Development of High Temperature Resistant Stereocomplex PLA for Injection Moulding

Abstract

In this study, the production of stereocomplex PLA formulations (sc-PLA) by compounding and subsequent injection moulding at different mould temperatures was investigated. Several selective nucleating agents were identified and compounded with different poly(L-lactide)/poly(D-lactide) (PLLA/PDLA) ratios on a co-rotating twin screw extruder. The effect of nucleating agents (NA) on the crystallisation behaviour of the compound was systematically investigated by DSC analysis. The crystallisation behaviour of NA-21 (aluminium complex of a phosphoric ester), also in combination with talc, under cooling rates of up to 70 K/min was analysed. The wide-angle X-ray diffraction (WAXD) results showed a complete stereocomplex (sc) crystal formation on all specimens containing NA-21 even at the highest cooling rates. The thermo-mechanical testing of sc-PLA shows a Young’s modulus of approx. 3 GPa, yield stress of 30–40 MPa, elongation of 1%, and a heat deflection temperature (HDT-B) up to 180 °C. Processing sc-PLA exclusively via the processing route of compounding and injection moulding will open new areas of application for PLA at higher temperatures.

Scalable Continuous Manufacturing Process of Stereocomplex PLA by Twin-Screw Extrusion

“…Results were statistically processed and compared to PLLA (L175) base resin used in the production of SC PLA. Previous studies have reported an improvement in the tensile properties, particularly the elastic modulus and ultimate tensile strength, of stereocomplex PLA compared to its PLA homopolymer counterpart [ 32 , 34 , 38 , 75 ]. Theoretically, there should be an increase in the mechanical properties of the polymer as the crystallinity increases, as the densely packed crystallites improve the strength of the polymer, whereas the amorphous regions are responsible for the elastic behavior of the polymer [ 76 , 77 ].…”

“…Stereocomplex PLA extruded with 1% Finn nucleator produced the highest mechanical properties, as seen in Figure 12 . They conclude that increasing mold temperature coincides with decreasing nucleation frequency and increasing crystal growth [ 38 ]. The results shown are when SC PLA with 1% Finn is molded at 33 °C.…”

“…In order to improve the thermal/mechanical performance as well as the hydrolysis resistance of PLA, researchers have conducted many studies on PLA stereocomplexation using a variety of additives, nucleating agents, rheometers, and/or solvent-based techniques [ 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 ]. Table 1 summarizes the results in comparison to our technique.…”

“…Without additives/nucleators, only ~25% of stereocomplex formation occurred, with a stereocomplex melting peak temperature of ~220 °C. Only the use of two additives (aluminum complex with phosphoric ester NA-21 and NA-21 + talc) resulted in >99.99% of stereocomplex formation, but the melting peak temperature was still ~220 °C and the crystallinity was ~40% [ 38 ]. The most successful results in achieving exclusive stereocomplexation with excellent thermal properties originated from a recent study in Hungary.…”

“…The large extent of stereocomplex formation was due to the high shearing force caused by high-velocity air from melt blowing, leading to improved stereoselective interaction between the PLLA and PDLA chains [ 48 ]. However, this was a two-step process to produce stereocomplex PLA and resulted in a lower melting temperature, most likely due to the molecular weights of the homopolymers—PLLA (M n = 72 kDa) and PDLA (M n = 50 kDa) [ 35 , 38 ]. It is clear that no scalable, solventless, additive-free one-step process to produce exclusive stereocomplex PLA using high molecular weight PLA (M n >90 kDa) has been reported in journal articles.…”