关于效应的纤维浓度、温度及模具厚度及其在完整熔接线性的共聚物复合材料@中英文翻译@外文翻译

Effect of fibre concentration, temperature and mould thicknesson weldline integrity of short glass-fibre-reinforced polypropylenecopolymer compositesS. Hashemi P. OnishiReceived: 29 May 2009 / Accepted: 29 July 2009 / Published online: 11 August 2009C211 Springer Science+Business Media, LLC 2009Abstract The effect of fibre concentration, temperatureand mould thickness on tensile strength of single- anddouble-gated injection-moulded polypropylene copolymerreinforced with 0, 10, 20, 30 and 40 wt% short glass fibrewas studied at a fixed strain-rate of 7.58 9 10-3s-1between 23 and 100 C176C. It was found that tensile strengthof single-gated mouldings, rc, increased with increasingvolume fraction of fibres, /fin a nonlinear manner anddecreased with increasing temperature in a linear manner.However, for /fvalues in the range 010% a simpleadditive rule-of-mixtures adequately described the varia-tion of rcwith /fover the entire temperature range23100 C176C studied here. Tensile strength of double-gatedmouldings like their single-gated counterparts decreasedlinearly with increasing temperature. The presence ofweldlines significantly reduced tensile strength of double-gated composite mouldings but had little effect on tensilestrength of the matrix. Weldline integrity factor, Fr,defined as weldline strength divided by unweld strength,decreased with increasing /fbut increased with increasingtemperature. A linear dependence was found between Frand temperature. Mould thickness had no significant effectupon weld and unweld tensile strengths and consequentlyhad no significant effect upon weldline integrity factor.IntroductionTensile strength of short fibre composites is derived froma combination of the fibre and matrix properties and theability to transfer stresses across the interface between thetwo constituents. Tensile strength is affected by a numberof parameters, most importantly, concentration, length andorientation of the fibres as well as the degree of interfacialadhesion between the fibre and the matrix 112. How-ever, as most short fibre composites are fabricated by aninjection-moulded process, a major design concern is theeffect that weldlines may have on tensile strength of thepolymer matrix and its composites. Weldlines areobserved in injection-moulded components due to multi-gate moulding, existence of pins, inserts, variable wallthickness and jetting and are classified as either being coldor hot. A cold weldline is formed when two melt frontsmeet head on and this type of weldline is the worst-casescenario as far as tensile is concerned. A serious reductionin strength has been reported for many polymers and theircomposites in the presence of cold weldlines 19.However, very little information is available regarding theinfluence of temperature and to some extent mouldthickness on weldline strength and therefore the integrityof the welded components. To this end, this study wasundertaken to examine the influence of temperature andmould thickness on weld and unweld tensile strengths ofinjection-moulded short glass-fibre-reinforced polypro-pylene copolymer (PPC) composite with fibre concentra-tion in range 040 wt%.Experimental detailsMaterialsPolypropylene copolymer and its composites containing10, 20, 30 and 40 wt% short glass fibres were supplied byPolyOne in the form of injection-moulded compounds.S. Hashemi (&) C1 P. OnishiLondon Metropolitan Polymer Centre, London MetropolitanUniversity, London, UKe-mail: s.hashemilondonmet.ac.uk123J Mater Sci (2009) 44:55885594DOI 10.1007/s10853-009-3786-zInjection mouldingMaterials were injection moulded in a Klockner FerromatikF-60 injection-moulding machine at the processing condi-tions listed in Table 1 to produce a series of dumbbell-shaped test specimens. The moulds used consisted of asingle-gate (SG) and a double-gate (DG) cavities as illus-trated in Fig. 1 with nominal dimensions 4 9 10 9120 mm3and 1.5 9 10 9 120 mm3(thickness, width andlength, respectively). In the case of double-gated cavities,the two opposing melt fronts met to form a cold weldlineapproximately mid-way along the gauge length of thespecimen.Differential scanning calorimetric (DSC)Thermal characterisation was carried out by DSC using aPerkin-Elmer DSC-6 modulated instrument. Samples ofPPC and PPC composites, in the range 910 mg, were cutfrom the injection-moulded specimens and submitted to thethermal cycles of heating from -60 to 220 