Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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1
WATERMARK D POL ~;'zj3~ ~~ET
r~.l~T~3S?~? ~?.1E~11~A~~~% SAME
The present invention relates to a watermarked polymeric sheet ~.d a. method
of making
the same. In particular, but not exclusively, the invention relates to a
watermarked
synthetic paper and a method of making a watermarked synthetic paper.
In this specification, the term "watermark" mesas a mark formed. from areas of
increased
and/or decreased translucency that is visible by transmitted light and
resembles a
conventiotsal watermark in a sheet ofcellulose-based papar. Th.e terms
"watermarked" and
"watermarking" should be construed accordingly.
The term "synthetic paper" is used herein and throughout the specification to
mean plastics
film and sheet products having a feel and printability similar to cellulose
paper. It has been
recognised that plastics sheet of these types can provide an iznproved
alternative to paper
where durability and toughness are .required, Plastics sheets produced from
polyolefins
have several advantages over other plastics since they offer UV resistance,
good tear
strength and the ability to be recycled in many post-consumer waste
applications.
Synthetic papers have been produced commercially by the plastics industry for
many years
and have taken a cumber of different fornas. They have included products
having voided
(i.e. multicellular) or wwoided structures, some of which ha.vc been caatEd
with filler
and/or pigment-containing surface coatings to improve printing qualities. The
voiding
technique has frequently been used to reduce the density of the synthetic
paper produced.
A voiding agent such as zinc-calcium rrsinate is generally used, which causes
voiding
when a heated sheet of synhhetic paper is stretched. This technique produces a
very
ZS servi.cEable sheet that has gained widespread commercial acceptance.
Watermarking may be useful as a security feature, to make copying more
difficult and so
prevent forgery. This may be valuable for itarrs such as banknotes, ch~ucs,
share
certificates and identity cards, and labels tbr high value products such as
wine, perfume and
pharmaceuticals. Watermarking may also be useful for decorative purppses.
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Z
A process for making a watermarked synthetic paper product is described in EP
0655316.
Iz~. that process, a synthetic paper product is made in a conventional manner
by extruding
a film of high density polyethylene and then stretching the filin in the
machine direction
and the transverse direction to produce biaxial orientation of the polymer
molecules. Prior
to stretch.uag, the film i.s passed between a pair of rollers, ono of which
has a patterned
surface in relief or of hollows, to produce an impression on the surface of
the film. This
produces a corresponding pattern of light and dark areas in the film after
stretching, which
can be seen by transmitted light. It has been shown that the dark areas, which
correspond
to the in relief portions of the roller, are caused by an increase in
substance in those areas,
and vice versa for the light areas of the film.
Although the process described in ~P 0655316 may be used to make a watonmarked
product, we have .found that the watornnark is rather indistinct and not well
defined. The
pattern also generally has a short repeat length, depending on the
circumference of the
patterned roller, and it must therefore be relatively simple. If a different
watermark or
pattern is required, the roller must bo changed, which is a complex and time-
consuming
process. Another disadvantage is that the watermark cannot include variable
information
ar data, such as an identii;cation code, date or serial number.
It is an object of the present invention to provide a watermarked polymeric
sheet and a
method of making a watermarked polymeric sheet, that mitigates at least some
o~f the
aforesaid disadvantages.
According to the present invention there is provided a method of making a
watermarked
polymeric sheet, the method including forming a web of a polymeric material,
selectively
irradiating portions of the web with elec;tramagnetic radiation, and
stretching the web to
farm a stretched sheet having areas of increased translucency that earrespond
to the
irradiated portions ofthe web, the areas of increased translucency fonning a
watermark that
is visible by transmitted light.
The watermarked polymeric sheet hoe a plurality of indentations in areas
corresponding to
the irradiated portions of the web. The indentations comprise the areas of
increased
translucency that form the watermark.
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We have found that watermarks produced according to the present invenkiozr arc
distinct
and well defined. The pattern may have any repeat length and can be simple or
complex.
Different watemo,arks can be generated very easily without having to alter or
reconfigure
the apparatus, simply by controlling the light source. The watermark may. be
adapted
readily to include logos, pictures, text and earl abl a information such as
identification codes,
dates and serial numbers.
The polymeric material may include at least one polyolefin, which as
preferably
polyethylene.
