NO341227B1 - Improved composition and processes for paper production - Google Patents

Description

IMPROVED COMPOSITION AND PROCESSES FOR PAPER PRODUCTION

Technical area

The present invention relates to methods for improving brightness and optical properties, preventing loss of brightness and for improving resistance to thermal yellowing in pulp and paper production.

Background for the invention

Pulps produced by either mechanical or chemical pulping methods have a color that can range from dark brown to cream depending on the type of wood and the defibrillation process used. The pulp is bleached to produce white paper products for a variety of applications.

Bleaching is the inhibition of the light-absorbing substances found in unbleached pulp. In bleaching mechanical pulp, the purpose is to decolorize the pulp without dissolving the lignin. Either reducing (eg sodium hydrosulphite) or oxidising (eg hydrogen peroxide) bleaches are usually used. Bleaching is often a multi-step process. The bleaching of chemical pulps is an extension of the delignification that started in the cooking step. The bleaching is often a multi-step process, which steps may include chlorine dioxide bleaching, oxygen-alkaline delignification and peroxide bleaching.

Discoloration, most often attributable to thermal aging, results in yellowing and loss of brightness in various stages of the papermaking process that use bleached pulp and in the resulting paper products. The industry invests considerable resources in chemicals such as bleaching agents and optical whiteners that improve the optical properties of the finished paper or paper products. So far, however, the results have been unsatisfactory and financial losses resulting from discoloration and yellowing represent significant ongoing challenges for the industry. There is therefore a need for a satisfactory and practical solution for loss of brightness and unwanted yellowing of pulp and paper.

WO 02/46522 describes a method for final bleaching of cellulose-containing pulp which has previously been subjected to bleaching with a peroxide component. The method includes the sequential steps of exposing the pulp to an amount of a reducing agent to remove residues of the peroxide compound from the pulp and then finally bleaching the pulp by exposing it to an amount of a formamidine sulfinic acid.

WO 05/103135 describes treatment of mechanical or chemical pulp with approx. 0.01 mass-% to approx. 5% by mass of a mixture. The mixture is water,

diethylene triamine pentakis (methyl) phosphonic acid or its known salts, polyacrylic acid or its known salts, and optionally one or more inert compounds. The mixture acts to either maintain the brightness level of the pulp at the target value for brightness even though the amount of other ingredients, such as hydrogen peroxide, hydrosulphite, sodium silicate and magnesium that are typically added to the pulp, is reduced.

JP 09-049195 describes a process in which a stilbene-based fluorescent bleaching agent is added to the raw pulp slurry together with a reducing agent such as sodium thiosulphate or sodium sulphite. The slurry is then subjected to a papermaking process. Alternatively, the reducing agent can be added to the paper during the papermaking process but after the addition of the stilbene or it can be applied after the papermaking process. This method is aimed at improving the photoyellowing stability of paper containing optical brighteners.

US 2002/059999 describes a method for reducing or inhibiting alkaline darkening of mechanical pulp caused by the presence of calcium carbonate fillers during manufacture. The method includes the addition of a sulfite, or sulfur-containing reducing agent, especially a sulfite to the mechanical pulp and filler. The method is most usefully used in the production of mechanical pulp and paper where calcium carbonate is used as filler and also in the production of paper under neutral or slightly alkaline conditions.

The present invention provides compositions and methods for improving and stabilizing lightness and increased resistance to yellowing in the papermaking process.

In one aspect, the invention is a method of making bleached pulp material having improved brightness and improved resistance to thermal yellowing comprising (i) providing a bleached pulp material; (ii) contacting the bleached pulp material with an effective amount of one or more reducing agents that improve the brightness and resistance to thermal yellowing of the pulp compared to pulp that has not been treated with the reducing agent or agents, and (iii) contacting the bleached pulp material in contact with one or more optical brighteners selected from the group consisting of azoles, bifenthyls, coumarins, furans, naphthalimides, pyrazenes, and salts of such compounds including alkali metal salts, alkaline earth metal salts, transition metal salts, organic salts and ammonium salts; and combinations thereof.

The applicant has also discovered that the reducing agents used in combination optical brighteners improve the effect of the optical brighteners and improve the color composition.

The reducing agent, optical brighteners and possibly chelating agents can be used alone or in combination with known additives to improve the quality of the desired paper product.

Detailed description of the invention

The present invention provides an improved method for the production of paper and paper products which exhibit high optical brightness. Brightness stabilization against thermal yellowing, color improvement and brightness improvement of bleached pulp and the paper product made from the bleached pulp can be achieved by adding one or more reducing agents and one or more optical brighteners as defined here pulp, paper, board and tissue, anywhere in the papermaking process.

Lightness is a term used to describe the whiteness of the pulp or paper, on a scale from 0% (absolute black) to 100% (relative to an MgO standard, which has an absolute lightness of about 96%) by reflectance with blue light (457 nm) from the paper. "Thermal lightness loss" is a lightness loss in paper and pulp under the influence of time, temperature and humidity (non-photochemical lightness loss). "Light loss during storage" is thermal light loss over time under storage conditions.

