Rheological properties and crystallization kinetics of polypropylene block copolymer with repeated extrusion

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1 Kore-Austrli Rheology Journl Vol. 17, No. 1, Mrch 2005 pp Rheologicl properties nd crystlliztion kinetics of polypropylene block copolymer with repeted extrusion Yu-tek Sung, Won Jin Seo, Jong Sung Kim, Woo Nyon Kim*, Dong-Hwn Kwk 1 nd Te-Won Hwng 1 Deprtment of Chemicl nd Biologicl Engineering, Applied Rheology Center, Kore University, Anm-dong, Seoul , Kore 1 Reserch & Development Division for Hyundi Motor Compny & Ki Motors Corportion, Jngduk-dong, Hwseong-si, Gyeonggi-do , Kore (Received, November 11; finl revision received Mrch 3, 2005) Abstrct Rheologicl properties nd crystlliztion kinetics of the polypropylene (PP) block copolymer nd recycled PP block copolymer were studied by dvnced rheometric expnsion system (ARES), differentil scnning clorimetry (DSC), nd opticl microscopy. In the study of the dynmic rheology, it is observed tht the storge modulus nd loss modulus for the PP block copolymer nd recycled PP block copolymer did not chnge with frequency. In the study of the effect of the repeted extrusion on the crystlliztion rte, hlf crystlliztion time of the PP smples ws incresed with the number of repeted extrusion in isotherml crystlliztion temperture (T c ). From the isotherml crystlliztion kinetics study, the crystlliztion rte ws decresed with the increse of the number of repeted extrusion. Also, from the result of Avrmi plot, the overll crystlliztion rte constnt (K) ws decresed with the increse of the number of the repeted extrusion. From the study of the opticl microscopy, the size of the spherulite of the PP smples did not chnge significntly with the number of repeted extrusion. However, it ws clerly observed tht the number of the spherulite growth sites ws decresed with the number of repeted extrusion. From the results of the crystlliztion rte, isotherml crystlliztion kinetics, Avrmi plots, nd opticl microscopy, it is suggested tht the crystlliztion rte of the PP block copolymer is decresed with the increse of the number of repeted extrusion. Keywords : polypropylene, crystlliztion kinetics, extrusion, rheology 1. Introduction The properties of semi-crystlline polymer such s polypropylene (PP) re ultimtely dependent on the structure of the polymer (Almo nd Mndelkern, 1991; Almo et l., 2003; Chen et l., 1997; Chun et l., 2000; Kim et l., 1994; Mreu nd Prud homme, 2002; Mrreu nd Prud homme, 2003; Schönherr et l., 2002). The structure of the semi-crystlline polymer is controlled by the mechnism of the crystlliztion nd crystlliztion kinetics. The crystlliztion mechnism is influenced by severl fctors such s therml tretment nd geometricl fctors (Mreu nd Prud homme, 2002; Mrreu nd Prud homme, 2003; Schönherr et l., 2002). For exmple, Schönherr et l. (Schönherr et l., 2002) reported the effects of the therml tretment on the crystllinity of the PP elstomer *Corresponding uthor: kimwn@kore.c.kr 2005 by The Koren Society of Rheology nd Mreu nd Prud homme (Mreu nd Prud homme, 2002) reported the dul growth rte mechnism in the crystlliztion of the poly(ε-cprolctone)/poly(vinyl chloride) blend which ws cused by the geometric fctor of the polymer smple. Also, the crystlliztion kinetics ws extensively studied by severl reserchers (Almo nd Mndelkern, 1991; Almo et l., 2003). Almo nd Mndelkern (Almo nd Mndelkern, 1991) studied the crystlliztion kinetics for the ethylene-hexne rndom copolymer by the overll crystlliztion. Almo et l. (Almo et l., 2003) reported the crystlliztion kinetics of the isottic PP using the liner growth rte (G). PP hs been widely used in utomobile industry. In some cses, the recycled PP re used s the utomobile prts for environmentl concern. For its industril importnce, mny reserches hve studied the properties of the recycled PP (Aurrekoetxe et l., 2001; Aurrekoetxe et l., 2001b). Aurrekoetxe et l. (Aurrekoetxe et l., 2001) reported Kore-Austrli Rheology Journl Mrch 2005 Vol. 17, No. 1 21

