EP1444399B1

EP1444399B1 - Method and product in the manufacturing of paper cardboard or paperboard

Info

Publication number: EP1444399B1
Application number: EP02763163A
Authority: EP
Inventors: Philip HÅKANSSON
Original assignee: Stora Enso AB
Current assignee: Stora Enso AB
Priority date: 2001-09-25
Filing date: 2002-09-02
Publication date: 2011-11-16
Anticipated expiration: 2022-09-02
Also published as: SE0103171L; SE0103171D0; EP1444399A1; SE520012C2; ATE533893T1; WO2003042455A1

Abstract

A method to treat a sizing agent in connection with its use in a sizing composition in the manufacturing of sized paper, cardboard or paperboard, which sizing agent is constituted by at least one alkyl keten dimer (AKD), and which sizing agent exhibits a melting jpoint above 20 DEG C. According to the invention, said sizing agent is heated to a temperature above its melting point, where after the sizing agent is maintained at a temperature above a phase transition temperature for phase transition to solid state at least up to charging of the sizing composition, that comprises the sizing agent, to a stock for the paper or paperboard. The invention also relates to a method for the manufacturing of sized paper, cardboard or paperboard by use of the sizing cmposition, and the paper or paperboard manufactured accordingly.

Description

TECHNICAL FIELD

The present invention relates to a method to treat a sizing agent that is used in a sizing composition in connection with the production of paper, cardboard, paperboard and similar products. The invention also relates to a method for the production of sized paper, sized cardboard or sized paperboard.

