HIGH SPEED TWIN SCREW EXTRUSION FOR BIODEGRADABLE POLYMER BLENDS: ANALYSIS OF COMPATIBILITY AND RHEOLOGY PREDICTION

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1 HIGH SPEED TWIN SCREW EXTRUSION FOR BIODEGRADABLE POLYMER BLENDS: ANALYSIS OF COMPATIBILITY AND RHEOLOGY PREDICTION Bárbara A. Calderón, Margaret J. Sobkowicz, Department of Plastics Engineering, University of Massachusetts Lowell, One University Ave., Lowell, MA 01854, USA Abstract This work examines the effects of high shear on the degradation and compatibility of blends of poly(propylene carbonate) and poly(butylene succinate) (PPC/PBS) in twin screw extrusion. Also, since solid PPC has poor flowing capabilities, different feeding methods for the TSE trials were compared for their ability to produce consistent results. The blends were compounded at 200, 500, 1000 and 2000 rpm. Viscosity measurements were used to estimate degradation, and it was found that the Maron - Pierce model for viscosity of composites accurately predicted blend viscosity at low shear. The viscosity change was inversely proportional to the screw speed, indicating matrix degradation. Moreover, the blend was more sensitive to thermomechanical degradation than the neat PBS. However, the molecular weight loss did not exceed 22% even at the highest screw speed of 2000 rpm. Finally, morphology investigation showed that the TSE blends had smaller droplet size with a broader shape distribution than the batch mixed blends. All results supported the idea that the high levels of shear stress are the governing factor in the morphology and the degradation of blends in twin screw extrusion. Introduction A plausible solution to greenhouse gas emissions and disposal of excessive plastic waste our world faces nowadays is to expand the market of biobased and biodegradable polymers. Poly(propylene carbonate) (PPC) is a fairly new thermoplastic copolymer. PPC is an amorphous polymer and it comes from the polymerization of propylene oxide and carbon dioxide. In this way, PPC can help mitigate the impact of greenhouse gases in the atmosphere. PPC is also a good fit with sustainability concepts due to its biodegradability [1]. Poly(butylene succinate) is a promising polyester that is semicrystalline and is able to safely degrade into the environment. It is synthesized using succinic acid and 1,4 butanediol, both of which are available from renewable resources [2]. Although these polymers have many advantages over fossil fuel based plastics they also have a few drawbacks that limit their widespread adoption in the industry. Their thermomechanical properties are marginal in comparison to the commodity plastics. Moreover, PPC s electrostatic nature and its low glass transition and thermal decomposition temperatures compromise its handling. Therefore, it is necessary to gain knowledge in their processing to enhance their utility and manufacturability. One of the most practical and convenient methods to achieve this goal is via melt compounding by means of twin screw extrusion (TSE). This process is attractive because it is customizable through changing the processing parameters, the screw program and including multiple feed ports for reactive extrusion and devolatilization [3]. In this study, application of a novel, ultra-high speed twin screw extruder is explored. A Technovel TSE capable of reaching screw speeds up to 4500 rpm was used to prepared blends of biobased and biodegradable polymers. The high level of shear is purported to improve the distributive and dispersive mixing as well as to trigger chemical reaction (enhance compatibility) between the polymer phases. However, the high shear can also produce thermomechanical degradation and hence the deterioration of the blend properties [4]. The mixing efficiency and the degradation of blend systems can be characterized using morphology studies and rheological analysis, respectively. It is known that the zero shear viscosity of a polymer follows a power law with molecular weight (h ~ M 3.4 w ). Therefore, if there is a reduction in molecular weight due to chain scission from the high shear and viscous heating, the zero-shear viscosity will display a similar fractional decrease. The resultant change in viscosity can also alter the morphology of the blend system and hence the performance of the final product. In the case of blend systems the rheological behavior also depends on the nature of the interface and the deformation of the droplets when the system is exposed to shear [5]. However, if one can predict the viscosity of a blend under minimal mechanical perturbation (when exposed to little shear), it is possible to draw conclusions about the degradation and compatibility of the system. There are several models for the prediction of the rheological behavior of blend systems. However, the models derived for filled systems such as composites have never been used to predict the viscosity of polymer blends. The Maron-Pierce equation relates the relative viscosity of the composite (viscosity of the composite divided by the viscosity of the matrix) and the volume fraction of the filler to the nature of the filler such as glass fiber, talc and glass spheres [6]. SPE ANTEC Anaheim 2017 / 1088