Advantageously, a plurality of voids are formed in the stretched sheet. The
polymeric
material may include a voiding agent to help promote t>~e formation of voids.
Advantageously, at least two polymeric materials are co-extruded to form a
mufti-layer web
having s base layer and at least one outer layer. The pol.ymeuc materials may
include a
first material containing a voiding agent that forms the base layer and a
second materiel
containing substantially no voiding agent that forms the outer layer.
The energy of the radiation incident on the irradiated portions of tl;se web
tadiatioz~ zaay be
in the range 0.04 - 4 J/mm2, preferably 0.1 - 1 _6 Jlmmz, zrr~ore preferably
0_2 - 0.8 J/mmz.
The irradiating radiation may be concentrated onto a spot on the web surface
with an area
in the range 0.05 - 5mm2, preferably 0.1 - 2.Smrn2, more preferably 0.25 -
lmm2. To
achieve this spat size, the radiation may be focussed, or a narrow beaxn of
radiation may
be used, or a small light source may located in close proximity to the pl.an.e
of the web.
Preferably, the web is irradiated using a laser.
The incident radiation may be scanned and/or pulsed to create a pattern of
irradiation on
the surface of the web. The scanning and/or pulsing may be coz.~troiled, for
cxarxxple by a
comQuter, to produce different patterns, images, logos or text_
Advantageously, the web is irradiated after it has been conditioned and before
the stretching
operation has been completed. By "conditioned" we mean an operation whereby
the
temperature of the web is stabilised and made uniform across its width,
Preferably, the web
is irradiated substantially at the start of the stretching operation.
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Advantageously, the web is stretched by a ratio of between 1:2 az~d 1:10,
preferably
approximately 1:4. The web may be stretched biaxially, and preferably
simultaneously.
The polymeric sheet is preferably a synthetic paper.
The surface of the polymeric sheet may be treated chemically and/or by corona
discharge
for improved print acceptance.
Advantageously, the polymeric material includes a copolymer of HDPE, a rosin
dezived
voiding agent, polystyrene, HDPE hornopolymer, calcium Carbonate filler,
titanium
dioxide, styrene butadiene anal calciux~n. oxide.
According to another aspect of the invention there is gravided a
watarrna~z'1<ed polymeric
sheet, comprising a stretched sheet of a polymeric material. having a
plurality of
indentations in at least one surface thereof, the iz~dsntatxons comprising
areas of increased
translucency, which form a watema.ark that is visible by transmitted light.
The weight per unit area of the polymeric sheet may be reduced in the
indentations. The
indentations may have an average depth in the range 4 - 100Wm, preferably 10 -
40~m.
The polymeric material may include at least one polyolefin, which is
preferably
polyethylene. The stretched sheet may include a plurality of voids and th,e
polymeric
material may include a voiding agent. The number of voids may be reduced in
the
indentations.
Preferably, the stretched sheet has multiple co-extruded layers, including a
base layer and
at least one co-extruded outor layer. Advantageously, the base layer includes
a pl,urslity o.f
voids and at least one co-extruded outer layer includes substantially no
voids.
The sheet is preferably biaxially oriented. The polymeric sheet ma.y be a
synthetic paper.
Advantageously, the surface of the polymeric sheet includes a coating and/or
is treated
chemically and/or by corona discharge.
~5 The polymeric sheet is suitably a synthetic paper that comprises at least
one printable
surface layer and a base layer (which can also be termed the core layer if
there is more than
one surface layer e.g. one on either side of the base layer). The synthetic
paper may be
formed either:
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A. by single extrusion of a single composition in which the surfaces and the
core poitiozi
of the single extrudate rEprcsent the surface and base layers respectively, or
$, by co-extrusion of the composite from two or more compositions where the
relatively
thicker of the two layers forms the base layer and the relatively thinner of
the two layers
5 represents the suzface layer, or
C. by lamination of a plurality of layers whereby at least ozie of the
outermost layers
represents the surface layers anal the layers) below said surface layer or in
between the two
outer surface layers represents the base layer, or
D. by applying a coating of a printable layer on the surface layer of a sheet
produced by any
of the methods (A) to (C) above.
Sheet produced by co-extrusion and having the coating of a printable layer on
the surface
thereof is preferred. Metla.ods of lanriin.ation and co~extrusion are well
known in the art.
Descriptions of formulations connprising a polyolefin and methods for
producing synthetic
papers based on poIyolefins can be found in GB-A-1470372, GB-A-1492771 and GB-
A-
I 5 1490512. Further, a description of pazticularly advantageous coatings can
be found in GB-
A-2177413. 'The disclosures of all the aforementioned specifications are
included herein
by reference.
A voi.dirig agent can be used both in the surface layer and in the base layer
but is
particularly effective in the base layer.
Fillers may be used in films/sheets such as synthetic paper intended for
printing to provide
an appropriate opaque white surface. These fillers are usually selected from
inert minerals
such as chalk, siLi.ca or clay. In addition, minor additives may be used to
render the
film/sheet anti-static and/or to lower its density.
It is well recogziised that polyolefin filins have low surface energies and
this generally
means that prinring is difficult because the ink does not readily wet the
surface and the
dried xnk does not adhere sufficiently to the surface thereof. In order to
overcozrie these
problems, the surfa.cc of polyolefin films/sheet may be subjected to various
treatments such
as e.g. a corona discharge treatment. Such treatments improve ii~il. laydown
and adhesion
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sufficiently to provide a useful material. The material so treated may, in
some cases, lack
absorption and require specialised printing techniques.
The lack of absorption of films/sheets such as synthetic paper can be overcome
by applying
a coating comprising a major amount of a absorbent filler and a minor amount
of. an
adhesive binder. The coating can be incorporated during the manufacturing
process. Such
a method yields a product that is receptive to print and such produ.ets have
gained wide
commercial acceptance. Where such coatings are inconvenient and expensive to
apply and
require a sEparate manufacturing process, or render the surface so treated
susceptible to the
adverse efFects of water and solvents, a higher amount of a filler such as
silica can be
ezup.loyed.
The base layer in the film or sheet of the synthetic paper znay also include
other
components such a5 pigments, other fillers, rubbers and the like. Thus, the
base 1$yer may
be of any a>mposition such as are described in GrB-A~1470372 and GB-A-1492771.
In a
preferred embodiment, the composition of the base layer is as follows:
T Component Parts by weight
5
High deztsity polyethylene (copolymer) 100
Calcium-zinc resinate 5-15
Polystyrene 4.5-S.5
High density polyethylene (homopolymer) 17.5-21
Calcium carbonate filler 15-25
Titanium dioxide 5-10
Styrene-butadiene copoly~nor 0-1.0
Calciuzzx oxide 0.4-1.0
Embodiments of the invention will now be described by way of example, with
reference
to the accompanying drawings, in which:
Figure 1 is a. schematic sectional side view of an apparatus for manufacturing
a
watermarked product;
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Figure 2 is a schematic plan view of a web passing throw. a stretching machine
in the
apparatus shown in Figure 1;
Figure ~ is an image of a first watermarked product, viewed by transmitted
light;
Figures 4 and 5 are magnified images of the first watermarked product, viewed
by reflected
S and transrni.tted light respectively;
Figure 6 is a graphical representation of the pmfile of the first watermark
pattern;
Figure 7 is an image of a second watermarked product, viewed by transmitted
light;
Figures 8 and 9 are magnified images of the second watermarked product, viewed
by
reflected and transmitted light respectively;
Figure 10 is a plan view showing pan of a modified apparatus for manufacturing
a
waterzn.arked product, and
Figure 11 is a magnified image of a third watermarked p~coduct, viewed by
transmitted
light.
An apparatus for making wat~xmarked synthetic paper is shown in figures 1 and
2. The
apparatus includes an extrusion apparatus 2, a first set of. conditioning
rol)ers 4, a
simultaneous biaxial stretching machine 6 that is xta.ounted in an oven 8, a
light source 10,
a second set of conditioning rollers 12 and a take up reel 14.
The extrusion apparatus 2 may for example be a conventional three-layer co-
extrusion
apparatus comprising first and second extruders (only the first extruder 16
being shown),
a distributor I 8 and a sheeting die 20. The ca-extrusion 3pp~urt~tus enables
athree-layer co-
extruded polym.eri c extrudate comprising a core layer and two surface layers
to be produced
continuously.
The first conditioning rollers 4 are mounted immediately downstream of the
extrusion die
20. The polymeric material is exi~ruded into the nip between a first pair of
cooled rollers
24, which cool and solidify the extntdate 22 to fown a web 22. The web 22 then
passes
around the other conditioning rollers 4, which. condition the web prior to
deliveri.n.g it to the
stretching machine 6, to ensure that the temperature ofthe web is stable and
uniforrr~ across
its width.
