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IIT and Politecnico di Torino Develop Composite CNT Photopolymer Materials for DLP 3D Printing

Italian researchers from the Istituto Italiano di Tecnologia Center for Sustainable Future Technologies (IIT-CSFT) in Turin and Politecnico di Torino University have developed photopolymer based composite CNT (Carbon NanoTubes) materials that can be 3D printed by DLP stereolithography to make functional parts. The study was published on the Science Direct journal.

The need for 3D printable materials with high performance and smart-functionalities is constantly increasing since they could move the core of polymeric 3D printing business from rapid prototyping to a wider range of more sophisticated applications: from medicine to electronics and further.

The Italian reseachers Annalisa Chiappone and Ignazio Roppolo from the Istituto Italiano di Tecnologia Center for Sustainable Future Technologies (IIT-CSFT) in Torino, with the supervision of Prof. Candido Fabrizio Pirri and in collaboration with Prof. Marco Sangermano from Politecnico di Torino have been studying new, functional 3D printable materials since the beginning of 2015,

Exploiting their background and the knowledge in the photopolymerzation field, their research is focused on the printing technologies using light induced reactions i.e. DLP and SL. With such techniques, by choosing the right ingredients for the photocurable liquid formulations and taking care to maintain a good printability, it is possible to tailor the final properties of the produced objects.

The most common strategy for the modification of the materials properties is the addition of a filler, as the researchers did in a recently published work1 in which carbon nanotubes (CNTs) were added to an acrylic formulation. The addition of CNTs causes dispersion and stability problems, a large increase of the formulation viscosity and a strong reduction of light penetration depth, thus the choice of the most suitable reactive diluent and of the right amount of initiator resulted fundamental to achieve the printability of the material. The printed structures, containing up to 0,5wt% of CNTs, showed enhanced electrical conductivity and mechanical properties.

We are able to print very simple shapes containing 0.5%wt of CNT and more complex geometries containing 0.1 or 0.3 wt%,” said Professor Marco Sangermano of Politecnico di Torino. “The introduction of CNT in the printed polymer induces an increase of conductivity  (from 2,00E-09 S/cm for the polymer to 2,00E-05 S/cm for the 0.5 wt% CNTs composite) but the value is still too low to claim to have conductive pieces. The use of more powerful light sources could allow to print composites containing higher amounts of CNT and thus reach higher conductivity values.”

Aiming to avoid viscosity and stability problems linked to the direct addition of the filler, the IIT researchers envisaged an alternative bottom-up approach in which liquid or soluble precursors of the desired nanoparticles can be added to a light-sensitive mixture in order to obtain the required functionality directly into the printed piece through a post processing step.

In this perspective, silver nitrate was added to a reactive mixture in order induce the in situ generation of silver nanoparticles during the UV-post processing of the printed parts2 or through a thermal post treatment.3 This approach enabled the formation of extremely precise polymer-silver structures, with promising electrical properties and thus it could become a competitive alternative for the production of composites 3D structures in the electronics market.

Following the same strategy silica nanodomains were directly generated in a photocured 3D structured matrix dispersing metal alkoxide precursor in the initial formulation and submitting the printed part to a sol-gel post-process in acidic vapors.4 This procedure allowed to generate a reinforcing phase in a polymeric medium without meeting the above-mentioned drawbacks. The resulting hybrid materials presented improved mechanical properties and could have a huge potential for applications in a variety of advanced technologies.

The studies developed in the IIT/PoliTO laboratories open the possibility of developing functional objects with complex geometries through a simple but very precise process.

Credits:

  1. Gonzalez, G.; Chiappone, A.; Roppolo, I.; Fantino, E.; Bertana, V.; Perrucci, F.; Scaltrito, L.; Pirri, F.; Sangermano, M., Development of 3D printable formulations containing CNT with enhanced electrical properties. Polymer 2017, 109, 246-253.
  2. Fantino, E.; Chiappone, A.; Roppolo, I.; Manfredi, D.; Bongiovanni, R.; Pirri, C. F.; Calignano, F., 3D Printing of Conductive Complex Structures with in Situ Generation of Silver Nanoparticles. Advanced Materials 2016, 28 (19), 3712-3717.
  3. Fantino, E.; Chiappone, A.; Calignano, F.; Fontana, M.; Pirri, F.; Roppolo, I., In situ thermal generation of silver nanoparticles in 3D printed polymeric structures. Materials 2016, 9 (7).
  4. Chiappone, A.; Fantino, E.; Roppolo, I.; Lorusso, M.; Manfredi, D.; Fino, P.; Pirri, C. F.; Calignano, F., 3D Printed PEG-Based Hybrid Nanocomposites Obtained by Sol-Gel Technique. ACS Applied Materials and Interfaces 2016, 8 (8), 5627-5633.

Highlights

• The feasibility of DLP-3D printing acrylic-CNTs nanocomposite objects is demonstrated.

• A suitable formulation is developed adjusting viscosity and dispersion stability.

• The best composition and printing parameters were studied by FT-IR and photorheology.

• 3D shaped nanocomposite objects with a content up to 0.3 wt % of CNTs were printed.

• Electrical conductivity measurements on the printed samples give promising results.

Abstract

This study demonstrates the feasibility of printing 3D composite objects based on acrylic photocurable formulations containing CNTs, by using an unmodified commercial DLP-printer. In the preliminary investigations, the most suitable formulation was developed. Viscosity and dispersion stability were adjusted by the addition of a reactive diluent to the acrylic formulation. FT-IR analyses in real time and photorheology tests allowed finding the best composition and printing parameters.

Printing conditions were adjusted to get 3D structures from formulations with a content up to 0.3 wt % of CNTs. The presence of the filler causes a decrease of the crosslinking density, which could be overcome using higher intensity light sources. Electrical conductivity measurements performed on the printed samples give promising results for the use of the developed formulation for the building of 3D structures with electrical properties.

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