This invention relates to security paper.
Security documents frequently carry variable information
applied by means of a toner printer, most usually a laser printer. The variable
information can be, for example, a monetary amount, or the name of a payee, account
holder or document owner. Bank cheques, social security or welfare cheques, travellers
cheques, money orders, postal orders, bankers drafts, share certificates and other
certificates, identity documents, registration documents, driving licences, vehicle
road tax licences and other licences or permits, savings or bank account passbooks,
travel tickets, vouchers and shipping and other transport documents are examples
of such security documents.
Good toner adhesion is particularly important when toner
printers are used to fill out details of security documents such as cheques. If
the toner is not firmly bound to the paper, there is a risk that toner-printed characters
can be removed from the security document using a scalpel or other means and then
replaced by other characters. Good toner adhesion is also important if the printed
characters are not to become dislodged or abraded on handling or folding of the
The need for good toner adhesion is not of course confined
to the security paper field, and various paper treatments have been proposed for
enhancing toner adhesion for general applications. For example, US patent No. 5017416
and International (PCT) Patent Publication No. WO 90/13064 each disclose latex treatment
of papers to be printed by an ion deposition process (a type of toner printing in
which the toner is bonded to the paper by pressure only, i.e. without the more usual
thermal fusion bonding which is a feature of laser printing and most xerographic
copying processes). WO 90/13064 expressly refers to potential benefits in relation
to prevention of intentional defacement or alteration of printed information.
European Patent Application No. 507998A discloses the use
of a low coatweight (3 gm-2 or less) coating of silica and a binder for
improving toner adhesion. This proposal has the drawback of increasing raw material
and process costs, particularly if both surfaces of the paper are to be coated.
International (PCT) Patent Publication No. WO 96/30811
discloses the use of a thin uniform coating of a thermoplastic primer compound applied
to at least parts of one side of a security document. The thermoplastic material
fuses with toner applied by a hot fusion toner-printing method and so makes fraudulent
alteration more difficult.
The use of latex or silica coatings can be effective in
creating a better bond between the toner and the paper, but they are not necessarily
effective to defeat the most skilful attempts at fraudulent alteration. Furthermore
the fact that a fraudulent alteration has been made will not necessarily be readily
apparent, because the bond between the coating and the paper to which it is applied
may be strong enough to permit toner to be removed whilst leaving the underlying
fibrous layer intact.
There is therefore a need for a security paper which not
only makes fraudulent removal of toner-printed characters difficult to achieve but
also reveals or makes evident that an alteration has been made or attempted, and
which does not require costly additional coating operations.
We have now found that the key to this is appropriate manipulation
of the internal cohesion of the base paper so as to lower it to a level at which
attempts to remove toner from the paper result in visually evident disruption of
the cellulose fibres in the sheet (for example, lifting up of fibres, and disruption
of watermark features, when present). This critical cohesion.level is quantifiable
as a maximum Scott bond strength of 150 J m-2. Additionally, the inherent
toner adhesion must be increased by (a) ensuring that the paper has a sufficiently
smooth surface i.e. a Bendtsen roughness not more than 150 ml min-1 and
(b) by treatment of the paper with a thermally-soft.enable polymer composition.
A size .press is advantageously used for carrying out this treatment, since it avoids
the need for special coating equipment and operations, but coating, spraying or
other treatment methods can be used instead if desired.
Accordingly, the present invention provides, in a first
aspect, security paper adapted to resist fraudulent alteration of toner-printed
information thereon and to make such alteration or attempts at alteration evident
- (a) has a maximum Scott-Bond strength of 150 J m-2;
- (b) has a Bendtsen roughness of not more than about 150 ml min-1;
- (c) has been treated, in particular size-pressed, with a thermally-softenable
polymer composition, derived from a latex or a colloidal dispersion, comprising
an acrylic polymer or copolymer, a polyvinyl acetate polymer, a vinyl acetate/ethylene
copolymer, or a vinyl acetate/vinyl chloride/ethylene terpolymer.
In a second aspect, the present invention resides in a
security document produced using security paper according to the first aspect of
In a third aspect, the present invention resides in the
use of a security paper according to the first aspect of the invention in the production
of a security document for the purpose of making toner-printed information subsequently
applied to the security document more resistant to fraudulent alteration and of
making such alteration or attempts at alteration evident.
The measurement of Scott-Bond strength is described in
Routine Test Method No. 39, Issue No. 1 November 1998 of The Paper
Federation of Great Britain (Rivenhall Road, Swindon, SN5
7BD, United Kingdom). Preferably the Scott-Bond strength of the present paper is
not more than 140 J m-2, even more preferably not more than 130 J m-2.
