Method and arrangement for on-site producing of alkyl ketene dimer emulsion, alkyl ketene dimer emulsion and its use

Abstract

Method for on-site producing alkenyl ketene dimer (AKD) emulsion, in which method an AKD wax and a heated emulsifying agent solution comprising starch and / or a synthetic polymer is premixed in a two-stage premixing unit, which comprises a first low-shear premixing unit, where the heated emulsifying agent solution is mixed with the AKD wax to obtain a mixture comprising melted AKD wax and the emulsifying agent, and a second high- shear premixing unit, where the mixture comprising melted AKD wax and the emulsifying agent is mixed to obtain a pre-emulsion of AKD, and then the pre-emulsion of AKD is emulsified in an emulsifying unit. Alkenyl ketene dimer emulsion produced by the method comprises AKD and an emulsifying agent comprising starch and / or a synthetic polymer starch, and AKD has a medium value of the particle size distribution of more than 0.5 µm, typically in the range of 0.5 – 5 µm.

Description

[0001] METHOD AND ARRANGEMENT FOR ON-SITE PRODUCING OF ALKYL KETENE DIMER EMULSION, ALKYL KETENE DIMER EMULSION AND ITS USE

[0002] Field of the invention

[0003] The present invention relates to a method and an arrangement for on-site producing of alkyl ketene dimer (AKD) emulsion according to the independent claims presented below. The invention also relates to AKD emulsion produced by a method according to the present invention, and its use.

[0004] Background of the invention

[0005] Emulsions or dispersions of hydrophobic sizing agents are used in paper or paperboard manufacturing to decrease the rate of liquid penetration into the structure of a paper or paperboard. The most commonly used hydrophobic, cellulose-reactive sizing agents in paper or paperboard manufacture are alkyl ketene dimer (AKD) or alkenyl succinic anhydride (ASA). In order for the said hydrophobic agents to be retained to the anionic cellulose fibre material, a polymer is added in the emulsifying process of them. In most cases, the polymer consists of starch and / or synthetic polymers, which also have a stabilizing effect on the emulsion.

[0006] Emulsions of ASA are produced according to a so-called on-site process, i.e. in close proximity to the ASA emulsion’s application at paper or paperboard manufacturing plant. This is due to the fact that ASA has a tendency to hydrolysis. The finished ASA emulsion cannot therefore be stored, especially not under unfavourable conditions such as high temperature. AKD emulsions are more stable and can be stored for a long time without losing the sizing effect compared to ASA emulsion. Hence, AKD sizes are currently supplied to paper and paperboard manufacturers as ready-made size emulsions, i.e. AKD emulsions are off-site produced. However, these emulsions have several disadvantages. For example, they are often transported over considerable distances to the paper or paperboard manufacturing plant which is uneconomical. Further, the conventional process for off-site produced AKD emulsion uses expensive high-pressure homogenizer, which is needed to make emulsion with relatively small particle size (median value of the particle size distribution of about 0.4 pm). A small particle size of AKD is necessary for emulsion to maintain stability during storage. The small particles size is also considered to be beneficial for good sizing efficiency.

[0007] A conventional process for producing AKD emulsion is presented in Figure 1. The conventional method for producing AKD emulsion comprises firstly gelatinizing starch and melting of AKD wax, and then mixing the gelatinized starch with the melted AKD wax. After mixing an emulsification is carried out by a high-pressure homogenizer, and then the obtained AKD emulsion is cooled thereby obtaining the AKD emulsion to be transported to at the paper or paperboard manufacturing plant.

[0008] To avoid disadvantages of AKD emulsions, it has already been proposed in the past to prepare the AKD emulsion directly at the place of application, namely at the paper or paperboard manufacturing plant. However, the problem is that emulsifying of AKD is not easy and requires considerable know-how and expensive mixing equipment, namely high-pressure homogenisers to make small particle size AKD emulsion like off-site made product. It is not realistic to install a high-pressure homogeniser in every paper or paperboard plant. Therefore, there is a need for a cost-efficient method for producing AKD emulsions on-site at the paper or paperboard plant.

[0009] Summary of the Invention

[0010] It is an object of the present invention to reduce or even eliminate the above- mentioned problems appearing in prior art.

[0011] It is especially an object of the present invention to provide a novel method for producing alkyl ketene dimer (AKD) emulsion on-site at paper or paperboard manufacturing plant.

