WO1993001940A1

WO1993001940A1 - Heat sensitive recording paper with image stability

Info

Publication number: WO1993001940A1
Application number: PCT/US1992/005362
Authority: WO
Inventors: Roberta R. Arbree
Original assignee: Nashua Corporation
Priority date: 1991-07-22
Filing date: 1992-06-24
Publication date: 1993-02-04
Also published as: AU2269592A

Abstract

A heat sensitive recording material is provided having a thermosensitive layer with an additive to improve image stability. Images generated on the heat sensitive recording material of this invention have good image stability and fade resistance. Preferably, the image stabilizing compound is the butylated reaction product of p-cresol and dicyclopentadiene.

Description

HEAT SENSITIVE RECORDING PAPER WITH IMAGE STABILITY

Background of the Invention^'

This invention relates to heat sensitive recording material which is able to generate more stable images.

Heat sensitive recording materials are well known in the art and are fully described in patent and other technical literature. Generally, these materials consist of a support or a substrate (e.g., paper) having a light or colorless color developing layer formed either directly upon the substrate or upon a pre-coat layer formed upon the substrate. The color developing layer comprises one or a mixture of color forming materials, such as leuco dyes, and a developer material such as an acidic substance. The color forming materials and the developer materials react together upon the application of heat (via a thermal printhead) to produce a dark or black image in a selected area. These heat sensitive recording materials are widely used in facsimile and other printing technology where an image is generated by heating selected areas of recording materials with, for example, a thermal printhead, to generate images.

One common drawback of heat sensitive recording materials is the tendency of developed images to lose their intensity over the course of time. The degree to which such .images may fade depends upon environmental conditions, particularly heat and humidity. Accordingly, it would be advantageous to provide heat sensitive recording materials which are able to form more stable images and are not unduly affected by environmental conditions.

One approach to stabilizing images formed on heat sensitive recording paper, disclosed in U.S. Patent No. 4,473,831 (Watanabe), involves including a particular type of phenol compound within the thermosensitive coating of the paper. While it may be useful to formulate a thermosensitive coating including a phenol compound of the type disclosed in the Watanabe patent, it is desirable to provide alternative thermosensitive coatings which form stable images using stabilizing compounds other than those disclosed in the Watanabe patent.

Accordingly, one object of the invention is to provide heat sensitive recording materials which maintain good contrast between developed images and background. Another object is to provide heat sensitive recording material having developed images which have good resistance to fading. A further object is to provide a thermosensitive coating composition, for use with heat sensitive recording material, which yields environmentally^' stable images. Other objects will be apparent to one of ordinary skill in the art upon reading the disclosure which follows. Summary of the Invention

The present invention provides a heat sensitive recording material, such as facsimile paper, which yields images with good stability and fade resistance. The recording material comprises a substrate, such as paper, which is coated with a thermosensitive layer. The thermosensitive layer comprises a polymeric binder material within which is dispersed an electron-donating, colorless leuco dye and an electron-accepting developer material. Upon the application of heat, the electron accepting developer material reacts with the dye to produce a visible image. The thermosensitive coating also has dispersed therein an antioxidant which enables the developed image to resist fading which often results through exposure to a variety of common environmental conditions such as heat and humidity. The antioxidant preferably is a compound having the general formula:

wherein R_x represents a branched alkyl group having 3 to 8 carbon atoms, R₂ represents a hydrogen or an alkyl group having 1 to 3 carbon atoms, and x represents whole number from 1 to 6. Most preferably, the antioxidant is butylated reaction product of p-cresol and dicyclopentadiene (C.A.S. Reg. No. 68610-51-5). The thermosensitive coating may also include other additives such as pigments, fillers, lubricants and sensitizers.

Detailed Description

The heat sensitive recording material of the present invention comprises a substrate material such as paper which has coated thereon a thermally sensitive, imagewise exposable layer. This coating may be virtually any type of thermal coating known in the art, and preferably contains one or more electron-donating materials (e.g., leuco dyes) and electron-accepting developer materials (e.g., acidic materials) disposed in a binder together with various additives including whitening agents, dispersants, fillers, sensitizers, and lubricants. The thermal coating also includes a specific type of stabilizing compound which improves the fade resistance of generated images. Upon imagewise exposure to heat, such as occurs during transmission in a facsimile machine, the electron-donating and electron-accepting materials melt and react to form a desired image which is readily visible. The stabilizing compound included within the thermal coating acts to maintain the visibility of this image over time.

