JPH08272083A - Plate making method for planographic printing plate - Google Patents

Abstract

PURPOSE: To provide a plate making method by which a large amt. of printing plates can stably be produced over a long period of time without generating insoluble matter or gel. CONSTITUTION: In a plate making method in which a photosensitive planographic printing plate is imagewise exposed, developed with an alkali developer and rinsed with a rinsing soln. contg. an anionic surfactant and/or a nonionic surfactant, an acid and its salt (buffer) rinsing is carried out with plural rinsing baths, and in accordance with the progress of rinsing, the acid content of the rinsing soln. is increased and the buffer content is reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rinsing treatment performed on a lithographic printing plate obtained by subjecting a photosensitive lithographic printing plate to image exposure and development.

[0002]

2. Description of the Related Art A method is known in which a photosensitive lithographic printing plate is subjected to imagewise exposure and development, followed by rinsing with running water or rinsing water circulated, and further rinsing with a multi-stage rinsing consisting of a plurality of baths. However, when the developing solution brought into the washing bath is diluted with washing water, the components of the photosensitive layer which have been dissolved and dispersed in the developing solution are often precipitated to form insoluble matter. Especially in the method of circulating the washing water with an automatic processor,
These insoluble substances generated in the rinsing water often block the spray nozzle or adhere to pipes or tanks to cause inconvenience in circulation of the rinsing water. Further, the insoluble matter may adhere to the printing plate to cause stains on the non-image area.

On the other hand, a method of circulating and using a so-called rinse solution containing a surfactant and a pH buffering agent to perform a rinse treatment after development is described in JP-B-62-16200, but a large number of printing plates are used. When the treatment is carried out, insoluble matter is generated in the rinse liquid, and at the same time, problems such as printing stains due to stains on the non-image areas of the printing plate and defective ink replenishment occur. Further, JP-A-63-172270 discloses a method of performing a rinsing treatment with a plurality of rinsing baths and lowering the pH of the rinsing solution to a later stage. However, even with this method, the above-mentioned problems are still unsolved. Sufficient and limited throughput.

[0004]

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a plate-making method capable of stably producing a large amount of printing plates for a long period of time without producing insoluble matter or gel.

[0005]

The above object is to provide a rinse containing an anionic surfactant and / or a nonionic surfactant, an acid and a buffer after the photosensitive lithographic printing plate is imagewise exposed and developed with an alkaline developer. In the plate making method of a lithographic printing plate which is rinsed with a liquid, the rinse treatment is carried out in a plurality of rinse baths, and the content of the acid in the rinse liquid is made higher and the content of the buffer is made lower as the latter stage. It is achieved by the method of making a planographic printing plate. Hereinafter, the present invention will be described in detail.

Examples of the acid to be contained in the rinse liquid of the present invention include acetic acid, oxalic acid, tartaric acid, benzoic acid, molybdic acid, boric acid, nitric acid, sulfuric acid, phosphoric acid, and the like, and the buffer (acid salt) is used. Water-soluble alkali metal salts and ammonium salts of these acids, such as ammonium acetate, sodium acetate, potassium acetate, sodium molybdate, potassium molybdate, sodium borate, ammonium borate, lithium nitrate, sodium nitrate, potassium nitrate, primary phosphorus. Preferred are sodium phosphate, dibasic sodium phosphate, tribasic sodium phosphate, monobasic potassium phosphate, dibasic potassium phosphate, tribasic potassium phosphate, tribasic ammonium phosphate, sodium polyphosphate and the like. Details of the buffering agent are described in, for example, “Chemical Handbook Basic Edition II Revised 2nd Edition”, Maruzen, pages 1490 to 1499, edited by The Chemical Society of Japan, and these can be directly applied to the present invention.

The content of the acid and the buffer in the rinsing solution of the present invention (percentage by weight) is preferably 0.01 to 1% by weight in the acid contained in the rinsing solution of the first rinsing bath.
.About.0.5% by weight is more preferable, and the buffer is 0.1 to 10.
The range of wt% is preferable, 1 to 5 wt% is more preferable, and the acid contained in the rinse liquid of the last rinse bath is 0.1 to 5.
The range of 10 wt% is preferable, the range of 0.3-5 wt% is more preferable, the range of 0.01-2 wt% of the buffer is preferable, and the range of 0.05-1 wt% is more preferable.

