Compositions for sizing paper

Abstract

 An aqueous sizing composition can be made by mixing into water a concentrate that comprises a substantially anhydrous dispersion of a polyelectrolyte in a liquid reactive size.

Description

[0001] The invention relates to the sizing of cellulosic fibres and to compositions for use in this, and to their manufacture.

[0002] During the manufacture of paper it is necessary to render the naturally hydrophilic cellulosic fibres hydrophobic so that penetration of aqueous liquids into the formed sheets is limited thereby making writing and printing on the sheets possible. This process, known as sizing, can be carried out by adding a sizing agent to the pulp slurry (usually termed internal sizing) or the sizing agent can be applied to the formed paper sheet. This invention is concerned with the internal sizing process.

[0003] There are two types of sizing agent in general use. One of these is based on rosin which is used in conjunction with alum. The rosin is added as a soap solution or as an emulsion and alum is added afterwards just prior to sheet formation to precipitate the rosin as a fine particulate which is retained by the sheet.

[0004] The second type of size is a reactive size, such as a ketene dimer or an anhydride-based size, which reacts chemically with the cellulosic fibres. Preferably it is applied in combination with a polyelectrolyte which will help to retain the size in the sheet.

[0005] The reactive size is generally added to the pulp in the form of an aqueous emulsion, generally a cationic emulsion. The emulsion can be prepared at the mill but this necessitates the mill having emulsifying equipment and so it would be more convenient if a concentrated emulsion could be supplied to the mill ready for dilution and use. Unfortunately reactive sizes tend to react with water so that an aqueous emulsion is liable to be rather unstable.

[0006] Anhydride based sizes, such as alkenyl succinic anhydride sizes, are so reactive that their emulsions have to be prepared at the mill just prior to use. These sizes are normally supplied to the mill with a cationic starch which generally has to be precooked before emulsification, thus making it even less convenient for the emulsion to be formed at the mill.

[0007] Ketene dimer sizes often are supplied to the mill in the form of an emulsion but these emulsions have only limited shelf-life and the maximum concentration of ketene dimer in the emulsion is rather low, generally below 6%, so that very large volumes of emulsion have to be supplied to the paper manufacture.

[0008] Emulsification of liquid ketene dimers can be achieved using conventional emulsification equipment but some of the preferred ketene dimers are solids at ambient temperature. As described in U.S. Patent Specification 3,046,186, emulsification of these necessitates initially either melting the solid (so that upon cooling the emulsion is converted to a dispersion) or dissolving the solid in a solvent, generally benzene. A typical important ketene dimer is distearyl ketene dimer and this only has relatively low solubility in organic solvents with the result that the solution of it that is emulsified must be rather dilute. For instance we have found that this dimer precipitates from a 40% by weight solution in benzene (weight ratio benzene:dimer of 1:0.67) and so any solution in benzene must be much more dilute than this. Also this dimer is less soluble in other organic solvents than it is in benzene.

[0009] As described in U.S. 3,046,186 the emulsions are generally prepared by emulsifying the dimer into an aqueous solution of cationic dispersing agent although that patent does mention that "in certain instances the emulsifying agent may be predispersed in the ketene dimer". It is stated that the emulsions may be prepared at any convenient solids content but are used at 1 to 5% solids by weight.

[0010] In each of the examples in U.S. 3,046,186 the initial composition that was prepared and that contained both size and polyelectrolyte was very dilute. For instance in Example 1 the initial concentration is about 9% by weight size based on the total composition.

[0011] In every instance the emulsifying or dispersing agent in U.S. 3,046,186 was introduced as an aqueous solution. Also, in every instance, substantial amounts of additional water are included in the initial composition. The introduction of the emulsifying or dispersing agent as an aqueous solution inevitably incorporates substantial amounts of water into the composition. With ketene dimer sizes, as in U.S. 3,046,186, this is tolerable provided the initial composition is not stored for too long.

[0012] As mentioned above, it is preferred to provide a polyelectrolyte with the reactive size and it might be thought that some of the disadvantages associated with providing emulsions of reactive size and polyelectrolyte could be minimised if the reactive size and the polyelectrolyte were supplied separately. However this incurs other disadvantages.

[0013] It would therefore be very desirable if it was possible to supply a stable concentrated composition that contained both reactive size and polyelectrolyte and which was readily dilutable with water at the mill.

