GB2502955A

GB2502955A - Soil resistant paper substrate with pore spaces bridged by microfibrillated cellulose

Info

Publication number: GB2502955A
Application number: GB1209516.2A
Authority: GB
Inventors: Rohan Ratnakumar; Paul Howland
Original assignee: De la Rue International Ltd
Current assignee: De la Rue International Ltd
Priority date: 2012-05-29
Filing date: 2012-05-29
Publication date: 2013-12-18
Anticipated expiration: 2032-05-29
Also published as: CN104350203A; GB201209516D0; RU2014153157A; BR112014029228A2; WO2013178986A1; EP2855772A1; KR20150024346A; US20150191036A1; EP2855772B1; GB2502955B; IN2014DN09839A; PL2855772T3

Abstract

A soil resistant paper substrate is made from a stock comprising a suspension of paper fibres and treated with microfibrillated cellulose such that the microfibrillated cellulose bridges pore spaces formed by and in between the paper fibres in at least a surface of the substrate to provide soil resistance. The microfibrillated cellulose may be produced from cotton pulp or wood pulp, and may be added to the stock or applied as a coating before or after printing. The coat weight may be 0.1 to 5 gsm, and may have a solids content of 0.1 to 2% weight for weight. The length of the fibres may be up to 100 microns, and their width may 1 to 100nm. The paper may form a security paper with an overt security feature.

Description

tM:;: INTELLECTUAL .*.. PROPERTY OFFICE Application No. 0B1209516.2 RTM Date:15 August2013 The following terms are registered trade_marks and should be read as such wherever they occur in this document: Platinum De La Rue Crane & Co

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Intellectual Properly Office is an operaling name of Ihe Patent Office www.ipo.gov.uk

A SUBSTRATE FOR SECURITY DOCUMENTS

The present invention provides a durable substrate for security documents such as banknotes, cheques, identification documents etc. In particular, the present invention relates to durable substrates that are resistant to the build-up of soil on their surfaces, thereby reducing the rate at which documents are out-sorted by sorting machines or otherwise rejected for further use as information provided on the document becomes unreadable.

Security documents such as banknotes, identification documents and other such multi-use documents are subjected to regular handling and storage in places in which soil (e.g. oils and dirt)can accumulate and be transferred to the surface of the document. Soil can eventually build up to such an extent as to render security features or security information provided thereupcn difficult to read by either human or machine scrutiny. At this point, the document must be taken out of circulation, destroyed and replaced, at a cost borne by the bearer or the bank note issuing authority.

Durable security documents are already known. Banknotes in some countries, such as Australia and Canada, are printed on polymeric substrates which have an enhanced lifespan over conventional paper-based substrates. Whilst these polymeric substrates offer improved physical durability, this comes with a number of disadvantages, such as increased initial manufacturing costs and the increased complexity in trying to incorporate certain types of security devices, which would be incorporated into paper-based substrates at the time of their manufacture, e.g. watermarks, embedded or

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windowed security elements etc. . Additionally polymer substrates have a polymer tactility, which means that such banknotes no longer have the traditional feel and sound of a banknote.

Composite paper-polymer substrates are also known in the art, and laminar substrates having paper-polymer-paper or polymer-paper-polymer structures are commercially available. These go some way to addressing the security limitations of purely polymer-based substrates, but again at very high manufacturing costs. Furthermore they can suffer particularly in humid environments where differences in hygro-expansivity between the paper and polymeric layers result in inherent weaknesses in the laminar substrate that present opportunities to the counterfeiter.

Soil-resistant traditional paper security substrates are also commercially available, such as the Platinum®-coated paper made by Dc La Rue (UK), or the AST-coated paper from Crane & Co (USA) . In these systems, synthetic polymer-based soil-resistant coatings are applied to the surface of the substrate to size and seal it against the ingress of oils and dirt encountered in circulation.

