Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02482255 2004-10-15
- I~THOD AND APPARATUS FOR MAN~TEACTURING
A RETROREFLECTIVE DEYTCE
The present invention relates to a method and
apparatus fo.r manufacturing an agglomeration of glass
beads. In particular, the present invention relates to
method and apparatus far manufacturing an agglomeration of
glass beads for use as a retrorefleetive device. Such a
device may be used to create retroreflective surfaces, for
IO example reflective markings and delineators, and high
visibility coatings having reflective characteristics.
The present invention finds particular use on roads and
road signs.
I5 Markings for highway (road) marking axe usually
required to be reflective at night. Light emitted from
vehicle headlights is reflected back in the direction of
the source, i.e, retroreflected. from the surface of the
marking or other reflective surface. The retroreflective
20 characteristic of the marking material is typically
improved by use of added retroreflective elements or
devices. In road markings, spherical glass beads are
often added to the sux.f,3ce of the marking during
application, or sometimes premised in the body of the
25 marking material, and by this means the retroreflective
characteristics are significantly improved over the
natural reflective praperty of the marking surface.
However, road markings are usually applied in locations
likely to be exposed to traffic, i.e, contacted by vehicle
30 wheels, and such contact leads to deterioration, through
abrasion and other effects, of the reflective material,
thereby reducing its retroreflective properties.
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CA 02482255 2004-10-15
2
Spherical glass beads have also been used to form a
retroreflective element that consists of a plurality of
glass beads disposed about a central core material.
However, the retroreflective properties of such device
will be lost should the glass beads become removed or
damaged, as will often be the case when they are exposed
to frictional forces, for example by the action of wheel
abrasion.
Accordingly, it is desirable to provide a
retroreflective device which, when used in combination
with a road marking paint or coating, will impart very
good reflectivity characteristics and be durable under the
action of traffic.
Previously, it has been proposed to provide a
retroreflective device comprising an agglomerate of light-
reflecting spheres joined together by an adhesive. Such a
device represents a considerable improvement over the
prior art discussed above, since it will consist of a
multi-layer structure of light-reflecting spheres. Should
the outer layer become removed or damaged a new layer of
spheres will be exposed.
However, despite the apparent desirability of such a
device, considerable problems have been experienced in
trying to manufacture agglomerations of glass beads of
consistent quality and size. and on a scale large enough
to allow retroreflective devices to be produced on a
3o commercial level.
One previously proposed method of manufacture involves
the use of a spray device which sprays adhesive binder
CA 02482255 2004-10-15
3
onto the surface of a moving, agitated bed of glass beads.
A number of spray technigues have been proposed, such as
air assisted atomisation, spinning disk (prilling) etc.
However, any advantages assaciated with the production
rate of this techniques are outweighed by the lack of
precision that it affords. In particular, the quantity
and size of binder droplets cannot be controlled to a
sufficient degree, so that the resultant agglomerations of
beads are of varying size. Furthermore, when spraying
binder onto a bed of glass beads, some of the beads do not
corns into contact with the binder, whereas some beads are
°'double coated". Thus, the quality and size of
agglomerates produced by this method is inconsistent.
US 3,254,563 discloses a method of forming reflective
spheroids. Agitated hot glass spheres on a conveyor come
into contact with droplets of a binder material. The heat
from this contact lowers the viscosity of the binder
sufficiently to allow the glass spheres to become embedded
approximately halfway in the droplet. The droplet then
cures to a solid sphericoid.
In US 4,609,587 coated glass spheres are dispersed in
wet paint just after paint is applied to a highway
surface.
DE 1952 184'7 discloses joining individual spheres by
small drops of adhesive into a container holding small
spheres with the size of the drops being adjustable by
rapid lateral movement of the adhesive dispenser. An
alterraative disclosure is using an atomising gun to spray
the adhesive sideways onto a layer of stationary or
downwardly drizzling spheres.
CA 02482255 2004-10-15
4
US 6,398,359 discloses elastomeric particles
containing glass heads. The particles are formed in a
mould.
