Microfibrillar Reinforced Composites Research Articles

Rheology–morphology interrelationship in high-density polyethylene/polyamide-6 microfibrillar composites

Due to the importance of rheology and morphology of microfibrillar reinforced composites (MFCs), a study has been conducted on MFCs based on high-density polyethylene (HDPE) and polyamide-6 (PA-6) using scanning electron microscope and dynamic rheometer. In the absence of an interfacial modifier and at 10 wt% concentration of the dispersed phase (PA-6), two principal factors, elongational deformation and coalescence, were found to be controlled by viscosity and elasticity ratios of the pure constituents accounting for the observed morphology of MFCs. The developed microfibrillar morphology remarkably influenced the viscoelastic properties of MFCs. It was observed that when the viscosity ratio approached unity and the elasticity ratio was minimal, the extent of coalescence in composite was significant. The rheological assessment showed enhanced storage modulus and complex viscosity upon microfibrils formation. This was more pronounced for samples with thinner microfibrils due to more strongly entangled network, compared to the composites with thicker microfibrils. The length-to-diameter ratio of the microfibrils could not be estimated due to their highly asymmetric length scale.

Effect of nanoparticles on the morphology and properties of PET/PP in situ microfibrillar reinforced composites

In this work, the effect of SiO2 nanoparticles on the morphology and properties of PET/PP in situ microfibrillar reinforced composites (MFC) obtained from slit die extrusion and hot stretching‐quenching was investigated. The scanning electron micrographs revealed that the nanoparticles could have different effects on microfibril formation of PET/PP MFC depending on their concentration. Addition of appropriate content of nanoparticles (e.g., 1.6 and 8 wt%) facilitated the PET droplet‐fibril transition during stretching due to the presence of nanosilica in the PP matrix, which increased the viscosity of PP matrix and then improved the droplet deformability. However, at higher loading (e.g., 12 wt%), the aggregation of silica nanoparticles around the PET droplets prevented disperse phase coalescence during drawing and then reduced the fibrillation ability of PET minor phase. The dynamic rheological test performed at 190°C showed that the increased spatial restriction of the nanoparticles improved the viscoelastic moduli and complex viscosity of PET/PP MFC. DSC results indicated that nanosilica had little heterogeneous nucleation effect on the PP matrix of PET/PP MFC. Additionally, nanoparticles could present drastic improvement in the degradation behavior of PET/PP MFC under thermo‐oxidative conditions and increased the modulus of PET/PP MFC. POLYM. COMPOS., 38:2718–2726, 2017. © 2015 Society of Plastics Engineers

Cost-Effective Microfibrillar Reinforced Composites for Lightweight Applications

Microfibrillar reinforced composites (MFC) are self-reinforced polymer-polymer composites, consisting of a cold drawn (fibrillized) phase in an isotropic matrix. They are manufactured via melt blending of two immiscible polymers with different melting temperatures, followed by a subsequent cold drawing and thermal annealing step. The present study examines the manufacturing of composite material out of melt-spun microfibrillar reinforced filaments. Polypropylene (PP) and Polyethylene terephthalate (PET) were chosen as the low-melting matrix and the high-melting reinforcement phase, respectively.The filaments were woven to flat textile structures and processed to composites via hot pressing. They represent a bidirectional reinforced composite, comparable to other fiber reinforced polymers. To ensure optimized processing the influence of relevant parameters has been investigated with respect to mechanical properties of the MFC‑filaments and the derived composites. In addition, the morphology was visualized by SEM imaging after all manufacturing steps. An important observation was that the reinforcing fibrils are still intact after thermal processing, leading to a significant increase in mechanical properties of the resulting composites. Quasistatic tensile tests show more than 100 % higher modulus and more than 50 % higher strength of the only 20 wt-% reinforced PET‑PP composites compared to neat PP. The influence of the amount of PET reinforcement, the variation in processing conditions and composite layup were investigated. Additionally, an outlook on the melt-spinning of blends with Polyamide (PA) is given. In future work it is meant to show that a broad spectrum of tailored properties can easily be achieved by such polymer blends and composites outperforming existing homopolymers as well as thermoplastic composites like short glass‑fiber‑reinforced Polypropylene.The material cost reduction thanks to adding cheaper mass‑production polymers and the transfer onto conventional manufacturing lines is meant to ensure the feasibility of industrial production. The low density and excellent recycling options of these composites underline their potential for automotive and aircraft applications.

