United States Patent
`
`[19]
`
`[11] Patent Number:
`
`5,384,658
`
`Ohtake et a1.
`
`[45] Date of Patent:
`
`Jan. 24, 1995
`
`USOOS384658A
`
`[54] PLASTIC OPTICAL MIEIVIBER AND
`LIGHT-QUANTITY-CONTROLLING
`MEIVIBER EACH HAVING A
`LIGHT-DIFFUSING LAYER 0N ITS
`SURFACE
`
`[75]
`
`Inventors: Genji Ohtake, Yokohama; Hisao
`Morobashi, Kawasaki; Souichi
`Hasegawa, Yokohama, all of Japan
`
`[73] Assignee: Ohno Research & Development
`Laboratories Co. Ltd., Yokohama,
`Japan
`
`[21] Appl. No.:
`
`849,398
`
`[22] PCT Filed:
`
`Sep. 3, 1991
`
`[86] PCT No.:
`
`PCT/JP91/01177
`
`§ 371 Date:
`
`Apr. 27, 1992
`
`§ 102(e) Date:
`
`Apr. 27, 1992
`
`[87] PCT Pub. No.: won/04400
`
`PCT Pub. Date: Mai-.19, 1992
`
`[30]
`
`Foreign Application Priority Data
`
`Sep. 4, 1990 [JP]
`Oct. 30, 1990 [JP]
`
`Japan .................................. 2-233978
`Japan .................................. 2.292991
`
`[51]
`
`Int. 0.6 ........................ G02B 13/20; (30213 5/02;
`A61G 13/00
`
`[52] US. Cl.
`
`........................... 359/707; 359/599;
`362/33
`[58] Field of Search ............... 359/599, 707, 893, 894, -
`359/619; 362/26, 31, 33
`
`[56]
`
`A
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`6/1972 Finch .................................. 359/599
`3,671,101
`3,850,667 11/1974 Tani et a1.
`ll7/36.7
`
`4,114,983
`9/1978 Maffitt et a1.
`359/599
`1/1982 Jaccard ...........
`4,310,219
`359/599
`
`359/599
`4,427,265
`1/1984 Suzuki et a1.
`
`4,529,563 7/1985 McGinniss ..........
`.. 264/84
`
`362/26
`4,573,766 3/1986 Bournay, Jr. et a1.
`..
`..... 359/599
`
`4,989,933
`2/1991 Duguay ...............
`
`' FOREIGN PATENT DOCUMENTS
`
`2054433 5/1972 Germany .
`349167 11/1959 Japan .
`498711
`2/1974 Japan .
`57-128383
`8/1982 Japan .
`58-155947 9/1983 Japan.
`59-8809 2/1984 Japan.
`61-27683] 12/1986 Japan.
`2126501
`5/1990 Japan .
`
`OTHER PUBLICATIONS
`
`English language translation of claim of JP-A—34—9167.
`English language translation of claim of JP—A—49—871 1.
`English language translation of claim of JP—B—59—8809.
`English language translation of claims of JP—A-S-
`7—128383.
`English language Abstract of JP reference No.
`2—126501.
`English language translation of Claims of JP—A—5-
`8—155947.
`
`English language Abstract of JP Reference No.
`61—276831.
`Publications Ltd. London, GB; AN
`Derwent
`89—183663 & JP 1 123 201 May 16, 1989.
`Derwent Publications Ltd. London, GB AN 86—058934
`& JP 61 010499 Jan. 17, 1986.
`
`Primary Examiner—Scott J. Sugarman
`Assistant Examiner—Thong Nguyen
`Attorney, Agent, or Firm—Ladas & Parry
`
`[57]
`
`ABSTRACT
`
`There is provided a plastics optical member having on
`its surface a light-diffusing layer composed of a layer of
`micro-joints formed by immersing a plastics material
`successively in a good solvent and a poor solvent, and a
`plastics light-quantity-controlling member having on its
`surface a white pattern composed of the micro-joints.
`The superiority in light diffuse reflection, transmission
`and ' light-quantity-controlling property permit
`their
`incorporation into various optical systems such as
`screens for projection, various displays, light-source-
`direct]y-under-an-illumination—surface type light boxes,
`edge light type light boxes, etc.
`
`8 Claims, 6 Drawing Sheets
`
`
`
`' MBI_OO1592
`
`Mercedes-Benz Ex. 1013
`
`Mercedes-Benz Ex. 1013
`
`MBI_001592
`
`

