长玻璃纤维增强聚丙烯的反常流变行为【中文4390字】
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长玻璃纤维增强聚丙烯的反常流变行为【中文4390字】,玻璃纤维,增强,聚丙烯,反常,流变,行为,行动,中文
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2012 The Korean Society of Rheology and Springer 307 Korea-Australia Rheology Journal, Vol.24, No.4, pp.307-312 (2012)DOI: 10.1007/13367Anomalous rheological behavior of long glass fiber reinforced polypropyleneDong Hak Kim1, Young Sil Lee2and Younggon Son3,*1Department of Chemical Engineering, Soonchunhyang University, Asan, Chungnam 336743, Republic of Korea2FD Group, Cheil Industries Inc, 332-2 Gocheon-dong, Uiwang, Gyeonggi 437-711, Republic of Korea3Divsion of Advanced Materials Science and Engineering, College of Engineering, Kongju National University,Cheonan, Chungnam 331717, Republic of Korea(Received April 26, 2012; final revision received October 12, 2012; accepted October 15, 2012)Dynamic rheological properties of PP-based long glass fiber-reinforced thermoplastics (LFT) were inves-tigated. Weight fractions of the glass fibers investigated in the present study ranged from 0.15 to 0.5, whichare higher than those of previous studies. We observed very abnormal rheological behavior. Complex vis-cosity (*) of the LFT increased with the glass fiber content up to 40 wt. %. However, the * with a weightfraction of 0.5 is observed to be lower than that of LFT with a weight fraction of 0.4 in spite of higher glassfiber content. From various experiments, we found that this abnormal behavior is analogous to the rheo-logical behavior of a lyotropic liquid crystalline polymer solution and concluded that the abnormal rheo-logical behavior for the LFT is attributed to the formation of a liquid crystal- like structure at highconcentrations of long glass fibers.Keywords: long glass fiber reinforced thermoplastic, polypropylene, abnormal rheological behavior 1. IntroductionLong fiber reinforced thermoplastics (LFT) have beenattracting great attention because they provide bothincreased stiffness and toughness whereas short fiber rein-forced thermoplastics (SFT) provide increased stiffness butdecreased toughness (Thomason and Vlug, 1996). LFTmanufacturers generally use a pultrusion process to man-ufacture LFT, which entails pulling continuous fiber rovingsthrough a polymer melt in a specialized processing die. Theresulting composite strands are cooled and chopped intopellets. Thus, the length of fiber in LFT is same with that ofchopped pellets and it is approximately one order of mag-nitude longer than that of the short glass fiber (GF). Due tothe longer fiber length, the rheological behaviors of LFT areconsequently much different from those of SFT. The rheological properties of fiber reinforced compos-ites are affected by many factors including, volume frac-tion of fiber, aspect ratio of fiber, fiber alignment, surfaceconditions of the fiber, etc (Han, 1981; Park at al., 2011;Tucker et al., 1994; White, 1990). Among these prop-erties, the volume fraction of fiber has most profoundeffect on the viscosity of fiber-reinforced thermoplastics.The steady shear viscosity (and/or storage and loss mod-ulus) of fiber-reinforced thermoplastics generally increaseswith the volume fraction of fiber at lower shear rates (and/or frequency), while the viscosity at higher shear rates isnot significantly affected by the volume fraction of GF(Greene et al., 1995).The fiber orientation also exhibits a large effect on therheological prosperities of the fiber-reinforced thermo-plastics. It has been observed that the orientation of thefiber affects an overshoot of the shear stress and normalstress in a time dependent start-up experiment (Laun,1984). The sample having an initial fiber orientation thatis perpendicular to the flow direction showed the greatestovershoot, while the sample with a initial fiber orientationparallel to the flow direction showed the lowest overshoot.A randomly orientated sample lied between these twoextreme cases (Laun, 1984). This observation is more pro-nounced when the aspect ratio and the volume fraction ofthe fiber are increased (Laun, 1984). Pre-shearing a sample before the dynamic test affectsthe complex viscosity of the fiberreinforced thermoplas-tics (Kim et al., 1997). Pre-shearing increases the orien-tation of the fiber to the flow direction, and thus thecomplex viscosity of the composite decreases with the ori-entation induced by the pre-shearing (Kim et al., 1997).The orientation of the fiber affects not only the rheologicalproperties but also the mechanical properties