Tamper resisting holographic security seal

A tamper resistant security seal is a laminated tape having a transparent carrier layer (2); an optical, diffraction pattern defining layer (3,4); and an adhesive layer (6) for adhering the tape to a substrate. The optical pattern, such as a hologram, defined by the optical pattern defining layer is visible from outside the laminate. The optical diffraction pattern defining layer (3) is formed by a polymeric layer permanently bonded to the transparent carrier layer which, when heated, causes the diffraction pattern to undergo an irreversible change. The adhesive is a pressure sensitive adhesive. An additional removable support layer may be provided on the carrier. The laminate is constructed so that a reduction in temperature below 0.degree. C. will cause an irreversible change in the diffraction pattern, or is such that subsequent to such a temperature reduction, attempted removal of the tape from a substrate will cause an irreversible change in the diffraction pattern.

BACKGROUND 
1. Technical Field 
The invention relates to a tamper resistant security seal, for example for 
sealing containers such as security pouches and the like which are used 
for conveying items of value such as banknotes, letters of credit and the 
like. 
2. Description of the Prior Art 
It is extremely important that any such pouches possess tamperproof or 
tamper-evident seals. Various attempts have been made in the past to make 
such seals. 
Generally these seals are adhesive backed tape constructions which are 
applied under or over the edge of the pouch flap so that the flap is held 
in place against the body of the pouch in a firm and tamperproof manner. 
They can also be in the form of labels. 
In recent years the use of metallised films has been replaced by the use of 
optically embossed metallised films, which have a holographic or 
diffractive image. Such a construction was recently described by Advanced 
Holographics in GB-A-2211760. The advantage of using holographic films is 
that their counterfeiting is extremely difficult. The construction of the 
general purpose tape is similar to tapes used for hot stamping, for 
example as described in GB-A-2129739. 
Very generally these tapes consist of a supporting film, a wax release 
layer, and a coating of an embossable thermoplastic polymer which has been 
subsequently diffractively embossed. Vapour deposited aluminum is then 
applied with an optional protective layer. The adhesive is then applied 
from a coating solution. WO-A-88/05728 introduces the general concept of a 
holographic protective film having a wax interlayer. Then a general 
purpose pressure sensitive adhesive layer is applied which is in turn 
protected by a peelable release paper. In use, the tape is fixed to a 
substrate using the adhesive. It is difficult to copy or alter. 
JP-A-63106780 also describes another general purpose holographic tape. The 
tape is designed with weaker bonding between a protective layer over the 
holographic layer and an adhesive layer than between a transparent film on 
which the tape is formed and the holographic layer. 
Similar constructions are known for covering large areas of, say, carton 
card in which an holographic transfer foil is rolled onto the card to 
produce a card having a diffractive metallic appearance. Such transfer 
foils are not known to have been used for security pouch seals. However, 
they are of similar structure to the tapes mentioned above except that 
instead of a wax release layer the embossable layer is chosen to have 
release properties from the carrier film. 
Searle (GB-B-2136352) discloses holographic seals in which locally embossed 
areas of thermoplastic polymer are covered by a metallised film which is 
then demetallised. This leaves areas which are unprotected by the 
holographic image which is undesirable in case forgery is attempted. 
Dai Nippon Insatsu in U.S. Pat. No. 4,856,857 discloses transparent 
embossed holographic structures in which the holographic impression is 
supplemented by a partial appearance of the underlying surface which may 
be a photograph. 
Makowka (U.S. Pat. No. 4,834,552) describes making tamper-evident seals for 
plastic envelopes. The seal is double sided requiring two adhesives and is 
concealed under the flap in use. Inspection of tamper evidence can only be 
by folding back the flap and looking at the edges. Paper or cloth having a 
porous structure is used to protect against low temperature attack. 
The use of holographic effects for security purposes is thus well known. 
The fineness of optical embossing and the nature of the holographic image 
make it very difficult to alter such devices or manufacture them afresh. 
The term "counterfeiting" may be taken to mean the copying of an article by 
fresh but fraudulent manufacture. 
Holographic devices are counterfeit resistant and may be counterfeit 
indicating. It is relatively difficult to construct an holographic image 
by "copying" it on a holographic table even if one were available. Slight 
variations in image quality would also be readily detected in any copy 
because of the fineness of the surface relief structures employed. The 
counterfeiter would need to have access to holographic equipment, 
embossing equipment and metallising equipment to manufacture copies, in 
practice this would be very difficult. 
Holographic seals are also forgery resistant by which is meant alteration 
resistant. They are also readily alteration indicating, as it is very 
difficult for a forger to replace accurately any cut away or altered area: 
the fineness of optical relief embossing acts as a considerable deterrent. 
