DE2134114C3 - Pressure sensitive recording papers - Google Patents

Description

The invention is concerned and particularly relates to pressure sensitive recording papers pressure-sensitive recording papers, their coating layer contains an ether derivative of an aromatic hydroxyl compound.

Pressure-sensitive recording papers, such as those used e.g. From U.S. Patents 2,712,507,2,730,465 and 2,730,457 are based on the application of microcapsules, which are a solution of a practically colorless organic compound, which is hereinafter referred to as color former, and an electron accepting adsorbing substance, hereinafter referred to as a color developer is included.

In the context of the description, a carrier with a raised on top of an upper arch is used Layer of microcapsules, which contain a color former, with an interleaf, a carrier with a layer of microcapsules containing a color former, coated surface and with an opposite surface containing a layer of a color developer and as an under sheet Understood carrier with a coating of a color developer thereon. Furthermore, under a simple Sheet a carrier with a coating layer thereon made of microcapsules and at the same time understood by the color developer.

Examples of color formers are malachite green lactone, which is 3,3-bis - (- p-dimethylaminophenyl) phthalide, Benzoylleucomethylene blue, crystal violet lactone, the 3,3-bis - (- p-dimethylaminophenyl -) - 6-dimethylaminophthalide represents Rhodamine-B-lactam, 3-dialkylamino-7-dialkylaminofluorane and 3-methyl-2,2'-spirobi- (benzo [f] chromene).

On the other hand, the color developer capable of forming a color upon contact with the Color former is capable of an active clay substance, for example, acid clay, active clay, attapulgite, zeolite or bentonite or from acidic organic substances such as succinic acid, tannic acid, bile acid, Pentachlorophenol and phenolic resins exist.

The light resistance of the dye formed when the color former and the color developer are reacted depends on the structure of the color former as well as that of the color developer. The now im Color developers widely used are clays such as acid clays. The lightfastness of the formed Dye on a sub-sheet using this developer is, albeit a function of varies depending on the type of color former, generally poor, except for benzoylleucomethylene blue. Thus, when such a paper is left standing indoors or in sunlight is exposed, the density of the dye formed on the paper is slightly decreased or the Color is easily changed, so that the technical usefulness of the pressure-sensitive recording papers is greatly reduced.

The color former, which is used in the largest quantities as a standard item, consists of one Mixture of crystal violet lactone and benzoylleucomethylene blue. Since crystal violet lactone is a very If it has resistance to light, the dye image formed on the color developing sheet will be bleached off when left indoors or exposed to sunlight. Consequently All that remains is the light blue color due to the methylene blue and the technical value of the recording paper is thereby reduced. To make a black color or a green one Color is used 3-dibenzylamino-7-diethylaminofluoran, which is a green color on acidic clay which, however, turns reddish brown when standing indoors or when exposed to sunlight changes. Therefore, the black or green formed with 3-benzylamino-7-diethylaminofluoran takes Paint a reddish tinge when standing indoors or when exposed to sunlight, causing There is also a decrease in the technical value of the pressure-sensitive recording paper adjusts.

An object of the invention is to provide a pressure-sensitive recording paper with excellent Lightfastness.

The above objects are achieved by incorporating at least one ether derivative of an aromatic one Hydroxyl compound achieved in a coating layer for pressure-sensitive recording papers. Examples of ether derivatives of aromatic hydroxyl compounds which are used according to the invention can include