of the fiber-reinforced thermoplastics. Therefore, many researchgroups have dedicated their efforts to establish a model forthe relationship between the fiber orientation and pro-cessing conditions (Advani et al., 1990; Dinh et al., 1984;Ranganathan et al., 1993). Most of the studies on the rheological properties offiber-reinforced thermoplastics noted above have beenconducted with SFTs, and these studies were limited tolow volume fractions of fiber. Studies on volume fractions*Corresponding author: sonygkongju.ac.krDong Hak Kim, Young Sil Lee and Younggon Son308 Korea-Australia Rheology J., Vol. 24, No. 4 (2012)higher than 0.2 are very rare.There have been few studies on the rheological prop-erties of LFT. Bush et al. (Bush et al., 2000) and Servaiset al. (Servais et al., 2002) developed an improvedsqueeze flow rheometer to measure the shear viscosity ofLFT, since conventional rheological testing methods arenot appropriate to produce reliable data. Thomasset et al.(Thomasset et al., 2005) measured the shear viscosity ofPP-based LFT by a homemade capillary rheometermounted on an injection molding machine. These studiesshowed that the shear viscosity of LFT is much higherthan that of SFTs at the same volume fraction of fiber. Theeffect of the volume fraction of the fiber on the viscositybecomes stronger with the aspect ratio of the fiber.In this study, we investigated the dynamic rheologicalproperties of two different PP-based LFT. The weightfractions of the long GF investigated in the present studyranged from 0.15 to 0.5, which are higher than the fibersconsidered in other studies. We observed very abnormalrheological behavior. Complex viscosity of LFT with aweight fraction of 0.5 is observed to be lower than that ofLFT with a weight fraction of 0.4 in spite of higher GFcontent. By various experiments, we proposed a new the-ory on this abnormal behavior. 2. Experiments2.1. Materials Base polymers employed in this study were commercialgrade polypropylene (PP). One is a general PP producedby Honam Petrochemical Co. (grade name: JH-380, meltflow index = 8.5 g/10min; hereafter, referred to as homoPP).The other PP is co-polymerized with a small amount ofethylene and exhibits a typical rubber/matrix biphasicmorphology consisting of the PP matrix and poly(ethyl-ene-co-propylene) (EPR) rubber. (Hereafter, this PP isreferred to as coPP). coPP was produced by PolyMirae(grade name: EP300R, melt flow index = 30 g/10min).Long glass fiber reinforced (LGFR) PPs with variousamounts (15-50 wt.%) of fiber were prepared using a pul-trusion process equipped with specialized T-die config-urations in Honam Petrochemical Co. Hereafter, thefollowing code is used to distinguish various LGFR PPsinvestigated in present study. The code LFGR15-homoPPrepresents a composite prepared with homoPP and longGF of 15 wt.%. By the same rule, the code LFGR50-coPPrepresents a composite prepared with coPP and GF of50 wt.%. Pelletized LGFR PP has a cylindrical shape of11 mm (length), and thus the length of the long GF in LFTwas 11 mm, too. 2.2. Rheological testComplex viscosities (*) as a function of frequency ()were determined with a strain-controlled rheometer (Phys-ica MCR301 from Anton Paar) in dynamic oscillatorymode with a parallel-plate configuration of 25 mm diam-eter and 1 mm gap size at 200oC. Before each frequencysweep, samples were pre-sheared at various shear rates(0.05, 0.1, and 1.0 s-1) for given times (1000, 500 and 50 s)so that the total strain applied to the samples was 50regardless of the shear rates. 3. Results and DiscussionFig. 1 shows the complex viscosities of long glass fiber-reinforced (LGFR) coPPs with various glass fiber (GF)contents () as a function of the frequency at 200oC. Theeffect of GF contents on * shows typical rheologicalbehavior of concentrated suspensions of interactiveparticles (Carreau et al., 1997). * increases with throughout the range of investigated, and is especiallypronounced at low . Unbounded complex viscosities areclearly seen at low for high GF content. This is anindication of yield stress. As the GF content increases,individual GFs become closer and the interactionsbetween individual GFs increase. The concentrated GFsmay form a network-like structure at low and behavelike a solid; i.e., all GFs contact each other and the wholecomposite becomes a solid-like structure. As increases,the network-like structure is broken by high external forceand * decreases sharply. The rheological tests describedabove and afterward were carried out at a strain of 1%,which was in the linear viscoelastic region and determinedby strain sweep. The critical strain for the linear vis-coelastic region was found to be very small compared tothat of unfilled polymers (typical values are 30 100%.).This is due to the fact that the network-like structurebreaks easily at relatively low strain. Fig. 2 shows * of the LGFR homoPPs with various GFcontent as a function of . To facilitate understanding, *as a function of LGF content is also shown in Fig. 3. *increases with the GF content up to 40 wt.