MFC-structured biodegradable poly(l-lactide)/poly(butylene adipate-co-terephatalate) blends with improved mechanical and barrier properties

Both polylactide (PLA) and poly(butylene adipate-co-terephthalate) (PBAT) are biodegradable polymers. They are thermoplastics which can be processed using conventional polymer processing methods. In this study, microfibrillar-reinforced composites (MFC) based on PLA/PBAT (PLA/Ecoflex®) blends in different weight ratios were prepared under industry-relevant conditions by melt extrusion followed by continuous cold drawing of the extrudates. Strip-like specimens (films) and plates (laminates) of the drawn blends were prepared by compression molding (CM) at processing temperature above the melting temperature (T m) of PBAT, but below T m of PLA. SEM and WAXS observations show that the extruded blend components are isotropic, but become highly oriented after drawing, and they are converted into MFC-structured polymer–polymer composites after CM. An effect of PLA microfibrils on the non-isothermal crystallization of the Ecoflex during cooling from the melt, associated with the formation of crystalline regions of the matrix around the fibrils, was observed. Depending on the blend composition, the compression-molded samples possess a 3- to 7-time higher tensile strength as well as a 15–30 higher modulus than the neat Ecoflex. In addition, the MFC-structured plates exhibited superior barrier properties compared to the neat Ecoflex, e.g., the oxygen permeability decreased by up to 5 times.

In situ fibrillation of polymeric nucleating agents in polypropylene and subsequent transcrystallization propelled by high-pressure water penetration during water-assisted injection molding

In situ microfibrillar reinforced composites (MFCs) parts of polypropylene (PP) and a special polymeric nucleating agent (acrylonitrile–styrene copolymer, SAN) with four weight ratios (PP/SAN; 98/2, 96/4, 94/6, and 92/8) were molded via water-assisted injection molding (WAIM). The phase morphology and crystalline structure of the WAIM MFCs parts were studied. It was found that high shear stress and cooling rate caused by the high-pressure water penetration during the WAIM could result in the in situ fibrillation of the SAN droplet, followed by the formation of transcrystals at the interface between the SAN microfiber and PP matrix. Moreover, β-form spherulites can be induced by the SAN. The contents of the transcrystals and β-form spherulites were dependent on the SAN content and melt temperature. Interestingly, a WAIM PP/SAN MFCs part with nearly half of its thickness totally dominated by transcrystals and the other half containing high contents of β-form spherulites can be obtained at proper SAN content and melt temperature. This work is expected to provide a promising way to tailor the crystalline structure of the molded parts.

Mechanical and Tribological Properties of PET/HDPE MFCs

Microfibrillar reinforced composites (MFCs) were prepared from polyethylene terephthalate (PET) and high density polyethylene (HDPE). The mechanical and tribological properties of the MFCs were investigated. Static and dynamic mechanical tests revealed that the tensile strength, tensile modulus, and dynamic modulus of the HDPE matrix were improved greatly. Wear mechanism analysis demonstrated that the ability of HDPE to form transfer films on the counter rings was strengthened, which was responsible for the variation of friction and wear properties of the matrix.

Structure and properties of microfibrillar‐reinforced composites based on thermoplastic PET/LDPE blends after manufacturing by means of pultrusion

AbstractIn situ microfibrillar‐reinforced composites (MFC) based on blends from poly(ethyleneterephthalate) (PET) and low‐density polyethylene (LDPE) were prepared under industrial relevant conditions by melt extrusion, followed by continuous cold drawing in weight ratios of PET/LDPE equal to 50/50. Test specimens were prepared by pultrusion (Pult) of the drawn blend at a processing temperature below the melting temperature of PET. This was the first attempt to pultrude such a material. By varying the Pult parameters, rectangular cross‐sectional profiles have been successfully produced using a self‐designed Pult line. For comparison, plates were also prepared by compression (CM) and injection molding (IM). Samples of each stage of MFC manufacturing and processing were characterized by means of scanning electron microscopy (SEM), wide‐angle X‐ray scattering (WAXS), and mechanical testing. SEM and WAXS showed that the highly oriented blends are converted into MFC‐structured polymer–polymer composites during the Pult, CM, and IM process. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers

Comparative study on phase and properties between rPET/PS and LCP/PS in situ microfibrillar-reinforced composites

Microfibrillar-reinforced composites based on two dispersed phases, liquid crystalline polymer (LCP) and recycled poly(ethylene terephthalate) (rPET), and polystyrene (PS) were prepared using extrusion process. The rheological behavior, morphology, and thermal stability of LCP/PS and rPET/PS blends containing various dispersed phase contents were investigated. All blends and LCP exhibited shear thinning behavior, whereas Newtonian fluid behavior was observed for rPET. The incorporation of both LCP and rPET into PS significantly improved the processability. The potential of rPET as a processing lubricant by bringing down the melt viscosity of the blend system was as good as LCP. The elongated LCP domains were clearly observed in as-extruded strand. Although the viscosity ratio of rPET/PS system was lower than that of LCP/PS system, most rPET domains appeared as small droplets. An addition of LCP and rPET into PS matrix improved the thermal resistance in air significantly. The obtained results suggested the high potential of rPET as a processing aid and thermally stable reinforcing-material similar to LCP. The mechanical properties of the LCP-containing blends were mostly higher than those of the corresponding rPET-containing blends when compared at the same blend composition.

SAXS-Fiber Computer Tomography. Method Enhancement and Analysis of Microfibrillar-Reinforced Composite Precursors from PEBA and PET

The nanostructure gradient along the fiber radius inside polymer strands is uncovered by small-angle X-ray scattering (SAXS) microbeam-scanning experiments and X-ray scattering fiber computer-tomography (XSF-CT) analysis. We notice restricted visibility of scattering features within the series of patterns. The reason is violation of local fiber symmetry (LFS) in the irradiated volume elements (voxels). For its theoretical treatment, a set of elementary topologies (tangential grain, radial grain) is introduced. Systematic aberrations (ultrareconstruction, infrareconstruction) generated by tomographic reconstruction of affected series are described. A concept for handling and utilization of these aberrations for nanostructure analysis is devised. Precursors of polymer microfibrillar-reinforced composites (MFC) containing poly(ether)-block-amide (PEBA) and poly(ethylene terephthalate) (PET) with varying cold-draw ratio are studied. We compare results from a direct analysis of the smeared measured patterns to...

Microfibrillar reinforced composites from PET/PP blends: processing, morphology and mechanical properties

Microfibrillar reinforced composites (MFC) based on blends from recycled poly(ethyleneterephthalate) (PET), polypropylene (PP) and a compatibilizer (ethylene-glycidyl methacrylate (E-GMA)) were prepared under industry-relevant conditions by melt extrusion, followed by continuous cold drawing. The PET/PP/E-GMA weight rations amounted to 40/60/0, 40/59/1, 40/57/3, 40/54/6 and 40/51/9, respectively. Test specimens were prepared by compression- and injection molding (CM and IM) at processing temperatures either below or above the melting temperatures of PET. Samples of each stage of MFC manufacturing and processing were characterized by means of scanning electron microscopy (SEM), wide-angle X-ray scattering (WAXS), and mechanical testing. SEM and WAXS showed that the extruded blends are isotropic, but become highly oriented after drawing, and they are converted into MFC structured polymer–polymer composites during CM and IM. The MFC structures are characterized by an isotropic PP matrix, which contains reinforcing PET fibrils of different orientations after IM or of uniaxial orientation in the case of CM. The flexural modulus and the flexural strength of the IM samples are by 50% better than those of the neat PP, without expressing any clear effect of the amount of E-GMA in the blend. This was different from the CM samples, in which both the flexural modulus and the flexural strength increased with an increase of E-GMA part.