BACKGROUND OF THE INVENTION

For most paper, cardboard and paperboard qualities, there is a need to decrease the penetration rate of liquid into the paper, cardboard or paperboard structure, by adding hydrophobic substances during the paper-making process. Examples of such paper, cardboard and paperboard qualities are photocopying paper, writing and printing paper, paper for so called ink-jet printers and liquid board for packages for juice, milk e.g. or other beverages and liquids. For their use, these qualities need to have certain liquid repelling properties. Several different methods exist to achieve this effect. One of these is to add an emulsion or a dispersion of a hydrophobic material, during the paper-making process. Many different hydrophobic materials can be used. Among the most efficient can be mentioned so called hydrophobic, cellulose reactive sizing agents. When using this type of agents, it is believed that the hydrophobic effect is achieved by a reaction between the hydrophobic material and the hydroxyl groups of the cellulose.
The predominantly most used hydrophobic, cellulose reactive materials at neutral or weakly alkaline conditions during the paper- or paperboard-making process are alkyl keten dimer (AKD) or alkenyl succinic acid anhydride (ASA). Accordingly, alkenyl succinic acid anhydride (ASA) is an example of a so called cellulose reactive hydrophobing agent. The anhydride can react with the hydroxyl groups of the cellulose, whereby a trans-esterification occurs. As compared to AKD, ASA has a much higher reactivity with the cellulose and thereby also with water, whereby hydrolysis of the product is a much bigger problem for ASA as compared to AKD.
Normally, ASA is delivered to the paper mill as an oil, sometimes with an emulsifier added. Emulsification takes place in a special emulsifying equipment in connection with the paper- or paperboard machine (so called in situ production) and cationic, anionic or amphoteric starch or other polymer is used as a stabiliser and charge carrier. The particle size in the emulsion is usually 1 - 5 micrometers. Larger particles are hydrolysed more slowly, but with smaller particles there is a better distribution on the cellulose fibres, which means that an optimal, intermediate particle size exists. Accordingly, ASA is hydrolysed fast and the hydrolysis product and its salts with Ca²⁺ are sticky and quite often result in depositions in the paper- or paperboard making process. Both hydrolysis and reaction rate increases with an increasing pH, but the reaction is fast over the entire pH scale. ASA is quite seldom used for hydrophobation of liquid board, where the requirements of strong sizing are large.
These above mentioned disadvantages of ASA result in that one often is reduced to use AKD in order to obtain a desired degree of hydrophobation.
Alkyl keten dimers , that are mainly used when the requirements of strong and long-lasting hydrophobation are large, may react with the cellulose in the drying of the paper or paperboard during formation of beta-keto-esters.
At low pH values, the reaction is slow and in practice, AKD can not be used other than in the neutral or weakly alkaline pH range. Instead of reacting with the fibre during the paper- or paperboard production, AKD may also be hydrolysed and will then give an instable beta-keto-acid, which is decarboxylised to the corresponding ketone. The hydrolysis products formed during the paper-making process, i.e. the ketone products formed, are very disturbing in use of the produced paper in certain types of photocopying machines where these ketones form deposits. The extent of the hydrolysis reaction for AKD is however considerably less as compared to ASA. This means that AKD usually is delivered to the paper or paperboard mill in the form of a dispersion that has a particle size of between 0.2 and 2 micrometers. The dispersion is usually stabilised by a cationic polymer. This polymer, that also gives the particles their charge, most often consists of cationic starch but anionic or amphoteric starches or synthetic polymers are also used. AKD has a melting point of between 15 - 60 °C, depending on the length of the fatty tail. The most common commercial products so far most often have a mixture of C14, C16 and C18 hydrocarbons, resulting in a melting point of between 40 - 60 °C. It is however also known to use AKD products having shorter carbon chains or having branched carbon chains, that are liquid at room temperature and therefore may be delivered to the paper or paperboard mill in the form of an emulsion. Such AKD products however require that very large amounts are used in order for an acceptable sizing level to be obtained.
One of the most noticeable disadvantages of AKD is that the particles of the dispersion become sticky as their temperature starts to approach the melting point. This results in adhesion problems, in particular in the press section of the paper or paperboard machine. This stickiness also results in depositions in the wet section of the paper or paperboard machine and in the process that precedes the actual paper or paperboard machine, i.e. in the so called short circulation. This stickiness may also result in breaks in the paper or paperboard machine, with costly losses in efficiency as a consequence.
In the paper- or paperboard making process, the temperature is between 40 and 55 °C, with no or very few exceptions, all the way to the drying section of the paper or paperboard machine, where the temperature is increased additionally. This means that in the process, the AKD particles reach the temperature at which they normally are sticky, given that the sizing composition consists of sizing efficient AKD wax. In order to avoid that the particles become sticky, two alternative methods have hitherto been used in the manufacturing of the sizing compositions:

1. Starting out with AKD wax having such a high melting point that the particles in the dispersion do not become sticky at the temperature that they are exposed to in the stock and in the wet and dry section of the paper or paperboard machine. Despite certain advantages, this method has obvious drawbacks:
1. a/ Since the liquid distribution on the fibre gets slower for a particle having a higher melting point, the reaction between fibre and AKD is slower too.
2. b/ Somewhere in the paper or paperboard machine, the AKD particles will, eventually reach their sticky temperature, with the problems mentioned above following.
2. Starting out with AKD wax having such a low melting point that the particles are not at solid state but liquid already at room temperature. Accordingly, the sizing composition is an emulsion. Despite that the tendency for deposition is minimal, this is not an advantageous method since very large amounts of sizing agent are required to reach an acceptable level.

As a conclusion, the hitherto available techniques has not been able to combine a good sizing economy, i.e. a low consumption of the sizing agent, in order to achieve a good sizing, with a low stickiness that minimises deposits and running problems in the paper or paperboard machine.

