Surface Science and Adhesion in Cosmetics. Группа авторов
3.1 Free radical polymerization: photoinitiators absorb light energy that generate free radicals (initiation), the conversion of the product into a cured solid material proceeds as a normal bulk free radical polymerization and continues to propagate (propagation), chain transfer reaction then occurs in which an active center is transferred from a growing oligomer molecule to another molecule (chain transfer) and then termination reaction occurs involving the growing polymer sites reacting together (termination).
3.2.3 Chain Transfer Reaction
Chain transfer reaction then occurs in which an active center is transferred from a growing oligomer molecule to another molecule.
3.2.4 Termination Reaction
Termination reaction involves the growing polymer sites reacting together. Free radical polymerization can also be terminated or retarded by the presence of atmospheric oxygen. Several techniques are used to prevent this termination or retardation of the free radical cure, especially at the interface between the coating and ambient air. We will discuss in a later section methods and techniques to minimize the retardation or termination of the free radical cure by oxygen [2].
3.2.5 Photoinitiation
The most significant method of cure for acrylated oligomers and monomers is through the use of UV light and a PI. The PI acts as the catalyst to free-radically cure the system instantaneously. An example of a PI is hydroxypropiophenone shown in Figure 3.2.
The cleavage reaction of 2-hydroxy-2-methyl-1-phenyl-propan-1-one when it is photolyzed is shown in Figure 3.2. This proposed cleavage goes through a very short duration triplet state and decomposes by α-splitting to give a benzoyl radical and a 2-hydroxy-2-propyl radical which cross-link the system [2, 3].
Figure 3.2 Photoinitiation: UV light hits the 2-hydroxy-2-methyl-1-phenyl-propan-1-one resulting in a cleavage reaction that goes through a short duration triplet state and then decomposes by splitting to give a benzoyl radical and a 2-hydroxy-2-propyl radical which cross-link the system.
3.3 UV Cure Light Sources: Gallium-Doped Low-Wattage Long Wavelength Fluorescent (FL) Bulbs and Light Emitting Diodes (LEDs)
3.3.1 UV Light Spectrum
To activate the PI a UV light source is needed that is tailored to the proper wavelength. To understand this better we need to review the wavelengths of UV light sources that are available commercially.
As one can see in Figure 3.3, UV light sources are available in the UV-A (long wavelength: starts at 365 nm), UV-B (midrange wavelength: starts at 302 nm) and UV-C (short wavelength: starts at 254nm).
Figure 3.3 Electromagnetic spectrum. UV-C (shortwave) starts at 254 nm, UV-B (midrange) starts at 302 nm and UV-A (longwave) starts at 365 nm. UV cure light sources used in the UV nail gel area use the UV-A source.
3.3.2 Matching the PI with the UV Light Source and Pigments Absorption/Transmission
It is important when selecting a PI that every effort is made to match the PI to the wavelength of the UV light source to obtain maximum crosslinking of the coating. In pigmented coatings this becomes even more important due to the absorption of UV light by most pigments. As can be seen in Figure 3.4 a rutile version of titanium dioxide ((TiO2 )) has the following absorption/transmission spectrum.
This absorption/transmission spectrum is critical when using certain PIs. In fact, UV light sources that are in the UV-B and UV-C ranges as shown in Figure 3.3 cannot fully activate the PIs through to the bottom of the applied coating. The formulator then needs to find a light source that operates in the UV-A wavelength since the rutile (TiO2) will block the absorption of the UV light.
Figure 3.5 shows the proper source of UV-A wavelength that operates above 365 nm using gallium to spectrally shift a traditional low-wattage long-wavelength fluorescent (FL) bulb to be above the absorbance of the rutile (TiO2).
It is important to utilize PIs that are not blocked by the rutile (TiO2) as can be seen in Figure 3.6. Typically, PIs (PIs a, b & c shown in Figure 3.6) that are used in unpigmented coatings will not work with pigmented systems since the UV light is not able to penetrate the coating and will result in a cured upper surface and an uncured lower surface. However, to resolve this issue PIs that operate in the 380 nm and above cut-off will activate and fully cure the coating all the way to the substrate as shown in Figure 3.6 (PI-d).
UV light sources and their intensities also play a significant role in curing and through-curing the pigmented coating. As can be seen in Figure 3.7 pendulum hardness values vary greatly depending on the UV light source utilized.
Figure 3.4 Absorption/transmission spectrum of rutile ((TiO2)) that is important to consider when attempting to cure pigmented UV nail gel formulations.
Figure 3.5 Emission spectrum of gallium-doped (Phillips-60 W TL03) low-wattage long-wave fluorescent bulb. Gallium is used to give a spectral shift upwards so it matches the PI.