Field of the Invention
[0001] This invention generally relates to radiation sensitive compositions used in lithographic
printing plates.
Description of Prior Art
[0002] The feature of lithographic printing is that the image area retaining ink and non-image
area retaining water are on a lithographic printing plate, wherein the image area
retains oil and repels water and the non-image area retains water and repels oil.
The operation of lithographic printing uses the immiscibility of oil (oily materials
or ink) and water. A non-image of a lithographic printing plate is firstly moistened
with water to form a water-retained and oil-repelled water film. Then ink is applied
to the lithographic printing plate wherein the image area accepts ink. Pressure is
applied during printing process such that the ink on the image area containing images
or texts is transferred to the surface of papers or printable objects via a blanket.
Typically, the ink is firstly transferred to the blanket, the intermediate material.
Then the ink transferred onto the blanket is further transferred to the printable
surface of a material such as papers, cloths, plastics or metals.
[0003] The preparation of pre-sensitized plate (PS plate) may start with electrolyzing or
polishing a surface of an aluminum plate to form a plurality of grains required for
printing. The light sensitive or heat sensitive radiation sensitive composition is
coated on the grains of the aluminum plate to form a radiation sensitive coating.
The radiation sensitive coating is baked and cured. Following that, a negative having
pre-determined images and texts is transferred to the radiation sensitive coating
via exposure and develop process to produce a printing plate for printing the images
and texts on printable objects.
[0004] In recent years, digital technologies using computer for processing and outputting
image data are widely utilized. Various applicable new image output methods are utilized
in the market. There are lithographic printing technologies skipping process using
a negative. Digital images in a computer are directly transferred to a radiation sensitive
coating via expose and develop process to produce printing plate for printing images
and texts on printable objects. Thus the operation of plate production is significantly
simplified, which increases the printing efficiency and lowers production cost. Such
technologies are called Computer to Plate (CTP). The methods for producing the printing
plate applicable to CTP have new issues to resolve.
[0005] Upon a radiation sensitive composition coated on a surface of an aluminum plate used
for lithographic printing is exposed, the exposed portion is alkali soluble and is
removed during the develop process to form a positive plate. On the other hand, exposed
portion is cured and becomes alkali non-soluble to form a negative plate. Under the
above two circumstances, the image area accepts ink or is lipophilic and the non-image
area accepts water or is hydrophilic. Weather a light-sensing plate is categorized
as positive or negative type depends on the radiation sensitive composition coated
on the aluminum plate.
[0006] The radiation sensitive composition mentioned above mainly composed of the following
materials: radiation absorptive dyes (a light sensitive agent or a heat sensitizing
agent), acid generators, novolac resins (film-forming resins), copolymers, solvents,
and other assistants.
[0007] The prior art disclosed in the
U.S. Pat. No. 6,063,544 is a positive plate coated with mixtures of novolac resin or cresol-formaldehyde
or polyhydroxystyrene and infrared light absorptive dyes. In addition, the
U.S. Pat. No. 5,372,907,
5,372,915,
5,340,699,
5,491,046 disclose plates coated with mixtures of novolac resin, cresol novolac resin, infrared
light absorptive dyes or colorant and latent Bronsted acid. When the plates are exposed
to infrared light radiation, the latent Bronsted acid is decomposed to generate a
material catalyzing crosslinking reactions of different novolac resins (such as a
novolac resin and a cresol resin) in order to cure the mixtures in the exposed areas.
Further, the exposed printing plate is heated to enhance curing of the exposed .coating
and becomes insoluble to alkaline developer solution, where the non-exposed area remains
soluble to developer solution. The
U.S. Pat. No. 5,919,601 1 discloses a radiation sensitive composition composed of binder resins, crosslinking
agents, thermal-activated acid generators and infrared light absorbents. The composition
is exposed to infrared light. Then the acid generators are decomposed to release acid
ions to catalyze crosslinking reactions of binder resins via crosslinking agents.
