Method for selective lamination of thermoplastic layers

Disclosed is a method and apparatus to facilitate the method of discontinuously laminating at least two thermoplastic layers together. The layers are heated and pressed together over all but a selected window portion of the layers to facilitate lamination between the layers at all points except the selected window portions. Relief grid images can be embossed in the selected window portion or holograms or other difficult to forge inserts can be provided in these window areas prior to lamination. During lamination, the reduced or absence of heating in these window portions permits lamination of the card without destruction of the selected window portion insert.

BACKGROUND OF THE INVENTION 
The present invention relates generally to a process and an apparatus for 
laminating layers of partially thermoplastic films by action of heat and 
pressure on the layers. 
Laminating thermoplastic film under pressure with the action of temperature 
on other films or other layers, for example paper, creates a firm bond. 
Along with other uses, this process is used for the preparation of 
identity cards which have a core layer which can have security prints, 
personal data, further information and details, and, if appropriate, an 
image of the cardholder, with the core layer laminated between transparent 
films for the protection against unauthorized alterations. 
As far as identification cards, such as check cards, identity cards or 
passport cards, are concerned, a multiplicity of security features, such 
as guilloche prints or watermarks make an imitation more difficult and 
thus contribute to its avoidance. Security features such as microfilm 
images, holograms and relief grid images can be used for this purpose as 
well. These security features are generally inserted in a window hole in 
the card core and protected by the outer transparent films. 
From West German Offenlegungsschrift No. 2,308,876 a plasticized identity 
card is known, which comprises a thicker, transparent film, a printed 
special paper of high quality and as far as the securing technology is 
concerned, which shows differences in thickness resulting from watermarks 
and/or additionally incorporated security features, and of a thinner 
transparent film. The thicker transparent film acts as supporting film and 
confers to the identity card the necessary stability and rigidity as well 
as the desired thickness. The supporting film can alternatively represent 
the front or the rear of the plasticized identity card. The thinner 
transparent film acts as a covering film and may alternatively compose the 
rear or the front of the identity card. 
A photograph in the form of a film transparency can additionally be 
plasticized in this identity card. In this case the exposed, developed and 
fixed film transparency is alternatively inserted between the supporting 
film and the special paper or between the special paper and the covering 
film and is plasticized together with the films. In addition a signature 
field of a special paper can be incorporated in the identification card 
and be plasticized with it. In another embodiment of the identity card, no 
additional special paper strip is introduced as signature field but an 
opening in either the supporting film or covering film is left in the 
covering film during the plastification process and acts as signature 
field. 
During the plastification or lamination process, high pressures and 
temperatures are required, which endanger the inserted images or 
holograms. A slight melting of the gelatin layer may occur which could 
lead, in a photographic information carrier, to image distortions which 
can end in illegibility of the stored data and information. Holograms are 
even more sensitive to layer displacement. Layers with relief images are 
endangered even to a greater extent than are photographic layers during 
this lamination process. These relief images are produced by exposing and 
developing photolacquer layers, by embossing thermoplastic films or in an 
electrophotographic process, by electrostatically charging, exposing and 
developing a photoconductive, thermoplastic recording layer until a relief 
image is formed. 
To avoid melting of the security features certain precautions have been 
taken, including eg. embossing the films at temperatures which are as high 
as possible and are higher than the laminating temperature, crosslinking 
substances such as aromatic azides being added, thermally or 
photochemically, to the photoconductive, thermoplastic recording layers. 
However, these measures are in general insufficient for the image 
stabilization during the lamination process. 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide a process and an apparatus for 
carrying the process out where pressure and temperature sensitive securing 
features such as grid images, microfilm images, holograms etc. can be 
laminated in a bond of films in such a way that the image remains stable, 
without any destruction of the information carried in the image. 
The above and other objects are achieved in accordance with the present 
invention by the process of: aligning at least two thermoplastic layers; 
pressing the two thermoplastic layers together; and heating the external 
surfaces of the thermoplastic layers in a discontinuous manner across the 
surface, the discontinuous heating occurring at a selected window portion 
of the layers wherein the temperature at the window portion is at least 
10.degree. C. lower than the rest of the external surface of the layers. 
The above and other objects are achieved through the use of a laminating 
apparatus which includes: first and second laminating plates; means for 
discontinuously heating the first and second laminating plates; and means 
for pressing the plates together with the thermoplastic layers to be 
laminated therebetween, with the discontinuous heating means heating the 
thermoplastic layers less at a window portion than elsewhere on the 
external surface of the layers.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
The problem of laminating identity cards with images was solved by the 
process, mentioned above, in such a way that the laminating temperature 
acting on the layers, is lowered by 10.degree. to 70.degree. C. in the 
window area compared to the remaining areas of the layers. Additionally, 
the pressure in the window area may be lowered, compared to the laminating 
pressure on the remaining surface of the layers which are to be laminated. 
