JPH0363490B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION 1. Technical Field of the Invention The present invention relates to a method for manufacturing a synthetic resin optical element having a refractive index distribution in the axial direction.

2 Description of the Prior Art In recent years, it has been proposed to correct aberrations of spherical lenses by creating a refractive index distribution in the optical axis direction. Then, the refractive index distribution in the optical axis direction is as follows (1) or (2), n(z)=n ₀ (1-tz) (1) n(z)=n ₀ √1- (2 ) (In the formula, n(z) is z in the optical axis direction from the center of the spherical surface.
, _n0 is the refractive index at the center of the spherical surface, t is a positive constant, and z is the distance from the center of the spherical surface in the optical axis direction) Theoretically, it has been shown that this can be dramatically reduced.

The above-mentioned axially graded index lens generally has a three-dimensional distribution in which all of the equirefractive index surfaces are perpendicular to the optical axis, but it is also possible to consider lenses in which the equirefractive index surfaces are curved.

Expanding this concept, it is thought that aberrations can be determined freely by giving a spherical lens a refractive index distribution such that the equirefractive index surface is a flat surface or a spherical surface of a certain curvature. is very useful not only for correcting aberrations of a single lens, but also for realizing all combinations of focal length and aberration required for a certain lens in a lens system consisting of a combination of multiple lenses. It is.

3 Problems with the prior art In order to form such a refractive index distribution, it is sufficient to form a composition distribution that exhibits a predetermined refractive index distribution in the material. For example, when using inorganic glass, ion exchange or It can be formed by methods such as the CBD method. However, when using inorganic glass, both ion exchange and CVD require enormous amounts of heat and vacuum equipment, which is not very practical. Therefore, organic glasses are advantageous for manufacturing curved lenses with a refractive index distribution.

On the other hand, the production of lenses for use in optical devices using synthetic resins has become popular in recent years, and cameras using plastic lenses have also recently been produced.

In particular, attempts have been made recently to manufacture aspherical lenses that reduce spherical aberration by precision injection molding, but it is extremely difficult to manufacture molds with precise shape accuracy, and furthermore, injection molding is difficult. Since it is difficult to remove the resulting distortion, the current situation is that the desired performance cannot be obtained. Such defects can be significantly improved by manufacturing a spherical lens having the above-mentioned refractive index distribution. A method for manufacturing a spherical lens with the above-mentioned refractive index distribution is to form the above-mentioned refractive index distribution on a flat or cylindrical synthetic resin base material, and then perform post-processing (cutting, polishing, etc.). A method is already known in which a spherical lens is formed by using a spherical lens. However, such a method has disadvantages in that post-processing such as cutting or polishing requires time and cost, and furthermore, the formed refractive index distribution cannot be used effectively.

By the way, a well-known method for manufacturing a self-focusing optical element having a radial refractive index distribution using organic glass includes a two-step process as described in Japanese Patent Publication No. 52-5857. One example is the polymerization method, which involves semi-polymerizing the monomers that form the crosslinkable polymer into a gel state, and then diffusing another monomer from the surface into the gel body to form the monomer. The monomer composition distribution is fixed by forming a composition distribution and then completing the polymerization, and if this method is applied directly to the manufacture of axially distributed lenses, the shape of the surface with a uniform distribution of refractive index will be obtained. The surface is not uniformly flat or spherical, and local distortions tend to occur.

In other words, since the surface of an object in a gel state has relatively large irregularities, when a monomer is diffused from this gel surface, the isoconcentration surface of the diffused monomer, in other words, the equirefractive index distribution surface, is the same as that of the above gel. There is a problem that the distribution surface is distorted according to the unevenness of the body surface, and furthermore, the gel body swells as the monomer diffuses, and the distortion of the uniform refractive index distribution surface is further expanded.

4. Object of the Invention The object of the present invention is to provide an integral molding method that can solve the above-mentioned difficulties in molding in a method of manufacturing a spherical lens having such an axial refractive index distribution.

5 Summary of the Invention The method of the present invention which achieves the above objects comprises: (a) a monomer containing at least two types of monomers having different refractive indexes and monomer reactivity ratios when turned into polymers; (b) holding the mixture in a container having a pair of opposing planar walls and a spherical wall of constant radius of curvature; The polymerization reaction is allowed to proceed ubiquitously by standing under conditions that are substantially uniform except for those caused by and (c) a step of finally heating the monomer mixture in the gel state to complete the polymerization. The gist is a manufacturing method that relies on molding.