Yellowing of a bleached pulp material (brightness reversion) is a loss of brightness of bleached pulp, paper, board, paper tissues and related materials made from the bleached pulp over a period of time.

The reducing agents described herein are applicable for use on any bleached pulp material used in papermaking processes, and any paper product made from the bleached pulp. As used herein, "bleached pulp material" means bleached pulp and paper products made from the bleached pulp including paper, cardboard, tissue and the like.

The reducing agents according to the present invention include chemical substances which are capable of transforming functional groups in the bleached mass from a higher oxidation state to a lower oxidation state. The benefits of this transformation included increased brightness stability in the paper machine and improved performance of the optical brighteners.

In one embodiment, the reducing agents are selected from the group consisting of sulfites, bisulfites, metabisulfites (pyrosulfites), sulfoxylates, thiosulfates, dithionites (hydrosulfites), polythionates, formamidine sulfinic acid and salts and derivatives thereof, formaldehyde-disulfite adducts and other aldehyde bisulfite adducts, sulfinamides and ethers of sulfinic acids, sulfenamides and ethers of sulfenic acids, sulfamides, phosphines, phosphonium salts, phosphites and thiophosphites.

As used herein, "sulfites" means dibasic metal salts of sulfuric acid, H 2 SO 3 , including dibasic alkali and alkaline earth metal salts such as sodium sulfite (Na 2 SO 3 ), calcium sulfite (CaSCb) and the like.

"Bisulfites" means monobasic metal salts of sulfuric acid, H2SO3, including alkali and alkaline earth metal monobasic salts such as sodium bisulfite (NaHSCb), magnesium bisulfite (Mg(HS03)2) and the like.

"Sulfoxylates" means salts of sulfoxylic acid, H 2 SO 2 , including zinc sulfoxylate (ZnSO 2 ) and the like.

"Metabisulfites (Pyrosulfites") means salts of pyrosulfuric acid, H2S2O5, including sodium metabisulfite (Na2S20s) and the like.

"Thiosulfates" means salts of thiosulfuric acid, H 2 S 2 O 3 , including sodium thiosulfate (Na 2 S 2 O 3 ) and the like.

"Polythionates" means salts of polythionic acid, H 2 SnO 6 (n=2-6), including sodium trithionate (Na 2 S 3 O 6 ), salts of dithionic acid, H 2 S 2 O 6 , such as sodium dithionate Na 2 S 2 O 6 , and the like.

"Dithionites (hydrosulphites)" means salts of dithion (hydrosulphuric, hyposulphuric) acid, H2S2O4, including sodium dithionite (hydrosulphite)

(Na2S204), magnesium dithionite (MgS204) and the like.

"Formamidine sulfinic acid (FAS)" means a compound of formula H2NC(=NH)SO2H and its salts and derivatives including sodium salt H2NC(=NH)SO2Na.

"Aldehyde bisulfite adducts" to compounds with the formula R1CH(OH)SO3H and metal salts thereof, where Ri is selected from alkyl, alkenyl, aryl and arylalkyl. Representative aldehyde bisulfite adducts include formaldehyde bisulfite adduct HOChteSOsNa, and the like.

"Sulfinamides and ethers of sulfinic acid" means compounds of formula Ri-S(=O)-R2, wherein Ri is defined herein and R2 is selected from OR3 and NR4R5, where R3-R5 are independently selected from alkyl, alkenyl, aryl and arylalkyl. Representative sulfinamides include ethyl sulfindimethylamide (CH 3 CH 2 S(=O)N(CH 3 ) 2 ) and the like.

"Sulfenamides and ethers of sulfenic acid" means compounds of the formula R1-S-R2, where R1 and R2 are defined above. Representative sulfenamides include ethyl sulfendimethylamide (CH 3 CH 2 SN(CH 3 ) 2 ) and the like.

"Sulfamides" means compounds of the formula Ri-C(=S)-NR4Rs, where Ri, R4 and R5 are defined above. Representative sulfonamides include CH 3 CH 2 C(=S)N(CH 3 ) 2 and the like.

"Phosphines" means derivatives of phosphine, PH3, normally organically substituted phosphines of the formula R6R7R8P, where R6-Rs are independently selected from H, alkyl, alkenyl, aryl, arylalkyl and NR4R5, where R4 and R5 are defined above. Representative phosphines include (HOCH 2 ) 3 P (THP) and the like.

"Phosphites" means derivatives of phosphoric acid P(OH)3 including organically substituted phosphites of the formula (R30)(R40)(RsO)P, where R3-R5 are defined above. Representative phosphites include (CH 3 CH 2 O) 3 P and the like.

"Thiophosphites" means derivatives of phosphorothioacid HSP(OH)2, including organically substituted thiophosphites of the formula (R30)(R40)(R5S)P, where R3-R5 are defined above. Representative thiophosphites include (CH 3 CH 2 O) 2 (CH 3 CH 2 S)P and the like.

