Relationship between fiber degradation and residence time distribution in the processing of long fiber reinforced thermoplastics

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1 express Polymer Letters Vol.2, No.8 (28) Avalable onlne at DOI: /expresspolymlett Relatonshp between fber degradaton and resdence tme dstrbuton n the processng of long fber renforced thermoplastcs H. Zhuang *, P. Ren, Y. Zong, G. C. Da State Key Laboratory of Chemcal Engneerng, East Chna Unversty of Scence and Technology, 2237 Shangha, Chna Receved 14 May 28; accepted n revsed form 29 June 28 Abstract. Long fber renforced thermoplastcs (LFT) were processed by n-lne compoundng equpment wth a modfed sngle screw extruder. A pulse stmulus response technque usng PET spheres as the tracer was adopted to obtan resdence tme dstrbuton (RTD) of extruson compoundng. RTD curves were ftted by the model based on the supposton that extruson compoundng was the combnaton of plug flow and mxed flow. Characterstc parameters of RTD model ncludng P the fracton of plug flow reactor (PFR) and d the fracton of dead volume of contnuous strred tank reactor (CSTR) were used to assocate wth fber degradaton presented by fber length and dsperson. The effects of screw speed, mxng length and channel depth on RTD curves, and characterstc parameters of RTD models as well as ther effects on the fber degradaton were nvestgated. The nfluence of shear force wth dfferent screw speeds and varable channel depth on fber degradaton was studed and the man mpetus of fber degradaton was also presented. The optmal process for obtanng the balance of fber length and dsperson was presented. Keywords: polymer compostes, renforcements, fber degradaton, RTD, LFT 1. Introducton Fber renforced thermoplastcs are wdely used for the advantages of weght savngs, low producton costs, and freedom of desgn. Long fber renforced thermoplastcs (LFT) have been confrmed to possess sgnfcant mprovement n the propertes of stffness, strength, and toughness over ther counterparts of short fber renforced thermoplastcs. Because of excellent mechancal propertes, low producton cost and good envronmental protecton, LFT has been broadly appled n the automotve ndustry over the last decade [1 2]. Longer fber length and better fber dsperson are the key factors to obtan excellent mechancal propertes of LFT. The nstance s that longer fber length s n favor of enhancng strength and toughness, and the mprovement of fber dsperson s benefcal to ncrease stffness [3]. However, fber length was reduced serously by undergong nteracton between fber-equpment, fber-fber, and fber-matrx durng extruson compoundng of LFT [4]. As to fber degradaton ndcated by fber length and dsperson, the reducton of fber length had been nvestgated, but few studes were focusng on fber dsperson so far [5]. There s a conflct of factors whch nfluence fber length and dsperson durng extruson compoundng of LFT, and how to obtan the optmal balance of fber length and dsperson s the purpose of ths study. In the mxng process of polymer system, resdence tme dstrbuton (RTD) plays a sgnfcant role n determnng mxng capacty as well as components *Correspondng author, e-mal: zhuanghu@vp.ctz.net BME-PT and GTE 56

