Jul 9, 2018 | By Thomas
The Technical University of Vienna (TU Wien) in Austria has developed a new approach for high-resolution 3D printing of tailored, homogenously crosslinked, tough, methacrylate-based photopolymers.
Credit: Wiley
Many coatings, including tooth fillings, varnishes and printing inks, are cured with light, but homogenous, tailored, polymer networks cannot be produced. Even if you manage it, the materials tend to be brittle, which limits the ability to use photopolymers for applications such as 3D printing, biomedicine, and microelectronics.
In the journal Angewandte Chemie, researchers published a paper titled “Vinyl Sulfonate Esters: Efficient Chain Transfer Agents for the 3D Printing of Tough Photopolymers without Retardation,” which explains a method by which methacrylate-based, homogenously crosslinked, tailored, tough polymers can be made – even at high resolution for 3D printing.
Abstract
The formation of networks through light‐initiated radical polymerization allows little freedom for tailored network design. The resulting inhomogeneous network architectures and brittle material behavior of such glassy‐type networks limit the commercial application of photopolymers in 3D printing, biomedicine, and microelectronics. An ester‐activated vinyl sulfonate ester (EVS) is presented for the rapid formation of tailored methacrylate‐based networks. The chain transfer step induced by EVS reduces the kinetic chain length of the photopolymer, thus shifting the gel point to higher conversion, which results in reduced shrinkage stress and higher overall conversion. The resulting, more homogeneous network is responsible for the high toughness of the material. The unique property of EVS to promote nearly retardation‐free polymerization can be attributed to the fact that after the transfer step no polymerizable double bond is formed, as is usually seen in classical chain transfer agents. Laser flash photolysis, theoretical calculations, and photoreactor studies were used to elucidate the fast chain transfer reaction and exceptional regulating ability of EVS. Final photopolymer networks exhibit improved mechanical performance making EVS an outstanding candidate for the 3D printing of tough photopolymers.
Light-curing is usually a radical chain polymerization, where light energy splits an initiator into radicals that attack the monomer. Then, a new radical is formed which then becomes starting point of a growing polymer network by attacking more monomers and binding to them.
Newer methods to better control radical photopolymerization and the material properties of the products tend to slow the curing process, which is not ideal for 3D printing. A short irradiation phase is critical for high spatial resolution and economical production times.
A new approach for the tailored production of methacrylate-based photopolymers without inhibiting the curing process has been developed by a team led by Robert Liska at the Technical University of Vienna (Austria). Their approach uses an ester-activated vinyl sulfonate ester (EVS) as a chain transfer agent, because it can easily splits off one portion of itself to activate the process.
If a growing polymer network attacks EVS instead of another monomer, an intermediate will form, and quickly split to form a terminated polymer chain in the network and a highly reactive radical (tosyl radical), which will in turn start a new chain reaction. The more EVS is added, the shorter the average length of chain the polymer network will have. Because shorter polymer chains remain mobile longer, the danger of shrinkage cracks during curing is significantly reduced.
In contrast to conventional chain transfer agents, the polymerization is not inhibited in this new process, as there aren't stable intermediates or reversible reaction steps involved. The splitting off of the tosyl radical is favored.
To test it, the researchers set up a scaffold-like sample structure using a methacrylate copolymer. Individual layers with a thickness of 50 µm were spatially well resolved in the structure. The material is very homogenous, solid but elastic and impact resistant with high tensile strength. These properties can be adjusted by changing how much EVS is added. Without EVS, the material was too brittle to be 3D printed. This new approach prepares tough photopolymers for uses in biomedicine applications, such as shape-memory polymers for tissue growth and dental fillings.
Posted in 3D Printing Technology
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