WO2006061460A1 - Starch acetate composites, method for the production thereof, and use in paper and board production - Google Patents

Starch acetate composites, method for the production thereof, and use in paper and board production Download PDF

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Publication number
WO2006061460A1
WO2006061460A1 PCT/FI2005/050447 FI2005050447W WO2006061460A1 WO 2006061460 A1 WO2006061460 A1 WO 2006061460A1 FI 2005050447 W FI2005050447 W FI 2005050447W WO 2006061460 A1 WO2006061460 A1 WO 2006061460A1
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Prior art keywords
starch acetate
paper
composite
light scattering
silicate
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PCT/FI2005/050447
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French (fr)
Inventor
Petri Silenius
Kimmo Koivunen
Lars Gädda
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M-Real Oyj
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Application filed by M-Real Oyj filed Critical M-Real Oyj
Priority to EP05817784A priority Critical patent/EP1819769A1/en
Publication of WO2006061460A1 publication Critical patent/WO2006061460A1/en

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    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J3/00—Processes of treating or compounding macromolecular substances
    • C08J3/20—Compounding polymers with additives, e.g. colouring
    • C08J3/205—Compounding polymers with additives, e.g. colouring in the presence of a continuous liquid phase
    • C08J3/21—Compounding polymers with additives, e.g. colouring in the presence of a continuous liquid phase the polymer being premixed with a liquid phase
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08L—COMPOSITIONS OF MACROMOLECULAR COMPOUNDS
    • C08L3/00—Compositions of starch, amylose or amylopectin or of their derivatives or degradation products
    • C08L3/04—Starch derivatives, e.g. crosslinked derivatives
    • C08L3/06—Esters
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J2303/00—Characterised by the use of starch, amylose or amylopectin or of their derivatives or degradation products
    • C08J2303/04—Starch derivatives
    • C08J2303/06—Esters

Definitions

  • Starch acetate composites Method for the production thereof, and use in paper and board production
  • the present invention relates to novel starch acetate composites useful for instance in paper and board production, and to a method for producing such composites.
  • the invention is also directed to the use of said composites as fillers for paper and board and as coating pigments.
  • the invention relates to methods for producing, and for coating paper and board.
  • a coating consists of pigments, e.g. kaolin, ground calcium carbonate (GCC) and talc, and further, a binder such as a latex and starch, and it may also contain additives such as dispersing agents, agents for pH adjustment, lubricants, and anti- microbial agents.
  • Pigments normally comprise from 80 to 95 % by weight of the coating, so the pigments have a particularly significant influence on the optical properties of the coating, e.g. opacity, brightness, and gloss. Brightness is improved by the low light absorption capacity and high light scattering coefficient of the pigment, opacity being also improved by the latter.
  • Factors influencing the gloss include particle size of the pigment, and post-coating treatment of paper and board, such as calendering.
  • fillers are added to the pulp.
  • the amount of the filler to be added depends on the product being produced, the proportion thereof being for instance between 4 and 10 % for LWC papers and between 15 and 30 % for chemical pulp papers, relative to the base paper weight.
  • Fillers include e.g. kaolin, calcium carbonate and titanium dioxide.
  • the optical properties and printa- bility of paper and board are influenced also by the filler. A considerable proportion of the filler may comprise reject material from paper production.
  • the optical properties of paper and board may be improved by increasing the proportion of pigments in the coating, and the amount of fillers in base paper. This, however, deteriorates the strength characteristics of paper and coating.
  • starch is typically added to the pulp as an aqueous solution primarily for improving the strength properties of the product.
  • the thus improved strength is, among other things, based on its gelatinization in the drying section.
  • Use of cooked starch is limited for instance by limited adsorption, rannability problems, and cost.
  • Strength properties of paper may also be improved by refining of pulp, and addition of fines, but this is, however, accompanied by opacity reduction.
  • FI2004074J discloses a method for producing a pigment or filler based on starch, and further, a starch based pigment or filler product.
  • the products presented are light and biodegradable, and have a particle size or structure making them suitable as coating pigments and fillers.
  • One product disclosed in said document is a spherical starch derivative, such as starch acetate having a mean particle size ranging from 90 to 1000 nanometers.
  • WO 99/16972 discloses the use of alkali metal silicates, particularly sodium silicate in combination with starch to improve the strength of paper and board. Both the silicate and the starch are added in a finely dispersed form to the fibre suspension at the wet end prior to web formation.
  • the starch is uncooked and the alkali silicate is selected from silicates insoluble in water at the temperatures of the wet end, the silicates being soluble only at temperatures encountered in the drying section.
  • US 6,623,555 describes a method for producing a composite pigment from pre- cipitated calcium carbonate (PCC) and silicate.
  • Said composite pigment is produced by adding a soluble silicate compound to an aqueous medium containing precipitated calcium carbonate obtained by carbonation of lime milk.
  • An insoluble silicon compound is precipitated on the calcium carbonate thus formed by carbon- ating the reaction mixture, that is, by bubbling CO 2 through the reaction mixture.
  • Optical properties and bonding strength are some of the most important properties of printing papers.
  • Scott Bond value Optical properties and bonding strength
  • Incineration of waste papers containing inorganic mineral pigments for energy production results in high amounts of ash, the disposal of which causes problems.
  • aims set for the proportion of bioenergy in the total energy production should be reached until 2010. For these aims, it is also desir- able to use in papers and boards as much renewable organic materials as possible.
  • Inorganic mineral pigments also increase the weight of paper and board. There is an ever growing need for increasingly lighter papers for magazines, catalogues and the like, furnished, however, with high quality printing properties.
  • An object of the invention is to provide novel starch acetate composites.
  • An object of the invention is also to provide a method for producing starch acetate composites.
  • an object of the invention is the use of starch acetate composites as fillers of paper and board, and as coating pigments in the production thereof.
  • Another object of the invention is to provide a method for producing paper and board.
  • Still another object of the invention is to provide a method for coating paper and board.
  • the invention relates to starch acetate composites.
  • the composite of the invention comprises starch acetate particles having a light scattering material on the surface thereof.
  • the term light scattering material refers for instance to silica, silicate, precipitated calcium carbonate (PCC), gypsum, calcium oxalate, titanium dioxide, aluminium hydroxide, barium sulphate, or zinc oxide, or a modification or a combination thereof, or any other light scattering material.
  • the starch acetate is coated with the light scattering material defined above.
  • the coating with the light scattering material may be performed by precipitation, adsorption, gas phase coating technique, i.e. CVD (chemical vapour deposition), or spin coating technique, or in a similar manner.
  • the starch acetate composite obtained may be used as a filler of paper and board for improving the combination of optical and strength properties of the product.
  • the composite of the invention may be used as a coating pigment for paper and board.
  • Figure Ia shows an electron micrograph (magnification xl200) of uncoated starch acetate.
  • Figure Ib shows an electron micrograph (magnification xl200) of starch acetate composite coated with silicate, prepared according to Example 1.
  • Figure 2a is a graphical presentation of the light scattering coefficient of a sheet containing 10 % and 20 % by weight of a starch acetate composite filler SA4 (prepared according to Example 1) or SA2 (prepared according to a modification of Example 1) as a function of the tensile strength index.
  • Sheets containing respec- tively 4 % and 8 % by weight of uncoated starch acetate (API) as a filler, and a sheet without a filler serve as controls.
  • SA2 90°C, SA4 90°, and API 90 °C refer to sheets treated at 90 0 C prior to drying.
  • Figure 2b is a graphical presentation of the light scattering coefficients of sheets containing starch acetate composite filler of the invention as a function of the bonding strength. Filler contents used and symbols are as in Figure 2a.
  • Figures 3a, 3b and 4 are graphical presentations of the tensile strength indices, the light scattering coefficients, and the bonding strength of sheets containing starch acetate composite filler of the invention as a function of the filler content. The symbols are as in Figure 2a.
  • starch acetate may be coated with a light scattering material such as silica, silicate, precipitated calcium carbonate (PCC), gypsum, calcium oxalate, titanium dioxide, aluminium hydroxide, barium sulphate, zinc oxide, or a like, or with modifications or combinations thereof, to give a starch acetate composite useful both as a coating pigment and as a filler in the production of paper and board.
  • a light scattering material such as silica, silicate, precipitated calcium carbonate (PCC), gypsum, calcium oxalate, titanium dioxide, aluminium hydroxide, barium sulphate, zinc oxide, or a like, or with modifications or combinations thereof, to give a starch acetate composite useful both as a coating pigment and as a filler in the production of paper and board.
  • the particle size of the starch acetate to be used in the composite of the invention ranges between 0.05 and 10 ⁇ m.
