eventually spilled over into the developing UV nail gel market with certain problems and issues. With refined newer formulations, new UV light sources and techniques were developed. This chapter will build the foundation of this unique chemistry and other ways of using the technology for other than human nail beautification.
3.2 UV Cure Chemistry
The use of Ultra Violet (UV)-cure nail polishes had its roots in the industrial coatings market where UV-cure technologies were first introduced for the protection of wood substrates in the office furniture and kitchen cabinet industry to reduce the emissions of formaldehyde as a hazardous air pollutant (HAP). These technologies were based on the uses of oligomers (resins) and monomers in combination with PI to free-radically cure the coating system.
The use of UV radiation provides very fast and controlled generation of highly reactive chemical species which initiate polymerization. Most of the advantages of UV/Electron Beam curing result from a high degree of control over the initiation process.
3.2.1 Initiation Reaction
Photoinitiators (PIs) are used to absorb the light energy and generate the free radicals as shown in Figure 3.1, the initiation step. These initiators are added in the range of 1 to 12 % of the total formulation.
The initiation process occurs only during exposure of the material to UV energy. Since the existing free radicals have very short life spans, all of the propagation reactions also stop when the material is no longer under the UV energy source. Consequently, the final product’s properties are achieved immediately after removal from the UV energy source. Exposure to the UV can range from seconds to minutes.
3.2.2 Propagation Reaction
After initiation, the conversion of the product into a cured solid material proceeds as a normal bulk free radical polymerization and continues to propagate as shown in Figure 3.1, the propagation reaction.
Figure 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