[0008] The disadvantage of the above technologies in common is that the developed image
area of the printing plate lacks of integrity and is not durable for printing for
a long time which leads to undesirable image resolution and printing qualities.
[0009] In order to improve commercial competiveness, printing operators are required to
increase plate making efficiency and printing qualities, there is a demand for faster
sensitivity of the radiation sensitive coating on a printing plate. In other words,
the required exposure energy is desired to be lower and the required exposure time
is desired to be shorter. In addition, the exposed portion is desired to be easier
removed by a developer solution during develop process. Non-exposed portion is desired
to have higher resistance to soak in and washed by a developer solution. That means
the non-exposed portion is desired to have higher tolerance on temperature, concentration
of the developer solution as well as soaking and washing time during develop process.
Whilst, it is also desired to maintain fine resolution, ink absorption and abrasion-resistance
of the radiation sensitive coating. Further, it is desired to increase adhesiveness
between the radiation sensitive coating and aluminum plate.
SUMMARY OF THE INVENTION
[0010] The objective of the invention is to provide a radiation sensitive composition for
producing images. The radiation sensitive composition may effectively improve hardness,
abrasion-resistance, adhesiveness and print durability without interfering exposure
sensitivity and developing latitude.
[0011] To achieve the above objective, the inventor conducted researches in order to provide
a copolymer having metal ion structural unit represented by formula (I) in the following.
The copolymer is added as part of the formula of a radiation sensitive composition
to offer high sensitivity and developing latitude and effectively increases hardness,
abrasion-resistance, adhesiveness and print durability of the radiation sensitive
coating.

wherein M is a zinc , magnesium, or copper divalent metal ion, X is O, S, N, or polyalkylene
oxide group , and n≥1.
[0012] The radiation sensitive composition of the present invention comprises copolymers
polymerized with specific monomers for enhancing hardness and adhesiveness, which
significantly increases hardness, abrasion-resistance and adhesiveness of radiation
sensitive coating while maintaining exposure sensitivity and developing latitude.
Thus, print durability of lithographic printing is considerably enhanced.
DETAILED DESCRIPTION OF THE INVENTION
[0013] The present invention provides a radiation sensitive composition used for forming
an image layer. The composition is used as a coating on a positive and negative plate.
The composition is composed of a mixture comprising (a) 30∼95 weight percent of a
novolac resin (film forming resin), (b) 4∼70 weight percent of a copolymer, (c) 0.1∼45
weight percent of a radiation absorptive dye, and (d) 0.1∼15 weight percent of an
acid generator. The composition of the invention further comprises solvents and additive
agents.
[0014] The present invention also provides a method for producing an image. The method comprises:
(I) coating an imaging layer on a substrate, wherein the imaging layer is composed
of (a) a novolac resin, (b) a copolymer, (c) a radiation absorptive dye, (d) an acid
generator, wherein the imaging layer further comprises a solvent and an additive agent;
(II) drying the imaging layer; (III) exposing an image forming area of the imaging
layer under an energy source emitting sufficient light beams, wherein the light beams
can be UV/Vis light beams with a wave length of 320nm∼750nm or IR light beams with
a wave length of 750nm∼1350nm; and (IV) soaking the image forming area in a developer
solution for removing the exposed portion from the substrate.
[0015] The novolac resins applicable to the present invention include phenol, o-cresol,
m-cresol , p-cresol , 2-naphthol, phenol derivatives or a mixtures composed of at
least two above mentioned compounds, and aldehyde condensation products such as formaldehyde,
acetaldehyde, other aliphatic or aromatic aldehyde etc. The product, the novolac resin,
preferably has a weight average molecular weight in a range between 300 ∼ 400,000.