While all of these apprcaches go some way to solving the problem of providing durable security substrates, there still remains a need for documents with enhanced circulation lifetimes. In particular, there still remains a need for security substrates having improved soil resistance and physical integrity combined with reduced environmental impact including end-of-life biodegradability or facile re-pulping of production spoil.

As cellulose is one of the most commonly found natural polymers, much research has been carried cut over the years into ways of processing cellulose fibres to improve their usefulness. This lead to research into miorofibrillating cellulose. Micrcfibrillation is the process of opening up the fibre structure to increase the surface-to-volume ratio thereof. It also results in the shortening of the fibres, resulting in a fine particle size of the order of microns to tens of microns such that the microfibrils exhibit a gel-like characteristic in water with pseudo plastic and thixotropic properties. The manufacture of microfibrillated cellulose (IWO) first came about in the late 1970s. However the aforementioned properties make MFO a difficult material to handle so commercial uses of IWO have been slow to develop.

In the packaging industry, it has been found that IWO enhances the properties of a water vapour barrier of a dispersion coating made from colloidal particles of a polymer. This is described in WO-A-20l1/056130. The addition of the MFO to the dispersion coating has been shown to improve the water holding capacity and reduces the brittleness of the coating.

WO-A-20l1/078770 describes the use of IWO in a layered arrangement to provide a paper or paperboard substrate having barrier properties against liguids, vapour and gases.

To provide this the paper or paperboard substrate has a first fibre based layer, a second layer comprising IWO and a third layer comprising a polymer. The MFO layer is provided to increase the density of the fibre layer and to smooth the surface thereof, which in turn increases the smoothness and the adherability of the polymer layer which provides the known barrier to liquids/vapour. It has been found that the combination of the MFC and the polymer layers provides good oxygen barrier properties which are not provided by the use of the polymer coating by itself.

The prior art is concerned with using MFC as part of a polymer based barrier coating. Polymer based barrier coatings are not ideal for use on security documents particularly as a coating for a security paper which is to be printed on as the time taken for typical security inks to dry (oil based lithographic and intaglic inks) will be slower than on paper where the ink can be absorbed into rough paper surface.

Thus the use of MFC is known in the paper and paperboard industry for some limited applications as described above. The present invention has arisen through the surprising discovery that, when used by itself with a paper substrate, and not in conjunction with a polymer layer, it advantageously provides an unexpected level of soil resistance. In addition the use of the MFC material in this manner enable the characteristics of the paper surface to be maintained which provides the improved soil resistance without significantly impacting on the ink drying characteristics of the security paper.

The invention lies in the use of a particular form of cellulose fibre known as microfibrillated cellulose (MFC) which is incorporated into and/or applied to the surface of a paper substrate, to improve the strength of security documents, such as banknotes, made from the substrate and to reduce their uptake of soil due to day to day handling.

The invention therefore provides a soil resistant paper substrate made from a stock comprising a suspension of paper fibres and treated with microfibrillated cellulose such that the microfibrillated cellulose bridges pore spaces formed by and in between the paper fibres at at ieast a surface of the substrate to provide soil resistance.

The invention further comprises a security paper formed from the aforementioned soil resistant paper comprising an overt security feature to which a transparent micro-fibrillated cellulose based soil resistant coating or varnish is applied.

The invention additionally comprises a security document comprising the aforementioned soil resistant paper substrate wherein the document is printed before the micro-fibrillated cellulose is applied as a coating to the surface of the substrate.

The invention also comprises a method of manufacturing a soil resistant paper substrate comprising the steps of forming an intermediate paper substrate from a stock comprising suspension of paper fibres and coating the substrate with microfibrillated cellulose suoh that the miorofibrillated cellulose bridges pore spaces formed by and in between the paper fibres at at least a surface of the substrate to provide soil resistance.

The aforementioned problems are thus substantially addressed by the present invention. The microfibrillated cellulose (MEt) has the advantage that it is inherently low cost compared to polymer coatings as it is produced from a common raw material, such as wood or cotton pulp, rather than by a complex chemical synthesis process based on petrochemicals.

