Note: Descriptions are shown in the official language in which they were submitted.
16
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METHOD OF C~RING A COMPRESSIBLE PRINTING BLANKET
~ND A Co~PRESSIBLE PRINTING BLANKET PRODUCED THEREBY
This invention relates to a method of curing
printing blankets and printing blanket produced thereby,
and in particular ralates to a compreseible printing
blanket of the type uged in offset lithographic printing.
The u8~ of blankets in offset lithograpy i8 well
known and ha~ a primary function of tran~ferring ink from
a printing plate to paper. Printing blanket~ are ~ery
carefully designed 80 that the surface of the blanket is
not damaged either by the mechanical contact of the
blan~et with the part~ of the press or by chemical
reaction with the ink ingredients. Repeated mechanical
contacts cause a certain amount of compression of the
blanket which must be within acceptable limits 80 that the
image is properly reproduced. It is also important that
the blanket have resiliency, i.e. be capable of eventually
returning to its original thicknes6, and that it provide
constant image transfer regardless of the amount of use to
which it is put.
Printing blankets are normally composed of a
substrate base material which will give the blanket
integrity. Woven fabrics are preferred for this base.
The bas0 may consist of one, two, three, or more layers of
fabric. The working surface, by which is meant the
surface that actually contacts the ink, is usually a layer
of elastomeric material ~uch as rubber. The blanket is
conventionally made by calendaring or spreading rubber in
layers until a desired thickness of rubber has been
deposited, after which the assembly is cured Ol vulcanized
to provide the finlshed blanket. Such a blanket is
acceptable for many applications, but often lacks the
nece66ary compressibility and re~iliency needed for other
'
~9~6
applications. It i8 de~irable, ~herefore, to produ~e more
highly compreffsible blankets with improved resiliency.
It is difficult to obtain an ~mproved
compressibility factor by the standard construction
described above because the rubber material, while it i8
highly elastomeric, i8 not compres~ible and cannot be
compressed in a direc~ion at right angle~ to its surface
without causing a di~tortion or stretch of the blanket in
areas adjacent to the point of compression. If
irregularitie~ exist in ehe printing plate, the presseR,
or the paper. the compression tO which the blanket is
expo~ed will vary during the printing operation and the
irregularities in the plate~, presses, or paper will be
magnified by the lack of compression in the printing
blanket.
The key to providing a printing blanket having
the desired compressibility and resiliency i6 in providi~g
a compressible layer therein. In particular, it has been
found that by including at least one layer of material
comprising a compressible layer of resilient polymer in a
printing blanket that erinting problems such as those
described above as well as "blurring~ (lack of
definition), caused by a small standing wave in the
blanket printing surface adjacent to the nip of the
printing press, can be avoided. Also, a compre~sible
layer can 6erve to absorb a "6mash", that is a substantial
deformation in the blanket caused by a temporary increase
in the thickne~s in the material to be printed, for
example, the accidental introduction of more than one
sheet of paper during the printing operation. By
incorporating a compressible layer in the blanket, a
"6mash" can be ab60rbed without permanent damage to the
blanket or impairment to the printing quality of the
blanket. In addition, a re~ilient, compressible layer
hel2s to maintain the evenne~s of ~he printing ~urface and
the thickne~s of the blanket during the printing operation
by restoring the nor~a~ thickne~s of the blanket ater
compression at the nip of the pre~.
Many different means of producing a compressible
layer within a pr~nting blanket are known in the art. ~or
example, compre~ible layers have been formed by mixing
granular salt particle~ with the polymer u~ed to prcduce
the layer, and thereafter leaching the salt f~om the
polymer to create void~. The voids in the layer make
possible the positive displacement of the surface layer
without distortion of the surface layer since volume
compression occur~ and displacement takes place
substantially perpendicular to the impact of the press.
Such a method is disclosed in Haren et al, U.S. Patent
4,025,685. Other method~, such as using compressible
fiber structures have been used heretofore to produce
compressible layer~. Examples are found in Duckett et al,
U.S. Patents 3,887,750 and 4,093,764. Rodriguez, U.S.
Patent 4,303,721, teaches a compressible blanket made
using blowing agents to create voids in the compressible
layer. The use of rubber particles to create voids is
disclosed in Rhodarmer, U.S. Patent 3,795,568.
The forming of voids using blowing agents has the
disadvantages that the size of the voids formed, and the
interconnecting of the voids is not easily controlled.
Oversizsd voids and interconnected voids cause some areas
of the printing blanket to be more compressible and less
resilient than adjacent areas of the printing blanket,
which re6ults in deformations during printing. The
leaching of salt~ from a polymer matrix has the
disadvantages that the particle sizes u~ed are limited,
o~
--4--
and that the leaching 8tep i8 difficult, time con~uming
and expensive.
More recently, it ha~ been found pre~erable to
produce printing blanket~ having a compres~ible layer
comprising a cellular re~ilient polymer having cells or
voids in the compressible layer in ~he form of di6crete
microcell~. It has been found particularly advantageou~
to produce a compre~sible layer by incorporating hollow
microcapsules in the polymer, as illustrated by Shrimpton
et al, U.S. Patent No. 3,700,541, and corresponding
Briti6h patent 1,327,758; and by Larson, U.S. Patent
4,042,743.
In prior art methods of producing a compressible
lay~r for a printing blanket employing microcapsules, it
has been found that the thickness of the compre6sible
layer formed iB not easily controlled since microcapsules
most suitable for this use will melt at a temperature
lower than the vulcanizing temperature used for
vulcanizing the printing blankets. Since the
microcapsule6 melt before the vulcanization i6 complete,
and before the compre6sible layer achieve6 a ~et
6tructure, agglomeration of the voids created by the
microcapsules occurs, and size variations in the voids
also occur. This can affect overall performance
~5 properties of the blanket. Al~o, the variation~ in the
sizes of the voids can weaken the printing blanket and
cause the printing blanket to wear out prematurely.
It is a feature of the present invention to
overcome the deficiencies in the prior art printing
blanket6 described above. The present invention provide6
a me~hod of making a laminated printing blanket by forming
a base ply and a surface layer and dispo~ing therebetween
an intermediate compres6ible layer formed by incorporating
microcap~ule6 in an elastomeric matrix.
.?~
--5--
In accoedance with one aspect of the method of
the present invention, at least one fabric substrate layer
iB provided. An inte~mediate compres6ible layer of an
elastomeric material i8 formed thereon. The elastomeric
material has a substantially uniform thickness and has
microcapsules incorporated therein and su~tantially
uniform distributed throughout the compressible layer.
The compressible layer i8 maintained at a temperature
below the melting point of the microcapsules for a time
~ufficient to cause the elastomeric material to vulcanize
to a degree sufficient to substantially fix the positions
of the microcapsules within the compressible layer. A
surface layer iB provided over the compressible layer to
form a printing blanket construction which is vulcanized
to cure all of the layers and provide the compressible
layer with sub6tantially uniformly distributed void6 of
substantially uniform size. In a preferred form, the
elastomeric material including the microcapsules iB
initially fixed by adding an accelerator capable of
permitting vulcanization of the elastomeric material at a
temperature ranging from 43 to 77C (110F-170F), over a
time period ranging from 1 to 12 hours. These temperature
conditions, which are below the melting point of the
microcapsules, cau~e an initial vulcanization of the
elastomer compound to form the intermediate layer. Then
all of the layers are laminated to create a unitary
printing blanket by finally vulcanizing the assembly under
controlled heat and pressure. The final product comprises
a laminated printing blanket including at least one
substrate layer, a surface layer, and a compres~ible
intermediate layer disposed therebetween. The
intermediate layer ha~ a closed-cell cellular structure
having substantially uni~orm thickness and sub~tantially
evenly distributed void6 of 6ubstantially uniform 6ize, in
which the void~ a~e not interconnected. Additional
reinforcing fabcic layer~ may also be incorporated therein.
Othe~ details, features, ob~ect~, uses and
advantages of this invention will become apparent from the
embodiments thereof presented by the way of example in the
following detailed description, the accompanying drawings,
and the appended claims.
In o~der that the invention may be more readily
under~tood, reference will now be made by example to the
accompanying dcawings in which:
Fig. 1 illustrate~ a cross section of a printing
blanket of the present invention, indicating the
relationship of the layers; and
Fig. 2 iB a flow diagram, illustrating the stees
comprising the method of the present invention.
Reference is now made to Fig. 1 which illustrates
a printing blanket made according to the present
invention. The printing blanket 10 comprises a surface
layer 20, a reinforcing fabris layer 22, a compressible
layer 24, and at least one additional layer of fabric as a
substrate or base. For purpose~ of illustration, two
substrate fabric layers 26 and 27 aLe shown in the
drawing. Those skilled in the art will recognize that the
number and types of layers used can vary depending on the
uses intended. Adhesive layers 30, 32, 34 and 36 are
preferred to ensure sufficient bonding between the
different layers in the blanket. Voids 28 in the
compressible layer 24 make possible di~placement of the
surface layer 20 without distortion thereof under
operating conditions. As illustrated in the drawing, the
voids 28 in the compressible layer 24 have sub6tantially
uniform size and subtantially uniform distribution and
are not inter-connected. It ha6 been found that the
1;~89~ 6
dimen~ions of the voids 28 produced in the comeressible
layec 24 are qenerally in ~he same range as the dimen6ions
of the microcapsule~ u~ed to create the voids. In
general, the voids will be in the range of 10-125 microns
in diameter, and preerably 20-60 microns.
The compr2ssible layer 24 is formed by an
elastomeric material having the usual processing,
stabilizing, strenqthening and curing additives and is
formulated with reference to its specific application. A~
is known in the art, this formulation i~ different from
the one used for the surface layer 20. Any suitable
polymeric material which i~ considered a curable or
vulcanizable material can be used for example, natural
rubbee, styrene-butadiene rubber (SBR), EPOM
(ethylene/propylene/non-conjugated diene ter-polymer
rubber), butyl rubber, butadiene, acrylonitrile rubber
(NBR), polyurethanes etc. An elastomer which is resi6tant
to solvents and ink is preferable.
The adhesive layers 30, 32, 34 and 36 may be any
suitable elastomeric adhesive known in the art.
Preferably the adhesive will be a rubber cement. The
fabric layers 22, 26 and 27 should be made of plain woven
fabric of lower extensibility in the warp direction (in
the direction longitudinal to the machining of the
blanket), and are typically high grade cotton yarns, which
are free from slubs and knots, weaving defects, seeds,
etc. The fabeic may also be rayon, nylon, polye6ter, or
mixtures thereof. When aæplying adhesive to any of the
fabric layers, it is usually spread with a
knife-over-roll-spreader. The adhesive i8 apelied in
layers until the desired thickness is obtained.
Typically, a fabric layer will be about 0.076 to 0.41 mm
(0.003 to 0.016 in.) thick.
The compres~ible layer 24 is formed by applying
the elastomeric compound as described above, containing
microcap~ules, to the fabric substrate 26, also using a
knife-over-roll-spreader. The elastomeric compound is
brought to the desiIed consistency ~or spreading by adding
a ~olvent. In genesal, a number of layer~ of the compound
are required to make an intermediate layer 24 of the
de~ired thickne~. As each layer is applied, it is
solidified, but not crosslinked, due to the evaporation of
the solvent. In general, the compressible layer will be
about 0.20 to 0.38 mm (0.008 to 0.015 in.) thick. It is
preferred that the layer be about 0.2B to 0.30 mm (0.011
to 0.012 in.) thick.
The surface layer 20 is made usinq the 6a~e
lS procedure as is described in the making of the
intermedia~e layer, using an elastomeric compound suitable
for the working face of the printing blanket 10. In
general, a number of layers of compound are required to
make a surface layer 20 of the desired thickness. In
general, the surface layer 20 will be about 0.005 to 0.25
in. thick, preferably be about 0.010 to 0.015 in. thick.
It is most preferred to provide a surface layer in the
range of about 0.30 to 0.38 mm (0.012 to 0.015 in.) thick.
According to the present invention, conventional
re~inous microcapsules that are known in the art may be
used in the intermediate layer. Any microcapsules having
the properties described herein will be suitable for use
in the present invention. Microcapsules having a melting
point of about 74 to 132C (165F to 270F) can be used.
Preferably, the microcapsules will melt at about 8Z
(180F). Some of ~he materials suitable for use in the
microcapsules are phenolic re6in, and thermoplastic
materials such as polyvinylidene chloride. Preferably,
? ~1 ~
the material~ used in making the microcapsules will be
thermoplastics. Example6 of such materials are
polyvinylidene chloride, poly methyl methacrylate,
polyvinyl chloride, polyacrylonitril2, and copolymers
thereGf. Preferably, a copolymer of acryloni~rile and
vinylidene chloride will be used. Mixtures of different
kinds of thermoplastic microcapsules may be used if
desired.
In addition, any of the resins having the
properties that are described herein that are mentioned in
U.S. Patent 2,797,201 may be used. Of course, it is
understood that these materials named are illustrative,
and the present invention i6 not limited thereby.
The microcapsules used in the method claimed are
approximately seherical in shape and range from 10 to 100
micron6 in diameter, with an average particle size of
about 30-50 microns. These capsules are formed, for
example, with a thin, elastic, thermoplastic shell which
may be composed of vinylidene chloride/acrylonitrile
copolymer or similar material. These cap~ules may be
formed as i8 generally de~cribed in U.S. Patent 2,797,201
or U.S. Patent 3,615,972. The capsules may contain an
inert gas to maintain their shape. The capsules may be
mixed with the elastomeric matrix in any conventional
method, such as dispersing them in a dough of the
elastomer. The capsules are dispersed by mixing them
uniformly throughout the elastomeric matrix for a
controlled time period. Because the act of mixing
generate~ a certain amount of heat, it is preferred not to
mix longer than 30 minute~. Studies made utilizing an
electron micro6cope, of the preferred embodiment, appear
to indicate that a major portion, if not substantially all
of the microcap~ules u6ed in making the compressible layer
--10--
are recognizable in the completed printinq blanket when
utilizing the method of the eresent invention.
The a~ount and the size of the specific capsules
u6ed may be based on the desired compres~ibility of the
blanket. For example, microcapsule~ having an average
diameter of 40 micIons may be used in a catio representing
50 percent of the ela~to~eric material used in the
compressible layer to produce a compressible layer having
50 percent voids.
Preferably the presence of water is avoided
during the incorporation of the microcap~ules in the
elastomer in order to avoid water vapor blowing during any
subsequent heating of the polymer. For this reason, the
microcapsules are preferably dried before mixing the
elastomer.
Reference is now made to Fig. 2 which is a flow
diagram illustrating the steps in one method of preparing
the printing blanket of the invention. The reference
numbers used in Fig. 2 are the same as those used in Fig.
2~ 1 where the same elements are described. As noted in the
flow diagram adhesive layer 34 is spread on the upper
surface of substrate layer 26; compressible layer 24 i~
formed by mixing microcapsules and an activator or
accelerator with an elastomeric compound for about 30
minutes: and the compound is spread onto the fabric
substrate 26. In general, a number of layers of compound
are required to obtain the desired thickness of the
compressible layer. A layer of adhesive 32 is then spread
on the upper surface of the layer 24, and the fabric layer
22 as bonded to the layer 24. The resulting assembly is
then subjected to sufficient heat to initially vulcanize
layer 24 to a degree sufficient to set the structure of
the polymer ma~rix with the microcap6ules fixed in
position therein, 6aid vulcanization being made po6sible
by the addition of an accelerator that promote~ the
vulcanization at a low temperature. Vulcanization at 56C
(150F) for ~ hours is preferred, however, a temperature
range of 43 to 77C (110 to 170F) for period~ of 1 to
12 hours may also be used. Theoretically, ambient
temperatures for longer periods of time are also
pos6ible. The preferred accelerator ~eferred to in this
example iB a dithiocarbamate such a6 the one 601d under
the name "Butyl-Eight" (trademark), which is available
from R.T. Vanderbilt Co. Other examples of accelerators
u6eful in this invention are piperidinium pentamet ffllene
dithiocarbamate (Accel 552 (trademark), available from
Dupont Corp), zinc dibenzyl dithiocarbamate (Arazate
(trademark), available from ~niroyal), zinc dibutyl
dithiocarbamate (Butyl Ziram (trademark), available from
Pennwalt). Thi~ low temperature vulcanizing 6tep create6
a permanent cell structure which iB not changed throughout
the rest of the manufacturing process.
In the preferred embodiment of the invention, the
low temperature initial vulcanization, using the
acccelerator, will cause optimum final vulcanization of
the csmpressible layer. That is, substantially all the
~ites in the polymer, which are most 6usceptible to
cros61inking, are cro~61inked in thi6 6tep, said
crosslinking providing the preferred ela6tic modulus and
re~iliency and other elastic properties of the elastomer.
Of cour6e, tho6e 6killed in the art will recognize that,
in a rubber product, cros61inking i6 a continuing proces6,
and that no rubber material i6 ever completely
cro6slinked. Therefore, those skilled in the art will
recognize that the vulcanization in the low temperature
cure of the compressible layer may be interrupted prior to
-12-
optimum vulcanization as long as the elastomeric matrix
containing the microcapsules has ~et up ~ufficiently to
"freeze~ the microcap~ules in po~ition and still obtain an
acceptable product. A thus ~partially~ vulcanized
compressible layer may obtain better cros~linking with the
base layer and the printing surface. One skilled in the
art will al~o recognize, however, that a compres~ible
layer, which has been substantially completely vulcanized,
may be cro~linked to the base layer and the surface layer
by means of an adhesive specifically formulated for that
purpose.
After curing comeressible layer 24, a second
sub~trate fabric layer 27 is laminated to the lower
surface of layer 26, by means of adhesive layer 36, on
laminating pinch collers. Following this step, the
working surface layer 20, compounded as described above,
is applied to the upper surface of reinforcing fabric
layer 22, using adhe~ive layer 30 to achieve a bond. The
resultant assembly i~ then subjected to the final
vulcanization process known in the art, at a temperature
range of 132 to 160C (270F to 320F), and preferably
143 to 149~ (290F to 300F), for one-half hour to 16
hours under pressures ranging from atmospheric to 5.2 bars
(75 psi). These variables will depend on the exact
compounding.
In the final vulcanizing ~tep, paper having a
fine finish i8 disposed in contact with the face of the
printing blanket, together with a fine talc prior to
placing the blanket in the vulcanizing oven. The paper,
contacting the printing blanket surface, assures the
smoothne6s of the printing blanket, since the smoothness
of the paper is imparted to the printing blanket. For
most applications, the finish 80 provided to the printing
blanket will be sufficient for its use, and grinding of
the surface will not be required.
It has been found that the ~tep of ~ubjecting the
intermediate layer to a partial vulcanization or semi-cure
causes the microcapsules to be captured in stationary or
~et po~itions in the elastomeric matrix. Since the
po6itions of the microcap~ule~ are ~et in the matrix, the
portions of the void~ created br the microcapsules are
predetermined by the position of the microcap~ules in the
matrix. When the assembled blanket undergoes the final
vulcanization ~tep, the already set structure of the
in~ermediaee layer holds its shape and prevents the
agglomeration of void6 or the collapse of voids in the
layer. Thi6 fixed position will not change under final
processing of the blanket.
It has been pointed out that the melting
temperature of the microcapsule6 is below the final
vulcanizing temperature. Obviously most, if not all of
the microcap~ule6 probably melt, but since they are
retained within closed cells, they appear to re-form, and
act as a coating of the walls of the voids. It is al~o
possible that some of the material will partially interact
with the elastomer to form a new coating substance.
Electron micro~cope studies have confirmed that the
thermoplastic material from which the microcapsules are
made remain the voids of the completed printing blanket.
Of course, as long as the voids in the compres6ible layer
are formed as de~cribed, the presence or absence of the
microcap~ule material in the voids will not affect the
performance of the blanket. Regardles6 of what happen~
the previously fixed void structure remain6 unchanged.
The exact construction of the blanket may be
varied according to its final use. For example, a ~ingle
-14-
fabric ~ubstrate layer 26 may be utilized without layer
27, or a third or additional similar layers may be
incorporated. It may also be desired to provide
additional reinforcing fabric layers similar to layer 22.
Since, Por production purposes, it i8 alBO
po6sible to prepare a compre6sible layer for u~e as an
intermediate layer in a printing blanket in one location,
and to ~hip the compressible layer to another location for
fabrication of the blanket, the comeressible layer, by
itself, represents a separate feature of the of the
instant invention. The compressible intermediate layer
may be prepared by the same method decribed above in the
description of the preparation of the printing blanket,
and will have generally the same paramaters with regard to
dimen6ions and distribution of voids. In such a case, it
would be preferable to aeply the compressible layer to at
least one fabric substrate layer prior to vulcanizing and
shipping. However, it is also possible to utilize a
fabric or release paper as the sub6trate layer and
reinforcing layer and strip off the compressible layer for
shipping. In that case, the6e layers will be added later.
Also, it will be recognized by those skilled in
the art that it is po6sible to make a complete blanket
construction, including a compressible layer containing
microcapsules and a dithiocarbamate accelerator, and
subject the construction to a low temperature vulcanizing
step in order to fix the microcapsules in the compressible
layer as before, before 6ubjecting the blanket
construction to final vulcanization.
It i8 to be noted that the above description i~
merely illustrative of the invention, and other parameter6
and embodiments may be used without departing from the
inventive concept herein. Accordingly, the pre6ent
invention is only limited by the appended claims.
