BIMODAL POLY(ETHYLENE TEREPHTHALATE) BLENDS: EFFECT OF MOLECULAR WEIGHT DISTRIBUTION ON MATERIAL PROPERTIES

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1 BIMODAL POLY(ETHYLENE TEREPHTHALATE) BLENDS: EFFECT OF MOLECULAR WEIGHT DISTRIBUTION ON MATERIAL PROPERTIES Artemis Ailianou, Sudheer Bandla, Jay C. Hanan* Research & Development, Niagara Bottling, Ontario, CA *Corresponding author: Abstract This study presents an investigation of bimodal poly(ethylene terephthalate) blends using injectionmolded parts. The blends were characterized for intrinsic viscosity, molecular weight distribution, crystallization, and tensile mechanical properties. The data shows that injection molding preserves the intended bimodal molecular weight distribution, despite typical inherent material degradation from processing. Our results show that the maximum effect was observed when the high molecular weight component is at 10 wt.%. Introduction Poly(ethylene terephthalate) (PET) is a semicrystalline polyester that is widely used to make fibers, films, and packaging. PET is well known for its straininduced crystallization behavior and mechanical properties. With lightweight packaging initiatives currently on the rise, tailoring the polymer properties to the application is increasingly important. The various PET grades are characterized by their Intrinsic Viscosity (IV), the viscosity of the polymer in dilute solution, which is related to the molecular weight (MW) through the Mark-Houwink equation: [η] = KM α (1) where η is the intrinsic viscosity, M is the numberor weight-average weight, and K and α are factors dependent on polymer type, solvent, and temperature [1]. Typical fiber grade PET has a M n of 15,000 to 20,000 g/mol, which refers to an IV between 0.55 and 0.67 dl/g. Bottle-grade PET has an average M n of 24,000 to 36,000 g/mol, which refers to an IV between 0.75 and 1.00 dl/g [2]. The majority of industrial processes yield PET with an initial IV of Increasing the IV is done through a solid state process (SSP) where short PET chains (low molecular weight) grow in length (high molecular weight) through polycondensation in vacuum. Polymer blends are an attractive solution for tailoring material properties to a specific application, improving properties of lower grade resins, and improving properties after recycling [3]. Chain extenders have been used to increase the molecular weight and viscosity of both virgin and recycled PET [4]. Blending with both chain extenders and a small amount (1.3%) of nanoclay has been shown to increase impact strength [5]. Other nano-additives have also shown benefits for mechanical properties in PET [6]. Recycled PET grades can be blended with polyethylene-glycidylmethacrylate (an impact modifier) and low levels of talc to improve impact strength, tensile properties, and heat deflection [7]. Bimodal blends combine the benefits of two different molecular weight distributions (modes) for improved performance and processability. Bimodal blends have been known and used in polyolefins, particularly polyethylene, by combining various grades to change the crystallization and injection properties of the base resin [3, 8-10]. In the last two decades, advances in catalytic technology have enabled commercially available bimodal HDPE without the need for blending. The advantage of bimodal HDPE for injection molding is that it offers the faster melt flow profile of the low molecular weight mode and retains a relatively higher stiffness for the injection molded part from the high molecular weight mode [3]. In this study, we applied the concept of bimodal blends to produce PET blends with two distinct molecular weight populations using injection molding. Materials and Methods A commercially available poly(ethylene terephthalate) (PET) resin with nominal intrinsic viscosity of 0.76 ± 0.02 (origin USA) was used as the base resin for all blends. The high molecular weight PET component was a commercially available resin with nominal 1.10 ± 0.03 IV (origin USA). SPE ANTEC Anaheim 2017 / 143