Microfibrillar Reinforced Composites from PET/LDPE Blends: Morphology and Mechanical Properties

Microfibrillar reinforced composites (MFC) comprising an isotropic matrix from a lower melting polymer, i.e., low density polyethylene (LDPE), reinforced by microfibrils of a higher melting polymer, recycled from bottles, i.e., poly(ethylene terephthalate) (PET), were processed under industrially relevant conditions via injection molding in a weight (wt) ratio of PET/LDPE equal to 30/70 and 50/50, respectively. Dogbone samples with MFC structure were characterized by means of light microscopy (LM), scanning (SEM), and transmission (TEM) electron microscopy, and mechanical testing. The SEM observations of cryogenic fracture surfaces show an isotropic LDPE matrix reinforced by more or less randomly distributed PET microfibrils while LM and TEM on thin slices indicate on orientation in machine direction. By means of TEM on stained ultrathin slices, one observes the formation of transcrystalline layers of the LDPE matrix on the surface of the PET microfibrils where the crystalline lamellae are parallel to each other and are placed perpendicularly to the fibril surfaces. By this transcrystallization and the relatively high aspect ratio of the fibrils (at least 50), the impressive mechanical properties of the injection‐molded samples can be explained. The elastic modulus is about 10 times higher than that of LDPE and about three times higher than LDPE reinforced with glass spheres, approaching the modulus of LDPE reinforced with 30 wt% short‐glass fibers (GF). The tensile strength is at least two times higher than that of LDPE or of LDPE reinforced with glass spheres, approaching that of LDPE reinforced with 30 wt% GF.

Design and characterisation of microfibrillar reinforced composite materials based on PET/PA12 blends

Microfibrillar reinforced composites (MFCs) have been prepared by reactive melt-extrusion of poly(ethylene terephthalate) (PET) and polyamide 12 (PA12) in the presence of a catalyst. These composites have been generated by a series of processes, which include reactive extrusion in a mini twin-screw extruder, followed by drawing (fibrillation) and finally the isotropisation of the lower-melting PA12 phase. During the preparation of MFCs additional condensation and transreactions occur at high temperature at the interface between PET and PA12 which leads to the formation of a copolymer layer at the blend interface playing the role of an in situ compatibiliser for the composite system. These composites have been investigated by differential scanning calorimetry (DSC), X-ray diffraction, scanning electron microscopy (SEM) and static and dynamic mechanical testing. The melting and crystallisation behaviour could be understood from DSC analysis. Wide-angle X-ray diffraction measurements have been made use of to characterise the orientation and crystallisation of the blends upon the drawing and isotropisation processes. The microstructure of MFCs, as revealed by SEM, showed fine fibrils of the PET phase in the matrix of PA12. Finally, the static and dynamic mechanical properties of the blends have been evaluated. The MFCs showed excellent ultimate mechanical properties.

Elastic properties of oriented polymers, blends and reinforced composites using the microindentation technique

The elastic behaviour of poly(ethylene terephthalate) (PET) and nylon 6 (PA6), their blends (1:1 by weight) and microfibrillar-reinforced composites of the previously mentioned homopolymers has been investigated by means of load–displacement analysis from indentation experiments. The dependence of the elastic modulus of the homopolymers upon the degree of crystallinity and the crystal size, as derived from indentation experiments, is discussed. A linear correlation between the elastic modulus anisotropy and the microindentation hardness anisotropy values is also found to apply for the oriented materials. The results reveal that the indentation modulus values of the PET/PA6 blends follow the parallel additivity model of the individual components. The use of the additivity law is also shown to provide a value, otherwise not accessible from direct measurements, of the modulus of the microfibrils within the microfibrillar-reinforced composites.

Direct electron microscopic observation of transcrystalline layers in microfibrillar reinforced polymer-polymer composites

Microfibrillar reinforced composites (MFC) comprising an isotropic matrix from a lower melting polymer, i.e., low density polyethylene (LDPE), reinforced by microfibrils of a higher melting polymer, recycled from bottles, i.e., poly(ethylene terephthalate) (PET), were processed under industrially relevant conditions via injection molding in a weight ratio of PET/LDPE = 50/50. Dog bone samples with MFC structure were characterized by means of scanning (SEM) and transmission (TEM) electron microscopy. SEM observations on cryogenic fracture surfaces show an isotropic LDPE matrix reinforced by more or less randomly distributed PET microfibrils. By means of TEM on stained ultrathin slices one observes the formation of transcrystalline layers of LDPE matrix on the surface of the PET microfibrils. In these layers the crystalline lamellae are aligned parallel to each other and are placed perpendicularly to the fibril surfaces. This is in contrast to the bulk matrix where the lamellae are quasi-randomly arranged.

