DE3237247C2 - Thermosensitive recording sheet - Google Patents

Abstract

A heat-sensitive recording sheet obtained by coating a support with a normally colorless or light colored dye precursor and benzyl p-hydroxybenzoate as a color developer (which reacts with the dye precursor to form a color when heated) together with benzyl p-benzyloxybenzoate. By using the benzyl p-hydroxybenzoate and benzyl p-benzyloxybenzoate in combination, the image stability is increased, the fogging is prevented and the sensitizing effect is improved.

Description

35

The invention relates to a heat sensitive Recording sheet, and particularly a heat-sensitive one Recording sheet with improved recording stability.

Thermosensitive recording systems are in the past driving for numerous purposes has been used, e.g. B. for printing, telecommunications etc .. because such systems during of the recording do not generate any noises caused by impact, because the recordings · Ι5 does not need to first develop and fix and because the whole mechanism is easy to handle and to monitor is. In image telegraphy in particular, there is a constant need for thermosensitive ones Recording systems, and the recording speed is constantly increasing while reducing the transmission cost. Because of the higher speeds in image telegraphic transmission there is a growing need for higher sensitivity of the heat-sensitive recordings used.

When high speed telephoto transmission occurs the sending and receiving of an original with a size of DIN A4 in general in one Period of time from 20 s to 1 min. With the thermal head f> o the transmission unit acts in short intervals of only 1 to 2 milliseconds, and the resulting Heat energy is transferred to the heat-sensitive recording sheet to form the image there to effect. That is why it is extremely important. * s that a thermosensitive recording sheet has a very good heat sensitivity to the Image recording reaction due to the applied heat

to bring about meenergie within a very short time.

Usually, heat-sensitive paper consists essentially of a dye precursor and one Dye developing agent and, if necessary, a sensitizer. The sensitizer serves to reduce the Speed up the color reaction when it is released by the heat transfer energy and then the Dissolves or includes dye precursors and the developing agent. The thermal responsiveness of the sensitizer is therefore an effective means of increasing the sensitivity of a thermosensitive recording paper to increase.

The sensitizer itself does not contribute directly to color formation. He has a thinning effect and causes the initiation of sensitization and often also a sticking. That is why it is essential advantageous.die heat responsiveness.ii ^ s dye precursor or to improve the color developing agent as this is directly involved in the color forming reaction take part. However, due to the numerous problems involved, such as color formation efficiency, limits the properties after color formation, the costs, the manufacturing process, etc., and therefore it is difficult to obtain a dye precursor with excellent thermal responsiveness from them limited number. As a result, it seems best to get the This improves sensitivity in a heat-sensitive recording system. that one Selects color developing agents with good thermal responsiveness.

Have thorough studies on color developing agents with the aforementioned properties reveal that the one described in Japanese Patent Laid-Open No. 74 762/79 Benzyl p-hydroxybenzoate has excellent thermal response. Finds a similar disclosure can also be found in) P-OS 1 44 193/81.

Benzyl p-hydroxybenzoate is actually one for producing a highly sensitive heat-sensitive one Recording paper suitable color developer, however, have sheets that use of this ester are poor in image stability, and the image stability is one of the most essential requirements for a heat-sensitive recording sheet. Because of the easy sublimability benzyl p-hydroxybenzoate can also sublime on the recorded image and crystallize, resulting in a white haze and shadow on the picture (this phenomenon becomes referred to as fogging). Investigations into these parts of p-Hydroxybenzoic acid benzyl ester to eliminate the use as a dye developing agent, have now shown that the combined Use of the p-hydroxybenzoic acid benzyl ester and a p-benzyloxybenzoic acid benzyl ester, the has not previously been used as a dye developer, can markedly improve the image stability and the Prevents fogging and has sensitization.

In a process in which the benzyl p-hydroxybenzoate and benzyl p-benzyloxybenzoate together in the preparation of a When using heat-sensitive recording sheets according to the invention, the two esters are separated or at the same time pulverized and dispersed in the form of fine particles by a pulverizer. The dispersion obtained is then coated with a dye pulverized and dispersed in the same way.

precursors. Binder. Pigment and / or others Components mixed and, if necessary, you can also use a suitable additive or additives for Add preparation of coating color. This coating color is then applied to a support applied and dried to obtain the desired thermosensitive recording sheet.