"Phosphonium salts" means organically substituted phosphines of the formula RiR3R4R5P<+>X", where Ri and R4-R5 are as defined above and X is any organic or inorganic anion. Representative phosphonium salts include (H02CCH2CH2)3P<+>HCI- (THP), [(HOCH2)4P<+>]2(SO4)<2->(BTHP) and the like.

"Alkenyl" means a monovalent group derived from a straight or branched hydrocarbon containing at least one carbon-carbon double bond by the removal of a single hydrogen atom. The alkenyl can be unsubstituted or substituted with one or more groups selected from amino, alkoxy, hydroxy and halogen.

"Alkoxy" means an alkyl group attached to the molecular head group via an oxygen atom. Representative alkoxy groups include methoxy, ethoxy, propoxy, butoxy and the like. Methoxy and ethoxy are preferred.

"Alkyl" means a monovalent group derived from a straight or branched chain saturated hydrocarbon by the removal of a single hydrogen atom. The alkyl may be unsubstituted or substituted with one or more groups selected from amino, alkoxy, hydroxy and halogen. Representative alkyl groups include methyl, ethyl, n- and iso-propyl, n-, sec- and tert-butyl and the like.

"Alkylene" means a divalent group derived from a straight or branched chain saturated hydrocarbon by the removal of two hydrogen atoms, for example methylene, 1,2-ethylene, 1,1-ethylene, 1,3-propylene, 2,2-dimethylpropylene and the like.

"Amino" means a group of the formula -NY 1 Y 2 , wherein Y 1 and Y 2 are independently selected from H, alkyl, alkenyl, aryl and arylalkyl. Representative amino groups include amino(-NH 2 ), methylamino, ethylamino, isopropylamino, diethylamino, dimethylamino, methylethylamino and the like.

"Aryl" means aromatic carboxyl radicals and heterocyclic radicals with approx. 5 to approx. 14 ring atoms. The aryl may be unsubstituted or substituted with one or more groups selected from amino, alkoxy, hydroxy and halogen. Representative aryls include phenyl, naphthyl, phenanthryl, anthracyl, pyridyl, furyl, pyrrolyl, quinolyl, thienyl, thiazolyl, pyrimidyl, indolyl and the like.

"Arylalkyl" means an aryl group attached to the molecular head group via an alkylene group. Representative aryl groups include benzyl, 2-phenylethyl and the like.

"Halo" and "halogen" mean chlorine, fluorine, bromine and iodine.

"Salt" means the metal, ammonium, substituted ammonium or phosphonium salt of an inorganic or organic anionic counterion. Representative metals include sodium, lithium, potassium, calcium, magnesium and the like. Representative anionic counterions include sulfite, bisulfite, sulfoxylate, metabisulfite, thiosulfate, polythionate, hydrosulfite, formamidine sulfinate, and the like.

In one embodiment, the reducing agent is selected from the group consisting of substituted phosphines, sulfites, bisulfites and metabisulfites.

In one embodiment, the reducing agent is sodium bisulfite.

The method according to the present invention can be practiced on conventional papermaking equipment. Although papermaking equipment varies in terms of operation and mechanical design, the processes by which the paper is produced on different equipment may contain common steps.

Papermaking typically includes a pulping step, bleaching step, a pulping step, a wet end step and a dry end step.

In the pulping steps, individual cellulose fibers are released from a cellulose source either chemically or mechanically, or both. Representative sources of cellulose include, but are not limited to, wood and similar "woody" plants, soy, rice, cotton, straw, flax, manila hemp, hemp, sugarcane, lignin-containing plants and the like as well as virgin and recycled paper, paper tissue and paperboard . Such pulps include, but are not limited to grinding pulp (GWD), bleached grinding pulp, thermomechanical pulps (TMP), bleached thermomechanical pulps, chemithermomechanical pulps (CTMP), bleached chemithermomechanical pulps, deinked pulps, kraft pulps, bleached kraft pulps, sulphite pulps and bleached sulphite pulps. Recycled pulps may optionally be bleached in the recycling steps, but they are assumed to be originally bleached. Each of the pulps described above which has not previously been subjected to bleaching can be bleached as described herein to give a bleached pulp material.

In one embodiment, the bleached pulp material is selected from the group consisting of virgin pulp, recycled pulp, kraft pulp, sulfite pulp, mechanical pulp, any combination of such pulps, recycled paper, paper tissue, and any paper made from specified pulps or combinations thereof.

Several advantages of the present invention are that it allows the exchange of lower priced mechanical pulp for higher priced kraft pulp in kraft mechanical paper of printing quality. Use of the chemistry and methods described herein increases brightness and stability against yellowing and allows higher amounts of mechanical pulp, with associated cost reductions, without loss of quality in the resulting paper product.

