Artigo publicado a convite da Society for Polymer EngineersThe behavior of pristine and functionalized particles within polypropylene was investigated with the use of a prototype small-scale mixer.Matepro (NORTE-07-0124-FEDER-000037); Foundation for Science and Technology (project PEst-C/CTM/LA0025/2011); PhD grant FRH/BD/87214/201

Covas, J. A.

Cunha, Eunice Paula Freitas

Hilliou, L.

Paiva, M. C.

Universidade do Minho: RepositoriUM

10.2417/spepro.006072Dispersion of graphitenanoplates duringmelt mixingJose´ Covas, Eunice Cunha, Maria C. Paiva, and Loic HilliouThe behavior of pristine and functionalized particles within polypropy-lene was investigated with the use of a prototype small-scale mixer.Graphite nanoplates (GnPs) are exfoliated graphite particles in the formof thin flakes—with large surface areas and nanosized thicknesses—that tend to agglomerate because of van der Waals interactions. GnPsare commonly used to reinforce composite materials. It is expectedthat maximum composite performance levels should occur when GnPagglomerates are dispersed within the polymer matrix as primary 2Dparticles and they have good interfacial adhesion. Interfacial bondingbetween the GnPs and the polymer can be improved by tailored chem-ical functionalization of the graphite surfaces.1–3 Dispersion of GnPagglomerates in polymers, however, is complex.Melt compounding is an economically attractive process that is usedsuccessfully in industrial applications to prepare polymer composites.Previous studies of melt compounding with carbon nanofibers4 and car-bon nanotubes5, 6 used a variety of mixing equipment (e.g., extruders,internal mixers, and prototype mixers). These investigations demon-strated that the intensity and type of hydrodynamic stresses, residencetime, and interfacial adhesion play a major part in nanoparticle dis-persion. Although the dispersion mechanism of carbon nanotubes inpolymers is well studied, the equivalent GnP literature is still mostlyfocused on composite properties and applications.7 Few studies con-cerning the dispersion of GnPs through melt mixing methods have sofar been reported.We have monitored the progression of dispersion for ‘as-received’(i.e., pristine) and functionalized GnPs within a polypropylene (PP)matrix to investigate this complex process.8 Our prototype small-scalecontinuous mixer, which we used for our experiments, is shown inFigure 1. With our device we create repetitive converging/divergingflow sequences along its length. As such, a strong extensional stresscomponent is generated. We are also able to precisely control flowrate and temperature.4, 5 Once the experiments are completed, we canquickly dismantle the equipment so that the material samples at the var-ious axial locations can be collected for subsequent characterization.We obtained the functionalized graphite nanoplates (F-GnPs) throughFigure 1. Schematic diagram of the prototype small-scale continuousmixer. : Diameter.covalent functionalization of the GnPs9 and through grafting with PPthat was modified with maleic anhydride.We monitored how the pressure drop varied with the number of ringpairs (see Figure 1) that we used in the mixer. We conducted these testsfor PP, PP with 2wt% GnP, and PP with 2wt% F-GnP matrices. Forall these materials, our results exhibit a linear pressure increase withthe number of pairs of rings. This behavior demonstrates that append-ing pairs of rings has an additive effect, i.e., there is no stress build-up in the mixer, nor any significant structural orientation at the testedshear rate. Adding pairs of rings is therefore equivalent to increasing themixing time. We constructed shear–flow curves from the pressure dataand equations for capillary rheometry.10 We find that the pressure dropshould predominantly be caused by flow along smaller channels (i.e.,those with a radius of 0.5mm) and the apparent length (which dependson the number of stacked pairs of rings). The flow curve we obtainedfor the PP matrix—as shown in Figure 2(a)—closely matches the curvethat is calculated using