Note: Descriptions are shown in the official language in which they were submitted.
WO 95/02515 PCT/US94/05413
_ ~j~~~~9
HOT STAMPING FOIL
Field of Invention
The present invention relates to a hot stamping
foil, a method for using such foils to form desired
' graphic patterns on substrates, and articles made with
such foils.
Backc round
It is known to form graphic patterns on
substrates using transfer articles bearing
predesignated designs. G.B. Patent No. 1,218,058
(Hurst et al.) discloses transfers with an adhesive
layer applied to only those areas intended to be
transferred to the substrate. U.S. Patent Nos.
4,786,537 (Sasaki) and 4,919,994 (Incremona et al.)
disclose transfer graphic articles wherein the graphic
design is formed via imagewise differential properties
within the transfer film itself. One problem with such
approaches is that a large and varied inventory must be
maintained in order to provide a variety of graphic
patterns.
Hot stamping foils comprising a carrier, one or
more color layers, and an adherence layer have been
known for some time. Such films have been used to
provide imagewise graphic patterns, e.g., alphanumeric
or decorative legends, to substrates. In some
embodiments, additional members such as release layers
are used to facilitate desired performance. In some
embodiments, so-called "texture layers" and/or "ticks",
metal layers, etc. are used as well to yield desired
appearance. Hot stamping foils are also sometimes
called hot stamp tapes or thermal transfer tapes.
To form a graphic pattern on a desired
substrate, the adherence layer of the transfer film is
contacted to the substrate and heat applied in
imagewise fashion to the carrier. This leads to'
bonding of the adherence layer to the substrate where
heat was applied. The carrier is then removed,
resulting in selective separation of the color layers)
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therefrom in the region or regions corresponding to the
image in which the heat was applied. The carrier and
non-heated portions of the color layers) and adherence
layer are removed from the substrate leaving the
desired graphic pattern thereon. The color layer(s),
adherence layer, and any other layers that are
selectively applied to the substrate should split or
fracture in desired manner in order for the applied
graphic pattern to have a desired edge appearance.
Some illustrative examples of previously known
hot stamping foils are disclosed in U.S. Patent Nos.
3,770,479 (Dunning) 3,953,635 (Dunning), and 4,084,032
(Pasersky).
It has also been known to transfer graphic
patterns using means in addition to or other than heat
to achieve imagewise separation of imaging material
from a carrier and adhesion to a substrate. For
example, U.S. Patent No. 3,834,925 (Matsumura et al.)
discloses a transfer material that utilizes solvent
action to achieve imagewise deposition.
An advantage of the foregoing techniques is
that the transfer film may be made as a uniform sheet,
i.e., with no specific latent image embodied therein.
The applicator defines the graphic pattern by
controlling the application process, e.g., imagewise
application of heat. This permits the maintenance of a
smaller inventory of material.
One well known use of hot stamping foils is to
print legends on vehicle identification plates. For
example, license plates produced using hot stamping
foils have been used in Austria, Australia, Finland,
Germany, Ireland, Portugal, and Switzerland. One
commercially available hot stamping foil currently used
on license plates with polyvinyl chloride cover films
is believed to comprise a polyester carrier, about 28
micrometers thick; a color layer based on acrylic
resins such as polymethyl methacrylate and containing
carbon black pigments, about 5 micrometers thick; and
an acrylate-based adherence layer, about 5 micrometers
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WO 95/02515 PCT/US94/05413
~~~~649
thick. Examples of resins that are believed to have
been used in adherence layers include polyvinyl alcohol
copolymers, nitrocellulose, and methyl
methacrylate/butyl methacrylate copolymers.
Recently improved retroreflective sheetings
have been made available which have cover films made of
olefin-based materials or polyurethane-based materials
to improve certain performance. As disclosed in U.S.
Patent No. 4,896,943 (Tolliver et al.), olefin-based
cover films, e.g., ethylene/acrylic acid copolymers,
can provide superior properties including abrasion and
dirt resistance. Conventional hot stamping foils do
not achieve good adherence to such cover sheets,
however, resulting in graphic patterns having
unsatisfactory durability and performance. The need
exists for improved hot stamping foils which can be
used to,form durable graphic patterns on such cover
sheets and a method for forming such graphic patterns.