Despite many holographic seal variants disclosed in the art, these all 
being directed towards enhancing in various ways the anti-counterfeiting 
properties and/or anti-forgery properties, the importance of providing 
substitution resistance, which is the third form of attack which a 
criminal may make, has not hitherto been maximised. 
The prior art recognises that holographic seals should not be readily 
detachable from the substrate to which the seals are attached. Thus for 
example it is recognised in GB-B-2136352 that the holographic layer should 
be weak so that attempted removal of the carrier will destroy the 
holographic embossing. 
Similarly in GB-A-2211760 the removal of the carrier film (aided by the 
strength of the wax it is assumed) will cause damage to the holographic 
layers. 
While such structures have been used previously, they are unlikely to have 
provided substitution prevention or tamper prevention and possibly tamper 
indicating properties for example when such substitution or tampering is 
undertaken at extremes of ambient temperature during freezing or heating. 
Neither is there any indication in the prior art as to how a superior 
holographic tape possessing such properties may be made. 
By substitution is meant the detachment of all or part of the seal allowing 
its replacement without giving evidence of that having happened. For 
example if a seal on a security pouch could be temporarily detached and 
then resealed without trace, this would be particularly undesirable. Yet 
many of the prior art seals are susceptible to such action. 
By tampering is meant unauthorised interference with the seal whether for 
the purposes of counterfeiting, forgery or substitution. 
In this specification, by printing is meant the application of readable 
markings of dyes and/or pigments such as those delivered during printing 
operations, especially thin ink film printing operations such as occur in 
lithographic, flexographic and gravure printing. The marking may be 
employed under electronic control such as during laser printing of toners, 
ink jet printing, thermal transfer printing, impact ribbon printing and 
the like. Markings may take the form of fine line security indicia, such 
as alpha numerical characters, symbols, geometrical designs, obliterating 
coatings and the like. 
Markings may also be made caused by printing small shapes which pattern the 
embossed surface before or after metallisation or by gross embossing 
number shapes. The printed markings a take the form of single images which 
may require registration for labels, or the printed markings may take the 
form of multiple repetitions of a particular design in the form of an 
endless pattern. Serial or batch numbering may be used to identify 
individual seals. 
SUMMARY OF THE INVENTION 
In accordance with a first aspect of the present invention a tamper 
resistant security seal comprises a laminate having a transparent carrier 
layer; an optical, diffraction pattern defining layer; and an adhesive 
layer for adhering the seal to a substrate, wherein an optical pattern 
defined by the optical, diffraction pattern defining layer is visible from 
outside the laminate, and wherein an attempt to tamper with the seal will 
cause an irreversible change in the optical pattern characterized in that 
the optical, diffraction pattern defining layer is formed by a polymeric 
layer; in that the optical, diffraction pattern defining layer is 
permanently bonded, directly or indirectly, to the transparent carrier 
layer, such that any attempt to delaminate the carrier from the optical, 
diffraction pattern defining layer will irreversably damage the optical 
pattern; in that the adhesive is a pressure sensitive adhesive; and in 
that the laminate is such that a reduction in temperature below 0.degree. 
C. will cause an irreversible change in the optical pattern, or is such 
that subsequent to such a temperature reduction, attempted removal of the 
seal from a substrate will cause an irreversible change in the optical 
pattern. 
The permanent bond between the carrier and optical pattern defining layer 
allows the layers to be kept very thin. Thus, any attempt to peel the 
layers apart will cause the optical pattern defining layer to fragment 
destroying the optical pattern. 
The invention provides a security seal which exhibits a high degree of 
deterrance to each of counterfeiting, forgery and substitution attempts 
within an integrated structure which is suitable for manufacture on a 
large scale using conventional production equipment. 
These new security seals, which are resistant to a wide range of criminal 
challenges, may be made with a structure which is so fragile that it will 
very readily fail. 
We have realised that it is important to make an improved seal which has 
high resistance to counterfeiting, forgery and substitution, yet which 
will degrade irreversably and readily under many conditions to which the 
criminal may subject it. 
We have devised a new type of tamper resistant security seal which is 
capable of resisting not only a high temperature challenge but also a 
challenge at low temperature. Such a challenge could result in complete 
destruction of the optical pattern defining layer, a variation in that 
optical pattern, or a variation in the laminate structure such that any 
attempt to remove the seal from a substrate will cause the optical pattern 
to be varied or destroyed. Preferably, the laminate is constructed so as 
to withstand a challenge at -50.degree. C. 
