Chemical-free Reactive Melt Processing of Biosourced Poly(butylene-succinate-adipate) for Improved Mechanical Properties and Recyclability

Abstract

Biosourced and biodegradable polyesters like poly(butylene succinate-co-butylene adipate) (PBSA) are gaining traction as promising alternatives to oil-based thermoplastics for single-use applications. However, the mechanical and rheological properties of PBSA are affected by its thermomechanical sensitivity during its melt processing, also hindering PBSA mechanical recycling. Traditional reactive melt processing (RP) methods use chemical additives to counteract these drawbacks, compromising sustainability. This study proposes a green reactive method during melt compounding for PBSA based on a comprehensive understanding of its thermomechanical degradative behavior. Under the hypothesis that controlled degradative paths during melt processing can promote branching/recombination reactions without the addition of chemical additives, we aim to enhance PBSA rheological and mechanical performance. An in-depth investigation of the in-line rheological behavior of PBSA was conducted using an internal batch mixer, exploring parameters such as temperature, screw rotation speed, and residence time. Their influence on PBSA chain scissions, branching/recombination, and cross-linking reactions were evaluated to identify optimal conditions for effective RP. Results demonstrate that specific processing conditions, for example, twelve minutes processing time, 200 °C temperature, and 60 rpm screw rotation speed, promote the formation of the long chain branched structure in PBSA. These structural changes resulted in a notable enhancement of the reacted PBSA rheological and mechanical properties, exhibiting a 23% increase in elastic modulus, a 50% increase in yield strength, and an 80% increase in tensile strength. The RP strategy also improved PBSA mechanical recycling, thus making it a potential replacement for low-density polyethylene (LDPE). Ultimately, this study showcases how finely controlling the thermomechanical degradation during reactive melt processing can improve the material's properties, enabling reliable mechanical recycling, which can serve as a green approach for other biodegradable polymers.

Figure 1

Time-dependent evolution of the torque recorded during PBSA processing in an internal mixer under different processing conditions. In particular, (A) at T = 180 °C and different screw speeds and (B) at 60 rpm and different processing temperatures.

Figure 2

(A) Dispersity of unprocessed PBSA, and RP reacted PBSAC1, PBSAC2, PBSAC3, and PBSAC4 and (B) Number average molar mass (M_n) and weight averaged molar mass (M_w) as a function of processing time, measured by SEC. (C) scheme of branching/recombination.

Figure 3

(A) FT-IR ATR spectra of PBSAC1, PBSAC2, PBSAC3, and PBSAC4 normalized to the intensity of mode centered at 750 cm^–1. (B) Close-up view of the range 4000–2500 cm^–1, showcasing the time-evolution of specific spectral features. (C) Close-up view of the range 1800–1500 cm^–1.

Scheme 1. Closed-Loop Kinetic Scheme (CLS) Proposed for PBSA Modification during Green REx

Figure 4

Rheological characterization of the samples processed at different times at 200 °C and 60 rpm: (A) Complex viscosity as a function of the angular frequency ω and (B) viscoelastic storage moduli, recorded during the frequency sweep tests in the molten state (T = 120 °C); (C) Van Gurp-Palmen plot, constructed by plotting the phase angle as a function of the complex shear modulus; and (D) Cole–Cole plots, obtained by diagramming log (G′) vs log (G″).

Figure 5

(A) Representative tensile stress–strain curves of PBSAC1, PBSAC2, PBSAC3, and PBSAC4; (B) digital photographs of Dumbbells’ specimens after tensile tests; (C) Young’s Moduli; (D) tensile strength; (E) yield stress; (F) elongation at break of the different materials; (G) cross sectional SEM micrographs of tensile fractured PBSAC1; and (H) PBSAC3.

Figure 6

DSC analysis of PBSAC1, PBSAC2, PBSAC3 and PBSAC4: (A) scans collected during second heating, (B) cooling scans, and (C) WAXD patterns.

Figure 7

(A) Representative tensile stress–strain curves of PBSA neat and PBSAC3 after 10 cycles of mechanical recycling; (B) digital photographs showing the Dumbbells’ specimens appearance after 10 mechanical recycling processes; (C) Young’s Modulus; (D) tensile strength; and (E) elongation at break.