[0014] In EP 0141641 A2 (not-published at the priority date hereof) we have described concentrated compositions comprising a substantially anhydrous dispersion of a polyelectrolyte in an organic liquid comprising a solution of a reactive size in a hydrophobic solvent. The reactive size concentration in such compositions is generally above 20% and although it may be up to, for instance, 85% it is usually lower.

[0015] A concentrate composition according to the present invention is a substantially anhydrous dispersion of a polyelectrolyte in a liquid reactive size. The composition is substantially free of solvent. Preferably there is no solvent but if solvent is present its arount is insufficient to contribute significantly to the physical properties of the composition, e.g., below 5% and usually below 1% solvent by weight of the composition.

[0016] Whereas in EP 0141641 A2 the size could be solid or liquid and was present in the composition in solution in the hydrophobic solvent, in the present invention the size must be liquid at the temperature at which the composition is to be used and stored, typically ambient temperatures such as 20 to 25°C. Preferably the size is liquid at O°C.

[0017] The preferred liquid sizes for use in the invention are anhydride reactive sizes, including especially alkenyl succinic anhydride sizes. Suitable materials are described in U.S. Patent No.3,102,064.

[0018] Various other reactive sizes are liquid and can be used in the invention, for instance certain ketene dimer sizes.

[0019] The concentrate composition must be substantially anhydrous in order that the reactive size does not react in the composition upon storage. The amount of water is generally net more than the equilibrium moisture content of the polyelectrolyte (i.e., the water content of the polyelectrolyte if it is exposed in the form of dry powder to the ambient atmosphere) and is preferably below this. Normally the water content is not more than 1%, or at the most 2%, by weight of the composition.

[0020] The polyelectrolyte will generally be water soluble and an advantage of the invention is that it can have a any desired molecular weight and in particular can have a molecular weight that is higher than is conveniently possible with existing compositions. For instance the intrinsic viscosity can typically be above 1 and generally above 3, e.g., above 6. Although it is generally below 9 it can be higher, e.g., up to 20 or more.

[0021] The polyelectrolyte may be formed from a water soluble ethylenically unsaturated monomer (or a water soluble blend of ethylenically unsaturated monomers) and may be cationic, anionic or non-ionic, the cationic polyelectrolytes generally being preferred.

[0022] Preferred cationic electrolytes include homopolymers or copolymers of diallyl dialkyl (generally dimethyl) ammonium chloride and homopolymers and copolymers of dialkylaminoalkyl acrylates and methacrylates (preferably dimethylaminoethyl acrylates and methacrylates) present as acid addition salts or quaternary ammonium salts, generally quaternised with methyl chloride or dimethyl sulphate. Copolymers of such monomers may be formed with acrylamide or methacrylamide and will typically contain at least 10%, and usually at least 30%, by weight of the cationic monomer. Other cationic acrylamides and methacrylamides can be used. Other cationic polymers that can be used are polyamines and polyimines such as polyamine-epi-halohydrin polymers and dicyandiamide condensates and polyethylene imines.

[0023] Suitable non-ionic polymers include polyacrylamide.

[0024] Suitable anionic polymers include polymers formed from monomers including carboxylic or sulphonic acid groups. These groups may be present as free acid or, more usually, as a water soluble ammonium or alkali metal (generally sodium) salt. Suitable acids are acrylic acid, methacrylic acid and 2-acrylamido-2-methyl-propane sulphonic acid. Sodium allyl sulphonate may be used. The anionic polymers may be homopolymers of such acids, or mixtures thereof, or copolymers with, for instance, acrylamide. A suitable polymer is polyacrylamide containing up to 25% or more acrylic acid groups.

[0025] The particles of polyelectrolyte are generally below 500 µm and preferably below 200 µm in size.

[0026] The novel compositions can be obtained by mixing powdered polyelectrolyte into the liquid reactive size. In these compositions the polyelectrolyte generally has a particle size above 5, and usually above 10 nm, and often the maximum particle size is within the range 20 to 100 µm.

