Note: Descriptions are shown in the official language in which they were submitted.
D-4546
PHOTOPOLYMER PRINTING PI~TES HA~IING
A DIMPLED PRINTING SURFACE
BACKGROUND OF THE I~3VENTION
1. Field_of the In~ention
This invention relates to photopol~mer printing pla~es
having a printing surface sf enhanced printing ink
retention properties and to a process for producing
such plates.
2. escriPtion o~ Related Art
Flexography is a method to print flexible materials
such as paper, plastic films and metal ~oils or to
print irregular surfaced material such as corrugated
board. In recent years photopolymer ~lexographic
printing plates have become increasingly accepted in
the industry because they are quicker and less costly
to make than conventional molded rubber plates which
are prepared by vulcanizin~ rubber in a mold under high
temperature and pressure.
Photopolymer plates may be de~ined as sheets, films or
laminates composed of a solvent soluble polymeric
binder material and a photochemical system which
undergoes polymerization or crosslinking when exposed
to actinic radiation o~ the wavelength to which thP
photochemical system is sensiti~e. Thus, when such a
system is exposed to light throu~h a negative
transparent mask containing opaque image areas, the
photochemical reaction on those areas of the surface of
the plate which are exposed to light initiates a
photopolymerization or crosslinking reaction, whereas
those areas of the plate surface which are masked and
not exposed to liyht remain essentially unchanged. The
polymeric material in the exposed portion of the plate
is thereby rendered less soluble or insoluble in
solvent while the polymeric material in the non-exposed
portions of the plate remains solvent soluble. A
raised relief image may then be obtained by development
of the plate to dissolve or wash away the soluble
portions o~ the plate surface.
Polymer binder materials which are commonly employed in
photopolymer printing plates include cross linkable
polyamide binders such as~disclosed in U.S. Patents
3,884,702 and 3,695,887. Elastomeric polymers are also
used in such applications ~uch as co-polymers of
butadiene, partioularly butadliene and acrylonitrile
such as disclosed in U.5. Patents 4,177,074 and
4,41S,649, polyurethanes such as disclosed in U.S.
~atent 3,948,665, and block copolymers of butadiene or
isoprene and a vinylaromatic monomer such as styrene,
as disclosed in U.S. Patents 4,686,172 and 4,369,246.
Most of these systems are based on compo~itions
containing a binder material, a diethylenically
unsaturated monomer and a photoinitiator. These
systems are all processed in a similar fashion which is
to expose the presensit~zed elastomeric plate with
ultraviolet light through a negative transparency.
Addition polymerizati~n occurs selectively in the
exposed areas which corresponds to ~he clear areas of
the transparency, and substantially no polymerization
occurs in unexposed areas which correspond to the
opaque areas of the transparency. A relief image is
produced when the plate is developed by brushing in a
solvent. The system is designed so that the developing
solvent dissolves the unexposed, unpolymerized areas
and does not dissolve the exposed, polymerized areas.
Whereas flexographic plates prepared by exposing
photopolymers to light are much easi~r to prepare than
their molded vulcanized rubber counterparts, and offer
the additional advantage that they can be expssed by
computer laser, it is o~ten more difficult to control
the softness, flexibility and ink wetting properties o~
the printing surfaces o~ such plates. This can be a
particular problem in those arleas of the raised relie~
which contain printing cha~acters having a relatively
large printing sur~ace area~ In many applications,
particularly ~lat bed printing, the printing ink tends
to spread to ~he edges of large solid areas resulting
in poor or uneven print density in the central regions
of the printed character and diminishment of contrast
along the edges of the printed character.
