Influence of hydroxyethyl acrylamide addition to dental adhesive resin
Introduction
Dental adhesive systems’ bond strength to tooth substrate is related to a wide range of variables, such as the chemical structure [1], polymerization process [2] and interactions between functional monomers [3]. Contemporary adhesives contain hydrophobic and hydrophilic functional monomers – such as acrylates, methacrylates and vinyl esters [3], [4] – to achieve polymers with reliable properties [2], [5].
Methacrylate monomers, such as 2-hydroxyethyl methacrylate (HEMA), exhibit low reactivity [6], high water sorption [7] and high solubility [1]. In these systems, hydrolysis of the methacrylate ester group can occur, either when these monomers are combined with water from the formulation of commercial adhesive systems or at the adhesive–dentin interface. As a result, polymer properties [8], [9] and monomer shelf life [3] are compromised. Low molecular weight and high hydrophilicity of HEMA can also lead to an adhesive layer more prone to water sorption, leading to decreased properties in the formed polymer [9]. Consequently, a decrease in physicochemical properties is observed, affecting the long-term behavior of the adhesive interface [6]. Furthermore, leaching of degraded polymer chains through dentinal tubules can lead to toxicity effects on pulp cells [10].
In order to improve the hydrolysis resistance of adhesive systems, alternatives have been proposed such as HEMA-free adhesive systems [4], [11], [12] and methacrylates monomers substitution by acrylamides and methacrylamides [3]. Acrylamides and methacrylamides have a more stable amide group (RNH–CO– or –CO–NR1R2) instead of an ester group (–COOR) [13]. Moreover, acrylamides are similar to the amino acids that compose collagen fibrils, with carboxylate groups [14] which could facilitate hydrogen bond between collagen and amide groups (N–H) of acrylamides [15]. Thus, in theory, acrylamide components could also provide hydrolytic stability to the hybrid layer without compromising biocompatibility [16].
Despite the great variety of studies of adhesive systems development regarding different characteristics such as radiopacity [17], anticariogenic potential [18], [19], and increased bond strength to dental tissues [1], [20], few studies evaluate the development of more hydrolytically stable polymers [12], [21], [22] and the replacement of HEMA by methacrylamide monomers in adhesive systems are barely studied [23]. Therefore, the purpose of this study is to determine the resultant physicochemical properties – polymerization behavior (degree of conversion and maximum rate or polymerization), residual monomers elution, hardness and the resultant softening, glass transition temperature (Tg), storage modulus, reticulation degree (ρ), and flexural strength – of experimental adhesive resins containing hydroxyethyl acrylamide (HEAA).
Section snippets
Formulation
Three experimental adhesive resins were produced in order to analyze the influence of HEAA incorporation at adhesive resins: GHEAA33% (33.3% hydroxyethyl acrylamide—HEAA + 66.6% Bisphenol A glycerolate dimethacrylate-BisGMA), GHEAA50% (50% HEAA + 50% BisGMA) and GHEAA-FREE (33.3% 2-hydroxyethyl methacrylate—HEMA + 66.6% BisGMA). The photoinitiator:co-initiator system was composed by 0.8 mol% camphorquinone (CQ) and 0.4 mol% ethyl 4-dimethylaminobenzoate (EDAB). The base homopolymers, BisGMA, HEAA,
Results
Degree of conversion values and polymerization kinetic parameters for homopolymers and experimental groups are shown in Fig. 1, Fig. 2. Among the homopolymers the highest value (79.51%) was associated with the HEAA homopolymer (p < 0.001). In contrast, the HEMA* without EDAB did not polymerize. All experimental adhesive resins exhibited a high degree of conversion value with significant differences among groups (GHEAA50% = 64.07% > GHEAA33% = 55.82% > GHEAA-FREE = 49.02%—p = 0.008). Addition of HEAA
Discussion
Many factors may affect the polymerization kinetics of adhesive resins, including photoinitiator concentration [26], viscosity [27], exposure to light [28], the monomeric composition and the ratio [3]. Physicochemical properties were evaluated in this study and the addition of hydroxyethyl acrylamide (HEAA) influenced the mechanical properties, polymerization behavior and the type of residual eluted monomers of our experimental adhesive resins.
BisGMA presents two aromatic rings and hydroxyl
Conclusion
The incorporation of HEAA affected the polymerization behavior, the residual monomers elution and the physicochemical properties of experimental resins. HEAA groups showed higher BisGMA elution than GHEAA-FREE and decreased flexural strength. However, the addition of HEAA increased the materials reactivity and, consequently, improved the maximum rate of polymerization, degree of conversion and the storage modulus of experimental adhesive resin.
Acknowledgements
The authors gratefully acknowledge CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior) for the scholarship (S.B.R.); Julio C.P. Vaghetti of the LAMAT (Laboratório Multiusuário de Análise Térmica); Vinicius Pistor for the DMA test and Vagner M. Costa of the LAMEF (Laboratório de Metalurgia Física).
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2021, Dental MaterialsCitation Excerpt :Despite inherently being also hydrophilic, acrylamide monomers are thought to be more hydrolytically stable than HEMA. Hydroxyethyl acrylamide (HEAA) has recently been suggested as HEMA alternative [14,15], but solely its rheological properties were investigated, while HEAA was not yet added to an adhesive formulation. Diethyl acrylamide (DEAA) is another candidate acrylamide monomer to replace HEMA in dental adhesives.