Even the addition of small amounts of benzyl p-benzyloxybenzoate to benzyl p-hydroxybenzoate gives an improvement in image retention and prevents fogging, but the effect is greater when the amount is increased to 2% by weight and this effect is further increased if the amount added is increased to 5% by weight. The amount of p-Benzyloxybenzoic acid benzyl ester to be added should, according to the invention, preferably be 10% by weight or be more and in particular 15% by weight or more, with 20% by weight or more being particularly preferred.

The relationship between the amount and the effect of the ester will be more clear from the following facts. The addition of the ester in an amount of 2% by weight or more already brings about a sufficient prevention of fogging compared with the prior art, the addition of 15% by weight or more practically excluding a fogging effect and the addition of 20 Csw .-% or more prevents the fogging effect completely. The image retention rate is approximately 70% when not adding the p-Benzyloxybenzoesäurebenzyiesters after standing for 24 hours at 6O ⁰ C, while after the addition of the ester in an amount of 2 wt .-% or more of the image retention rate under the gleiche-. Conditions is increased to about 80%. The image retention rate is increased to around 90% when the ester is added in an amount of 15% by weight or more, and it exceeds 95% when the ester is added in an amount of more than 20% by weight.

It should be noted, however, that the addition of the benzyloxybenzoate p-benzyloxybenzoate in an amount of more than 60% by weight to the benzyl p-hydroxybenzoate gave no further improvement in terms of fogging and image retention. In addition, adding too much of the benzyl p-benzyloxybenzoate increases the undesirable phenomena described below: The transferred heat energy is consumed, and since the ester itself does not take part in color formation, it has a thinning effect and reduces the color density of the printed Parts and also causes sticking. Due to these facts, ^ ^0, and the cost, the amount should be added to the p-hydroxybenzoic acid benzyl ester p-Benzyloxybenzoesäurebenzylesters preferably not more than 100 wt .-%, particularly not more than 70 wt .-% and most preferably not more als60Gew .-% amount .

It was also found that you are special good results with the combination of p-benzyloxybenzoic acid benzyiester obtained with p-hydroxybenzoic acid benzyl ester when one p-benzyloxybenzoe- eo benzyl ester beforehand into the fine particles of benzyl p-hydroxybenzoate instead of powdering and pulverizing both esters separately or simultaneously disperse, because in the former case you have an equal or achieved even greater effect than in the latter case (mixing the particles) with a smaller amount.

The second method for combined use composed of p-benzyloxybenzoic acid benzylic acid with benzyl p-hydroxybenzoate in the preparation of the heat-sensitive of the present invention The recording sheet consists in dividing the former ester into the fine particles of the second Esters included.

After the first addition process, in which particles from both esters are mixed, one achieves one sufficient anti-fogging effect in practical use, when using benzyl p-benzyloxybenzoate in an amount of 2% by weight or more, but in the second method, in the benzyl p-benzyloxybenzoate was previously included in p-hydroxybenzoic acid benzyl ester, a satisfactory prevention of Fogging can already be achieved if you add the benzyl p-benzyloxybenzoate in an amount of 0.5% by weight or more. In the former method, 15% by weight or more can be used p-Benzyloxybenzoic acid benzyl ester essentially avoided the formation of fog, while in the latter If only 0.2% by weight or more of the ester is added, the same effect can be achieved. in the the former case, 20 wt .-% or more can inhibit fogging very well, while in the in the latter case, the same effect is achieved with 10% by weight or more.

With respect to the image retention degree after standing for 24 hours at 6O ⁰ C is obtained as a degree of 80% by the addition of 2 wt .-% or more in the former case, however, even where m is the same level may also be achieved by the addition of 03 wt .-% in the latter case. An image retention level of about 90% is achieved in the former case by adding 15% by weight or more and in the latter case by adding 5% by weight or more.

The developed color density when 20% by weight or more is added by the first method is 1. 2 and in the latter method one can get the same level of color density by adding before. 5 Gev .- '/ o or more achieve.