[0012] Further, the invention provides a novel AKD emulsion. In order to achieve among others the objects presented above, the invention is characterized by what is presented in the characterizing parts of the enclosed independent claims.

[0013] To achieve these aims, the invention is characterized in what will be presented in the independent claims.

[0014] The embodiments and advantages mentioned in this text relate, where applicable, both to the method, the arrangement as well as to the AKD emulsion product and its uses according to the invention, even though it is not always specifically mentioned.

[0015] Typical method according to the present invention for on-site producing alkenyl ketene dimer (AKD) emulsion comprises

[0016] - premixing a solid alkenyl ketene dimer (AKD) wax and a heated emulsifying agent solution comprising starch and / or a synthetic polymer in a two-stage premixing unit, which comprises a first low-shear premixing unit, where the heated emulsifying agent solution is mixed with the solid AKD wax to obtain a mixture comprising a melted AKD wax and the emulsifying agent, and a second high-shear premixing unit, where the mixture comprising the melted AKD wax and the emulsifying agent is mixed to obtain a pre-emulsion of AKD, and

[0017] - emulsifying the pre-emulsion of AKD in an emulsifying unit.

[0018] Typical arrangement according to the present invention for on-site producing alkenyl ketene dimer (AKD) emulsion comprises

[0019] - a two-stage premixing unit, which comprises a first low-shear premixing unit and a second high-shear premixing unit, and

[0020] - an emulsifying unit, preferably an ambient or atmospheric pressure emulsifying unit.

[0021] Typical alkenyl ketene dimer (AKD) emulsion according to the present invention comprises alkenyl ketene dimer (AKD) and an emulsifying agent comprising starch and / or a synthetic polymer, and AKD has a median value of the particle size distribution of more than 0.5 pm or more than 1 pm, typically in the range of 0.5 - 5 pm or 1 - 5 pm and more typically in the range of 0.5 - 2 pm or 1 - 2 pm. According to one preferred embodiment of the present invention, alkenyl ketene dimer (AKD) emulsion comprises alkenyl ketene dimer (AKD) and starch, more preferably AKD and cationic starch, and AKD has a median value of the particle size distribution of more than 0.5 pm or more than 1 pm, typically in the range of 0.5 - 5 pm or 1 - 5 pm and more typically in the range of 0.5 - 2 pm or 1 - 2 pm

[0022] Typical AKD emulsion according to the present invention is produced by the method according to the present invention.

[0023] AKD emulsion according to the present invention is typically used as a sizing agent in a manufacturing of paper or paperboard.

[0024] It has now been surprisingly found that AKD emulsion can be produced onsite at a paper or paperboard manufacturing plant in a simple manner by using a specific premixing of a heated emulsifying agent solution comprising starch and / or a synthetic polymer and AKD wax. A typical method according to the invention comprises two steps in premixing of AKD wax and heated emulsifying agent solution. In the method of the present invention, AKD wax is melted by a heated emulsifying agent solution comprising starch and / or a synthetic polymer in a premixing stage. In one preferred embodiment of the present invention, AKD wax is melted by a cooked starch solution. More particularly, in an embodiment according to the on-site method according to the present invention an existing cooked starch in a paper or paperboard manufacturing plant, typically cooked cationic starch, is used to melt solid AKD wax. Premixing is carried out in a two-stage premixing unit, which comprises a low-shear premixing stage of AKD wax with a heated emulsifying agent solution, wherein the heated emulsifying agent solution melts solid AKD wax, and a high-shear premixing stage of the mixture of melted AKD wax and the emulsifying agent to obtain a pre-emulsion of AKD. After premixing stages, the pre-emulsion of AKD is fed to an emulsifying unit, typically an ambient or atmospheric pressure emulsifying unit to make an AKD emulsion, which can be used as a sizing agent in a production of a paper or paperboard. The premixing stages of the method according to the invention make possible to use various emulsifying units selected from the group comprising low-shear emulsifying units and high-shear emulsifying units. However, there is no need to use expensive high-pressure homogenisers for emulsifying since pre-emulsion is already made in two- stage premixing. On-site method according to the present invention uses typically inexpensive ambient or atmospheric pressure emulsifying units rather than expensive high-pressure homogenizer unit.