p-chloroanisole,

p-methyl anisole,

p-acetoanisole,

p-acetaminoanisole,

o-chloroanisole,

m-chloroanisole,

N - (- /? - cyanoethyl -) - p-anisidine,

p-methoxyacetophenone,

p-methoxybenzophenone,

2,2-Bi - (- p-methoxyphenyl -) - propane,

Catechin dimethyl ether,

Resorcinol dimethyl ether,

Hydroquinone dimethyl ether,

Catechin diethyl ether,

Resorcinol dipropyl ether,

Hydroquinone dibutyl ether,

ethyl p-methoxybenzoate,

p-ethoxybenzenesulionamide,

l-phenoxy-2-cyanoethane,

1-phenoxy-2-hydoxyethane,

l-phenoxy-2-chloroethane,

Diphenyl ether,

Hydroquinone dibenzyl ether,

l, 2-dimethoxy-4 - (- p-methoxyphenoxy -) - benzene,

1 ^ -Dimeihoxy ^ -octadecylbenzene,

H-a, a-DimethyIbenzylphenoxy -) - 4-ethoxy benzene,

l-ethoxy ^ .S-methylenedioxybeniol,

2,5-dimethoxybenzenesulfonamide,

l ^ dibutyl ^ S-dimethoxybenzoI,

1,4-diethoxyanthracene,

S-methyl-l ^ -diethoxyanthracene,

8-Chloi-l ^ -diethoxyanthracene,

9,10-dimethoxyanthracene,

4-methy! -9,10-dimethoxyanthracene,

T-chloro-QJO-dimethoxyanthracene,

l-methoxynaphthalene,

2-ethoxynaphthalene,

4-chloro-l-methoxynaphthalene,

5-methyl-l-methoxynaphthalene,

S-chloro ^ -ethoxynaphthalene and

ö-propyl ^ -ethoxynaphthalene.

These and similar ether derivatives of the aromatic hydroxy compounds can be contained in the microcapsules be encapsulated. In this case, the compounds can also be used with at least one color former the microcapsules are encapsulated. It is also possible to use these compounds in a color developer to add.

The suitable amount of the ether derivatives of the aromatic hydroxy compounds is 10 to 200 percent by weight, based on the weight of the color former used, or 0.2 to 10%. related to the weight of the color developer used. The microcapsules containing the color former can for example, by the method of U.S. Patent 2,700,458, but is the method of making the microcapsules is not limited to this as the invention is by incorporation of the ether derivative is marked.

The following examples serve to further illustrate the invention, all parts of which are based on are based on weight.

example 1

10 parts of an acid-treated pigskin gelatin and 10 parts of gum arabic were dissolved in 400 parts of water at 40 ⁰ C. 0.2 part of Turkish red oil was added as an emulsifier. 40 parts of a color former in which 3% of the hydroquinone dimethyl ether was dissolved were added to the solution, and the mixed solution was emulsified and dispersed. The Farbbildneröl consisted of a solution of 4 parts of diphenyl chloride and 1 part of kerosene, wherein 2 ° / ₀ of crystal violet lactone were dissolved.

When the average size of the oil droplets reached 5 microns, the emulsification was stopped. Water at 40 ^° C. was added to the emulsion to a total of 900 parts and stirring was continued. It is important to ensure that the temperature is not lowered below 4O ⁰ C. Then, 10 ° / ₀ acetic acid to adjust the pH of the liquid was added to 4.0, which initiates the coacervation. The mixture was stirred for a further 20 minutes and then the mixture was cooled with ice water to gel the coacervate films deposited around the oil droplets. After the temperature of the liquid was 20 ° C, 7 parts of 37% formaldehyde were added. After the temperature had dropped to 10 ⁰ C, an aqueous 15 ° / _o sodium hydroxide solution to adjust the pH-value was added in. 9 The sodium hydroxide was added extremely carefully so that the system did not coagulate. With continued stirring, the liquid was heated to 5O ⁰ C. It was cooled to 3O ⁰ C, after which the resulting microcapsule liquid onto a paper having a unit weight of 40 g / m ² in an amount of 6 g / m ^2, as a solid content, raised and dried so that a top sheet of a pressure-sensitive recording paper was obtained.

Example 2

In 40 parts of a color former oil, which consisted of a solution with 2% crystal violet lactone in an oil of 4 parts diphenyl chloride and 1 part kerosene, 3%, based on the color former oil, of N - (- / 3-cyanoethyl -) - p- anisidine dissolved. The solution was encapsulated in microcapsules in the same manner as in Example 1. The obtained microcapsule liquid was coated on a paper having a unit weight of 40 g / m ² in an amount of 6 g / m ^{2 in} solid content and dried to obtain an upper sheet of a pressure-sensitive recording paper.

Example 3

3 percent by weight, based on the oil, of p-methyl anisole were dissolved in 40 parts of a color former oil which had been obtained by dissolving 2% crystal violet lactone in an oil composed of 4 parts diphenyl chloride and 1 part kerosene. The solution was encapsulated in microcapsules in the same manner as in Example 1. The obtained microcapsule liquid was drawn up on a paper having a unit weight of 40 g / m ² in an amount of 6 g / m ^{2 in} solid content and dried to obtain an upper sheet of a pressure-sensitive recording paper.

