Note: Descriptions are shown in the official language in which they were submitted.
~3;~7~
~ his invention relates to methods of preparing screen
printing stencils, to coating compositions and copolymers for
use in preparing screen printing stencils, and to improved screen
printing stencils and methods of using them. More particularly,
this invention relates to the use of certain ultraviolet-sensitive
copolyacrylate/polyurethane block copolymeric compositions in
preparing screen printing stencils.
The art of screen printing was developed around 1900.
This art was originally referred to as "silk-screen printing,"
that term being derived from the fine mesh silk which was origi-
nally used as the screen. According to this method of printing,
a fine~mesh fabric, such as silk, is stretched across a frame
and an image is formed on the fabric. In its simplest form,
the so-called "silk-screen stencil" is formed by gluing a hand
cut paper stencil to the silk screen. The side of the silk~screen
stencil to which the paper stencil is attached is then placed
; against the surface to be imprinted, and ink is forced through
the screen onto the surface. When the ink is forced through
the openings in the silk mesh, it imprints on those areas of
the surface which are not covered by pieces of the paper. Thus
the printing operation is based on the ability of the ink to
flow through only that part of the screen mesh which is not
; blocked by the paper, i.e., through the "cut image."
A more advanced method for preparing a screen printing
stencil is the "tusche and glue" method. In accordance with
this method, a design or image is formed on the screen with
tusche, a type of lithographic ink, by painting the tusche on
the screen. After the tusche has dried, the entire screen is
covered with a glue, and when the glue dries, the entire screen
is washed with an organic solvent, such as kerosene or turpentine.
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The solvent washes out the glue covering the tusche, but the
remainder of the glue is retained on the screen. Thus, the only
mesh areas which remain open for ink transfer are those defining
the original tusche design.
~ n even more advanced method of forming screen printing
stencils involves the use of photosensitive compositions to
form images in response to light transmitted through a master.
; This method is discussed in U.S. Patent 3,246,986. In accordance
with this method, a screen is coated with a colloid solution
containing a light sensitizer. The coated screen is allowed
to dry and then expcsed to light through a master (such as a
photographic negative). The light which passes through the master
and onto the coated screen causes the colloid to be hardened
in a pattern corresponding to that of the master. Those portions
of the coated screen which are protected from the light by the
nontransparent part of the master are unaffected by the light,
and the coating thereon remains unhardened. Since the unhardened
colloid material is soluble in water and the hardened colloid
is insoluble, an image can be "developed" on the screen by wash-
ing it with water. When this is done, the unhardened colloidmaterial is washed away, while the image-defining hardened mate-
rial remains on the screen, thus orming a screen printing stencil.
The screen printing stencil so prepared may then be used for
printing the image on receptor surfaces, such as paper, fabric
and the like. Various colloids and light sensitizers have been
used to prepare the light-sensitive coatings used in this method.
Typical colloids used in preparing screen printing stencils by
this method are listed in U.S. Patent 3,246,986 and include
polyvinyl alcohol, partially acrylated polyvinyl alcohol, par-
tially hydrolyzed polyvinyl acetate, partially acetylated
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polyvinyl alcohol, polyacrylic acid, methyl cellulose and gela-
tin. Typical light-sensitizing compounds used are potassium
dichromate, ammonium dichromate and condensation products of
diazo diphenylamines with aldehydes.
Unfortunately, these methods of preparing screen print-
ing stencils and the screen pxinting stencils prepared by such
methods often have undesirable characteristics. For example,
manv of the screen printing stencils prepared by the prior art
photochemical methods do not perform well with water-based inks,
because the indicia-defining materials on the screen printing
stencils prepared by these methods are generally sensitive to
the water-based inks, which are usually alkaline in nature.
Although screen printing stencils which are compatible with
water-based inks may be prepared using certain lacquers to define
indicia thereon, we are aware of no photochemical method for
preparing screen printing stencils w:ith such materials.
; Even where water insensitivity is not required, the
~` prior art photochemical methods of preparing screen printing
stencils are not completely ~atisfactory. In this regard, the
use of dichromate light sensitizers in the prior art methods
is of particular concern, since they are generally environmentally
unacceptable, poisonous compounds.
Therefore a need exists for a new method of preparing
screen printing stencils, for new coating compositions and co-
polymers useful in preparing screen printing stencils, for new
screen printing stencils and for new screen printing methods.
It is therefore a primary object of this invention
to provide a new photochemical method of preparing screen print-
ing stencils without the use of the prior art diazo or dichromate
light sensitizing compounds. It is another object of this
invention to provide new ultraviolet-cross-linkable liquid
coating compositions for preparing screen printing stencils
for printing with either water-based or oil-based inks. It
is another primary object of this invention to provide new
copolyacrylate/polyurethane block copolymers which, when cross-
linked, form materials having physical characteristics and
solvent resistance suitable for coa~ings on screen printing
stencils for use with either water-based or oil-based inks.
It is yet another object of this invention to provide screen
printing stencils and screen printing methods for printing
with either water-based or oil-based inks. Other objects o~
the invention will in part be obvious and will in part be
apparent hereinafter.
The invention accordingly comprises the several
steps and the relation of one or more of such steps with res~
pect to each of the others, the composition possessing the
features, properties and the relation of constituents and the
article possessing the ~eatures, properties and the relation
; of elements, all as exemplified in the detailed disclosure
hereinafter set forth, and the scope of the invention will be
indicated in the claims.
In accordance with one aspect of this invention
there is provided a method of preparing a screen printing
stencil, characterized by the step of affixing to a printing
screen substrate an indicia-defining coating, the coating
comprising an ultraviolet-cross-linked block copolymer formed
of blocks of copolyacrylate and polyurethane. The copolyacry-
late is a copolymer of a hydroxy-containing acrylate and an
acrylate which is partially substituted with bromine or iodine.
In preferred embodiments of the method of this
invention, a predetermined indicia pattern is produced on a
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substrate coated with an ultraviolet~cross-linkable coating by
exposing the coated substrate to ultraviolet radiation through
a master defining the indicia as a transparent pattern for a
time sufficient to partially cross-link the coating in areas
corresponding to the transparent pattern, and developing the
indicia pattern by removing from the substrate that portion
of the coating which is not cross-linked by the exposure to
the ultraviolet radiation.
In another aspect of this invention there is prov-
ided an ultraviolet-cross-linkable liquid coating composition
comprising a block copolymer formed of blocks of a copoly-
acrylate and a polyurethane, the copolyacrylate comprising a
copolymer of a hydroxy-containing acrylate and an acrylate
partiall~ substituted with bromine or iodine, a solvent for
the block copolymer, an ultraviolet initiator and a cross-
` linking promoter.
