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
  • 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
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C9/00—After-treatment of cellulose pulp, e.g. of wood pulp, or cotton linters ; Treatment of dilute or dewatered pulp or process improvement taking place after obtaining the raw cellulosic material and not provided for elsewhere
  • D21C9/001—Modification of pulp properties
  • D21C9/002—Modification of pulp properties by chemical means; preparation of dewatered pulp, e.g. in sheet or bulk form, containing special additives
  • D21C9/005—Modification of pulp properties by chemical means; preparation of dewatered pulp, e.g. in sheet or bulk form, containing special additives organic compounds
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C9/00—After-treatment of cellulose pulp, e.g. of wood pulp, or cotton linters ; Treatment of dilute or dewatered pulp or process improvement taking place after obtaining the raw cellulosic material and not provided for elsewhere
  • D21C9/10—Bleaching ; Apparatus therefor
  • D21C9/1063—Bleaching ; Apparatus therefor with compounds not otherwise provided for, e.g. activated gases

Definitions

• the present invention relates to a method that can be used in pulp mills for whitening lignin-containing pulps. More particularly it teaches the use of fluorescent whitening agents instead of bleach chemicals to obtain desired target brightness values while maintaining desirable fiber characteristics of pulps that contain significant amounts of lignin.
• Pulps from mechanical processes are bleached, if desired, prior to the paper-making step, with chemicals that do not remove lignin, such as alkaline hydrogen peroxide or sodium dithionite, resulting in paper having a brightness value of up to about 80.
• paper from a mechanical pulping process has lower light stability, strength and permanence compared to paper prepared from a chemical pulping process.
• a major market for paper prepared by mechanical pulping processes is paper for newspapers.
• pulp which is a blend of chemical and mechanical pulps is advantageously employed.
• recycled paper usually contains paper made from both chemical and mechanical pulps, but often predominantly the latter.
• the amount of lignin can vary greatly, at one extreme being about the same as that found in the wood chips from which the pulp was prepared and, at the other end, being close to zero in high brightness bleached chemical pulps suitable for high quality white paper manufacture.
• the brightness standard is measured as the reflectance of light in the blue range (457 nm) in comparison to magnesium oxide as 100% white. In the United States brightness is usually measured with the General Electric brightness meter. Thus a GE brightness value of 80 corresponds to 80% of the brightness of magnesium oxide, as measured with the GE meter.
• the color, or more precisely the lack of color of the pulp is a critical parameter. It is therefore highly desirable to be able to increase the whiteness or brightness of lignin-containing pulps, and hence the whiteness of the resulting products, in a cost-effective nianner. It is also highly desirable to be able to do this without brightness being the primary determinant of other pulp properties.
• pulps are bleached to the desired brightness and the fiber furnish is chosen to give roughly appropriate properties, often with the desired physical properties taking a back seat to brightness.
• paper prepared from chemical pulping processes in combination with bleaching i.e. substantially lignin-free paper
• fluorescent whitening agents are marketed for this express purpose.
• fluorescence is inhibited by lignin. This effect has precluded the use of fluorescent whitening agents in making paper from pulps containing significant amounts of lignin, such as those from mechanical pulping processes.
• fluorescent whitening agents also referred to as optical brighteners
• optical brighteners Traditional have not been used directly with at the pulp mill, even though the process would be technically straightforward. Rather it has been left to the papermaker (where there is better understanding of the fiber and fluorescent whitening agent chemistry) to purchase pulps having an appropriate starting brightness and to add fluorescent whitening agents and other materials to obtain the desired target brightness levels in the resulting paper.
• optical brighteners as an alternative to bleaching during the manufacture of traditional kraft type chemical pulps is not typically practiced.
• Copending application 08/766,909 discloses that it is possible to increase the whiteness of paper made from a lignin-containing pulp by adding to an aqueous slurry of the lignin-containing pulp, in the paper-making step, a fluorescent whitening agent.
• a fluorescent whitening agent such as sodium bicarbonate
• optical brighteners as a bleaching alternative in high-lignin-containing pulps during manufacture at the pulp mill is unknown.