[0025] The method according to the present invention eliminates separate AKD wax melting stage using a heated emulsifying agent solution comprising starch and / or a synthetic polymer to melt AKD wax. In one preferred embodiment according to the present invention, a cooked starch available in a paper or paperboard manufacturing plant, typically cooked cationic starch used as a strength additive in the paper or paperboard manufacturing, can be used in the method according to the present invention. Starch available at the paper or paperboard manufacturing plant has typically a higher molecular weight than the starch used in off-site emulsification process of AKD. In the on-site method according to the present invention, starch having a higher molecular weight can be used since the larger particle size of AKD emulsion is not a problem, where the obtained AKD emulsion is not stored long time, but it is used within substantially short time period after the emulsification. Hence, an increasing of particle size and its possible effect on loss of storage stability of AKD emulsion can be ignored. On-site produced AKD emulsion according to the present invention gives freedom not to be concerned about storage stability, since it can now be acceptable even with big particle size.

[0026] AKD emulsion produced by a method according to the present invention differs from commonly used off-site produced AKD emulsion. AKD emulsion according to the present invention comprises relatively big particles of AKD compared to off-site produced AKD emulsion without loss of sizing efficiency. Relatively larger particle size of AKD emulsion according to the present invention compared to conventional AKD emulsion (median value of the particle size distribution of about 0.4 pm) unexpectedly has an improved sizing efficiency. An improved sizing efficiency may be due to an improved retention into the fiber web. Bigger particle size of AKD emulsion compared to the conventional smaller particle size AKD emulsion may affect retention.

[0027] Description of the drawings

[0028] In the following, some embodiments of the invention will be described in detail. The appended drawings are part of the description. In the drawings, Fig. 1 shows a process chart of conventionally off-site manufacturing method of AKD emulsion,

[0029] Fig. 2 shows a process chart of the method according to an embodiment of the present invention, and

[0030] Fig. 3 shows the results of sizing tests of Example 3.

[0031] Detailed description of the invention

[0032] The present invention relates to a method for producing alkenyl ketene dimer (AKD) emulsion on-site at paper or paperboard manufacturing plant. On-site produced AKD emulsion according to the present invention refers to an emulsion produced at paper or paperboard manufacturing plant, which is typically used instantly without any long-term storage steps. AKD emulsion refers to an emulsion comprising, or consisting of, alkenyl ketene dimer (AKD) and an emulsifying agent comprising starch and / or a synthetic polymer. According to one preferred embodiment of the present invention, AKD emulsion comprises, or consists of, alkenyl ketene dimer (AKD) and starch, typically cationic starch. In one preferred embodiment of the present invention, AKD emulsion is prepared using starch, typically cationic starch as an emulsifying agent.

[0033] Figure 2 shows a process chart of the method according to an embodiment of the present invention. In a method according to the present invention for on-site producing AKD emulsion, an AKD wax and a heated emulsifying agent solution comprising starch and / or a synthetic polymer is firstly premixed in a two-stage premixing unit, which comprises a first low-shear premixing unit and a second high-shear premixing unit. In the first low-shear premixing unit, a heated emulsifying agent solution comprising starch and / or a synthetic polymer is mixed with solid AKD wax to obtain a mixture comprising melted AKD wax and the emulsifying agent, and in the second high-shear premixing unit the mixture comprising melted AKD wax and the emulsifying agent is mixed to obtain a pre-emulsion of AKD. Then, the preemulsion of AKD is emulsified in an emulsifying unit to obtain AKD emulsion. After emulsification the obtained AKD emulsion may be cooled prior to use, or it may be directly conveyed to a paper or paperboard machine for use as an additive, such as a sizing agent. An arrangement according to the present invention for producing AKD emulsion on-site at paper or paperboard manufacturing plant comprises at least a two-stage premixing unit, which comprises a first low-shear premixing unit and a second high-shear premixing unit, and an emulsifying unit. The premixing is divided in two parts. Further, the arrangement comprises means for supplying a heated emulsifying agent solution comprising starch and / or a synthetic polymer to the first low-shear premixing unit and means for supplying the mixture comprising melted AKD wax and the emulsifying agent from the first low-shear premixing unit to the second high-shear premixing unit, and further means for supplying the pre-emulsion from the second high- shear premixing unit to the emulsifying unit.

[0034] In a method according to the present invention, a heated emulsifying agent solution comprising starch and / or a synthetic polymer melts solid AKD wax in a first low-shear premixing unit. Simultaneously, the melted AKD wax can be dispersed homogeneously in the emulsifying agent solution. In a method of the invention, the heated emulsifying agent solution melts AKD wax and there is no need for a separate AKD melting step, where solely AKD wax is melted in a melting chamber and only after melting the melted AKD wax is mixed with the emulsifying agent solution. Therefore, the present invention simplifies the emulsification process of AKD.

