Technical Field

The present invention relates to a method for the surface treatment of paper or board. The invention further relates to a method for manufacturing paper or board and a paper or board produced by the method.

Background

Paper, board and other fibre-based webs are often treated with surface treatment agents, such as surface sizing

compositions, in order to improve the surface strength and the printability of the paper or board. Usual surface

treatment agents used in the art are e.g. starch,

carboxymethyl cellulose (CMC), and/or polyvinyl alcohol. The predominant method of applying surface treatment agents to the surface of a paper is by use of a size press.

WO2010019302 describes a method for making paper, in which method a layer of surface treatment agent is applied onto the surface of a base paper by means of a roll comprising recesses that are filled with the treatment agent. The amount of surface treatment agent applied to the paper is, according to WO2010019302, at most 0.8 g/m ² /side of the paper.

WO2010018302 describes a method for manufacturing paper, in which method the paper is calendered and coated in order to improve the printing properties. The coating is applied in an amount of about 3 g/m ² /side by film transfer or by spray coating.

In the art, there has been a demand of reducing the amount of surface treatment agents applied to the paper. However, reduced amount of surface treatment agents oftentimes affects the quality of the printing negatively since it is hard to achieve an even coverage of the surface treatment agents and an acceptable sizing profile.

Summary of he invention

One object of the present invention is to provide a method for surface treating a paper or board with a small amount of a surface treatment agent, but yet achieving a high printing quality at the printing of the surface treated paper or board.

This, and other advantages are achieved with the present invention. The present invention relates to a method for the surface treatment of paper or board, which method comprises adding a surface treatment agent to the paper or board and treating said paper or board in at least one calender or breaker stack nip formed by a first and a second calender or breaker stack roll. The method is characterized in that the surface treatment agent is applied onto the surface of at least one side of the paper or board immediately prior to or after the calender or breaker stack nip, while the paper or board is in contact with one of the calender or breaker stack rolls . It has been shown that when the surface treatment agent is applied onto the surface immediately prior to or after the calender or breaker stack nip, while the paper or board is in contact with one of the calender or breaker stack rolls, the surface treatment agent stays on the surface of the paper or board and no significant amount of surface treatment agent penetrates into the paper or board. Consequently, less amount of the surface treatment agent can be applied to the surface while a high printing quality still can be achieved. The surface treatment agent may be applied onto the paper or board e.g. by contactless film transferring or by

spraying. Preferably, the surface treatment agent is applied onto the paper or board by spraying. Spraying the treatment agent onto the paper or board gives rise to a more even coverage and spray pattern. In one preferred embodiment, the surface treatment agent is sprayed onto the paper or board in an atomized state. This may be achieved by atomizing an aqueous solution of the treatment agent at an enhanced pressure and an enhanced temperature. Spraying of the high temperature treatment agent onto the surface of the paper/board gives rise to a high temperature at the surface of the paper/board. Preferably, the atomized medium comprising the surface treatment agent has a temperature of at least 80°C, preferably between 80- 110°C even more preferably between 90-100°C _f most preferably a temperature of 95°C. In this way, a heat and moisture gradient is formed in the paper/board, whereby the

temperature at the surface of the paper/board is higher than the temperature inside the paper/board. This improves the effect of the treatment in the calender or breaker stack, whereby the gloss and smoothness of the paper/board is increased and the bulk is increased.

The surface treatment agent may be applied onto the surface of the paper or board in an amount of below 0,5 g/m2/side of the paper or board, preferably between 0,1-0,5 g/m2/side, and more preferably between 0,1-0,2 g/m ² /side, calculated as dry weight of the surface treatment agent applied to the paper or board. Surprisingly, it has been found that it is possible to apply such small amount of a surface treatment agent to the paper or board and yet to achieve an excellent printing quality.

The temperature of the paper or board being sprayed with the surface treatment agent is preferably at least 50°C, more preferably at least 55°C, and most preferably at least 60°C The paper or board achieves the desired temperature by being heated by the calender or breaker stack roll in contact with the paper or board and/or by the heat transferred to the paper/board with the atomized surface treatment agent. The high temperature of the paper or board makes water applied together with the surface treatment agent to evaporate quickly whereby the surface treatment agent is kept on the surface of the paper or board.