Summarv of Invention
The present invention provides a novel hot
stamping foil that provides previously unattainable
performance. The present invention also provides a
novel method utilizing the hot stamping foils described
herein to form graphic patterns on retroreflective
substrates and also provide articles made with such
transfer films.
Briefly summarizing, hot stamping foils of the
invention comprise, in order, a carrier, optionally a
release control layer, a color layer, and an adherence
layer. In an important distinction over previously
known hot stamping foils, the adherence layer of hot
stamping foils of the invention comprises, and may
consist essentially of, a mixture of an ethylene
. 35 copolymer dispersion and an acrylic dispersion.
In brief summary, the method of the invention
. for forming a graphic pattern on a substrate comprises
the steps of:
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a) providing a retroreflective sheet with a face on
which a graphic pattern is to be formed;
b) providing a hot stamping foil comprising, in
order, a carrier, optionally a release control
layer, a color layer, and an adherence layer
wherein the adherence layer comprises a mixture of
ethylene copolymer dispersion and acrylic
dispersion;
c) contacting the adherence layer to the face of the
retroreflective sheeting such that carrier of the
hot stamping foil is exposed;
d) applying heat in imagewise fashion to the carrier
of the hot stamping foil such that corresponding
portions of the adherence layer are activated; and
e) removing the carrier from the retroreflective
sheet such that an imagewise portion of the
adherence layer and the color layer remain bonded
to the face of the retroreflective sheet and the
remainder of the adherence layer and said color
layer are removed with the carrier.
Hot stamping foils of the invention provide
good adhesion to a variety of materials (such as
polyvinyl chloride, polyurethanes, polyolefins, alkyd-
melamines, etc.) that are used as cover films on
retroreflective sheetings. One of the advantages of
the present invention is that the hot stamping foils
described herein can be used to form graphic patterns
exhibiting good adhesion to high performance cover film
materials such as olefin-based cover films, e.g.,
ethylene/acrylic acid copolymers. Graphics formed with
foils of the invention do not need additional drying or
curing time and achieve durable, abrasion-resistant
state quickly. No special post application brushing or
treatment is required. High resolution images with
sharp edge definition may be achieved. The graphics
are easily applied in solvent free manner and the foils
are polyvinyl chloride, heavy metal free.
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WO 95/02515 PCT/US94/05413
Brief Description of Drawincr
The invention will be further explained with
reference to the drawing, wherein:
Figure 1 is a cross-sectional illustration of
an illustrative hot stamping foil of the invention; and
Figure 2 is a cross-sectional illustration of
an illustrative retroreflective article of the
invention.
These figures, which are idealized, are not to
scale and are intended to be merely illustrative and
non-limiting.
Detailed Description of Illustrative Embodiments
An illustrative hot stamping foil of the
invention is shown in Figure 1. Hot stamping foil 10
comprises carrier 12, optional release control layer
14, color layer 16, and adherence layer 18. In some
instances an optional release liner (not shown) may be
provided over adherence layer 18.
Foils 10 of the invention are typically wound
into roll form for shipping and handling and are
typically sufficiently flexible to be wound around a
2.5 centimeter (1 inch) diameter core without cracking
or breaking. In many instances, foils of the invention
will be used to apply graphics to substantially planer
surfaces, but if appropriate application equipment is
used they can also be used to apply graphics to non-
planar substrates.
Carrier 12 provides means for handling foil 10.
Carrier 12 is preferably sufficiently heat resistant to
remain dimensionally stable (i.e., substantially
without shrinking, curling, or stretching) when heated
to a sufficiently high temperature to achieve adherence
of the adherence layer to the desired substrate, e.g.,
- 35 typically at least about 200°C in most instances.
Also, carrier 12 preferably provides desired adhesion
to the color layer during shipping and handling as well
as desired release properties from the color layer
after contact to the substrate and heating. Finally,
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PCT/US94/05413
carrier 12 and the other components of foil 10
preferably exhibit sufficient thermal conductivity such
that heat applied in imagewise fashion will heat a
suitable region of the adherence layer such that a
graphic of desired resolution is transferred.
Carrier 12 is typically between about 10 and 40
micrometers thick. Carriers which are too thin may
tend to provide insufficient support to foil 10 and
carriers which are too thick may tend to provide
insufficient thermal conductivity such that resolution
of the graphic is impaired.
Suitable carriers 12 can be selected by those
skilled in the art. Illustrative examples of suitable
carrier materials include the following: polyester
film (e.g. , a sheet of MYLAR~'''' 23A Polyester from E.I.
DuPont De Nemours Company), aluminum foil, and coated
papers. The carrier should be able to withstand the
temperature encountered during application, for
instance, MYLAR~ polyester films are useful for
application temperatures under 200°C with other
polyester films being preferred for use under higher
temperatures.
Depending upon the characteristics of carrier
12 and~color layer 16, optional intermediate release
control layer 14 may be desired. Release control layer
14 may be selected to provide the desired adhesion and
release characteristics between carrier 12 and color
layer 16. Suitable release control layers may be
readily selected by those skilled in the art.
Illustrative examples of typically suitable materials
include wax or lacquer. Typically, release control
layers will be relatively thin, e.g., about 0.1
micrometer or so.
Color layer 16 is selected to provide desired
adhesion and release from carrier 12, or, if used,
release control layer 14. Color layer 16 also
essentially defines the appearance of the resultant
graphic and is formulated to provide the desired color.
Color layer 16 may be made up of an essentially
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WO 95/02515 ~ PCT/US94/05413
homogenous or uniform layer of desired color, or may be
segmented with two or more different colors if desired.
Color layer 16 and adherence layer 18 are typically
coextensive with one another. They may each be
substantially continuous over substantially the entire
foil or they may be made up of discontinuous segments
corresponding to the desired graphic pattern.
Typically color layer 16 comprises one or more
inks selected from among the following: cellulose
inks, polyvinyl chloride inks, polyurethane inks, and
acrylic inks. Suitable color layers may be readily
formulated by those skilled in the art.
Typically color layer 16 is between about 2 and
about 15 micrometers thick, preferably between about 4
and about 6 micrometers thick. Color layers which are
too thick may tend to undesirably increase the thermal
conductivity of foil 10 such that graphic resolution is
impaired. Color layers which are too thin may tend to
yield graphics which do not exhibit desired durability,
hiding power, etc. Color layer 16 can be formed by
known techniques, e.g., coating or printing.
As will be understood, the surface texture and
appearance of color layer 16 is defined at least in
part by the surface of carrier 12, or release control
layer 14 if used, to which it is contacted.
A critical feature of the present invention is
the use of a novel adherence layer that provides
surprising results. In accordance with the invention,
adherence layer 18 comprises a mixture of ethylene
copolymer dispersion and acrylic dispersion, as well as
certain optional ingredients, if desired.
Advantageously the ethylene copolymers used
herein display hot melt adhesive properties. The
utility of such materials in this regard is influenced
- 35 by their melt index and the binder copolymer is
preferably selected so as to provide good quality hot
melt adhesive characteristics. Generally, copolymers
having a melt index (measured according to ASTM 1238-
88) between about 50 and 2000 will be preferred, with
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WO 95/02515 ~ ~ ~ ~ PCT/US94/05413
those having a melt index between about 50 and 1500
being more preferred for use as binders herein. If the
melt index is too high, the resulting polymer may tend
to be too soft, have reduced temperature resistance,
and be subject to undesirable creep. If the melt index
of the selected copolymer is too high, the resultant
adherence layer may be too stiff and may not form a
good bond to substrates to which graphics are applied.
The most preferred binders are ethylene/acrylic
acid copolymers and ethylene/methacrylic acid
copolymers. In these materials the ethylene monomer
preferably makes up about 91 to 97 mole percent of the
copolymer with the acrylic or methacrylic acid monomer
correspondingly providing about 3 to 9 mole percent.
Ethylene copolymer dispersions having a pH of
at least 7 have been found to be generally stable and
to yield a stable mixture with many acrylic dispersions
used herein. It has been observed that using ethylene
copolymer dispersions having a ph of less than 7 tend
to lead to flocculation when mixed with some acrylic
dispersions.
The viscosity of the ethylene copolymer
dispersion should be such as to yield a mixture with
the acrylic dispersion that can be formed into the
adherence layer in desirable manner. For instance, if
the adherence layer is to be formed via gravure
printing, the viscosity of the ethylene copolymer
dispersion should generally be below about 500 milli
Pascals x seconds (mPa's). If the adherence layer is
to be formed via rotary screen printing or coating,
higher viscosities, e.g., up to about 5,000 mPa's are
generally useful.
An example of a commercially available suitable
ethylene copolymer is Dow Chemical Company's PRIMACOR~
4990, a 35 weight percent solids water-based dispersion
of ethylene/acrylic acid copolymer. Other illustrative
alternative water-based ethylene/acrylic acid copolymer
dispersions include ADCOTE~ 50T4983 (made from Dow
PRIMACOR~ 5980) having approximately 20 weight percent
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WO 95/02515 PCT/US94/05413
(9 mole percent) acrylic acid and a melt index of 300,
ADCOTE~ 50T4990 (made from Dow PRIMACOR~ 5990) having
approximately 20 weight percent (9 mole percent)
acrylic acid and a melt index of 1300, and ADCOTE"''
t 5 56220 having 13 weight percent (7.5 mole percent)
methacrylic acid and a melt index of 60. Each of these
materials is available from Morton. Other useful
examples include MICA G-927 and Byk Cera's AQUACER'~"~
6RC684.
The ethylene copolymer is blended or
supplemented with another resin to improve adhesive
characteristics of the adherence layer. Particularly
useful in this regard are acrylic emulsions resulting
from the radical polymerization of methacrylic acid,
acrylic acid, butyl acrylate, and/or other acrylic
monomers.
Acrylic dispersions having a pH of at least 7
and a Brookfield viscosity of less than 200 mPa's have
been found useful. It is believed that acrylic
dispersions that yield films capable of being heat
laminated to polyvinyl chloride or non-crosslinked
polyurethanes at temperatures of under about 120°C will
be useful herein. An example of a commercially
available suitable acrylic dispersion is ICI's NEOCRYL'~"s
A45, a 37.5 weight percent solids water-based
dispersion of acrylic copolymers. Also useful are
polyurethane dispersions, resulting from the reaction
of an aliphatic isocyanate with a polyol. An example
of a commercially available suitable polyurethane
dispersion is ICI's NEOREZTM 8962.
Commercially available waterborne polymer
emulsions and dispersions are typically provided as a
salt in which the carboxylic acid functionality has
been neutralized with a base so as to improve the
aqueous dispersibility of the resin. Consequently, it
will be understood the adherence layer formulations may
contain basic salts of the above polymers. Useful
bases for neutralizing the carboxylic acid
functionality include ammonia and other amines. Non-
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WO 95/02515 ~ ~~ ~ ~ ~ PCT/US94/05413
volatile bases such as sodium hydroxide or potassium
hydroxide may also be used, but are less preferred due
to concerns of moisture sensitivity.
Ethylene copolymer dispersions and acrylic
dispersions that yield mixtures that are of suitable
viscosity for formation of an adherence layer via
desired technique, stable mixtures, and soften at an
appropriate temperature for hot stamping (e. g.,
typically at least 55°C or more to withstand exposure
in the application equipment and conditions to which
the finished substrate is likely to be subjected and
preferably within the range of about 100 to 120°C) are
suitable.
Typically the ethylene copolymer dispersion and
acrylic dispersion are present in adherence layer 18 in
a weight ratio between about 10:90 and about 60:40,
preferably in a weight ratio between about 3:2 and
about 2:3, and most preferably in a weight ratio
between about 55:45 and about 45:55. Formulations
outside these ranges tend to provide less effective
results. For instance, it has been observed that
adherence layers comprising essentially NEOCRYL~ A45
alone would provide satisfactory adherence to polyvinyl
chloride cover films but not to other cover films such
as polyolefins such as ethylene/acrylic acid copolymers
(e. g., SURLYN~ and PRIMACOR~ materials), crosslinked
polyurethanes, and polyurethane/acrylic blends. Also,
its has been observed that adherence layers comprising
essentially PRIMACOR~ 4990 alone do not provide
satisfactory adherence to any of the following cover
films: polyvinyl chloride, polyolefins such as
ethylene/acrylic acid copolymers (e.g., SURLYN~ and
PRIMACOR~ materials), crosslinked polyurethanes, and
polyurethane/acrylic blends; and it is expected that
such adherence layers would not provide satisfactory
adherence to cover films of alkyd-melamine/polyester
blends or polyvinylbutyral.