A paper with the desired Scott-Bond value can be obtained
by suitable adjustment of the filler content of the paper (the higher the filler
content, the lower the internal bond strength). As a rough guide, filler content
of the order of 15 to 20% is likely to give the desired Scott-Bond strength in a
paper which is otherwise similar to a conventional cheque paper.
Although the present security paper can have a Bendtsen
roughness of up to 150 ml min-1, a Bendtsen value in the range 60 to
100 ml min-1 is preferred. This is readily attainable by standard papermaking
techniques, for example by wet pressing and/or calendering.
In addition to being resistant to attempts at fraudulent
alteration, most security papers have to comply with well established tensile and
tear strength specifications. It is important that manipulation or adjustment of
the Scott-Bond strength as described above should not compromise compliance with
such specifications. Thus it may be necessary to compensate for the effect of increased
filler content on the tensile and tear strength of the paper by, for example, increasing
the proportion of softwood in the furnish and by increasing the level of refining
of the furnish. For example, for bank cheque paper which has to meet UK Clearing
Bank Specification No. 1, the proportion of softwood in the furnish may have to
be raised from a level of about 10% to about 20% w/w (based
on dry fibre content only, in both cases).
The thermally softenable polymer composition is typically
a latex or a colloidal dispersion. Sytrene-acrylic copolymers are preferred, but
alternative possibilities include other acrylic polymers or copolymers, polyvinyl
acetate polymers, vinyl acetate/ethylene copolymers, or vinyl acetate/vinyl chloride/ethylene
polymers, all in latex or colloidal dispersion form. It should be noted that the
chemical composition of the latex has a marked influence on the degree of toner
adhesion enhancement obtained.
The thermally softenable polymer is preferably incorporated
in a conventional starch surface sizing composition. In the case of a styrene-acrylic
copolymer material, the ratio of starch: styrene-acrylic copolymer is preferably
about 90:10 on a dry weight basis. Smaller proportions of styrene-acrylic copolymer
can be used, down to a minimum of about 5% (i.e. a starch : copolymer ratio of 95:5).
Amounts above 10% styrene-acrylic copolymer give increased toner adhesion, up to
a limit of about 15%, beyond which no further increase is observed. The increased
toner adhesion obtained has to be balanced against the increased raw materials cost
When a starch : styrene-acrylic copolymer size press mixture
as described above is used, the solids content should typically be in the range
of from about 4-12%, preferably 8-9%. For a typical base paper of, say, 90 gm-2
basis weight before sizing, the target wet pick up should be ca. 50-60%, or ca.
5-6% on a dry basis, giving a total dry pick up of ca. 4-5 gm-2, i.e.
ca.2-2.5 gm-2 per side, and a final dry paper of ca. 95 gm-2
The guidelines given above in relation to the amount and
mode of application of styrene-acrylic copolymer apply also to latexes or colloidal
dispersions of the alternative polymers or copolymers referred to above.
The invention will now be illustrated by the following
Examples and a Comparative Example (control), in which all parts and percentages
are by weight unless otherwise stated, and an asterisk indicates a proprietary trade.mark.
A white woodfree cheque base paper was made on a full-size
Fourdrinier paper machine at 100 m min-1, without any wet end starch
addition. The furnish was 20% softwood fibre, 50% hardwood fiber (eucalyptus) and
30% broke, together with kaolin filler. The paper was surface sized at the size
press with a composition comprising (on a dry basis) about 90% starch ("Amylofax"
00 supplied by Avebe UK Ltd of Ulceby, North Lincolnshire, United Kingdom) and 10%
styrene-acrylic, copolymer ("Dow* DSP 70" supplied as a colloidal dispersion at
15% solids content by Dow Europe S.A., Horgen, Switzerland). Small amounts of sensitising
security chemicals as usually used in cheque base paper production were also present.
The size press pick-up was about 5-6% in total on a dry mass basis. The final paper
had a basis weight of ca. 95 gm-2, a Bendtsen roughness of 100 ml min-1,
a Scott-Bond strength of 128 J m-2 and a filler content (ash content)
of 17.8%. The double-sided tape used in the Scott-Bond value determinations was
a general purpose grade supplied by Viking Direct, Leicester, UK (catalogue reference
G22-DS 10325), and the determinations were carried out in a 50% relative humidity/23°C
An attempt was made to measure toner adhesion using an
IGT AIC 2/5 Tester, in accordance with IGT Toner Adhesion Test Method EN 12283 (Version:
Concept 4, November 1999), after samples had been printed using a Hewlett Packard
Laserjet 4 Plus Desktop laser printer. However, the sample delaminated i.e. it failed
within the paper, rather than at the toner/paper interface, and so no toner adhesion
value could be obtained.
