Radiation imagery chemistry: process – composition – or product th – Microcapsule – process – composition – or product
Reexamination Certificate
2000-03-17
2003-09-16
Baxter, Janet (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Microcapsule, process, composition, or product
C428S402210, C428S402220
Reexamination Certificate
active
06620571
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an improved method for producing microcapsules and, more particularly, to a method for producing microcapsules in which the microcapsule walls are substantially impermeable and exhibit improved temperature and humidity stability. While the method of the present invention is useful in producing microcapsules generally, microcapsules prepared in accordance with the present invention are particularly useful in photosensitive imaging materials of the type which employ a layer of microcapsules containing a radiation sensitive composition.
Photohardenable imaging systems employing microencapsulated radiation sensitive compositions are the subject of commonly assigned U.S. Pat. Nos. 4,399,209; 4,416,966; 4,440,846; 4,766,050; 4,962,010 and 5,283,015. These imaging systems are characterized in that an imaging sheet including a layer of microcapsules containing a photohardenable composition in the internal phase is image-wise exposed to actinic radiation. In the most typical embodiments, the photohardenable composition is a photopolymerizable composition including a polyethylenically unsaturated compound and a photoinitiator and is encapsulated with a color former. Exposure to actinic radiation hardens the internal phase of the microcapsules. Following exposure, the imaging sheet is subjected to a uniform rupturing force by passing the sheet through the nip between a pair of pressure rollers. An image transfer system in which the developer material is coated on a separate substrate as a separate developer or copy sheet is disclosed in commonly assigned U.S. Pat. No. 4,399,209. A self-contained imaging sheet in which the encapsulated color former and the developer material are co-deposited on one surface of a single substrate as one layer or as two interactive layers is disclosed in commonly assigned U.S. Pat. No. 4,440,846. An imaging system comprising a support, a layer containing microcapsules, a layer of developer material, and a layer containing an opacifying agent is disclosed in commonly assigned U.S. Pat. No. 4,766,050. The opacifying agent can form a separate layer or can be part of the layer containing the microcapsules or both but is interposed between the microcapsules and the developer to hide the microcapsules when viewing the image. Methods for forming amino-formaldehyde microcapsules for use in photosensitive imaging systems are disclosed in U.S. Pat. Nos. 4,962,010 and 5,283,015.
SUMMARY OF THE INVENTION
In accordance with the present invention, microcapsules containing amine-formaldehyde shell walls are prepared by emulsifying an oily material internal phase containing an isocyanate pre-wall former in an aqueous medium and subsequently forming amine-formaldehyde walls around the internal phase by in situ polycondensation. The invention is particularly characterized by adding a hydrophilic polymer into at least one of the internal phase or the continuous aqueous phase. It has been found that when a hydrophilic polymer is present in either the internal phase or the continuous aqueous phase, and microcapsules are formed in an otherwise conventional manner, the hydrophilic polymer becomes incorporated into the microcapsule wall. Microcapsules produced in accordance with the present invention are substantially impermeable and exhibit improved temperature and humidity stability as a result of the hydrophilic polymer in the microcapsule wall. Furthermore, the presence of the hydrophilic polymer in the microcapsule wall provides for microcapsules which retain moisture better and therefore are less likely to dry out upon storage. Microcapsules which have dried out upon storage are more likely to prematurely rupture and cause undesirable discoloration or image darkening with time.
Another embodiment of the present invention resides in the production of microcapsules in which a hydrophilic polymer is used in place of at least a portion of the amount of methylated polygalacturonic acid, typically pectin, in the continuous aqueous phase. Previously, amine-formaldehyde capsules having a desired uniform small size and wall characteristics have been obtained by using a methylated polygalacturonic acid, typical pectin, as a system modifier. However, pectin is a naturally occurring product and therefore its composition and effectiveness as a system modifier can vary depending upon the time and the source from which the pectin is derived. Furthermore, pectin tends to gel in the presence of zinc, thereby creating problems when used in the preparation of microcapsules incorporated into imaging systems utilizing zincated developers. The hydrophilic polymers of the invention, especially synthetic hydrophilic polymers, tend to be more consistent as far as composition and functional properties as compared to the naturally occurring pectin which can vary depending upon the season and the source from which it is derived. Furthermore, substituting a hydrophilic polymer for some or all of the pectin in a conventional system can minimize problems associated with pectin gelling when used with zincated developers.
Moreover, it is hypothesized that by incorporating a hydrophilic polymer into the microcapsule wall, a substantially impermeable barrier is formed around the internal phase which prevents the unwanted migration of volatile components from the developer resin into the encapsulated contents which can cause premature dye development and discoloration. Accordingly, the substantially impermeable capsule walls provide superior results in terms of temperature and humidity stability.
Accordingly, the present invention provides a process for producing microcapsules having substantially impermeable microcapsule walls comprising the steps of: forming an emulsion of an internal phase in a continuous aqueous phase, the internal phase including a photosensitive composition and a polyvalent isocyanate, the continuous aqueous phase including a sulfonated polystyrene and at least one of the internal phase or the continuous aqueous phase further including a hydrophilic polymer; and enwrapping particles of the internal phase in an amine-formaldehyde condensation product produced by in situ condensation of an amine and formaldehyde.
The microcapsules may contain a photohardenable or photosoftenable material as the photosensitive composition. In the most typical case, the capsules contain a polyethylenically unsaturated monomer, a photoinitiator, and a dye precursor. Imaging is conducted as described in the aforementioned commonly assigned patents which are incorporated herein by reference.
DETAILED DESCRIPTION OF THE INVENTION
In accordance with the present invention, an improved method for producing amine-formaldehyde microcapsules is provided. The method includes the steps of forming an emulsion of an internal phase in a continuous aqueous phase and enwrapping the particles of the internal phase in an amine-formaldehyde wall produced by in situ condensation of an amine and formaldehyde. The invention is particularly characterized by the presence of a hydrophilic polymer in at least one of the internal phase or continuous aqueous phase. The presence of the hydrophilic polymer functions to enable the production of microcapsules having improved wall characteristics in terms of impermeability and temperature/humidity stability.
The microcapsules produced in accordance with the present invention preferably are used as photographic materials, and in particular, in microencapsulated imaging systems such as those described in U.S. Pat. Nos. 4,399,209 and 4,440,836. The microcapsules may also be used for a number of different practical applications such as in agricultural chemicals, medicines, perfumes, chemicals, adhesives, liquid crystals, detergents, dyes, dye precursors, developers, catalysts, rust-preventing agents as well as photographic materials.
In one embodiment of the invention, a hydrophilic polymer is incorporated into the internal phase which contains a chromogenic material and a photosensitive composition. The hydrophilic polymer is dissolve
Camillus Joseph C.
Katampe Ibrahim
Baxter Janet
Eastman Kodak Company
Thompson Hine LLP
Thornton Yvette C.
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