Note: Descriptions are shown in the official language in which they were submitted.
lO9~Z69
BACKGROUND OF THE INVENTION
This invention relates to laminates containing a
rubber surface and in particular to laminates which are useful
as printing blankets for the printing industry. The invention
also relates to a method of preparing such laminates which
are of the compressible variety.
The use of blankets in offset printing for transfer- -
ring the ink from a printing plate to the paper is well known.
Printing blankets must be carefully designed so that the
surface of the blanket is not damaged either by the mechanical
contact with the parts of the press or by chemical reaction
with the ink or ink ingredients. These repeated contacts
cause a certain amount of compression of the blanket w!hich
must be within proper limits so that the image is properly
transferred without causing permanent deformation of the
blanket. An important feature of offset printing blankets,
therefore, is the ability of the blanket to return to its
original thickness upon repeated use and to provide constant
image transfer.
In addition to the problems caused by variations
of the printing apparatus, conventional printing blankets
may contain variations in thicknesses which are formed during
the manufacturing steps. The existence of high or low spots
in blankets may create uneven reproductions on the finished
product.
The printing blankets which are currently used in
the industry generally comprise a base material which pro-
vides integrity to the blanket and a working surface or top
layer of an elastomeric material made of natural or synthetic
rubber. The base material may comprise one or more fabric
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plies and a layer of a soft highly resilient rubber backing
layer. The working surface is a void-free layer of elastomeric
material which actually contacts the ink.
The usual method for applying the working layer
of elastomeric material to the laminate is by calendering
or spreading rubber in successive thin layers until a desired
thickness of rubber has been deposited. The assembly then
is cured to provide the finished blanket. Such blankets are
acceptable but often lack the necessary compressibility or
compression set characteristics required in many applications.
Moreover, the spreading of successive thin layers of rubber
to form the top coat is tedious and often results in surface
irregularities which cannot be completely removed by surface
treatments such as grinding.
Although blankets have been manufactured with im-
proved compression set characteristics through the use of
special plasticizers, textile fibers, foam backing, etc.,
ideal results have not been obtained becase of problems such
as uneven compressibility of the surface coating resulting
from nonuniform closed cell structure in the backing. Also,
backing materials containing cell structures formed by blowing
processes have not always contained uniform cellular structures.
The problem with the quality of the printing obtained
through the use of the known printing blankets has been magni-
fied by the recent developments in printing technology suchas by the design and introduction of very large presses running
at ever-increasing speed and new developments in inks and
paper.
U.S. Patent 3,795,568 describes a typical procedure
which has been used for preparing printing blankets. The
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normal procedure involves the formation of the laminate by
successive formation of the various layers. For example,
two plies of fabric are bonded together by a neoprene cement
and placed upon a conventional blanket manufacturing machine.
A compressible intermediate layer is formed on the second
fabric ply by spreading a mixture of a rubber compound, solvent
and foam rubber particles onto the fabric and exposing the
mixture to an elevated temperature to remove the solvent.
A third adhesive coated fabric ply is placed over the spread
layer and the assembly is passed through a spreading machine
where layers of rubber are applied to the surface by repeated
steps until a layer of about 0.01 inch is built up. The assem-
bly then is cured at an elevated temperature.
Printing blankets which have been used in the past
have exhibited moderate resistance to the vehicles used in
the inks. The blankets have not been easy to clean, swell
considerably after extended cleaning, and are not impervious
to ozone attack.
Thus, there continues to be a need in the print-
ing industry for a printing blanket which overcomes these
problems and deficiencies without a compromise in the desirable
and necessary properties of printability, ink release and
dimensional stability.
SUMMARY OF THE INVENTION
The present invention provides an improvement in
the manufacture of printing blankets particularly suitable
for offset printing and an improvement in the blankets them-
selves. The improvements are obtained by preparing a composite
of the various plies of material desired to be incorporated
into the printing blanket and thereafter subjecting the composite
_ . . . . . . .. , , . . . _ _ . _ . ~ . _
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to molding pressures at elevated temperatures. More particularly,
the composite comprises 1) a fibrous or pressure sensitive ad-
hesive backing material, 2) a layer of a foamable and cross-
linkable elastomeric material over the backing material, 3)
a second fibrous material over the layer of foamable material,
and 4) a top layer of a non-foamable cross-linkable elastomeric
material over said second layer of fibrous material. Molding
the composite under pressure at an elevated temperature results in
in vulcanization of both elastomeric layers and chemical foaming
of the foamable elastomer in situ. The final molded object
has a surface which is substantially free of voids and a finish
which is substantially free of surface irregularities. The
use of fluoroelastomers as the ealstomeric material of the
composite provides improved printing blankets characterized by
improved dimensional stability, resistance to swelling, print-
ability and cleanability.