[0035] According to an embodiment of the present invention, the first low-shear premixing unit comprises a reservoir and an agitator arranged in the reservoir. In an exemplary embodiment of the present invention, the first low- shear premixing unit comprises an impeller agitator, but also other types of agitators can be used in order to provide adequate shear to get a homogenous mixture comprising melted AKD wax and the emulsifying agent. The mixing speed and the duration of mixing can be adjusted as desired. In an embodiment according to the present invention low-shear premixing may be achieved, for example, by an agitator by using rotational speed of below 500 rpm. According to an embodiment of the invention, low-shear premixing may be achieved, for example, by an agitator by using rotational speed of 100 - 500 rpm or 100 - 400 rpm. According to an embodiment of the present invention temperature of the mixture comprising melted AKD wax and the emulsifying agent is typically more than 70 °C or 80 °C during the first low- shear premixing. In a second high-shear premixing unit, a mixture comprising melted AKD wax and the emulsifying agent from the first low-shear premixing unit is mixed using high-shear mixing, wherein the melted AKD wax is homogeneously dispersed in the emulsifying agent solution and further making a preemulsion of AKD. According to an embodiment of the present invention, the second high-shear premixing unit comprises a reservoir and an agitator arranged in the reservoir. In an embodiment of the present invention, the second high-shear premixing unit comprises an impeller agitator or an impeller agitator comprising multiple impellers, but also other types of agitators can be used. The second high-shear premixing unit is used to continue mixing of the melted AKD wax and the emulsifying agent and to obtain a homogenous pre-emulsion of AKD. In an embodiment according to the present invention high-shear premixing may be achieved, for example, by an agitator by using rotational speed of 500 - 1000 rpm. According to the present invention, rotational speed in the second high-shear premixing unit is higher than in the first low-shear premixing unit. Therefore, according to an embodiment of the invention, high-shear premixing may be achieved, for example, by an agitator by using rotational speed of 500 - 1000 rpm, wherein rotational speed is selected to be higher than in the first low-shear premixing unit. Mixing is continued in the second high-shear premixing unit until a preemulsion is obtained. Long mixing time is not required due to the previous mixing in the first low-shear premixing unit. In an embodiment of the present invention, the duration of mixing in the second high-shear mixing unit may preferably be at most 5 minutes, typically 1 - 5 minutes.

[0036] From the second high-shear premixing unit the pre-emulsion of AKD is supplied to an emulsification unit to make an emulsion of AKD. The pre- emulsion of AKD may be subjected to emulsification in any suitable emulsification unit, apparatus or device, which is able to create appropriate shear power in aqueous systems. For example, the pre-emulsion of AKD may be subjected to the shear treatment in a homogenizer, a high-speed mixer, a disperser, a rotor-stator mixer, a mixer with two counterrotating rotors, a centrifugal pumping device providing a straight flow or back rotation flow, high pressure devices, shear pumps or the like. In one preferred embodiment according to the present invention, an ambient or atmospheric pressure emulsifying unit, such as an atmospheric pressure mixing unit can be used as an emulsification unit. Suitable apparatuses and homogenizers are well-known for a person skilled in the art and commercially available, for example, under tradenames Cavitron™ and Burks pumps™. Emulsification according to the present invention does not require the use of high-pressure homogenization apparatus, but the emulsification can be carried out at ambient or atmospheric pressure. Thus, more inexpensive emulsification units can be used, since the pre-emulsification of AKD is already made in the two-stage premixing stage.

[0037] According to an embodiment of the present invention, the pre-emulsion is subjected to an emulsification where a rotational speed of 500 - 1000 rpm, is applied on the pre-emulsion comprising or consisting of AKD and the emulsifying agent comprising starch and / or a synthetic polymer, preferably AKD and cationic starch.

[0038] The preparation of the AKD emulsion according to the present invention can be carried out continuously or batch-wise.