`

`US. Patent
`
`Jan. 24, 1995
`
`Sheet 1 of 6
`
`5,384,658
`
`FIG.
`
`FIG. 2
`
`MBI 001593
`
`MBI_001593
`
`
`

`

`US. Patent
`
`Jan. 24, 1995
`
`Sheet 2 of 6
`
`5,384,658
`
`FIG.3
`
`
`
`MBI_OO1594
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`MBI_001594
`
`

`

`US. Patent
`
`Jan. 24, 1995
`
`Sheet 3 of 6
`
`5,384,658
`
`FIG. 4
`
`-
`
`131'";
`
`
`, \
`flir-_
`
`5 \jLCQDIAK‘\
`
`
`\- '~_-——..-—-
`
`
`2
`
`MBI_OO1595
`
`MBI_001595
`
`

`

`US. Patent
`
`Jan. 24, 1995
`
`Sheet 4 of 6
`
`5,384,658
`
`FIG. 5
`
`
`
`
`
`-
`
`""1...7......an
`
`MBI_OO1596
`
`MBI_001596
`
`

`

`US. Patent
`
`Jan. 24, 1995
`
`Sheet 5 of 6
`
`5,384,658
`
`FIG. 8
`
`l3
`
`4
`
`Wlllwnnnu
`
`
`
`MBI_OO1597
`
`MBI_001597
`
`

`

`US. Patent
`
`~
`
`Jan. 24, 1995‘
`
`Sheet 6 of 6
`
`' 5,384,658
`
`FIG.
`
`ll
`
`'"lnnnnlfinnnnm
`
` I8
`
`MBI_OO1598
`
`MBI_001598
`
`