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The stretching machine 6 stretches the conditioned web biaxi.ally: i.e. in the
longitudinal
or machine direction (MD) and the transverse direction (TD). The web 22 is
thus converted
into a stretched sheet 26. The stretching machine 6, which is not shown in
detail, may be
of a conventional type, for example as described in GB 1442113, the content of
which is
incorporated by reference herein. Briefly, the stretching machine 6 includes
two endless
articulated chains that are driven along predetermined paths, the links of the
chains being
caused to expand and contract as the chains rotate. A series of gripping
devices attached
to the chain links successively engage the edges of the web 22 as it enters
the stretching
machine 6 and move apart from one another in the longitudinal and transverse
directions
as the web advances, thereby stretching the web 22 simultaneously in both
directions.
After completxag the stretching operation, the gripping deices release the
stretched sheet
26 and rotate back to the start of the stretching machine. The stretching
machine 6 is
n,oun.ted irz a circulating air oven 8, which is used to control the
temperature of the web
throughout the stretching process.
The stretching operation consists of. three distinct stages, the oven 8 being
divided into
three zones to maintain the web 22 and the stretched sheet 26 at the correct
temperatures
during each of those stages. In the first stage, the clamped web 22 is heated
to the correct
temperature for stretching in the first (pre-heat) zone Sa of the oven. The
web 22 is not
stretched during this first stage but it is clasnped by the gripping devices
to maintain it in
a flat condition. The pre-heated web 22 is then stretched as it passes through
the second
(stretch) zone 8b of the oven, which controls the temperature of the web
during the
stretching operation. Finally, in the third stage of the stretching operation,
the stretched
sheet 26 is annealed by being held in the stretched condition as it passes
through the third
(annealing) gone 8c of the oven, before being released by the gripping
devices.
Downstream of the stretching machine 6, the second set of conditioning rollers
12 is
provided to condition the stretched sheet 26 as it emerges from the oven 8.
The conditioned
sheet is then wound onto the take-up reel 14. Optionally, the apparatus may
include an air
blower for cooling the sheet and edge trimmers for removing waste material
from the side
edges of the sheet. A corona discharge machine may also be provided for
trcati.ng the
surface of the sheet 26 to make it more receptive to print.
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A light source 10 is mounted above the stretching machine 6 and arranged to
direct a beam
of radiation 28 through a window in the oven 8 onto the web 22. The beam of
radiation 28
is focussed onto a spot 30 on. th,e surface of the web 22 approximately at the
point where
stretching of the web commences: i.e. at the tTansi.txowfrom th.e pre-heat
zone 8a to the
stretching zone 8b of the ova. The light source 10 is preferably a medium or
high power
laser, for example a COZ laser with an output power of SOW to 2kW.
In use, a polymeric substance, for example polyethylene or polypropylene, is
extruded
through the die 20 to form a layer of extrudate, typically with a width of
about 400znm and
a thickness of Smm. The extrudate may have a layered composition, consisting
of a base
layer and two co-extruded outer layers_ 1.n a preferred embodirrient, the base
layer and the
outer layers are both made primarily of polyethylene, the base layer (but not
the outer
layers) also including a voiding agent.
Iznmedi.ately after lea~itng the extru.Rion die, the axtrudste passes between
the cooled rollers
24, which cool and solidify the extrudate to form the v~reb 22, which
typically has a
thickness of about 1.5mm. In the case of a layered web, the base layer
typically a thickness
of 1.32mm and the two outer layers are each about 0.9mm thick. The web 22 then
passes
around the other conditioning rollers 4 to prepare it for strctchins.
The conditioned web 22 is delivered to the stretching machine 6, where it is
stretched as
described above to form a stretched sheet 26. The stretching ratios in the
longitudinal and
transverse directions may be different but usually they arc similar.
Generally, the
dimensions of the web are increased in both directions by a ratio of bEtween
1:2 and 1:10,
a ratio of 1:4 being typical. The overall thickness of the sheet is at the
same time reduced,
typically to approximately 100~m. The stretching operation produces biaxial
orientation
of the polymer molecules and causes microscopic voids to be formed in the base
layer of
the sheet. This reduces the density of tlae sheet and increases its rigidity,
making it suitable
for use as a synthetic paper.
