By BYann-Jiun Chen 1,2, Matthieu Fischer 1, Chao-Chang A. Chen 2 , and Ines Kuehnert 1*, 1Department of Processing Leibniz Institute for Polymer Research, Dresden, 01069, Germany 2 Department of Mechanical Engineering National Taiwan University of Science and Technology, Taipei, 10607, Taiwan
Poly (lactic acid) (PLA) plastics have been popularly applied on many bio-degradable products and claimed as a green polymer materials for environmental concerns. In this study, a poly(lactic acid) (PLA)/paraffin wax (PW) composites with blends containing different amounts of PW and different compounding times have been developed and investigated. These composites blends were prepared by a micro-compounder with twin screw. Then, a neat PLA and the PLA/PW composites have been used to fabricate tensile specimens by micro injection molding machine. Effects caused by different compounding time and PLA/PW ratios, the thermal behavior and mechanical properties have been tested and investigated. Moreover, distribution and dispersion of PW in the PLA matrix have been observed in optical microscope and then calculated for comparison. Experimental results showed that the addition of PW yields significant improvements in ductility and toughness compared to that of neat PLA. The crystallinity and complex viscosity have also been improved. Finally, the samples of PLA/PW made by longer compounding time exhibits better distribution. Results of this study can be used for developing PLA/PW composites for bio-prosthesis for implants applications.
Introduction
In the past decades, polymers materials have contributed to popularity of technical products to human life. Some polymers such as polyethylene, polystyrene, polypropylene, and poly (vinyl chloride) are widely used in disposable products suitable for mass production. However, polymer pollution has become a serious issue to living environment 1. Nowadays, poly (lactic acid) (PLA) has become one of the most popular biomaterials since it is made from agricultural sources such as starch, cellulose, roots, or sugarcane, which makes it widely available for use in food packaging, disposable utensils, medical devices, and structural applications 2-6.
However, the biodegradable properties of PLA also limit its potential applications. PLA is inherently brittle and has a slow crystallization rate, low thermal resistance, and higher viscosity when molten 7, 8. Thus the PLA composites with other additive materials need to be investigated. In previous studies, paraffin wax (PW) has been used as a filler for the PLA matrix to investigate the thermal properties, mechanical behavior, and morphology of PLA/PW blends [9]. This is because paraffin wax is chemically inert, is an excellent lubricant, is commercially available at a low cost [10], undergoes ductile failure at low strain rates [11], and is water resistant. Paraffin wax has also been used in research as a sacrificial material 12-15, phase change material [16-18], and food additive [19, 20].
This study focuses on the effects of the compounding time and the paraffin wax ratio in the PLA/PW composites. Due to the physical properties of paraffin wax, it is expected that the hydrolysis degradation and water resistance of the blends can be superior to that of the neat PLA.
Experimental Methods
Processing
Materials
Materials for experimental study including PLA and PW. The Ingeo 3001D, an injection grade poly(lactic acid) (PLA) in pellet form, was purchased from NatureWorksm (Minnetonka, MN) and it had a MFR of 22 g/10 min (ASTM D1238), a density of 1.26 g/cm3, and a 1.4% D-LA content. Purified paraffin wax beads were obtained from LorAnn Products (Michigan, MI) with a density of 0.88 g/cm3. They arrived as white, odorless beads and were used as received.
Material Preparation and Process Conditions
PLA was melt compounded with paraffin wax at ratios of 15%, 20%, and 25%, and the compounding times were 1, 5, and 10 minutes as shown in Table 1. After pelletizing, solid tensile bars were injection molded. An investigation of the thermal properties, mechanical behaviors, water absorption, and rheological behaviors of as-injected specimens have been undertaken to study the effects of the compounding time and the ratio of paraffin wax. A polarized optical microscope is used to characterize the distribution and dispersion of the paraffin wax in the PLA matrix. Mixing of the blends was performed using a Daca micro-compounder (Goleta, CA). The total amount of processed volume was 5cc. Before extrusion, PLA pellets were dried in a vacuum oven at 40 °C for 24 hours to remove any moisture. Prior to injection molding process, the extruded pellets were again dried in a vacuum oven at 40 °C for 24 hours. Tensile test bars with a diameter of 0.6 mm were injection molded using a Desma FormicaPlast 2K microinjection molding machine. Some major processing conditions are listed in Table 2.
Polarized Optical Microscopy (POM)
The extruded pellets were cut into 10 mm slices via a Leica RM 2265 microtome to study the distribution and dispersion of paraffin wax in the PLA matrix using a ZEISS microscope. The middle parts of the extruded pellets were sliced and sandwiched between glass slides. Given the obtained POM images, the amount of paraffin wax, as well as its distribution and dispersion in the PLA matrix, can be calculated using a Matlab program.
Rheology
The rheological properties of the extruded pellets were investigated on a TA Instruments ARES-G2 rheometer. The extruded pellets were placed between 25 mm parallel plates with a fixed gap of 1.5 mm. 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.
