This invention relates to a board, in particular a writing board
comprising a base layer and a cover layer provided for erasably writing thereon.
Such boards are generally known. There are for example known white
boards on which erasable texts or drawings can be written or drawn by means of
filt-tip pens even of different colours.
These known boards are not suited for being used in the dark or if
the surrounding lighting is too weak, since in these circumstances the texts or
drawings applied thereon are not clearly visible any more.
An object of the present invention is to provide a board on which
the texts or drawings present thereon are still visible in the dusk or even in
To this end, a board according to the invention is characterized
in that a photo-luminescent layer is disposed between said base layer and said
cover layer made of a transparent material, which photo-luminescent layer comprises
photo-luminescent particles embedded in a polymeric layer. A text or drawings applied
on this board can be seen clearly in the dusk or even in the dark since said photo-luminescent
layer provides a light emitting plane behind the transparent cover layer. Moreover,
that photo-luminescence light is agreable to look at and thus offers an advantage
from an aesthetical point of view.
In a particular embodiment of the board according to the invention,
said photo-luminescent layer comprises from 30 to 95 % by weight and preferably
from 40 to 60 % by weight of said photo-luminescent particles. Even if a relatively
small amount of photo-luminescent particles is used, the photo-luminescent activity
of said photo-luminescent layer is sufficient to make the applied texts or drawings
in an agreable way visible in the dark. Moreover, a smaller amount of photo-luminescent
particles improves the mechanical properties of the photo-luminescent layer.
In a preferred embodiment of the board according to the invention
said photo-luminescent particles have a diameter comprised between 5 and 100 microns.
Such small particles offer the advantage that they allow to obtain a homogeneous
distribution of the photo-luminescent material over the photo-luminescent layer
and they have a larger specific surface resulting in a higher photo-luminescent
activity and a more agreable appearance.
In a further preferred embodiment of the board according to the invention,
said photo-luminescent layer is an extruded layer. An advantage of an extrusion
is that it allows to obtain a very homogeneous distribution of the photo-luminescent
particles into the photo-luminescent layer.
Other advantages and particularities of the invention will appear
from the following description of a board in particular a writing board according
to the invention. This description is only given by way of example and does not
limit the scope of the invention. The reference numerals relate to the annexed
drawings wherein :
- Figure 1 shows a perspectif view of a particular embodiment of a writing board
according to the invention;
- Figure 2 shows schematically a sectional view according to line II-II in Figure
- Figure 3 shows a diagramatical scheme of a method for manufacturing a board
according to the invention.
In the different figures, the same reference numerals relate to the
same or analogous elements.
Figure 1 shows a preferred embodiment of a writing board according
to the invention comprising a frame 1 applied at least around the actual writing
The frame 1 can be made of any suitable material and can have any
shape and dimensions according to its destination. In Figure 1 it shows for example
a recess 3 provided for applying a sticker and a groove 4 for carrying a pen 5
to be used for writing on the board.
As shown in Figure 2, the board 2 is composed of different superimposed
layers and comprises at least a base layer 6, a transparent cover layer 7 and a
photo-luminescent layer 8 disposed between the base layer 6 and the cover layer
The base layer 6 constitutes usually a support layer for the writing
board 2. Preferably it is a dull layer in order to avoid that too much light would
reflect on the writing board 2. The base layer 6 may be formed for example of a
film, a plate, a sheet, a woven or a non wover fabric, a felt or a wadding. The
base layer 6 may further comprise a more rigid layer which has not been shown
in the figures and which is for example a part of the frame 1.
The photo-luminescent layer 8 comprises photo-luminescent particles
embedded in a polymeric layer. These photo-luminescent particles are made of a photo-luminescent
material. A large group of such materials are suitable for that purpose and they
comprise for example zinc sulphide, selenide and telluride; cadmium sulphide, selenide
and telluride; alkaline earth sulphide, selenide and telluride and mixtures thereof.
The photo-luminescent activity of all these materials is due to the presence of
a metallic activator such as for example copper, cobalt or bismuth powders and
mixtures thereof. All of these photo-luminescent materials have a particular body
colour in the dark. Preference is given to the following photo-luminescent materials
: zink sulphide with copper activator (ZnS:Cu), zink and cadmium sulphide with
copper activator ((Zn,Cd)S;Cu), zink sulphide with copper and cobalt activator
(ZnS:Cu,Co), strontium sulphide with bismuth activator (SrS:Bi), calcium sulphide
with bismuth activator (CaS;Bi), strontium and calcium sulphide with bismuth activator
((Sr,Ca)S:Bi) and strontium and calcium sulphide with bismuth activator mixed with
zink sulphide with copper activator ((Sr,Ca)S:Bi) + (ZnS):Cu).
The photo-luminescent particles are embedded in the polymeric layer
in the form of a fine powder, the particles of which have a diameter comprised
between 2 and 200 microns and preferably between 5 and 100 microns. This can be
done by means of an extrusion. The thus obtained photo-luminescent layer 8 comprises
from 30 to 95 % by weight and preferably from 40 to 60 % by weight of said photo-luminescent
particles and has a thickness of 400 to 1200 microns. Two or more of such photo-luminescent
layers can be superimposed.
