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© 2018 The Korean Society of Rheology and Springer 47 Korea-Australia Rheology Journal, 30(1), 47-53 (February 2018) DOI: 10.1007/s13367-018-0006-x www.springer.com/13367 pISSN 1226-119X eISSN 2093-7660 Water-assisted extrusion of bio-based PETG/clay nanocomposites Naeun Lee 1 and Sangmook Lee 2, * 1 RPAT PJT Platform Technology, Corporate R&D, LG Chem, Daejeon 34122, Republic of Korea 2 Division of Chemical Engineering, Dankook University, Yongin 16890, Republic of Korea (Received December 14, 2017; final revision received January 18, 2018; accepted January 20, 2018) Bio-based polyethylene terephthalate glycol-modified (PETG)/clay nanocomposites were prepared using the water-assisted extrusion process. The effects of different types of clay and clay mixing methods (with or without the use of water) and the resulting nanocomposites properties were investigated by measuring the rheological and tensile properties and morphologies. The valuable properties were achieved when Cloisite 30B was mixed in a slurry state. The results of the X-ray diffraction (XRD) and transmission elec- tron microscopy (TEM) studies showed that the nano-clay was well dispersed within the PETG matrix. This shows that the slurry process could be an effective exfoliation method for many nanocomposites systems as well as for bio-based PETG/clay nanocomposites. Keywords: water-assisted extrusion, nanocomposite, PETG, nano-clay, exfoliation 1. Introduction Polymer nanocomposites have attracted a great deal of interest during the last few decades as they exhibit remarkable properties such as thermal stability, mechani- cal properties, flame retardancy, and even gas permeabil- ity, which can be achieved by mixing polymers with only a small amount of nano-fillers (Ray et al., 2003). It is known that the final properties of nanocomposites are affected by shape, size, size distribution, content, disper- sion of clays, and adhesion between the clay and polymer matrix (Pannirselvam et al., 2008). PETG is a widely used amorphous thermoplastic poly- mer, which has good impact and tear strength, barrier properties, chemical resistance, and transparency (Bryd- son, 1999). Bio-based PETG is a type of PETG that con- tains natural bio-mass. Because of its excellent transparency and barrier properties, it can be used for many applica- tions, particularly for storing materials that are sensitive to the environment without the concerns that accompany the use of bisphenol-A. Recently, the effects of organo-clay platelets in PET on water vapor permeability have been reported (Hayrapetyan et al., 2012; Li et al., 2012). The desirable properties of the nanocomposites can be enhanced using well-dispersed nano-clays, which show good affinity with the polymer matrix (Paul and Robeson, 2008). Water-assisted extrusion was first reported as a process to prepare polyamide/untreated clay nanocomposites (Korbee and Van Geenen, 2002). In the middle of the pro- cess, water is injected into the twin screw extruder and drained through a vacuum pump placed at the end of the system. This new method was successfully used to mix molten polyamide 6 and unmodified nano-clays with water using a degassing system (Fedullo et al., 2006; Molajavadi and Garmabi, 2011; Stoeffler et al., 2013; Yu et al., 2005). Another method commonly that is employed involves mixing the clays and water, feeding them into the extruding system, and blending them with the molten polyamide in the slurry state (Hasegawa et al., 2003). Other researchers also reported significant effects by injecting water into the twin screw extruder during the preparation of nanocomposites based on polypropylene and polyethylene terephthalate (Lecouvet et al., 2011; Li et al., 2012). In this study, the efficiency of water-assisted extrusion methods for dispersing clays and enhancing the advanta- geous properties of the bio-based PETG is presented along with the water injection methods used for the preparation of nanocomposites of various types of clays. 2. Experimental 2.1. Materials Bio-based PETG (9% biomass content, ECOZEN-T95, SK Chemicals Co.) was used as a base polymer. Three types of modified clay (Cloisite Na + , Cloisite 30B, South- ern Clay Products, and Somasif ME100, Co-op Chemicals Co.) were used as nanoparticles. Cloisite 30B has methyl, tallow, bis-2-hydroxyethyl quaternary ammonium cations in place of the interlayer sodium cations of the purified pristine clay. Somasif ME100 is hydrophilic expandable mica with a cation-exchange capacity (CEC) of 120 meq/ 100 g. 2.2. Nanocomposite preparation An intermeshing co-rotating twin screw extruder (KZW15, Technovel Corporation) with a 15 mm in screw *Corresponding author; E-mail: [email protected]
Transcript
Page 1: Water-assisted extrusion of bio-based PETG/clay nanocomposites · 2018. 2. 27. · Water-assisted extrusion of bio-based PETG/clay nanocomposites Korea-Australia Rheology J., 30(1),

