Sizing emulsion

Abstract

 The invention refers to a novel sizing emulsion comprising a substituted succinic anhydride and positively charged colloidal silica particles or positively charged colloidal aluminia particles or positively charged colloidal zirconia particles.

Description

[0001] The present invention relates to an aqueous sizing emulsion and more specifically to emulsions containing a substituted succinic anhydride as sizing agent, commonly referred to as ASA, and colloidal cationic silica particles.

[0002] Aqueous emulsions of substituted succinic anhydrides are used for sizing of neutral to slightly alkaline stocks at which pH range calcium carbonate effectively can be used as filler material. As substituted succinic anhydrides are hydrophobic, the size has to be evenly distributed in an aqueous phase before being added to the cellulosic stock. Usually such sizing emulsions are prepared in the vicinity of the end user, i.e. in the paper mill, in the presence of surfactants and/or polyelectrolytes such as starches.

[0003] Substituted succinic anhydrides generally provide satisfactory sizing of the paper. Though, sizing properties of substituted succinic anhydrides commonly deteriorates with cellulosic stocks having a high conductivity due to a high amount of charged particles and/or stocks having a significant amount of dissolved organic substances, i.e. lipophilic wood extractives, such as resin acids, fatty acids, fatty esters, etc. High conductivity stocks and/or stocks containing dissolved organic substances are even more pronounced in paper mills where the white water is extensively recirculated with the introduction of only minor amount of fresh water into the process, thereby increasing the amount of conductivity and the accumulation of organic substances/lipophilic extractives and non-retained sizing agents in the white-water and the stock to be dewatered.

[0004] Moreover, intimately linked with the high reactivity of succinic anhydride-based sizing agent are problems with deposits in the paper-making machine due to hydrolysed sizing material. Problems which are accentuated when recirculating the white water.

[0005] Furthermore, it is desirable to further improve sizing properties of substituted succinic anhydride size emulsions.

[0006] WO-A1-9731152 refers to sizing emulsions comprising a reactive sizing agent and an anionic microparticulate material rendering the emulsion anionic. Furthermore, it is stated that the inclusion of cationic compounds is undesirable, thus, the sizing dispersions are essentially free from cationic compounds.

[0007] From EP-A1-564994 it is known to use sizing compositions comprising a ketene dimer and positively charged compounds. When using ketene dimers as sizing agent the slipperiness of the surface of the produced paper is increased. However, by using a substituted succinic anhydride the problem of slipperiness is circumvented due to the fact that substituted succinic anhydride sizing agents do not introduce slipperiness to the paper surface. Thus, the object of the compositions of EP-A1-564994 is to increasing the coefficient of friction of the paper surface. Furthermore, ketene dimer sizing agents are slow reacting sizes compared to substituted succinic anhydrides based sizing agents making it impossible to achieve full sizing on the paper machine. Accordingly, paper sized with ketene dimer needs to be post-cured at ambient temperatures before being used in printing processes.

[0008] In accordance with the present invention it has been found that improved sizing characteristic can be obtained with aqueous emulsions according to the claims. More specifically, the invention provides an aqueous emulsion comprising a substituted succinic anhydride and positively charged colloidal silica particles or positively charged colloidal aluminia particles or positively charged colloidal zirconia particles. Additionally, the invention provides a process for the production of cellulose-based products in which a sizing emulsion is added to the stock or applied as a surface size.

[0009] The present size emulsion makes it possible to produce paper with improved sizing over conventional size emulsions at a corresponding dosage and to use a lower dosage of substituted succinic anhydride sizing agent to attain a corresponding level of sizing. The possibility of using lower amounts of sizing agent to obtain in-specification sizing reduces the risk of accumulation of non-adsorbed sizing agents in the white water recirculating in the process, thereby reducing the risk of aggregation and deposition of the sizing agent on the paper machine.