Formulations comprising between about 20 and
about 30 parts by weight of PRIMACOR'~'' 4990, between
WO 95/02515
PCT/US94I05413
about 30 and about 20 parts by weight of NEOCRYL'~"~ A45,
about 25 parts water, and about 25 parts ethanol, have
been found to yield adherence layers that provided good
adhesion to retroreflective sheetings with cover films
made of polyvinyl chloride, polyolefins such as
ethylene/acrylic acid copolymers (e.g., SURLYNT"~ and
PRIMACOR~ materials), crosslinked polyurethanes,
polyurethane/acrylic blends, alkyd-melamine/polyester
blends, and polyvinylbutyral. Dilution with water and
ethanol has been found to improve mixing of the two
components to yield a uniform mixture and facilitates
processing to form the adherence layer. A formulation
containing equal parts by weight of PRIMACOR~ 4990,
NEOCRYL~ A45, water, and ethanol has been found to be
suitable for most applications.
Optimum mixing ratios for specific applications
can be readily determined by trial and error. For
instance, weight ratios of PRIMACOR~ 4990 to NEOCRYL~
A45 from about 2:3 to 1:9 have yield adherence layers
that provided satisfactory adhesion to polyvinyl
chloride and polyolefin cover films, but provided
satisfactory adhesion to only some polyurethane-based
cover films.
In some instances it may be desirable to
include a water-based dispersion of polyurethane, e.g.,
up to about 20 to 35 parts of ICI~s NEOREZ~ 8962, in
the adherence layer to improve adhesion to cover films
made of polyvinyl chloride, polyolefins, and at least
some polyurethanes.
Adherence layer 18 is typically between about 1
and about 5 micrometers thick, preferably between about
1 and about 3 micrometers thick. If adherence layer 18
is too thick, it may tend to yield a resultant graphic
which has an undesirably high profile and it may also
tend to reduce the thermal conductivity of foil 10 such
that the resolution of the resultant graphic is less
than desired. If adherence layer 18 is too thin, color
layer 16 may not be securely bonded to the substrate
(not shown) and thus may exhibit less durability than
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WO 95/02515 ~,~~~ ~ PCT/US94/05413
desired. Adherence layer 18 can be formed by known
techniques, e.g., coating or printing.
Hot stamping foil 10 of the invention may be
used to form a graphic pattern on the face of a
retroreflective sheeting by the steps of:
a) providing a retroreflective sheet;
b) providing a hot stamping foil as described herein;
c) contacting the adherence layer of the hot stamping
foil to the face of the retroreflective sheeting
such that carrier of the hot stamping foil is
exposed;
d) applying heat in imagewise fashion to the carrier
such that corresponding portions of the adherence
layer are activated; and
e) removing the carrier from the retroreflective
sheet such that an imagewise portion of the
adherence layer and the color layer remain bonded
to the face of the retroreflective sheet and the
remainder of the adherence layer and the color
layer are removed with the carrier.
An advantage of the present invention is that
hot stamping foils of the invention may be used to form
graphic patterns on retroreflective sheetings and other
substrates with face comprising olefin-based materials
or polyurethane-based coatings. Conventional hot
stamping foils do not adhere well to such substrates,
particularly when no priming treatment is used. For
instance, hot stamping foils of the invention may be
used to great advantage to provide graphic patterns on
cover films or faces that comprise ethylene/acrylic
acid copolymer. If desired, hot stamping foils of the
invention may be used to provide graphic patterns on
substrates to which no priming treatment has been
applied.
A typical article produced in accordance with
the method of the invention using hot stamping foils of
the invention is shown in Figure 2. Retroreflective
article 20 comprising face layer 22 made of an olefin-
based material or a polyurethane-based coating with
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WO 95/02515 ~ PCT/US94105413
graphic pattern 24 adhered to face layer 22. Graphic
pattern 24 comprises color layer 16 and adherence layer
18 as described herein. Typically retroreflective
article 20 will comprise a layer of retroreflective
elements (e. g., a monolayer of microspheres 28, spacing
layer 30, and reflective layer 32) disposed under face
layer 22 and adhesive layer 34. Illustrative examples
of such articles include motor vehicle license plates,
retroreflective stickers, or retroreflective emblems.
Various modifications and alterations of this
invention will become apparent to those skilled in the
art without departing from the scope and spirit of this
invention.
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