  • the starch acetate is preferably spherical starch acetate.
  • the degree of substitution of the starch acetate may vary between 0.5 and 3, being preferably from 1.5 to 3. The higher the degree of substitution of the starch acetate, the better the dimensional stability of the composite prepared therefrom will be at higher process temperatures, and the better the light scattering properties, among other things, will be preserved during reject recycling.
  • the starch acetate useful in the invention may be produced by any known production method, starting for instance from native, hydrolyzed, oxidized, cross-linked, or gelatinized starch, e.g.
  • the starch may be any starch found in nature, such as starch from potato, wheat, corn, barley, durum, tapioca, or rice.
  • Starch acetate may be produced for instance according to patent publications FI 107386, FI20040741, and FI20040743.
  • the material used to coat the starch acetate is selected among any light scattering materials. Suitable light scattering materials include silica, silicate, precipitated calcium carbonate (PCC), gypsum, calcium oxalate, titanium dioxide, aluminium hydroxide, barium sulphate, zinc oxide, or a like, and modifications or combina- tions thereof.
  • a preferable light scattering material is silicate.
  • the silicate to be used in the composite is selected from the group consisting of metal silicates such as alkaline earth metal silicates, alkali metal silicates, alkaline earth metal, and alkali metal aluminium silicates, and modifications thereof in- eluding mixed salts with alkaline earth metal salts and hydroxides, and further, the mixed salts and combinations of said compounds.
  • the silicate is preferably calcium silicate, magnesium silicate, sodium aluminium silicate, sodium magnesium silicate, sodium silicate, or aluminium silicate, and particularly sodium aluminium silicate.
  • the composite of the invention contains the coating material from 5 to 95 % by weight, preferably from 5 to 20 %, or from 50 to 80 % by weight of the composite.
  • the content of the coating material preferably varies between 5 and 20 % by weight of the composite in products, the disposal of which may desirably be achieved by burning, thus minimizing the ash formation.
  • the coating material content ranging from 5 to 20 % by weight allows for the improvement the combi- nation of optical and strength properties of the product according to the invention.
  • the thickness of the coating may vary within wide limits, ranging typically from 1 nm to 5 ⁇ m.
  • a starch acetate is coated with a light scattering material, said coating being carried out by precipitation, adsorption, gas phase coating, or spin coating processes, or a like. It is thus possible to coat the starch acetate using a modification of said coating processes, e.g. using a modified gas phase coating, for instance atomic layer epitaxy (ALE) process.
  • ALE atomic layer epitaxy
  • the coating may be accomplished by adding the substance to be precipitated to a suspension containing starch acetate particles, the pH and temperature values being optionally adjusted to suitable ranges.
  • the suspension containing starch acetate is optionally combined with an aqueous solution of the precipitating com- pound, and eventually with an adjuvant salt prior to the addition of the substance to be precipitated.
  • the precipitating compound in an aqueous, alcoholic or alcoholic/aqueous solution, or in a gaseous form and/or an acid or seed crystals of the precipitate substance is/are added.
  • the coating may also be carried out by contacting the starch acetate particles with the light scattering material, and by allowing the adsorption thereof on the starch acetate particles.
  • the coating may further be carried out by contacting the starch acetate particles with gaseous starting materials that dissociate and/or chemically react in the gas phase, thus forming a solid coat on the surface of the starch acetate particles.
  • the coating may be carried out by depositing aqueous layers compris- ing starch acetate particles and the light scattering material with the spin coating process, the layers are allowed to solidify, and the solidified layers are crushed to the desired grain size.
  • the starch acetate to be used in the method of the invention comprises starch ace- tate particles having a particle size between 0.05 and 10 ⁇ m.
  • the light scattering material to be used in the method of the invention may comprise e.g. silica, silicate, precipitated calcium carbonate (PCC), gypsum, calcium oxalate, titanium dioxide, aluminium hydroxide, barium sulphate, zinc oxide, or a like, or a modification or a combination thereof.
  • Silicon dioxide or silica (SiO 2 ) may be precipitated according to the reaction equation (1) below.
  • a suitable substance to be precipitated that is a basic metal silicate, in this example an aqueous solution of sodium silicate (water glass) is reacted with a precipitating compound, in this case with a mineral acid, typically H 2 SO 4 .
  • Precipitated silica is also obtained by reacting an alkali metal silicate with sulphurous acid, or with sulphur dioxide. An aqueous solution of alkali metal sulphite or bisulphite is also formed.
  • silicates are obtained by reacting a silicon compound acting as the substance to be precipitated with a precipitating compound.
  • the precipitating compound may also be formed in situ during the reaction.
  • silicates such as sodium aluminium silicate, calcium silicate, and aluminium silicate are obtained.
  • sodium aluminium silicate is the most widely used silicate used in papermaking.
  • Suitable substances to be precipitated include precipitated silicas, metal silicates e.g. alkaline earth metal silicates, and alkali metal silicates, alkaline earth, and alkali metal aluminium silicates, and modifications thereof, including mixed salts with alkaline earth metal salts and hydroxides, and further, the mixed salts and combinations of said compounds.
  • a silicate such as sodium aluminium silicate may be precipitated according to the following reaction equation (2). Aluminium sulphate, or alum, reacts with an aqueous solution of sodium silicate.
  • an alkali metal silicate may be reacted with an aqueous solution of aluminium sulphite to give precipitated alkali metal aluminium silicate and an aqueous phase containing alkali metal sulphite, or bisulphite depending on the pH in the final reaction stage.
  • Precipitated alkali metal aluminium silicate is also obtained by treating an alkali metal silicate solution with an alkali metal aluminate in the presence of sulphur dioxide, sulphurous acid solution, or sulphuric acid solution.
  • an aque- ous phase containing alkali metal sulphite is obtained.
  • the precipitating aluminium sulphite reagent is formed in situ during the reaction.
  • Zinc silicate may be precipitated by mixing sodium silicate solution and zinc chloride solution, replacing the zinc chloride solution by a sulphuric acid solution at the end of the reaction.
  • Precipitated calcium carbonate, or PCC is obtained for instance according to following reaction equations (3) - (5).
  • reaction (3) limestone is heated, thus dissociating it to give lime, CaO, and carbon dioxide.
  • lime is mixed with water in the reaction (4), thus obtaining slaked lime, Ca(OH) 2 .
  • any impurities may be removed for instance by screening.
  • Calcium carbonate is precipitated in the carbonization step wherein carbon dioxide is passed to an aqueous slurry of the slaked lime in reaction (5).
  • the particle size, and the particle size distribution of the precipitated calcium carbonate, and further, the shape, and the surface properties of these particles may be influenced by adjusting the reaction conditions.
  • Calcium carbonate may also be precipitated according to the reaction equation (6).
  • slaked lime is reacted with sodium carbonate.
  • the alkaline solution produced in the reaction is neutralized prior to using the CaCO 3 in papermak- ing.
  • Calcium carbonate may further be precipitated by reacting sodium carbonate with calcium chloride according to equation (7):
  • Calcium sulphate is found in various hydrated and anhydrous forms, of which the calcium sulphate dihydrate, CaSO 4 -2H 2 O, is commonly called gypsum.
  • This di- hydrate is the most stable form of calcium sulphate, and thus, it is used in coating pigments.
  • the spontaneous precipitation of the dihydrate form is a common phenomenon in case of boiler sediments, and the precipitation takes place in oversatu- rated solutions according to the reaction equation (8).
  • the dihydrate is also precipitated according to the reaction equation (9) from cal- cium sulphate hemihydrate, CaSO 4 - 1 ⁇ H 2 O once it is slurried in water.
  • the particle size distribution and particle shape of the precipitating gypsum may be influenced by adjusting the precipitation conditions. 2CaSO 4 - !/2H 2 O + 3H 2 O -» 2CaSO 4 -2H 2 O (9)
  • the dihydrate form is also precipitated once calcium phosphate is reacted with sulphuric acid in an aqueous solution according to the reaction equation (10). Also phosphoric acid is formed in the reaction.
  • Ca 1 o(P0 4 ) 6 F 2 reacts with sulphuric acid in an aqueous solution, the dihydrate form of calcium sulphate, phosphoric acid, and hydrofluoric acid are formed according to the reaction equation (11).
  • Calcium oxalate may be produced by precipitation from oxalic acid in the presence of a compound containing calcium.
  • the compound containing calcium may for instance be calcium carbonate, calcium hydroxide, or calcium chloride.
  • the production of calcium oxalate from calcium carbonate and oxalic acid is presented in reaction equations (13)-(14).