Monitoring the Evolution of Morphology of Polymer Blends Upon Manufacturing of Microfibrillar Reinforced Composites

Microfibrillar reinforced composites (MFC) based on HDPE/PET blends were prepared under conditions relevant for direct scale-up to an industrial process. The evolution of the morphology and of the linear viscoelastic response of the blend along the axis of a co-rotating twin screw extruder and at several locations along the extrusion line was monitored. Major changes in the average particle size and size distribution of the disperse phase occurred upon melting of the components, whilst a much slower evolution rate was evident downstream in the extruder. Simultaneously, G′ and G″ increased along the extruder. Pellets showing well oriented PET fibrils embedded in a HDPE matrix with poor adhesion between both were obtained. This MFC showed the typical improvement expected in mechanical performance when compared with the matrix.

Structure-property relationships of injection- and compression-molded microfibrillar-reinforced PET/PA-6 composites

Structure-property relationships of injection- and compression-molded microfibrillar-reinforced PET/PA-6 blends with different weight ratio of the components have been studied. The blends were prepared by melt mixing using a single-screw extruder. Thermal, X-ray (WAXS), morphological, and static mechanical studies of the bulk samples were carried out in order to determine an optimum processing window. The upper processing temperature was established by the onset of the melting of the fibrillized PET component in the blend. Morphological studies of the injection-molded samples indicate skin-core morphology. The skin layer is composed of fibrils that are well oriented along the flow axis. The core region is composed of a small amount of randomly oriented, fibrillar PET bundles, as well as a large amount of spherical PET domains in the isotropic PA-6 matrix. SEM observations of the fracture surface of compression-molded samples show well-extended PET fibrils embedded in an isotropic PA-6 matrix. The reason for this significant difference in the morphology of the samples is the fact that in the case of compression molding it is possible to keep very accurately the desired processing temperature in a quite narrow interval. On the other hand, an overheating of the system due to non-isothermal shear and a viscosity dissipation are the main reasons for the melting of a part of the PET fibrils and their transformation into spherical domains in the core of the injection-molded samples. The morphology strongly affects the tensile properties of the bulk samples. Compared to the values of injection-molded neat PA-6, the elastic modulus and the strength of injection-molded MFC (Microfibrillar reinforced composites) blends have been increased by a factor of 2.5 and 1.7, respectively. In the case of compression-molded samples, both modulus and strength were about four times higher than those of the neat PA-6. © 2000 John Wiley & Sons, Inc. Adv Polym Techn 19: 249–259, 2000

Microfibrillar reinforced composites - the role of the chemical interactions at the fibril - matrix interface

The preparation, structure and some of the properties of microfibrillar reinforced composites (MFC) based on polyamide-6 (PA6) as a polymer matrix and poly(ethylene terephthalate) (PET) in the form of reinforcing microfibrils are briefly discussed. It is demonstrated that the PA6-PET MFC displays comparable or even better mechanical properties than a conventional composite consisting of short glass fibers in a PA6 matrix. This effect is attributed to the interface between the PA6 polymer matrix and the PET microfibrils resulting in the formation of a new phase - a PET-PA6 copolymer which plays the role of a selfcompatibilizer. The progress of the chemical interactions between PET and PA6 amorphous phases leads ultimately to the involvement of the entire amount of PA6 into a block copolymer, thus changing the orginal homopolymeric PA6 matrix into a copolymeric PET-PA6 one. In such a way it is possible to overcome the immiscibility of the MFC components and to obtain control over mechanical properties of the PA6-PET MCF.