The laser 10 is mounted to irradiate the web approximately at the point where
the stretching
process begins: i.e. at the transition between the first and second zones of
the oven. The
surface of the web is heated by the laser beam, producing a small localised
increase in
temperature (for example of about 2°C)_ This results in differential
stretching of the web,
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the slightly warmer irradiated regions stretching mare readily than the
remainder of the
web.
After the leaving the stretching machine, the stretched sheet passes around
the second set
of conditioning rollers 12 and is then wound onto the take-up reel 14.
Optionally, the sheet
5 may be cooled by an air blower and waste material. may be removed from th.e
side edges
of the sheet by edge trimmers. The surface of. the sheet may be treated with a
corona
discharge machine.
The sheet may subsequently be coated to increase its ability to be printed.
Many coating
substances commonly used in the paper industry may be used, including aqueous
coatings,
10 latex-based coatings and in particular coatings of the fiype described in
GB A-2177413, the
content of which is incorporated by reference herein. It rnay th.er~ be
pri,x~ted. Az~. adhesive
coating such as a pressure-sensitive or heat-sensitive coating may
altero,atively or
additionally be applied to one of the sheet surfaces, allowing it to be
converted into self
adhesive labels or tags.
The finished sheet carries markings that are visible by transmitted light in
regions of the
sheet, corresponding to the areas of the web that were in:adiated by the laser
prior to
stretching. These markings consist of shal)ow indentations in the surface
ofthe sheet. The
amount of material per unit area is less iaa th.e indentations than in the
zest of the shEet and,
in addition, less voiding of the base layer is found in the indentations. We
believe that
these effects both result from the increased temperature caused by the
incident radiation,
which allows the web to stretch more readily. Botla of these factors affect
the translucency
of the sheet, the translucency being greater in the irradiated regions where
the sheet is
thinner and has fewer voids. The process of irradiation followed by
st~retck~.ing therefore
produces a watermark that is visible by transmitted light.
The laser may be pulsed or continuous and may be either fixed or moveable,
allowing it to
irradiate different parts of the web as the web passes through the stretching
maclxi.ne.
Alternatively, scanning equipment may be providod tv direct the laser beam
onto different
parts of the web.
By pulsing the laser it is possible to produce a watermark comprising a line
of dots running
3U along the length of the sheet, the distance between the dots being
dependent on the pulse
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frequency of the laser and the line speed of tb.c machine. if the point at
which the laser
bEam hits the web is moved during operation, a. different pattern may be
produced: for
example, by moving the point back and forth in the transverse direction an
oscillating wave
or snake-like pattern cax~ be produced. A. more complicated pattern or image
can be
produced by pulsing the laser beam and scanning it across the width of the
sheet to build
up a pattern or image, in a manner similar to that employed in a laser
printer. The scanning
equipment can be digitally controlled, for example by a computer, allowing a
variety of
images to be generated and/or allo~u'ving variable information such as a
serial number or date
to be ixacorporated into the watermark.
Examples of some watermarked products and the processes used to make those
products
are provided below.
Example 1
The following composition was used to produce Comgound A (used in the
production of
the base material):
Component Parts
by
Wt
Rigidex TM 002/55 HDPE copolymer (MFI 0.2g/10 min 100
& density 0.955
K m3 ex BP Chemicals Ltd
Rigidex TM HD60?OEA HDPE (Ml~I 7.5g/10 min & density17.6
0.96 Kg/m3,
ex BP Chemicals Ltd
Pol s ene Grade HF888 ex HP Chemicals Ltd 4.8
DERTOLINE TM MP 170 6,0
Cariflex TM TR1102 Styrene-butadiene-styrene copolymer0.6
(ex Shell UK
Ltd
Anh d.CaC03 2 5 article size, OMYA ex Craxton & 21.0
G
Ti02 Rutile RCR2 ex Tioxide 5.8
Armostat TM 400 antistat ex Akzo Chemicals Ltd 0.14
Armostat TM 375D antistat ex Akzo Chemicals Ltd 0.35
Caloxal TM CPA Ca0 ex Stun a Lifford Ltd 0.58
Calcium Stearate ex TtT~ Chemicals Ltd 0_04
1r anox TM $215 antiox. ex Ciba-Gei Ind Ltd 0.29
HDPE ~ High density polyethylene
MFI = Melt flow index
Compound A was prepared from the above components as follows:
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Separate, meltblended, cooled and diced masterbatches (A 1 and B) were
obtained from the
above with the calcium carbonate and titanium dioxide respectively and
cornpri.sed:
_..__. --._.. _- ....