2 η " = $ %&'() $ *+,-./ = 0 [02( 4 5 )]8 (1) where η r is the relative viscosity of the composite, ϕ the volume fraction of the filler and A the filler constant which depends on the shape of the filler. In order to fit this model to polymer blends it is assumed that the dispersed phase acts as solid filler during the melt processing. In the case of the blend studied PPC/PBS it is known that PPC, at this melt compounding conditions, is extremely viscous in comparison to the PBS, therefore, it was assumed that PPC behaves as particulate filler in the system. In this study, the empirical Maron- Pierce model was used for the prediction of polymer blends viscosity and to investigate the extent of degradation and compatibility of the biobased and biodegradable blend system. Moreover, attempts were made to find the optimal feeding method for this blend system due to the thermal and electrostatic characteristic of the PPC that makes this an onerous task. The aim of this work is to enhance the performance of PPC and PBS by tailoring their properties using empirical models that can predict the morphology and the viscosity of the system. Materials Experimental The poly(butylene succinate) compounded in this study was a Bionolle 1020 supplied by Showa Highpolymer Co, LTD, with molecular weight of g/mol, melt index of 4.0 g/10min at 190 o C/216g, density of 1.26g/cm 3 and a glass transition temperature of -32 o C. The poly(propylene carbonate) used was a QPAC40 supplied by Empower Materials. The molecular weight is 150, ,000 Da, melt index 0.9 g/10min at 150 o C/216g, density of 1.26g/cm 3 and the glass transition temperature is around 40 o C. Melt Compounding PPC and PBS were melt compounded by means of two different machines. The batch mixer was used to prepared blends exposed to low levels of shear (hence, less degradation). The twin screw extruder was used to compound blends at different screw speeds in order to evaluate its effect on the properties of the blend systems. Polymer blends containing 25 and 50 wt % PPC were compounded by means of a Brabender batch mixer with sigmoidal mixing elements (C.W. Brabender Instruments, USA) at 50 rpm, 130 o C for 7 minutes. Also, neat PBS was processed at the same conditions to serve as a reference for further analysis. The extrusion trails were conducted on a twin screw extruder KZW15 from TECHNOVEL Corporation of Osaka Japan. The equipment contained two fully intermeshed corotating screws of 15 mm diameter and L/D of 60:1. The targeted weight percent of PPC was 25. Four different screw speeds were tested: 200, 500, 1000 and 2000 rpm. The temperature was set 130 o C along the barrel but in the first two zones (feed zones) were it was set to 120 o C. Also, batches of neat PBS (100 wt%) were made as reference for further analysis. Feeding Method Due to PPC s electrostatic nature and because its glass transition temperature (T g ) is very close to room temperature the feeding of this polymer into the extruder barrel is very complicated. The as received PPC comes in blocks of pellets that are glued together; hence it is necessary to break up the blocks into small pieces or single pellets. Besides, PPC is very static and tends to stick inside the feeder making its flow very inconsistent. In order to find the best method to feed this resin, three different techniques were tested. The first one involved the feeding of a premix of PBS and PPC using only the main feeder. The second one consisted of feeding both polymers separately; the main feeder was used to feed the PBS pellets and a side feeder with an agitator inside was installed to feed the PPC pellets. The third method consisted of milling the PPC pellets into powder and grounding the feeder using a copper gauze to dissipate the electrostatic charges of the resin. A laser thermometer was used throughout the compounding trials to make sure the feeder temperature was below the glass transition temperature of PPC. In order to study the accuracy of the feeding methods, a volumetric feeder calibration was conducted. The feeder rate (feeder screw speed) was set to a specific value and resin was collected for one minute, this process was done 4 times for each of the screw speeds selected. Characterization Techniques Proton Magnetic Resonance (NMR) spectroscopy was conducted on the polymer blends to verify their weight ratio by means of a Bruker Avance DPX 500 spectrometer with field strength of 500 MHz. Solutions of 45wt% resin in deuterated chloroform (CDCl 3 ) were tested under ambient temperature. The rheological behavior of neat PBS and the blends were measured by means of a parallel-plate rheometer (ARES-G2, TA Instruments) set with 25 mm diameter stainless steel parallel disks. The runs were conducted at 130 o C. Prior to the tests explained above, the samples were dried in a vacuum oven at 80 C for 24 hours. To analyze the morphology development of the blends a scanning electron microscope (Jeol JSM-6330F) was used at 7kV. Samples were cryogenically fractured to SPE ANTEC Anaheim 2017 / 1089