`

`1
`
`5,384,658
`
`PLASTIC OPTICAL NIEMBER AND
`LIGHT-QUANTITY-CONTROLLING MEMBER
`EACH HAVING A LIGHT-DIFFUSING LAYER 0N
`ITS SURFACE
`
`TECHNICAL FIELD
`
`The present invention is a technique utilizing light
`diffusion, and relates to a plastic optical member having
`a specific micro-joints layer on its surface, and a light-
`quantity—controlling member having a white pattern of
`the aforementioned micro-joints on its surface.
`Here, the term “optical member” refers to an optical
`member used in any part of various optical systems in
`which light is desired to be subjected to diffuse reflec-
`tion or diffuse transmission. Specific examples thereof
`are optical members used in planar light sources, pro-
`jection screens, window materials for building, various
`displays, office automation appliances, and precision
`optical instruments. The term “light-quantity—controll—
`ing member” refers to a specific optical member used
`for controlling the quantity of light. Specific examples
`thereof are optical members for homogenizing the light-
`ness of the illumination surface in the so-called “thin
`light box” such as a backlight used in liquid crystal
`displays of personal computers, a schaukasten for medi-
`cal treatment, an electrically illuminated display, etc., to
`render the brightness uniform. The optical member and
`the light-quantity—controlling member commonly uti-
`lize the light-diffusing properties imparted by the provi-
`sion of the aforementioned specific micro-joints layer
`on the whole or a part of their surfaces.
`BACKGROUND ART
`
`Using conventional such as surface chemical treat-
`ment, sandblasting method (see JP-B-49-8711), coating
`with a light-diffusing white coating material (see, for
`example, JP-B-34-9168), etc., it is difficult to obtain on
`the surface of a plastic material a thin light—diffusing
`layer which can be formed partly or in the shape of a
`pattern, permits adjustment of light diffuse reflection
`prOperties and light diffuse transmission prOperties, and
`can be formed not only on planar surfaces but also on
`any curved surfaces. The reason for the difficulty is
`explained below for each method.
`In the surface chemical treatment, a violent reaction
`is carried out using a strong acid, a strong base, etc., so
`that the Spectral absorption characteristics of the sur-
`face of the resulting light-diffusing layer are changed by
`the chemical reaction. In other words, coloring and the
`like are caused. Moreover,
`the chemical
`treatment
`merely forms depressions and protuberances in the sur-
`face and hence does not impart significant light diffuse
`reflection properties or light diffuse transmission prop-
`erties to the surface. Furthermore, when the treatment
`is carried out so as to form a pattern, it is difficult to
`prepare a resist which is resistant to the above-men-
`tioned violent chemical reaction.
`In the sandblasting method, sand grains are caused to
`collide with a plastic material at high speeds. For the
`collision of the grains, high energy is required. In addi-
`tion, the finer the sand grains, the greater the air resis-
`tance, so that the fineness of the plastic material surface
`achieved by the treatment has its own limit and it can-
`notbe expected to impart very high light diffuse reflec—
`tion properties or light diffuse transmission properties
`to the surface of the plastic material.
`
`5
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`MBI_001599
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`2
`When a light-diffusing white coating material is ap-
`plied on a plastic material, the spectral absorption char-
`acteristics of the coated material tend to be imbalanced
`because the light-diffusing white coating material
`is
`composed of a white pigment, a binder, various addi-
`tives, etc. The reason is that in the case of such a coating
`material, coating properties should be given priority. It
`is diffith to make the content of the white pigment in
`the coating material much higher than that of the binder
`from the viewpoint of the film-forming properties of the
`coating material. In addition,
`in this case,
`it is the
`boundary surface between the binder and the white
`pigment that scatters light. A small difference between
`their refractive indexes results in a small 1ight-scattering
`effect. Therefore, if a large light diffuse reflection effect
`is desired, it is necessary to thicken the coating of the
`light-diffusing white coating material, so that a thin
`light diffuse reflective layer cannot be realized. More-
`over, the light-diffusing white coating material is diffi-
`cult to be applied on a curved surface.
`On the other hand, conventional thin light boxes
`requiring a uniform illumination surface includes the
`following two types of light boxes: the so-called edge
`light type (see, for example, JP—A-57-128383 and JP-A-
`2-126501) which comprises a light-source provided at a
`side end of an illumination surface, and a light-quantity-
`controlling member with a light guide panel having a
`reflective pattern formed thereon, which member dif-
`fuses light from the light source uniformly on the illumi-
`nation surface; and the so-called lighting curtain type
`(see, for example, JP-B-59-8809) which comprises a flat
`housing with an illumination surface on its open side, a
`light source provided in the flat housing and a light-
`quantity—controlling member provided right above the
`light source and containing a translucent reflector
`called “lighting curtain” which has a reflecting pattern,
`so as to allow light from the light source to reflect
`repeatedly between the inner surface of the housing and
`the light-quantity controlling member to achieve uni-
`form illumination.
`
`The edge light type light box is disadvantageously
`heavy because it is used together with a light guide
`panel made of transparent plastic, glass or the like. The
`edge light type light box is disadvantageous in that the
`area of light guide portion cannot be increased in pro-
`portion to the enlargement of the illumination surface
`since light is introduced through the end face of the
`light guide panel, so that the light box is dark.
`The lighting curtain type light box is advantageous in
`that the light box is lightweight and very light because
`no light guide panel is used therein. However, its assem-
`bling is complicated because the positional relationship
`between the illumination surface and the reflective sur-
`face at the inner surface of the housing must be strictly
`set to obtain a uniform illumination.
`In both the edge light type light box and the lighting
`curtain type light box, a regularly reflecting pattern
`such as a conventional aluminum-deposited pattern as
`well as a printed pattern with a white pigment is con—
`ventionally used as the reflecting pattern on the light-
`quantity-controlling member. However, the regularly
`reflecting pattern has the shortcoming that parallax is
`caused on the illumination surface, depending on the
`angle from which the illumination surface is looked at,
`due to its directivity, although the regularly reflecting
`pattern has a high reflectance. The printed pattern with
`a white pigment is disadvantageous in that the reflection
`wavelength characteristics of the pigment are shifted by
`
`MBI_001599
`
`

`

`5,384,658
`
`4
`layers in the direction of the depth of the plastic mate-
`rial. The cracks provide a very large boundary surface
`area between air and the constituent plastic of the plas-
`tic material. Since light is scattered at the boundary
`surfaces, there are an extremely large number of por-
`tions from which light is scattered. Therefore, a large
`light-diffusing effect can be obtained even by using a
`thin light-diffusing layer.
`When such a light-diffusing layer comprising micro-
`10 joints as described above is formed on the surface of a
`plastic material by treatment with the solvents, the
`surface portion of the plastic material is at first swollen
`by the immersion of the plastic material in a first solvent
`which is a good solvent for the constituent plastic of the
`plastic material. The swelling is caused by the expansion
`of the surface portion of the plastic material. This ex-
`pansion is caused by the intrusion of the molecules of
`the solvent between the molecules of the constituent
`plastic. When the plastic material in this state is im-
`mersed in a second solvent which is a poor solvent for
`the plastic, the molecules of the first solvent present
`between the molecules of the plastic are replaced by the
`molecules of the second solvent, so that the molecules
`of the plastic are rapidly shrunk and solidified. There-
`fore, a large number of cracks are formed and become
`micro-joints.
`In this case, unless the first solvent and the second
`solvent are compatible with each other, the molecules
`of the first solvent are not smoothly replaced by the
`molecules of the second solvent, and hence no uniform
`micro-joints are formed. Therefore, the first solvent and
`the second solvent must be compatible with each other.
`Owing to this compatibility, the molecules of the first
`solvent are gradually replaced by the molecules of the
`second solvent as the ratio of the former to the latter is
`changed, so that uniform micro-joints are formed.
`Thus, the micro-joints according to the present in-
`vention have a structure composed of very small cracks
`and spaces integrated closely with high density. The
`appearance of the micro-joints slightly varies depending
`on the kind of the constituent plastic to be treated, the
`kinds of the first and second solvents,
`immersion
`method, immersion temperature, immersion time, etc.
`For example, the micro-joints appear as assemblies of
`very small cracks or appear spongy or felt-like. How-
`ever, the micro-joints serve as the light-diffusing layer
`of an optical member, irrespective of their appearance.
`The average thickness (depth) of the micro-joints
`layer obtained in the present invention is l to 30 pm. A
`micro-joints layer having an average thickness of 10 to
`20 pm is particularly preferable for a light-diffusing
`layer. The average distance between the cracks or
`spaces formed is 0.01 to 50 um, and in particular, in an
`optical member is preferably 0.02 to 20 um.
`The outstanding advantages of the micro-joints are as
`follows: the micro-joints provide a very high light dif-
`fuse reflectance (a high whiteness degree of the sur-
`face); they do not peel off because they are integral with
`the substrate (they have the same quality as that of the
`substrate) and provide a higher surface strength as com-
`pared with the coating with a white pigment, etc. The
`micro-joints also have advantages in their production.
`For example, their characteristics can easily be chosen
`by changing the kinds of the solvents employed and the
`conditions of the treatment.
`
`3
`various additives added to an ink for printing in order to
`maintain the printability, resulting in the so-called color
`shift. Furthermore, in the case of a white pattern ob-
`tained by printing, the size of dots constituting the pat-
`tern is restricted by printing techniques and cannot be
`made very small. Therefore, the illumination surface
`requires an additional light diffuser panel for hiding a
`dot image since printing of dots with an invisible size is
`difficult.
`
`SUMMARY OF THE INVENTION
`
`The present invention seeks to solve the various prob-
`lems in the formation of a thin light-diffusing layer on
`the surface of a plastic as present material in the prior
`art, and provides a plastic optical member comprising a
`plastic material as present having on the surface a light-
`diffusing layer which has adjustable diffuse reflection
`properties or light diffuse transmission properties, can
`be formed partly or in the shape of a pattern, on any
`curved surfaces, and has a flat spectral absorption distri-
`bution. In addition, the present invention is seek to solve
`the above-mentioned various problems in conventional
`light-quantity—controlling means in light-quantity-con-
`trolling members for adjusting the quantity of transmit-
`ted light and the quantity of reflected light, and pro-
`vides a light-quantity-controlling member which makes
`it possible to obtain a bright box that is lightweight,
`very light, and easy to construct and hardly causes
`parallax, and which member is free from color irregu-
`larities and has a simple structure.
`Further in the present invention, these objects are
`achieved by forming a light-diffusing layer on the sur-
`face of a plastic material. The light-diffusing layer com-
`prises micro-joints formed on the surface of the plastic
`material.
`
`For forming such micro-joints, the plastic material is
`immersed in a good solvent (hereinafter referred to also
`as first solvent) for the constituent plastic of the plastic
`material, and then in a poor solvent (hereinafter re-
`ferred to also as second solvent) for the constituent
`plastic which is compatible with the first solvent. By
`this procedure, the light-diffusing layer is formed on the
`surface of the plastic material. Each solvent may be
`either a single solvent or a mixed solvent. In general,
`when an object is increased in volume by swelling,
`dissolution, melting, etc., and returns to its original
`state, it shrinks toward the centers of shrinkage scat-
`tered in the object. Owing to the cracks or spaces
`formed by the shrinkage between the centers of shrink-
`age, numerous laminated structures in a pillar shape, a
`string shape, a plate shape or a spherical shape are
`formed near the surface of the object, and they are
`called joints.
`In some cases, the joint structures become spongy or
`felt-like depending on the shrinkage rate and the flexi-
`bility and brittleness of the object. Their sizes reach
`several micrometers to several tens of centimeters in
`some cases.
`
`The present inventors found that by the above-men-
`tioned immersion of the plastic material in the good
`solvent and then in the poor solvent, extremely minute
`joints are formed on the surface of the plastic material.
`These micro-joints are assemblies of very small
`cracks formed by expansion of a surface portion of the
`plastic material,
`followed by the rapid shrinkage
`thereof. The cracks are formed in a very small size with
`very high density in the surface portion of the plastic
`material. Furthermore, the cracks are formed in multi-
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`As regards the plastic material used as the optical
`member or the light-quantity—controlling member of the
`present invention, the micro-joints layer can be formed
`
`MBI_001600
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`