In accordance with another aspect of this inventionthere is provided a new block copolymer formed of blocks of
copolyacrylate and polyurethane, the copolyacrylate comprising
a copolymer of a hydroxy-containing acrylate and an acrylate
which is partially substituted with bromine or iodine.
In accordance with yet another aspect of this
invention there is provided a new screen printing stencil
having indicia defined thereon by a cross-linked block co-
polymer formed of blocks of a copolyacrylate and of a poly-
urethane, the copolyacrylate comprising a copolymer of a
hydroxy-containing acrylate and an acrylate partially substi-
tuted with bromine or iodine.
In accordance with still another aspect of th;s
invention there is provided an improved method of screen
printing wherein indicia are printed onto a receptor surface by
' ~ - 5 -
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bringing the indicia-bearing side of a screen printing stencil
having indicia defined thereon by an indicia-defining coating
into contact with the receptor surface and forcing ink
through those areas of the screen printing stencil not blocked
by the indicia-defining coating, the improvement comprising
preparing the screen printing stencil by the method of this
-invention.
In accordance with a paritcular embodiment of the
invention, a method of preparing a screen printing stencil -
is characterized by the step of affixing to a printing
screen substrate an indicia-defining coating, said coating
comprising an ultraviolet cross-linked block copolymer
formed of blocks of copolyacrylate and polyurethane, said
block copolymer comprising from about 60 to about 90
percent by block copolymer weight of copolyacrylate and from
about 10 to about 40 percent by weight of polyurethane, said
copolyacrylate weight of a hydroxy-containing acrylate and
from about 5 to about 50 percent by weight of an acrylate
substituted with bromine or iodine.
In accordance with a further embodiment of the
invention, a method of preparing a screen printing stencil
comprises the steps of a. affixing to a transparent support
sheet a coating composition comprising (1) a block copolymer
formed of blocks of a copolyacrylate and a polyurethane said
block copolymer comprising from about 60 to about 90 percent
by block copolymer weight of copolyacrylate and from about 10
to about 40 percent by weight of polyurethane, said copoly-
acrylate comprising from about 5O to about 95 percent by
copolyacrylate weight of a hydroxy-containing acrylate and from
about 5 to about 50 percent by weight of an acrylate substituted
with bromine or iodine, (2) an ultraviolet initiator, and
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(3) a cross-linking promoter, (b) drying said coating
composition to form an ultraviolet cross-linkable coating on
said support sheet thereby to provide a coated support sheet;
(c~ producing in said coating a predetermined indicia pattern,
(d) transferring said indicia pattern to a printing screen
substrate, and (e) exposing said indicia pattern on said
screen substrate to ultraviolet radiation thereby to form a
printing screen stencil.
In accordance with a still further embodiment of -
the invention, a method of preparing a screen printing stencil
comprises the steps of (a) affixing to a printing screen sub-
strate a coating composition comprising (1) a block copolymer
formed of blocks of a copolyaerylate and a polyurethane said
block eopolymer comprising from about 60 to about 90 pereent
by bloek eopolymer weight of eopolyacrylate and from about 10
: to about ~0 pereent by welght of polyurethane, said copoly-
acrylate eomprising from about 50 to about 95 percent by copoly-
: acrylate weight of a hydroxy-containing acrylate and ~ro~ about
5 to a~out 50 percent by weight of an acrylate substituted
with bromine or iodine, (2) an ultraviolet initiator, and (3)
a cross-linking promoter' (b) drying said coating composition
to form an ultraviolet cross-linkable coating on said printing
screen substrate, (c) producing in said coating a predetermined
indicia pattern; and (d) exposing said indicia pattern on said
screen subs~rate to ultraviolet radiation thereby to form a
screen printing stencil.
From a different aspect, and in accordance with
the invention, there is ~rovided a block copolymer formed of
blocks of copolyacrylate and polyurethane, said block
copolymer comprising from about 60 to about 90 percent by
block copolymer weight of copolyacrylate and from about 10
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to about 40 percent by weight of polyurethane, said copoly-
acrylate comprising from about 50 to about 95 percent by
copolyacrylate weight of a hydroxy-containing acrylate and
from about 5 to about 50 percent by weight of an acrylate
substituted with bromine or iodine.
In accordance with a further embodiment of the
invention, a screen printing stencil has indicia defined
thereon by a cross-linked block copolymer formed of blocks of
a copolyacrylate and a polyurethane, said block copolymer com-
prising from about 60 to about 90 percent by block copolymerweight of copolyacrylate and from about 10 to about 40 percent
by weight of polyurethane, said copolyacrylate containing from
: about 50 to about 95 percent by copolyacrylate weight of a
hydroxy-containing acrylate and from abou-t 5 to about 50
percent by weight of an acrylate substituted with bromine or
iodine.
In accordance with a still further embodiment of
the invention, an ultraviolet cross-linkable liquid coating
composition comprises (a) a block copolymer formed of blocks
of a copolyacrylate and a polyurethane, said block copolymer
comprising from about 60 to about 90 percent by block ~:
copolymer weight of copolyacrylate and from about 1~ to about
40 percent by weight of polyurethane, said copolyacrylate
comprising from about 50 to about 95 percent by copolyacrylate
weight of a hydroxy-containing acrylate and from about 5 to
50 percent by weight of an acrylate substituted with bromine or
iodine (b) a solvent for said block copolymer, (c) an ultra
violet initiator and (d) a cross-linking promoter~
For a fuller understanding of the nature and object
of this invention, reference should be had to the following
detailed description taken in connection with the accompany-
6b -
ing drawings in which
Figs. 1-11 illustrate the steps of one method
embodiment of this invention'
Fig. 12 illustrates the use of a screen printing
stencil of this invention in printing on a receptor surface
, ..
Figs. 13-17 illustrate the steps of another
method embodiment of this invention'
Figs. 18-20 illustrate the steps of yet another
method embodiment of this invention and
Fig. 21 illustrates the steps of still another
method embodiment of this inventionO
The preparation of screen printing stencils in
accordance with the method of this invention is illustrated
diagrammatically in Figs. 1-11 in which the same reference
numerals are used to identify the same components.
Fig. 1 illustrates one type of master 10 which is
used in the method of this invention. This master is a
negative transparency having a nontransparent background 11
and transparent indicia pattern 1~. It is also, of course,
within the scope of this invention to use a positive trans-
parency as a master which is the reverse of that shown in -~
Fig. 1.
As shown in Fig. 2, the ultraviolet-sensitive coat-
ing composition 21 of this invention is applied to one side of a
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light transparent support sheet 20, such as polyethylene tere-
phthalate. The coated support sheet 20 is then (Fig. 3) exposed
to an ultraviolet radiation source 25 through master 10, having
ultraviolet-transparent indicia 12 and nontransparent background
11. The ultraviolet radiation passing through indicia 12 of
master 10 causes the partial cross-linking 22 (Fig. 4) of a por-
tion of the coating 21 in a pattern corresponding to that of
indicia 12, while the remainder of coating 21, which is protected
from irradiation by nontransparent background 11 of master 10,
is unaffected. Indicia 22 are then "developed" on support sheet
20 by placing support sheet 20 in an etching bath containing
a water/alcohol solution, such as 1/3 (by volume) water/ethyl
alcohol, to dissolve away the unexposed coating composition 21
leaving on the support sheet only the exposed, partially cross-
; linked indicia-defining coating 22 as shown in Figs. 5 and 6.
After drying indicia-defining coating 22, the coated side of
support sheet 20 is brought into contact with a suitable screen
23 so that indicia-defining coating 22 is hetween screen 23 and
support sheet 20, thus forming an assembly having indicia-defining,
partially cross-linked coating 22 interposed between support
sheet 20 and screen 23 as shown in Fig. 7. The indicia-de~ining
coating 22 extends up into the interstices of screen 23 to form
a good bond. Screen 23 is then moistened with an appropriate
solvent, such as the water/alcohol solution used in the develop-
ment step, and allowed to stand for about 30 seconds to improvQ
the adhesion of indicia-defining coating 22 to screen 23. Alter-
natively, coated support sheet 20 may be brought into contact
with screen 23 while indicia-defining coating 22 is still wet
from the development step, in which case it will not be neces-
sary to moisten the screen. Excess solvent is then removed by
33~
blotting with a water-absorbent surface such as newsprint. The
assembly shown in Fig. 7 is then air dried for about 15 minutes,
after which support sheet 20 is removed (Fig. 8) to leave indicia-
defining coating 22 on screen 23 to form screen printing stencil
24 as shown in Figs. 9 and 10. The coating-bearing side of
screen printing stencil 24 is dried for about 10 minutes, and
then coating 22 on screen printing stencil 24 is post-cured by
exposing it to ultraviolet radiation (Fig. 11), which completes
the cross-linking of the partially-cross-linked, indicia-defining
coating composition 22 and promotes its bonding to screen 23.
Fig. 12 illustrates the use of the screen printing
stencil of Fig. 11 in printing indicia onto a suitable receptor
surface 30. As illustrated, the indicia-bearing side of screen
printing stencil 24 is contacted with receptor surface 300
Printing ink is applied to the opposite side of screen printing
stencil 24 and forced through those areas 26 (Fig. 12) of screen
printing stencil 24 not blocked by indicia-defining coating com-
position 22~ The ink passing through the screen printing stencil
and onto receptor surface 30 defines indicia on receptor surface
30 which thereby becomes a right reading copy of master 10.
Figs1 13-18 illustrate the preparation of screen print-
ing stencils in accordance with another embodiment of the method
of this invention. As shown in Fig. 13, transparent support
sheet 20, which is coated with coating composition 21, is brought
into contact with screen 23 in such a mannex as to create an
assembly having coating composition 21 interposed between support
sheet 20 and screen 230 Screen 23 is then moistened with an
appropriate solvent, such as a water~alcohol solution or some
of the original coating solution, to help bond screen 23 to coat-
ing 21 on support sheet 20~ Excess solvent is removed by blotting
with newsprint or the like. The assembly is then dried in amanner not to induce premature cross-linking after which it is
exposed to ultraviolet radiation through indicia master lO, from
the direction of the support sheet side (Fig. 14). The ultra-
violet radiation passes through transparent indicia 12 of master
lO, then through transparent support sheet 20 to coating 21.
This causes the partial cross linking of those areas of coating
21 which correspond to indicia 12 of master 10 (Fig. 15). The
remainder of coating 21, being protected from irradiation by
nontransparent area ll of master 10, is unaffected.
Support sheet 20 is then removed (Fig. 16) leaving
the coating (both cross-linked 22 and noncross-linked 21) on
screen 23 (Fig. 17). lndicia are then "developed" from the
coating on the screen by placing the screen in an etching bath
containing a water/alcohol solution which dissolves away noncross-
linked coating 21 leaving only partially cross-linked indicia-
defining coating 22 on the screen as previously shown in Fig.
9. The resulting unfinished screen printing stencil 24, carrying
partially cross-linked indicia-defining coating 22 adhered thereto,
is then dried and exposed to ultraviolet radiation to complete
the cross-linking of indicia defining coating 22 and promote
its bonding to the screen, as previously shown in Fig. ll. This
completes the screen printing stencil.
Figs. 18-20 illustrate the preparation of screen print-
ing stencils in accordance with still another method embodiment
of this invention. In accordance with this embodiment coating
composition 21 is applied directly onto both sides of screen
23, as shown in FigO 18. Then, as shown in Fig. l9, coated screen
23 is exposed to ultraviolet radiation source 25 through master
10. The ultraviolet radiation passing through transparent
indicia 12 of master 10 causes partial cross-linking 22 tFig.
20) of that part of coating composition 21 corresponding to the
pattern of indicia 12, while the remainder of coating 21, being
protected from the ultraviolet radiation by the nontxansparent
background of master 10, is unaffected. Indicia are then "de-
veloped" from the coating on the screen by placing the screen
in an etching bath containing a water/alcohol solution which
dissolves away the noncross-linked coating 21 leaving only the
partially cross-linked indicia-defining coating 22 on the screen
as previously described. The screen is then dried and exposed
to ultraviolet radiation to complete the cross-linking of
coating 22 and promote its bonding to the screen, as previously
illustrated in FigO 11. The screen printing stencil is thus
completed.
Fig. 21 illustrates the preparation of scxeen printing
stencils in accordance with yet another method embodiment of
this invention. In accordance with this embodiment, coating
composition 21 is applied to the surface of support sheet 20.
After drying, indicia pattern 40 is formed from coating 21 on
support sheet 20 by cutting out a portion of the coating corre-
sponding to the pattern and removing that portion 41 of the coat-
ing which is excess to the pattern. The indicia pattern is
then transferred to screen 23 and the screen printing stencil
completed as previously shown in Figs, 7~
The objects of this invention are attained through
the use of a unique class of block copolymers and coating com-
positions made therefrom. The unique block copolymers of this
invention are copolyacrylate/polyurethane block copolymers in
which the copolyacrylate blocks are copolymers of at least one
hydroxy-containing acrylate and at least one acrylate which
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is partially substituted with bromine or iodine. Preferably,
the polyurethane blocks are polyether urethane or polyester ure-
thane, although polyether urethane is more preferred.
The block copolymers of this invention are prepared
by the same general method disclosed by Tobolsky in U.S. Patents
3,865,898, 3,291,859, and 3,257,476. In accordance with the
general method disclosed in those patents, a polymeric constit-
uent such as a polyether, for example, is reacted with a diiso-
cyanate to form what is referred to as a "diisocyanate-capped"
10 prepolymer. The diisocyanate-capped prepolymer is then reacted
with a hydroperoxide or a dihydroperoxide, such as 2 r 5-dimethyl-
2l5-bis(hydroperoxy)hexane to form a peroxycarbamate, such as
bist2,5-dimethyl-2-dihydroperoxyhexane-5-peroxycarbamate) poly-
ether. Chain extension of the polyether polymer constituent
may be effected in this second reaction if desired by addition
; of a suitable chain extender, such as butane diol, to the reac-
tion mixture. The peroxycarbamate is then reacted with one or
more ethylenically-unsaturated monomers, the active sites in
the peroxycarbamate serving to initiate the free radical polym-
erization of the monomer.
The reaction forming the peroxycarbamate is best con-
ducted in a solvent, such as toluene, at about 50 per cent by
weight solids. The product of this reaction (i.e., the peroxy-
carbamate solution) is then mixed with additional solvent and
the acrylate monomers which form the copolyacrylate blocks of
the block copolymers of this invention. These components are
then reacted by heating and stirring, to form the copolyacrylate/
polyurethane block copolymers which may then be used directly
as a block copolymer-containing syrup. Alternatively, the block
3~.~
copolymer may be precipitated from the product syrup and recov-
ered, as will ~e understood by those skilled in the art~
The polymeric constituents which are reacted with
diisocyanates to form diisocyanate-capped prepolymers in the
practice of this invention are hydroxy-terminated polyethers
or hydroxy-terminated polyesters. Preferred hydroxy-terrninated
poyethers are hydroxy-terminated polyoxypropylene and hydroxy-
terminated polyoxyethylene and preferred hydroxy-terminated
polyesters are those formed by reacting adipic acid with propyl- -
ene glycol, ethylene glycol or mixtures thereof.
The diisocyanates which may be used in the practice
of this invention include, but are not limited to toluene diiso-
cyanate, bis(4-isocyanatocyclohexyl)methane, hexamethylene diiso-
cyanate, 4,4'diphenylmethane diisocyanate, isophorone diisocyanate
and trimethylhexamethylene diisocyanate. Bis(4-isocyanatocyclo-
hexyl)methane is preferred.
The hydroperoxides and dihydroperoxides used in pre-
paring peroxycarbamates in the practice of this invention are
those having one or more terminal -OOH groups and include, but
are not limited to, cumene hydroperoxide, t-butyl hydroperoxide,
tetralin hydroperoxide, phenylcyclohexane hydroperoxide, diiso-
propyl benzene hydroperoxide~ p-menthane hydroperoxide, pinane
hydroperoxide and 2,5-dimethyl-2,5-bis(hydroperoxy)hexane.
Of these, 2,5-dimethyl-2,5-bis(hydroperoxy)hexane is preferred.
The hydroxy-containing acrylates present in the copoly-
acrylate blocks of the block copolymers of this invention include,
but are not limited to, 2-hydroxyethyl acrylate, hydroxypropyl
acrylates, 2-hydroxyethyl methacrylate and hydroxypropyl
methacrylates. Of these, 2-hydroxyethyl methacrylate is
preferred.
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The acrylates which are partially substituted with
bromine or iodine and which are present in the copolyacrylate
blocks of the block copolymer of this invention include, but
are not limited to dibromopropyl methacrylate and dibxomopropyl
acrylate. Of these, dibromopropyl methacrylate is preferred.
The preparation of the copolymers of this invention
is illustxated in greater detail in the following nonlimiting
examples.
500 grams of dry polyoxyethylene having a molecular
weight range of 3000-4000, 292 grams of toluene, 93.7 grams of
bis(4-isocyanatocyclohexyl)methane and 0.5 gxam dibutyltin di-
laurate were added to a 1.5 liter 3-necked reaction flask equipped
with an agitator and a nitrogPn purge and heated in a controlled
temperature oil bath. The contents were purged with dry nitrogen
and heated rapidly to 75C while being agitated. An isocyanate
anaylsis (ASTM Test No. D2572-67T) was made after 5 hours at
75C and, based upon the results of this analysis, 16.3 grams
of 2,5-dimethyl-2,5-bis(hydroperoxy)hexane, 7~9 grams of butane
diol, 2.6 grams dibutyltin dilaurate and 189 grams toluene were
added to the flask, and the tempera~ure maintained a~ 50C for
a period of 4 to 5 hours. At that time isocyanate analysis indi-
cated an isocyanate content of less than 0.05 per cent by weight,
which was taken as an indication that the reaction was complete.
The clear, viscous peroxycarbamate solution was transferred to
glass bottles and refrigerated. Upon cooling, the produc~ became
a soft, waxy, opaque solidO Total solids were determined to
be 54 per cent by weight.
Example 2
54.1 grams of the peroxycarbamate from Example 1, 71.8
grams of ~-hydroxyethyl methacrylate and 250 ml of ethylene
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glycol monomethyl ether were added to a 1000 ml 3-necked flask,
equipped with an agitator, a carbon dioxide purge, a thermometer
and a pressure equalizing addition funnel, and heated by a con-
trolled-temperature oil bath. The contents were mixed until
all ingredients were dissolved. 43.1 grams of 2,3-dibromopropyl
methacrylate and 94 ml of ethylene glycol monomethyl ether were
then added to the addition funnel. Both the flask and the addi-
tion funnel were purged with carbon dioxide gas for 3 minutes,
after which time a slow carbon dioxide purge was maintained by
subsurface bubbling through the liquid in the flask, with the
purge gas escaping through a fine-bore stopcock on top of the
addition funnel. The temperature of the flask contents was
brought to 80C and the 2,3-dibromopropyl methacrylate of the
addition funnel was uniformly added over a 5-hour period. The
temperature was maintained at 80C for an additional three hours
after the last of the 2,3-dibromopropyl methacrylate had been
added to assure completion of the reaction.
The resulting polymer syrup had a viscosity of 500
cps. at 24C and a total solids content of 29 per cent by weight.
- 20 ~
The same procedure was followed as that of Example
2, except that 2,3-dibromopropyl acrylate on an equal weight
basis was substituted for the 2,3-dibromopropyl methacrylate
of Example 2. The resultinq polymer syrup had a viscosity of
738 cps. at 16C and a total solids content of 2801 per cent
by weight.
As will be understood by those skilled in the artl
the relative amounts of polyuxethane to copolyacrylate, as well
as the relative amounts of the acrylates making up the copoly-
acrylate blocks of the block copolymers of this invention, may
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be varied by varying the relative amounts of the corresponding
reactants used in preparing them.
In the practice of this invention the relative amounts
of the various components making up the block copolymers are
balanced to achieve desired characteristics in coatings made
with the block copolymers. Thus, the relative amount of poly-
urethane in the block copolymer must be adjusted to achieve
a balance of properties in the coating films when used to prepare
screen printing stencils. For example, if too much polyurethane
is present in the block copolymer, the coating films will be
flexible but will lack durability; if too little polyurethane
is present, the coating films will exhibit durability, but will
be too brittle. The relative amount of bromine- or iodine-
substituted acrylate to hydroxy-containing acrylate must also
be balanced in order to control the amount of cross-linking
which may be effected in the coating film. The extent of cross-
linking of the coating film is determined hy the number of cross-
linking sites, such as bromine or iodine substituents on the
; block copolymer, and therefore, on the amount of bromine- or
iodine-substituted acrylate present in the block copolymer.
The relative amount of hydroxy-substituted acrylate affects the
solubility of the noncross-linked copolymer in polar solvents
such as water/alcohol mixtures. This characteristic is of par-
ticular importance during the "development" step of the method
of this invention for preparing screen printing stencils.
We have found that block copolymers having charac~er-
istics which are desirable for the practice of this invention
preferably comprise from about 60 to about 90 per cent by block
copolymer weight of copolyacrylate and from about 10 to about
40 per cent by weight of polyurethane. The copolyacrylate
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preferably comprises from about 50 to about 95 per cent by copoly
acrylate weight of hydroxy~containing acrylate and frsm about
5 to 50 per cent by weight of acrylate substituted with bromine
or iodine. A preferred block copolymer for making screen print-
ing stencils according to this invention is one comprising about
80 per cent copolyacrylate and about 20 per cent polyurethane
by weight, the copolyacrylate comprising about 60 per cent
2-hydroxyethyl methacrylate and about 40 per cent 2,3-dibromo-
propyl methacrylate by weight.
In addition to flexibility and durability, it is also
possible to introduce other specific characteristics into the
block copolymers by including in the copolyacrylate blocks certain
other acrylates, in amounts of up to about 15 per cent by copoly-
acrylate weight. Thus, for example, the substitution of acrylic
acid for some of the hydroxy-contain:ing acrylate will impart
to the total copolymer a sensitivity to basic solvents, such
as ammonia and sodium hydroxide solutions. This may be an impor-
tant characteristic where it is desired to recycle used printing
screens by washing the coatings from them with basic solvents.
This particular substitution should only be made, however, in
block copolymers used for preparing screen printing stencils
for use with nonwater-based inks. This is because the water
sensitivity of coating films made from block copolymers having
this substitution is increased, thereby rendering them less dura-
ble in the presence of water than they would otherwise be. In
a similar manner, the substitution of methyl methacrylatQ for
some of the hydroxy-containing acrylate increases the hardness
and water resistance of coating films made from the block copoly~
mers, and substitution of butyl methacrylate for some of the
hydroxy-containing acrylate imparts improved flexibili~y to the
-16-
~L33~
coating films. Therefore, where such specific characteristics
are desired, the block copolymers comprise from about 60 to about
90 per cent by block copolymer weight of copolyacrylate and from
about 10 to about 40 per cent by weight of polyurethane, the
copolyacrylate comprising from about 50 to about 95 per cent
by copolyacrylate weight of hydroxy-containing acrylate, from
about 5 to about 50 per cent by weight of acrylate substituted
with bromine or iodine and up to about 15 per cent by weight
of another acrylate, such as acrylic acid, methyl methacrylate
or butyl methacrylate.
Although polymer syrups, such as those prepared in
Examples 2 and 3, may be used directly in preparing the coating
compositions of this invention, it may be preferable in some
circumstances to recover the block copolymer from the syrup and
use it in a purified form. The block copolymer may be recovered
from the syrup in which it is prepared by precipitating it in
an excess of water, removing the water and vacuum drying the
precipitate, as is well known in the art. When such purified
copolymer is used instead of the syrup, some improvement in the
quality of developed indicia may result; and an improved response
to ultraviolet radiation may also be observed. It is, of course,
necessary to weigh the benefits of these improvements against
the additional processing step required to achieve them.
In practicing this invention, we prefer to apply the
aforedescribed block copolymers to substrates as solutions in
suitable solvents. It is also preferred to add to the solution
at least one cross-linking promoter and at least one ultraviolet
initiator. Thus, another emodiment of this invention is a print-
ing screen coating composition comprising a solution of the
block copolymer in a suitable solvent containing a cross-linking
-17-
~L33~
promo~er and an ultraviolet initiatorO
When the purified form of the copolymer is used in
preparing the coating compositions of this invention, the copoly-
mer is dissolved in a solvent to prepare a coating solution.
When, however, the copolymer syrup is used directly, it is gener-
ally not necessary to add any additional solvent, since the
solids level of the syrup itself is usually in the range desired.
That is to say, the cross~linking promoter and ultraviolet initi-
ator and any other desired additives may be added directly to
the polymer syrup~
Although we have found it convenient to use coating
compositions having solids contents of about 30 per cent by
weightl the desired solids level and therefore viscosity may
vary over a relatively wide range depending upon such factors
as the technique used in applying the coating composition to
the substrate, the nature of the solvent used, the amount of
cross-linking promoter used, the nature of the substrate being
coated, and the like. Therefore, although solids levels of from
about 20 to about 40 per cent by weight of the coating composition
are preferred, the practice of this invention is not limited
to any particular range of solids levels or viscosities. The
formulation of the coating compositions of this invention to
achieve solids levels or viscosities appropriate to the particular
use contemplated is well within the expertise of those skilled
in the art and need not be further defined.
The solvents used in preparing coating compositions
according to this invention may be important factors in the ulti-
mate performance of the coating compositions. Thus, where a
relatively nonvolatile solvent is used, the time required to
dry the coating, once applied to the substrate, could be so long
-18-
~ ~33~7~ `
as to present a processing disadvantage. On the other hand,
if a highly volatile solvent is used, it may evaporate so rapidly
as to make the application of a uniform coating to the substrate
being coated rather difficult. Therefore, we prefer to use sol-
vents of intermediate volatility such as ethylene glycol mono-
methyl ether, ethyl alcohol, n-propyl alcohol, isopropyl alcohol,
blends of ethylene glycol monomethyl ether with methylethyl ke-
tone, ethyl alcohol, dimethyl formamide, isopropyl alcohol and
the like. It is, of course, also within the scope of this inven-
tion to choose other organic solvents.
In practicing this invention, we have found it conven-
ient to add small amounts of ultraviolet initiators to the coating
compositions. The ultraviolet initiators contribute to the rapid
cross-linking or curing of the coatings in response to exposure
to ultraviolet irradiation. When exposed to ultraviolet radiation,
these initiators are thought to generate free radicals which
initiate the cross-linking of the copolymer chains in the coating.
While there are many ultraviolet initiators known to those skilled
in the art, persons so skilled also know that certain ultraviolet
initiators may perform better in particular chemical systems
than others. Thus, while there may be other ultraviolet initia-
tors which may be used in the coating compositions of this inven-
tion, we prefer to use phenanthrenequinone or uranyl nitrate~
While the actual amounts of ultraviolet initiator used in formu-
lating the coating compositions of this invention may vary,
depending on the particular formulation and needs of the formu-
lator, we have found that the ultraviolet initiator concentration
preferably ranges from about 0.01 to about 5.0 per cent by block
copolymer weight. However, concentrations ranging from about
0.1 to about 3 per cent by weight are preferred.
--19~
~ ;33~
The coating compositions of this invention also gener-
ally include small amounts of one or more cross-linking promoters
in their formulation. These compounds serve to extend the degree
of cross-linking which may be achieved by providing bridges or
links between reactive sites of different block copolymer mole-
cules. The nature of the cross-linking promoter and the amount
used influence the degree of cross-linking, and therefore, the
hardness and durability of the final coating. The hardness of
the final coating, in turn, influences the performance character-
istics of screen printing stencils having indicia defined bysuch final coatings. For example, if an insufficient degree
of cross-linking is achieved, the indicia-defining coating may
be too soft, and may not wear well during printing operations.
If, on the other hand, too much cross-linking is achieved, the
indicia-defining coating may be too hard and brittle, and may
crack during printing operations thereby resulting in defective
prints. There are many cross-linking promoters known to those
skilled in the art which may be advantageously used in formulating
the coating compositions of this invention, and it is well within
the exper~ise of those skilled in the art ~o determine the rela-
tive amounts to use in the formulation. Preferred cross-linking
promoters for use in preparing the coating composi~ions of this
invention include, but are not limited to, polyethylene glycol
diacrylates, pentaerythritol triacrylate, trimethylolpropane
triacrylate and mixtures thereof. These are preferably used
at concentrations of up to about 20 per cent by weight based
on the total weight of copolyacrylate/polyurethane block copoly-
mer used.
We have also found it helpful to add small amounts
of pigmenting material to the coating compositions of this
--~0--
~33~7~
invention. The pre~ence of such piymenting material improves
the visibility of the coating composition during the preparation
of the screen printing stencils of this inven*ion as well as
that of the indicia on the final screen printing stencils. This
can be an important factor in discovering errors or defects
during the preparation of the screen printing stencils, and prior
to their use in printing operations. Although there may be other
pigments which are useful for the purposes of this invention,
we prefer to use phthalocyanine blue at concentrations of from
about 0.1 to about 3.0 per cent by block copolymer weight.
We have also observed that, for reasons not fully
understood, the solvent resistance of coatings formed from coat-
ing compositions based on block copolymers in which the bromine-
or iodine-substituted acrylate is dibromopropyl acrylate, is
improved by the presence in the coating composition of small
amounts (i.e., about 1 per cent by block copolymer weight) of
dimethylaminoethyl acrylate.
The coating compositions of this invention may be for-
mulated from their individual components using conventional
techniques. Thus, for example, the block copolymer may be added
to the solvent and stirred until it is all in solution; then
the ultraviolet initiator and cross~linking promoter may be added.
A conventional mixer, such as a high-gpeed mixer may be used,
and the whole operation may be conducted at room temperature.
Since the coating compositions of this invention are generally
noncorrosive, no special materials of construction are required,
and conventional processing equipment may be used; however, we
prefer to use stainless steel or glass lined equipment to minimize
the possibility of contamination. The preparation of the coating
compositions of this invention is further illustrated by Example
-21-
~33~
4, it being understood that this example is illustrative only
and in no way limits the scope of the invention.
The polymer syrup prepared in Example 2 was mixed
with phenanthrenequinone (ultraviolet initiator~, trimethylpro-
pane triacrylate (cross-linking promoter) and tetraethylene gly-
col diacrylate (cross-linking promoter) and filtered through
glass fiber in a pressure filter~ Phthalocyanine blue pigment
(as a dispersion in ethylene glycol monomethyl ether) was added
to the filtered solution which was then thoroughly mixed with
a high-speed mixer to assure adequate pigment dispersion~
Since the resulting coating composition was light
sensitive, the formulation was carried out in amber glass con-
tainers and the final product was stored in amber glass containers.
A second coating composition was prepared using the
same procedure as described above except that the polymer syrup
prepared in Example 3 was used and a small amount of dimethylamino-
ethyl acrylate was added to the formulation. The actual formula-
tion amounts for these two coating compositions are shown under
~A" and "B", respectively in Table I.
Each of the two coating compositions thus prepared
was used to prepare screen printing stencils by the method of
this invention. The resulting screen printing stencils were
used to print with a standard laboratory printing press and pro-
duced satisfactory results.
A third coating composition was prepared by the same
procedure as the first two and using a polymer syrup such as
that prepared in Example 2, except that uranyl nitrate was used
as the ultraviolet initiator instead of phenanthrenequinone.
This coating composition was then used to prepare a screen
-22-
r
~3.
printing stencil. The actual formulation amounts for this coat-
ing composition are shown under l'C'~ in Table I.
Table I
% by Weight
of Total Composition
Composition A B C
polymer syrup from Example 2
(29% solids) 91.8
polymer syrup from Example 3
(28.1% solids) - 93.4
pol~mer syrup (29~ solids) 96.4
phenanthrenequinone 1.9 2.6
uranyl nitrate 0.1
trimethylolpropane triacrylate 2.1 0.6 0.6
triethylene glycol diacrylate 2.1 0.6 0.6
phthalocyanine blue solution* 2.1 2.1 2.3
dimethylaminoethyl acrylate - 0.7
*8.7% solids in ethylene glycol monomethyl ether
As will be understood by those skilled in the art,
it is important that the temporary support sheet te.g., 20 of
Fig. 3) used in the practice of the invention be transparent
to ultraviolet radiation. The ~emporary support sheet should
also have favorable adhesion and release characteristics with
respect to the coating compositions of this invention. The adhe-
sion between the coating and the support sheet should be suffi-
cient to enable the engraving of certain parts of the coating
film without disturbing the remainder of the film. It ~hould
also be strong enough to hold the image-defining cross-linked
coating material during the development process, when the support
sheet is washed with a water/alcohol solution. On the other
-23-
~L33~L ,7~
hand, the adhesion between the coating and the support sheet
should not be so strong as to interfere with transfer of indicia
from the support sheet to the printing screen, i.e., the coating
should be released to the printing screen by the support sheet.
The temporary support sheet should be sufficiently
flexible to enable good contact with the printing screen, and
should also be compatible with solvents, such as water/ethanol
mixtures used in the development step of the method of this
invention, and with the solvents used in preparing coating
lQ solutions.
While there may be many materials which are suitable
for use as temporary support sheets in the practice of this in-
vention, we prefer to use thin sheets of polyethylene terephthal-
ate coated with an appropriate adhesion/release agent. The poly-
ethylene terephthalate sheets are generally from 2-7 mils thick
and preferably, about 3 mils thick. The adhesion/release agents
with which the sheets are coated include, but are not limited
- to, thin films of natural or synthetic rubber, which may be ap-
plied to the support sheets as dilute (0.5-3 per cent by weight)
solutions in volatile mixed aliphatic/ketone solvents.
The printing screens used in the practice of this in-
vention are highly porous carrier materials, especially screens
or fabrics made of polyamides, polyesters, silk, or metals such
as copper, brass, bronze and stainless steel. The screens used
generally have a mesh number of 60 to 450, or more. The mesh
number refers to the openings per linear inch and is measured
from the center of any given wire to a point one inch away there-
from. For example, number 80-square mesh cloth would have 80
x 80 openings per square inch or 6400 openings per square inch.
The coating compositions of this invention may be ap-
pli~d to the appropriate substrates by any suitable conventional
~24-
~L~33~6~
means. For example, they may be applied by painting, knife
coating, rod coating or flow coatiny. Subsequent to the applica-
tion of the coating composition to the substrate, it may be air
dried, vacuum dried or oven dried.
In practicing this invention, we have found that satis-
factory results may be achieved with dry coating thicknesses
of from 0.3-2.0 mils, although this range is by no means critical.
As will be apparent to those skilled in the art, the coating
may be applied in several layers to obtain the desired thickness.
The amount of ultraviolet radiation used in forming
the screen printing stencils of this invention will vary accord-
ing to several parameters, such as coating thickness, amount
of ultraviolet initia~or used, degree of cross-linking desired,
etc. In this regard, it should be noted that less radiation
is required for some steps than for others. Thus, where the
coated support sheet is irradiated through a master, as shown
in Fig. 3, to define an indicia with partially cross-linked mate-
rial, it will suffice to irradiate it only to the extent required
to render the affected part of the coating insoluble in the sol-
vent used for the subsequent development step. Where1 on the
other hand, the irradiation is used to complete cross-linking
and bond the indicia-defining coating to the printing screen,
as shown in Fig. 8, somewhat more irradiation may be required.
Thus, in practicing this invention, the coatings are irradiated
with ultraviolet radiation of sufficient intensity for sufficient
time to achieve the desired degree of cross-linking. As will
be recognized by those skilled in the art, there are many ultra-
violet radiation sources which may be used in the practice of
this invention. These include, but are not limited to, carbon-
arc lamps, high-intensity tungsten filament lamps, metal-halide
-~5-
~13~17CD
lamps and mercury-vapor lamps. While the radiation intensity
and exposure time will vary for different materials and different
radiation sources, we have found that when using a standard 12-
ampere carbon-arc source in the practice of this invention, expo-
sure at a distance of 18 inches ~or periods of from about 2 to
; 4 minutes are sufficient to partially polymerize the coating
compositions and render them insoluble in the development solvent,
and exposures at a distance of 12 inches for periods of from
about 5 to 10 minutes are usually sufficient to post~cure the
coatings on the printing screens.
While there are many solvents which may be used as
developer solutions to wash away the noncross-linked coating
composition in the practice of this invention, we prefer to
use water/alcohol solutions such as water/ethanol or water/iso-
propanol mixtures. The ratios of water/alcohol in these solu-
tions is usually in the range of from 1/1 to l/30
The practice of this invention for preparing screen
printing stencils is further illustrated by the following non-
limiting examples.
Example 5
An 8 x 10-inch support sheet of 3-mil polyethylene
terephthalate film was coated with a thin layer of an adhesion/
release agent applied as a dilute tO.75~ by weight solids) solu-
tion of a white transparent rubber cement in a volatile mixed
aliphatic/ketone solvent using a No. 3 wire wound Meyer rod.
After the adhesion/release agent coating was allowed to dry,
a coating of the printing screen coating composition prepared
in Example 4 (Formulation "A" of Table l) was applied using a
coating knife having a 10-mil clearance. The resulting coated
support sheet was then air dried for about 2 hours and oven dried
at 6GC for an additional 2 hours.
-26-
~33~;~
The coated support sheet was then exposed through
a standard test positive ~from the direction of the uncoated
side) to a standard 12-ampere carbon-arc light at a distance
of about 18 inches for a period of about 3 minutesO The exposed
coated support sheet was then placed in an etching bath of 1/3
(volume) water/ethyl alcohol solution and the unexposed coating
was dissolved away leaving a printed negative image of exposed,
~` partially cross-linked coating composition on the support sheet.
The image-bearing support sheet was then air dried in front of
a fan for about 10 minutes. The image-defining coating was then
transferred from the support sheet to a clean, frame-mounted
printing screen as follows: The support sheet was placed, coated
side up, on a flat, raised surface slightly smaller than the
inside dimension of the printing screen frame. The frame-mounted
printing screen was then lowered onto the support sheet with
; the ink or squeegee side up. The screen was swabbed with a 1/1
water/ethyl alcohol blend and allowed to stand for about 30 sec-
onds to soften the surface of the coating. The screen was then
covered with newsprint and gently rolled to press the surface
of the screen against the coated support sheet while soakin~
up excess solvent with the newsprint at the same time. Care
was taken not to exert excessive pressure which would distort
the image.
The screen/support sheet assembly was then placed
in front of a fan and air dried for about 15 minutes, after
which the support sheet was peeled off of the printing screen
leaving the coating (i.e., the design) on the printing screen.
The coating-bearing side of the printing screen was then air
dried in front of the fan for about another 10 minutes. The
printing screen was then exposed to a standard 12-ampere carbon
-27-
~L~3;~
arc ultraviolet source at a distance of about 12 inches for
about 10 minutes on e~ch side to complete the cross-linking of
the image-defining coating and promote its bonding to the screen,
thus completing the screen printing stencil.
The resulting screen printing stencil was used to print
with a standard laboratory screen printing press and produced
satisfactory prints.
Exam~le 6
An 8 x 10-inch support sheet of 3 mil clear polyethyl-
ene terephthalate film was coated with a thin layer of an adhe-
; sion/release coating as a dilute (0.75 per cent by weight solids)
solution of a white, transparent rubber cement in a volatile
mixed aliphatic/ketone solvent using a No. 3 wire-wound Meyer
rod. After the adhesion/release coating was dried, two succes-
sive castings of the coating composition of Example 4 (Formulation
"A" of Table I) were applied using a casting knife with a clear-
ance of 6 mils. After each layer of the coating composition
was applied, the coated support sheet was air dried for 2 hours
and oven dried at 60C for 2 additional hours.
A clean, frame-mounted printing screen was then placed
on top of the coated side of the support sheet. A thin film
of the printing screen coating composition of Example 4 was then
cast on top of the screen as an adhesive to bond the screen to
the coating on the support sheet, after which the entire assemhly
was air dried in front of a fan for about 10 minutes. The assem~
bly was then exposed through a standard test negative to a stand~
ard 12-ampere carbon-arc ultraviolet source at a distance of
about 18 inches for a period of about 3 minutes. The support
sheet was then removed, leaving the coating on the printing
screen. The printing screen was placed in an etching bath of
-28-
3~
1/1 (by volume) water/ethyl alcohol solution~ and the unexposed
coating composition was washed away leaving a printed positive
image of exposed, partially cross-linked coating composition
on the printing screen. The developed printing screen was then
air dried in fxont of a fan for about 10 minutes on each side
and then exposed to a standard 12-ampere carbon-arc ultraviolet
source at a distance of about 12 inches for about 10 minutes
on each side. The resulting screen printing stencil was used
to print with a standard laboratory screen printing press and
produced a number of good prints.
Exam~le ?
A coating of the printing screen coating composition
of Example 4 (Formulation "A" of Table I) was applied to both
sides of a clean, frame-mounted printing screen and allowed to
dry. The coated screen was then exposed, developed, dried and
post cured as in the previous example. The resulting screen
printing stencil was used to print with a standard laboratory
screen printing press and produced good prints.
Example 8
An 8 x 10-inch support sheet of 3-mil clear polyethyl-
ene terephthalate film was coated with a thin layer of an adhesion/
release coating applied as a 3 per cent by weight solution of
a white transparent rubber cement in a solvent comprised of
a heptane/hexane/methyl ethyl ketone mixture using a No. 7 wire-
wound Meyer rod. After the adhesion/release coating was dried,
two successive castings of the coating composition of Example
4 (Formulation "A" of Table I) were applied using a casting knife
having a clearance of 6 mils. The coated sheet was air dried
for 2 hours and oven dried for 2 hours at 60C aft~r each layer
of the coating composition was applied.
-29-
~3~ 7~
Once the coatings were dry, a sharp stencil-cutting
knife was used to cut a design in the coating material, taking
care not to score the polyethylene terephthalate film itself.
The unwanted portion of the coating material was peeled away
from the support sheet and discarded.
The completed design was transferred to a printing
screen by the technique described in Example S, and then cured
by exposure to ultraviolet irradiation as described in Example 5.
The resulting screen printing stencil was used to print
400 copies of the design using a standard laboratory screen print-
ing press. Of the 400 copies, 200 were made with a commercially
available water-based ink and, after cleaning the screen printing
stencil, 200 more copies were printed using a commercially avail-
able ketone-based ink.
Visual inspection of the screen printing stencil after
printing 400 copies with it showed no sign of wear, and the sten-
cil appeared to be in substantially the same condition as it
was prior to being used to print the 400 copies.
Although the coated support sheets and printing screens
of the preceeding examples were air dried for two hours and oven
dried for an additional 2 hours following the application of
the coating material to them, these drying times may be reduced
so long as the solvents are essentially all removed.
The novel screen printing stencil of this invention
is a screen printing stencil having indicia defined ~hereon in
the form of a cross-linked block copolymer having alternating
blocks of copolyacrylate and polyurethane, the copolyacrylate
being a copolymer of at least one hydroxy-containing acrylate
and at least one acrylate which is partially substituted with
bromine or iodine. Typically, the surfaces upon which indicia
-30
are printed using the screen printing stencils of this invention
are paper, fabric, plastic or the like.
~ he screen printing stencils of this invention repre-
sent a substantial advancement of the art in that they are com-
patible with both oil-based and water-based inksO More specifi-
cally, these screen printing stencils combine the precision of
photochemically prepared screen printing stencils with the dura-
bility of alkaline and water-resistant coatings for use with
such water-based inks. As will be understood by those skilled
in the art, the use of water-based inks in screen printing pxo~
cesses, enabled by the screen printing stencils of this invention,
is particularly advantageous with respect to the environmental
consequences of the drying of the ink after printing. Thus,
where water~based inks are used instead of oil-based inks, the
; environmental problems associated with evaporation of the ink
solvent into the atmosphere are substantially lessened.
Although this invention has been described in terms
of specific embodiments which are set forth in considerable
detail, it should be understood that this is by way of illustra-
tion only, in that the invention is not necessarily limitedthereto, since alternate embodiments and techniques will become
apparent to those skilled in the art in view of the disclosure.
Accordingly, modifications are contemplated which can be made
without departing from the spirit of the described inven~ion.
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