DETAILED DESCRIPTION OF THE INVENTION 
The film forming carrier layer may comprise a Sun, Ault and Wiborg VHL16157 
lacquer. This is preferably applied at 2 to 4 microns thickness from a 
first solvent/reverse roll using a coater. The composition may be a 
polyvinyl butyral and polyacrylate mixture in solvent. The film forming 
carrier layer has much more cohesive strength than the subsequently 
applied optical pattern defining layer and provides surface protection to 
a linear extent when there is no support. Additionally, the carrier is 
selected to bond securely to the subsequent coating and it contracts 
proportionally much more than the other layers on prolonged freezing to 
low temperatures such as by liquid nitrogen. 
The optical pattern defining layer comprises a transparent film forming 
polymeric coating and a metallic layer provided on the surface of the 
polymeric coating remote from the carrier layer. The pattern comprises a 
transmission hologram or diffraction pattern which is viewed by reflection 
against a metallic surface. 
To provide resistance to heat attack, the polymeric coating, such as an 
embossable lacquer, will typically have a Tg of below the boiling point of 
water but above ambient transporting conditions ie in the range 50.degree. 
to 90.degree. C. preferably 60.degree. to 80.degree. C. Embossing may 
occur at temperatures of 20.degree. C. degrees above the Tg of the given 
lacquer. 
For example, the optical pattern defining layer may be applied to the 
carrier layer at 4 to 12 microns dry thickness and comprise Holden's 3190 
lacquer. The optical pattern defining layer will be thermoplastic and may 
have some elastomeric properties. Chemically it may be a polyurethane or a 
polyester which when applied to the carrier layer will exhibit 
significantly greater adhesion than that between the carrier layer and 
additional support layer. 
The exposed side of this optical pattern defining layer is preferably 
embossed against a nickel or similar master shim, in order to impart 
optically diffracting characteristics. 
A surface of the carrier layer or optical pattern defining layer may be 
printed or otherwise marked. The metallic layer is applied generally 
completely to the embossed surface, such that substantially all the 
embossed diffracting pattern is covered, and this layer may be of 
aluminium and for example be 20 nm thick. The metallic layer is preferably 
continuous but may alternatively be partial for example in a half tone 
pattern which may in turn depict larger shapes, the embossing normally 
covering the complete area of the seal. It is preferable that the embossed 
area extends over the complete surface without interruption. 
At least one of the materials of the carrier and embossing layers is 
preferably susceptible to common solvents such that it will swell or 
dissolve on solvent challenge, often causing irreversable change to the 
delicate holographic layer. The use of alkali is likely to affect any 
aluminum reflector. The structure is preferably acid and water resistant. 
The pressure sensitive transfer adhesive may comprise a National Adhesive 
Company pressure sensitive transfer adhesive. The adhesive must be a 
pressure sensitive adhesive preferably protected by a siliconised release 
paper, applied by transfer i.e. after drying it is rolled at ambient 
temperature under mild pressure against the remainder of the construction. 
The use of solvent borne pressure sensitive adhesives coated onto the 
metal is impossible because of solvent sensitivity of the embossable 
layer. Hot stamping adhesives cannot be used because of the inbuilt 
temperature sensitivity. 
The release paper may be continuous, but is preferably releasable in more 
than one section. Generally the pressure sensitive adhesive is chosen to 
retain its adhesive properties over a temperature range of -10.degree. C. 
to +60.degree. C., preferably the range is from -50.degree. C. to 
+60.degree. C., and to have a Tg from 50.degree. C. to 150.degree. C. It 
is well known that general purpose pressure sensitive adhesives will 
harden during chilling, causing them to adopt a glassy state exhibiting no 
adhesion. Thus by providing an adhesive which retains its tack at low 
temperatures, freezing delamination can be avoided. Similarly adhesives 
may soften and thus be susceptible to heat delamination and become 
peelable. 
Such adhesives may be obtained commercially from adhesive suppliers and 
suitability for particular applications may be tested experimentally so as 
to ensure a high degree of adhesion at the lowest temperatures to the 
substrate and to the metallic layer. 
The pressure sensitive adhesives are generally made from polymers which 
have a high surface energy. While relatively pure polymers having a low Tg 
may be employed and the Tg may equate approximately to the change between 
tackiness and the non-tacky glass-like state, it will often be found that 
plasticising or tackifying agents may be incorporated with the polymer to 
render, it tacky at temperatures below the Tg of the polymer. Such 
tackifying agents may be non-volatile organic molecules having structual 
similarity to the polymer, or at least compatibility, or there may be 
included very low molecular weight polymers. 
The pressure sensitive adhesives will generally be acrylic polmers and the 
like. Many examples may be found in the art. 