It is al50 known to employ photopolymer plates in
planographic or lithographic printing processes. In
such processes, the photopolymer layer on the plate is
generally quite thin (no~ more than 0.015 mm thick) and
the developed printing image is in the form of a series
of hal~ tone dots of varying sizes. Such plates are
generally produced by ~orming a hal~ tone image on a
transparent mask hy photographically exposing the mask
to an original through a line screen, which mask is
then subsequently used to e~pose the photopolymer plate
2 ~ 2 ~
to produce a halftone dot imaye on the plate after
de~elopment. Density of the solid printing surfaces on
the printing plate may be controlled as a function of
the dot size and dot density, or by etching the
halftone dots to reduce their size. Examples of such
techniques are disclosed in U.S. Patents 4,369,239 and
4,173,673. Since the printing surfaces of the plate
are composed of a plurality of discrete half tone dots
even in the denser printing areas, the printing of
large surface areas is not as much of a problem as it
is with flexographic processes, since the large
printing surfaces are not composed of a contiguous mass
of photopolymer, but rather a plurality of closely
spaced, discrete dots.
SUMMARY OF THE INVEN~ION
The present invention provides a photopolymer printing
plate ~or use in flexographic printing wherein the
photopolymer relie~ structure is composed of printing
characters of uniform printing height, the planar
printing surfaces of which characters contain a
plurality of well-like dimples or depressions present
at a density within the range o~ from about 25 to about
500 depressions per linear inch of planar printing
surface. The invention also provides a process for
preparing such photopolymer plates by expo~ing the
photopolymer layer to actinic radiation through a
photographic mask containing optically transparent
ar~as and optically opaque image areas, the tra~sparant
areas of said mask charact~rized by the prPsence o~ a
plurality o~ opaque discrete dots or other geometric
shapes present at a density of from about 25 to about
500 dots per linear inch. These dots sexve to `olock a
portion of the light passing through the transparent
areas of the mask thereby forming discrete regions in
the underlying photopolymer layer which are not exposed
or are only partially exposed. Upon development of the
plate, the regions of the photopolymer layer underlying
the dot pattern are removed resulting in the formation
of a plurality of depressions in the relie~ planar
surface of the exposed portions of the photopolymer
layer.
Upon the application of printing ink to the relief
printing surface of the plates of this invention, the
depressions on the relief surface act as ink reservoirs
resulting in the application of a more even
distribution of ink to surfaces to ba printed,
particularly where large solid areas are printed.
BRIEF DESCRI~ D~ L~GS
Fig. 1 is a vertical schematic (not to scale) v~ew of a
portion of a dot pattern present on a transparent base
film or transparent areas o~ a negative mask film.
Fig. 2 is a side view o~ a laminate o~ the base film of
Fig. 1 superimposed over a negative transparency, both
of which are superimposed over a photopolymer layar.
Fig. 3 is a cross sectional side view of an exposed and
developed photopolymer layer bearing well;like
depressions on the raised printing surface.
Fig. 4 is a schematic diagram lllustrating the
methodology by which the well-like depressions on the
printing surfaca ~hown in Fig. 3 are formed.
~l~3~Ll~
DETAILED DESCRIPTION OF THE_INVENTION
As indicated above, the photopolymer compositions which
may be employed in the preparation of printing plates
of this invention are known materials, and generally
consist of at least about 30% by weight, more
preferably at least about 70% by weight of a polymeric
binder material and generally from about 1 to about 30%
by weight of a photocrosslinkable monomer. Many of
these compositions also contain an amount of
photoinitiator sufficient to effect polymerization o~
the photocrosslinkable monomer when exposecl to actinic
radiation,said amount generally ranging from about 0.1
to about 10~ by weight. The compositions may also
contain fillers such as carbon black, silica or talc:
dyes and pigments; plasticizers; polymerization
inhibitors; and like additiv~s.
The preferred binder materials for use in this
invention include the elastomeric block copolymers of
styrene and butadiene and the photochemical system as
disclosod in U.S. P~tents 4,369,24~ and 4,~86,172, the
disclosures o~ which patents are incorporated herein by
reference. Other syst2ms include elastomeric binders
based on copolymers of butadiene and a copolymerizable
monomcr such as acrylonitrile and the photochemical
system such AS disclosed ln U.S. Patents 4,177,074 and
4,415,649; binders based on polyamides as disclosed in
U.S. Patents 3,884,702 and 3,6~5,887; and binders based
on polyurethanes as disclosed in U.S. Patent 3,946,665,
the disclosures o~ each of which patents are
incorporated herein by re~erence.
