Resins for Frontal Photopolymerization: Combining Depth-Cure and Tunable Mechanical Properties

Abstract

Photopolymerization has undergone significant development in recent years. It enables fast and easy processing of materials with customized properties and allows precise printing of complex surface geometries. Nevertheless, photopolymerization is mainly applied to cure thin films since the low curing depth limits the fast production of large volumes. Frontal photopolymerization (FPP) is suitable to overcome these limitations so that curing of centimeter-thick (meth)acrylic layers can be accomplished within minutes by light induction only. Prerequisites, however, are the low optical density of the resin and bleaching ability of the photoinitiator. To date, tailored FPP-resins are not commercially available. This study discusses the potential of long-chain polyether dimethacrylates, offering high-temperature resistance and low optical density, as crosslinkers in photobleaching resins and investigates the mechanical properties of photofrontally-cured copolymers. Characteristics ranging from ductile to hard and brittle are observed in tensile tests, demonstrating that deep curing and versatile material properties are achieved with FPP. Analyzed components display uniform polymerization over a depth of four centimeters in Fourier transform infrared spectroscopy and swelling tests.

Effects of Color Modifier on Degree of Monomer Conversion, Biaxial Flexural Strength, Surface Microhardness, and Water Sorption/Solubility of Resin Composites

“…The current study showed that composites with an added color modifier of greater than 1 wt% exhibited DC values lower than 40%, even after being light-cured for 40 s. It is known that the maximum curing depth in light-activated free-radical polymerization is limited by the attenuation of curing light. This could be explained using Beer-Lambert's law [33,34] (Equation ( 6))…”

Frontal‐Photopolymerization of Fully Biobased Epoxy Composites

“…[40,41] The cationic frontal photopolymerization, known as radical-induced cationic frontal polymerization (RICFP) technique, is very promising for curing thick epoxy samples, as already reported in the literature. [42][43][44] The mechanism involves the photo-cleavage of an iodonium salt which generates a superacid that can start the cationic ring-opening epoxy polymerization. The heat release will cleave the thermo-labile initiator generating reactive radicals, which are oxidized in the presence of iodonium salt, forming a reactive carbocation.…”

Fused carbazole–coumarin–ketone dyes: high performance and photobleachable photoinitiators in free radical photopolymerization for deep photocuring under visible LED light irradiation

“…Photobleaching PIs undergo photochemical reactions under irradiation, causing the absorbance in the visible-light region to decrease or even disappear completely; this effect can not only realize the colorlessness of materials but also increases light penetration to obtain thick materials. 41,42 Therefore, the development of visible LED PIs with photobleaching properties is of great importance for photocuring. Considerable work has been done on photobleaching PIs, including type I PIs, such as tetrakis (2,4,6-trimethylbenzoyl) stannane 43 and coumarin oxime esters, 44,45 and type II PIs, such as cinna-moyl, 46 curcumin, 47 and thiacarbocyanines.…”