According to the development of the process, the laminating temperature is 
increased in a narrow area around the window area, compared to the 
laminating temperature in the other areas outside the window area of the 
layers which are to be laminated. 
For carrying out the process a pressing apparatus with two plates is used; 
each plate is heated and has an opening with an inserted plug, the 
temperature of which is lower than the temperature of the plate, and in 
this apparatus both plates are arranged in axial symmetry and the layers 
which are to be laminated can be inserted between them. 
By this invention excellent laminations of the security features in the 
identity card can be obtained with no influence on their information 
content and without any destruction of the films in the window area of the 
identity card. 
Referring now to the drawings wherein like reference characters designate 
like parts throughout the several views, the identity card 1 shown in 
FIGS. 1a and 1b comprises a card core 4 which has a window area 3 or a 
window hole, in which a security feature 2, for example, a relief grid 
image, a hologram or a microfilm image, is inserted. The card core 4 is 
protected on the front and on the rear by outer transparent films 5. The 
card core 4 is provided with personal data of the cardholder which may 
include the family and christian name of the cardholder and a reference 
number as shown in the plan view of FIG. 1a. 
It is technically extremely difficult to produce a jump of some 10.degree. 
C. in temperature over distances in the order of magnitude of 1 mm and to 
keep it up for a determined period. Two temperature profiles 6 and 7 
extending along the line AB in FIG. 1a are represented in FIG. 2. The 
desired, theoretically obtainable, temperature profile 6 in the window 
area 3 is represented by an interrupted line and has a rectangular-shaped 
form in the area of the window. In actuality, the temperature profile 7 
occurs because the changes in temperature are not discontinuous. It is 
desirable that the edges of the temperature graph be as vertical as 
possible when passing in the window area 3. To obtain this a pressing 
apparatus with two plates is used as shown in FIG. 3 in which the 
temperature and the pressure in the window area 3 of the layer which is to 
be laminated can be lowered in comparison to the temperature and pressure 
used for lamination in the other areas of the identity card. 
The pressing apparatus comprises two plates 8 which have an axial 
symmetrical arrangement and between which the layers which are to be 
laminated are inserted. Each plate 8 is heated and has an opening with an 
inserted plug 9. The layers which are to be laminated to a bond 26 have 
two core layers 27', 27" which enclose a supporting layer 28. The card 
core, comprising both core layers and the supporting layer, is covered by 
an upper protection layer 29 and a lower protection layer 30. The core 
layers 27' and 27" include window openings 25' and 25", and the supporting 
layer 28, also has a window opening 25'" which is coincident with the 
window openings 25', 25". 
The upper core layer 27' comprises a diagrammatically indicated relief grid 
in the window opening 25'. Before the process of lamination the card-bond 
26 is inserted in such a way in the pressing apparatus that the plugs 9 of 
both plates 8 are brought to coincide with the window openings 25' and 
25". The plugs 9 consist of a material of good thermal conductivity, such 
as copper or brass, and include a system of channels 10, one of which is 
diagrammatically indicated in the respective plug 9. Cooling liquid, 
compressed air or preferably carbon dioxide gas, is conducted through the 
channels 10. Thus, it is obtained that the temperature in plug 9 is from 
10.degree. to 70.degree. C. lower than in the other areas of plate 8. The 
respective temperature reduction depends on the thermal conductivity of 
the plug material as well as the type and quantity of the cooling agent 
passing through the channels. 
The plugs 9 can also be prepared of pressure resistant material of a low 
thermal conductivity having no cooling channels therein, and instead of 
this would be removable from the openings of the plates 8. To obtain a 
sufficient temperature reduction in the area of the openings, the plugs 9 
are then removed from the openings of the plates 8 after a determined 
number of working hours in order to cool off. 
The cooled plugs 9 are thermally insulated against the plates 8 by a 
temperature and pressure resistant filling layer 11. Polyfluoroethylene, 
such as .RTM. Teflon, of a thickness of from 0.5 to 2 mm may be used for 
this filling layer. It would also be possible to provide an air gap 
between plug 9 and plate 8 to aid in the thermal insulation of the plug 9 
although this may have the disadvantage that laminating material could 
flow into this air gap. In the embodiment shown in FIG. 3, the heating of 
the plates 8 result indirectly by the physical contact of the plate with 
heating elements in the form of resistance wires 12 which are connected to 
voltage sources 18. The resistance wires 12 are inserted together with a 
thermometer probe 13 in the plates 8 in order to measure and control the 
temperature of the plates. The resistance wires 12 are more closely packed 
around the plugs 9 than in the other areas of the plates 8. Thus the 
edge-steepness of the jump in temperature between plugs 9 and plates 8 is 
especially large, because the plugs 9 are intensively cooled and, in order 
to compensate the resulting heat loss to the coolant the plates 8 around 
the plugs 9 are heated to a larger extend than the other areas of plates 8 
by the closer packing of the resistance wires 12. The plates 8 are 
insulated in a suitable manner against the supporting plates 15 of the 
pressing apparatus by insulating layers 14. A further embodiment of the 
pressing apparatus is shown in FIG. 4 and comprises plates 8 to which a 
laminar heating resistance layer 16 acting as heating element is connected 
in series. The heating resistance layer 16 faces the thermoplastic layers 
which are to be laminated and which are to be laminated and the resistance 
layers are also thermally insulated from the plates 8 by an insulating 
layer 14. Contacts 17 which are linked to a heating voltage source 31, are 
connected to the heating resistant layer 16. This heating voltage source 
can be a source which applies pulse-shaped voltages to the heating 
resistant layer. The rest of the structure of the pressing apparatus 
corresponds generally to the embodiment shown in FIG. 3 and will therefore 
not be described again. 