From this it follows that one. if you have the p-Benzyloxybenzoic acid benzyl ester previously included in the p-hydroxybenzoic acid benzyl ester. the effect of the present invention is increased and one also has an equal or even greater effect in the Compared to the mixing obtained even if the amount of benzyl p-benzyloxybenzoate was high is decreased.

There are several methods to p-Benzyloxybenzoic acid benzyl ester to be included in benzyl p-hydroxybenzoate. For example, you can do both First melt the ester and mix it in the heat, then allow it to solidify and pulverize. Man You can also first dissolve them in a solvent that is suitable for both and then a nonsolvent or add a solvent that dissolves only to a small extent in order to precipitate them out of the mixture. It is It is also possible to produce a molten mixture of the two esters by first adding benzyl p-benzyloxybenzoate forms as a by-product in the production of benzyl p-hydroxybenzoate and then precipitating both esters on a molten mixture or by making a partially eutectic mixture forms.

Those used in the manufacture of the heat-sensitive recording sheet of the present invention The most important components are explained below, although this explanation is only an example

wise isL

(1) dye precursors

One can use any of the dye precursors used in the manufacture of thermosensitive paper be e.g. B .:

Crystal violet lactone,

3-Diethylamino-7-methylfluoran, S-Diechylamino-e-chloro ^ -methylfluoran, S-Diethylamino-ö-methyl ^ -chlorofluoran, 3-Diethylamino-7-anilinofluorane, 3-Diethylamino-7- (2-chloroanilino) fluoran.

3-dibutylamino-7- (2-chloroanilino) fluorane, 3-Diethylamino-7- (3-chIoroanilino) fluoran, S-Diethylamino-ö-melhyl ^ -anilinofluoran, 3- (N-Ethyl-P-toIuidino) -6-methyl-7-aniiino-

fluoran,
3- (N-methylcyclohexylamino) -3-methyl-

7-anilinofluoran,
S-piperidinoO-methyl ^ -aniünofluoranetc.

(2) binder

Various types of starch, hydroxyethyl cellulose, methyl cellulose, polyvinyl alcohol, styrene-maleic anhydride copolymers, Styrene-butadiene copolymers, polyacrylamide, etc. can be used as binders in the present invention can be used.

(3) pigment

Diatomaceous earth, talc, kaolin, calcined kaolin, potassium carbonate, magnesium carbonate, titanium oxide, zinc oxide, silicon exide, Aluminum hydroxide, urea-formaldehyde resin etc. can be used as the pigment in the present invention can be used.

The invention is described below in the examples and comparative examples.

Example I.

(1) Solution A (dispersion of

p-hydroxybenzoic acid benzyl ester)

p-Hydroxybenzoic acid benzyl ester 25% aqueous solution of the
Sodium salt of styrene-maleic anhydride copolymer water

The aforementioned materials are and in a ball mill 48 h to form a Solution A dispersed.

(2) Solution B (dispersion of
p-Benzyloxybenzoic acid benzyl ester)

p-Benzyloxybenzoic acid benzyl ester 175 g of 10% strength aqueous solution of the
Ammonium salt of a styrene-maleic anhydride copolymer 87.5 g

Water 237.5 g

The aforementioned materials are pulverized and dispersed in a ball mill for 48 hours to form a solution B.

(3) Solution C (dispersion of the dye precursor)

3-Dicthylarnino-6-methyl-

7-anilinofluor.ir 200 g

Malon MS-25 24 g

Water 276 g

The aforementioned materials are pulverized and dispersed for 4½ hours in a ball mill to obtain a solution C.

Using the three solutions A, B and C, a thermosensitive recording color was obtained the following composition:

Parts Potassium carbonate 6th Solution a 14.3 Solution b 5.7 Solution C 5 25% aqueous solution of the sodium salt of styrene Maleic anhydride copolymer 15th water 29

This coating color was coated on a base paper having a basis weight of 50 g / m ² , the coating weight after drying being 5.6 g / m ² , and the base paper thus coated was treated in a supercalender to obtain a heat-sensitive paper a Bekk smoothness of approximately 400 s.

Examples 2 to 10

Coating colors were prepared in the same way as in Example 1, except that the amount of Solution B, as shown below, was changed and these coating colors were applied to the respective base papers so applied "that the coating weight after drying the following Had values. This was followed by supercalendering to obtain the heat-sensitive papers according to Examples 2 to 10.

