EP1196503A1

EP1196503A1 - Kaolin products and their preparation and use in paper making and coating

Info

Publication number: EP1196503A1
Application number: EP00929693A
Authority: EP
Inventors: John Claude Husband
Original assignee: Imerys Minerals Ltd
Current assignee: Imerys Minerals Ltd
Priority date: 1999-05-18
Filing date: 2000-05-17
Publication date: 2002-04-17
Also published as: BR0010630A; AU754382B2; AU4769700A; WO2000069977A1

Abstract

A method for the preparation of a composition for use in paper making or pigment coating compositions containing also an optical brightening agent (OBA) includes the steps of (a) preparing an aqueous suspension of a clay mineral comprising a kaolin particulate material; and (b) treating the clay mineral by addition of a treatment agent which reduces the harmful effect of metal ions present in the clay mineral on the activity of the OBA, the treatment agent being substantially free of multivalent cations and comprising (i) a complexing agent or (ii) formamidine sulfinic acid (FAS).

Description

KAOLIN PRODUCTS AND THEIR PREPARATION AND USE IN PAPER MAKING AND COATING

The present invention relates to kaolin products and their preparation and use in paper making and coating. In particular, the invention is concerned with improved kaolin products to be used in compositions for paper making and especially for paper coating in which an optical brightening agent is employed.

Kaolin particulate materials are one of a number of kinds of mineral material employed as property enhancing fillers in compositions for making paper and like products and as property enhancing pigments in compositions for coating paper and like products.

Whiteness is a key factor in the appearance of more expensive printing and writing grades of paper and, in addition to pigments such as kaolin, optical brightening agents (OBAs) are frequently added to coating compositions and in some cases to paper making compositions. OBAs absorb ultraviolet radiation present in daylight and emit visible light as fluorescence making the paper appear whiter and brighter and bluer in shade.

It is believed that the fluorescence of OBAs in the presence of kaolin is quenched due to the presence of trace impurities in the kaolin. This causes an undesirable reduction in the effectiveness of the OBA when used in conjunction with a kaolin particulate material. This problem has not been solved satisfactorily in the prior art .

One purpose of the present invention is to provide a kaolin particulate material which shows enhanced fluorescence when used in compositions containing an OBA.

According to the present invention in a first aspect there is provided a method for the preparation of a kaolin product for use in paper filling or pigment coating applications in the presence of an optical brightening agent which includes the steps of (a) preparing an aqueous suspension of a clay mineral comprising a kaolin particulate material; and (b) treating the clay mineral by addition of a treatment agent which reduces the harmful effect of metal ions present in the clay mineral on the activity of the OBA, the treatment agent being substantially free of multivalent cations and comprising (i) a complexing agent or (ii) formamidine sulfinic acid (FAS) .

We have found that by preparing a kaolin product by the method of the first aspect of the invention that surprisingly and beneficially, paper filling or coating compositions may be produced as demonstrated later which show improved fluorescence brightness compared with clay minerals treated in accordance with prior art methods . In this specification the expression ^Λpaper' embraces products which are of paper, board, card and the like.

Although complexing agents have been used in the prior art to improve the whiteness and brightness of a natural white mineral material they have not been used to enhance the fluorescence produced by an OBA in the presence of a clay mineral pigment or filler by preventing the quenching effect of metals such as iron on an OBA.

The processes described in GB-2268484-B, US-3899343 and US-3193344 make use of complexing agents to improve the brightness and whiteness of clay but there is no suggestion that complexing agents could be used to prevent the harmful effect of metal ions in the clay mineral on the activity of an OBA. Also the process of GB-2268484-B requires the addition of a low oxidation state transition metal ion with the complexing agent which is not required in the method of the invention.

WO-9707076-A1 relates to a method of producing whitened layer silicates using formamidine sulfinic acid as a bleaching agent. There is however no suggestion that a clay mineral bleached with FAS will exhibit less of a quenching effect on the fluorescence produced by an OBA. It has been found that complexing agents produce good gains in the fluorescence that may be produced by an OBA compared to a conventional reductive bleaching agent such as sodium hydrosulfite which gives only slight gains. The improvement in fluorescence when an OBA is used with a clay mineral treated in this way could not have been expected. Furthermore the improvement in fluorescence caused by the treatment with FAS which, like sodium hydrosulfite, is a reductive bleaching agent is highly unexpected. Where the treatment agent comprises a complexing agent, the complexing agent may comprise one or more known chelating agents for metal ions, especially ions of transition metals such as iron. For example, the complexing agent may comprise one or more of triethanolamine

(TEA) , ethylenediaminetetra-acetic acid (EDTA) or one of its salts, diethylenetriaminepentacetic acid (DTPA) or one of its salts, citric acid or one of its salts and sodium glucoheptonate . The treated kaolin product produced in step (b) may be treated, prior to OBA addition, to remove impurities, such as iron, solubilised by the addition of the treatment agent. For example the treatment to remove impurities may include the steps of dewatering and optionally washing, e.g. diluting with water and reconcentrating. The dewatering step and washing and reconcentrating steps, if required, may be carried out in a well known manner, for example by sedimentation, decantation, centrifugation or by the use of a filtration membrane. Alternatively, the impurities solubilised by the treatment in step (b) , especially where the treatment agent comprises TEA, mayu be retained in the suspension and the suspension may subsequently be employed in an OBA-containing composition, e.g. for paper making or paper coating.

The clay mineral which is used in this invention may have considerable iron staining in its untreated state but in its pure state it may be substantially free of any such staining. The clay mineral may contain at least 50% by weight kaolinite. For example, most commercially important china clays and kaolin minerals contain greater than 75% by weight kaolinite and may contain greater than 90%, in some cases greater than 95% by weight of kaolinite.

