Additives Application
Additives Application News Make PLA better Improving Processing and Properties By Connie Lo and Zuzanna Donnelly Arkema Inc. King of Prussia, Pennsylvania, USA It took over a decade to get to the present day, and scientists are continually working to improve the processing and properties of PLA. Two areas of interest include impact modification to improve toughness of PLA to overcome the extremely brittle nature of the polymer; and increasing the melt strength of the molten polymer in order to access applications such as blown film and foaming which require a high degree of melt strength and melt elasticity. Toughening of PLA PLA is a brittle polymer compared to many traditional petroleum-based plastics. However, with the addition of core shell impact modifiers, these modifiers can dramatically increase the impact toughness of PLA by as much as several orders of magnitude. Core shell impact modifiers have been observed to impart the highest degree of toughening in PLA. These modifiers mitigate cracking and chipping problems during processes such as thermoforming as well as improving the performance of the finished article. Impact toughening becomes increasingly critical for durable goods applications that require higher impact strength and good low temperature impact. The decrease in tensile and flexural modulus is proportional to the amount of modifier added and can decrease the stiffness of PLA in applications such as blown film. 4000 Flexural Modulus Modulus [MPa] 3000 2000 1000 0 0 2 4 6 8 10 12 wt % Impact Modifier 12 Gardner Impact (1mm (40 mil) molded disk) 10 Figure 2: 1 mm (40 mil) thick injection molded PLA disk without impact modifier (right) and with 5% rubber based core shell impact modifier (left). Impact [J] 8 6 4 2 0 0 2 4 6 8 10 12 wt % Impact Modifier Figure 1. Mechanical properties of PLA core-shell impact modifier 36 bioplastics MAGAZINE [02/12] Vol. 7
Additives Improving Melt Processing Another area of interest for PLA modification is increasing the melt strength of the polymer. PLA has very low melt strength resulting in difficulties in processing the polymer with techniques such as blown film, deep draw thermoforming or foaming which rely on large draw down ratios or rapid controlled expansion of the melt. Melt strength of PLA can be improved by the addition of small amounts of linear high molecular weight acrylic copolymers. These copolymers are highly miscible with PLA resulting in a blend that is optically transparent. In this manner the melt strength of the blend can be increased by 50-100% over the neat PLA. Figure 2B qualitatively illustrates the effect of addition of acrylic melt strengthener on the PLA melt. The melt containing additive is noticeably stiffer and holds its shape better than the neat PLA. The melt strength of PLA decreases with decreasing PLA molecular weight. As with other condensation polymers, PLA is subject to degradation through hydrolysis when melt processed in the presence of moisture. Thus drying of PLA pellets as well as PLA regrind scrap is required prior to processing. When drying equipment is not available, the use of melt strengthening additives has been shown to compensate for losses in melt strength due to hydrolysis as illustrated in Figure 3 below. Thus the addition of 4% of an acrylic melt strengthener can improve the melt strength of PLA that has been processed without drying to levels above the virgin resin. Future Trends Bioplastics only contribute to 1% of the plastics used in the world today. However, its range of applications is rapidly growing and developing as processors look towards using bioplastic in areas traditionally dominated by petroleum based resins. In addition to the additives presented here, much work is being devoted to addressing the issues of the low heat distortion temperature of PLA and increasing the rate of crystallization of PLA from the melt. There are also trends towards making blends of PLA with starch and other degradable bioplastics for completely biodegradable articles. On the other end of the spectrum, manufacturers are looking to blend PLA with thermoplastics such as polycarbonate or PMMA for making durable goods with an increased biobased content. With the current push towards sustainability coupled with the steadily increasing global PLA production capacity the applications and innovations around PLA will undoubtedly grow in the coming years. Force [N] Force [N] Rheotens Analysis of PLA 0.18 0.16 neat PLA PLA with 2% additive 0.14 PLA with 4% additive 0.12 0.10 0.08 0.06 0.04 0.02 0.00 0 100 200 300 400 Pull-Off speed [mm/s] Figure 2 A) PLA strands analyzed on a Rheotens apparatus with and without acrylic copolymer. Figure 2 B) PLA without additive (left) and with 4% melt strengthener (right) 0.20 0.18 undried PLA processed with 4% additive Unprocessed PLA 0.16 PLA processed without drying 0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00 0 100 200 300 Pull-Off speed [mm/s] Figure 3. Rheotens data showing effect of acrylic melt strengthening additive on PLA processed without drying www.arkema-inc.com bioplastics MAGAZINE [02/12] Vol. 7 37