[0016] The copolymers applicable to the present invention are composed of a (meth)acrylic
acid monomer such as methyl methacrylate (MMA), methacrylic acid (MAA), lauryl (meth)acrylate,
isobutyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, ethylhexyl (meth)acrylate,
dodecyl 2-(meth) acrylate etc., styrene and the derivatives such as α-methylstyrene,
4-hydroxystyrene, 4-hydroxymethylstyrene, Methylstyrene with halogen, maleic anhydride,
acrylonitrile and N-substituted maleimide monomer, wherein N-substituted maleimide
monomer is N-phenylmaleimide, 4-hydroxyphenylmaleimide, N-(2,3-dimethylphenyl) maleimide.
The above monomers are polymerized based on different composition combination and
ratios used as a light sensing coating. The copolymers of the present invention have
metal structural units, wherein structure of a reactive monomer having metal ions
is represented as formula (II):

wherein R
1 and R
2 are H or CH
3, M is a zinc , magnesium, copper divalent metal ion, X and Y are O, S, or N polyalkylene
oxide group. Among which, a di(meth)acrylate monomer and the derivatives of a diacrylate
monomer having metal ions is zinc diacrylate (ZDA), zinc dimethacrylate (ZDMA), magenesium
diacrylate, copper diacrylate, zinc thiodiacrylate and zinc thiodimethylacrylate,
etc.
[0017] The radiation absorptive dyes applicable to the present invention largely include
light sensitive dyes and heat sensitive dyes. The light sensitive dyes mainly include
1,2-naphthoquinonediazide-4-sulfonyl chloride or 1,2- naphthoquinonediazide-5-sulfonyl
chloride and esterification condensates of polyhydroxy compounds. The heat sensitive
dyes mainly include dyes with wave length of 700-900nm, such as cyanine dyes, polymethine
dyes, naphthoquinone dyes, phthalocyanine dyes, anthraquinone dyes, or indoaniline
metal complex dyes, etc.
[0018] Acid generators applicable to the present invention generate protic acids after exposing
to heat or light generated by light within an infrared light spectrum. The generated
protic acid is used in photo decomposition of light sensing materials of the copolymers
in the exposed area. Compounds generating protic acids via thermo chemistry are disclosed
in
U.S. Pat. No. 5,466,557. The US patent is incorporated in the present application as references. Applicable
compounds include haloalkyl compounds such as haloalkyl-substituted s-triazinehaloalkyl,
haloalkyl 2-pyrone, haloalkyl oxazole, haloalkyl oxadiazole andhaloalkyl thiazole,
etc. These compounds generally have functional group such as trihalomethyl, which
may produce polyhalomethyl-substituted hydrohalic acid when receiving the heat of
infrared radiation. The most applicable acid generators may be selected from a variety
of (trichloromethyl) -triazine ,that is, it is preferred to select from, but not to
limit to 2,4,6-(trichloromethyl)-1,3,5-triazine, 2,4-(trichloromethyl)-6-phenyl-1,3,5-triazine,
2-(4-Methoxyphenyl)-4,6-bis(trichloromethyl)-1,3,5-triazine, 2,4-(trichloromethyl)-6-(3,5-methoxyphenyl)-1,3,5-triazine,
2,4-(trichloromethyl)-6-naphenyl-1,3,5-triazine, 2,4-(trichloromethyl)-6-(methoxynaphenyl)-1,3,5-triazine,
2,4-(trichloromethyl)-6-(styryl)-1,3,5-triazine and 2,4-(trichloromethyl)-6-(4-methoxystyryl)-1,3,5-triazine,
etc.
[0019] The compounds generally have trihalomethyls and generate polyhalomethyls with halogen
acids upon a compound is under heat from IR radiation. The most frequently used acid
products are selected from various trichloromethyltriazines, preferably selected from
but not limited to tris-trichlorotriazine, bis(trimethylphenyl)triazine, bis(trichloro)-o-methoxyphenyltriazine
or bis(trichloro)-p-methoxyphenyltriazine,bis(tr ichloro)(3,5-dimethoxylphenyl)triazine,
is(trichloro)naphthyltriazine,bis(trichloro)(5-methoxynaphthyl)triazine, bis(trichloro)styryltriazine,
bis(trichloro)styryl)triazine and bis(trichloro)(4-methoxylstyryl)triazine, etc.