2 Cylindrical parts with 1.5 mm radial thickness were produced using a 90-ton injection molding system in a two-cavity configuration with valve gates (HyPET 90, Husky Injection Molding Systems). The mold temperature was maintained at 8 C to minimize the crystallinity of the injection-molded parts. Blends with 0 to 30 wt.% and 100 wt.% high MW component were made and were designated H0 to H100 (fraction of high MW PET). The molecular weight distribution (MWD) for each blend was determined using high temperature gel permeation chromatography (GPC). Intrinsic viscosity (IV) measurements were done using dilute solution viscometry. Both MWD and IV measurements were performed on as-produced injection molded parts at a third party laboratory. Rheological characterization of the resins and injection-molded parts was done using a rotational rheometer (AR2000ex, TA Instruments) at 260 C using Nitrogen atmosphere. All samples were dried at 170 C for 12 hours prior to testing. Strain sweeps at 260 C were performed to determine the linear viscoelastic regime for each blend. Dynamic modulus data was collected for the frequency range of 100 rad/s to 0.1 rad/s at 260 C using 1% strain. The data was then transformed to steady shear viscosity using the Cox- Merz rule. After transformation, the data was fitted to the Williamson model using TA Instruments Rheology Advantage software to calculate the shear viscosity. Young s Modulus and Ultimate Tensile Strength were determined using an Instron Universal Testing Machine at an extension rate of 5 mm/min at an ambient temperature of 22 C [11]. Eight as-produced injectionmolded parts were tested for each condition, 24 to 48 hours post-molding. stable injection cycles. The injection molding cycle time was kept the same for all blends. As a result, the fill pressure and recovery torque increased with increasing fraction of high MW PET, as shown on Table 1. The fill pressure of H100 was 73.3% higher than H0. Addition of 30% high MW component increased the fill pressure by only 18.7%. The screw recovery torque was similarly affected by increasing the high MW content. The recovery torque of H100 was 85% higher than H0. Addition of 30% high MW PET increased the recovery torque by only 14.7%. Molecular Weight Distribution and Intrinsic Viscosity The measured intrinsic viscosity values of the injection molded parts using H0 and H100 were 0.73 and 0.96, respectively. Using the weighted average method, the IV for each blend was calculated based on the measured H0 and H100 IV values. The measured IV values matched the calculations for blends H5 and H10 but were lower for H20 and H30 (Fig. 1). We attribute this to increased degradation with increasing high MW fraction due to the higher fill pressure and screw torque. Table 1: Fill pressure and screw recovery torque for the injection molding process of the blends. Blend (%H) Fill Pressure (bar) Recovery Torque (Nm) ± ± ± ± ± ± ± ± ± ± ± ± 1 The percent crystallinity and crystallization temperatures of the blends were measured using differential scanning calorimetry with a heating/cooling rate of 10 C/min (Q2000, TA Instruments) between 30 and 260 C. Crystallinity content was calculated for the as-produced parts, as well as the neck of the tensiletested parts. Results Injection Molding Process Pre-blended mixtures of low and high MW PET were dried for 4 hours prior to injection molding. Samples for testing were collected after 30 minutes of SPE ANTEC Anaheim 2017 / 144

3 Figure 1. Measured vs. Expected weight-average Intrinsic Viscosity. The molecular weight distribution reflects the increase in IV, except in the case of H5 (Table 2). For the 5% blend, both M n and M w are lower than the 0% blend, which we attribute to higher than expected degradation during injection molding. However, H5 M z is very similar to that of H0 indicating that degradation did not preferentially affect the long chains of the high MW fraction. For the rest of the blends, M n, M w and M z increase linearly with increasing fraction of high MW PET. Table 2: Molecular weight distribution of the blends. Blend (%H) M n M w M z M w /M n 0 38,083 68,429 96, ,787 66,133 95, ,118 69,706 97, ,020 72, , ,354 75, , ,619 94, , Viscosity The complex viscosity of the blends increases at frequencies less than 1 rad/s (Figs. 2-3). H5 has lower viscosity throughout the entire shear rate range; this decrease indicates that degradation happened uniformly. H10 has the highest viscosity increase in the low frequency range, which suggests a higher degree of entanglements. Figure 2. Viscosity increases at low shear rates with increasing high MW content. Figure 3. Enlarged view of the increase in viscosity at low shear rates. Crystallization The thermal properties of the blends change with increasing high molecular weight component. The crystallinity of injection molded parts increases with small amounts of high MW content but then decreases with increasing amounts. The as-produced injectionmolded parts are primarily amorphous with less than 10% crystallinity. Interestingly, as-produced H10 has the highest crystallinity of all blends at 8% versus the SPE ANTEC Anaheim 2017 / 145

