Poly (lactic acid) (PLA) 3052D and 2003D exhibit a semicrystalline structure (the degree of crystallinity of about 33 %), have a similar glass transition temperature (Tg) (62 °C), melting temperature (Tm) (150 °C) and cold crystallization temperature (116–118 °C). There was no effect of the CE on the glass transition and melting temperature of both PLA grades. With an increasing content of the CE an increase in the cold crystallization temperature and decrease in the degree of crystallinity of PLA samples was observed (Table 1). The epoxy groups of Joncryl can react with the carboxyl and hydroxyl groups in the polyester, which brings about formation of branched polymer chains. That makes difficult a dense packing of polymer chains, which resulted in a decrease of crystallinity. Reduced mobility of polymer chains due to an increase in the molecular weight, enhanced the temperature of cold crystallization, which is consistent with the literature data. Higher drop in the crystallinity degree that has been observed for PLA 3052D may be related to a difference in the chain length of the two polymers, as evidenced by the difference in viscosity of tested materials (Figs. 2, 3). Shorter chains of PLA 3052D are more easily bound in long chains due to reaction with the CE.
Rheological properties play an important role during the foaming process of polymers. Low melt viscosity and elasticity allow growing of the pores whereas a high melt strength avoids coalescence of neighboring cells. However, if the melt viscosity is very low, an uncontrolled growth of pores may occur. Rheological studies have shown that both grades of PLA exhibit a pseudoplastic character of flow and PLA 3052D has a lower melt viscosity than PLA 2003D (Figs. 2, 3). Addition of the CE influenced markedly the viscoelastic characteristics of PLA. High increase in viscosity observed for both types of PLA has evidenced that Joncryl is an efficient CE of PLA. PLA 3052D was used in further studies because of its lower viscosity. Results of the elongational flow of molten polymer strands have shown that addition of the CE brought about an increase in the melt strength confirming a positive impact of the coupling agent on rheological properties. The force that causes a rupture of the polymer strand is 1.7 cN (Fig. 4), whereas addition of 1.5 % of the CE caused 10 fold increase in the melt strength (Table 2).
The results presented in Fig. 5 have shown that with an increased content of the CE the oxygen permeability of PLA 3052D became higher. That may be interpreted in terms of a lower crystallinity and higher free volume of the polymer with extended/branched chains.
The structure in PLA 3052D obtained during foaming extrusion has been presented in Fig. 6. The pore size equals to ca. 250 μm (Fig. 6a). Addition of the CE caused generation of a large number of small cells. The finest cellular structure (pore size of 20–50 μm) of PLA was observed after addition of 1.0 and 1.5 % of the CE (Fig. 6d, e). The foam morphology can be correlated with the viscoelastic characteristics of the melts.
Neat PLA is a low viscous fluid within which the cells may grow easily to a large diameter. Additionally a low melt strength facilitates merging of neighbouring cells into large pores. In contrary, PLA with addition of the CE exhibited 10-folds higher melt viscosity, therefore the cells growth is more difficult and high melt strength protects the cells of coalescence.
Density of all foamed materials is lower than that of the solid polymer (1.24 g/dm3) for about 30–40 % (Fig. 7). Foamed PLA 3052D has a density of 0.8 g/dm3 due to a high number of large cells. Addition of the CE brought about formation of much smaller pores in PLA matrix. The lowest density (0.7 g/dm3) has been observed for a polymer modified with 1.5 % of the CE.
Thermal Insulating Properties
Foaming allowed to reduce the thermal conductivity coefficient of foamed PLA 3052D for about 36 % and about 42 % of foamed PLA with 1.5 % CE compared to a solid polymer (Fig. 8). The results are in a good agreement with the cell morphology—the best thermal insulating properties have been obtained for the foams with a large number of small cells.