Injection-molded Composites Research Articles

Micro-CT based structure tensor analysis of fibre orientation in random fibre composites versus high-fidelity fibre identification methods

The fibre orientation distribution controls the mechanical properties of random fibre composites. Generally accepted methods for its characterisation involve identification of fibres or their ellipsoidal cross sections as individual objects, requiring high image resolution and high computational resources. This paper investigates whether structure tensor analysis can be an alternative and whether it can work with lower resolution images. Micro-computed X-ray tomography images of random glass fibre/polypropylene injection moulded composites were processed using ellipsometry on 2D slices, 3D fibre identification (Avizo software) and analysis of the structure tensor (VoxTex software). The images had resolutions of 1.4, 3.2, 8 and 16 µm per pixel, compared to an average glass fibre diameter of 17 µm. All the methods yielded similar results for high-resolution images (1.4 and 3.2 µm). The high-fidelity, direct identification of fibres failed for low-resolution images, but the structure tensor analysis still yielded results close to the high-resolution scans.

Evaluation of injection moulded natural fibre-polyolefin for potential in automotive applications

This paper presents results from measurements of the interfacial adhesion of coir fiber with various polypropylene matrices along with the mechanical properties of their related injection molded composites. Fiber-matrix adhesion, including the effect of polymeric coupling agent, was characterized using the fiber pull-out method. The addition of coupling agent significantly increased the interfacial shear strength of coir fiber-polypropylene. Results from tensile and impact testing of coir reinforced polypropylene composites showed that the coupling agent increased composite tensile strength and unnotched impact strength but reduced notched impact performance. The potential of these materials to replace existing automotive materials is discussed. It is concluded that coir reinforced-polypropylene is not a suitable candidate to replace glass fiber – polypropylene but that it may have some potential as a replacement in lower performance talc reinforced polypropylene applications.

Biobased ‘Mid-performance’ composites using losses from the hackling process of long hemp – A feasibility study as part of the development of a biorefinery concept –

The Multihemp biorefinery concept combines processing systems that, starting from harvesting, transform hemp biomass into a spectrum of marketable intermediate and final products. Hackling loss sorted out during production of long hemp was used for the compounding process to produce hemp fibre-reinforced Polylactide (PLA) granules for the injection moulding process. Even though only side stream fibres were used, the overall mechanical properties lie within the range of properties reported for PLA – bast fibre composites cited in the literature or even above. The use of hemp from an earlier harvest at full flowering (harvest 1) significantly increases Young’s modulus values compared to the pure PLA polymer and shows a slightly better reinforcing potential than hemp harvested later at seed maturity (harvest 2). A significant influence of harvest time on the composite properties was found, which is stronger than the variety influence. Based on the presented results, the poly-product MultiHemp system showed a proof of concept for the use of hackling loss for injection moulded composites.

Resistance of Injection Molded Wood-Polypropylene Composites against Basidiomycetes According to EN 15534-1: New Insights on the Test Procedure, Structural Alterations, and Impact of Wood Source

In this study, we investigated injection molded wood-polypropylene composites based on various wood sources and their decay resistance against white rot (Trametes versicolor) and brown rot (Coniophora puteana) in a laboratory test according to EN 15534-1:2014. The manufactured composites consisted of poplar (Populus spp.), willow (Salix spp.), European beech (Fagus sylvatica L.), Norway spruce (Picea abies (L.) H. Karst.), and a commercial wood source (Arbocel® C100), respectively. All formulations were compounded on a co-rotating twin screw extruder and subsequently injection molded to wood–PP specimens with a wood content of 60% or 70% by weight. It was found that the test procedure had a significant effect on the mechanical properties. Loss in mechanical properties was primarily caused by moisture and less by fungal decay. Moisture caused a loss in the modulus of rupture and modulus of elasticity of 34 to 45% and 29 to 73%, respectively. Mean mass and wood mass losses were up to a maximum of 3.7% and 5.3%, respectively. The high resistance against fungal decay was generally attributed to the encapsulation of wood by the polymer matrix caused by sample preparation, and enhanced by reduced moisture uptake during the preconditioning procedure. Notable differences with respect to the wood particle source and decay fungi were also observed. Structural characterization confirmed the decay pattern of the fungi such as void cavities close the surface and the deposition of calcium oxalates.

Modeling the Thermal Conductivity Inhomogeneities of Injection-Molded Particle-Filled Composites, Caused by Segregation.

