EP0512696A1

EP0512696A1 - Improved thermosensitive recording material

Info

Publication number: EP0512696A1
Application number: EP92303356A
Authority: EP
Inventors: Maurice Wendell Lewis; Stephen Douglas Lakes
Original assignee: NCR International Inc
Current assignee: NCR International Inc
Priority date: 1991-05-06
Filing date: 1992-04-14
Publication date: 1992-11-11
Also published as: FI921990A; NO921770D0; FI921990A0; NO921770L; IE921451A1

Abstract

A thermosensitive recording material comprises a thermally reactive layer (14) separated from a receiver stock (10) by an undercoating layer (12). The undercoating (12) has ingredients which inhibit the penetration of a thermally reactive layer (14) into the receiver stock (10) and which also enhances the opaqueness and brightness of the receiver stock (10). The undercoating (12) comprises a water resistant binder, a hollow sphere pigment, and a viscosity modifier. As such, the undercoating (12) comprises about 10-50% styrene-butadiene, about 35-85% styrene-acrylic, and about 1-20% viscosity modifier selected from the group consisting essentially of polyvinyl alcohol, zinc stearate, sodium polyacrylate, paraffin wax, fumed silica, carboxy methyl cellulose and Natrasol.

Description

The present invention relates to a thermosensitive recording material.
In the field of direct thermal printing, a thermal printhead prints data by heating or energizing selective portions of a thermosensitive recording material having a thermosensitive layer which has been coated onto a receiver stock. A problem with this type of thermal printing is that the heat applied to the thermosensitive layer can dissipate into the receiver stock. This results in an undesirably poor print quality due to an inadequate amount of heat being applied to the thermosensitive layer. In order to obtain an improved print quality, the thermal printhead must thermally energize the thermosensitive layer for a longer period of time. This results in a decrease in the useful life of the thermal printhead.
It is known to provide an undercoat, or insulating layer, between the receiver stock and the thermosensitive layer in order to inhibit the disipation of heat into the receiver stock. The resulting images printed on the thermosensitive layer are darker, and so the sensitivity of the thermosensitive layer is increased. Although the provision of known undercoat layers increases the sensitivity of the thermosensitive layer, the brightness and opaqueness of the receiver stock are disadvantageously restricted. Also, known undercoat layers are disadvantageous in that the thermosensitive layer can readily penetrate into the receiver stock.
It is, therefore, an object of the present invention to provide a thermosensitive recording material having a thermosensitive layer separated from a receiver stock by an undercoat layer and which can provide good brightness and opaqueness characteristics and also inhibit the penetration of the thermosensitive layer into the receiver stock.
According to the present invention there is provided a thermosensitive recording material comprising a thermally reactive layer separated from a receiver stock by an undercoating layer, characterized in that said undercoating layer comprises a water resistant binder, a hollow sphere pigment and a viscosity modifier.
The invention is described further hereinafter, by way of example only, with reference to the accompanying drawing which is a sectional view of a base sheet of paper or receiver stock 10. The receiver stock 10 may be any suitable stock which is capable of supporting an undercoating or subcoating 12 and a thermosensitive coating or layer 14.
The following examples show subcoating means providing a subcoating 12 for a receiver stock 10 which improves the opaqueness, brightness, whiteness, and print density of the receiver stock 10 and also for inhibiting the penetration of the thermosensitive layer 14 into the receiver stock 10.

EXAMPLE I

Example I is a composition which provides a subcoating 12 which could be used with any thermosensitive layer 14.

Subcoating 12

Material % Dry Range of % dry

Binder 15 10-50

Pigment 77 35-85

Viscosity Modifier 8 1-20

$\bar{100}$
In this example, the binder is a carboxylated styrene-butadiene water resistant binder. In a preferred embodiment, a suitable binder is the carboxylated SBR Emulsion known as 76 Res 4470. The sources from which the various materials described herein are available are described below. The pigment used in this Example I is a hollow sphere styrene-acrylic emulsion known as Ropaque OP-90. The viscosity modifier is a polyvinyl alcohol which is soluble in water. Suitable viscosity modifiers are the polyviol LL-603 and PVA 107.
The binder, pigment and viscosity modifier were mixed together and coated on 45.6 gsm receiver stock 10 at 7.4 gsm (5.0 lbs/3300ft²). The thermosensitive layer 14 was then coated on the subcoating 12 at 6.8 gsm (4.6 lbs/3300 ft²).