The clay mineral for use in the invention may be treated by one or more other processes which are well known to those skilled in the art, for example by known refining or benefication steps. For example, the clay mineral may be bleached with a reductive bleaching agent, such as sodium hydrosulfite, prior to step (b) and optionally prior to step (a) . If sodium hydrosulfite is used, the bleached clay mineral may optionally be dewatered, and optionally washed and again optionally dewatered, after the sodium hydrosulfite bleaching step. Alternatively the clay mineral used in step (a) of the first aspect of the invention may be untreated in the form of a solid or as an aqueous suspension and after treatment in step (b) with FAS or a complexing agent it may optionally undergo further refining and benefication steps, for example bleaching with a reductive bleaching agent, such as sodium hydrosulfite, prior to its use in an OBA containing composition.

The production of a paper making or coating composition containing a kaolin product produced in accordance with the method of the first aspect of the invention and also an OBA allows paper products of improved fluorescence to be obtained. Such products are brighter in daylight and this is a highly desirable property, particularly in high quality papers. Furthermore the effect of the improved fluorescence may be shown by a kaolin product which may or may not be made significantly brighter by further bleaching. A wide variety of organic molecules containing delocalised π electrons can be used as optical brightening agents though for economic reasons the OBAs generally used are dye molecules which are derivatives of 4 : 4 ^x -bistriazinyl aminostilbene-2 : 2 " -disulfonic acid which has the generalised structure shown below;

The identity of the groups x, y, w and z vary and are known to affect the substantivity of the molecule to the substrate. In coating grades, molecules contain at least 4 and sometimes 6 sulfonic acid groups. The OBA used in in conjunction with the kaolin product produced by the method of the first aspect of this invention may comprise one or more OBAs known in the art, for example it may comprise one or more molecules containing between 2 and 6 sulfonate groups, e.g. one or more substituted stilbenes. In addition to the dye itself, solutions of commercially available OBAs sometimes contain one or more additives, e.g. a stabiliser such as urea. An activator (or carrier) may be used in conjunction with the OBA. This interacts with the OBA molecules and allows greater levels of fluorescence to be achieved. Self activating OBA are also commercially available in which an activator has been combined with an OBA.

The aqueous clay mineral suspension employed in step (a) of the method according to the first aspect of the invention may have a solids content of between 10% and 70% by weight, e.g. from 12% to 50%, especially from 15% to 30% based on the total weight of the suspension. Mixing of the suspension may be for a period of time e.g. 2 to 30 minutes, especially 5 to 15 minutes to help to attempt to form a homogenous suspension. The process of forming a suspension of the required solids content is well known to those skilled in the art .

A gas may be bubbled through the clay mineral suspension before and during the treatment process of step (b) to help to minimise the reversion of the iron to the oxidised ferric state which causes the undesirable discolouration of the clay mineral and quenches the fluorescence produced by the OBA when the OBA is added to the clay mineral. The gas may be a non-oxidising gas, especially an inert gas, e.g. nitrogen. The addition of the gas before the treatment step may be carried out during the formation of the suspension described earlier or as a separate step once the suspension is formed and is relatively homogenous. If it is added once the suspension is formed it may be added with the suspension still being mixed. The addition of gas prior to the treatment process of step (b) may last for a period of time e.g. 2 to 20 minutes, especially 5 to 15 minutes. The suspension may also be stirred during the treatment in step (b) of the first aspect of the invention as well as before and during the addition of the gas. The OBA added to the kaolin product or suspension containing it may be added either alone or together with other ingredients, e.g. in the preparation of a pigment coating composition to incorporate the kaolin product as a pigment ingredient. The preparation of coating compositions is described later. If the OBA is used with other ingredients these may be added to the coating composition either before or after the OBA or they may be combined with the OBA prior to its addition into the pigment coating composition. The OBA itself may be in solution, for example in an aqueous solution, which may contain other additives such as urea. The OBA could alternatively be added in a dry form. The other ingredients added before, with, or after the OBA or combined with it may include one or more ingredients selected from binders, activators, dispersants and other ingredients commonly used in coating compositions.

The treatment agent used to treat the kaolin product produced by the method according to the first aspect of the invention may be either (i) a complexing agent or (ii) FAS. Step (b) may involve adding the treatment agent and leaving the suspension for a period of time to allow the reaction with impurity species provided by the treatment to take place. The suspension may be stirred during this time.

Where the treatment agent is (i) a complexing agent the following conditions may be used.

The complexing agent may be added in dry form or in an aqueous solution. If an aqueous solution of the complexing agent is used this may be adjusted to a suitable pH before it is added to the clay mineral suspension and once the complexing agent is added the pH of the clay mineral suspension may then be adjusted to the pH required for the reaction.

The complexing agent may be added in an active amount of up to 80 kg.t^-1, e.g. in an amount from O.lkg.t^-1 to 30kg. t^"1 especially

2.5kg.t^_1 to 20kg. t^"1 based on the dry weight of the clay mineral.

The treatment of the clay mineral suspension with the complexing agent may be carried out for between 10 minutes and 5 hours, e.g. from 30 minutes to 4 hours, especially from 45 minutes to

3 hours. The suspension may be mixed by stirring during this time.

Treating with the complexing agent may be carried out at a temperature of between 10°C and

80°C, e.g. between 20°C and 70°C, especially between 40°C and 65°C.

