Optical component

This application is based on application No. H11-362664 filed in Japan on Dec. 21, 1999, the entire contents of which are hereby incorporated by reference.

1. Field of the Invention

The present invention relates to an optical component produced by forming an anti-reflection film on a surface of a molding of a norbornene resin, and to a method for producing such an optical component.

2. Description of the Prior Art

In recent years, synthetic resins have come to be widely used as materials for optical components such as lenses, mirrors, and prisms. This is because synthetic resins are lighter and cheaper than glass materials, and can easily be molded into complex shapes. Conventionally preferred kinds of such synthetic resins are polymethyl methacrylate (PMMA) and polycarbonate (PC). However, both PMMA and PC are insufficiently heat resistant and in addition moisture-absorbent. Therefore, in practical use, moldings of those resins pose the serious problem of easily changing their shapes and refractive indices.

This has encouraged development of satisfactorily heat-resistant and non-moisture-absorbent synthetic resins, of which one outcome is norbornene resins having no polar group. Norbornene resins have proved to be excellent in transparency, heat resistance, and non-moisture-absorbency, and are thus effective as optical materials.

However, forming a conventional anti-reflection film on a molding of a norbornene resin results in rather low adherence between them, which inconveniently facilitates exfoliation of the anti-reflection film from the molding. The adherence declines particularly sharply at higher ambient temperatures and humidities and with greater film thicknesses. This has prompted attempts to improve the adherence of the anti-reflection film by adding a polar group to the norbornene resin. However, no satisfactory results are known to have been obtained to this date.

An object of the present invention is to provide an optical component having an anti-reflection film adhering to a surface of a molding of a norbornene resin so strongly as not to exfoliate therefrom, and to provide a method for producing such an optical component.

To achieve the above object, according to one aspect of the present invention, an optical component is provided with: a molding formed out of a material containing a norbornene resin having a polar group; and an anti-reflection film formed on a surface of the molding. This anti-reflection film is composed of, from the molding side to the air side: an equivalent film of which the overall refractive index as calculated according to formula (1) below is 1.52 to 1.8 and that includes, from the molding side to the air side, an intermediate layer measuring 10 to 50 nm in mechanical film thickness and containing Al 2 O 3 , a first high-refractive-index layer, and a first low-refractive-index layer containing SiO 2 as a main ingredient, N = n g + ∑ i = 1 3     [ 4  ( n i - n g )  n i  n d ] ( 1 )

where n g represents a refractive index of the molding and n i and d i represent a refractive index and a mechanical film thickness, respectively, of an i-th layer of the equivalent film as counted from the molding side; a second high-refractive-index layer measuring 0.4λ to 0.6λ (where λ is 480 to 550 nm) in optical film thickness and containing TiO 2 as a main ingredient; and a second low-refractive-index layer measuring 0.2λ to 0.3λ (where λ is 480 to 550 nm) in optical film thickness and containing SiO 2 as a main ingredient.

According to another aspect of the present invention, an optical component is provided with: a molding formed out of a material containing a norbornene resin having a polar group; and an anti-reflection film formed on a surface of the molding. This anti-reflection film is composed of, from the molding side to the air side: a first high-refractive-index layer containing TiO 2 as a main ingredient and formed with the molding heated to 70 to 130° C.; a first low-refractive-index layer containing SiO 2 as a main ingredient; a second high-refractive-index layer measuring 0.4λ to 0.6λ (where λ is 480 to 550 nm) in optical film thickness and containing TiO 2 as a main ingredient; and a second low-refractive-index layer measuring 0.2λ to 0.3λ (where λ is 480 to 550 nm) in optical film thickness and containing SiO 2 as a main ingredient.

In these optical components according to the present invention, an anti-reflection film composed of specifically defined layers is formed on a surface of a molding of a norbornene resin having a polar group. This ensures that the anti-reflection film adheres to the surface of the molding so strongly as not to exfoliate easily therefrom.

According to another aspect of the present invention, a method for producing an optical component includes: a molding step of forming a molding having a desired shape out of a material containing a norbornene resin having a polar group; a heating step of heating the molding to 70 to 130° C.; and a vapor-deposition step of vapor-depositing an anti-reflection film as a first layer on a surface of the molding.