In EP 0 322 671 a particle is coated in film with
microspheres being dipped in a transparent binder to
provide a cluster. The clusters are formed in a rotary
mixer.
GH 2,164,762 disperses glass spheres contained in a
softened steel from which granules are formed by passing
the sheet through nip rollers, one of which has
indentations in its surface to cut off the granules from
the sheet.
US 3,093,196 discloses beads being attached to tacky
coated granules in a tumbler in a batch process.
US 5,942,280 describes glass flakes being coated with
a barrier layer. Then coated optical elements are mixed
with the coated glass flakes prior to a heat treatment
occurring to partially encbed the optical elements into the
core.
All of these methods are time consuming and produce
inconsistent retroreflective devices.
It is therefore desirable to provide a method of
manufacturing an agglomeration of glass beads which
results in the production of agglomerations of consistent
size and quality and which may be implemented on a
sufficiently large scale.
CA 02482255 2004-10-15
According to one aspect of the present invention a
method of manufacturing an agglomeration of retroreflected
pieces includes forming a bed of the pieces, depositing a
plurality of droplets of binder material from a plurality
5 of spaced outlets of a droplet dispenser onto the bed of
pieces thereby causing a plurality of the pieces to be
held together in discrete agglomerations by each droplet,
and causing relative movement in a first direction between
the bed of pieces and the dispenser whereby droplets from
each dispenser outlet are only applied at a discrete
location onto the bed of beads.
The droplets from each dispenser may only be applied
at a single discrete lacation, that is one droplet only at
each location.
The method may comprise causing the discrete location
of at least two dispensed droplets from the dispenser to
be spaced from each other in a second direction transverse
to the first direction. The method may comprise causing
the discrete location of at least two droplets from the
dispenser to be spaced from each other in the extent of
the first direction. At least two outlets of the droplet
dispenser may be arranged to be supplied with binder
material from a common passage prior to the binder
material in that common passage being divided into
separate outlet passages for the outlets.
There may be a plurality of droplet dispensers each
dispensing a plurality of droplets of binder material at
discrete locations.
CA 02482255 2004-10-15
6
Each droplet dispenser may dispense a number of spaced
droplets onto the bed of pieces. At least three dispensed
droplets in adjacent rows may be deposited first upstream
with respect to the first direction, secondly downstream
and thirdly upstream. The first and third upstream
locations may be arranged to be at different upstream
locations with respect to the first direction. All
droplets may be caused to be at different locations in the
first direction in adjacent rows.
15
The method may compri9e causing further retroreflected
pieces to be deposited onto the bed of pieces after the
depositing of the dxoplets thereby causing further pieces
to be binded to the already bound pieces.-
The method may comprise treating the binder material
to harden the binder material.
The method may comprise forming the bed of pieces on a
2o conveyor moving in the first direction.
The method may comprise causing at least two droplets
to be dispensed simultaneously.
25 The method may comprise depositing the droplets on the
dispenser by means of a plurality of a channels, each
channel having substantially the same internal dimensions.
According to a further aspect of the present invention
3o apparatus adapted to form an agglomeration of
rectroreflected pieces includes a binder dispenser
arranged, in use, to dispense the binder material from a
plurality of spaced outlets onto a bed of pieces and
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CA 02482255 2004-10-15
7
movement means arranged, in use, to cause relative
movemen t between the bed and the dispenser whereby
droplets from each outlet are applied at a single discrete
location onto the bed of pieces.
The apparatus may include a plurality of the dispenser
outlets each arranged to be supplied by a separate
channel. Each channel may have the same internal
configuration along at least part and preferably the whole
l0 extent. The cross-sectional area of each separate channel
may be constant along the length of each channel. A
plurality of separate channels ma5r be connected to a
common inlet. The apparatus may include a plurality of
dispensers each having a plurality of outlets and each
being connected to a common inlet, each inlet being
connected to a single binder distribution device.
Where retroreflective pieces are referred to herein it
will be appreciated that they may comprise beads such as
glass beads.
According to one aspect of a first aspect of the
present invention there is provided a method of
manufacturing an agglomeration of glass beads, wherein the
method comprises: i) forming a bed of glass beads; ii)
depositing droplets of a binder material onto the bed of
glass beads by means of a plurality of channels. each
channel being of substantially identical length and
diameter.
Preferably, the agglomerates are highly reflec~ive,
strong, abrasion resistant and weather resistant. It is
particularlypreferable to achieve a drop yield whereby
CA 02482255 2004-10-15
a
900 of the drops have a size tolerance of at least +/- 2~
by weight.
Preferably, each of the channels are in fluid
communication with a single binder inlet.
Preferred methods include the step of applying a
further layer of beads after deposition of the binder
material, so as to form a substantially spheroid or ovoid
agglomeration of glass beads.
The drops of binder material will diffuse into the
glass beads such that as the binder material hardens, or
is cured by the application of heat or UV radiation,
1S groups of the~glass beads will bind. together. A curing
oven may be provided Which applies heat to cure the binder
compound or can otherwise be used to Cure by W radiation
or any ether radiation for a suitable binder activated by
radiation other than UV.
Preferably, the bed of glass beads is moved from a
first position at which the binder material is deposited
to a Second position at which the agglomerations of glass
beads are removed from the bed, preferably by means of
separation techniques, and any loosed beads are returned
to the first position.
The glass beads are preferably approximately spherical
and have a diameter preferably selected to be within one
of the following ranges: from 100 microns to 300 microns,
from 200 microns to 400 microns, or from 400 microns to
700 microns. Zarger beads may be used to form
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CA 02482255 2004-10-15
9
agglomerations, but the ranges specified are preferred
sizes for the application.
The bed of beads may be of any depth but is preferably
not less than l0mm deep. Advantageously, selected
properties of each glass bead. for example its refractive
w index, may be chosen in accordance with the desired
retroreflectivity of the device. Furthermore, the size of
each glass bead may be selected.
1~
Desirably, the binder material, which may consist of
more than one component, comprises an adhesive material,
for example epoxy resin, acrylic, polyurethane or a hot
me7,t adhesive, or any other suitable adhesive such as
polyureides or polyesters. Furthermore, numerous blends
or combinations of these adhesives are envisaged.
The adhesive material may be pigmented, thereby to
colour retroreflected light from the device. The adhesive
material may include a metallic pigment which may be a
coloured pigment. Preferably, each of the components of
the binder material are separately de-aerated and
conditioned in a low pressure chamber prior to being
supplied to the dispensing device- Furthermore, they are
preferably mixed to a homogeneous consistency before being
supplied, under pressure, to the dispensing device. This
may ba achieved by means of a dynamic mixing blade running
at speeds of between 100rpm to 5000rpm. Hinder components
may be separately transferred from a low pressure chamber
3o to a mixing device via pumps and pneumatically controlled
dispensing valves which accurately inject predetermined
amounts of material. A particular advantage df preferred
CA 02482255 2004-10-15
embodiments of the present invention is the ability to mix
liquid components of differing viscosities.
The size of the binder droplet, physical properties of
5 the binder material (particularly its viscosity and cure
rate) and the size/gradation of the glass beads are key
factors which determined the quality of the agglomeration
of beads produced.
10 The channels may be disposed such that the paths of
the binder droplets do not overlap.
According to an embodiment of a second aspect of the
present invention, there is provided an ,apparatus for
manufacturing ~an agglomeration of glass beads comprising a
plurality of glass beads and a binder material, the
apparatus comprising at least one binder dispensing
device, wherein the or each dispensing device comprises a
plurality of channels along which, in use, the binder
material flows, each channel terminating in an outlet and
being in fluid communication with a single binder inlet,
and wherein the channels are of substantially identical
length and diameter.