The monomer mixture used in the present invention may contain any number of monomers as long as they are plural, but at least two of them must satisfy the following conditions.

(1) Different monomer reactivity ratios.

(2) The refractive index of homopolymers obtained from each monomer is different.

Hereinafter, the present invention will be explained in detail. The present invention was basically completed based on the findings in the polymerization reaction of two or more types of monomers having different refractive indexes and monomer reactivity ratios when turned into polymers.

That is, X types (X is at least 2) of monomers M ₁ , M ₂ , M ₃ ... that satisfy the following conditions.
The mixture of M _x is polymerized, for example, by the polymerization method described in the aforementioned Japanese Patent Publication No. 52-5857. In general, multicomponent copolymerization reactions are as follows.

The rate constant of the growth reaction ~ ~ ~ ~ Mi * + Mj * is Kij
Then, the reactivity ratio Rij is defined as Rij=Kii/Kij, and there are X (X-1) reactivity ratios in the X element copolymerization.

The monomer combinations of the present invention indicate the conditions to be satisfied. Now, two integers i and j are 1≦i and j≦
When there is a relationship x,i<j, (1) Regarding the reactivity ratio (Rij (Mi/Mj) _n +1)/{(Mi/Mj) _n +Rij}>1.
1 Here, (Mi/Mj) _n is monomer Mi and monomer Mj
The mixing molar ratio of

(2) Regarding refractive index (2a) Ni (refractive index of Mi homopolymer) <Nj
(Mj homopolymer refractive index) or (2b)Ni(¨¨¨¨¨¨)>Nj(¨¨¨¨¨).

The case where X=3 will be specifically explained. In ternary copolymerization, the following nine types of growth reactions occur in competition.

〜〜〜M ₁ ※ ＋M ₁ →〜〜〜M ₁ ※ (Rate constant K ₁₁ ) 〜〜〜M ₁ ※ ＋M ₂ →〜〜〜M ₂ ※ ( 〃 K ₁₂ ) 〜〜〜M ₁ ※ ＋M ₃ → 〜〜〜M ₃ * ( 〃 K ₁₃ ) 〜〜〜M ₂ ※ ＋M ₁ →〜〜〜M ₁ ※ ( 〃 K ₂₁ ) 〜〜〜M ₂ ※ ＋M ₂ →〜〜〜M ₂ ※ (rate constant K ₂₂ ) ～～～M ₂ * +M ₃ →～～～M ₃ * ( 〃 K ₂₃ ) ～～～M ₃ * +M ₁ → ～～～M ₁ * ( 〃 K ₃₁ ) ～～～M ₃ * +M ₂ →〜〜〜M ₂ * ( 〃 K ₃₂ ) 〜〜〜M ₃ * +M ₃ →〜〜〜M ₃ * ( 〃 K ₃₃ ) The monomer reactivity ratio is defined by equation (3).

R ₁₂ =K ₁₁ /K ₁₂ R ₂₁ =K ₂₂ /K ₂₁ R ₁₃ =K ₁₁ /K ₁₃ R ₃₁ =K ₃₃ /K ₃₁ R ₂₃ =K ₂₂ /K ₂₃ R ₃₂ =K ₃₃ /K ₃₂ (3 ) The conditions that the combination of monomers M ₁ , M ₂ , and M ₃ of the present invention must satisfy are (1) Regarding the reactivity ratio (R ₁₂ (M ₁ /M ₂ ) _n +1)/{(M ₁ /M ₂ ) _n + R ₂₁ }＞1.
1(4) (R ₁₃ (M ₁ /M ₃ ) _n + 1) / {(M ₁ /M ₃ ) _n + R ₃₁ }＞1.
1(5) (R ₂₃ (M ₂ /M ₃ ) _n + 1) / {(M ₂ /M ₃ ) _n + R ₂₃ }＞1.
1(6) where (Mi/Mj) _n is the mixing molar ratio of monomer Mi and monomer Mj.