40 Example 2 Amount of Coating weight 45 Example 3 Solution b 175g Example 4 (Parts) ' (g / m ² ) Example 5 0.3 5.0 35 g Example 6 0.7 5.1 290 g Example 7 1.4 5.1 like example 8 2.1 5.3 pulverized Example 9 2.9 5.3 Young one Example 10 8.6 5.9 10.0 6.1 12.1 6.3 14.3 6.5

Comparative example 1

A coating color was prepared as in Example I except that Solution B was not used, and this coating color was applied to a base paper having a coating weight after drying of 5.0 g / m ^{2 to} obtain a comparative heat-sensitive paper.

Experimental example 1

The heat-sensitive papers of Examples I to 10 and that of Comparative Example 1 were combined in one commercially available thermal paper device and then tested as shown below.

Exams:

(a) Color density

Measured with a Sakura densitometer PDA-45 using an amber filter.

(b) Degree of image retention after storage at 60 ° C for 24 hours.

(c) Degree of image retention after storage for 24 hours at 40 ° C and 90% relative humidity. The degrees of image retention of (b) and (c) were calculated using the following formula:

Degree of image retention (%) = ^{density after the experiment} χ loo
Density before trying

(d) Degree of fogging after standing in a room for 2 days.
X: strong

Δ: easy

O: very little

©: not available.

The test results are shown in Table I below.

Table I.

A * ¹ test test - χ inn test test (%) (a) (b) (c) (d) Comparative example 1 0 1.12 76.2 74.4 X Example 2 2 1.13 80.1 76.5 Δ Example 3 5 1.14 83.4 80.1 Δ Example 4 10 1.15 85.6 84.2 Δ Example 5 15th 1.18 93.2 90.8 O Example 6 20th 1.22 95.1 92.3 © example 1 40 1.24 97.6 95.0 © Example 7 60 1.24 98.0 95.5 © Example 8 70 1.24 98.0 95.5 © Example 9 85 1.23 97.8 95.5 © Example 10 100 1.20 97.8 95.0 © ^ ₁ amount of benzyl p-benzyloxybenzoate used

Amount of benzyl p-hydroxybenzoate used

The table above shows that the I3I dissolved.6g of water. The obtained dispersion was Image retention by the combined application of powdered and dispersed in a ball mill for 48 h p-Benzyloxybenzoic acid benzyl ester, which has so far not been obtained with a solution D.

was used as a color developer, with p-hydroxy Using the thus obtained solution D and

befizoesäurebenzyles ^ r, a known color developer 50 of the solution C used in Example 1 was a curler, after the first addition process, significantly heat-sensitive coating color is subsequently improved. It is found that to achieve the composition prepared: an image retention rate of 90% or more
benzyl p-benzyloxybenzoate in an amount
of 15% by weight or more based on p-hydroxybene-55
benzyl ester, must use. It was too
a significant awareness raising effect and a
considerable anti-fogging effect
determined by the combined use of the p-benzyloxybenzoic acid benzyl ester. 00

Example 11

2.0 g of p-Benzyioxybenzoic acid benzyl ester were The coating paint was on a base paper with

mi? 4.0 g of p-hydroxybenzoic acid benzyl ester applied in a basis weight of 50 g / m ² , the

Heat melted and with stirring to a b5 coating after drying was 5.1 g / m ² and so

given aqueous solution that had been obtained, coated base paper was supercalendered under

by 8.4 g of 25% aqueous solution of sodium formation of a heat-sensitive paper with a

Salt of the styrene-maleic anhydride copolymer in Bekk smoothness of approximately 400 s.

