GB2060920A - Thermographic paper - Google Patents

Abstract

A thermographic paper suitable for use in electronic alphanumerical printing devices which use a semiconductor printing head and has a large shelf life, is highly sensitive and allows more contrast to be obtained comprises a paper base, a thermosensitive coating and a protective film of cellulose ether or polyvinylbutyral. The paper base comprises prehydrolysis sulphate cord pulp having a particular ground particle size, together with an amino-aldehyde resin, preferably a melamine-formaldehyde resin. The thermosenstive coating may comprise ferric stearate pyrocatechol- hexamethylenetetramine complex, zinc oxide, thuiourea and polyvinyl butyral (Example 1).

Description

SPECIFICATION Thermographic paper The present invention relates to paper manufacture, in particular to manufacture of thermographic paper.

The invention is particularly useful for the production of paper for data-carrying, as used in recording systems which involve the principle of thermal recording. This is based on the phenomenon of paper colour transformation as a result of thermal impact of electric current.

Recording on the paper of the invention can be accomplished in a variety of ways, for example by means of a linear member, such as a stylus, or by means of a semiconductor heater of the flattened type. When the heater is energized, localized areas of the effective surface of the paper can be heated to a temperature of from 200 to 250"C, causing the surface of the paper to change colour within a few milli-seconds.

Such systems rely on the thermal principle of record, and this is where the invention can be beneficially used. Examples of these systems include thermoprinting devices having a semiconductor printing head, and these are rapidly gaining an ever increasing scope of application as output units for electronic computers and as terminal units for data communication systems. The invention is also suitable for use in automatic recording facilities which use a heated stylus designed specially for a variety of both medical uses (such as electrocardiographic and electroencelographic apparatus) and industrial uses.

Furthermore the invention can be used in thermocopying systems to obtain reproductions of, for example, drawings, textual materials and graphic representations by the method of reflection.

In this copying process, a sheet of thermographic paper is placed over the original and subjected to irradiation by infra-red rays for a short period. The image is formed due to the absorption of the IR rays by the opaque character of the original lines, letters and figures. The release of heat at these areas induces heating of the corresponding areas of the thermographic paper, thus causing their colouration.

The development of thermographic paper has been encouraged by the advent of a principally novel type of electronic alphanumerical thermoprinting device which has a number of considerable advantages over mechanical devices of the impact type, including minimum overall dimensions, structural simplicity, high reliability, noise-free operation, fast response and lower cost.

Thus, the present invention consists in a thermographic material, which comprises: 77-90% by weight of base; 8-15% by weight of thermosensitive coating; and 2-8% by weight of protective film, comprising cellulose ether or polyvinylbutyral; in which the base comprises 95-97% by weight of prehydrolysis sulphate cord pulp having a ground particle size of 40 to 70 SR, and 3-5% by weight of an amino-aldehyde resin.

We prefer that the amino-aldehyde resin is a melamino-formaldehyde resin, in the form of a solution in 2% hydrochloric acid. The addition of resin aids reduction in the pH of the paper base to 3.5-5.0, and also enables the production of damp-resistant paper suitable for further processing on a coating machine. One component of the thermosensitive coating is preferably pyrocatechol. This compound is fairly readily oxidized by oxygen of the air to form dark coloured compounds, for example p-benzoquinones, especially under neutral or alkaline conditions, and therefore the acidic nature of the paper base tends to stabilize the applied thermosensitive coating and, therefore, stabilize the product thermographic paper. It is preferred that the protective layer comprises ethylcellulose, acetylcellulose, acetobutyrate cellulose or polyvinylbutyral.These substances offer good film-forming properties, and a sufficiently high softening temperature. The protective layer promotes further fastening of the thermosensitive coating, improves smoothness of the working surface of the thermographic paper, and also partially keeps the components of the thermosensitive coating from being oxidized by oxygen of the air.

The quoted relative amounts of the three components of the product makes possible the obtaining of clear detail, high-contrast recording on thermographic paper with a minimum of energy consumption, and a minimum of smudging of thermal heads of printing devices.

The use of paper manufactured from prehydrolysis sulphate cord pulp with an addition of melamino-formaldehyde resin as a base can give the resulting thermographic paper high sensitivity and high-quality recording contrast together with an enhanced shelf life.

High heat sensitivity of the paper results from the fact that the composition of the thermosensitive coating contains ferric stearate and pyrocatechol as major chromogenic components; a combination of these colour-forming substances is one of the most sensitive in thermography. However, a main disadvantage inherent in compositions based on ferric stearate and pyrocatechol is believed to be the lack of stability, and the spontaneous colouration of thermosensitive coatings during storage of the paper.

