Diisopropylbenzene containing solvent and method of developing flexographic printing plates

Abstract

This invention describes photopolymer printing plate developing solvents comprising diisopropylbenzene, alone or in combination with one or more co-solvents. Compared with solvents of the prior art, the developing solvents of the present invention have lower toxicity, higher flash points, a milder and more pleasant odor, increased resistance to thermal decomposition, higher solvency, faster dry time, and pose reduced threat to the environment. The solvents of the present invention can exhibit azeotropic-like properties resulting in improved recycling characteristics and reduced drying times. Also, the solvents of the present invention exhibit a degree of selective solvency towards non-crosslinked photopolymers vs. crosslinked photopolymers that is not seen in solvents of the prior art.

Description

[0001] This application is a divisional of Application Ser. No. 10 / 243,061, filed Sep. 13, 2002, now allowed, which is a continuation of application Ser. No. 09 / 696,277, filed Oct. 25, 2000, which is a continuation of application Ser. No. 09 / 427,212, filed Oct. 26, 1999, which issued on Dec. 19, 2000, as U.S. Pat. No. 6,152,593. The disclosure of the prior applications and issued patent is incorporated herein by reference.BACKGROUND--FIELD OF INVENTION[0002] This invention relates to the development of flexographic (photopolymer) printing plates using an improved developer solvent. More specifically, this invention relates to the use of diisopropylbenzene alone, or in combination with a co-solvent(s), as a washout solvent for the non-crosslinked polymer material in the printing plates to develop the relief image. Diisopropylbenzene is non-toxic, can develop plates quickly, causes minimal plate swelling and distortion, dries quickly, is easy to reclaim by distillation, has a very mil...

AI Technical Summary

Benefits of technology

0015] It is the object of the present invention to provide a solvent that overcomes all of the above limitations and disadvantages of the prior art. Specifically, the objects and advantages of the present invention are:

0016] (a) to provide a solvent that develops a wide range of solvent-developed photopolymer plates;

0017] (b) to provide a solvent that has a flash point in

Problems solved by technology

This is because most solvents that dissolve the non-crosslinked material also absorb into the crosslinked portions of the plate, causing these areas to soften and swell, thus changing the shape of the plate.

This erosion results in the final image on the plate becoming different from the target image on the negative.

Hence printing quality suffers.

However, the process or solvents used in these patents all suffer from numerous drawbacks, as explained below.

However, this solvent mixture is toxic and dangerous, and therefore is no longer used (perchloroethylene is widely recognized to be carcinogenic, and butanol is flammable).

A number of replacement solvents have been proposed to replace perc / butanol, but virtually all known mixtures have serious disadvantages.

For instance, in U.S. Pat. No. 4,267,260 Miura disclosed as early as 1981 that glycol ethers, DMSO, NMP, diethylformamide, xylene, cyclohexane, and monochlorobenzene can be used to develop plates, but they cause the plates to "swell" resulting in poor image quality during printing.

), xylene (flash point 25.degree. C.) and cyclohexane (flash point -18.degree. C.) are flammable, making them dangerous or expensive to use, and DMSO has an intensely strong odor that is offensive.

Further, glycol ethers are only effective on certain plate types, and therefore are restricted to a very small portion of the industry.

However, acids are corrosive and therefore attack machine parts, and are severe skin and eye irritants.

Further, gamma-butyrolactone is not effective on many plate types.

In U.S. Pat. No. 4,806,452 Hoffmann teaches the use of terpene hydrocarbons such as d-limonene in developer solvents, but terpene hydrocarbons have intense odors, are moderate skin and eye irritants, and have flash points below 141.degree. F., making the waste solvent mixture "hazardous" by RCRA guidelines.

This leads to increased disposal cost and high regulatory compliance costs.

All of these solvents cause excessive plate swelling and / or cause plates to delaminate (come apart) during developing, and therefore are not useful.

Low toxicity saturated hydrocarbons are also discussed, but they exhibit very poor solvency and therefore cannot process plates quickly enough.

In U.S. Pat. No. 5,061,606 Telser discloses the use of hydrogenated petroleum fractions in combination with low levels of alcohols (to remove the covet layer) and monoterpenes (added as odorants), but this combination has poor solvency and therefore develops conventional plates much too slowly to be practical.

In U.S. Pat. No. 5,077,177 Frass discloses the use of phenyl ethers as developers, but these solvents also have very strong odors which make them undesirable, and they generally have flash points too low to be practical (<141.degree. F.).

Further, Frass explains that d-limonene has the additional deficiency of not being able to develop plates containing nitrile rubber.

Unfortunately, these mixed aromatic solvents are relatively toxic and distinctly odoriferous, and both butanol and 2-ethyl butanol are very odoriferous and have flash points below 141.degree. F., making the waste solvent hazardous.

Therefore this combination is impractical.

1. The developer solvent contains 25-70% of mixed aromatic solvents such as Solvesso 150 (product of Exxon Chemical Japan Limited) that have high odor intensity and moderate toxicity. These solvents are so odoriferous that an odor masking agent is required;

2. The mixed low molecular weight aromatic solvents are so aggressive (dissolve the polymer so quickly) that concentrations above 70% excessively swell the photopolymer plate. For this reason a third component, that acts as a diluent, is added to "control the swelling action" of the active ingredient;

3. The developer solvent is a non-azeotropic blend of three components, all with different boiling points. Because spent solvent is always reclaimed by distillation in which components preferentially vaporize in boiling point order, the distilled solvent can easily end up being depleted of one or more critical components. For this reason it is desirable to have a solvent system with as few components as possible, preferably no more than two, or to have a solvent system which is azeotropic;

4. The so-called odor masking agent, isobutyl isobutyrate, has a flash point of only 99.degree. F., classifying it as a flammable liquid. Addition of significant amounts of this solvent to the blend will reduce the flash point of the mixture below 141.degree. F., causing the waste solvent to be classified as a RCRA hazardous waste. And although isobutyl isobutyrate is described as an odor-masking agent, it actually has a strong, unpleasant odor itself.

To date, no solvent has been able to achieve all these objectives.

This reduction in swelling and distortion is surprising because while other aggressive flexographic developer solvents (such as terpene hydrocarbons, lower molecular weight aromatic hydrocarbons or chlorinated hydrocarbon solvents) may quickly remove the unexposed polymer in the development process, they also attack the exposed crosslinked polymer resulting in plate swelling, distortion of the image, actual damage to the plate and much longer drying times when compared to the diisopropylbene-based solvents.

Flexographic solvents of the prior art have not exhibited this degree of selectivity for non-crosslinked polymer vs. crosslinked polymer.

Important issues to be considered in this reclamation process are the boiling points of the different constituents in the solvent blend and the boiling point of the last material to come over in the distillation.

If this final boiling point is too high, then distillation will be difficult because of the excessive temperatures required.

Also, if the boiling point of the first material to come over is significantly lower than that of the last, then there is the potential to leave some of the higher boiling solvent behind in the distillation pot with the waste, resulting in an unbalanced developer solvent that is ineffective.

As is recognized in the art, it is not possible to predict the formation of azeotropes.