As the anionic surfactant to be contained in the rinse liquid of the present invention, fatty acid salts, abithienoic acid salts, hydroxyalkanesulfonic acid salts, alkanesulfonic acid salts, dialkylsulfosuccinic acid ester salts, linear alkylbenzenesulfonic acid salts, Branched-chain alkylbenzene sulfonates, alkylnaphthalene sulfonates, alkylphenoxy polyoxyethylene propyl sulfonates, polyoxyethylene alkylsulfophenyl ether salts, N-methyl-N-oleyl taurine sodium salts, N-alkylsulfosuccinic acid monoamides Disodium salts, petroleum sulfonates, sulfated castor oil, sulfated beef leg oil, sulfates of fatty acid alkyl esters, alkyl sulfates, polyoxyethylene alkyl ether sulfates Tellurium salts, fatty acid monoglyceride sulfate ester salts, polyoxyethylene alkylphenyl ether sulfate ester salts, polyoxyethylene styrylphenyl ether sulfate ester salts, alkyl phosphate ester salts, polyoxyethylene alkyl ether phosphate ester salts, polyoxyethylene alkylphenyl Examples thereof include ether phosphate ester salts, partially saponified styrene-maleic anhydride copolymers, partially saponified olefin-maleic anhydride copolymers, and naphthalenesulfonate formalin condensates.

The nonionic surfactant contained in the rinse liquid of the present invention includes polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene polystyryl phenyl ether,
Polyoxyethylene polyoxypropylene alkyl ether, glycerin fatty acid partial ester, sorbitan fatty acid partial ester, pentaerythritol fatty acid partial ester, propylene glycol monofatty acid ester, sucrose fatty acid partial ester, polyoxyethylene sorbitan fatty acid partial ester, polyoxy Ethylene sorbitol fatty acid partial ester, polyethylene glycol fatty acid ester, polyglycerin fatty acid partial ester, polyoxyethylenated castor oil, polyoxyethylene glycerin fatty acid partial ester, fatty acid diethanolamide, N, N-bis-2-hydroxy Examples thereof include alkylamines, polyoxyethylene alkylamines, triethanolamine fatty acid esters, trialkylamine oxides and the like.

It is preferable that the rinse liquid of the present invention contains a lipophilic substance. As a result, the image area of the lithographic printing plate becomes more oil-sensitive, and not only the development ink buildup becomes easier, but when the plate surface protective agent treatment is carried out after the treatment with the rinse solution, A decrease in oil sensitivity can be strongly suppressed.

Preferred lipophilic substances include, for example, organic carboxylic acids having 5 to 25 carbon atoms such as oleic acid, lauric acid, valeric acid, nonyl acid, capric acid, mistyric acid and palmitic acid, castor oil and the like. Be done. These lipophilic substances can be used alone or in combination of two or more. The content of the lipophilic substance contained in the rinse liquid of the present invention is preferably 0.005% by weight to about 10% by weight, more preferably 0.05 to 5% by weight.

The rinse solution of the present invention further comprises sorbic acid and p-
Preservatives such as ethyl oxybenzoate, fungicide, propyl gallate, 2,6-di-t-butyl-4-ethylphenol, 2,6
-An antioxidant such as di-t-butyl-4-methylphenol may be included. By adding a small amount of these preservatives, antifungal agents, and antioxidants as preservatives, it is possible to prevent the rinsing liquid from deteriorating due to storage, but the preferable addition amount is 0.001 to 5% by weight.

Further, the rinse liquid of the present invention preferably contains an antifoaming agent such as an organic silane compound.

The photosensitive lithographic printing plate to which the present invention is applied is
Photosensitive lithographic printing plates of the type developed with an aqueous alkaline solution, for example, British Patent Nos. 1460978 and 15
Negative-type photosensitive material having a photosensitive layer made of a mixture of a diazo resin described in each specification of No. 05739 and a polymer containing a hydroxyethyl methacrylate unit or a hydroxyethyl acrylate unit as a main repeating unit on an aluminum plate. A lithographic printing plate and a positive-working photosensitive lithographic printing plate in which a photosensitive layer comprising an o-quinonediazide compound is provided on an aluminum plate are included. The plate-making method of the present invention is particularly applicable to the latter positive-working photosensitive lithographic printing plate. In the plate making method for producing a printing plate, since a particularly remarkable effect is exhibited, this aspect will be described in detail below.