© 2018 The Korean Society of Rheology and Springer 47

Korea-Australia Rheology Journal, 30(1), 47-53 (February 2018)DOI: 10.1007/s13367-018-0006-x

www.springer.com/13367

pISSN 1226-119X eISSN 2093-7660

Water-assisted extrusion of bio-based PETG/clay nanocomposites

Naeun Lee1 and Sangmook Lee

2,*1RPAT PJT Platform Technology, Corporate R&D, LG Chem, Daejeon 34122, Republic of Korea

2Division of Chemical Engineering, Dankook University, Yongin 16890, Republic of Korea

(Received December 14, 2017; final revision received January 18, 2018; accepted January 20, 2018)

Bio-based polyethylene terephthalate glycol-modified (PETG)/clay nanocomposites were prepared usingthe water-assisted extrusion process. The effects of different types of clay and clay mixing methods (withor without the use of water) and the resulting nanocomposites properties were investigated by measuringthe rheological and tensile properties and morphologies. The valuable properties were achieved whenCloisite 30B was mixed in a slurry state. The results of the X-ray diffraction (XRD) and transmission elec-tron microscopy (TEM) studies showed that the nano-clay was well dispersed within the PETG matrix. Thisshows that the slurry process could be an effective exfoliation method for many nanocomposites systemsas well as for bio-based PETG/clay nanocomposites.

Keywords: water-assisted extrusion, nanocomposite, PETG, nano-clay, exfoliation

1. Introduction

Polymer nanocomposites have attracted a great deal ofinterest during the last few decades as they exhibitremarkable properties such as thermal stability, mechani-cal properties, flame retardancy, and even gas permeabil-ity, which can be achieved by mixing polymers with onlya small amount of nano-fillers (Ray et al., 2003). It isknown that the final properties of nanocomposites areaffected by shape, size, size distribution, content, disper-sion of clays, and adhesion between the clay and polymermatrix (Pannirselvam et al., 2008).

PETG is a widely used amorphous thermoplastic poly-mer, which has good impact and tear strength, barrierproperties, chemical resistance, and transparency (Bryd-son, 1999). Bio-based PETG is a type of PETG that con-tains natural bio-mass. Because of its excellent transparencyand barrier properties, it can be used for many applica-tions, particularly for storing materials that are sensitive tothe environment without the concerns that accompany theuse of bisphenol-A. Recently, the effects of organo-clayplatelets in PET on water vapor permeability have beenreported (Hayrapetyan et al., 2012; Li et al., 2012). Thedesirable properties of the nanocomposites can beenhanced using well-dispersed nano-clays, which showgood affinity with the polymer matrix (Paul and Robeson,2008).

Water-assisted extrusion was first reported as a processto prepare polyamide/untreated clay nanocomposites(Korbee and Van Geenen, 2002). In the middle of the pro-cess, water is injected into the twin screw extruder anddrained through a vacuum pump placed at the end of thesystem. This new method was successfully used to mix

molten polyamide 6 and unmodified nano-clays withwater using a degassing system (Fedullo et al., 2006;Molajavadi and Garmabi, 2011; Stoeffler et al., 2013; Yuet al., 2005). Another method commonly that is employedinvolves mixing the clays and water, feeding them into theextruding system, and blending them with the moltenpolyamide in the slurry state (Hasegawa et al., 2003).Other researchers also reported significant effects byinjecting water into the twin screw extruder during thepreparation of nanocomposites based on polypropyleneand polyethylene terephthalate (Lecouvet et al., 2011; Liet al., 2012).

In this study, the efficiency of water-assisted extrusionmethods for dispersing clays and enhancing the advanta-geous properties of the bio-based PETG is presented alongwith the water injection methods used for the preparationof nanocomposites of various types of clays.

2. Experimental

2.1. MaterialsBio-based PETG (9% biomass content, ECOZEN-T95,

SK Chemicals Co.) was used as a base polymer. Threetypes of modified clay (Cloisite Na+, Cloisite 30B, South-ern Clay Products, and Somasif ME100, Co-op ChemicalsCo.) were used as nanoparticles. Cloisite 30B has methyl,tallow, bis-2-hydroxyethyl quaternary ammonium cationsin place of the interlayer sodium cations of the purifiedpristine clay. Somasif ME100 is hydrophilic expandablemica with a cation-exchange capacity (CEC) of 120 meq/100 g.

