Conventionally, it has been very useful to use (meth)acrylate having a fluorinated alkyl group independently or to use a curable composition comprising said acrylate as an optical material for optical lenses or the like, in order to achieve both optical characteristics such as reducing a refractive index and mechanical characteristics (strength) caused by a crosslinking reaction. Accordingly, many kinds of said (meth)acrylates, manufacturing methods thereof, and compositions comprising said (meth)acrylate have been proposed. The (meth)acrylate having a fluorinated alkyl group is also useful as a surface-active agent, a surface-modifying agent and a raw material for the agents, which can achieve surface characteristics such as water repellent and/or oil repellent property, smoothing property, chemical resistance and stain guard property due to a high electronegativity, that is, low polarizability peculiar to a fluorine atom in the compound.
For example, (meth)acrylate which is obtained by a condensation reaction between trimethylol propane, perfluoroalkyl carboxylic acid, and (meth)acrylic acid has been proposed as a polyfunctional (meth)acrylate having a fluorinated alkyl group (for example, please refer to Japanese Unexamined Patent Application, First Publication, 9-157326, pages 2 to 4. Hereinafter, the document is cited as patent document 1). However, this (meth)acrylate of patent document 1 is a compound wherein the perfluoroalkyl group is directly combined with a carbon atom of a carbonyl group in an ester bond. Accordingly, the electron density of said carbon atom tends to be decreased and hydrolysis affects the (meth)acrylate easily, and thus, an obtained cured product has problems in that performances such as the optical characteristics, mechanical characteristics and the like may not be maintained over a long period of time. Moreover, accumulative property of perfluoroalkyl carboxylic acid (CnF2n+1COOH), which is a raw material or product produced after the hydrolysis, in a living body and in the environment is recognized as a problem particularly in U.S. and therefore perfluoroalkyl carboxylic acid is a compound which should be avoided from the viewpoint of safety.
Furthermore, as polyfunctional urethane acrylate having a fluorinated alkyl group, for example, a polycondensed reaction product of isophorone diisocyanate, mono- or di-alcohol having a fluorinated alkyl group, and polyfunctional acrylate monomer having a hydroxyl group has been proposed (for example, please refer to Japanese Unexamined Patent Application, First Publication, 2002-145936, pages 3 to 5. Hereinafter, the document is cited as patent document 2). Since the compound proposed in patent document 2 is a compound wherein a perfluoroalkyl group is not combined directly with a carbon atom in a carbonyl group, deterioration of a cured product due to hydrolysis is not caused easily. However, the compound has a structure in which an acrylate portion (crosslinking point) bonds with a fluorinated alkyl group via isophorone diisocyanate. Accordingly, a molecular weight of the compound per molecule is large, and the functional group density of a (meth)acryloyl group is low and the mechanical characteristics of a cured product become insufficient.
Furthermore, as a method for manufacturing a polyfunctional urethane acrylate having a fluorinated alkyl group, methods are proposed in which said acrylate is produced by using diepoxide having a fluorinated alkyl group or diol having a fluorinated alkyl group as a raw material (for example, please refer to Japanese Unexamined Patent Application, First Publication, 9-301925, pages 3 to 5, and Japanese Unexamined Patent Application, First Publication, 2001-072646, pages 3 to 4.) Hereinafter, the documents are cited as patent documents 3 and 4). However, since there are acryloyl groups (crosslinking point) at both terminal ends of a fluorinated alkylene chain via the residues of a hydroxyl group or an epoxy group in both documents, a fluorinated alkylene chain is incorporated in a network structure of a cured product. Accordingly, the surface properties originated from a fluorine atom are not sufficient, since it is difficult for a fluorine atom to exist on the surface of the cured product effectively and —CF3, which can contribute to decrease surface tension, does not exist on the surface.
Moreover, for example, a reactant which is obtained by reacting fluorine-containing acrylate, which has a hydroxyl group, and isocyanate having an acryloyl group has been proposed as fluorinated alkyl group containing (meth)acrylate which has two (meth)acryloyl groups (for example, please refer to Japanese Unexamined Patent Application, First Publication, 2000-044650, pages 3 to 7. Hereinafter, this document is cited as patent document 5).
The compound proposed by patent document 5 is excellent in surface characteristics as compared with the compounds proposed by patent documents 3 and 4, since a fluorinated alkyl group of patent document 5 is pendant from the three dimensional structure generated by the curing reaction. However, since the crosslinking points are bonded to each other by the chain structure wherein a flexible alkylene chain is a main structure, therefore, an obtained three dimensional network structure itself is also flexible, and mechanical characteristics of the cured product obtained become insufficient.
In view of the aforementioned circumstances, problems to be solved by the present invention are to provide a fluorine-containing photocurable composition including (meth)acrylate having a fluorinated alkyl group, wherein the composition can be used suitably for an optical material, coating material and the like, and a cured product obtained from the composition having excellent surface characteristics, optical characteristics and mechanical characteristics, and also excellent long-term stability of such characteristics due to excellent hydrolysis resistance thereof.