The pressure sensitive adhesive should be chosen to maintain its tackiness 
for prolonged periods at the specified minimum operative temperature for a 
given application. 
In some examples the seal further comprises a support layer joined to the 
carrier layer. 
The support layer may, comprise a transparent film, for example biaxially 
oriented polyester film of the ICI plc type sold under the trademark 
"MELINEX". Its thickness will be typically 23 microns or 50 microns but 
could be very thin such as 12 microns. Generally the polyester will be 
colourless although it may be tinted. It may carry security printing or 
other markings on either surface if the support layer is intended to 
remain on the affixed seal. In other embodiments however the support layer 
may be stripped off from the remainder of the seal after it has been 
affixed to the pouch. The remaining layers are however usually too fragile 
to withstand much handling and usually the support layer is left in place. 
Its removal does not destroy the holographic layer in this embodiment. It 
must be left in place during affixing. 
In accordance with a second aspect of the present invention a method of 
manufacturing security bags comprises providing a length of tamper 
resistant security seal according to the first aspect of the invention, 
the seal including a releasable, film forming protective layer over the 
adhesive layer, wherein the releasable, protective layer is releasable in 
more than one section, folding a length of security bag material to form a 
bag with an opening, removing one section of the protective layer, and 
affixing the length of security seal to one edge of the opening via the 
portion of the adhesive layer thereby exposed. 
This is a particularly important aspect of the invention enabling security 
bags to be mass produced, as explained below. 
In accordance with a third aspect of the invention, a tamper resistant 
security seal comprises a laminate having a transparent carrier layer; an 
optical, diffraction pattern defining layer; and an adhesive layer for 
adhering the seal to a substrate, wherein an optical pattern defined by 
the optical diffraction pattern defining layer is visible from outside the 
laminate, and wherein an attempt to tamper with the seal will cause an 
irreversible change in the optical pattern characterized in that the 
adhesive layer defines a pattern of areas with and without adhesive and in 
that the optical diffraction pattern defining layer is permanently bonded, 
directly or indirectly, to the transparent carrier layer, such that any 
attempt to delaminate the carrier from the optical, diffraction pattern 
defining layer will irreveribly damage the optical pattern; 
Preferred seals are constructed according to both the first and third 
aspects of the invention. 
In accordance with a fourth aspect of the present invention, a method of 
manufacturing a tamper resistant security seal comprises preparing a 
laminate of a carrier layer, optical diffraction pattern defining layer 
and metallic layer; and applying transfer pressure sensitive adhesive, 
supported on a releasable protective layer, under mild roller pressure. 
The security seal is visible at all times, as distinct from being concealed 
under a flap or the like, to enable easier detection of tampering, and 
either remains intact or rapidly degrades on being subjected to a variety 
of attacks. 
A new method of manufacturing such extremely delicate structures and 
methods of application have been devised such that the tape can be made 
using relatively conventional manufacturing equipment. Relatively 
inexpensive security products which incorporate the seals of the invention 
may also be made.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The seal shown in FIG. 1 comprises a smooth transparent support 1 formed by 
a polyester film, biaxially oriented for strength, with a thickness in the 
range 12-50 microns. Typically 19 or 23 micron polyester film is used. 
This support I provides support for the remaining layers and can be 
removed. This removal can be done without damage to a holographic image 
embossed on a subsequent layer. The removal of the support 1 would be done 
only after the seal was affixed to its resting place as the construction 
is not otherwise self supporting. The support could be left in place and 
indeed would be left in place for many applications because it imparts 
scuff resistance to the seal. In its absence the holographic image could 
be irreversibly damaged during normal handling. The use of corona 
discharge treatment provides fine control of the bonding characteristics 
to a degree greater than wax could provide, so that a balance of 
properties may be achieved. 
Instead of biaxially oriented polyethylene terephthalate (polyester) the 
support 1 may be made of a thermoplastic film material which has a lower 
Tg e.g. an ethylenic polymer such as polypropylene. Polypropylene has the 
advantage that it is easier to cut with a hot wire than paper or 
polyester. The support 1 may have printing on one or both surfaces or be 
tinted. Polymer surfaced paper may be used as a support provided its 
surface is relatively smooth: this would be subsequently removed. Corona 
treatment levels of 50 dynes per cm of polyester film will give a useful 
degree of releasable bonding to the preferred lacquer which is used, while 
balancing handling and security requirements. 
A smooth transparent carrier film forming polymer 2 is coated on the 
support 1. The carrier 2 has intermediate release properties relative to 
the support 1 such that the support may be removed later without 
delaminating any of the layers in the construction. 