[0039] In a method according to the present invention, solid AKD wax is melted by a heated emulsifying agent solution in a first low-shear premixing unit. Typically, a solid AKD wax is in the form of fine particles, e.g., flakes, prills, powder or the like. Characteristics of the AKD wax can vary. Different kind of AKD waxes have different melting temperatures. Melting temperature of AKD wax may be, for example in the range of 40 - 50 °C, 40 - 65 °C or 80 - 95 °C. Suitable AKD wax can be selected based on the application. According to an embodiment of the present invention, temperature of the emulsifying agent is typically more than 70 °C or 80 °C in the first low-shear premixing unit and it is dependent on the AKD wax.

[0040] In a method according to the present invention, an emulsifying agent comprising starch and / or a synthetic polymer is used as an emulsifier. In one preferred embodiment of the present invention, the emulsifying agent solution comprises starch, preferably cationic starch. Starch solution, preferably cationic starch solution is prepared by cooking a starch mixture comprising starch and water. Thus, cationic starch solution is a water solution of starch that has been cooked according to methods that are as such well-known for a person skilled in the art. Cooking may be performed with a jet cooker or batch wise or with any other suitable method for cooking starch. The temperature during the starch cooking is typically 95 - 150 °C. Any starch type is acceptable for preparing AKD emulsion in the present invention, but cationic starch is preferred. In an exemplary embodiment, starch comprises tapioca, corn, maize and / or potato starches. In a typical embodiment according to the present invention, starch is in a form of starch solution which comprises 2 - 6 weight-% of starch, calculated from the total weight of the solution. In an embodiment of the present invention, a cooked starch solution, preferably a cooked cationic starch solution is used to melt AKD wax.

[0041] In a preferred embodiment according to the present invention, starch comprises cationic starch. The starches occurring most commonly in sizing agent compositions used in paper or paperboard manufacturing are of a cationic character. However, anionic and amphoteric starches may also be used. The starch is thus substituted by chemical modification with cationic and / or anionic groups, preferably with cationic groups. Therefore, in a typical embodiment according to the present invention, AKD wax and a cooked cationic starch solution is firstly premixed in a two-stage premixing unit. In the first low shear premixing unit, cationic starch solution is mixed with solid AKD wax to obtain a mixture comprising melted AKD wax and cationic starch, and in the second high-shear premixing unit the mixture comprising melted AKD wax and starch is mixed to obtain a pre-emulsion of AKD. Then, the preemulsion of AKD is emulsified in an emulsifying unit, preferably low-shear emulsification unit to obtain an AKD emulsion.

[0042] According to an embodiment of the present invention a starch having a higher molecular weight than the starch used in off-site emulsification process of AKD can be used. Starch already available at the paper or paperboard manufacturing plant has typically been used in a method according to the present invention. Cationic starch is commonly used in papermaking, for example as a strength agent. According to an exemplary embodiment of the present invention, the starch in a starch solution may have a weight average molecular weight (MW) over 5 000 000 g / mol, preferably over 7 500 000 g / mol, more preferably over 10 000 000 g / mol. Using higher molecular weight starch, typically cationic starch available at paper or paperboard manufacturing plant (so-called mill starch), more shear power or energy is not required to make AKD emulsion since it has been observed that there is no need to make small particle size AKD emulsion because on-site AKD emulsion according to the present invention is used directly without storage or it is stored only a short time.

[0043] Polymer molecular weights may be measured by various methods known to skilled persons. For example, molecular weight may be measured using gel permeation chromatography (GPC). Alternatively, or additionally, molecular weights of the polymers may be measured by GPC / Light Scattering / Viscometry also known as Triple Detection GPC which employs Refractive Index Detector (with or without UV Detector), Dilute Solution Viscometry and Light Scattering all in series to determine molecular weights, distribution and related solution parameters.

[0044] According to an embodiment of the present invention, starch is a cationic starch comprising cationic groups. In an embodiment of the invention cationic starch has a degree of substitution (DS), which indicates the number of cationic groups in the starch on average per glucose unit, at least 0.02. Typically, DS is in the range of 0.02 - 0.06. According to an embodiment of the invention cationic starch has a charge density of 0.4 - 1 .5 meq / g.

[0045] According to an embodiment of the method of the present invention, temperature of cooked starch is typically over 80 °C, wherein it can melt AKD wax in a first low-shear premixing unit. Required temperature is dependent on melting temperature of AKD wax.

[0046] According to an embodiment of the present invention, an emulsifying agent solution may comprise a synthetic polymer. In an embodiment of the present invention, a synthetic polymer comprises a cationic synthetic polymer or an amphoteric polymer. A synthetic polymer may comprise, for example polyamine or amphoteric polyacrylamide.