conventional rheometry. Our approach for ob-taining rheological information has thus been validated. We also findthat the viscosity of the PP material is similar to that of the compositesmade with 2wt% GnP and F-GnP, which is relevant for extrusion andContinued on next page10.2417/spepro.006072 Page 2/3compounding. We measured the extrudate swell (when polymeric ma-terial is forced through a die) of the filaments—computed as the ratiobetween their diameter and the diameter of the die orifice—for differ-ent shear rates. As expected, we observe—see Figure 2(b)—that theextrudate swell increases with increasing shear rate for all our materi-als. In addition, the surface of the extruded material remained smoothand rheological defects were absent over the range of applied shearrates. We also observe (as for other polymer composites that containdifferent filler types)11–14 that the addition of GnPs causes a decreasein extrudate swell.We have also monitored the progression of GnP and F-GnPdispersion along the mixer, using a setup that consisted of eightpairs of stacked rings. We forced the composites—with 2wt% ofnanoparticles—through the mixer at 500 and 2000s 1, whereas thecomposites that contained 4wt% of nanoparticles were only tested at500s 1. We defined an agglomerate area ratio (AR) as the ratio of thetotal area of all the surviving agglomerates to the total area of the an-alyzed composite.4, 6 The highest dispersion levels thus correspond tothe lowest computed AR values. Our AR results—shown in Figure 3(a)and (b)—reveal that the dispersion of GnPs develops gradually alongthe mixer. The dispersion of F-GnPs, however, appears to level off forsamples that were collected between the fourth and sixth ring pairs.The AR values of the latter are systematically higher than those fortheir pristine counterparts. We observe these trends consistently alongthe mixer for the 2 and 4wt% GnP and F-GnP compositions.Dispersion of GnPs in solid suspensions—as with polymer blends—may occur when sufficient stress levels and residence times areattained.15 In our experiments, higher shear rates promoted greaterstress levels at the cost of residence time (i.e., 202s for a shearrate of 500s 1 and 54s for a shear rate of 2000s 1). We find thatFigure 2. Shear flow behavior of polypropylene (PP) and PP compos-ites, made with 2wt% functionalized graphene nanoplates (F-GnP) andnon-functionalized graphene nanoplates (GnP). Shear flow curves areshown in (a). The variation of the extrudate swell with shear rate, forPP and the PP nanocomposites, is shown in (b). These measurementswere conducted using the prototype mixer equipped with six pairs ofrings.Figure 3. Measures of the progression of GnP dispersion in the PPmatrix along the mixer. The area ratio (AR) variation for the compos-ites with (a) 2wt% GnP and F-GnP and (b) 4wt% GnP and F-GnPis shown. Scanning electron images of cross sections through thenanocomposites are also shown, for the composites reinforced with (c)2wt% GnP and (d) 2wt% F-GnP. These materials were collected fromthe eighth pair of rings in the mixer.residence time is therefore the dominant factor under the conditionswe tested. In addition, higher levels of graphite incorporation cause anincrease in AR. The evolution of the dispersion indices, however, isqualitatively (and in some cases quantitatively) similar to that of theless-filled composite. Scanning electron microscope images of exam-ple cryo-fractured samples, with 2wt% GnP and F-GnP, are shown inFigure 3(c) and (d), respectively. These images demonstrate that thereis improved interfacial adhesion of F-GnPs within the PP matrix com-pared with the non-functionalized GnP composite.We have conducted a set of experiments to test how pristine andfunctionalized graphene nanoplates disperse within a polypropylenematrix. Our work demonstrates the importance of selecting and apply-ing effective stress levels and residence times to optimize GnP disper-sion and achieve maximum composite performance levels. In our futurework we will explore the dispersion of GnPs in polymer