Comparative example 1

40 parts of a color former oil, which had been obtained by dissolving 2% of crystal violet lactone in an oil of 4 parts of diphenyl chloride and 1 part of kerosene, were encapsulated in microcapsules in the same manner as in Example 1. The obtained microcapsule liquid was drawn up on a paper having a unit weight of 40 g / m ² in an amount of 6 g / m ^{2 in} solid content and dried to obtain an upper sheet of a pressure-sensitive recording paper.

To carry out the comparative tests with the upper sheets obtained according to Examples 1 to 3 and Comparative Example 1, a lower sheet was used manufactured in the following way:

Method of making a sub-arch

8 parts of 20 ° / ₀ sodium sodium hydroxide were added to 300 parts of water, and with stirring, 100 parts of an acid clay was added gradually.

After thorough stirring, stirring was slowed down and 20 parts, as a solid content, of a slyrole butadiene rubber latex was gradually added to obtain a coating solution. The coating solution was coated on a base paper having a unit weight of 40 g / m ² in an amount of 10 g / m ^{2 in} solid content and dried to obtain a base sheet for pressure-sensitive recording papers for experimental purposes.

Comparative experiment 1

Each top sheet according to Examples 1, 2 and 3 and Comparative Example 1 was placed on the bottom sheet and a pressure of 600 kg / cm ^{2 was applied} to form a color. After the specimen had been left to stand in the dark for 1 hour, the spectral absorption curve (fresh) was determined in a wavelength range from 700 to 380 μm with the developed color. The results are shown in Fig. 1, (1) to (4).

The determination of the spectral absorption curve was carried out using a spectrophotometer. The lightfastness values were obtained in the following way:

Lightfastness =

Density at the absorption maximum after exposure to sunlight fresh density at the absorption maximum

100.

The results are shown in Table I below.

Table I.

Lightfastness values of crystal violet lactone at the absorption maximum after exposure

of sunlight

One hour Three hours Designation Suspension on Suspension on tion sunlight sunlight according to C / o) ( ⁰ A.) Fig. 1 Example 1.. 65.1 41.3 (D Example 2 .. 59.1 32.6 (2) Example 3 .. 54.2 32.1 (3) Comparison example 1 .. 46.4 21.6 (4)

In Fig. 1 shows the fresh density with A, the density after exposure during one hour of sunlight with B and the density after exposure to sunlight during 3 hours shown with C, the same also being shown on the F i g. 2 to 6 apply.

From the above values it follows that when the ether derivative is used, the aromatic Hydroxyl compound an increase in light resistance from crystal violet lactone.

Example 4

In 40 parts of a color former oil, which by dissolving 2% 3-methyl-2,2'-spirobi- (benzo [f] -chromen) had been obtained in an oil of 4 parts of diphenyl chloride and 1 part of kerosene, 3%, based on the oil, dissolved in hydroquinone dimethyl ether. The solution was made in the same way as encapsulated in microcapsules in example 1. The obtained microcapsule solution was on a paper with . a unit weight of 40 g / m * in an amount of 6 g / m * as solids content and dried and a top sheet for pressure-sensitive recording paper is obtained.

Example 5

In 40 parts of a color former oil, which by dissolving 2% 3-methyl-2,2'-spirobi- (benzo [f] -chromen) had been obtained in an oil of 4 parts of diphenyl chloride and 1 part of kerosene, 3%, based on the oil, on N - (- /? - CyanoethyI -) - p-anisidine solved. The solution was encapsulated in microcapsules in the same manner as in Example 1. The received Microcapsule liquid was put on a paper having a unit weight of 40 g / m * in an amount of 6 g / m * as solids content drawn up and dried and an upper sheet of a pressure-sensitive Recording paper formed.

Example 6

In 40 parts of a color former oil, which by dissolving 2 ⁰ I ₀ 3-methyl-2,2'-spirobi- (benzo [f] chromo-

men) had been obtained in an oil of 4 parts of diphenyl chloride and 1 part of kerosene, 3%, based on the oil, dissolved in p-methyl anisole. the Solution was encapsulated in microcapsules in the same manner as in Example 1. The obtained micro-

ao capsule liquid vurde mounted as a solid content and dried to obtain an upper sheet of a pressure-sensitive recording paper on a paper having a unit weight of 40 g / m ² in an amount of 6 g / m ^z.