% throughoutthe range of . Surprisingly, * of LGFR50-homoPP isanomalously lower than that of LGFR40-homoPP in spiteof higher GF content. Because this is very abnormalbehavior, we repeated the experiments several times andobtained reproducible results. The most likely explanationthat can be considered is thermal degradation of the matrixpolymer due to viscous heating. We therefore repeated thesame tests several times with the same sample, and * athigher was almost identical. Thus, the possibility ofthermal degradation was excluded. The abnormal behavior for the LGFR-homoPP observedin this study is similar with the rheological behavior of alyotropic liquid crystalline polymer (LCP) solution, theviscosity of which increases with the concentration ofLCP at low concentrations but suddenly decreases at acritical concentration (c*). Indeed, there are analogiesAnomalous rheological behavior of long glass fiber reinforced polypropyleneKorea-Australia Rheology J., Vol. 24, No. 4 (2012) 309Fig. 1. (Color online) Complex viscosity of long glass fiber rein-forced coPP with various amounts of fibers. Strain amplitude (o)= 1 %. Each frequency sweep experiment was carried after con-stant pre-shearing at (a) 0.05, (b) 0.1, and (c) 1.0 s-1 for (a) 1000,(b) 500, and (c) 50 seconds.Fig. 2. (Color online) Complex viscosity of long glass fiber rein-forced homoPP with various amounts of fibers. Strain amplitude(o) = 1%. Each frequency sweep experiment was carried afterconstant pre-shearing at (a) 0.05, (b) 0.1, and (c) 1.0 s-1 for (a)1000, (b) 500, and (c) 50 seconds.Dong Hak Kim, Young Sil Lee and Younggon Son310 Korea-Australia Rheology J., Vol. 24, No. 4 (2012)between the LFT and lyortopic liquid crystalline polymersolution. Both GF and lyotropic LCP are rigid. Further-more, the aspect ratios of the long GF and the lyotropicLCP are high. Thus, we speculate that the abnormalbehavior of viscosity for the LGFR-homoPP is due to theformation of a liquid crystal (LC)-like structure of thelong GF. According to Florys theoretical work (Flory, 1956), asolution of rod-like molecules forms a liquid crystal phasewhen the concentration of rod-like particles reaches up to40% by volume and the aspect ratio of the particles is onthe order of 200 400. In the present system, the diameterof the LGF is about 30 m and the length is 11 mm, pro-viding an aspect ratio of about 370. Therefore, it is mostlikely that the ordered structures for the LGFR-homo PPare formed at the fiber content near 40 wt.%. As a result,the viscosity of the composite decreases sharply above thecritical concentration of GF, which is between 40 and50 wt.% for this system. When looking at * of LGFR40-homoPP and LGFR50-homoPP as a function of pre-shear rate (Fig. 3(b), it isnoticed that * decreases with the shear rate at the pre-shearing step. * of LGRR50-homoPP pre-sheared at 1 s-1iseven lower than that of neat homoPP at of 0.4 16 s-1(Fig 2(c). As increases further above 16 s-1, * of theLGFR50-homoPP approaches that of LGFR40-homoPP.This is most likely due to breakage of the LC-likestructure by high frequency. The * of LGFR40-homoPPpre-sheared at 1 s-1is almost equal to that of LGFR25-homoPP or slightly lower, which is not observed for theruns pre-sheared at 0.05 and 0.1 s-1. Shear stress applied tothe GFs at the pre-shearing steps tends to align the GFs,and the degree of alignment increases with the shear stressand shear rate. As a consequence of the increased fiber-alignment, * decreases as the shear rate in the pre-shear-ing step increases.Fig. 3 (b) suggests that the critical concentration (c*),where * shows a local maximum value, in the * versusconcentration plot is between 30 and 40 wt.