Many applications require new materials that have good thermal conductivity, are electrical insulators and can be processed easily and with relatively little energy. A new innovative solution for this problem is thermally conductive composites, which can replace metals in many cases. Many papers have focused on the prediction of their thermal conductivity. At the same time segregation has to be taken into account in the case of composites because it affects the distribution of thermally conductive particles, and thus local thermal conductivities. In this paper, we examined and modeled segregation during injection molding and its effect on thermal conductivity. We injection-molded samples from polypropylene with glass beads of different sizes and analyzed their filler content as a function of the flow path. We described the distribution of the filler with a mathematical model. Using this, we created a new, segregation-dependent model that describes the local thermal conductivity of polymer composites as a function of filler content with great accuracy.

Distribution of fillers and reinforcements in injection moulded thermoplastic composites: Case study of glass bubbles

Apart from the complex nature of natural fibers, studying glass bubbles, as reinforcement phase, is considered a good start to understand the flow behaviour of fillers or reinforcements and hence their distribution in the final products after injection moulding. Glass bubbles are thermally stable within the processing temperatures of Polypropylene. They are also void from the effect of fibre aspect ratio. This work explores the uneven glass bubbles content along injection moulded spirals out of polypropylene (PP). Glass bubbles are experimented at 10 and 30% volume fraction Vf levels. The processing temperature during the injection moulding varies in the range of 180-200°C. Injection pressure is changed from 250 to 1000 bar. Flow speed is adjusted at two levels of 60and 100 cm3/s. The response measurement is the distribution of the glass bubbles across the spiral at different distances from the injection point. The experimental results show the significance of the nominal Vf, pressure, temperature and flow speed on the distribution homogeneity where the bubbles move further in case of 30% at high pressure of 1000 bar and higher temperature of 200°C that eases the flow. At 10% Vf, the fluctuations in the measured Vf is minimal. The reason of the fluctuation in case of 30% Vf at high temperature and pressure is the multi-phase flow where the investigation shows that bigger size of bubbles acquires greater momentum to flow. Scaling-up of this study to the synthetic fibres will cover fragmentation. More up-scaling to the case of the natural fibre thermoplastic composites NFTC can deal with other factors like thermal stability and fibre splicing or branching.

Milkweed floss fiber/PLA composites: effect of alkaline and epoxy-silanol surface modifications on their mechanical properties

ABSTRACTThe aim of this study was to enhance the mechanical properties of milkweed floss (MW)/polylactic acid (PLA) composites by compatibilizing the interface between MW and PLA. MW floss is a natural fiber exhibiting a wide lumen and a thin cross-section. The addition of high specific surface area MW fibers to polymer matrices can lead to significant improvements of composites’ mechanical performance. However, MW floss presents a poor bonding with PLA. This study evaluates the improvement of the adhesion and, consequently, of the composites’ mechanical properties after MW modification with alkaline treatment or grafting of epoxy-silane. First, the influence of both treatments on the geometry, mechanical properties and roughness of MW fibers was investigated. Injection-molded composites made of 1wt.% treated MW were then characterized. Results showed that the mechanical properties of treated MW significantly decreased. Diminutions of 68 and 60% of Young’s modulus (E) and ultimate tensile stress (UTS) were respectively measured for alkali treated MW (NaOH-MW). Reductions of 30 and 60% were respectively calculated for E and UTS of epoxy-silane treated MW (Si-MW). Still, improvements in the composites’ mechanical properties were measured, such as increases of the UTS of 9 and 17% for PLA reinforced with NaOH-MW and Si-MW, respectively.

Method to determine the required microstructure size to be represented by a second order fibre orientation tensor using X-ray micro computed tomography to evaluate compression moulded composites

The second order fibre orientation tensor has been used to predict the mechanical response of injection and compression moulded composites with the capability to couple mould filling with structural finite element simulations to account for the microstructure. In order to accurately model these materials the mesh size used simulations must be representative of the actual microstructure of the composite. Herein a method to determine the microstructure size required to achieve a near constant value of the orientation tensor based on its derivative is presented. The method is demonstrated with the microstructure of a sheet moulding compound composite which has been evaluated using X-ray micro computed tomography. The average values of the derivative of the orientation tensor and its standard deviation approach constant values with increasing microstructure size. For the material considered a microstructure size of 5 mm square is sufficient to achieve a near constant value of the fibre orientation tensor, which is called a microstructural unit.

Novel approaches to treat preeclampsia: from new drugs to innovative delivery.