2.2. Nanocomposite preparationAn intermeshing co-rotating twin screw extruder

(KZW15, Technovel Corporation) with a 15 mm in screw*Corresponding author; E-mail: [email protected]

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Naeun Lee and Sangmook Lee

48 Korea-Australia Rheology J., 30(1), 2018

diameter and a 45 in length to diameter ratio was used toprepare the nanocomposites at 200 rpm. From hopper todie, the set temperatures of six heating blocks were 250,255, 260, 255, 255, and 255ºC, respectively, and the dietemperature was kept 255ºC. PETG were melt-mixed withthe clays in the extruder using three different methods, asshown in Table 1. The clay content in all the nanocom-posites was fixed at 2 wt.%. In the C process, dry-mixedPETG and clay was fed into the extruder and com-pounded. The W process was performed similarly to the Cprocess, except that water was injected into the extruderafter the sealing process. The last S process was used tointroduce clays into the extruder in the slurry state.

Slurries were prepared by mixing water and clay in anagitator at 500 rpm for 2 h. Slurry with a clay content of5 wt.% was introduced into extruder using a liquid pump.The state of slurry including Cloisite Na+ or SomasifME100 seemed to be stable during the experiment withoutany flocculation or sedimentation. On the other hand, theslurry including Cloisite 30B was required continuous stir-ring to keep the slurry state stable. For all three processes,water introduced was removed using a vacuum pumpthrough an open vent. Process S is schematically shown inFig. 1. The extruded nanocomposites were cooled in awater-bath and pelletized. The pellets were compression-molded at 255ºC using a hot press (Laboratory press,

Carver Inc.) into 25 mm disks and dog-bone shapes tomeasure the rheological and tensile properties.

2.3. InstrumentsThe complex viscosities were measured using stress-

controlled rheometer (AR-G2, TA Instruments) at fre-quency range 0.05 ~ 500 rad/s, strain amplitude 5% andtemperature 255ºC. To estimate the degree of delamina-tion and dispersion, a structural characterization was car-ried out utilizing an XRD (D/Max-A. Rigaku) with CuKα

radiation (λ = 1.54056 Å). The 2θ angles range from 1o to10o at a scanning rate of 0.5o/min. The generator was oper-ated at 30 kV and 15 mA. The state of the clay in thematrix polymer was also investigated using a TEM (TalosF200S, FEI). The specimens were ultra-microtomed usinga microtome. The tensile strength was measured by a uni-versal testing machine (LR-5K, Lloyd Instruments) at acrosshead speed of 20 mm/min. Barrier properties weredetermined using a water vapor transmission rate tester(Perme W3/060, Labthink) under a relative humidity of90% and a temperature of 38ºC for 24 h. The dimensionsof samples were 33 cm2 with a thickness of 500 µm.

3. Results and Discussion

3.1. Rheological behaviorThe dispersion state of the filler in polymer nanocom-

posites can be validated indirectly with rheological char-acterization (Lecouvet et al., 2011). Figure 2 shows theplot of complex viscosities of PETG/Closite Na+ nano-composites as a function of angular frequency in oscilla-tory rheometer at 255oC. For comparison p-PETG (processedPETG) and W (feeding water without clay) were also pre-sented. The complex viscosities of the nanocompositeswere slightly higher than that of p-PETG and amongthem, CNa showed the highest value. The zero shear vis-cosity is a constant melt viscosity at low frequencies and

Table 1. Composition of samples and clay feeding method.

Process Sample code Clay type Remark

- p-PETG - Processed PETG (PETG only)W - Feeding water without clay

C30B Cloisite-30BC CNa Cloisite-Na+ Feeding clay without water

CME Somasif ME100

W30B Cloisite-30BW WNa Cloisite-Na+ Feeding clay and water separately

WME Somasif ME100

S30B Cloisite-30BS SNa Cloisite-Na+ Feeding clay in slurry state (clay 5 wt.% in water)

SME Somasif ME100

Fig. 1. Water-assisted extrusion with the attached clay slurryinjection system.

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Korea-Australia Rheology J., 30(1), 2018 49

known to be proportional to molecular weight of a poly-mer (Fetters et al., 1994). In this study, introduction ofwater during the extrusion resulted in the reduction ofzero-shear viscosity from 430 (p-PETG) to 320 (W) Pa·s.It is known due to that the molecular weight of PETGdecreased by the hydrolysis reaction.

In Fig. 3, the complex viscosities of PETG/SomasifME100 nanocomposites show about 10% higher thanthose of PETG/Cloisite Na+ nanocomposites but their gen-eral trends are similar. The increase of viscosities isthought to be due to physical interactions of both PETG-clay and clay-clay. The complex viscosities of SMEbecame highest with increasing angular frequency. It isbelieved due to that the water led a better distribution ofthe clay in the polymer matrix and particularly in theslurry state, the viscosity reduction caused by hydrolysiswas covered by better clay distribution.