The method of the invention does not wet the paper

significantly, and, consequently, no additional drying of the paper after the application of the surface-treatment agent is needed.

The moisture content of the paper or board may be between 3-12%, preferably between 4-10% by weight at the point of spraying the treatment agent onto the surface. This further contributes to the prevention of the treatment agent to penetrate into the paper/board, and thus, being kept on the surface of the paper/board.

The surface treatment agent applied onto the surface in accordance with the invention may be any water soluble surface treatment agent. Preferably, the treatment agent is selected from the group consisting of starch, carboxymethyl cellulose (CMC), nano-pigment, salt of multivalent metals such as calcium chloride, casein, protein, lignin,

saccharide, hemicelluloses, wax or resin, polyvinyl alcohol (PVA), latex and Optical brightening agents (OBAs) , water- based release agents or a combination of any of these.

The invention further relates to a method of manufacturing a paper or board comprising the steps of forming a fibrous web from pulp, dewatering the formed web, spraying a surface treatment agent onto the surface of the web and treating the web in at least one calender or breaker stack nip formed by a first and a second calender or breaker stack roll. The method of manufacturing a paper or board is characterized in that the surface treatment agent is sprayed onto the surface of at least one side of the paper or board immediately prior to or after the calender or breaker stack nip, while the paper or board is in contact with one of the calender or breaker stack rolls. The invention further relates to a paper or board

manufactured by said method. The paper or board manufactured by said method exhibit excellent printing and surface

strength properties. The paper or board treated by the method of the invention is most suitable for offset printing, but can for example also advantageously be printed in gravure or ink-jet printing methods.

Detailed description

In accordance with the present invention, a surface

treatment agent is sprayed onto the surface of a paper or board immediately prior to or after a calender or breaker stack nip while the paper or board is in contact with a calender or breaker stack roll. It has been found that by choosing said point of application, a small amount of surface treatment agent may be applied to the paper/board while an excellent printing quality still may be obtained. The contact with the calender or breaker stack roll provides the paper or board with a relative high temperature, preferably of above 50°C, more preferably of above 55°C, and most preferably of above 60°C, whereby the water added together with the surface treatment agent is instantly evaporated. Moreover, if the treatment agent is added after the last drying unit and in the calender or breaker stack, the paper or board exhibits a comparatively low Bendtsen roughness, as compared to

application points of prior art. The low Bendtsen roughness of the paper further contributes to keeping the applied surface treatment agent on the surface of the paper or board and prohibits it to penetrate into the paper or board. In this context, immediately prior to or after the calender or breaker stack nip means milliseconds prior to or after the calender or breaker stack nip, preferably between 5-100 ms, even more preferably between 10 - 50 ms, or 10 30 ms prior to or after the calender or breaker stack nip.

The surface treatment agent is preferably applied to the surface of the paper or board by spraying it in an atomized state. This can, e.g., be achieved by mixing an aqueous solution of the treatment agent with steam at increase pressure and temperature whereby an atomized aqueous

solution/steam mixture is formed. Preferably, the atomized aqueous solution/steam mixture has a temperature of at least 80°C, preferably between 80 - 110 °C even more preferably between 90 - 100 °C, most preferably of 95 °C. Application of a high temperature, atomized medium comprising a surface treatment agent onto the paper/board surface at the calender or breaker stack gives the paper/board a high surface

temperature which improves the effect of the treatment in the calender or breaker stack. A temperature gradient is formed in the paper/board with the highest temperature at the surface, which is beneficial for the treatment.

The aqueous solution comprising the treatment agent may have a concentration of above 5%, more preferably of above 10% by weight. This allows an efficient transfer of the surface treatment agent to the paper or board while the strength of the surface of the paper/board is not negatively affected by the water in the aqueous solution. Preferably, the viscosity of the aqueous solution does not exceed 100 mPas. The water amount added by applying the aqueous solution onto the paper/board is preferably below 4.0 g/m ² , e.g.

between 0.5 and 4.0 g/m ² . This low amount of water applied to the paper/board does not affect the surface strength of the paper/board negatively.

