Laser imaging materials comprising carbon black in overlayer

The laser imaging material comprises a laser-transparent support layer of, for example, polyester. A first overlying layer is laser-transparent and formed from a binder that is instantaneously thermally-chemically-decomposable with the production of a gas, for example, an acrylic or methacrylic homopolymer or copolymer, a cellulosic polymer, or a homopolymer or copolymer of stryene. A second overlying layer is formed from such a material, the same as or different from that of the first overlying layer, pigmented with carbon black. Upon laser irradiation through the support layer 10, selected portions of the carbon black are transferred to a suitable receptor surface such as paper or a lithographic plate.

This invention relates to laser imaging materials and particularly concerns 
coated materials which are usable for forming positive and negative images 
by exposure, through a transparent carrier film, to radiation produced by 
a laser. Provided they are sufficiently opaque to ultra-violet and visible 
radiation, film transparencies of such negative and positive images can be 
used as film masters. 
Film masters are commonly used in the printing industries when carrying out 
operations in which printing plates coated with a photosensitive 
composition are exposed to actinic radiation. Depending upon the nature of 
the composition, the radiation selectively solubilises or insolubilises 
the coating in the areas struck by light. Development of the resultant 
image is then carried out, whereby soluble material remaining in the 
coating as a result of the exposure is removed, usually by the action of a 
solvent. It is important for the image areas of film masters used for 
exposing printing plates to have a sufficiently high optical density in 
order to act as efficient photomasking materials. It is also important for 
those areas of the film master which are not covered by the coating to 
have a very low optical density to the radiation used during the exposure. 
The non-image areas of the film master are preferably completely 
transparent to the exposing radiation, so that the exposure times used can 
be as short as possible in practice. 
Coated film materials which are suitable for imaging printing plates and/or 
for producing film masters by means of laser beams have been proposed. 
GB-A-1,385,533 discloses a method for the recording of data in visible 
form by directing the laser beam through a transparent film support, 
carrying a layer comprising a radiation-absorbing pigment such as carbon 
black, dispersed in a self-ignitible binder. The heat generated by 
absorption of the laser energy causes the layer to separate from the 
support in the irradiated areas. The usual procedure is to effect exposure 
with the coated surface in close contact with a receptor sheet, to which 
the coating removed by the laser is transferred. If the laser beam is 
modulated so as to form a positive image on the coated film, a negative 
image is formed on the receptor sheet and vice versa. A coated film 
material operating in a generally similar way is disclosed in U.S. Pat. 
No. 3,964,389 and comprises a transparent support carrying a coating of 
particles which absorb laser energy, dispersed in a mixture of a 
self-oxidising binder and one or more cross-linkable additives. This film 
material, in use, is exposed to laser energy through the transparent 
support while the coating is in contact with a lithographic printing 
surface. In the areas exposed to the laser radiation, the coating is 
transferred to the lithographic printing surface. The transferred coating 
is then rendered durable by means of heat, which causes a crosslinking 
agent to cure the additive(s) present. In this way, a durable lithographic 
printing plate is stated to be formed, which is a positive or negative of 
the image. 
When either of the above-mentioned known film materials is used as 
described, transfer of the coating from the transparent substrate leaves a 
negative or positive transparency of the transferred image. Such 
transparencies can be useful in imaging photographic materials, especially 
litho and letterpress plates utilising photosensitive coatings. It is 
important to ensure when positive or negative film masters are produced in 
this way, for use as exposure masters for producing printing plates, that 
the laser beam removes the layer as cleanly and completely as possible. 
The efficiency of removal of the layer depends not only on the composition 
of the coating, but also on the power and duration of the laser exposure. 