The positive-working photosensitive lithographic printing plate used in the preferred embodiment of the plate-making method of the present invention is o-coated on a support.
A photosensitive layer comprising a quinonediazide compound, more preferably an o-naphthoquinonediazide compound, is provided.
A preferred support is an aluminum plate, which is grained, for example sodium silicate, potassium fluorozirconate,
Surface treatment such as immersion treatment in an aqueous solution of phosphate or the like and anodization treatment is preferable. The above-mentioned anodizing treatment is, for example, an inorganic acid such as phosphoric acid, chromic acid, sulfuric acid or boric acid, or an organic acid such as oxalic acid or sulfamic acid, or an aqueous solution or non-aqueous solution of these salts, or an electrolysis in which two or more kinds are combined. It is carried out by passing an electric current in a liquid using an aluminum plate as an anode. Also, US Pat. No. 365
The silicate electrodeposition as described in the specification of 8662 is also effective.

Further, JP-B-46-27481, JP-A-52-58602, and JP-A-52-30503.
It is also useful to use the above-mentioned anodizing treatment of a support which has been subjected to electrolytic graining as disclosed in Japanese Patent Laid-Open Publication No. 2003-242242.

The photosensitive layer provided on the hydrophilic surface of the support comprises an o-naphthoquinonediazide compound. Such o-naphthoquinonediazide compounds are described in many publications, and these can be used preferably. Among these, o-naphthoquinone diazide sulfonic acid ester or o-naphthoquinone diazide carboxylic acid ester of an aromatic hydroxy compound, and o-naphthoquinone diazide sulfonic acid amide or o-naphthoquinone diazide carboxylic acid amide of an aromatic amino compound are particularly preferable. Further, esterification reaction of o-naphthoquinonediazide sulfonic acid with a condensate of pyrogallol and acetone described in U.S. Pat. No. 3,635,709,
A polyester obtained by esterifying o-naphthoquinonediazide sulfonic acid or o-naphthoquinonediazide carboxylic acid with a polyester having a hydroxy group at the terminal described in U.S. Pat. No. 4,028,111; British Patent No. 1
Ester reaction of o-naphthoquinone diazide carboxylic acid or o-naphthoquinone diazide carboxylic acid with a homopolymer of p-hydroxystyrene as described in US Pat. No. 4,940,43 or a copolymer thereof with another copolymerizable monomer. It may be made to be.

These o-naphthoquinonediazide compounds can be used alone, but it is preferable to mix them with an alkali-soluble resin and provide this mixture as a photosensitive layer. Suitable alkali-soluble resins include novolak-type phenol resins, and specifically include phenol formaldehyde resin, o-cresol formaldehyde resin, m-cresol formaldehyde resin, and the like. Further, as described in JP-A-50-125806, together with the phenol resin as described above, a carbon number of 3 to 8 such as t-butylphenol formaldehyde resin is used.
It is even more preferable to use a condensate of formaldehyde and phenol or cresol substituted with an alkyl group of Alkali-soluble resin is about 50 in the photosensitive layer.
% To about 85% by weight, more preferably 60 to 80% by weight.

If necessary, the photosensitive layer comprising the o-naphthoquinonediazide compound may further contain additives such as dyes, plasticizers, and components that give printout performance. Further, as described in JP-A-52-80022, the sensitivity can be increased by adding a cyclic acid anhydride to the photosensitive layer.

The photosensitive composition comprising such o-naphthoquinonediazide is coated on a support from a solution of a suitable solvent. Suitable solvents include ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, glycol ethers such as 2-methoxyethyl acetate, acetone, methyl ethyl ketone, ketones such as cyclohexanone, and chlorinated hydrocarbons such as ethylene dichloride. .

The coating amount of the photosensitive layer comprising the o-naphthoquinonediazide compound provided on the support is from about 0.5 to about 7.
a g / m ^2, more preferably from 1.5~3g / m ^2.

