Note: Descriptions are shown in the official language in which they were submitted.
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Title of the Invention
All-weather type pavement marking sheet material
Background of the Invention
.
This invention relates to a pavement marking sheet
material and, more particularly, to an all-weather type
pavement marking sheet material having an excellent
visibility during night and particularly when it is
raining.
Pavement marking materials which are presently used
for pavement marking purposes all have glass
microspheres dispersed on the surface thereof or mixed
in the material for affording visibility during
night. These pavement marking materials have the common
disadvantage that, when the surface of the marking
material is covered with a film of water when it
is raining, upper hemisphere portions of the glass
microspheres exposed above the surface
of the marking material and performing
- a light reflecting function are mostly submerged in the
water film with a result that the light reflecting
function is impaired and visibility during night when
it is raining becomes extremely poor.
For improving visibility of the marking materials
during night when it is raining, some proposals have
been made in the past. For example, Japanese Patent
Publication No. 29060/1970 discloses a glass microspheres
marking material comprising glass microspheres of low
lZi36269
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refractive index below 1.55 and colored or uncolored glass
microspheres of medium refractive index of 1.65 - 1.85
of 10% - 60~ in weight percent mixed together and
further comprising adhesive mixed in these glass microspheres.
Japanese Patent Publication No. 48735/1977
discloses an all-weather type pavement marking material
which comprises glass microsphleres having refractive index of
1.55 or less and glass microspheres having a high
refractive index of 2.0 - 2.3 mixed together or on
which such glass microspheres having refractive index
of 1.55 or less and those having refractive index of 2.0
- 2.3 are dispersed on the surface of the marking
material substantially simultaneously when the marking
material is applied on the pavement. Japanese Patent
Publication No. 9460/1978 discloses a method of applying
an all-weather type pavement marking line according to
which, in dispersing glass microspheres on a coated
surface of a pavement marking material by
flowing glass microspheres onto the pavement marking
material, there are provided a surface
on which glass microspheres having refractive index of
1.6 or less are dispersed and a surface on
which glass microspheres having refractive index of 1.9
or more are dispersed, both surfaces being provided in
the form of a belt extending in the longitudinal
direction of the pavement marking material.
Japanese Patent Publication No. 9461/1978 discloses a
method of applying an all-weather type pavement marking
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3 --
line according to which, in dispersing glass
microspheres for light-reflection during night on an
undried coated surface of a pavemet marking material,
10% - 60% in weight percent of the glass microspheres to
be dispersed consists of high refractive glass
microspheres having refractive index of 1.9 or more and
the rest of the glass microspheres consists of low
refractive glass microspheres having refractive index of
1.6 or less and these two kinds of glass microspheres
are dispersed one after the other on the entire surface
of the coated surface of the marking material.
The techniques disclosed in these publications are all
intended to provide a pavement marking method exhibiting
visibility in all weather conditions except for snow by
` the combined use of relatively high refractive glass
microspheres which have sufficient retro-reflective
property, i.e., brightness in reflection, when they
are covered with~a water film due to rain during night
but are expensive and have a relatively low brightness
in reflection in weather conditions except for rain and
relatively low refractive glass microspheres which lose
brightness in reflection when they are covered with a
water film due to rain during night but are cheap and
have a sufficient brightness in reflection in weather
conditions except for rain, thus making most of the
merits of the respective glass microspheres and making
up for the defects of the respective glass microspheres
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by the merits of each other.
These techniques all disclose an art in which glass
microspheres are bonded directly to the pavement surface
with a traffic paint. Aside from these techniques,
Japan~se Preliminary Patent Publication No. 238505/1985
discloses a pavement marking sheet material which is a
pavement marking material preformed in the form of a sheet
and is bonded on the pavement surface in use. In this
sheet material, a surface sheet of a thickness of 1 mm
or less consisting of a base layer in which reflex-
reflecting elements such as glass microspheres are
disposed on the surface thereof and a transparent and
flat surface layer covering the base laer is bonded to
the entire surface or a part of the surface of a base
sheet of a thickness of 3 mm or less made of a material
having an excellent conformability to the pavement
surface selected from materials such as rubber,
synthetic resin and the like. This pavement marking
sheet material is intended to maintain good visibilty during
night when it is raining by reflection of light by the
reflex-reflecting elements covered with the cover layer
while holding the total thickness of the sheet material
at 4 mm or less.