A particularly useful laser source which has found extensive commercial 
application is the solid-state yttrium aluminium garnet or YAG laser, 
which produces infra-red radiation of wavelength 1.06.times.10.sup.-3 mm 
(1.06 microns). A practical operating power range for YAG lasers is from 5 
to 50 watts. At present, commercially available equipment for graphic arts 
use, usually operates over a power range of 5 to 15 watts. At such power 
values, scan speeds are used which result in an area of 0.2-0.5 m.sup.2 
undergoing exposure in a time of 2 to 5 minutes. The laser exposes the 
heat-sensitive coated film material by scanning it through the transparent 
plastics support in a helical raster pattern which may be varied in 
periodicity. The laser energy transmitted during the short but intense 
exposure is effectively absorbed at the interface between the transparent 
support and the layer containing IR absorbing material. The sudden 
generation of heat caused by this absorption thermally decomposes the 
layer at this interface and the gases produced in the exposed areas lift 
the coating from the support and allow it to transfer to a suitable 
receptor surface held in intimate contact with the coated surface of the 
film material during the exposure. The scanning laser beam can be 
modulated so as to form a positive or negative image, as desired. 
The production of film masters in this manner suffers from the disadvantage 
that incomplete transfer of the layer can occur, resulting in the open 
areas of the film master being incompletely transparent. U.S. Pat. No. 
4,245,003 describes the use of graphite instead of carbon black as the 
radiation-absorbing pigment, in order to overcome this difficulty. Layers 
containing graphite and ethyl cellulose are said to be more easily 
removable by the laser beam, enabling lower laser powers or faster 
exposure times (writing speeds) to be used. Although such 
graphite-containing coatings give satisfactory transfer at high scan 
speeds and commercially-useful laser power ratings, as shown by the 
acceptably low amounts of coating left on the support, as indicated by 
measuring its optical density, graphite can give coating with poor 
adhesion, cohesive strength and abrasion resistance. These defects can 
make such coatings difficult to handle, due to their tendency to scratch 
and rub off. More importantly, the lack of adhesion and abrasion 
resistance which such coatings may show can cause the positive and 
negative film masters produced to be unsuitable for use as exposure 
masters with printing plates, in the way desired, because of removal of 
the coating from the film master as a result of adhesion to or abrasion by 
the printing plate surface during the contact exposure operation. 
The poor performance of coatings containing graphite as the 
radiation-absorbing pigment can be explained as follows. 
The pigmented organic coating of the prior art is very thin, preferably 
having a thickness of 0.5 to 3.0.mu., in order to be completely removable 
during laser exposure. It is also generally true that the thinner the 
coating layer, the less laser energy is required to transfer it from the 
film support to the receptor surface. Coatings thinner than about 0.5.mu. 
are not useful because, in practice, their optical density is not high 
enough for them to function as exposure masters in subsequent platemaking 
operations. Optical densities greater than 2.0 are required in the 
ultra-voilet and blue spectral regions, as the photosensitive coatings of 
most commercially used printing plates undergo their photochemical 
reactions when exposed to radiation of wavelength less than 500 nm. A 
lower limit to the wavelength of the radiation used for plate exposure is 
imposed by the absorption of wavelengths less than about 320 nm by the 
plate glass used in the exposure apparatus. When carbon black is used as 
the radiation-absorbing pigment, coatings having an approximate thickness 
of 1 micron can be formulated so as to have optical densities to radiation 
in the ultra-voilet and visible wavelength ranges in the range from 2.0 to 
4.0 and such coatings are hard and have good abrasion and scratch 
resistance. 
Graphite has a much larger particle size range than carbon black, namely 
1-5.mu. for the finest graphite dispersions, as compared with 1-50 nm for 
carbon black, and so has much poorer light-stopping powder than carbon 
black. Consequently greater amounts must be used to give coatings with the 
minimum optical densities required. The much higher pigment-to-binder 
ratios required for graphite-based coatings compared to carbon black-based 
coatings can even lead to them being "under-bound", that is, insufficient 
binder being present to form a cohesive coating layer. Moreover, it is 
generally very difficult to obtain graphite dispersions having an average 
particle size of 1.0 micron or less, whereas it is comparatively easy to 
form dispersions of carbon black having an average particle size of 0.10 
micron or less. Thus, in coated layers having a thickness of 1 micron, any 
oversize graphite particles protrude from the surface and so may readily 
be rubbed off the coating, causing blackening of hands and equipment and 
giving rise to dust problems during exposure operations. 