The calender used in accordance with the invention may be an on-line or an off-line calender, depending on the paper or board quality to be produced. The calender may be any kind of a calender, such as a machine calender, or a super calender. Even though it may be preferably to use a calender, it is also possible to use a breaker stack which is a kind of calender placed in the drying section of the paper machine.

The surface treatment agent may be sprayed onto the surface of the paper/board immediately prior to, or immediately after, the first calender or breaker stack nip. The surface treatment agent may also be sprayed onto the surface of the paper/board immediately prior to, or immediately after a second or a third calender or breaker stack nip. Preferably, the paper or board is treated in at least one nip after the surface treatment agent has been applied to the paper/board. Most preferably, the paper or board is also, or

alternatively, treated in at least one nip before the surface treatment agent is applied to the paper/board. In this way, the surface roughness of the paper/board may be decreased even further before the surface treatment agent is applied, which contributes to the prevention of the treatment agent to penetrate into the paper/board.

In case a machine calender is used in accordance with the invention, the nip pressure may be at least 60 kN/m,

preferably at least 65 kN/m and most preferably between 70-85 kN/m.

Both sides of the paper/board may be treated in accordance with the method of the invention. This may be accomplished by, e.g., spraying a treatment agent onto the first side of the paper or board immediately prior to a first nip, while the paper or board is in contact with a first calender or breaker stack roll and spraying a treatment agent onto the second side of the paper or board immediately prior to a second nip, while the paper or board is in contact with a second calender or breaker stack roll. The paper or board treated by the method according to the invention may be any kind of paper or board. Preferably, the paper or board is a mechanical pulp based, un-coated, base paper.

The invention is further described by way of an example with reference to fig 1.

Example

An uncoated base paper, 52 gsm, comprising a mix of

chemical and mechanical pulp, is treated in an on-line 2-nip calender as shown in fig. 1. The web speed is 1260 m/min and the machine width is 8550 mm. The calender comprises a first (1) and a second (2) calender roll, forming a first

calendering nip (Nl) and a third (3) and a fourth (4) calender roll, forming a second calendering nip (N2) . The nip (NO) between the second (2) and the third (3) calendering roll is an open nip. Thus, the stack of rolls in the calender consists of four rolls placed one above the other. The first (1) and forth (4) calender rolls are cambered thermo rolls which are heated to a temperature of approximately 58 °C. The second (2) and third (3) rolls are metal covered rolls. The nip pressures are between 78 - 82 kN/m. A calcoil unit (not shown) may be arranged on the fourth (4) calender roll.

The web of the base paper (W) is guided from the last drying unit into the first nip (Nl) with the aid of a guiding roll (5) . The web is supported and guided by the calendering rolls (2) and (3) through the open nip (NO) into the second calendering nip (N2) . After the second calendering nip (N2), the web (W) is passed to another calendering stack or to reeling. A first spray unit (6) is arranged at the second calender roll (2) . A second spray unit (7) is arranged at the third calender roll (3) . The spray units (6 and 7) mix an aqueous solution of the treatment agent with steam at enhanced temperature and pressure, whereby the aqueous solution sprayed onto the surface of the web (w) in an atomized state. The distance between the nozzles of the spray units and the paper is approximately 10 cm. The first spray unit (6) sprays a surface treatment agent, for example CMC and/or PVA, to one side of the web (W) immediately after (26 ms) the first calendering nip (HI) , while the web (W) is in contact with the second calender roll (2) . The second spray unit (7) sprays a surface treatment agent to the other side of the web immediately prior to (26 ms) the second calendering nip (N2), while the web (W) is in contact with the third calendering roll (3) .

While the above example has been shown for purpose of illustrating the invention, it will be apparent to those skilled in the art that various changes in the invention disclosed herein may be made without departing from the scope of the invention, which is defined in the appended claims. For example, the number of calender or breaker stack rolls and calender or breaker stack nips may be increased and decreased and the type of the rolls, the length and the pressure of the nips may be varied.