[0010] Starch cannot be used in emulsions without being pretreated by cooking, a treatment which greately affect the properties of starch. Accordingly, it is important how the cooking is implemented in order to obtain a starch with adequate characteristics. Furthermore, there is always a possibility that the level of bacterial activity may increase in cooked starch impairing the properties. It is therefore desirable to be able to reduce the amount of starch or even omit starch in sizing emulsions of ASA.

[0011] It is also desirable to facilitate the emulsification of the sizing agent by the preparation of the emulsion in the presence of suitably one compound acting both as dispersing agent and stabilising agent, which enables a simplified preparation at the end user.

[0012] The sizing agent according to one preferred embodiment of the invention is a substituted succinic anhydride, commonly referred to as ASA. Suitable acid anhydrides can be characterized by the general formula (I) below, wherein R³ and R⁴ can be identical or different and represent saturated or unsaturated hydrocarbon groups suitably containing from 8 to 30 carbon atoms, or R³ and R⁴ together with the -C-O-C- moiety can form a 5 to 6 membered ring, optionally being further substituted with hydrocarbon groups containing up to 30 carbon atoms. Examples of acid anhydrides which are used commercially include alkyl and alkenyl succinic anhydrides and particularly isooctadecenyl succinic anhydride. Further examples are: iso-octadecyl succininc anhydride, n-hexadecenyl succinic anhydride, dodecenyl succinic anhydride, decenyl succininc anhydride, octenyl succinic anhydride, triisobytenyl succinic anhydride, 1-octyl-2-decenyl-succinic anhydride and 1-hexyl-2-octenyl-succinic anhydride.

[0013] Suitable acid anhydrides include the compounds disclosed in US 3102064, US 3821069, US 3968005, US 4040900 and Re 29960 which are incorporated herein by reference.

[0014] The sizing dispersions according to yet another preferred embodiment of the invention comprise positively charged colloidal silica or positively charged colloidal alumina or positively charged colloidal zirconia. Stable cationic aquasols of colloidal silica, alumina, or zirconia are well known in the art, for example from US 3007878, US 3620978, 3719607, 3754126 and US 3956171 which all are incorporated herein by reference. Typically, these sols contain colloidal, dense, finely divided particles, suitably silica. Typical known sols are those containing positively charged particles having a dense silica core coated with a hydroxyl of other oxygen compound of a polyvalent metal such as aluminium, along with an anionic counter ion such as a chloride, acetate or nitrate. Preferred positively charged silica particles or sols are available from Eka Chemicals.

[0015] Additionally, suitably inorganic positively charged colloidal silica particles, also referred to as colloidal cationic silica particles, are contained in the emulsions. Preferably, the positively charged colloidal silica particles are inorganic silica particles which suitably are aluminium-modified. It is preferred that the particles are surface-modified with aluminium. The general method for preparing positively charged cationic silica sols starts suitably from aqueous sols of silica which are reacted with a basic salt of a polyvalent metal to give the sol particles a positive surface charge and stabilisers such as boric acid, alkali metal bases, alkaline earth metal bases, ammonia etc. The polyvalent metal salt is usually an aluminium salt, however, it is also possible to use basic salts of other polyvalent metals for preparing cationic silica based sols, such as chromium, zirconium. Any basic salt which is water soluble and renders the desired positively charged surface can be used and typically the cationic silica is prepared using chlorides, nitrates or acetates of the metal. Preferably, poly aluminium chloride is used as basic salt.

[0016] According to another preferred embodiment of the present invention the emulsion suitably consists essentially of a substituted succinic anhydride and positively charged colloidal silica particles or positively charged colloidal aluminia particles or positively charged colloidal zirconia particles.

