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Description  |
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The present invention relates to an assembly which is adapted to receive a
photopolymerisable material to produce a relief printing plate.
A relief printing plate consists essentially of a photopolymerisable
material mounted on a suitable substrate. In use the photopolymerisable
material is irradiated with actinic e.g. ultraviolet light through which
an image-bearing transparency e.g. a photographic negative film, whereby
the exposed portion representing the image is hardened and the non-exposed
portion is subsequently removed e.g. by solvent washing. In more detail,
the process of plate production involves irradiating the plate from
opposite sides to effect simultaneous exposure of the back of the plate
and the image on the front, followed by removal of unpolymerised material
and final curing of the plate by further irradiation.
One important factor in the construction of such a relief printing plate is
the adhesion of the photopolymerisable material to the substrate. Thus,
after irradiation the photopolymerisable material should not tend to peel
from the substrate nor should parts of the image slide on the substrate as
a result of handling the plate after the "wash-out" process both before
and after final curing irradiation because in such a case the relief plate
will not provide a true representation of the transparency. Another
desirable property in the assembly is flatness, i.e. on lying flat there
should be no curling of the edges.
In order to achieve satisfactory adhesion between the substrate and the
photopolymerisable material it has proved necessary to pre-treat the
substrate with one or more layers of substances capable of providing a
keying effect between the substrate and the photopolymerisable material.
Such substances are generally applied to the substrate in layers a few
microns thick to give an assembly which is then adapted to receive the
photopolymerisable material.
The present invention is a new assembly of this type which confers an
excellent resistance to peel on a variety of photopolymerisable materials
and at the same time has a reduced tendency to curl.
Accordingly, the invention comprises an assembly adapted to receive a
photopolymerisable material in the production of a relief printing plate
which comprises
(a) a polymeric substrate
(b) a priming layer on the substrate, and
(c) a resin layer on the priming layer, wherein the resin layer is at least
2 microns thick and preferably at least 6 microns thick and comprises a
resin which is vinyl chloride/vinyl acetate copolymer of average molecular
weight in the range 1,000 to 100,000 preferably 5,000 to 25,000 and which
contains 60 to 98 wt% vinyl chloride, 2 to 20 wt% vinyl acetate and,
optionally, up to 20 wt% of a termonomer.
The vinyl chloride-vinyl acetate copolymer resin is a relatively low
molecular weight material, and the average molecular weight (number
average) thereof is more preferably in the range 6,000 to 16,000,
especially 6,000 to 10,000.
The termonomer, when present, preferably amounts to 10 to 20% especially
15% by weight of the resin polymer and may be an ethylenically unsaturated
monomer, such as maleic acid or, notionally, vinyl alcohol produced by
hydrolysing a proportion of the vinyl acetate units in the polymer chain
but is preferably a hydroxyl group-containing ethylenically unsaturated
termonomer such as a hydroxyl-containing alkyl acrylate or methacrylate,
particularly a lower alkyl acrylate or methacrylate in which the alkyl
group contains from 1 to 6 carbon atoms--especially a methyl or ethyl
group.
When a hydroxyl-containing termonomer is present, the total hydroxyl
content, based on the weight of the terpolymer, is desirably from 1.5 to
2.5 wt%, particularly from 1.8 to 2.2, e.g. 2.0, wt%.
A preferred resin comprises a terpolymer containing from 75 to 85 weight %
of vinyl chloride, from 2 to 8 weight % of vinyl acetate and the balance
(to 100 weight %) of an unsaturated termonomer.
It is also desirable to incorporate a filler, especially a particulate
filler, into the resin layer in order to roughen the surface and for this
purpose particulate oxides of metals or metalloids, such as alumina and
silica, especially the latter, have proved to be very satisfactory.
The resin layer is at least 2 microns, preferably at least 6 microns thick
and is more preferably about 12 microns thick. There is no upper limit to
the thickness of the layer from the point of view of optimum anti-peel
properties of the assembly but for practical and economic reasons there is
no cause to apply a resin layer greatly exceeding 30 microns.
