A Simple and Efficient Model to Determine the Photonic Parameters of a Photopolymerizable Resin Usable in 3D Printing

Digital Light 3D Printing of Double Thermoplastics with Customizable Mechanical Properties and Versatile Reprocessability

Digital light processing (DLP) is a 3D printing technology offering high resolution and speed. Printable materials are commonly based on multifunctional monomers, resulting in the formation of thermosets that usually cannot be reprocessed or recycled. Some efforts are made in DLP 3D printing of thermoplastic materials. However, these materials exhibit limited and poor mechanical properties. Here, a new strategy is presented for DLP 3D printing of thermoplastics based on a sequential construction of two linear polymers with contrasting (stiff and flexible) mechanical properties. The inks consist of two vinyl monomers, which lead to the stiff linear polymer, and α-lipoic acid, which forms the flexible linear polymer via thermal ring-opening polymerization in a second step. By varying the ratio of stiff and flexible linear polymers, the mechanical properties can be tuned with Young's modulus ranging from 1.1 GPa to 0.7 MPa, while the strain at break increased from 4% to 574%. Furthermore, these printed thermoplastics allow for a variety of reprocessability pathways including self-healing, solvent casting, reprinting, and closed-loop recycling of the flexible polymer, contributing to the development of a sustainable materials economy. Last, the potential of the new material in applications ranging from soft robotics to electronics is demonstrated.

Results of an interlaboratory study on the working curve in vat photopolymerization

The working curve informs resin properties and print parameters for stereolithography, digital light processing, and other photopolymer additive manufacturing (PAM) technologies. First demonstrated in 1992, the working curve measurement of cure depth vs radiant exposure of light is now a foundational measurement in the field of PAM. Despite its widespread use in industry and academia, there is no formal method or procedure for performing the working curve measurement, raising questions about the utility of reported working curve parameters. Here, an interlaboratory study (ILS) is described in which 24 individual laboratories performed a working curve measurement on an aliquot from a single batch of PAM resin. The ILS reveals that there is enormous scatter in the working curve data and the key fit parameters derived from it. The measured depth of light penetration Dp varied by as much as 7x between participants, while the critical radiant exposure for gelation Ec varied by as much as 70x. This significant scatter is attributed to a lack of common procedure, variation in light engines, epistemic uncertainties from the Jacobs equation, and the use of measurement tools with insufficient precision. The ILS findings highlight an urgent need for procedural standardization and better hardware characterization in this rapidly growing field.

Triplet Upconversion under Ambient Conditions Enables Digital Light Processing 3D Printing

The rapid photochemical conversion of materials from liquid to solid (i.e., curing) has enabled the fabrication of modern plastics used in microelectronics, dentistry, and medicine. However, industrialized photocurables remain restricted to unimolecular bond homolysis reactions (Type I photoinitiations) that are driven by high-energy UV light. This narrow mechanistic scope both challenges the production of high-resolution objects and restricts the materials that can be produced using emergent manufacturing technologies (e.g., 3D printing). Herein we develop a photosystem based on triplet-triplet annihilation upconversion (TTA-UC) that efficiently drives a Type I photocuring process using green light at low power density (<10 mW/cm2) and in the presence of ambient oxygen. This system also exhibits a superlinear dependence of its cure depth on the light exposure intensity, which enhances spatial resolution. This enables for the first-time integration of TTA-UC in an inexpensive, rapid, and high-resolution manufacturing process, digital light processing (DLP) 3D printing. Moreover, relative to traditional Type I and Type II (photoredox) strategies, the present TTA-UC photoinitiation method results in improved cure depth confinement and resin shelf stability. This report provides a user-friendly avenue to utilize TTA-UC in ambient photochemical processes and paves the way toward fabrication of next-generation plastics with improved geometric precision and functionality.