3 prevent any deformation on the surface to be imaged. Also, the specimens were sputter coated with gold to increase their electrical conductivity. Results Feeding Method Analysis The feeder calibration results using the different configurations are shown in Table 1. The flow rate of the premix trials appeared more consistent than the ones using the side feeder and PPC pellets. This is because the PBS pellets are very free flowing and tend to feed well in the barrel. However, during this process the PPC pellets tended to agglomerate at the end of the feeder screw and did not go down inside the barrel. This happened because the PBS pellets are uniform in size whereas the PPC pellets are non homogeneous due to the breakup process. This caused the resins to flow at different rates, hence, not achieving the targeted blend composition. The feeding of the PPC pellets using the side feeder showed very inconsistent feeding evidenced in Table 1. In contrast, the flow rate of the powdered PPC grounded with copper gauze appeared steadier; the error associated with the measurements is very small in comparison to the other two feeding methods. This is enough evidence to state that this last configuration provided a more uniform feeding which will lead to the targeted blend weight fraction. However, the morphology and degradation results presented in this article are those obtained using a side feeder with PPC pellets because trials using PPC powder are still in process. Table 1. Feeding Calibration of the three different feeding methods used for twin screw extrusion. Feeding Methods Premix with Main Feeder PPC Pellets with Side Feeder PPC Powder with Side Feeder Speed [rpm] Blend Composition Analysis Flow rate [g/min] Error In order to determine the blend composition of the melt compounding trials, a curve fitting on the NMR spectra was done using Origin 8. Table 2 displays the weight fraction of the samples obtained using the two feeding methods explained earlier; with the premix of PBS/PPC and using a side feeder to feed the resins individually. The targeted weight fraction was not achieved in any of the systems. This is evidence of the difficulties in accurately feeding the PPC pellets into the hopper. Despite the inaccurate blend ratios, analysis of the blend properties was carried out bearing in mind the measured weight fraction. The samples prepared using the side feeder were chosen to carry out the analysis because they are generally closer to the targeted weight than the premix samples. Table 2. Blend Composition Results. Blends [rpm] PPC [%] Premix (pellet form) PPC [%] Side feeder (pellet form) Rheology Study In order to analyze the effect of the screw speed on the rheological properties of the blends and the neat PBS, parallel plate rheology was conducted. Using the Cross Model and a routine written in Origin 8, the zero shear viscosity (η) at different screw speeds was calculated for every sample. Then, the relative viscosity (η r ) was determined by dividing the viscosity of the blend by the viscosity of the matrix (neat PBS). In order to account for the potential degradation of the PBS matrix, the experimental relative viscosity (ηr Exp ) was determined using the zero shear viscosity of the neat PBS processed at the different screw speeds. For example, the ηr Exp of the 500 rpm blend was obtained dividing the zero shear viscosity of the blend by the neat PBS both extruded at 500 rpm. Moreover, the Maron-Pierce model for composites was used to predict the viscosity of the blend systems and compare it to the experimental values. For this analysis it was assumed that the PPC phase behaves as a solid filler in the system. The filler constant was chosen to be the one for spheres, which has a value of The NMR composition results were employed to calculate the theoretical relative viscosity ηr Theo. Table 3 displays the results of samples obtained with the batch mixer and the twin screw extruder. The viscosity prediction provided an excellent fit with the experimental data for all the Brabender batch mixer and TSE samples. This proves that the Maron - Pierce equation can model the behavior of this blend system at these processing conditions. This also serves as evidence of the incompatibility of the system; a discrepancy between the results will mean the blend has some degree of miscibility due to deviation from a spherical droplet morphology. The relative viscosity of the 1000 rpm sample happened to be the highest because it contains the highest PPC content of all the samples. In contrast, the 2000 rpm run had the lowest h r because it contains less PPC. Even though the model seemed to fit the experimental data, one SPE ANTEC Anaheim 2017 / 1090