ACCOUNT OF THE INVENTION

Water based dispersions of hydrophobic, cellulose reactive kinds have been used in the production of paper or paperboard since the end of the 1950's. The present invention relates to a method of treating a known sizing agent in or for known sizing compositions, whereby a lot of the above described disadvantages for prior art sizing compositions are eliminated or considerably reduced.
According to the invention, there is presented a method according to claim 1, for treating a sizing agent that consists of at least one alkyl keten dimmer (AKD) and which sizing agent exhibits a melting point above 20 °C.
According to the invention, there is also presented a method according to claim 2, for the production of sized paper, cardboard or paperboard, in which a sizing composition comprising a sizing agent according to claim 1 is used.
A sized paper, cardboard or paperboard, that has been produced by the method according to claim 2 exhibits at least the advantages in comparison with prior art paper, cardboard or paperboard, that the sizing is strong and efficient also at relatively low contents of the sizing agent, at carbon chain lengths of the AKD that are optimal for the sizing, and that such a strongly and efficiently sized paper can be used without problem in all types of copying machines.
The new main feature of the claimed treatment is that the sizing agent that consists of AKD is rapidly heated to a temperature that exceeds the melting point, so that the sizing agent is transferred from solid to liquid phase, where after the sizing agent is maintained at a temperature above a phase transition temperature for phase transition from liquid state to solid state at least up to the addition of the sizing composition, that comprises the sizing agent, to a stock for the paper or paperboard. It has namely surprisingly been shown that the sizing agent that consists of AKD stays in liquid state also when it is cooled down to the temperature at which it previously underwent a phase change. In other words, at cooling, the said phase transition temperature is lower than the melting point. In the invention, this discovery is used in the following way: A sizing composition based on a sizing efficient AKD wax, i.e. having carbon chain lengths of C14, C16 or longer, is rapidly heated to a temperature that exceeds the melting point of the particles. Directly thereafter, the size is charged to a paper or paperboard stock. Before the time of charging, the temperature of the sizing emulsion is not allowed to decrease to the temperature at which the particles once again are transferred to solid state. In order to avoid this, it is important e.g. that a possible dilution of the sizing emulsion before the charging to the stock takes place with adequately hot water. Given that the temperature in the paper or paperboard manufacturing process does not fall below the phase transition temperature for the chosen type of AKD, the size particles will remain in liquid state when they have been deposited on the fibre surfaces. In this way, there is achieved a good spreading/running out of the size particles on the cellulose fibres, whereby the reaction with the cellulose becomes more efficient. This also gives conditions for the use of longer carbon chain lengths than is conventional, e.g. C20, which will give an increased hydrofobing degree, i.e. a stronger sizing. Moreover, the sizing particles are not sticky at any time during the paper or paperboard manufacturing process, which eliminates or greatly reduces the tendency for deposition.
According to one aspect of the invention, said AKD is constituted by AKD with mainly un-branched carbon chains with lengths of at least 14 carbon atoms, e.g. C14 and/or C16 and/or C18, but also C20 being conceivable, optionally in combination with C14 and/or C16 and/or C18. Also, a pure C18 AKD or a mixture of C16 and C18 is possible, e.g. 50/50. An upper limit for the carbon chain lengths is in practice that the phase transition temperature at cooling must not be higher than the temperature that is used in the paper or paperboard manufacturing process.
According to another aspect of the invention, said sizing agent that is constituted by AKD is chosen among the AKD waxes, that may be mixtures of different carbon chain lengths, that exhibit a melting point above 25 °C, preferably above 30 °C and even more preferred above 35 °C and most preferred above 40 °C.
According to yet another aspect of the invention, the heating is done to a temperature that is at least 5 °C above the melting point for the AKD, preferably 5 to 10 °C above the melting point.
According to a first, preferred embodiment of the invention, the AKD is heated above the melting point, where after the melted AKD is emulsified in a base for the sizing composition, i.e. normally water, together with a polymer that preferably is constituted by anion active, cation active or amphoteric starch, and an emulsifier. In this connection it is secured that all components in the sizing composition have a temperature that exceeds said phase transition temperature for the AKD and that such a temperature is maintained during the emulsifying and up to charging of the sizing composition to a stock for the paper or paperboard.
According to a second, preferred embodiment of the invention, said sizing agent that is constituted by AKD may initially be emulsified in the base for the sizing composition, at enhanced temperature and together with the other components of the sizing composition, where after the emulsion is cooled to a more durable dispersion. At the heating according to the invention, the particles of the dispersion are transferred back to liquid state. In this embodiment, such a fully prepared dispersion may be delivered to the mill, in order then to be in-situ heated and maintained at temperature at the mill, preferably in direct connection with the machine or the machines that manufacture(s) the paper, the cardboard or the paperboard.
According to one aspect of the invention, the sizing composition may also comprise a second sizing agent, preferably in the form of at least one alkenyl succinic acid anhydride (ASA), whereby said sizing agent that is constituted by AKD constitutes 20 - 100 % by weight of the total amount of sizing agent in the sizing composition, preferably 30 - 100 % by weight and even more preferred 40 - 100 % by weight. In this connection it is preferred to use a sizing in the form of a dispersion or an emulsion where the separate particles are constituted by a mixture of AKD and ASA (according to Swedish patent SE 9601225-7 ). This may be achieved independent of which of the above mentioned embodiments that is used. In the first case, melted AKD and heated ASA in oil form are mixed before the emulsification is performed. In the second case, a dispersion in which the separate particles consist of a mixture of AKD and ASA has already been manufactured (according to Swedish patent SE 9601225-7 ), which dispersion is rapidly heated to a temperature that exceeds the melting point for the particles, before charging it to the stock.
Independent of the embodiment, the heating is suitably performed in direct connection with the machine or the machines that manufacture(s) the paper, cardboard or paperboard, i.e. in-situ in the mill. The reason for this is that the enhanced temperature increases the risk of hydrolysis of the AKD (and optional ASA) in the water. Therefore, the time between contact, between heated sizing agent that is constituted by AKD and water in the sizing composition, and the charging to the stock, is minimised. This time is preferably not allowed to exceed 2 minutes, is preferably not allowed to exceed 1 minute and is even more preferred not allowed to exceed 30 seconds. In the first embodiment, this time is the time between emulsification and the charging to the stock. In the second embodiment, this time is the time between the point at which the dispersion reaches the melting point for the AKD and the charging to the stock.