Comparative example 2

40 parts of a color former oil obtained by dissolving 2% of 3-methyl-2,2'-spirobi- (benzo [f] chromene) in an oil of 4 parts of diphenyl chloride and 1 part of kerosene were used in the same manner as encapsulated in microcapsules in example 1. The obtained microcapsule solution was coated on a paper having a unit weight of 40 g / ^m2 in an amount of 6 g / m2 * in solid content and dried to obtain an upper sheet of a pressure-sensitive recording paper.

Comparative experiment 2

Each of the top sheets obtained in Examples 4, 5 and 6 and Comparative Example 2 was placed on a test bottom sheet and a pressure of 600 kg / cm ^{8 was applied} to develop the color. After the paper had been left to stand in the dark for one hour, the spectral absorption curve (fresh) of the developed color image was determined in a wavelength range from 700 to 380 μm. The determination was also carried out after the test pieces were exposed to sunlight for one hour and 3 hours. The results are shown in Fig. 2, (1) to (4). The light resistance values were determined in the same manner as in Comparative Experiment 1. The results are shown in Table II.

Table II

Light resistance values of 3-methyl-2,2'-spirobi- (benzo [f] chronu: n) at the absorption maximum after exposure to sunlight

6o One hour
Suspension on
sunlight
(7o) Three hours
Suspension on
sunlight
(%) Designation
tion of the
Fig. 2 Example 4 ..
S ₅ example 5 ..
Example 6 ..
Comparative
r cexample 2 .. 87.6
86.5
78.6
73.5 83.8
84.7
75.4
67.2 (D '
(2)
(3)
(4)

It can be seen from the above values that the light resistance of 3-methyl-2,2'-spirobi- (bcnzo [f] chromene) with the use of the ether derivative of the aromatic hydroxyl compound is increased and its color changes very little.

Example 7

In 40 parts of a Farbbildneröles which ⁰ "3-dibenzylamino-7-diäthylaminofluoran was obtained in an oil of 4 parts of diphenyl chloride and 1 part of fuel by dissolving 5 /, 3%, based were dissolved in the oil to Hydrochinondimethyläther. The solution was encapsulated in microcapsules in the same manner as in Example 1. The obtained microcapsule liquid was coated on a paper having a unit ^{weight of 40 g / m 2} in an amount of 6 g / m ^{2 in} solid content and dried to obtain a top sheet for pressure-sensitive recording paper.

Example 8

In 40 parts of a Farbbildneröles which ° ₀ 3-dibenzylamino-7-diäthylaminofluoran was obtained in an oil of 4 parts of diphenyl chloride and 1 part of fuel by dissolving 5 /, were 3%, based on the oil, of N - (- / J-cyanoethyl -) - p-anisidine dissolved. The solution was encapsulated in microcapsules in the same manner as in Example 1. The obtained microcapsule liquid was coated on a raw paper having a unit weight of 40 g / m ² in an amount of 6 g / m ^{2 in} solid content and dried to obtain a top sheet for pressure-sensitive recording paper.

Example 9

3%, based on the oil, of p-methylanisole were dissolved in 40 parts of a color former oil which had been obtained by dissolving 5 "/ o 3-dibenzylamino-7-diethylaminofluoran in an oil composed of 4 parts of diphenyl chloride and 1 part of kerosene The solution was encapsulated in microcapsules in the same manner as in Example 1. The obtained microcapsule liquid was ^{coated and dried on a paper having a unit weight of 40 g / m 2} in an amount of 6 g / m ^{2 in} solid content, and an upper sheet for obtained a pressure-sensitive recording paper.

Comparative example 3

40 parts of a color former, which had been obtained by dissolving 5% of 3-dibenzylamino-7-diethyiaminofluoran in an oil of 4 parts of diphenyl chloride and 1 part of kerosene, were encapsulated in microcapsules in the same manner as in Example 1. The obtained microcapsule liquid was coated on a paper having a unit weight of 40 g / m ² in an amount of 6 g / m ^{2 in} solid content and dried to obtain a top sheet for pressure-sensitive recording paper.