Thus, commercially-available and other known imaging materials suffer from 
important disadvantages and this invention seeks to provide materials 
which avoid all these problems, by providing a laser removable coating on 
a transparent film base which has good adhesion and resistance to abrasion 
and is clean and trouble-free in handling. Moreover, the laser imaging 
materials of this invention have coatings which can be completely removed 
where struck by laser radiation through the transparent film base. 
SUMMARY OF THE INVENTION 
According to the present invention, a laser imaging material is provided, 
which comprises a thermally-stable transparent support, a first overlying 
layer which is transparent and thermally-decomposable and is carried on a 
surface of the support, and a second overlying layer which comprises 
thermally-decomposable material pigmented with carbon black and which 
extends over and is adhered to the first overlying layer. 
DETAILED DESCRIPTION OF THE INVENTION 
The first and second layers may comprise the same or different 
thermally-decomposable polymeric materials, but in either case it is 
necessary that the first layer has good adhesion to the transparent film 
support. A further layer may be applied over the pigmented layer, if 
desired, in order to provide some additional desirable property in the 
laser-removable composite coating comprising the superimposed layers on 
the film support, eg enhanced slip properties. Preferably, the total 
thickness of the black pigmented layer and any overlying layers shoul not 
exceed 3.0.times.10.sup.-3 mm (3.0.mu.) and most preferably not more than 
2.0.times.10.sup.-3 mm (2.0.mu.). The black-pigmented layer desirably has 
a weight ratio of carbon black to thermally-decomposable binder in the 
range from 8:1 to 1:8; preferably 4:1 to 1:4. In practice, carbon blacks 
vary widely in particle size and oil absorption values, so that more 
specific proportions cannot be given, although in any material embodying 
this invention the carbon black to binder ratio is preferably selected so 
as to give hard abrasion-resistant coatings having an optical density 
greater than 2.0. 
Thermally-decomposable binders which are suitable for use alone or in 
admixture with one another in the products of this invention include 
cellulosic polymers, especially nitrocellulose and ethylcellulose; acrylic 
and methacrylic homopolymers and copolymers; homopolymers and copolymers 
of styrene, particularly solution grade polystyrene polymers and 
copolymers. Also suitable are acrylic and methacrylic copolymers 
containing as monomers at least two of styrene, butadiene and 
acrylonitrile, and some vinyl homopolymers and copolymers. It is necessary 
for the suitability of the polymeric components to be established 
experimentally, the above list being given by way of example and not in 
any limiting sense. 
The transparent film support is preferably dimensionally-stable. The film 
preferably comprises a polyester, polystyrene, PVC or polycarbonate, 
particularly a polyester or polycarbonate. The most preferable material is 
biaxially-oriented polyethylene terephthalate. Polyester film of this type 
is the standard film base used for photographic and photoresist products 
in the reprographic industries. Its high degree of dimensional stability, 
clarity and chemical resistance make it ideal for use in this invention. 
It is available in a wide variety of grades and gauges with or without 
adhesion promoting treatments. Suitable transparent films other than those 
mentioned above may also be used if desired. 
The energy-sensitive coated film materials of this invention are 
particularly useful for use in the graphic arts fields. They may be used 
to form positive or negative film transparencies, proof copies or 
lithographic printing plates. It is of especial importance that the 
positive and negative film transparencies produced from the coated film 
materials of this invention have sufficient opacity to the passage of 
ultra-voilet and visible radiation to enable them to be used as film 
masters. Laser imaging material according to the invention can be made by 
a process which comprises applying transparent thermally-decomposable 
material to a surface of a transparent, thermally-stable support to form a 
first layer, and applying a thermally-decomposable material pigmented with 
carbon black to the first layer to form a second layer extending over and 
adhered to the first layer. The thermally-decomposable polymers in the 
pigmented and any overlying layer may be used in admixture with modifying 
agents, in order to improve one or other property of the layer(s). Such 
modifying agent may be selected from one or more of polymers, resins, 
dyestuffs, pigments, plasticizers, matting agents, slip agents and 
crosslinking agents, for instance. One particularly advantageous modifying 
agent is a particulate matting material such as polymethylmethacrylate 
beads or matting silica which prevents adhesion in use to the receptor 
material and the formation of Newtons rings. 