Because of its surprising ability to bridge the inherent surface pore structure of paper, significantly less MFC coating is required in order to obtain the same effect as an equivalent polymer coating which provides a processing benefit. This is due to the fibril nature of the MFC which enables it to bridge the pore structure of the paper. Unlike with the known polymer based soil resistant coatings, the MFC coatings of the current invention will not flow into the paper substrate when heated and therefore all of the MFC will be used to bridge the pcre structure and therefore improve the soil resistance. With the polymer coating, on the other hand, a significant volume of the coating will flow into the paper struoture and only fraction will function as a soil resistant coating on the surface.

Similarly, when MFC is incorporated into the substrate during the paper making process (rather than by means of a ooating process which requires additional processing steps), this leads to a reduction in the porosity of the substrate, which improves the resistance of the substrate to soil compared to paper made in the usual way.

Advantageously, with the MFC consisting predominantly of physically-modified cellulose, it undergoes the same biological degradation as the bulk cellulose of the substrate, and spoil can be incorporated directly back into papermaking stock by standard re-pulping processes.

Methods for producing microfibrillated cellulose are described in, for example, GB-A-2066l45, in which a liquid suspension of cellulose at high pressure is passed through an orifice to cause an explosive decompression of the suspension and the fibres contained therein. Unfortunately, the energy expenditure required to produce Ji4FC's by mechanical means is very high, requiring approximately 30000kWh/tonne of product. Alternative approaches are described in, inter alia, WO-A-2007/091942, which discloses an enzymatic process by which microfibrillation of wood pulp can be performed, resulting in a product comparable to that described in GB-A-2066145 but at drastically reduced energy expenditure. The MFC's of the present invention can be prepared from any source of cellulosic material, including wood pulp or preferably cotton fibres. Wood pulp contains 40-50% cellulose, while cotton fibres contain up to around 90% cellulose.

The following table gives some typical values for the dimensions of cellulose fibres prior to, and following, the above cited microfibrillation process: Length Width Thickness Non-0.8-1.2mm 10-20 microns 3-20 microns microfibrillated Cotton Mioroflbrillated 1-100 microns 5-10 nm 5-10 nm Cellulose The length of the fibres In the MFC may be up to 100 microns, and preferably 50 microns or less and is most preferably 10 microns or less.

The width of the fibres in the MFC may be in the range 1 to lOOnm, preferably 2 to SOnm, preferably 5 to 2Onm and most preferably approximately Snm.

The thickness of the fibres may lie in the range 2 to SCum, preferably 5 to 2Cnm and is most preferably approximately 5nm.

Note in particular the three or more orders of magnitude by which all of the dimensions of the fibres are reduced during the process.

In one embodiment of the present invention, MFC is used as a coating applied to the external surface of a paper substrate, to provide a substrate from which security documents, such as banknotes can be made, in order to increase soil resistance. Traditionally used polymer-based coatings require a coat weight of approximately 2 grammes/m2 (gsm) to provide a soil index of 15-30%. Using MFO, a similar level of soil resistance can be obtained by a significantly lower coat weight in the range of 0.1 to 5gsm, preferably 0.5 to 3gsm and is most preferably lgsm. In the present context, the soil index is defined as the ratio of the differences in the lumincsities of uncoated and coated substrates, subjected to standard soiling procedures, expressed as a percentage. The skilled practitioner will be familiar with the so-called FIRA (Furniture Industry Research Association) Soil test, referred to in WO-A- 9628610. In this test, a sample of the paper is placed at one end of a cylinder along with a reference sample placed at the opposite end and 20 felt cubes impregnated with artificial sweat and colloidal graphite. The cylinder is rotated in alternate directicns for a period of 30 minutes.

The change in reflectance of the printed samples is measured and the relative soil pickup is calculated by comparing the results of the test. In such tests, soil-resistant substrates such as Platinum-coated paper supplied by Dc La Rue (United Kingdom) and AST Paper supplied by Crane & Co (USA) achieve soil indices of between 20-30%. The MET coated paper substrate of the present invention achieves an equivalent soil index.