Comparative experiment 3

Each of the top sheets obtained in Examples 7, 8 and 9 and Comparative Example 3 was placed on a test bottom sheet and a pressure of 600 kg / cm ^{2 was applied} to develop the color. The paper was left to stand in the dark for 1 hour and then the spectral absorption curve (fresh) of the developed color in the wavelength range from 700 to 380 μm was determined. The spectral absorption curve of the paint after exposure to sunlight for 1 hour and 3 hours was also determined. The results are shown in FIG. 3, (1) to (4). The light resistance values were determined in the same manner as in Comparative Example 1. The results are summarized in Table III.

ίο Table III

Light resistance value of 3-benzylamino-7-diethyIaminofluoran at the absorption maximum
after exposure to sunlight

EinslUndigc Three hours tion in
Fig. 3 Suspension on Suspension on sunlight sunlight (D (7.) (7 ") (2) Example 7 .. 84.8 75.8 (3) Example 8 .. 84.1 76.1 Example 9 .. 83.5 72.6 (4) Comparison example 3 .. 69.1 59.9

The above values show that the light resistance of 3-dibenzylamino-7-diethylaminoHuoran likewise increased when the ether derivative of the aromatic hydroxyl compound is used will.

On the basis of the spectral absorption curves according to FIG. 3, (1) to (4), the shift in the absorption maximum (A ₁ ) on exposure to sunlight was determined, the values of which are listed in Table IV below.

Table IV

Shift in the absorption maximum A ₁ of 3-dibenzylamino-7-diethylaminofluoran on exposure to sunlight during the specified period

45 Fresher
worth
λ,
(Γημ) A ₁ after
one hour
Suspension on
sunlight
(ηιμ) λ, after
three hours
Suspension on
sunlight
(πιμ) Example 7 ..
Example 8 ..
Example 9 -.
50 comparative
example 3 .. 600
600
600
600 580
580
585
545 560
550
550
485

It can be seen from these values that the shift in the absorption maximum of 3-dibenzylamino-7-diethylaminofluoran considerably reduced when exposed to sunlight when using the ether derivative of the aromatic hydroxyl compound was. This means that there was only a slight change in color.

Compared to other color formers, the ether derivatives show the aromatic hydroxyl compound the effect of increasing the light resistance of the color formers and of preventing their discoloration.

Example 10

8 parts of 20% sodium hydroxide as a dispersant was added to 300 parts of dispersion water

609 617/394

ίο

was added, and while stirring, 100 parts of sour clay was gradually added. A solution of 2 g of Ilydroquinone dimethyl ether in 30 ecm of methanol was gradually added with stirring. Then the stirring was slowed down and 20 parts (solids content) of a styrene-butacliene compound was gradually added to form the coating solution. The coating solution was coated on a paper having a unit ^{weight of 40 g / m 2} in an amount of 10 g / m ² as a plastic content and dried, to obtain an under sheet for pressure-sensitive recording paper.

Comparative example 4

The same test sub-arches were used as in »5 Examples 1 to 9 and Comparative Examples I. to 3 used.

Upper sheet arch for experimental purposes

20th

For comparative tests with the lower arches, the upper arches of Comparative Examples 1, 2 and 3 were used.

Comparative experiment 4

The top sheet according to Comparative Example 1 was placed on top of a respective bottom sheet according to Example 10 and Comparative Example 4, and a pressure of 600 kg / cm ^{2 was applied} to develop the color. The paper was left to stand in the dark for 1 hour, and the spectral absorption curve (fresh) in a wavelength range from 700 to 380 μm was determined. The spectral absorption curve of the developed color after exposure to sunlight of the hydroquinone dimethyl ether in the presence of a color developer for one hour or 3 hours, the light resistance of 3-methyl case was determined. The results are shown in Fi g. 4, (1) ent-2,2'-spirobi- (benzo [f] chromene) improved and keep it. The result of comparative example 4 is the alteration v.rt; -j-. same as in comparative example 1. The light resistance

Same example 4 was applied and a pressure of 600 kg / cm ² was applied for the Karbenlwicklung. The paper was left to stand in the dark for 1 hour, after which the spectral absorption curves (fresh) were determined in a wavelength range from 700 to 380 μm. The spectral absorption curves of the developed color after exposure to sunlight for 1 hour and 3 hours, respectively, were also determined. The results are shown in FIG. 5, (1) reproduced. The light resistance values were calculated in the same manner as in Comparative Experiment 1. The results are shown in Table VI. The result in Comparative Example 4 was the same as in Comparative Example 2.

Table Vl

Light fastness value of 3-methyl-2,2'-spirobi- (benzo [f] chromene) to A at the absorption maximum of sunlight USPENSION

example

Comparative Example 4 One hour

Suspension on

sunlight

CL)
92.9

73.5 three hours

Suspension on

sunlight

CA,)
87.4

Figuring

the
drawing

Fig. 5 (1)

Fig. 2 (4)

Prevents discoloration.

w r ...... ι / ωιοιί-

Durability values were calculated in the same way as in Comparative Experiment 1. The results are in Table V summarized.

Table V
Lightfastness Value of Crystal Violet Lactone at

ility value of crystal violet lactone at Maximum absorption after exposure to sunlight

45 Comparative experiment 6

The top sheet according to Comparative Example 3, the microcapsules of B-Benzylamino ^ -diäthylaminofluoran

BeJn nn ^Urd ! ^{placed on a respective} sub-sheet according to Example 10 and Comparative Example 4 and a

£, «T ™ ° ^ ^cm * used to develop the color. The paper was in the dark for 1 hour

knrvwf ^lasS . \ ⁿ . ^{and da} ™ the spectral absorption curve (fresh) in a wavelength range from 700 to 380 mu. definitely. The spectral absorption curve of the developed color after exposure for 1 hour and 3 hours was also determined. The results are given in δ. The result of comparative Ί · K »u ^{ar the same as in} comparative example 3. The. Light resistance values were calculated in the same manner _as in Comparative Experiment 1. The results are listed in Table VI.

From the above values it is evident that Au ^"erschlebun g of the absorption maximum λ is in Tjk η vf ™ ^Sonnenlic ht, based on Fig. 6 in the Hydrochinondimethyläther in the presence of the dyes 60 developer, the light resistance of Cristal violet A _1!, J ^Wled given.

from the following values it follows that vnn, ^yd _i r? _u ^quinone dimethyl ether increases the resistance to light, nH ^ ^lbenz y ^lamin ° - ⁷ -diethylaminofluoran and significantly prevents its discoloration. .
The same issP ",;" U-. ^ ._ comparative tests 1 to 6,

One hour Three hours drawing 1 Suspension on Suspension on sunlight sunlight Fi g. CL) <%,) (1) 1 Example 10 55.0 32.1 Fig
(2) Comparison example 4 .. 46.4 21.6

j "j · -» ■ .— .u ^ VgmnoiiucsrarD-

developer improves the lightfastness of crystal violet lactone.

Comparative experiment 5

An upper arch, the microcapsules of 3-methyl-2,2'-spirobi- (benzo [f] chromene) contained, was on each a sub-sheet according to Example 10 and Ver

Table VII

Lightfastness value of 3-l) ibcnzylamino-7-diethylaminofluoran at the absorption maximum

Exposure to sunlight

üinhoursc Three hours Uczeichen- Suspension on Suspension on tion of the sunlight sunlight Character ("/") (7n) nings Example 10 80.2 71.4 F i g. 6th Comparative each example 4 .. 69.1 59.9 Fig. 3 (4)

Table VIII

Shift of the absorption maximum λ, from 3-Dibenzylamino-7-diethylaminofluoran on exposure to sunlight

10 Example 10
Comparative
example 4 .. Fresher
worth
(Γημ) A ₁ after
one hour
Suspension on
sunlight
(mti) A ₁ after
three hours
Suspension on
sunlight
(ιημ) 600
600 580
545 550
485

In addition 8 sheets of drawings

Claims (3)

Patent claims: 1. Pressure-sensitive recording paper that has a rail of microcapsules that form a Containing color formers, and containing a layer of a color developer, these layers are mounted on the same carrier or on different carriers, characterized in that, that at least one of these layers is an ether derivative of an aromatic hydroxyl compound contains, this ether derivative being the oil to be encapsulated or the color developer is incorporated. 2. Pressure-sensitive recording paper according to claim 1, characterized in that the amount of the ether derivative of ¹ O to 200 weight percent ¹ S based on the color former, is. 3. Pressure-sensitive recording paper according to claim 1, characterized in that the amount of the ether derivative is 0.2 to 10 percent by weight, based on the color developer. ^a0