1

Introduction

All contemporary dental adhesives contain hydrophilic methacrylates and hydrophobic dimethacrylates to bond to wet vital dentin and to restorative materials, respectively. Solvents and polymerization initiator systems are added to complete the adhesive formulations with the attempt to increase wettability and create a single-phase solution. Three-step etch-and-rinse and two-step self-etch primer adhesives cover their primers with a separate layer of unsolvated hydrophobic resin. In contrast, in simplified adhesives (two-step etch-and-rinse and one-step self-etch formulations, respectively), the manufacturers have combined the hydrophilic primers with the hydrophobic dimethacrylates and solvent to increase their hydrophilicity . The lack of an unsolvated, relatively hydrophobic layer in simplified adhesive interfaces allows water to flow from the underlying dentin substrate to the top of the adhesive layer , thus confirming that simplified adhesives behave as permeable membranes after polymerization .

Besides the hydrophilicity of the resin blends, the permeability of the adhesive interface has also been correlated to solvent content and degree of cure (DC) . That is, higher solvent percentages blended within simplified adhesives produced lower DC and higher permeability . During the curing reaction, hydrophilic and hydrophobic domains containing polymer chains should sufficiently cross-link to make the resin strong enough to serve as bonding agent. Previous studies revealed that the polymerization reaction of these amphiphilic resin blends is frequently incomplete due to several factors affecting the DC: monomer structure and functionality , comonomer composition , adhesive hydrophilicity , solvent type and concentration and presence of water are some of the most investigated factors affecting polymerization.

Nano-phase separation phenomena have been frequently revealed, mainly within simplified adhesives, due to the simultaneous presence of hydrophilic and hydrophobic domains . Ye et al. using tapping mode atomic force microscopy imaging, confirmed a nano-scale phase separation for all cross-linked polymethacrylates related to BisGMA/HEMA and water concentration. The most hydrophilic domains (poly(HEMA)-rich segments linked to water) separate from poly(BisGMA)-rich hydrophobic areas creating heterogeneous resin blends. As camphorquinone (CQ), the most common photo-initiator, is hydrophobic, hydrophilic monomers may suboptimally polymerize due to inappropriate polymerization initiation, and this may explain why simplified adhesives exhibited lower degree of cure if compared to unsimplified counterparts.

To improve the DC of simplified adhesives, the use of additional hydrophilic photo-initiators, in addition to standard CQ activation, has been proposed . Indeed, the addition of a water-compatible component to photo-initiators reduced the detrimental effect of phase separation and improved the performances of the adhesives promoting polymerization of both hydrophilic and hydrophobic domains .

However little is known on the use of CQ-alternative co-initiators (such as TPO: ethyl 4-dimethylaminobenzoate and diphenyl(2,4,6-trimethylbenzoyl)-phosphine oxide) on the polymerization of hydrophilic dental adhesives. Thus the aim of the present study was to investigate polymerization of three experimental adhesive blends with respect to their hydrophilicity, solvent and water content. The hypotheses tested were that the DC of the adhesive blend is insensitive to (1) resin hydrophilicity, (2) solvent/water content and (3) incorporation of a CQ-alternative hydrophilic photo-initiator.

2

Materials and methods

Three experimental resin blends with increasing hydrophilicity were investigated. R2 (70% BisGMA, 28.75% TEGDMA) is similar to a nonsolvated hydrophobic resins used in the formulation of a two-step self-etch adhesive; R3 (70% BisGMA, 28.75% HEMA) is representative of a two-step etch-and-rinse adhesive, while R4 (40% BisGMA, 30% TCDM, 28.75% TEGDMA) is the most hydrophilic blend tested, and is similar to the formulation of one-step self-etch adhesives. All compositions expressed in terms of total weight percentage are listed in Table 1 .

Table 1

Composition (wt%) of experimental resins 2–4 and Hoy’s solubility parameters ( δ ) for the comonomers.

Resin #	Neat resin composition		IS 1				IS 2				IS 3
			δ d	δ p	δ h	δ t	δ d	δ p	δ h	δ t	δ d	δ p	δ h	δ t
R2	70% BisGMA 28.75% TEGDMA	Neat resin	15.86	12.38	6.53	21.15	15.89	12.42	6.47	21.19	15.87	12.40	6.51	21.17
		10% ETH	15.41	12.21	8.40	21.38	15.44	12.25	8.35	21.4	15.42	12.23	8.38	21.39
		20% ETH	15.00	12.06	10.12	21.74	15.02	12.09	10.07	21.75	15.00	12.07	10.10	21.75
		10% ETH + 10% H 2 O	15.02	13.33	12.03	23.41	15.04	13.36	11.99	23.42	15.02	13.35	12.02	23.42
R3	70% BisGMA 28.75% HEMA	Neat resin	15.58	13.02	8.68	22.08	15.62	13.07	8.62	22.11	15.59	13.04	8.67	22.10
		10% ETH	15.17	12.76	10.24	22.32	15.20	12.81	10.19	22.34	15.18	12.78	10.24	22.33
		20% ETH	14.79	12.53	11.68	22.64	14.82	12.57	11.64	22.65	14.8	12.54	11.68	22.64
		10% ETH + 10% H 2 O	14.81	13.81	13.64	24.41	14.84	13.85	13.59	24.43	14.81	13.83	13.63	24.42
R4	40% BisGMA 30% TCDM 28.75% TEGDMA	Neat resin	16.41	12.81	6.99	21.96	16.45	12.86	6.92	21.99	16.42	12.83	6.97	21.98
		10% ETH	15.87	12.58	8.84	22.10	15.91	12.62	8.78	22.12	15.88	12.59	8.82	22.10
		20% ETH	15.38	12.36	10.52	22.36	15.41	12.40	10.47	22.38	15.39	12.38	10.51	22.37
		10% ETH + 10% H 2 O	15.41	13.67	12.49	24.09	15.44	13.71	12.44	24.11	15.42	13.69	12.47	24.10

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