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
  • D21H19/00—Coated paper; Coating material
  • D21H19/36—Coatings with pigments
  • D21H19/38—Coatings with pigments characterised by the pigments
  • D21H19/42—Coatings with pigments characterised by the pigments at least partly organic
• • 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
  • D21H19/00—Coated paper; Coating 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
  • 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
  • D21H21/50—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 form
  • D21H21/52—Additives of definite length or shape

Definitions

• the invention relates to microcrystalline cellulose for use in paper-coating applications.
• the invention further relates to an improved coating composition which contains a "carrier" substance for additives, particularly optical brighteners.
• coated printing papers are prepared using paper-coating compositions which comprise a pigment (for example kaolin, calcium carbonate or titanium dioxide) which is dispersed in water, and a binder.
• a pigment for example kaolin, calcium carbonate or titanium dioxide
• binders used were exclusively high molecular weight, naturally-occurring products, such as starch or casein.
• these have largely been replaced entirely or partially by synthetic high molecular weight copolymers in the form of aqueous dispersions.
• additives commonly found in coating compositions are dispersants, water-soluble polymers, optical brighteners and alkali for pH adjustment.
• anionic dispersants are used as deflocculants, causing particle repulsion and reduced viscosity.
• the water-soluble polymers previously mentioned act as cobinders and impart water retention to the paper coating composition. They may be naturally-occurring polymers such as starch and casein or alternatively synthetic compounds such as polyvinyl alcohol (PVOH), carboxymethylcellulose (CMC) and water soluble polyacrylates.
• PVOH polyvinyl alcohol
• CMC carboxymethylcellulose
• water soluble polyacrylates water soluble polyacrylates
• optical brightening agents are added to increase the level of whiteness of the coated paper and the efficiency of these compounds depends very much on the coating composition. It is known that if the brightener molecule can bond to a substance by means of adsorption or hydrogen bond formation, the brightener performance is appreciably enhanced.
• Some water-soluble polymers used in coating compositions such as PVOH and CMC, do adsorb optical brighteners to some extent, but the water-insoluble binders, in aqueous dispersion, have a negative influence on brightener performance.
• the invention relates to a paper coating composition containing a substance which allows the easy addition of additives to a paper, and which in particular greatly enhances the performance of commercial optical brighteners.
• the substance is cellulose, preferably microcrystalline cellulose, having a particle size of 1 to 10 microns, preferably 1 to 3 microns, in aqueous suspension. It is well known that cellulose swells in water and that the "dry" particle size of a cellulose particle will be considerably smaller than the "wet” particle size.
• optical brighteners are very substantive to microcrystalline cellulose and once adsorbed they appreciably improve the whiteness of the coated paper.
• Microcrystalline cellulose sold in powder form, has been commercially available for many years and is used extensively in the pharmaceutical industry. It has been used to a limited extent in the paper industry but its potential has been restricted by its large particle size and lack of surface area.
• the present invention provides a method for reducing the particle size of cellulose (preferably microcrystalline cellulose) in water and/or an organic solvent (preferably water) comprising
• the present invention enables the additive to be adsorbed on to the carrier substance.
• the process according to the invention comprises
• the additive may be a surface sizing agent and/or a dyestuff.
• the preferred additive is an optical brightener. This particular aspect of the invention will be discussed further hereinunder.
• the present invention also provides an aqueous slurry or paste (a carrier), for use in a paper-coating composition, containing microcrystalline cellulose having an average particle size of 1 to 10 microns, preferably 1 to 3 microns. This greatly enhances the performance of commercial optical brighteners that are often used in paper coating compositions.
• the slurry, paste or dispersion may be given several passes through the mill until the wet particle size averages 1 to 10 microns, preferably 1 to 3 microns.
• optical brighteners are very substantive to microcrystalline cellulose, and once adsorbed they improve the whiteness of the coated paper substantially. It is a particular feature of this invention that the use of optical brighteners results in an unexpectedly high increase in whiteness.
• optical brighteners are commercially available and therefore do not require more detailed comment here. However, a more detailed characterization of this category of compounds may be found in German Laid-Open Application DOS No. 2,628,878 and German Published Application DAS No. 1,795,047 (which are incorporated herein by reference).
• the viscosity of the cellulose increases during milling and, if desired, compounds may be added which reduce viscosity.