According to a particularly preferred embodiment, the
apparatus comprises thxee binder dispensing devices and
binder material is supplied to each of the binder inlets
from a single distribution device. The distribution
device preferably comprises a distributor inlet and three
distributor channels, each of the distributor channels
being of substantially identical length and diameter and
each distributor channel terminating at one of the binder
inlets.
CA 02482255 2004-10-15
11
The use of a binder dispensing device embodying the
present invention exhibits a number of advantages.
Importantly, the binder dispensing device allows a
plurality of drops of a binder material to be dropped onto
the bed of glass beads at different positions
substantially simultaneously, thereby significantly
increasing the production rate of agglomerates as compared
to the methods known from the prior art. Tndeed,
embodiments of the present invention will allow the mass
production of agglomerates of glass beads.
Furthermore, the provision of a p3.urality of channels,
each of substantially identical length and diameter,
ensures that the size of the droplets from each of the
channels are substantially identical. In addition, the
binder dispensing device allows the .flow of binder in each
of the plurality of channels to be controlled by adjusting
the quantity of binder applied to a single binder inlet.
Thus, agglomerates manufactured according to preferred
methods of the present invention are advantageously of a
consistent size and quality.
Furthermore, the rate of discharge of binder material
from the channel outlets can be controlled by adjusting
the pressure applied to the single inlet, and will be
substantially the same from each outlet. This enables the
optimum drop rate to be selected according to the chosen
speed of movement of the glass bead.
The rate of discharge may typically ,range from 5 to
100 milligrammes per second per nozzle outlet and the
channels may typically range from 15 to 20mm long.
CA 02482255 2004-10-15
~Z
However, the length of the channels are not critical
provided that the channels (and nozzle outlet) are all of
substantially equal diameter and length so as to balance
the internal pressures and flow rates. Depending on the
desired droplet size, the channel diameters may range from
0.2 or 0.3mm to 5mm, or an equivalent cross-sectional area
of any configuration other than a diameter such as a semi-
circle, and generally the channel diameter should match
the nozzle outlet diameter.
to
According to an embodiment of a third aspect of the
present invention, there is provided an agglameration of
glass beads manufactured according to a method embodying
the first aspect of the present invention.
According to an embodiment of a fourth aspect of the
present invention, there is provided a retroreflective
device comprising an agglomeration of glass beads
manufactured according to a method embodying the .first
aspect of the present invention. .
According to wn embodiment of a fifth aspect of the
present invention, there is provided a retroreflective
device far use in creating a retroreflective surface,
which devzce comprises an agglomeration of glass beads
manufactured according to a method embodying the first
aspect of the present invention.
Retroreflective devices comprising agglomerates of
glass beads manufactured in accordance with preferred .
embodiments of the present invention can advantageously be
used to enhance the reflectivity of road surfacing
CA 02482255 2004-10-15
13
materials and road markings, including coloured road
surfacing, traffic claming surfaces, etc.
According to an embodiment of the present invention
there is provided the use of a plurality of
retroreflective devices, comprising agglomerates of glass
beads manufactured according to methods embodying the
present invention, in combination eaith road marking
material as a retroreflective road marking coating or road
surfacing material.
According to an embodiment of the present invention
there is provided the use of a plurality of
retrore'lective devices comprising agglomerates of glass
beads manufactured according to methods embodying the
present invention, in combination with a binder material
as a retroreflective surface dressing.
According to an embodiment of the present invention
fo there is provided a retroreflective road marking coating
comprising a road marking material applied to the surface
of a road and a p3urality of retroreflective devices,
manufactured according to an embodiment of the first
aspect of the present invention, embedded in the road
marking material so as to protrude partially therefrom.
The retroreflective devices may be premixed or otherwise
immersed in the road marking material.
According to an embodiment of the present invention
there is provided a retroreflective surface dressing
comprising a binder material coating the surface to be
dressed and a plurality of retroreflective . devices,
manufactured according to an embodiment of the first
CA 02482255 2004-10-15
I9
aspect of the present invention, adhering to the binder
material so as to protrude partially thereform.
