MECHANICAL PROPERTIES OF LIGHT WEIGHT HYBRID THERMOPLASTIC BASED COMPOSITES

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1 MECHANICAL PROPERTIES OF LIGHT WEIGHT HYBRID THERMOPLASTIC BASED COMPOSITES Réd Ourhmoune, Michelle Slvi Lortoire de triologie et dynmique des systèmes Ecole Centrle de Lyon, Ecully, Frnce Emil: ABSTRACT Comintion of thermoforming process of thermoplstic lmintes reinforced with continuous fiers ssocited with injection molding of discontinuous reinforced thermoplstic in order to dd stiffeners or specific functions t precisely defined loctions is incresingly used for pplictions which require structurl lightweight prts. This technique is well suitle to use in sustituting metls nd thermosetting composite mterils y thermoplstics sed composites prticulrly in utomotive industries llowing to the vrious dvntges which cn offer thermoplstics composites like: lightness (compred to metls), toughness nd recyclility (compred to thermosets). In this study which is mde in the frmework of FUI project ARIZONA, complex shped components were fricted from thermoplstic prepreg comined with injected composite sed on PA6 mtrix. The prepreg ly-up ws consolidted s plte, pre-heted with n IR heting system, nd put inside the mold. The plte ws then thermoformed y closing the mold nd overmolded in one step. The ojective of this study ws to chrcterize the therml-mechnicl nd mechnicl properties of the lminte, injected nd over-molded zones in reltion with the processing prmeters for different environments use (temperture nd moisture). Therml-mechnicl properties were investigted y the Mechnicl Dynmicl Anlysis (DMA) in rnge of temperture etween C to 200 C nd dynmicl frequency ws vried from 0.1 to 30 [Hz]. Three-point ending nd tensile tests were used to investigte the mechnicl properties. Micro-tomogrphy ws relized to investigte in 3D reinforcement rchitecture nd filure scenrii. The results show clerly the influence of the moisture sorption on the mechnicl properties of ech composite. 1

2 1 INTRODUCTION In recent decdes, the utomotive industry hs experienced considerle technologicl development, ccording to demnd for more nd more chllenging. Tody, due to the gloliztion cr mnufcturers re innovting every dy, in sense of growing competition in the mrket y introducing vehicles more comfortle, sfe, economic nd environmentlly friendly throughout. In this wy the choice of mterils ecme n importnt step in the eco-conception, which cn contriute significntly in environment preservtion. Composite mterils with orgnic mtrix cn e good lterntive to replcement of clssicl metllic mterils. During the lst century, the introduction of dvnced composite mterils in vrious industries (erospce, erospce, utomotive...) ws striking success. The strengths of these mterils reside in: their lightness, their chemicl stility nd their implementtion fcilities. However, the physicl properties of these mterils vry depending on severl fctors: the nture of the mtrix nd the reinforcement, the rchitecture nd the reinforcement rte... etc. In n economic context the use of mid-rnge mterils is preferred in utomotive mss production. In this context, the thermoplstic mtrix sed composites reinforced with glss fier re good cndidtes. Recyclility of thermoplstics is one of their vrious dvntges. In ddition, these mterils re chrcterized y their low density which gives considerle lightness in comprison to metllic mterils. This significnt weight sving hs direct impct on the surrounding, y the considerle decrese of emission of non-desirle gses such s (CO 2 ). One of these mterils mterils tht re used incresingly in utomotive field is polymide-6 (PA-6), which is semi crystlline polymer chrcterized y cceptle mechnicl properties, good resistnce to ftigue, chemicls nd hydrocrons. However, it hs poor resistnce to wter nd its implementtion requires drying [1]. The use of this mteril s mtrix constitutes n idel cndidte for the mnufcture of thermoplstic composites reinforced with glss fiers for the utomotive industry. However, t thickness nd shpe equivlent composite mterils sed on PA-6 reinforced with glss fiers re not equivlent to steels in terms of overll rigidity. In order to overcome this difficulty, new technique of composite mnufcture hs een developed; this technique is composite over-molding. This study is mde in the frmework of FUI project ARIZONA, which consists in new methodology of composite mnufcturing from thermoplstic prepreg comined with injected composite sed on PA6 mtrix The ojective of this work is to chrcterize therml-mechnicl nd mechnicl properties of different zones of the composite prt: lminte, injected nd over-molded zones in reltion with the processing prmeters nd moisture content. 2 MATERIALS & METHODES 2.1 Mterils The mterils used in this study were three Polymide PA6 sed composites: - Continuous glss fiers reinforced PA6, commercilly nmed Tepex Dynlyte 102 RG 600(2); the reinforcement of this mteril is lnced fric with 0 nd 90 oriented mde from two plies of 0.5 mm oriented t [0, 90 ] given totl thickness of 1mm.The reference of the mteril in this study is CGFR-PA6. Fier weight frction is out 64% (cf. fig.1.1). 2

3 - Injected composite reinforced y discontinuous glss fiers with nominl length of 250µm; this mteril is mde with the sme grde of PA6 nd commercilly nmed Durethn BKV 60 chrcterized y fier weight frction of 62% (cf. fig.1.2). The reference of the mteril in this study is DGFR-PA6. - Over-molded composite which is i-lyered composite etween continuous nd discontinuous fier reinforced composite. All mterils were supplied y Bond-Lmintes GmH nd Lnxess. Discontinuous glss fiers reinforced PA6 (DGFR-PA) Continuous glss fiers reinforced PA6 (CGFR-PA) Fig.1. Numericl opticl microgrph showing over-molded composite lyers. In the im to chrcterize the effect of process mnufcturing prmeters, prototype prt with complex shpe ws designed in ARIZONA project. The design ws crried out y Mecplst fter shpe optimiztion given y LTDS. The mold ws relized y Compose. All prototype prts were mnufctured t the PEP. The prepreg ly-up ws consolidted s plte, pre-heted with n IR heting system, nd put inside the mold. The plte ws then thermoformed y closing the mold nd over-molded y injecting the DGFR-PA6 (Fig.1.) in one step. Temperture of pre-heting ws vried s follow: 250, 270, 285 nd 295 C. Mold temperture ws 110 C. All specimens were otined y wter jet cutting from prototypes. 2.2 Thermomechnicl nlysis Rheologicl chrcteristics of mterils were performed with the use of DMA50 0.1dB from METRAVIB on rectngulr (20mm 2mm thickness mm) specimens in the tension/compression mode t controlled lternting strin. The temperture rnge ws -130 C to 200 C with heting rte of 1 C/min nd the frequency ws vried from 0.1 to 30 Hz. 2.3 Tree point ending test The three point ending lodings were crried out using n INSTRON 4206 electromechnicl mchine with lod cells of 100 kn nd cross hed speed ws 2.5 mm/min. The rdius of the lod roller ws 5 mm nd the distnce etween supports ws 60 mm. Experimentl procedure ws mde ccording to the Interntionl stndrd ISO t room temperture (T=23±1 C, RH = 40±5%). CGFR-PA6 smples hve the following dimensions: 100 mm long, 25 mm wide nd 1 mm thick. In the cse of DGFR-PA6 nd over-molded composite the smples dimensions re 100 mm long, 15 mm wide nd 3 mm thick. The pprent flexurl elstic modulus (E f ) nd the ultimte stress (σ mx ) were the mechnicl properties evluted. 3

4 Figure (fig.2) shows n exmple of three point ending curves of DGFR-PA6 composite. The liner prt corresponds to the elstic ehvior nd the mximum recorded stress is considered s n ultimte flexurl stress. Fig.2. Flexurl stress vs flexurl strin curve (three onding test): Discontinuous glss reinforced PA Computed tomogrphy nlysis The computed tomogrphy is non-destructive method sed in the 3D X ry scn which llows three-dimensionl structure informtion of mterils without contct. The microtomogrph used is Nnotm reserch edition mnufctured y Generl Eclectic inspection Technology. It llows nlysis of smples size: height from 1 mm to 120 mm, mximum 100 mm width nd smple cquisition in queous medium. The X-ry source is n open tue GE Phoenix nno-focus, providing mximum voltge of 180 kv nd mximum power of 15 W. The voltge rnge opertes etween 10 nd 180 kv. The pprtus detector is digitl detector with dimensions 115 mm x 115 mm, with pixel size of 50 microns x 50 microns (2300 x 2300 mtrix px). It provides sumicron resolution (minimum pixel size of 0.5 microns). However the pixel resolution is highly dependent on the specimen size. Detector X-ry source Specimen Fig.3. Micro-tomogrphy device. The test prmeters used in this study re s follows: smple dimensions 15 mm long, 15 mm width nd 4 mm thick nd the em operted t 80 kev nd 120 µa. According to the smple size the voxel resolution chieved in this study ws 8µm. The result otined y X-ry tomogrphy ws stck of slices in 3 directions which includes out 2400 imges. These slices were nlysed using VG-studio nd ImgeJ to generte 3D representtion of the specimens. 4

5 3 RESULTS & DISCUSSIONS 3.1 Mterils conditioning Effect of moisture content on PA6 sed composite on mechnicl properties were investigted. Specimens were dried for 8 dys in stove t 60 C, in the ojective to remove ll residul moisture due to ir conditioning nd trnsporttion. After surfce polishing of smple the sorption of wter ws chieved y immersion in distillted wter t room temperture (T = 23 C ± 2 C). Uptke moisture of specimens ws mesured y differentil weighting using n electronic lnce of ccurcy 10-5 g. Difference etween the weight of dried specimens nd the weight fter wter immersion ws clculted. Supposing tht in the composite only the mtrix PA6 uptkes wter, the moisture content M(t) sored y ech specimen ws clculted from its initil weight (w 0 ) of dried mtrix PA6 nd its weight fter sorption (w t ) s follows: w 0 = w 0c. (1 X f ) w t = w tc w 0c (eq.1) (eq.2) Where, (w 0c ) is the initil weight of the composite, (X f ) is the weight frction of the fier (0.64%) nd (w tc ) the weight of conditioned composite t the instnt t. M(t) = 100. ( w t w 0 w 0 ) (eq.3) Figure VI.1 shows the weight gin M(t) s function of the squre root of time (t½) during geing in distilled wter for the studied composites. Three specimens were studied for ech cse nd the presented wter uptkes re verge vlues. Fig.4. Moisture content evolution s function of time for the three composites CGFR-PA6 1 mm thick, CGFR-PA6 2 mm thick nd CGFR-PA6 3 mm thick All composite curves showed similr profiles nd one cn distinguish the presence of two zones. The first one is liner corresponding to rpid increse of moisture content. The second region corresponds to plteu t which the ged mteril reches sturtion. In this cse, nd for ll studied composites the kinetics of wter diffusion follow the one-dimensionl Fick s second lw. The coefficient of diffusion of wter cn e clculted from the slope (S) of the liner prt of the lst curves s follows [2]: D = π ( h ) 2 (S) 2 (eq.4) 4M By considering the coefficients D nd Mm, the moisture content s function of time M(t) cn e expressed ccording to the following eqution [3]: 5

6 M(t) M 8 = 1 exp ( D(2n+1)2 π 2 t n=0 ) (eq.5) π²(2n+1)² h 2 where M is the moisture content t sturtion, D is the pprent diffusion coefficient, t is the ging time nd h is the smple thickness. The pprent diffusion coefficient nd the mximum moisture content vry with the nture of composite. The continuous glss reinforced PA-6 composites chieved moisture sturtion t 8.7% with n pprent coefficient of wter diffusion equl to 7.01 e-7 mm²/s wheres the discontinuous glss reinforced PA-6 composite (DGFR-PA6) showed etter hygrotherml properties since the wter diffusivity nd the moisture content t sturtion were lower thn for the CGFR-PA6 (M = 7.7% nd D= 5.1 e-7 mm²/s). 3.2 Dynmicl mechnicl nlysis The storge moduli (E ) nd the loss fctor (tn δ) s function of the temperture nd frequency for ll composites were mesured y DMA. In order to determine different relxtion tempertures, CPGR-PA6 composite were oriented t ± 45 ccording to the direction of loding. Figure 3 shows n exmple of the evolution of storge modulus (E ) nd dmping fctor (Tn ( s function of temperture nd frequency for CPGR- PA6. These figures compre two moisture content sttes: dried nd wet t sturtion. T α T α Shift due to the presence of wter T α T β T γ T β Fig. 5. Evolution of viscoelstic properties of CGFR-PA6 oriented t ±45 mesured y DMA s versus temperture nd frequency : ) Storge modulus E ) dmping fctor (tn(δ)). (----) dried composite ( ) wet composite. In the rnge of temperture studied ( C) decrese of storge modulus is oserved with temperture increse. The structurl chnge occurring in polymeric mterils is result of the moleculr moility t different time scles. These motions re known s moleculr relxtions. In the cse of dried composites three cler relxtions cn e distinguished: the min relxtion or relxtion- corresponding to the drsticlly decrese of the storge modulus nd the highest pek of the tn(δ); this relxtion is ssocited to the glss trnsition, nd corresponds to the coordinted motion of reltively long chin segments y deonding of low energy onds (hydrogen onds) [4]. This relxtion occurs pproximtely t T α = C depending on frequency. two su-t g relxtions [5-6]: the relxtion-β nd relxtion- corresponding to the motions of smll chin segments or moleculr functions. The relxtion-β involves the rottion of the mide functions nd occurs in the rnge of -80 / -40 C pproximtely; the 6

7 relxtion- γ corresponds to the virtionl motions of the methylene functions in the chins nd it occurs t round -130 C. After conditioning the composites y wter immersion, the DMA results show tht the presence of wter in the PA6 mtrix involves shift of mechnicl moleculr relxtions to low temperture s oserved in fig.5. In fct, the Tα decreses from 80 C in dried composite to -10 C for wet composite t 1Hz s shown in Figure (fig.6). Fig.6. Temperture of min relxtion versus moisture content for different loding frequencies. In the cse of the min relxtion (α) the shift of temperture is due to the fct tht the presence of wter increses the locl volume y seprting the polymer chins [11, 12]. 3.3 Three-point ending tests Mechnicl properties of ll composites ccording to the process conditions nd moisture content were studied using 3-point ending test. The pprent flexurl modulus ws tken s min mechnicl property discriminnt etween different composites. Figures (fig.7. & fig.7.) compre the flexurl properties of Continuous GFR-PA6 nd Discontinuous GFR-PA6 respectively for different processing conditions nd smpling zones. Prepreg heting ws vried from 250 to 295 C. In ll cses the composite filure occurred on the tensile side. The results show there is not significnt vrition in the pprent flexurl modulus for the two composites whtever the processing conditions. Nevertheless there is lrge modulus rnge for discontinuous fier reinforced composite (7-16 GP) due to the fier rchitecture which vries with smpling zone. Fig.7. Evolution of pprent flexurl modulus ccording to the temperture process vrition: ) CGFR-PA6 nd ) DGFR-PA6 (zone1 lue & 2 red). 7

8 Figures (Fig.8. & Fig.8.) compre the flexurl properties of over-molded composites for different processing tempertures. Due to the multi-lyering feture of these composites, two ending configurtions were investigted: in the first, continuous glss fiers on tensile side nd in the second on compressive side. Fig.8. nd Fig.8. show the homogenized pprent flexurl modulus nd the ultimte flexurl strength respectively. CGFR-PA6 on tensile side c DGFR-PA6 on tensile side d Fig.8. Evolution of flexurl properties for the over-molded composite: ) pprent flexurl modulus ) Ultimte flexurl strength c) nd d) illustrtion of different ending configurtions nd filure fetures. The results show tht there is no cler tendency concerning the pprent flexurl modulus ccording to the temperture processing nd ending configurtion. This ehvior is proly ssocited to the complex fier rchitecture ccording to smpling zone (fig.8.). The ultimte flexurl strength evolution ccording to the different mode of ending is shown in figure (fig.8.). When the CGFR-PA6 is on the tensile side the flexurl strength is the highest nd qusi-stle whtever the processing conditions. For the system with the discontinuous fiers on the tensile side the ultimte stress increses with the processing conditions proly due to etter interfce etween the two composites. The computed tomogrphy technique ws used to investigte filure mode of the over-molded systems. Fig.9 shows n exmple of 3D reconstitution computed from X-ry slices for continuous glss fiers on the tensile side (Fig.9.) nd discontinuous glss fiers on the tensile side (Fig.9.). These figures show typicl filure fetures. In the cse when CGFR- PA6 is on the tensile side the filure growths through the two composites (Fig.9.). However, in the second cse (discontinuous fiers on the tensile mode) the filure occurs through the discontinuous glss fier composite nd t the interfce etween the two mterils (Fig.9.). Fig.9. 3D micro-tomogrphy representtion of over molded composite fter 3-point ending test: ) CGFR-A6 on the tensile side nd ) DGFR-A6 on the tensile side. 8

9 This nlysis confirms tht the scttering of the results is due to the fier rchitecture of the injected composite. In fct, the injection flow mkes the fier orienttion: in some cses the skin/core morphology is dominnt with high thickness of skin lyer nd with fiers oriented principlly in longitudinl direction, leding to higher strength. Figure (fig.10) shows two overmolded composites with the sme process prmeters ut presenting different ultimte flexurl strength ( : σmx = 190 MP, : σmx = 312 MP). Skin lyer thickness Skin lyer thickness Fig.10. Micro-tomogrphy slices showing discontinuous fiers orienttion for the sme process conditions ut different smpling zones. Effect of moisture on flexurl properties Figure (fig.11) illustrtes the evolution of pprent flexurl modulus (fig.11.) nd ultimte flexurl strength (fig.11.) s function of moisture content for ll studied composites for given smpling zone. The results show shrp decrese in the flexurl modulus s well s in the ultimte flexurl strength. The continuous glss fires reinforced PA6 oserves decrese of 50% in pprent flexurl modulus nd the ultimte flexurl strength for the moisture sturtion stte. At 5% of moisture content the decrese of flexurl modulus is out 50% nd the flexurl strength ws reduced of out 36%. Fig.11. Evolution of flexurl properties ccording to the moisture content for ll composites: ) Apprent flexurl modulus nd ) ultimte flexurl stress. As shown previously with DMA, the sored wter decreses the min relxtion of PA6 due to the plsticizing ction of wter molecules. This phenomenon leds to drop of the modulus of the mtrix t room temperture, nd therefore to decrese of the composite one (mixture lw). Moreover, the increse in moleculr moility of the wet mtrix reduces the yield stress nd increses the ductility nd elongtion t rek of the composite. 9

10 4 CONCLUDING REMARKS In this study the thermomechnicl nd mechnicl properties of PA6 composites ws investigted. This work demonstrtes tht the conditioning history nd environment conditions re importnt prmeters to e tken into ccount in design of structurl prt composites sed on PA6. The PA6 mtrix is very sensitive to humidity; the sorption of wter cn decrese drsticlly the therml nd mechnicl properties of glss fier reinforced PA6 composites. The overmolding of prepreg composite cn e solution to enhnce the rigidity nd therefore the mechnicl properties of composite prts y locl ddition of discontinuous glss fier reinforced composite. 5 ACKNOWLEDGEMENTS The uthors would like to cknowledge ll the prtners of ARIZONA project, Guillume Huguet from Mecplst, Mthieu Schwnder from PEP, Guillume Slun from Compose nd Jen-Mrie Olive from Lnxess nd our collegues in ECL for their help: Jerome Lorde, Jen-Michel Vernet nd Benoit Ponsrd. 6 REFERENCES [1] L. Monson, M. Brunwrth, nd C.W Extrnd, Moisture sorption y vrious polymides nd their ssocited dimensionl chnges, Journl of Applied Polymer Science, (107): , [2] C.H. SHEN, G.S. SPRINGER, Moisture Asorption nd Desorption of Composite Mterils, Journl of Composite Mterils, (10) : 2-20, 1976 [3] W. Jost, Diffusion in Solids, Liquids, Gses. New York: Acdemic Press, [4] A. Owen, R. Bonrt, Coopertive relxtion processes in polymers, Polymer, (26):1034, [5] Y. Prk, J. Ko, T-K. Ahn, nd S. Choe, Moisture Effects on the Glss Trnsition nd the Low Temperture Relxtions in Semiromtic Polymides, J. Polym. Sci. Prt B: Polym. Phys., (35): 807, [6] G. Rotter nd H. Ishid, Dynmic Mechnicl Anlysis of the Glss Trnsition: Curve Resolving Applied to Polymers, Mcromolecules, (25): 2170, [7] N.S. Murthy, Hydrogen Bonding, Moility, nd Structurl Trnsitions in Aliphtic Polymides, J. Polym. Sci. Prt B: Polym. Phys., (44):1763, [8] S. Rstogi, A.E. Terry, nd E. Vinken, Dissolution of Hydrogen-Bonded Polymers in Wter: A Study of Nylon-4,6, Mcromolecules, (37): 8825, [9] E. Vinken, A.E. Terry, O. vn Asselen, A.B. Spoelstr, R. Grf, nd S. Rstogi, Role of superheted wter in the dissolution nd perturtion of hydrogen onding in the crystlline lttice of polymide 4,6, Lngmuir, 24, 6313 (2008). [10] Ach et l. express Polymer Letters, (3) 4 : , [11] N. S. Murthy, M. K. Akkpedi, Anlysis of lmellr structure in semicrystlline polymers y studying the sorption of wter nd ehylene glycole in nylons smll ngle neutron scttering, Mocromelecules, (3): ,

11 [12] A. Bergeret, L. Ferry, P. Ienny. Veillissement hygrothermique des composites thermoplstiques renforcés pr des fires de verre, Revue des composites et des mtériux vncés, (18)1: 33-49, COPYRIGHT NOTICE Copyright 2015 y the uthors. Distriution of ll mteril contined in this pper is permitted under the terms of the Cretive Commons license Attriution-NonCommercil-NoDerivtives 4.0 Interntionl (CC-y-nc-nd 4.0). 11