4 must bear in mind it does not explicitly account for the molecular weight loss of either phase due to thermomechanical degradation during processing. However, since it is assumed that the PBS degradation is taken into account in the calculation of the relative viscosity, it is possible to compare both theoretical and the experimental relative viscosities. Table 3. Experimental and theoretical relative viscosity of the batch mixer and TSE samples. Blend ηr Exp ϕ PPC ηr Theo Batch Mixer runs (50 rpm) 25 wt% PPC wt% PPC Extruder runs 200 rpm rpm rpm rpm In order to analyze the effect of the screw rotational speed on the degradation of neat PBS and the blends the change in viscosity η/η o and the molecular weight loss ΔM w were calculated. η corresponds to the zero shear viscosity of the processed samples (extruded at different screw speeds) and η o the zero shear viscosity of the virgin unextruded PBS, in the case of the neat PBS samples. For the blends, η o is the blend zero shear viscosity calculated by the Maron-Pierce model using the viscosity of the virgin neat PBS and the determined PPC volume fractions. In essence, a small η/η o indicates a more significant reduction in viscosity and a high η/η o (close to 1) means the viscosity of the sample is similar to the one of the virgin resin. For polymers above their entanglement molecular weight (usually around 3-8 kda), the Fox- Flory equation (2) shows a power law with the melt viscosity and the average molecular weight M w [7]. η~m w 3.4 Using this relation the molecular weight for each of the processed samples was estimated and the molecular weight loss percent (ΔM w =1 - M w /M wo ) was calculated. Taking the initial molecular weight (M wo ) as that for virgin PBS, M w and DM w were calculated using the viscosity from the Maron-Pierce model. These results are depicted in Figure 1 for the neat PBS and the blends processed by TSE. In the case of the neat PBS the viscosity change remained pretty much the same for the first screw speeds but decreased a bit more, in the case of the 2000 rpm (solid diamonds). This resin is very liquid like, having viscosity values around 400 to 240 Pa.s from the lowest to the highest screw speeds. It appears that the degradation effects become dominant only at high screw (2) speeds. These results correlate with what found by Chen et al. [8], regarding the molecular weight loss of PBS/fumed silica composites, which stated the molecular weight loss becomes significant only at very high levels of shear. In contrast, the viscosity and the molecular weight loss of the blends seemed to follow a linear trend with the screw speed variation. Since the molecular weight of the blends has been normalized by its theoretical unprocessed molecular weight at the specific weight fraction, it is possible to explore the influence of high viscosity PPC on PBS degradation from these results. Even though the PPC imparts high viscosity to the system due to its solid like nature it also triggers more degradation to the matrix, perhaps because of shearing forces on the PBS, or because of specific chemical interactions between PBS and PPC during extrusion. For example, the viscosity of the 40 wt% PPC blend processed at 1000 rpm was Pa.s; this is 7 times higher than the viscosity of the neat PBS extruded at the same conditions. Therefore the blends molecular weight loss (empty triangles) seems to be a little bit higher than the neat PBS. Finally, the linear trend found for the case of the blend viscosity provides evidence for the incompatibility of the system. Deviations of the tendency will demonstrate some degree of compatibility due to interfacial adhesion caused by grafting between the phases [9]. η/ηo Figure 1. Relative viscosity and molecular weight of neat PBS and its blends versus screw speeds. Morphology Study Blend Neat PBS Screw Speed [rpm] 20% 15% 10% 5% The morphology of polymer blends establishes the bulk properties of the system. The microstructure of two immiscible polymers blended during melt extrusion is a matrix-droplet morphology. Equation 3 describes the relationship between the zero shear viscosity, the volume fraction of the blends and their morphology. V 1 /V 2 η 2 /η 1 =X (3) V represents the volume fraction and η the zero shear viscosity of each of the phases. When X>1 phase 1 is the matrix, X<1 phase 2 is the matrix and when phase ΔM w SPE ANTEC Anaheim 2017 / 1091