• a process which comprises adding to an aqueous slurry comprising a lignin-containing pulp, during pulp manufacture, prior to the drying step or paper making step if the pulp is not isolated, an effective amount of a fluorescent whitening agent.
• the fluorescent whitening agent can be added to the aqueous slurry comprising the lignin-containing pulp at any processing step, to minimize losses, it is advantageously added in the latter stages of pulp manufacture, prior to the final dewatering and drying steps. Preferably it is added after completion of the last bleaching step.
• a lignin-containing pulp any pulp that still contains 5% or more of lignin by weight on a dry basis.
• lignin is that portion of the pulp which is insoluble in 72 weight percent sulfuric acid. Suitable test procedures for lignin content are given in TAPPI T 223 and ASTM D 1106.
• the process of this invention is useful to produce significant whitening of pulps containing from about 5% lignin on a dry weight basis up to 100% of the lignin present in an equivalent amount of wood chips.
• the process can be employed, e.g. on relatively low-lignin-containing pulps such as certain bleached kraft pulps up to and including higher lignin content pulps such as thermomechanical pulps, bleached chemi-thermomechanical pulps, and even deinked bleached thermomechanical pulps.
• the pulps contain at least 10% of lignin by weight on a dry weight basis; most preferably they contain at least 15%.
• the range of brightness that can be obtained varies from about 50 to 90+ depending on starting pulp brightness and the type of pulp employed.
• chelating agents in processes to bleach pulps from mechanical pulping processes. See V. N. Gupta , Pulp Paper Mag. Can., 71 (18), T391-399 (1970).
• a chelating agent to an aqueous pulp slurry controls the natural yellowing tendency of glucuronic acids, extractives and lignin present in the pulp by removing or minimizing iron and other heavy metals such as copper, zinc and manganese metals that catalyze color-forming side reactions.
• the iron and other heavy metals are converted into the form of their highly soluble chelates and largely removed in the dewatering steps. This decreases the incorporation of the heavy metal ions into the pulp.
• the chelating agent sequesters the salts of iron and other heavy metals which remain and which, in their own right would otherwise relax the excited state of fluorescent whiteners and render them ineffective .
• this metal control step may be done as matter of course in pulping processes where reductive bleaching (e.g., bisulfite, hydrosulfite, or formamidine sulfite bleaching) or oxidative bleaching (e.g., peroxy- or peroxide bleaching) is employed.
• reductive bleaching e.g., bisulfite, hydrosulfite, or formamidine sulfite bleaching
• oxidative bleaching e.g., peroxy- or peroxide bleaching
• the background level of residual iron and other heavy metals and their ions in wood chips is generally about 10-25 ppm, although it is rather dependent on geography and species considerations.
• the amount of iron and other heavy metals and their ions in the water used in pulping mills varies widely. Significant additional amounts of iron and other heavy metals and their ions are introduced during mechanical pulping of wood chips as well as in recycling newsprint.
• the amount of iron and other heavy metals and their ions in the aqueous pulp during manufacture is may be several hundred parts per million by weight, based on the dry weight of the pulp, at some stages of pulp manufacture.
• a chelating agent is advantageously employed if the aqueous slurry comprising the lignin-containing pulp still contains from 25 to 500 ppm by weight, based on the dry weight of the pulp, of salts of iron and other heavy metals at the processing stage where the fluorescent whitening agent is to be added.
• Heavy metal contents can be determined by standard analytical procedures such as atomic absorption spectroscopy or inductively coupled plasma analysis. Once the type and amounts of the various heavy metals are known, the amount of the chelating agent to employ to reach 100 ppm or less, preferably about 25 ppm or less, can readily be calculated or determined from tables. It is not harmful to use a small excess.
• the chelating agent selected and the degree of whiteness improvement desired from 0 up to about 1% by weight, based on the dry weight of the pulp, of a chelating agent may be advantageously employed
• An additional and substantial benefit of chelate treatment is to open the fiber matrix to make it more accessible to the fluorescent whitening agent.