The electron-donating dye materials for use in the present invention may be of the type generally known in the art. Typically, these are pale or colorless materials which may be activated (i.e., develop color) alone or in combination with other such materials when, as a result of heating, they contact an electron-accepting (acidic) substance. Such electron-donating materials may include all of the leuco dyes commonly used for heat sensitive recording materials, including triphenylmethane leuco dyes, fluoran-type leuco dyes, phenothiazine-type leuco dyes, auramine-type leuco dyes, spiropyran-type leuco dyes, and rhodamine-type leuco dyes. Generic and specific examples of such electron-donating materials (e.g., dyes) are disclosed in U.S. Patent Nos. 4,370,370; 4,593,298; 4,473,831; and 4,839,332, all of which are hereby incorporated by reference. Exemplary electron-donating materials include 3-N-cyclohexyl-N-methyl-amino, 6-methyl-7-anilino fluoran and 3-pyrrolidino-6-methyl-7-anilino fluoran. Preferred electron-donating materials include the following dyes which may be used in combination or separately: 3-isopentyl ethyl amino-6-methyl-7-anilino fluoran; 3-isobutyl ethyl amino-6-methyl-7-anilino fluoran; 3-dibutylamino-6-methyl-7-anilino fluoran; 3-N-ethyl-N-tetrahydrofurfurylamino-6-methyl-anilino fluoran; and crystal violet lactone.

The electron-donating material is typically present in the coating at a range of about 5-15 percent by weight. Most preferably these materials are present at about 10 percent .by weight.

Many electron-accepting (acidic) substances which function as developer materials in the thermosensitive coating of the heat sensitive recording material are well known in the art. A number of such compounds are disclosed in U.S. Patent Nos. 4,473,831; 4,721,700; and 4,885,271, which are hereby incorporated by reference. Generally, such developer compounds include organic electron acceptors such as phenolic compounds, and aliphatic and aromatic carboxylic acids and their polyvalent metal salts. These materials are characterized by their ability to induce a color change in a leuco dye. Specific examples of preferred developers which may be used with the present invention are 4-hydroxy-4'isoρropoxy diphenol sulfone; bisphenol A; bisphenol S; and benzyl p-hydroxybenzoa e.

The developer compounds may be present in the thermosensitive coating at a range of approximately 15-45 percent by weight. Most preferably these materials are present at the range of about 25-30 percent by weight.

A stabilizing compound is included in the thermal coating layer at a range of approximately 3-10 percent by weight to provide stability to images developed in the heat sensitive recording materials. Most preferably a stabilizing compound is used at the range of about 5-7 percent by weight. Stabilizing compounds useful with this invention are represented by the following general formula:

wherein R₁ represents a branched alkyl group having 3 to 8 carbon atoms and R₂ represents a hydrogen or an alkyl group having 1 to 3 carbon atoms, and x represents a whole number from 1 to 6. In a preferred embodiment, the antioxidant is such that Ri is tert-butyl, R2 is methyl and x is between 2 and 4. The molecular weight of the antioxidant is preferably in the range of 600-700. A preferred stabilizing compound is the butylated reaction product of p-cresol and dicyclopentadiene (C.A.S. Registration No. 68610-51-5). An example of such a compound is sold by R.T. Vanderbilt Company, Inc. of Norwalk, CT. , under the mark VANOX L. A similar compound is sold by Goodyear Chemicals of Akron, OH under the mark WINGSTAY L.

The chemical structure of the butylated reaction product of p-cresol and dicyclopentadiene, assuming x (shown with the general formula) to be 2, is as follows:

The stabilizing compound has a more pronounced effect where the electron-accepting substance (i.e., developer) does not itself contribute to image stability. For example, bisphenol A contributes some stability to the image, so the use of a stabilizer is not essential. However, developers such as benzyl p-hydroxybenzoate contribute little or no stabilizing effect and thus where compounds such as these are used there is a greater need for a stabilizing compound.

The polymeric binder used in the thermosensitive coating comprises one or a mixture of resinous materials which act to hold the other constituents of the layer together, and to bind them to the substrate. The binder typically comprises about 3 to 8 percent of the total coating composition. The currently preferred binder material is a low viscosity polyvinyl alcohol. Other known binders which may be used include polyvinyl pyrrolidone, polyacrylamide, starches, modified starches and modified cellulosics. Latex emulsions such as acrylics, styrene butadiene rubbers and polyurethanes may be used as well. Exemplary polyvinyl alcohols which may be used in the present invention include AIRVOL™ 107, 125, 165, 203, 205, 325, 350, 425, 523, 540, 704, 714, and 736, all of which are available from Air Products and Chemicals, Inc., Allentown, Pennsylvania. Other types of polyvinyl alcohol may be used as well, including ELVANOL™ 75-15, available from DuPont. Polyvinyl alcohol binders used with the present invention may be partially or fully hydrolyzed.