In accordance with one aspect of the present invention,
there is provided a process for preparing a printing blanket
useful for offset printing which comprises; preparing a comp-
osite by, sheeting out a layer of a foamable and cross-linkable
elastomeric material over a fibrous or pressure sensitive
adhesive backing material, applying an intermediate layer of
fibrous material over the layer of foamable material, and sheeting
out a top layer of a non-foamable cross-linkable elastomeric
material over said intermediate layer of fibrous material, and
molding the composite at an elevated temperature under pressure
whereby both layers of elastomeric material are cross-linked in
situ and the foamable elastomeric material is foamed in situ.
In accordance with a further aspect of the present
invention, there is provided a process for preparing a com-
pressible printing blanket for offset printing which comprises,
preparing a composite by, sheeting out a layer of a foamable
and cross-linkable fluoroelastomeric material over a fibrous
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backing material, applying an intermediate layer of fibrous
material over the layer of foamable elastomeric material, and
sheeting out a top layer of non-foaming cross-linkable fluoro~
elastomeric material over said intermediate layer of fibrous
material, and molding the composite under a pressure of from
one to about 500 psi at a temperature of from about 150C to
about 200C for a period of from about one to 20 minutes where-
by both layers of elastomeric material are cross-linked in situ
and the foamable elastomeric material is foamed in situ.
In accordance with a further aspect of the present
invention, there is provided a printing blanket, useful for
offset printing, comprising in combination: a) sheeted-out
lower layer of foamed elastomeric material over a fibrous back-
ing material, b) an intermediate layer of fibrous material over
said layer of foamed elastomeric material, and c) a sheeted-out
top layer of non-foamed elastomeric material over said inter-
mediate layer of fibrous material, said combination comprising
a molded product wherein both layers of elastomeric material
are cross-linked and whereby the combination results from the
molding being carried out at an elevated temperature and under
pressure whereby both layers of elastomeric material are cross-
linked in situ and the foamable elastomeric material is foamed
in situ.
BRIEF DESCRIPTION OF THE DRAWING
The drawing illustrates one embodiment of the com-
pressible printing blanket of the present invention which com-
prises an elastomeric top surface printing layer l, and inter-
mediate layer of a fibrous material 2, a layer of a foamed
elastomeric material 3 and a backing layer of a fibrous
material 4.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
It has now been found that printing blankets for
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offset printing can be prepared by a molding technique which
provides for a printing blanket having improved properties,
and particularly an improved substantially void-free smooth
printing surface. It also has been found that printing
4b-
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las~z6s
blankets having a top printing surface of a fluoroelastomer,
and preferably a printing blanket having a top surface of
a fluoroelastomer and an intermediate foamed layer of a
fluoroelastomer exhibit improved resistance to swelling and
image distortion, good ink release and transfer, good print-
ability and improved cleanability after use without glaze
buildup.
The process for preparing the improved printing
blankets of this invention comprises
a) preparing a composite of
1) a fibrous or pressure sensitive ad-
hesive backing material,
2) a layer of a foamable and cross-
linkable elastomeric material over
the backing material,
3) a second fibrous material over the
layer of foamable material, and
4) a top layer of a non-foamable cross-
linkable elastomeric material over
said second layer of fibrous material,
and
b) molding the composite at an elevated
temperature under pressure.
The conditions for molding the composite, such as the tempera-
ture, pressure and time of molding will vary depending upon
the particular elastomeric composition selected for the top
layer and the intermediate foamable layer since curing and
foaming occur while the composite is molded.
The preparation of the composite is effected by
sheeting out a layer of the foamable and cross-linkable
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~ 94269
elastomeric material on the backing material followed by
applying the second fibrous material on top of the foamable
layer, and finally applying or sheeting out a top layer of
the non-foamable cross-linkable elastomeric material which
is to form the top layer with a printing surface.
The amounts of foamable or non-foamable cross-
linkable elastomeric materials utilized for the top layer
and the intermediate foamable layer will be determined by
a consideration of the desired characteristics of the print-
ing blanket including the desired thickness and compressi-
bility characteristics of the printing blanket. Generally,
printing blankets may be utilized having thicknesses of from
about 55 to about 75 mils. The precise thickness of a print-
ing blanket will be determined by its end use and the par-
ticular requirements which may be dictated by the end user.The cover layer and intermediate foam layer thicknesses may
range from about 5 to about 30 mils or more with the fabric
layers making up the difference.
