LIGHT WEIGHT MULTIFUNCTIONAL COMPOSITES

LIGHT WEIGHT MULTIFUNCTIONAL COMPOSITES WITH ENHANCEDMECHANICAL PROPERTIESDiego Pedrazzoli, G. W. Woodruff School of Mechanical Engineering - Georgia Institute of Technology, Atlanta, GA & Department of Industrial Engineering, University of Trento, Trento, Italy&Alessandro Pegoretti, Department of Industrial Engineering, University of Trento, Trento, ItalyKyriaki Kalaitzidou, G. W. Woodruff School of Mechanical Engineering & School of Materials Scienceand Engineering - Georgia Institute of Technology, Atlanta, GAAbstractIn this study, polypropylene (PP) composites reinforced with short glass fibers (GF) and expanded graphite nanoplatelets (xGnP) were produced by melt compounding and injection molding. Quasi-static tensile tests and morphological observations were carried out in order to investigate how the morphology and the mechanical properties of the composites were affected by the combined effect of two fillers of rather different size scales (i.e. micro- and nano- scale). The results indicate that it is possible to introduce the nano-materials at the GF-PP interphase and significantly improve the tensile modulus of the composites, leading to lighter and stronger composites, as part of the higher density GF can be replaced with a small amount of the nano-materials. In addition to decreasing the weight of the composite, the processability is significantly improved as the increase in polymer viscosity reduces with decreasing the GF content. In conclusion, the results lead to hybrid composites that combine the advantages of nano-materials and micro-size reinforcements.IntroductionFiber reinforced polymer composites have been widely used in structural applications such as aerospace, automotive, civil and marine structures, especially due to their high specific stiffness and strength, chemical and weather resistances, tailorable mechanical, thermal and electrical properties [1,2]. In most cases, high loadings of GF are required to achieve the desired performance; this leads to an undesirable increase in specific gravity, decreased melt flow, and increased brittleness [3]. Short-fiber reinforced composites have been recently applied in a growing number of applications in engineering and consumer products in order to improve or tailor certain thermo-mechanical properties of the matrix for specific applications or to reduce the cost of the article.In general, short-fiber reinforced composites are much less resistant to mechanical load and fatigue damage than the corresponding continuous-fiber-reinforced materials, mainly because the weak matrix has to sustain a greater proportion of the load [5]. On the other hand, polymer nanocomposites based upon nanofillers such as silica, expanded graphite and carbon nanotubes have recently attracted great interest due to the considerable enhancement in stiffness realized at very low filler loadings [6,7]. Moreover, substantial improvements in mechanical, thermal, and flammability properties have been reported, while maintaining similar density and optical properties to those of the neat polymer matrix [8,9]. Furthermore, there are some clear indications that nanoparticles could play a beneficial role on the interfacial properties of structural composites [10,11].Although polymer nanocomposites exhibit improved thermal and mechanical properties at very low filler contents, loadings of more than 10 wt% usually lead to poor dispersion and processing characteristics [11]. Nevertheless, glass fiber loadings of 30 to 50 wt% are quite common [12]. It is of great interest to explore whether combining two fillers of rather different size scales (i.e. micro- and nano- scale) would give the desired performance at low to intermediate filler loadings. Only few preliminary studies have been reported on the structure and properties of glass fiber reinforced polymer nanocomposites [13,14]. The aim of this study is to investigate how the morphology and the mechanical properties of short-glass fiber reinforced PP nanocomposites are affected by the combined effect of nanofiller and reinforcing