[0020] The solvent used by the present invention is selected from ethanols, esters, ketones,
ethers, carboxamides, aromatics and mixtures thereof and in particular 1-methoxyl-2-
ethanol, 1-methoxyl-2- propanol, ethyl glycol acetate, ethyl acetate, acetone, butanone,
diisobutyl ketone, methyl isobutyl ketone, cyclohexanone, toluene, xylene, 1-propanol,
isopropanol, tetrahydrofuran, butyrolactone, methyl lactate, dimethyl amide and mixtures
thereof.
[0021] The additive agents used by the present invention include additives such as surfactants,
coloring agents and wetting agents. The coloring agent is used for differentiating
image on the printing plate after the develop process. The coloring agent applicable
to the present invention include oil-soluble dyes or basic dyes such as crystal violet,
malachite green, victoria blue, methylene blue, ethyl violet, oil blue 603/613, or
rhodamine B etc. and mixtures thereof.
[0022] The present invention is detailed by the following examples:
<Example 1> The copolymer synthesis
[0023] The synthesis method of the copolymer and the monomer ratio are listed but not limited
to the following.
|
monomer composition (wt%) |
copolymer No. |
PMI |
MMA |
St |
AN |
ZDA |
P-1 |
35 |
30 |
0 |
35 |
0 |
P-2 |
35 |
25 |
0 |
35 |
5 |
P-3 |
35 |
20 |
0 |
35 |
10 |
P-4 |
35 |
0 |
30 |
35 |
0 |
P-5 |
35 |
0 |
25 |
35 |
5 |
P-6 |
35 |
0 |
20 |
35 |
10 |
[0024] Note:PMI=N-phenyl maleimide; MMA= methyl methacrylate; St= styrene; AN= acrylonitrile;
ZDA=zinc diacrylate
(The synthesis method of the copolymer)
[0025] 35g PMI, 30g MMA, 35g AN, 1g initiator (Azobisisobutyronitrile, AIBN) and 200g solvent
(Dimethyl Formamide, DMF) are placed in a 500ml four-necked flask. The four-necked
flask equipped with a stirring blade, a thermometer, a reflux condenser and a nitrogen
inlet. The reaction temperature is controlled at 70°C for 24 hours. After cooling
to room temperature, the resulting reaction mixture was poured into 6kg of water to
precipitate the copolymer in the form of powder. The copolymer is filtered and dried
to give the copolymer (copolymer P-1). Copolymers with other different compositions
can also be obtained by the above method.
<Example 2> Preparation of the radiation sensitive composition and a heat sensitive
CTP printing plate
[0026] The heat sensitive radiation sensitive composition of the present example is generated
by stirring the following components until the components are totally dissolved in
the solvents. The components include 20g novolac resin -Rezicure5200 (high purity
degree of a m-cresol novolac resin from SpecialCheme Company), 6g novolac resin -DURITE®
SD-1508 (a novolac resin composed mainly of bisphenol from Hexion Company), 9.0g copolymer
(copolymer P-1), 0.8g radiation absorptive dyes -cyanine dyes IR dye 23b[PCAS] (dyes
reacted under IR light from PCAS Company), 0.24g 2,4-(trichloromethyl)-6-(4-methoxystyryl)-1,3,5-triazine,
0.5g coloring agent -victoria blue, 0.5g coloring agent -crystal violet lactone, 6ml
solvent ethyl acetate, and 104g solvent ethyl glycol acetate.
[0027] Provide an aluminum plate with hydrophilic treatment. The above mentioned heat sensitive
radiation sensitive composition is coated on the aluminum plate with coating thickness
of 1.8 g/m
2. The coated aluminum plate is baked under 100°C for 4 minutes and cured under 50°C
for 24 hours to generate a positive heat sensitive CTP printing plate.