The pavement marking methods disclosed in the above
describved Japanese Patent Publications No. 29060/1970,
No. 48735/1977, No. 9460/1978 and No. 9461/1978
according to which glass microspheres are bonded
directly on the pavement surface with traffic paint
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require a strict temperature control in applying the
material on the pavement surface and, in addition,
require complicated apparatus and the speed of
application is rather slow. The pavement marking
sheet material disclosed in the above described
Japanese Preliminary Patent Publication No. 238505/1985
realizes a simplified application as compared with the
above described pavement marking methods since the
material is preformed in the form of a sheet. This
pavement marking sheet material, however, requires an
extra manufacturing step for adhereing two kinds of
sheets produced separately together with a resulting
high manufacturing cost.
It is, therefore, an object of the invention to
provide an all-weather type pavement marking sheet
material having an excellent visibility during night
wnen it is raining which can be applied on the pavement
surface more easily and quickly and can be manufactured
at a reduced cost.
Summary of the Invention
The pavement marking sheet material achieving the
above described object of the invention is characterized
in that it comprises a base sheet made of rubber,
synthetic resin or the like, and glass microspheres
having refractive index of 1.5 - 1.9 and glass
microspheres having refractive index of 2.0 - 2.4
embedded mixeclly and at least partially
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in a portion of the base sheet, said portion
being formed at intervals in the longitudinal direction
o~ the base sheet or continuously in the longitudinal
direction of the base sheet in a part of the base sheet
as viewed in the transverse direction of the base sheet.
According to the invention, since the portion in
which the glass microspheres having refractive index of
2.0 - 2.4 which can maintain reflex-reflectivity during
night even if they are covered with a water film due to
rain are embedded in the base sheet is provided at
intervals in the longitudinal direction of the base
sheet or continuously in the longitudinal direction of
the base sheet in a part of the base sheet as viewed in
the transverse direction of the base sheet, this portion
has an excellent visibility during night when it is
raining and, accordingly, drivers of automobiles can
readily recognize the pavement marking sheet material
in such weather condition if the pavement marking sheet
material is used as, e.g., a center line of the
pavement. In weather conditions other than rain during
night, the glass microspheres having refractive index of
1.5 - 1.9 contribute to brightness in reflection in
addition to the glass microspheres having refractive
index of 2.0 - 2.4.
Since the pavement marking sheet material according
to the invention is so constructed that the two
different kinds of glass microspheres are dispersed on
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the preformed base sheet, the strict temperature control
and the complicated applicating appratus as required in
applying the prior art pavement marking methods causing
the glass microspheres to be bonded directly on the
pavement surface can be obviated and the application
work can be done with a remarkably increased speed.
Moreover, according to the invention, the two different
kinds of microspheres are dispersed not in a state in
which the two kinds of microspheres are superposed one
upon the other or in paralle-to each other as has been
proposed in the prior art methods but in such a manner
that the high refractive glass microspheres which are
relatively expensive are dispersed at intervals in the
longitudinal direction of the base sheet or dispersed
continuously in the longitudinal direction of the base
sheet in a part of the base sheet as viewed in the
transverse direction of the base sheet. This contributes
greatly to reduction in the manufacturing cost of the
pavement marking sheet material without sacrificing
brightness in reflection during night when it is
raining. If compared with the prior art pavement
marking sheet material in which two separately produced
sheets are adhered to each other in manufacturing, the
pavement marking sheet material according to the
invention does not require such separate production of
the two kinds of sheets so that the sheet material can
be manufactured at a reduced cost.
Embodiments of the invention will now be described
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in conjunction with the accompanying drawings.
Brief Description of the Drawings
In the accompanying drawings,
Fig. 1 is an enlarged sectional view schematically
showing an embodiment of the pavement marking sheet
material according to the invention; and
Figs. 2, 3, 4 and 5 are views showing examples of
manner of disposing the portion in which the glass
microspheres having refractive index of 2.0 - 2.4 are
dispersed.
Description of Preferred Embodiments
The glass microspheres having refractive index of
1.5 - 1.9 used in the sheet material of the present
invention (hereinafter referred to as "glass
microspheres A") have a diameter of 50 ~m - 1,000 ~ m,
preferably 100 ~m - 500 ~m (average diameter of 350~m).