The usefulness of prehydrolysis sulphate cord pulp in the composition of the paper base is due to its high chemical purity, particularly as far as iron is concerned. Iron is known to advance pyrocatechol oxidation, and to be capable of forming coloured complexes with pyrocatechol in the type of composition used for thermosensitive coatings.

The following table presents a comparison of the major physico-technical indices of prehydrolysis sulphate cord pulp and bleached sulphate pulp of pine wood.

Table 1 Prehydrolysis Bleached Index Pulp Pulp Mechanical Strength as breaking length, metres (at ground particle size 60 SR and casting weight 75 g/m2) 7000 7800 Fracture as no. double bends 1000 1600 Whiteness (%) 87 12 Dirtiness as no. dirt particles per m2 of area greater 0.06-1.5 mm2 40 80 Ash Content (%) 0.08 not rated Iron (% wt) 0.0008 Ca (% wt) 0.007 , Si (% wt) 0.008 , This type of pulp also shows fairly high mechanical strength indices, which ensure the production of durable paper of small bulk, say 30-40 g/m2. A decrease in the bulk, and therefore in the thickness of the paper base, has a favourable effect on the quality of the final print (recording) due to improved heat conduction from, for example, the heated elements of a thermal head to the paper under the action of a fixed pressure.

Paper base manufactured from prehyrolysis sulphate cord pulp and melamino-formaldehyde resin is distinguished by a low iron content and by a rather low pH value, which exerts a stabilizing effect, and provides stability of the thermosensitive composition and the product paper during storage.

The thermographic paper of the present invention can be made by applying successively a thermosensitive coating and a protective coating to a paper base. The paper base should be firm under both dry and wet conditions, fairly transparent and thin, it should contain a minimum amount of iron (preferably up to 30 mg/kg), and have its pH preferably within the range 3.5 to 5.0.

The paper can be manufactured by known processes from milling prehydrolysis sulphate cord pulp, followed by casting, pressing, drying and calendering.

The pulp can be ground in rolls or in mills with the use of desalinated water to a particle size of 40-70" SR and an average fibre length of from 40 to 80 decigrams. Melaminoformaldehyde resin can be added at the casting stage in an amount of 3-5%, reckoned in terms of the weight of totally dry fibre, but added in the form of a solution in 2% hydrochloric acid.

The paper is then dried by means of dryer rolls at a progressive increase of temperature from 110-120"C, after which it is calendered to a density of at least 0.8 g/cm3. A recommended bulk of the paper base is 30-40 g/m2.

A preferred thermosensitive coating comprises the following components, in per cent by weight: ferric stearate 55-56 complex of pyrocatechol with hexamethylene-tetramine 18-22 thioureas 0.5-0.6 filler 10-12 binder 5.5-10.4.

The major chromogenic components of the thermosensitive coating are ferric stearate and pyrocatechol. To preclude premature reaction of pyrocatechol with iron, pyrocatechol can be used in the form of a complex salt with hexamethylenetetramine. Thiourea is here used to obtain deeper colouring of the print, and a filler is employed to absorb the components of the thermosensitive coating which melt during recording. White pigments, for example, kaolin, zinc oxide, titanium dioxide and talc, may be used as the filler. The addition of the filler improves the degree of whiteness of the thermosensitive coating. Polyvinylbutyral, ethylcellulose, acetylcellu lose or acetobutyrate cellulose are preferred binding materials, and ethyl alcohol, isopropyl alchohol, ketones and ethyl acetate are suitable solvents.

Grinding of the components of the thermosensitive coating is preferably accomplished by means of ball or head mills to yield a particle size of not more than 10 mcm. Ferric stearate mixed with the filler and binder, and a complex of pyrocatechol with hexamethylenetetramine are ground separately. Then, the pyrocatechol-hexamethylenetetramine complex is intermixed with the suspension of ferric stearate in an amount of 1 part by weight of the complex to 3 parts by weight of ferric stearate, reckoned on a dry basis.

Thiourea can be added to the resulting suspension in the form of a solution in ethyl alcohol.

The finished suspension is properly stirred, filtered, and brought by the addition of solvent to the concentration required for use on a coating machine.

The procedure of applying the thermosensitive suspension or composition to the paper base can be effected in a variety of ways, for example, by means of a wire blade (Mayer's blade), a roller blade or a roller arrangement.

The paper together with its thermosensitive coating is then dried, under a drying temperature that should not exceed 60"C. A recommended coating weight is 3-7 g/m2. The protective layer is preferably applied by means of a coating machine over the thermosensitive coating, and preferably as a 2.5-5% solution of a film-forming polymer in a solvent.

The protective layer applied can comprise solutions of polyvinylbutyral, ethylcellulose, acetylcellulose, and acetobutyrate cellulose. It is preferred that polymers having a sufficiently high softening temperature (preferably about 200"C) are used, in order to avoid adhesion of the paper to the heated elements of the thermal printing head during recording.