[0027] It is necessary that the composition should be used whilst the polyelectrolyte is substantially uniformly dispersed throughout the liquid reactive size. If the composition has a tendency towards settlement it should therefore be used before serious formation of a non-redispersable sediment occurs. Settlement can be minimised or prevented by the inclusion of an effective amount (e.g., 0.1 to 10%) of a dispersion promoter. For instance dispersion stabilisers that are insoluble in the reactive size, such as the clays and silica based dispersion stabilisers known for stabilising dispersions in oil, may be used. Preferred stabilisers for this purpose are organophilic clays such as the materials sold under the trade name Bentone. Stabilisers that are soluble in the size may also be used.

[0028] Preferred compositions according to the invention however have the polyelectrolyte with a maximum particle size below 10 µm, preferably below 3 um. Although a dispersion promoter can be added to the composition the required amount is usually low. Deliberate addition of, for instance, clays is generally unnecessary. The particles _!are preferably made by reverse phase polymerisation preferably in the presence of a dispersion stabiliser of the type known for use in reverse phase polymerisation. The presence of residues of this on the polymer particles will generally be sufficient to stabilise the compositions of the invention.

[0029] Particularly preferred compositions are made by blending into the liquid size a substantially anhydrous dispersion of polyelectrolyte in a volatile organic liquid and then evaporating this organic liquid. The volatility of the liquid must be such that it can be evaporated from the dispersion in liquid reactive size at a temperature below that at which undesired chemical reaction might occur.

[0030] The particle size of this dispersion in organic liquid should be below 10, and often below 3 µm. Preferably it is below 2 µm, often mainly in the range . 0.05 to 1 µm. It may be made by reverse phase polymerisation, generally reverse phase suspension polymerisation, of water soluble monomer or monomer blend dispersed in water immiscible organic liquid. The reverse phase polymerisation may be conducted in the presence of an oil soluble polymer, generally an amphipathic polymer, as a dispersion stabiliser and this stabiliser may also promote the stability of the final dispersion in the liquid size. The reverse phase polymerisation may also be conducted in the presence of a water in oil emulsifier. Materials and processes for reverse phase polymerisation are well known and are described in, for instance, EP 0126528. Suitable organic liquids that can be used for the dispersion include volatile aliphatic hydrocarbons.

[0031] After making the dispersion of polyelectrolyte in organic liquid by reverse phase polymerisation the dispersion is dehydrated to a substantially anhydrous state in conventional manner, generally by azeotropic distillation.

[0032] If desired the content of continuous organic phase can be altered after polymerisation in known manner in order that the organic liquid (that has to be evaporated from the dispersion of polyelectrolyte in liquid size and organic liquid) has the optimum volatility.

[0033] The compositions according to the invention are substantially free of solvent or other diluents and at least 80%, preferably at least 85% or at least 90%, consists of the polyelectrolyte and the liquid size. Preferred compositions contain from 45 to 90%, preferably 60 to 80%, by weight reactive size, 10 to 50%, preferably 20 to 40%, by weight polyelectrolyte and 0 to 15%, preferably 5 to 10% of additives such as the described insoluble or soluble stabilisers or water-in-oil emulsifiers.

[0034] Preferably the additives include oil-in-water emulsifying agent that serves to facilitate emulsification of the concentrate into water at the point of use. Instead of incorporating oil-in-water emulsifying agent into the concentrate it may be present in the water into which the concentrate is emulsified. If appropriate mixing apparatus is available, adequate emulsification may be achieved without the use of an oil-in-water emulsifying agent, but this is usually less preferred.

[0035] The concentrate is converted to an emulsion in water prior to use.

[0036] The water in which the emulsion is formed may be the water of the cellulosic pulp suspension that is to be treated but preferably the concentrate is first converted into an aqueous emulsion to give a reactive size concentration of from 0.01 to 5%, preferably 0.05 to 1%, based on the weight of the aqueous solution.

[0037] This emulsion may then be added to the aqueous cellulosic pulp, and paper may be made from it, in the usual way. The amount of reactive size in the aqueous pulp is generally from about 0.01 to about 1% by weight based on the dry weight of the pulp. Upon addition to the pulp slurry, the active size/oil droplets are retained by the polymer on the fibres and the size reacts with the fibres. The size released from an emulsion in this way produces results at least as good as those obtained with the conventional ketene dimer emulsions.

[0038] Thus by the invention we obtoin sizing results at least as good as those obtained using known compositions and yet for the first time we have the ability to supply storage stable concentrated compositions that the user can easily convert into aqueous solutions, and that contain both the polyelectrolyte and the size and that are substantially free of components other than the size and the polyelectrolyte.