It is believed that the microfibrillated cellulose, with its chemically identical structure, has stronger interactions with, and is more efficient at bridging, the pores between the non-miorofibrillated fibres of the substrate than the polymeric coatings typically employed.

This means that the number of loose fibre ends and the overall surface area of the substrate is reduced, producing a concomitant reduction in soiling. The MFC becomes crystalline as it cures, which helps to seal the pores and to resist oil based soil. Furthermore, the elimination of the differences in hygroscoplclty between the coating and the substrate resolves the weaknesses in existing laminar security substrates identified above.

Paper derives its mechanical strength from hydrogen bonding between cellulose microfibrils. The MET, which is also cellulose, will also have the ability to form hydrogen bonds, not only between the nanofibrils of the MFC but with the cellulose mlcrofibrils of the paper fibres. It wIll therefore adhere well to the base paper fibres.

F4FC can be applied to a paper substrate by any known coating method such as doctor blades, dip roll coating, gravure, flexography etc. with dip coating and gravure being -10-preferred techniques. The JYIFC is typically delivered to the substrate as a suspension of fibres, preferably in an aqueous medium, the suspension having a solids content of approximately 1% weight for weight (w/w) . Suspensions with higher solids levels tend to become too pasty for application, while suspensions with significantly lower solids contents impose greater reguirements on post-coating drying apparatus. A particular advantage of using MFC as a coating on a secure paper substrate is that coatings produced from suspensions having a solids content in the range 0.1 to 2% weight for weight (w/w) are transparent and therefore do not affect the appearance of security feature incorporated into the paper substrate such as watermarks or embedded or partially embedded security threads.

In a second embodiment of the present invention, MFC is incorporated throughout the body of the paper substrate by mixing it with standard cotton fibre stock in the papermaking stage of production. In a typical example, the addition of 10% MFC to the bulk of a cotton fibre-based substrate affords a soil index according to the same test as described above of the order of 15%. The stock is preferably formed by adding microfibrillated cellulose to the suspension of paper fibres in a guantity of up to 30% by weight. The MFC is a suspension of fibres in an aqueous medium which preferably has a solids content of 0.01 to 1% w/w, and more preferably 0.05 to 0.5% w/w.

In a third embodiment of the present invention, MFC is used as a post-print varnish to further improve the circulation durability of security documents coated therewith. Printing inks are formulated to optimise their adhesion to the substrate onto which they are to be printed. -11-

As the cotton-based substrate and the MFC-based post-print varnish share identical chemistries, the adhesion between varnish, ink and substrate is also optimised. Post-print varnishes may be applied by any suitable coating technique S known to the skiiled practitioner. As with the pre-print coating, the technique should be suitable to handle coatings of a high viscosity such as a 1% w/w aqueous suspension of MFC. Preferred techniques include flexography, which can be used to deposit coat weights of approximately lgsm from a 1% w/w suspension of MFC. The fcrmulation of the MFC coating must be selected to be sufficiently transparent not to detract from the underlying print and other security features on the finished security document. A coat weight of approximately lgsm from a 1% w/w suspension of MFC would be transparent. The preferred range for the solids content of the suspension of MFC would be from 0.1 to 2% w/w, and more preferably 0.5 to 1.5% w/w.

The fibres which are present In the InItIal paper stock may be all natural fibres or a mixture of natural and synthetic fibres, or all synthetic fibres. The fibres used may be, for example, PVOH, Pclyamide, polyester, or other poly olefins.

Although the principal required benefit of using MFC in the present Invention is to provide Improved soil rosistanco, It was also found that as tho ratio of MFC usod in, or added to, the paper was increased, there was a coriseguential increase in the strength of the paper substrate, a decrease in porcslty and an Improvement in double folds tests. Double fcld tests measure the durability of paper when repeatedly folded under constant load. A Schopper double fold tester may be used to determine the -11_ number of times a paper can be folded until it breaks. The folding strength is gucted as the number of double folds until the paper breaks (at 230 and 50% RH) These improvements are illustrated by the test results given below.

