Tuning the functionalization chemistry of polypropylene for polypropylene/clay nanocomposites

New polypropylene (PP)-graft-maleic anhydride (PP-g-MA) samples have been successfully synthesized by adding N-bromosuccinimide (NBS) during the reactive extrusion process. These NBS-mediated PP-g-MAs possess higher graft content than classic PP-g-MAs (i.e. without NBS) while they keep acceptable molar masses. NBS-mediated PP-g-MAs were used as matrices in model PP-g-MA/organoclay nanocomposites and compared with commercial and home-made classic PP-g-MAs in order to evaluate their ability to disperse the clay. Significantly better degrees of clay delamination and dispersion were reached using NBS-mediated PP-g-MAs than with classic PP-g-MAs. As expected, PP-g-MAs having high graft content showed the best clay dispersion. Within the examined range of molar masses, the PP-g-MA molar mass had no influence on the clay dispersion. However PP-g-MAs exhibiting important reduction of crystallinity lead to poor clay dispersion whatever the graft content. The PP-g-MA/organoclay nanocomposite prepared using the selected “optimized” NBS-mediated PP-g-MA exhibited the best improvement of thermal properties and one of the best clay dispersions. PP/PP-g-MA blends were prepared to evaluate the miscibility between PP and selected PP-g-MAs. No problem of miscibility between the selected NBS-mediated PP-g-MA and PP was noticed. Finally the PP/organoclay prepared using the selected NBS-mediated PP-g-MA as compatibilizer showed much better clay dispersion and thermal stability than the one prepared with the corresponding classic PP-g-MA, thus establishing the interest to use such new NBS-mediated PP-g-MAs as compatibilizers.     

Preparation and properties of polypropylene/org-attapulgite nanocomposites

Polypropylene (PP)/org-attapulgite (ATP) nanocomposites were prepared by melt blending in a mixer apparatus. Org-attapulgite was attained by silane coupling agent modification first and then graft-polymerization with butyl acrylate. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to assess the clay morphology and the dispersion of the org-attapulgite, respectively. The changes of crystalline structure for PP nanocomposites were characterized by X-ray diffraction (XRD). The mechanical properties of PP/attapulgite nanocomposites were studied through tensile and impact tests. The thermal and dynamic mechanical properties were characterized by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). The strength and stiffness of PP/org-ATP nanocomposites were both improved significantly in the presence of organic attapulgite. In addition, the incorporation of org-ATP also gave rise to an increase of the storage modulus and the changes of the glass transition temperature for PP composites. TEM and XRD results revealed the addition of attapulgite did not change the crystal structure of PP, however org-attapulgite acted as nucleating agents for the crystallization of PP.     

Chlorosulfonated polypropylene: preparation and its application as a coupling agent in polypropylene-clay nanocomposites

Polypropylene nanocomposites containing organophilic layered silicate were prepared by melt mixing. In order to increase polypropylene polarity, Cl and SO₂Cl groups were introduced by reaction with sulfuryl chloride under UV irradiation. Chlorosulfonated polypropylene was subsequently melt-compounded with organophilized montmorillonite clay to produce a masterbatch. The masterbatch was then blended with commercial isotactic polypropylene. An organophilized silicate (Cloisite 15A) and three chlorosulfonated polypropylenes with different degrees of functionalization were used in this study. The effect of various processing procedures was examined as well. The morphology of nanocomposites obtained was examined using TEM and X-ray diffraction. It has been shown that the presence of polar groups leads to an increased gallery distance and partial exfoliation. Nevertheless, full exfoliation of clay platelets has not been achieved. The observed morphologies affected the resulting tensile mechanical behaviour: both stiffness and strength significantly increased.     

Use of polymeric compatibilizers in polypropylene/calcium carbonate composites

The effects of compatibilizing agents on the mechanical properties, viscoelastic properties, and morphology of polypropylene filled with calcium carbonate composites are investigated. It is found that the use of PP-g-MA and PP-g-AA significantly increases the tensile strength and improves particle dispersion and interfacial adhesion. The higher effect of compatibilization is obtained by using PP-g-MA as a compatibilizer. The results on the dynamic thermomechanical properties, viscoelastic properties, and SEM pictures also support the improved interfacial characteristics. It is also found that there exists a limiting amount of PP-g-MA at about 5% beyond which a further increase in the tensile strength is not obtained. The use of untreated calcium carbonate or SEBS-g-MA does not allow films to be drawn for the purpose of testing.     

Effect of processing conditions on morphology and mechanical properties of compatibilized polypropylene nanocomposites

This work focuses on the influence of processing conditions on the nanocomposites structure, i.e. intercalated or exfoliated, and on the enhancement of mechanical properties of polypropylene (PP) nanocomposites. These nanocomposites were prepared using the melt intercalation technique in a co-rotating intermeshing twin screw extruder. In order to optimise processing conditions, both screw speed and barrel temperature profile were changed. The role of the compatibilizer (maleic anhydride grafted polypropylene) was also studied. The results obtained show that the barrel temperature is a very important parameter: using lower processing temperature, the apparent melt viscosity and, consequently, the shear stress are higher and, therefore, the exfoliation of the clay is promoted. Even using optimised processing conditions, exfoliation of clay can be achieved only when an high compatibility between polymer and clay exists: the PP nanocomposites containing maleic anhydride show an exfoliated structure and a sensible enhancement of mechanical properties while PP nanocomposites without compatibilizer show a structure mainly intercalated and a lower improvement of mechanical properties.     

The toughening mechanism of polypropylene/calcium carbonate nanocomposites

The toughening mechanism of polypropylene (PP) filled with calcium carbonate (CaCO₃) nanoparticles is described. In a previous study (Macromolecule 2008;41:9204), we observed that intensive ligament-stretching following debonding of nanoparticles was responsible for the significant improvement in the impact toughness of the annealed PP/CaCO₃ nanocomposites. Furthermore, we hypothesized that strong ligaments, which have high fracture stresses, are needed to stabilize the crack-initiation process and to increase the energy dissipation in the crack-initiation stage. In this study, we used a high-molecular-weight PP to test this hypothesis because strong ligaments could be created from this high-molecular-weight PP. The notched Izod impact strength of the nanocomposites containing the high-molecular-weight PP and 20 wt% CaCO₃ nanoparticles with a monolayer coating of stearic acid was measured to be about 370 J/m, whereas the impact strength of the unfilled PP was 50 J/m. The size of the plastic deformation zone was found to be dependent on the molecular weight of the PP matrix because the strong ligaments of the high-molecular-weight PP enabled the expansion of the plastic deformation zone, leading to a considerable increase in the impact strength. The synergic effect of the high-molecular-weight PP and the monolayer-coated nanoparticles produced nanocomposites with high impact strength, which is much greater than the inherent impact strength of the unfilled polymer. In addition, the effect of the high-molecular-weight PP on the dispersion of the nanoparticles was investigated.     

Ultrasonic oscillations induced morphology and property development of polypropylene/montmorillonite nanocomposites

Polypropylene/montmorillonite nanocomposites (PPCNs) with 3% organophilic montmorillonite (OMMT) content were prepared via ultrasonic extrusion. The objective of present study was to investigate the effects of ultrasonic oscillations in processing on the morphology and property development of PPCNs. XRD and TEM results confirmed the intercalated structure of OMMT in conventional nanocomposite (without ultrasonic treatment) and ultrasonicated nanocomposite, but ultrasonic oscillations could make silicate layers finely dispersed and a little exfoliated. According to SEM, the OMMT particles were evenly and finely dispersed in the ultrasonicated nanocomposite via ultrasonic oscillations, and the aggregation size of clay particles was about 100 nm, which is less than that in conventional nanocomposite. The crystalline dimension, crystalline morphology and the growth rate of crystallization in PPCNs were investigated by DSC and PLM, it was found that the OMMT particles and ultrasonic oscillations played an important role in the nucleation rate, crystallization temperature and spherulite size of PP matrix in nanocomposites. Compared with conventional nanocomposite, the mechanical properties of the ultrasonicated nanocomposite increased due to the improved dispersion of OMMT and diminished spherulite size. The thermal stability and the rheological behavior of PP and its nanocomposites were both studied by thermogravimetry and high pressure rheometer, respectively.     

Morphological and structural characterization of polypropylene based nanocomposites

Nanocomposites based on isotactic polypropylene and montmorillonite were studied, investigating the polymorphism of the polymer, and examining the interaction between polypropylene and silicate. Wide angle X-ray diffraction (WAXD) was used to study the crystallization of the matrix, particularly the influence of additives and processing. Small angle X-ray scattering (SAXS) was employed to assess the lamellar morphology, estimating how silicate dispersion could affect this factor. Interaction between polymer and clay was quantified from WAXD and SAXS profiles; by the position and shape of the clay basal peaks, two different degrees of interaction were identified, characterized by a different affinity between the constituents. Observation of the samples' morphology by transmission electron microscopy (TEM) confirmed the results of X-ray diffraction methods. A fitting procedure was applied to SAXS profiles and the dimensions of the clay stratifications were estimated. These results were successfully compared with those drawn by application of Scherrer equation on (001) WAXD peaks.     

Water-assisted extrusion as a novel processing route to prepare polypropylene/halloysite nanotube nanocomposites: Structure and properties

Naturally occurring halloysite nanotubes (HNTs) are used to prepare Polypropylene (PP)/HNTs nanocomposites using a novel “one step” water-assisted extrusion process with and without the use of a PP-graft-maleic anhydride (PP-g-MA) as compatibilizer. In order to analyze the influence of PP-g-MA and/or water injection on the HNTs dispersion and therefore on nanocomposite properties, structural analysis (SEM and TEM) is combined with rheological and thermo-mechanical experiments. The best clay dispersion is obtained when compatibilizer and water injection are combined together (synergistic effect). As a consequence, the linear viscoelastic properties and the dynamic storage modulus are dramatically enhanced for this system. A mechanism explaining the interaction between HNTs and PP-g-MA in presence of water is proposed. The thermal stability and flame retardant property are also investigated. Thermal analyses reveal two opposite effects of HNTs on the thermal behaviour of PP. A surface catalytic action of the halloysite speeds up thermal degradation of PP. However, this effect is reduced with improved HNTs dispersion, presumably via an entrapment mechanism of the decomposition products inside the lumens. Finally, cone calorimeter results show that low flammability of nanocomposites is only achieved when combining water injection and PP-g-MA. In view of these results, PP/HNTs nanocomposites prepared using this novel processing route are promising candidates for flame retardant applications.     

Preparation of exfoliated isotactic polypropylene/alkyl-triphenylphosphonium-modified montmorillonite nanocomposites via in situ intercalative polymerization

Alkyltriphenylphosphonium-modified montmorillonite(PMMT) was used to prepare TiCl₄/MgCl₂/PMMT compound catalyst and exfoliated i-PP/PMMT nanocomposites were prepared by in situ intercalative polymerization of propylene with TiCl₄/MgCl₂/PMMT catalyst. The catalytic efficiency of the above catalyst under optimum polymerization condition could reach as high as 1300 kg/(molTi h) and the combining of PMMT with Z-N catalyst do not change the stereo-regulation catalytic properties of the Z-N catalyst. The synthesized PP possessed high isotacticity, melting point and molecular weight. Wide angle X-ray diffraction (XRD) and transmission electron microscopy (TEM) examinations evidenced the nanocomposites obtained were exfoliated ones.     

The structure and physical properties of polypropylene and thermoplastic olefin nanocomposites containing nanosilica

The morphology and physical properties of thermoplastic olefin blend (TPO) based nanocomposites containing nanosilica are reported. Addition of maleated PP resulted in improved filler dispersion within the PP matrix, where the filler resided exclusively. This separated morphology resulted in selective reinforcement of the PP matrix without compromising ductility, as demonstrated by mechanical property characterization. The tensile moduli, impact and flexural properties of TPO/nanosilica composites showed improvements at low loadings of nanosilica, indicating a good balance of stiffness and toughness. The addition of nanosilica into the TPOs decreased the size of the dispersed elastomer phase, which was a factor in the observed improvements of impact strength. Silane-modified nanosilica dispersed more efficiently in the polymer matrix, giving rise to improved impact properties of the TPO composites, compared to the unmodified filler.     

Effects of organoclay Soxhlet extraction on mechanical properties, flammability properties and organoclay dispersion of polypropylene nanocomposites

The organic treatment on a layered silicate used in nanocomposite synthesis is the interface between the hydrophilic layered silicate (clay) and hydrophobic polymer in the case of polypropylene. However, the typical synthesis of an organoclay can result in excess organic treatment which can hinder mechanical and flammability benefits. This excess organic treatment may result in plasticization of the polymer matrix, possibly removing some of the mechanical and flammability property benefits provided by the nanocomposite. In this paper, the effects of using Soxhlet Extraction on the Organoclays after synthesis was investigated. Soxhlet extraction times on organoclays were found to have an effect on the mechanical and flammability properties of the resulting polypropylene nanocomposite. The removal of excess organic treatment by Soxhlet extraction resulted in improvements in flex modulus, improved clay dispersion, delayed time to ignition, and lowered heat release rate during burning.     

Preparation and characterization of polypropylene/layered silicate nanocomposites using an antioxidant

Polypropylene (PP)/layered silicate nanocomposites were prepared via simple melt mixing of three components, PP, layered silicates modified with octadecylamine (C18-MMT) and antioxidant, to investigate the role of antioxidant. TEM and X-ray scattering results confirmed the intercalated state of silicates in PP/layered silicate nanocomposites with antioxidant. In rheological and mechanical study, the nanocomposites with antioxidant showed higher properties than those of the unfilled PP. The nanocomposite with 5 wt% C18-MMT and 0.5 phr antioxidant exhibited about 1.4 times higher tensile modulus and 1.3 times higher storage modulus than the unfilled PP. However, PP/C18-MMT without antioxidant showed lower rheological values owing to the thermal decomposition of PP and the poor compatibility between PP and C18-MMT. It could be concluded that antioxidants played an important role in enhancing the compatibility between PP and C18-MMT. According to the real time X-ray diffraction, the nanocomposite showed the weak ordering of PP crystals than the unfilled PP in the load-extension plateau region of elongation.     

Evaluation of different amine‐functionalized polyethylenes as compatibilizers for polyethylene film nanocomposites

The compatibilization effects provided by different amine‐functionalized polyethylenes (PEs) versus those provided by a maleated polyethylene (PEgMA), for forming PE‐based film nanocomposites, were studied. Amine‐functionalized PEs were prepared by reaction of PEgMA with two primary amines, 2‐aminoethanol (EA) and 1,12‐aminododecane (D12), and a tertiary amine, 2‐[2‐(dimethylamino)ethoxy]ethanol (DMAE), in the melt to form the corresponding PEgEA, PEgD12 and PEgDMAE. Nanocomposites were prepared by melt mixing in a twin‐screw extruder PE and these three functionalized compatibilizers with a modified montmorillonite clay. The purpose of the current work was to determine the effect of the various amine‐functionalized PEs on the degree of exfoliation and optical properties of PE–clay nanocomposites in order to obtain nanocomposite films for greenhouse cover applications. Fourier transform infrared analysis confirmed the formation of the amine‐modified PE compatibilizers. Structural, morphological, mechanical, rheological and optical properties of film samples were used to characterize the nanocomposites. All the amine‐modified PE‐compatibilized nanocomposites had better clay exfoliation compared to uncompatibilized PE composites. Results showed that PEgDMAE formed highly exfoliated morphology and a favorable balance between mechanical (stiffness and ductility), optical and thermal insulating film properties even at higher clay contents. It was determined that nanocomposites with greater exfoliated structure showed better optical and thermal insulating film properties. PEgEA and PEgD12 compatibilizers did not provide a better interaction for exfoliation of the organoclay than the PEgMA material. The PEgDMAE compatibilizer led to a highly exfoliated morphology and a favorable balance between mechanical, optical and thermal insulating film properties even at higher clay contents. The PEgDMAE film nanocomposites could be used ideally for greenhouse cover applications. Copyright © 2010 Society of Chemical Industry     