[0047] In a method according to the present invention, a synthetic polymer is heated prior to mixing with AKD wax in order that the heated synthetic polymer solution melts AKD wax in the first low-shear premixing unit.

[0048] In an embodiment according to the present invention, AKD wax and an emulsifying agent comprising starch and / or a synthetic polymer, are mixed in a first low-shear premixing unit in a weight ratio from 10:1 to 6:1. In an exemplary embodiment AKD wax and an emulsifying agent are mixed in a first low-shear premixing unit in a weight ratio of about 5:1 .

[0049] Alkenyl ketene dimer (AKD) emulsion according to the present invention comprises alkenyl ketene dimer (AKD) and an emulsifying agent comprising starch and / or a synthetic polymer, and AKD has a median value of the particle size distribution of more than 0.5 pm or more than 1 pm, typically in the range of 0.5 - 5 pm or 1 - 5 pm. In an embodiment of the present invention 90 % of AKD particles in the AKD emulsion have a diameter in the range of 0.5 - 5 pm or 1 - 5 pm, and more typically in the range of 0.5 - 2 pm or 1 - 2 pm AKD emulsion is produced by the method of the present invention.

[0050] AKD emulsion according to the present invention is typically used as a sizing agent in a manufacturing of paper or paperboard, preferably internal sizing agent. A typical method for the manufacturing of sized paper or sized paperboard from cellulosic stock, comprising adding AKD emulsion according to the present invention as a sizing agent during manufacturing of said paper or paperboard. Suitably, the sizing agent composition may be dosed at any suitable dosage point. The amount of dosage may vary depending on different process parameters, such as the type used in pulps and fillers, temperature in the system and existence of disturbing substances.

[0051] EXPERIMENTAL

[0052] Example 1: On-site AKD emulsion production test

[0053] A two-stage premixing unit according to the invention is used to produce a pre-emulsion of AKD instead of AKD wax melting chamber. In a first premixing unit with an impeller agitator, AKD wax (AKD-1840) is homogenously mixed into a cooked cationic starch solution (corn starch; DS = 0.02; temperature more than 80 °C) with relatively low shear (around 100 rpm). A second premixing unit with impeller agitator and relatively high shear (500 - 1000 rpm) is used to mix further melted AKD wax and cationic starch and expect pre-emulsion of AKD before entering Cavitron™ emulsifying unit (emulsifying unit used for making ASA emulsion). For emulsifying the preemulsion of AKD, two different Cavitron™ running conditions were used: high shear (40A) and low shear (30A).

[0054] In the experiments, the ratio of AKD and cationic starch was about 5:1. The final solid content of AKD emulsion was 12.62 w-%.

[0055] The particle size of AKD (D90) after emulsification with high shear treatment (40A) was 3 pm and after emulsification with low shear treatment (30A) was 2.15 pm. The 40A condition showed a slightly bigger particle size than the 30A condition even though the 40A condition provides more circulation in emulsification.

[0056] The sizing of on-site AKD emulsions is tested on laboratory compared to the commercial AKD emulsion (12,5%) as a reference. Mill refined leaf bleached kraft pulp (LBKP) is used in producing 100g / m2handsheets. Stoeckigt sizing degrees are presented at Table 1 .

[0057] Table 1 .

[0058] Compared to commercial AKD emulsion, two on-site AKD emulsions showed better sizing efficiency, and AKD emulsion 40A with slightly bigger particle size showed better sizing efficiency than AKD emulsion 30A. Example 2: On-site AKD emulsion production test

[0059] Batch making test for producing on-site AKD emulsion was made, where a pre-emulsion of AKD was made using two-stage premixing unit according to the invention. Emulsifying unit was Burks pump™ type small ASA unit. Sizing efficiency of on-site AKD emulsion was compared to commercial AKD emulsion (12.5%). Also, the effect on sizing efficiency was tested when 50% of commercial AKD emulsion was replaced by on-site AKD emulsion. Stoeckigt sizing degrees are presented at Table 2.

[0060] In the experiments, the ratio of AKD (AKD-1840) and cationic starch (corn starch; DS = 0.02) was about 5:1. The final solid content of AKD emulsion was 13.15 w-%.

[0061] Table 2.

[0062] Sizing efficiency of on-site produced AKD was substantially same as commercial off-site made AKD emulsion used as a reference.