• chelating agents are suitable in the present invention, i.e. those that offer thermodynamic or kinetic control of metal ions. However preference is given to chelating agents that offer thermodynamic control, that is, chelating agents that form a stable, isolable, complex with a heavy metal ion. Within this group it is particularly preferred to use aminocarboxylic acid chelates.
• Well known and commercially available members of this class include ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), hydroxyethylethylenediaminetriacetic acid (HEDTA) and nitrilotriacetic acid (NTA).
• thermodynamic and kinetic-controlling chelating agents e.g. citrates, keto acids, gluconates, heptagluconates, phosphates, and phosphonates
• thermodynamic and kinetic-controlling chelating agents also work well in reducing the content of free heavy metal ions in the pulp to acceptable levels.
• kinetic-controlling chelating agents are also commercially available.
• Kinetic controlling chelating agents are those which do not form a stable, isolable, complex with a heavy metal ion.
• Fluorescent whitening agents are substances that absorb light in the invisible ultraviolet region of the spectrum and reemit it in the visible portion of the spectrum, particularly in the blue to blue violet wavelengths. This provides added brightness and can offset the natural yellow cast of a substrate such as pulp or paper made from it.
• Fluorescent whitening agents useful in the present invention may be selected from a wide range of chemical types such as 4,4'-bis-(triazinylamino)-stilbene-2,2'-disulfonic acids, 4,4'-bis-(triazol-2-yl)stilbene-2,2'-disulfonic acids, 4,4'-dibenzofuranyl-biphenyls, 4,4'-(diphenyl)stilbenes, 4,4'-distyryl-biphenyls, 4-phenyl-4'-benzoxazolyl-stilbenes, stilbenyl-naphthotriazoles, 4-styryl-stilbenes, bis-(benzoxazol-2-yl) derivatives, bis-(benzimidazol-2-yl) derivatives, coumarins, pyrazolines, naphthalimides, triazinyl-pyrenes, 2-styryl-benzoxazoles or -naph
• Preferred 4,4'-bis-(triazinylamino)-stilbene-2,2'-disulfonic acids are those having the formula: in which R 1 and R 2 , independently, are phenyl, mono- or disulfonated phenyl, phenylamino, mono- or disulfonated phenylamino, morpholino, -N(CH 2 CH 2 OH) 2 , -N(CH 3 )(CH 2 CH 2 OH), -NH 2 , -N(C 1 -C 4 alkyl) 2 , -OCH 3 , -Cl, -NH-CH 2 CH 2 SO 3 H, CH 2 CH 2 OH or ethanolaminopropionic acid amide; and M is H, Na, Li, K, Ca, Mg, ammonium, or ammonium that is mono-, di-, tri- or tetra-substituted by C 1 -C 4 alkyl, C 1 -C 4 hydroxy
• each R 1 is 2,5-disulfophenyl and each R 2 is morpholino, -N(C 2 H 5 )2, -N(CH 2 CH 2 OH) 2 or ethanolaminopropionic acid amide; or each R 1 is 3-sulfophenyl and each R 2 is NH(CH 2 CH 2 OH) or N(CH 2 CH 2 OH) 2 ; or each R 1 is 4-sulfophenyl and each R 2 is N(CH 2 CH 2 OH) 2 , N(CH 2 CHOHCH 3 ) 2 , morpholino, or ethanolaminopropionic acid amide; or each R 1 is phenylamino and each R 2 is morpholino, NH(CH 2 CH 2 OH), N(CH 2 CH 2 OH)CH 3 , N(CH 2 CH 2 OH) 2 or ethanolaminopropionic acid amide, and, in each case, the sulfo group is
• Preferred 4,4'-bis-(triazol-2-yl)stilbene-2,2'-disulfonic acids are those having the formula: in which R 3 and R 4 , independently, are H, C 1 -C 4 alkyl, phenyl or monosulfonated phenyl; and M is H, Na, Li, K, Ca, Mg, ammonium, or ammonium that is mono-, di-, tri- or tetra-substituted by C 1 -C 4 alkyl, C 1 -C 4 -hydroxyalkyl or a mixture thereof.