[0017] The colloidal cationic silica particles can have a size less than about 500 nm and the size is usually greater than 1.0 nm. The particles of the positively charged silica preferably have a small average particle size, usually below 100 nm and the size is generally in the range of from 2 nm up to 100 nm, suitably in the range from 2 nm up to 80 nm. Preferably, the particle range is within the range from 2.5 nm up to 50 nm. The specific surface area of the silica particles can be in the range of about 5 to about 1800 m²/g, suitably the specific surface area is in the range of from about 30 to about 1200 m²/g. Preferably, the specific surface area of the particles is from 50 to 1000 m²/g. The specific surface area can be measured, after removal of the polyvalent metal, by means of titration with NaOH in conventional manner, for example according to the method described by Sears in Analytical Chemistry 28(1956):12, 1981-1983.

[0018] The cationic silica particles can have positively charged species of the polyvalent metal, preferably aluminium, on their surface and the weight ratio of Al₂O₃ to SiO₂ can be in the range from 1:20 to 4:1, suitably within the range of from 1:10 to 2:1 and preferably within the range of from 1:5 to 1:1.

[0019] The colloidal particles are suitably contained in aqueous sols.

[0020] The emulsions according to the invention can have contents of substituted succinic anhydride sizing agents from about 0.1% by weight up to about 50% by weight. The content of substituted succinic anhydrides sizing agent is suitably within the range of from 1 up to 40% and preferably from 2 up to 30% by weight.

[0021] In the emulsions/dispersions according to the invention the weight ratio of substituted succinic anhydride sizing agent to positively charged colloidal particles, preferably positively charged colloidal silica particles, can be within the range of from 1:1 to 100:1. The weight ratio is suitably within the range from 1.5:1 to 30:1 and preferably within the range from 2:1 to 20:1. The solids content of the emulsions preferably exceeds 1% by weight and can reach 50% by weight. The solids content suitably exceeds 5% by weight. The upper limit is suitably 40% and preferably 30% by weight.

[0022] Preferably, the overall charge of the colloidal particles and optional protecting colloids and/or dispersing agents used are cationic.

[0023] If desired, non-ionic, anionic, amphoteric or cationic protective colloids and non-ionic, amphoteric or cationic dispersing agents may be included in the emulsions, preferably in minor amounts and provided that the overall charge of the total amount of colloidal silica particles and/or optional protective colloids and/or dispersing agents which are present in the dispersions is positive or cationic. Such compounds, i.e. dispersing agents, can advantageously be included in emulsions of higher dry contents. As examples of suitable protective colloids can be mentioned water-soluble cellulose-derivatives such as hydroxyethyl- and hydroxypropyl-, methylhydroxypropyl- and ethylhydroxyethylcellulose, methyl- and carboxymethylcellulose, gelatin, starch, guar gum, xanthan gum, polyvinyl alcohol, etc.. Non-ionic dispersing agents can for example be selected from ethoxylated fatty alcohols, fatty acids, alkyl phenols or fatty acid amides, ethoxylated or non-ethoxylated glycerol esters, sorbitan esters of fatty acids, etc.. Suitable cationic dispersing agents and protective colloids can for example be selected from nitrogen-containing compounds such as quaternary ammonium compounds, salts of tertiary amines, water-soluble nitrogen-containing epichlorohydrin resins and cationic starches, etc.. The emulsion may also contain other additives such as preservative agents.

[0024] However, the emulsions according to the invention can preferably be prepared by simply mixing a substituted succinic anhydride sizing agent with a sol of positively charged colloidal silica particles, such as any of those described above.