The substrate may comprise any suitable polymeric material, particularly in
the form of a self supporting film or sheet. Suitable polymeric materials
include
cellulose esters, e.g. cellulose acetate, polystyrene, polyamides, polymers
and copolymers of vinyl chloride, polymers and copolymers of olefines,
e.g. polypropylene, polysulphones and particularly linear polyesters which
may be obtained by condensing one or more dicarboxylic acids or their
lower alkyl diesters, e.g. terephthalic acid, isophthalic, phthalic, 2,5-,
2,6- and 2,7-naphthalene dicarboxylic acid, succinic acid, sebacic acid,
adipic acid, azelaic acid, diphenyl dicarboxylic acid, and
hexahydroterephthalic acid or bis-p-carboxyl phenoxy ethane, optionally
with a mono-carboxylic acid, such as pivalic acid, with one or more
glycols, e.g. ethylene glycol, 1,3-propanediol, 1,4-butanediol, neopentyl
glycol and 1,4-cyclohexane-dimethanol. A biaxially oriented and heat-set
film of polyethylene terephthalate is particularly useful for the
production of an assembly according to the invention and may be produced
by any of the processes known in the art, e.g. as described in British
patent specification 838,708.
The polymer substrate is suitably from 50 to 3,000, particularly from 50 to
300 especially from 100 to 175 microns thick.
The priming layer is achieved by treating the surface of the polymeric
substrate with an agent having a solvent or swelling action on the
substrate polymer which is known in the art for this purpose. Examples of
such agents, which are particularly suitable for the treatment of
polyester substrates, are halogenated phenols dissolved in common organic
solvents e.g. a solution of p-chloro-metacresol, 2,4-dichlorophenol,
2,4,5, or 2,4,6-trichlorophenol or 4-chlororesorcinol in acetone or
methanol. In addition, and preferably, the priming solution may contain a
partially hydrolysed vinyl chloride/vinyl acetate copolymer such as has
been described for use in the resin layer, particularly a copolymer
containing 0.5 to 3 wt% hydroxyl units. The molecular weight of this
polymer is preferably greater than that of the vinyl chloride-vinyl
acetate copolymer comprising the resin layer, and suitably lies in the
range of from 10,000 to 30,000 but is preferably from 16,500 to 25,000.
The thickness of the priming layer is however considerably less than that
of the subsequent resin layer being at most less than 2 microns, and
usually less than 1 micron, thick.
An optional yet preferred feature of the assembly according to the
invention is the incorporation of a photoinitiator. The photoinitiator may
be included in the resin layer, but preferably is deposited as a further
layer superimposed on the resin layer. The Photoinitiator has the function
of accelerating the polymerisation of the photpolymerisable material when
it is irradiated after being applied to the assembly. A wide range of
photoinitiators may be used for this purpose e.g. benzoin, a benzoin alkyl
ether such as benzoin methyl-, ethyl-, or isopropyl-ether, alpha-methyl-
benzoin, benzil, substituted benzils such as p-nitro and
p-methoxyl-benzil, biacetyl, fluorenone, anthraquinone, phenanthraquinone,
camphorquinone, diphenyl disulphide, 2-napthalene-sulphonyl chloride,
omega-bromoacetophenone, eosin, thionin, chloro- or alkyl-substituted
thioxanthone, dibenzosuberone, or alkyl esters of dialkylamino- benzoic
acid.
As with the priming layer, the resin layer and photoinitiator, if applied
as an independent layer, may conveniently be applied in solution, the
solvent being evaporated to leave behind the resin or photoinitiator. The
common organic solvents are used for this purpose e.g. acetone,
methyl-ethylketone, methanol and chlorinated solvents such as methylene
chloride and 1,1,1-trichloroethane.
The invention is illustrated by reference to the accompanying drawings in
which
FIG. 1 is a fragmentary, schematic, cross-sectional elevation (not to
scale) of an assembly comprising a substrate, primer and resin layer,
FIG. 2 is a similar assembly including a further photoinitiator layer,
FIG. 3 is a similar assembly including a further photopolymerisable layer,
and
FIG. 4 is a schematic cross-sectional elevation of a portion of a relief
printing plate.
Referring to the drawings, a substrate layer 1, such as a biaxially
oriented, polyethylene terephthalate film, is provided on a surface 2
thereof with a priming layer 3. On surface 4 of the priming layer is
provided a resin layer 5.
In FIG. 2, a photoinitiator layer 7 has been deposited on surface 6 of the
resin layer.
