BACKGROUND OF THE INVENTION
In general, the present invention relates to systems and methods for
delaminating laminates, and more particularly, to systems and methods for delaminating
protective overcoating layers on an image bearing medium in a manner which minimizes
the -formation of artifacts in the image bearing medium.
A recent development in the image forming arts employs the use of
thermal imaging laminates for achieving high quality, high resolution images, such
as for radiological images useful in the medical arts. Examples of such media are
described in WO 88/04327; and U.S.Patent No. 5,200,297. More particularly, the noted
International Patent Application describes a thermal imaging medium and a process
for forming an image. The medium is a laminate in which an image forming layer thereof
is a porous or particulate imaging material, preferably, a layer of carbon black
that is deposited on a heat-activatable image-forming surface of a first sheet-like
element. The image forming layer has an adhesive strength to a first sheet-like
element of the laminate that is a function of its exposed state. The first sheet-like
element carrying the imaging material is covered with a second sheet-like element
that is laminated to the first so that the imaging material is confined between
the first and second sheets.
This medium can be imagewise exposed as by laser scanning, whereby
exposed portions of the imaging material are firmly attached to the first sheet,
and unexposed portions of the imaging material are firmly attached to the second
sheet. The result is a first image surface which comprises exposed portions of an
image-forming substance that is more firmly attached to the first sheet and a complementary
second image surface which comprises non-exposed portions of the image-forming substance
carried or transferred thereto.
After imaging in the manner noted, the sheets are then peeled or delaminated
with the first sheet carrying exposed imaging material portions, and the second
element carrying unexposed portions. As a result of the peeling, a pair of complementary
or binary image layers is obtained, either one of which may for reasons of informational
content be considered the principal image area. Such image forming materials and
processes are capable of producing extremely high quality and high resolution images.
However, there are possibilities for damaging the image layer by physical
contact, physical elements or the like. Therefore, it is desirable to protect the
image forming layer. One known approach is through the application of a protective
overcoating material, e.g. a thin, transparent, but durable layer, such as described
in WO 92/09930. Lamination of protective overcoats, such as those described in the
cited patent applications, have been accomplished by using a continuous roll, i.e.
carrier web, to transfer the durable protective layer to the image carrying sheets.
Activation energy is necessary for fusing the durable layer to the imaged sheet
at a nip formed by and between a pair of compression rollers.
While such laminating approaches are successful, nevertheless possibilities
exist for artifacts, such as pinholes, being formed in the laminated image sheet
when the latter is delaminated from the carrier web. Pinholes are considered to
be disruptions in the image forming or bearing layer which permit the undesired
passage of light therethrough. For instance, pinholes can vary in size from about
10 to 300 µm. During delamination of the imaged sheet, some of the image bearing
particles can be physically removed because of being adhered to the release layer
of the overcoating material on the carrier web. While pinholes are not necessarily
large, their presence can otherwise diminish achievement of the high resolution
achievable by the foregoing type of imaging media. As a result of such pinholes
of this removal type, the final imaged product may not be commercially acceptable.
Accordingly, there is a continuing desire for improving upon known efforts to enhance
the protection of the image layer while reducing the formation of undesirable pinholes
which might adversely affect image quality.
US-A-4,631,110 discloses an apparatus for delaminating a carrier web
from an image bearing medium wherein the laminate is passed through means for advancing
the carrier in a first direction past a heated delaminating bar, such that thereafter
the carrier web and image bearing medium are advanced in different directions. The
apparatus further includes means for controlling the temperature of the delaminating
assembly to a temperature range from 30 to 40°C.
SUMMARY OF THE INVENTION
According to the present invention, provision is made for improving
upon known methods and systems for reducing pinhole formation in image bearing material
on an image carrying medium, for reducing the formation of pinholes in the image
bearing layer upon release of the overcoating material during delamination, while
minimizing physical distortions to the image bearing medium, such as minimizing
fringing of the sheet and/or minimizing curling and rippling of the image bearing
Accordingly, the invention relates to a system and a method for delaminating
a carrier web laminate carrying a protective overcoating material after the overcoating
material has been laminated onto an image bearing layer carried on an image bearing
medium by heat and pressure applying means, as defined in claims 1, 4 and 5, respectively.