Effects of polypropylene-g-(maleic anhydride/styrene) compatibilizer on mechanical and rheological properties of polypropylene/clay nanocomposites

To enhance the dispersibility of clay in polypropylene (PP) matrix, PP-g-(maleic anhydride/styrene) (MA/ST) was prepared as a compatibilizer by graft copolymerization of maleic anhydride (MA) and styrene (ST) with PP. The addition of ST was known to be effective in improving the graft degree. PP/clay nanocomposites with the compatibilizer were prepared by melt intercalation. The X-ray diffraction (XRD) peaks of (0 0 1) plane of the organo-modified montmorillonite (O-MMT) were shifted to lower angles by an addition of PP-g-(MA/ST), indicating the intercalation capability of PP-g-(MA/ST) in the silicate layers. Transmission electron microscopy (TEM) photographs showed that the O-MMT in the presence of PP-g-(MA/ST) was intercalated and partly exfoliated during melt mixing. The addition of O-MMT and PP-g-(MA/ST) improved the thermal stability, tensile and rheological properties of the nanocomposites.     

Metallocene‐based functionalized polyolefins as compatibilizers in polyolefin nanocomposites

This article reports a study of some functionalized polyolefins evaluated as compatibilizers in polyethylene nanocomposites. The functionalized polymers were prepared by direct metallocene‐mediated copolymerizations of ethylene and a functional comonomer. The prepared nanocomposites were evaluated for mechanical and barrier property enhancement. A good combination of mechanical and barrier properties was obtained with the metallocene‐based functionalized polyethylene. The toughness–stiffness balance was better than or comparable to that achieved with conventional functionalized polymers such as maleic anhydride grafted polyethylene. The results also indicated that these metallocene‐based functionalized polyolefins, when used as compatibilizers, could have relatively higher molar masses and lower functionality than those of conventional post‐reactor‐modified compatibilizers, and so the drawbacks associated with the latter could be avoided. Their inherent properties could also further improve the final nanocomposite properties. This was attributed to the more homogeneous nature of metallocene‐catalyzed polymers in comparison with post‐reactor‐modified products. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 1094–1100, 2004     

Adhesion of olefin block copolymers to polypropylene and high density polyethylene and their effectiveness as compatibilizers in blends

Adhesion of four ethylene-octene block copolymers (OBCs) to polypropylene (PP) and high density polyethylene (HDPE) was studied by peeling PP/OBC/HDPE microlayered tapes. The four OBCs had different comonomer composition, mechanical properties and phase morphology. Through the Irwin damage zone analysis, it was found that the stress-strain behavior of OBC was the primary factor that determined the adhesion strength. Effectiveness of these OBCs as compatibilizers in PP/HDPE blends was also investigated. Toughness of all OBC-compatibilized blends was effectively improved. The OBCs having higher adhesion strength also resulted in better mechanical performance for the compatibilized blends. A quantitative correlation was established between the adhesion strength and mechanical performance of the blends.     