By this method according to the present invention, first a molding of a norbornene resin is heated to 70 to 130° C., and then a first layer is vapor-deposited on the norbornene-resin molding. This ensures that an anti-reflection film adheres to the surface of the molding so strongly as not to exfoliate therefrom.

This and other objects and features of the present invention will become clear from the following description, taken in conjunction with the preferred embodiments with reference to the accompanying drawings in which:

FIG. 1 is a sectional view of an optical component according to the invention;

FIG. 2 is a sectional view of a conventional optical component;

FIG. 3 is a diagram showing the spectral reflectance characteristic of the anti-reflection film of Example 1;

FIG. 4 is a diagram showing the spectral reflectance characteristic of the anti-reflection film of Example 2;

FIG. 5 is a diagram showing the spectral reflectance characteristic of the anti-reflection film of Example 3;

FIG. 6 is a diagram showing the spectral reflectance characteristic of the anti-reflection film of Example 4; and

FIG. 7 is a diagram showing the spectral reflectance characteristic of the anti-reflection film of Comparative Example 1.

The inventors of the present invention investigated why an anti-reflection film formed on a surface of a molding of a norbornene resin having a polar group tends to exfoliate easily therefrom, and learned the fact described below. When an anti-reflection film is formed on a surface of a molding of a resin such as PMMA or PC conventionally used as a material for optical components, as shown in FIG. 2, the anti-reflection film is generally composed of, from the molding surface side, a medium-refractive-index layer 5 , a high-refractive-index layer 3 ′, and a low-refractive-index layer 4 ′ that are laid over one another. Here, to enhance the adherence between the molding and the anti-reflection film, the medium-refractive-index layer 5 is generally made of a silicon oxide SiO x (2>x>1). However, with a molding made of a norbornene resin having a polar group, if a silicon oxide SiO x is used as the material of the medium-refractive-index layer 5 that makes contact with the molding, this layer does not afford sufficient adhesion.

An optical component according to the present invention is devised on the basis of this newly found fact, and is characterized principally by having instead of a conventional medium-refractive-index layer an equivalent film composed of three specifically defined layers. More specifically, in the present invention, an equivalent film is composed of an intermediate layer measuring 10 to 50 nm in mechanical film thickness and containing Al 2 O 3 , a first high-refractive-index layer, and a first low-refractive-index layer containing SiO 2 as a main ingredient, and has an overall refractive index N in the range from 1.52 to 1.8 as calculated according to formula (1) below. N = n g + ∑ i = 1 3     [ 4  ( n i - n g )  n i  d i ] ( 1 )

where

n g represents the refractive index of the molding; and

n i and d i represent the refractive index and the mechanical film thickness, respectively, of the i-th layer of the equivalent film as counted from the molding side.

FIG. 1 shows a sectional view of an optical component according to the present invention. On a surface of a molding 1 , an equivalent film 2 is formed that is composed of an intermediate layer 21 containing Al 2 O 3 , a first high-refractive-index layer 22 , and a first low-refractive-index layer 23 containing SiO 2 as a main ingredient, and, on top thereof, a second high-refractive-index layer 3 and a second low-refractive-index layer 4 are formed in the order named. First, the equivalent film will be described.

By forming first the intermediate layer containing Al 2 O 3 , which excels in adherence to a norbornene resin having a polar group, on the surface of the molding, it is possible to enhance the adherence between the anti-reflection film and the molding. Here, the mechanical film thickness of the intermediate layer needs to be in the range from 10 to 50 nm. With a mechanical film thickness smaller than 10 nm, sufficient adhesion cannot be obtained, with the result that crack development is very likely in the anti-reflection film. On the other hand, with a mechanical film thickness greater than 50 nm, the stress in the intermediate layer is too strong to be ignored, with the result that crack development is very likely in the anti-reflection film when it is left in high-temperature, high-humidity conditions. Preferably, the mechanical film thickness of the intermediate layer is in the range from 20 to 30 nm.