DESCRIPTION OF THE DRAWINGS

In the following, the principle of phase transition for AKD, in certain cases mixed with ASA, will be described at heating and cooling, with reference to the drawings, of which:

Fig. 1: shows the phase transition curve for pure AKD wax (100 %),
Fig. 2: shows the phase transition curve for a mixture of 20 % ASA and 80 % AKD,
Fig. 3: shows the phase transition curve for a mixture of 40 % ASA and 60 % AKD,
Fig. 4: shows the phase transition curve for a mixture of 60 % ASA and 40 % AKD.

When determining the melting point for different AKD waxes and mixtures of AKD and ASA, one may use an equipment that is called TAP (Thermal Analysis Processing).
Fig. 1 shows a melting point determination for a 100 % AKD wax, by aid of TAP equipment. The heating took place at 2 °C per minute and is shown in the lower part of the graph. The wax becomes sticky at about 42 °C and in completely liquid phase only at about 58 °C. In this example, the wax has thereafter been additionally heated to about 70 °C, where after the wax has been allowed to cool. The cooling is shown in the upper part of the graph. It is then surprisingly discovered that the wax returns from liquid phase to solid phase only at 38 °C. When such a wax is used according to the invention, the size particles will remain in liquid phase during the entire paper or paperboard manufacturing process, given that the temperature in the paper or paperboard manufacturing process is not 39 °C or below, which never or very seldom is the case.
Fig. 2 shows how a mixture of 80 % AKD and 20 % ASA (according to Swedish patent SE 9601225-7 ) behaves in melting point context. The mixture transfers to liquid state at about 56 °C but at cooling, the transition takes place only at 34 °C. In this example too, the size particles may remain in liquid state if the sizing dispersion is rapidly heated to a temperature above 56 °C, even if the size particles thereafter are cooled during the paper or paperboard manufacturing process, given that the temperature does not sink below about 35 °C.
Fig. 3 shows the behaviour for a mixture of 60 % AKD and 40 % ASA. The mixture is transferred to liquid state at about 52 °C but at cooling, the transition back to solid state takes place only at about 33 °C. In this example too, the size particles may remain in liquid state if the sizing dispersion is rapidly heated to a temperature above 52 °C, even if the size particles thereafter are cooled during the paper or paperboard manufacturing process, given that the temperature does not sink below about 34 °C.
Fig. 4 shows the behaviour for a mixture of 40 % AKD and 60 % ASA. The mixture is transferred to liquid state at about 50 °C but at cooling, the transition back to solid state takes place only at about 29 °C. In this example too, the size particles may remain in liquid state if the sizing dispersion is rapidly heated to a temperature above 50 °C, even if the size particles thereafter are cooled during the paper or paperboard manufacturing process, given that the temperature does not sink below about 30 °C.
The invention is not limited by the described embodiments, but may be varied within the scope of the claims.

