MECHANICAL PROPERTIES AND THERMAL CHARACTERISTICS OF POLY(LACTIC ACID) AND PARAFFIN WAX BLENDS PREPARED BY CONVENTIONAL MELT COMPOUNDING AND SUB-SUPERCRITICAL GAS- ASSISTED PROCESSING (SGAP) Yann-Jiun Chen, An Huang, and Lih-Sheng Turng Polymer Engineering Center, Department of Mechanical Engineering University of Wisconsin Madison, Madison, Wisconsin, 53706, USA Abstract In this study, poly(lactic acid) (PLA)/paraffin wax (PW) blends containing different amounts of PW were investigated. The blends were prepared by a twin-screw extruder in two ways: conventional melt compounding extrusion and sub-supercritical gas-assisted processing (SGAP). Then, these blends and the neat PLA were injection molded into ASTM 638 Type V tensile bars for evaluation. To observe the effects of the different melt compounding processes and the amounts of PW on the blends, the thermal stability and mechanical properties of the tensile bars were characterized. The results showed that the addition of paraffin wax changed the fractural mechanism and yielded tremendous improvements in elongation compared to neat PLA. In addition, samples made by the sub-supercritical gas-assisted processing (SGAP) extrusion exhibited better and more consistent results. Introduction In anticipation of eventual depletion of petroleum resources and in consideration of the environments, the plastics industry is increasingly interested in biomaterials. Poly(lactic acid) (PLA) is one of the most widely used bioplastics for various end-use applications, including food packages, medical devices, and disposable utensils. PLA is a renewable, biocompatible, and biodegradable polyester that can be made from starch, cellulose, roots, or sugar [1]. Unfortunately, PLA exhibits low impact strength and slow crystallization rate, is very viscous, and has low thermal resistance, all of which limit its potential applications. As such, PLA has been modified in several ways to increase its processability, mechanical properties, and crystallization [2]. Such modifications include blending it with other polymers and/or adding fibers, fillers, or additives, such as organic montmorillonite [3], poly-tetrafluoroethylene (PTFE) [4], and graphene [5]. Waxes like jojoba oil have been used as plasticizers in PLA [6]. In this study, paraffin wax (PW) was chosen as a filler to improve the processability and properties of PLA. Paraffin wax has been widely used in lost-wax casting [7] and investment casting [8] to improve precision and surface finish of metallic components. This is because paraffin wax is chemically inert, has stable mechanical properties, is an excellent lubricant, is commercially available at a low cost [9], and undergoes ductile failure at low strain rates [3]. In order to achieve a better dispersion of paraffin wax in PLA, a twin-screw extruder was employed to melt compound PLA and paraffin wax at different weight ratios. In addition, a special process called subsupercritical gas-assisted processing (SGAP) extrusion was employed. The idea is to introduce nitrogen (N 2) or carbon dioxide (CO 2) at the sub-supercritical state to facilitate the dispersion of paraffin wax by adding elongational stresses through bubble expansion during extrusion [10]. In this study, N 2 was employed as the blowing agent in the SGAP method. SGAP is a low cost process that only requires simple equipment such as a standard gas cylinder, regulator, and metal hose compared to relatively more expensive supercritical fluid injection equipment. PLA was melt compounded with paraffin wax at ratios of 20%, 35%, and 50% (by weight) to modify PLA s mechanical properties. After pelletizing, two different kinds of pellets were obtained: conventionally melt compounded pellets and SGAP foamed pellets. Pellets prepared by SGAP are labelled with a prefix F-. Solid tensile bars were then injection molded with these two different pellets. Although foamed pellets from the SGAP process were used for injection molding, the premolding drying and plastication allowed the N 2 to diffuse out and pack/hold pressure would eliminate any preexisting voids, thereby producing solid tensile test bars. Thermal investigations were performed to study the effects of paraffin wax as a filler of PLA. Stress strain behavior and SEM were also used to characterize the tensile properties and morphology of the PLA/PW blends. Experimental Methods Processing Materials Ingeo 3001D an injection grade PLA in pellet form was purchased from NatureWorks (Minnetonka, MN). Purified paraffin wax beads were obtained from LorAnn Products (Michigan, MI). Equipment SPE ANTEC Anaheim 2017 / 1526
