CA2505586C

CA2505586C - Watermarked polymeric sheet and method of making the same

Info

Publication number: CA2505586C
Application number: CA2505586A
Authority: CA
Inventors: Jonathan James Cooper
Original assignee: Arjobex Ltd
Current assignee: Arjobex Polyart Ltd
Priority date: 2005-04-28
Filing date: 2005-04-28
Publication date: 2010-08-31
Anticipated expiration: 2025-04-28
Also published as: CA2505586A1

Abstract

A method of making a watermarked polymeric sheet includes forcing a web (22) of a polymeric material, selectively irradiating portions of the web (22) with electromagnetic radiation, and stretching the web to Form a sheet (26) having increased length and/or width.
The watermarked polymeric sheet (26) has a plurality of indentations in at least one surface thereof, in areas corresponding to the irradiated portions of the web. The indentations form a watermark comprising areas of increased translucency, which is visible by transmitted light.

Description

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.
S-P550706CA.vupd 26 April 2005 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.
S-P55070f~CA.wpd 26 APn12005 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.
S-P550706CA.wpd 26 April 2005 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:
s-P550'70bCA.wpd 26 April 2005 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 S-P550706CA.wpd 26 April 2005 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 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;
S-P550706CA.wpd 26 April 2005 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.
S-P550706CA_wpd 26 April 2005 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.
S-P550706CA.wpd 26 April 2005 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, S-PSSD7D6CA.wpd 26 April 2005 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 S-P55070t5CA.wpd 26 April 2005 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:
S-P550706CA.wpd 26 Apri12005 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

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).
S-P550706CA.wpd 26 April 2UU5 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 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 S-P550706CA.wpd 26 April 2005 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 S-P550706CA.wpd 26 April 2005 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.
S-P550706CA.wpd 26 April 2005 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.
S-P55070fiCA.wpd 26 April 2005

Claims

1. 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 electromagnetic radiation, and stretching the web to form a stretched sheet having areas of increased translucency that correspond to the irradiated portions of the web, the areas of increased translucency forming a watermark that is visible by transmitted light.

2. A method according to claim 1, in which the polymeric material includes at least one polyolefin.

3. A method according to claim 2, in which the polymeric material includes polyethylene.

4. A method according to any one of the preceding claims, in which a plurality of voids are formed in the stretched sheet.

5. A method according to claim 4, in which the polymeric material includes a voiding agent.

6. A method according to any one of the preceding claims, in which at least two polymeric materials are co-extruded to form a multi-layer web having a base layer and at least one co-extruded outer layer.

7. A method according to claim 6, in which the polymeric materials include a first material containing a voiding agent that forms the base layer and a second material that includes substantially no voiding agent that forms an outer layer.

8. A method according to any one of the preceding claims, in which the energy of the radiation incident on the irradiated portions of the web is in the range 0.04 -J/mm2, preferably 0.1 - 1.6 J/mm2, more preferably 0.2 - 0.8 J/mm2.

9. A method according to any one of the preceding claims, in which the irradiating radiation is concentrated onto a spot on the web surface with an area in the range 0.05 - 5mm2, preferably 0.1 - 2.5mm2, more preferably 0.25 - 1mm2.

10. A method according to any one of the preceding claims, in which the web is irradiated using a laser.

11. A method according to any one of the preceding claims, in which the incident radiation is scanned and/or pulsed to create a pattern of irradiation on the surface of the web.

12. A method according to any one of the preceding claims, in which the web is irradiated after it has been conditioned and before the stretching operation has been completed.

13. A method according to claim 12, in which the web is irradiated substantially at the start of the stretching operation.

14. A method according to any one of the preceding claims, in which the web is stretched by a ratio of between 1:2 and 1;10, preferably approximately 1:4.

15. A method according to any one of the preceding claims, in which the web is stretched biaxially.

16. A method according to any one of the preceding claims, in which the web is simultaneously stretched biaxially.

17. A method according to any one of the preceding claims, in which the polymeric sheet is a synthetic paper.

18. A method according to any one of the preceding claims, in which the surface of the polymeric sheet is treated chemically and/or by corona discharge.

19. A method according to any one of the preceding claims, wherein the polymeric material includes a copolymer of HDPE, a rosin derived voiding agent, polystyrene, HDPE homopolymer, calcium carbonate filler, titanium dioxide, styrene butadiene and calcium oxide.

20. A watermarked polymeric sheet, comprising a stretched sheet of a polymeric material having a plurality of indentations in at least one surface thereof, the indentations comprising area of increased translucency, which form a watermark that is visible by transmitted light.

21. A sheet according to claim 20, in which the weight per unit area of the polymeric sheet is reduced in the indentations.

22. A sheet according to claim 20 or claim 21, in which the indentations have an average depth in the range 4 - 100µm, preferably 10 - 40µm.

23. A sheet according to any one of claims 20 to 22, in which the polymeric material includes at least one polyolefin.

24. A sheet according to claim 23, in which the polymeric material includes polyethylene.

25. A sheet according to any one of claims 20 to 24, in which the stretched sheet includes a plurality of voids.

26. A sheet according to claim 25, in which the polymeric material includes a voiding agent.

27. A sheet according to claim 25 or claim 26, in which the number of voids is reduced in the indentations.

28. A sheet according to any one of claims 20 to 27, in which stretched sheet has multiple co-extruded layers, including a base layer and at least one co-extruded outer layer.

29. A sheet according to claim 28, in which the base layer includes a plurality of voids and at least one co-extruded outer layer includes substantially no voids.

30. A sheet according to any one of claims 20 to 29, in which the sheet is biaxially oriented.

31. A sheet according to any one of claims 20 to 30, in which the polymeric sheet is a synthetic paper.

32. A sheet according to any one of claims 20 to 31, in which the surface of the polymeric sheet includes a coating and/or is treated chemically and/or by corona discharge.

33. A sheet according to any one of claims 20 to 32, wherein the polymeric material includes a copolymer of HDPE, a rosin derived voiding agent, polystyrene, HDPE
homopolymer, calcium carbonate filler, titanium dioxide, styrene butadiene and calcium oxide.