Parts Potassium carbonate 6th Solution D 22.8 Solution C 5 25% aqueous solution of the sodium salt of styrene Maleic anhydride copolymer 15th water 29

ίο

Examples 12 to

Over / .iig colors were applied in the same way as in Example 11 prepared, but with the amounts; in p-BenzyloxybenzoesiHircbenzylestcr (2.0g in example II) and p-Hydronybzoic acid benzyl ester (40.0 g in Example II) as shown in the table below were changed, and where the solutions D in the

Table II

amounts given in the table below are used together with solution C of Example I. became. The same procedure was used as in the example, but with the coating weight (5.1 g / m- in Example II) as shown in the table, was changed. Heat-sensitive recording papers according to Examples 12 bis were obtained in this way

p-benzyloxybenioe- p-hydroxybenzoic Men acid benzyl ester acid benzyl ester (B) («) (Part 0.2 41.8 21.8 0.4 41.6 22.0 0.8 41.2 22.2 3.8 38.2 23.9 5.5 36.5 25.0 7.0 35.0 26.1 12.0 30.0 30.4 15.7 26.3 34.8 17.3 24.7 36.9 19.3 22.7 40.2 21.0 2t, 0 43.5

Example 12 Example 13 Example 14 Example 15 Example 16 Example 17 Example 18 Example 19 Example 20 Example 21 Example 22

5.0 5.0 5.1 5.2 5.3 5.3 5.6 5.9 6.1 6.3 6.5

Experimental example 2

The heat-sensitive recording sheets of Examples 11 to 22 were used in a commercially available thermal copier and then subjected to the same tests as in Experimental Example 1, the results shown in Table III were obtained.

Table III

* ¹

Test (a)

test (b)

test (c)

Test (d)

Example 12 0.5 1.13 80.2 77 Δ I. Example 13 1 1.14 82.5 79 Δ Example 14 2 1.17 85.8 82.5 O Example 11 5 1.22 91.3 88.6 O i Example 15 10 1.28 93.7 92.5 ® I. Example 16 15th 1.30 95.7 94.4 ® 5 Example 17 20th 1.31 96.2 95.8 ® Ϊ Example 18 40 1.31 96.8 96.3 ® Example 19 60 1.31 96.8 96.3 ® % Example 20 70 1.30 96.5 95.9 © Example 21 85 1.28 96.3 95.0 © Example 22 100 1.25 96.2 94.5 © ti '
I. ", Lot of the p-benzyloxybenzoic acid benzyl ester used _v . " I.
ψ

Amount of benzyl p-hydroxybenzoate used

π 12

From Table III it can be seen that when a benzyl ester is added in the form of a molten mixture with

Smaller amount of benzyl-p-benzyloxybenzoic acid benzyl-p-llydroxybenzoate. A very minor one

; .. i esters more pronounced effects in terms of the emp- Schleicrffekt is at 2 wt .-% addition, and the

'Sensitivity, the image retention and the prevention of the fogging effect is completely due to IO wt .-%

> Schlcierbildung achieved if the ester is inhibited in 5 p-benzyloxybenzoic acid benzyl ester. The access

',''■ p-Hyclroxybenzoic acid benzyl ester incluclicrt instead. if there were 10% by weight or more of p-benzyloxybcnzoe-

i | that both esters are added separately (Table I). acid benzyl ester is sufficient to

i'fi ■ The veil problem is prevented by achieving a degree of 90% or more according to test (c).
^r .i addition of 0.5% by weight of p-benzyloxybenzoic acid

Claims (5)

Patent claims: 1. A thermosensitive recording sheet containing on a support a coating of a normally colorless or light colored one Dye precursors and p-hydroxybenzoic acid benzyl ester as color developers, characterized in that that in combination with the dye developer p-Benzyloxybenzoic acid benzyl ester present 2. Heat-sensitive recording sheet according to claim 1, characterized in that the Benzyloxybenzoic acid benzyl ester in an amount of 2 to 100% by weight, based on p-hydroxyben- ise benzyl benzoate. 3. Heat-sensitive recording sheet according to claim 1, characterized in that the Benzyloxybenzoic acid benzyl ester in the fine particles of the p-hydroxybenzoic acid benzyl ester is incuded. 4. Heat-sensitive recording sheet according to claim 3, characterized in that the Benzyloxybenzoic acid benzyl ester in the fine particles of the p-hydroxybenzoic acid benzyl ester is incised by heat fusion. 5. Heat-sensitive recording sheet according to claim 3, characterized in that p-Benzyloxybenzoic acid benzyl ester in an amount of 2 to 60% by weight, based on p-hydroxybenzoic acid benzyl ester, has been admitted.