Ernbedment of glass beads in a pigmented adhesive or
binder is known to give a reflected colour depending on
the type and properties of the pigment and binder/adhesive
used. However, agglomerates of glass beads manufactured
in accordance with preferred embodiments of the present
invention, and which comprise pigmented adhesive or
to binder, have been found to exhibit far superior
reflectance of colour when compared to know products.
Indeed, the use of glass beads of a specific
quality/refractive index and a predetermined uniform size,
enables a reflective device to be produced which has a
high density of glass spheres on the surface which are in
contact with a large surface area of colour. This
achieves far superior colour density and intensity of
reflected light and is demonstrably better than known
products comprising ordinary glass beads embedded in a
coloured binder. This superior colour reflectance is also
a result of the highly reflective properties of the
agglomerate which is a result, not only of using high
quality glass beads, but also of its closely packed
construction, i.e. the glass beads are bound together in
very close proximity. In addition to close packing of
glass beads throughout the body of the agglomerate, the
glass beads on the surface of the bead cluster are also
close packed thereby achieving optimum reflective
performance and resistance to traffic and/or weathering.
The glass beads are preferably spherical and formed of
good quality clear glass substantially free from faults
CA 02482255 2004-10-15
and inclusions. They preferably exhibit a refractive
index of 1.5, 1~.9 or 2.1.
Any of the methods referred to herein may be combined
5 and any of the features referred to may be substituted for
any of the other features.
For a better understanding of the present invention
and to show how the same may be carried into effect,
10 reference will now be made, by way of example, to the
accompanying drawings in which:
Figure I shows an apparatus for manufacturing an
agglomeration of glass beads embodying the present
15 invention;
Figure 2 shows a dispensing device for use with an
apparatus embodying the present invention;
Figure 3 shows a schematic illustration of a method of
manufacturing an agglomeration of glass beads embodying
the present invention; anti
Figure 4 shows a retroreflective device manufactured
in accordance with methods embodying the present
invention.
Figure 1 shows an apparatus for manufacturing an
agglomeration of glass beads embodying the present
invention comprising: a distribution device 2, having a
distribution inlet 10, coupled to three binder dispensing
devices 3a, 3b and 3c by means of distributor channels Via,
4b and 4c of substantially identical length and diameter.
CA 02482255 2004-10-15
16
Each of the distribution devices comprises seven dispenser
channels 5 of substantially identical length and diameter
and being connected to a binder inlet 6, The channels
each terminate in an outlet 8.
In use, a predetermined quantity of binder material,
which may comprise a homogeneous mix of two or more
components, is supplied by means of pumps to the
distribution inlet 10 under pressure. The speed of the
l0 pumps and the pressure can be controlled so that, in
combination with the distribution channels 4a, 4b and 4c
and the dispenser channels 5, drops of a precisely defined
size are produced at a controlled rate of discharge. The
paths of the droplets of binder material have been traced
by lines 9 for il~.ustrative purposes. It can be seen that
the positions of the outlets have been chosen relative to
each other so as to ensure that he paths of the drops do
not overlap. In this way, when using the apparatus in
accordance with method embodying 'the present invention,
2o the drops may be deposited onto a moving bed of glass
beads without double coating any areas of the bed.
Figure 2 shows, in more detail, the dispensing device
3a as shown in Figure 1. The outlet nozzles B are screw
fitted to the dispensing device 3a which is itself screwed
to the distribution device 2. The dispensing device 3a
consists of an upper wnd a .lower. casing, zla and 11b
respectively, which is bolted together. In this
embodiment, the dispensing device is usefully designed so
that the upper and lower casing can be easily separated
for cleaning and maintenance. The nozzles are arranged
such that the distance between the central axes of
adjacent nozzles, shown by ~c in Figure 2, is 9mm.
