Micromechanical Deformation Processes Research Articles

A Coupled Electro-Mechanical Homogenization-Based Model for PVDF-Based Piezo-Composites Considering α → β Phase Transition and Interfacial Damage.

Polyvinylidene fluoride or polyvinylidene difluoride (PVDF) is a piezoelectric semi-crystalline polymer whose electro-mechanical properties may be modulated via strain-induced α → β phase transition and the incorporation of polarized inorganic particles. The present work focuses on the constitutive representation of PVDF-based piezo-composites developed within the continuum-based micromechanical framework and considering the combined effects of particle reinforcement, α → β phase transition, and debonding along the interface between the PVDF matrix and the particles under increasing deformation. The micromechanics-based model is applied to available experimental data of PVDF filled with various concentrations of barium titanate (BaTiO3) particles. After its identification and predictability verification, the model is used to provide a better understanding of the separate and synergistic effects of BaTiO3 particle reinforcement and the micromechanical deformation processes on the electro-mechanical properties of PVDF-based piezo-composites.

Super-Tough PLA-Based Blends with Excellent Stiffness and Greatly Improved Thermal Resistance via Interphase Engineering.

Super-tough poly(lactic acid)/polycarbonate (PLA/PC) (50/50) blends with an excellent balance of stiffness, toughness, and thermal stability were systematically designed and characterized. Poly(methyl methacrylate) (PMMA) was utilized as a novel, highly effective nonreactive interphase to promote PLA-PC phase compatibility. Partial miscibility of PMMA with both PLA and PC produced strong molecular entanglements across the PLA-PC phase boundary followed by an excellent phase adhesion. This was predicted from interfacial energy measurements and supported by dynamic mechanical thermal analysis, morphological observations, and mechanical tests. Ternary PLA/PC/PMMA blends exhibited an exceptional set of stiffness, tensile and flexural strength, tensile and flexural ductility, and thermal stability together with improved impact strength compared with neat PLA and uncompatibilized PLA/PC blends. Addition of nonreactive polybutadiene-g-styrene-co-acrylonitrile (PB-g-SAN) impact modifier to the compatibilized blend resulted in further dramatic improvements in the dispersion state of PC and PMMA phase domains followed by the development of an interconnected structure of PC, PMMA, and PB-g-SAN domains in the PLA matrix. Such a network-like morphology, with rubbery particles percolated at the interface between the dispersed structures and surrounding PLA matrix, produced a tremendous increase in impact resistance (≈700 J/m) and tensile ductility (≈200% strain) while maintaining excellent stiffness (≥2.1 GPa). The combined effects of interfacial localization of impact modifier particles, network-like morphology (extended over the entire volume of the blend), and strong phase interactions between the components (due to mutual miscibility) are described to be responsible for super-tough behavior. The role of PMMA as an efficient interphase adhesion promoter in the toughened quaternary blends is also clarified. Impact fractography revealed multiple void formations, plastic growth of microvoids, and the formation of void-fibrillar structures around as well as inside the dispersed structures as the main micromechanical deformation processes responsible for massive shear yielding and plastic deformation of blends. Blends designed in this work offer remarkable improvements in tensile and flexural ductility, impact resistance, and heat deflection temperature compared with neat PLA resin. The overall characteristics of these blend systems are comparable and/or superior to those of several commercial thermoplastic resins.

Micromechanical Deformation Processes and Failure of PBS Based Composites Containing Ultra-Short Cellulosic Fibers for Injection Molding Applications.

The use of biobased thermoplastic polymers has gained great attention in the last years as a potential alternative to fossil-based thermoplastic polymers. Biobased polymers in fact offer advantages not only in terms of reduced dependence on fossil resources but they also lower the CO2 footprint in accordance with sustainability and climate protection goals. To improve the properties of these materials, reinforcement with biobased fibers is a promising solution; however, it must be kept in mind that the fibers aspect ratio and the interfacial adhesion between the reinforcement and the matrix plays an important role influencing both physical and mechanical properties of the biocomposites. In this paper, the possibility of producing composites by injection molding, based on polybutylene succinate and ultra-short cellulosic fibers has been explored as a potential biobased solution. Thermo-mechanical properties of the composites were investigated, paying particular attention to the local micromechanical deformation processes, investigated by dilatometric tests, and failure mechanisms. Analytical models were also applied to predict the elastic and flexural modulus and the interfacial properties of the biocomposites. Good results were achieved, demonstrating the that this class of biocomposite can be exploited. Compared to pure PBS, the composites with 30 wt.% of cellulose fibers increased the Young's modulus by 154%, the flexural modulus by 130% and the heat deflection temperature by 9%.

Utilization of wheat-bran cellulosic fibers as reinforcement in bio-based polypropylene composite

Polymer composites reinforced with natural fibers have received widespread attention due to the harmful effects of the petroleum derived synthetic polymer waste. In this study, wheat bran (WB), as a new source of natural fiber, has been investigated in manufacturing bio-based composites. Wheat bran was pretreated with sodium hydroxide (NaOH) and the compositions were analyzed. Pretreatment removed fat, starch, and crude protein significantly and resulted in an increase of cellulose content from 10.9% to 29.6%. Polypropylene (PP) was melt-blended with bran in a screw-extruder with and without using a chemical compatibilizer, and test specimens were produced by an injection molder. Composite specimens tested for characterization of different thermophysical properties. A 16.3 % increase in flexural strength was obtained at fiber loading of 20 % or more, while tensile strength was comparable to neat PP using compatibilizer. Moduli increased and impact strength decreased with fiber addition. The highest tensile and flexural moduli were 2838 and 1702 MPa, respectively for WB/PP composite. The highest impact strength reduction was 66 % with WB/PP composite at 30 % loading rate. Thermal expansion coefficient of the WB/PP biocomposites decreased with a highest reduction of 64 %; meaning that WB, a grain by-product, is very promising as functional fibers for polymer materials. The interactions of WB fiber in bio-based polymer matrix for failure mechanism and composite strength were also studied by manufacturing WB/polylactic acid (PLA) composites using fiber loading optimized for WB/PP composite. Inferior mechanical performance of WB/PLA composites was observed compared to neat PLA. Poor adhesion, debonding, and fiber fracture were the dominating micromechanical deformation processes in WB/PP composites, whereas debonding, cavity, and crack formation due to fiber orientation resulted in poor performance of WB/PLA composites. Prospects of WB-based composites present opportunities for growers having beneficial uses of bran and compensating reduced profit margins.

Volume Change during Creep and Micromechanical Deformation Processes in PLA-PBSA Binary Blends.

In this paper, creep measurements were carried out on poly(lactic acid) (PLA) and its blends with poly(butylene succinate-adipate) (PBSA) to investigate the specific micromechanical behavior of these materials, which are promising for replacing fossil-based plastics in several applications. Two different PBSA contents at 15 and 20 wt.% were investigated, and the binary blends were named 85-15 and 80-20, respectively. Measurements of the volume strain, using an optical extensometer, were carried out with a universal testing machine in creep configuration to determine, accompanied by SEM images, the deformation processes occurring in a biopolymeric blend. With the aim of correlating the creep and the dilatation variation, analytical models were applied for the first time in biopolymeric binary blends. By using an Eyring plot, a significant change in the curves was found, and it coincided with the onset of the cavitation/debonding mechanism. Furthermore, starting from the data of the pure PLA matrix, using the Eyring relationship, an apparent stress concentration factor was calculated for PLA-PBSA systems. From this study, it emerged that the introduction of PBSA particles causes an increment in the apparent stress intensity factor, and this can be ascribed to the lower adhesion between the two biopolymers. Furthermore, as also confirmed by SEM analysis, it was found that debonding was the main micromechanical mechanism responsible for the volume variation under creep configuration; it was found that debonding starts earlier (at a lower stress level) for the 85-15 blend.

Structural biomaterials engineered from lignocellulosic agricultural waste

AbstractMilled sunflower husk was added to three types of polyolefins (polypropylene, low‐density and high‐density polyethylene) in a wide composition range. Composite series were prepared with or without maleic anhydride‐grafted polyolefin coupling agents to investigate the effect of coupling on the mechanical properties and micromechanical deformation processes. Several independent approaches were followed to estimate interfacial adhesion qualitatively and quantitatively, respectively. The results show that interfacial adhesion is significantly improved in the presence of a coupling agent. As a result, coupling changes the dominant deformation process from matrix/filler debonding to the fracture of sunflower husk particles. Milled sunflower husk can enhance significantly the modulus of polyolefins; however, elongation‐at‐break values are very small, especially at larger filler content, which represents the largest obstacle of practical application.

Poly(lactic acid) (PLA)/Poly(butylene succinate-co-adipate) (PBSA) Compatibilized Binary Biobased Blends: Melt Fluidity, Morphological, Thermo-Mechanical and Micromechanical Analysis.

In this work poly(lactic) acid (PLA)/poly(butylene succinate-co-adipate) (PBSA) biobased binary blends were investigated. PLA/PBSA mixtures with different compositions of PBSA (from 15 up to 40 wt.%) were produced by twin screw-extrusion. A first screening study was performed on these blends that were characterized from the melt fluidity, morphological and thermo-mechanical point of view. Starting from the obtained results, the effect of an epoxy oligomer (EO) (added at 2 wt.%) was further investigated. In this case a novel approach was introduced studying the micromechanical deformation processes by dilatometric uniaxial tensile tests, carried out with a videoextensometer. The characterization was then completed adopting the elasto-plastic fracture approach, by the measurement of the capability of the selected blends to absorb energy at a slow rate. The obtained results showed that EO acts as a good compatibilizer, improving the compatibility of the rubber phase into the PLA matrix. Dilatometric results showed different micromechanical responses for the 80–20 and 60–40 blends (probably linked to the different morphology). The 80–20 showed a cavitational behavior while the 60–40 a deviatoric one. It has been observed that while the addition of EO does not alter the micromechanical response of the 60–40 blend, it profoundly changes the response of the 80–20, that passed to a deviatoric behavior with the EO addition.

Experimental investigation of the nonlinear quasi-static and dynamic mechanical behaviour of novel PA6/XHNBR thermoplastic vulcanizates: Linking mechanical nonlinearities to microstructural features

Driven by the need to design environmentally friendly and sustainable polymeric materials that achieve superior mechanical properties, this work is centred on the relationship between the microstructure and the nonlinear quasi-static and dynamic mechanical behaviour of novel polyamide 6 (PA6)/carboxylated hydrogenated acrylonitrile butadiene rubber (XHNBR) thermoplastic vulcanizates (TPVs). In particular, with the aim to contribute to the further understanding of the physical mechanisms involved in the complex nonlinear mechanical behaviour of TPV compounds, the impact of different microstructural features of the PA6 matrix and the rubber network on the mechanical behaviour has been investigated. Interestingly, stronger rubber network interactions and so better elastic recovery capacity of the rubber network can hinder the growth of voids more efficiently. The observed micromechanical deformation processes are interpreted in terms of the role of the yielding, buckling and bending of thin PA6 ligaments at low strain levels and the rupture of covalent and non-covalent bonds presented in the rubber network at high strain levels.

Mitsui Chemicals developing technology to produce bio-based polypropylene

Polymer composites reinforced with natural fibers have received widespread attention due to the harmful effects of the petroleum derived synthetic polymer waste. In this study, wheat bran (WB), as a new source of natural fiber, has been investigated in manufacturing bio-based composites. Wheat bran was pretreated with sodium hydroxide (NaOH) and the compositions were analyzed. Pretreatment removed fat, starch, and crude protein significantly and resulted in an increase of cellulose content from 10.9% to 29.6%. Polypropylene (PP) was melt-blended with bran in a screw-extruder with and without using a chemical compatibilizer, and test specimens were produced by an injection molder. Composite specimens tested for characterization of different thermophysical properties. A 16.3 % increase in flexural strength was obtained at fiber loading of 20 % or more, while tensile strength was comparable to neat PP using compatibilizer. Moduli increased and impact strength decreased with fiber addition. The highest tensile and flexural moduli were 2838 and 1702 MPa, respectively for WB/PP composite. The highest impact strength reduction was 66 % with WB/PP composite at 30 % loading rate. Thermal expansion coefficient of the WB/PP biocomposites decreased with a highest reduction of 64 %; meaning that WB, a grain by-product, is very promising as functional fibers for polymer materials. The interactions of WB fiber in bio-based polymer matrix for failure mechanism and composite strength were also studied by manufacturing WB/polylactic acid (PLA) composites using fiber loading optimized for WB/PP composite. Inferior mechanical performance of WB/PLA composites was observed compared to neat PLA. Poor adhesion, debonding, and fiber fracture were the dominating micromechanical deformation processes in WB/PP composites, whereas debonding, cavity, and crack formation due to fiber orientation resulted in poor performance of WB/PLA composites. Prospects of WB-based composites present opportunities for growers having beneficial uses of bran and compensating reduced profit margins.

Micromechanics-based constitutive modeling of plastic yielding and damage mechanisms in polymer–clay nanocomposites: Application to polyamide-6 and polypropylene-based nanocomposites

Abstract The present work focuses on the continuum-based micromechanical modeling of the elastic–plastic stress–strain response including damage mechanisms of polymer–clay nanocomposites. The micromechanical elastic–plastic-damage model includes both the actual microstructure of the said composites using a multi-scale approach and the microstructural evolution related to damage accumulation under applied macroscopic deformation. The interfacial debonding between clay nanoparticles and polymer matrix, and the polymer matrix voiding are the two prevalent damage events considered. The tensile stress–strain response and micromechanical deformation processes of polyamide-6 and polypropylene-based systems reinforced with modified montmorillonite clay at various concentrations are experimentally investigated by a video-controlled technique. The usual shear yielding deformation mode of neat polyamide-6 is altered by the presence of clay platelets which induce a dilatational process due to interfacial debonding. In addition to matrix shear yielding, a dual-dilatational deformation mechanism by crazing and interfacial debonding is revealed in polypropylene–clay nanocomposites. Using the intrinsic deformation micromechanisms and elastic–plastic properties of the polymer matrix, and the nanocomposite structural characteristics, the micromechanical model is found to successfully describe the experimental results of the two nanocomposite materials in terms of tensile stress–strain response and inelastic volumetric strain.

Factors determining the performance of thermoplastic polymer/wood composites; the limiting role of fiber fracture

Thermoplastic polymer/lignocellulosic fiber composites were prepared with a considerable range of matrices and fibers in an internal mixer. Tensile properties were determined on bars cut from compression molded plates. Local deformation processes initiated around the fibers were followed by acoustic emission testing supported by electron and polarization optical microscopy. The analysis of results proved that micromechanical deformation processes initiated by the fibers determine the performance of the composites. Debonding usually leads to the decrease of composite strength, but decreasing strength is not always associated with poor adhesion and debonding. The direction of property change with increasing wood content depends on component properties and interfacial adhesion. Good interfacial adhesion often results in the fracture of the fibers. Depending on their size and aspect ratio, fibers may fracture parallel or perpendicular to their axis. At good adhesion, the maximum strength achieved for a particular polymer/wood pair depends on the inherent strength of the fibers, which is larger for perpendicular than parallel fracture. Inherent fiber strength effective in a composite depends also on particle size, larger particles fail at smaller stress, because of the larger number of possible flaws in them. A very close correlation exists between the initiation stress of the dominating local deformation process and composite strength proving that these processes lead to the failure of the composite and determine its performance.

PLA/lignocellulosic fiber composites: Particle characteristics, interfacial adhesion, and failure mechanism

ABSTRACTPoly(lactic acid) (PLA) composites were prepared using six lignocellulosic fibers with widely varying particle characteristics. The composites were characterized by tensile testing, scanning electron (SEM), and polarization optical microscopy (POM). Micromechanical deformation processes during loading were followed by acoustic emission measurements. Interfacial adhesion was estimated by three independent methods. Contrary to most claims published in the literature, interfacial adhesion between PLA and natural fibers was found to be rather strong, a result confirmed by the quantitative estimation of adhesion strength, acoustic emission measurements, and SEM study. Strong interfacial adhesion results in weak dependence of the extent of reinforcement on the particle characteristics of the reinforcing fibers. Both acoustic emission measurements and microscopy indicated that the dominating micromechanical deformation process is the fracture of the fibers and close correlation was found between the initiation stress of fiber fracture, reinforcement, and the ultimate strength of the composites. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 39902.