• the cellulose may be milled in the presence of dispersing agent, suitable examples of which are mentioned hereinunder in connection with pigments, optionally in the presence of a filler such as calcium carbonate or clay.
• Dispersants may be used in a quantity of up to 10% by weight (on dry cellulose).
• An especially effective and preferred dispersant for the purposes of this invention is carboxymethyl cellulose (CMC)
• sequestering agents may be added. Depending on the construction of the bead mill, a grey colour resulting from metal pick-up may be observed in the milled cellulose. If sequestering agents are used, they are used in a quantity of up to 0.4% by weight, (based on the weight of dry cellulose), usually in the form of commercially-available material which has a 40% concentration.
• Preferred sequestering agents are ethylene diamine tetraacetic acid (EDTA), diethylene triamine pentaacetic acid (DTPA), diethylene triamine pentamethylene phosphonic acid (DTPMA) and nitrilotriacetic acid (NTA) or their sodium salts.
• EDTA ethylene diamine tetraacetic acid
• DTPA diethylene triamine pentaacetic acid
• DTPMA diethylene triamine pentamethylene phosphonic acid
• NTA nitrilotriacetic acid
• the surface area of the cellulose is greatly increased, creating much higher adsorption capacities for the additive.
• the additive which may be in liquid or powder form, is then added to the carrier.
• the amount used depends on the substantivity of the additive and should be adjusted so that once the cellulose particle size is reduced, almost all the adsorption sites are taken up and very little additive is left free in the water.
• hardness salts such as magnesium sulfate may be used to increase the substantivity of the additive. Normally an addition of 10 to 100 ppm magnesium sulfate to the water is sufficient.
• the preparation of the aqueous carrier according to the invention may also be carried out by adding the additive and any dispersant and/or hardness salts to the water, prior to the addition of the cellulose powder. The mixture is then milled as before.
• the preferred dry microcrystalline cellulose powder that is added to water to form the carrier has an average particle size of 2 to 100 microns. It is relatively simple to bring the size distribution to the required level using classification techniques. Once mixed with water, however, the particles swell and their size is increased by a factor of 10 to 100. It is therefore important that the particle size be reduced after the powder has been mixed with the water.
• a process for coating paper comprising applying the carrier defined above, optionally containing one or more additives, to the surface of paper.
• the fine particle size cellulose carrier after application to the paper, dries to form a film.
• the surface properties of the paper are enhanced in such a way that if the coated paper is to be printed, the printing properties are far superior to those of paper which has not received this surface treatment.
• the cellulose particles shrink on drying to give a smooth, tough film.
• the interstices in the paper surface are filled with the carrier, and when dry the paper surface has a smooth feel and appearance.
• the large hydrogen bonding capacity of the micronised cellulose ensures a high surface strength for the coated paper. This surface modification is particularly noticeable if the cellulose carrier is applied on its own or together with the additive, using any commercial coating technique or a size press.
• any commercial optical brightener, dyestuff or other art-recognised additive, having affinity for cellulose can then be added to the modified microcrystalline cellulose carrier.
• the amounts of additive are adjusted so that almost total adsorption is achieved.
• the carrier with additive is then added to the coating composition, thereby imparting to the coated paper whatever property the additive confers, for example, high whiteness, high colour yield or surface wet/dry strength.
• a pigment-containing composition for use in paper coating comprising per 100 parts by weight of pigment,
• Suitable pigments for use in the coating composition include, china clay (kaolin), calcium carbonate, sulfate or silicate, titanium dioxide, talc, barium sulfate, zirconium oxide and alumina. These pigments may be used independently or in admixture.
• Appropriate copolymers for use in the binder mixture are all commercial synthetic binders which are available in the form of an aqueous dispersion. These polymers have a glass transition temperature of from -40° to +50°C. Examples of typical monomers from which these copolymers may be prepared are esters of acrylic acid and of methacrylic acid, acrylonitrile, methacrylonitrile, acrylamide, methacrylamide, vinyl chloride, vinylidene chloride, ethylenically monounsaturated or polyunsaturated hydrocarbons, e.g.
• alkalis e.g. sodium hydroxide, potassium hydroxide or ammonia.
• alkalis e.g. sodium hydroxide, potassium hydroxide or ammonia.
• a dispersant e.g. a low molecular weight polymer of acrylic acid and preferably an ammonium salt or an alkali metal (e.g. sodium) salt of a polyacrylic acid having a K value of from 10 to 35
• the K values of the polymers are measured by the method of H. Fikentscher, Cellulose-Chemie 13 (1932), 58-64 and 71-74, in 0.5% strength aqueous solution at 25°C.
• the water-soluble polymers may be regarded as co-binders because they themselves possess pigment binding properties.
• the carrier (as herein defined) is mixed with the other coating composition ingredients, preferably once the pigment/binder mixture has been prepared, in a manner as described below.
• the final solids of the coating composition is 50 to 70%.
• the whiteness of the sheet is far superior to that where cellulose is not used.
• Traditional water-soluble polymers, used as "carriers" for additives, are not able to generate the increased levels of whiteness seen with the present invention.
• This method may be used with any other suitable additive replacing the optical brightener.
• the traditional method of preparation for coating compositions is to slurry the pigments in water containing dispersant and then add copolymer binder, water soluble cobinder and, say, optical brightener After pH adjustment to 8.0 to 9.0, the mix is ready for use.
• the additive is added as a liquid and, since very little of the additve is actually adsorbed on to a composition component, most of it migrates with the water phase into the base sheet or is left in the coating after drying.
• high levels of brightener are used to produce low levels of whiteness, showing that the traditional use of additives in coating compositions is far from efficient.
• the amount of optical brightener-containing coating mix applied to the paper surface has a direct influence on the level of whiteness, measured on the finished paper.
• a coat weight i.e., the amount of dry weight added to the original base paper and measured in grams per square meter (gsm) of say 5 gsm almost always develops a higher measured whiteness than one with an 18 gsm coat weight. This is due to the "free" optical brightener migrating into and adsorbing on the base sheet. If the coat weight is low, e.g. 5 gsm, the whiteness of the base sheet shows through the coating and contributes to the measured value. On the other hand, a coat weight of 18gsm tends to mask the base sheet, resulting in a lower measured whiteness.
• the present invention using the novel carrier substance allows a high whiteness to be achieved, even with high coat weights.
• additive is a commercial optical brightening agent
• many more additives may be used in conjunction with micronised microcrystalline cellulose.
• the carrier paste is dyed to the required colour and then applied to the paper surface.
• This proposal has the same effect as coating using coloured pigments.
• the colouration media is particulate, albeit in a very fine form.
• dyestuff however, recycling of paper is much easier due to the ease with which decolourization can be carried out using oxidative bleach. This is not the case where coloured pigments have been used; many are resistant to oxidative bleaches.
• additives which can also be successfully applied to the cellulose carrier include cationic sizes, cationic water repellent agents, dry and wet strength resins and other surface modification chemicals, which, even if not substantive to cellulose, can be "fixed” using a cationic chemical.
• microcrystalline cellulose (“Avicel” PH-105) having an average dry particle size of 20 ⁇ m, is mixed with 380 g water and 0.8g carboxymethylcellulose (CMC)using a high shear laboratory blender.
• CMC carboxymethylcellulose
• the resulting dispersion is then passed through a laboratory bead mill having a chamber size of 250 ml and a glass grinding bead size of 1 mm and with a shaft rotation speed of 4000 rpm, until the average wet particle size of the carrier is 1 to 5 ⁇ m. (measured with a Horiba LA 500 particle analyzer using laser diffraction) : Yield is 400g.
• the resulting white viscous paste hereinafter referred to as the carrier paste, is now ready for mixing with an additive.
• the solid content of the coating composition is approximately 60%. After mixing for 10 minutes, the coating composition is applied to paper, using a metering bar coater.
• the solids content of the coating composition is approximately 55%. After mixing for 10 minutes, the coating composition is applied to paper, using a metering bar coater.
• the viscosity is measured using a Brookfield viscometer, spindle No 4 speed 100 rpm.
• Delta reflectance is a measure of the difference in whiteness with the UV filter in and the UV filter out. With the UV filter in, the effect of additive is not observed and therefore the measurement can be taken to be that of the base coating.
• a medium even shade of red is produced on the paper, compared to a severe mottle if the liquid dye is mixed with 8% Amylox P 45 and applied to the paper using the size press.

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Cited By (17)

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• 1991
  • 1991-01-30 GB GB919101965A patent/GB9101965D0/en active Pending
• 1992
  • 1992-01-21 EP EP92810035A patent/EP0499578B1/fr not_active Expired - Lifetime
  • 1992-01-21 AT AT92810035T patent/ATE138436T1/de not_active IP Right Cessation
  • 1992-01-21 DE DE69210856T patent/DE69210856T2/de not_active Expired - Lifetime
  • 1992-01-28 NO NO92920372A patent/NO920372L/no unknown
  • 1992-01-28 AU AU10487/92A patent/AU1048792A/en not_active Abandoned
  • 1992-01-28 CA CA002060105A patent/CA2060105A1/fr not_active Abandoned
  • 1992-01-28 FI FI920370A patent/FI920370A/fi not_active Application Discontinuation
  • 1992-01-29 KR KR1019920001305A patent/KR920014993A/ko not_active Application Discontinuation
  • 1992-01-30 JP JP4014478A patent/JPH04289294A/ja active Pending

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