Classifications

• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21H—PULP COMPOSITIONS; PREPARATION THEREOF NOT COVERED BY SUBCLASSES D21C OR D21D; IMPREGNATING OR COATING OF PAPER; TREATMENT OF FINISHED PAPER NOT COVERED BY CLASS B31 OR SUBCLASS D21G; PAPER NOT OTHERWISE PROVIDED FOR
  • D21H17/00—Non-fibrous material added to the pulp, characterised by its constitution; Paper-impregnating material characterised by its constitution
  • D21H17/71—Mixtures of material ; Pulp or paper comprising several different materials not incorporated by special processes
  • D21H17/72—Mixtures of material ; Pulp or paper comprising several different materials not incorporated by special processes of organic material
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21H—PULP COMPOSITIONS; PREPARATION THEREOF NOT COVERED BY SUBCLASSES D21C OR D21D; IMPREGNATING OR COATING OF PAPER; TREATMENT OF FINISHED PAPER NOT COVERED BY CLASS B31 OR SUBCLASS D21G; PAPER NOT OTHERWISE PROVIDED FOR
  • D21H23/00—Processes or apparatus for adding material to the pulp or to the paper
  • D21H23/02—Processes or apparatus for adding material to the pulp or to the paper characterised by the manner in which substances are added
  • D21H23/04—Addition to the pulp; After-treatment of added substances in the pulp
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21H—PULP COMPOSITIONS; PREPARATION THEREOF NOT COVERED BY SUBCLASSES D21C OR D21D; IMPREGNATING OR COATING OF PAPER; TREATMENT OF FINISHED PAPER NOT COVERED BY CLASS B31 OR SUBCLASS D21G; PAPER NOT OTHERWISE PROVIDED FOR
  • D21H17/00—Non-fibrous material added to the pulp, characterised by its constitution; Paper-impregnating material characterised by its constitution
  • D21H17/20—Macromolecular organic compounds
  • D21H17/33—Synthetic macromolecular compounds
  • D21H17/46—Synthetic macromolecular compounds obtained otherwise than by reactions only involving carbon-to-carbon unsaturated bonds
  • D21H17/47—Condensation polymers of aldehydes or ketones
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21H—PULP COMPOSITIONS; PREPARATION THEREOF NOT COVERED BY SUBCLASSES D21C OR D21D; IMPREGNATING OR COATING OF PAPER; TREATMENT OF FINISHED PAPER NOT COVERED BY CLASS B31 OR SUBCLASS D21G; PAPER NOT OTHERWISE PROVIDED FOR
  • D21H21/00—Non-fibrous material added to the pulp, characterised by its function, form or properties; Paper-impregnating or coating material, characterised by its function, form or properties
  • D21H21/14—Non-fibrous material added to the pulp, characterised by its function, form or properties; Paper-impregnating or coating material, characterised by its function, form or properties characterised by function or properties in or on the paper
  • D21H21/30—Luminescent or fluorescent substances, e.g. for optical bleaching

Definitions

• the invention relates to a method for producing paper and cardboard by dewatering a paper stock in the presence of at least one cationic polymer on a wire.
• paper consists essentially of fibers consisting of wood and / or cellulose, and optionally of mineral fillers, in particular calcium carbonate and / or aluminum silicate, and that the essential process in paper manufacture consists of a separation these fibers and fillers from a dilute aqueous
• the cationic polyelectrolyte is to have undesired or desired substances, such as B. anionic oligomers and polymers, resins, adhesive contaminants, dyes, sizing agents, strengthening agents, etc., bind to the fibers, this is known as a fixing agent. If the cationic polyelectrolyte improves the strength-related property of the paper, then it is a hardener.
• a high whiteness not only conveys the impression of cleanliness and harmlessness, but also increases the legibility of the font due to the stronger contrast to the font color, particularly in poor lighting.
• a particular advantage of high whiteness is shown when the paper or cardboard is to be printed, written on or painted in color. The whiter the background, the better and more natural the color contrast, especially when writing, printing or painting with light or translucent colors or pastel tones. Due to the increasing use of waste paper in the production of graphic papers in recent times, the product of the paper manufacturer is significantly grayer than when using fresh fiber materials.