Furthermore, the invention relates to a system for protecting an image
bearing medium carrying an image bearing layer by a heat softenable protective overcoating
material carried on a carrier web laminate therefore, as defined in claim 6, and
a process of protecting an image bearing layer on an image carrying medium with
a heat softenable overcoating layer, as defined in claim 8.
Preferred embodiments are recited in the dependent claims.
The temperature controlling means may comprise at least a surface
on the delaminating member having a thermal heat transfer of coefficient which is
effective for controlling heat exchange so as to maintain the temperature within
The temperature controlling means may also comprise means for directing
air past the delamination assembly for controlling its temperature, or may include
a heating element for actively heating the delaminating assembly.
Other objects and further scope of applicability of the present invention
will become apparent when reading the following detailed description thereof when
taken in conjunction with the accompanying drawings wherein like parts are represented
by like reference numerals throughout the several views.
BRIEF DESCRIPTION OF THE DRAWINGS
- Figure 1 is a diagrammatic cross-sectional side elevational view of a web and
an image carrying medium prior to lamination;
- Figure 2 is a diagrammatic cross-sectional side elevational view of the web
and the image carrying medium of Figure 1 during lamination;
- Figure 3 is a diagrammatic cross-sectional side elevational view of a protective
overcoat bonded to an image carrying medium in the process of delamination;
- Figure 4 is a diagrammatic side view of a laminating and delaminating system
according to a preferred embodiment of the invention shown in one mode of operation;
- Figure 5 is a diagrammatic side view of another embodiment of the system similar
to Figure 4 in which there are shown a blower and heating element for controlling
temperature of a delaminating bar.
In one preferred embodiment, as illustrated in Figs. 1-4, there is
provided a laminating sheet 10 comprising a carrier web 11 and in juxtaposed relationship
thereto a protective overcoating material 12 adapted to be laminated onto an image
forming or bearing layer 14 carried on a substrate 15 of an image bearing sheet
or medium 16 for purposes of protecting the latter. The web 11 and its integral
protective coating material 12 form a laminating sheet which can be like that described
in the last noted patent applications. Essentially, the web 11 forms a support layer
for the overcoating materials 12 which overcoating materials comprise an exterior
adhesive layer 18, a barrier layer 19, an intermediate durable layer 20, and a release
layer 22. The protective overcoat, i.e. a thermal transfer overcoat, is thermally
bonded to the image bearing medium 16 including the image bearing layer 14 in a
laminating process to be described. Preferably, the release layer would be ,completely
removed from the durable layer following lamination. However, in practice, the release
layer does not cleanly separate from the durable layer, so that upon delamination
of the carrier web some of the porous particles forming the image forming layer
14 are unnecessarily removed therewith. Accordingly, undesirable pinholes or openings
can be formed in the image forming layer, thereby permitting light to pass therethrough.
These pinholes, although not shown, usually range in shape and size from about 10
to about 300 µm.
Fig. 1 illustrates the laminating sheet 10 disposed in juxtaposed
relationship over the image carrying medium 16. In the illustrated embodiment, the
image carrying or bearing medium referred to as a keeper includes an image bearing
layer 14 which is made of, for example, carbon particles formed on a transparent
substrate layer 15 made of, for example, polyester. It will be understood that the
image forming medium 16 has had another polyester layer (not shown) and complementary
layer (not shown) of carbon particles removed therefrom, the removed particles are
referred to as a throwaway layer. In the illustrated embodiments, the thicknesses
of the keeper or image forming medium 16 and the noted throwaway layer can be about
0,013 to 0,25 mm (0.5 to 10 mil) and 0,013 to 0,18 mm (0.5 to 7 mil); respectively.
For a more detailed description of this type of thermal imaging media, reference
is made to the aforementioned WO 88/04327. Examples and methods of obtaining an
image carrying medium 16 may be had from the description in U.S. Pat. No. 5,155,003;
and, U.S. Pat. No. 5,200,297; which descriptions are incorporated herein by reference.
While these examples all relate to imaging media wherein the image forming or bearing
surfaces are porous or the particulate image bearing surfaces are developed by laminar
separation, use of the present invention is not limited to developed thermal imaging
media, but rather, can also be used advantageously for the protection of images
prepared by resort to other known imaging methods including, but not limited to,
those prepared by thermal dye transfer, ink jet, and laser ablation transfer methods.
