Supercritical CO2-assisted impregnation of PLA films with R-carvone. II: Effect of process variables on mechanical behavior

MURATORE, FLORENCIA; MIRANDA VILLA, PATRICIA P.; LUJÁN, LAUTARO; GAÑAN, NICOLÁS A.; GOÑI, MARÍA L.; MARTINI, RAQUEL E.

Los Cocos

Congreso; VI Iberoamerican Conference on Supercritical Fluids (PROSCIBA 2023); 2023

IPQA-UNC-CONICET; PLAPIQUI-UNS-CONICET

Supercritical CO2-assisted impregnation has been investigated for several years as an emerging technology for the incorporation of active compounds into polymers for different applications. In particular, the incorporation of natural-occurring compounds with antimicrobial, antioxidant, and/or insecticidal activities into flexible polymer films appears as an interesting strategy to obtain active materials for food preservation. In addition, the food packaging industry has shown an ever-increasing interest in the use of biodegradable materials, such as polylactic acid (PLA), aiming to gradually decrease the use of petroleum-based polymers, which nowadays represents an environmental problem. In this sense, the supercritical impregnation of PLA films with R-(–)-carvone, a compound with known antimicrobial and insecticidal activity, was studied as a route to develop an active material for food packaging applications. The influence of process variables on the impregnation yield, release kinetics, and thermal and crystallinity properties was previously studied [1,2]. In this work, a systematic study of the effect of the supercritical processing and the incorporation of R-(–)-carvone on the mechanical behavior of PLA films was assessed. Commercial films of PLA were kindly provided by Converflex (ARCOR Group, Argentina), while R-(–)-carvone (98% purity) was purchased from Sigma-Aldrich (Germany) and industrial extra-dry carbon dioxide (water content ≤ 10 ppm v/v) was from Linde (Argentina).The supercritical processing of PLA films with and without the addition of carvone was performed in batch mode, using a lab-scale high-pressure system described before [1] Three process variables were evaluated: scCO2 density (278-683 kg.m-3), temperature (40-60 °C), and depressurization rate (0.6-6.0 MPa.min-1). All runs were performed in duplicate, using a contact time of 2 h and a carvone/PLA mass ratio of 0 or 0.5. Tensile properties were analyzed in a universal testing machine (Instron, USA), according to ASTM D 882–12. The statistical effect of the process variables on elastic modulus (EM), tensile strength (TS), and elongation at break (E%) was assessed by ANOVA testing (p < 0.05).Neat PLA films showed a brittle behavior, presenting high EM (1606.7 ± 168.9 MPa), and low E% and TS (E% = 11.6 ± 8.3 %, TS = 25.4 ± 2.0 MPa). The samples treated with scCO2, in absence of carvone, presented similar tensile behavior (EM = 1285 – 1749 MPa), but with higher TS (25.5 – 38.9 MPa) and E% (45.4 – 123.4 %) values. The increase in tensile strength could be attributed to the higher crystallinity degrees of processed samples (~3% for neat samples and ~25–30% for treated samples, as reported in the first part of this study [2]). On the other hand, higher elongation values, opposite to that expected for higher crystallinities, can be explained by the relaxation of tensions imposed by stretching during the PLA film manufacturing, due to the plasticization induced by scCO2. ANOVA testing suggested that depressurization rate has no statistical effect on the mechanical behavior of pressurized samples. On the other hand, scCO2 density showed a significant effect on EM and TS, decreasing their values for higher densities. Furthermore, temperature presented a significant effect on TS and E%, decreasing both properties as temperature increases. The incorporation of carvone was also studied, obtaining films with ~6–30 carvone wt% [1]. These samples showed higher E% and lower TS, compared to the pressurized samples without the addition of carvone. The higher changes were observed at lower CO2 density, conditions that favor the carvone impregnation. According to ANOVA testing, for these samples, scCO2 density showed a significant positive effect on all responses, increasing the value of EM, TS, and E% for higher densities. On the other hand, the temperature presented a significant effect only on EM and TS, decreasing both properties as temperature increases. Finally, depressurization rate showed an effect only on EM and E% obtaining higher EM and lower E% when fast depressurization is used. Furthermore, a decrease in EM was observed as the carvone amount increased in the polymer matrix, probably due to an increase of free volume in the amorphous phase introduced by the higher plasticizer effect due to the presence of carvone. Particularly, EM suddenly decays at a carvone amount of 10 wt%, obtaining values around 200 MPa and lower than 50 MPa for higher additive content, suggesting that phase separation of the additive starts at this concentration, as observed in the thermal behavior study [2]. Finally, the results obtained here may provide useful information for broadening the knowledge of the individual effect of the process variables and the amount of additive incorporated into the polymer matrix on the mechanical behavior of PLA films.