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Now that we have reviewed technical aspects of the UV cure nail gel coatings technology, we must now look into what the human nail plate presents as a substrate to be coated.

Table 3.4 Properties of the bio-based high solids UV nail gels.

	Acetone double rubs	König hardness (Oscillations)	Pencil hardness
Method of curing	UV-Mercury	UV-LED	UV-Mercury	UV-LED	UV-Mercury	UV-LED
Base coat	170	180	126	110	H	2H
Polish	>200	>200	120	114	F	F
Top Coat	>200	>200	136	120	3H	5H

Table 3.5 Properties of the bio-based UV-PUD formulations.

	Acetone double rubs	König hardness (Oscillations)	Pencil hardness
Method of Curing	UV-Mercury	UV-LED	UV-Mercury	UV-LED	UV-Mercury	UV-LED
Polish	15	12	86	90	HB	HB
Polish including 10 wt.% TMPTA	45	40	87	94	F	F
Polish including 10 wt.% Bomar BR 952	15	20	76	76	F	F
Non-pigmented formulation including 10 wt.% TMPTA	40	38	85	90	H	H

1 a. Before one can coat the human nail plate, one must understand the conditions in which the human nail exists. Researchers determined the surface free energy of the nail plates in vivo. They found that the surface free energy of healthy human fingernail was 34 mJ/m2. Contact angle measurements were accomplished utilizing water, formamide, diiodomethane and glycerol. There are many ways to determine surface free energy of solids using contact angle measurements [18, 19] but here we have used the Lifshitz-van der Waals/acid-base (LW-AB) approach, also known as the van Oss, Chaudhury and Good approach. The in vivo method was performed on 8 females, 9 males who were 23 to 51 years old.

2 b. As can be seen in Table 3.6 the surface free energy values for in vivo subjects nail plates are determined using the water-formamide-diiodomethane (WFD) and water-glycerol-diiodomethane (WGD) liquids combinations. These values will be important to understand later in this chapter when we describe the application of UV cure nail gels based on acrylated oligomers and acrylated monomer systems as well as UV curable polyurethane dispersions [20].

The mechanical behavior of the nail is another important factor in understanding how to coat the human nail. Grigale-Sorocina et al [21] evaluated the mechanical properties of the human nail. The formulations in the test conformed to the requirements of the EU Cosmetic Regulations. The formulations included the following: tri-functional urethane acrylate oligomer and a hexa-functional urethane acrylate oligomer. The viscosity was achieved by the addition of the following acrylate monomers for the proper ‘use viscosity’ at various levels: tertisobutyl cyclohexyl acrylate (TBCHA), ethylene glycol dimethacrylate (EGDMA) tetrahydrofurfuryl acrylate (THFA) and hydroxypropyl methacrylate (HPMA). The PI used was TPO that has been shown in Figure 3.15. The UV nail gel coating should match the natural nail plate mechanical properties so that there is no loss of adhesion. Mechanical stress due to shrinkage in the applied UV nail gel coating is the normal cause for this adhesion loss.

As can be seen in Table 3.7 Grigale-Sorocina et al. [21] evaluated different monomers and looked at the gel fraction, surface gloss and micro-hardness of the UV cured coatings. They varied the monomer concentration by 30 and 40%. Several of the formulations met the nail coating application viscosity of 3,000 to 4,500 cPa.s. Degree of conversion as shown as gel content was the highest with the formulation using monomer EGDMA with gel fraction (GF) of 96.4% to 98.5%.

Table 3.6 Surface free energy of untreated, hydrated and abraded nail plates.

Liquid combination used	State of the nail	Total surface energy (mJ/m2) γ	Surface energy components (mJ/m2)
Lifshitzvan der Waals γLW	Acid-base (polar) γAB	Acid i.e. electron acceptor γ+	Basic i.e. electron donor γ
WFD	untreated	35.5 ± 4.7	34.0 ± 3.9	1.6 ± 4.0	0.4 ± 0.9	11.0 ± 7.0
	hydrated	34.2 ± 3.6	33.6 ± 3.8	0.7 ± 3.3	0.5 ± 1.0	11.8 ± 8.7
	abraded	39.2 ± 3.9	37.0 ± 4.2	2.2 ± 3.9	0.7 ± 1.1	9.5 ± 6.5
WGD	untreated	32.6 ± 6.2	34.1 ± 3.9	-1.3 ± 5.6	-0.1 ± 1.3	13.7 ± 7.6
	hydrated	31.5 ± 6.0	33.4 ± 3.8	-1.9 ± 4.9	-0.2 ± 0.6	15.0 ± 8.2
	abraded	32.9 ± 4.8	37.0 ± 4.0	-4.0 ± 5.4	-0.1 ± 1.4	15.4 ± 9.2

Table 3.7 Monomers effect on the gel fraction (GF), surface gloss (G) and micro-hardness (HV) of the UV cured cross-linked coatings. Monomers 1-TBCHA, 7-HPMA and 8-HPMA fall into the ɛ_β (elongation at break) range of 20-50% so that they exhibit reduced brittleness. AGF=GF₀ (is after 72 hours of storage) –GF (is immediately after cure).

Monomer	Conc.	Gel fraction	Surface gloss	Microhardness
GF	GF0	AGF*/GF0 x 100	G	G0	AG**/G0 x 100	HV	HV0	AHV***/HV0 x 100
		%	%	%	%	GU	GU	%	MPa	MPa	%
1	TBCHA	30	97,4	98,0	0,61	46	68	32	104	144	38
2	TBCHA	40	94,0	94,1	0,11	51	72	29	69	83	20
3	EGDMA	30	98,5	99,3	0,81	47	59	20	204	281	38
4	EGDMA	40	96,4	98,9	2,53	47	57	18	220	314	43
5	THFA	30	96,0	96,3	0,31	59	73	19	25	47	88
6	THFA	40	95,7	96,7	1,03	54	61	11	28	32	14
7	HPMA	30	97,0	97,6	0,61	50	63	21	99	151	53
8	HPMA	40	95,0	99,9	4,90	43	58	26	65	134	106

* AGF = (GF₀ – GF)

** AG = (G₀ – G)

*** AHV = (HV_Q – HV)

**** Characteristics in bold are in required range of close to that

Figure 3.17 Elastic modulus (E)and (%) elongation at break (ɛ_B) values for the coatings with 30% and 40% of monomers measured immediately after cure (0) and after 3 days of storage.

They also found that the human nail at 55% relative humidity has a Young’s modulus of 2.32 GPa. Systems containing TBCHA that they tested showed the most stable E values with the post-cure having no effect on the post-cured coatings. So, for the natural nail the coating should have at least an ɛ_B (elongation at break) value of 0.6%. As can be seen in Figure 3.17 TBCHA and HPMA fall within the range of the recommended 20-50% for a coating performance that was not too brittle yet not too soft for a coating to perform on the nail plate [21].