CA 02482255 2004-10-15
17
Figure 3 shows a schematic illustration of a method of
manufacturing an agglomeration of glass beads embodying
the presen t invention. Glass beads axe stored in
containers 21a, 21b and 21c according to their
size/refractive index and may be transported to a moving
bed 22 by means of a conveyor system 23. The binder
components are separately deaerated and conditioned in
low-pressure chambers 24a, 24b and 24c and are transferred
to a mixing device before being supplied to a distribution
to unit 25. The distribution unit comprises 4 distribution
devices 26a-d, each having three binder dispensing
devices. The positions of the outlets have been chosen
relative to each other so as to ensure that the paths of
the drops do not overlap.
The rate of discharge of the binder droplets is
controlled in accordance with the speed of the moving bed
of glass beads.
2o The binder coated beads are passed into a curing oven
27 so as to shorten the time it takes for the binder to
harden and for agglomerates of glass beads to be formed.
Although not specifically illustrated, a means for
depositing a second layer of beads, after the binder
deposition has taken place and before the moving bed
enters the oven, may be provided in accordance with
preferred embodiments of the present invention. The
agglomerates and uncoated glass beads are then separated
in a separation unit 2~ and any loose glass beads are
3o recycled. The time between application of the binder and
the collection and separation process needs to be
controlled to enable setting/curing of the binder 3 to a
CA 02482255 2004-10-15
18
sufficient degree to allow handling of the product without
damage or disruption to the agglomeration of beads.
As shown in Figure 4, a retroreflective device 30,
produced by methods embodying the present invention, is
manufactured by bznding a quantity of spherical glass
beads 32 of a desired size with an adhesive 33 so as to
form a spherical or ovoid agglomeration or cluster 30,
preferably 2 to 4mm in diameter (although other sizes may
7.0 be useful according to the application). The size of the
glass beads 32 is preferably selected to be within one of
the following ranges, from 100 microns to 300 microns,
from 200 microns to 400 microns, or from 400 microns to
700 microns, although larger beads may also be used to
form agglomerations where appropriate. In this example,
twa different sizes of glass beads are used, howevex, in
many instances it will be preferably to use glass beads of
a single uniform size. The adhesive 33 may, for example,
be epoxy resin, acrylic, polyurethane or hot melt
adhesive. The cluster 30 of beads 32 so formed has
retroreflective properties as its surface is made up of a
number of glass spheres in close packed formation
throughout the cluster presenting a large number of
reflecting elements. Light entering a bead 32 is
reflected internally and re-emitted in the direction of
the source. The light returning to the source (e.g. the
vehicle) can be modified in colour by using a pigmented
adhesive 33a, 33b or 33c to bind the beads 2, as shown in
Figures 4A to 4C. The pigmented adhesive 33a, 33b, 33c
forms a coloured backing to the glass beads 32. Light
entering the glass beads 32 is subject to internal
reflection and allows some diffusion into the pigmented
adhesive 33a, 33b, 33c. By this means the light colour is
CA 02482255 2004-10-15
19
modified by the effect of the pigmented adhesive 33a, 33b,
33c and is thus modified before it returns in the
direction of the source. The adhesive material 33 may be
pigmented with white, red, yellow, green, or indeed any
strong colour, to produce a reflected colour as required.
Alternatively, the glass may itself be coloured to modify
the light, either by the chemical composition of the glass
or by a suitable coating treatment. By this means the
bead clusters 30 may be used in road markings and other
road surfacing to produce a coloured appearance as an aid
to driver safety and to provide information about road
layout and possibly hazardous situations.
The prapert,ies of the glass used to make the beads
32, such as its chemical formulation, may be varied to
achieve a greater degree of reflectivity. In particular,
glass of different refractive index, for example values of
1.5, 1.9 and 2.1, may be used, since glass beads 32
manufactured from higher refractive index glasse s return
more light and therefore improve the retroreflective
performance. Additionally, a mixture of glass beads of
different refractive indices may be used.