fibers.Materials and Composite FabricationThe matrix of the composite samples used in this work was an isotactic homopolymer polypropylene (MFI of 6.9 g/10’ at 190 °C and 2.16 kg ; density=0.904 g·cm-3) produced by Polychim Industrie S.A.S. (LOON-PLAGE, France) and provided by Lati Industria Termoplastici S.p.A (Varese, Italy) with the commercial code PPH-B-10-FB. FUSABOND®P M-613-05 maleic anhydride modified polypropylene (PPgMA) (MFI at 190 °C and 2.16 kg = 106.8 g/10’, density=0.903 g·cm-3, with maleic anhydride content in the range of 0.35 - 0.70 wt%), was supplied by DuPont™ de Nemours (Geneva, Switzerland).Exfoliated graphite nanoplatelets xGnP®-M5 have been supplied from XG SCIENCE Inc. (East Lansing, USA). This filler is platelets with an average diameter of ~5 µm and thickness in the range of 10~20 nm.E-glass fibers, designed as RO99 P319, were supplied by Saint-GobainVetrotex (Chambèry Cedex, France) and were used as-received. These GF are indicated as treated with a silane based coupling agent specifically designed for polypropylene matrices. Chopped strand glass fibers (single fiber diameter of 15.3 ± 1.5 µm, and average length of 6.50 ± 0.44 mm) were obtained by chopping long glass fibers using a chopper gun CDA-08 provided by GlasCraft (Graco®, Bury, England).xGnP-coated glass fibers were prepared by sonication of xGnP-M5 in isopropanol with a filler concentration of 5 mg/ml. Sonication was carried out using a Misonix S-4000-010 (Farmingdale, NY) for 1 h (30% amplitude, 8 W power) equipped with a probe of 12.5 mm diameter. After adding the glass fibers to the solution, a second sonication was performed for ½h. Coated glass fibers were finally rinsed in isopropanol and left under hood overnight to let the solvent completely evaporate. The xGnP content added onto the fibers’ surfaces through sonication was measured around 0.1-0.2 wt%.Composite samples were produced by melt mixing and injection molding. A vertical, co-rotating, bench-top twin-screw micro-extruder (DSM Micro 15 cm3 Compounder) connected to a micro-injection molding unit (DSM) were used to obtain dogbone specimens. The compound was mixed for 3 min, at 190 °C and a screw speed of 250 rpm. After the polymer compound had melted and homogenized, short GF strands were added to the melt and further mixing for 2 min and final injection moulding were carried out. The temperature of the mold was 80 °C, while the injection molding pressure was about 800 KPa. Ternary nanocomposites were prepared by adding 5 wt% of PPgMA as a compatibilizer to the systems containing 5 wt% of nanofiller.Composites were designated indicating the matrix, the compatibilizer (if any) with its content, the kind of filler with its amount. For instance, the composite based on unfilled matrix loaded with 10 wt% GF was denoted as PP/GF-10. On the other hand, a sample filled with 5 wt% of PPgMA, 5 wt% of xGnP-M5 and 10 wt% GF was indicated as PP-PPgMA-5-xGnP-5/GF-10. Coated glass fibers were indicated as GFc.Experimental techniquesFracture surfaces of nanocomposite samples were observed at various magnifications by using a Phenom G2 Pro (Phenom-World BV., Eindhoven, The Netherlands) bench-top scanning electron microscope (SEM), at an acceleration voltage of 5 kV. Prior to the SEM observations, a thin gold coating was applied on the surface by plasma sputtering in order to minimize the charging effects.Uniaxial ramp tensile tests were performed according to ASTM D638 with an Instron model 33R 4466 (Norwood, USA) tensile tester equipped with a 500 N load cell, on samples consisting of at least five dogbone specimens. Tests were carried out at a crosshead speed of 5 mm/min. Axial strain was recorded by using a resistance extensometer Instron® model 2630-101 with a gauge length of 10 mm. The elastic modulus was measured as secant modulus between longitudinal deformation levels of 0.05 % and 0.25 %.Dynamic mechanical analyses (DMA) were carried out at a DMA Q800 (TA Instruments, New Castle, Delaware, USA) testing machine over a temperature range between -20 °C and 160 °C, imposing a heating rate of 5 °C/min and a frequency of 1 Hz. A preload of 0.2 MPa and a maximum strain of 0.05 % were imposed on rectangular samples 25 mm long, 3.30 mm wide and 3.27 mm thick. The most important viscoelastic functions (E’, E’’, tan(δ)) were recorded at different temperatures.Results and discussionAs the amount of xGnP increases both tensile modulus (E) and ultimate tensile strength (UTS) increase, as reported in Figure 1, while the elongation at ultimate tensile strength (εUTS) decreases. This trend is consistent with observations for adding fillers to a relatively brittle matrix. In addition, the higher yield strength exhibited by the hybrid samples with respect to the sample PP/GF-10, reflects the greater interfacial sheat strength between fiber and matrix which promotes a better stress transfer between matrix and fiber.Figure 1. Elastic modulus of hybrid PP compositesThe superiority of hybrid composites with respect to just GF composites (i.e. PP/GF-10 and PP/GF-30), see Figure 2, is more evident when the specific properties are taken into account (specific elastic modulus and the specific ultimate tensile strength, E* and UTS*, respectively). In particular, the values of UTS* exhibited by hybrid composites greatly overcome those of PP/GF-10 andPP/GF-30.Figure 2. Specific Elastic modulus of hybrid PPcompositesHybrid composites are thus lighter and stronger than GF composites. Interestingly, the composites of coated GF i.e. PP/GFc-10 and PP-xGnP-5/GFc-10 exhibit slightly higher E* and UTS* compared to PP/GF-10 and PP-xGnP-5/GF-10 composites respectively, where the graphite was added directly in the polymer instead of being introduced at the GF-polymer interface, probably indicating an enhanced adhesion between fiber and matrix. Improved interfacial properties can also be observed when the compatibilizer PPgMA is added to the matrix (i.e. PP-PPgMA-5-xGnP-5), as the UTS is significantly higher that of the PP-xGnP-5 composite that does not contain compatibilizer. Concurrently, the incorporation of xGnP in composites produces a substantial increase in glass transition temperature (T g), as evaluated at the tan(δ) peak during DMA experiments (Figure 3). Since changes in T g are related to the primary relaxation of polymer chains and the extent of the immobilized chains, the alteration of polymer chain mobility promoted by polymer-xGnP physical interactions might significantly contribute to the reinforcing mechanisms. Therefore xGnP reinforces PP not only because it is stiffer but also because it remarkably alters locally the physical properties of the polymer.In the same way, lower values of tanδrecorded upon nanofiller addition in hybrid composites can be attributed not only to the enhanced fiber-matrix adhesion, but also to polymer-xGnP physical interactions, resulting in lower loss modulus and enhanced elastic modulus of PP. Figure 3. Thermomechanical properties of the composites SEM micrographs of fracture surfaces for PP composites loaded with 10 wt% GF are shown in Figure 4a. The glass fibers are generally well dispersed in the PP matrix, and many fibers are pulled out from the matrix. The interfacial debonding appears to be the dominant failure mechanism, indicating low-adhesion condition. On the other hand, a different failure behavior is observed for hybrid PP composites that contain 5 wt% xGnP and 10 wt% glass fibers (Figure 4b) and 5 wt% PPgMA, 5 wt% xGnP and 10 wt% glass fibers (Figure 4c). In this case, a matrix crack occurred, showing a significantly better fiber-matrix adhesion. Furthermore, the substantial chemical affinity between fiber and matrix is evidenced by the presence of residual matrix on the fiber surface after the pull-out.(a)(b)(c)Figure 4. SEM micrographs of (a) PP/GF-10, (b) PP-xGnP-5/GF-10 and (c) PP-PPgMA-5-xGnP-5/GF-10.ConclusionsIn this study, the mechanical properties of the hybrid GF/PP composites are determined as a function of the graphite nanoplatelets content, and physical properties of PP including glass transition, as well as the dispersion of the nano-materials within the polymer, are investigated. 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