The coating compositions used to form the layers may be solvent-based or 
water-based lacquers or dispersions, but are preferably solvent-based 
lacquers. It can be advantageous to use different thermally-decomposable 
polymers or combinations of thermally-decomposable polymers in the two (or 
the first two) coated layers, in order that different coating solvents may 
be used. By judicious selection of solvent/polymer blends, it is possible 
to ensure that the first layer does not redissolve the second layer before 
the layers are dried. However, some interfacial mingling of the two layers 
may occur and it may be necessary to provide it, so as to ensure good 
adhesion between the layers. This can usually be done by appropriate 
selection of the solvents used. Where polystyrene is used as the film 
support, the solvent(s) used for the first layer must be selected so as 
not to attack the polystyrene film too severely, although a limited amount 
of solvent etching can be useful for ensuring good adhesion between the 
polystyrene and the overlying coated layer. 
In the laser imaging materials of the invention, the thickness of the clear 
and transparent first layer can be varied between wide limits but is 
preferably about 0.1.mu. and 1.0.mu.. The second layer, pigmented with 
carbon black, preferably has a thickness of 0.5.mu. to 2.0.mu.. These 
materials are useful for producing imaged transparencies by directing 
laser energy through the transparent support into the radiation-absorbing 
coating appropriately while scanning and modulating the laser beam, such 
process commonly being termed "laser writing". Absorption of the laser 
energy by the black pigment causes a very rapid rise in temperature to 
occur, which in turn causes the continuous phase in which the carbon 
particles are dispersed to undergo thermal decomposition, lifting the 
overlying pigmented layer and decomposing some of the underlying layer in 
the areas scanned by the laser. It is thought that the decomposition of 
the unpigmented transparent heat-decomposable layer underlying the black 
pigmented layer helps to ensure the substantially complete removal of the 
latter from the transparent support. The reasons for this are not 
completely understood, but it may be that, during the rapid and violent 
decomposition of the thermally-decomposable binder, particles of carbon 
black are forced on to the surface of the support at the same time as the 
overlying coating material is forced upwards off the support. The 
formation of gaseous decomposition products at the surface of the 
underlying unpigmented layer may oppose this effect, thereby reducing the 
amount of carbon left behind, and better-defined images result. 
During exposure, the coated surface of the film material is held in 
intimate contact with a receptor sheet, to which the coating exposed to 
the laser beam is transferred. The receptor sheet can have an important 
influence on the completeness of image transfer. Best results are obtained 
when the receptor sheet is slightly porous or microscopically rough, so as 
to enable the transferred coating to embed into its surface and hence 
adhere firmly. Most commonly used receptor sheets are paper and anodised 
aluminium litho plates.

The accompanying drawing is a cross-sectional view of a laser imaging 
material embodying the present invention. The material comprises a 
transparent support layer 10 of, for example, biaxially-orientated 
polyethylene terephthalate, of about 75.mu. thickness. The support layer 
10 carries a first thermally-decomposable layer 20 of, for example, 
polystyrene and a second thermally-decomposable layer 30 of, for example, 
ethyl cellulose and pigmented with carbon black. The 
thermally-decomposable layers 20,30 are each about 1.0.mu. in thickness. 
The invention will now be described further by way of following examples 
of the preparation of laser imaging materials embodying the invention. 
