Chitin Nanocomposites for Fused Filament Fabrication: Flexible Materials with Enhanced Interlayer Adhesion.

In this work, we present a series of nanocomposites for Fused filament fabrication (FFF) based on polycaprolactone (PCL) and chitin nanocrystals (ChNCs). The ChNCs were synthesized by acid hydrolysis using HCl or lactic acid (LA). The approach using LA, an organic acid, makes the ChNCs synthesis more sustainable and modifies their surface with lactate groups, increasing their compatibility with the PCL matrix. The ChNCs characterization by X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, and transmission electron microscopy revealed that both ChNCs presented similar morphologies and crystallinity, while differential scanning calorimetry and thermogravimetric analysis proved that they can bear temperatures up to 210 °C without degrading, which allows their processing in the manufacturing of PCL composites by twin-screw extrusion. Therefore, PCL composites in the form of filaments containing 0.5-1.0 wt % ChNCs were produced and used as feedstock in FFF, and standard tensile and flexural specimens were printed at different temperatures, up to 170 °C, to assess the influence of the ChNCs in the mechanical properties of the material. The tensile testing results showed that the presence of ChNCs enhances the strength and ductility of the PCL matrix, increasing the elongation at break around 20-50%. Moreover, the vertically printed flexural specimens showed a very different bending behavior, such that the pure PCL specimens presented a brittle fracture at 7% strain, while the ChNCs composites were able to bend over themselves. Hence, this work proves that the presence of ChNCs aims to improve the interlayer adhesion of the objects manufactured by FFF due to their good adhesive properties, which is currently a concern for the scientific community and the industrial sector.

Additive Manufacturing of Thermoplastic Polyurethane-Cork Composites for Material Extrusion Technologies.

Among the material extrusion technologies of additive manufacturing, fused granular fabrication is playing a bigger role in the industry. The increase in the size of printers demands extrusion systems with higher deposition rates that facilitate printing larger parts in shorter times with a need for cost reduction. This cost reduction in fused granular fabrication systems is due to the utilisation of pellets as the material source for the prints, such as pellets that are the most common way of distributing polymeric materials in industry and do not need the usual previous transformation into filaments. Most of the polymers in the industry can be found in the shape of pellets, so the opportunities for developing new materials beside the traditional filaments found in the market are expanding. In this research, a novel composite material has been developed based on the blending of commercial thermoplastic polyurethane (TPU) and cork particles obtained from industrial waste at different concentrations. These materials have been processed at a laboratory scale, and their mechanical, thermal and rheological properties have been studied. Despite a 53.52% reduction in the maximum stress on the x-axis, an 81.82% decrease in the values obtained with specimens oriented on the z-axis and a shortage in the deformation values, the results reveal a remarkable weight reduction leading to 21.31% when compared to the TPU of the blends,. These results may open a path to further explore these blends and find suitable applications in industry as proposed.

Influence of the Carbon Fiber Length Distribution in Polymer Matrix Composites for Large Format Additive Manufacturing via Fused Granular Fabrication.

Many studies assess the suitability of fiber-reinforced polymer composites in additive manufacturing. However, the influence of the fiber length distribution on the mechanical and functional properties of printed parts using these technologies has not been addressed so far. Hence, in this work we compare different composites based on Acrylonitrile Styrene Acrylate (ASA) and carbon fiber (CF) suitable for large format additive manufacturing (LFAM) technologies based on fused granular fabrication (FGF). We study in detail the influence of the CF size on the processing and final properties of these materials. Better reinforcements were achieved with longer CF, reaching Young's modulus and tensile strength values of 7500 MPa and 75 MPa, respectively, for printed specimens. However, the longer CF also worsened the interlayer adhesion of ASA to a greater extent. The composites also exhibited electrical properties characteristic of electrostatic dissipative (ESD) materials (105-1010 Ω/sq) and low coefficients of thermal expansion below 15 µm/m·°C. These properties are governed by the CF length distribution, so this variable may be used to tune these values. These composites are promising candidates for the design of elements with enhanced mechanical and functional properties for ESD protection elements or molds, so the products can be manufactured on demand.

Basalt Fiber Composites with Reduced Thermal Expansion for Additive Manufacturing.