2 Yu-tek Sung, Won Jin Seo, Jong Sung Kim, Woo Nyon Kim, Dong-Hwn Kwk nd Te-Won Hwng the crystlliztion behvior nd mechnicl properties of the recycled PP nd they lso reported the frcture behvior of the recycled PP (Aurrekoetxe et l., 2001b). In this study, rheologicl properties nd crystlliztion kinetics of the virgin PP copolymer nd recycled PP copolymer were studied using the dvnced rheometric expnsion system (ARES) for the dynmic rheologicl properties nd differentil scnning clorimetry (DSC) for the overll crystlliztion rte, nd the opticl microscopy for the spherulite morphology. 2. Experimentl 2.1. Polymers The polymer used in this study ws obtined from commercil source. PP (JI-360 grde) ws provided by Honm Petroleum Corportion. The PP ws synthesized using the MgCl 2 -supported Ziegler Ntt ctlyst. The JI-360 grde is the 6 wt% ethylene contining PP block copolymer, which is widely used in the utomobile industry. The PP contins phosphite ntioxidnt (3,000 ppm). The chrcteristics of the PP used in this study re shown in Tble Preprtions The recycled PP smples were prepred by extrusion of the virgin PP using Prism twin screw extruder. The 1-time extrusion of the virgin PP, 2-times repeted extrusion of the virgin PP, nd 3-times repeted extrusion of the virgin PP were designted s PP-A, PP-B, nd PP-C, respectively. The PP smples were dried under vcuum (< 1mmHg) t 90 o C for 12 hrs before use. The tempertures of the extruder were set t 180 nd 220 o C in feeding nd brrel zones, respectively Differentil scnning clorimetry The isotherml crystlliztion of the PP smples ws nlyzed by Perkin-Elmer DSC-7. Temperture clibrtion ws performed using indium (T m =156.6 o C, H f = 28.5 J/g). For the isotherml crystlliztion of the PP smples, the smples were melted t 200 o C for 5 min, nd then rpidly cooled to the isotherml crystlliztion temperture (115, 120, 125, 130 o C). During isotherml crystlliztion of the PP smples, the het flow ws recorded until the het flow is not chnged Opticl microscopy The microscopy tests were crried out with opticl microscopy (Olympus BX-60F) for the spherulite growth. For the microscopy test of the isotherml crystlliztion of the PP smples, the PP smples were melted on hot stge (Mettler FP-90) t 200 o C for 5 min, nd then cooled 130 o C by 5 o C/min. 3. Results nd discussion Fig. 1 shows the storge modulus (G ) nd loss modulus (G ) with frequency for the virgin PP, PP-A, PP-B, nd PP- C t 190 o C. It is observed tht the storge modulus nd loss modulus for the virgin PP, PP-A, PP-B, nd PP-C do not chnge with frequency. From Fig. 1, it is lso observed tht the relxtion time of the virgin PP, PP-A, PP-B, nd PP- C from the cross-over point of the storge modulus nd loss modulus does not chnge. It is known tht the dynmic rheologicl properties such s storge modulus, loss modulus, nd relxtion time reflect on the moleculr structure such s moleculr weight, moleculr weight distribution, nd brnching degree (Peón et l., 2001; Shroff 2.3. Rheology Dynmic mesurements were crried out on dvnced rheometric expnsion system (ARES) in oscilltory sher t 10% strin in the prllel-plte rrngement with 25 mm plte under dry nitrogen tmosphere. The PP smples were fbricted in disk with 2 mm in thickness. The frequency sweeps from 0.05 to 100 rd/s were crried out t 190 o C. For ll mesurements, it hs been verified tht the behvior of the PP smples were liner viscoelstic. Tble 1. Chrcteristics of the ethylene contining polypropylene block copolymer used in this study T m Ethylene Smple M n M w MWD ( o C) content (wt%) PP b Mesured in our lbortory by DSC. b Supplied by Honm Petroleum Corp. Fig. 1. Storge modulus (G ) nd loss modulus (G ) vs. frequency for the virgin PP, PP A, PP-B, nd PP-C t 190 o C. 22 Kore-Austrli Rheology Journl

3 Rheologicl properties nd crystlliztion kinetics of polypropylene block copolymer with repeted extrusion Fig. 2. Hlf crystlliztion time (τ 1/2 ) of the virgin PP, PP-A, PP- B, nd PP-C with isotherml crystlliztion temperture (T c ). Tble 2. Hlf crystlliztion time (τ 1/2 ) of the virgin PP, PP-A, PP-B, nd PP-C with isotherml crystlliztion temperture (T c ) Smple τ 1/2 (min) b τ 1/2 (min) τ c 1/2 (min) d τ 1/2 (min) Virgin PP PP-A PP-B PP-C τ 1/2 ws mesured by DSC t 115 o C. b τ 1/2 ws mesured by DSC t 120 o C c τ 1/2 ws mesured by DSC t 125 o C d τ 1/2 ws mesured by DSC t 130 o C Fig. 3. Crystlliztion frction (X t ) of the virgin PP, PP-A, PP-B, nd PP-C t 130 o C. nd Mvridis, 1999; Veg