The nanocomposites filled with Cloisite 30B, which isorgano-modified nano clay, shows strong shear thinningbehavior as presented in Fig. 4. Because even at low fre-quencies, shear force exceeds the weak interactions

between PETG-clay and between clay-clay, the shear thin-ning behavior starts from at low frequency range. Thecomplex viscosities of C30B and W30B are higher thanthose of p-PETG at low frequencies but they reverse as afrequency increases. Dini et al. (Dini et al., 2014) reportedabout the reduction of viscosity for nanocomposites filledwith organoclay, and they suggested that the strong deg-radation caused by the presence of the nanoparticles canbe a possible reason. PETG is more exposed to the surfaceof Cloisite 30B than Somasif ME100 or Cloisite Na+,which leads to higher degree of polymer degradation. Par-ticularly, however, the complex viscosities of S30B arehigher than those of p-PETG over the entire frequencyrange. Therefore, introducing clay in slurry state would bemore effective for dispersion of clay in polymer matrixand enough to compensate the reduction of complex vis-cosities caused by hydrolysis reaction.

The Cole-Cole plots of PETG and its nanocompositeswith clay type were shown in Figs. 5-7. At low frequen-cies, the storage moduli of the nanocomposites werehigher than those of p-PETG. It is believed due to that theinterconnected or network structure of clay was formed in

Fig. 2. (Color online) Complex viscosities of PETG/Closite Na+

nanocomposites at 255oC.

Fig. 3. (Color online) Complex viscosities of PETG/SomasifME100 nanocomposites at 255oC.

Fig. 4. (Color online) Complex viscosities of PETG/Closite 30Bnanocomposites at 255oC.

Fig. 5. (Color online) Cole-Cole plots of PETG/Closite Na+

nanocomposites at 255oC.

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50 Korea-Australia Rheology J., 30(1), 2018

the polymer matrix, which constrained the molecularchain mobility and led to an increase of the chain rigidity(Chae and Kim, 2007). At high frequencies, however, thestorage moduli of p-PETG and its nanocomposites showedalmost same. It could be explained that the physical net-work structure built by nano clay was not strong enoughto bear a high shear condition and collapsed. It has beenknown that as frequencies increase, the slopes of the Cole-Cole plots for the PET/clay nanocomposites graduallyincrease and approach to neat PET whose slope is approx-imately 2 (Kim et al., 2007). This reflects that the hetero-geneity system caused by nanoparticles becomes homo-geneous gradually with increasing shear force. From theFigs. 5-7, the nanocomposite samples of S process showedthe lowest slope close to p-PET and therefore it can beconcluded that the slurry process is the best way todisperse clay in the polymer matrix. In Fig. 7, the networkof S30B was kept even until at 20,000 Pa of loss modulus,whereas SNa and SME were until at 3,000 and 4,000 Pa,respectively. It corresponded to the trends of shear thin-ning in Figs. 2-4.

3.2. Tensile propertiesThe tensile strength and Young’s modulus of the PETG

nanocomposites are presented in Figs. 8 and 9, respec-tively. The use of organo-clay increased the tensile strength,and S30B, particularly, had as notably increased tensilestrength (63.4 MPa) by 16% compared to that of p-PET(54.7 MPa) and even C30B and W30B showed 6% and10% higher values than p-PET, respectively. This can beattributed to better dispersion and distribution of the claywhen processed in the slurry state, which results in betterstress distribution and less bundle formation and, conse-quently, enhanced mechanical properties (Dini et al., 2014).However, quite different tendencies are seen for the nano-composites filled with non-modified clays. Due to poorcompatibilities of non-modified clays with non-polarpolymer, the tensile strengths of PETG/Cloisite Na+, andPETG/Somasif ME100 nanocomposites are not as high asthose of PETG/Cloisite 30B nanocomposites. This is mostlikely caused by clay agglomeration and hydrolyzed PETG.The Young’s modulus results didn’t show any pronouncedtendency, but generally, when water was used, the betterresults were obtained.

3.3. Barrier propertiesIt has been reported that nano-fillers enhance the barrier

properties for both oxygen and carbon dioxide (Avella et

Fig. 6. (Color online) Cole-Cole plots of PETG/Somasif ME100nanocomposites at 255oC.

Fig. 7. (Color online) Cole-Cole plots of PETG/Closite 30Bnanocomposites at 255oC.

Fig. 8. Tensile strength of PETG and its nanocomposites.

Fig. 9. Young’s modulus of PETG and its nanocomposites.

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Korea-Australia Rheology J., 30(1), 2018 51

al., 2006) because of the impermeability of clay layers,which create a tortuous path for the diffusing moleculesand reduce the gas permeability (Pannirelvam et al., 2008).Therefore, exfoliation of clays is crucial in order to improvethe barrier properties of nanocomposites.

In Fig. 10, as expected, PETG/Closite 30B nanocom-posites (C30B, W30B, S30B) show the most significantreduction in the water vapor permeability, which are 53-57% of neat PETG. However, the permeability of C30Bwas slightly lower than that of those processed with S30B,although S30B had better dispersion and distribution ofthe clays compared to C30B according to the results ofXRD and TEM. This result can be attributed to a smallamount of water remaining in the polymer matrix, causingmore polymer degradation.

3.4. MorphologyFigures 11 and 12 show the XRD results of the neat

nano clays and their nanocomposites used to determinethe degree of clay dispersion, respectively. Neat Cloisite30B, Cloisite Na+, and Somasif ME100 had peaks at 4.8,7.7, and 9.3o of 2θ, respectively (Gennen et al., 2016;Mallakpour and Moslemi, 2012; Souza et al., 2011). Onedistinct peak was observed for all the nanocomposites.The peak height and position of the C process samplesseem similar to those of the W process samples. Galleryspacing of the Cloisite Na+ sample seemed to be indepen-dent of water usage. This is likely because water fails towiden the gap between the layers of Cloisite Na+ as wellas because of the poor compatibility between Cloisite Na+

and PETG. However, the peaks for SME and S30B sam-ples show small peaks at a lower angle. This implies thatsome exfoliation of clay by the PETG chains has occurredand can be attributed to Cloisite 30B being well dispersedand distributed in the PETG matrix by the relativelystrong interaction to PET and the slurry state feeding.Water increases the inter-gallery distances and acts as aswelling agent and increases the chain mobility of PETGas a hydrolysis agent (Yalcinyuva et al., 2000; Yu et al.,

2005). Nevertheless, the diffusion of PETG chains into thegalleries of the polar clay is restricted by the poor affinitybetween Cloisite Na+ and PETG as well as by strong elec-trostatic forces between the adjacent platelets of Cloisite-Na+ (Dini et al., 2014).

To investigate the affinity of clay with the polymermatrix and the effect of each process, TEM analysis wasconducted. Nano-clays tend to agglomerate and forminterconnected structures due to its small size, high aspectratio, and large surface area (Kim et al., 2007). In Fig. 13,for the CNa and CME samples, the low degree of disper-sion can be observed, and the collapsed and stacked layerof the clays shows that they are not expected to have evenintercalated. However, the dispersion of C30B improvedsignificantly compared to that of the non-organo-clay. Even

Fig. 10. Water vapor permeability of PETG and its nanocom-posites. Fig. 11. (Color online) XRD patterns of nano clays.

Fig. 12. (Color online) XRD patterns of PETG/clay nanocom-posites.

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Naeun Lee and Sangmook Lee

52 Korea-Australia Rheology J., 30(1), 2018

though some stacks of clay platelets are still observed inthe image, most are broken up and have delaminated intosingle layers. The best clay dispersion and distributionresults were obtained when water and clay were injectedin the slurry state (S30B). All clay existed individually assingle layers and was uniformly distributed in the matrix.

The proposed mechanism supporting the results of thisstudy is shown in Fig. 14. When PETG was melt-mixedwith clay in the slurry state, small size droplets in whichthe clays were fully exfoliated were distributed and dis-persed in the PETG matrix and the clays were exfoliatedin the PETG matrix when water removed by vacuum ven-tilation.

4. Conclusions

In water-assisted extrusion, water functioned as a swell-ing agent by increasing the interlayer spacing of the clayand delaminating it into single layers. However, the effectof water varied with the affinity between clay and PETGmatrix. Water did not show its effect on the nanocompos-ites filled with non-modified clays. S30B showing exfo-

liated morphology had the highest tensile strength of 64MPa, which was 16% higher value than that of p-PETG.From the results of XRD patterns and TEM images, it wasdue to enhanced dispersion and distribution of the clay inthe polymer matrix. The water vapor permeability ofS30B was slightly higher than that of C30B due to resid-ual water, but still showed 43% improvement compared tothat of p-PETG. Therefore, it is supposed that water-assisted extrusion has a direct and positive impact on theproperties of bio-based PETG without producing anyenvironmentally harmful chemicals.

Acknowledgement

The present research was conducted by the researchfund of Dankook University in 2016 (No: R201600374).

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