The pH of the clay mineral suspension is preferably adjusted during or immediately after the addition of the complexing agent. The most beneficial pH for the reaction depends on the complexing agent used although it may be in the range of between pH 1.5 and 9, especially between pH 2 and 7.5. With EDTA the best results have been achieved at a low pH of between 2 and 3.

With citric acid it was found that a higher pH of between 5 and 8 gave better results. With TEA it was found that a pH of between 5.5 and 8.5 gave suitable results. The pH adjustment of the clay mineral suspension may be carried out using dilute solutions of an acid, such as a 5% to 15% solution of sulfuric acid, or dilute solutions of a base, such as a 5% to 15% solution of sodium hydroxide. If the complexing agent is used as an aqueous solution, the pH of this may be adjusted in the same manner.

When the treatment in step (c) is complete the suspension may preferably be dewatered, preferably using filtration to remove the complexed impurities, especially the complexed iron impurity, from the clay mineral. The clay mineral may be filtered at the pH of the reaction. Alternatively, if the reaction has been carried out at high pH, the pH may be lowered, e.g. to between pH 3 and 6, especially to between pH 3.5 and 4.5, preferably using a dilute solution of an acid, such as a 5% to 15% solution of sulfuric acid. This process allows the complexing agent to be used at a pH of above 7 without the disadvantage of the clay mineral deflocculating on filtering.

Where the treatment agent used to treat the clay mineral in step (b) according to the first aspect of the invention is (ii) FAS the following conditions may be used:

FAS may be used in an amount of up to 40kg. t^"1 especially from O.lkg.t^"1 to 20kg. t^'1 , e.g. from lkg. t^"1 to 10kg. t^"1.

The treatment may be carried out for up to 1.5 hours, e.g. for between 10 minutes and 1 hour, especially for between 20 and 50 minutes. Treatment with FAS may be carried out at a temperature of between 10°C and 90°C, e.g. between 15°C and 70°C, e.g. between 20°C and 60°C. The clay mineral suspension may have its temperature adjusted to that required for the reaction prior to the addition of the FAS.

FAS is a reductive bleaching agent and works via the typical reaction of;

H₂N _χO H₂N

)c— s + NaOH *^{► X}C=0 + NaHSO,

// \ / ³

HN OH H₂N

A base such as sodium hydroxide must therefore be present in an amount of at least a 1:1 molar ratio with the FAS, in order to use the FAS most effectively, when FAS is used to treat the clay mineral. However it has been found that a considerably higher molar ratio of base: FAS of 9:1 gives superior results and when low doses of FAS are used the molar ratio of base: FAS used may be as high as 70:1. The base: FAS molar ratio may therefore be between 1:1 and 70:1, e.g. between 2:1 and 35:1, especially from 4:1 to 15:1. The base may be sodium hydroxide. For convenience it may be found advantageous to use the sodium hydroxide in a solution of between 10% and 40% by weight e.g. from 15% to 25%. It may be beneficial to form an aqueous clay mineral suspension and to form a solution of the FAS and an alkali such as sodium hydroxide which is added to the clay mineral suspension to give a suspension with a suitable solids content during the treatment process. The solids content of the clay mineral suspension during the treatment process may be between 10% and 70% by weight, e.g. from 12% to 50%, especially from 15% to 30%. The solid FAS or an aqueous solution of FAS alternatively be added separately to the clay mineral suspension without first forming a solution of FAS with alkali such as sodium hydroxide, the alkali being added separately.

After the treatment process the pH of the suspension may be lowered to between pH 3 and 6, e.g. to between pH 3.5 and 4.5, for example using an acidic solution, e.g. an aqueous solution of sulfuric acid, e.g. a 5 to 15% solution, although more concentrated solutions could also be used. The suspension may then preferably be dewatered, for example by filtration.

If the treated clay mineral produced in step (b) of the method according to the first aspect of the invention, using either (i) a complexing agent or (ii) FAS as the treatment agent, has been dewatered it may optionally be resuspended at a solids level similar to that used for the treatment process e.g. between 10% and 70% by weight, e.g. from 12% to 50%, especially from 15% to 30% and mixed for a period of time, e.g. from 1 to 30 minutes, especially from 10 to 20 minutes before it is again optionally dewatered to remove any remaining impurities. If the kaolin product produced in step (b) is initially dewatered by filtering, the filter cake so formed may be washed through with water whilst on a filter rather than by being re-suspended and again dewatered in the process previously described. The dewatered clay mineral cake produced by either of the methods above may then be dried, e.g. thermally dried. Thermal drying may be carried out at a temperature of at least 60°C, especially at least 80°C for a period of time, e.g. 1 to 10 hours especially for 2 to 6 hours, The dry treated clay mineral may optionally then be ground or milled to give a fine powder suitable for use in paper filling or pigment coating applications . The dewatered kaolin product may also be used in a paper making or coating composition without the drying step, The kaolin product may be used as a wet solid after the initial dewatering step or after the optional washing and reconcentrating/dewatering steps and water may be added to it after dewatering to form an aqueous suspension. Alternatively the amount of water removed in either of the dewatering stages may be controlled so that the solids content of the aqueous suspension is adjusted to that required for use in the application, e.g. coating composition. The kaolin product could also be used without any dewatering or washing but it is usual to carry out the initial dewatering. The application suspension in which the kaolin product is used may comprise a paper making composition, e.g. a dilute aqueous suspension of cellulose fibres the kaolin being used as a filler material optionally with other materials. Alternatively, the composition in which the kaolin product is used, together with an OBA may comprise a coating composition, e.g. for coating paper products. The other components of the composition may be conventional. For example, the composition for coating may contain a dispersant, for example one of the well known dispersants employed in the art. For example the dispersant may be a polyelectrolyte, e.g. a polyacrylate dispersant, e.g. sodium polyacrylate . According to the present invention in a second aspect there is provided a composition for coating paper or the like which composition comprises : (A) an aqueous suspension of pigment material; (B) a binder; and (C) an OBA; wherein the pigment material comprises a kaolin product produced by steps (a) and (b) of the method according to the first aspect. The kaolin product produced by the method according to the first aspect of the present invention may be the sole component of the pigment material. Alternatively the kaolin product may be blended with one or more additional pigment materials well known to those skilled in the art. The other pigment materials may for example comprise a mineral selected from kaolinite containing minerals, calcined kaolin, titania, talc, so called plastic pigment, calcium sulphate, silica, mica or calcium carbonate. If calcium carbonate is used as the additional pigment material it may be either a natural ground calcium carbonate or a synthetic precipitated calcium carbonate. If the kaolin product of the method according to the first aspect of the present invention is blended with one or more other pigment materials then it may be present in an amount of between 1% and 99% by weight in the blended pigment material. For example the blended pigment material may contain between 10% and 90%, especially between 25% and 75%, by weight of the kaolin product.

