Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Finishing or perfecting composition or product
Reexamination Certificate
2002-09-13
2004-01-27
Le, Hoa Van (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Finishing or perfecting composition or product
Reexamination Certificate
active
06682877
ABSTRACT:
BACKGROUND
1. Field of Invention
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 mild odor, has a reasonably low cost, is thermally stable, and works on virtually all known solvent-developed photopolymer plate types.
Ease of recycling of flexographic developer solvents is important to reduce waste, and because it is often more cost-effective to recycle than to continually replace used developer solvent with new. In this regard, azeotropic or azeotrope-like compositions that exhibit reduced boiling points, enhanced recovery rates and limited fractionation upon distillation are desirable. Thus, there is a need for an environmentally-friendly, easily recyclable, preferably azeotropic or azeotrope-like, low vapor pressure, high flash point, solvent that effectively develops flexographic printing plates. The present invention addresses this need.
The diisopropylbenzene-based solvents show selective solubility for non-crosslinked photopolymer material vs. crosslinked photopolymer material when compared to solvents of the prior art. This results in a reduction of plate swelling, image distortion and drying times.
2. Description of Prior Art
Flexography is a type of relief printing that uses flexible sheets of photopolymer to transfer an image onto a substrate. In the flexography process, photopolymer sheet is exposed to light through a negative or mask that blocks selected portions of the photopolymer from the light. The portions of the photopolymer that are exposed to light crosslink. The non-crosslinked photopolymer is soluble in many organic solvents, whereas the crosslinked photopolymer is harder and much more resistant to chemical solvent attack. By “washing” the selectively-exposed photopolymer sheet in a solvent bath, the unexposed portions of the plate are removed, a process known as developing. The resulting developed plate will contain a raised relief image in those areas where light passed through the negative. The flexible relief plate is wrapped around a cylinder on a printing press, and used to transfer ink onto a substrate, such as paper, film, bags, etc.
While many organic solvents are capable of dissolving non-crosslinked material from the plate, only a few are considered good developer solvents. 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 softening and swelling encourages erosion of the relief image by the brush in the developer tank. This erosion results in the final image on the plate becoming different from the target image on the negative. Hence printing quality suffers. The ideal developer solvent would dissolve and remove the non-crosslinked portions of the plate while at the same time not softening or swelling the crosslinked portions.
The general process of developing flexographic plates is well-known and described in detail in a number of U.S. patents including Miura (U.S. Pat. No. 4,267,260), Shimizu (U.S. Pat. No. 4,271,261), Merrem (U.S. Pat. No. 4,539,288), Brault (U.S. Pat. No. 4,665,009), Hoffmann (U.S. Pat. No. 4,806,452), Kobayashi (U.S. Pat. No. 4,844,832), Worns (U.S. Pat. No. 4,847,182), Telser (U.S. Pat. No. 5,061,606), Frass (U.S. Pat. No. 5,077,177), Frass (U.S. Pat. No. 5,116,720), Telser (U.S. Pat. No. 5,128,234), Telser (U.S. Pat. No. 5,176,986), Larimer (U.S. Pat. No. 5,204,227), Telser (U.S. Pat. No. 5,240,815), Bach (U.S. Pat. No. 5,252,432), Schlosser (U.S. Pat. No. 5,312,719), Schober (U.S. Pat. No. 5,354,645), and Takagi (U.S. Pat. No. 5,578,420). However, the process or solvents used in these patents all suffer from numerous drawbacks, as explained below.
As Telser discusses in U.S. Pat. No. 5,061,606 most modern photopolymer printing plates generally have a multilayer structure. Typically the topmost layer is composed of a thin (5 micrometers in thickness) layer of polyamide referred to as the “cover layer” that serves to protect the delicate photopolymer during handling and exposure. Because this layer is resistant to some photopolymer developer solvents, an additive co-solvent is often incorporated into the developing solution to facilitate removal of this cover layer in the developing bath. Because polyamide is soluble in most organic alcohols, developing solutions typically contain a low percentage (10-30%) of an organic alcohol for this purpose.
Originally, exposed photopolymer plates were developed with a solvent mixture consisting of perchloroethylene to dissolve uncured photopolymer and butanol to remove the protective cover layer. 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. In addition, diethylformamide (flash point 60° C.), xylene (flash point 25° C.) and cyclohexane (flash point −18° 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. In U.S. Pat. No. 4,271,261 Shimizu teaches the use of glycol ethers in combination with acids and/or gamma-butyrolactone (GBL). 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° F., making the waste solvent mixture “hazardous” by RCRA guidelines. This leads to increased disposal cost and high regulatory compliance costs. Other inventors discuss the use of chlorinated solvents (which are toxic to people or are dangerous to the environment), mixed low-molecular-weight aromatic solvents (which are odoriferous and relatively toxic), and ketones (which are overly aggressive and odiferous). 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. 4,847,182 Worns discloses the use of d-limonene in combination with water, benzyl alcohol, and/or butanol as a developer solvent, but this invention suffers from the strong odor of d-limonene, as well as the low flash point of d-limonene that makes the waste solvent a RCRA hazardous waste. 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 cover 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
Gallagher R. Scott
Wyatt Marion F.
Le Hoa Van
Saitta Thomas C.
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