et l., 1998). Therefore, it is thought tht the moleculr weight nd moleculr weight distribution mong the PP smples do not chnge significntly with repeted extrusion. Fig. 2 shows the hlf crystlliztion time (τ 1/2 ) of the virgin PP, PP-A, PP-B, nd PP-C with isotherml crystlliztion temperture (T c ). The hlf crystlliztion time of the PP smples is incresed with incresing the number of the extrusion in the crystlliztion temperture rnge (115~ 130 o C). The increse of the hlf crystlliztion time of the PP smples is clerly observed t the slow crystlliztion temperture (130 o C). At 130 o C, the hlf crystlliztion time is incresed from 10.3 to 13.6 min for the virgin PP nd PP-C, respectively. In Tble 2, the hlf crystlliztion time (τ 1/2 ) of the virgin PP, PP-A, PP-B, nd PP-C with isotherml crystlliztion temperture (T c ) is represented. Fig. 3 shows the crystlliztion frction (X t ) of the virgin PP, PP-A, PP-B, nd PP-C t 130 o C, which obtined form the therml nlysis results of the differentil scnning clorimetry. The crystlliztion frction (X t ) ws determined s follows: t 0 dh dt X t = ( )dt (1) ( dh dt 0 )dt where X t is the crystlliztion frction fter time t. A numertor is the het generted fter time t nd denomintor is the het generted fter time goes infinite. From Fig. 3, it is observed tht the crystlliztion frction shifts to longer crystlliztion time with the increse of the number of repeted extrusion, indicting tht the PP smples crystllize slowly with the increse of the number of repeted extrusion. For the het of fusion ( H f ) of the virgin PP, PP-A, PP-B, nd PP-C, however, does not chnge significntly, which shows 88 ± 5J/g. The crystlliztion kinetics of the PP smples under isotherml crystlliztion were nlyzed by the following Avrmi eqution (Avrmi, 1940): log[ ln(1 X t )] = logk + nlogt (2) where n is the Avrmi exponent nd K is the overll crystlliztion rte constnt, respectively. Fig. 4 shows the Avrmi plots of the virgin PP, PP-A, PP- B, nd PP-C t 130 o C. From Fig. 4, the overll crystlliztion rte constnt (K) is decresed from to for the virgin PP nd PP-C, respectively (Tble 3). The Avrmi exponent (n) of the PP smples, however, do not chnge with the number of repeted extrusion (Tble 3). In generl, the Avrmi exponent (n) is relted to the type nd geometry of the nucletion nd growth. Fig. 5 () nd (b) show the opticl microgrphs of the virgin PP nd PP-C under the isotherml crystlliztion t 130 o C, respectively. The microgrphs were tken fter 5 min isotherml condition t 130 o C. From Fig. 5, it is observed tht the virgin PP nd PP-C show the spherulite Kore-Austrli Rheology Journl Mrch 2005 Vol. 17, No. 1 23

4 Yu-tek Sung, Won Jin Seo, Jong Sung Kim, Woo Nyon Kim, Dong-Hwn Kwk nd Te-Won Hwng Fig. 4. Avrmi plots of the virgin PP, PP-A, PP-B, nd PP-C t 130oC. Tble 3. Overll crystlliztion rte constnt (K) nd Avrmi constnt (n) of the virgin PP, PP-A, PP-B nd PP-C t 130oC Smple K Virgin PP PP-A PP-B PP-C n K nd n were clculted from the Avrmi eqution (Avrmi, 1940). type crystlliztion morphology. From Fig. 5() nd (b), the size of the spherulite of the virgin PP nd PP-C does not chnge significntly between the smples. However, it is clerly observed tht the number of the spherulite growth sites is decresed for the smple of PP-C, which suggests tht the crystlliztion rte of the PP-C is slow compred to the virgin PP. The decrese of the spherulite growth sites my be due to the increse of impurities cused by the increse of number of repeted extrusion. This result is consistent with the results of the isotherml crystlliztion nd Avrmi plot of the PP smples t 130oC, which suggests tht the crystlliztion rte is slow with the increse of the number of repeted extrusion. From the bove isotherml crystlliztion, Avrmi plot, nd the opticl microscopy studies, it is suggested tht the crystlliztion rte of the PP is decresed with the increse of the number of repeted extrusion time, which cused by the decrese of the number of the spherulite growth sites. 