The pulp is dissolved in water in the pulp production step. Additives such as brighteners, dyes, pigments, fillers, antimicrobials, anti-foaming agents, pH controlling agents and drainage aids may also be added to the pulp at this step. As the term is used in this specification, "pulp preparation" includes such operations as dilution, screening and purification of the pulp suspension which may occur before the web is formed.

The wetting end step of the papermaking process involves depositing pulp suspensions or the pulp slurry on a wire or felt in the paper machine to form a continuous web of fibers, draining the web, and consolidating the web ("pressing") to form a sheet. Any known paper machine is applicable for use with the method according to the present invention. Such machines may include cylinder machines, Fourdrinier machines, double-wire forming machines, tissue machines and the like and modifications thereof.

At the dry end of the papermaking process, the web is dried and may be subjected to further processing such as size pressing, calendering, spray coating with surface modifiers, printing, cutting, corrugation and the like. In addition to a glue press and calender box, the dried paper can be spray coated using a spray boom.

In one embodiment, one or more chelating agents are added to this bleached pulp or paper product. Suitable chelating agents according to the present invention include compounds capable of chelating transition metals that form colored products with the pulp constituents and catalyzing color forming reactions in the bleached pulp or paper products.

In one embodiment, the chelating compound is a compound selected from the group consisting of organic phosphonate, phosphate, carboxylic acids, dithiocarbamates, salts of any of these aforementioned compounds, and any combination thereof.

"Organic phosphonates" means organic derivatives of phosphonic acid, HP(0)(OH)2, containing a single C-P bond such as HEDP (CH3C(OH)(P(0)(OH)2), 1-hydroxy-1, 3-propanediylbisphosphonic acid ((HO)2P(0)CH(OH)CH2CH2P(0)(OH)2)); preferably containing a single C-N bond close (vicinal) to the C-P bond, such as DTMPA

((HO)2P(0)CH2N[CH2CH2N(CH2P(0)(OH)2)2]2), AMP (N(CH2P(0)(OH)2)3),

PAPER PAPER

((HO)2P(0)CH2)2NCH(CH3)CH2(OCH2CH(CH3))2N(CH2)6N(CH2P(0)(OH)2)2), HMDTMP ((HO)2P(0)CH2) 2N(CH2)6N(CH2P(0)(OH)2)2), HEBMP (N(CH2P(0)(OH)2)2CH2CH2OH) and the like.

"Organic phosphates" means organic derivatives of phosphoric acid, P(0)(OH)3, containing a single C-P bond, including triethanolamine tri(phosphate ester) (N(CH2CH2OP(0)(OH)2)3) and the like.

"Carboxylic acids" means organic compounds containing one or more carboxylic acid groups, -C(0)OH, preferably aminocarboxylic acids containing a single C-N bond close (vicinal) to the C-CChH bond, such as EDTA ((H02CCH2)2NCH2CH2N(CH2C02H)2 ), DTPA

((H02CCH2)2NCH2CH2N(CH2C02H)CH2CH2N(CH2C02H)2) and the like and alkali and alkaline earth metal salts thereof.

"Dithiocarbamates" include monomeric dithiocarbamates, polymeric dithiocarbamates, polydiallylamindithiocarbamates, 2,4,6-trimercapto-1,3,5-triazine, disodium ethylene bisdithiocarbamate, disodium dimethyldithiocarbamate and the like.

In one embodiment, the chelating agent is ethphosphonate.

In one embodiment, the phosphonate is

diethylenetriaminepentamethylenephosphonic acid (DTMPA) and salts thereof.

In one embodiment, the chelating agent is a carboxylic acid.

In one embodiment, the carboxylate is selected from

diethylenetriaminepentaacetic acid (DTPA) and its salts and ethylenediaminetetraacetic acid (EDTA) and its salts.

Applicant has also discovered that reducing agents used in combination with optical brighteners ("OBA") enhance the effect of the optical brighteners (OBA). The reducing agents also improved the color composition. This allows reduction of the amount of OBA and lighteners such as blue dyes required to achieve comparable lightness and color. Replacing some of the OBA and colorants with reducing agents allows the pulp and paper manufacturers to reduce production costs and reduce the total amounts of OBA and colorants present, while maintaining an acceptable level of lightness in the paper product and achieving the color target. In some cases, it may be possible to eliminate the dyes completely and retain the color.

Two or more optical brighteners ("OBA") are therefore added to the bleached pulp or paper product.

"Optical brighteners" are fluorescent dyes or pigments that absorb ultraviolet radiation and re-emit it at a higher frequency in the visible spectrum (blue), therefore imparting a white, bright appearance to the paper sheet when added to the pulp composition. Representative optical brighteners that can be used in the present invention include azoles, biphenyls, coumarins, furans, naphthalimides, pyrazenes, and salts of such compounds, including but not limited to alkali metal salts, alkaline earth metal salts, transition metal salts, organic salts, and ammonium salts of such brighteners, and combination is of one or more of the preceding compounds.