In order to obtain a retroreflective surface, a
plurality of retroreflective devices are applied to the
still liquid or semi-liquid surface of a road marking
material painted onto a road and become embedded in the
surface so that they are anchored in the surface with a
portion of each retroreflective device 30 protruding above
the surface of the marking, such that the exposed part of
the bead clusters 30 can become illuminated with light
from head lamps of vehicles and reflect light back to the
driver, The bead clusters 30 embedded into the surface
are firmly held by the road marking material, the surface
CA 02482255 2004-10-15
f0
structure of each cluster 30 being textured by the
presence of glass beads 32 so that the road marking
ma~.erial is absorbed into the textured surface of the
cluster 30, this keying effect increasing retention and
strength of adhesion of the bead cluster 30.
As mentioned above, the size of the cluster 30 is
usefully in the range from 2mm to 4mm diameter; however,
larger or smaller clusters 30 may be used in accordance
with the thickness of the coating for which they are
intended and the degree of embedment. Thus a road marking
paint line nominally 500~tm in thickness could use clusters
30 in the size range lmm to 2mm diameter, whereas a
thicker line such as a thermoplastic road marking
nominally 3mm in depth would require clusters 30 of 4mm to
Cmm diameter to be effective,
An alternative use of the retroreflective devices 30
would be in a road surface dressing, coloured road
2o surfaces for hazard warning, or on vertical surfaces, for
example safety barriers, road signs (vertical), etc.
These applications would require a relatively low
thickness of binder material to allo~r a large exposed area
of reflective material. Such usage requires a
particularly strong and durable binder to hold the
clusters 30 to the substrate, for example (but no
exclusively) two component materials epoxy resin, acrylic
and polyurethane.
Unlike prior art road markings whose reflectivity is
provided by individual glass beads and which therefore
lose reflectivity as the beads become damaged or are
dislodged from the surface due to the action of traffic,
CA 02482255 2004-10-15
21
retroreflective devices 30 manufactured according to
present techniques comprise an agglomeration of glass
beads 32 having a multilayer structure which enables
continuity of reflectivity by exposing a new, inner layer
of glass beads 32 after the original outer layer of beads
32 has been removed, for example by the action of road
traffic.
Rather than being applied on a surface, the
retroreflective devices 30 can also be advantageously used
a premixed additives to a road marking material, in a
quantity proportional to the thickness of the coating to
be applied, the devices becoming exposed as the road
marking material wears away.
Thus, retroreflective devices manufactured by method
embodying the present invention have a retroreflectivity
performance providing efficient retroreflection of
incident light. When used in road marking or surfacing
2o materials to increase visibility in low light or nigh-time
conditions the devices have higher durability under
traffic than the individual glass beads used in the prior
art, owing to the mufti-layering of glass beads in the
cluster and the keying effect of the surface
characteristics of the cluster 1. Larger bead clusters
are likely to give extra visibility performance in so-
called "wet night conditions", because the clusters stand
proud of the road marking line and are more visible when
there is water on the road.
Attention is directed to all papers and documents
which are filed concurrently with or previous to this
specification in connection with this application and
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CA 02482255 2004-10-15
22
which- are open to public inspection with this
specification, and the contents of all such papers and
documents are incorporated herein by reference.
All of the features disclosed in this specification
(including any accompanying claims, abstract and
drawings), and/or all of the steps of any method or
process so disclosed, may be Combined in any Combination,
except combinations where at least some of such features
2o and/or steps are mutually exclusive.
Each feature disclosed in this specification
(including any accompanying claims, abstract and drawings)
may be replaced by alternative features serving the same,
equivalent or~ similar purpose, unless expressly stated
otherwise. Thus, unless expressly stated otherwise, each
feature disclosed is one example only of a generic series
of equivalent or similar features.
The invention is not restricted to the details of the
foregoing embodiment(s). The invention extends to any
novel one, or any novel combination, of the features
disclosed in this specification (inczuding any
accompanying claims, abstract and drawings), or to any
novel one, or any novel combination, of the steps of any
method or process so disclosed.