4 5% and 2% crystallinity of H0 and H100. Following the same trend, tensile-tested H5 and H10 have the highest crystallinity at 37% versus the 32% and 28% crystallinity of H0 and H100 blends (Fig. 4). The melt crystallinity (Fig. 5) is overall lower than the cold crystallinity (Fig.4). The trend of higher crystallinity at lower fractions of high MW and lower crystallinity with higher concentration of high MW is observed again. The cold crystallization temperature of the blends increases with increasing high MW content (Fig. 6). Conversely, the melt crystallization temperature decreases with increasing high MW fraction (Fig. 7). Figure 5. Melt crystallinity (%) increases for H5 and H10 but then decreases with increasing high MW content, similar to the cold crystallinity. Figure 4. Cold crystallinity (%) increases for H10 but then decreases with increasing high MW content. Error bars overlap with symbols. Figure 6. Cold crystallization temperature increases with increasing high MW content. Error bars are smaller than the symbols. SPE ANTEC Anaheim 2017 / 146

5 Discussion In this study, we have successfully produced injection-molded parts by blending PET resins with two different molecular weight distributions. Processing the blends showed an increase in fill pressure and screw torque recovery for the same cycle time and barrel temperature due to the high molecular weight content. Applying the method of weighted averages, the measured IV matches the expected IV assuming the amount of degradation for H0 and H100 remains the same for all blends. There is a slight deviation at higher fractions, most likely because of additional degradation due to the difficulty in plasticizing the high MW component of the blends. Figure 7. Melt crystallization temperature decreases with increasing high MW content. Error bars are smaller than the symbols. Mechanical Properties The largest improvement in Young s Modulus was observed for H10, which had a 5.1% increase over H0. The largest improvement in Ultimate Tensile Strength was observed for H30 at 3.2% improvement over H0. However, none of the changes were significant compared to the experimental deviation (Fig. 8). Figure 8. Blending with high MW material shows a small increase in Young s modulus and Ultimate Tensile Strength. The melt viscosity measurements reveal that the degradation from injection molding did not preferentially happen on the long chains of the high MW component. At high shear rates, the viscosity of the H10-H30 blends matches that of H0. The decreased viscosity of H5 in the entire shear rate range, including the high end, indicates degradation in the base resin. Therefore, we infer that degradation is either uniform or primarily influencing the smaller chains of the base component. At low shear rates, where the additional modes of longer chains are probed, the viscosity increases to eventually match that of H100. It is surprising that H10 exhibits an increase in viscosity even higher than that of H100 at low shear rates. This can be attributed to a disproportionately larger degree of entanglements: the concentration is favorable for the 10% long chains to dictate the viscoelastic behavior of the blend [12]. Whether this is optimal depends on several factors related to the process including the amount of chain scission of the PET melt for a particular set of processing conditions. One of the benefits of bimodal blends is the change in crystallization kinetics and crystallinity content. As molecular weight increases, the rate of crystallization is slower and requires higher undercooling for the same amount of crystallization [13-14]. The amount of cold crystallization provides some insight about the effect of the long chains from the high MW fraction because the amount of cold crystallinity is the result of crystal formation from the nuclei generated during injection molding. For the as-produced and tensile-tested parts, the crystallinity increased by a maximum of 3% and 6% over H0, respectively, for H10 (Fig. 3). This increase for small amounts of long chains is compatible with the theory that during injection molding, long chains undergo a coil-stretch transition to create crystallization nuclei that will then grow under the right conditions [15]. Both the as-produced and tensile-tested parts produced SPE ANTEC Anaheim 2017 / 147