Tensile Properties
Tensile tests were carried out on a Zwick/Roell universal testing machine with a 100 kN load cell at a crosshead speed of 3 mm/min. Five samples of each composition were tested to obtain the tensile stress vs. strain curves and to examine the consistency of the results.
Results and Discussion
Thermal behavior by DSC
The DSC thermograms recorded during the second heating scan of the paraffin wax, PLA, and PLA/PW blends are shown in Figure 1. It can be seen from Figure 1 and Table 3 that, with the addition of paraffin wax, the cold crystallization temperature decreased slightly and the crystallinity increased. The presence of paraffin wax served as a lubricant in the PLA matrix leading to an improved crystallinity which can be observed on the lower cold crystallization temperature compared to neat PLA. This increase in the degree of crystallinity of PLA in the PLA/PW blends was due to the nucleating effect of the paraffin wax phase on the PLA matrix.
DSC thermograms of (a) paraffin wax and neat PLA, and (b–d) PLA/PW blends.
Conceivably, the molecular mobility and crystallinity of the blends strongly depend on the paraffin wax content and its distribution in the PLA matrix. As the temperature exceeded the melting temperature of paraffin wax, the intramolecular interactions of the paraffin wax collapsed and the low molecular weight chains of paraffin wax began to disentangle and rearrange in the PLA matrix, 22, eventually contributing to the change in crystallinity of the PLA in the PLA/PW composites.
From the DSC curves, it can be seen that there are two melting temperatures. This indicates that PLA and paraffin wax are immiscible. Additionally, the glass transition temperature of PLA and the melting temperature of paraffin wax are 56.2 °C and 56.8°C; thus, they overlap one another. For this peak, the one-minute mixing blends acquired higher enthalpy than the five- and ten-minute mixing blends, thus demonstrating that the paraffin wax was not well mixed in the PLA matrix. As the content of the paraffin wax increased, this peak was even more significant.
Distribution and dispersion of PW in PLA matrix observed by POM
The dispersion of the paraffin wax in the PLA matrix was slightly affected by the mixing time. With a higher content of paraffin wax, the paraffin wax percentage could be increased, as shown in Figure 2. Since a material such as PLA takes a longer time to crystalize, the POM image of neat PLA showed only a few crystals, as can be seen in Figure 3 (a). The nucleating effect caused by the paraffin wax introduced a large number of crystals into the PLA matrix. Moreover, the longer mixing time and greater shear force yielded a better distribution of the paraffin wax, as shown in Figure 3 (b–d).
Rheological Properties
Under shear stress, paraffin wax lowers the viscosity of molten polymer composites as shown in Figure 4. As is well known, PLA is very sensitive to high temperatures. Thus, longer processing times during PLA compounding caused PLA to undergo severe thermal degradation and chain scission, leading to a lower molecular weight and broadening the molecular weight distribution curve 23, 24. Therefore, as expected, the ten-minute processing curves exhibited the lowest complex viscosities in each group. In addition, when paraffin wax was added to the polymer matrix, a decrease in complex viscosity values was observed.
Furthermore, since paraffin wax acted as a lubricant in the polymer matrix, it is expected that the addition of paraffin wax will further improve the fluidity and processability of the blend as compared to neat PLA.
Mechanical Properties of PLA Blends
Figure 5 plots the Young’s modulus, tensile strength, ductility (elongation-at-break), and toughness (area underneath the stress vs. strain curve) of neat PLA and the blends obtained through tensile tests. The incorporation of paraffin wax exhibited dramatic improvements in ductility and toughness compared to neat PLA. This is due to plasticizer and lubricating effect of the paraffin wax. The Young’s modulus, ductility and toughness of neat PLA slightly decreased with the longer compounding time, however, the blends showed different tendency. The elongation at break and the toughness of the blends were increased with the increasing compounding time. This is because the agglomerated paraffin wax disperse more uniformly in PLA matrix as well as lower the standard deviation.
Conclusion
This study has developed and investigated PLA/PW blends and compared with mechanical properties of micro tensile specimens by injection molding process. The key that greatly improved ductility and toughness under tensile loading is the incorporation of percentage of paraffin wax in PLA/PW composites. Paraffin wax also served as nucleating agent, plasticizer and lubrication in PLA matrix, increasing the crystallinity, ductility, toughness and lowering the complex viscosity. From experimental results, the distribution of the secondary phase material in polymeric matrix can be improved by longer compounding time.
Acknowledgements
The authors would like to acknowledge the support of the colleagues at the Leibniz Institute for Polymer Research, Dresden, Germany, Dr. Andreas Leuteritz, Martin Zimmermann, Yvonne Spörer, Pascal Pöhlmann as well as the Ministry of Science and Technology (MOST) of Taiwan and DAAD for financial support.
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