% dependingon the pre-shear rate and appears to decrease with theshear rate in the pre-shearing step. The viscosity of thelytropic LCP solution increases with the concentration ofthe LCP at low concentrations. This is indeed the behaviorof a conventional polymer solution. However, at a criticalconcentration, a dramatic drop in viscosity occurs and thehomogeneous solution (i.e. transparent) is transformed toa nematic phase (i.e. turbid). It is frequently observed thatthe critical concentration, which shows the maximum vis-cosity, is affected by the shear rate (Gabor et al., 1978).Generally, the critical concentration decreases with theapplied shear rate. This is due to the aligned effect. Again,there is an analogy between the LFT and lyotropic LCPsolution system in the regard that the critical concentrationdecreases with the applied shear rate. Such abnormal behaviors are not seen in the LGFRcoPP regardless of the shear rate at the pre-shearing step.As shown in Fig. 1, although the shear rate at the pre-shearing strep slightly decreases the complex viscositiesof LGFR40-coPP and LGFR50-coPP, the abnormalbehavior of the viscosity at a specific range of GF contentis not seen. The major difference between the two poly-mers is the existence of a rubbery phase. The coPP is firstpolymerized with pure propylene at an early stage of reac-tion and then ethylene is introduced to the reactor at a laterstage, with the result that EPR rubber is formed. By thenature of the polymerization process, the reaction productFig. 3. Complex viscosity of long glass fiber reinforced as a func-tion of fiber content. Data for = 1 s-1were collected from Fig.1 and Fig. 2. Strain amplitude (o) = 1%. (a) coPP (b) homo PP.Anomalous rheological behavior of long glass fiber reinforced polypropyleneKorea-Australia Rheology J., Vol. 24, No. 4 (2012) 311exhibits a typical rubber/matrix biphasic morphology con-sisting of the PP matrix (with a very small amount of eth-ylene units) and EPR rubber instead of forming a singlephase of poly(ethylene-co-propylene). The viscosity andelasticity of this rubbery phase are much higher than thoseof the PP matrix. Thus, the GF does not easily penetratethe rubbery phase and, consequently, the orientation of theGF is hindered, as demonstrated schematically in Fig. 4. The formation of a LC-like structure of the LFT is ourbest speculation, and this can explain the abnormalexperimental results that we obtained. We acknowledgethat more experimental evidence supporting our spec-ulation is needed. In order to observe the formation of aLC-like structure of long GF for LGFR50-homoPP, opti-cal microscopic observation was performed. Unfortu-nately, it was very difficult to identify GFs from thematrix PP in both the transparent and reflection modes ofthe optical microscope. To confirm the existence of theLC-like structure, another experimental technique suchas the use of a light scattering device coupled with arotational rheometer is needed. This will be the focus ofour next study.As indicated above, there is no direct proof for the for-mation of the LC-like structure. There may be anothermechanism for the abnormal viscosity decrease. Thus, wesuggest another possible cause other than the formation ofLC-like structure. Slip can cause the viscosity reduction ofsuspension at a high solid content. We performed variousmeasurements with different gap size of the parallel platesupon same sample (50%). The complex viscosity of thecomposites was not influenced by the gap size, and thisindicates that there is no wall slip between polymer andthe plates. Though there seems to be no massive slipbetween wall and polymer melts, there may be smallamount of slip between polymer and surface of glassfibers. The amount of slip in this case is thought to be verylow but overall viscosity can be altered because there aremany interfaces (polymer-glass fiber) across the gap of theparallel plates. 4. ConclusionsThe rheological behavior of long glass fiber reinforcedpolypropylene melts has been examined using differenttypes of polypropylene. Because of the high aspect ratioand the rigidity of the long glass fiber, the viscosity of theLGFR homoPP above the critical concentration of LGFdecreases with LGF concentration. This is presumably dueto flow induced liquid crystal phase formation. The reduc-tion of the viscosity can provide easy processing and highmechanical properties at high concentrations of the LGF.However, for the case of LGFR coPP, abnormal behaviorof the viscosity was not observed due to hindrance of theorientation of the long glass fiber by the rubbery phases. AcknowledgementsThis research was supported by the Basic ScienceResearch Program through the National Research Foun-dation of Korea (NRF) funded by the Ministry of Edu-cation, Science and Te
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