The carrier 2 is coated very thinly indeed. The dry thickness limits, which 
are critical, are between 2 and 6 microns, preferably between 2 and 4. The 
carrier 2 is tougher than the embossable lacquer 3, which is useful for 
scuff resistance, but the carrier is less tough than the support 1. The 
carrier 2 can be cut with a hot wire as it is thermoplastic. The material 
is generally not as susceptible to attack from solvent as the embossable 
lacquer 3 but the combined differential solubility of the two layers 
provides a useful defence against solvent assisted tampering. 
The relative adhesion between the carrier 2 and the support 1 is controlled 
by selection of the materials and also surface treatment of the support 1, 
such as with a corona discharge. Wax is not used as the bonding between 
the support 1 and the carrier 2 can be more easily and cost effectively 
controlled to make it release easily or not at all, depending on the 
corona setting. This of course is readily controllable. 
The embossable lacquer 3 is applied to the carrier 2. The surface of this 
embossable lacquer 3 remote from the carrier 2 is embossed to define a 
diffraction pattern such as a hologram. 
The dry thickness of the embossable lacquer 3 is from 4 to 12 .mu.m and 
preferably 6 to 8 .mu.m. It is thermoplastic and cuttable with a hot wire. 
Thermoset, highly crosslinked coatings are not used as they are too tough. 
Typically a non-crosslinkable polyurethane or polyester is chosen. Solvent 
soluble polymers are employed as they cannot withstand solvent attack 
later. 
The embossable lacquer 3 is generally selected to have a Tg between 
50.degree. C. and 90.degree. C. The lower limit is rather too low for hot 
countries but generally allows the embossable lacquer not to "melt" under 
normal working conditions. The higher limit is selected to be low enough 
to cause deterioration of the holograhic embossing pattern on hot air 
(hair drier) or steam challenge. If subjected to these temperatures the 
embossable lacquer 3 would relax and the holographic quality would 
deteriorate to a noticeable extent, providing tamper evidence. 
Embossing takes place at a temperature such that the lacquer 3 permanently 
accepts the embossing pattern. 
On solvent attack the embossable lacquer 3 which is soft and solvent 
soluble is quickly irreversibly damaged resulting in loss of holographic 
image quality. If heated with hot air or steam to a temperature above the 
softening point of the embossable lacquer (50.degree. C. to 90.degree. C.) 
the embossable lacquer 3 relaxes and the holographic properties degrade 
irreversibly. 
The embossable lacquer 3 is embossed at a temperature as described above 
and under pressure and then metallised with a thin metallic film 4 of 
aluminum or other metal typically 20 .mu.m thick. The embossable lacquer 3 
may be metallised then embossed but this is not usually done in practice. 
The holographic layers 3,4 are very thin and fragile. The seal can be 
handled well at room temperature provided that it is on its support 1. 
This is very important for automatic application of lengths of tape. 
The embossable lacquer 3 may be metallised partially, balancing holographic 
reflectance and see-through transmission in this use. Transmissions of 75 
to 80% are typical. 
The embossed diffraction patterns can include holograms of objects, two 
dimensional graphical diffraction patterns (which give the perception of 
none or one or more layers of depth to the viewer) , stereoholograms, 
kinoforms, diffractive mosaic patterns including computer generated 
diffracting patterns and the like alone or in any combination. The images 
are preferably white light viewable. The images may be individual perhaps 
surrounded by plain metal or continuously repeating in register in an 
overall geometric design. The holographic features may alternatively be 
viewable only on monochromatic light including visible and infrared light. 
Machine readable and verifiable diffraction patterns may be included in 
the holographic embossing. 
A protective polymeric coating (not shown) may be applied to the metallic 
film 4 before applying a pressure sensitive transfer adhesive 6. The 
adhesive is not coated on as its solvent or drying would possibly attack 
the carrier 2 or embossable lacquer 3. Rather the adhesive is transferred 
already releasably adhered to a release paper (or film) 7, and the two 
surfaces are brought together under mild pressure of rollers to bond the 
pressure sensitive adhesive 6 firmly and irreversibly to the metallic film 
4. 
The bond strength between the pressure sensitive adhesive and the release 
paper 7 is less than that between the support 1 and the carrier 2. This 
allows the release paper 7 to be stripped away and the seal to be adhered 
in place. The support 1 may then be removed as the bond to the carrier 2 
is weaker than the bonding among the carrier 2, embossable lacquer 3, 
metallic film 4 and pressure sensitive adhesive 6 (otherwise the tape 
would split apart). 
The support 1 must be kept in place while the seal is being affixed because 
it is too soft to remain intact while being peeled from the release paper 
7 over the pressure sensitive adhesive 6 without the support. 