• a known method of increasing the whiteness and brightness of paper is the use of so-called “whiteners” or “optical brighteners”, which, according to the current state of the art, are added to the paper pulp in various paper production and paper finishing steps or on the paper.
• optical brighteners used in the paper industry are mostly 1,3,5-triazinyl derivatives of 4,4'-diaminostilbene-2,2'-disulfonic acid, which can carry additional sulfonic acid groups, for example a total of 2, 4 or 6.
• An overview of such brighteners can be found, for example, in: Ullmann's Encyclopedia of Industrial Chemistry, Sixth Edition, 2000 Electronic Release, OPTICAL BRIGHTENERS - Chemistry of Technical Products.
• newer types of brightener are also possible, e.g. B. Derivatives of 4,4'-distyrylbiphenyl, as are also described in the aforementioned literature Ullmann's Encyclopedia of Industrial Chemistry.
• the white toners can be used in various phases of paper production and paper finishing. You can see the optical brightener z. B. add the pulp, but also in a size press together with surface sizing agents or strengthening agents, such as. B. starch, or together with other aids. Optical brighteners are most often used in paper coating slips, with which paper and cardboard are coated. Use in bulk is particularly advantageous when it comes to the uniformity of the lightening and good fastness to bleeding ⁇ see BW Bieber, A. Brockes, B. Hunke, J. Krüsemann, D. Loewe, F. Müller, P.
• optical brighteners only leads to optimal success if they are not only present in the paper in an optimal distribution, but also in an optimal chemical structure and conformation, since only the trans form is optically active in stilbenes and only then maximum fluorescence if it is monomolecularly distributed and held in one plane (see above, KP Kreutzer). When added to the pulp, this is generally due to the adsorption on the cellulose.
• the brighteners used are chemically modified in such a way that they have a high affinity for cellulose and therefore do not require any additional fixatives or enhancers, known as carriers.
• care must be taken to ensure that no other chemicals in the pulp reduce the effects of the brighteners (see literature above).
• Stable solutions of optical brighteners are known from EP-A-0 192 600. They contain 10 to 500 parts by weight of a polyethylene glycol with an average molecular weight of 1,000 to 3,000 and at least 20% by weight of water, based on the mixture as a whole, per 100 parts by weight of a brightener. The mixtures are used as optical brighteners in paper coating slips.
• Linear, basic polymers are known from EP-A-0 071 050, which contain 90-10 mol% vinylamine units and 10-90 mol% N-vinylformamide units in copolymerized form. They are produced by radical polymerization of N-vinylformamide and subsequent partial hydrolysis of the polymer thus obtained.
• the partially hydrolyzed poly-N-vinylformamides are used, for example, as retention aids, drainage aids and flocculants in the manufacture of paper.
• the object of the invention is to provide an improved process for the production of paper, cardboard and cardboard, products with increased whiteness being obtained compared to the known processes.
• the object is achieved according to the invention with a process for the production of paper, cardboard and cardboard by dewatering a paper stock in the presence of at least one cationic polymer on a sieve if the paper stock is mixed with at least one essentially linear, cationic polyelectrolyte and one before the sheet is formed optical brightener is added, with at least two parts by weight of the polyelectrolyte being used per part by weight of the optical brightener.
• Cationic polyelectrolytes are understood to mean polymers which carry positive charges which are distributed over the polymer chain.
• Cationic polyelectrolytes are also to be understood as meaning those substances which, when dry, can be non-ionic, but because of their basic character are protonated in water or other solvents and thus carry positive charges.
• the mixtures which are added to the paper stock before sheet formation usually contain
• optical brighteners are also suitable for the process according to the invention which, because of a lack of substantivity, cannot be used in the pulp, such as.
• B. stilbene with 6 sulfonic acid groups SG Murray, Dyes and fluorescent whitening agents for paper in Paper Chemistry, ed. JC Roberts, 2nd edition, Blackie Academic & Professional, Glasgow (1996), page 187. No fluorescence is detectable in white water when used.
• the cationic polyelectrolytes (ii) which can be used in the process according to the invention are known.
• polymers that are known by the chemical trivial names can be used: polyvinylamine, polyallylamine, poly (diallyldimethylammonium chloride), cationic polyvinylformamide, cationic polyvinylpyrrolidone, cationic polyvinyl acetamide, cationic polyvinyl methyl formamide, cationic polyvinyl methylacetopropyl amide, poly (diamine)
• Poly (amidoamine-epichlorohydrin) and their salts if they are basic polymers.