C176Cat10C176C/minand cooling from 220 to -60 C176Cat5C176C/min to obtaincrystallisation data. The DSC curves were used to obtainthe melting temperature (Tm), crystallisation temperature(Tc), enthalpy of crystallisation (DHc) and the enthalpy ofmelting (DHm). Values of DHc, DHmand percentagecrystallinity (vc) were calculated from the followingrelationships:DHmDHmwp1aDHcDHcwp1bvc%DHomDHmC2 100 1cwhere DHomis the heat of fusion for 100% crystallinepolypropylene taken as 148 J/g and wpis the weight frac-tion of PPC in the composite (wp= 1 for 100% PPC).Fibre concentration and length measurementsThe exact weight fraction of the fibres in as received com-pounds (ARC) and in injection-moulded dumbbell speci-mens (IMDS) was determined by ashing a pre-weighedamount of material in a muffle furnace at 550 C176C for at least1 h. After cooling, the remnant was weighed and weightfraction of fibres wfwas determined. It can be seen fromTable 2 that the measured weight fractions for both ARC andIMDS are within 1% of the manufacturers specification.The measured weight fractions, wf, were subsequentlyconverted into volume fractions, /f, using Eq. 2:/fqcqfwf: 2Table 2 shows values of /fobtained via Eq. 2, takingdensity of glass fibre, qf, as 2540 kg m-3and compositedensities, qc, as provided by the manufacturer.The ashes of fibrous material were subsequently spreadon glass slides and placed on the observation stage of amicroscope. Approximately 500 fibre lengths were mea-sured for each composite (ARC and IMDS) using animage-processing system. From the fibre-length distribu-tions examples of which are given in Fig. 2, the effect offibre concentration and processing conditions on the aver-age fibre length (Lf) was assessed.Table 1 Processing conditionfor injection-moulded PPC andits compositesProcessing condition 0 wt% 10 wt% 20 wt% 30 wt% 40 wt%Barrel temperature (C176C)Nozzle 220 220 220 210 220Zone 2 210 210 210 207 210Zone 3 210 210 210 207 210Zone 4 210 210 210 207 210Mould temperature (C176C) 30 30 30 30 30Injection pressure (%) 95 95 95 95 95Injection speed (%) 85 85 85 85 85Cooling time (s) 15 15 15 15 15Fig. 1 Single- and double-gated cavities with nominal dimensions of4 9 10 9 120 mm3(thickness, width and length, respectively)J Mater Sci (2009) 44:55885594 5589123Tensile testsThe effect of fibre concentration, moulding thickness andtemperature on weld and unweld tensile strengths wasstudied using an Instron testing machine. All the tests wereperformed at a constant crosshead of 50 mm/min (strain-rate of 7.58 9 10-3s-1). At least six specimens weretested for determining an average value.Results and discussionAnalysis of DSC thermograms shows that melting tem-perature (Tm) of the PPC matrix is not affected by theaddition of fibres or by the heating cycle. However, asshown in Table 3, the heat of fusion (DHm) in the secondheating cycle is always greater than in the first heatingcycle, and much closer to DHcvalue. It is also evident thatDHmand DHcboth decrease initially with increasing fibreconcentration showing similar trend to that of the per-centage crystallinity (vc) versus fibre volume fraction (/f)as shown in Fig. 4. The striking feature of Fig. 3 is higher,vc,for composite containing 30 wt% fibres, being 9%higher than for the unreinforced PPC.The average length of the fibre length, Lf, in IMDS andARC are compared in Fig. 4 as a function of fibre con-centration, /f. It can be seen that Lfin IMDS is consistentlylower than in ARC. It is also evident that fibre concen-tration plays a major role in the shortening of the fibresparticularly in IMDS where reduction in Lfdue to pro-cessing increases from 5 to 19%, as fibre concentrationincreases from 10 to 30 wt%. This reduction in fibre lengthwith increasing /fis attributed to increased probability offibrefibre and fibremachine interactions as well asTable 2 Fibre concentrationand the average fibre lengths inARC and in IMDSValues given in the parenthesisare the total number of fibrelengths measuredComposites PPC PPC PPC PPC10% w/w GF 20% w/w GF 30% w/w GF 40% w/w GFARC (%w/w) 9.50 20.03 30.05 39.70IMDS (%w/w fibre) 8.60 20.10 28.30 39.70Density (kg m-3) 960 1030 1130 1220%v/v (IMDS) 3.80 8.10 13.30 19.20Average fibre length, ARC (mm) 0.389 (485) 0.353 (453) 0.350 (500) 0.347 (492)Average fibre length, IMDS (mm) 0.374 (534) 0.326 (479) 0.309 (424) 0.281 (400)Reduction in fibre length (%) 5 8 12 196005004003002001000.0050.0040.0030.0020.0010.000Fibre lengthFrequency374.0 105.6 534325.8 98.66 479308.6 84.42 424280.5 94.42 394Mean StDev N10%GF20%GF30%GF40%GFVariableFig. 