Structure development in PET/PA6 microfibrillar-reinforced composites as revealed by revealed by microhardness

Homopolymer poly(ethylene terephthalate) (PET) and nylon-6 (PA6) and a blend (1 : 1 by weight) of these polymers, were extruded as strips and ultraquenched from the melt. After zone drawing and additional annealing at temperatures, Ta, of 220 or 240 °C for 5 or 25 h in vacuum, the samples were studied by scanning electron microscopy (SEM), wide-angle X-ray scattering, solubility and microhardness, H, tests. In conformity with previous studies of the same system, the present SEM observations show that mechanical drawing results in the formation of a highly oriented fibrillar structure of PET which is preserved even after annealing above the melting point of PA6. Furthermore, raising of both annealing temperature and duration up to 240 °C and 25 h, respectively, results in a strong decrease of the solubility of the PA6 fraction in formic acid (five-fold). This is attributed to intensive chemical interactions between components, drastically improving the adhesion between matrix and reinforcing microfibrils. From the dependence of H on degree of crystallinity, wc, the hardness values for completely amorphous, Ha, and fully crystalline, Hc, neat homopolymers were extrapolated (HaPET = 128 MPa, HcPET = 294 MPa, HaPA = 52 MPa and HcPA = 283 MPa). Using these values and applying the additive law, the H-value of the microfibrils is derived. The high value obtained for PET fibrils (360 MPa) is explained by the peculiarity in the structure formation of these microfibrils. The effect of crystal size on the formation of H is also discussed. The H-value of infinite large PA6 crystals is derived to be H∞ = 460 MPa. It is shown that the type and extent of the mutual dispersion of the components, as well as the adhesion between them, are important factors for the proper applicability of the additive law.

Morphology of microfibrillar reinforced composites PET/PA 6 blend

Extruded isotropic films of a poly(ethylene terephthalate) (PET) and polyamide 6 (PA 6) blend are subjected to drawing and annealing at different temperatures and durations in order to produce microfibrillar reinforced composites (MFC). In conformity with previous X-ray studies of the same system, the present scanning electron microscopic (SEM) observations show that drawing results in the formation of a highly oriented fibrillar structure of PET which is preserved even after annealing above the melting point of PA 6. Further raising of both annealing temperature and duration up to 240°C and 25 h, respectively, results in a strong decrease of the solubility of the PA 6 fraction in formic acid (by 5×) as well as in a continuous increase of the intensity of the infra-red spectral band at 3300 cm−1 which is characteristic of the amide group. SEM observations of these samples reveal larger aggregates of fibrillized PET arising from the gradual incorporation of PET in a copolymer matrix, the latter being insoluble in formic acid. The observed changes are explained by the occurrence of chemical interactions between PET and PA 6 under these conditions, as concluded earlier from the complete loss of crystallinity of the PA 6 component. Starting from the present and previous results on MFC, a model is proposed, illustrating the morphological changes on the macro- and molecular levels, during their preparation.

Dynamic mechanical study of amorphous phases in poly(ethylene terephthalate) /nylon-6 blends

The changes in the thermomechanical behaviour of a poly(ethylene terephthalate)/nylon-6 (PET/PA) blend (1:1 by weight) subjected to mechanical and thermal treatments are examined by means of dynamic mechanical measurements. It is established from previous studies that PET/PA blends are incompatible in the isotropic state, but form so-called microfibrillar-reinforced composites upon extrusion, drawing and suitable annealing. This study focuses mainly on the amorphous component of these blends and thus complements that recently performed ( Polymer 1993, 34 , 4669) in which the crystalline phases were analysed. The orientation and crystallization of the homopolymers induced by drawing improve the dispersion of components and induce some compatibility as far as one glass transition temperature is observed. Yet, by annealing the drawn blend at temperatures below the melting temperatures of both components (220°C, for instance) the biphasic character of the composite is enhanced in as much as the microstructures of both the crystalline and amorphous phases are improved and the reorganization of species within separate phases is favoured. The components of the heterogeneous blend become compatible provided that the annealing is performed at a sufficiently high temperature (240°C). This temperature is intermediate between the melting temperature of the two components and allows for the isotropization of the low-melting component PA. The increase of compatibility is attributed to transreactions producing compatibilizing layers of PET/PA copolymers at phase boundaries between microfibrils and the amorphous matrix. Prolonged annealing (25 h) leads to the randomization of the original block copolymers and results in the complete participation of PA in the copolymer, which is evidenced by the disappearance of the glass transition peak of PA.