Reference is made back again to the laminating sheet 10, which in
this embodiment is in the form of a continuous web having a width generally wider
than the image bearing or carrying medium 16 for ensuring complete lamination coverage
of the image carrying surface. The web 11 can be formed of any material, such as
a filled polyester film base, which supports the thermal transfer overcoating material.
Some characteristics of the web 11 are that it has no subcoats. The web widths can
vary from about 560 to 1600 mm (22 inches to 63 inches) with roll lengths being
6100 to 12200 m (20,000 to 40,000 linear feet). Of course, other dimensions for
the laminating sheet can be employed given the particular medium being laminated.
Film roughness can be approximately 0.2µm RMS. Unrestrained heat shrinkage values
are about 4% in both the machine and transverse directions when measured at 150°C
for 30 min. The thickness can be about 0.023 mm (0.92 mil), but other thickness
dimensions can be used consistent with the principles of the present invention.
The web 11 may be formed from any material, besides the noted polyester material,
so long as it can withstand the conditions which are required to laminate the protective
overcoat material 12 to the image carrying medium 16. If desired, the web 11 may
be treated with a subcoat or other surface treatment, as well-known, to those skilled
in the coating art, to control its surface characteristics, for example, increase
or decrease the adhesion of the durable layer 20 to the web 24 by means of the release
layer 22. The web 11 should be sufficiently coherent and adherent to the durable
layer 20 to permit displacement of both the web 11 and part of the release layer
22, away from the protected laminated image carrying medium including removal of
those portions of the laminating sheet 10 which extend beyond the periphery of the
With reference to the thermal overcoat material, the durable layer
20 may be formed from any material (such as a cured acrylic polymer or a polymethacrylate)
which confers the desired properties for protecting the image. For example, the
aforenoted WO 92/09930 describes an embodiment wherein the durable layer 20 is coated
as a discontinuous layer from a latex which clears during lamination to produce
a clear durable layer. As described, the durable layer is comprised 80% by weight
acrylic polymer, 10% by weight polyethylene/paraffin wax, and 10% by weight aqueous-based
polyamide binder, and was prepared by mixing the polymer and wax lattices, adding
the binder, then adding a silicone surfactant. In general, it is preferred that
the overcoating material 12, when laminated over the binary image bearing layer,
not have a thickness greater than about 30 µm, since thicker overcoating layers
may, in some cases, cause problems in viewing the image due to optical effects within
the overcoating material 12. Desirably, the thickness of the durable layer 20 does
not exceed 10 µm, and, more desirably, this thickness is in the range of 3 to 6
µm. The durable layer 10 should of course be abrasive and chemically resistant to
materials with which it is likely to come into contact, including the materials
which may be used to clean the protected laminated image carrying medium. Although
the exact materials which may contact the image will vary with the intended uses
of the protected laminated image carrying medium, in general it is desirable that
the material for the durable layer 20 should be resistant to and substantially unchanged
by any materials with which it may come into contact, such as water, isopropanol
and petroleum distillates.
It will be appreciated that the protection of the image carrying medium
16 conferred by the protective overcoat is improved with increased lubricity. Therefore,
at least one of a wax, a solid silicone and silicone surfactant is, preferably,
included in the durable layer 20 to increase the lubricity of this layer. Also,
the release layer 22 can be composed of a material having high lubricity.
Referring back to the release layer 22, it may break unevenly so that
part of the release layer having a discontinuous thickness remains with a discard
or throwaway layer or sheet 36 and another part of the release layer 22 remains
attached to the durable layer 20 on the keeper substrate sheet or medium 16. As
noted, however, pinholes in the image forming layer, which are referred to as the
removal type, are caused when particulate pieces or chunks (not shown) of the carbon
of the image forming layer 14 tend to adhere to and go with the part of the release
layer 22 remaining with the throwaway layer 36; see Fig. 3. It will be seen that
the throwaway layer 36 will include the entire laminating sheet when it is not laminated
to the image bearing medium 16.
Now referring to the adhesive layer 18 of the coating material 12,
it is disposed on the surface of the durable layer 20 remote from the web 11. During
lamination, the durable layer is adhered to the image layer 14 by means of the adhesive
layer 18. The use of an adhesive layer 18 is desirable to achieve strong adhesion
between the durable layer 20 and the image carrying medium 16. Various types of
adhesive may be used to form the adhesive layer 18. For example, the adhesive layer
18 might be formed from a thermoplastic adhesive having a glass transition temperature
in the range of about 85°C (185°F), in which case bondability is effected by the
conductive heating of the adhesive layer above its glass transition temperature.
An example of a suitable adhesive layer 18 is designated X95-180. The barrier layer
19 is preferred to be an aqueous barrier coating which performs solvent resistance
functions. It can be a PVDC material, such as Daran ® 158. A laminating sheet
10 which comprises the above laminar constructions is available from Polaroid Corporation,
Cambridge, Massachusetts, USA.
In the laminating system of Fig. 4, the laminating sheet 10 is juxtaposed
to the image carrying medium 16 and both are fed together at a suitable rate, such
as about 13 mm (.5 inches) per second to a laminating unit. Both the sheet 10 and
the image bearing medium 16 travel through a compression nip 28 formed between a
heated roller assembly 30 which is about 90 mm (3.5 inches) in diameter and is actively
heated by a heating device (not shown), and a cold roller assembly 32 which is also
approximately 90 mm (3.5 inches) in diameter, and is actively cooled by a cooling
device (not shown). As will be noted hereinafter, the sheet 10 and the medium 16
can be prewrapped onto an angular portion of the cold roller assembly 32. A variety
of heating devices can be used to heat the heated roller assembly 30. For instance,
the heating device can take the form of an interior resistance cartridge controlled
by an external thermistor spaced near the top surface of the hot roller. The heated
roller assembly 30 is preferably maintained at a temperature of about 166 ± 3°C
(320 ±5°F) and the cold roller assembly 32 is, preferably, maintained at a temperature
of about 32°C (90°F) or less in order to minimize ripple and curl in the protected
laminated image carrying medium 34; as described in greater detail in the last noted
applications. Both the hot roller assembly 30 and the cold roller assembly 32 should
be constructed from conductive materials, such as aluminum, and at least one of
the rollers should have a compliant elastomeric layer to evenly distribute a nip
loading of about 363 kg (800 lb.), Referring back to the cold roller assembly 32,
any commercially available cooling unit can be used to actively cool the temperatures
which are desired. The cold roller assembly 32 can be cooled either internally,
such as by circulating cool air or a liquid coolant through the interior of the
roller, or externally, such as by fanning cooled air over the cold roller surface.
The structure of the cold roller assembly 32 can be designed to maximize the cooling
effect of the cooling unit. For instance, a cold roller cooled by air flow could
be designed as a hollow roller with internal fins.
As the laminating sheet 10 and the image carrying medium 16 are fed
through the nip 28, a bonded image carrying medium 34 is formed due to the adhesive
layer 18 softening, molding to, and adhering to the image carrying medium under
a compressive force for a time sufficient to promote adhesion of it and the barrier
layer, the durable layer and portions of the release layer.
After passing through the nip 28, the bonded sheet and image carrying
medium 16, designated jointly as the bonded image carrying medium 34, are postwrapped
along the cold roller assembly 32 for an arcuate distance defined by the angle &thetas;1,
where &thetas;1 is ideally about 20 degrees. However, this angle can vary for the
reasons noted in the above noted application for eliminating some types of laminating
artifacts, such as longitudinal curl and ripples, in the protected laminated image
carrying medium 34. The purposes for postwrapping the laminated or bonded image
carrying 34 are noted in the last noted application. Basically, the first is to
counter a curl tendency when the sheet 10 is prewrapped along the hot roller assembly
30; the second is extracting heat from the bonded image carrying medium 34 along
the cold roller assembly 32 for eliminating ripples in the protective overcoat;
a third is to prevent thermal expansion from buckling the sheet 10 and thereby imparting
ripples thereto; and, the fourth is to maintain a bond between the sheet 10 and
the cold roller assembly 32 during a time in which the web temperature is high enough
to otherwise distort the web dimensions, compromising registration quality. The
degree of postwrap angles at which the sheet and the medium 16 contact the lower
cold roller do not form part of the present invention and will not be discussed
herein in further detail. Also, the sheet 10 and the image bearing medium 16 are
prewrapped and the prewrap angles can also vary.
For purposes of understanding curl, it is defined as any curvature
of the protected laminated image carrying medium 34 away from the plane of its major
surface area. Curl can occur in either the longitudinal direction which is the direction
of feeding of the sheets, or in the transverse direction which is perpendicular
to the longitudinal direction. Rippling which generally occurs in the transverse
direction, i.e. the direction perpendicular to the feed direction of the web is
defined as oscillating elevations of the protected laminated image carrying medium
above or below the plane of the major surface area of the protected laminated image
After postwrapping the bonded web and image carrying medium 34 throughout
the arcuate distance of &thetas;1, the throwaway layer 36, consisting of the web
11 and a part of the release layer 22, is separated from the protected laminated
image carrying medium 34 by a delaminating assembly which in the preferred embodiment
is in the form of an elongate delaminating bar 50 extending generally parallel to
the laminating roller assemblies. In this regard, the throwaway layer 36 is wound
onto take-up roller 52 (as shown in Figure 4) with the assistance of the tension
supplied by a pair of pull rolls 53. In the process, the throwaway layer 36 is brought
against a delaminating surface 54 defined by the outside surface of the bar 50 with
sufficient tension so as to effect separation or delamination of the throwaway layer
36 from the laminated image carrying medium 34. As noted, the protected laminated
image carrying medium 34 includes the image bearing substrate 15, the image forming
layer 14, the adhesive layer 18, the durable layer 20, and part of the release layer
22. The image bearing medium 34 is pulled under constant tension in a direction
different from the throwaway layer by a pair of eject rolls 55.
As noted the sheet 10 can have a variety of widths and can be a continuous
81.3 mm (32 inch) wide member which spans between an idle supply roller 56 and a
driven take-up roller 52. The width of the web 10 is set to ensure its registration
with the width of the image carrying medium 16. For instance, the image carrying
medium 16 can vary in widths which vary from about 203 to 762 mm (eight inches to
about 30 inches).
In accordance with the present invention it has been determined that
for reducing the formation of pinholes in the image forming layer 14 during delamination
at the delaminating bar 50, the heated temperature of the delaminating bar be-controlled
to be within a predetermined range which has been effective to reduce pinhole formations.
The type of pinholes reduced are those of the removal type which are formed by the
removal of carbon particles from the image forming layer 14 during delamination
of the carrier web 11 and the release layer 22 from the laminated medium 34. It
is believed that the significant reduction of the size of the pinholes, by as much
as 90% when compared to other approaches in delaminating without the temperature
being controlled as indicated, is due to the fact that the controlled heated temperature
affects the release forces of the release layer 22, such that they are more uniform
and thus, the adherence forces on the carbon by the adhesives on the medium 16 are
overcome. As a result, the carbon is not readily pulled away with the throwaway
layer 36. It has been found that the temperature range which is essential for effecting
the pinhole size reduction for the materials above is in a range of about 51 to
71°C (125 to 160°F). If the temperatures are too high there might be a problem with
fringing. Fringing occurs when a clean break between the protected laminated image
carrying medium 34 and the throwaway layer 36 is not realized, so that pieces or
strips of durable layer 20 and adhesive layer 18 remain attached in a stringy form
to the edge of the protected laminated image carrying medium 34. In the illustrated
embodiment, the delaminating bar 50 can be an elongated and hollow piece of anodized
aluminum which extends generally parallel to the roller assemblies. It has been
determined that such an aluminum bar has a thermal heat transfer coefficient which
is effective for 5 controlling heat exchange of the medium and layer 34 and 36;
respectively so as to maintain the temperature within the noted temperature range.
Of course, other materials besides aluminum can be used.
In the illustrated embodiment of Fig. 5, the delaminating bar 50 can
have its temperature controlled so as to be actively heated by an electrical heating
element 60 which is disposed therein. Temperature sensors, not shown, can regulate
the temperature provided by the heating element so that the delaminating bar 50
remains in the desired temperature range for effecting the desired pinhole reduction.
Also, depicted in Fig. 5, is a blower unit 70 which in this embodiment can be a
fan which will be operated to blow air passed the delaminating bar 50 for controlling
the temperature thereof. The blower unit need not be used in conjunction with heater
for effecting the desired temperature control although it is contemplated that such
an arrangement is possible. Of course, appropriate temperature sensors, not shown,
can be oppressively connected to the blower for controlling the latter.