Biocomposite from polylactic acid and lignocellulosic fibers: Structure–property correlations

PLA biocomposites were prepared using three corn cob fractions and a wood fiber as reference. The composites were characterized by tensile testing, scanning electron microscopy (SEM) and polarization optical microscopy (POM). Micromechanical deformation processes were followed by acoustic emission measurements. The different strength of the components was proved by direct measurements. Two consecutive micromechanical deformation processes were detected in composites containing the heavy fraction of corncob, which were assigned to the fracture of soft and hard particles, respectively. The fracture of soft particles does not result in the failure of the composites that is initiated either by the fracture of hard particles or by matrix cracking. Very large particles debond easily from the matrix resulting in catastrophic failure at very low stresses. At sufficiently large shear stresses large particles break easily during compounding, thus reinforcement depending on interfacial adhesion was practically the same in all composites irrespectively of initial fiber characteristics.

Effect of Clay Modification on the Mechanism of Local Deformations in PA6 Nanocomposites

AbstractPA nanocomposites are prepared from clays organophilized with a phosphonium and an ammonium salt, and sodium montmorillonite is used as reference. The analysis of mechanical and micromechanical properties of the composites reveal that several micromechanical deformation processes occur in the PA/MMT composites. The matrix cavitates at relatively small stress. Processes related to non‐exfoliated clay structural units are initiated at larger stresses. Sound is emitted mainly by the fracture of particles, but debonding may also occur. The plastic deformation of the matrix dominates at larger stresses and deformations. The various local deformations are independent of each other and composite properties are not determined by silicate related processes but by the deformation of the matrix.magnified image

Adhesion and micromechanical deformation processes in PLA/CaSO4 composites

PLA/CaSO4 composites were prepared from uncoated and stearic acid coated filler particles in a wide composition range. The strength of interfacial adhesion was estimated quantitatively with three independent methods. Structure was characterized by DSC, XRD and SEM measurements, while mechanical properties by tensile and instrumented impact tests. The results proved that adhesion is twice as strong in composites prepared with the uncoated particles than in those containing the coated filler. Coating changes also local deformation processes around the particles. Although debonding is the dominating micromechanical deformation process in all composites, local plastic deformation is larger around coated particles. The extent of this deformation depends very much also on the local distribution of particles. The final properties and performance of the composites depend unambiguously on the micromechanical deformation processes occurring during loading, on debonding and the subsequent plastic deformation. Stearic acid used for the coating of the filler seems to dissolve in the polymer and locally change its properties.

Tensile and impact properties of three-component PP/wood/elastomer composites

Polypropylene (PP) was reinforced with wood flour and impact modified with elastomers to increase stiffness and impact resistance simultaneously. Elastomer content changed in four (0, 5, 10 and 20 wt%), while that of wood content in seven steps, the latter from 0 to 60 wt% in 10 wt% steps. Structure and adhesion were controlled by the addition of func- tionalized (maleated) polymers. Composites were homogenized in a twin-screw extruder and then injection molded to ten- sile bars. Fracture resistance was characterized by standard and instrumented impact tests. The results showed that the components are dispersed independently of each other even when a functionalized elastomer is used for impact modifica- tion, at least under the conditions of this study. Impact resistance does not change much as a function of wood content in PP/wood composites, but decreases drastically from the very high level of the PP/elastomer blend to almost the same value obtained without impact modifier in the three-component materials. Increasing stiffness and fiber related local deformation processes led to small fracture toughness at large wood content. Micromechanical deformation processes depend mainly on the strength of PP/wood interaction; debonding and pull-out take place at poor adhesion, while fiber fracture dominates when adhesion is strong. Composites with sufficiently large impact resistance cannot be prepared in the usual range of wood contents (50-60 wt%).