Sensitizing compounds commonly are used in association with developer compounds to lower the melting point of the developer, thus reducing the required energy input. Many sensitizers are known in the art and may be used in the recording material of the present invention. Preferred sensitizers include parabenzyl biphenyl, dimethyl benzyl oxalate, M-terphenyl, diphenoxyethane, benzyl ester of p-nitro benzoic acid (e.g., "Nipafax BPN", available from Nipa Laboratories, Ltd. of Glamorgan, U.K.), 2-benzyloxy-naphthalene, dibenzyl terephthalate, dibenzyl oxalate, and diphenyl carbonate. These sensitizers are generally used in weight percentages in the range of about 2.0 to 25.0 percent by weight of the entire coating composition, and most preferably at about 4 to 10 percent by weight of the entire coating composition.

Preferred fillers (pigments) include alumina trihydrate, calcium carbonate, calcined clay, silicon dioxide, talc and clay. Other filler materials well known in the art may be included as well. These materials may be present alone or in combination with each other. More typically, these materials are present as dispersed particles having a diameter of about one micron. Fillers typically are present in the coating at about 25 to 65% by weight of total coating composition depending upon the amount and the identity of the filler composition.

The dispersants and lubricants which may be used with the present invention are well known in the art and are widely used in the manufacture of thermal paper. Exemplary dispersants include polyacrylate compounds, polyvinylpyrrolidone and the ammonium salt of styrene acrylic acid copolymer. A generally preferred dispersant is styrene-maleic anhydride-monomethyl maleate polymer, sodium salt, available as SCRIPSET from Monsanto Company. Exemplary lubricants include metallic (e.g., zinc, calcium, magnesium) stearates, amide wax, paraffin and polyolefin waxes, fluorocarbon lubricants, and carnuba wax.

The heat sensitive recording materials of the invention may be prepared by coating a substrate (e.g., paper) with color developing layers. General methods of applying coatings to a paper substrate to produce a heat sensitive recording material, which are well known in the art, are applicable to the present invention.

The following examples illustrate coating compositions which may be used to prepare the thermosensitive layer of the heat sensitive recording material of the present invention.

Coating compositions for use in forming the thermosensitive layer of a heat sensitive recording material were formed by preparing the Mix A and Mix B dispersions for the examples which follow, in which all ingredients are disclosed- in parts by weight. Example 1

Mix A Parts

Component

3-butylamino-6-methyl-7-anilino fluoran 220.0

Polyvinyl alcohol binder (AIRVOL 540) 45.8

Dispersant (Scripset) 9.2

Water 1125.0 Mix B Parts

Component

10% solution of Polyvinyl alcohol

(Elvanol 75-15) 66.0

Water 144.0

Benzyl hydroxy benzoate 31.0

Alumina trihydrate 56.0 butylated reaction product of p-cresol and dicyclopentadiene (VANOX L) 6.5

Parts

3-N-ethyl-N-isoamyl-amino-6- 220.0 methyl-7-anilino-fluoran

Polyvinyl alcohol binder (AIRVOL 540) 45.5

Dispersing Agent (Scripset) 9.2

Water 1125.0

Mix B Parts Component

10% solution of Polyvinyl alcohol

(Elvanol 75-15) 66.0

Water 84.6

Benzyl hydroxy benzoate 31.0

Alumina trihydrate 56.0 butylated reaction product of p-cresol and dicyclopentadiene (VANOX L) 6.6 Example 3

Mix A Parts

Component

3-N,N-diethylamino-6-methyl-7- anilino fluoran 220.0 Polyvinyl alcohol binder (AIRVOL 540) 45.8 Dispersing Agent (Scripset) 9.2 Water 1125.0

Mix B Parts

- Component

10% solution of Polyvinyl alcohol

(Elvanol 75-15) 66.0

Water 84.6

Benzyl hydroxy benzoate 40.0

Calcium carbonate 53.5 butylated reaction product of p-cresol and dicyclopentadiene (VANOX L) 6.5

Example 4

Mix A Parts

Component

3-N,N-dibutylamino-6-methyl-7- anilino 220.0 fluoran Polyvinyl alcohol binder (AIRVOL 540) 45.8 Dispersing Agent (Scripset) 9.2 Water 1125.0 Mix B Parts