[0039] The following are examples of the invention.

Example 1

[0040] A sizing concentrate was prepared, including alkenyl succinic anhydride as the active sizing constituent, by dispersing a powdered polyelectrolyte into the liquid sizing component to which had been added an oil-in-water emulsifying surfactant.

[0041] The chosen alkenyl succinic anhydride was a liquid at ambient temperature and did not require melting to allow the dispersion to take place.

[0042] The polyelectrolyte, a copolymer of methyl chloride quaternised dimethyl aminoethylmethacrylate and acrylamide (25.75 by weight), was prepared by bulk solution polymerisation. The resulting polymer gel was cut into particles less than 5 mm in dimension dried on a fluid bed drier and then ^gound down to the required dimension of less than 53 microns.

[0043] The composition comprised 65 g of alkenyl succinic anhydride, 10 g of oil-in-water emulsifying surfactant, 12.5 g of polyelectrolyte to give a 74.3% active sizing composition. The composition was ostensibly free from water.

Example 2

[0044] 2.7 g of the concentrate composition made in Example 1 was added to 197.3 g of water with stirring to give a 1% active alkenyl succinic anhydride emulsion which was used to carry out 1 minute Cobb tests. This emulsion was labelled A.

[0045] For this test 100 gsm handsheets were prepared, from a bleached sulphate/bleached birch stock containing 10% calcium carbonate loading, on a Standard Laboratory sheet making machine. Prior to sheet formation the required amount of 1% alkenyl succinic anhydride emulsion as prepared above was added to 600 mls of 1.0% consistency stock. After stirring, handsheets were prepared on the standard sheet-making machine. The sheets were couched off the sheet-machine in the normal manner placed on glazing plates and pressed at 50 psi for 5 minutes prior to drying on rings at 110°C for 1 hour. After conditioning at room temperature the degree of sizing ' achieved was measured by the standard 1 minute Cobb test. The results are shown in the table below.

Example 3

[0046] A concentrate similar to Example 1 is made using, instead of the comminuted gel polymer, a bead polymer made by reverse phase bead polymerisation followed by azeotropic distillation and separation of the beads from the oil in which they were formed.

Example 4

[0047] In order to impart long term storage stability to the concentrate of Example 1 an appropriate amount of Bentone 38 can be added.

Example 5

[0048] Preparation of a Dispersion of a Cationic Co-Polyacrylamide in Alkenyl Succinic Anhydride

[0049] A copolymer of 75 parts by weight acrylamide and 25 parts by weight of trimethyl 6-acryloxyethyl ammonium chloride was first prepared in a hydrocarbon liquid of boiling range 154-168°C (Shell SBPll) by conventional reverse phase polymerisation as follows. 287.4 gms of a 52.2% aqueous solution of acrylamide, 0.05 gms of azo-bis-isobutyronitrile and 160.2 gms of water were mixed to form a solution whose pH was adjusted to 4.6 with sodium hydroxide solution (46% wt/wt) then 71.0 gms of a 70.4% aqueous solution of trimethyl β-acryloxyethyl ammonium chloride was mixed in to form the aqueous monomer solution. An oil phase was prepared comprising 363.3 gms of SBPll, 14.2 gms at a 2 to 1 molar copolymer of stearyl methacrylate and methacrylic acid as suspension polymerisation stabiliser (as described in GB 1,482,515) and 7.8 gms Span 80.

[0050] The aqueous phase was homogenised with the oil phase and deoxygenated with nitrogen gas then polymerised by stirring in 1.5 mls of a 5% solution of sodium metabisulphite in water followed by a 1% solution of tertiary butyl hydroperoxide in SBP11 added at a rate of 0.25 mls per minute until polymerisation was complete.

[0051] The resulting aqueous polymer gel dispersion was azeotropically dehydrated under reduced pressure by recycling the SBP11. Part of the SBPll was then distilled off resulting in an anhydrous polymer dispersion at a concentration of 40% polymer by weight in SBP11.

[0052] 250 gms of this 40% copolymer dispersion was mixed with 500 gms of alkenyl succinic anhydride and subject to distillation under reduced pressure to remove the SBP11. Final distillation conditions were 95°C at a pressure of 10 Torr. The resultant product was a stable dispersion of 100 grams polymer in 500 grams liquid reactive size. It could be rendered self emulsifying by the addition of high HLB surfactants.