• M is Na, Li or K.
• Especially preferred compounds of formula (2) are those in which R 3 is phenyl, R 4 is H and M is sodium.
• Preferred 4,4'-dibenzofuranyl-biphenyls are those of the formula: in which R a and R b , independently, are H or C 1 -C 4 alkyl, and M is H, Na, Li, K, Ca, Mg, ammonium, or ammonium that is mono-, di-, tri- or tetra-substituted by C 1 -C 4 alkyl, C 1 -C 4 -hydroxyalkyl or a mixture thereof.
• M is Na, Li or K.
• the 4,4'-distyryl-biphenyls used are those of the formula: in which R 5 and R 6 , independently, are H, SO 3 M, SO 2 N(C 1 -C 4 alkyl) 2 , O-(C 1 -C 4 alkyl), CN, Cl, COO(C 1 -C 4 alkyl), CON(C 1 -C 4 alkyl) 2 or O(CH 2 ) 3 N®(CH 3 ) 2
• An - in which M is H, Na, Li, K, Ca, Mg, ammonium, or ammonium that is mono-, di-, tri- or tetra-substituted by C 1 -C 4 alkyl, C 1 -C 4 -hydroxyalkyl or a mixture thereof, An - is an anion of an organic or inorganic acid or a mixture thereof, and n is 1.
• M is Na, Li or K and
• An - is a formate, acetate, propionate, glcolate, lactate, acrylate, methanephosphonate, phosphite, dimethyl or diethyl phosphite anion, or a mixture thereof.
• each R 6 is H and each R 5 is a 2-SO 3 M group in which M is sodium or each R 5 is O(CH 2 ) 3 N ⁇ (CH 3 ) 2 An - , in which An - is acetate.
• Most especially preferred is the compound of the formula:
• Preferred 4-phenyl-4'-benzoxazolyl-stilbenes have the formula: in which R 7 and R 8 , independently, are H, Cl, C 1 -C 4 alkyl or SO 2 -C 1 -C 4 alkyl.
• the stilbenyl-naphthotriazoles used are those of the formula: in which R 9 is H or Cl; R 10 is SO 3 M, SO 2 N(C 1 -C 4 alkyl) 2 , SO 2 O-phenyl or CN; R 11 is H or SO 3 M; and M is H, Na, Li, K, Ca, Mg, ammonium, or ammonium that is mono-, di-, tri- or tetra-substituted by C 1 -C 4 alkyl, C 1 -C 4 hydroxyalkyl or a mixture thereof.
• M is Na, Li or K.
• Especially preferred compounds of formula (6) are those in which R 9 and R 11 are H and R 10 is 2-SO 3 M in which M is Na.
• the 4-styryl-stilbenes used are those of the formula: in which R 12 and R 13 , independently, are H, SO 3 M, SO 2 N(C 1 -C 4 alkyl) 2 , O-(C 1 -C 4 alkyl), CN, Cl, COO(C 1 -C 4 alkyl), CON(C 1 -C 4 alkyl) 2 or O(CH 2 ) 3 N ⁇ (CH 3 ) 2
• Especially preferred compounds of formula (7) are those in which each of R 12 and R 13 is 2-cyano or 2-SO 3 M in which M is sodium or O(CH 2 ) 3 N ⁇ (CH 3 ) 2 An - in which An - is acetate.
• M is Na, Li or K.