5 cocontinuity exists X 1. In the case of this blend system, when phase 1 is PBS and phase 2 is PPC, X<1 which means that PPC will form the droplets and PBS will become the matrix. However, the size and shape of the dispersed phase depends on several material and processing parameters including rheology, interfacial interactions, blend composition, residence time and shear rate distribution [10]. Figure 2 displays the morphology of the blends processed at different screw speeds by TSE and batch mixer at the same magnification. The blends compounded at 200 (Figure 2a) and 500 rpm (Figure 2 b) resulted in similar weight fraction (both are around 30 wt% PPC). They present a matrix droplet morphology with elongated domains of irregular geometry. The domains seem to be isolated from the matrix which is an evidence of the repulsion between the phases due to high interfacial tension. The 25wt% PPC blend prepared with the batch mixer (Figure 2c) can be compared to these images since the difference in weight ratios is small. The Brabender blend has a more uniform droplet shape but they are also bigger in size. In contrast the blends processed at higher speed seem to be smaller in size and less uniform in geometry. This also confirms that when the screw speed is faster, the higher levels of shear trigger the breakup of droplets during extrusion. These results correlate with those of Gug et al. [11] in their morphology (a) analysis of high speed reactive extrusion, where the droplets tended to become smaller with higher rpms. The 1000 rpm sample (Figure 2d) presented a broader domain size distribution and different domain geometries. The smaller droplets seem to have a more spherical shape and the bigger agglomerates are more elongated and nonuniform. This blend composition is around 45% of PPC which explains why it seems to contain more PPC phase. This blend can be compared to the 50 wt% prepared using the batch mixer (Figure 2e). In this image the droplets appeared to be elongated but also bigger in size due to the higher PPC fraction in this case. However, the difference in weight percent between Figure 2d and 2e is just 5%. Another point worth mentioning is that degradation of PPC can cause a viscosity reduction that can increase coalescence of its particles. Moreover, the 2000 rpm blend (Figure 2f), which contains the lowest PPC content seemed to have smaller droplets and more uniform domain shapes. Its morphology looked similar to the 25wt% PPC Brabender sample; however, the droplet size is smaller in the case of the 2000 rpm. The domains also seemed to have a better interaction with the matrix since there are no large gaps observable at the interfaces, as is the case for the other images. (b) (c) (d) (e) (f) Figure 2. SEM images of blends at (a) 200 rpm (30 wt% PPC), (b) 500 rpm (30 wt%) (c) 50 rpm (25 wt%), (d) 1000 rpm (44 wt%), (e) 50 rpm (50 wt%) and (f) 2000 rpm (15 wt%) at 3000X. SPE ANTEC Anaheim 2017 / 1092

6 Conclusions 100% biodegradable and biobased polymer blends were melt compounded by means of a Brabender batch mixer and a high speed twin screw extruder at different screw speeds. It was found that milling the PPC and using copper gauze on the feeder to dissipate the electrostatic charges helped to obtain a more consistent feeding process. The Maron-Pierce model validated the assumption that PPC behaves as a solid-like filler in this blend system. The model was also used to obtain the viscosity change and the molecular weight loss of the blends to compare it to that of neat PBS. It was found that the blend experienced higher levels of degradation due to the higher shear stress when PPC was present. Moreover, the blend viscosity change was inversely proportional to the screw speed, indicating that there was no interfacial interaction between the polymer phases. Finally, the morphology study showed a two-phase morphology for all the blend systems. It was concluded that the two dominant factors affecting the morphology are the blend ratio and the screw speed. The screw speed helped the breakup of the droplets but also produced agglomerates with less uniform shapes. This work was intended to provide a better understanding of the viscosity behavior, molecular weight loss and morphology development on the properties of biodegradable and biobased polymer blends in order to help expand their market and their utility. Acknowledgments References [1] X. H. Li, Y. Z. Meng, G. Q. Chen, and R. K. Li, Journal of Applied Polymer Science, 94, 711 (2004). [2] B. Ahn, S. Kim, Y. Kim, and J. Yang, Journal of Applied polymer Science, 82, 20808, (2001). [3] T. A. Osswald, Understanding polymer processing: processes and governing equations, Cincinnati: Hanser Publications, (2011). [4] A. Faranchi, R. Malloy, and M. Sobkowicz, Journal of Applied Polymer Science, 56, 743 (2016). [5] L. A. Utracki, and M. R. Kamal, Polymer Engineering and Science, 22, 96 (1982). [6] S.H. Maron, and P. E. Pierce, Journal of Colloid Science, 11, 80, (1956). [7] L.H. Sperling, Introduction to Physical Polymer Science, Wiley, Hoboken, NJ (2015). [8] X. Chen, and M. Sobkowicz, Journal of Polymer Science Part B: Polymer Physics, 54, 1820, (2016). [9] C. Koning, M. Van Duin, C. Pagnoulle, and R. Jerome, Progress in Polymer Science, 23, 707, (1998). [10] U. Sundararaj, and C. W. Macosko, Macromolecules, 28, 2647 (1995). [11] J. Gug, Ph.D Thesis, University of Massachusetts Lowell, (2016). The authors wish to acknowledge the funding from the National Science Foundation (NSF) CMMI Also, the authors wish to thank Azadeh Farahanchi and JeongIn Gug for their help with the twin screw extruder and the characterization techniques. SPE ANTEC Anaheim 2017 / 1093