The effective amount of reducing agent added to the bleached pulp or paper product is the amount of reducing agent that improves the brightness and resistance to thermal yellowing of the pulp or paper compared to pulp or paper that has not been treated with the reducing agents. The methods for determining lightness and resistance to thermal yellowing are described here.

Typically approx. 0.005 to approx. 2, preferably approx. 0.05 to approx. 0.25 mass percent, based on oven-dried pulp of reducing agent added to the bleached pulp or paper product.

In a typical application, approx. 0.001 to approx. 1, preferably approx. 0.01 to approx. 0.1 mass percent phosphonate, phosphate or carboxylic acid chelating agent and/or approx. 0.002 to approx. 0.02 mass percent dithiocarbamate chelating agent based on oven-dried pulp added to the bleached pulp or paper product.

Optical brighteners are typically added in a quantity of approx. 0.005 to approx. 2, preferably 0.05 to approx. 1 mass percent of optical brightener based on oven-dried pulp.

The reducing agents and optical brighteners can be added to the bleached pulp or paper at any point in the paper or tissue manufacturing process. Representative addition points include, but are not limited to

(a) the pulp slurry in the waiting box; (b) to the pulp after the bleaching step in the storage, mixing or transfer case; (c) to the pulp after bleaching, washing and dewatering followed by cylinder or flash drying; (d) before and after the cleaners;

(e) before and after the fan pump to the paper machine inlet box:

(f) to the white water of the paper machine; (g) to the silo or save all; (h) in the press section by, for example, using a glue press, coating

or syringe rod;

(i) in the drying section using, for example, a glue press, coater or

spray rod;

(j) on the calender using a disc jacket; and/or (k) on the paper in an off-machine coater or glue press; and or

(I) in the curl control unit.

The exact position where the reducing agents and optical brighteners are to be added will depend on the particular equipment used, the exact process conditions used, and the like. In some cases, the reducing agents and the optical brighteners can be added at one or more positions for optimum effectiveness.

The addition may be in any conventional manner used in the papermaking process, including by "split-feeding", whereby a portion of the reducing agent and optical brightener is added at one point in the papermaking process, for example to the pulp or a wet sheet (before the dryers) and the the remaining part is added at a subsequent point, for example in the glue press.

The optical brightener can be added to the bleaching pulp or paper product before, after or simultaneously with the reducing agent. The optical brightener can also be formulated with the reducing agent.

In one embodiment, one or more reducing agents and one or more optical brighteners can be formed with the surface adhesive solution and applied in the adhesive press.

In one embodiment, the reducing agent is added to the bleached pulp after the bleaching step in the storage, mixing or transfer case.

At these different positions, the reducing agents and the optical brighteners can also be added with a carrier or additive that is typically used in papermaking, such as retention aids, sizing aids and solutions, starches, precipitated calcium carbonate, ground calcium carbonate, and other clays or fillers and brightening additives.

In one embodiment, the reducing agents, chelating agents and/or optical brighteners are used in combination with one or more partially neutralized polycarboxylic acids, preferably polycarboxylic acids such as polyacrylic acid (CH3CH(CO2H)[CH2CH(CO2H)]nCH2CH2CO2H, where n is about 10 to about 50,000. The polycarboxylic acid be neutralized to the pH target (typically 5-6 as discussed below) with alkali such as sodium hydroxide.

In one embodiment, the invention is a formulation including one or more chelating agents, one or more reducing agents and one or more polycarboxylic acids. The formulation preferably has a pH of approx. 4-7, more preferably 5-6.

The reducing agents and optical brighteners may be used in addition to other additives commonly used in papermaking to improve one or more properties of the finished paper product, aid in the process of making the papers themselves, or both. These additives are generally characterized as either functional additives or control additives.

Functional additives are typically those additives used to improve or impart certain particularly desired properties to the final paper product, and include, but are not limited to, brighteners, dyes, fillers, sizing agents, starches and adhesives.

Control additives, on the other hand, are additives included during the process of making the paper to improve the overall process without significantly affecting the physical properties of the paper. Control additives include biocides, retention aids, antifoam agents, pH control agents, pitch control agents and drainage aids. Paper and paper products produced using the process according to the present invention may contain one or more functional additives and/or control additives.

Pigments and dyes give color to the paper. Dyes include organic compounds with conjugated double bond systems; azo compounds; metallic azo compounds; anthraquinones; triaryl compounds, such as triarylmethane; quinoline and related compounds; acid dyes (anionic organic dyes containing sulfate groups, used with organic compounds such as alum); basic dyes (cationic organic dyes containing amine functional groups); and direct dyes (acid-type dyes with a high molecular weight and a specific, direct affinity for cellulose); as well as combinations of the above indicated appropriate dye compounds. Pigments are finely divided minerals that can be either white or colored. The pigments commonly used in

the papermaking industry is clay, calcium carbonate and titanium dioxide.