• Polymers containing vinylamine units such as cationic polyvinylformamides and polyvinylamine, and also cationic polyacrylamide and poly (diallyldimethylammonium chloride) are preferred.
• Polymers containing vinylamine units in the form of the free bases or as salts are particularly preferred.
• the cationic polyelectrolytes (ii) which can be used in the process according to the invention have different molecular weights, which are described in the text below with the aid of the Fikentscher K values. be characterized.
• the molecular weights of the cationic polyelectrolytes which can be used according to the invention are not restricted. As a rule, they correspond to K values of 20 to 200, preferably 30 to 150, particularly preferably 40 to 100 (the specified K values are determined according to H. Fikentscher in 5% aqueous sodium chloride solution at pH 7, 25 ° C. and one Polymer concentration of 0.1% by weight).
• the very particularly preferred polymers containing vinylamine units such as cationic polyvinylformamides, contain vinylamine and vinylformamide units according to the general formula (I),
• the vinylamine units of the polymers can be present either in whole or in part as salts with mineral acids such as hydrochloric acid, sulfuric acid or phosphoric acid or as salts with organic acids (e.g. formic acid, acetic acid, propionic acid, toluenesulfonic acid, benzenesulfonic acid or methanesulfonic acid).
• the polymers containing vinylamine units are prepared by e.g. N-vinylformamide of the formula (II)
• the N-vinylcarboxamide units contained in the polymer are preferably cleaved in the presence of bases, such as, for example, sodium hydroxide, potassium hydroxide, alkaline earth metal hydroxides, ammonia or amines.
• Cationic polymers of N-vinylformamide can be obtained particularly easily by hydrolytically cleaving homopolymers of N-vinylformamide with defined amounts of acid or base to the desired degree of hydrolysis, as in EP-B-0 given in the prior art 071 050.
• the resulting amino groups on the polymer chain are more or less protonated depending on the pH of the solution and thus give the polymer a more or less cationic character.
• the cleavage of the formyl group during the hydrolysis usually takes place at temperatures in the range from 20 to 200, preferably 40 to 180 ° C., in the presence of acids or bases.
• the hydrolysis in the presence of acids or bases is particularly preferably carried out in the temperature range from 70 to 90 ° C.
• acidic hydrolysis For each formyl group equivalent in poly-N-vinylformamide you need e.g. for acidic hydrolysis about 0.05 to 1.5 equivalents of an acid such as hydrochloric acid, hydrobromic acid, phosphoric acid, sulfuric acid.
• the pH in acidic hydrolysis is, for example, in the range from 2 to 0, preferably from 1 to 0.
• the hydrolysis of the formyl groups of the poly-N-alkyl-N-vinylformamide can also be carried out in an alkaline medium, for example in the pH range from 11 to 14.
• This pH is preferably determined by adding alkali metal bases, such as, for example, sodium hydroxide solution or potassium hydroxide solution set.
• alkali metal bases such as, for example, sodium hydroxide solution or potassium hydroxide solution set.
• ammonia, amines and / or alkaline earth metal bases for the alkaline hydrolysis, 0.05 to 1.5, preferably 0.4 to 1.0 equivalents of a base are used.
• the cleavage can also be carried out at high temperatures, for example above 100 ° C., preferably 120 to 180 ° C., particularly preferably 140 to 160 ° C. in the presence of a solvent, for example water, in the absence of acids or bases. This is preferably carried out under conditions above the critical point of the solvent, for example with supercritical water.
• hydrolysis i.e. the formyl group is split off from the poly-N-vinylformamide in water in the presence of acids or bases, and carboxylic acid, for example formic acid, or its salts is obtained as a by-product.
• carboxylic acid for example formic acid, or its salts
• the solutions thus obtainable can be used in the process according to the invention without further working up, but the hydrolysis or solvolysis products can also be separated off.
• the solutions obtained are treated with ion exchangers, for example, or subjected to ultrafiltration.
• cationic polyacrylamides which can be used for the process according to the invention are known, cf. D. Horn, F. Linhart, in Paper Chemistry, ed. Roberts JC, 2 nd edition, Blackie Academic & Professional, Glasgow (1996) pp 66-67, and references cited therein).