(2) Regarding refractive index (2a) ni (refractive index of Mi homopolymer) < n ₂
(Refractive index of M ₂ homopolymer) < n ₃ (Refractive index of M ₃ homopolymer) or (2b) n ₂ > n ₂ > n ₃ . where |n ₃ −n ₂ | and |n ₂ −n ₁ |
are preferably both at least 0.005.

Condition (1) is that as the terpolymerization progresses, the initial monomer
M ₁ rapidly polymerizes, then monomer M ₂ polymerizes,
It shows that monomer M ₃ polymerizes most slowly. In other words, the copolymer formed at the initial stage of polymerization contains a large amount of monomer _M1 , but as the polymerization progresses, the content of _M1 rapidly decreases until the content of monomer _M2 decreases. To increase. As the polymerization progresses further, the content of M ₂ also decreases, and the content of monomer M ₃ increases. If condition (2a) is satisfied, the refractive index of the copolymer produced will increase as the polymerization progresses, but by adjusting the type of monomer and the monomer charging ratio, the refractive index of the copolymer will increase as the polymerization progresses. The refractive index of the polymer can be gradually increased with the polymerization conversion rate over a wide range of conversion rates. If condition (2b) is satisfied, the refractive index of the copolymer decreases with the polymerization conversion rate.

In the case of a combination of two types of monomers M ₁ and M ₂ , it is sufficient that equation (4) holds regarding the reactivity (However, in this case, R ₁₂ and R ₂₁ are usually expressed as R ₁ and R ₂ , respectively. ).

In the present invention, it is desirable to select a monomer system that has a good affinity between the produced copolymer and the remaining monomers. Since the polymerization reaction proceeds at the same rate throughout the system, the generation of polymerization heat is the same everywhere in the system, creating no temperature non-uniformity, and therefore no convection can occur.

The precipitated copolymer is not transported by convection. Since the precipitated copolymer settles under the action of gravity and/or inertia, a polymer layer (containing monomers) is formed as the polymerization reaction progresses, forming a composition distribution and, ultimately, a refractive index distribution. be done. Regarding the refractive index, in a monomer system that satisfies the condition (2a), the refractive index of the part that is precipitated and precipitated first is low, and the refractive index of the part that is precipitated last is high.

The opposite is true for condition (2b).

Monomers that can be used in the present invention are preferably monomers that form transparent polymers, but even if a single polymer tends to become opaque due to, for example, a high degree of crystallinity, If it becomes transparent when copolymerized, it can be used. As monomers that can be used in the present invention, compounds having one or more types of polymerizable double bonds such as vinyl groups, acrylic groups, methacrylic groups, and allyl groups are suitable; vinyl, vinyl acetate,
Styrene, phenyl vinyl acetate, vinyl benzoate, vinyl fluoride, vinylnaphthalene, vinylidene fluoride, vinylidene chloride, methyl acrylate, ethyl acrylate, 2,2,2-trifluoroethyl acrylate, benzyl acrylate, acrylic acid Phenyl, naphthyl acrylate, methyl methacrylate, ethyl methacrylate, acrylonitrile, 2,2,2-trifluoroethyl methacrylate, phenyl methacrylate, benzyl methacrylate, naphthyl methacrylate, 1,1,3-trihydro methacrylate perfluoropropyl, allyl benzoate,
phenyl allyl ether, methacrylonitrile,
α-methylstyrene, parachlorostyrene, butadiene, 1,5-hexadiene, vinyl acrylate, vinyl methacrylate, divinyl phthalate, divinyl isophthalate, divinylbenzene, divinylnaphthalene, ethylene glycol divinyl ether, vinyl naphthoate, β-vinyl naphthoate, diallyl phthalate, diallyl isophthalate, allyl acrylate, allyl methacrylate, β-methallyl methacrylate, methacrylic anhydride, diethylene glycol bisallyl ether, diethylene glycol bisallyl carbonate, tetraethylene glycol dimethacrylate, bisphenol A dimethacrylate, triallyl trimellitate, triallyl phosphate, triallyl phosphite,
These include diphenyldiallylsilane and diphenyldivinylsilane.

Among these monomers, select monomers that satisfy the conditions of reactivity ratio and refractive index, and add to the mixture,
A thermal radical initiator such as a peroxide or an azo compound may be added. Examples include benzoyl peroxide, azobis-t-butane, and azobisisobutyronitrile.

