Transfer imaging elements

Details Transfer imaging elements Transfer imaging elements

1995-06-13

2003-12-16

Letscher, Geraldine (Department: 1752)

Radiation imagery chemistry: process, composition, or product th

Transfer procedure between image and image layer, image...

Image layer portion transfer and element therefor

C430S252000, C430S254000, C430S255000, C430S627000, C430S631000, C430S635000, C430S636000, C430S200000, C430S201000

Reexamination Certificate

active

06664020

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to imaging elements and in particular to imaging elements which involve the transfer of colourant from a substrate.
BACKGROUND TO THE INVENTION
There is a continuing need for imaging materials that are dry-processed, i.e., which do not need aqueous or other chemical treatments in order to develop or fix the image. There is a particular need in the area of colour reproduction, e.g., colour proofing, where the demands in terms of resolution are extremely high. Peel-apart and thermal transfer are attractive technologies for this purpose, but poor resolution has hitherto been a problem.
Peel-apart imaging is well-known in the art, and is described, for example, in “Imaging Systems” (Jacobson & Jacobson, Focal Press, 1976, p.213), and in patent literature. Suitable materials generally comprise a photosensitive colourant layer sandwiched between a substrate and a stripping sheet or layer. Light exposure through a suitable mask, followed by careful removal of the stripping sheet, causes an imagewise partitioning of the colourant layer between the substrate and stripping sheet. Both positive- and negative-acting systems are known, i.e., the colourant may be selectively removed from or retained by the substrate in exposed areas.
Because the resulting image consists solely of areas of maximum density and areas of zero density, such systems are ideally suited to half tone reproduction. Full colour images may be provided by generating separate (y,m,c,k) images on transparent substrates and assembling them in register (overlay), as described, for example in U.S. Pat. No. 4,282,308, U.S. Pat. No. 4,286,043 and EP385466, and exemplified by the Du Pont “Cromacheck” system. There is increasing interest in applying peel-apart technology to the formation of integral proofs, in which successive y,m,c,k images are assembled on a common substrate, as described in U.S. Pat. No. 4,489,153 and EP 385466, and British Patent Application No. 92 25687.4
Typically, the colourant layers described in the prior art comprise one or more pigments dispersed in a photocurable medium comprising suitable monomer(s), photo initiator(s) and binder(s). Other layers may be present, such as adhesive layers, release layers, etc, but the prime cause of the image differential is photopplymerisation.
It is known to add surfactants, including fluorinated surfactants, to the photocurable colourant layers described above. Fluorinated surfactants are frequently used as coating aids in the imaging industry generally, e.g., to promote even wetting of substrates, enhance coating uniformity and prevent striations, as described, for example, in U.S. Pat. No. 4,504,567, EP 230995, EP 239082, JP 61-292137 and JP 62-036657, and peel-apart imaging elements are no exception. Generally, the quantities. of surfactant involved are small. For example, EP385466 (table.2, p.16) shows the use of fluorosurfactant in peel-apart colourant layers to the extent of 0.068wt % of the total solids. Of the formulations disclosed, the cyan layer has the lowest pigment content, and this still represents a pigment: surfactant ratio of about 70:1. Likewise EP373532 discloses the use of fluorosurfactants in the photocurable colourant layers of peel-apart imaging elements, but does not give details of preferred quantities and does not mention any effects on the speed or resolution of the media. The Examples disclose pigment:surfactant ratios ranging from 57.8:1 to 20.9:1.
Thermal transfer imaging is also well known in the art. Two types of donor media may be distinguished, namely sublimation (or diffusion) transfer media, and mass transfer media. In the former, dyes are caused to migrate from a donor to a receptor in amounts proportional to the energy applied, giving continuous tone images, while in the latter, zero or 100% transfer of the colourant layer takes place according to whether the applied energy exceeds a given level. The latter type is highly suited to half tone reproduction since the resulting images consist of areas of zero or maximum optical density.
Two modes of address may also be distinguished for thermal transfer imaging, namely print head and IR address. In the former, heat is supplied to the donor-receptor assembly via an external print head comprising an array of microresistors, while in the latter the energy is supplied by a radiant source (usually an IR emitter such as a laser) and converted to heat within the donor-receptor assembly by a suitably-placed radiation absorber. Print heads have inherent limitations with respect to resolution (e.g., 400 dpi max.), so that the resolution of the media is less of a problem. Laser scanners can readily achieve >2000 dpi resolution, and so the capabilities of the media become critical. Print head addressable mass transfer media generally comprise a substrate bearing a colourant layer of a waxy resin containing dispersed dyes or pigments, a typical commercially-available example being TLP OHPll, available from Mitsubishi.
IR addressable thermal mass transfer media are well known in the literature see, for example, JP 63-319192, WO 90/12342 and WO 92/06410 and additionally comprise a suitable IR absorbing dye or pigment either within the colourant layer or in a separate layer underneath the colourant layer. It is also possible for the IR absorber to be located in the receptor, as described in our copending PCT Application No. GB 92/01489.
Fluorinated surfactants have also found utility as coating aids in the preparation of transfer media. In addition, such surfactants have been used for other purposes.
U.S. Pat. No. 5,034,371 describes a dye-diffusion transfer donor for printhead address comprising a fluorocarbon additive in addition to dye(s) and binder. The relevant Example teaches about 5 wt % fluorocarbon (FC). Improvements are shown in terms of non-sticking to the receptor and lack of creasing.
JP 61-206694 discloses a mass-transfer donor for printhead address comprising a pigmented layer overcoated with a layer of surfactant, preferably fluorosurfactant. The purpose is to achieve better transfer to poor quality paper.
WO 88/04237 discloses a thermal imaging medium which is laser-addressable, comprising a support sheet having a surface of a material which may be temporarily liquified by heat and upon which is deposited a particulate or porous layer of an image forming substance which is wettable by the material in its liquefied state. Either the image forming substance is itself IR-absorbing, or a separate absorber is present. In exposed areas, the liquefiable material melts, wets the particles of the image forming substance, then resolidifies, thus anchoring the particles to the substrate. Removal of a stripping sheet (either present from the outset or applied subsequent to exposure) then causes selective removal of the particles in the non-exposed areas. The image forming layer may comprise pigment particles with or without binder, but when binder is used the pigment:binder ratio is high—in the range 40:1 to 1:2, preferably 5:1. Surfactants, including fluorosurfactants, may be added, and the examples show the use of Fluorad FC-120 at the level of 2.7 wt % of total solids, which corresponds to a pigment:FC ratio of about 30:1. The stated purpose is to improve coating quality, with no mention of resolution enhancement. This patent application also teaches the addition of submicroscopic particles to the image forming layer in order to improve the abrasion resistance of the final image. Polytetrafluoroethylene is said to be particularly useful, but this embodiment is not demonstrated in the Examples.
It has now been found that the inclusion of relatively large amounts of fluorocarbon compounds in the colourant layer of a mass transfer sheet leads to improved properties, particularly improved resolution.
BRIEF SUMMARY OF THE INVENTION
Therefore according to the present invention there is provided a mass transfer imaging element comprising a substrate bearing a colourant layer containing a pigment which is adapted to be imagewise transferred, in whi

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