Fillers are added to the paper to increase opacity and lightness. Fillers include but are not limited to calcium carbonate (calcite); precipitated calcium carbonate (PCC); calcium sulfate (including various hydrated forms); calcium aluminate; zinc oxides; magnesium silicates; such as talc; titanium dioxide (TiCh), such as anatase or rutile; clay, or kaolin, consisting of hydrated SiCte and AI2O3; synthetic clay; mica; vermiculite; inorganic aggregates; perlite; sandy; gravel; sandstone; glass balls; aerogels; xerogels; sea gel; fly ash; alumina; microspheres; hollow glass spheres; porous ceramic balls; cork; seed; lightweight polymers; xonotlite (a crystalline calcium silicate gel); pumice stone; painted stone; waste concrete products; partially hydrated or non-hydrated hydraulic cement particles; and diatomaceous earth as well as combinations of such compounds.

Sizing agents are added to the paper during the manufacturing process to help develop a resistance to the penetration of liquids through the paper. The adhesives can be internal adhesives or external (surface) adhesives and can be used for hard-gluing, impact-gluing or both. More particularly, the adhesives include resins; resin precipitated with alum (Al2(SO4)3); abietic acid and abietic acid homologues such as neoabietic acid and levopimaric acid; stearic acid and stearic acid derivatives; ammonium zirconium carbonate; silicone and silicone-containing compounds, such as RE-29 available from GE-OSI and SM-8715, available from Dow Corning Corporation (Midland, MI), fluorochemicals of general structure CF3(CF2)nR, where R is anionic, cationic or other functional group, such as Gortex; alkyl ketene dimers (AKD), such as Aquapel 364, Aquapel (I 752, Heron) 70, Hercon 79, Precise 787, Precise 2000, and Precise 3000, all of which are commercially available from Hercules, Incorporated (Willmington, DE); and alkylsuccinic anhydride (ASA); emulsions of ASA or AKD with cationic starch; ASA including alum; starch; hydroxymethyl starch; carboxymethyl cellulose (CMC); polyvinyl alcohol; methyl cellulose; alginates; growing; wax emulsions; and combinations of such adhesives.

Starch has many uses in papermaking. For example, it acts as a retention agent, dry-strengthening agent and surface adhesive. Starches include, but are not limited to, amylose; amylopectin; starches containing different amounts of amylose and amylopectin; such as 25% amylose and 75% amylopectin (corn starch) and 20% amylose and 80% amylopectin (potato starch); enzymatically treated starches; hydrolyzed starches; heated starches; also known in the field as "starch pastes"; cationic starches, such as those resulting from the reaction of a starch with a tertiary amine to form a quaternary ammonium salt; ainoic starches; ampholytic starches (containing both cationic and anionic functionalities); cellulose and cellulose-derived compounds; and combinations of these compounds.

The method according to the present invention provides paper products with a bright surface.

The foregoing can be better understood with reference to the accompanying examples, which are given for illustrative purposes only and which are not intended to limit the scope of protection of the invention.

EXAMPLES

In these examples, sufficient 50% aqueous sodium hydroxide was added to achieve the appropriate pH for the agent or mixture under investigation. All percentages in these examples are given as mass percent on a dry mass basis.

In these examples, the following terms shall have the specified meaning.Br for ISO lightness R457 (TAPPI 525); Ye for E313 yellowness; Im Br for R457 brightness after application; TA Br for R457 brightness after thermal ageing; TA loss for loss of brightness after thermal ageing; %lnh. For %inhibition of brightness loss: %lnh. = 100-100<*>(lmBr-TABr)/(lmBr-TABr) control; Wl for E313 whiteness: TMP for thermomechanical pulp; CTMP for chemithermomechanical pulp; RMP for refined mechanical pulp; OBA for optical brighteners; FAS forformamidine sulfinic acid; TCP for (HOCH2CH2)3PHCI, tris-carboxyethylphosphonium hydrochloride; BTHPfor [(HOCH 2 ) 4 P] 2 (SO 4 ), tetra-hydroxymethylphosphonium sulfate; THP for (HOCH2)3P, tris-hydroxymethylphosphine; EDTA for (H02CCH2)2NCH2CH2N(CH2C02H)2, ethylenediaminetetraacetic acid; DTPA for

(H02CCH2)2NCH2CH2N(CH2C02H)CH2CH2N(CH2C02H)2, diethylenetriaminepentaacetic acid; DTMPA for

H2O3PCH2N[CH2CH2N(CH2PO3H2)2]2, diethylenetriaminepentamethylenephosphonic acid; and DTC for sodium dimethyldithiocarbamate.

Treatment

Hand sheets of bleached pulp were produced and then used in the experiments, where the reducing agents were added either to a wet sheet (before or after the press) before drum drying or drum drying (temperature during drum drying: 100°C). The third option was split feed application. Surface adhesive application was followed by one or more rounds on a drum dryer.

The loading of the tested agent or mixture solution was determined based on the dry weight of the pulp sample. The agent or mixture solutions were applied using a stand, as evenly as possible, as solutions in water. The test sheets were dried using a laboratory tumble dryer under uniform conditions (one round) and then, after measuring lightness, subjected to accelerated aging tests as described below.