2 unformty [6 7]. The extruson compoundng of polymer reacton, mmscble blend and renforced plastcs had been nvestgated by RTD analyss. Measurements of RTD n a sngle-screw extruder were carred out durng expermental studes of controlled chemcal degradaton of polypropylene (PP). A radoactve tracer method was employed, and the effect of screw speed, temperature, and reacton on RTD curves were examned [8]. In an extruder, the effect of screw confguraton on resdence tme and mxng effcency was studed for an mmscble PA6/PP blend. RTD was used to ndcate the total mxng effcency, and ts assocaton wth the dsperson of mnor phase [9]. The mechansm of fber fracture and methods to reduce t through process mprovement and machne desgn n fber renforced compostes were studed by Karthk Raman et al. It s found that the resdence tme, fll-up, and the ntensty of mxng durng extruson compoundng have a predomnant effect on fber fracture [1]. RTD can be studed by the ntroducton of a pulse tracer or by ntroducton of a step change n the concentraton [11]. Also RTD can be determned by an analyss of melt flow, but t s very complcated. An alternatve analyss s the constructon of a flow pattern wth conceptual models. The most wdely reported models nclude a combnaton of plug flow and mxed flow [12 13]. Mean resdence tme (MRT) was used for evaluatng the extent of fber fracture, but no paper was reported usng the RTD model for the evaluatons between processng condton, machne desgn and fber fracture. In-lne compoundng equpment modfed wth a sngle screw extruder was used for processng LFT. PP and glass fber were added nto the extruder from prmary hopper and downstream hopper respectvely. Three factors of screw speed, mxng length, and channel depth whch affected RTD curves were studed. Two characterstc parameters of RTD model were used for characterzng RTD curves. The effects on fber degradaton presented by fber length and fber dsperson were studed. Fber degradaton was assocated wth two characterstc parameters of RTD model, and also wth above three factors. The nfluence of shear force wth dfferent screw speeds and varable channel depth on fber degradaton was studed and the man mpetus of fber degradaton was also presented. The optmal process for obtanng the balance of fber length and dsperson was the purpose of the study. 2. Experment 2.1. Materals Homo-polymer polypropylene (Y16, Shangha Petrochemcal Co., Ltd., Chna) was used as matrx resn. Its melt flow ndex was 16. g/mn at 23 C. Glass fber of drect rovng wth flament dameter 17 μm (GF362, Jush Group Co., Ltd., Chna) was used as the renforcement Expermental condton A modfed sngle screw extruder SJ-45B from Bejng Plastcs Machnery Insttute (screw dameter was 45 mm and length to dameter rato was 3) was used for processng LFT. Free flow screw of three leads was used for the study of extruson compoundng shown n Fgure 1. Three screws of dfferent channel depth (H 1 3 mm, H 2 6 mm and H 3 9 mm) were used n the study. Channel depth of 6 mm would be used unless otherwse noted. As shown n Fgure 2, three hoppers were equpped on the extruder, and the length from the frst and the second hopper to the ext was 5 mm (L 1 5 mm) and 25 mm (L 2 25 mm) respectvely. Mxng length was defned by the length from the frst hopper or the second hopper to the ext. Unless otherwse noted, mxng length was 5 mm. PP was added nto the extruder from the prmary hopper. Extruson temperature was controlled between 2 and 22 C. Glass fber, at 3% Fgure 1. Schematc dagram of free flow screw Fgure 2. Dagram of the modfed sngle screw extruder 561

3 of both PP and glass fber by weght, was added nto the extruder from the frst hopper or the second hopper. The extrudate was processed nto plates of requested sze by a calender, for analyss and testng purpose. Four screw speeds (n rpm, n rpm, n rpm, and n rpm) were nvestgated Fber length and dsperson The fber length measurements were conducted wth a sem-mage analyzer system. The matrx PP was removed by burnng the plate off n a muffle furnace mantaned at 5 C for 4 hr. The recovered glass fbers were dspersed n slcone-water suspenson, and then the lengths of at least 8 fbers were measured by the sem-mage analyzer system. Fber length presented by number average (L n ) was determned n Equaton (1): L n N L (1) Fber dsperson s defned as the weght fracton of fber flaments n total glass fber. After the matrx PP was drect burnt n a muffle furnace mantaned at 5 C for 4 hr, the weght of resdual ash was defned as W, and fber bundles, whch were not dspersed nto fber flaments selected manually was defned as W 1. Fber dsperson was determned n Equaton (2): W W1 Dsperson W N (2) 2.4. RTD test RTD curve was obtaned usng a pulse stmulus response technque, wth black PET/poly (ethylene terephthalate) spheres as the tracer. 5 partcles of the tracers were added nto the extruder from the hopper that glass fber had ntroduced, and the tme was recorded smultaneously. The extrudng plate of 1 mm wdth and 8 mm thckness was collected and black tracers n translucence PP can be seen clearly. The number of the tracers was counted at same tme nterval, and the fracton of the tracers n dfferent tme was obtaned for RTD analyss. RTD curves were descrbed as F( versus θ curve. Mathematcally, these relatons are descrbed by Equatons (3) and (4): C C E( t) CΔt Cdt F( t) (3) (4) where C s the tracer concentraton at tme t. The characterstc parameter of RTD curve, mean resdence tme (MRT) was calculates by Equaton (5): MRT (5) In order to compare RTD curves under dfferent processng condtons, normalzed tme θ was defned by Equaton (6): t θ MRT and F( was defned by Equaton (7): F ( F( t) E( t)dt te( t)dt C Δt C Δt t C Δt C Δt (6) (7) RTD curves of extruson compoundng at four screw speeds, two mxng length and three channel depth values have been measured. The analyss results of RTD curves were used to explan flow behavor of extruson compoundng, and more were assocated wth fber degradaton RTD model The RTD curves were frst modeled by consderng extruson compoundng as the combnaton of mxed flow and plug flow by Wolf and Rescnck (1963). The model named Wolf model s expressed by Equaton (8) [14]: θ P F( u( 1 e 1 P u(, θ < P; u( 1, θ p t where P s the fracton of PFR (plug flow reactor). (8) 562

4 The flow n CSTR was taken for mxed flow and the flow n PFR was consdered as plug flow, so Yeh et al. ntroduces d, the fracton of dead volume n CSTR nto Wolf model, so the model named Yeh model s expressed by Equaton (9) [15]: θ P d P F u (1 )(1 ) ( ( 1 e u(, θ < P; u( 1, θ p (9) The parameters d and P of above two models can be obtaned by non-lnear regresson calculaton, and were assocated wth fber degradaton. 3. Results and dscusson 3.1. RTD model RTD curve was shown as F( versus θ for 81. rpm screw speed and 6 mm channel depth n Fgure 3. The RTD curve suggested that flow behavor was between plug flow and mxed flow. There was some declnaton between Wolf model and expermental data; but the consstency between Yeh model and expermental data was good. Parameter values derved from Wolf model and Yeh model were lsted n Table 1. In Yeh model, addtonal parameter d, the fracton of dead volume n CSTR was presented besdes P, the fracton of PFR ndcted n Wolf model. As shown n Fgure 3, Yeh model was better than Wolf model for fttng expermental data. The exstence of parameter d ndcated there was some flow-lmted area durng extruson compoundng Influence of processng factors Screw speed RTD curves wth 6mm channel depth was llustrated n Fgure 4 by F( versus θ for dfferent screw speeds. It suggested that the flow behavor of extruson compoundng was between plug flow and mxed flow. Parameters values derved from Yeh model at dfferent screw speeds were lsted n Table 2. The fracton of PFR P and the fracton of dead volume Fgure 3. RTD curves as F( versus θ wth expermental data and dfferent models Fgure 4. RTD curves as F( versus θ at dfferent screw speeds Table 1. Parameter values derved from dfferent models Screw No. Channel depth [mm] Screw speed [rpm] P d S2 Wolf model S2 Yeh model Table 2. Parameter values at dfferent screw speeds Screw No. Channel depth [mm] Screw speed [rpm] Mxng dstance [mm] P d S S S S

5 Table 3. Fber length and dsperson at dfferent screw speeds Screw No. Channel depth [mm] Screw speed [rpm] Fber length [mm] Fber dsperson [%] S S S S n CSTR d have no bg varaton at dfferent screw speeds, whch showed that the flow behavors of extruson compoundng were almost the same. The results ndcted that mxng capacty was slghtly changed at dfferent screw speeds. Table 3 lsted fber length and dsperson at dfferent screw speeds wth 6mm channel depth. As the screw speed ncreased from 4.5 rpm to 81. rpm, the fber length changed from mm to mm, and the dsperson ncreased from 3.58 to 31.36%. There were slghtly change of fber length and dsperson wth ncreasng of screw speed. The results ndcated that fber length and dsperson would be somehow related to the value of P and d Mxng length Fgure 5 lsted RTD curves were presented by F( and θ for dfferent mxng length. RTD curves showed that flow behavor of extruson compoundng was between plug flow and mxed flow, and flow behavor of longer mxng length trended towards mxed flow. Parameters values at dfferent mxng length derved from Yeh model were lsted n Table 4. The fracton of PFR P was decreased wth the extenson of mxng length, and the fact showed that the trend of the flow behavor was towards mxed flow. The fracton of dead volume n CSTR d remaned unchanged wth the ncrement of mxng dstance. Table 5 lsted fber length and dsperson wth dfferent mxng length at 81. rpm screw speed and 6 mm channel depth. Fber length was not changed Fgure 5. RTD curves as F( versus θ at dfferent mxng length wth the addton of mxng length, and fber dsperson ncreased dstnctly from to 3.58%. As to Yeh model, the extruder s consdered as the combnaton of PFR and CSTR, extruson compoundng s regarded as the combnaton of plug flow and mxed flow. The results showed that fber dsperson was ncreased wth the ncrement of P, and the ncrement of P would mpar fber flament. Fber length was not affected by the change of P, and the flow behavor of PFR,.e. plug flow would not brng fber to fracture severely Channel depth Mxng length of 25 mm RTD curves presented as F( versus θ were shown n Fgure 6 at 25 mm mxng length, wth dfferent channel depth. It ndcted that flow behavor of extruson compoundng was between plug flow and Table 4. Parameter values at dfferent mxng length Screw No. Channel depth [mm] Mxng length [mm] Screw speed [rpm] P d S S Table 5. Fber length and dsperson at dfferent mxng length Screw No. Channel depth [mm] Screw speed [rpm] Mxng length [mm] Fber length [mm] Fber dsperson [%] S S

6 mxed flow, and flow behavor of bgger channel depth was close to plug flow. Parameters values wth dfferent channel depth at 25 mm mxng length derved from Yeh model were lsted n Table 6. The fracton of PFR P was not changed wth the addton of channel depth. The fracton of dead volume n CSTR d was ncreased wth the addton of channel depth, and the fact showed that flow behavor of bgger channel depth tended to plug flow. Table 7 lsted fber length and dsperson wth dfferent channel depth and 25 mm mxng length. Wth the ncreasng of channel depth from 3 to 9 mm, fber length ncreased from 1.1 to mm and fber dsperson reduced from 21.4 to 7.42%. The results showed that the ncrement of the fracton of dead volume n CSTR d mpled the mprovement of fber length and the mparment of fber dsperson. As to Yeh model, the extruder s consdered as the combnaton of PFR and CSTR, extruson compoundng s consdered to be a combnaton of plug flow and mxed flow. The mxng n a dead volume of CSTR s lmted. Wth same value of P, the addton of d reduced mxng capacty of extruson compoundng. The varaton of fber length and dsperson was due to the reducton of mxng capacty presented by the ncreasng of the fracton of dead volume n CSTR d. Fgure 6. RTD curves as F( versus θ at dfferent channel depth for 25 mm mxng length Mxng length of 5 mm RTD curves as F( and θ wth dfferent channel depth at 5 mm mxng length were shown n Fgure 7. RTD curves suggested that flow behavor of extruson compoundng was between plug flow and mxed flow. Wth the ncreasng of channel depth flow behavor approached to plug flow. Parameter values wth dfferent channel depth at 5 mm mxng length derved from Yeh model were lsted n Table 8. The fracton of PFR P was ncreased from.44 to.68 and showed that the flow behavor tended to plug flow wth the ncreasng of channel depth. The fracton of dead volume n CSTR d was ncreasng from.2 to.2 wth the ncreasng of channel depth, whch showed that the screw of deeper channel behaved wth less mxng capacty. The value of d (.2) for S1 screw Fgure 7. RTD curves as F( versus θ at dfferent channel depth for 5 mm mxng length Table 6. Parameter values of P and d at dfferent channel depth for 25 mm mxng length Screw No. Channel depth [mm] Screw speed [rpm] P d S S S Table 7. Fber length and dsperson at dfferent channel depth for 25 mm mxng length Screw No. Channel depth [mm] Screw speed [rpm] Fber length [mm] Fber dsperson [%] S S S

7 Table 8. Parameter values of P and d at dfferent channel depth for 5 mm mxng length Screw No. Channel depth [mm] Screw speed [rpm] P d S S S Table 9. Fber length and dsperson at dfferent channel depth for 5 mm mxng length Screw No. Channel depth [mm] Screw speed [rpm] Fber length [mm] Fber dsperson [%] S S S ndcated that there exsted severe back mxng besdes forward mxng. Table 9 lsted fber length and dsperson wth dfferent channel depths at 5 mm mxng length. Fber length ncreased from 7.61 to mm and fber dsperson reduced from to 3.58% wth the ncreasng of channel depth. Those changes of fber degradaton were due to mutual effect of the addton of both P and d Influence of shear force In sngle screw extruder, shear rate could be expressed wth constant channel depth and constant screw ptch by Equaton (1) [16]: π D n γ& (1) H where γ& s smple shear rate, D s screw dameter, n s screw speed, and H s channel depth. Shear force brought by the melt under dfferent screw speeds was lsted n Table 1. When screw speed ncreased from 4.5 to 81. rpm, shear force rose from to s 1, glass degradaton was not changed sgnfcantly based on the data lsted n Table 3. Shear force brought by the melt under dfferent channel depth was shown n Table 11. Whereas channel depth ncreased from 3 to 9 mm wth 25 and 5 mm mxng length, shear force rose from 62.4 to s 1, glass degradaton changed dstnctly based on the data lsted n Table 7 and Table 9. From the dfferent changes of fber degradaton brought by the ncreasng of shear force, the t was concluded that shear force was not man mpetus of fber degradaton. In the process of LFT-PP extruson, temperature range was durng 2 22 C, and correspondng η was about 1 5 Pa s, sngle shear rate was about 1 s 1 from the data n Table 1 and Table 11, see Table 8. So that the force γ&η of glass fber underwent durng the process of LFT-PP extruson would be about 1M Pa, as shown by Equaton (11): 2 ( γ&η ) (11) Salnas et al. posted essental condton presented by Equaton (12) that fber would fracture durng the compoundng [17]. As for glass fber, γ&η 1 6 MPa. The value of γ&η that would brng glass fber fracture s far more than that n the process of LFT-PP extruson. Table 1. Shear rate of S2 screw at dfferent screw speeds Screw No. Channel depth [mm] Screw speed [rpm] Smple shear rate [s 1 ] S S S S Table 11. Shear rate under dfferent channel depth wth 5 mm mxng dept Screw No. Channel depth [mm] Screw speed [rpm] Smple shear rate [s 1 ] S S S