When the positive type photosensitive lithographic printing plate thus obtained is exposed through a transparent original image by a light beam rich in actinic rays such as a carbon arc lamp, a mercury lamp, a metal halide lamp, a xenon lamp and a tungsten lamp, the portion is alkali-soluble. Turns into. Therefore, the exposed portion of the photosensitive layer is eluted by the alkaline aqueous solution, and the hydrophilic surface of the support is exposed.

The developing solution for the positive type photosensitive lithographic printing plate is an aqueous alkaline solution containing a silicate dissolved therein. Preferred silicates are those which show alkalinity when dissolved in water, and include, for example, alkali metal silicates such as sodium silicate, potassium silicate and sodium metasilicate, and ammonium silicate. The amount of silicate in the developer is generally about 1 to about 10% by weight, more preferably 1 to 8% by weight, most preferably 2 to 6% by weight, based on the total weight of the developer. It

The developer may be alkaline, and preferably has a pH at 25 ° C. of about 10.5 to about 13.5.
As the pH becomes lower than 10.5, the elution of the exposed photosensitive layer of the positive-working photosensitive lithographic printing plate becomes insufficient. On the other hand, as the pH becomes higher than 13.5, the required amount of acid in the aqueous solution containing the surfactant increases. Therefore, the most preferable pH is 12 to 13.5.

The developing solution may further contain an organic solvent in an amount of 5% by weight or less based on the total weight. Examples of the organic solvent include benzyl alcohol, 2-butoxyethanol, triethanolamine, diethanolamine, monoethanolamine, glycerin, ethylene glycol, polyethylene glycol, polypropylene glycol and the like.

The developer may further contain a surfactant. As a result, the developer can be well wetted on the surface of the photosensitive layer of the photosensitive lithographic printing plate, and the processing ability of the developer (the amount of the photosensitive layer capable of dissolving and removing a unit volume of the developer) can be improved. The range of development conditions (such as temperature and processing time) that gives optimum results can be broadened. Preferred such surfactants include anionic and amphoteric surfactants. Specific preferred examples of the anionic surfactant include alkylbenzene sulfonates such as sodium dodecylbenzenesulfonate (wherein the alkyl group has 8 to 18 carbon atoms, more preferably 12 to 16 carbon atoms), for example, isopropylnaphthalene sulfone. Alkylnaphthalene sulfonates such as sodium acid salt (wherein the alkyl group has 3 to 1 carbon atoms)
0), formalin condensates of naphthalene sulfonate, dialkyl sulfosuccinates (wherein the alkyl group has 2 to 18 carbon atoms), dialkyl amide sulfonates (wherein the alkyl group has 11 to 17 carbon atoms), etc. Be done. Preferred specific examples of the amphoteric surfactant include imidazoline derivatives,
For example, N-alkyl-N, N, N-tris (carboxymethyl) ammonium (wherein the alkyl group has 12 to 12 carbon atoms).
18), N-alkyl-N-carboxymethyl-N, N
-Betaine type compounds such as dihydroxyethylammonium (the number of said alkyl groups is 12 to 18) are included. The amount of such a surfactant used is not particularly limited, but is generally about 0.003 to about 3% by weight, more preferably 0.00.
It is contained in the developer at a concentration of 6 to 1% by weight.

The developer may further contain an antifoaming agent. Suitable defoamers include US Pat. No. 32,507.
No. 27, No. 3545970, and British Patent No. 13829.
No. 01, No. 1387713 and the like are compounds described in each specification. Among these, organosilane compounds are preferable.

It is desirable that the developer has good wettability with respect to the surface of the photosensitive layer of the photosensitive lithographic printing plate. In particular, the contact angle of the developer with respect to the surface of the photosensitive layer of the photosensitive lithographic printing plate is preferably 85 ° or less as measured by the water drop in air method.
It is more preferably 60 ° or less. When the present invention is carried out using a developer having a contact angle of more than 85 °, the ink tends to be unevenly attached to the image area of the lithographic printing plate which should be uniformly attached. Therefore, it is difficult to obtain a good printed matter from such a lithographic printing plate. In order to obtain a developing solution having a low contact angle with the surface of the photosensitive layer of the photosensitive lithographic printing plate, the above-mentioned surfactant is contained in the developing solution.