[0025] The emulsions or dispersions according to the invention can be used in a conventional manner in the production of cellulose-based products, including paper, board and cardboard. They can be used both for surface sizing and internal or stock sizing at the production of such products. The present invention also relates to a method for the production of cellulose-based products using an aqueous emulsion containing a substituted succinic anhydride sizing agent and colloidal cationic silica particles, as defined above, as surface or stock sizing agents. The stock contains cellulosic fibres, optionally in combination with mineral fillers, and usually the content of cellulosic fibres is at least 50% by weight, based on dry stock. Examples of fillers of conventional types include kaolin, china clay, titanium dioxide, gypsum, talc and natural and synthetic calcium carbonates such as chalk, ground marble and precipitated calcium carbonate. The method is of course advantageous to the papermaker in that the sizing dispersion has high sizing efficiency and improved stability which reduces the tendency of the sizing agent to form deposits and thus simplifies high shearing operations such as pumping and dosing. Suitably, the amount of sizing agent either added to the stock containing cellulose fibers, and optional fillers, or applied on the cellulose-based product as a surface size, usually at the size press, is from 0.01 to 1.0% by weight, based on the dry weight of cellulose fibers and optional fillers, preferably from 0.05 to 0.5% by weight, where the dosage is mainly dependent on the quality of the pulp or cellulose-based product to be sized, the substituted succinic anhydride sizing agent used and the level of sizing desired.

[0026] Chemicals conventionally added to the stock in papermaking such as retention aids, aluminium compounds, dyes, wet-strength resins, optical brightening agents, etc., can of course be used in conjunction with the present dispersions. Examples of aluminium compounds include alum, aluminates and polyaluminium compounds, e.g. polyaluminium chlorides and sulphates. Examples of suitable retention aids include cationic polymers, anionic inorganic materials in combination with organic polymers, e.g. bentonite in combination with cationic polymers, silica-based sols in combination with cationic polymers or cationic and anionic polymers. Particularly good stock sizing can be obtained when using the dispersions of the invention in combination with retention aids comprising cationic polymers. Suitable cationic polymers include cationic starch, guar gum, acrylate-based and acrylamide-based polymers, polyethyleneimine, dicyandiamide-formaldehyde resins, polyamines, polyamidoamines and poly(diallyldimethyl ammoniumchloride) and combinations thereof. Cationic starch and cationic acrylamide-based polymers are preferably used, either alone or in combination with each other or with other materials. In a preferred embodiment of the invention, the emulsions are used in combination with a retention system comprising at least one cationic polymer and anionic silica-based particles such as retention systems sold under the name COMPOZIL®. The present emulsions can be added before, between, after or simultaneously with the addition of the cationic polymer or polymers. It is also possible to pre-mix the size emulsion with a retention aid, e.g. a cationic polymer like cationic starch or a cationic acrylamide-based polymer, prior to introducing the mixture thus obtained into the stock.

[0027] The invention is further illustrated in the following examples, which, however, are not intended to limit the same. Parts and % relate to parts by weight and % by weight, respectively, unless otherwise stated.

Example 1

[0028] The sizing efficiency of the novel emulsion was evaluated in this example using a standard HST test on paper with a grammage of 78 g/m². To a furnish of birch and pine kraft pulp (60:40 weight %) with a consistency of 0.5% containing 20 weight % calcium carbonate based on dry fibres having a conductivity of 520 µS/cm and a COD value of 15 mg/l the novel emulsion comprising alkenyl succinic anhydride to colloidal cationic silica particles in a weight ratio of 15:1 was added at an amount of 2 kg/ton dry pulp based on ASA (test 1 and 2). The used retention system contained cationic starch with a D.S. of 0.035 and anionic silica sol. For comparison sizing efficiency of prior art emulsions comprising alkenyl succinic anhydride (ASA) and a cationic starch were evaluated (test 3 and 4) having a weight ration ASA to starch of approximately 2,7:1. The prior art emulsions were added at an amount of 2 kg/ton dry solids based on ASA.

Table 1

Emulsion	Cationic starch* kg/ton	Anionic silica sol* kg/ton	ASA kg/ton	HST
Test 1, novel emulsion	10	1	2	318
Test 2, novel emulsion	10	0.5	2	257
Test 3, prior art emulsion	9	1	2	223
Test 4, prior art emulsion	12.5	1	2	201

* retention system

[0029] Obviously, an emulsion according to the present invention shows significantly higher sizing values than the prior art emulsions.