• Preferred coumarins are those of the formula: in which R 18 is H, Cl or CH 2 COOH, R 19 is H, phenyl, COO-C 1 -C 4 alkyl or a group of the formula: and R 20 is O-C 1 -C 4 alkyl, N(C 1 -C 4 alkyl) 2 , NH-CO-C 1 -C 4 alkyl or a group of the formula: in which R 1 and R 2 , independently, are phenyl, mono- or disulfonated phenyl, phenylamino, mono- or disulfonated phenylamino, morpholino, -N(CH 2 CH 2 OH) 2 , -N(CH 3 )(CH 2 CH 2 OH), -NH 2 , -N(C 1 -C 4 alkyl) 2 , -OCH 3 , -Cl, -NH-CH 2 CH 2 SO 3 H or -NH-CH 2 CH 2 OH,
• Especially preferred compounds of formula (10) are those having the formula: or
• the pyrazolines used are those having the formula: in which R 22 is H, Cl or N(C 1 -C 4 alkyl) 2 , R 23 is H, Cl, SO 3 M, SO 2 NH 2 , SO 2 NH-(C 1 -C 4 alkyl), COO-C 1 -C 4 alkyl, SO 2 -C 1 -C 4 alkyl, SO 2 NHCH 2 CH 2 CH 2 N ⁇ (CH 3 ) 3 or SO 2 CH 2 CH 2 N ⁇ H(C 1 -C 4 alkyl) 2
• An - , R 24 and R 25 are the same or different and each is H, C 1 -C 4 alkyl or phenyl, R 26 is H or Cl,
• An - is an anion of an organic or inorganic acid
• M is H, Na, Li, K, Ca, Mg, ammonium, or ammonium that is mono-, di-, tri- or tetra-substituted by C 1 -C 4 alkyl
• Especially preferred compounds of formula (13) are those in which R 22 is Cl, R 23 is SO 2 CH 2 CH 2 N ⁇ H(C 1 -C 4 alkyl) 2 An - in which An - is phosphite and R 24 , R 25 and R 26 are each H; or those having the formula: or
• Preferred naphthalimides are those of the formula: in which R 27 is C 1 -C 4 alkyl or CH 2 CH 2 CH 2 N ⁇ (CH 3 ) 3 An - in which An - is an anion of an organic or inorganic acid, R 28 and R 29 , independently, are O-C 1 -C 4 -alkyl, SO 3 M or NH-CO-C 1 -C 4 alkyl; and M is H, Na, Li, K, Ca, Mg, ammonium, or ammonium that is mono-, di-, tri- or tetra-substituted by C 1 -C 4 alkyl, C 1 -C 4 hydroxyalkyl or a mixture thereof.
• M is Na, Li or K.
• Especially preferred compounds of formula (16) are those having the formula: or
• Preferred 2-styryl-benzoxazole- or -naphthoxazole derivatives are those having the formula: in which R 31 is CN, Cl, COO-C 1 -C 4 alkyl or phenyl; R 32 and R 33 are the atoms required to form a fused benzene ring or R 33 and R 35 , independently, are H or C 1 -C 4 alkyl; and R 34 is H, C 1 -C 4 alkyl or phenyl.
• Preferred benzimidazole-benzofuran derivatives are those having the formula: in which R 36 is C 1 -C 4 alkoxy; R 37 and R 38 , independently, are C 1 -C 4 alkyl; and An - is an anion of an organic or inorganic acid.
• a particularly preferred compound of formula (21) is that in which R 36 is methoxy, R 37 and R 38 are each methyl and An - is methane sulfonate.
• Preferred oxanilide derivatives include those having the formula: in which R 39 is C 1 -C 4 alkoxy, R 41 is C 1 -C 4 alkyl, C 1 -C 4 alkyl-SO 3 M or C 1 -C 4 alkoxy-SO 3 M in which M is H, Na, Li, K, Ca, Mg, ammonium, or ammonium that is mono-, di-, tri- or tetra-substituted by C 1 -C 4 alkyl, C 1 -C 4 hydroxyalkyl or a mixture thereof, preferably Na, Li or K, and R 40 and R 42 are the same and each is hydrogen, tert. butyl or SO 3 M, in which M is as defined for R 41 .
• fluorescent whitening agents In the above classes of fluorescent whitening agents, it is advantageous to employ those that have a high affinity for the cellulosic or the lignin portion of the pulp fibers.
• a preferred group of such fluorescent whitening agents are those that are substituted by sulfonic acid groups, especially 2 to 6 sulfonic acid groups. It is especially preferred to employ those that have 2 sulfonic acid groups as the primary fluorescent whitening agents.
