Use of Hoy’s solubility parameters to predict water sorption/solubility of experimental primers and adhesives Yoshihiro Nishitani 1 , Masahiro Yoshiyama 1 , Keiichi Hosaka 2 , Junji Tagami 2 , Adam Donnelly 3 , Marcela Carrilho 4 , Franklin R. Tay 3 , David H. Pashley 3 1 Department of Operative Dentistry, Okayama University, Graduate School of Medicine, Dentistry and Pharmaceutical Sciences, Okayama, Japan; 2 Department of Cariology and Operative Dentistry, Graduate School, Tokyo Medical and Dental University, Tokyo, Japan; 3 Department of Oral Biology & Maxillofacial Pathology, School of Dentistry, Medical College of Georgia, Augusta, GA, USA; 4 Department of Restorative Dentistry, Piracicaba School of Dentistry, University of Campinas, Brazil Self-etching primers contain 20% water and 20% eth- anol or acetone, together with 60% acidic monomers and dimethacrylates (1). Many self-etching primers also contain various concentrations of 2-hydroxyethyl meth- acrylate (HEMA). After the self-etching period, the sol- vents are evaporated, thereby raising the concentration of the monomers (2, 3). The combined concentrations of acidic monomers and HEMA make these resin blends very hydrophilic (3, 4). During self-etching, the solvated primers etch through the smear layer and into the underlying dentin by removing apatite crystallites from around and within mineralized collagen fibrils (5, 6). After the recommended self-etching time, the solvents are evaporated, leaving the newly formed interfibrillar spaces of the hybrid layer completely filled with primer comomers that are more viscous than the original sol- vated primer. There is much uncertainty regarding how much (if any) mixing occurs between viscous primer comomers (7) and the subsequently placed neat adhesive during the typical 15–30 s bonding time. The use of different fluorescent dyes in primers vs. adhesives to follow potential interdiffusion is equivocal because the dyes are not covalently linked to the monomers (7). If there is little mixing of primer and adhesive comono- mers, then, after polymerization, primer polymers may make significant contributions to the total bond strength of self-etching primer adhesives. If there is little mixing of primer comonomers with adhesive constituents, it is uncertain whether the strength of the resin–dentin bond is contributed by the strength of the polymerized primer or the polymerized adhesive, or both. As all-in-one adhesives and self-etching primers are intrinsically hydrophilic owing to the presence of acidic, highly polar functional groups substituted on methacry- lates, they will rapidly absorb water, which results in polymer swelling, plasticizing (8, 9), and weakening of the polymer network (8, 10). Water absorption into polymers is assumed to be directly related to the hydrophilicity of the polymers. To test this idea, one needs to rank the hydrophilicity of polymers. The use of Hoy’s solubility parameters to rank the degree of hydrophilicity of copolymers is increasing (11–15). Hoy’s solubility parameters provide estimates of the relative contribution of dispersion forces (d d ), polar forces (d p ), and hydrogen bonding forces (d h ) to the total cohesive forces (d t ) that hold the polymers together. They are based on the molar attraction values of their individual molecular compo- nents and can be calculated according to the method of Hoy (16) using commerically available software (Com- puter Chemistry Consultancy, http://www.compchem- cons.com). Polymers lacking polar groups or constituents incapable of forming hydrogen bonds are relatively hydrophobic and have high d d values relative to their d p and d h values. Polymers containing polar groups that cannot form ionic attractions or hydrogen bonds (i.e. halogen-substituted polymers) are moderately hydrophi- lic and have relatively high d p values. Polymers containing hydroxyl groups, amines, carboxylic acids or phosphates Nishitani Y, Yoshiyama M, Hosaka K, Tagami J, Donnelly A, Carrilho M, Tay FR, Pashley DH. Use of Hoy’s solubility parameters to predict water sorption/solubility of experimental primers and adhesives. Eur J Oral Sci 2007; 115: 81–86. Ó 2007 The Authors. Journal compilation Ó 2007 Eur J Oral Sci Self-etching primers and adhesives contain very hydrophilic acidic monomers that result in high water sorption/solubilities of their polymers. However, the chemical composition of these products varies widely. The purpose of this work was to vary the chemical composition of experimental self-etching primers and adhesives to determine if the water sorption/solubility of the polymers were affected in a predictable manner. The Hoy’s solubility parameters of these mixtures were calculated to permit ranking of the degree of hydrophilicity of the polymers. Water sorption/solubility was measured according to ISO 4049. The results showed highly significant (R 2 ¼ 0.86, P < 0.001) correlations between water sorption and Hoy’s solubility parameter for polar forces (d p ) of the polymers. Similar correlations were obtained between polymer solubility and d p . When these results were compared with previously published results obtained with more hydrophobic resins, excellent correlations were obtained, indicating that Hoy’s d p values may be used to predict the water sorption behavior of methylmeth- acrylate polymers. Dr David H. Pashley, Department of Oral Biology & Maxillofacial Pathology, School of Dentistry, Medical College of Georgia, 1120 15th Street, CL-2112, Augusta, GA, 30912– 1129, USA Telefax: +1–706–7216252 E-mail: dpashley@mail.mcg.edu Key words: HEMA; hydrophilic monomers; solubility parameters; water sorption Accepted for publication November 2006 Eur J Oral Sci 2007; 115: 81–86 Printed in Singapore. All rights reserved Ó 2007 The Authors. Journal compilation Ó 2007 Eur J Oral Sci European Journal of Oral Sciences