• A represents oxygen or an NH group and in which R 1 can denote hydrogen or a lower alkyl group having 1-3 C atoms, R2 and R3 lower alkyl groups having 1-5 C atoms or a benzyl group and R5 hydrogen or a Can mean methyl group, and in which the ratio of n: m can take on values from 99: 1 to 0: 100, in which g can take on the values 1 and 2 and p can take on values between 50 and 50,000, and in the X- for can be any anion, such as. B.
• the poly (diallyl-dimethylammonium chloride) which can be used in the process according to the invention can be characterized, for example, using the formula (V)
• n can have values between 30 and 30000.
• Such polymers have been known for many years cf. D. Horn, F. Linhart, in Paper Chemistry, ed JC Roberts, 2 nd edition, Blackie Academic & Professional, Glasgow (1996), pp. 70; G. Butler, in Polymeric Amines and Ammonium Salts, ed. EJ Goethals, Pergamon Press, Oxford 125, (1980).
• diallyldialkylammonium chlorides are also possible, e.g. B. those with the general polymer formula (VI),
• R 1 and R 2 can independently of one another be alkyl groups with 2 to 4 carbon atoms, where R 1 or R 2 can also be hydrogen, and in which n can take on values from 30 to 30,000.
• Cationic polyelectrolytes which are preferably used are hydrolyzed homopolymers of N-vinylformamide with a degree of hydrolysis of 1 to 99 mol%, copolymers of acrylamide and cationic monomers (for example monomers containing amino groups or ammonium groups), polymers of diallyldimethylammonium chloride and the polyamidoamine which can be used as wet strength agents epichlorohydrin resins.
• cationic polyelectrolytes which can be obtained by copolymerizing starting monomers of the polyelectrolytes already mentioned.
• copolymers can also be made from Use vinylformamide (formula (II)) and diallyldimethylammonium chloride or from vinylformamide and basic acrylates, as described in EP-B-0 464 043, and also copolymers of acrylamide and diallyldimethylammonium chloride or other diallyldialkylammonium chlorides.
• the solubility of the cationic polyelectrolytes in the solvent (iii) used for example at 20 ° C., is generally at least 1% by weight until complete solubility.
• Brighteners and the cationic polyelectrolytes can range between 1: 2 and 1: 100, but a clear excess of cationic polyelectrolytes is advantageous.
• the use of optical brighteners and cationic polyelectrolytes in a weight ratio of 1: 2 to 1:50 and very particularly preferably from 1: 5 to 1:20 is particularly advantageous.
• optical brighteners (i) can be used for the method according to the invention.
• brighteners such as those described in Ullmann's Encyclopedia of Industrial Chemistry, Sixth Edition, 2000 Electronic Release, OPTICAL BRIGHTENERS - Chemistry of Technical Products, chap. 2.1 to chap. 2.9 are described.
• Suitable optical brighteners (i) include, for example
• distyrylbenzenes for example cyano-substituted 1,4-distyrylbenzenes with cyano groups in positions 2 'and 3' '[CAS RegNr. 79026-03-2], or in position 2 'and 2' '[13001-38-2], 3' and 3 "[36755-00-7], 3 'and 4" [79026-02-1] as well 4 'and 4 "[13001-40-6], or amphoteric compounds, such as [84196-71-4], each in the 2' and 2" position, one group
• distyrylbiphenyls for example 4,4'-di (2-sulfostyryl) biphenyl disodium salt [27344-41-8], 4,4'-di (3-sulfostyryl) biphenyl disodium salt [51119-63-2], 4, 4 '-Di (4-chloro-3-sulfostyryl) biphenyl disodium salt [42380-62-1], 4, 4' -Di (6-chloro-3-sulfostyryl) biphenyl disodium salt [60477-28-3 ], 4, 4 '-Di (2-methoxystyryl) biphenyl [40470-68-6] or a 4, 4' -di (styryl) biphenyl which has a group in position 2 on the styryl radical
• N-bis (hydroxyethyl) amine o- and additionally a sulfonic acid group in position 3 on the anilino group (CAS RegNr. [12224-02-1]), N-bis (2-hydroxypropyl) amino and additionally on the anilino group a sulfonic acid group in position 4 (CAS Reg. No. [99549-42-5]), N-bis (hydroxyethyl) amino - and additionally on the anilino group a sulfonic acid group in position 4 (CAS Reg. [16470-24-9]), N-hydroxyethyl-N-methylamino and in addition on the anilino group a sulfonic acid group in position 4 (CAS RegNr.