The second order fibre orientation tensor has been used to predict the mechanical response of injection and compression moulded composites with the capability to couple mould filling with structural finite element simulations to account for the microstructure. In order to accurately model these materials the mesh size used simulations must be representative of the actual microstructure of the composite. Herein a method to determine the microstructure size required to achieve a near constant value of the orientation tensor based on its derivative is presented. The method is demonstrated with the microstructure of a sheet moulding compound composite which has been evaluated using X-ray micro computed tomography. The average values of the derivative of the orientation tensor and its standard deviation approach constant values with increasing microstructure size. For the material considered a microstructure size of 5 mm square is sufficient to achieve a near constant value of the fibre orientation tensor, which is called a microstructural unit.

Thermal and mechanical properties of injection moulded heat-treated oil palm empty fruit bunch fibre-reinforced high-density polyethylene composites

ABSTRACTOil palm empty fruit bunch (OPEFB) was heat treated at 180°C using a vacuum oven for one hour, extruded and compounded with high-density polyethylene at 10%, 20% and 30% weight fraction. The composites then were injection moulded into dumb-bell shaped specimens. The effect of composition and heat treatment on the thermal properties of composites were investigated using Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC). The tensile and flexural properties were also tested using an Instron Universal Testing Machine. TGA shows an increase in the degradation peak temperature of the heat-treated composites. DSC revealed an increasing trend in the degree of crystallinity (Xc) of the matrix as the heat-treated empty fruit bunch was used as a filler. An increment in the tensile modulus and tensile strain were observed for the treated fibre composites. In addition, the tensile strength value was increased for treated fibre composites with lower fibre loading.

Morphology, thermal and mechanical properties of extruded injection molded kenaf fiber reinforced polypropylene composites

The aim of this study is to explore the effect of coupling agent and surface treatment of natural fibers on the correlations among structural, thermal and mechanical behavior of injection molded composites. Polypropylene (PP)-based composites reinforced with kenaf fiber as a biodegradable filler were fabricated. Kenaf composites were prepared by adding 10, 20, 30, and 40 wt% of kenaf fibers into the matrix using melt mixing and injection molding considered as a simple, scalable, conventional, and efficient process, that is. Different concentrations of NaOH were applied to modify kenaf fibers. The scanning electron microscope (SEM), Fourier Transform Infrared (FTIR), and x-ray Diffraction (XRD) analysis were conducted in order to find the optimum concentration of NaOH for the chemical treatment and to study the morphology of the fabricated specimens. Maleic anhydride polypropylene (MAPP) was employed as the coupling agent. Moreover, the tensile, flexural and impact tests were conducted to evaluate the mechanical properties of the specimens. Results showed that by the incorporation of kenaf fibers of up to 40 wt%, the tensile and flexural strength significantly improved by 100% and 120%, respectively, with respect to those exhibited by the neat PP. It was shown however that the impact strength decreased by around 18%. Moreover, the results of thermal analysis using the differential scanning calorimetry (DSC) showed that the addition of kenaf fibers into the PP matrix increases the melting and crystallization temperature, and the degree of crystallinity. The XRD analysis of composites further confirmed the influence of fibers on the PP crystallization behavior. The results of this work can further extend the use of natural fiber-reinforced PP composites for structural applications such as automotive interior parts.

Improving thermal and mechanical properties of injection moulded kenaf fibre-reinforced polyhydroxy-butyrate composites through fibre surface treatment

Ecofriendly biodegradable composites were developed to mitigate the impact they have on the environment. Composites made from completely biodegradable materials were prepared using kenaf fibres and polyhydroxybutyrate (PHB). The composites were mixed using a twin screw extruder and injection moulded into dumbbell-shaped specimens. The composites were subjected to thermal analysis via differential scanning calorimetry (DSC) and mechanical testing (tensile and flexural tests). The addition of alkali-treated kenaf fibre reduced the crystallinity of PHB (up to 6% reduction), making it more ductile. The rheological behaviour of PHB was modified with the addition of kenaf fibres. Additionally, the flexural modulus was improved by up to 11% with the addition of kenaf fibres, for both the treated and untreated kenaf fibres. However, it was also observed that to improve fibre matrix adhesion, an alkali treatment alone is insufficient, as shown by the 16% reduction of tensile strength in composites with 20% fibre loading.

The influence of matrix crystallinity on the mechanical performance of short-fibre composites – Based on homo-polypropylene and a random polypropylene copolymer reinforced with man-made cellulose and glass fibres

This research shows the influence of the crystallinity of thermoplastic polymer matrix on the mechanical properties of short-fibre composites. It proves that crystal phase affects not only the properties of matrix but predominantly the transfer of stresses to fibres. The injection moulded composites were based on homo-polypropylene and random polypropylene copolymer matrices with glass and man-made cellulose short fibre reinforcements. They were also subjected to annealing to induce growth of crystal phase in the matrices, which was investigated using XRD, DSC and density measurements. The mechanical performance of applied polypropylenes and their composites was compared with annealed counterparts. This comparison shows that matrix crystallinity is important for the mechanical performance of composite and may cause its inconstancy induced by factors like variation of molecular mass distribution, processing conditions and cold crystallisation happening over time. This effect should be heeded at the production, exploitation and research on composites with semicrystalline matrices.

Fiber migration in shear flow: Model predictions and experimental validation

Understanding transient flow behavior of polymer composites is crucial for improving the predictions of final composite properties in polymer processing operations. This work is aimed at measuring concentration migration of fiber reinforcing particles in addition to fiber orientation evolution during simple shear flow. Center‐gate injection molded samples using Nylon‐6 containing 33 wt% short glass fibers (PA6‐33GF) were prepared. Samples were imaged using X‐ray computed tomography (X‐CT) to obtain fiber orientation tensors and fiber volume fraction distributions after different strain units were imposed. With higher applied strains, the orientation of fibers evolved from a shell–core structure typical of injection molded composites to a preferential alignment in the shearing direction. As for fiber concentration, an initial shell–core concentration profile established during injection molding was found to diminish towards a uniform concentration distribution upon shearing. This experimentally observed concentration migration was modeled using the suspension balance model (SBM) which predicts concentration changes based on changes in particle stress contributions. The SBM uses a particle stress contribution model that is designed for spherical particles. Predictions from this model gave reasonably accurate steady state results and the evolution of the concentration profile. A modified fiber stress model that incorporates fiber orientation effects was also tested for its applicability for fiber‐filled systems. Predictions from this model yielded reasonable agreement with measured concentration evolution data but with a slower attainment of a uniform concentration profile. Further investigation on the empirical constants used in the fiber stress model warrants a rheological assessment to improve concentration evolution predictions for use in constitutive rheological modeling. POLYM. COMPOS., 40:3573–3581, 2019. © 2019 Society of Plastics Engineers

Cork-wood hybrid filler system for polypropylene and poly(lactic acid) based injection molded composites. Structure evaluation and mechanical performance

Presented work discusses comparative studies of the use of a hybrid filler system on the properties of polypropylene and poly(lactic acid) composites. Wood flour have been added to improve the initially poor material performance of cork based composites. The hybrid cork/wood filler system used during this study was introduced to polymer system by two step extrusion/injection molding process in the amount of 10, 20 and 30 wt%. Samples were subjected to detailed thermo-mechanical and structural analysis. The investigated hybridization method was successfully implemented, leading to the enhancement of mechanical characteristics comparing to pure cork composites. Density analysis in combination with structural observations, confirmed significant changes in the morphology of cork, including the loss of the cellular structure, due to high shearing of the injection molding process. The favorable results of water absorbency tests indicate the potential use of cork filler to improve the dimensional stability of composites.

Effects of incorporation of polyester on the electrical resistivity of polycarbonate/multi-walled carbon nanotube nanocomposite

In this work, we present the effect of incorporation of polyester on the electrical properties of injection-molded polycarbonate/multi-walled carbon nanotube nanocomposites. The study was conducted by melt-mixing polycarbonate, multi-walled carbon nanotube, and three types of polyesters: polybutylene terephthalate, polyethylene terephthalate, and liquid crystal polymer. It was found that the volume resistivities of injection-molded composites containing 2 phr polyester significantly decreased because of the transesterification reaction between the polycarbonate and polyester. The resulting polycarbonate-polyester random block copolymer kept the conductive networks intact because of the preferential affinity of multi-walled carbon nanotubes with polyester. This study showed that incorporating polyester with polycarbonate–multi-walled carbon nanotube increases the electrical conductivity of injection-molded polycarbonate/multi-walled carbon nanotube nanocomposites to a great extent.

Investigation of Long Cellulose Fibre Reinforced and Injection Moulded Poly(lactic acid) Biocomposites

We investigated injection moulded composites of a polylactic acid matrix reinforced with cellulose fibers. We produced long fiber reinforced granules (preforms) with the use of two technologies: extrusion coating and film stacking. We examined the effect of fiber reinforcement and manufacturing technology on the properties of the composites. 30 wt% fiber reinforcement caused an increase in both strength and modulus compared to the reference PLA, and we also managed to improve creep resistance.