Mixing of the blends was performed in a Leistritz ZSE-18 co-rotating twin screw extruder with a standard N 2 cylinder and regulator purchased from Airgas. A metal hose (1/8 diameter) with Swagelok fittings was used to connect the regulator to the barrel of the extruder. Before extrusion, PLA pellets were dried in a vacuum oven at 80 C for 6 hours to remove any moisture. Prior to the injection molding process, the pellets were again dried in a vacuum oven at 40 C for 24 hours. ASTM 638 Type V tensile bars were injection molded using an Arburg Allrounder 270A. Characterization Differential Scanning Calorimetry (DSC) Thermal characterization of extruded PLA and blends was performed using a TA Instruments Q20 with sealed aluminum pans and a constant nitrogen flow of 50 ml/min. All samples were taken from the extruded (conventional and SGAP) pellets and were within 5 to 10 mg. The DSC profile was of a heat/cool/heat cycle from 25 C to 200 C at a rate of 5 C/min. The crystallinities (X c) of the neat PLA and its blend were evaluated from their corresponding melting enthalpies using Eq. 1 [11]. For PLA, the 100% crystalline melting enthalpy was taken as 93.0 J/g [12], Xc = H m H cold H m100 W f 100% (1) where ΔH m was the apparent melting enthalpy, ΔH cold was the cold crystallization enthalpy, ΔH m100 was the melting enthalpy of the 100% crystalline polymer matrix, and W f was the weight fraction of PLA in the blend. Thermogravimetric Analysis (TGA) TGA analyses were performed with a TA Instruments Q50 with a platinum pan at a constant nitrogen flow rate of 50 ml/min with a temperature ramping from 25 C to 600 C at a rate of 10 C/min. Tensile Properties ASTM 638 Type V samples were tested on an Instron 5967 with a 30 kn load cell. The crosshead speed was 1 mm/min and five samples from each group were tested to examine the consistency of the results. After tensile testing, the cross-sections taken at the fracture location were dipped in liquid nitrogen and examined via SEM for phase morphology. Scanning Electron Microscopy (SEM) Neat PLA and blend morphologies were obtained on a NeoScope JCM-5000 with a 10 kv accelerating voltage. The surfaces of the samples were gold coated for 60 s at 50 ma in a Denton Vacuum Desk V sputter coater. Rheology Rheological measurements were done on a TA Instruments AR 2000ex rheometer. A 25 mm parallel plate was used with a fixed gap of 500 m. In the frequency sweep mode, the strain amplitude was set at 1%. The frequency sweep was run from a high of 100 Hz to a low of 0.1 Hz at 180 C. DSC Results Results and Discussion The DSC thermograms recorded during the second heating of PW, PLA, and their blends are shown in Fig. 1. According to Fig. 1, PW has two endothermic peaks during heating (40.08 C and 57.45 C) and two exothermic peaks during cooling (35.58 C and 51.92 C). Previous studies denoted the first peak as the solid solid transition, and the second peak as the solid liquid transition. Before melting, the phase transformation of the solid solid transition will change from an ordered phase to a more disordered phase [13]. The enthalpies of the paraffin wax during melting and crystallization were 99.54 J/g and 101.1 J/g, respectively. The melting temperature is 57.5 C. The results of neat PLA and its blends are shown in Figs. 1 and (c). There are four additional thermal characteristic temperatures in addition to the transition temperatures of paraffin wax, namely, the glass transition (T g), cold crystallization peak (T cc), recrystallization peak (T c), and melting peak (T m) temperatures of PLA [14]. T m1 indicates the melting temperature of paraffin wax and T m2 of PLA. With an increase in the content of paraffin wax, the cold crystallization peak shifted to a lower temperature as compared to neat PLA. This indicates that PLA crystallization was affected by the presence of paraffin wax. The degree of crystallinity can be calculated via Eq. 1. As shown in Table 1, the degree of crystallinity ranged from 22.18% to 29.01% for conventional PLA/PW blends and 24.01% to 28.60% for SGAP foamed F- PLA/PW blends. With increasing amount of paraffin wax, the crystallinity increased proportionally. However, the conventional PLA/50%PW blend specimen showed an unexpected drop in crystallinity. This can be attributed to the non-uniform dispersion of paraffin wax in the PLA matrix. On the other hand, SGAP foamed specimens showed proportional results, indicating that there was more uniform distribution of paraffin wax in PLA. As the content of paraffin wax increased, a new endothermic peak appeared around 60 C related to the solid liquid transition of paraffin wax in the PLA matrix. TGA Results The thermal stability of neat PLA, conventional PLA/PW blend pellets, and SGAP foamed F-PLA and F- SPE ANTEC Anaheim 2017 / 1527
PLA/PW blend pellets were measured via TGA with the results presented in Fig. 2 and Table 1. Thermal Figure 2. TGA curves of conventional PLA/PW blend and SGAP foamed PLA/PW blend. Effect of SGAP on Melt Compounding For traditional extrusion, mixing predominantly relies on shear flows, which sometimes are inadequate to obtain a sufficiently dispersed and distributed secondary phase. During shear flow, for spherical or semi-spherical particles, like polymer droplets, the magnitude of the force to break up the droplets is given by F Shear = 3πη s γ r 2 (2) (c) Figure 1. DSC curves of paraffin wax, conventionally melt compounded PLA/PW blend, and (c) SGAP foamed PLA/PW blend. degradation was assessed by examining the onset temperature of degradation, T onset, at 5% weight loss, and the maximum degradation temperature, T max, at the point of maximum slope. Figures 2 and illustrates the differences between conventional PLA/PW blend and SGAP foamed F-PLA/PW blend. The samples processed with SGAP showed a higher T onset and T max. This behavior was due to better mixing and dispersion of the paraffin wax. Also, it can be seen that the thermal stabilities of the blends decreased compared to the neat PLA. where η s is the shear viscosity, γ is the magnitude of the rate of deformation tensor, and r is the radius of the particles. When using the SGAP process, the polymer or blend foams due to gas expansion, which incurs an equi-biaxial extensional flow in the material as the gas bubbles expand. The magnitude of the force on a polymer droplet in this process is given by F Extension = 6πη eb γ r 2 (3) where η eb is the equi-biaxial extensional viscosity [15] For a linear viscoelastic material, Trouton s ratio is the ratio of elongational extensional viscosity to shear viscosity, which is 3 [16, 17]. In the case of equi-biaxial extension, it is doubled, thus giving Tr = 6 SPE ANTEC Anaheim 2017 / 1528
η eb = 6η s (4) 70.1%. Note that both the tensile (4) stress and strain value were higher in the foamed samples. At 20% paraffin wax content, the Young s moduli of the blend are even slightly higher than that of the neat PLA. After increasing the content of paraffin wax to 35% and 50%, the Young s modulus decreased markedly due to the characteristics of paraffin wax. Overall, the most dramatic influence of paraffin wax on PLA was the increased ductility of PLA. Furthermore, increasing the paraffin wax content increased the elongation-at-break of the blends. SEM micrographs of fractured surfaces of PLA and PLA/PW blends tensile test bars are shown in Figs. 4 and 5. In Figs. 4 and 5, due to the brittleness of neat PLA, the fracture surface shows a typical brittle, smooth surface topography. The surface roughness of Figs. 4 (d) s and 5 (d) increased with the addition of paraffin wax. In Fig. 4, with the increased amount of paraffin wax, larger paraffin wax particles and voids can be seen on the cross-section. The voids were caused by large paraffin wax particles falling out during tensile testing. Figure 5 shows smaller voids as well as a more uniform structure. This suggests a more uniformly dispersed paraffin wax phase, which contributed to the greater elongation it can achieve. Both Equations [2] and [4] refer to the forces on a droplet in their respective flows. However, both shear and elongation flows are occurring during the melt compounding inside the extruder. With additional equibiaxial elongational flow in the SGAP process, it can be expected that the dispersion of the secondary phase in SGAP will be better than conventional melt compounding because additional forces are applied in the melt, aiding in breakup of suspended polymer droplets. Mechanical Properties and Phase Morphology of PLA Blends The mechanical property results of neat PLA and its blends are shown in