• Stilbenyl-2H-triazoles for example stilbenyl-2H-naphtho [1,2-d] triazoles such as the sodium salt of 4- (2H-naphtho [1,2-d] triazol-2-yl) stilbene-2-sulfonic acid [6416 -68-8] or those in position 6 carry a sulfonic acid on the naphthol ring and at position 2 of the stilbene structure [2583-80-4], or carry a cyano group in position 2 on the stilbene structure and a chlorine group in position 4 '[5516-20-1] or eg bis (1, 2 , 3-triazol-2-yl) stilbenes, such as, for example, 4,4′-bis (4-phenyl-1,2,3-triazol-2-yl) stilbene-2,2′-disulfonic acid dipotassium salt [52237-03- 3] or 4,4'-bis (4- (4'-
• Furans, benzo [b] furans and benzimidazoles such as e.g. Bis (benzo [b] furan-2-yl) biphenyls, for example sulfonated 4,4'-bis (benzo [b] furan-2-yl) biphenyls or cationic benzimidazoles, for example 2,5-di (1-methyl-benzimidazole -2-yl) furan [4751-43-3], [72829-17-5], [74878-56-1], [74878-48-1] or [66371-25-3], or 1,3 -Diphenyl-2-pyrazolines, e.g.
• Bis (benzo [b] furan-2-yl) biphenyls for example sulfonated 4,4'-bis (benzo [b] furan-2-yl) biphenyls or cationic benzimidazoles, for example 2,5-di (1-methyl-benzimidazole -2-yl)
• 4,4'-distyrylbiphenyl derivatives or stilbene derivatives which are substituted with up to 6, particularly preferably with 2, 4 or 6, sulfonic acid groups can preferably be used, preferably the Blankophor® brands from Bayer AG, particularly preferably
• (R) ® are Blankophor P and Blankophor PSG, further preferred are the Tinopal® brands from Ciba Specialty Chemicals, particularly preferred Tinopal® MC liquid, Tinopal® ABP-Z liquid, Tinopal®
• SPP-Z liquid and Tinopal® SK-B liquid and further preferred are the Leukophor® brands from Clariant AG, particularly preferably Leukophor® APN, UO, NS or SHR.
• optical brighteners and cationic polymers which contain vinylformamide units in copolymerized form can also be added to the paper stock separately from one another, metering in the cationic polyelectrolyte first and then the optical brightener or reversing the order of addition.
• Suitable solvents (iii) for the mixtures are, for example, water, methanol, ethanol, isopropanol, n-propanol, n-butanol, dimethylformamide or N-methylpyrrolidone; water is preferred.
• the concentration should be chosen so that the respective dosing and subsequent dilution process for example, the viscosity of the mixture can be optimally carried out. Optimal viscosities for various dosing methods and dilution methods are known to the person skilled in the art. Usual concentrations of the mixtures are between 2 and 20% by weight.
• the molecular weight of the cationic polyelectrolytes which can be used according to the invention should be adapted to the particular target profile of the polyelectrolyte. If the cationic polyelectrolyte is to act as a retention aid in papermaking, for example, cationic is preferably used
• cationic polymers with a very high molecular weight. If the cationic polymers are to act as fixatives or as strengthening agents, cationic polymers with medium to low molecular weights are used. It is generally state of the art to add retention and drainage agents to the paper stock before dewatering on the wire. The mostly used cationic polymers have very high molecular weights in the range between 2 and 20 million Daltons (see F. Linhart, retention, PTS seminar - basics of chemistry for paper rings! Eure, part 2, J. Weigl and R. Grenz (ed.), Kunststoff: PTS Kunststoff, 1991, CP - SE 111, chapter 7, p. 9).
• cationic polymers with molecular weights between 500,000 daltons and 2 million daltons have also been successfully used as retention aids.
• the application rates of these polymers, based on dry paper stock, are between 50 g / t and 5 kg / t, preferably between 100 g / t and 2 kg / t.
