced by Innovene (INN), Spheripol (SPH) and Unipol (UNI) technologies were subjected to multiple extrusion using a twin-screw extruder W&P ZSK25 at 220 °C. Processing stability and changes in properties induced by extrusion were investigated. The materials were of similar MFR ∼6 dg/min, similar ethylene contents ∼7.5 wt. % and the same type and level 1200 ppm of phenol/phosphite stabilizer system was used. Ranking INN < UNI < SPH in processing and long-term (LTHA) stabilities observed was primarily related to the reactivity of catalyst residues rather than to other factors, such as contents of ethylene, quantity of extractables, EPR phase composition, levels of ash or individual elements in it. The mechanically demanding multiple extrusion conditions and consequently different extent of processing degradation, however, induced only minimum changes in morphology and impact strength of the solid ICPP matrix. Thus, regardless of changes in melt-flow properties induced by extrusions, all the three grades even after 5th extrusion at 220 °C exhibited Charpy notched impact strength at 23 °C only minimally changed.     

Thermo-oxidative stability of polypropylene/layered silicate nanocomposites

Several series of experiments were carried out to check the effect of components on the stability of PP/layered silicate nanocomposites. The amount of organophilic montmorillonite (OMMT) changed between 0 and 6, while that of maleated polypropylene (MAPP) between 0 and 50 vol%. The composites were prepared in an internal mixer at 190 °C. Mixing speed and time were changed to study the effect of processing conditions on stability. The structure of the samples was characterised by various methods, while stability by the induction time of oxidation (OIT), the onset temperature of degradation (OOT) and by colour. Contrary to numerous claims published in the literature, which indicate the positive effect of layered silicates on the stability of polymer nanocomposites, our results clearly proved that both OMMT and MAPP accelerate degradation during processing and deteriorate the properties of PP composites. Residual stability decreases drastically with increasing amounts of both components, chain scission leads to the decrease of viscosity and to inferior strength and deformability. In spite of expectations, the effect of the components is independent of each other. Discoloration is caused mainly by the inherent colour of the filler and it decreases with increasing exfoliation. The most probable reason for decreased stability is the reaction of the components with the stabilisers, but this explanation needs further verification. Processing conditions influence degradation considerably, increasing shear rate and longer residence times lead to more pronounced degradation. The basic stabilization of commercial grade polypropylenes is insufficient to protect the polymer against degradation and without additional stabilization processing under normal conditions results in products with inferior quality.     

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.     

Fabrication of microporous membranes from melt extruded polypropylene precursor films via stretching: Effect of annealing

In this work, the effects of annealing conditions on the microstructure of polypropylene(PP) precursor films and further on the porous structure and permeability of stretched membranes were investigated. Combinations of WAXD, FTIR, DSC and DMA results clearly showed the crystalline orientation and crystallinity of the precursor film increased with annealing temperature, while the molecular chain entanglements in the amorphous phase decreased. Changes in the deformation behavior suggested more lamellar separation occurred for the films annealed at higher temperatures. Surface morphologies of the membranes examined by SEM revealed more pore number and uniform porous structure as the annealing temperature increased. In accordance with the SEM results, the permeability of the membranes increased with annealing temperature. On the other hand, it was found that 10 min was almost enough for the annealing process to obtain the microporous membranes with an optimal permeability.     

Temperature and molecular weight dependence of fracture behaviour of polypropylene films

The effect of temperature on fracture behaviour of isotactic polypropylene films has been studied on two PP samples of molecular weights M_W=270 kg mol⁻¹ and M_W=150 kg mol⁻¹, using the Essential Work of Fracture method. Two ductile-brittle transitions as a function of temperature are in evidence at respectively 10 and 60 °C. The former transition occurs for the highest molecular weight and the latter one for the lowest molecular weight.

Three processes are involved in the temperature effect on PP toughness: (1) The decrease of yield stress with temperature according to Eyring’s law; (2) The role of cooperative motions in the amorphous phase: the ductile-brittle transition of the sample of highest M_W corresponds to the glass transition; and (3) The role of the mobility of the crystalline phase: the ductile-brittle transition of the sample of lowest M_W corresponds to the C transition.     

Effect of thermal history on the mechanical properties of three polypropylene impact-copolymers

The effect of thermal history on static mechanical properties and impact fracture behavior of three reactor polypropylene impact-copolymers (ICPPs) was investigated for three ICPPs prepared using commercial Innovene^®, Unipol^® and Spheripol^® polymerization technologies. Multiple extrusion employing a co-rotating twin-screw extruder resulted in a significant reduction of the molecular weight of the PP homopolymer phase evidenced by the increasing melt flow index (MFI). Neither cross-linking of the ethylene-propylene rubber (EPR) phase nor EPR particle coarsening was detected for any of the ICPPs after 5 consecutive extrusions. Decreasing molecular weight of the PP homopolymer phase caused change in the crystalline morphology of injection molded specimens due to the change in crystallization kinetics and reduction of the number of tie molecules, however, the overall degree of crystallinity did not change, significantly. The static tensile mechanical properties (E, σ_y, ?_b), critical strain energy release rate, G_c, and the Charpy notched impact strength, a_k, decreased with increasing MFI in a monotonous manner for all the ICPPs investigated. Despite significant differences between the absolute values of the mechanical properties for the three ICPPs, the MFI dependence of the σ_y and G_c relative to that for the unaffected ICPP fell on a single master curve for all of them. High-speed digital camera, used to follow the fracture process during the instrumented impact test, revealed no significant change of the small scale yielding fracture process with increasing MFI. This was in an agreement with the negligible change in the size of the crack tip plastic zone, R_p, predicted using simple mixed mode fracture model. The plane strain value of the critical strain energy release rate, G_1c, calculated from the measured G_c for the INN (2.4 kJ/m²), UNI (2.8 kJ/m²) and SPH (3.5 kJ/m²) using a simple LEFM model did not exhibit significant dependence on the number of extruder passes. The observed differences between the three ICPPs were ascribed to the significantly larger EPR content in UNI compared to the other two ICPPs and significantly larger content of isotactic PP homopolymer in the INN compared to the remaining two ICPPs.     

The role of microstructure in the pyrolysis of polypropylene. A preliminary study on the syndiotactic stereoisomer

The pyrolysis of two syndiotactic polypropylene samples with different molar mass and microstructure has been studied by means of TGA. The volatiles evolved have been analysed as a whole by mass spectrometry. The relative content of the different low mass alkenes, alkanes and dienes provides a fingerprint which has been used to asses the similarity of the mechanism. The changes in the apparent E_a with conversion have been shown by means of the Friedman’s method.The results show that, even though the chemical pattern is identical in both samples, important differences in the E_a trends are found. Such disparity in the energy requirements for the pyrolysis to take place can be reasonably attributed to the different microstructure of the samples.     

Effect of multiple extrusion on molecular structure of polypropylene impact copolymer

Neat and multiple processed polypropylene impact-copolymer (ICPP) were fractionated using series of hydrocarbon solvents with increasing solvent power. The analyses of the fractions obtained in successive extractions showed significant decrease in weight-average molecular weight (Mw) and narrowing the molecular weight distribution (MWD) of investigated samples after extrusions. Although the changes due to thermooxidation were observed in all phases of the system, the most intensive degradation was found in the prevailing PP homopolymer phase.     

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.     

Effect of carbon nanotubes on the photo-oxidative durability of syndiotactic polypropylene

Polymer nanocomposites with carbon nanotubes as fillers have attracted more attention than any other nanomaterials. A full development of these materials requires a deep understanding of the way they behave in the use conditions. In this work, the resistance to accelerated photooxidation of syndiotactic polypropylene/multi-walled carbon nanotube (sPP/MWCNTs) films was compared to the photooxidation behaviour of unfilled polypropylene films with the same structural organization. The chemical and structural modifications resulting from photooxidation have been followed using infrared spectroscopy and diffractometric analysis. It was found that a good dispersion degree of the nanofiller, evaluated by atomic force microscopy, contributes to reduction in the rate of photooxidation and an increase in the oxidative thermal stability of the polymeric matrix. Different concomitant effects are considered to explain these results, among which morphology and structure of the nanocomposites together with the MWCNT capacity to interact with oxygen molecules making them unavailable in the first stages of photooxidation.     

Effect of natural and synthetic antioxidants on oxidative stability of FAMEs obtained from hevea brasiliensis

Biodiesel(fatty acid alkyl esters),an alternate to fossil fuel,has the tendency of autoxidation and hence requires antioxidants for long term storage.The influence of synthetic and natural antioxidants on the oxidative stability was analysed for fresh FAMEs(fatty acid methyl esters)obtained from hevea brasiliensis at 140 C.Higher activity was observed for synthetic antioxidants following the order of GA>BHT>DTBP>Q>GT-M>PH-M>GT-C,whereas the oxidative stability of stored FAMEs samples measured at 110 C reveals a nearly inverse trend.Storage stability was tested for the FAMEs obtained from hevea brasiliensis stored at 30 C,after addition of synthetic and natural antioxidants—butylated hydroxytoluene(BHT),2,6-di-tert-butylphenol(DTBP),quercetin(Q),gallic acid(GA),methanol extracts from green tea(GTM),pomegranate hull(PH-M),and chloroform extract of green tea(GT-C).Antioxidant activities above 1500 ppm was in the order of DTBP>BHT>GA>GT-C>GT-M>Q>PH-M.Synthetic antioxidants have been found more efficient to improve the storage stability of FAMEs obtained from hevea brasiliensis.DTBP in particular has the highest protection factor.     

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.