Fusion-bonding behavior of plasticized corn proteins in fused deposition modeling process

The processing of natural biopolymers by Fused Deposition Modeling (FDM) opens perspectives for applications in food and health domains by taking advantages of their edibility, biocompatibility and bioresorbability. Glycerol plasticized zeins (proteins from maize kernels) present thermomechanical properties matching the extrusion step requirements of FDM (at T_printing=130°C for 20% of glycerol). The present work focuses on the fusion-bonding step of the process between adjacent filaments. Mechanisms at the root of the thermal bonding of amorphous polymers at T>Tg are governed by melt’s surface tension (Γ, driving force) and viscosity (η, limiting force). In addition, healing phenomenon, assessed by the degree of healing, Dh, increases with time as Dh∝t^1/4. It originates from the diffusion of polymer chains across the interface in accordance with the reptation theory. Dynamic rheological properties of molten extruded filaments of plasticized zeins were determined in a pre-heated oscillatory rheometer at 130°C, with |η ∗ |$γ˙$ =1.6s⁻¹ ranging from 0.6 to 0.8kPa.s. Γ was estimated from the evolution of the fusion-bonding neck growth between two extrudates (polymer sintering model). The 0.1mm.s⁻¹ bonding rate observed at 130°C allowed estimating a melt surface tension of 30-40mN.m⁻¹. Concurrently surface energy measurements were conducted on solid plasticized zein at 20°C using the sessile drop method: By varying liquids deposited on zein-based surface and following Owens and d⁻¹ Wendt’s approach, ySV was found to amount to 39.2±1.6mN.m⁻¹, with the dispersive component $γSVd=4.2±0.4mN.m−1$ and the polar one $γSVp=35.0±1.2mN.m−1$ These values were used to extrapolate a melt surface tension using the typical surface tension dependence dγ/dT≈-0.05mN.m⁻¹.K⁻¹ like for synthetic polymers following the Eötvös’ law. The extrapolated values at 130°C were in agreement with those obtained from fusion-bonding experiments.