• the cationic fixing agents added to the paper stock in many cases have significantly lower molecular weights, which are very different depending on the chemical nature and the function of the polymer and are in a range between 10,000 and 500,000 daltons.
• the application rates of the fixative, based on dry paper stock, are between 100 g / t and 2 kg / t.
• the molecular weights of polymeric cationic solidifiers vary in the very wide range between a few hundred daltons, as is known, for. B. is possible with so-called reactive wet strength agents, over 100,000 to 500,000 Daltons with synthetic dry strength agents (J. Marton, Dry-Strength Additives, in Paper Chemistry, J.
• the mixtures can be a dispersion or, preferably, a solution.
• the paper stock is diluted continuously or discontinuously with water to concentrations between 0.01% by weight and 1% by weight in order to achieve a faster and more uniform mixing with the paper stock.
• concentrations between 0.01% by weight and 1% by weight in order to achieve a faster and more uniform mixing with the paper stock.
• the point of addition of the mixtures depends on the requirements and the target profile of the cationic polyelectrolyte in the mixture. If the mixture is not only to increase the whiteness of the paper, but also to increase retention and dewatering speed at the same time, an addition in the thin stock is appropriate just before the headbox, before or after the pressure sorter.
• the mixture can be used in the entire area of the substance preparation, e.g. B.
• the cationic polyelectrolyte in the mixture is to act as a solidifier, it is advisable to add the mixture at a point which is customary for the addition of solidifiers, e.g. B. in the mixing chest or machine chest, but also before the material grinding or in the thin material area.
• the optimal dosing point must be determined in every single case through practical tests.
• the process according to the invention is preferably used in the production of bright white paper and cardboard, the starting materials of which should already have a sufficient basic whiteness. Therefore, mainly fiber and fillers of high whiteness are used for this purpose.
• the pulps that can be used primarily include cellulose, eg. B. bleached
• Bleached wood pulps such as. B. wood pulp, pressure sanding (PGW), refiner mechanical pulp (RMP), thermomechanical pulp (TMP) or chemithermomechanical pulp (CTMP, APTMP and other variants
• fibers from annual plants such as. B. cotton, cotton sintered, bleached straw pulp from straw of various types of grain, bleached bagasse pulp, hemp, flax, kenaf, etc.
• a very important fiber for the process according to the invention is a bleached fiber freed from printing inks and based on graphic waste paper, the so-called deinked pulp (DIP).
• the fillers suitable for the process according to the invention are generally aluminum silicates, such as, for. B. kaolin or modifications obtained by further treatment thereof to magnesium silicates, such as. B. talc, calcium carbonate in the form of ground marble or limestone or in the form of natural or ground chalk or in the form of precipitated calcium carbonate, calcium sulfate in the form of gypsum or titanium dioxide.
• the paper, cardboard or cardboard produced by the process according to the invention can be produced in conventional processes, e.g. Offset, gravure or gravure printing, flexographic printing or digital printing, such as Laser printing or ink jet printing processes are printed, but also processed or refined in another way, e.g. B. coated.
• the method according to the invention makes it easier for the person skilled in the art to carry out the difficult task of producing paper, cardboard and cardboard with relatively simple means and with great flexibility in an improved process or with higher quality and at the same time increased whiteness.
• the parts given in the examples are parts by weight.
• the K values of the polymers were determined according to H. Fikentscher, Cellulose-Chemie, Vol. 13, 58-63 and 71-74 (1932) in a 5% strength by weight aqueous solution at pH 7, 25 ° C. and one Polymer concentration of 0.1% by weight.
• the whiteness and color values of the finished paper were determined using the CIELAB system (DIN 6174).
• the finished paper was also visually checked for fluorescence by illumination with an ultraviolet lamp.
• the full fixation of the brightener in the paper was examined by soaking a strip of highly absorbent white wood-free paper with white water and visually testing for fluorescence when illuminated with ultraviolet light.
• the process according to the invention gives very good retention and at the same time increases the whiteness of the paper considerably, the desired shift in hue towards blue and red occurring.
• the extent of the whiteness increase is particularly surprising when one considers that only 0.005 part of optical brightener, based on about 122 parts of solid paper stock, is used as the proportion of the mixture.

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