Formation of stretched fibrils and nanohybrid shish-kebabs in isotactic polypropylene-based nanocomposites by application of a dynamic oscillatory shear

The hierarchical structures from micro- to nano-scale in polymeric materials can dramatically affect their physical properties. Controlling the formation of hierarchical structures in polymers through processing remains a challenge. We present here a green and convenient polymer processing approach to prepare isotactic polypropylene (iPP)/polytetrafluoroethylene (PTFE) nanocomposites with hierarchical superstructures via recently developed loop oscillating push-pull molding (LOPPM) method. By taking full advantage of intense oscillatory shear, the in situ generated PTFE fibrils are refined into nanoscale and well aligned, serving as the central shish to induce the formation of nanohybrid shish-kebab (NHSK) superstructure with iPP crystalline lamellae (kebabs). The structure evolution of NHSK superstructures was further examined using molecular dynamics simulations. The stretched fibrils can promote the kebabs growing into large domains with a more compact and ordered molecular chain configuration, compared to that in systems without fibrils or with flexible fibrils. The well-aligned fibrils and NHSK superstructure play a crucial role in enhancing properties. The prepared iPP/PTFE nanocomposites containing 3 wt% PTFE achieved 197.9% and 36.0% improvement in impact and flexural strength, respectively, as compared with the conventional injection-molded composites. Meanwhile, the LOPPM iPP/PTFE nanocomposites exhibits excellent resistance to thermal deformation under harsh environment (over 150 °C) while fabricates a hydrophobic surface. These findings provide a facile strategy to produce high-performance polymers from judiciously controlled sample preparations.

Temperature-dependent constant fatigue life diagram for press-formed short carbon fiber reinforced polyamide composite

Fatigue strength of a short carbon fiber reinforced polyamide (SCF/PA) composite fabricated using a press forming technique is examined. Unlike an injection-molded SCF/PA composite, this one exhibits in-plane isotropy as well as through-thickness uniformity. Fatigue tests on the SCF/PA composite are carried out at different stress ratios and temperatures, respectively. The fatigue test results show that the S–N relationship for the SCF/PA composite significantly depends on stress ratio, and fatigue degradation occurs most rapidly at the critical stress ratio, regardless of test temperature. The fatigue stress for a given life decreases with temperature. The reduction in fatigue strength with temperature correlates well with the reduction in static strength with temperature. Accordingly, the effect of temperature on fatigue life for each of different constant values of stress ratio can approximately be removed by normalizing fatigue stress with respect to static strength. The full shape of constant fatigue life diagram for the press-formed SCF/PA composite is identified for each of different test temperatures. It is shown to incline toward the direction of positive mean stress, regardless of test temperature. A difference in shape of CFL diagram between the press-formed and injection-molded composites is addressed. This study demonstrates that the anisomorphic constant life diagram approach with temperature as the parameter allows adequately predicting the constant fatigue life diagram and thus the S–N curve for the SCF/PA composite over the whole range of mean stress at any temperature in the tested range.

The effect of kenaf filler reinforcement on the mechanical and physical properties of injection moulded polypropylene composites

Natural fibres potentially offer better reinforcement to improve the mechanical and physical properties of polymer composites. However, these natural materials at this stage are not fully explored yet due to the fibres themselves have limited heat resistance and are quite sensitive to moisture. This limitation will weaken the adhesion when interacting with thermoplastic matrices during the processing of composites. Therefore, the main purpose of this study is to investigate inherent strength characteristics among kenaf (core and bast) fillers as a reinforcement in polypropylene composites at various geometries and loadings via the injection moulding process. The composite materials consisted of kenaf with the geometric core filler of the 20 mesh (992 μm), 40 mesh (460 μm) and bast filler (166.9 μm) were mixed with polypropylene based on the filler loadings of 10 up to 40 wt. %. The results showed that bast filled composites had the highest tensile strength of 19.52 MPa at 30 wt. %, compared to core filled composites. Instead, 20 mesh core filled composites were obtained had the highest flexural strength which values were 25 MPa and 29 MPa at 20 wt. % and 30 wt. %, respectively. While 40 mesh core filled composites had the highest values of 25.35 MPa at 40 wt. % of filler loading compared to bast filled composites. SEM micrograph images showed the good interfacial bonding of core filler which surrounded by PP leading to diffusion and permeation of bonding. In conclusion, the use of kenaf (core and bast) fillers as a reinforcement in composite materials is reasonable to maximise the use of fibre from natural sources.