Post-yellowing test (thermal ageing, paper):

3x9 cm samples were cut out of the test sheets and kept in a water bath at 70°C, 100% humidity for approx. 3 days. The samples were equilibrated in a room with constant humidity before measuring lightness.

Post-yellowing test (thermal ageing, mass):

The pulp samples (10% consistency, 5 g of pulp on a similar basis) were sealed in plastic bags and kept in a water bath at 70°C for 3-6 hours. Hand sheets were produced which were leveled in a room with constant humidity before measuring lightness.

Experimental equipment:

Laboratory tumble dryer

"Elrepho 3000", "Technidyne Coltor Teouch 2 (Model ISO)" or another instrument for brightness measurements. Hitachi F-4500 fluorescence spectrometer or other instrument for measuring relative fluorescence intensity.

Micropipette

Surface adhesive application kit (pad and adhesive 3-applicator bar). Room with constant humidity (23°C, 50% humidity). Water bath/thermostat with space for a floating plastic box with paper samples. 100 ml application cuvette for the soaking method.

Dry surface application procedure (surface bonding):

1. Prepare 8x8 inch hand sheets according to the standard procedure. Target dry weight is 2.5 g. Run the wet hand sheets through a cycle on the tumble dryer.

2. Cut the sheets into 4 smaller squares (approximate weight of 0.625g each).

3. Glue one side of the smaller squares (test sheet) to a glass pad using Scotch tape with a length greater than the side of the sheet. 4. The application rod is placed on the scotch tape and a volume of 0.2 ml of this mixture is applied to the tape against the rod using a micropipette. 5. The agent in solution was applied in such a way that it was evenly distributed over the tape to cover the entire test sheet. 6. Quickly draw the solution from the tape over the sheet using the rod so that the reducing agent compound solution is evenly distributed over the entire sheet.

7. Tumble dry the test sheet and level at room temperature.

8. Measure lightness and yellowness.

Procedure for application on a dry surface (surface bonding, soaking method):

1. Prepare 8x8 inch hand sheets according to standard procedure. Target dry weight is 2.5g. Run wet hand sheets through a cycle to the dryer.

2. Cut 1/8 strip of the sheet (0.31 g).

3. In a 50 ml test tube, prepare solution of pre-boiled starch (if necessary) and the reducing agent compound solution based on the predetermined uptake rate and target dosage. 4. Dip the paper strip in the solution for 10 seconds, let it drip for 35 seconds and then pass it through the press.

5. Tumble dry the test sheet and smooth at room temperature.

6. Measure lightness and yellowness.

Wet End Application Procedure:

1. An 8x8 inch sheet was prepared and dewatered using the press with two pullers at the bottom and one puller at the top. The consistency of the pressed sheet is around 40%. 2. The upper puller and the lower puller are removed from the sheet after the press. 3. The sheet together with a bottom extractor is cut into 4 smaller test sheets of equal size (approximate dry weight of the sheet is 0.625 g). 4. The test sheet is taped together with the tracing paper to the glass pad as described in "dry surface application procedure". 5. Solution 1 is applied as described on a dry surface

the application procedure.

6. After application, the test sheet together with the wet tracing paper is removed from the glass pad, the tape is removed and the tracing paper is removed from the test sheet. The tracing paper is thrown away. 7. The test sheet is then drum-dried and leveled at room temperature.

Split feeding application procedure:

1. An 8x8 inch sheet is prepared according to the standard procedure.

2. The sheet formed on the screen is then covered with 4 tracing papers.

3. The sheet as with the tracing papers is then rolled using a heavy metal roll. This process removes excess water from the sheet and increases the consistency of the sheet to around 20%.

4. The three upper tracing papers are removed from the sheet.

5. The sheet and a tracing paper are then removed from the wedge and cut into 4 smaller pieces as described in the "wet end application procedure". 6. The sheet and tracing paper are then taped to the glass pad as described in the "wet end application procedure". 7. Solution 2 is applied as described in "wet end application procedure".

8. The test sheet is then pressed with 2 tracing papers on each side.

9. After pressing, all the backing papers are removed and the sheet is drum-dried.

Solution 2 is then applied to the sheet, dried and measured as described in steps 3 to 8 "dry surface application procedure".

Mass Application Procedure:

The chemicals were added directly to the pulp (thin pulp or thick pulp) and mixed with the pulp in sealed bags. In a pulp application procedure for OBA improvement, the chemicals were directly added to the bleached kraft pulp at 20% consistency, mixed with the pulp in the sealed bags and held at 45-80°C for 30 min. The pulp was diluted to 5% consistency, OBA was added, mixed with the pulp and the slurry was held at 50°C for 20 min. The slurry was then further diluted and hand sheets were prepared according to standard procedures.