Example 6

[0053] The concentrate described in Example 5 was used to prepare a corresponding aqueous emulsion, having a 1% by weight active size content, by stirring the appropirate amount of dispersion into water. This emulsion was further diluted to 0.1% by weight active size content and labelled B.

[0054] As control, a conventional alkenyl succinic anhydride emulsion was prepared as follows. A 12% aqueous dispersion of a cationic starch was cooked at 95°C for 20 minutes with constant stirring. The cooked starch was cooled and diluted to 9% activity. 2 parts by weight of alkenyl succinic anhydride was added to 3 parts by weight of cationic starch with agitation. High shear mixing with a Silverson mixer was continued to achieve a fine particle size emulsion. This emulsion was diluted with water to 0.1% by weight active size content and labelled C.

[0055] 100 g.s.m. handsheets were prepared from a bleached sulphate/bleached birch stock containing 10% calcium carbonate loading on standard laboratory sheet making machine. Prior to sheet formation, the required amount of 0.1% emulsion labelled B was added to 600 mls of 1.0% consistency stock. After stirring, handsheets were prepared, pressed at 50 p.s.i. for 5 minutes prior to

[0056] drying at 110°C for 1 hour. After conditioning at room temperature, the degree of sizing was determined by the standard 1 minute cobb test.

[0057] Control sheets were prepared in the same manner as described above, but with emulsion C replacing emulsion B. The results are shown below:

Claims

1. A concentrate composition suitable, upon dilution with water, for sizing cellulosic fibres and which is a substantially anhydrous dispersion of a polyelectrolyte in a liquid reactive size in the substantial absence of solvent.

2. A composition according to claim 1 containing fram 45 to 90% by weight of the reactive size and from 10 to 50% by weight of the polyelectrolyte and in which the combined weight of polyelectrolyte and size is at least 85% by weight.

3. A composition according to claim 1 containing 60 to 80% by weight reactive size, 20 to 40% by weight polyelectrolyte and 5 to 10% by weight additives selected from emulsifiers and dispersion stabilisers.

4. A composition according to any preceding claim in which the polyelectrolyte has a particle size of from 5 to 500 µm and the composition is made by blending the polyelectrolyte with the liquid reactive size and with an added dispersion stabiliser.

5. A composition according to any of claims 1 to 3 in which the polyelectrolyte has a particle size below 10 µm. _,

6. A composition according to claim 5 in which the polyelectrolyte has been made by reverse phase polymerisation in the presence of a dispersion stabiliser which serves as the only dispersion stabiliser in the composition.

7. A composition according to any preceding claim which has been made by blending into the liquid size a substantially anhydrous dispersion of polyelectrolyte in a volatile organic liquid and then' evaporating the organic liquid.

8. A composition according to claim 7 in which the substantially anhydrous dispersion of polyelectrolyte in a volatile organic liquid is a dispersion made by reverse phase polymerisation of a water soluble ethylenically unsaturated monomer, or a water soluble blend of ethylenically unsaturated monomers,in a water immiscible organic liquid and the polyelectrolyte has a particle size below 2 um.

9. A composition according to any preceding claim in which the size is an alkenyl succinic anhydride size that is liquid at 20°C.

10. A composition according to any preceding claim in which the polyelectrolyte is selected from polymers formed from at least one monomer selected from dialkylaminoalkyl acrylates and methacrylates and their acid addition salts and their quaternary ammonium salts; diallyl dialkyl ammonium chlorides; acrylamide; acrylic acid, methacrylic acid and 2-acrylamido-2-methyl propane sulphonic acid and water soluble salts; and sodium allyl sulphonate, and from polyamine and polyimine polymers.

11. A composition according to any preceding claim additionally including an oil-in-water emulsifier.

12. An aqueous composition obtained by dispersing into water a concentrate according to any preceding claim an amount sufficient to give a reactive size concentration in the water of from 0.01 to 5% by weight.

13. A method in wh-ch cellulosic fibres are sized by treating aqueous cellulosic pulp or cellulosic paper made from such pulp with an aqueous composition according to claim 12.

14. A method of making a composition according to claim 1 comprising blending into the liquid reactive size a substantially anhydrous dispersion of the polyelectrolyte in a volatile organic liquid and then evaporating the organic liquid.