SynerPI® is the amide of acetic acid and dibenzylamine. It can be used with most LED based photinitiators and with both Norrish Type I and Norrish Type II photoinitiators. Materials that improve photoinitiation efficiency are known as synergists. They are known as synergists since they are not actually photoinitiators themselves but improve the kinetics of photoinitiated polymerization. Traditional synergists are amines and exhibit synergy by coupling with the photoinitiator and lowering the activation energy of the photoinitiation reaction and/or by acting as an antioxidant.
The effects of SynerPI® on the Norrish Type Iphotoinitiator benzyl dimethyl ketal (BDK) compared to diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO) were determined in a pigmented UV-curable gel nail polish formulation. The formulation and results are shown in the tables below. The results show that there is an optimal ratio of SynerPI® to BDK which is 1.25:1 (Sample 9). Increasing the ratio to 2:1 actually results in a significant reduction in cure depth (Sample 11 vs 9). Likewise, reducing the ratio to 1:1 also results in a significant reduction in cure depth (Sample 6 vs 9). It is important to experimentally determine the optimal ratio of SynerPI® to photoinitiator by determining the effect of varying the ratio of SynerPI® to a given photoinitiator on cure depth. The results also show that when using SynerPI® the amount of BDK could be reduced by 50% (Sample 4 vs 9) while increasing the cure depth to be as good, or better, than that with TPO (Sample 1 vs 9). The same is true for the Norrish Type II photoinitiator benzophenone in the presence of an amine synergist and only 1-1.5% SynerPI®. Without SynerPI® curing was incomplete. Alternatively, by removing the amine synergist surface curing was incomplete consistent with SynerPI® not being an antioxidant and operating by a mechanism different than conventional amine synergists. One possibility is that SynerPI® reduces the probability of the initially formed excited state returning to ground state before subsequent radical formation and polymerization initiation can occur.
Procedure for preparing UV-curable nail polish films
Films were prepared by mixing powder photoinitiator and synergist, by hand, into a pigmented UV-curable nail polish formulation. Once mixed thoroughly, a 10-mil static drawdown bar was used to apply a wet film to a 6" x 3" Q-PANEL and placed under a Nail Flair UV lamp for 90 seconds. The films were then wiped with acetone and allowed to dry for 18 hours. They were then removed from the panel and tested for dry film thickness using a digital caliper micrometer.
TPO is by far the most commonly used photoinitiator because of its efficiency. There are few photoinitiators that can come close to matching TPO in its ability to cure thick layers and avoid the development of color – postcure yellowing, and even fewer would function at the long and safe ultraviolet wavelengths (ca 390 nm) of LED lamps.
A major drawback with TPO is that it has been reclassified as a reproductive toxicant by the EU. So, there is a need for alternatives to TPO. Therefore, the combination of BDK and SynerPI® is a safer alternative to TPO without compromising efficiency.
Contact Ethox Chemicals to learn more about how SynerPI® shows improvements in rate of cure, depth of cure, yellowing, and safety in gel nail polish.