Flexographic printing precursors and methods of making

Abstract

A mixture of a high molecular weight EPDM rubber with a low molecular weight (liquid) EPDM rubber provides a highly useful laser-ablatable flexographic printing plate precursor formulation. This formulation is sensitive to infrared radiation by the incorporation of an IR absorbing compound such as a carbon black. The inclusion of the liquid EPDM rubber avoids the need for plasticizers such as process oils during manufacturing, and provides improved image sensitivity, print quality, and run length. Both flexographic printing plates and printing sleeves can be made using the mixture of EPDM rubbers.

Description

FIELD OF THE INVENTIONThis invention relates to flexographic printing precursors in the form of either plates or sleeves that contain an IR-ablatable relief-forming layer comprising a mixture of rubbery resins. This invention also relates to a method of preparing these flexographic printing precursors in either plate or sleeve form.BACKGROUND OF THE INVENTIONFlexography is a method of printing that is commonly used for high-volume printing runs. It is usually employed for printing on a variety of substances particularly those that are soft and easily deformed, such as paper, paperboard stock, corrugated board, polymeric films, fabrics, plastic films, metal foils, and laminates. Course surfaces and stretchable polymeric films can be economically printed by the means of flexography.Flexographic printing plates are sometimes known as “relief printing plates” and are provided with raised relief images onto which ink is applied for application to the printing substance. The raised relief...

AI Technical Summary

Benefits of technology

The present invention provides a laser engravable flexographic printing precursor that is readily manufactured without using process oils, and having improved image sensitivity, print quality, and run length.

Whereas prior art researchers have used high molecular weight EPDM rubber as well as other rubbery materials, they have failed to appreciate that its non-polar nature has made it particularly suitable as a basis for laser-engravable flexographic printing precursors and superior to other materials that are described in patents. Furthermore, we have found advantages from the inclusion of a low molecular weight EPDM polymer with the high molecular weight EPDM rubber, as a replacement for plasticizers. Low molecular weight EPDM provides the benefits of process oils in manufacture without the problems of leaching out either during grinding, printing or precursor storage.

We have also found that the use of the low molecular weight EPDM polymer causes an increase in crosslinking density in the rubber mixture with consequent advantages. For example, there is an improvement in compression set and mechanical properties such as tensile strength and elongation to the length at which the material breaks or snaps into at least two pieces (see ASTM D3759).

The present invention provides improved flexographic printing precursors that can be in the form of plates or sleeves. These precursors can be cleanly engraved using infrared radiation (lasers) to provide very sharp features in the resulting printed images. In addition, these precursors have improved run length and can be used for many high quality prints without degradation. These advantages are also provided by using a specific mixture of solid (high molecular weight) EPDM and liquid (low molecular weight) EPDM to formulate the infrared radiation ablatable layers.

Problems solved by technology

However, the requirement of relief depths in excess of 500 μm challenges the speed at which these flexographic printing plate precursors can be imaged.

While they are generally slow and expensive to use and have poor beam resolution, they are used because of the attractions of direct thermal imaging.

These lasers provide better beam resolution, but are very expensive.

Formulations designed for ablation by carbon dioxide lasers cannot be easily modified for laser diode ablation by simply adding a suitable infrared radiation absorbing material.

These have been found to be of limited suitability for laser engraving because ablation of thermoplastic materials results in melted portions around the ablated areas and sometimes re-deposition of ablated material onto the ablated areas.

Such printing plate precursors do not contain infrared absorbing dyes or pigments and therefore are unsuitable for use with IR absorbing laser diode systems.

), such curing would be defective due to the high absorption of UV as it traverses through the thick precursor layer.

The use of inert plasticizers or mineral oils can present a problem as they leach out either during precursor grinding (during manufacture) or during storage, or under the pressure and contact with ink during printing.

What is especially difficult is to simultaneously improve the flexographic printing precursor in all directions.

Throughput for flexographic engraving is somewhat determined by the equipment that is used but if this is the means for improving imaging speed, then cost becomes the main factor.

Improved imaging speed is related to equipment cost.

There is a limit to what the market will bear in equipment cost in order to have faster imaging.