The solids content of the coating composition according to the second aspect may be from 40% to 90%, e.g. 50% to 80%, in many cases from 60% to 75% by weight. The binder may be added, on an active basis, in an amount of up to 30% by weight based on the dry weight of the pigment material. Usually, the active binder content will be less than 10% by weight, e.g. from 4% to 6% by weight based on the dry pigment weight .

Such a composition could be prepared by any person skilled in the coating art in a known manner and may also contain other known additives. The OBA in the composition according to the second aspect may be added, on an active basis, in an amount of up to 2% by weight, e.g. from 0.01% to 1%, especially 0.05% to 0.5%, more especially 0.1% to 0.2% by weight based on the dry weight of the kaolin product present in the suspension. The OBA could be added to a dry powder of the clay mineral. It may though be found most convenient to add the OBA to an aqueous suspension formed from the dried kaolin product which is to form a coating composition as previously described.

Known additives may be associated with the OBA. For example, a known activator (or carrier) may be included to allow greater levels of fluorescence to be achieved. Alternatively the OBA may be self activating. The OBA may comprise one or more OBA compounds known in the art, for example it may comprise one or more stilbene compounds containing between 2 and 6 sulfonate groups. The activator may comprise a polyvinyl alcohol (PVOH) and the binder may comprise a latex binder, although other OBAs, water-borne binders and activators known in the art may be used.

The use of OBAs to produce pigment coating compositions with improved fluorescence is widely known and in order to generate greater fluorescence an activator may be used in conjunction with the OBA, as previously described, in the pigment coating composition. Coating pigments and latex binders, with the exception of PVA latices do not act as OBA activators so, in order to achieve the required levels of fluorescence, an activator may be present within the coating composition. Suitable activators are water soluble polymers such as polyvinyl alcohol (PVOH) , sodium carboxymethyl cellulose (NaCMC) starch, or proteins. Low molecular weight polyethylene glycol (PEG) may be used as a non-thickening activator although it is less effective than NaCMC or PVOH. Most of these polymers also function as cobinders, thickeners and possibly water retention aids.

The coating composition according to the second aspect may be prepared in a known manner by first forming an aqueous suspension. This may be done by dispersing the dry kaolin product into a solution of water and dispersant. Alternatively, dispersant and if necessary additional water may already have been added to the wet clay mineral at some point prior to the end of step (b) and the wet dispersed product may be employed.

The dispersant may be used, on an active basis, in amounts of up to 3% by weight, e.g. from 0.01% to 2% by weight, e.g. from 0.05% to 1%, especially 0.1% to 0.5% based on the dry weight of the pigment material in the coating composition. The pH of this coating composition may be adjusted, e.g. to between pH 6 and 10, especially to between pH 7.5 and 8.5. This may be done using an aqueous solution of a base such as sodium hydroxide, especially a dilute solution, which may be a 5% to 15% solution. The pigment coating composition may also optionally contain other well known additives additional to those hereinbefore described to control other properties of the composition, in a well known manner .

Embodiments of the present invention will now be described by way of example with reference to the following Examples which include, for comparison purposes, examples of prior art procedures .

Example 1

As a comparative example, a coating composition was prepared from an untreated

English kaolin clay which had a particle size distribution such that about 70% by weight consisted of particles smaller than 2μm and a powder brightness of 77.1 ISO brightness units. The coating composition was made using 50g of the dry clay, 0.3% by weight, on an active basis, of sodium polyacrylate dispersant based on the dry weight of the pigment material and 22.5g of water to which about 0.5cm³ of a 10% solution of NaOH and a small amount of water were added to produce a lump free suspension. These were all mixed together using an impeller with the clay added slowly to the water and dispersant solution. The suspension of pigment material was weighed so that reagents could be added in the correct concentrations. Latex sold under the trade name Dow 950 in an amount of 5%, on an active basis, by weight based on the dry weight of the pigment material was added. After mixing 2%, on an active basis, by weight of PVOH sold under the trade name Polyviol LL603 was added based on the dry weight of the pigment material. After further stirring 0.5% by weight of the commercially available OBA sold under the trade name Blankophor P was added based on the dry weight of the pigment material and the coating composition was further stirred. The pH was adjusted to between 7.8 and 8.0 and then the coating composition was screened using a 53μm screen before after which it was coated onto an impervious, non fluorescent plastics substrate made of material sold under tha trade name Synteape using wire wound hand draw down rods. Various coatweights between about β g.rrf² and about lβg.rrf² were produced. These samples were then tested for brightness and fluorescence. The reflectance at 457nm was measured on a Datacolour Elrepho E3000 series reflectance spectrophotometer using a UV cut-off filter which could be placed in the light beam to allow measurements to be taken with and without UV. The difference between the readings with and without UV was recorded as the fluorescence (and can be referred to as the Delta R457 or UV gain) . The values of fluorescence were normalised in each case to the equivalent for a coat weight of lOg.πf². The pigment, which had a powder brightness of 77.1 ISO brightness units, produced a coating with a fluorescence of 2.46 ISO brightness units at a coat weight of lOg.m . All measurements of powder brightness given in this specification were carried out using a reflectance spectrophotometer calibrated using an ISO level 2 standard, using a barium sulfate tablet, as the primary reference. All measurements of the paper brightness which were used to determine the fluorescence of the coated paper were carried out using a reflectance spectrophotometer calibrated with ISO method 2469 (Methods for determining optical properties of pulp, paper and board) level 3 reference standards and fluorescence reference standards.

