A facile manufacturing method to enable the in situ foam 3D printing of thermoplastic materials is reported. An expandable feedstock filament was first made by incorporation of thermally expandable microspheres (TEMs) in the filament during the extrusion process. The material formulation and extrusion process were designed such that TEM expansion was suppressed during filament fabrication. Polylactic acid (PLA) was used as a model material, and filaments containing 2.0 wt % triethyl citrate and 0.0–5.0 wt % TEM were fabricated. Expandable filaments were then fed into a material extrusion additive manufacturing process to enable the in situ foaming of microcellular structures during layer deposition. The mesostructure, cellular morphology, thermal behavior, and mechanical properties of the printed foams were investigated. Repeatable foam prints with highly uniform cellular structures were successfully achieved. The part density was reduced with an increase in the TEM content, with a maximum reduction of 50% at 5.0 wt % TEM content. It is also remarkable that the interbead gaps of mesostructure vanished due to the local polymer expansion during in situ foaming. The incorporation of TEM and plasticizer only slightly lowered the critical temperatures of PLA, that is, glass-transition, melting, and decomposition temperatures. Moreover, with the introduction of foaming, the specific tensile strength and modulus decreased, whereas the ductility and toughness increased severalfold. The results unveil the feasibility of a novel additive manufacturing technology that offers numerous opportunities toward the manufacturing of specially designed structures including functionally graded foams for a variety of applications.

This work reports the fabrication and characterization of entirely biobased composites made with polylactic acid (PLA) and deep eutectic solvent (DES)-extracted lignin. White fir sawdust and corn stover were used as feedstocks to extract sawdust lignin (SDL) and corn stover lignin (CSL). Commercial alkali lignin (CAL) was also used as a baseline. PLA/lignin composites were fabricated using a twin-screw extrusion process followed by compression molding. Characterizations of the composites were conducted using scanning electron microscopy (SEM), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and tensile testing. Overall, the results revealed that PLA/DES lignin biocomposites significantly outperform their PLA/CAL counterparts. SEM results indicated the absence of microcracks in PLA/DES lignin morphologies and better lignin dispersion and smaller lignin agglomerates compared with PLA/CAL composites. The initial thermal degradation temperature of PLA slightly dropped by 8−27 °C with the addition of 5−15 wt % DES lignin, compared with a significant reduction of 89−124 °C when incorporating CAL. DSC analysis showed a maximum drop of about 5 and 15 °C in the melt temperature of PLA when DES lignin and CAL lignin, respectively, were added. Moreover, SDL increased the PLA's crystallinity several folds. The tensile strength and elongation at break of PLA/DES lignin composites were significantly greater than those of PLA/CAL composites, up to a maximum of 1 order of magnitude. The better performance of PLA/DES lignin biocomposites was associated with the high purity, ultrafine particle size, low heterogeneity, and low molecular weight of DES lignin. The results suggest DES lignin as a potential feedstock for entirely biobased high-performance materials.

Fused deposition modeling (FDM) is highly commercialized Rapid Prototyping (RP) technology for its ability to build complex parts with low cost in a short period of time. The process parameters in the FDM play a vital role in the mechanical properties of the polymeric parts. Most of the research studies show that the variable parameters such as orientation, layer thickness, raster angle, raster width, and air gap are some of the key parameters that affect the mechanical properties of FDM-processed polymeric parts. However, no reports have been made regarding the influence of nozzle diameter with raster width on the tensile properties of FDM fabricated polymeric parts. This work was devoted to achieving improved and isotropic mechanical properties in polycarbonate (PC) and PC/carbon nanotube (PC/CNT) nanocomposites by investigating the effect of printing parameters in FDM process. The nozzle diameter to raster width ratio, α was found to significantly affect the mechanical properties. The printing direction dependency in tensile properties were studied with the ratio α < 1 and α≥ 1 at three different raster angles of 0°, 45°/−45° and 90°. For α < 1, Ultimate tensile strength and modulus of elasticity were higher for 0°, compared to 45°/−45° and 90° raster angles. However, for α ≥ 1, the ultimate tensile strength and the modulus of elasticity showed little dependency to print direction. This certainly determines the decrease in anisotropy at higher values of α. Mesostructure characterization with microscopy and image analysis were used to further explain the printing behavior and the resultant properties of the printed samples.

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Layered multi-materials of dissimilar polymers and their nanocomposites offer new opportunities as smart materials and structures. A critical aspect of such structures is the quality of interlayer adhesion between dissimilar polymer matrices. This work reports the development of asymmetric double cantilever beam (ADCB) specimens of dissimilar polymers and its use in the analysis and understanding of their interlayer adhesion in 3D-printed rigid/soft interfaces. Acrylonitrile butadiene styrene (ABS), polycarbonate (PC), polylactic acid (PLA) were chosen as the rigid polymers and combined with thermoplastic polyurethane (TPU) as the soft component. 3D-printed ADCB specimens were loaded under opening mode, until fracture, to obtain the load-displacement data and the fracture surfaces were analyzed using optical microscopy. ABS/TPU/ABS and PC/TPU/PC material combinations resulted in a more stable crack growth with a high peak load indicating a relatively good interfacial adhesion. The high nozzle temperatures of ABS and PC and their amorphous nature contributed to a good layer-to-layer fusion during 3D printing. However, PLA/TPU/PLA specimens exhibited an unstable crack growth behavior with a pure adhesive failure mode and a significantly lower peak load. This poor interfacial bond strength was correlated to the relatively low nozzle temperature of PLA and its semi-crystalline structure. The maximum loads in ABS/TPU/ABS and PC/TPU/PC specimens were found to be ∼2.5 times greater than that of PLA/TPU/PLA ones. The method provides a valuable tool in quantifying interlayer adhesion quality in printed dissimilar polymers and their functional nanocomposites.