Fig. 3, and the stress strain values are listed in Table 2. In comparing neat PLA and conventional PLA/PW blends in Fig. 3, although the tensile stress decreased from 58.63 MPa to 40.36 MPa, the elongation improved significantly from 11.6% to 49.9%. Figure 3 showed the same trend for SGAP foamed F-PLA/PW blends, with tensile stress decreasing from 57.99 MPa to 46.72 MPa and elongation increasing from 9.3% to Figure 3. Stress strain curves of conventional PLA/PW blends and SGAP foamed F-PLA/PW blends. Table 1. Different DSC and TGA results of neat PLA, conventional PLA/PW blends, and SGAP foamed F-PLA and F- PLA/PW blends (with a prefix F- ). Sample T g( C) T cc( C) T m( C) H c(j/g) H m(j/g) X c(%) T onset( C) T max( C) PLA 58.67 103.81 169.16 26.31 41.94 19.32±1.29 327.0 371.30 PW 35.8 45.58 57.5 99.54 101.1 1.68±0.23 241.5 316.12 PLA/20%PW NA 100.20 168.96 19.09 44.42 28.43±1.85 301.6 376.16 PLA/35%PW NA 100.18 168.46 20.18 47.16 28.44±1.07 299.3 376.34 PLA/50%PW NA 100.26 168.59 21.54 46.20 26.25±1.53 268.2 372.11 F-PLA 58.62 101.63 169.92 21.39 43.72 23.31±0.91 336.5 378.94 F-PLA/20%PW NA 100.13 169.61 21.64 46.11 27.40±0.76 330.6 378.94 F-PLA/35%PW NA 99.15 169.27 21.00 45.92 28.35±0.81 330.9 380.26 F-PLA/50%PW NA 99.98 169.59 17.51 44.11 33.46±0.89 329.0 379.42 SPE ANTEC Anaheim 2017 / 1529
Table 2. Mechanical properties of PLA and its blends. Young s modulus (MPa) Tensile strength (MPa) Elongation at break (%) PLA 810.25±10.41 58.63±0.50 11.6±0.06 PLA/20% PW 832.65±12.22 46.49±1.30 36.6±0.17 PLA/35%PW 790.92±21.82 44.79±2.51 44.7±0.16 PLA/50%PW 758.55±17.55 40.36±1.19 49.9±0.15 F-PLA 886.56±7.62 57.99±0.67 9.3±0.03 F-PLA/20%PW 893.20±22.47 49.07±0.95 58.7±0.14 F-PLA/35%PW 849.40±17.66 48.06±1.17 70.1±0.15 F-PLA/50%PW 797.14±15.21 46.72±1.52 58.4±0.27 Rheological Properties The rheological properties of the blends were studied to investigate the effects of paraffin wax on the PLA matrix. Figure 6 shows the complex viscosity (η*) of the PLA matrix and its blends. Both PLA and foamed PLA showed Newtonian behavior at frequencies lower than 1 Hz. Beyond that, they exhibited shear thinning behavior. It was also observed that neat PLA presented a higher viscosity across the whole frequency range. When paraffin wax was added to the polymer matrix, a decrease in complex viscosity values was observed. Therefore, with the addition of paraffin wax in the PLA matrix, the fluidity can be improved [18]. Conclusion Paraffin wax (PW) has been found to greatly improve the ductility of PLA/PW blends. It is also beneficial to apply the sub-supercritical gas-assisted processing (SGAP) with nitrogen as the blowing agent during melt compounding. As the polymer foams due to gas expansion, better mixing could be achieved through the additional break-up of the PLA and paraffin wax domains. The higher degree of PLA crystallinity is an indication that the PLA polymer matrix gained mobility via the addition of paraffin wax. For SGAP foamed PLA/PW blends, the DSC results suggested better and more uniform dispersion of paraffin wax, which led to higher T onset and T max temperatures and greater elongation-atbreak for the tensile test bars. The rheology tests showed that paraffin wax also improved the fluidity of the blend. (c) (d) Figure 4. Cross-sections of SEM images after tensile testing: neat PLA, PLA/20%PW blend, (c) PLA/35% blend, and (d) PLA/50% PW blend. SPE ANTEC Anaheim 2017 / 1530
(c) (d) Figure 5. Cross-sections of SEM images after tensile testing: neat F-PLA, F-PLA/20%PW blend, (c) F-PLA/35% blend, and (d) PLA/50% PW blend. Figure 6. Complex viscosity (η*) verses frequency of conventionally compounded PLA/PW blend and SGAP foamed PLA/ PW blend. References 1. T. Mekonnen, P. Mussone, H. Khalil, and D. Bressler, Journal of Materials Chemistry A, 1, 43, (2013). 2. N. Kawamoto, A. Sakai, T. Horikoshi, T. Urushihara, and E. Tobita, Journal of Applied Polymer Science, 103, 1, (2007). 3. J. Wang, S.J. Severtson, and A. Stein, Advanced Materials, 18, 12, (2006). 4. K. Bernland and P. Smith, Journal of applied polymer science, 114, (2009). 5. G. Mittal, V. Dhand, K.Y. Rhee, S.-J. Park, and W.R. Lee, Journal of Industrial and Engineering Chemistry, 21, (2015). 6. M. A. Elsawy, J. De C. Christiansen, and C.-G. Sanporean, Optoelectronics and Advanced Materials Rapid Communications, 8, (2014). SPE ANTEC Anaheim 2017 / 1531
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