As long as sufficient adherence to the norbornene resin is obtained, the intermediate layer may contain any proportion of Al 2 O 3 . For maximum adherence, the intermediate layer may even be made solely of Al 2 O 3 .

Next, the first high-refractive-index layer and the first low-refractive-index layer will be described. The refractive index and the film thickness of each of these two refractive-index layers are determined appropriately in such a way that the value N calculated according to formula (1) noted above is in the range from 1.52 to 1.8. That is, the equivalent film is so formed as to act just like a conventional medium-refractive-index layer.

The reason that the main ingredient of the first low-refractive-index layer is limited to SiO 2 here is that the main ingredient of the second high-refractive-index layer formed on top thereof is TiO 2 , which exhibits inward membrane stress, and therefore selecting SiO 2 , which exhibits outward membrane stress, as the material for the former helps cancel membrane stress and thereby suppress film distortion.

The first high-refractive-index layer is given a refractive index higher than 1.8. The specific values of the refractive index and the mechanical film thickness of the first high-refractive-index layer are determined on the basis of the overall refractive index and the desired film thickness of the equivalent film as a whole. The first high-refractive-index layer may be made of any conventionally known material as long as its refractive index is higher than 1.8. For example, TiO 2 , ZrO 2 , CaO 2 , or Ta 2 O 5 can be used singly or in combination. Among these, using TiO 2 , which exhibits membrane stress in the direction opposite to the main ingredient SiO 2 of the first low-refractive-index layer, is recommended because doing so helps cancel membrane stress between the first low-refractive-index layer and the first high-refractive-index layer and thereby reduce film distortion.

The first low-refractive-index layer is given a refractive index lower than 1.52. The specific values of the refractive index and the mechanical film thickness of the first low-refractive-index layer are determined on the basis of the overall refractive index and the desired film thickness of the equivalent film as a whole.

The overall refractive index N as calculated according to formula (1) noted previously is set to be in the range from 1.52 to 1.8. If the overall refractive index N is lower than 1.52 or higher than 1.8, sufficient anti-reflection effect cannot be obtained. Preferably, the overall refractive index N is in the range from 1.6 to 1.75.

From the viewpoint of anti-reflection effect and membrane stress, the equivalent film is given an optical film thickness preferably in the range from 0.2λ to 0.4λ, and further preferably in the range from 0.2λ to 0.3λ.

Next, before a description of the second high-refractive-index layer, the second low-refractive-index layer will be described. The second low-refractive-index layer contains SiO 2 as a main ingredient and measures 0.2λ to 0.3λ in optical film thickness. The reason that SiO 2 is used as a main ingredient here is that SiO 2 has a low refractive index and can be formed into a hard layer even by low-temperature vapor deposition, and is therefore ideal as the outermost layer of an anti-reflection film, where satisfactory anti-reflection effect and resistance to marring are required. Here, the optical film thickness needs to be in the range from 0.2λ to 0.3λ. With an optical film thickness smaller than 0.2λ or greater than 0.3λ, sufficient anti-reflection effect cannot be obtained.

Now, the second high-refractive-index layer will be described. This high-refractive-index layer contains TiO 2 as a main ingredient and measures 0.4λ to 0.6λ in optical film thickness. The reason that TiO 2 is used as a main ingredient here is that TiO 2 has a high refractive index and exhibits membrane stress in the direction opposite to the main ingredient SiO 2 of the second low-refractive-index layer formed on top of this layer, and therefore acts to cancel distortion caused by stress in the second low-refractive-index layer. Here, the optical film thickness needs to be in the range from 0.4λ to 0.6λ. With an optical film thickness smaller than 0.4λ or greater than 0.6λ, sufficient anti-reflection effect cannot be obtained.

In the present invention, as a molding of a norbornene resin having a polar group, it is possible to use a molding of such a norbornene resin alone or a composite body thereof with another material or molding. The molding may be of any shape, and its shape is determined according to the use of the optical component as a whole, which can be, for example, a lens for spectacles or for a camera.