[0028] Following exposure, several developer solutions are used for develop process and
the comparison result is listed in the following table. The exposure process and develop
process use the positive heat sensitive CTP printing plate generated in the Example
2, a Screen PTR-8600 900rpm exposure machine (from Screen Company). The range of exposure
percentage is 50%∼90% and the exposure step condition is 5%. Among which, the developer
solutions Kodak Premium, Kodak Plus are from Kodak Company, and Agfa Energy is from
Agfa Company.
developer solution |
Kodak Premium |
Kodak Plus |
Agfa Energy |
develop temperature (°C ) |
23.0 |
23.0 |
24.0 |
develop time (seconds) |
30 |
35 |
20 |
clarity (%) |
60 |
65 |
60 |
all pass range (%) |
60∼90 |
70∼90 |
60∼90 |
[0029] <Example 3> The preparation of a radiation sensitive composition and a heat sensitive
CTP printing plate printing plate in the present example is largely identical with
the preparation of the positive heat sensitive CTP printing plate in the example 2,
wherein the difference is that the copolymer used is the sample P-2. The procedure
of Example 2 was repeated except that copolymer P-1 was replaced with copolymer P-2.
<Examples 4> The preparation of a radiation sensitive composition and a heat sensitive
CTP printing plate
[0030] The preparation of the printing plate in the present example is largely identical
with the preparation of the positive heat sensitive CTP printing plate in the example
2, wherein the difference is that the copolymer used is the sample P-3. The procedure
of Example 2 was repeated except that copolymer P-1 was replaced with copolymer P-3.
<Example 5> The preparation of a radiation sensitive composition and a heat sensitive
CTP printing plate
[0031] The preparation of the printing plate in the present example is largely identical
with the preparation of the positive heat sensitive CTP printing plate in the example
2, wherein the difference is that the copolymer used is the sample P-4. The procedure
of Example 2 was repeated except that copolymer P-1 was replaced with copolymer P-4.
<Example 6> The preparation of a radiation sensitive composition and a heat sensitive
CTP printing plate
[0032] The preparation of the printing plate in the present example is largely identical
with the preparation of the positive heat sensitive CTP printing plate in the example
2, wherein the difference is that the copolymer used is the sample P-5. The procedure
of Example 2 was repeated except that copolymer P-1 was replaced with copolymer P-5.
<Example 7> The preparation of a radiation sensitive composition and a heat sensitive
CTP printing plate
[0033] The preparation of the printing plate in the present example is largely identical
with the preparation of the positive heat sensitive CTP printing plate in the example
2, wherein the difference is that the copolymer used is the sample P-6. The procedure
of Example 2 was repeated except that copolymer P-1 was replaced with copolymer P-6.
<Example 8> The preparation of a radiation sensitive composition and a heat sensitive
CTP printing plate
[0034] The preparation of the printing plate in the present example is largely identical
with the preparation of the positive heat sensitive CTP printing plate in the example
2, wherein the difference is that no copolymer is used for serving as a blank example
for comparing effects adding copolymers. The present example uses 24.5g novolac resin
-Rezicure5200, 10.5g novolac resin -DURITE® SD-1508, 0.8g radiation absorptive dyes
-cyanine dyes IR dye 23b[PCAS], 0.24g 2,4-(trichloromethyl)-6-(4-methoxystyryl)-1,3,5-triazine,
0.5g coloring agent victoria blue, 0.5g coloring agent -crystal violet lactone, 6ml
solvent ethyl acetate, and 104g solvent ethyl glycol acetate.
[0035] The positive heat sensitive CTP printing plates generated in Example 2 to Example
8 are put through exposure and develop tests. The develop test uses developer solution
Kodak premium, at temperature of 23°C and develop time is 30 seconds. The 50% dot
formation test uses iCPlate2 [X-rite] printing plate measuring apparatus (from X-Rite
Company). The result is categorized and listed in the following.