After removal of the support I the holographic layer is so weak that it 
rips apart on attempted peeling. The support 1 can be detached without 
pulling off the holographic layers. Overall the construction is very thin, 
typically the carrier and embossable layers taking up about 8-10 .mu.m. 
The release paper 7 could be siliconised paper, siliconised plastic, or 
releasable plastic such as polyester (if necessary surface treated), 
polythylene, polypropylene or the like. Plastic is useful since it allows 
the completed seal to be cut with a hot wire during plastic security bag 
manufacture. The Tg of the thermoplastic release layer will not usually be 
greater than 180.degree. C. to allow hot wire cutting. The completed 
material can then be cut into reels or sheets for use as tape or 
individual labels. The release paper 7 may be partially slit. 
As a variant the adhesive can be supplied in a patterned format, covering 
at least half of the available surface. The advantage of this is that 
tearing attempts will encounter differential adhesion. It is however 
somewhat of a disadvantage as the adhesive pattern can be seen against the 
holographic layer where the level changes. If patterned adhesive is to be 
used then the adhesive has to be placed in tramline fashion to span where 
the slitting knives will cut, otherwise the seal will destruct on 
slitting. In between the tramline's partial coverage a series of small 
blocks may be used. 
The patterned adhesive gives differential failure variation of the seal. 
The seal described above breaks down readily on tampering, especially 
peeling. This breakdown can be enhanced by providing some irregularity in 
the flap of a bag being sealed, e.g. by serrating the edge of the flap. 
Regular failure to a geometric design is attractive but security can be 
enhanced by providing greater degrees of irregularity than simply by 
serrations. This can be achieved by placing a pattern of adhesive using 
patterned adhesive printing rollers. Either the adhesive is laminated in 
place rather than coated or the bond between the continuous adhesive and 
the metal is broken by printing a release coating onto the metal in a 
patterned form. 
FIG. 4 illustrates a patterned adhesive for use on a seal, the adhesive 
layer being arranged with adhesive areas 20 and non-adhesive areas 21. Two 
longitudinally uninterrupted tracks of adhesive 22 and 23 are provided 
where the tape is to be slit along lines 24-25 and 26-27 so as to prevent 
the soft coatings prematurely detaching. 
The net effect is that when the support is peeled away those areas with 
adhesive are kept affixed to the substrate whilst those areas which are 
adhesive free are pulled by the carrier. The weak layers are therefore 
subjected to contrary forces and as the adhesion to adhesive and adhesion 
to the carrier are greater than the cohesion and adhesion of the carrier 
and embossable layers, these layers tear irregularly and cannot be 
reinstated. 
The adhesive pattern also causes local variation in the thickness of the 
seal and this effect manifests itself in the holographic layer. This is 
otherwise completely flat but it is tilted by the adhesive. 
Thus, patterned adhesives may be used where extra breakdown and tamper 
evidence is required. Solvent readily wicks under the coatings where there 
is no adhesive and because of the differential thermal conductivity of the 
structure, rapid cooling and heating might result in additional visible 
changes to the holographic image. 
Another adaptation is to print a security bag with a patterned releasable 
flexographic ink where the seal is to be sealed. The pattern is applied by 
standard printing techniques and when the seal is peeled away by trying to 
lift it, for example with adhesive tape, the holographic layer tears in 
the pattern of the ink. The ink may be made to have release properties by 
including wax or other compatible low surface energy material. 
In a further variant the corona field intensity may be varied across the 
web so as to provide differential adhesion. 
On freezing in a freezer at -10.degree. C. or lower temperature, according 
to the adhesive's properties the adhesive 6 will not debond from the 
substrates which have been used because of the choice of adhesive. The 
adhesive 6 has a low hardening temperature. On regaining room temperature 
no deterioration of holographic quality need be evident. On prolonged 
freezing or on very low temperature challenge such as at liquid nitrogen 
temperatures the hologram will irreversably deteriorate as thermal 
stresses develop between the securely bonded layers. Additionally it is 
thought that the presence of ice crystals forming within the holographic 
structure contribute to the effect. The aluminum layer appears to lose 
reflectance and this is readily noticeable. 
The seal, which may be a continuous tape or comprise individual labels, is 
applied to a flat surface for example to protect an underlying feature, 
over the joint between two flat overlapping surfaces such as a bag flap or 
envelope flap, or over a short gap in a surface. The pressure sensitive 
adhesive will be varied depending on the end use. 
The seal may be used as an edge seal for example spanning part of one edge 
of a photograph or visa affixed to a passport page or to seal a gap 
completely, for example security bag flaps. 
