Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Radiation sensitive composition or product or process of making
Reexamination Certificate
2001-05-03
2003-03-11
Hamilton, Cynthia (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Radiation sensitive composition or product or process of making
C430S285100, C430S286100, C430S283100
Reexamination Certificate
active
06531263
ABSTRACT:
The present invention relates to a photopolymerizable composition which comprises a mixture of thermoplastic-elastomeric SIS and SBS block copolymers as binder, ethylenically unsaturated monomers, plasticizer and photoinitiator. It furthermore relates to photopolymerizable flexographic printing elements whose photopolymerizable layer comprises said photopolymerizable composition, and to the use of photopolymerizable flexographic printing elements of this type for the production of flexographic printing plates.
Thermoplastic-elastomeric block copolymers have been used for some time for the production of photopolymerizable flexographic printing elements. Thus, for example, DE 22 15 090 and U.S. Pat. No. 4,323,636 disclose photosensitive compositions comprising binder, ethylenically unsaturated monomers and photoinitiator in which the binder employed is either polystyrene-polybutadiene-polystyrene block copolymers (SBS) or polystyrene-polyisoprene-polystyrene block copolymers (SIS). Block copolymers of this type can advantageously be processed by melt extrusion, thus avoiding complex casting processes. Furthermore, they are inexpensive and readily commercially available. Although other (co)polymers have frequently also been proposed as binder, virtually all commercially available photopolymerizable flexographic printing elements that can be developed in organic media nowadays comprise thermoplastic-elastomeric block copolymers as binder.
For the production of commercially available photopolymerizable flexographic printing elements, hitherto only block copolymers either of the SBS type or of the SIS type have been employed. The SIS and SBS block copolymers which can be employed for the production of photopolymerizable flexographic printing elements are generally not compatible with one another and tend to separate. The photopolymerizable mixtures become cloudy owing to separation processes of this type and are unsuitable for the production of high-quality flexographic printing plates.
Flexographic printing plates based on SIS block copolymers or SBS block copolymers each have very characteristic advantages and disadvantages. SIS rubbers are softer and more plastic than SBS rubbers. They are therefore very particularly suitable for the production of relatively soft flexographic printing plates.
However, flexographic printing plates based on SIS rubbers tend to form cracks on exposure to ozone. For this reason, ozone protection waxes, such as, for example, selected paraffins or microwaxes, are therefore usually added to the formulations. Ozone protection waxes of this type form a nonpolar protective film on the surface of the flexographic printing plate, which can have a disadvantageous effect on ink transfer, in particular in the case of printing with water-based printing inks. Furthermore, flexographic printing plates based on SIS rubbers usually have higher surface tack and greater abrasion that those based on SBS.
SBS rubbers can be employed without the addition of ozone protection waxes. Flexographic printing plates based on SBS rubbers therefore have very good ink transfer, especially with water-based printing inks. The surface of flexographic printing plates based on SBS rubbers is less tacky and has greater abrasion resistance, which is very advantageously evident in particular in the case of printing on rough stock. However, printing plates comprising SBS rubbers have the disadvantage of greater hardness. In order to obtain soft flexographic printing plates, larger amounts of plasticizer are employed than in the case of SIS-based plates. However, the amount of plasticizer in the formulation naturally cannot be increased as desired without adversely affecting other properties. Also disadvantageous is the tendency of SBS printing plates toward anisotropy. This is taken to mean that direction-dependent mechanical, optical and printing properties are obtained during conventional production by melt extrusion and calendering. Since printing plates are not always clamped to the printing cylinder in the same orientation, adverse effects occur, in particular in the case of multicolor printing. Excessively high anisotropy is totally intolerable.
It would be desirable to have available a flexographic printing plate having the advantages of SBS rubbers without having to accept the disadvantages of the excessive hardness and anisotropy. It is therefore an object of the present invention to provide photopolymerizable flexographic printing elements for the production of flexographic printing plates of this type.
We have found, surprisingly, that this object can be achieved by using certain SBS rubbers in a mixture with SIS rubbers. This was all the more surprising as SIS and SBS rubbers are usually incompatible with one another.
Accordingly, the present invention provides photopolymerizable compositions which comprise a mixture of certain thermoplastic-elastomeric SIS and SBS block copolymers as binder, ethylenically unsaturated monomers, plasticizer and photoinitiator. The present invention furthermore provides photopolymerizable flexographic printing elements whose photopolymerizable layer comprises this composition, and the use of photopolymerizable flexographic printing elements of this type for the production of flexographic printing plates.
The SIS component employed is a conventional thermoplastic-elastomeric SIS block copolymer which is suitable for the production of photopolymerizable flexographic printing elements. The polystyrene content of such polymers is usually less than 40% by weight. Preference is given to block copolymers having a polystyrene content of from 10 to 35% by weight, particularly preferably block copolymers having a polystyrene content of from 10 to 20% by weight. The apparent molecular weight of the SIS block copolymers is usually from 80,000 to 300,000, preferably from 100,000 to 250,000. The term “apparent molecular weight” as used in the present specification is taken to mean the mean molecular weight M
w
determined with the aid of gel permeation chromatography (GPC) using polystyrene standards. The SIS block copolymers can either be linear block copolymers or radial block copolymers. They are usually three-block copolymers of the A-B-A type, but can also be two-block copolymers of the A-B type or those comprising a plurality of alternating elastomeric and thermoplastic blocks, for example A-B-A-B-A. It is also possible to employ mixtures of two or more different SIS block copolymers. Commercially available SIS block copolymers often have an SI two-block content. The SI two-block content is preferably from 0 to 40% by weight, particularly preferably from 10 to 30% by weight.
Preferably, the SIS block copolymers employed are linear block copolymers. In the photopolymerizable composition according to the invention, the SIS block copolymer is employed in the form of a mixture with one or more thermoplastic-elastomeric SBS block copolymers which are compatible with the SIS block copolymer. For the purposes of this invention, the term compatibility is taken to mean that the SBS and SIS block copolymers, together with the other components of the photopolymerizable mixture, can be converted into a clear, non-hazy mixture which does not scatter actinic light. Haziness, streaks or the like in the photopolymerizable layer have an adverse effect on the photopolymerization process, which means that no flexographic printing plates at all or at least only those of poor quality are obtained.
The SBS block copolymers can be linear three-block copolymers, two-block copolymers, polymers having a plurality of elastomeric and thermoplastic blocks, radial or quasi-radial block copolymers or mixtures thereof. The polystyrene content of the SBS block copolymers is from 5 to 25% by weight. At higher polystyrene contents, transparent mixtures are no longer obtained with the SIS block copolymers described above. The polystyrene content of the SBS block copolymer is preferably from 10 to 25% by weight, very particularly preferably from 15 to 20% by weight. The apparent molecular
BASF Drucksysteme GmbH
Hamilton Cynthia
Keil & Weinkauf
LandOfFree
Photopolymerizable flexographic printing elements comprising... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Photopolymerizable flexographic printing elements comprising..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Photopolymerizable flexographic printing elements comprising... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3021757