3D Printing New Dyes for Photoinitiators in Three-Component Systems

June 2, 2020 by Bridget O'Neal 3D Printing 3D Printing Materials

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Researchers from France and Australia worked together recently to develop new 3D printing materials, apprising us of the details in the recently published ‘Free Radical Photopolymerization and 3D Printing Using Newly Developed Dyes: Indane-1,3-Dione and 1H-Cyclopentanaphthalene-1,3-Dione Derivatives as Photoinitiators in Three-Component Systems.’

In an effort to encourage free radical polymerization in mild conditions like low light intensity, the researchers developed new dyes for use in multicomponent photoinitiating systems at 405nm LED. Two series of dyes were used as they offered the first study centered around the ‘structure–performance relationship for irradiation.’

As research into photopolymerization continues, better performance from photoinitiators is required. In this study, the authors concentrated on refining the process for greater efficiency, examining critical parameters, properties, energy levels, stability, and more. For developing improved dyes, the research team focused on those with strong electronic delocalization.

Two series of dyes were explored and evaluated regarding absorption wavelengths:

“… hence, the comparison of their efficiency for a same wavelength (405nm) will allow a comparison of their performance.”

Chemical structures of dyes 1–21 examined in this study: (a) indane-1,3-dione and (b) 1H-cyclopentanaphthalene-1,3-dione series.

Twenty-one dyes were assessed, with all of them addressing problems regarding photoinitiator design.

“Notably, the different dyes could be prepared using one of the simplest reactions known in organic chemistry, namely, the Knoevenagel reaction which was reported for the first time in 1896,” explained the researchers. “As for specificity, the concepts of Green chemistry could also be applied. Thus, ethanol could be used as a safe solvent and no expensive transition metal complexes are required to condense the aldehyde onto the activated methylene groups of the different electron acceptors.

“Parallel to this, the different dyes reported in this work could be prepared in high reaction yields (ranging between 74 and 94% yields), without extensive purification processes since all dyes precipitated in ethanol after reaction.”

The researchers used basic filtration for setting the molecules in ‘pure form,’ noting that such an exercise was actually of ‘crucial importance’ because the filtration was cheap, simple, and could be easily used within the industry. The dyes also become attractive for use due to the potential for using green solvents in the fabrication process. Another point of interest is that the dyes were also used for the first time in photoinitiating systems made up of a tertiary amine, ethyl dimethylaminobenzoate (EDB), and iodonium salt.

Chemical structures of the iodonium salt (Iod; Speedcure 938), the amine (ethyl dimethylaminobenzoate (EDB); Speedcure EDB) and the functional benchmark monomer (Ebecryl 40).

“Among all dyes, some of them showed high final monomer conversions and remarkable photoinitiating abilities,” stated the researchers.

Photopolymerization profiles of Ebecryl 40 (conversion rate of C=C bonds vs irradiation time) initiated by iodonium (Iod) and amine (EDB) upon exposure to LED@405nm under air in the presence of dyes 1–21 for curves 1–10 (a) and 11–21 (b) at the same weight ratio: dye:Iod:amine = 0.1%:2%:2% in Ebecryl 40; the curve number indicated the investigated dye. The reference curve without dye is curve 0 for Iod:amine = 2%:2%. The irradiation starts for t = 10 s.

Summary of the final acrylate function conversions (FCs) at 405 nm for Ebecryl 40 using three-component photoinitiating systems: dyes (0.1%, w/w), iodonium salt (Speedcure 938, 2%, w/w) and amine (Speedcure EDB, 2%, w/w).

Dyes 2, 5, 14, 16, 17 and 20 were all noted as ‘the most promising candidates,’ while 5, 14, 16, 17 and 20 were considered to be superior in terms of polymerization rates.

The most appealing properties of the promising dye were found in dye 5, as follows:

Synthesis in high reaction yield, using a Green synthesis and simple workup
Suitable solubility for monomers
High molar extinction coefficient
Low oxidation potential
Unique bleaching properties

UV-visible absorption spectra of dyes 9, 19 (1.30 × 10−5 M; 1.11 × 10−5 M, respectively.) (left) in the presence of iodonium salt (Speedcure 938, 1.46 × 10−4 M) upon exposure to LED@405nm under air in the solvent of acetonitrile: (a) dye 9, (c) dye 19 and (right) in the presence of amine (Speedcure EDB, 4.07×10−4 M) upon exposure to LED@405nm under air in the solvent of acetonitrile: (c) dye 9, (d) dye 19.

Dye 17 was also considered to be a good photoinitiator.

Contour plots of HOMOs and LUMOs for dye 5 and dye 17 (isovalue = 0.02).Contour plots of HOMOs and LUMOs for dye 5 and dye 17 (isovalue = 0.02).

“The electron transfer between dyes, iodonium salt and amine in the FRP process are confirmed by both free energy change calculations and ESR experiments. Interestingly, very good photobleaching properties and fluorescence quenching of dye 5 were detected,” concluded the researchers. “It was also found that both good light absorption properties and excellent photochemical reactivity in the excited state processes (redox reactions) are required for the dyes to reach efficient photoinitiating systems.

“Finally, some 3D patterns were written using these three-component photoinitiating systems. To conclude, our research contributes to improve the knowledge of photopolymerization carried out at 405 nm with LEDs by the development of new PISs comprising push–pull chromophores. By improving the electron accepting ability of the naphthalene-based electron acceptor, PISs operating in the near infrared region could be developed and the synthesis of such dyes is currently under progress.”

A variety of different photoinitiators are being used today in 3D printing, to include plant-based, water-soluble, and more. What do you think of this news? Let us know your thoughts! Join the discussion of this and other 3D printing topics at 3DPrintBoard.com.

[Source / Images: ‘Free Radical Photopolymerization and 3D Printing Using Newly Developed Dyes: Indane-1,3-Dione and 1H-Cyclopentanaphthalene-1,3-Dione Derivatives as Photoinitiators in Three-Component Systems’]