6 for this study show that with small amounts (around 10%) of high MW fraction, the melt crystallinity increases by at least 2% (Fig. 4). However, crystallinity content decreases rapidly with increasing high MW fraction. The overall crystallinity for tensile-tested parts is higher than as-produced parts due to strain-induced crystallization. It is interesting to note that there is no difference between H30 and H100 for melt crystallization (Fig.5). This suggests that by 30 wt.%, the long chains start to dominate the crystallization behavior. The higher cold crystallization temperature and the lower melt crystallization temperature with increasing high MW component (Figs. 6-7) shows that crystallization kinetics are highly influenced by the long chains present in the melt. Previous studies have investigated the effect of intrinsic viscosity and molecular weight on the material properties of PET. It has been shown that for PET with IV of 0.7 to 0.9, the tensile properties do not change substantially [16]. A similar overall trend was observed in this study. Elastic properties are highly dependent on the crystallinity of the material. We believe that the blends did not result in a substantial increase in tensile properties due to the small increase in crystallinity from the addition of the high MW component. Conclusions This study has shown that bimodal PET blends are possible without excessive degradation during injection molding. There is a favorable concentration of high MW component around 10 wt.% where viscosity and crystallinity increase. Above that fraction, there appears to be little or no benefit in blending a high MW PET with a lower MW PET. Acknowledgements The authors would like to thank Mark Aceves and Dydier Dominguez Moguel of Niagara Bottling, LLC for their assistance in making the injection molded parts and the rheological measurements, respectively. References 1. N.B. Sanches, M.L. Dias, E.B.A.V. Pacheco Comparative techniques for molecular weight evaluation of poly(ethylene terephthalate), Polymer Testing, 24, (2005) 2. J. Scheirs, T.E. Long, Modern polyesters: Chemistry and Technology of Polyesters and Copolyesters, John Wiley & Sons, Ltd (2003) 3. L. A. Utracki, C. A. Wilkie, Handbook of Polymer Blends, Second Edition, Springer (2014) 4. I.S. Duarte et al. Chain Extension of virgin and recycled poly(ethylene terephthalate): rapid estimate of molecular weight increase, SPE-ANTEC Tech. Papers (2013) 5. Y. Srithep, L-S. Turng. Solid and Microcellular Recycled Poly(ethylene terephthalate) (PET) Blends with Chain Extenders (CE) and Nanoclay. SPE- ANTEC Tech. Papers (2011) 6. S. Bandla, J.C. Hanan. Microstructure and elastic tensile behavtior of polyethylene terephthalateexfoliated graphene nanocomposites. J. Mater. Sci. 47, (2012) 7. K. Yamada, H. Hamada, S. Tamada, N. Kunimune Effect of talc filler on recycled PET blends injection moldings SPE-ANTEC Tech. Papers (2013) 8. R.K. Krishnaswami, Q. Yang. Influence of comonomer distribution profile on the crystallization characteristics and physical properties of High Density Polyethylene, SPE-ANTEC Tech. Papers (2004) 9. M. Yamaguchi, M. Mieda. Enhancement of melt elasticity of long-chain branched polyethylene by blending a linear polyethylene. SPE-ANTEC Tech Papers (2012) 10. M. Diop, J. M. Torkelson Effective Blending of Ultrahigh Molecular Weight Polyethylene with Polyethylene via Solid-State Shear Pulverization, SPE-ANTEC Tech Papers (2013) 11. Bandla, S. MS Thesis, Oklahoma State University, Stillwater, OK (2010) 12. H. A. Barnes, Handbook of rheology, University of Wales (2000). 13. J.P. Jog, Crystallization of Polyethyleneterephthalate, Rev. Macromol. Chem. Phys., C35 (3), (1995) 14. L.A. Baldenegro-Perez et al. Molecrular weight and Crystallization Temperature Effects on Poly(ethylene terephthalate) (PET) Homopolymers, an Isothermal Crystallization Analysis, Polymers, 6, (2014) 15. H. Janeschitz-Kriegl, Crystallization Modalities in Polymer Melt Processing, Springer-Verlag (2010) 16. E.R. Dixon, J.B Jackson, The Inter-Relation of Some Mechanical Properties with Molecular Weight and Crystallinity in Poly(ethylene terephthalate), Journal of Materials Science, 3, (1968) SPE ANTEC Anaheim 2017 / 148