Fused filament fabrication (FFF) is gaining attention as an efficient way to create parts and replacements on demand using thermoplastics. This technology requires the development of new materials with a reliable printability that satisfies the requirement of final parts. In this context, a series of composites based on acrylonitrile styrene acrylate (ASA) reinforced with basalt fiber (BF) are reported in this work. First, several surface modification treatments are applied onto the BF to increase their compatibility with the ASA matrix. Then, once the best treatment is identified, the mechanical properties, coefficient of thermal expansion (CTE) and warping distortion of the different specimens designed and prepared by FFF are studied. It was found that the silanized BF is appropriate for an adequate printing, obtaining composites with higher stiffness, tensile strength, low CTE and a significant reduction in part distortion. These composites are of potential interest in the design and manufacturing of final products by FFF, as they show much lower CTE values than pure ASA, which is essential to successfully fabricate large objects using this technique.

High-Resolution 3D Fabrication of Glass Fiber-Reinforced Polymer Nanocomposite (FRPN) Objects by Two-Photon Direct Laser Writing.

This paper reports on the nanofabrication of a fiber-reinforced polymer nanocomposite (FRPN) by two-photon direct laser writing (TP-DLW) using silica nanowires (SiO2 NWs) as nanofillers, since they feature a refractive index very close to that of the photoresist used as a polymeric matrix. This allows for the best resolution offered by the TP-DLW technique, even with high loads of SiO2 NWs, up to 70 wt %. The FRPN presented an increase of approximately 4 times in Young's modulus (8.23 GPa) and nanohardness (120 MPa) when compared to those of the bare photoresist, indicating how the proposed technique is well-suited for applications with higher structural requirements. Moreover, three different printing configurations can be implemented thanks to the use of silicon chips, on which the SiO2 NWs are grown, as fabrication substrates. First, they can be effectively used as an adhesive layer when the laser beam is focused at the interface with the silicon substrate. Second, they can be used as a sacrificial layer, when the laser beam is focused in a plane inside the SiO2 NW layer. Third, only the outer shell of the object is printed so that the SiO2 NW tangle acts as the internal skeleton for the structure being fabricated in the so-called shell and scaffold printing strategy.

Itaconic-Acid-Based Sustainable Poly(ester amide) Resin for Stereolithography.

Material science is recognized as a frontrunner in achieving a sustainable future, owing to its primary reliance upon petroleum-based chemical raw materials. Several efforts are made to implement common renewable feedstocks as an alternative to common fossil resources. For this purpose, additive manufacturing (AM) represents promising and effective know-how for the replacement of high energy- and resource-demanding processes with more environmentally friendly practices. This work presents a novel biobased ink for stereolithography, which has been formulated by mixing a photocurable poly(ester amide) (PEA) obtained from renewable resources with citrate and itaconate cross-linkers and appropriate photopolymerization initiators, terminators, and dyes. The mechanical features and the relative biocompatibility of 3D-printed objects have been carefully studied to evaluate the possible resin implementation in the field of the textile fashion industry.

Straightforward functionalization of breath figures: Simultaneous orthogonal host-guest and pH-responsive interfaces.

Herein, we report the design and preparation of multireversible smart porous surfaces combining two different abilities. On the one hand, either neutral or negatively charged surfaces can be formed by formation/disruption of host-guest complexes. On the other hand, these surfaces have the capability of alternating negatively and positively charge upon complexation of a polycation. Moreover, these two functionalization steps were demonstrated to be reversible so that the initial surface can be recovered and employed again. For this purpose, first, a copolymer was prepared by polymerization of two different monomers, i.e. styrene (S) and a styrene modified with cyclodextrin (SCD) by click chemistry. Blends of this copolymer and polystyrene were employed to fabricate porous surfaces with controlled pore sizes and chemical distribution by the breath figures technique. More precisely, the cyclodextrin (CD) moieties, specifically located inside the holes of the surface, interact reversibly with adamantane end-terminated poly(acrylic acid) chain (Ada-PAA85). The latter served to establish electrostatic interaction with a polycation (poly-L-lysine, PLL), leading to positively charged surface. These interactions, both host-guest and electrostatic, can be inverted obtaining again the original surface, proving the full reversibility of the system.

Honeycomb patterned surfaces functionalized with polypeptide sequences for recognition and selective bacterial adhesion.

We report on the preparation of functional polymer surfaces with controlled topography by using the breath figures approach. The resulting surfaces prepared from a mixture of a PS-b-PAA diblock copolymer and a homopolymer (PS) exhibit pores that are mainly composed of diblock copolymer whereas the rest of the surface is formed by homopolymer. The formation of a hexagonal assembly of pores was achieved by controlling several parameters during the casting process including relative humidity, composition of the blend and polymer concentration. A selective modification of the pore inner part by using appropriate polypeptide sequences permitted the use of these surfaces as scaffolds for pattern and display of active biomolecules, as ordered templates for specific recognition processes and finally for the micropatterning of bacterial cells.