Stock material or miscellaneous articles – Structurally defined web or sheet – Discontinuous or differential coating – impregnation or bond
Reexamination Certificate
2001-02-01
2002-12-31
Kelly, Cynthia H. (Department: 1774)
Stock material or miscellaneous articles
Structurally defined web or sheet
Discontinuous or differential coating, impregnation or bond
C428S325000, C428S327000, C428S500000
Reexamination Certificate
active
06500527
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to image receptor sheets that may be stacked without image offset and more particularly to crosslinkable coating compositions applied to film and paper substrates to provide absorbent, ink receiving layers bearing images of high fidelity and color saturation. Image bearing sheets are durable and scuff resistant in the presence or absence of water.
BACKGROUND OF THE INVENTION
The presentation of pictorial and textual images requires materials, as image receptors, that retain evidence of the color, tone, resolution and brightness of the original pictorial subject or textual message. Certain characteristics are required of image receptor materials to provide suitable contrast and fidelity of image reproduction. This need applies particularly to receptors used for recording images formed from colored droplets such as those delivered by ink jet printers and copiers. An image recorded as liquid droplets requires a receptor on which the recording liquid dries quickly without running or spreading. High quality image reproduction using ink jet printing techniques requires receptor substrates, typically sheets of paper or opaque or transparent film, that readily absorb ink droplets while preventing droplet diffusion or migration. Ready absorption of ink encourages image drying while minimizing image migration maintains the sharpness of the appearance of the recorded image.
Advances in ink jet printing technology yielded ink jet printers requiring less time for complete image generation. Improvements in printer technology translate into the need for improved receptor materials that satisfy a number of important requirements related to increasing speeds of multicolor printers. In addition to the need for image sharpness and rapid absorption of ink droplets, there is a demand for image receptors that satisfy quality standards with respect to brightness, opacity, internal strength, and resistance to picking and scuffing. An effective receptor material also protects an image from water damage that would extract color from an image or distort its appearance by image bleeding due to spreading of an area of colored dye.
Solutions to problems associated with the use of aqueous ink jet inks include the use of water swellable polymers, and hydrophilic additives to improve liquid absorption and drying rate. The use of mordants provides control of liquid droplets, as deposited, to enhance image sharpness and limit droplet migration that appears as image bleeding.
Despite improvement in the performance of ink jet image receptors a challenge exists to provide sheet materials so that multiple imaged sheets may be stacked in the output tray of ink jet color printers, without evidence of image transfer between sheets in the stack. Such image transfer is also known as image offset or blocking.
Receptor sheet durability is an issue related to the preservation of recorded images which may be damaged by picking and scuffing. Picking and scuffing could occur during stacking of multiple sheets during high speed printing or copying. Therefore, increased durability may allow stacking of imaged sheets with less blocking, in less time. This is becoming important in response to the increasing image generation speed of ink jet printers.
It is known that introducing crosslink sites into a polymer structure may increase its toughness and durability. The benefits of durability of crosslinked polymers may be applied to image receptor layers. In the case of ink jet imaging, coatings for receptor layers typically comprise water soluble polymers that upon crosslinking become less water soluble with reduced tack and increased durability, especially in imaged areas that are saturated with aqueous-based ink.
A known crosslinking reaction that may be applied to image receptor layers involves the interaction of carboxy terminated materials and multifunctional aziridines. Information of crosslinking of carboxylate species using multifunctional aziridines is available in a number of references exemplified by United States Patents including U.S. Pat. No. 3,470,136; U.S. Pat. No. 3,507,837; U.S. Pat. No. 3,817,945; and U.S. Pat. No. 3,959,228. Further evidence of the use of multifunctional aziridine crosslinking agents with carboxylic acid groups is given in U.S. Pat. No. 4,490,505. This patent teaches the benefit of reacting aziridines with carboxylic acid groups rather than other types of groups, such as primary and secondary amine groups, hydroxyl groups and phenolic groups, which react more slowly with the crosslinking agent.
U.S. Pat. No. 5,208,092 describes the use of multifunctional aziridine crosslinkers for carboxylic acid groups in liquid ink-receptive recording sheets. The crosslinking reaction in this case involves compositions containing from 0.5% to about 20% by weight of at least one ethylenically unsaturated monomer having acidic groups in ammonium salt form. Such groups react to form crosslinks in the presence of a multifunctional aziridine, present from 0.5% to about 8.0% by weight. Illustration is provided of a crosslinking scheme. U.S. Pat. No. 5,389,723 also describes ink receptive layers crosslinkable through reaction of acid groups with polyfunctional aziridines such as propane-tris(&bgr;-(N-aziridinyl)propionate. The ink receptive layers may contain pendant ester groups, and these can be rendered crosslinkable by hydrolysis.
As indicated, known crosslinking reactions involving multifunctional aziridine crosslinkers appear to emphasize the benefits of the ready reaction of such crosslinkers with carboxylic acid functionality. Reference to the relative slowness of reactions involving other functional groups suggests the potential inferiority of the crosslinked structures they produce. It could also be inferred that accelerating such reactions, e.g. using more vigorous high temperature conditions, may adversely affect performance properties of the resulting product.
Crosslinked systems derived from carboxylic acid group reaction with multifunctional aziridines have produced useful ink receptor materials. Although apparently less reactive, the possibility exists that other crosslinkable compositions may provide ink receptors having similar or improved properties compared to those operating through the reactivity of carboxylic acid functionality. For this reason there is a need to explore other crosslinking schemes involving multifunctional aziridine crosslinkers as well as their application to image recording products.
SUMMARY OF THE INVENTION
The present invention provides a coated receptor for images formed from droplets of colorants issuing from discharge elements of image reproducing equipment, such as the nozzles of ink jet printers. A coated receptor, according to the present invention, includes an ink receptive layer comprising up to 80% of a homopolymer of a protonated polyvinylpyridine to provide crosslinked polymer networks by reaction with multifunctional aziridines. As an alternative the polymer requirement for an ink receptive layer may be satisfied using a copolymer including protonated pyridinium substituents. The ink receptive layers comprising pyridine-containing homopolymers or copolymers exhibit improved performance with respect to durability, scuff resistance, and image fidelity. They also exhibit water and moisture stability and limit migration that leads to image bleeding. The use of pigmented inks, applied to receptor layers according to the present invention, typically produces images of higher density with less “mud cracking.” Coating compositions, used for ink receptive layers possess improved stability for extended periods of time, compared with previously known, carboxylic acid-containing, polymer structures crosslinked using multifunctional aziridines. Receptor layer compositions according to the present invention dry efficiently, after coating, at temperatures that minimize damage to substrate materials including paper and film substrates.
Preparation of a durable receptor layer according to the present invention relies upon a crosslinke
3M Innovative Properties Company
Bell Alan
Kelly Cynthia H.
Shewareged B.
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