In a preferred embodiment, the thermosensitive layer 14 is a composition as follows:

Thermosensitive Layer 14
Material	% Dry	Range of % dry
Water	--	--
Leuco dye	10	5-15
Binder	10	5-15
Developer	20	15-25
Sensitizer	25	10-30
Fillers	30	25-40
Lubricants	5	1-10
	$\bar{100}$

In this example, the Leuco dye was selected from the group consisting of S-205, PSD-150, N102, and ODB-2. In a preferred embodiment, the ODB-2 dye was selected. The binder was selected from the group consisting of polyvinyl alcohol, starch and its derivatives, CMC, methyl cellulose and ethyl cellulose (with water as the carrier). In a preferred embodiment, the polyvinyl alcohol was selected. The developer can consist of any suitable acid material, such as Bisphenol A or Bisphenol S. In the preferred embodiment, Bisphenol A was selected. The filler was selected from the group consisting of magnesium carbonate, calcium carbonate, silicone dioxide, or clays. In a preferred embodiment, the calcium carbonate and silicone dioxide were selected. The lubricants was selected from the group consisting of zinc stearate and paraffin waxes. In a preferred embodiment, the zinc stearate was selected as the primary lubricant.

EXAMPLE II

Example II shows another composition for a subcoating system. In this example, the thermosensitive layer 14 is the same as that for Example I. The subcoating 12 in this subcoating system has the following characteristics:

Subcoating 12

Material % Dry Range

Binder 15.0 10-50

Pigment 80 35-80

Viscosity Modifier 5 5-20

$\bar{100}$
In this example, the binder and pigment are the same as was used in Example I. However, the viscosity modifier used in this Example II was selected from the group consisting of zinc stearate, sodium polyacrylate, paraffin wax emulsions, fumed silica, carboxy methyl cellulose, and Natrasol. In a preferred embodiment, zinc stearate was selected.
The subcoating 12 was applied to a 45.6 gsm receiver stock 10 at 7.4 gsm (5.0 lbs/3300ft²). The thermosensitive layer 14 was applied on the subcoating 12 at 6.9 gsm (4.0 lbs/3300 ft²) using conventional coating methods.

EXAMPLE III

In Example III only the the thermosensitive layer 14 mentioned in Example I was applied on the 45.6 gsm receiver stock. The coating weight was 6.4 gsm (4.3 lbs/3300 ft²)

TEST PROCEDURE AND RESULTS

A procedure was set up to test the whiteness, opaqueness and brightness of the receiver stock 10 as well as the ability of the subcoating 12 to prevent the thermosensitive layer 16 from penetrating the receiver stock 10. Three test samples were prepared in accordance with examples I, II and III by applying the subcoating 12 at room temperature. The test samples were then dried with a conventional forced air dryer and calendered to a smooth finish. The whiteness, brightness and opaqueness were then measured using a Brightimeter Micro S4-M from Technidyne Corporation. The higher the measurement indicated by the Brightimeter, the higher whiteness, brightness and opaqueness. Table 1 is a listing of the results found. TABLE 1