~he photosensitive compositions are formulated as
described in these patents and then calendared, or
extruded, or cast from sol~ent into sheet material
having a thickness in the range of from about 0.020 to
about 0.250 inches. The sheet material is then cut to
the desired dimensions prior to further processing
steps.
Particularly preferred photopolymers are used in the
form of a self supporting sheet and includ~ dual layer
photopolymer sheets marketed by DuPont under the trade
name "CYREL PLS" (based on an ABA block copolymer o~
styrene and butadiene), elastomer sheets marketed by
Uniroyal under the trade name I'FLASKOR", and polyamide
sheet material such as marketed by Toyobo under the
trade name "P~INTIGHT UF" or hy Toray under the trade
~name "T~ORELIEF SS".
In accordance with the proce5s of this invention, a
photopolymer relie~ structure is prepared by exposing
the photopolymer ~ilm or sheet to ac~inic radiation
through a transparent mask ~ontaining an image negative
having optically transparent areas and optically opaque
image areas, a portion of the optically transparent
areas on the film beiny blocked by a pattern o~
optically opaque discrete dots having a dot density of
from about 25 to about 500 dots per linear inch. These
dots serve to partially mask the transparent areas of
the negative such ~hat intermittent regions of the
photopolymer underlying the dots are not exposed or
only partially exposed to radiation while the
~urroundin~ regions of the photopolymer are exposed.
Upon development o~ the photopolymer plate, non-exposed
regions o~ photopolymer are removed resul~in~ in an
imaged relief structure having relief or printing
characters of uniform printing height wherein the
printing surfaces of said character~ contain a
plurality of well-like depressions representing areas
of the printing sur~ace where unexposed photopolymer
has been removed.
The series of opaque dots may be present on the
negative transparency itself or may be provided by
laminating a second transparent film screen containing
the dots between the negative transparency and either
the light source or the underlying photopolymer film,
with the films arranged such that their surfaces are
contiguous, i.e., in direct sur~ace contact.
A transparency having the opa~le dots directly on the
photographic surface may be~ px~pared at the time that
the negative is prepared by the utiliæation of an
appropriate contact screen or by computer controlled
techniques wherein both the imalge and the pattern dots
are created by computer Gontrolled Pxposure using a
laser beam. In su~h a case, ~ separate screen bearing
the opaquc dot pattern is not needed.
In the simpler embodiment, a separate transparent film
bearing the opaque dot pattern is employed. Fig. 1 is
a vertical schematic (not to scale) ViQW of a corner of
an optically transparent film material 10 having
printed thereon a plurality of opaque dots 20.
Suitable film materials include optically transparent
cellulose acetate and oriented polyester having a
preferred ~ilm thickness o~ less than 1 mil. The
opaque dot pattern may be composed of silver halide
photograph.ic residue or the residue o~ a pigmented
photosensitive material. Alternatively, the ~ot
pattern may be created on the film surface by printing
or by electrophotographic techniques.
The dot pattern may be arranged in a linear array (as
in Fig. 1), a rectilinear array on in a randam array,
with a dot fxequency ranging from about 2S to about 500
dots per linear inch. In the more preferred
embodiments, the dot frequency ranges from about 100 to
about 400 per linear inch, with a frequency of from
about 150 to about 300 per linear inch being most
pre~erred. The screen dots generally all have about
the same diameter which generally ranges from about
0.000~ to about 0.01 inches, with 0.000~ to about 0.002
inches being most preferred.
The term "dot" as used herein is intended to include
not only circular geometry, but also other ~eometries
as well such as diamond shapeas, square shapes, oval
shapes and the like, each of such size that they are
essentially equivalent in surface area to circular dots
having the diameters set forth above.
Fig. 2 shows a cross sectional view o a portion of the
dot screen of Fig. 1 shown superimposed over negative
transparency 30. The shaded area of transparency 30
represents a portion of an opaque image area present on
the transparency whereas the non-shaded area represents
a transparent area. Transparency 30 is in turn
superimposed over photopolymer layar 40. The Figure
also shows exposure of the structure to actinic
radiation, depicte~ by ver~ical arrows.