A1 B _ _
__
Calcium carbonate 60%w/w Titanium dioxide 60%w/w
5Ri idcx TM HD6070EA 39.6% Ri idex TM HD6070EA 39.6% w/w
w/w
Armostat TM 400 0.4%w/w Calcium Stearate 0.4%w/w
Masterbatches A1 and B were than intermixed in appropriate proportions with
the
remainder of the ingredients of the composition and fed to a compounding
extruder. The
co.rn~position was zn.eltbl.ended at approximately 200°C, extruded,
cooled and diced. to form
Compound A.
Compound A was fed to an in-line extruder of a twin extruder-distributor-
sheeting di.e co-
extrusion arrangement and Compound B was mixed at 20% with Rigidex TM H..D
002/55
HDPE and fed to a secondary extruder. The sheeting die and distributor were of
conventional type enabling a three-layer co-extrudate to be produced
continuously
comprising a layer of Compound B on each side of a layer of Compound A.
The extruders were arranged to enable each to forth az~d feed a substantially
homogencou.s
melt into the distributor, which was maintained at a temperature of
210°C. The die lips
were adj usted to approximately Smm and the flow of each of the melts was
adjusted to ,give
a composite layered extzudate about 395mm wide at an extrusion rate of 360
kglhr.
The composite extrudate was then fed directly onto anal around a set of
cooling a:nd
conditioning rollers running at a peripheral speed whereby the core material
was brought
to a temperature of approximately 122°C and the outer layers each to a
temperature of
approximately 118°C. This resulted in a conditioned composite web
having an overall
thickness of l.Smm, comprising a core thickness of 1.32mm and two outer layers
each
about 0.09mm thick.
The conditioned composite web was then fed into a simultaneous biaxiai
stretching
machine arranged to provide a 4 : 1 stretch in each of the longitudinal or
machine direction
(MD) aad the transverse direction (TD).
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The stretching apparatus was provided with a cltree zone circulating air oven,
tho zonos
comprising preheat Zone 1, stretching Zone 2 and annealing zone 3 _ The
temperatures and
lengths of the respective zones and the sheet speed are tabulated. below:
Zones Temperature ~ Length (metres)Speed in Speed out
'C ~ metres/min metres/min
5Zone 120 1 10_4
1
Zone 12Q 1.5 X13.8
2 - _ _ _
Zone 140 2 43.8
3
The web gripping devices were initially at a pitch of about 38mm and were
hoatcd to
approximately 10(?°C prior to contacting the web.
The web was irradiated with a 50W COZ laser, arranged perpendicular to the web
and
focussed to produce a spot with a diameter of 0_8~nm on the surface of the
web,
approximately at the transition between th.e first and second zones ofthe oven
(i.e. just prior
to stretching). The laser was pulsed at a frequency of 160Hz, with an on time
of 3.75ms
and an offtime of 2.Sms. Each pulse therefore had an energy of about 0.2J and
produced
an energy density on the surface of the web of about 0.4J/mm2, which raised
the
temperature of the irradiated portion of the web by about 2°C.
The composite plastics sheet thus produced had an average thickness of 0.094mm
aid
notn_inal substance of 75 gsm. This sheet was cooled, edge trimmed and then
reeled.
The watermark produced by the above xnothod is shown in. figures 3, 4 and 5.
As can be
seen in figure 3, the watermark cvnsi.sts of a line of dots runni.zxg in the
machine direction,
the dots having a greater translucency than the surrownding areas of the
sheet. Each dot
consists of an oval indentation in the surface of the sheet, having a width of
about 3.3mm
and an average depth of about 24Wm. The profiles of a number of the
indentations are
shown in figure 6: it can be seen that the profiles are of a fairly consistent
width and depth.
Magnified views of the indentations are provided in figures 4 (by reflected
light) and 5 (by
transmitted light).