In this resin molding, the anti-reflection film is formed on a surface thereof that the norbornene resin forms. Thus, the anti-reflection film may be formed all over the entire surface of the molding or only on a portion thereof where anti-reflection effect is required.

In the present invention, as a norbornene resin having a polar group, any conventionally known type can be used. For example, it is possible to use a norbornene polymer having a polar group added thereto or a ring-opening polymer of a norbornene monomer having a polar group, or a hydrogenated version thereof, or an addition polymer of a norbornene monomer having a polar group, or an addition copolymer of a norbornene monomer having a polar group and an olefin.

The polar group added to the norbornene monomer may be, for example, a polarity substitution product of a halogen, hydroxyl group, ester group, alkoxy group, cyano group, amido group, imido group, silyl group or the like, or a halogen, hydroxyl group, ester group, alkoxy group, cyano group, amido group, imido group, silyl group, epoxy group, or the like.

The norbornene monomer may be, for example, 2-norbornene, 5-methyl-2-norbornene, 5,5-dimetyl-2-norbornene, 5-ethyl-2-norbornene, 5-butyl-2-norbornene, 5-ethylidene-2-norbornene, 5-methoxycarbonyl-2-norbornene, 5-cyano-2-norbornene, 5-methyl-5-methoxycarbonyl-2-norbornene, 5-phenyl-2-norbornene, 5-phenyl-5-methyl-2-norbornene, 5-hexyl-2-norbornene, 5-octyl-2-norbornene, 5-octadecyl-2-norbornene, or the like.

Alternatively, the norbornene monomer may be a monomer obtained by adding one or more cyclopentadiene to a norbornene, or a derivative or substitution product thereof, for example, 1,4:5,8-dimethano-1,2,3,4,4a,5,8,8a-octahydro-2,3-cyclopentadienonaphthalene, 6-methyl-1,4:5,8-dimethano-1,4,4a,5,6,7,8,8a-octahydronaphthalene, 1,4:5,10:6,9-trimithano-1,2,3,4,4a,5,5a,6,9,9a,10,10a-dodecahydro-2,3-cyclopentadienoanthracene, or the like; a polycylic monomer obtained as a polymer of cyclopentadiene, or a derivative or substitute product thereof, for example, dicyclopentadiene, 2,3-dihydrodicyclopentadiene, or the like; an adduct of cyclopentadiene with tetrahydroindene or the like, or a derivative or substitution product thereof, for example, 1,4-methano-1,4,4a,4b,5,8,8a,9a-octahydrofluorene, 5,8-methano-1,2,3,4,4a,5,8,8a-octahydro-2,3-cyclopentadienonaphthalene.

The norbornene resin used in the present invention has a number-average molecular weight preferably in the range from 10,000 to 200,000, and further preferably in the range from 15,000 to 100,000, as measured by a GPC (gel permeation chromatography) method using toluene as a solvent and converted into a polystyrene basis. A norbornene resin having an unsaturated bond in its main chain structure, such as a ring-opening polymer of a norbornene monomer, can be hydrogenated to be turned into a saturated norbornene resin. In this case, hydrogenation is accomplished typically to 90% or more, preferably to 95% or more, and further preferably to 99% or more. Insufficient hydrogenation results in an unduly large proportion of unsaturated bonds left in the main chain structure of the norbornene resin, and thus makes the norbornene resin poorly light-resistant and prone to loss of transparency or of strength or other problems in particular when used in applications involving direct exposure to weather.

The norbornene resin here has a glass transition temperature (Tg) of preferably 110° C. or more, further preferably 120° C. or more, and particularly preferably 130° C. or more. If the norbornene resin has an unduly low Tg, the anti-reflection film cannot be vapor-deposited at a sufficiently high temperature, and thus insufficient adherence may result.

To the norbornene resin, various additives may be added as long as the object of the present invention is satisfactorily achieved. Such additives include, for example, an anti-aging agent such as one based on phenol, phosphorus, or the like; an anti-heat-deterioration agent such as one based on phenol or the like; an ultraviolet-stabilizer agent such as one based on benzophenone or the like; an anti-static agent such as one based on amine or the like; a lubricant such as an ester of an aliphatic alcohol, or a partial ester or partial ether of a polyhydric alcohol. Moreover, the norbornene resin may be mixed with another resin or the like as long as the object of the present invention is satisfactorily achieved.