Claims

A method to treat a sizing agent in connection with its use in a sizing composition in the manufacturing of sized paper, cardboard or paperboard, which sizing agent is constituted by at least one alkyl keten dimer (AKD), and which sizing agent exhibits a melting point above 20 °C, characterised in that said sizing agent is heated to a temperature above its melting point, where after the sizing agent is maintained at a temperature above a phase transition temperature for phase transition from liquid phase to solid state at least up to charging of the sizing composition, that comprises the sizing agent, to a stock for the paper or paperboard.
A method according to claim 1, wherein said sizing agent is charged to a stock for said paper, cardboard or paperboard, where after the sizing agent is maintained at a temperature above a phase transition temperature for phase transition to solid state at least up to charging of the sizing composition, that comprises the sizing agent, to said stock for the paper or paperboard.
Method according to claim 1 or 2, characterised in that said heated sizing agent is charged to a base for the sizing composition and emulsified therein at said maintained temperature, whereby the temperature of the sizing composition is brought to exceed said phase transition temperature during the actual emulsification and up to charging of the sizing composition to said stock for the paper or paperboard.
Method according to claim 1 or 2, characterised in that said sizing agent initially is emulsified at enhanced temperature in a base for the sizing composition, where after the thus formed emulsion is allowed to cool to form a dispersion, where after said heating is performed.
Method according to any one of the preceding claims, characterised in that a second sizing agent too is used in the sizing composition, preferably in the form of at least one alkenyl succinic acid anhydride (ASA), whereby said sizing agent that is constituted by AKD constitutes 20 - 100 % by weight of the total amount of sizing agent in the sizing composition, preferably 30 -100 % by weight and even more preferred 40 - 100 % by weight.
Method according to claim 3 and 5, characterised in that said heated sizing agent that is constituted by AKD is mixed with said sizing agent that is constituted by ASA, before emulsification of the sizing agents in the base for the sizing composition.
Method according to any one of the preceding claims, characterised in that said sizing agent that is constituted by AKD consists of AKD with in the main un-branched carbon chains having lengths of at least 14 carbon atoms.
Method according to any one of the preceding claims, characterised in that said sizing agent that is constituted by AKD exhibits a melting point above 25 °C, preferably above 30 °C, even more preferred above 35 °C and most preferred above 40 °C.
Method according to any one of the preceding claims, characterised in that said heating takes place in situ in a mill for the manufacturing of the paper or paperboard, the time between contact, between heated sizing agent that is constituted by AKD and water in the sizing composition, and the charging to the stock, is minimised and preferably is not allowed to exceed 2 minutes, is preferably not allowed to exceed 1 minute and is even more preferred not allowed to exceed 30 seconds.
Method according to claim 2, characterised in that said sizing composition is maintained at a temperature above the phase transition temperature for phase transition to solid state of the sizing agent that is constituted by AKD even after charging to the stock, in the process line for the manufacturing of paper, cardboard or paperboard, up to and including a drying section for the paper or paperboard.