Example 2
A composite co-extruded sheet was made using the same process and with the
same
composition as in Example 1 _ In this case, however, the laser was pulsed at a
frequency of
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14
5001-Iz, with an on time of l.6ms and an off time of 0.4ms. The angle of the
laser was
adjusted during operation, to cause lateral movement of the dot over the
surface of the web
(in the transverse direction).
The watermark produced by this process is shown in hgures 7, 8 and 9. As can
be seen in
figure 7, the watermark consists of a wavy line o.f. dots nunnin:g xz~ the
machine direction.
The dots are closer together than in Example 1 and consist of elongate
i.ndez~.tation,s in the
surface of the web, having a width of about 3mm and an average depth of about
l8p,m.
Magnified views of the indentations are provided in figures $ (by reflected
light) and 9 (by
transmitted light).
Example 3
A composite eo-extruded sheet was made using the same process and with the
same
composition as in Example 1. 1n this case, however, the laser 10 was mounted
on s frame
40 above the oven and the laser beam 28 was directed onto the web using a
scanner unit
42. The layout of the optical components was as shown in figure 10.
1 S The laser 10 was mounted so that the laser beam 28 emerged in a direction
parallel to the
longitudinal axis of the oven. The beam was passed through a beam expander 44
and then
reflected through 90° by a mirror 46 into the scanner unit 42, which
was mounted above
an access window 48 in the top wall of the oven. The scanner unit 42 was
arranSed to scan
the laser beam in a transverse direction: i.e. perpendicular to the direction
of travel of the
web through the oven. The arrangement allowed for beam control, scanning and
focussing
of the beam onto the moving web. The beam expander 44 was adjusted to provide
a spot
size of 0.3-0.4mm diameter on the surf~ac:e of the web. Other features of the
apparatus ware
as follows:
Laser: Rofin SC x30 CO,, laser
Wavelength: 10.6 N.m
Power: 10-300W (attenuated to ~ 80% of the output power at the workpiecc)
Pulse length: 5-400 ~.s
Peak power: 220-750W
Repeat rate: 0-62.5 kl-lz
Scannca: GSl Lumonics
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The image definition was controlled by changing the laser repetition rate,
pulse width.,
scanned width and laser spot size. These parameters were found to change the
i.m.age size
and opacity with variations in laser pulse overlap and pulse power density
adjusted to
optimise the marking process.
5 The laser was used to produce patterned "watcnnarks" in the shape of the
Euro symbol °~,
with a maximum repetition rate of 62.SkHz and minimum pulse length of S~sec.
These
were the typical conditions used, with the "watermarking" quality determined
by viewing
the web opacity using a light source behind the moving web on the production
line. An
example of a laser waterrnark seen by transxnitked light i.s shown i.n figure
1.1.
10 Although the exact mechanism of the process that creates watermarks is not
entirely certain
at present, we believe that the slight increase in the temperature of the web
surface
produced by the incident radiation increases the elasticity of the web,
allowing the
irradiated portions to stretch more readily fhan th.e remainder of the web.
This produces a
slight decrease in the thickness and the amount of material in the irradiated
regions of the
15 sheet, resulting in an increased translucency. The increased elasticity
also appears to cause
reduced voiding in the irradiated portions of the sheet, which contributes to
the increased
translucency of the sheet in the affected regions.
The exact point of irradiation is not critical and may be slightly ahead of or
behind the point
where stretching corrunences, providing that it is not so far ahead that any
heating of the
surface of the web produced by the incident: radiation has been dissipated
prior to the
commencement of the stretching, or so tar behind that the stretching operation
has akesdy
been substantially completed. Ideally, the irradiation point should be as
close as possible
to the start of the stretching operation.
The laser may be continuous or pulsed and scanning apparatus may be provided
to move
the point at which the radiation strikes the web, in the transverse andJor
longitudinal
directions. Pulsing and/or scanning of the laser ma.y be controlled, for
example by a
computer, to create watermarks containing images, logos or text or variable
data. Different
light sources may be used, providing they are sufficiently powerful to heat
the surface of
the web reasonably quickly and can be focussed onto a sufficiently small spot
to provide
good definition. The radiation may be of visible or infrared wavelengths.
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16
The web m.~y be stretched biaxially or i.n only one direction. In the case of
biaxial
stretching, this is preferably simultaneous, although sequential stretching
operations are
also possible. In this latter case, the web may be irradiated before either or
both of the
separate stretching operations.
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