Ethyl alcohol, isopropyl alcohol, ketones and ethyl acetate can be used as the solvent, and we prefer that the maximum drying temperature is again 60"C. A recommended weight of protective layer is 1.0-3.0 g/m2.

The present invention will be further illustrated by the following example.

Example I A thermographic paper was produced which contained the following components, in per cent by weight: paper base 77 thermosensitive coating 1 5 protective layer 8.

The paper base weighing 30 g/m2, was fabricated from prehydrolysis sulphate cord pulp having a ground particle size of 70 SR and a fibre length of 40 decigrams. The pulp was milled using desalinated water. Melamino-formaldehyde resin, in the form of a solution in 2% hydrochloric acid was added to the paper pulp in a paper-making machine in an amount of 5%, based on the weight of dry fibre. The paper was dried by means of dryer rolls at a gradually increasing temperature up to 110-1 20'C. Then, it was calendered to a density of not less than 0.8 g/cm3. The finished paper had a pH value of 3.5.

The thermosensitive coating which was applied to the paper base had the following composition, in per cent by weight: ferric stearate 60 pyrocatechol-hexamethylenetetramine complex 20 zinc oxide 1 2 thiourea 0.5 polyvinylbutyral 7.5.

The thermosensitive composition was prepared as follows. Ferric stearate with zinc oxide and a solution of polyvinylbutyral were ground in a bead mill to produce a suspension having a degree of dispersion of 10 mcm or less. The pyrocatechol-hexamethylenetetramine complex was ground in a ball mill for 10 hours and then added to this suspension in an amount of 1 part by weight of the complex to 3 parts by weight of ferric stearate, reckoned on a dry basis. Thiourea was added to the mixture in the form of a 5% alcohol solution. The resulting suspension was filtered, and its viscosity adjusted by the addition of solvent.

The resulting thermosensitive composition was applied in a uniform layer to a paper base by means of a coating machine. Drying was then carried out at a temperature below 60"C. The thermosensitive coating had a weight of 6 g/m2.

The protective film was applied to the thermosensitive layer by means of a coating machine in the form of 3.5% solution of polyvinylbutyral in isopropyl alchohol. After drying at a temperature below 60"C, the protective film has a weight of 3 g/m2.

Recording on the product thermographic material was carried out using a thermal head employing a voltage of 1 2 and a pulse length of 8 ms. Sharp clear-detail prints of black colour were produced having an optical density of 0.1, with a background optical density of 0.2. The paper has a shelf life greater than 2 years.

Example 2 A thermographic paper was produced having the following composition, in per cent by weight: paper base 90 thermosensitive coating 8 protective layer 2.

The paper base, of bulk 40 g/cm2, was fabricated from prehydrolysis sulphate cord pulp having a ground particle size of 40 SR and a fibre length of 80 decigrams together with a melamino-formaldehyde resin, in an amount of 3% by weight based on the weight of totally dry fibre at pH 5. This paper base was covered with a thermosensitive coating having the following composition, in per cent by weight: ferric stearate 66 pyrocatechol-hexamethylenetetramine complex 18 zinc oxide 10 thiourea 0.5 ethylcellulose 5.5.

The preparation of the thermosensitive composition and the application of this composition to the paper base were carried out substantially as described for Example 1. The thermosensitive coating has a weight of 3.5 g/m2.

The protective layer applied included a 4% solution of ethylcellulose in ethyl alcohol, and it was dried at a temperature below 60"C. The protective layer after drying had a weight of 1 g/m2.

In this example, recording was carried out with a thermal printing at a voltage of 10 and at a pulse length of 10 ms. Clear-detail prints of a black colour having an optical density of 1.0, against a background of optical density 0.2 were produced. The paper has a shelf life greater than 2 years.

Example 3 A thermographic paper was produced which contained the following components, in per cent by weight: paper base 83 thermosensitive coating 1 2 protective layer 5.

The paper base, of bulk of 35 g/m2, was fabricated from prehydrolysis sulphate cord pulp having a ground particle size of 50 SR and a fibre length of 65 decigrams together with a melamino-formaldehyde resin in an amount of 4% by weight based on the weight of dry fibre of pH 4.1. This base was covered with a thermosensitive coating having the following composition: ferric stearate 58 pyrocatechol-hexamethylenetetram ine complex 22 zinc oxide 10 thiourea 0.6 polyvinylbutyral 9.4.

Again, preparation of the thermosensitive composition and the application of this composition to the base were accomplished as described for Example 1.

The thermosensitive coating has a weight of 5 g/m2, and the protective film that was applied contained a 3% solution of acetycellulose in acetone. The protective layer has a weight of 2 g/m2.

