Improving mechanical performance of injection molded PLA by controlling crystallinity

Abstract

Currently, use of poly(lactic acid) (PLA) for injection molded articles is limited for commercial applications because PLA has a slow crystallization rate when compared with many other thermoplastics as well as standard injection molding cycle times. The overall crystallization rate and final crystallinity of PLA were controlled by the addition of physical nucleating agents as well as optimization of injection molding processing conditions. Talc and ethylene bis-stearamide (EBS) nucleating agents both showed dramatic increases in crystallization rate and final crystalline content as indicated by isothermal and nonisothermal crystallization measurements. Isothermal crystallization halftimes were found to decrease nearly 65-fold by the addition of only 2% talc. Process changes also had a significant effect on the final crystallinity of molded neat PLA, which was shown to increase from 5 to 42%. The combination of nucleating agents and process optimization not only resulted in an increase in final injection molded crystallinity level, but also allowed for a decreased processing time. An increase of over 308C in the heat distortion temperature and improved strength and modulus by upwards of 25% were achieved through these material and process changes.

Estudo do comportamento térmico e mecânico do PLA modificado com aditivo nucleante e modificador de impacto

“…Onde: ∆Hm é a entalpia endotérmica de fusão, ∆Hc é a entalpia exotérmica associada aos cristais durante o aquecimento, ou seja, a entalpia da cristalização a frio do PLA e ∆H m ∞ é a entalpia teórica do PLA 100% cristalino que é assumida ser 93 J/g [10] . O grau de cristalinidade foi significativamente modificado pelos aditivos.…”

“…Após recozimento [10] . A associação dos dois aditivos teve um efeito muito positivo com aumento de 50% no valor de E. A adição de MT e MI mostra pequena influência sobre a Resistência à tração e sobre o Alongamento dos corpos de prova testados.…”

“…A maior cristalinidade observada deveria ter aumentado o Módulo elástico, bem como a Resistência à tração [10] , o que não ocorreu. Isto poderia ser explicado por uma possível degradação causada pelas condições empregadas no tratamento térmico.…”

Composites of High-Temperature Thermomechanical Pulps and Polylactic Acid

“…The limited crystallization of PLA in injection molding can cause the impairment of mechanical properties (Harris and Lee 2008;Drieskens et al 2009;Sanchez-Garcia and Lagaron 2010), but our data indicate that the introduction of fibers speeds up the crystallization process in the used experimental setup. Transcrystallization, the formation of oriented crystals at the fiber-matrix interface, has been found to dominate in PLA/wood flour composites, where the fiber surface is rough (Mathew et al 2006).…”

Amino acids and poly(amino acids) as nucleating agents for poly(lactic acid)

“…Tsuji et al [17] proposed the following gradation of nucleation efficiency in PLLA (T c = 115.9°C) when 3 wt% of nucleating agent was added: 52 kg mol -1 PDLA (T c = 143.9°C) > talc (T c = 128.3°C) > fullerenes (T c = 126.9°C) > montmorillonite (T c = 120.4°C). Furthermore, other research groups focused on developing new additives such as self-assembling or chemical nucleating agents [23] like CaCO3, TiO2, BaSO4 [24], hydrazide compounds [25] or ethylene-bis-stearamide [26].…”