Example 2

As a further comparative example with the treatment process of our invention a prior art method for treating a clay mineral by reductive bleaching with sodium hydrosulfite was carried out. A coating composition was prepared according to the procedure described in Example 1 using a kaolin clay mineral which had been bleached with sodium hydrosulfite in a concentration of 2.5 kg.t^-1 relative to the dry weight of the clay mineral. For the hydrosulfite bleaching an aqueous suspension was prepared containing 20% by dry weight of the same feed clay as used in Example 1. The suspension was mixed by stirring using an impeller for 20 minutes to achieve a homogeneous suspension in a water bath at 20°C and then the pH was lowered to 2.3 with a 10% solution of sulfuric acid. A commercially available phosphate was added in an amount of 1 kg.t^-1 relative to the dry weight of the clay mineral. The suspension was stirred for a further 15 minutes. Sodium hydrosulfite was then added in an amount of 2.5 kg.t^-1 relative to the dry weight of the clay and the suspension was stirred for 20 minutes to help to ensure complete and homogenous bleaching. At the end of this time the pH of the suspension was raised to pH 4 with a 10% solution of sodium hydroxide and then dewatered by filtering using a Buchner funnel. The wet clay mineral cake so produced was washed by re- suspension in clean water to give a suspension of approximately 20% solids which was stirred for 20 minutes to give a homogenous suspension. This was then further dewatered by filtering using a

Buchner funnel and left overnight before it was dried in an oven at 80°C for 4 hours to produce a bleached clay mineral of around 98% solids content. This was ground using a pestle and mortar ready for incorporation into the coating composition and for testing for powder brightness. The coating composition was prepared and applied to a substrate, and the brightness and fluorescence of the coated substrate was measured, as described in Example 1.

The experiment was repeated with larger doses of sodium hydrosulfite and the results are set forth in Table I as follows;

It is clear from Table I that despite the powder brightness being improved by 6 ISO units by the use of at least 2.5 kg.t^"1 ofthe fluorescence was improved by only 0.4 ISO units

Example 3

An aqueous suspension was prepared using 150g of dry kaolin clay and 600g of distilled water to produce a suspension containing 20% by dry weight of the same feed clay as used in

Example 1. The suspension was mixed by stirring using an impeller for 20 minutes to achieve a homogeneous suspension and the suspension was then stirred for a further 15 minutes while nitrogen gas (oxygen free) was bubbled through it to exclude oxygen. This was carried out in a sealed reaction vessel equipped with a nitrogen inlet tube to bubble nitrogen into the clay suspension and an outlet hole in the top of the vessel equipped with a condenser through which the nitrogen could leave the reaction vessel.

Ethylenediaminetetra-acetic acid (EDTA) in the form of its disodium salt was then added to the suspension in an amount of 1 kg.t^-1 relative to the dry weight of the clay mineral and the pH was adjusted using a 10% solution of sulfuric acid to give a pH of 2.3. The reaction vessel was resealed and then immersed in a water bath at 60°C and left for a nominal reaction period of four hours with continual stirring. Nitrogen was passed through the suspension during this period.

On completion of the four hour period the suspension was dewatered by filtering using a Buchner funnel and the wet clay mineral cake so produced was washed by re-suspension in clean water to form a suspension of approximately 20% solids and this was stirred for 20 minutes until it was homogeneous. The suspension was then further dewatered by filtering using a Buchner funnel and left overnight to dry. The clay mineral cake produced was placed in an oven at a temperature of 80°C for a period of 4 hours to produce a bleached clay mineral of approximately 98% solids content. This product was ground using a pestle and mortar ready for incorporation in a coating composition and for testing.

A coating composition was prepared and applied to a substrate, and the brightness and fluorescence of the coated paper were measured, as described in Example 1.

The experiment was repeated using varying amounts of EDTA, and with a pH for the reaction of 7 instead of 2.3, which was achieved by raising the pH of the clay suspension to pH 7 with a 10% solution of sodium hydroxide immediately after the addition of the EDTA. The experiment was also repeated at 20°C instead of 60°C using 10kg. t^-1 of EDTA. The results are set forth in Table II as follows:

From Table II it can be seen that surprisingly, for an improvement in powder brightness similar to that produced by sodium hydrosulfite in Example 2 (6 ISO units), fluorescence improvements of up to 1.5 ISO units and even 1.8 ISO units are seen with the use of EDTA in the clay mineral treatment. Similar results are obtained when the coating composition is applied to a paper substrate.