As mentioned above, the cover or top printing layer
may comprise any material having rubbery or compressible
properties and which will cure and, optionally, foam under
the conditions of molding. Examples of elastomeric materials
which have been utilized previously as a top printing layer
include elastomeric materials having various hardnesses such
as can be prepared from a copolymer of acrylonitrile with
butadiene, epichlorohydrin rubbers which can be 100% of
epichlorohydrin or can contain up to about 50% of ethylene
oxide, polyurethane elastomers, acrylonitrile/epichlorohydrin
elastomers, polysulfide rubbers and fluoroelastomers. The
fluoroelastomers include elastomers such as fluorosilicones
and fluorocarbons with the fluorocarbons being preferred.
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Among the fluorocarbons which can be utilized in the
preparation of the printing blankets of this invention are
those which are commercially available from a variety of
sources under such trade names as "Viton"* (E.I. duPont de
Nemours and Company) and "Fluorel"* available from the
Minnesota Mining and Manufacturing Company. Elastomers
available under these two trade names are highly fluorinated
synthetic rubbers which generally consist essentially of a
polymeric compound containing a major portion of units of
hexafluoropropylene and vinylidene fluoride. Copolymers
of these monomers containing 30 to 60% by weight of hexa-
fluoropropylene monomer and 70 to 40% by weight of vinylidene
fluoride monomer are useful. Terpolymers thereof with tetra-
fluoroethylene in which there is from about 65 to 97% by
weight of units of hexafluoropropylene and vinylidene fluoride
also can be utilized.
The elastomeric material utilized as the top print-
ing layer also can be utilized as the intermediate foamed
layer of the printing blanket although it is not essential
that the elastomeric material utilized in the top layer be
identical to the material used in the foam layer. It is
preferred, however, that fluoroelastomers be utilized as
the top layer and the reinforcing foam layer since these
materials exhibit excellent cleanability and solvent resistance.
The elastomeric formulations utilized for the
preparation of the top printing layer and the intermediate
foam layer will contain the usual additives such as acid
acceptors, fillers, pigments, antioxidants, processing aids,
cross-linking agents, etc., and the formulation for the
foamable elastomer also will contain blowing agents normally
*Registered Trademark
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utilized in conjunction with the particular elastomer~ Acid
acceptors which may be utilized include materials such as
zinc oxide, magnesium oxide, lead oxide, and calcium oxide.
Magnesium oxide is preferred. Fillers include a variety
of materials such as mineral fillers including calcium car-
bonate, calcium silicate, silica, etc. Pigments such as
carbon black, titanium dioxide, zinc oxide, various dyes
and read lead may be included to provide a desired color
to the elastomeric layers.
A variety of processing aids are well known in
the elastomer art and the particular processing aids to be
included in the elastomeric compositions depend on the nature
of the elastomer. Processing aids are utilized in the present
invention to improve mixing and mold release. Examples of
processing aids include plasticizers and other materials
which can be utilized to control the viscosity of the formula-
tion such as polyethylene, carbauba wax, stearic acid, dibasic
lead phosphate, etc.
The cross-linking agents to be utilized with par-
ticular elastomers are well known in the art. Among the
cross-linking agents preferred in this invention for fluoro-
elastomers are the various peroxides and diamines such as
ethylene diamine carbamate, hexamethylene diamine carbamate,
benzoyl peroxide, dicumyl peroxide, etc. The preferred
blowing agents for the fluoroelastomers utilized in the
formation of the foamed layer include the blowing agent
available under the general trade designation "Celogen"*
available from the Naugatuck Chemical Company and "Kempore"*
available from National Polychemicals Company. Specific
examples include "Celogen OT" which has been identified as
*Registered Trademark
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principally p,p'-oxy-bis (benzene sulfonyl) hydrazide and
"Kempore 200"* which i5 believed to be azo-dicarbonamide.
Thus, the elastomeric materials utilized in the
formulation of the top layer and the intermediate foamed
layer will comprise generally:
Parts by Weiaht
Elastomer lOO.O
Acid acceptors1-20
Fillers and pigments 0-60
Processing aids 0-20
Cross-linking agent l-lO
Blowing agents0-5
The several ingredients are mixed until homogeneous and
sheeted out to the desired thickness.
In addition to the two layers of elastomeric material,
the printing blankets of the invention contain a layer of
backing material which may be either a fibrous material or
a pressure sensitive adhesive although fibrous materials
are preferred. Woven as well as non-woven fabrics can be
utilized as the fibrous backing material. The printing
blankets of the invention also contain an intermediate layer
of a second fibrous material, and this fibrous material may
be the same as or different from the fibrous material of
the bacXing layer. Typically, twills, drills, ducks and
other types of weaves of natural fibers such as cotton or
synthetic fibers such as rayon are utilized as the backing
material and the second fibrous material. Other synthetic
fibers such as nylon, polyesters, polypropylene or blends
of fibers also can be utilized. ~on-woven spun bonded fabrics
of polyester or polyolefins such as polyethylene can be
utilized as the backing material.