matrices un-der different flow conditions, as well as the effect of melt relaxation onnanoparticle dispersion and re-agglomeration. We will also comparethe GnP dispersion that we achieve by twin-screw extrusion process-ing with that achieved using the prototype equipment described in thiswork.Continued on next page10.2417/spepro.006072 Page 3/3The authors acknowledge financial support provided by Matepro(NORTE-07-0124-FEDER-000037), the Foundation for Science andTechnology (project PEst-C/CTM/LA0025/2011), and PhD grantFRH/BD/87214/2012.Author InformationJose´ Covas, Eunice Cunha, Maria C. Paiva, and Loic HilliouInstitute for Polymers and CompositesUniversity of MinhoGuimara˜es, PortugalJose´ Covas is a full professor in the Department of Polymer Engi-neering. His scientific interests include polymer processing and com-pounding. He has published more than 170 papers in international peer-reviewed journals, several book chapters, and has filed nine patents. Heis an associate journal editor for International Polymer Processing, andhe works closely with numerous companies in Portugal and around theworld.Eunice Cunha has a degree in chemistry and an MSc in micro- andnanotechnologies. She is currently a PhD student, working on the pro-duction of graphene-based polymer composites.Maria Paiva is an assistant professor in the Department of Polymer En-gineering. From 2000 to 2003 she was a visiting assistant professor atClemson University, SC. Her research interests include the modifica-tion of carbon surfaces and interfaces, as well as nanocomposite prepa-ration and optimization techniques.Loic Hilliou is a researcher in the Department of Polymer Engineer-ing. His research focuses on the relationships between process-inducedstructures and material properties. He has a special interest in the rhe-ological properties of polymer nanocomposites, as well as natural andbiodegradable polymer-based materials.References1. H. Bai, C. Li, and G. Shi, Functional composite materials based on chemically con-verted graphene, Adv. Mater. 23, p. 1088, 2011. doi:10.1002/adma.2011900252. J. Ma, Q. Meng, I. Zaman, S. Zhu, A. Michelmore, N. Kawashima, C. H. Wang, and H.-C. Kuan, Development of polymer composites using modified, high-structural integritygraphene platelets, Compos. Sci. Technol. 91, pp. 82–90, 2014.3. M. Naebe, J. Wang, A. Amini, H. Khayyam, N. Hameed, L. H. Li, Y. Chen, and B.Fox, Mechanical property and structure of covalent functionalised graphene/epoxynanocomposites, Sci. Rep. 4, p. 4375, 2014. doi:10.1038/srep043754. R. M. Novais, J. A. Covas, and M. C. Paiva, The effect of flow type and chemicalfunctionalization on the dispersion of carbon nanofiber agglomerates in polypropylene,Compos. Part A: Appl. Sci. Manuf. 43, pp. 833–841, 2012.5. S. Jamali, M. C. Paiva, and J. A. Covas, Dispersion and re-agglomeration phenomenaduring melt mixing of polypropylene with multi-wall carbon nanotubes, Polym. Test.32, pp. 701–707, 2013.6. G. R. Kasaliwal, S. Pegel, A. Go¨ldel, P. Po¨tschke, and G. Heinrich, Analysis of agglom-erate dispersion mechanisms of multiwalled carbon nanotubes during melt mixing inpolycarbonate, Polymer 51, pp. 2708–2720, 2010.7. H. Kim and C. W. Macosko, Processing-property relationships of polycarbon-ate/graphene composites, Polymer 50, pp. 3797–3809, 2009.8. E. Cunha, M. C. Paiva, L. Hilliou, and J. A. Covas, Tracking the progression of disper-sion of graphite nanoplates in a polypropylene matrix by melt mixing, Polym. Com-pos., 2015. doi:10.1002/pc.236579. M. C. Paiva, F. Simon, R. M. Novais, T. Ferreira, M. F. Proenc¸a, W. Xu, and F. Besen-bacher, Controlled functionalization of carbon nanotubes by a solvent-free multicom-ponent approach, ACS Nano 4, pp. 7379–7386, 2010.10. C. W. Macosko, Rheology: Principles, Measurements, and Applications, p. 568,Wiley, 1994.11. K. 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Dispersion of graphite nanoplates during melt mixing

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Universidade do Minho: RepositoriUM

Universidade do Minho: RepositoriUM