Component

10% solution of Polyvinyl alcohol

(Elvanol 75-15) 66.0

Water 84.6

Benzyl hydroxy benzoate 31.0

Calcium carbonate 56.0

Butylated reaction product of p-cresol and dicyclopentadiene (VANOX L) 3.3

Example 5

Mix A Parts

Component

3-N,N-dibutylamino-6-methyl-7- anilino fluoran 220.0 Polyvinyl alcohol binder (AIRVOL 540) 45.8 Dispersing Agent (Scripset) 9.2 Water 1125.0

Mix B Parts

Component

10% solution of Polyvinyl alcohol

(Elvanol 75-15) 66.0

Water 84.6

Benzyl hydroxy benzoate 31.0

Alumina trihydrate 56.0 Butylated reaction product of p-cresol and dicyclopentadiene (VANOX L) 1.7 For each of the above examples coating mixes were prepared by combining the A and B mixes specified for each example at 9.5 parts A mix and 30.0 parts B mix. The resulting coating mixes were applied to paper substrate at about 5 to 7g/m² to form heat sensitive recording materials.

Comparative Example

The stability of an image formed on heat sensitive recording materials prepared with the coating mixes of examples 1 through 5 was compared to that of an image formed on heat sensitive recording materials formed with coating mixes identical to those of examples 1 through 3, except that the comparative coatings were formed while omitting from the B Mix the stabilizer compound (butylated reaction product of p-cresol and dicyclopentadiene) .

An image was formed on each paper prepared from the coatings of examples 1 through 5, and the comparative, non-stabilized coatings by contact for 5 seconds with a metal bar heated to 210°F. The activated samples were then placed in a fade test environment at a temperature of 115°F at 75% relative humidity. The optical density (OD) of each activated sheet was measured before residence in the fade test environment and after periods of 24 hours and 1 week in the fade test environment. The results are shown below in Table 1, in which larger numbers indicate a superior image. TABLE 1 - Comparative Testing

Test Coating OD Before OD @ 24hrs OD . 1 wk

Example 1 1.31 1.24 1.15 with stabilizer

Example 1 no stabilizer 1.31 1.00 0.40

Example 2 with stabilizer 1.25 1.18 0.96

Example 2 no stabilizer 1.29 0.44 0.17

Example 3 with stabilizer 1.31 1.31 1.22

Example 3 no stabilizer 1.33 0.93 0.25

Example 4 1.30 1.18 0.96 with stabilizer

Example 5 1.31 1.25 0.94 with stabilizer

What is claimed is:

Claims

1. A heat sensitive recording material, comprising: a substrate; a thermosensitive layer disposed on the substrate comprising a polymeric binder having dispersed therein an electron-donating, colorless dye material, an electron-accepting developer which reacts with the colorless dye upon exposure to heat to produce a visible image, and an antioxidant which provides stability to a developed image, represented by the formula

wherein R_r represents a branched alkyl group having 3 to 8 carbon atoms and R₂ represents hydrogen or an alkyl group having 1 to 3 carbon atoms, and x is an integer between 1 and 6.

2. The recording material of claim 1 wherein R_x is t-butyl and R₂ is methyl.

3. The recording material of claim 2 wherein x is an integer between 2 and 4.

4. The recording material of claim 1 wherein the antioxidant has a molecular weight in the range of about 600-700.

5. The recording material of claim 1 wherein the antioxidant is the butylated reaction product of p-cresol and dicyclopentadiene, having a molecular weight in the range of about 600 to 700, and the formula

6. The recording material of claim 5 wherein the colorless dye material is present in the thermosensitive coating at about 5 to 15% by weight.

7. The recording material of claim 5 wherein the developer material is present in the thermosensitive coating at about 15 to 45% by weight.

8. The recording material of claim 5 wherein the antioxidant is present at about 3 to 10% by weight.

9. The recording material of claim 1 wherein the binder is a polyvinyl alcohol.

10. A heat sensitive recording material, comprising: a substrate; f a thermosensitive layer disposed on the substrate comprising a polymeric binder having dispersed therein an electron-donating, colorless dye material, an electron-accepting developer which reacts with the colorless dye upon exposure to heat to produce a visible image, and a fade resisting additive comprising the butylated reaction product of p-cresol and cyclopentadiene, having a molecular weight in the range of about 600 to 700 represented by the formula:

wherein x is between 2 and 4.

11. The recording material of claim 10 wherein the fade resisting additive is present at about 3 to 10% by weight.