WHITENESS BRIGHTNESS OPAQUENESS

EXAMPLE I subcoating 83.81 90.57 92.58

EXAMPLE II subcoating 84.50 89.33 93.30

EXAMPLE III no subcoating 79.10 86.27 88.64

As indicated by the results in Table 1, the application of the subcoating 12 on the receiver stock 10 caused the whiteness, brightness and opaqueness of the printed receiver stock 10 to increase when compared to the control sample.
A wax test procedure was also set up to test the ability of the subcoating 12 to inhibit the penetration of the thermosensitive layer 14 into the receiver stock 10. A plurality of Dennison wax picks (not shown), manufactured by Dennison Mfg. Co. of Framingham, Massachusetts were used to test the penetration of the thermosensitive layer 14 into the receiver stock 10. The test samples were prepared in accordance with Examples I, II and III, and the Dennison wax picks were melted and adhered to each sample. After the molten wax solidified, the wax was pulled away from the receiver stock. The degree to which the thermosensitive coating 14 came off with the wax indicated the degree to which the thermosensitive coating 14 penetrated the receiver stock. For example, if the thermosensitive coating 14 came off of the test sample as the wax was removed, then the thermosensitive coating 14 did not penetrate the test sample. Conversely, if the thermosensitive coating 14 did not come off of the test sample, then the thermosensitive coating 14 penetrated the test sample. The wax picks have a Dennison rating from 2 to 26 which provides an indication of the degree to which the thermosensitive coating 14 penetrated the test sample. A number of wax picks having different ratings were used on each test sample until a wax pick was found that could remove the thermosensitive layer 14 from the test sample. If the Dennison rating was high, this indicated that it was more difficult to remove the thermosensitive layer 14 from the test sample, thereby indicating a higher amount of penetration by the thermosensitive layer 14 into the test sample. If the Dennison rating was low, this indicated that it was easy to remove the thermosensitive layer 14 from the test sample, thereby indicating a lower amount of penetration by the thermosensitive layer 14 into the test sample. The data for the thermosensitive layer 14 penetration test was as follows: TABLE 2

PENETRATION

EXAMPLE I 2

EXAMPLE II 2

EXAMPLE III 10

Finally, a density test procedure was used to test the density of the developed colour on the thermal receiver stock 10 after imaging using an Atlantic Thermal Response Tester. For this test procedure, a McBeth densitometer Model Rd-400 was used to measure the optical density of a reacted area which was developed at a given pulse time on the Tester. Table 3 provides the results of the Density tests.

TABLE 3

Example	Pulse time (milliseconds)
	1.0	.9	.8	.7	.6	.5	.4	.3	.2	.1
I	1.25	1.25	1.24	1.23	1.17	1.03	.69	.31	.08	.05
II	1.31	1.30	1.29	1.26	1.20	1.02	.70	.32	.08	.05
III	1.26	1.24	1.21	1.15	1.03	.77	.43	.15	.06	.05

As is illustrated by the data, the optical density of the test samples prepared in accordance with Examples I and II generally improved over the control sample of Example III. Thus, a receiver stock 10 having the subcoating 12 coated thereon in accordance with Examples I and II improved the density of the characters printed on the receiver stock.
The various ingredients used in the present invention are available from the sources listed in the following Table 4.

Claims

A thermosensitive recording material comprising a thermally reactive layer (14) separated from a receiver stock (10) by an undercoating layer (12), characterized in that said undercoating layer (12) comprises a water resistant binder, a hollow sphere pigment and a viscosity modifier.
A material according to claim 1, characterized in that said undercoating (12) comprises 10-50% water resistant binder, and in that said water resistant binder comprises a styrenebutadiene.
A material according to claim 2, characterized in that said undercoating (12) comprises about 25% water resistant binder.
A material according to claim 1, characterized in that said undercoating (12) comprises 35-85% hollow sphere pigment, and in that said hollow sphere pigment comprises a styrene-acrylic.
A material according to claim 4, characterized in that said undercoating (12) comprises about 75% hollow sphere pigment.
A material according to any one of claims 1 to 5, characterized in that said viscosity modifier is selected from the group consisting essentially of polyvinyl alcohol, zinc stearate, sodium polyacrylate, paraffin wax, fumed silica, carboxy methyl cellulose and Natrasol.
A material according to claim 6, characterized in that said undercoating (12) comprises 1-10% viscosity modifier, and in that said viscosity modifier comprises polyvinyl alcohol.
A material according to claim 6, characterized in that said undercoating comprises 1-10% viscosity modifier, and in that said viscosity modifier comprises a zinc stearate.
A material according to any one of the preceding claims, characterized in that said thermally reactive layer includes 5-10% dye, 5-10% binder, 15-25% sensitizer, 30-40% fillers and 1-10% lubricants.