  • Titanium dioxide may be produced for instance with the known sulphate process, that is, by dissolving dried and ground ilmenite, or titanium slurry using concentrated sulphuric acid, and heating to produce a solid reaction product cake.
  • the reaction product cake is dissolved in water or diluted sulphuric acid, and further, solid impurities are removed from the titanium sulphate solution for instance by filtering.
  • the iron content of the solution may be further reduced by cooling, thus precipitating the iron as an iron sulphate heptahydrate that may be removed by filtering.
  • the solution is concentrated to precipitate the titanium as tita- nium(rV)oxyhydroxide, followed by filtering, washing, and conversion to the desired crystal size and shape by calcination, if necessary.
  • Starch acetate may then be coated with the titanium dioxide thus obtained using e.g. adsorption, or spin coating processes.
  • Titanium dioxide may also be produced with the procedure disclosed in the document US 6,001,326, that is by adding ice cubes made of water, or icy water to an undiluted titanium tetrachloride solution, and diluting the aqueous solution of titanyl chloride thus obtained to give the desired concentration, followed by heating thus precipitating finely divided titanium dioxide.
  • Aluminium Hydroxide also known as aluminium trihydrate
  • aluminium trihydrate may be produced from bauxite by dissolving the aluminium contained therein, followed by separation of the other minerals.
  • the aluminium compounds of the solution are extracted with sodium hydroxide and then insoluble impurities are separated by sedimentation and filtration.
  • the clear sodium aluminate filtrate is cooled, followed by the addition of fine aluminium hydroxide crystals, specifically prepared as seed crystals for this purpose, if necessary, and starch acetate.
  • the aluminate contained in the filtrate is precipitated on the seed crystals and starch acetate added.
  • Barium sulphate may be precipitated from barium compounds soluble in water using compounds containing a sulphate group, also being soluble in water.
  • Said barium compound may for instance be barium nitrate, sulphide, hydroxide, or chloride, whereas the compound containing a sulphate group is sodium, or magnesium sulphate, or sulphuric acid.
  • the preparation of barium sulphate from bar- ium chloride and sodium sulphate is illustrated by the reaction equation (15).
  • Zinc oxide may be precipitated by heating zinc nitrate, thus resulting in zinc oxide, nitrogen dioxide, and oxygen. Zinc oxide may also be precipitated by heating zinc carbonate, thus giving zinc oxide, and carbon dioxide. Moreover, zinc oxide may be precipitated with calcium oxide, or with calcium hydroxide from a solution containing zinc ions, or by hydrolysis of zinc acetate with lithium hydroxide, or with tetramethylammonium hydroxide in an alcoholic or alcoholic/aqueous solution.
  • an aqueous suspension containing the starch acetate is optionally combined with an aqueous solution containing the precipitating compound, followed by the optional addition of another salt as an adjuvant, and thereafter, the substance to be precipitated is added as such, or as an aqueous solution or slurry, and further, optionally the pre- cipitating compound as an aqueous, alcoholic, or alcoholic/aqueous solution, or in a gaseous form, and/or an acid or seed crystals of the precipitate substance are added. pH and temperature values are optionally adjusted to a suitable range.
  • the precipitating compound is selected from the group consisting of inorganic acids, sulphur dioxide, as well as alkaline earth metals, alkali metals, earth metals, salts of zinc and aluminium, preferably sulphate, sulphite, nitrate, and ammonium sulphate salts.
  • the precipitation is particularly preferably carried out using aluminium sulphite, or alkali metal alumi- nate in the presence of sulphur dioxide, sulphurous acid, or sulphuric acid.
  • the precipitation may also be accomplished with zinc chloride that will be replaced by a sulphuric acid solution in the final stage of the reaction.
  • the precipitating compound may for instance be gaseous carbon dioxide, or sodium carbonate.
  • the precipitating compound will be sulphuric acid.
  • gaseous oxygen is used as the precipitating compound.
  • any compound releasing sulphate ions when dissolving in water may be used as the precipitating compound.
  • the precipitating compound may be left out.
  • the precipitating compound is oxalic acid.
  • the substance to be precipitated may be heated instead of adding a precipitating compound, thus giving finely divided titanium dioxide.
  • aluminium hydroxide seed crystals are added instead of the precipitating compound, if necessary.
  • the precipitating compound is a compound con- taining sulphate groups, such as sodium, or magnesium sulphate, or sulphuric acid.
  • the precipitating compound is for instance a calcium oxide, hydroxide, lithium hydroxide, or tetramethylammonium hydroxide.
  • zinc nitrate, or zinc carbonate is used as the substance to be precipitated, the addition of any precipitating compound is not necessarily needed.
  • the salt serving as an adjuvant is selected from a group consisting of alkaline earth metal salts, and hydroxides.
  • Suit- able salts include the chlorides, sulphates, and carbonates of alkaline earth metals such as magnesium, or calcium. Magnesium hydroxide is preferably used.
  • the substance to be precipitated is selected from the group consisting of precipitated silicas, alkali metal, and alkaline earth metal silicates, alkali metal, and alkaline earth metal aluminium silicates, and modifications thereof including mixed salts with alkaline earth metal salts and hydroxides, and further, the mixed salts and combinations of said compounds.
  • the substance to be precipitated is selected from the group consisting of alkali metal, and alkaline earth metal silicates.
  • the substance to be precipitated is for instance calcium hydroxide, or calcium chloride.
  • Calcium hydroxide is obtained by mixing burnt lime in water, said lime thus reacting to give calcium hydroxide.
  • the substance to be precipitated is calcium phos- phate, calcium sulphate hemihydrate, raw phosphate, calcium hydrogen sulphite, or any compound releasing calcium ions when dissolved in water.
  • the substance to be precipitated is any compound containing calcium, for instance calcium chloride, calcium carbonate, or calcium hydroxide.
  • the substance to be precipitated is for instance titanyl chloride.
  • the substance to be precipitated is sodium aluminate.
  • the substance to be precipitated is a barium compound, e.g. barium nitrate, sulphide, hydroxide, or chloride.
  • the substance to be precipitated may for instance be zinc nitrate, zinc carbonate, or zinc acetate.
  • an aqueous suspension of the dispersed or slurried starch acetate is optionally combined with an aqueous solution of the precipitating compound, and an adjuvant salt as necessary.
  • the substance to be precipitated as such, or as an aqueous solution, or slurry, and optionally the precipitating compound as an aqueous or alcoholic or alcoholic/aqueous solution, or in a gaseous form, and/or an acid, or seed crystals of the precipitate substance.
  • the pH and temperature values of the reaction mixture are adjusted to a suitable range, according to the precipitating compound and the substance to be precipitated.
  • the precipitating compound (alumi- nium sulphate (AI 2 (SCM) 3 X 14.3H 2 O)) is dissolved in an excess amount of water having a temperature varying between 10 and 99 °C, preferably between 20 and 60 °C. Thereafter, starch acetate dispersed in an excess amount of water is added to the solution.
  • a salt, preferably magnesium hydroxide may be optionally added as an adjuvant to improve the efficiency of the precipitation, followed by mixing of the mixture at 10 - 99 °C.
  • a compound to be attached by precipitation metal silicate, in this case sodium silicate (Na 4 SiO 4 )
  • an aqueous solution of a precipitating compound aluminium sulphate
  • metal silicate in this case sodium silicate (Na 4 SiO 4 )
  • a precipitating compound aluminium sulphate
  • the viscosity of the suspension is adjusted to a suitable value by water addition.
  • the pH value of the suspension is adjusted to a value of 9 or below, preferably between 4 and 8, using the precipitating compound (aluminium sulphate).
  • the suspension may be used as such without any post treat- ment/filtering/drying, and it may also optionally be filtered, washed, and dried. If necessary, the particle size may be optimized for instance by grinding. If desired, adjuvants such as dispersing agents may be added to the suspension.
  • the starch acetate may also be coated by adsorbing a light scattering material on the starch acetate particles.
  • a light scattering material is thus contacted with starch acetate particles, and allowed to adsorb on the starch acetate particles.
  • starch acetate may be coated with a light scattering material using a gas phase coating technique, or modified gas phase coating, for instance atomic layer epitaxy.
  • the coating is formed from gaseous starting materials with chemical reactions.
  • the reaction may for instance comprise pyrolysis, reduction, oxidation, hydrolysis, or synthesis.
  • Halides, hydrides, metal carbonyls, metal organic compounds, and the like may be used as precursors.
  • the light scattering material is preferably zinc oxide, silicon oxide, or titanium dioxide, the production of which with the gas phase coating technique is illustrated by the reaction equation (16).