*Registered Trademark
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For some printing applications, a pressure sensitive
adhesive may be utilized as the backing in place of the
fibrous backing. Typical pressure sensitive adhesives that
can be utilized are those based on natural or synthetic
elastomers modified with various resins and other compounding
agents. These pressure sensitive adhesives can be applied
as a latex or in a solvent or the foamable and cross-linkable
elastomeric material can be applied over the adhesive contained
on a release paper.
Cements or adhesives are not required for the
formation of the printing blankets of this invention since
the composites are prepared utilizing uncured elastomeric
materials which form tight bonds with the fibrous material
or adhesive backing material when cross-linked or cured under
pressure at elevated temperatures. An important result which
is obtained by the method of this invention is that the
elastomeric layers, and particularly the foamed elastomeric
layer partially fills the interstices of the base and inter-
mediate layers of fibrous materials. The presence of the
foamed elastomeric layer in the interstices of the backing
layer results in a high degree of adhesion of the printing
blanket to the printing cylinder upon which it is mounted,
and this reduces the possibility of the printing blanket
slipping on the printing cylinder during operation.
After the composite described above is prepared,
it is molded under pressure such as between steel plates
or in a calender. Molding temperatures between about 125
to about 225C. or higher and pressures of from about one
to about 500 pounds per square inch may be utilized. The
duration of the molding operation will vary but generally
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can be effected in a period between about one to 20 minutes.
During the molding operation, any entrapped air is
removed while vulcanization or cross-linking and chemical
foaming takes place in situ. The final molded object has a
top surface substantially free of voids and a finish which
replicates that of the steel plate or the calender. The
foamed intermediate layer contains a multitude of individual
closed cells.
The following example illustrates the method of
this invention utilizing a fluorocarbon as the elastomer
of the top printing layer and the foamed intermediate layer.
Unless otherwise indicated, all parts and percentages are
by weight. The elastomeric materials are compounded in
accordance with standard procedures published for compounding
fluoroelastomers on a rubber mill.
Formulation -- Top Printina Layer
Parts by Weiqht
Viton E-60C (a fluoroelastomer
available from DuPont and having
a Shore "A" durometer hardness when
cured of about 60 to 95 75
Viton LD 3193-90 (a fluoroelastomeric
copolymer available from DuPont) 25
Calcium Carbonate 35
Precipitated Silica (5 micron) 5
Titanium Dioxide 5
Magnesium Oxide 3
Polyethylene (Natrocel*PE) 2
Vanfre*AP-2 (processing aid from
Vanderbilt Co.) 2
Monastral Blue 0.5
Calcium Hydroxide 3.0
*Registered Trademark
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\
Formulation of Foamable Elastomer
, .
Parts by Wei~ht
Viton ~ (copolymer of hexa-
fluoropropene and vinylidene
fluoride, available from DuPont) 50
Viton E-60C 37.5
Viton LD 3193-90 12.5
zinc Oxide 4.0
Dibasic Lead Phosphite ("diphos") ~.0
Calcium Carbonate 23.5
Precipitated Silica (5 micron) 2.5
Titanium Dioxide 2.5
Medium Thermal Black 4.0
Magnesium Oxide (Maglite D from
Merck & Company) 1.5
Kempore 200 1.0
Vaseline* 2.5
Hexamethylenediamine Carbamate
(Diak*No. 1) 1.2
Calcium Hydroxide 1.5
The base fabric utilized in this example is a double
warp wigan cotton duck, 15 mil, six ounces per square yard
f~bric. The foamable el~stomeric formulation described above
is sheeted onto the base fabric to provide an elastomeric
layer thickness of about 15-17 mils. Over the foamable material
is placed a second layer of double warp wigan cotton duck
fabric of about 11 mils thickness and weighing approximately
five ounces per square yard. The top layer elastomeric material
described above then is sheeted on top of the second fabric
to a thickness of about 15-17 mils.
The composite prepared in this manner then is molded
between steel plates at a temperature between about 150C.
*Registered Trademark
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and 200C. under a pressure of about 110 psi for a period
of about five minutes. During this molding operation, entrapped
air escapes from the four sides of the composite while vulcaniza-
tion and chemical foaming takes place in situ. The printing
blanket prepared in accordance with this example has a surface
which is substantially free of voids and a finish which repli-
cates that of the steel plate. The foam layer is character-
ized by a multitude of individual closed cells.
The novel printing blankets of this invention exhibit
the following properties:
1. They are compressible in design which minimizes
heat buildup;
2. They are highly resistant to swelling and image
distortion from inks and cleaning solvents;
3. They exhibit excellent resistance to ozone
attack such as found in ultra-violet dryers;
4. They are easily cleaned in a wash bath without
glaze buildup;
5. They exhibit good ink release and transfer;
6. They exhibit good printability and mechanical
strength.
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