8 2σ (ln 2r 1.75) γ&η (12) 2 r As descrbed by Wolf, durng mxng glass fber length was reduced by undergong three nteractons of fber-equpment, fber-fber, and fberpolymer [6]. Normally shear force of glass fber appled by the melt would not brng glass fber to be fractured because actual shear force resulted by the melt was far less than theoretcal shear force derved by Salnes as descrbed above. But there are some regons such as the gap between screw edge and barrel or hghly turbulent flow where fber fracture would happen, the extent that brought glass fber to fracture should be small. The man mechansm whch brngs glass fber to facture would be fber-equpment, and fber-fber nteracton, and above two nteractons would be mproved by the enhancement of mxng capacty that could be presented by the characterstc parameters of RTD model such as P and d. 4. Conclusons c Fber degradaton expressed by fber fracture and fber flament could be nterpreted by characterstc parameters of RTD model such as the fracton of PFR P and the fracton of dead volume n CSTR d. Fber fracture characterzed by fber length was only ncreased wth the ncrement of the fracton of dead volume n CSTR d. Fber flament evaluated by fber dsperson was mpared wth the ncreasng of the fracton of PFR P and the fracton of dead volume n CSTR d. P and d were almost unaffected by the addton of screw speed, and there was a small reducton of fber length and mld ncrement of fber dsperson at same tme. P was decreased from.74 to.57 and d showed no change wth the addton of mxng length.there was a dstnct ncrease of fber dsperson from to 3.58% and no change of fber length wth the ncrement of P. Wth the ncreasng of channel depth from 3 to 9 mm for 25 mm mxng length, P was almost same whle there was sgnfcant ncreasng of d from.4 to.29. Fber length was ncreased from 1.1 to mm and fber dsperson was decreased from 21.4 to 7.42% wth the addton of d. Wth the ncreasng of channel depth from 3 to 9 mm for 5 mm mxng length, P and d showed sgnfcant change. P was added from.44 to.68 and d was ncreased from.2 to.21 respectvely. Fber length was ncreased from 7.61 to mm and fber dsperson was reduced from to 24.44% wth the ncrement of P and d. Screw speed and channel depth would brng some change of shear force. Shear force rse from to s 1 wth screw speed ncreasng from 4.5 to 81. rpm, glass degradaton was not changed sgnfcantly. Whereas shear force rse from 62.4 to s 1 wth channel depth ncreasng from 3 to 9 mm, glass degradaton was changed dstnctly. The result was derved that shear force was not man mpetus of fber degradaton. Fber degradaton was brought by undergong three nteractons of fber-equpment, fber-fber, and fber-polymer durng the extruson. The man mechansm whch brngs glass fber to facture would be fber-equpment, and fber-fber nteracton mproved by the enhancement of mxng capacty that could be presented by the characterstc parameters of RTD model such as P and d. From the nfluence of process condton and screw confguraton on fber length and dsperson, 6mm channel depth and 5 mm mxng length of free flow screw was apprecate for the optmal balance of fber length and dsperson. References [1] Karan H. G.: Handbook of polypropylene and polypropylene compostes. Marcel Dekker, New York (1999). [2] Schemme M.: Long fbre renforced thermoplastcs. Kunststoffe, 93, (23). [3] Baley R., Kraft H.: A study of fbre attrton n the processng of long fbre renforced thermoplastcs. Internatonal Polymer Processng, 2, (1987). [4] Fsa B.: Mechancal degradaton of glass fbers durng compoundng wth polypropylene. Polymer Compostes, 6, (1985). [5] Kuroda M. M. H., Scott C. E.: Intal dsperson mechansms of chopped glass fber n polystyrene. Polymer Compostes, 23, (22). [6] Wolf D., Holn N., Whte D. H.: Resdence tme dstrbuton n a commercal twn-screw extruder. Polymer Engneerng and Scence, 26, (1986). [7] Ldor G., Tadmor Z.: Theoretcal analyss of resdence tme dstrbuton functons and stran dstrbuton functons n plastcatng screw extruders. Polymer Engneerng and Scence, 16, (1976). 567

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