The molding used in the present invention may be formed in any manner and thus by any conventionally known method such as injection molding, melt extrusion, heat pressing, solvent casting, or drawing.

Moreover, to further enhance the adherence between the norbornene resin molding and the anti-reflection film, the surface of the molding may be subjected to surface reform treatment such as energy beam irradiation treatment or chemical treatment. Energy beam irradiation treatment includes corona discharge treatment, plasma treatment, electron beam irradiation treatment, ultraviolet ray irradiation treatment, and the like.

The anti-reflection film may be formed on the surface of the norbornene resin molding in any manner and thus by any conventionally known method such as vacuum deposition, sputtering, ion plating, or ion-beam assisting. Among these, vacuum deposition is recommended because it offers high productivity, and particularly preferable is a production method according to the present invention as will be described below.

Now, a method for producing an optical component according to the present invention will be described. This production method is characterized in that first a molding of a norbornene resin having a polar group is heated to 70 to 130° C., and then the first layer of an anti-reflection film is vapor-deposited thereon. When an anti-reflection film is formed on a glass member, the layers of the anti-reflection film can be vapor-deposited with the glass member heated to about 300° C. However, with a member made of a conventionally used resin such as PMMA or PC, since it has a low TG and cannot be heated, vapor deposition needs to be performed at ordinary temperatures. This leads to insufficient adherence between the anti-reflection film and the resin molding in particular when the former is composed of multiple layers.

To overcome this problem, the production method according to the present invention exploits the fact that a norbornene resin has a higher TG than a conventionally used resin such as PMMA or PC. Specifically, the first layer of an anti-reflection film, where adherence to a resin molding matters, is vapor-deposited thereon with the resin molding heated to 70 to 130° C., i.e. temperatures higher than conventionally practiced, so that enhanced adherence is obtained between the resin molding and the anti-reflection film. With the norbornene resin molding heated to below 70° C., the adherence between the molding and the anti-reflection film is not enhanced satisfactorily. On the other hand, with the norbornene resin molding heated to above 130° C., the resin may start melting and cause deformation of the molding. The molding needs to be heated in such a way as to reach the above noted temperature range at least before vapor deposition is started and then remain within that temperature range preferably until completion of the vapor deposition of the first layer, and especially preferably until completion of the vapor deposition of all of the two or more layers of which the anti-reflection film is composed. In reality, once the resin molding is heated to within the above noted temperature range, even if the heating is stopped before vapor deposition is started, the molding is usually very likely to remain within that temperature range well until completion of vapor deposition.

From the viewpoint of anti-reflection effect in as wide a range of the spectrum as possible, the production method according to the present invention requires the use of an anti-reflection film composed preferably of multiple layers that are laid over one another, and further preferably of four or more layers for higher reliability. The anti-reflection film may have any layer structure as long as it exerts anti-reflection effect, and is preferably composed of high-refractive-index layers and low-refractive-index layers laid over one another alternately. The main ingredient of the high-refractive-index layers may be, for example, ZrO 2 , TiO 2 , CaO 2 , or Ta 2 O 5 , among which TiO 2 is preferable because it offers a high refractive index. The main ingredient of the low-refractive-index layers may be, for example, MgF 2 or SiO 2 .

The first layer of the anti-reflection film may be of any type, and, from the viewpoint of adherence to the substrate, its main ingredient is preferably a metal oxide, and further preferably TiO 2 because, as compared with other metal oxides, TiO 2 can be formed into a thinner layer and offers excellent adherence.

Optical components according to the present invention, having an anti-reflection film formed on a surface of a molding of a norbornene resin, are used mainly in spectacles and cameras.

Hereinafter, the present invention will be described in more detail by presenting practical examples thereof. It is however to be understood that the specific examples described below are not intended to constrain the present invention in any way and therefore that various modifications are possible within the purport of what is described in the present specification and recited in the appended claims.