The cover layer 7 is a transparent layer and is in particular transparent
for the light emitted by the photo-luminescent layer 8. The cover layer is made
of a polymeric material. It may consist of one or several layers which have all
to be transparent. A particular characteristic of this cover layer 7 is that text
or drawings written thereon with the pen 5 can be wiped out. It's thickness is
preferably comprised between 10 and 100 microns.
The polymeric material of the photo-luminescent layer 8 as well as
of the cover layer 7 can be selected from the following polymers or copolymers
: polyethylene, polypropylene, ethylene-propylene copolymers and other olefinic
polymers and copolymers; vinyl-acetate polymers and copolymers (especially copolymers
with ethylene and/or propylene); methyl-methacrylate polymers and copolymers and
other resins coming from acrylic or methacrylic esters and nitriles; aromatic polycarbonates;
thermoplastic polyurethanes; polyethylene terephthalate, polybutylene terephthalate
and other crystalline aromatic polyesters; vinyl or vinylidene fluoride polymers
and copolymers; polyvinyl-chloride; elastomeric polymers and copolymers and in
particular ethylene-propylene elastomeric copolymers (EPM) or ethylene-propylene-diene
elastomeric terpolymers and polybutadiene; silicon rubbers and other natural or
synthetic rubbers; vinyl aromatic polymers and copolymers, like polystyrene and
especially high impact polystyrene, where the high impact properties come from
the presence of an elastomeric component, for instance polydienic units, EPM or
EPDM rubbers and so on; styrene-acrylonitrile (SAN) and styrene-butadiene-acrylonitrile
(ABS) copolymers; polyamides such as nylon 6, nylon 6,6 and so on; blends thereof.
From this series, polypropylene, polymethylacrylate, polystyrene
and high impact polystyrene are especially preferred.
The polymeric material of the photo-luminescent layer 8 may be different
from the one of the cover layer 7. Both layers 7 and 8 can be glued to each other.
If the compatibility of the layers with each other is however so that a good adhesion
between the layers can be obtained, said board can be made by means of lamination,
extrusion or co-extrusion or by means of a combination thereof.
According to the lamination method, two or more layers are applied
to each other by leading them between the hot rollers of a rotary press (calender).
According to the extrusion method, the photo-luminescent layer 8 is
extruded for example on the cover layer 7 or possibly on the base layer 6. The
co-extrusion method, on the contrary, comprises the simultaneous extrusion of several
layers which flow together into a single head wherein the stratified streams of
the different components are superimposed to form a multilayer object.
The photo-luminescent layer 8 is preferably extruded. In order to
get a product of good quality, this extrusion has to be carried out very carefully
as will be explained hereunder. In particular the following successive steps are
- pre-mixing the photo-luminescent particles with said polymeric material, both
in the powder form, the difference between the particle size of both of these powders
being smaller than 10 % of the smallest size, preferably said particle sizes are
substantially equal; this pre-mixing can be carried out for example in a rotary
drum or in a tumbling drum;
- feeding the obtained mixture to an extruder;
- extruding this mixture very smoothly, i.e. with a low shear stress, in order
to avoid crushing and/or abrasion of the photo-luminescent particles which would
thus lose their luminescence properties;
- pressing the extruding mixture in the molten state through a perforated die;
- cutting the product into pellets or granules;
- cooling them down preferably to room temperature; and
- extruding the cooled pellets or granules in the form of a photo-luminescent particles
containing layer in a second extruder.
A particular example of a method for manufacturing a writing board
according to the invention is given hereunder with reference to Figure 3.
In this example there is started from an isotactic polypropylene
powder and a powder of the photo-luminescent material ZnS:Cu (or possibly (Zn,Cd)S:Cu)
showing substantially the same particle size as the polypropylene powder. These
powders are gently mixed in a tumbling drum and fed via a hopper to a mono-screw
extruder kept at 180°C. The extruding mixture is led in the molten state through
a perforated die, the obtained product is cut into pellets or granules which are
cooled down to room temperature.
These pellets or granules are fed via a second hopper, indicated
in Figure 3 by reference 9, to a sheet co-extrusion unit 10 wherein a first extruder
11 forms the photo-luminescent layer 8.
To a second extruder 12 is fed via a third hopper 13 the same isotactic
polypropylene powder as used for the photo-luminescent layer 8. The extruded sheet
obtained by means of this second extruder 12 forms the transparent cover layer
7 and is led together with the extruded photo-luminescent layer 8 into a co-extrusion
head 14 wherein a composite layer 15 consisting of the photo-luminescent layer
8 and the cover layer 7 is formed. This composite layer 15 is led together with
an opacified polypropylene layer 6, forming the base layer 6 and coming from a
first roller 16 of a calender 17, between rollers 18, 19 and 20 and is at the same
time cooled down. The formed layer is then stretched by means of a train 21.
The thus obtained board 2 shows a wonderfull yellow afterglow persistent
in the dark after exposure to daylight even for a time of 5 hours or more. Texts
or drawings applied thereon are in clear contrast with the light-emitting photo-luminescent
A board according to the invention can have many different shapes
and can be used for various applications. It can con stitute the whole or part
of differently shaped objects, for example toys, especially toys for teaching purposes;
warning signals; direction signals; machine parts; dials or portions of a dials;
sport equipments; switches and furnishings of rooms for example of dancing rooms.
It will be clear that the invention is not limited to the above described
embodiments but that many modifications for example concerning the shape or the
number of layers can be taken into consideration without leaving the scope of the
present patent application.