Bis(trimethylsilyl)amino modified Type Ⅱ photoinitiators can alleviate oxygen inhibition of near-UV LED photopolymerization due to moisture induced decomposition of initiators

Abstract

Near UV-LED photopolymerization of acrylates can find many crucial applications in industrial fields, such as coatings, adhesives and 3D printing of hydrogels. However, developing the photoinitiators (PIs) well matching with the commercial LED light sources and reducing oxygen inhibition effect remain challenges. Herein, we have synthesized three kinds of type Ⅱ PIs containing bis(trimethylsilyl)amino groups; Molar extinction coefficients of these PIs are all at the 103 ∼ 104 (L·mol−1·cm−1) order of magnitude at the 365 ∼ 395 nm, comparable to the commercial PIs including Michler’s ketone (MK) and 2-isopropylthioxanthone (ITX); However, their photoinitiation capacity is much better than the MK and the ITX. In addition, these PIs have better resistance to the oxygen inhibition effect than the MK. Some researchers think the PIs containing rich Si/F elements can self-migrate towards upper layers of curing systems to reduce the oxygen inhibition effect, however, these obtained PIs with high Si fraction did not have such effect. Based on the obtained experimental results, we have proposed a new mechanism explaining their good ability to reduce the oxygen inhibition: The moisture induced decomposition of the initiators can play a key role in alleviating the oxygen inhibition; The generated Si-containing fragments after PIs’ hydrolysis can self-float towards the top surfaces to hinder the oxygen penetration, not the self-floating effect of the PIs by themselves.