In FIG. 3, a photopolymerisable layer 9 has been deposited on surface 8 of
the photoinitiator layer, and in FIG. 4 the assembly of FIG. 3 has been
exposed to ultra-violet light through a pattern mask (not shown), and
washed in a suitable solvent to remove unpolymerised regions of layer 9,
thereby providing a printing plate with recesses 10 and relief regions 11.
The invention is further illustrated by reference to the following Examples
in which a range of assemblies were made and tested.
EXAMPLES 1-13
The substrate used in every case was polyethylene terephthalate of 100
microns thickness (MELINEX 0 Grade, MELINEX is a registered trademark).
The priming layer applied was an acetone solution of p-chlorometacresol
(3.75% weight/vol) and VINYLITE VAGH (0.75% weight/vol), VINYLITE VAGH is
a copolymer of vinylchloride (90 wt %) and vinyl acetate (4 wt %) with 2.3
wt % hydroxyl content and of average molecular weight 23,000. After
coating the acetone was removed by heating at 100.degree. C. for 1 min
leaving a priming layer of less than 1 micron thickness applied to the
substrate.
Following the application of the priming layer the substrate was next
coated with various resins to give assemblies in accordance with the
invention or, by using resins other than vinyl chloride/vinyl acetate
copolymers to give assemblies characteristic of those used in the prior
art. Typically, the coating solution was a solution in methylethyl ketone
of VINYLITE VROH (45% wt/vol). VINYLITE VROH is a terpolymer of vinyl
chloride (81 wt%), vinyl acetate (4 wt %), and a hydroxyl-containing
alkylacrylate, with a total hydroxyl content of 2.0% by weight of the
terpolymer, and an average molecular weight of 8,000. The coating solution
also contained 2% wt/wt of a silica filler (GASIL EBN) and had a viscosity
of 1 to 2 poise. The solvent was finally removed from the substrate by
heating at 80.degree. to 90.degree. C. with impingement of air for 20
seconds leaving a resin coating on the substrate of thickness 11 microns.
In some of the Examples a third, photoinitiator layer was also applied from
1,1,1-trichloroethane by a similar method to the resin layer, drying being
carried out at 80.degree.-90.degree. C. for 7 minutes. The photoinitiators
used were 2-isopropyl-9H-thioxanthene-9-one. (QUANTACURE ITX) or a mixture
of isopropyl-benzion ether and p-dimethyl-aminobenzaldehyde.
Scratch Peel Test
The experimental assemblies were tested by first scratching to give a
number of approximately parallel scratches of one inch spacing and then
coating with a commercial photopolymerisable material manufactured by
Hercules Inc and known as FFFW to give a photopolymerisable layer of
approximately 40 microns thickness. Test assembly was then also applied to
the free surface of the photopolymerisable material so that the latter
formed the center of a sandwich with the scratched coated side of the
assembly in contact with the two surfaces of the photopolymerisable
material.
The sandwich was next placed flat on a glass plate and illuminated through
the plate by means of a battery of six Thorn UV lamps at an intensity of
800 micro watts/sqcm for 30 seconds. The sandwich was then turned over and
the other side illuminated at 3,000-4,000 microwatts/sqcm for 12 minutes.
The cured test sandwich was then tested for its adhesive strength by
placing it in the jaws of an INSTRON Tensile Tester and pulling the
sandwich apart, the peeling action being across the scratch marks at a
peel rate of 5cm/minute. The weight at which delamination occurred was
recorded and the results are given in the following Table.