As a method for developing the imagewise exposed photosensitive lithographic printing plate with the above-mentioned developing solution, various conventionally known methods can be used. Specifically, a method of immersing an image-exposed photosensitive lithographic printing plate in a developing solution, a method of spraying the developing solution from a large number of nozzles onto the photosensitive layer of the photosensitive lithographic printing plate, the developing solution is wet. Examples thereof include a method of wiping the photosensitive layer of the photosensitive lithographic printing plate with the formed sponge, a method of roller coating a developing solution on the surface of the photosensitive layer of the photosensitive lithographic printing plate, and the like. Further, after the developing solution is applied to the photosensitive layer of the photosensitive lithographic printing plate in this manner, the surface of the photosensitive layer can be lightly rubbed with a brush or the like. The developing conditions can be appropriately determined by those skilled in the art according to the above developing method. As an example, in the developing method by immersion, for example, it is immersed in a developing solution at about 15 to 35 ° C. for about 20 to 80 seconds.

The lithographic printing plate obtained by imagewise exposing and developing the photosensitive lithographic printing plate as described above is preferably squeezed so that the amount of the developing solution on the plate surface is reduced. In the present invention, the amount of the developing solution remaining on the plate surface of the lithographic printing plate after squeezing is 10 ml /
It is preferably m ² or less, and the amount is preferably as small as possible. Therefore, the residual amount of the developing solution is more preferably 5 ml / m ² or less, and most preferably 3 ml / m ^2.
m ² or less. Volume after squeezed is 10ml /
When it is more than m ² , the rinse solution deteriorates more quickly, and the processing capacity thereof deteriorates earlier.

As a method of squeezing the plate surface of the planographic printing plate, the planographic printing plate is passed between elastic roller pairs in which elastic members such as rubber are coated on the roller surface, and the developing solution for the plate print is removed by the nip pressure. It is possible to employ a method, or a method in which an elastic plastic material having a smooth surface is arranged along the conveyance path of the lithographic printing plate and is brought into sliding contact with the plate surface to scrape off the developing solution of the print.

The lithographic printing plate thus obtained by development is treated with the treating agent of the present invention. A specific treatment method is to pour an appropriate amount of the above aqueous solution on the plate surface, rub it with a sponge so as to coat the entire plate surface, immerse in a container filled with the aqueous solution, and apply the aqueous solution with a roller. A method, a method of spraying the above aqueous solution on the plate surface, or the like can be used.

[0033]

EXAMPLES Examples of the present invention will be described below together with comparative examples.

Example 1 Novolak type cresol formaldehyde resin 1, 2
1 part by weight of naphthoquinone-2-diazide-5-sulfonic acid partial ester and 2 parts by weight of novolac type cresol formaldehyde were dissolved in 20 parts by weight of ethylene glycol monomethyl ether to prepare a photosensitive solution. Thickness 0.
Approximately 2 g / m ² of oxide film is formed on a 3 mm grained aluminum plate by electrolytic anodic oxidation, thoroughly washed and dried, and then the above photosensitive solution is applied and dried to obtain approximately 2 g / m 2.
A photosensitive lithographic printing plate having a photosensitive layer of m ² was obtained. The printing plate was exposed using a halftone dot positive film.

Next, development processing was carried out by using a developing solution and a rinsing solution as shown below and the automatic developing machine described in the Example of JP-A-63-172270.

(Developer) A developer having the following composition (pH
12.8) 21 l was charged into a developing bath, and 1030 mm x 8
Every time a 00 mm printing plate was processed, a developing replenisher having the following composition was replenished by 40 ml.