The monomer mixture is placed in a mold and held in a predetermined shape. For example, a liquid monomer mixture is filled in a molding space surrounded by a flat wall and a spherical wall of a lens mold made of metal, glass, etc., and the lens is left standing with the center axis perpendicular to the center, or Rotate around the axis. The mold filled with the monomer mixture is placed under conditions such that the energy applied to each part of the mold is substantially uniform except for gravitational and inertial forces. That is, the conditions are such that light is irradiated from one direction and there is no heating element nearby. When applying heat, make sure that it is applied uniformly from the surrounding area, and that the rate of temperature rise is not so slow that a temperature gradient occurs between the center of the container and the parts that are in contact with the heating medium at the periphery. Must not be. Under such conditions, the polymerization reaction will proceed substantially uniformly in each part of the container, and at a certain point, the same composition will be found everywhere within the container. A polymer will be forming.

In a polymerization system under such conditions, only gravity and/or inertia force act as directional forces, and the polymer formed within the system moves in the direction of the resultant force. It begins to precipitate.

After precipitation of the polymer in the monomer mixture in the polymerization system is completed, heating is performed to complete the polymerization. This is because the precipitation of the polymer occurs in a form containing not only the polymer but also the monomer.

5 Effects of the Invention According to the method of the present invention, not only a spherical lens with small spherical aberration, but also a lens with the required focal length and spherical aberration can be integrally molded, and there is no need for post-processing steps, so A spherical lens with a precise refractive index distribution can be manufactured at almost the same cost as manufactured plastic molded lenses.

Furthermore, according to the method of the present invention, an initial precipitated copolymer layer is formed on the wall surface of the container, and later, polymer layers having different refractive indexes are sequentially deposited and laminated on this polymer layer. If the wall is made sufficiently smooth, it is possible to obtain a homogeneous refractive index distribution surface without local distortion along the wall of the container, and as mentioned above, making the wall of the container a highly smooth surface can be achieved. This method is much easier than the two-step copolymerization method in which the surface of the gel body is smoothed.

6 Examples The present invention will be explained in more detail below using examples.

Various examples of axial graded index lenses that can be formed by the method of the present invention are shown in FIGS. 1A to 1F.

Each of these lenses 10 has one refractive surface 11A that is a flat surface and the other refractive surface 11B that is a convex or concave spherical surface, and is provided with a refractive index distribution that changes in the lens optical axis direction. The equirefractive index surfaces 12 in FIGS. 1B and E are perpendicular to the optical axis,
The lenses shown in A and F have a spherical shape with the center of curvature almost coinciding with the spherical refractive surface 11B, and the lenses shown in C and D have a spherical shape with the center of curvature in the opposite direction to the spherical refractive surface 11B. are doing.

Furthermore, in each of the lenses shown in FIGS. 1A to 1F, the direction of increase/decrease in the refractive index is such that there is a distribution in which the maximum is on the spherical refractive index surface side and the distribution decreases sequentially toward the flat refractive surface 11A, and vice versa. There are two cases in which the distribution has a maximum value of 0 and gradually decreases toward the spherical refractive surface 11B side.

The combination of the above lens shape and refractive index distribution can be used as a single lens with low aberrations, or as a lens with positive or negative aberrations that can correct the aberrations of other lenses used in combination, depending on the application. If selected appropriately, the method of the present invention can produce any of these types of lenses. Next, a preferred example of the manufacturing method will be described.

Planar mold surface 2A and concave spherical mold surface 2 as shown in Fig. 2
Mixture 1, for example, a mixture of methyl methacrylate and vinyl benzoate to which a small amount of benzoyl peroxide is added, is placed in a mold 2 having a molding space surrounded by B, and the mixture 1 is left standing with the flat mold surface 2A facing down. In this system, methyl methacrylate is polymerized first, and as the methyl methacrylate monomer decreases, the proportion of vinyl benzoate polymerized increases. The generated polymer gradually accumulates on the plane mold surface 2B, and finally the entire filling in the mold becomes a gel-like substance. When this is heat-treated to complete polymerization and taken out from the mold 2, it becomes a convex lens whose refractive index gradually decreases from the spherical side toward the optical axis.

This lens has extremely small spherical aberration. If you do the same thing with the spherical surface 2B facing down as shown in FIG. 3, you will end up with a convex lens whose refractive index gradually increases from the spherical side toward the optical axis.
Such lenses are suitable for use in lens systems that produce spherical aberrations of equal absolute value and opposite sign.