4. Conclusions Rheologicl properties nd crystlliztion kinetics of the 24 Fig. 5. Opticl microscopy of the spherulite of the virgin PP nd PP-C t 130oC: () virgin PP; (b) PP-C. ethylene contining polypropylene (PP) block copolymer nd recycled PP block copolymer were investigted. In the study of the dynmic rheology, it is observed tht the storge modulus nd loss modulus for the PP block copolymer nd recycled PP block copolymer did not chnge with frequency. In the study of the effect of the repeted extrusion on the crystlliztion rte, hlf crystlliztion time of the PP smples ws incresed with the increse of the number of repeted extrusion in the crystlliztion temperture rnge (115~130oC). In the study of the isotherml kinetics of the PP smples t 130oC, the crystlliztion rte of the PP smples ws decresed with the number of repeted extrusion. Also, from the Avrmi plots results, the overll crystlliztion rte constnt (K) of the PP smples ws decresed with the increse of the number of the repeted extrusion. From the bove results of the isotherml crystlliztion nd Avrmi plots, it is concluded tht the crystlliztion rte is decresed with the increse of the number of repeted extrusion. Kore-Austrli Rheology Journl

5 Rheologicl properties nd crystlliztion kinetics of polypropylene block copolymer with repeted extrusion From the result of the opticl microscopy study, the spherulite size of the PP smples did not chnge significntly between the PP smples. However, it is clerly observed tht the number of the spherulite growth sites ws decresed with the increse of the number of repeted extrusion time. This result is consistent with the results of the isotherml crystlliztion nd Avrmi plots of the PP smples t 130 o C. From the results of the hlf crystlliztion time, isotherml crystlliztion, Avrmi plots, nd opticl microscopy studies, it cn be concluded tht the crystlliztion rte of the PP block copolymer is decresed with the incresing the number of repeted extrusion. Acknowledgement This study ws supported by reserch grnts from the Kore Science nd Engineering Foundtion (KOSEF) through the Applied Rheology Center (ARC), n officil KOSEF-creted engineering reserch center (ERC) t Kore University, Seoul, Kore. Also, finncil support from Hyundi Motor Compny & Ki Motors Corportion is grtefully cknowledged. References Almo, R. G. nd L. Mndelkern, 1991, Crystlliztion kinetics of rndom ethylene copolymers, Mcromolecules 24, Almo, R. G., J. A. Blnco, P. K. Agrwl nd J. C. Rndll, 2003, Crystlliztion rtes of mtched frctions of MgCl 2 -supported Ziegler Ntt nd metllocene isottic poly(propylene)s. 1. The role of chin microstructure, Mcromolecules 36, Aurrekoetxe, J., M. A. Srrionndi, I. Urrutibesco nd M. L. Mspoch, 2001, Effects of recycling on the microstructure nd the mechnicl properties of isottic polypropylene, J. Mter. Sci. 36, Aurrekoetxe, J., M. A. Srrionndi, I. Urrutibesco nd M. L. Mspoch, 2001b, Frcture behviour of virgin nd recycled isottic polypropylene, J. Mter. Sci. 36, Avrmi, M., 1940, Kinetics of phse chnge. II, J. Chem. Phys. 8, Chen, H., L. Li, W. Ou-Yng, J. C. Hwng nd W. Wong, 1997, Spherulitic crystlliztion behvior of poly(ε-cprolctone) with wide rnge of moleculr weight, Mcromolecules 30, Chun, Y. S., M. Hn, J. Prk nd W. N. Kim, 2000, Therml behvior nd rheology of polypropylene nd its blends with poly(ε-cprolctone), Kor. Aust. Rheol. J. 12, Kim, W. N., S. Hong, J. Choi nd K. Lee, 1994, Morphology nd mechnicl properties of bixilly oriented films of polypropylene nd HSPE blends, J. Appl. Polym. Sci. 54, Mreu, V. H. nd R. E. Prud homme, 2002, Dul growth rtes nd morphologies of isothermlly crystllized miscible polymer blends, Mcromolecules 35, Mreu, V. H. nd R. E. Prud homme, 2003, Growth rtes nd morphologies of miscible PCL/PVC blend thin nd thick films, Mcromolecules 36, Peón, J., J. F. Veg, M. Aroc nd J. Mrtínez-Slzr, 2001, Rheologicl behviour of LDPE/EVA-c blends. I. On the effect of vinyl cette comonomer in EVA copolymers, Polymer 42, Schönherr, H., W. Wiytno, J. Pople, C. W. Frnk, G. G. Fuller, A. P. Gst nd R. M. Wymouth, 2002, Morphology of thermoplstic elstomers: elstomeric polypropylene, Mcromolecules 35, Shroff, R. N. nd H. Mvridis, 1999, Long-chin-brnching index for essentilly liner polyethylenes, Mcromolecules 32, Veg, J. F., A. Sntmrí, A. Muñoz-Esclon nd P. Lfuente, 1998, Smll-mplitude oscilltory sher flow mesurements s tool to detect very low mounts of long chin brnching in polyethylenes, Mcromolecules 31, Kore-Austrli Rheology Journl Mrch 2005 Vol. 17, No. 1 25