If the diameter is less than 50~ m, the glass
microspheres will be embedded too deeply in the base
sheèt in the embossing step with resulting decrease in
brightness of reflection and, besides, they will tend to
be readily covered with a water film when it rains with
resulting decrease in brightness in reflection. If, on
the other hand, the diameter exceeds 1,000 ~m, the base
sheet tends to be injured by the embedded glass micro-
spheres so that such diameter is undesirable. The glass
microspheres having refractive index of 2.0 - 2.4 (here-
inafter referred to as 'lglasc microspheres B") have a
" - 9~LZ8~i269
diameter of 50 ~m - 350 ~m (average diameter of 70 ~m). If
the diameter is less than 50 ~m, the glass microspheres
will be embedded too deeply in the base sheet in the
embossing step whereas if the diameter exceeds 350 ~m,
they will be too expensive to allow a commercial use in
such sheet material.
For attaining a high brightness in reflection, the
glass microspheres A should preferably be dispersed
uniformly and continuously over the entire surface of
the base sheet. If, however, an adequate brightness in
reflection is obtainable owing to conditions of the
pavement, the glass microspheres A may be partially
dispersed at intervals in the longitudinal direction of
the base sheet or may be continuously dispersed in the
longitudinal direction of the base sheet in a
part of the base sheet as viewed in the transverse
direction of the base sheet. The glass microspheres B are
dispersed not uniformly over the entire surface of the
base sheet but at intervals in the longitudinal
direction of the base sheet or, alternatively,
continuously in the logitudinal direction of the base
sheet in a part of the base sheet as viewed in the
transverse direction of the base sheet. For this purpose,
a portion S in which both the glass microspheres A and B
exist may be disposed in various manners such, for
example, as in the form of a belt-like section extending
normally to the longitudinal direction of the base sheet
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as shown in Fig. 2, as in the form of a belt-like
section extending obli~uely with respect to the longi-
tudinal direction of the base sheet as shown in Fig. 3,
as in the form of a broken line in the longitudinal
direction of the base sheet as shown in Fig. 4 and as in
the form of a belt extending continuously in the
longitudinal direction of the base sheet in a part of
the base sheet as viewed in the transverse direction of
the base sheet as shown in Fig. 5.
There is no particular rimitation on the width and
interval of the portion S in which both the glass micro-
spheres A and B exist. The width and interval of the
portion S may be suitably determined so that a
sufficeint brightness in reflection may be obtained
depending upon conditions of the pavement on which the
pavement marking sheet material is applied.
Preferable ratios of dispersion of the glass
microspheres A and B in the portion S in which the
glass microspheres A and s coexi-st are ones in which
the glass microspheres A are dispersed over 30% - 40%
of the area of a flat sheet before embossing and the
glass microspheres B are dispersed over 50% - 85%
(preferably 70% - 80%) of the area of the flat sheet before
embossing remaining after the glass microspheres A have
been dispersed. If the amount of dispersion of the
glass microspheres A is less than 30%, a sufficient
brightness in reflection cannot be obtained whereas if
the amount of dispersion of the glass microspheres A
. . ..
.. . . .. -..... .. . .
2~36;i~
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exceeds 40%, vehicles running over the pavement marking
sheet material tend to slip and therefore is
undesirable. If the amount of dispersion of the glass
microspheres B is less than 50~, a sufficient brightness
in reflection cannot be obtained whereas the amount
of dispersion of 85~ is considered to be a maximum
possible value of dispersion.
The base sheet requires to be made of a material
having excellent conformability to the irregular
pavement surface and durability to various mechanical
forces. The base sheet therefore comprises, as its main
ingredient, unvulcanized synthetic rubber suchJ for
example, as acrilonitrile-butadiene rubber (NBR),
isobutylene-isoprene rubber ~IIR), chloroprene rubber
(CR), chloro-sulphonyl-polyethylene, a urethane rubber
or a synthetic resin such, for example, as polyvinyl
chloride. The base sheet may comprise, if n~cessary,
additives including a filler such as calcium carbonate
powder, a pigment for providing color to the base
sheet, small glass microspheres and wear resisting
particles. ~he mixture of these ingredients is formed
into a sheet with a thickness of 5 mm or less by passing
it through pressure rolls or by other convenient
methods. If the small glass microspheres are mixed in
the base sheet, small glass microspheres having a
diameter not exceeding about 500 ~m may be employed
and those of about 70 ~m - 100 ~m in average diameter
2~i9
may be conveniently employed.
An adhesive layer of a suitable thickness of 50 ~m
or more, preferably about 100 ~m - 200 ~m, is provided under
the lower surface of the base sheet. A release paper is
normally attached to the lower surface of the adhesive
layer but it may be omitted depending upon the type of
the adhesive employed in the adhesive layer.
For making the base sheet, the unvulcanized
synthetic rubber or synthetic resin which is an
ingredient of the base sheet is aaded with desired
amount of the filler, pigment, small glass microspheres
and other materials if required. The mixture is blended
and is formed into a sheet of a thickness of 5 mm or
less through heated pressure rolls and then is wound
into a roll. If necessary, the roll of the base sheet is
unwound and an adhesive coated on a release paper in a
thickness of 50 ~m or more is superposed on the lower
surface of th~ base sheet and then the base sheet is
wound again.
The base sheet may be composed of two layers,
i.e., a base layer made of the above described
ingredients and a thin colored layer provided on the
base layer. In this two layered structure, the
thickness of the base layer preferably is about 1.0 mm -
1.6 mm and the thickness of the colored layer
preferably is about 20 ~m - 25 ~m. The thin colored
layer on the base layer is required to have the same
features as those of the base layer described above, to
1~862~9
be adhered integrally to the base layer and to be
colored in a desired color for the pavement marking
purpose so that the same material as, or a material
which is different but has the same properties as, the
unvulcanized synthetic rubber used for the base layer
added with a suitable coloring agent is preferably
employed.
For forming the colored layer on the base layer,
the base layer which has once been wound into a roll is
unwound and the colored layer is formed thereon by
coating a liquidized material prepared by dissolving
an unvulcanized synthetic rubber material in a solvent
and adding a coloring agent affording a desired color
thereto on the upper surface of the base layer by an
amount sufficient for giving a desired thickness to the
coated film after drying.
The main ingredient and filler constituting tne
base layer and the colored layer need not be of the same
composition if these layers adhere well to each other.
The composition, thickness, the ratios of additives, the
diameter and amount of the small glass microspheres, the
kind and amount of the coloring agent etc. may be
suitably determined having regard to the purpose of the
sheet material.
The surface of the base sheet may be either flat or
formed with a continuous pattern of multiplicity
of protuberances and depressions. The depth of
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embedding of the glass microspheres may be either
uniform or not uniform. Most preferably, the base sheet
is formed on the surface thereof with a continuous
pattern of a multiplicity of protuberances and
depressions, the portion S in ~hich the glass
microspheres A and B are embedded is provided at
intervals in the longitudinal direction of the base
sheet or continuously in the longitudinal direction
of the base sheet in a part of the base sheet as
viewed in the transverse direction of the base sheet
and the glass microspheres A and B in this portion S
are embedded in such a manner that a depth of
embedaing of these glass microspheres A and B in the base
sheet from the surface is randomly different one from
another and a majority of the glass microspheres A and B
in these protuberances and depressions are partially
exposed from the surface of the base sheet.
According to this construction, a sufficient amount
of light can be reflected back toward its light source,
no matter which direction the light may be incident
from so that a high initial brightness in reflection
can be obtained in an initial stage of use of the
pavement marking sheet material. Further, since the
glass microspheres A and B are embedded in the surface
portion of the protuberances and depressions of the base
sheet in such a manner that the depth of embedding is
randomly different one from another, the loss or sharp
decrease in the brightness in reflection never occurs as
- 15 - 2i~
in prior art pavement marking sheet materials even when
the brightness decreases due to coming off of the glass
microspheres caused by contact with wheels of the
passing vehicle but a high brightness in reflection is
maintained for a long period of time. Since the depth
of embedding of the glass microspheres embedded in the
surface portion of each protuberance and depression is
randomly different, when the glass microspheres contact
wheels of the passing vehicle, the glass microspheres
come off one after another in the order of the depth of
embedding, i.e., a glass microsphere of a smaller depth
of embedding coming off earlier, and never come off
substantially all at once as in the prior art pavement
marking sheet materials. Accordingly, the sheet
material can maintain a high brightness in reflection
until a layer of glass microspheres of the largest depth
of embedding comes off.
In a case where the glass microspheres A are
dispersed continuously over the entire surface of the
base sheet and the glass microspheres B are dispersed
at intervals in the longitudinal direction of the base
sheet, it is preferable that the depth of embedding of
a majority of the glass microspheres embedded in the
surface portion of the base sheet should differ
randomly not only in the portion S in which the glass
microspheres A and B coexist mixedly but also in the
portion in which the glass microspheres A only are
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dispersed.