2 Fibre length distribution in IMDSTable 3 Thermal analysis results for PPC and PPC compositeswf(%)Tm(C176C) Tc(C176C) DHm(J/g) DHc(J/g)% vc1stcycle2ndcycle1stcycle1stcycle2ndcycle1stcycle0 163.44 166.47 119.25 85.85 89.69 91.61 60.6010 165.56 166.30 130.92 75.92 85.06 87.94 57.4720 165.41 165.76 129.99 74.16 84.75 82.86 57.2630 164.89 165.12 129.47 84.46 97.74 99.64 66.0440 164.43 164.82 130.92 75.20 88.07 88.07 59.500204060801000 0.05 0.1 0.15 0.20.25Fibre volume fraction, f% CrystallinityFig. 3 Effect of fibre concentration on percentage crystallinity5590 J Mater Sci (2009) 44:55885594123increased in the apparent melt viscosity which gives rise tohigher bending forces on the fibres during moulding.Effect of fibre concentration, temperature and mouldthickness on tensile strength of single-gated mouldings(unweld strength)Single-gated mouldings (i.e. unweld specimens) failedafter exhibiting a clear yield point referred to in the fol-lowing text as tensile strength. It was also noted that whisttensile strength increased, elongation to failure decreased,with increasing fibre concentration. This behaviour wasconsistently observed over the entire temperature rangeconsidered here.The effect of fibre volume fraction, /f, on tensilestrength of single-gated mouldings, rc, at test temperaturesranging from 23 to 100 C176C is shown in Fig. 5. Resultsindicate that unweld tensile strength, rc, increases nonlin-early with increasing /fand therefore does not conform torule-of-mixtures for strengths. The data presented here, asfor many polymer composite systems 4, 5, 10, can best betreated using a second-order polynomial function of theform:rc a0 a1/f a2/2f: 3The polynomial functions describing the data in Fig. 5are given in Table 4. These polynomials may be used toobtain some indication of the optimum value of /f(i.e./f,max) for achieving the maximum tensile strength, at agiven temperature. Values of, /f,max, calculated atdrc/f=0, suggest that it is advantageous to increase thefibre concentration in the composite up to 2832% byvolume. However, the processing difficulties and the pos-sible strength loss due to fibrefibre interactions at highfibre concentration may limit the optimum value of /fbelow that of /f,max.Figure 6 shows the data in Fig. 5 re-plotted for fibreconcentration values in the range of 030 wt%. Clearly,within this range rcis a linear function of /f(regressioncoefficients R2= 0.996) and therefore it is reasonable tosuggest that within this range, variation of rcwith /fconforms to rule-of-mixtures for tensile strengths as givenby Eq. 4.rc ksrf/f1 C0 /frm: 4In the above equation, rfand rmare tensile strengths ofthe fibre and the matrix, respectively. The term ksis fibreefficiency parameter taking into account the effects oncomposite strength due to shortness of the fibres and theirmisalignment in the moulded specimen. Rearranging Eq. 4givesrc rmksrfC0 rm/f: 5The average values of ksas obtained from the slope ofthe linear regression lines in Fig. 6 with rf= 2470 MPaare plotted in Fig. 7 as a function of temperature. As can beseen ksdecreases linearly (R2= 0.998) with increasingtemperature, thus indicating that fibres becoming less50.40 0.05 0.1 0.15 0.2 0.25Fibre volume fraction, fAverage fibre length (mm)IMDSARCFig. 4 Effect of fibre concentration and injection moulding on theaverage fibre length in PPC composites0204060801000 0.05 0.1 0.15 0.2 0.25Volume fraction of glass fibres, fUnweld tensile strength (MPa)23 4060 80100Fig. 5 Effect of fibre volume fraction on tensile strength of single-gated mouldings at 23, 40, 60, 80 and 100 C176CTable 4 Polynomial functions for tensile strengths and the optimumvolume fraction of fibres for single-gated mouldings at varioustemperaturesTemperature (C176C) Polynomial function /f,max23 rc 25:17 459:84/fC0 717:61/2f0.3240 rc 19:98 372:68/fC0 584:67/2f0.3260 rc 14:91 319:42/fC0 533:72/2f0.3080 rc 10:91 260:21/fC0 464:61/2f0.28100 rc 7:98 188:31/fC0 338:86/2f0.28J Mater Sci (2009) 44:55885594 5591123efficient as reinforcing fillers as temperature increases. Thetemperature dependence of kscan be expressed as:ksTA C0 BT 6where A = 0.192 and B = 1.28 9 10-3C176C-1.The effect of temperature on rcis shown more explicitlyin Fig. 8 where it can be seen that rcdecreases linearlywith increasing temperature. The effect of increasing /fisan upward