The fragility of the holographic layers means that the seal does not 
provide a significant degree of strength to the area being sealed. Thus in 
security bags which have a flap which is folded and sealed against the 
body of the bag, there is generally a separate adhesive strip which 
provides a strong bond. This adhesive may be a double sided adhesive strip 
protected by a removable release layer. After the flap is sealed in 
position the adhesive strip will not normally be visible even though it 
may have a tamper indicating construction. 
Although the seal is weak when the support 1 is left in position as will 
commonly be the case, the support 1 adds to the stress resistance of the 
holographic layer so that the seal is able to withstand minor flexing 
without damage. The support 1 also provides scuff resistance. If the 
support 1 is removed the carrier 2, which is tougher than the embossable 
lacquer, will provide limited impact protection. 
A stronger version of security seal can be made for example for use for 
lamination or sealing in place of passport photographs. This requires that 
there is a permanent backing which is not releasable. In practice this is 
done by using polyester as the carrier, which has been surface treated 
with a corona discharge so that it bonds strongly to the coating. The soft 
embossable lacquer 3 is then between the strong pressure sensitive 
adhesive 6 and the carrier 2. On peeling there will be metal 4 to 
embossable lacquer 3 failure or cohesive failure of the embossable lacquer 
3. 
While complete metallisation can be used for this seal for some purposes, 
partial metallisation has to be used for the passport photograph 
overlaminate application to allow the photograph to be seen. 
The passport overlaminate application has anticounterfeit and antiforgery 
properties. It possesses strong bonding with clear tamper resistance. 
The seal may be supplied in lengths so that it can be bound into a passport 
book, next to the photograph page. The release paper which is not be 
stitched would be peeled off to reveal the adhesive which would then be 
smoothed over the page holding the holder's photograph. The seal may be 
used to seal the edge of a visa and could be signed. 
The seal is designed to be resistant to freezing and high temperature 
attack as well as solvent or chemical eg alkali solution attack The seal 
if peeled causes irreversible irregular splitting of the soft holographic 
layers. 
An example of a higher strength seal for use with security bags is shown in 
FIG. 3. A transparent biaxially orientated polyester film 2, which may be 
between 19 .mu.m and 50 .mu.m thick, in this case 23 microns, was corona 
treated at approximately 50 dynes per centimetre to provide a surface on 
which the subsequent coating would exhibit clinging engagement. 
To the corona treated surface of the carrier 2 a transparent coating of 
embossable lacquer 3 of the aforementioned type is then applied at for 
example 8 microns dry thickness and gently dried. The lacquer may be 
applied from a volatile solvent which is subsequently removed, at a 
thickness of between 7 .mu.m and 12 .mu.m. 
A holographic pattern is then imparted to the surface of this lacquer 3, 
the holographic pattern comprising a series of abutting individual images 
separated by small plain margins. The surface of the lacquer 3 may be 
printed with a thin ink layer in a fine pattern. Embossing is undertaken 
under heat and pressure against a nickel shim which holds the holographic 
pattern on its surface at a temperature about 20.degree. C. degrees above 
the Tg of the lacquer, approximately 80.degree. C. to 110.degree. C. 
The embossed composite film is then metallised either completely or 
partially (to allow transparency), with aluminium under vacuum to deposit 
a layer of metal 4 approximately 20 .mu.m thick. 
To the surface of a roll of corona treated polypropylene film 7 is applied 
National Adhesives acrylic pressure sensitive adhesive Type 380-2819 or 
1825 at a dry thickness of approximately 12 microns. This is dried to form 
the pressure sensitive adhesive layer 6 which is then rolled against the 
metallised composite film under mild pressure to provide the final seal. 
This is then slit into rolls and at the same time the release layer 
covering the adhesive is provided with a longitudinal tearing line to 
allow part of the adhesive to be made available for affixing the seal to a 
security bag. 
The adhesion between this surface of the carrier 2 and the embossable 
lacquer 3 being sufficient to allow for manufacturing and automated seal 
affixing stresses. The adhesion between the carrier 2, embossable lacquer 
3, metal layer 4 and the adhesive 6 is greater than that between the 
pressure sensitive adhesive 6 and its release paper 7. The seal can be 
affixed to a substrate by removing the pressure sensitive adhesive's 
release paper 7. 
This seal may then be affixed in a continuous security bag manufacturing 
line to the surface of a thermoplastic security bag. Individual bags rom 
the continuous strip by means of a hot wire or guillotine which cuts and 
seals the bag edges and simultaneously cuts the security seal. 
FIG. 2 illustrates a continuous series of security bags bearing the seal of 
the invention, the bags having been vertically edge sealed by a hot wire 
which has cut through the thermoplastic bag material as well as the 
structural adhesive and the holographic security tape. 