Fig. 1 shows an example of the preferable form of
the base sheet. Fig. 1 is an enlarged sectional view
of the base sheet of Fig. 2 taken along lines I - I in
Fig. 2. A continuous pattern of a multiplicity of
protuberances 2 and depressions 3 of random shapes and
dimensions is formed on the surface of a base sheet 1.
A multiplicity of glass microspheres A and s are
embedded in the base sheet 1 in such a manner that a
depth of embedding of the glass microspheres A and B in
the base sheet 1 from the surface of each protuberance 2
and depression 3 is randomly different one from another.
A majority of the glass microspheres A and B in these
protuberances 2 and depressions 3 are partially exposed
from the surface of the base sheet 1 and the height of
the exposed portion of these partially exposed glass
microspheres is randomly different one from another.
Reference numerals 4, 5 and 6 designate respectively
an adhesive layer, small glass microspheres and wear
resisting particles.
A process for manufacturing the sheet material will
now be described about a sheet material in which the
glass microspheres A are dispersed over the entire
surface of the base sheet including both the portion S
and the rest of the base sheet and the depth of
embedding of the glass microspheres in the surface
portion of each protuberance and depression of the base
sheet is randomly different in both the portion S and the
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- 17 -
rest of the base sheet.
The base sheet 1 manufactured in the above
described manner is unwound from the roll and a solvent
is coated on the unwound base sheet 1 for facilitating
embedding of the glass microspheres A and B. Then,
the glass microspheres A are uniformly dispersed from a
hopper (not shown) over an undried surface of the moving
base sheet 1. Thereafter, the glass microspheres B are
dispersed from another hopper (not shown) disposed
downstream from the hopper containing the glass
microspheres A over the undried surface of the base
sheet intermittently at predetermined time intervals
or, alternatively, continuously over a part of the
base sheet as viewed in the transverse direction of
the base sheet. The base sheet then is dried. As
alternative means for facilitating embedding of the
glass microspheres A and B, a coating material which
is of a composition identical or similar to that of
the base sheet 1 may be coated on the surface of the
base sheet 1. Then, the base sheet 1 on which the glass
microspheres A and B are dispersed is passed through an
embossing device so that an embossed pattern of desired
shape and dimensions is formed and simultaneously the
glass microspheres A and B are embedded in the surface
portion of each protuberance 2 and depression 3 of the
base sheet 1 thus formed by the embossing.
The embossing device used in making the sheet
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material of the invention is one whose protuberances and
depressions respectively are not of a flat surface, if
viewed in an enlarged scale, but are formed with a
multitude of small depressions with depth and diameter
ranging from several tens microns to several hundred
microns. By virtue of employing this embossing device,
in carrying out the embossing process, many of the glass
microspheres A and B are engaged in these small
depressions formed on each of the sufaces of the
protuberances and depressions constituting the embossing
pattern of the embossing device and, accordingly, the
material of the base sheet 1 entering each of these
small deressions is stopped at a line along which the
glass microsphere is engaged with the inside wall of the
small depression and thereby is prevented from further
entering the inside of the small depression to enclose
the glass microsphere entirely by the material of the
base sheet 1. Since the shape, diameter and depth of
these small depressions are not uniform on one hand and
tne diameter of the glass microspheres A and B is varied
within a certain range on the other, position of
engagement of the glass microspheres A and B in the
small depressions is infinitely varied depending upon
combination of each individual small depression and each
individual glass microsphere.
Accordingly, as shown in Fig. 1, a multiplicity of
glass microspheres A and B whose depth of embedding is
randomly different one from another are embedded in the
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surface portion of the respective protuberances 2 and
depressions 3 of the base sheet 1 formed through the
embossing process. If necessary, the surface of the
embossed base sheet 1 is treated with a releasing
agent. The base sheet 1 thereafter is dried and wound
into a roll to provide a finished product.
The method of providing protuberances and
depressions on the surface of the base sheet 1 is not
limited to the above described embossing process but
any other method capable of forming a multitude of
continuous protuberances ana depressions may be
employed. In case the embossing device is employed,
it is effective in some case to apply the embossing
process twice depending upon the embossing pattern of
the embossing device used.
Example 1
The following ingredients were used for producing
the base sheet 1:
NBR85 (parts by weight)
TiO2 100
CaCO3 140
petroleum resin 15
small glass microspheres 140
(average diameter 100 ~m)
These materials were blended and formed into a
sheet having a thickness of 1.6 mm and a width of
1,000 mm through an extrusion process at a processing
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- 20 -
temperature of 90C. The sheet was once wound and then,
as the sheet was unwound, a release paper coated with
pressure sensitive adhesive was superposed on the lower
surface of the sheet. The sheet was wound again. Then,
the sheet was unwound and triol was coated on the
surface of the base sheet in a coating amount of
5 g - 6 g. Glass microspheres A having an average
diameter of 350~ m and refractive index of 1.50 were
uniformly dispersed over the undried surface of the base
sheet. Glass microspheres B having an average
diameter of 70 ~m and refractive index of 2.2 were
dispersed over the base sheet to form a belt-like
section of a width of 25 mm at an interval of 300 mm in
the longitudinal direction of the base sheet. The base
sheet was then dried at 80C for 5 minutes.
The base sheet was then passed through an embossing
device at an embossing temperature of 80C whereby the
glass microspheres were densely embedded in the surface
portion of protuberances and deprèssions formed on the
base sheet. For providing a layer of releasing agent on
the surface of the base sheet thus having the glass
microspheres embedded therein, a releasing agent
comprising a synthetic resin as a main ingredient was
coated on the base sheet. The base sheet was then dried
and wound into a roll to provide a finished product.
Since an excessive thickness of the releasing agent
layer decreases the brightness in reflection, the
coating thickness of the releasing agent on the glass
- 21 -
microspheres should be less than about 2 ~m.
In applying this sheet material to the pavement
surface to form, e.g., a center line thereon, the sheet
material is cut into a belt of a suitable width, e.g.,
150 mm.
Example 2
A base sheet of two layer structure consisting
of the base layer and the colored layer of the
following composition was manufactured:
Base layer
chloro-sulphonyl-polyethylene 80
(parts by weight)
BR 20
softener 20
filler 270
small glass microspheres 150
(average diameter of 100 ~m)
coloring agent 15
Colored layer
chloro-sulphonyl-polyethylene 100
pigment 1 50
solvent (triol) 500
These materials were blended and formed into a
sheet having a thickness of 1.6 mm and a width of
1,000 mm through an extrusion process at a processing
temperature of 90~C. The sheet was once wound and then,
as the sheet was unwound, the above described materials
12~3~;2~9
- 22 -
for forming the colored layer were coated on the sheet.
The sheet was dried to form a base sheet having the
colored layer of a thickness of 20~ m. After winding
the sheet into a roll, the processings including
superposition of a releasing paper, dispersion of the
glass microspheres A and B and embossing were carried
out in the same manner as described with respect to
Example 1 to produce a finished product.
Example 3
A base sheet of two layer structure of the
base layer and the colored layer of the following
compositions was manufactured:
Base layer
NBR 70
(parts by weight)
BR 10
chloro-sulphonyl-polyethylene 10
filler 270
stearic acid 1.5
small glass microspheres 150
(average diameter of 100~ m)
coloring agent 15
Colored layer
chloro-sulphonyl-polyethylene 100
pigment 150
solvent (triol) 500
A base sheet having a base layer of a thickness
of 1.6 mm and a width of 1,000 mm and a colored layer
~L2~36Z69
- 23 -
of a thickness of 20 ~m is formed by the same process as
in Example 2 and superposition of a release paper and
coating of solvent were carried in the same manner as in
Example 2. Then, glass microspheres s having an average
diameter of 80 ~m and refractive index of 2.2 were
dispersed over the base sheet in the form of a belt-like
section of a width of 30 mm at an interval of 350 mm in
the longitudinal direction of the base sheet. These
glass microsphers B were embedded substantially
uniformly into the base sheet by a depth of about 40 ~m
by applying a light pressure over them by means of a roll.
Thereafter, glass microspheres A having an average
diameter of 200 ~m and refractive index of 1.9 were
dispersed over the base sheet substantially uniformly
and these glass microspheres A were embedded into the
base sheet by a depth of about 60~ m by applying a
light pressure over them by means of a roll. The base
sheet was dried at 80~C for 5 minutes and then wound
into a roll. A sheet material in which the glass
microspheres A and B are embedded in a base sheet of
a flat surface was obtained.
~,