• fluorescent whitening agents may exhibit a green or bluish cast at high dosage levels, e.g. at dosage levels of about 2% by weight, based on the dry weight of the pulp. This is a normal expected effect and is unchanged by the presence of a metal chelating agent in the inventive process. This effect can be counteracted by the use of appropriate levels of mixtures of fluorescent whitening agents, particularly mixtures which contain fluorescent whitening agents having a more reddish cast.
• One preferred aspect of the present invention is to extend the effectiveness of the primary fluorescent brightener, in particular a fluorescent whitening agent that has 2 sulfonic acid groups, with a more highly active and lower affinity whitener such as a fluorescent whitening agent that has 4 or 6 sulfonic acid groups. This allows the tailoring of the brightener mix to optimize the development of fluorescence and shade, as well as the economics of the process.
• Use of a mixture of the fluorescent whitening agents of the formulae (1a) and (1b) is particularly preferred in this regard.
• a mixture comprising 30 to 90 parts by weight of a compound of the formula (1a) with 70 to 10 parts by weight of the compound of the formula (1b), most especially a mixture of 50 to 70 parts by weight of a compound of the formula (1a) with 50 to 30 parts by weight of the compound of the formula (1b).
• Judicious choice of the optical brighteners allows the manufacturer to achieve the desired brightness targets in the pulp mill while simultaneously balancing other bleach parameters to provide the desired physical properties such as bulk, stiffness, curl, opacity, fiber strength, drainage, etc.
• the use of fluorescent whitening agents also gives benefits in shade properties, for example in blueness (b* value), that are desirable in the marketing of these pulps.
• Another preferred aspect of the present invention is to employ mixtures of the same or different types of disulfo fluorescent whitening agents.
• mixtures comprising a compound of the formula (1a) and/or (1c) with the compound of the formula (4a).
• Such mixtures may comprise 5 to 95 parts by weight of a compound of the formula (1a) and/or (1c) with 95 to 5 parts by weight of the compound of the formula (4a), preferably a mixture of 60 to 90 parts by weight of a compound of the formula (1a) and/or (1c) with 40 to 10 parts by weight of the compound of the formula (4a) and especially a mixture of 75 to 85 parts by weight of a compound of the formula (1a) and/or (1c) with 25 to 15 parts by weight of the compound of the formula (4a), the sum of the parts being in each case 100.
• the fluorescent whitening agents comprising the mixtures may be added to the aqueous pulp separately or as a blend.
• the above fluorescent whitening agent mixtures give superior results, particularly with regard to blueness (b* value), compared to the same weight of the individual compounds.
• the amount of the fluorescent whitening agent to employ will vary from 0.01 up to about 2% by weight, based on the dry weight of the pulp, depending on the degree of whiteness improvement desired. Preferably from 0.1 to 1.5% is used; most preferably 0.2 to 0.8% is used.
• the fluorescent whitening agent is preferably added in the latter steps of the pulping process to minimize physical and chemical losses. Most preferably the fluorescent whitening agent is added after completion of the bleaching steps and prior to the final dewatering, fluffing and drying.
• the pulp usually has a reasonable pH (preferably between 4 and 10) and a very fluid consistency which promotes rapid mixing of the fluorescent whitening agent throughout the pulp.
• the solids content is 5-15% by weight, based on the dry weight of the pulp.
• the fluorescent whitening agent does work at high consistencies (> 50% solids by weight, based on the dry weight of the pulp) in the present invention
• the final pulp is more subject to mottling due to incomplete mixing relative to the time it takes for the fluorescent whitening agent to fix onto the fiber. Therefore, advantageously, the solids content based on the dry weight of the pulp is below 50%, preferably below 30%. Most preferably it is between 5-15% by weight when the fluorescent whitening agent is added.
• Another reason for preferring to add the fluorescent whitening agent after the final bleaching stage is that the amount of fluorescent whitening agent to add can easily be varied at this point in the pulping process to adjust the brightness to the appropriate level. While some residual peroxide or reductive bleaching chemicals may still be present at this stage, this does not adversely affect the inventive process.