`

`5
`on the surface of any kind of plastic so long as there is
`a good solvent for the plastic. Examples of plastic in-
`clude homopolymers
`such as acrylic
`resin (e.g.
`PMMA), polyester resin (e.g. PET), polycarbonate,
`polyolefin (e.g. polyethylene), polystyrene, polyamide,
`polyoxymethylene, polyvinyl chloride and the like, and
`copolymers such as ABS, AS and the like. These plastic
`can be used even when they are not particularly trans-
`parent.
`As a method for allowing the first solvent and the
`second solvent to act on the plastic, any methods can be
`employed, such as immersion in each of the solvents,
`exposure to the vapor of each of the solvents, spraying
`each of the solvents onto the surface, coating of the
`surface with each of the solvents, etc. Since the second
`solvent replaces the first solvent which has already
`swollen the surface portion of the plastic, it is necessary
`to supply a large amount of the second solvent to the
`surface.
`
`As described above, the plastic material having a
`light-diffusing layer comprising the micro-joints as a
`white surface for carrying out light diffuse reflection
`provides highly efficient and dense diffuse-reflected
`light. The plastic material can be employed for the
`production of a diffuse reflector used in various illumi-
`nators, a material constituting the inner surface of a
`light box, a projection screen, etc. Particularly when
`the material is utilized as a projection screen, a light and
`highly clear projected image can be obtained owing to
`the thin light-diffusing layer and the dense surface
`thereof.
`
`5
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`
`The light-quantity-controlling member of the present
`invention for adjusting the quantity of transmitted light
`and for adjusting the quantity of reflected light has a
`light-quantity-controlling means comprising a white
`pattern comprising micro-joints formed on the surface
`of a plastic material. Such a light-quantity-controlling
`means can be formed as follows. Before forming the
`white pattern comprising the micro-joints, the surface
`of the plastic material is masked with a resist such as
`photoresist which is insoluble in both the first and sec-
`ond solvents, wherein the first solvent is a good solvent
`for the constituent plastic, and the second solvent is a
`poor solvent for the constituent plastic and is compati-
`ble with the first solvent. The plastic material thus
`treated is immersed successively in the first solvent and
`then in the second solvent.
`7
`
`In the present invention, the surface of the plastic
`material in which the white pattern is to be formed is
`masked with a resist insoluble in both the first solvent (a
`good solvent for the plastic material) and the second
`solvent (a poor solvent for the plastic material), while
`leaving a portion to be patterned. Then, the plastic
`material thus treated is immersed in the first solvent. At
`this time, a portion not masked with the resist is allowed
`to swell and increase in volume.
`The immersion of the plastic material in the second
`solvent causes rapid replacement of the first solvent by
`the second solvent due to the compatibility between
`these solvents, and results in the rapid shrinkage and
`solidification of the swollen portion, which lead to the
`formation of numerous extremely minute cracks. Since
`the plastic material has low crystallinity, the cracks
`constitute a layer of the so-called micro-joints which
`are irregularly placed one upon another in a large num-
`ber- in the direction of the depth of the plastic material,
`as can be seen from FIG. 2 and FIG. 3 given hereinaf-
`ter.
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`MBI_001601
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`5,384,658
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`6
`The removal of the solvent on the surface and of the
`resist forms a sharp pattern composed of a micro-joints
`layer on the surface of the plastic material. The micro-
`joints in the white pattern constitute a multiple voids
`layer having an average distance between the voids of
`0.01 to 50 um and an average layer thickness of l to 30
`pm.
`
`When the pattern comprising the micro-joints layer
`formed on the surface of the plastic material in the
`manner as described above is irradiated with light, the
`cast light carries out repeated reflection and refraction
`due to the existence of a large number of the very small
`cracks which are densely and irregularly overlaid in the
`direction of the depth of the plastic material. Therefore,
`in spite of the thinness of the layer of the micro-joints,
`the pattern has a high whiteness and a very high non-
`directional reflectance.
`
`As described above, the white pattern comprising a
`micro-joints layer repeats reflection and refraction very
`many times in three dimensions in the thin layer. There-
`fore,
`the white pattern exhibits extremely excellent
`effective diffusion, per unit transmittance, than does an
`etched surface or a sandblasted surface, whose diffusion
`is mainly attributable to planar reflection and refraction.
`Accordingly, the white pattern layer can be made thin-
`ner, whereby light loss is reduced. Therefore, the light-
`quantity-controlling member of the present invention
`having the white pattern comprising micro-joints effi-
`ciently reflects or refracts the light thereby to enable
`accurate and non-directional control of the light.
`The light-quantity-controlling member of the present
`invention does not contain any factor which changes
`the wavelength characteristics of the light at the time
`when the member reflects or refracts the light. There-
`fore, the member does not cause color shift.
`Embodiments of the present invention are explained
`below.
`~
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a schematic view of one example of the
`surface of a light-diffusing layer comprising the micro-
`joints of the present invention observed under a micro-
`scope;
`FIG. 2 is a schematic view of one example of a sec-
`tion of the light-diffusing layer comprising the micro-
`Jomts;
`FIG. 3 is a schematic illustration of the micro-joints
`of the present invention at a magnification of 5,000;
`FIG. 4 is a partial perspective view of the light-quan-
`tity-controlling member of the present invention;
`FIG. 5 shows another example of the light-quantity-
`controlling member of the present invention. FIG. 6,
`FIG. 7, FIG. 8, FIG. 9, FIG. 10 and FIG. 11 are sec-
`tional side views of other examples of the present inven-
`tion.
`
`The symbols employed in these figures denote as
`follows.
`
`1 - - - housing,
`2 - - - light source,
`3 - - - light-quantity-controlling member,
`4 - - - light diffuser panel,
`5 - - - reflective surface,
`6 - - - white pattern,
`7 - - - light-quantity-controlling member,
`8 - - - white layer,
`9 - - - transparent member,
`10 - - - light-quantity-controlling member,
`11 - - - light diffuse transmission member,
`
`MBI_001601
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`

