For restoration of tooth structures (enamel, dentin, and cementum) damaged, for example, by dental caries, restorative filling materials such as filling composite resins and filling compomers, or crown restoration materials such as metal alloys, porcelains, and resin materials, are typically used. In general, however, restorative filling materials and crown restoration materials (both of these materials may collectively be referred to as “dental restorative materials” in the present description) themselves have no adhesive properties to tooth structures. This is why bonding between tooth structures and dental restorative materials conventionally employs various adhesive systems involving the use of adhesives. An example of conventionally-employed adhesive systems is an adhesive system of the so-called acid etching-type (total etching-type), in which the surface of a tooth structure is subjected to an etching treatment using an acid etching material such as an aqueous phosphoric acid solution, and then a bonding material, which is an adhesive, is applied to the tooth structure so as to bond the tooth structure and a dental restorative material.
Adhesive systems of the so-called self-etching type, which involve no use of any acid etching material, have also been known. Self-etching adhesive systems that had been predominantly used in the past are two-step adhesive systems in which a self-etching primer containing an acidic monomer, a hydrophilic monomer, and water is applied to the surface of a tooth structure and then a bonding material containing a crosslinkable monomer and a polymerization initiator is applied directly to the primer without rinsing with water. In recent years, however, one-step adhesive systems using a one-part dental adhesive (one-part bonding material) having functions of both a self-etching primer and a bonding material have been widely used.
In some applications, the self-etching adhesive system employs a technique called selective etching, in which only the enamel is subjected to phosphoric acid etching as a pretreatment to improve adhesiveness for enamel, including when restoring portions that are highly dependent on enamel bonding, such as an occlusal surface of a molar, and a fractured central incisor, or when high adhesiveness is needed for the enamel. In selective etching, however, treating enamel also treats the dentin at the same time at the enamel-dentin boundaries. Phosphoric acid etching, which is usually highly demineralizing, is known to expose collagen fibers through demineralization of hydroxyapatite when applied to dentin, which contains organic material such as collagen. The collagen in the exposed dentin contracts during rinsing and drying, and prevents smooth penetration of the polymerizable monomer component contained in the dental adhesive. It has accordingly been difficult with a self-etching dental adhesive to provide high adhesiveness for dentin subjected to phosphoric acid etching, and there is a demand for a self-etching material having improved adhesiveness to dentin subjected to phosphoric acid etching.
In general, a one-part bonding material contains monomer components such as an acidic monomer, a hydrophilic monomer, and a crosslinkable monomer, and (meth)acrylate compounds are usually used as such monomer components.
One-part bonding materials are required to have high adhesiveness to tooth structures (in particular, enamel and dentin), and further improvement of these properties is required. In response to these requirements, use of a (meth)acrylamide compound, which is a monomer component less susceptible to hydrolysis than a (meth)acrylate compound, has been reported to provide a dental composition with improved storage stability and high adhesiveness to dentin and enamel (see, for example, Patent Literatures 1 to 3).
Patent Literature 1 proposes a one-part dental adhesive composition containing: an acidic monomer; a bifunctional (meth)acrylamide compound represented by the general formula (5) having two (meth)acrylamide groups both of which are secondary amide groups, or a bifunctional (meth)acrylamide compound represented by the general formula (6) having two (meth)acrylamide groups both of which are tertiary amide groups; a solvent (for example, water); and a curing agent (for example, polymerization initiator) (hereinafter, in the present description, a (meth)acrylamide compound having two (meth)acrylamide groups both of which are secondary amide groups, and a (meth)acrylamide compound having two (meth)acrylamide groups both of which are tertiary amide groups may be referred to as symmetric (meth)acrylamide compounds, for the sake of convenience).
wherein Ra, Rb, Rc, and Rd are each independently a hydrogen atom or a methyl group, Re and Rf are each independently a group such as an alkyl group and an aryl group (Re and Rf do not represent a hydrogen atom), and Ya and Yb are each independently a divalent organic group optionally having an oxygen atom and/or a nitrogen atom.
However, most of the bifunctional (meth)acrylamide compounds represented by the general formula (5) have the following disadvantages. These compounds are solid in nature and have poor compatibility with other monomers. Therefore, in a dental composition containing this solid compound, deposition or phase separation of the monomers occurs, or phase separation of the components occurs when air-blowing is performed for use, resulting in low storage stability and poor adhesiveness to tooth structures. Some of the bifunctional (meth)acrylamide compounds represented by the general formula (5) are oily in nature and have good compatibility with other monomers, but a dental composition containing this oily compound has the disadvantage of low adhesiveness to tooth structures. Furthermore, the bifunctional (meth)acrylamide compounds represented by the general formula (6) are also oily in nature and have good compatibility with other monomers, but a dental composition containing this oily compound has the disadvantage of low adhesiveness to tooth structures.
Patent Literature 2 proposes a dental adhesive composition having a pH of at least 3 and suited for total etching. The dental adhesive composition proposed in this publication contains: a bifunctional (meth)acrylamide compound represented by the general formula (5) above having two (meth)acrylamide groups both of which are secondary amide groups, a bifunctional (meth)acrylamide compound represented by the general formula (6) above having two (meth)acrylamide groups both of which are tertiary amide groups, or a monofunctional (meth)acrylamide compound represented by general formula (7); a water-soluble polymerizable carboxylic acid; and a water-soluble organic solvent.

In general formula (7), Rg, Rh, and Ri each independently represent a hydrogen atom, or a C1 to C8 alkyl group.
However, a disadvantage of this dental adhesive composition is that, because it is a dental adhesive composition suited for total etching, the dental adhesive composition has poor adhesiveness when used for bonding without phosphoric acid etching, that is, when used as a self-etching adhesive composition. The dental adhesive composition also has the same disadvantages as Patent Literature 1 when it contains a compound represented by general formula (5). Specifically, deposition or phase separation of the monomers occurs, or phase separation of the components occurs when air-blowing is performed for use, resulting in low storage stability and poor adhesiveness to tooth structures. The dental adhesive composition also suffers from poor adhesiveness to tooth structures when it contains a compound represented by general formula (6).
Patent Literature 3 proposes a dental composition containing: an acidic monomer; an asymmetric bifunctional (meth)acrylamide compound represented by the general formula (8) having two (meth)acrylamide groups, one of which is a secondary amide group and the other of which is a tertiary amide group (hereinafter, in the present description, a (meth)acrylamide compound having two (meth)acrylamide groups, one of which is a secondary amide group and the other of which is a tertiary amide group may be referred to as an asymmetric (meth)acrylamide compound, for the sake of convenience).

In general formula (8), Ro and Rq are each independently a hydrogen atom or a methyl group, Rp represents a linear or branched C1 to C4 aliphatic group, and Yc is a divalent organic group optionally having an oxygen atom and/or a nitrogen atom.
The composition disclosed in Patent Literature 3 has good storage stability because its components are highly compatible with one another and thus difficult to separate from one another. This composition further has good initial bond strength to both dentin and enamel. This composition, however, has been found to have low bond durability, and poor adhesiveness to tooth structures subjected to phosphoric acid. Subsequent studies by the present inventors have revealed that this composition still has room for improvement.
Patent Literature 4 proposes an adhesive component containing a carboxamide group-containing (meth)acrylic acid ester and suitable for treatment of collagen-containing materials such as bones and teeth.
The composition disclosed in Patent Literature 4 is proposed as an alternative treatment agent to acid etching agents but the etching effect of this composition on tooth structures is not strong enough, and thus has the disadvantage of low adhesiveness to both enamel and dentin.