The following results were obtained in hand tests on 9ogsm hand sheets formed using waterleaf paper stock and MFC. Separate batches of MFC were formed from cotton pulp and wood pulp respectively, and these were added to separate batches of the paper stock in different proportions (0, 5, 10, 15 and 20% w/w as illustrated in the tables below. The MFC was diluted to provide a gel comprising 0.15 -0.17% microfibrils in water.

Wood Pulp MFC __________ g g Ml ml MFC Vol of MFC Vol of Dry wt of MFC dry addition to add @ stock hand sheet wt/sheet rate 0.15% used 0 1.8 0 0 460 1.8 0.09 60 435 1.8 0.18 120 410 1.8 0.27 180 385 1.8 0.36 240 360 Cotton Pulp MFC __________ g g Ml ml MFC Vol of MFC Vol of Dry wt of MFC dry addition to add @ stock hand sheet wt/sheet rate 0.15% used 0 1.8 0 0 425 1.8 0.09 60 405 1.8 0.18 120 385 1.8 0.27 180 365 1.8 0.36 240 345 -13-Eight sheets of each were dried and subjected to tensile strength, double folds and porosity tests with the average results shown below.

The Bendtsen test is a standard test and we oan quote 130 5636 3 MFC% 0 5 10 15 20 Beridtsen Porosity (ml/min) 107.7 45.7 26.7 6.7 3.3 Double Folds 2091 1327 2476 2335 2895 Terisiles (KgF) 7.2 6.5 10.1 10.0 11.1 Cotton Pulp MFC MFC% 0 5 10 15 20 Bendtsen Porosity (ml/min) 120 77 35 15 6 Double Folds 996 2652 2158 3290 3267 Terisiles (KgF) 7.2 7.9 9.1 10.1 11.5 -14-