`

`7
`
`5,384,658
`
`12 - - - reflective sheet,
`13 - - - light guide member,
`.
`14 - - - reflecting pattern,
`15 and 15' - - - white pattern comprising a micro-
`joints layer,
`16 - - - light-quantity-controlling member,
`17 - - - light-quantity-controlling member,
`18 - - - housing.
`
`BEST MODE FOR CARRYING OUT THE
`INVENTION
`
`The present invention is further illustrated below
`with examples.
`
`EXAMPLE 1
`
`A 30 mmX 80 mm transparent polymethyl methacry-
`late plate was immersed in dichloromethane (a first
`solvent) for 30 seconds and then in n-hexane (a second
`solvent) for 60 seconds, and dried to form a white light-
`diffusing layer on the surface of the polymethyl methac-
`rylate plate. In the formation of the light-diffusing layer,
`one side of the polymethyl methacrylate plate was cov-
`ered with an adhesive tape made of polyester so that the
`light-diffusing layer was formed only on the other side
`of the polymethyl methacrylate plate which was ex-
`posed to the solvents. After the formation of the light-
`diffusing layer, the adhesive tape was peeled off.
`The reflection optical density of the light-diffusing
`layer thus formed on the surface of the polymethyl
`methacrylate plate was measured with a photographic
`densitometer (Model P—2, mfd. by Fuji Photo Film Co.,
`Ltd.) [the reflection optical density is expressed by the
`equation D=log (Io/I)
`(log: common logarithm)
`wherein D is the value of the reflection optical density,
`10 is the intensity of an incident light, and I is the inten-
`sity of a reflected light]. A sheet of black felt was
`brought into close contact with the reverse side of a
`sample to be measured, whereby the light transmitted
`through the light-diffusing layer was absorbed.
`The reflection optical density of the light-diffusing
`layer formed on the surface of the polymethyl methac-
`rylate plate was 0.03, and the value corresponds to a
`reflectance of 93%. Thus, the light-diffusing layer has
`sufficient light diffuse reflection properties and hiding
`power as a white layer.
`Observation of the light-diffusing layer formed on the
`surface of the polymethyl methacrylate plate under a
`microscope demonstrated that there were micro-joints
`composed of numerous extremely minute and irregular
`cracks on the surface of the polymethyl methacrylate
`plate. As mentioned above, the micro-joints observed
`under the microscope are schematically shown in FIG.
`1.
`
`Microscopic observation of a section of the poly-
`methyl methacrylate plate having the light-diffusing
`layer on the surface revealed that the thickness of the
`light-diffusing layer was about 20 pm. The section ob-
`served under a microscope is schematically shown in
`FIG. 2. Each of FIG. 1 and FIG. 2 merely shows an
`example, and the size and shape of the micro-joints vary
`depending on the conditions at the formation of the
`light-diffusing layer.
`the light-diffusing layer
`When visually observed,
`formed on the surface of the polymethyl methacrylate
`plate was pure-white, had a very dense surface shape,
`and showed no directional property of surface profile at
`all. This is considered to be due to the non-crystalline,
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`45
`
`50
`
`55
`
`65
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`MBI_OO1602
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`8
`amorphous structure of the polymethyl methacrylate
`plate.
`For comparison, the same polymethyl methacrylate
`plate is sandblasted on one side and the above-men-
`tioned reflection optical density was measured. The
`reflection optical density was 0.62. When visually ob-
`served, the sandblasted surface was not so white and
`was rough.
`Next, a white coating material comprising titanium
`oxide as a white pigment was applied on one side of a
`polymethyl methacrylate plate to a thickness of 20 pm.
`The thickness was the same as that of the aforesaid
`light-diffusing layer composed of the micro-joints
`formed by the method of the present example. In this
`case, the reflection optical density was 0.08. For further
`reducing the reflection optical density,
`the coating
`thickness or the titanium oxide content in the white
`coating material may be increased. However, even
`when either of these factors is increased, the application
`of the white coating material becomes difficult and the
`formation of a dense layer on the surface of the poly-
`methyl methacrylate plate becomes impossible.
`In the present example, the transparent polymethyl
`methacrylate plate was used merely for showing the
`result of evaluation of the formed light-diffusing layer
`composed of the micro-joints in terms of the reflection
`optical density. As mentioned above, the plastic mate-
`rial used in the present invention is not limited to trans-
`parent plastic plates.
`In the method of the present example, it is sufficient
`to immerse the plastic material successively in the first
`solvent and then in the second solvent. Therefore,
`whatever shape the plastic material may have, the light-
`diffusing layer can be formed on the surface of the
`plastic material in the same manner as described above.
`
`EXAMPLE 2
`
`A light diffuse reflective layer of micro-joints was
`formed on the surface of each of 3 kinds of plastic plates
`in the same manner as in Example 1, except for replac-
`ing the first solvent and the second solvent. The reflec-
`tion optical density was measured in the same manner as
`in Example 1.
`For choosing solvents as the first solvent and the
`second solvent, respectively,
`the value of solubility
`parameter can be used.
`The definition of solubility parameter is shown be-
`low:
`
`,l C
`SP= T(AH—R7)
`
`‘
`
`SP: solubility parameter,
`AH: latent heat of vaporization,
`R: gas constant (cal/mole),
`C: density (g/cc),
`M: gram-molecular weight (g/mol),
`T: absolute temperature.
`When a polymethyl methacrylate plate is used as a
`transparent plastic plate, the value of solubility parame-
`ter (hereinafter referred to as SP value) of the first sol-
`vent, i.e. the good solvent, is preferably between 9.0 and
`9.8, more preferably between 9.3 and 9.7. It is sufficient
`that the SP value of the second solvent, i.e. the poor
`solvent, is 9.9 or more, or 8.8 or less. As the second
`solvent, there can be exemplified alcohols, glycols,
`chain hydrocarbons and cyclic hydrocarbons.
`
`MBI_001602
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`

`9
`Table 1 shows examples of the formation of a light
`diffuse reflective layer of micro-joints in the surface of
`a polymethyl methacrylate plate by immersion of the
`polymethyl methacrylate plate in each of the first sol-
`vents having different solubility parameters for 30 sec-
`onds and then in ethanol (SP value: 12.7) as the second
`solvent for 60 seconds. For the layers of micro-joints
`which remained transparent, the symbol T is written in
`the table instead of measuring the reflection optical
`density.
`
`TABLE 1
`Reflection
`optical density
`
`First solvent
`
`SP value
`
`T
`8.8
`Xylene
`0.83
`9.2
`Benzene
`0.32
`9.3
`chloroform
`0.28
`9.5
`Chlorobenzene
`Dichloromethane
`9.7
`0.34
`Dioxane
`9.9
`T
`
`
`Table 2 shows the results obtained under the same
`conditions as in Table 1, except that n-hexane (SP va-
`lue=7.3) was used as the second solvent.
`
`TABLE 2
`Reflection
`
`First solvent
`SP value
`optical density
`Xylene
`8.8
`T
`Benzene
`9.2
`1.24
`Chloroform
`9.3
`0.03
`Chlorobenzene
`9.5
`0.02
`Dichloromethane
`9.7
`0.03
`
`9.9Dioxane T
`
`
`Table 3 shows the results obtained under the same
`conditions as in Table 1, except that dichloromethane 35
`(SP=9.7) was used as the first solvent and each of vari-
`ous solvents was used as the second solvent.
`TABLE 3
`
`Reflection
`Second solvent
`SP value
`optical density
`12.7
`Ethanol
`0.340.08
`9.9
`Dioxane
`T
`8.8
`Xylene
`8.5
`0.98
`Carbon tetrachloride
`8.2
`0.03
`Cyclohexanone
`0.03
`7.3
`n-Hexane
`
`
`When water was used as the second solvent, only
`large cracks were formed and no light diffuse reflective
`surface was formed.
`From the above results, it was found that the first
`solvent can be chosen on the basis of SP value. The
`second solvents with relatively small SP value was able
`to form a micro-joints layer having higher light diffuse
`reflection property and more uniform structur