7~ ~ ~3 ~
Fig. 4 is a schematic illustrating the methodology by
which the depressions in the planar printing surfac~ of
photopolymer layer 40 are formed. For clarity, screens
10 and 30 are not shown. Dot area 20 blocks the light
which is directed perpendicularly upon the surface of
the photopolymer layer, but diffuse components of the
light represented by the conver~ing arrows tends to
expose regions o~ the photopolymer layer under the
border of the dot circumference resulting in a
graduated exposure in such regions. The triangular
shape depicts the regions o~ unexposed photopolymer
underlying an opaque dot after exposure of the
photopolymer layer through the dot screen.
Fig. 3 is a cross sectlonal illustration o~
photopolymer layer 40 after development. The edge of
the relie~ structure charaeter is æhawn by wall 41.
Planar printlng surf~ce 50 ~ontains a plurality of well
like and ink retaining depr.essions 51 which are
generated as a consequence of development of the
exposed photopolymer layer.
The depth and width of the well-like depressions in the
printing surface shown at Sl in Fig. 3 may be
controlled largely as a function of the size of the
masking do~, the intensity o~ the expo~ure source, and
the degree to which the masking dots are opa~ue to the
exposure light~ Generally speakin~, dot depths of from
a~out 0.0~05 to about 0.005 inches are preferred for
the purposes of this invention, with the most preferred
depths ran~in~ from about 0.001 to about 0.003 inches.
Actinic radiation Erom any ~ource and of any type can
be used in the process. The radiatian may e~anate from
~ 13t~
11
point sources or be in the form of parallel rays or
divergent beams. By using a broad radiation source,
relatively close to the image-bearing transparency, the
radiation passing through the clPar areas of the
transparency enters as divergent beams and thus
irradiates a continually diverging area in the
photopol~merizable layer underneath the clear portions
of the transparency, resulting in a polymeric relief
having its greatest width at the bottom of the
photopolymerized layer, i.e., a frustum, the top
surface of the relief being the dimensions of the clear
area. Inasmuch as the free-radical-~enerating
addition-polymerization initiators act:lvatable by
actinic radiation generally exhibit their maximum
sensitivity in the ultraviolet range, the radiation
source should furnish an efEecti~e amount of this
radiation. Such source~ i~clude carbon arcs,
mercury-vapor lamps, fluorescent lamps with special
ultraviolet-light-emitting phosphors, argon glow lamps,
and photographic flood lamps. Of these, the
mercury-vapor lamps, parti¢ularly the sunlamp or ~Iblack
light" type, and the fluorescent sunlamps are most
suitable.
The solvent llquid used ~or washing or developing the
photosensitive plates should have good solvent action
on the solvent-soluble portions of the exposed
photopQlymer plate and littlP or no action on the
photopolymerized portions o~ the plate. Suitable
solvents include water or aqueous solutions of a base
such as an alkali metal hydroxide or an alkali metal
meta silicate for use with those photopol~mers which
are developable by aqueous solutions. Plat~s based on
elastomeric polymer binders may be developed using
,;3 q~ 2 ~.
12
organic solvents such as methyl ethyl ketone, benzene,
toluene, trichloroethane, perchloroethylene and similar
materials.
The printing relie~s made in accordance with this
invention can be used in all classes of printing but
are most applicable to those classes of printing
wherein a distinct difference of height between
printing and non printing areas is required and
particularly to the flexographic printing class wherein
a resilient print area is required, e.g., for printing
on deformable printing surface. These classes include
those wherein the ink is carried by the raised portion
of the relief such as in dry-of~set printing, ordinary
let'erpress printing, the latter requiring greater
heiaht differences between the printing and nonprinting
areas. The height of the relief characters preferably
ranges from about 0.01 to about 0.05 .inches.
Inks which may be used in conjunction with the printing
plates of this invention may be aqueous based or
solvent based. Preferred inks for plates developed
with organic solvent are glycol based inks and
preferred inks for plates developed with aqueous based
solvents are ester based inks, such as di-octyl
pht~alate, and preferably contain ~luorescent dyes or
pigments and a small amount of surfactant.
The following Examples are illustrative of the
invention.
2 ~ 2
Example 1
An 11 inch by 8.5 inch sheet of elastomeric
photopolymer supplied by DuPont under the trade name
CYRE~ PLS which is based on a block ABA copolymer o~
styrene and isoprene and containing a diethylenically
unsat~rated monomer and a photoinitiator was pro~ided.
The sheet had a thickness of 0.067 inch.
A relief image on the surface o~ the photopolymer sheet
was prepared by first blanket exposing to actinic
radiation the side of the she~t opposite the side to
bear the relief image for 30 seconds using an exposure
unit supplied by Greig Machine, Inc., Model ~. The W
energy level was from about 4--6 milliwatts/cm2. This
initial reverse sid~ exposure initiates
photopolymerization which cont:rols the depth of the
relie~ image to be obtained on the oppos~te side of the
sheet. The sheet was next turned over and exposed to
VV li~ht through a negative transparency bearing
postage meter indicia havin~ superimpo~ed thereover a
transparent sheet having on its surface a plurality of
linear opaque dots having a frequency of about 167 dots
per linear inch. The exposure conditions were 25
minutes at 4-6 milliwatts/cm2.
The plate was then developed by brushing the
image-exposed surface in a solvent comprising a mixture
o~ perchlorethylene and 25~ by weight N-butanol for a
period of about 7 minutes. The plate was then heated
to about 70C for about 60 minutes to remove tha
solvsnt, followed by a 16 minute post exposure to
actînic light to cure and detacki~y the plate.
2 ~
14
The resulting plate had an indicia relief depth of
about 0.025 inch.
The postage meter relief image was cut from the sheet
and mounted on a Pitney Bowes B~2 postage meter. The
relief surface was inked using a glycol based red ink.
Flexographic print quality of the image on white paper
was excellent.
Examples 2-6
Five additional photopolymer sheets were exposed,
developed and tested for print quality. Process
parameters were the same as in Example 1 except that in
the case of Examples 4, 5, and 6, the back exposure
time was 1.75 minutes, the image exposure time was 3
minutes, development time waS 3.5 minutes, drying time
was 25 minutes post curing tlmle was 5 minutes, and the
developer was an aqueous alkali solution. The
materials and screen dot densit:y are shown in Table 1.
A control plate was also prepared using the m~terials
and the process of Example 1, except that the
transparent sheet bearing the opaque dot pattern was
not used in the image exposure step~ Print results
using this plate are also shown in ~able 1.
As is evident from this data, the print quality in
solid areas of the print i5 notably superior when
printing is c~nducted using the photopolymer plates
made in accordance with this invention.
TABLE l
Photo- Plate D~t~ Ink Print Relief
PolYmer Thickness ~E Inch Base ~uality DePth
Exl - DuPont PLS 0.067~ 167 Glycol Excellent 0.025"
Ex2 - DuPont PLS 0.045" 255 Glycol Excellent 0.035
Ex3 - Unlroyal
Flas~or 0.067" 255 Glycol ~xcell~nt 0,030"
Ex4 - Toyo~o
Printi~ht 0.048" 255 Diest~r Excellent 0.030"
Ex5 - Toray
Trorelief 95SS 0.038" 255 Dlester Exc~llent 0.025"
Ex6 - Toray
Trorelief 95SS 0.038~ 167 Diester Excellent 0.025"
Con~rol 0.067~ 167 Glycol Poor 0.030"
A print quality of excellent mean~ that areas printed
on paper and having a solid print dimension of greater
than about 1/16 inch exhibit substantially uniform
print density throughout the printed area. A print
quality of poor means that the same printed areas show
numerous white spoks representing void area~ and areas
of low print density where the paper shows through,
indicative o~ non-uniform ink application in those
areas.
