On Pressure and Temperature Affected Shear Viscosity Behaviour of Poly(Lactid) Acid Melt

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1 On Pressure nd Temperture Affected Sher Viscosity Behviour of Poly(Lctid) Acid Melt P. PIYAMANOHA, T. SEDLAEK, P. SAHA Polymer entre entre of Polymer System Toms Bt University in Zlin nm. T. G. Msryk 5555, Zlin ZEH REPUBLI Abstrct: Rheologicl properties of poly(lctid) cid (PLA) s one of the most importnt bio-bse sustinble polymer potentilly pplicble for medicl pplictions hd been studied. Sher flow behviour of PLA melt ws determined using cpillry rheometer modified by bckpressure device directly connected to the min rheologicl pprtus. The device employed increses pressure ctuting on polymer melt strem in cpillry reservoir through restricting vlve resulting in n increse of melt viscosity. The temperture nd pressure sensitivity coefficients were determined through the viscosity dt fitting with the rreu-ysud model. The study reveled tht sher viscosity of tested PLA melt is strongly ffected by temperture nd pressure. Key-Words: Rheology, Sher Viscosity, Pressure oefficient, Temperture oefficient, Poly(lctid) cid, rreu- Ysud model 1 Introduction As it is indicted by n incresing number of reserch ppers in pst decde, poly(lctic) cid (PLA) which is produced from nturl renewble sources become n lterntive sustinble mteril in ner future [1]. This fct is connected nmely with high reproducibility of PLA production (compring to common biopolymers), nd controllble degree of biodegrde-bility. PLA is liphtic polyester which cn be produced by ring opening polymeriztion of lctic cid (LA) monomer nturlly occurred in mny food production industries. It ws shown tht semi-crystlline structure of this thermo-plstic polymer depends on the content of l-la nd d-la stereo isomers, prticulrly d-la decreses the crystlliztion rte of PLA, note tht 12-15% of d-la could even mke PLA completely morphous. Physicl ppernce nd properties of PLA re usully compred with polystyrene (PS) or poly(ethylene terephthlte) (PET) in term of its strength, rigidity, trnsprency, nd processing conditions, while its biodegrdbility wrrnts n importnt potentil dvntge especilly for its singleuse pplictions exploitble, for exmple, in medicl ppliction, food pckging, nd griculturl films. Generlly, PLA cn be formed employing either of polymer process such s extrusion, injection moulding, blow-moulding, etc. Processing flow behviour of PLA melts hve been thus studied in severl investigtions [2-6] describing sher nd elongtionl viscosities with the focus to solve the flowing problems risen in conventionl processes, s it ws done before with other similr thermoplstic polymers. Since the precise simultions of polymer processes ply importnt role in industril problem solutions, the pressure coefficient, commonly omitted up to now in flow behviour simultion, is being more essentil, prticulrly for high pressure processes, such s injection moulding. An investigtion of the pressure coefficient hs been proposed by Mxwell nd Jung [7] nd studied continuously since 1950s. One of the most prcticl used methodologies re using conventionl cpillry rheometer with different length of dies [8-9] or die blockge t the exit [10-13] in order to generte different pressure build up in melt strem. Moreover, some mthemticl models hve been chosen for the pressure influence evlution. Reported vlues of pressure coefficient presented in reserch ppers proved cceptble devition concerning vrious methodologies currently employed for its determintion. Up to now, the mostly conventionl polymers such s polystyrene (PS), polypropylene (PP), polyethylene (PE), polymethyl methcrylte (PMMA), Polycrbonte (P), poly-methylstyrene-co-crylo-nitrile (PMSAN), nd crylonitrile-butdiene-styrene (ABS) [8-13] were investigted, while the rheologicl properties of PLA nd its blends hve been focused only on routine sher nd elongtionl viscosity behviour description t chosen tempertures conditions. ISBN:

2 In present work, the pressure ffected flow behviour of PLA ws investigted by mens of cpillry rheometer equipped with bckpressure device designed t Toms Bt University nd in detils described by Sedlcek in [11]. Obtined flow curves t vrious temperture nd pressure conditions were described by the rreu-ysud model determining in this wy the both temperture nd pressure sensitivity coefficients. 2 Experimentl 2.1 Mteril nd Therml Anlysis A commercil vilble PLA grde Ingeo4060D produced by NtureWorks (USA) ws chosen for n investigtion. The resin grnules hd been dried in n oven t 55 for 4 hrs prior to experiment nd then kept in the oven t 45 long the experiment in order to prevent moisture sorption cusing significnt degrdtion of PLA. Therml nlysis of used polymer ws crried out using differentil scnning clorimeter (DS 1, Mettler Toledo, Switzerlnd) t temperture rnge from -40 to 210 nd heting nd cooling rte of 20/min. While the first heting scn ws performed in order to remove stress nd therml history of the smple, chrcteristics temperture, nmely glss trnsition, melting temperture, nd temperture of crystlliztion, were determined from the second heting scn. 2.2 Flow Behviour Determintion The pressure dependent sher flow behviour of the PLA ws mesured using cpillry rheometer (Goettfert 2001, Goettfert Inc., Germny) modified with device consisting of regulted pressure chmber nd restriction needle vlve locted t the exit of the chmber. Applied bckpressure device for determintion of pressure influence on viscosity behviour ws designed t Toms Bt University in Zlin, while its functionlity ws successfully verified in severl reserch works [11, 14-16]. Pressure profile during experiments ws recorded by the mens of the pressure trnsducer MDA 462-1/2 (Dynisco, USA) with nominl rnge of 700 brs ttched to the lower chmber in order to record pressure build up (exit or bckpressure) nd the entrnce pressure trnsducer GFT 048B (Goettfert, Germny) with nominl rnge of 2000 br monitoring pressure conditions in the reservoir bove the cpillry die entrnce. Two dies of two L/D (length/dimeter) rtios were selected in order to evlute pproprite corrections of viscosity behviour: pressure profile determined using 1/20 cpillry ws corrected for n entrnce pressure loss utilizing 0.12/1 orifice die. The experiments were performed t two different tempertures 170, nd 190 keeping constnt long the reservoir nd cpillry zone. An dditionl externl temperture control ws used for the pressure regulted chmber. The experiment ws crried out t different constnt pprent sher rtes covering rnge from s -1. Pressure effect ws determined from the mesurements comprising five different djustments of needle vlve t ech temperture nd repeted severl times in order to clculte men vlue of viscosity in further evlution. The resin melting time of three minutes ws kept in order to prevent polymer degrdtion from long residue time. 2.1 Temperture nd Pressure oefficient Evlution The following equtions were used to clculte the true sher stress ( c ), nd sher rte ( ) ccording to Bgley nd Rbinowitch corrections, respectively, while the negligible exit pressure loss ws ssumed in this clcultions: P PE R (1) 2L 4Q 3n 1 (2) 3 R 4n where R nd L re the rdius nd length of cpillry, respectively, nd Q stnds for flow rte. The correlted entrnce pressure drop mesurement ws determined bsed on the difference in cpillry nd orifice die pressure profile, where cpillry pressure drop P = P en -P ex (3) nd entrnce pressure drop P E = P Oen -P Oex (4) re employed. While the subscripts O mens orifice die, stnds for cpillry die, en nd ex correspond to entrnce nd exit pressure, respectively. The prmeter n ws obtined from the slope of log-log plot between true sher stress nd pprent sher rte. The sher viscosity c cn then be clculted from: (5) Assuming liner pressure profile in the cpillry die, men pressure (P m ) cn be determined from: P m Pen Pex (6) 2 ISBN:

3 Endo Sher viscosity (P.s) Recent Reserches in Geogrphy, Geology, Energy, Environment nd Biomedicine Finlly, the determintion of temperture coefficient () nd pressure coefficient () were determined by fitting the viscosity dt with rreu-ysud model: f (7) K f n / 1 where 0,, nd re zero-sher viscosity, sher rte, nd sher rte dependent viscosity, respectively, K 1, n, re empiricl constnts. The simple explntion of temperture nd pressure function, fused in polymer engineering clcultion for determintion of the nd re: It is cler tht PLA4060D is n morphous polymer performing by both cooling nd heting rmps. There is only one trnsition which cn be determined during the both heting nd cooling periods. Next to the first scn, the second heting ws performed. Obviously smll shift of trnsition temperture compring to the first heting rmp ws reveled due to ersing of stress nd therml history in the polymer resin together with utter evportion of wter. The glss trnsition temperture evluted from the second heting scn is round Similr to the first scn, no further temperture effects were determined on the curve thus indicting n bsence of crystllinity portion in the resin. f f T T r e (8) P m P r e (9) men pressure (MP) where P, nd T, stnds for guge pressure nd temperture, respectively, while r mens reference point. Both pressure nd temperture functions could be combined s follow: f e T T P P r m r (10) This sensitivity function ws used to clculte the verge pressure nd temperture coefficients from the experimentl dt. 3 Results & Discussion The DS scnning microgrph of used PLA is shown in Figure 1. ooling Heting Temperture () Figure 1. DS dt of PLA4060D t 20/min heting nd cooling rte (the solid lines refer 2 nd scn nd the dsh line refer 1 st scn) Sher rte (s -1 ) Figure 2. Pressure ffected sher viscosity dt of PLA4060D t tempertures of 170, nd 190; symbols stnd for experimentl dt, while the solid lines represent dt fitting by the rreu-ysud model The flow behviours of tested PLA t different men pressures nd tempertures re presented in Figure2. While pressure melt growth induces the obvious viscosity increse, s it is cler on the contrst of experimentl sher viscosities dt t vrious men pressures s indicted by different symbols in the grph, flow dt set t temperture of 190 hs significntly lower vlues of viscosity in the whole rnge of sher rte compred to the flow behviour of tested PLA t 170. Furthermore, sher viscosity of tested PLA decline with incresing sher rte for the ech men pressure indicting preservtion of sher thinning behviour typicl for common thermoplstic polymer. Thus it cn be concluded tht the pressure increse in the PLA melt did not influence the sher thinning behviour while it effects only on the mgnitude of viscosity. The pressure-temperture-dependent flow curves fitting experimentl dt re presented by solid lines in Figure 2. The numericl clcultion results employing ISBN:

4 lest squres method were determined utilizing the rreu-ysud model (see Eq. 7) with the sensitivity function expressed by Eq. 10. Temperture of 170 nd pressure of 0.1 MP were used here s the reference vlues. The clculted prmeters of rreu-ysud model determined in the described procedure re summrized in Tble 1. TABLE 1. Fitting prmeter of rreu-ysud model determined for PLA (P.s) n (-) K 1 (s -1 ) (-) (GP -1 ) ( ) From the tble, it cn be concluded tht tested PLA hs reltively high sensitivity to both pressure nd, especilly, temperture. ompring the experimentl dt nd fitting curves it could seen tht they do not fit ech other in bsolute vlues. It should be mentioned here tht observed discrepncies could be firly withdrw by the mens of discrete fitting of individul investigted effects, in other words description of flow behviour under prticulr pressure nd temperture condition will ssure ccurte result. Nevertheless, in this wy the dvntge of unique set of fitting prmeters for further modelling purposes will be lost. lerly, pertinence of determined flow behviour description is sufficient enough for estimtion of flow behviour dependencies, moreover even for the purposes of flow sitution simultions in the rnge of common processing conditions. 4 onclusion Pressure nd temperture sensitivity of PLA4060D ws investigted by mens of the modified cpillry rheometer. The bckpressure device providing different pressure levels llowed determintion of pressure influence on flow behviour of the PLA melts. Determined sher viscosity dt ws evluted utilizing the rreu-ysud model with sensitivity function. The effect of pressure increse nd temperture chnge on flow behviour ws described by the men of the temperture nd pressure sensitivity coefficients. It ws proved tht even the PLA sher thinning behviour t selected temperture conditions ws not ffected by n ctuting pressure, the sher viscosity of tested PLA melt is significntly influenced by the both effects. Pressure nd temperture coefficients hving vlues of (GP -1 ) nd ( ), respectively, hve been found to be pproprite to describe the sher flow behviour of PLA4060D in dequte mnner. Acknowledgment: Present rticle ws creted within the frmework of project entre of Polymer Systems (reg. number: Z.1.05/2.1.00/ ). The Ministry of Industry nd Trde of the zech Republic (project 2A-1TP1/126) nd the Ministry of Eduction, Youth nd Sports of the zech Republic, (Grnt. No. MSM ) re cknowledged for the finncil support. References: [1] Rfel A., Loong-Tk L., Susn E.M. S., nd Hideto T., Poly(lctic cid): Synthesis, Structures, Properties, Processing, nd Applictions, Wiley Series on Polymer Engineering nd Technology, 2010 [2] Fng Q., nd Hnn M. A., Rheologicl properties of morphous nd semicrystlline polylctic cid polymers, Industril rops nd Products, Vol.10, 1999, pp [3] Plde L.-I., Lehermeier H. J., nd Dorgn R. J., Melt Rheology of High L-ontent Poly(lctid cid), Mcromolecules, Vol.34, 2001, pp [4] Lehermeier HJ., nd Dorgn JR., Melt rheology of poly(lctid cid): onsequences of blending chin rchitectures, Polymer Engineering nd Science,Vol.41, 2001, pp [5] Liu J., Lou L., Yu W., Lio R., Li R., nd Zhou., Long chin brnching polylctide: Structures nd properties, Polymer, Vol.51, 2010, pp [6] Mihi M., Huneult M. A., nd Fvis B. D., Rheology nd Extrusion Foming of hin-brnched Poly(lctid cid), Polymer Engineering nd Science, Vol.50, pp [7] Mxwell B., nd Jung A., Hydrosttic pressure effect on polymer melt viscosity, Modern Plstics, Vol.35, 1957, pp [8] Ling J.-Z., Pressure effect of viscosity for polymer fluids in die flow, Polymer, Vol.42, 2001, pp [9] Pntni R., nd Sorrentino A., Pressure Effect on Viscosity for Atctic nd Syndiotctic Polystyrene, Polymer Bulletin, Vol.54, 2005, pp [10] Binding D.M., ouch M.A., nd Wlters K., The pressure dependence of the sher nd elongtionl properties of polymer melts, Journl of Non-Newtonin Fluid Mechnics, Vol.79, 1998, pp [11] Sedlcek T., Ztloukl M., Filip P., Boldizer A., nd Sh P., On the Effect of Pressure on the Sher nd Elongtionl Viscosities of Polymer Melts, Polymer Engineering nd Science, Vol.44, 2004, pp ISBN:

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