Interaction of Microstructure and Interfacial Adhesion on Impact Performance of Polylactide (PLA) Ternary Blends

Polyactide (PLA) was blended with an ethylene/n-butyl acrylate/glycidyl methacrylate (EBA-GMA) terpolymer and a zinc ionomer of ethylene/methacrylic acid (EMAA-Zn) copolymer. The phase morphology of the resulting ternary blends and its relationship with impact behaviors were studied systematically. Dynamic vulcanization of EBA-GMA in the presence of EMAA-Zn was investigated by torque rheology, and its cross-link level was evaluated by dynamic mechanical analysis. Reactive compatibilization between PLA and EBA-GMA was studied using Fourier transform infrared spectroscopy. The dispersed domains in the ternary blends displayed a “salami”-like phase structure, in which the EMAA-Zn phase evolved from occluded subinclusions into continuous phase with decrease in the EBA-GMA/EMAA-Zn ratio. An optimum range of particle sizes of the dispersed domains for high impact toughness was identified. Also, the micromechanical deformation process of these ternary blends was also investigated by observation of the impact-fractured surfaces using the electron microscope. It was suggested that the low cavitation resistance of dispersed particles in conjunction with suitable interfacial adhesion was responsible for the optimum impact toughness observed.

Evaluation of mechanical and thermal properties of Poly (butylene terephthalate) (PBT) composites reinforced with wollastonite

Particulate filled polymer composites are becoming attractive because of their wide applications and low cost. Present study deals with the preparation and characterization of PBT/wollastomte composites and analyzing the effect of incorporation of varying concentration of wollastonite on the mechanical, thermal, and morphological properties of PBT thermoplastic composite for future industrial applications. In the composites wollastonite content was varied by 0–30 % by weight. Tensile strength exhibited a marginal increase whereas a dramatic increase was observed in modulus with an increase in filler content. The experimental results were also compared with theoretical predictions which revealed good level of interfacial interaction in the composites. Thermal analysis revealed an increase in percent crystallinity and also the onset crystallization temperature thereby indicating the nucleating efficiency of wollastonite. Study of the fracture surface by SEM revealed a change in the micromechanical deformation process with an increase in filler content.

Micromechanical deformation processes in PP/wood composites: Particle characteristics, adhesion, mechanisms

Polypropylene (PP) was reinforced with four natural fillers having different particle characteristics. Interfacial adhesion was changed by the introduction of maleated polypropylene (MAPP). The properties of the studied PP/wood composites depended strongly on interfacial adhesion and on the particle characteristics of the wood. Coupling with functionalized polymer is necessary for the preparation of composites with acceptable properties if the size of the particles is large and their aspect ratio is small. The effect of adhesion is smaller for particles with large aspect ratio. Several micromechanical deformation processes may occur in PP/wood composites including matrix yielding, debonding, fiber pull-out and fiber fracture both parallel and perpendicular to the fiber axis. The processes are competitive and may take place simultaneously and/or consecutively. The inherent properties of the reinforcement may limit the improvement of composite strength. Micromechanical deformation processes determine composite properties irrespectively of their mechanism.

Properties of Wood Fibre-Polypropylene Composites: Effect of Wood Fibre Source

This study examined the effect of type of wood fibre source on the physical and mechanical properties of wood fibre-polypropylene composites. Wood flour, fibres of heat-treated wood and pellets were used as sources of wood fibres in the manufacturing process. All studied wood fibre-polypropylene composites were made from 75% wood, 22% recycled polypropylene (PP) and 3% maleated polypropylene (MAPP). Wood fibre-polypropylene composites were compounded in a conical twin-screw extruder. Water absorption and thickness swelling were studied. Mechanical properties of the composites were characterised by tensile, flexural, and impact testing. Micromechanical deformation processes were investigated using scanning electron microscopy done on the fractured surfaces of broken samples. The durability of composites exposed to three accelerated cycles of water immersion, freezing and thawing was examined. The results showed that the density of the composites was a key factor governing water absorption and thickness swelling. A significant improvement in tensile strength, flexural strength, and Charpy impact strength was observed for composites reinforced with heat-treated fibre compared to composites reinforced with pellets and especially to wood flour reinforced composites. The flexural strength and dimensional stability performance reduced after exposure to freeze-thaw cycling for all composites, but the degree of these changes was dependent on the wood fibre source.