• the fluorescent whitening agents work well within the wide temperature range, typically 70-150° F (21-62° C), encountered during the bleaching and subsequent stages of pulp manufacture.
• a chelating agent in the process of the present invention, even if a chelating agent was employed in earlier steps, for example during bleaching, it is still advantageous to supply additional chelating agent to the lignin-containing pulp prior to the addition of the fluorescent whitening agent in order to control the amount of salts of iron and other heavy metals within the ranges taught above. Additionally, the presence of a chelating agent is also advantageous as an aid in dispersing the fluorescent whitening agent in the pulp to minimize mottling. This is especially important if the fluorescent whitening agent is added to high consistency pulps, for example after dewatering and just prior to going into the fluffer and drying stages.
• Additives which are known to enhance the effectiveness of fluorescent whitening agents may also be used in the present invention, provided they are within the limits of what is acceptable to the end user of the pulp.
• another preferred aspect of the present invention comprises using a fluorescent whitening agent in combination with an additive, for example a substance used to promote UV absorption and "bloom" of the fluorescent whitener in paper or a material that effectively allows the optical brightener to develop a higher degree of fluorescent whitening by cleaning the pulp fibers.
• Suitable additives include cationic starch, polyvinyl alcohol and enzymes.
• Suitable enzymes include cellulases and hemicelllulases. The addition of polyvinyl alcohol is particularly preferred.
• addition of polyvinyl alcohol to the pulp at the 1.25% level, based on the dry weight of the pulp, can increase the effectiveness of an optical brightener by up to four more GE units, compared to the same fluorescent whitening agent without use of the additive.
• parts are parts by weight.
• a bleached but not dewatered CTMP i.e. a peroxide bleached chemi-thermomechanical pulp having an iron content of about 10 ppm and a lignin content which typically corresponds to 85-90% of the lignin present in an equivalent amount of wood chips, at a temperature between 50-65 degrees Centigrade and a pH between 7 and 8, is continuously added, in the ratio of 2.3 parts per 1000 parts of slurry, both based on the "as is" weight of the pulp slurry, an aqueous liquid containing 12.5% by weight of the fluorescent whitening agent Tinopal® HW (Ciba Specialty Chemicals Corp, Consumer Care Division, High Point, NC) of the formula
• the time between the Tinopal HW addition to the pulp slurry before the mixture is dewatered and dried is less than ten (10) minutes.
• the brightness of the resulting pulp rises from its initial value of 86 to 90+.
• Example 1 is repeated, but using an aqueous slurry containing about 8% of a peroxide bleached chemi-thermomechanical pulp. With 5.5 parts per 1000 parts, both based on the "as is" weight of the pulp slurry, of an aqueous liquid containing 12.5% by weight of the fluorescent whitening agent of the formula
• the brightness rises from 86 to 91+ and stabilizes in a relatively short time (ca. 10 minutes).
• a peroxide bleached chemi-thermomechanical pulp having an iron content of about 10 ppm and a lignin content which typically corresponds to 85-90% of the lignin present in an equivalent amount of wood chips is mixed with 4 parts per 1000 parts, both based on the "100% dry basis” weight of the pulp slurry, of an aqueous liquid mixture containing an 80:20 “dry basis ratio” mixture of 12.5% by weight of the fluorescent whitening agent Tinopal® HW and 33% by weight of the fluorescent whitening agent Tinopal® SK (Ciba Specialty Chemicals Corp, Consumer Care Division, High Point, NC), of the formulas and respectively.
• the Tinopal blend is allowed to mix with the pulp slurry for less than ten (10) minutes before the mixture is dewatered and dried.
• the brightness of the resulting pulp rises from its initial value of 81 to 94+.
• the blueness as judged by the b* value increases 20% over what is obtained by the use of Tinopal HW alone.

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• 1998
  • 1998-08-19 EP EP98810809A patent/EP0899373A1/fr not_active Withdrawn
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  • 1998-08-26 CA CA002245966A patent/CA2245966A1/fr not_active Abandoned
  • 1998-08-27 AU AU81952/98A patent/AU739524B2/en not_active Ceased
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