Attempts at removing the toner using a scalpel blade resulted
in obvious damage to the paper surface.
The procedure of Example 1 was repeated except that the
filler content was 18.4%, and the softwood and hardwood contents were 10% and 60%
Bendtsen roughness was 70 ml min-1 and the Scott-Bond
strength was 113 J m-2. The IGT toner adhesion test showed delamination
and the scalpel test resulted in obvious surface damage.
The procedure of Example 1 was repeated, except that the
filler content was 15.4%, and the softwood and hardwood contents were 30% and 40%,
The Scott-Bond strength was 107 J m-2 and Bendtsen
roughness was about 80 ml min-1. The IGT toner adhesion and scalpel test
results were as in Example 2.
The procedure of Example 1 was repeated except that the
filler content was reduced to 6% and softwood and hardwood fibre contents were 10%
and 60%, respectively.
In the IGT toner adhesion test, samples (printed at the
same time as in Example 1) did not delaminate and had a measurable toner adhesion
of 84%. Scalpel tests showed that while difficult to remove, the paper surface was
only slightly damaged.
The key data obtained from Examples 1 to 3 and the Comparative
Example are summarised in Table A below:
Softwood Content (%)
Scott-Bond Strength (J m-2)
3000 sheets from each of the above Examples and the Comparative
Example were subjected to offset printing using a Heidelberg 2 colour press. All
gave good results.
This illustrates the use of a range of different thermally
Two sets of rosin-sized handsheets were made, at a target
grammage of 85gm-2, from a mixed 70% hardwood (eucalyptus) 30% softwood
furnish. The sets had target kaolin filler contents of 3% and 20% respectively,
and target Cobb sizing values of 25ml m-2min-1.
Four surface sizing compositions were made up at a solids
content of 10%. Each contained 90 parts starch and 10 parts thermally softenable
polymer (on a dry weight basis). The starch was "Cerestar*5590", supplied by Cerestar
UK Limited, Trafford Park, Manchester, and the polymers were as follows:
- (a) polyvinyl acetate ("Vinamul*8481," supplied as a 54-56% solids content emulsion
by Vinyl Products Limited of Carshalton, Surrey, UK);
- (b) vinyl chloride/vinyl acetate/ethylene terpolymer ("Vinamul*3525", supplied
as a 50% solids content emulsion by Vinyl Products Limited);
- (c) styrene-acrylic copolymer ("Baysynthol* BMP", supplied as a 26% solids content
emulsion by Bayer A.G.); and
- (d) styrene-acrylic copolymer ("Dow*DSP 70", as used in Examples 1 to 3).
Additionally, a control composition containing starch but
no thermally softenable polymer was prepared at 10% solids content.
Each of the above compositions was applied to both sets
of the handsheets described above, using a laboratory size press. The dry pick-up
was about 5 gm-2, so that the treated handsheets had a grammage of about
90 gm-2. The treated handsheets were then calendered to a target Bendtsen
roughness value well below 150ml min-1.
Scott-Bond strength, Bendtsen roughness, and, where possible,
toner adhesion values were then determined for each of the handsheets, and the results
obtained are set out in Tables B1 and B2 below for the 3% and 20% kaolin content
papers respectively using the same type of tape as in Example 1.
Table B1 - 3% Kaolin Content
Bendtsen Roughness (ml min-1)
Scott-Bond Strength (J m-2)
Toner adhesion (%)
Table B2 - 20% Kaolin Content
Bendtsen Roughness (ml min-1)
Scott-Bond Strength (Jm-2)
Toner adhesion (%)
Borderline (see below)
It was not possible to obtain numerical values for toner
adhesion for the 20% kaolin content paper because delamination occurred.
It will be seen that the sheets which had a Scott-Bond
strength above 150 J m-2, i.e the sheets with 3% kaolin content, did
not reveal evidence of tampering , even when they had been treated with thermally
softenable polymer and had a Bendtsen roughness below 100 ml min-1. By
contrast, the corresponding 20% kaolin content papers with Scott-Bond strengths
below 150 J m-2 all showed evidence of tampering and delamination under
the toner adhesion test. However, evidence of tampering for the control sheet with
no thermally softenable polymer present was patchy, with only a minor degree of
fibre disturbance apparent at intervals on the sheet surface. The control sheet
was therefore judged unacceptable.
Attempts at removing toner with a scalpel from the surface
of the 20% kaolin content polymer-treated papers, i.e. the papers according to the
invention, resulted in very obvious damage to the surface of the papers. By contrast,
it was possible to remove toner from the 3% filler content papers and the two untreated
control papers with little or no visible surface damage.