Morphology and properties of polypropylene nanocomposites based on a silanized organoclay

A silanized organoclay (s-M₂(HT)₂) was prepared by reaction of trimethoxyphenyl silane with an organoclay with a M₂(HT)₂ surfactant structure. Nanocomposites were formed from polypropylene (PP) and a blend of PP and maleic anhydride-grafted polypropylene (PP-g-MA) and the M₂(HT)₂ and s-M₂(HT)₂ organoclays by melt processing to explore the extent of exfoliation and the mechanical properties. Wide angle X-ray scattering (WAXS) and transmission electron microscopy (TEM) coupled with detailed particle analysis were used to determine the effect of the organoclay used and the PP-g-MA compatibilizer on exfoliation and mechanical, rheological, and thermal expansion properties. The PP/s-M₂(HT)₂ nanocomposites have higher particle densities than the PP/M₂(HT)₂ nanocomposites though the aspect ratio remains the same. Platelet dispersion is significantly improved by using PP-g-MA compatibilizer for both organoclays. The rheological properties and the relative modulus improve for the PP/s-M₂(HT)₂ nanocomposites but not to the same degree as either organoclay in a PP-g-MA compatibilized matrix. The thermal expansion properties, however, are not improved by using the s-M₂(HT)₂ organoclay. The s-M₂(HT)₂ organoclay is less prone to agglomeration during extrusion than the M₂(HT)₂ organoclay.     

Linear viscoelasticity and structure of polypropylene-montmorillonite nanocomposites

Polypropylene (PP)/clay composites were prepared by melt mixing in a thermoplastic mixer using a polypropylene grafted with maleic anhydride (PPg) as the compatibilizer. Concentrations of an organophilic montmorillonite (MMT) between 2 and 15 wt% and concentration ratios of PPg/clay between 1:3 and 3:1 were employed to investigate the relationship between the structural characteristics of the hybrids and their rheological properties. The structure was analyzed with electron microscopy, X-ray diffraction and melt rheology. Thermogravimetric analysis and infrared spectroscopy were also used. The clay interlayer spacing increases after mixing with PP while the addition of PPg only facilitates the partial exfoliation of the clay platelets without changing that spacing. When clay loadings of 8 wt% or larger were used, an important fraction of the original clay particles was found to remain unmodified. The dynamic moduli show little effect of the presence of the inorganic material when no compatibilizer is added or the amount of PPg or clay is too small. As the extent of exfoliation increases, the linear viscoelastic behavior of the composites gradually changes with time while in the molten state, mainly at low frequencies. Evidence of solid-like behavior appears as the concentration of clay increases, for a given PPg/clay or PP/PPg concentration, or as the PPg concentration increases (for a given clay concentration). The concentrations of PPg and clay that induce percolation were observed to have an inverse relation. Evidence of regions with large concentration of MMT was obtained in the annealed samples of composites with solid-like rheological behavior. Additionally, infrared spectra of these materials suggest the simultaneous occurrence of chemical reactions between the PPg and the surfactant or products derived from its thermal decomposition during the annealing process.     

Nanocomposites formed from polypropylene/EVA blends

Rubber toughened polypropylene nanocomposites using two types of modified montmorillonite (organoclay) were explored with the objective of achieving an improved balance between stiffness and toughness. The effect of three blending sequences on microstructure and properties of the ternary nanocomposites was also investigated. A commercial grade of ethylene/vinyl acetate copolymer (EVA) containing 18 wt% of vinyl acetate was used as the impact modifier for polypropylene and an acrylic acid grafted polypropylene was used to compatibilize the systems studied. The toughened nanocomposites samples were prepared by melt compounding in a twin-screw extruder; the morphology and mechanical properties of the resulting materials were characterized by X-ray scattering, electron microscopy and tensile and impact testing. The results show that incorporation of EVA increases the toughness of the polypropylene but its stiffness decreased markedly due to the incorporation of the low modulus component. The addition of organoclay increased the modulus slightly for all the ternary nanocomposites with respect to the blend, but it remains lower than that of neat PP. Surprisingly, addition of organoclay to the blends promoted a drastic increase in the notched Izod impact strength and a considerable alteration of the shape of the dispersed EVA phase when the organoclay is located in this phase. Moreover, it was found that the blending sequence effects on the morphology and properties of the mixtures are dependent on the organoclay used.