No/Energy (mJ/cm2) |
146 |
155 |
165 |
176 |
188 |
203 |
220 |
Example 2 |
52.5 |
52.0 |
51.3 |
50.4 |
49.5 |
49.1 |
48.0 |
Example 3 |
52.3 |
51.7 |
51.1 |
50.2 |
49.4 |
48.5 |
47.8 |
Example 4 |
51.8 |
51.6 |
50.8 |
50.0 |
49.7 |
48.6 |
47.5 |
Example 5 |
52.6 |
51.7 |
51.4 |
50.4 |
49.4 |
48.8 |
48.2 |
Example 6 |
52.4 |
51.5 |
51.1 |
49.9 |
49.2 |
48.3 |
47.9 |
Example 7 |
52.1 |
51.2 |
49.8 |
49.5 |
49.1 |
48.0 |
47.5 |
Example 8 |
53.0 |
52.5 |
51.4 |
50.8 |
50.1 |
49.8 |
48.5 |
[0036] The X-rite 528 color meter (from X-rite Company) measures thinning difference of
samples from Example 2 to Example 8 (which means the difference of light sensing layer
before and after the test). The reference point for measuring clean point is concluded
by adding alcohol drops onto the plate until there is no color change and then the
corresponding clean point is measured. Typically the light sensing layer has colorants,
such as blue colorant. When the light sensing layer has residues on the plate, the
alcohol dissolves the colorant within the light sensing layer. In other words, alcohol
is used for testing if there are residues of the light sensing layer on the plate.
In addition, finger touch is the main reason causing printing surface falling off
or scratching. A surface hardness test is applied to the product graded by standard
pencil hardness to differentiate product surface hardness. The test uses 3M#610 tape
sticking to printing patterns. The tape is pressed firmly to assure no air is left
between the sticking surface between the tape and the printing patterns. One minute
after the tape sticking to the printing patterns, the tape is peeled from the printing
patterns. If the printing patterns are not damaged, the adhesiveness is OK. If the
printing patterns are torn by the tape, the adhesiveness is NG. The abrasion-resistance
test uses abrasion tester and wet test method wherein the surface is rubbed repeatedly
for 100 times and observed if any damage occurred to the surface. The result is graded
by three categories undesirable, normal and desirable. Lastly, accelerated abrasion
printing conditions are applied to test the maximum printing amount with commercially
acceptable quality.
No. |
Thinning Difference Percentage (%) |
Clean point |
Hardness (H) |
adhesiveness |
abrasion-resistance |
print durability |
Example 2 |
0.11 |
146mJ/ cm2 |
4H |
NG |
undesirable |
45000 |
Example 3 |
0.13 |
146mJ/ cm2 |
≥4H |
OK |
desirable |
100000 |
Example 4 |
0.12 |
146mJ/ cm2 |
≥4H |
OK |
desirable |
120000 |
Example 5 |
0.12 |
146mJ/ cm2 |
4H |
NG |
normal |
50000 |
Example 6 |
0.12 |
146mJ/ cm2 |
≥4H |
OK |
desirable |
110000 |
Example 7 |
0.13 |
146mJ/ cm2 |
≥4H |
OK |
desirable |
130000 |
Example 8 |
0.27 |
146mJ/ cm2 |
3H |
NG |
undesirable |
30000 |
[0037] According to the above data, it is concluded that the positive heat sensitive CTP
printing plates of the present invention have high sensitivity and high alkali resistance.
The added copolymers synthesized with zinc diacrylate (ZDA) do not have impact on
the sensitivity and alkali resistance. of printing plate, where hardness, adhesiveness,
abrasion-resistance and print durability are significantly improved. Without the copolymers
added, the hardness, print durability and adhesiveness are undesirable.