In addition, when molding with the spherical side facing down using a mold with a flat mold surface 2A and a convex spherical mold surface 2C facing each other as shown in Fig. 5 using the same monomer system, the spherical aberration is extremely small. Can make concave lenses,
When molded with the flat side facing down as shown in Figure 4, it becomes a concave lens with large spherical aberration, but like a convex lens, such a lens has spherical aberration with equal absolute values and opposite signs. Useful in lens systems that require a concave lens to correct for.

In order to produce a convex lens having a refractive index distribution as shown in FIG. 1A, a monomer mixture having the same composition as above is put into a mold as shown in FIG. 3, and the inside becomes a gel state. Rotate the container around the central axis 3 until. If you choose the rotation speed appropriately,
It is possible to form a refractive index distribution of an equirefractive index spherical surface that is concentric with the spherical surface of the mold inner wall. The convex lens taken out after completing the polymerization is also a convex lens with low spherical aberration. The magnitude of spherical aberration can be controlled by adjusting the rotation speed of the mold. When the rotational speed is high, the inertial force acts more horizontally, so polymer precipitation is faster at the periphery than at the bottom of the mold, so the finished lens has a radius of curvature of its equi-index sphere that is closer to the inner wall of the mold. It is smaller than the radius of curvature of a spherical surface. The opposite is true if the rotation speed is slow.

To manufacture a lens as shown in FIG. 1C, a mold as shown in FIG. 2 is used and rotated around the central axis 3. Similarly, to make a lens like that shown in FIG. 1D, the mold shown in FIG. 5 is used, and to make a lens like that shown in FIG. 1 F, a mold shown in FIG. 4 is used. These are rotated around the central axis 3.

In carrying out the present invention, the monomer mixture may undergo volumetric contraction due to polymerization, and if a closed mold with a constant volume is used, air bubbles may be generated inside. To prevent this,
For example, it is advisable to use a container that shrinks in response to volumetric contraction, or to replenish the monomer mixture as in the following example.

As shown in FIGS. 6 to 9, the mold 2 is provided with a monomer replenishment hole 4 communicating with the molding space from the outside and an air vent hole (not shown) at one or more locations, and the replenishment hole 4 is provided with a monomer reservoir. 5 is connected, and the monomer mixture 1 is filled into the container 5. As a result, when the monomer mixture in the mold 2 polymerizes and shrinks in volume, the monomer mixture in the monomer storage container 5 sequentially flows into the mold under the head pressure, thereby preventing the generation of air bubbles in the mold. be done.

As a method of replenishing the monomer mixture into the mold, in addition to using the head pressure of the mixture liquid stored in the replenishment container as described above, there is also a method of forcibly injecting the monomer mixture by applying a pressure higher than atmospheric pressure to the monomer mixture. You may also do so.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view showing various examples of axially graded refractive index lenses obtained by the method of the present invention, and FIG.
The figure is a cross-sectional view showing one embodiment of manufacturing a convex lens using the method of the present invention, FIG. 3 is a cross-sectional view showing another example of the present invention, and FIG. 4 is a cross-sectional view of manufacturing a concave lens using the method of the present invention. Sectional view showing an example of the case, No. 5
The figure is a sectional view showing another embodiment of the concave lens manufacturing method, and FIGS. 6 to 9 are sectional views showing an example in which a monomer mixture supply device is attached to the lens mold. DESCRIPTION OF SYMBOLS 1... Monomer mixture, 2... Molding mold, 3... Central axis, 4... Monomer supply hole, 5... Monomer reservoir container, 10... Lens, 12... Equal refractive index surface.

Claims (1)

[Claims] 1 A monomer mixture containing at least two types of monomers having different refractive indexes and monomer reactivity ratios when turned into a polymer has a flat molding surface and a spherical molding surface with a constant radius of curvature. holding it in a mold, and
allowing the polymerization reaction to proceed ubiquitously in the mixture;
A step in which the precipitated polymer is precipitated using gravity and/or inertial force to make the entire monomer mixture into a gel state, and the monomer mixture in the gel state is finally heated to perform polymerization. A method of manufacturing a synthetic resin optical element having a refractive index distribution by integral molding, the method comprising: