Nanoclays can improve the performance of injection-molded polypropylene components likely to be subjected to impact in servic

Frontini, P. M.

Pettarin, Valeria

Pouzada, A. S.

Universidade do Minho: RepositoriUM

10.2417/spepro.004495Making an impact withnanocompositesPatricia Marı´a Frontini, Valeria Pettarin, and Anto´nio PouzadaNanoclays can improve the performance of injection-molded polypropy-lene components likely to be subjected to impact in service.Polymer nanocomposites are new materials that show great potentialin a variety of applications. By incorporating nanoscale particles of afiller material, the performance and properties of the bulk material canbe drastically altered. Indeed, quite exceptional improvements can beachieved with small amounts of filler.1, 2 For example, polypropylene(PP), when filled with less than 5wt% of clay nanoparticles (nanoclay),is suitable for engineering applications.3–5 Optimum performance re-lies on good dispersion of the clay nanoparticles and separation (exfo-liation) of the clay layers within the nanoparticles.6–8To make nanocomposites economically viable, their production mustuse common processing equipment, and additional processing stepsshould be avoided. It is possible to achieve this processing requirementby mixing a masterbatch of pure polymer with a nanoclay-polymer mixduring injection molding.9 We have used this method to prepare PP-nanoclay nanocomposites and have studied the performance of thesenanocomposites in impact force tests. Our results show the effect of themolding process and of the nanoclay filler on the properties of PP. Inparticular, we investigated the material’s structure and properties at theweldline, formed where the two injection flows meet.10Rectangular boxes, 1.4mm thick, were injection molded using a hotrunner mold with two injection points (see Figure 1, top). Emulating theexpected industrial practice,9 pure PP was mixed with various amountsof a commercially available masterbatch (MB) of PP containing 50% oforganoclay, and processed at 235ıC. Transmission electron microscopyimages of the resulting blocks show that the clay platelets are uniformlydispersed, without signs of aggregation, and are oriented in the direc-tion of the polymer flow (see Figure 2).The impact properties of the moldings were assessed by uniaxialtensile and biaxial flexural tests. The former were carried out on rec-tangular samples taken from the weldline area and on another sample,taken from an area away from the weldline and aligned with the injec-tion flow direction (see Figure 1, bottom). The biaxial impact tests werecarried out on circular samples that were also taken at, and away from,the weldline area. The amount of energy required to cause the samplesFigure 1. Molds (top) and location of test specimens (bottom). Therectangular sections indicate the location of the three weldline (wl)samples (wl1–3) and the bulk sample (wwl) used in tensile testing. Thecircular sections denote the weldline (WL) and bulk (F) samples usedin flexural testing.to fail was measured. This analysis gives a realistic view of in-serviceimpact situations.11, 12The two sets of test data give different information on the prod-uct’s toughness (see Figure 3). In tensile testing, the weldline samplesContinued on next page10.2417/spepro.004495 Page 2/4generally absorbed less energy before failing than the sample taken ata distance from the weldline. This is because, when the two injectionflows meet, the polymer chains and the nanoclay platelets (if present)are forced to align themselves parallel to the weldline, and are thus per-pendicular to the applied impact force. Moving away from the weldlinein the flow direction, the sample strength increases, in agreement withprevious results.13 Weldlines formed at higher temperature and pres-sure showed better impact performance. In the uniaxial tests, the inclu-sion of nanoclay increased the toughness of the material at points awayfrom the weldline.In contrast, in biaxial testing on pure PP the weldline zone samplewas tougher than the sample taken near the injection point.14 The fail-ures originate at the weakest point and propagate due to radial and hoopstresses.15 Fracture patterns at the weldline and close to the injectionpoints are significantly different (see Figure 4). In pure PP samples im-pacted near the injection points, there is bending along the clampingring and a single split occurs in the melt flow direction. Conversely,in fractures at the weldline zone cracks run radially from the point ofimpact, while others follow a circular path around the same point, withevidence of plastic deformation. The inclusion of nanoclay did not im-prove the strength of the bulk sample relative to the weldline, with neg-ligible differences in impact toughness between the weldline and thebulk for these samples.In general, incorporating nanoclay improves PP’s impact toughness,with best results for 3–5% nanoclay content. The nanoclay tougheningeffect is thought to be the result of the polymers that are intercalatedwithin the host nanoparticle galleries having conformations differentfrom those in the bulk, and also the mobility of the nanoparticles.16Figure 2. Transmission electron microscopy images of nanoclay dis-persion in the polypropylene (PP) nanocomposite containing 6% of thepolymer-nanoclay masterbatch.Figure 3. Impact energy as a function of location and nanoclay content(%MB) for (a) tensile loading and (b) biaxial loading.The formation of crazes is also important: as the matrix absorbs en-ergy on deformation, nanosized free surfaces are necessary to initiatecrazes. The delamination of clay particles provides those surfaces.17 Inthe case of biaxial impact tests, with in-plane stresses, these toughen-ing effects are negligible, and fractures propagate in the direction ofnanoclay orientation.Weldlines are detrimental to the impact performance of PP-nanoclaycomposites due to their effect on polymer and nanoparticle orienta-tions. In tensile testing, their orientation at the weldline is perpen-dicular to the applied force, whereas away from the weldline thepolymer molecules and clay platelets are oriented with that force. Un-der flexural testing, the moldings are more ductile and tougher closeContinued on next page10.2417/spepro.004495 Page 3/4Figure 4. Failure under biaxial impact loading of PP and nanocom-posite (PP with 2% MB) moldings.to the weldlines than away from them. At the weldline, the weaknesscaused by the molecular orientation does not affect the nanocompositetoughness, and the difference between failure energies at the bulk andweldline zones is negligible.The best impact performance was observed in moldings with 3% ofnanoclay. With higher contents, as in microcomposites, intercalation isreduced and the tactoids act negatively as defects. It is clear that un-der biaxial stresses, nanoclay inclusion does not improve PP’s impactperformance. Future work will investigate nanocomposites based ona blend of PP with a thermoplastic elastomer as the matrix, and howorganoclay exfoliation in these materials leads to significant improve-ment in stiffness and impact resistance.Author InformationPatricia Marı´a Frontini and Valeria PettarinInstitute of Materials Science and TechnologyUniversity of Mar del PlataArgentinaPatricia Marı´a Frontini is professor of mechanical behavior of poly-mers and composites. Her current research includes fracture mechanicsand constitutive modeling, optimization of injection molding parame-ters, and nanomechanics of plastics, biomaterials, and composites. Shehas published several articles in SPE journals and has contributed sev-eral papers to ANTEC conferences.Valeria Pettarin is professor of macromolecular chemistry. Her cur-rent research includes optimization of injection molding parameters,fracture mechanics, and tribological behavior of plastics, biomaterials,and composites.Anto´nio PouzadaInstitute for Polymers and CompositesUniversity of MinhoGuimara˜es, PortugalAnto´nio Se´rgio Pouzada was professor of polymer design at the Univer-sity of Minho until 2010. He now coordinates research at the Institutefor Polymers and Composites, Portugal. His current research includesthe design of plastics, micro-injection molding, and tribological aspectsin injection molding. He joined SPE in 1995 where he is now a seniormember and has, with his co-workers, participated in and contributedto various ANTEC conferences.References1. S. Sinha Ray and M. Okamoto, Polymer/layered silicate nanocomposites: a re-view from preparation to processing, Prog. Polym. Sci. 28, pp. 1539–1641, 2003.doi:10.1016/j.progpolymsci.2003.08.0022. M. Alexandre and P. 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Making an impact with nanocomposites

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Making an impact with nanocomposites

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