Claims

CLAIMS: - 1. A soil resistant paper substrate made from a stock comprising a suspension of paper fibres and treated with microfibrillated cellulose such that the microfibrillated cellulose bridges pore spaces formed by and in between the paper fibres at at least a surface of the substrate to provide soil resistance.
2. A soil resistant paper substrate as claimed claim 1 in which the microfibrillated cellulose is produced from cotton pulp.
3. A soil resistant paper substrate as claimed in claim 1 in which the microfibrillated cellulose is produced from
4. A soil resistant paper substrate as claimed in any one of the preceding claims in which the microfibrillated cellulose is added to the stock.
5. A soil resistant paper substrate as claimed in any one of the preceding claims in which the microfibrillated cellulose is a suspension of fibres in an agueous medium which is added to the stock, the suspension having a solids content of 0.01 to 1% weight for weight, and preferably 0.05 to 0.5% weight for weight.
6. A soil resistant paper substrate as claimed in claim 4 or claim 5 in which the microfibrillated cellulose is added to the stock in a guantity of up to 30% by weight.-15 -
7. A soil resistant paper substrate as claimed in any cne of claims 1 to 3 in which the microfibrillated cellulose is applied as a ooating to the surface of the substrate after formation of an intermediate paper substrate from the paper fibre suspension.
8. A soil resistant paper substrate as claimed in any one of claims 1 to 3 in which the micrcfibrillated cellulose is applied as a coating to the surface of the substrate after the substrate has been printed.
9. A soil resistant paper substrate as claimed in any one of claims 7 to 8 in which the coat weight of the coating lies in the range 0.1 to Sgsm, and preferably 0.5 to 3 gsm and is most preferably 1 gsm. r10. A soil resistant paper substrate as claimed in any one of claims 7 to 9 In which the micrcfibrillated cellulose is a suspension of fibres In an aqueous medium which is used to form a coating, the suspension having a solids content of 0.1 to 2% weight for weight, and preferably 0.5 to 1.5% weight for weight.11. A soil resistant paper substrate as claimed in any one of the preceding claims in which the length of the fibres in the microfibrillated cellulose is up to 100 microns, and preferably up to 50 microns and even more preferably up to microns.12. A soil resistant paper substrate as claimed in any one of the preceding claims in which the width of the fibres in the microfibrillated cellulose lies in the range 1 to lOOnm, -16 -preferably 2 to SOnm, more preferably 5 to 20nm, and is most preferably 5nm.13. A soil resistant paper substrate as claimed in any one of the preceding claims in which the thickness of the fibres in the microfibrillated cellulose lies in the range 1 to lOOnm, preferably 2 to 5Onm, preferably 5 to 2Onm, and is most preferably Snm.14. A security paper formed from the soil resistant paper as claimed in any one of claims 1 to 3 or 7 to 13 comprising an overt security feature to which a transparent microfibrillated cellulose based soil resistant coating or C') varnish is applied.15. A security document comprising the soil resistant paper substrate as claimed in any cne of claims 1 to 3, or 7, or 9 to 14 wherein the document is printed before the microfibrillated cellulose is applied as a coating to the surface of the substrate.16. A method of manufacturing a soil resistant paper substrate comprising the steps of forming an intermediate paper substrate from a stock comprising suspension of paper fibres and coating the substrate with microfibriliated cellulose such that the micrcfibriilated cellulose bridges pore spaces formed by and in between the paper fibres at at least a surface of the substrate to provide soil resistance.17. A method of manufacturing a soil resistant paper substrate as claimed in claim 16 in which the microfibrillated cellulose is produced from cotton pulp.-17 - 18. A method of manufacturing a soil resistant paper substrate as ciaimed in of claim 16 in which the rnicrofibrillated cellulose is produced from wood pulp.19. A method of manufacturing a soil resistant paper substrate as claimed in any cne of claims 16 to 18 in which the microfibriilated cellulose is applied as a coating to the surface of the substrate after the substrate has been printed.20. A method of manufacturing a soil resistant paper substrate as claimed in claim 19 in which the coat weight of the coating lies in the range 0.1 to Sgsm, preferably 0.5 to 3 gsm and Is most preferably 1 gsm.21. A method of manufacturing a soil resistant paper substrate as claimed in any one of claims 16 to 20 in which the microflbrillated cellulose is a suspension of fibres in an aqueous medium the suspension having a solids content of 0.1 to 2% weight for weight, and preferably 0.5 to 1.5% weight for weight.22. A method of manufacturing a soil resistant paper substrate comprising the step of adding microfibrillated cellulose to a stock comprising a suspension of paper fibres and forming the substrate such that the microfibrillated cellulose bridges pore spaces formed by and in between the paper fibres at at least a surface of the substrate to provide soil resistance.23. A method of manufacturing a soil resistant paper substrate as claimed In claim 22 in which the -18-miorofibrillated cellulose is added to the paper fibre suspension in a quantity of up to 30% by weight.24. A method of manufacturing a soil resistant paper substrate as ciaimed in claim 22 or claim 23 in which the rnicrofibrillated cellulose is a suspension of fibres in an aqueous medium the suspension having a solids content of 0.01 to 1% weight for weight, and preferably 0.05 to 0.5% weight for weight.25. A method of manufacturing a soil resistant paper substrate as claimed in any one of claims 16 to 24 in which the length of the fibres in the microfibrillated cellulose is up to 100 microns, and preferably up to 50 microns and even more preferably up to 10 microns.26. A method of manufacturing a soil resistant paper substrate as claimed in any cne of claims 16 to 25 in which the width of the fibres in the microfibrillated cellulose lies in the range 1 to lOOnm, preferably 2 to SOnm, preferably 5 to 2Onm, and is most preferably 5nm.27. A method of manufacturing a soil resistant paper substrate as claimed in any one of claims 16 to 26 in which the thickness of the fibres in the microfibrillated cellulose lies in the range 1 to lOOnm, preferably 2 to SCum, preferably 5 to 2Onm, and is most preferably Snm.