Example 4

The procedure of Example 3 was repeated using an EDTA dose of 10kg. t^-1 and the same feed clay as used in Example 1 which had first been bleached with sodium hydrosulfite in a concentration of 2.5 kg.t^-1 relative to the dry weight of the clay mineral. The hydrosulfite bleaching was carried out according to the process of Example 2 except that after the suspension was washed and dewatered by filtering using a Buchner funnel, the wet clay mineral cake produced was left on the funnel for 2 days to produce a relatively dry cake of around 80% solids which was crumbled by gentle grinding ready for treatment with EDTA.

This experiment was also repeated using 25 kg.t^-1 of EDTA and also with the clay being first treated with sodium hydrosulfite using the process of Example 2 (but without drying the clay in an oven) and also separately by bleaching the clay with sodium hydrosulfite according to the process used in Example 2 after EDTA tratment. The results are set forth in Table III as follows :

Similar results are achieved whatever the order of addition of the EDTA and sodium hydrosulfite. Table III shows that EDTA can clearly improve the fluorescence effect of a clay mineral that has already been bleached using sodium hydrosulfite. These results also illustrate that fluorescence may still be increased when the powder brightness appears to have reached a maximum.

Example 5

An aqueous suspension was prepared using 150g of dry clay and 500g of distilled water to produce a suspension containing 23% by dry weight of the same feed kaolin clay as used in Example 1. The suspension was mixed by stirring using an impeller for 20 minutes to achieve a homogeneous suspension and the suspension was then stirred for a further 15 minutes whilst nitrogen gas (oxygen free) was bubbled through it. This was carried out in a sealed reaction vessel equipped with a nitrogen inlet tube to bubble nitrogen into the clay suspension and an outlet hole in the top of the vessel equipped with a condenser through which the nitrogen could leave the reaction vessel. An amount of diethylenetriaminepentacetic acid (DTPA) to give a concentration of 1 kg.t^-1 relative to the dry weight of the clay was accurately weighed into a beaker and 75g of distilled water and 10% sodium hydroxide and also a small amount of 10% sulfuric acid were used to dissolve the DTPA and produce a pH of 7. Distilled water was then added to adjust the mass of the solution to lOOg and the solution was then added to the suspension to produce a suspension of 20% solids. The reaction vessel was resealed and then immersed in a water bath at 60°C and left for a nominal reaction period of four hours with continual stirring and nitrogen being passed through the suspension during the reaction. On completion of the four hour period the suspension was dewatered by filtering using a Buchner funnel and the wet clay mineral cake so produced was washed by re-suspension in clean water to form a suspension of approximately 20% solids and this was stirred for 20 minutes. When the suspension was homogenous it was further dewatered by filtering using a Buchner funnel and left overnight to dry. The clay mineral cake produced was placed in an oven at 80°C for 4 hours to produce a treated clay mineral of approximately 98% solids content. This was then ground using a pestle and mortar ready for incorporation in a coating composition and for testing. The coating composition was prepared, and a substrate was coated using the composition and the brightness and fluorescence of the coated substrate were measured, as described in Example 1.

The experiment was repeated using varying amounts of DTPA and using a pH of 2.3 instead of 7. It was also repeated using 30 kg.t^-1 of DTPA and adjusting the pH of the clay suspension to pH 7 immediately after the addition of the DTPA solution. The results obtained are set forth in Table IV as follows:

Table IV shows that high levels of fluorescence can be produced independently of a significant improvement in powder brightness using DTPA. Similar improvements can be obtained by coating papers.

Example 6

An aqueous suspension was prepared containing 20% by dry weight of the same feed clay as used in Example 1. The suspension was mixed by stirring using an impeller for 20 minutes to achieve a homogeneous suspension and the suspension was then stirred for a further 15 minutes while nitrogen gas (oxygen free) was bubbled through it to exclude oxygen. This was carried out in a sealed reaction vessel equipped with a nitrogen inlet tube to bubble nitrogen into the clay suspension and an outlet hole in the top of the vessel equipped with a condenser through which the nitrogen could leave the reaction vessel.

20 kg.t^-1 of triethanolamine (TEA) was then added to the suspension and the pH was adjusted using a 10% solution of sulfuric acid to give a pH of 6. The reaction vessel was resealed and then immersed in a water bath at 60°C for a nominal reaction period of four hours with continual stirring and nitrogen being passed through the suspension during the reaction. On completion of the four hour period the suspension was dewatered by filtering using a Buchner funnel and the wet clay mineral cake so produced was washed by re-suspension in clean water to form a suspension of approximately 20% solids and this was stirred for 20 minutes until it was homogeneous. When the suspension was homogenous it was again dewatered by filtering using a Buchner funnel and left overnight to dry. The clay mineral cake produced was placed in an oven at a temperature of 80°C for a period of 4 hours to produce a treated clay mineral of approximately 98% solids content. This was then ground using a pestle and mortar ready for incorporation in a coating composition and for testing.

The coating composition was prepared and applied, and the brightenss and fluorescence of the coated substrate were measured, as described in Example 1.

The experiment was repeated using varying amounts of TEA and a clay that had been treated by bleaching with sodium hydrosulfite. The hydrosulfite bleaching was carried out according to the process described in Example 2 except that after the suspension was washed and filtered using a Buchner funnel it was left on the funnel for 2 days to produce a relatively dry cake of around 80% solids which was crumbled by gentle grinding using a pestle and mortar ready for treatment with TEA. The experiment was also repeated using a hydrosulfite bleached clay and a temperature of

20°C for 2 hours instead of 60°C for 4 hours. It was also repeated using a hydrosulfite bleached clay, 10 kg.t^-1 of TEA and a pH of 7.9 for the reaction and on completion of the reaction the pH of the suspension was lowered to pH 4 with a 10% solution of sulfuric acid and then it was filtered and washed according to the procedure above .