Potassium silicate A (SiO ₂ 26%, K ₂ O 13.5%) 80 parts by weight 86% Potassium hydroxide 50 parts by weight Water 326 parts by weight (Development replenisher) Potassium silicate A (SiO ₂ 26% , K ₂ O 13.5%) 100 parts by weight 86% potassium hydroxide 50 parts by weight Water 350 parts by weight (first rinse solution) A rinse solution (pH 5.
8) Charge 8l into the first rinse bath, 1030mm x 80
Every time a 0 mm printing plate was processed, 30 ml of the rinse liquid was replenished. (The storage tank of the first rinse bath overflows at 8 liters) Sodium polyoxyethylene lauryl ether sulfate 2 parts by weight 85% phosphoric acid 0.1 parts by weight Sodium dihydrogen phosphate dihydrate 1 part by weight phosphorus Disodium hydrogen acid dihydrate / 12 salt 4 parts by weight Water 93 parts by weight (second rinse solution) A rinse solution (pH 6.
0) Charge 8l into the second rinse bath, 1030mm x 80
Every time a 0 mm printing plate was processed, 30 ml of the rinse liquid was replenished. (The storage tank for the second rinse overflows at 8 liters) Sodium polyoxyethylene lauryl ether sulfate 2 parts by weight 85% phosphoric acid 0.4 parts by weight Sodium dihydrogen phosphate dihydrate 0.05 parts by weight Disodium hydrogen phosphate dodecahydrate 0.8 parts by weight Water 97 parts by weight 500 pieces of 1030 mm × 800 mm lithographic printing plates were continuously treated under the above conditions, and both the first and second rinse baths were processed. No insoluble matter or gel was produced. Further, when printing was carried out on the 500th printing plate, a beautiful printed matter was obtained in which stains in the non-image area and ink inking defects in the image area were not seen.

Example 2 A treatment was performed in the same manner as in Example 1 except that the compositions of the first rinse liquid and the second rinse liquid were changed to the following ones. No gel was formed. Further, when printing was carried out on the 500th printing plate, a beautiful printed matter was obtained in which stains in the non-image area and ink inking defects in the image area were not seen.

(First rinse liquid) pH = 5.5 Sodium lauryl sulfate 1 part by weight Polyoxyethylene cetyl ether (HLB = 13) 0.5 part by weight 85% Phosphoric acid 0.1 part by weight Sodium dihydrogen phosphate.2 Water salt 2 parts by weight Sodium succinate 2 parts by weight Water 95 parts by weight (second rinse liquid) pH = 5.7 Sodium lauryl sulfate 1 part by weight Polyoxyethylene cetyl ether (HLB = 13) 0.5 parts by weight 85% Phosphoric acid 0.5 parts by weight Sodium dihydrogen phosphate dihydrate 0.01 parts by weight Sodium succinate 0.55 parts by weight Water 98 parts by weight Comparative Example 1 Tap water for both the first rinse bath and the second rinse bath Treatment was carried out in the same manner as in Example 1 except that 8 l were charged and no supplement was added according to the treatment. Introduction insolubles is floated on the liquid, adheres to the non-image area on the 15th Edition of the print plate, dirt that portion was observed was subjected to printing.

When the treatment was further continued, the ink fleshiness gradually deteriorated from the 70th plate to the image area at the start of printing, and the ink was also thinly attached to the non-image area to the printed matter. A thin stain was produced.

Comparative Example 2 Treatment was carried out in the same manner as in Example 1 except that the compositions of the first rinse liquid and the second rinse liquid were changed to the following, and 1
After the 50th plate treatment, insoluble matter began to float in the 1st rinse bath and adhered to the non-image part on the 150th plate of the printing plate, and when printing was performed, stains were found on the part.

When the treatment was further continued, the ink receptivity to the image area at the beginning of printing gradually deteriorated from around the 200th plate, and the ink adhered thinly to the non-image area. There was a light stain on the.

(First rinse solution) pH = 10.7 Phosphoric acid-sodium hydroxide (buffer solution having a pH of 10.7) 1000 parts by weight Sodium dodecyldiphenyl ether disulfonate 10 parts by weight (second rinse solution) pH = 6.5 Phosphoric acid-sodium hydroxide aqueous solution (pH 6.5 buffer solution) 1000 parts by weight Sodium dodecyldiphenyl ether sulfonate (40% by weight aqueous solution) 10 parts by weight

[0044]

According to the present invention, there is provided a plate making method capable of stably producing a large amount of printing plates for a long period of time without producing insoluble matters or gel.

Claims (1)

[Claims] 1. A lithographic printing plate which is subjected to imagewise exposure and development with an alkaline developer, and then rinsed with a rinse liquid containing an anionic surfactant and / or a nonionic surfactant, an acid and a buffering agent. In the plate making method, a lithographic printing plate making method is characterized in that the rinsing treatment is performed in a plurality of rinse baths, and the content of the acid in the rinse liquid becomes higher and the content of the buffer becomes lower at the later stage. .