  • the starch acetate particles are thus contacted with gaseous starting materials, followed by the dissociation and/or chemical reaction of the starting materials in the gas phase, thus forming a solid coating on the surface of the starch acetate particles.
  • the starch acetate may be coated with a light scattering material by forming aqueous layers of the starch acetate, and the light scattering material using spin coating process, or the like. The layers may be deposited in any order, and the number thereof is not limited. Once the layers are solidified, they may be crushed to the desired grain size according to the desired application.
  • aqueous layers comprising starch acetate particles, and the light scattering material are deposited us- ing spin coating process, followed by solidification of the layers, and crushing of the solidified layers to the desired grain size.
  • the starch acetate composite of the invention may be used as a filler in paper and board.
  • the particle size of the composite to be used as a filler preferably varies from 1 to 2 ⁇ m.
  • the composite of the invention is a suitable filler both for fine papers and for papers containing mechanical pulp, examples including LWC, ULWC, MWC, and SC.
  • the starch acetate composite of the invention may also be used as a coating pig- ment for papers containing mechanical pulp such as for LWC printing papers, as well as a coating pigment for boards, for instance FBB board.
  • the particle size of the composite to be used as a coating pigment preferably varies from 0.2 to 1 ⁇ m.
  • Starch acetate is used in the pharmaceutical industry for instance as a tablet ma- trix, as a tablet coating, and in microparticles containing a drug. Starch acetate is also a widely used additive in food products.
  • the starch acetate composite of the invention may also be used in the pharmaceutical and food industries with the proviso that it is coated with substances acceptable in these fields, such as with sodium aluminium silicate, potassium aluminium silicate, calcium aluminium silicate, aluminium silicate, calcium silicate, magnesium silicate, magnesium tri- silicate, silicon dioxide, calcium sulphate, calcium carbonate, or the like.
  • the coated starch acetate composite is added to the pulp during paper or board production at a suitable stage of the system prior to the press section, preferably in the short circulation and particularly preferably at the proximity of the head box, such as at the suction side of the mixing pump, or at the proximity of the feed pump of the head box, in amounts resulting in filler contents in the paper or board, that is the amount of the coated starch acetate composite varying between 1 and 50 % by weight, followed by producing the paper or board in a conventional manner.
  • the coated starch is not softened nor gelatinized at temperatures of the drying section. The drying may be carried out in a normal fashion using contact, or cylinder drying with cylinders with typical superficial temperatures between 100 and 160 0 C, but, however, the drying may also be carried out using any other drying procedure.
  • the coated starch acetate composite is applied using the above suspension either as such or as a mixture with known binders used in coating pigments such as with starch or a latex, thickening agents e.g. carboxymethyl cellulose, or other additives, in amounts resulting in the proportions of the pigment in the coating paste typically varying from 80 to 95 % by weight.
  • Application on a paper or board web may be accomplished with any known coating method.
  • the coated starch acetate composite of the invention has several advantages in comparison to fillers and coating pigments of the prior art.
  • Critical properties particularly optical properties such as opacity, but also the strength properties e.g. the bonding strength and tensile strength index of paper and board may be favourably influenced by the composite.
  • the ability of the composite to improve the combination of the light scattering coefficient and the bonding strength is particularly preferable.
  • the properties of reject pulp are improved by the composite, since the optical effectiveness thereof and capacity to bind to the pulp is not lost in reject circulation.
  • the grammages of paper and board may be reduced by using said composite.
  • the proportions of renewable organic materials in papers and boards may be increased, and thus the utilization of papers and boards removed from the recycling system by burning may be improved.
  • the disposal of compostable materials to landfills will be prohibited in the future, and thus burning will be one of the important alternatives for waste disposal.
  • a starch acetate composite coated with silicate was produced in laboratory scale by precipitating silicate on starch acetate particles having a particle size between 5 and 8 ⁇ m, in a precipitation vessel with a capacity of 4 liters.
  • the reagents were tempered prior to addition, and the pH of the reaction mixture was measured during the reaction.
  • 9 g of granular aluminium sulphate having a dry matter content of 76.2 %, 1009.8 g of deionized water, 180.8 g of 20.2 % starch acetate slurry diluted with 466 g of deionized water, and further, 0.97 g of magnesium hydroxide were added to the precipitation vessel.
  • the mixture was agitated at 17 °C.
  • Sheets were made of pulp consisting of 70 % of bleached birch pulp and 30 % of bleached softwood pulp, the sheets containing starch acetate composite fillers SA2 and SA4 of the invention. Sheets without any filler and sheets containing an uncoated starch acetate as the filler (API) served as controls, respectively. Sheets having grammages of 60 g/m 2 were made according to the standard SCAN C 26:76. The contents of the uncoated starch acetate in the control sheets were 4 % and 8 %, by weight, respectively. For the composite fillers, the total filler contents were 10 % and 20 % by weight.
  • Figure 2a is a graphical presentation of the light scattering coefficient as a func- tion of the tensile strength index. As figure 2a shows, the light scattering coefficient may be improved by the composites of the invention in comparison to uncoated starch acetate, without any substantial deterioration of the tensile strength index.
  • Figure 2b shows the light scattering coefficient as a function of the bonding strength. This figure clearly shows that the combination of the light scattering coefficient and bonding strength properties may be improved by the composite fillers of the invention in comparison to uncoated starch acetate. It is obvious that the optical properties are improved by the coating in comparison to uncoated starch acetate.
  • Figures 2a and 2b also clearly show that high light scattering coefficients, and superior strength properties are obtained both in thermally treated (90 °C) and untreated sheets using the composites of the invention.

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Abstract

The invention relates to starch acetate composites comprising starch acetate coated with light scattering material, to methods for the production thereof, and to the use thereof as fillers and as coating pigments for paper and board, and further to methods for producing, and for coating paper and board.

Description

Starch acetate composites, method for the production thereof, and use in paper and board production
Field of the Invention
The present invention relates to novel starch acetate composites useful for instance in paper and board production, and to a method for producing such composites. The invention is also directed to the use of said composites as fillers for paper and board and as coating pigments. Moreover, the invention relates to methods for producing, and for coating paper and board.
Prior Art
The purpose of coating is to furnish the surface of paper and board with maximum smoothness and uniform quality for improving optical properties and printability. A coating consists of pigments, e.g. kaolin, ground calcium carbonate (GCC) and talc, and further, a binder such as a latex and starch, and it may also contain additives such as dispersing agents, agents for pH adjustment, lubricants, and anti- microbial agents. Pigments normally comprise from 80 to 95 % by weight of the coating, so the pigments have a particularly significant influence on the optical properties of the coating, e.g. opacity, brightness, and gloss. Brightness is improved by the low light absorption capacity and high light scattering coefficient of the pigment, opacity being also improved by the latter. Factors influencing the gloss include particle size of the pigment, and post-coating treatment of paper and board, such as calendering.
In paper and board production, fillers are added to the pulp. The amount of the filler to be added depends on the product being produced, the proportion thereof being for instance between 4 and 10 % for LWC papers and between 15 and 30 % for chemical pulp papers, relative to the base paper weight. Fillers include e.g. kaolin, calcium carbonate and titanium dioxide. The optical properties and printa- bility of paper and board are influenced also by the filler. A considerable proportion of the filler may comprise reject material from paper production.
The optical properties of paper and board may be improved by increasing the proportion of pigments in the coating, and the amount of fillers in base paper. This, however, deteriorates the strength characteristics of paper and coating.
During paper and board production, starch is typically added to the pulp as an aqueous solution primarily for improving the strength properties of the product. When adding starch to the pulp in granular form, the thus improved strength is, among other things, based on its gelatinization in the drying section. Use of cooked starch is limited for instance by limited adsorption, rannability problems, and cost.
Strength properties of paper may also be improved by refining of pulp, and addition of fines, but this is, however, accompanied by opacity reduction.
FI2004074J discloses a method for producing a pigment or filler based on starch, and further, a starch based pigment or filler product. The products presented are light and biodegradable, and have a particle size or structure making them suitable as coating pigments and fillers. One product disclosed in said document is a spherical starch derivative, such as starch acetate having a mean particle size ranging from 90 to 1000 nanometers.
WO 99/16972 discloses the use of alkali metal silicates, particularly sodium silicate in combination with starch to improve the strength of paper and board. Both the silicate and the starch are added in a finely dispersed form to the fibre suspension at the wet end prior to web formation. The starch is uncooked and the alkali silicate is selected from silicates insoluble in water at the temperatures of the wet end, the silicates being soluble only at temperatures encountered in the drying section.
US 6,623,555 describes a method for producing a composite pigment from pre- cipitated calcium carbonate (PCC) and silicate. Said composite pigment is produced by adding a soluble silicate compound to an aqueous medium containing precipitated calcium carbonate obtained by carbonation of lime milk. An insoluble silicon compound is precipitated on the calcium carbonate thus formed by carbon- ating the reaction mixture, that is, by bubbling CO2 through the reaction mixture.
Optical properties and bonding strength, often referred to as Scott Bond value, are some of the most important properties of printing papers. Generally for boards and papers, and particularly for graphical papers, there is a need for the simultaneous improvement of both the optical and the strength properties.
Incineration of waste papers containing inorganic mineral pigments for energy production results in high amounts of ash, the disposal of which causes problems. Within the European Union, aims set for the proportion of bioenergy in the total energy production should be reached until 2010. For these aims, it is also desir- able to use in papers and boards as much renewable organic materials as possible.
Inorganic mineral pigments also increase the weight of paper and board. There is an ever growing need for increasingly lighter papers for magazines, catalogues and the like, furnished, however, with high quality printing properties.
As on the basis of the above teachings may be seen, there is an obvious need for lighter fillers and coating pigments of novel types for paper and board, particularly allowing for the simultaneous improvement of the optical and also the strength properties of paper and board, and further, the increase of the proportion of renewable and combustible organic materials therein. Object of the Invention
An object of the invention is to provide novel starch acetate composites.
An object of the invention is also to provide a method for producing starch acetate composites.
Further, an object of the invention is the use of starch acetate composites as fillers of paper and board, and as coating pigments in the production thereof.
Another object of the invention is to provide a method for producing paper and board.
Still another object of the invention is to provide a method for coating paper and board.
The characteristic features of the inventive starch acetate composites, the method for the production thereof, the use thereof, and the methods for producing, and for coating paper and board are presented in the Claims.
Summary of the Invention
The invention relates to starch acetate composites. The composite of the invention comprises starch acetate particles having a light scattering material on the surface thereof. For the purposes of the invention, the term light scattering material refers for instance to silica, silicate, precipitated calcium carbonate (PCC), gypsum, calcium oxalate, titanium dioxide, aluminium hydroxide, barium sulphate, or zinc oxide, or a modification or a combination thereof, or any other light scattering material. In the method for producing starch acetate composites of the invention, the starch acetate is coated with the light scattering material defined above. The coating with the light scattering material may be performed by precipitation, adsorption, gas phase coating technique, i.e. CVD (chemical vapour deposition), or spin coating technique, or in a similar manner.
The starch acetate composite obtained may be used as a filler of paper and board for improving the combination of optical and strength properties of the product. Moreover, the composite of the invention may be used as a coating pigment for paper and board.
The invention is now illustrated by means of figures and detailed description, and further, with examples that should not be understood as limiting the invention.
Figure Ia shows an electron micrograph (magnification xl200) of uncoated starch acetate.
Figure Ib shows an electron micrograph (magnification xl200) of starch acetate composite coated with silicate, prepared according to Example 1.
Figure 2a is a graphical presentation of the light scattering coefficient of a sheet containing 10 % and 20 % by weight of a starch acetate composite filler SA4 (prepared according to Example 1) or SA2 (prepared according to a modification of Example 1) as a function of the tensile strength index. Sheets containing respec- tively 4 % and 8 % by weight of uncoated starch acetate (API) as a filler, and a sheet without a filler serve as controls. SA2 90°C, SA4 90°, and API 90 °C refer to sheets treated at 90 0C prior to drying.
Figure 2b is a graphical presentation of the light scattering coefficients of sheets containing starch acetate composite filler of the invention as a function of the bonding strength. Filler contents used and symbols are as in Figure 2a. Figures 3a, 3b and 4 are graphical presentations of the tensile strength indices, the light scattering coefficients, and the bonding strength of sheets containing starch acetate composite filler of the invention as a function of the filler content. The symbols are as in Figure 2a.
Detailed Description of the Invention
Surprisingly, it was found that the problems of the prior art may be avoided, or at least substantially reduced with the procedure of the invention. The invention is based on the finding that starch acetate may be coated with a light scattering material such as silica, silicate, precipitated calcium carbonate (PCC), gypsum, calcium oxalate, titanium dioxide, aluminium hydroxide, barium sulphate, zinc oxide, or a like, or with modifications or combinations thereof, to give a starch acetate composite useful both as a coating pigment and as a filler in the production of paper and board.
The particle size of the starch acetate to be used in the composite of the invention ranges between 0.05 and 10 μm. The starch acetate is preferably spherical starch acetate. The degree of substitution of the starch acetate may vary between 0.5 and 3, being preferably from 1.5 to 3. The higher the degree of substitution of the starch acetate, the better the dimensional stability of the composite prepared therefrom will be at higher process temperatures, and the better the light scattering properties, among other things, will be preserved during reject recycling. The starch acetate useful in the invention may be produced by any known production method, starting for instance from native, hydrolyzed, oxidized, cross-linked, or gelatinized starch, e.g. by reacting with an acetic anhydride, or acetic acid chloride. The starch may be any starch found in nature, such as starch from potato, wheat, corn, barley, durum, tapioca, or rice. Starch acetate may be produced for instance according to patent publications FI 107386, FI20040741, and FI20040743. The material used to coat the starch acetate is selected among any light scattering materials. Suitable light scattering materials include silica, silicate, precipitated calcium carbonate (PCC), gypsum, calcium oxalate, titanium dioxide, aluminium hydroxide, barium sulphate, zinc oxide, or a like, and modifications or combina- tions thereof. A preferable light scattering material is silicate.
The silicate to be used in the composite is selected from the group consisting of metal silicates such as alkaline earth metal silicates, alkali metal silicates, alkaline earth metal, and alkali metal aluminium silicates, and modifications thereof in- eluding mixed salts with alkaline earth metal salts and hydroxides, and further, the mixed salts and combinations of said compounds. The silicate is preferably calcium silicate, magnesium silicate, sodium aluminium silicate, sodium magnesium silicate, sodium silicate, or aluminium silicate, and particularly sodium aluminium silicate.
According to the invention, it is also possible to use various combinations of the coating materials.
The composite of the invention contains the coating material from 5 to 95 % by weight, preferably from 5 to 20 %, or from 50 to 80 % by weight of the composite. The content of the coating material preferably varies between 5 and 20 % by weight of the composite in products, the disposal of which may desirably be achieved by burning, thus minimizing the ash formation. The coating material content ranging from 5 to 20 % by weight allows for the improvement the combi- nation of optical and strength properties of the product according to the invention.
The thickness of the coating may vary within wide limits, ranging typically from 1 nm to 5 μm.
In the method of the invention for producing starch acetate composites, a starch acetate is coated with a light scattering material, said coating being carried out by precipitation, adsorption, gas phase coating, or spin coating processes, or a like. It is thus possible to coat the starch acetate using a modification of said coating processes, e.g. using a modified gas phase coating, for instance atomic layer epitaxy (ALE) process.
The coating may be accomplished by adding the substance to be precipitated to a suspension containing starch acetate particles, the pH and temperature values being optionally adjusted to suitable ranges. The suspension containing starch acetate is optionally combined with an aqueous solution of the precipitating com- pound, and eventually with an adjuvant salt prior to the addition of the substance to be precipitated. If necessary, following the addition of the substance to be precipitated, the precipitating compound in an aqueous, alcoholic or alcoholic/aqueous solution, or in a gaseous form and/or an acid or seed crystals of the precipitate substance is/are added.
The coating may also be carried out by contacting the starch acetate particles with the light scattering material, and by allowing the adsorption thereof on the starch acetate particles.
The coating may further be carried out by contacting the starch acetate particles with gaseous starting materials that dissociate and/or chemically react in the gas phase, thus forming a solid coat on the surface of the starch acetate particles.
Moreover, the coating may be carried out by depositing aqueous layers compris- ing starch acetate particles and the light scattering material with the spin coating process, the layers are allowed to solidify, and the solidified layers are crushed to the desired grain size.
The starch acetate to be used in the method of the invention comprises starch ace- tate particles having a particle size between 0.05 and 10 μm. The light scattering material to be used in the method of the invention may comprise e.g. silica, silicate, precipitated calcium carbonate (PCC), gypsum, calcium oxalate, titanium dioxide, aluminium hydroxide, barium sulphate, zinc oxide, or a like, or a modification or a combination thereof.
Precipitation of Silica
Silicon dioxide, or silica (SiO2), may be precipitated according to the reaction equation (1) below. A suitable substance to be precipitated, that is a basic metal silicate, in this example an aqueous solution of sodium silicate (water glass) is reacted with a precipitating compound, in this case with a mineral acid, typically H2SO4.
[Na2O:xSiO2] + H2SO4 -» XSiO2 + Na2SO4 + H2O (1)
Precipitated silica is also obtained by reacting an alkali metal silicate with sulphurous acid, or with sulphur dioxide. An aqueous solution of alkali metal sulphite or bisulphite is also formed.
Precipitation of Silicates Synthetic silicates are obtained by reacting a silicon compound acting as the substance to be precipitated with a precipitating compound. The precipitating compound may also be formed in situ during the reaction. As a result, silicates such as sodium aluminium silicate, calcium silicate, and aluminium silicate are obtained. Of these, particularly sodium aluminium silicate is the most widely used silicate used in papermaking.
Suitable substances to be precipitated include precipitated silicas, metal silicates e.g. alkaline earth metal silicates, and alkali metal silicates, alkaline earth, and alkali metal aluminium silicates, and modifications thereof, including mixed salts with alkaline earth metal salts and hydroxides, and further, the mixed salts and combinations of said compounds. A silicate such as sodium aluminium silicate may be precipitated according to the following reaction equation (2). Aluminium sulphate, or alum, reacts with an aqueous solution of sodium silicate.
[Na2O:xSiO2] + A12(SO4)3 -» Na2O • Al2O3 • 4[xSiO2] • 4-6 H2O + Na2SO4 (2)
Alternatively, an alkali metal silicate may be reacted with an aqueous solution of aluminium sulphite to give precipitated alkali metal aluminium silicate and an aqueous phase containing alkali metal sulphite, or bisulphite depending on the pH in the final reaction stage.
Precipitated alkali metal aluminium silicate is also obtained by treating an alkali metal silicate solution with an alkali metal aluminate in the presence of sulphur dioxide, sulphurous acid solution, or sulphuric acid solution. In addition, an aque- ous phase containing alkali metal sulphite is obtained. In this case, the precipitating aluminium sulphite reagent is formed in situ during the reaction.
Zinc silicate may be precipitated by mixing sodium silicate solution and zinc chloride solution, replacing the zinc chloride solution by a sulphuric acid solution at the end of the reaction.
Precipitation of Calcium Carbonate
Precipitated calcium carbonate, or PCC, is obtained for instance according to following reaction equations (3) - (5). CaCO3 + energy -> CaO + CO2 (3)
CaO + H2O -» Ca(OH)2 + energy (4)
Ca(OH)2 + CO2 -> CaCO3 + H2O + energy (5)
In the reaction (3), limestone is heated, thus dissociating it to give lime, CaO, and carbon dioxide. Next, lime is mixed with water in the reaction (4), thus obtaining slaked lime, Ca(OH)2. In this step, any impurities may be removed for instance by screening. Calcium carbonate is precipitated in the carbonization step wherein carbon dioxide is passed to an aqueous slurry of the slaked lime in reaction (5). In this step, the particle size, and the particle size distribution of the precipitated calcium carbonate, and further, the shape, and the surface properties of these particles may be influenced by adjusting the reaction conditions.
Calcium carbonate may also be precipitated according to the reaction equation (6). In this equation, slaked lime is reacted with sodium carbonate. The alkaline solution produced in the reaction is neutralized prior to using the CaCO3 in papermak- ing. Ca(OH)2 + Na2CO3 -* CaCO3 + 2NaOH (6)
Calcium carbonate may further be precipitated by reacting sodium carbonate with calcium chloride according to equation (7):
Na2CO3 + CaCl2 -> CaCO3 + 2NaCl (7)
Precipitation of Gypsum
Calcium sulphate is found in various hydrated and anhydrous forms, of which the calcium sulphate dihydrate, CaSO4-2H2O, is commonly called gypsum. This di- hydrate is the most stable form of calcium sulphate, and thus, it is used in coating pigments. The spontaneous precipitation of the dihydrate form is a common phenomenon in case of boiler sediments, and the precipitation takes place in oversatu- rated solutions according to the reaction equation (8).
Ca2+ + SO4 2" + 2H2O -» CaSO4-2H2O (8)
The dihydrate is also precipitated according to the reaction equation (9) from cal- cium sulphate hemihydrate, CaSO4- 1^H2O once it is slurried in water. The particle size distribution and particle shape of the precipitating gypsum may be influenced by adjusting the precipitation conditions. 2CaSO4- !/2H2O + 3H2O -» 2CaSO4-2H2O (9) The dihydrate form is also precipitated once calcium phosphate is reacted with sulphuric acid in an aqueous solution according to the reaction equation (10). Also phosphoric acid is formed in the reaction.
Ca3(PO4)2 + 3H2SO4 + 6H2O -> 3CaSO4-2H2O + 2H3PO4 (10)
As the raw phosphate, Ca1o(P04)6F2, reacts with sulphuric acid in an aqueous solution, the dihydrate form of calcium sulphate, phosphoric acid, and hydrofluoric acid are formed according to the reaction equation (11). Ca10(PO4)6F2 + 10H2SO4 + 20H2O -■> 19CaSO4-2H2O + 6H3PO4 + 2HF (11)
The dihydrate form of calcium sulphate is also precipitated as calcium hydrogen sulphite reacts with oxygen in an aqueous solution according to the reaction equation (12). Ca(HSO3)2 (1) + O2 (g) + 2H2O (1) "* CaSO4-2H2O (s) + H2SO4 (12)
Precipitation of Calcium Oxalate
Calcium oxalate may be produced by precipitation from oxalic acid in the presence of a compound containing calcium. The compound containing calcium may for instance be calcium carbonate, calcium hydroxide, or calcium chloride. The production of calcium oxalate from calcium carbonate and oxalic acid is presented in reaction equations (13)-(14).
CaCO3 + 2HCl -> CaCl2 + H2O + CO2 (13)
CaCl2 + H2C2O4 -» CaC2O4 + 2HCl (14)
Precipitation of Titanium Dioxide
Titanium dioxide may be produced for instance with the known sulphate process, that is, by dissolving dried and ground ilmenite, or titanium slurry using concentrated sulphuric acid, and heating to produce a solid reaction product cake. The reaction product cake is dissolved in water or diluted sulphuric acid, and further, solid impurities are removed from the titanium sulphate solution for instance by filtering. The iron content of the solution may be further reduced by cooling, thus precipitating the iron as an iron sulphate heptahydrate that may be removed by filtering. The solution is concentrated to precipitate the titanium as tita- nium(rV)oxyhydroxide, followed by filtering, washing, and conversion to the desired crystal size and shape by calcination, if necessary. Starch acetate may then be coated with the titanium dioxide thus obtained using e.g. adsorption, or spin coating processes.
Titanium dioxide may also be produced with the procedure disclosed in the document US 6,001,326, that is by adding ice cubes made of water, or icy water to an undiluted titanium tetrachloride solution, and diluting the aqueous solution of titanyl chloride thus obtained to give the desired concentration, followed by heating thus precipitating finely divided titanium dioxide.
Precipitation of Aluminium Hydroxide Aluminium hydroxide, also known as aluminium trihydrate, may be produced from bauxite by dissolving the aluminium contained therein, followed by separation of the other minerals. The aluminium compounds of the solution are extracted with sodium hydroxide and then insoluble impurities are separated by sedimentation and filtration. The clear sodium aluminate filtrate is cooled, followed by the addition of fine aluminium hydroxide crystals, specifically prepared as seed crystals for this purpose, if necessary, and starch acetate. The aluminate contained in the filtrate is precipitated on the seed crystals and starch acetate added.
Precipitation of Barium Sulphate Barium sulphate may be precipitated from barium compounds soluble in water using compounds containing a sulphate group, also being soluble in water. Said barium compound may for instance be barium nitrate, sulphide, hydroxide, or chloride, whereas the compound containing a sulphate group is sodium, or magnesium sulphate, or sulphuric acid. The preparation of barium sulphate from bar- ium chloride and sodium sulphate is illustrated by the reaction equation (15).
BaCl2 (aq) + Na2SO4 (aq) -» BaSO4 (s) + 2NaCl (aq) (15) Precipitation of Zinc Oxide
Zinc oxide may be precipitated by heating zinc nitrate, thus resulting in zinc oxide, nitrogen dioxide, and oxygen. Zinc oxide may also be precipitated by heating zinc carbonate, thus giving zinc oxide, and carbon dioxide. Moreover, zinc oxide may be precipitated with calcium oxide, or with calcium hydroxide from a solution containing zinc ions, or by hydrolysis of zinc acetate with lithium hydroxide, or with tetramethylammonium hydroxide in an alcoholic or alcoholic/aqueous solution.
Accordingly, in an embodiment of the method of the invention, an aqueous suspension containing the starch acetate is optionally combined with an aqueous solution containing the precipitating compound, followed by the optional addition of another salt as an adjuvant, and thereafter, the substance to be precipitated is added as such, or as an aqueous solution or slurry, and further, optionally the pre- cipitating compound as an aqueous, alcoholic, or alcoholic/aqueous solution, or in a gaseous form, and/or an acid or seed crystals of the precipitate substance are added. pH and temperature values are optionally adjusted to a suitable range.
For the precipitation of silicates and silica, the precipitating compound is selected from the group consisting of inorganic acids, sulphur dioxide, as well as alkaline earth metals, alkali metals, earth metals, salts of zinc and aluminium, preferably sulphate, sulphite, nitrate, and ammonium sulphate salts. The precipitation is particularly preferably carried out using aluminium sulphite, or alkali metal alumi- nate in the presence of sulphur dioxide, sulphurous acid, or sulphuric acid. Alter- natively, the precipitation may also be accomplished with zinc chloride that will be replaced by a sulphuric acid solution in the final stage of the reaction.
For calcium carbonate precipitation, the precipitating compound may for instance be gaseous carbon dioxide, or sodium carbonate. In case gypsum is precipitated from calcium phosphate, or from raw phosphate, the precipitating compound will be sulphuric acid. In case gypsum is precipitated from calcium hydrogen sulphite in an aqueous solution, gaseous oxygen is used as the precipitating compound. In case the precipitation is carried out in an oversatu- rated solution, any compound releasing sulphate ions when dissolving in water may be used as the precipitating compound. Alternatively in cases where calcium sulphate hydrate is precipitated from an aqueous slurry of a hemihydrate, the precipitating compound may be left out.
For precipitating calcium oxalate, the precipitating compound is oxalic acid.
For the precipitation of titanium dioxide, the substance to be precipitated may be heated instead of adding a precipitating compound, thus giving finely divided titanium dioxide.
In cases aluminium hydroxide is precipitated, aluminium hydroxide seed crystals are added instead of the precipitating compound, if necessary.
For barium sulphate precipitation, the precipitating compound is a compound con- taining sulphate groups, such as sodium, or magnesium sulphate, or sulphuric acid.
For zinc oxide precipitation, the precipitating compound is for instance a calcium oxide, hydroxide, lithium hydroxide, or tetramethylammonium hydroxide. In cases zinc nitrate, or zinc carbonate is used as the substance to be precipitated, the addition of any precipitating compound is not necessarily needed.
For the precipitation of silicates and silicas, the salt serving as an adjuvant is selected from a group consisting of alkaline earth metal salts, and hydroxides. Suit- able salts include the chlorides, sulphates, and carbonates of alkaline earth metals such as magnesium, or calcium. Magnesium hydroxide is preferably used. For the precipitation of silicates, the substance to be precipitated is selected from the group consisting of precipitated silicas, alkali metal, and alkaline earth metal silicates, alkali metal, and alkaline earth metal aluminium silicates, and modifications thereof including mixed salts with alkaline earth metal salts and hydroxides, and further, the mixed salts and combinations of said compounds.
For the precipitation of silicates, the substance to be precipitated is selected from the group consisting of alkali metal, and alkaline earth metal silicates.
For the precipitation of calcium carbonate, the substance to be precipitated is for instance calcium hydroxide, or calcium chloride. Calcium hydroxide is obtained by mixing burnt lime in water, said lime thus reacting to give calcium hydroxide.
For the precipitation of gypsum, the substance to be precipitated is calcium phos- phate, calcium sulphate hemihydrate, raw phosphate, calcium hydrogen sulphite, or any compound releasing calcium ions when dissolved in water.
For the precipitation of calcium oxalate, the substance to be precipitated is any compound containing calcium, for instance calcium chloride, calcium carbonate, or calcium hydroxide.
For the precipitation of titanium dioxide, the substance to be precipitated is for instance titanyl chloride.
For the precipitation of aluminium hydroxide, the substance to be precipitated is sodium aluminate.
For the precipitation of barium sulphate, the substance to be precipitated is a barium compound, e.g. barium nitrate, sulphide, hydroxide, or chloride. For the precipitation of zinc oxide, the substance to be precipitated may for instance be zinc nitrate, zinc carbonate, or zinc acetate.
In the method of the invention, an aqueous suspension of the dispersed or slurried starch acetate is optionally combined with an aqueous solution of the precipitating compound, and an adjuvant salt as necessary. This is followed by the addition of the substance to be precipitated as such, or as an aqueous solution, or slurry, and optionally the precipitating compound as an aqueous or alcoholic or alcoholic/aqueous solution, or in a gaseous form, and/or an acid, or seed crystals of the precipitate substance. If necessary, the pH and temperature values of the reaction mixture are adjusted to a suitable range, according to the precipitating compound and the substance to be precipitated.
In a preferable embodiment of the invention, the precipitating compound (alumi- nium sulphate (AI2(SCM)3X 14.3H2O)) is dissolved in an excess amount of water having a temperature varying between 10 and 99 °C, preferably between 20 and 60 °C. Thereafter, starch acetate dispersed in an excess amount of water is added to the solution. To the suspension thus obtained, a salt, preferably magnesium hydroxide may be optionally added as an adjuvant to improve the efficiency of the precipitation, followed by mixing of the mixture at 10 - 99 °C. Then, a compound to be attached by precipitation (metal silicate, in this case sodium silicate (Na4SiO4)) and an aqueous solution of a precipitating compound (aluminium sulphate) is added to the suspension. This results in the precipitation of the desired silicon compound (sodium aluminium silicate) on the starch acetate particles. As a result, a suspension containing starch acetate composite coated with a silicate is obtained. The viscosity of the suspension is adjusted to a suitable value by water addition. The pH value of the suspension is adjusted to a value of 9 or below, preferably between 4 and 8, using the precipitating compound (aluminium sulphate). The suspension may be used as such without any post treat- ment/filtering/drying, and it may also optionally be filtered, washed, and dried. If necessary, the particle size may be optimized for instance by grinding. If desired, adjuvants such as dispersing agents may be added to the suspension.
In the method of the invention for producing a starch acetate composite, the starch acetate may also be coated by adsorbing a light scattering material on the starch acetate particles.
In another embodiment of the method of the invention, a light scattering material is thus contacted with starch acetate particles, and allowed to adsorb on the starch acetate particles.
In still another embodiment of the method of the invention for producing a starch acetate composite, starch acetate may be coated with a light scattering material using a gas phase coating technique, or modified gas phase coating, for instance atomic layer epitaxy.
In the gas phase coating, the coating is formed from gaseous starting materials with chemical reactions. The reaction may for instance comprise pyrolysis, reduction, oxidation, hydrolysis, or synthesis. Halides, hydrides, metal carbonyls, metal organic compounds, and the like may be used as precursors. For the coating with the gas phase coating technique, the light scattering material is preferably zinc oxide, silicon oxide, or titanium dioxide, the production of which with the gas phase coating technique is illustrated by the reaction equation (16). TiCl4 + 2O2 -» TiO2 + 2Cl2 (16)
In yet another embodiment of the method of the invention, the starch acetate particles are thus contacted with gaseous starting materials, followed by the dissociation and/or chemical reaction of the starting materials in the gas phase, thus forming a solid coating on the surface of the starch acetate particles. Moreover, in the method of the invention for producing a starch acetate composite, the starch acetate may be coated with a light scattering material by forming aqueous layers of the starch acetate, and the light scattering material using spin coating process, or the like. The layers may be deposited in any order, and the number thereof is not limited. Once the layers are solidified, they may be crushed to the desired grain size according to the desired application.
In still another embodiment of the method of the invention, aqueous layers comprising starch acetate particles, and the light scattering material are deposited us- ing spin coating process, followed by solidification of the layers, and crushing of the solidified layers to the desired grain size.
The starch acetate composite of the invention may be used as a filler in paper and board. The particle size of the composite to be used as a filler preferably varies from 1 to 2 μm. The composite of the invention is a suitable filler both for fine papers and for papers containing mechanical pulp, examples including LWC, ULWC, MWC, and SC.
The starch acetate composite of the invention may also be used as a coating pig- ment for papers containing mechanical pulp such as for LWC printing papers, as well as a coating pigment for boards, for instance FBB board. The particle size of the composite to be used as a coating pigment preferably varies from 0.2 to 1 μm.
Starch acetate is used in the pharmaceutical industry for instance as a tablet ma- trix, as a tablet coating, and in microparticles containing a drug. Starch acetate is also a widely used additive in food products. The starch acetate composite of the invention may also be used in the pharmaceutical and food industries with the proviso that it is coated with substances acceptable in these fields, such as with sodium aluminium silicate, potassium aluminium silicate, calcium aluminium silicate, aluminium silicate, calcium silicate, magnesium silicate, magnesium tri- silicate, silicon dioxide, calcium sulphate, calcium carbonate, or the like. In the method of the invention for making paper or board, the coated starch acetate composite is added to the pulp during paper or board production at a suitable stage of the system prior to the press section, preferably in the short circulation and particularly preferably at the proximity of the head box, such as at the suction side of the mixing pump, or at the proximity of the feed pump of the head box, in amounts resulting in filler contents in the paper or board, that is the amount of the coated starch acetate composite varying between 1 and 50 % by weight, followed by producing the paper or board in a conventional manner. Contrary to conventional fillers containing starch, the coated starch is not softened nor gelatinized at temperatures of the drying section. The drying may be carried out in a normal fashion using contact, or cylinder drying with cylinders with typical superficial temperatures between 100 and 160 0C, but, however, the drying may also be carried out using any other drying procedure.
In the method of the invention for coating paper, the coated starch acetate composite is applied using the above suspension either as such or as a mixture with known binders used in coating pigments such as with starch or a latex, thickening agents e.g. carboxymethyl cellulose, or other additives, in amounts resulting in the proportions of the pigment in the coating paste typically varying from 80 to 95 % by weight. Application on a paper or board web may be accomplished with any known coating method.
The coated starch acetate composite of the invention has several advantages in comparison to fillers and coating pigments of the prior art. Critical properties, particularly optical properties such as opacity, but also the strength properties e.g. the bonding strength and tensile strength index of paper and board may be favourably influenced by the composite. The ability of the composite to improve the combination of the light scattering coefficient and the bonding strength is particularly preferable. Moreover, in comparison to fillers consisting of conventional starches, the properties of reject pulp are improved by the composite, since the optical effectiveness thereof and capacity to bind to the pulp is not lost in reject circulation. In addition, the grammages of paper and board may be reduced by using said composite.
By means of the composite of the invention, the proportions of renewable organic materials in papers and boards may be increased, and thus the utilization of papers and boards removed from the recycling system by burning may be improved. Within the European Union, the disposal of compostable materials to landfills will be prohibited in the future, and thus burning will be one of the important alternatives for waste disposal.
Examples
Example 1
Production of a starch acetate composite coated with silicate
A starch acetate composite coated with silicate was produced in laboratory scale by precipitating silicate on starch acetate particles having a particle size between 5 and 8 μm, in a precipitation vessel with a capacity of 4 liters. The reagents were tempered prior to addition, and the pH of the reaction mixture was measured during the reaction. 9 g of granular aluminium sulphate having a dry matter content of 76.2 %, 1009.8 g of deionized water, 180.8 g of 20.2 % starch acetate slurry diluted with 466 g of deionized water, and further, 0.97 g of magnesium hydroxide were added to the precipitation vessel. The mixture was agitated at 17 °C. Thereafter, 414 g of sodium silicate having a dry matter content of 43.5 %, and 91.9 g of aluminium sulphate dissolved in 391.74 g of deionized water were added to the mixture within 4 minutes, at the starting temperature of 17 °C, followed by agitation of the mixture at 17 0C. The pH was adjusted during the reaction to a value of 7 or below. The SEM-graph of the starch acetate composite coated with sodium aluminium silicate thus produced is presented in the appended figure Ib clearly showing the silicate particles coated on starch acetate particles (marked with arrows). The particle size of the composite was determined with laser diffraction method. The mean particle size was about 9 μm, the size ranging from 6 to 30 μm, including the agglomerated particles. The composite contained about 80 % by weight of sodium aluminium silicate, and 20 % by weight of starch acetate.
Example 2
Use of the starch acetate composite coated with silicate as a filler in paper
Sheets were made of pulp consisting of 70 % of bleached birch pulp and 30 % of bleached softwood pulp, the sheets containing starch acetate composite fillers SA2 and SA4 of the invention. Sheets without any filler and sheets containing an uncoated starch acetate as the filler (API) served as controls, respectively. Sheets having grammages of 60 g/m2 were made according to the standard SCAN C 26:76. The contents of the uncoated starch acetate in the control sheets were 4 % and 8 %, by weight, respectively. For the composite fillers, the total filler contents were 10 % and 20 % by weight. Some of the sheets containing fillers were thermally treated at 90 0C for 10 minutes prior to drying of the sheets under conven- tional conditions. The light scattering coefficients, bonding strengths as Scott Bond values, and tensile strength indices were determined with methods according to SCAN-P 8:93, TAPPI T 569, and SCAN-P 67:93.
Figure 2a is a graphical presentation of the light scattering coefficient as a func- tion of the tensile strength index. As figure 2a shows, the light scattering coefficient may be improved by the composites of the invention in comparison to uncoated starch acetate, without any substantial deterioration of the tensile strength index. Figure 2b shows the light scattering coefficient as a function of the bonding strength. This figure clearly shows that the combination of the light scattering coefficient and bonding strength properties may be improved by the composite fillers of the invention in comparison to uncoated starch acetate. It is obvious that the optical properties are improved by the coating in comparison to uncoated starch acetate. Figures 2a and 2b also clearly show that high light scattering coefficients, and superior strength properties are obtained both in thermally treated (90 °C) and untreated sheets using the composites of the invention.

Claims

Claims
1. Starch acetate composite, characterized in that said composite comprises starch acetate coated with a light scattering material.
2. Composite according to Claim 1, characterized in that said starch acetate comprises starch acetate particles with a particle size ranging between 0.05 and 10 μm.
3. Composite according to Claim 1 or 2, characterized in that said starch acetate comprises spherical starch acetate.
4. Composite according to any of Claims 1 - 3, characterized in that said light scattering material is selected form the group consisting of silica, silicate, PCC, gypsum, calcium oxalate, titanium dioxide, aluminium hydroxide, barium sulphate, zinc oxide, or modifications or combinations thereof.
5. Composite according to any of Claims 1 - 4, characterized in that said composite contains light scattering material from 5 to 95 % by weight, preferably from 5 to 20 %, or from 50 to 80 % by weight.
6. Composite according to any of Claims 1 - 5, characterized in that the light scattering material is a silicate selected from the group consisting of alkaline earth metal silicates, alkali metal silicates, alkaline earth metal aluminium silicates, al- kali metal aluminium silicates, modifications thereof, the mixed salts and combinations of said compounds, the light scattering material preferably being sodium aluminium silicate.
7. Method for producing a starch acetate composite, characterized in that the method comprises steps wherein a starch acetate is coated with a light scattering material by precipitation.
8. Method according to Claim 7, characterized in that for precipitation, the substance to be precipitated is added to a suspension containing starch acetate, the pH and temperature values being optionally adjusted to suitable ranges.
9. Method according to Claim 8, characterized in that the suspension containing starch acetate is combined with an aqueous solution of the precipitating compound, and optionally with an adjuvant salt, prior to the addition of the substance to be precipitated.
10. Method according to Claim 8 or 9, characterized in that following the addition of the substance to be precipitated, the precipitating compound in an aqueous, alcoholic or alcoholic/aqueous solution, or in a gaseous form, and/or an acid or seed crystals of the precipitate substance is/are added.
11. Method according to any of Claims 7 - 10, characterized in that the starch acetate comprises starch acetate particles having a particle size between 0.05 and 10 μτn.
12. Method according to any of Claims 7 - 11, characterized in that the light scattering material is selected from the group consisting of silica, silicate, PCC, gypsum, calcium oxalate, titanium dioxide, aluminium hydroxide, barium sulphate, zinc oxide, or modifications or combinations thereof.
13. Use of the starch acetate composite according to any of Claims 1 - 6 as a filler of paper or board.
14. Use of the starch acetate composite according to any of Claims 1 - 6 as a coating pigment of paper and board.
15. Method for producing paper or board, characterized in that starch acetate composite according to any of Claims 1 - 6 is added to the pulp followed by producing the paper in a conventional manner.
16. Method for coating paper or board, characterized in that starch acetate composite according to any of Claims 1 - 6 is applied using known methods on a paper or board web as a suspension or as a mixture with the adjuvants of the coating.
PCT/FI2005/050447 2004-12-08 2005-12-05 Starch acetate composites, method for the production thereof, and use in paper and board production WO2006061460A1 (en)

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WO2014001628A1 (en) * 2012-06-28 2014-01-03 Nordkalk Oy Ab Light and smooth coating for paper or board or paint coating formed from a composite structure

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