vertical shift in rctemperature curve. Theeffect of temperature on rccan be expressed as:rTA C0 BT 7where A and B are dependent upon volume fraction offibres. The slope of the lines in Fig. 8 (i.e. B=dr/dT) areplotted in Fig. 9 as a function of /fwhere it can be seenthat variation is reasonably linear (R2= 0.983) andtherefore can be expressed as:B drdT A B/f8where A = 0.24 MPa C176C-1and B = 2.573 MPa C176C-1forthe PPC composites under investigation.The effect of mould thickness on rcis shown in Fig. 10as a function of /f. Results show that mould thickness hadlittle effect if any on the unweld tensile strength. Thisinsensitivity to specimen thickness may be attributed to thefact that the average fibre length is much smaller than themould thickness and therefore fibre orientation efficiencyparameter, ks, is unaffected by the mould thickness.Effect of fibre concentration, temperature and mouldthickness on tensile strength of double-gated mouldings(weldline strength)Stressstrain curves for double-gated specimens revealedthat the presence of weldline in the double-gated0204060801000 0.03 0.06 0.09 0.12 0.15Volume fraction of glass fibres, fUnweld tensile strength (MPa)23 4060 80100Fig. 6 Effect of fibre volume fraction on tensile strength of single-gated mouldings at 23, 40, 60, 80 and 100 C176C for fibre concentrationvalues in the range 030 wt%0.000.000 20 40 60 80 100 120Temperature, T (C)Fibre efficiency parameterFig. 7 Effect of temperature on fibre efficiency parameter, ksforunweld tensile strength0 20 40 60 80 100 120020406080100Unweld tensile strength (MPa)0 10%20% 30%40%Temperature, T (C)Fig. 8 Effect of temperature on tensile strength of single-gatedmouldings for composites containing 0, 10, 20, 30 and 40 wt% shortglass fibres0.00.81.00 0.05 0.1 0.15 0.2 0.25Volume fraction of glass fibres, fd/dT(MPa/C)UnweldWeldFig. 9 Effect of fibre volume fraction on dr/dT for both weld andunweld specimens5592 J Mater Sci (2009) 44:55885594123mouldings reduces elongation at failure (i.e. ductility) andin the case of composites causes a significant reduction inboth tensile strength and elongation at break.As illustrated in Fig. 11, for fibre concentration valuesin the range 010%, weldline strength, rcw, decreases withincreasing /fat lower temperatures (T B 40 C176C) butremains more or less independent of /fat higher temper-atures (T 40 C176C). At fibre concentration value of about12% (corresponding to 30 wt%), rcwshows a maximumvalue over the entire temperature range studied here; thisvalue is higher than weldline strength of matrix material,rmw. The striking similarity between the way in whichpercentage crystallinity and weldline strength vary with /fimplies that weldline strength is controlled to a large extentby the percentage crystallinity in the moulded specimens,i.e. increase in crystallinity increases weldline strength.The effect of temperature on weldline strength is shownmore explicitly in Fig. 12 where it can be seen that tensilestrength of the double-gated specimens like their single-gated counterparts, decreases linearly with increasingtemperature, and like wise can be expressed as:rwTAwC0 BwT 9where Awand Bware dependent upon volume fraction offibres. The slope of the lines in Fig. 12 (i.e. Bw) are plottedin Fig. 9 and compared with the corresponding values forsingle-gated mouldings (i.e. B). Clearly, variation of Bwwith /ffollows that of crystallinity versus /fas in Fig. 3.Itis also worth noting that Bw B and the differencebetween the two increases with increasing /f.The effect of weldline on tensile strength is expressedquantitatively in terms of weldline integrity factor fortensile strength, Fr, defined as:Frrwr10where r is tensile strength of single-gated moulding (i.e.unweld tensile strength) and rwis tensile strength of thedouble-gated counterpart (i.e. weldline strength). Figure 13shows the variation of Frwith fibre concentration. Resultsshow that Frdecreases significantly with increasing fibreconcentration, /f, and whilst the value of Fris near unityfor the matrix, it reduces to around 0.3 for the compositecontaining 40 wt% fibres, i.e. the presence of weldline hascaused 70% reduction in tensile strength. The reduction incomposite strength in the presence of weldline is attributedto the alignment of the fibres at weldline region beingpreferentially parallel to the weldline hence normal to thedirection of the applied stress. It is also notable that thevariation of Frwith /flik