A roll of heat sealable plastic film 8 such as opaque polypropylene, 
suitable for making security pouches, longitudinally printed on the 
outside with the agent's name, is folded longitudinally on film 
transporters such that an edge 9 of an upper surface 10 does not extend as 
far as another edge 11 of a lower surface 12. The flap portion comprises a 
numbered section (the number is on the other side of the flap) detachable 
along a perforated line 13 when the bag is about to be holographically 
sealed. The flap has a series of perforations 14 to cause tearing on 
tampering. 
Individual bag shapes are then prepared by cutting lengths of this 
continuous assembly with a heated plastic wire (orthogonal to the 
direction of the seal). This will also have the effect of sealing the 
edges of the bag. Alternatively, the edges of the bag may be heat sealed 
together to provide edge bands which are then cut in the middle of the 
bands with a hot wire or knife. 
The bag, any detachable flap and optionally the seal may be numbered, for 
example by ink jet printing, to provide individuality to the bags. 
In use the numbered section is detached and the flap is folded at line 9 
and affixed to the surface 10 with a strong double sided adhesive strip 15 
bearing its thermoplastic release layer. Holographic security seal 16 of 
the type described above is affixed to the bag by the adhesive on one half 
17 of the seal (following removal of half the protective layer) . The 
other half 18 of the seal still bears its protective layer so that the 
edge 13 may be sealed when the flap extends to its limit 19. 
Security envelopes are used for the secure transport and storage of 
valuable items. By providing a holographic seal which is difficult for the 
criminal to reproduce and which cannot be substituted or broken and 
sealed, they are made more visibly tamper evident. The seal supplied may 
be 25 mm wide and the release paper or film has a longitudinal tearing 
line so that one side can be stripped off. The seal may be applied to the 
bags during their in-line manufacture. 
In order to test the laminate shown in the drawing, delaminating tests have 
been carried out at several temperatures on the type of seal which has a 
removable support (FIG. 1). These are 70 degrees Celsius, ambient 
temperature of about 20.degree. C., -50.degree. C. and -180.degree. C. 
The test seal is applied to a polypropylene or other plastic pouch surface 
and picking off intact is attempted. While the support may be able to be 
removed without destruction of the optical layer, the seal could not be 
removed intact at any of these temperatures. On exposure to the high or 
very low temperature the optical structure was irreversibly deformed. This 
may occur because of the differential stresses inside the structure so 
that on freezing, say, the carrier contracts more than the other layers 
with the result that the internal stresses cause failure of the 
diffraction image. The construction exemplified with the materials above 
has not only withstood freezing to -50.degree. C. with freon spray but has 
also withstood integral peeling after exposure to liquid nitrogen. We have 
found that with the above construction the polyvinyl butyral layer seems 
to contract much faster than the optical layers to which is firmly adhered 
with the result that the holographic seal visibly fragments. 
The seal was also found to be resistant to removal or failed irreversably 
on exposure to cold water, hot water, steam, aqueous alkali, aqueous acid, 
common solvents such as methylated spirits, acetone, petroleum spirit, 
ethylacetate, peeling, bending. The seal was difficult to copy or alter. 
This improvement is very significant indeed in maintaining the integrity of 
pouches or at least showing that a tamper condition has arisen. 
Different grades of seal are suitable for different applications. A normal 
grade may be used for light duty labels (including crack back release), or 
tape for bags, envelopes, cassettes, small seals, or passport stickers. 
The heavier duty seal with lower temperature resistance is suitable for 
strips, tapes, and labels where more load resistance is required such as 
the passport photograph overlaminate which is partially metallised. The 
light duty patterned adhesive may be used for applications where extra 
breakdown and tamper evidence is needed. A heavy duty patterned adhesive 
may be used for strip seals, labels, bags envelopes, and containers. 
The term "crack back" is used to indicate a method of applying labels in 
which the release paper fixed to the adhesive is sharply folded over a 
right angle causing the front of the label to project with its adhesive 
surface exposed. The adhesive engages substrate and the substrate then 
pulls the label off the release paper. Crack back is necessary for 
automated label applications. Additional transparent layers having a 
thickness of less than 12 microns may be added within the laminate. The 
seal of the invention may be used on envelopes which are designed to hold 
computer discs such as 51/4" or 31/2" floppy discs. The seals may be 
numbered individually or in batches to provide enhanced levels of 
security. The seals may also be used to secure boxes containing magnetic 
recording media which are provided in reel form such as magnetic tape 
cartridges for use as computer storage media, video recording tapes, audio 
tapes and the like.