Test results

I. Machine tests

The experimental data were collected by a Southern power pulp machine. The table below provides the sample data. In several tests, application of the product (Blending A) in a size press, with an OBA in the size solution, at 5 Ib/h and higher dosages produced a 1.5 point lightness solution followed by improved color to the paper sheet (reflected in decreasing DE values) . Returning to standard machine conditions (no pentrant mixtures added) resulted in a reduction of brightness to the background level. This experiment was repeated 3 times.

Several mixtures were investigated which gave good results in the laboratory simulation of the PM application. The chemicals (mixtures) not listed in Table 1 were applied as 40% solutions.

2. Reducing agent: Sodium metabisulfite (30% solution)

Tables 3-8 show the effect of the reducing agent sodium bisulphite (metabisulphite) and a brightness improving mixture on paper brightness: the reducing agents improve brightness (Tables 3-8), partially compensate for brightness loss in the dryer (Table 8). The chemistry that further improves brightness in the presence of an OBA (Table 5).

3. Reducing agents other than sodium metabisulphite

Application in a model surface glue solution with starch

Tables 9-11 show the effect of reducing chemicals other than sodium metabisulphite, such as FAS and phosphorus (III) compounds.

4. Chelating metabisulphite mixtures

Tables 12-27 show the application of mixtures where a reducing agent is combined with chelating agents. Different combinations can be compared (all effective). The formulations improve long-term brightness stability against thermal aging (Tables 24-27). This data set also shows an OBA activation of the mixtures (Tables 12-14, 25). Application of the formulation makes it possible to reduce the dose of an optical brightener. Tables 16 and 17 show the effects of the formulations on fluorescence.

5. Wetting application: separate application of the mixture on mass which

leading to increased performance of OBA applied later

Mass application procedure (80°C) for subsequent OBA enhancement

Tables 28-30 show activation of an OBA by previous application of a mixture.

6. Wet end application: adding a low dose of dithiocarbamates to the mixture

The data (Tables 31-33) show lightness recovery and long-term stabilization when applying the proposed formulations.

As long as the present invention is described in connection with representative or illustrative embodiments, these embodiments are not intended to be complementary or limiting to the invention. The invention is intended to cover all alternatives, both modifications and equivalents, which fall within the scope of protection of the invention as defined in the appended claims.

Claims (11)

1. Process for producing a bleached pulp material with improved brightness and improved resistance to thermal yellowing, characterized in that it includes: i) providing a bleached pulp material; ii) contacting the bleached pulp material with an effective amount of one or more reducing agents which improve the brightness and resistance to yellowing of the pulp compared to pulp which has not been treated with the reducing agent or agents; and iii) contacting the bleached pulp material with one or more optical brighteners selected from the group consisting of azoles, biphenyls, coumarins, furans, naphthalimides, pyrazenes, and salts of such compounds including alkali metal salts, alkaline earth metal salts, transition metal salts, organic salts, and ammonium salts , and combinations thereof. 2. Procedure according to claim 1, characterized in that it further includes bringing the bleached pulp material into contact with one or more chelating agents. 3. Procedure according to claim 1, characterized in that the bleached pulp material is selected from the group consisting of new pulp, recycled pulp, kraft pulp, sulphite pulp, mechanical pulp, any combination of such pulps, recycled paper, paper tissue, and any paper or paper products produced from such pulps or combinations thereof. 4. Procedure according to claim 1, characterized in that the reducing agent is selected from the group consisting of sulfites, bisulfites, metabisulfites (pyrosulfites), sulfoxylates, thiosulfates, ditinites (hydrosulfites), polythionates, formamidine sulfinic acid and salts and derivatives thereof, formaldehyde bisulfite adduct or other aldehyde bisulfite adducts, sulfinamides and ethers of sulfinic acid, sulfenamides and ethers of sulfenic acid, sulfamides, phosphines, phosphonium salts, phosphites and thiophosphites. 5. Procedure according to claim 4, characterized in that the reducing agent is selected from the group consisting of substituted phosphines, sulfites, bisulfites and metabisulfites. 6. Procedure according to claim 5, characterized in that the reducing agent is sodium bisulphite. 7. Procedure according to claim 2, characterized in that the chelating agent is selected from the group consisting of organic phosphonates, phosphates, carboxylic acids, dithiocarbamates, salts of any of the preceding members and any combination thereof. 8. Procedure according to claim 7, characterized in that the chelating agent is selected from the group consisting of diethylene-triamine-pentamethylenephosphonic acid (DTMPA) and salts thereof, diethylenetriaminepentaacetic acid (DTPA) and salts thereof and ethylenediaminetetraacetic acid (EDTA) and salts thereof. 9. Procedure according to claim 1, characterized in that it further includes bringing the bleached pulp material into contact with one or more polycarboxylates. 10. Method according to claim 9, characterized in that the carboxylate is partially neutralized polyacrylic acid. 11. Procedure according to claim 1, characterized in that the reducing agents and optical brighteners are mixed with a surface sizing solution and applied to the bleached pulp material in a sizing press.