EXAMPLE 1 
Polystyrene (BASF Styrol 144C) was dissolved in toluene to produce a 5% 
weight solution. The solution was coated onto 75.mu. polyester film (ICI 
Melinex 505) with a 7 thou Mayer rod and dried to give a coating of 0.58 
g/m.sup.2. A dispersion of carbon black (27 nm Furnace Black) in ethyl 
cellulose solution (Hercules N4) was prepared with a weight ratio of 
carbon black to ethyl cellulose of 4:6. The dispersion was ballmilled for 
24 hours and the solids adjusted to 5% by weight with isopropanol. The 
dispersion was then coated onto the polystyrene-subbed polyester base and 
dried, to give a total dry coating weight of 1.38 g/m.sup.2 and at the 
same time coated onto an unsubbed 75.mu. polyester film base to give a dry 
coating weight of 0.86 g.m.sup.2. Both coated film samples were then 
imaged by scanning through the uncoated side of the polyester base with a 
YAG laser of 8 watts power. The scan rate was 1000 lines/inch, the beam 
was 23.mu. in diameter and the wavelength 1.06.mu.. During scanning the 
coated sides of the films were held in vacuum with an anodised aluminium 
receptor sheet. After scanning the areas of the film where the coating had 
been ablated and the carbon black transferred to the receptor sheet had 
their light transmission measured using a Macbeth densitometer fitted with 
a UV filter. The results are tabulated below: 
______________________________________ 
Percentage light Transmission 
Before Ablation 
After Ablation 
______________________________________ 
Sample A with polystyrene sub 
approx 0.1% 40% 
Sample B no sub approx 0.1% 29% 
______________________________________ 
When the negatives derived from scanning, Sample A and Sample B were used 
to expose UV sensitive photopolymer coating the exposure times were found 
to be in close accordance with the open area transmission densities but 
the negative obtained from the unsubbed coating required exposure times 
15-20% longer than the negative incorporating the polystyrene sub. In both 
cases the negatives had hard glossy surfaces in the unablated areas which 
were not prone to soiling but the negative incorporating the polystyrene 
sub had far better coating adhesion and abrasion resistance. 
EXAMPLE 2 
A 10% by weight solution of nitrocellulose (ICI Ltd. DHM 10-25) in a 2:1 
mixture of n-butyl acetate and xylene was coated on to 75.mu. polyester 
(Melinex 542) film base using a 20 thou Mayer rod, and dried. A 
black-pigmented coating lacquer was prepared by dispersing Microlith Black 
C-A (Ciba Geigy) and Surcol 860 (an acrylic copolymer resin) (Allied 
Colloids Ltd) in 99% IMS so as to give a solution containing 5% by weight 
of each component. Using a 7 thou Mayer rod, this solution was then used 
to coat the previously prepared nitrocellulose subbed polyester film, and 
at the same time an identical sample of unsubbed film base. The coating 
weight of the pigmented coating was approximately 0.9 gm.sup.2. 
The coatings were scanned under the same conditions as described in Example 
1. After ablation, the transmission of the coated film and the areas of 
the film from which coating had been removed were measured as in Example 1 
and gave the following results: 
______________________________________ 
Percentage light Transmission 
Before Ablation 
After Ablation 
______________________________________ 
Sample A with NC sub 
.about.0.1% 40% 
Sample B without NC sub 
.about.0.1% 31% 
______________________________________ 
When used in subsequent exposure operations with UV sensitive photopolymer 
coatings, negatives produced from the nitrocellulose subbed film required 
approx 10% shorter exposure times. 
EXAMPLE 3 
A 10% by weight solution of polyethylmethacrylate (Cole Polymers Ltd 
Colacryl P1101) in 1:1 MIBK/Toluene was coated onto Melinex 505 film base 
using a 10 thou Mayer rod. The black pigmented coating lacquer of Example 
1 was coated on top of the subbed base and a sample of unsubbed base was 
coated at the same time. The coatings were then scanned with the 8 watt 
YAG laser under identical conditions to those of Examples 1 and 2. The 
results of transmission tests identical to those of Examples 1 and 2 are 
shown below: 
______________________________________ 
Percentage light Transmission 
Before Ablation 
After Ablation 
______________________________________ 
Sample A .about.0.1% 46% 
Sample B .about.0.1% 30% 
______________________________________ 
Exposure times were approx 20% shorter using the negative produced from the 
subbed film and the coating was hard with good adhesion and abrasion 
resistance. 
It is evident that those skilled in the art will make numerous 
modifications of the specific embodiments and Examples described without 
departing from the present invention concepts. It is accordingly intended 
that the invention shall be construed as embracing each and every novel 
feature and novel combination of features present in or possessed by the 
materials and the methods of their production described herein and that 
the foregoing disclosure shall be read as illustrative and not as limiting 
except to the extent set forth in the claims appended hereto. 
Typical modifications include the use of materials of the types mentioned 
hereinbefore, for example methylmethacrylate: polystyrene copolymer, as 
the polymeric materials of the first and second overlying layers.