A recording was carried out with a thermal printing head using a voltage of 10 and a pulse length of 5 ms, to produce a clear-detail black recording of optical density 1.0, against a background of optical density 0.2. The paper has a shelf life of more than 2 years.

Example 4 A thermographic paper was produced having the following components, in per cent by weight: paper base 85.7 thermosensitive coating 11.4 protective layer 2.9.

The paper base, of bulk 30 g/m2, was fabricated from prehydrolysis sulphate cord pulp having a ground particle size of 60 SR and a fibre length of 70 decigrams together with a melaminoformaldehyde resin. This was carried out as in Example 1. The result was carried out as in Example. The result was covered with a thermosensitive coating having the following composition, in per cent by weight: ferric stearate 60 pyrocatechol-hexamethylenetetramine complex 20 titanium dioxide 12 thiourea 0.5 polyvinylbutyral 7.5.

As before, the preparation of thermosensitive composition and the application of it to the paper base were carried out as described for Example 1. The resulting thermosensitive coating had a weight of 4 g/m2.

The protective layer applied contained a 3.5% solution of polyvinylbutyral in ethyl alcohol, and this was dried at a temperature below 60"C to produce a protective layer having a weight of 1.0 g/m2.

Recording was carried out with a thermal head at a voltage of 10 and a pulse length of 10 ms, to give a paper showing black clear-detail printing at an optical density of 1.0, against a background optical density of 0.2. The paper has a shelf life of greater than 2 years.

Example 5 Here a thermographic paper was produced substantially as described for Example 1, except that the thermosensitive coating contained the following components, in per cent by weight: ferric stearate 55 pyrocatechol-hexamethylenetetramine complex 22 titanium dioxide 1 2 thiourea 0.6 ethylcellulose 10.4.

Recording was carried out with a thermal head at a voltage of 1 2 and at a pulse length of 3 ms, to give a paper having black clear-detail printing at an optical density of 0.9, against a background optical density of 0.16. The paper has a shelf life of more than 2 years.

Example 6 A thermographic paper was produced as in Example 1, except that the protective layer comprised acetobutyrate cellulose. Recording using a thermal head at a voltage of 14 and a pulse length of 10 ms, produced a paper having clear-detail printing at an optical density of 1.2, against a background optical density of 0.2. The paper has a shelf life of more than 2 years.

Example 7 In this example a thermographic paper was produced as in Example 1, except that the protective layer comprised celluloseacetate. The protective layer had a weight of 2 g/m2.

Recording was carried out using a thermal head at a voltage of 1 5 and a pulse length of 10 ms, to produce a paper having clear-detail printing at an optical density of 1.2, against a background optical density of 0.1 8. The paper has shelf life of more than 2 years.

Table 2 below provides a comparison between a thermosensitive paper of the invention and a prior art thermosensitive paper.

Table 2 New Old Index Paper Paper Weight in g of 1 sq m 35 50 Thermosensitivity, "C 80 100 Optical density of background 0.2 0.3 Optical density of print (at 1 2 V, pulse length toms) 1.0 0.7 Shelf life, years 2 0.5 As can be seen from this table, the new paper has a high thermosensitivity, allows a greater contrast to be achieved, and has a shelf life 4 times larger than that of the prior paper.

These properties of the new paper make possible its advantageous use in electronic alphanumerical printing devices using a semiconductor printing head.

Claims (5)

1. A thermographic material, which comprises: 77-90% by weight of a base; 8-15% by weight of a thermosensitive coating; and 2-8% by weight of a protective film, comprising cellulose ether or polyvinybutyral; in which the base comprises 95-97% by weight of prehydrolysis sulphate cord pulp having a ground particle size of 40-70" SR, and 3-5% by weight of an amino-aldehyde resin.

2. A thermographic paper material comprising a paper base, a thermosensitive coating and a protective layer, wherein said paper base is fabricated from a paper of prehydrolysis sulphate cord pulp having a ground particle size of 40-70" SR, and amino-aldehyde resin with the following ratio (p.b.w.): prehydrolysis sulphate cord pulp 95-97 amino-aldehyde rein 3-5, whereas said protective layer is fabricated from a film of cellulose ether or polyvinylbutyral, with said paper base, thermosensitive coating and protective layer being selected in the following ratio (p.b.w.): paper base 77-90 thermosensitive coating 8-1 5 protective layer 2-8.

3. A thermographic paper material according to Claim 2, wherein as said amino aldehyde resin use is made of melamino-formaldehyde resin.

4. A thermographic paper material according to Claims 2 or 3, wherein the protective layer is fabricated from ethylcellulose, acetobutyrate cellulose, acetylcellulose or polyvinylbutyral.

5. A thermographic paper material substantially as herein described with reference to any one of the foregoing examples.