To obtain the desired pressure reduction in the window openings 25',25" of 
the card bond 26, ech plug 9 in the window area is recessed a distance D 
from the surface of plate 8, as it can be seen in FIG. 5. The distance D 
is within the order of magnitude of 20 .mu.m to 150 .mu.m. This 
pressure-reduction contributes to the fact that the process of laminating 
the identity card takes place without compressing of the information on 
the prepared but pressure sensitive relief images. 
In the embodiments of FIGS. 3 and 4 the contact surfaces of plate 8 can be 
textured as it is shown in FIG. 5. In this case, the surfaces of plate 8 
comprise smooth areas 19,20,21,22,23,24. After the lamination process the 
given geometric arrangement of the smooth areas results in corresponding 
smooth surface areas on the outer thermoplastic films. The relief like 
texture on the contact surface of plate 8 can be arranged to form a 
reproducible geometric arrangement which results in corresponding matt 
areas on the outer thermoplastic film after the lamination process. 
It is evident that each continued forming of matt and glossy card-areas in 
certain geometric arrangements, for example, in the form of heraldic 
figures, represent a security feature which can be easily controlled, 
especially if the structure causing the matting comprises structures which 
are difficult to imitate, for example relief grid structures. 
Additionally, the plates 8 can be roughened on their surface, so that the 
laminated cards, with the exception of the clear window area, have a matt 
surface without disturbing reflection. 
With a pressing apparatus according to FIG. 3 in which the plugs 9 were 
recessed by 50 .mu.m, a 125.times.90 mm-card of polyvinyl chloride films 
was lamintated from top to bottom with the following components: a 
transparent glass-clear film 29 having a thickness of 100 82 m- a 
matt-whitefilm 27', 140 .mu.m thickness with a security print and an 
electrophotographically produced card-image--an intermediate layer 28, 80 
to 100 .mu.m thickness, as supporting layer of perforated paper--a 
matt-white film 27" with a thickness of 140 .mu.m and with a security 
print--and a transparent, glass-clear film 30 with a thickness of 100 
.mu.m. The lamination process took place at a laminating temperature of 
140.degree. C. and under a laminating pressure of 15 bars (Kp/cm.sup.2) 
for a period of 4 mn. The plug cooling was accomplished with carbon 
dioxide, which was introduced in the channels 10. 
The card had window openings 25', 25" which were brought into congruence 
with the plugs 9, when being positioned. A grid image of embossed 
transparent polyvinyl chloride film having a thickness of 80 to 120 .mu.m 
and a diameter of 1 cm was inserted in the window opening 25'. A disc 
having a diameter of 1 cm and comprising 60 .mu.m to 160 .mu.m thick 
transparent polyester film was sutuated in the window opening 25' on the 
side of the grid image. Intermediate layer 28 and film 27", representing 
the lower core-layer, contained in the window areas 25'" and 25" two or a 
single transparent film-inserts, preferably capable of lamination, with a 
thickness over all of 80 .mu.m to 240 .mu.m. The sum of the combined 
thicknesses of the inserts in the openings 25', 25" and 25'" is equal to 
the sum of the thickness of the layers 27',28 and 27". The plugs 9 had 
also a diameter of 1 cm, whereas the insulating layer 11 had a thickness 
of 1 mm. After lamnination the intact grid image extended in a circular 
area of approximately 9 mm diameter in the window openings 25" of the card 
bond 26. The specified higher values of the thickness-ranges of the 
inserts are top boundary values. 
Although the invention has been described relative to specific embodiments 
thereof, it is not so limited and many modifications and variations 
thereof will be readily apparent to those skilled in the art in light of 
the above teachings. It is therefore, to be understood that, within the 
scope of the appended claims, the invention may be practiced otherwise 
than as specifically described. 
The embodiments of the invention in which an exclusive property or 
privilege is claimed are defined as follows: