Reactive Extrusion of Nonmigratory Antioxidant Poly(lactic acid) Packaging

Abstract

Reactive extrusion of bio-derived active packaging offers a new approach to address converging concerns over environmental contamination and food waste. Herein, metal-chelating nitrilotriacetic acid (NTA) ligands were grafted onto poly­(lactic acid) (PLA) by reactive extrusion to produce metal-chelating PLA (PLA-g-NTA). Radical grafting was confirmed by attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy with the introduction of secondary alkyl stretches (2919 and 2860 cm–1) and by X-ray photoelectron spectroscopy (XPS) with an increase in the atomic percentage of nitrogen. Compared to films prepared from native, granular PLA (gPLA), PLA-g-NTA films had lower contact angles and hysteresis values (86.35° ± 2.49 and 31.89° ± 2.27 to 79.91° ± 1.58 and 21.79° ± 1.72, respectively), supporting the surface orientation of the NTA ligands. The PLA-g-NTA films exhibited a significant antioxidant character with a radical scavenging capacity of 0.675 ± 0.026 nmol Trolox(eq)/cm2 and an iron chelation capacity of 54.09 ± 9.36 nmol/cm2. PLA-g-NTA films delayed ascorbic acid degradation, retaining ∼45% ascorbic acid over the 9-day study compared to <20% for control PLA. This research makes significant advances in translating active packaging technologies to bio-derived materials using scalable, commercially translatable synthesis methods.

Antioxidant functionalization of biomaterials via reactive extrusion

“…To ensure sufficient mixing with powdered reactants granulated PLA 4044D was prepared as described previously. 39 Briefly, Ingeo™ Reactive Extrusion Grade Poly(lactic acid) 4044D was dried in a gravimetric oven at 80 C for 4 h to remove any excess moisture according to the manufacturers' guidelines. Dried Poly(lactic acid) 4044D was fed into a Process 11 Parallel Twin Extruder (ThermoFisher Scientific, Waltham, ME) with eight heating zones equipped with a 1 mm die and extruded at 30-50 RPM and the following temperature profile: Zone 2 -145 C, Zone 3-190 C, Zone 4 -200 C, Zone 5 -200 C, Zone 6 -200 C, Zone 7 -200 C, Zone 8 -200 C, and 210 C at the die.…”

“…Dynamic water contact angle measurements of granulated PLA, cross-linked PLA, and antioxidant functionalized PLA were performed on Attention Theta Optical Tensiometer (Biolin Scientific, Stockholm, Sweden) as previously reported. 39 Values of advancing contact angle, receding contact angle, and contact angle hysteresis were measured and analyzed in accordance with Korhonen et al 57 and McCarthy & Gao, 2006. 58 Briefly, advancing contact angle (θ A ) was measured by first depositing a 2 μl droplet of deionized water on the surface, inserting the needle in the droplet, and increasing the size of the droplet at a rate of 0.5 μl/s.…”

“…Quantification was performed using the TA Universal Analysis software (TA Instruments, New Castle, DE). Granulated PLA, cross-linked PLA, and antioxidant functionalized PLA films were assessed for their free radical scavenging capacity using a modified trolox equivalent antioxidant capacity (TEAC) method as described by Herskovitz & Goddard, 2020 39 and others. [60][61] Briefly, equivalent volumes of 7 mM 2,2 0 -azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS) in 18 MΩ.cm at 25 C Milli-Q water and 2.45 mM potassium persulfate in 18 MΩ.cm at 25 C Milli-Q water were reacted in the dark at room temperature for 16 h forming ABTS radical stock solution.…”

“…The ability of granulated PLA, cross-linked PLA, and antioxidant functionalized PLA films to prevent oxidative degradation of ascorbic acid was measured as performed in Herskovitz & Goddard, 2020 39 and others. 36,62 Briefly, 1 x 1 cm 2 film samples were incubated with 1 ml of 20 mM ascorbic acid solution in 10 mM sodium acetateimidazole buffer, pH 4 within 10 ml glass GC vials and aluminum septum caps at 37 C for 9 days.…”

“…We have previously described a method for the creation of a bio-based metal chelating nonmigratory active packaging utilizing the industrially employed reactive extrusion process. 39 In this method, radical polymerization was utilized for the grafting of nitrilotriacetic acid (NTA), a commercial metal chelator with strong affinity for iron, onto PLA to produce PLA-g-NTA, resulting in a packaging material with significant metal chelating capacity and antioxidant efficacy. While this work demonstrated that PLA-g-NTA could be prepared by reactive extrusion, the synthesis route (radical polymerization) presents significant limitations, e.g.…”