TABLE 1
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Resin layer Peel
Example
Priming layer
Resin layer Thickness
Photoinitiator layer
(lbs/inch)
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1 p-chorometacresol +
VINYLITE VROH +
11 microns
QUANTACURE ITX
7.7
VINYLITE VAGH
GASIL EBN
2 p-chorometacresol +
VINYLITE VROH +
" -- 5.5
VINYLITE VAGH
GASIL EBN
3 p-chorometacresol +
VINYLITE VROH
5 -- 3.3
VINYLITE VAGH
4 p-chorometacresol +
VINYLITE VAGH
5 -- 0.8
VINYLITE VAGH
5 p-chorometacresol +
Cellulose Acetate
6 Isopropylbenzoinether
0.7
VINYLITE VAGH
propionate p-Dimethylamino
benzaldehyde
6 p-chorometacresol +
Polyester Urethane +
6 -- 1.0
VINYLITE VAGH
isopopylbenzoinether
7 p-chorometacresol +
Polyvinylacetal
6 -- 1.0
VINYLITE VAGH
8 p-chorometacresol +
Ethyl cellulose
6 -- 1.0
VINYLITE VAGH
9 p-chorometacresol +
Polybutadiene
6 -- 1.0
VINYLITE VAGH
10 p-chorometacresol +
Polyamide 6 -- 1.0
VINYLITE VAGH
11 p-chorometacresol +
Polyethyene 6 -- 1.0
VINYLITE VAGH
terephthalate
12 p-chorometacresol +
Polyvinyl pyrrolidone
6 -- 1.0
VINYLITE VAGH
13 p-chorometacresol +
Ethylene/vinyl
6 -- 1.0
VINYLITE VAGH
acetate copolymer
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EXAMPLES 14-18
The test was repeated using the assembly defined in Example 1 but using
different photopolymers. In this instance the test assembly coated with
the photopolymer was compared with a commercial assembly coated with the
same photopolymer. The results are given in the following Table.
TABLE 2
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Peel lbs/inch
Assembly according
Commercial
Example
Photopolymer
Manufacturer
to Example 1
Assembly
__________________________________________________________________________
14 LR 14 OD
Hercules
9.2 8.4
15 F41 Asahi 5.7 4.8
16 F32 Asahi 3.7 0.4
17 FW 140 Hercules
8.8 0.2
18 FWO 35 Hercules
7.0 6.2
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EXAMPLE 19
Curl Test
In this test an assembly according to Example 1 which had been stored flat
in a stack of sheets for several weeks was compared with a commercial
assembly with a similar storage history. The samples were allowed to lie
flat for six hours under atmospheric conditions of 20.degree. C. and 60%
relative humidity. The lift of the edge of the sheet was then recorded.
The following figures are for millimeters lift of an A4 size sheet of
substrate thickness 100 microns.
______________________________________
Assembly according to Example 1
5
Commercial assembly 8
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EXAMPLES 20-27
After an interval of several months the procedure of Example 1 was repeated
using a variety of resin layers. Details of the formulations and test
results are provided in the accompanying Table 3.
TABLE 3
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Resin Layer Adhesion
thickness
Photoinitiator
Peel Mode of
Example
Priming Layer
Resin Layer (microns)
Layer (lbs/in)
Failure
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20 p-chlorometacresol +
VINYLITE VROH +
11 QUANTACURE ITX
7.9 ppc
VAGH VINYLITE
GASIL EBN
21 p-chlorometacresol +
VINYLITE VROH +
11 None 4.4 ppc
VAGH VINYLITE
GASIL EBN
22 p-chlorometacresol +
VINYLITE VROH +
5 QUANTACURE ITX
2.2 b
VAGH VINYLITE
GASIL EBN
23 p-chlorometacresol +
VINYLITE VROH +
5 None 1.3 b
VAGH VINYLITE
GASIL EBN
24 p-chlorometacresol +
VINYLITE VAGH +
11 QUANTACURE ITX
1.3 b
VAGH VINYLITE
GASIL EBN
25 p-chlorometacresol +
VINYLITE VAGH +
5 " 1.5 b
VAGH VINYLITE
GASIL EBN
26 p-chlorometacresol +
VINYLITE VROH
11 " 7.7 ppc
VAGH VINYLITE
27 p-chlorometacresol +
VINYLITE VROH
5 " 2.2 b
VAGH VINYLITE
__________________________________________________________________________
ppc = photopolymer cohesion
b = adhesion to base
Comparison of the results obtained in Examples 20 and 21 with those of
Examples 1 and 2 respectively illustrates the excellent reproducibility of
the system when repeated after an interval of time.
The data in Table 3 also demonstrate that peel strength depends on the
thickness of the resin layer, that the low molecular weight resin VINYLITE
VROH yields peel strengths superior to those obtained with the higher
molecular weight VINYLITE VAGH resin, that the inclusion of particulate
silica does not adversely affect the peel strength conferred by VINYLITE
VROH resin, and that peel strength is increased by the provision of a
further photoinitiator layer containing QUANTACURE ITX.
* * * * *
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Description |
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