[0063] Example 3: AKD emulsion with synthetic polymers

[0064] Synthetic polymers including cationic polyamine and amphoteric polyacrylamide were tested as emulsifying agents in on-site AKD production method and AKD emulsions are prepared in a laboratory scale.

[0065] Polyamine based emulsion comprises 39.9 g (dry) of AKD wax and 8 g (dry) of polyamine, and 260 g water. Amphoteric polyacrylamide based emulsion comprises 39.9 g (dry) of AKD wax and 8 g (dry) of polyamine, and 236 g water. Further, amphoteric polyacrylamide + Alum emulsion comprises 6.11 g (dry) of Alum.

[0066] Polymer was diluted with hot water (75 °C) to obtain a polymer solution, and then AKD wax was added to the polymer solution and mixed with slow mixing. Then the solution comprising melted AKD wax and the polymer was stirred in high shear blender (36,000 rpm, 3min). Solid content, particle size and charge were measured. The results are presented below at Table 3.

[0067] Table 3.

[0068] Sizing tests were also made by using commercial AKD emulsion (15% product) as a reference. Mill refined leaf bleached kraft pulp (LBKP) is used in producing 100g / m2handsheets. AKD dosage was varied. Stoeckigt sizing degrees are presented at Figure 3.

[0069] Compared to commercial AKD emulsion product, synthetic polymer based on-site concept AKD emulsion showed better performance in laboratory tests.

Claims

Claims1. Method for on-site producing alkenyl ketene dimer (AKD) emulsion, characterized in that the method comprises- premixing a solid alkenyl ketene dimer (AKD) wax and a heated emulsifying agent solution comprising starch and / or a synthetic polymer in a two-stage premixing unit, which comprises a first low-shear premixing unit, where the heated emulsifying agent solution is mixed with the solid AKD wax to obtain a mixture comprising a melted AKD wax and the emulsifying agent, and a second high-shear premixing unit, where the mixture comprising the melted AKD wax and the emulsifying agent is mixed to obtain a pre-emulsion of AKD, and- emulsifying the pre-emulsion of AKD in an emulsifying unit.

2. The method according to claim 1 , characterized in that the heated emulsifying agent solution comprises starch, preferably cationic starch.

3. The method according to claim 2, characterized in that the heated emulsifying agent solution comprises cooked cationic starch.

4. The method according to any of the preceding claims, characterized in that the starch in a starch solution has a weight average molecular weight (MW) over 5 000 000 g / mol, preferably over 7 500 000 g / mol, more preferably over 10 000 000 g / mol.

5. The method according to any of the preceding claims, characterized in that the AKD wax and the emulsifying agent are mixed in a weight ratio from 10:1 to 6:1.

6. The method according to any of the preceding claims, characterized in that the first low-shear premixing unit and / or the second high-shear premixing unit comprises an agitator, preferably an impeller agitator.

7. The method according to claim 6, characterized in that a rational speed of the agitator in the first low-shear premixing unit is 100 - 500 rpm.

8. The method according to claim 6, characterized in that a rational speed of the agitator in the second high-shear premixing unit is 500 - 1000 rpm.

9. The method according to any of the preceding claims, characterized in that the emulsifying unit comprises an ambient or atmospheric pressure emulsifying unit.

10. An arrangement for on-site producing alkenyl ketene dimer (AKD) emulsion, characterized in that the arrangement comprises- a two-stage premixing unit, which comprises a first low-shear premixing unit and a second high-shear premixing unit, and- an emulsifying unit, preferably an ambient or atmospheric pressure emulsifying unit.11 . Alkenyl ketene dimer (AKD) emulsion characterized in that the emulsion is produced by the method according to any of the preceding claim 1 - 9 and the emulsion comprises alkenyl ketene dimer (AKD) and an emulsifying agent comprising starch and / or a synthetic polymer, and AKD has a medium value of the particle size distribution of more than 0.5 pm, typically in the range of 0.5 - 5 pm.

12. AKD emulsion according to claim 11 , characterized in that the emulsion comprises starch, preferably cationic starch as the emulsifying agent.

13. AKD emulsion according to claim 11 or 12, characterized in that the AKD emulsion comprises AKD and the emulsifying agent in a weight ratio of from 10:1 to 6:1 .

14. Use of alkenyl ketene dimer (AKD) emulsion according to any of the preceding claims 11-13 as a sizing agent in a manufacturing of paper or paperboard.

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