The results are set forth in Table V as follows :

From Table V it may be seen that although

TEA has no bleaching effect alone, in conjunction with sodium hydrosulfite it gives an improvement in fluorescence compared with use of hydrosulfite alone . Example 7

An aqueous suspension was prepared containing 20% by dry weight of the same feed clay as used in Example 1 which had been bleached with sodium hydrosulfite. The sodium hydrosulfite bleaching was carried out according to the process of Example 2 except that after the suspension was washed and filtered using a

Buchner funnel it was left on the funnel for 2 days to produce a relatively dry cake of around 80% solids which was crumbled by gentle grinding using a pestle and mortar ready for treating with citric acid.

The suspension was mixed by stirring using an impeller for 20 minutes to achieve a homogeneous suspension and the suspension was then stirred for a further 15 minutes while nitrogen gas (oxygen free) was bubbled through it to exclude oxygen. This was carried out in a sealed reaction vessel equipped with a nitrogen inlet tube to bubble nitrogen into the clay suspension and an outlet hole in the top of the vessel equipped with a condenser through which the nitrogen could leave the reaction vessel.

10 kg.t^-1 of citric acid was then added to the suspension and the pH was adjusted using a 10% solution of sulfuric acid to give a pH of 3. The reaction vessel was resealed and then immersed in a water bath at 60°C for a nominal reaction period of four hours with continual stirring and nitrogen being passed through the suspension during the reaction.

On completion of the four hour period the suspension was dewatered by filtering using a Buchner funnel and the wet clay mineral cake so produced was washed by re-suspension in clean water to form a suspension of approximately 20% and this was stirred for 20 minutes. Once the suspension was homogenous it was again dewatered by filtering using a Buchner funnel and left overnight to dry. The clay mineral cake produced was then placed in an oven at a temperature of 80°C for a period of 4 hours to produce a treated clay mineral of approximately 98% solids content. This was then ground ready for incorporation in a coating composition and for testing.

The coating composition was prepared and applied to a substrate, and the fluorescence of the coated paper was measured, as described in Example 1.

The experiment was repeated using varying amounts of citric acid and different pH values using a 10% solution of sodium hydroxide being used in place of the sulfuric acid to produce higher pH slurries. The results are set forth in Table VI as follows:

Table VI shows that the use of citric acid can give significant improvements in the fluorescence of a clay mineral that has been bleached with sodium hydrosulfite and may also further improve the powder brightness.

Example 8

An aqueous suspension was prepared using 150g of dry clay and 550g of distilled water to give a suspension containing 21.5% by weight of the same feed clay as used in Example 1. The suspension was mixed by stirring using an impeller for 20 minutes to achieve a homogeneous suspension and the suspension was then stirred for a further 15 minutes while nitrogen gas (oxygen free) was bubbled through it to exclude oxygen. This was carried out in a sealed reaction vessel equipped with a nitrogen inlet tube to bubble nitrogen into the clay suspension and an outlet hole in the top of the vessel equipped with a condenser through which the nitrogen could leave the reaction vessel. The vessel was immersed in a water bath at 15°C during the whole of this time.

Formamidine sulfinic acid (FAS), in an amount of 2 kg.t^-1 relative to the dry weight of the clay, was weighed into a beaker and distilled water was added to this along with a sufficient amount of a 20% solution of sodium hydroxide so that the sodium hydroxide would be present in the suspension in an amount of 50 kg.t^-1 relative to the dry weight of the clay. The mass of this solution was adjusted with distilled water so that when it was added to the suspension it produced a suspension of total mass 680g with a solids content of 18.3%. The reaction vessel was again resealed and was still immersed in a water bath at a temperature of 15°C and left for a period of 30 minutes during which the reaction continued. The suspension was stirred throughout with nitrogen bubbled through the suspension.

On completion of the 30 minutes period, the pH of the suspension was reduced to pH 4 with a 10% solution of sulfuric acid and then the suspension was dewatered by filtering using a Buchner funnel. The wet clay mineral cake so produced was washed by re-suspension in clean water to form a suspension of approximately 18% solids and this was stirred for 20 minutes until it was homogeneous. When the suspension was homogenous it was further dewatered by filtering using a Buchner funnel and left overnight to dry. The clay mineral cake produced was placed in an oven at a temperature of 80°C for a period of four hours to produce a treated clay mineral of approximately 98% solids content. This was then ground using a pestle and mortar ready for incorporation in a coating composition and for brightness testing.

The coating composition was prepared and applied to a substrate, and the fluorescence of the coated substrate was measured, as described in Example 1.

The experiment was repeated using varying amounts of FAS and with a fixed molar ratio of NaOH:FAS of 9:1 and a reaction time of 45 minutes as well as with other NaOH:FAS ratios. The experiment was also repeated over a range of temperatures except that the stirring of the suspension to form a homogenous suspension lasted longer at higher temperatures to allow the suspension to reach the desired temperature before the addition of the FAS. This experiment was also repeated using a clay which had first been bleached with sodium hydrosulfite according to the process described in Example 2. Control (comparative) experiments were also carried out using sodium hydroxide without FAS. The results are set forth in Table VII as follows:

From Table VII it may be seen that surprisingly, use of FAS gives brightness improvements similar to those obtained using sodium hydrosulfite bleaching in Example 2 but also gives an improvement in fluorescence comapered with use of sodium hydrosulfite alone. This is more surprising since FAS is not thought to complex iron. It is believed to act as a reductive bleach in the same way as sodium hydrosulfite. It is therefore unexpected to find an improvement in fluorescence using FAS. Example 9

A pigment coating composition was prepared using a clay mineral which had been bleached using 2.5kg.t^-1 of sodium hydrosulfite according to the method of Example 2. A coating composition was made using 50g of the dry clay mineral, 0.3% by weight, on an active basis, of sodium polyacrylate dispersant, based on the dry weight of the pigment material, and 22.5g of water to which about 0.5cm³ of a 10% NaOH solution and a small amount of water were added to produce a lump free suspension. These ingredients were all mixed together using an impeller with the clay added slowly to the water and dispersant solution. An amount of 5%, on an active basis, by weight of Dow 950 Latex was added, based on the dry weight of the pigment material. An amount of 0.2% by weight of TEA was then added, based on the dry weight of the pigment material and, after mixing, an amount of 2%, on an active basis, by weight of PVOH Polyviol LL603 was added based on the dry weight of the pigment material. After further mixing 0.5% by weight (active) based on the dry weight of the pigment material of the commercially available OBA Blankophor P in the form of a dilute solution was added and the mixture produced was thoroughly stirred. The pH was adjusted to 7.9 and then the pigment coating composition was screened using a 53μm screen before it was coated onto a substrate as in Example 1 and fluorescence measurements were made as in Example 1. The results obtained are set forth in Table VIII below:

It can clearly be seen from Table VIII that even relatively low doses of TEA can produce substantial gains in fluorescence. Similar results are obtained when papers are coated with the TEA and OBA containing pigment coating compositions.

Claims

1. A method for the preparation of a composition for use in paper making or pigment coating compositions containing also an optical brightening agent (OBA) which includes the steps of (a) preparing an aqueous suspension of a clay mineral comprising a kaolin particulate material; and (b) treating the clay mineral by addition of a treatment agent which reduces the harmful effect of metal ions present in the clay mineral on the activity of the OBA, the treatment agent being substantially free of multivalent cations and comprising (i) a complexing agent or (ii) formamidine sulfinic acid (FAS) .

2. A method according to claim 1 and wherein the treated clay mineral is dewatered following step (b) to remove metal ion impurities solubilised by the addition of the treatment agent .

3. A method according to claim 2 and wherein the dewatered clay mineral is washed and optionally reconcentrated. . A method according to claim 1 and wherein the clay mineral is not dewatered and a coating composition is produced by adding a binder and an OBA to the treated suspension produced by step (b) .

5. A method according to claim 4 and wherein the treatment agent comprises triethanolamine .

6. A method according to any one of the preceding claims and wherein the treatment agent is used in an active amount of from 0.1 kg. tonne^-1 to 80 kg. tonne^-1 based on the dry weight of the clay mineral.

7. A method according to any one of the preceding claims and wherein the treatment of the clay mineral is carried out at a temperature of between 10°C and 90°C.

8. A method according to any one of claims 1 to 3 or claims 6 or 7 and wherein the treatment agent comprises a complexing agent and the complexing agent comprises one or more of ethylenediaminetetra-acetic acid (EDTA) or one of its salts, diethylenetriaminepentacetic acid (DTPA) or one of its salts, citric acid or one of its salts, triethanolamine (TEA) or sodium glucoheptonate .

9. A method according to any one of the preceding claims and wherein the treatment of the clay mineral in step (b) is carried out using a complexing agent and is carried out at a pH of between 1.5 and 9.

10. A method according to any one of claims 1 to 5 and wherein the treatment in step (b) includes addition of FAS and includes adding a base.

11. A method according to claim 10 and wherein the molar ratio of base: FAS is between 1:1 and 70:1.

12. A method according to any one of the preceding claims and wherein the pH of the suspension is adjusted to between 3 and 6 on completion of the treatment in step (b) .

13. A method according to any one of the preceding claims and wherein a gas is passed through the clay mineral suspension before and/or during the treatment in step (b) .

1 . A method according to any one of the preceding claims and wherein the clay mineral is treated by one or more other refining and/or benefication steps either before or after step (b) .

15. A method according to claim 14 and wherein the suspension is treated by addition of a reductive bleaching agent prior to or after step (b) .

16. A method according to claim 15 and wherein the reductive bleaching agent comprises sodium dithionite.

17. A composition for coating paper comprising:

(A) an aqueous suspension of pigment material which comprises a clay mineral comprising particulate kaolin which has been treated by the addition of a treating agent selected from (i) complexing agents; and (ii) formamidine sulfinic acid (FAS) ; (B) a binder; and

(C) an OBA.

18. A composition according to claim 17 and wherein the OBA is used in an amount of up to 2% by weight, on an active basis, based on the dry weight of the pigment material.

19. A composition according to claim 17 or 18 and wherein the OBA comprises one or more substituted stilbene compounds containing between 2 and 6 sulfonate groups.

20. A coating composition according to any one of claims 17 to 19 and which also includes an activator.

21. A coating composition according to claim 20 and wherein the activator is present in an amount of between 0.1% and 6%, on an active basis, based on the dry weight of the pigment material .

22. A coating composition according to any one of claims 20 or 21 and wherein the activator comprises polyvinyl alcohol (PVOH).

23. A coating composition according to any one of claims 17 to 22 and wherein the binder is present in an amount of between 0.1% and 30%, on an active basis, by weight based on the dry weight of the pigment material.

24. A coating composition according to any one of claims 17 to 23 and wherein the binder comprises a latex binder. 25. A coating composition according to any one of claims 17 to 24 and wherein a dispersant is present in the coating composition in an amount of between 0.01% and 3%, on an active basis, by weight based on the dry weight of the pigment material.

26. A paper or board coated with a composition according to any one of claims 17 to 25.