Example 2

[0030] In this example the sizing efficiency (Cobb₆₀ -test) of the novel emulsion was evaluated using the same furnish as in example 1, however, with a conductivity of 508 µS/cm. The retention system used contained a cationic polyacrylamide of medium molecular weight with 10% charge and an anionic bentonite. All emulsions contained alkenyl succinic anhydride as sizing agent and the amount of added sizing agent was 2kg/ton dry pulp based on ASA. The emulsion used in test 1 was in accordance with the invention comprising colloidal cationic aluminium modified silica particles and ASA in a weight ratio of 1:10. The emulsion in test 2 was according to prior art comprising anionic silica particles and ASA in a weight ratio of 1:10. The emulsion in test 3 comprised anionic bentonite and ASA in a weight ratio of 1:10.

Table 2

Emulsion	Cationic polyacrylamide* kg/ton	Bentonite* kg/ton	ASA kg/ton	Cobb60
Test 1	0,5	2,0	2,0	35
Test 2	0,5	2,0	2,0	100
Test 3	0,5	2,0	2,0	81

* retention system

[0031] Table 2 shows that an emulsion containing positively charged colloidal silica particles has excellent sizing properties compared to emulsions containing anionic particles.

Example 3

[0032] In this example the sizing efficiency using the Cobb₆₀ -test of the novel emulsion was evaluated. The furnish was a bleached mechanical pulp with higher amounts of dissolved organic substances (organic trash) and with a concentration of 0.5%, a conductivity of 503 uS/cm and a COD value of 65 mg/l. The retention system contained cationic starch and anionic silica sol. The emulsion in test 1 according to the invention contained colloidal cationic aluminium modified silica particles and ASA in a weight ratio of 1:6.7. The emulsion in test 2 according to prior art contained cationic starch and ASA in a weight ratio of 1:2.7. All emulsions were prepared with the same ASA sizing agent and added to the furnish in an amount of 2.0 kg/ton dry pulp based on ASA.

Table 3

Emulsion	Cationic starch* kg/ton	Anionic silica sol* kg/ton	ASA kg/ton	Cobb60
Test 1	10	1.0	2	29
Test 2	10	1.0	2	84

*retention system

[0033] Table 3 surprisingly shows that good sizing results are obtained when the emulsion according to the invention is used on a furnish with higher amounts of dissloved organic substances.

Claims

1. Aqueous sizing emulsion characterised in that the emulsion comprises a substituted succinic anhydride and positively charged colloidal silica particles or positively charged colloidal aluminia particles or positively charged colloidal zirconia particles.

2. Sizing emulsion according to claim 1, characterised in that the silica particles are colloidal inorganic silica particles.

3. Sizing emulsion according to claims 1 or 2, characterised in that the silica particles are colloidal aluminium-modified silica particles.

4. Sizing emulsion according to claim 3, characterised in that the weight ratio of Al₂O₃ to SiO₂ of the silica particles is in the range from 1:20 to 4:1.

5. Sizing emulsion according to claims 1, 2, 3 or 4, characterised in that the weight ratio of substituted succinic anhydride to positively charged colloidal particles is within the range of from 1:1 to 100:1.

6. Sizing emulsion according to any of the preceding claims, characterised in that the colloidal cationic silica particles have a size less than 500 nm.

7. Sizing emulsion according to any of the preceding claims, characterised in that the silica particles have a specific area within the range from 5 to 1800 m²/g.

8. Sizing emulsion according to any of the preceding claims, characterised in that the substituted succinic anhydride is present in an amount of from 0.1% by weight to 50% by weight.

9. Sizing emulsion according to any of the preceding claims, characterised in that the substituted succinic anhydride is alkenyl succinic alhydride.

10. A process for the production of cellulose-based products in which a sizing emulsion is added to the stock or applied as a surface size characterised in that as the sizing emulsion is used an aqueous sizing emulsion as defined in any of the preceding claims.