EP4575087A1

EP4575087A1 - Method for preparing a coated paperboard or a coated linerboard

Info

Publication number: EP4575087A1
Application number: EP23218618.9A
Authority: EP
Inventors: Hanna LILJEQUIST; Erik BOHLIN
Original assignee: Billerud AB
Current assignee: Billerud AB
Priority date: 2023-12-20
Filing date: 2023-12-20
Publication date: 2025-06-25
Also published as: WO2025132277A1

Abstract

Method of producing a coated paperboard or linerboard is provided. The method comprises the steps of:
- providing a paperboard or linerboard substrate;
- coating the paperboard or linerboard substrate with a coating composition comprising binder and pigment to obtain a coated paperboard or a coated linerboard having a coated surface;
- infrared (IR) drying of the coated paperboard or the coated linerboard to obtain a pre-dried coated paperboard or a pre-dried coated linerboard, wherein the temperature of the coated surface reaches a maximum of 60-75 °C in the IR drying step; and
- air drying of the pre-dried coated paperboard or the pre-dried coated linerboard to obtain a dried coated paperboard or a dried coated linerboard.

Description

TECHNICAL FIELD

The present disclosure relates to the field of coated board such as paperboard and linerboard and in particular to drying of the coated board.

BACKGROUND

Paperboard and linerboard is commonly coated with one or more coatings. Some coatings may induce barrier properties against oxygen or water while others induce surface properties such as improved printability and gloss. Paperboards and linerboards are most commonly coated with multiple coatings.
The paperboard or linerboard is often coated with a coating composition and then dried using infrared drying and/or air drying to obtain a final coated paperboard product.
In certain applications, there is a desire to have a good printability of the final packaging material.

SUMMARY

For coated paperboards and linerboards intended for printing, it is desired to reduce the print mottle of the coated paperboard or the coated linerboard in order to obtain good printing and improved visual appearance. The present inventors have realized that a certain optimization of the drying of coated paperboards or the coated linerabords can result in a reduced print mottle.
Accordingly, a method of producing a coated paperboard or a coated linerboard is provided, which method comprises the steps of:

providing a paperboard substrate or a linerboard substrate;
coating the paperboard substrate or the linerboard substrate with a coating composition comprising binder and pigment to obtain a coated paperboard or a coated linerboard having a coated surface;
infrared (IR) drying of the coated paperboard or the coated linerboard to obtain a pre-dried coated paperboard or a pre-dried coated linerboard, wherein the temperature of the coated surface reaches a maximum of 60-75 °C in the IR drying step; and
air drying of the pre-dried coated paperboard or the pre-dried coated linerboard to obtain a dried coated paperboard or a dried coated linerboard.

Paperboard and linerboard will henceforth be referred to as board and paperboard substrate and linerboard substrate will be referred to as board substrate.
The method is preferably an integrated part of a board production process, wherein the above-mentioned steps are carried out in a coating section of a board machine, preferably a full-scale board machine. This coating section is typically arranged downstream a drying section of the same board machine. In turn, a forming section and a pressing section are typically arranged upstream the drying section of the same board machine. In other words, the method is preferably an in-line coating method.
The method according to the present disclosure enables the production of a coated board having a reduced print mottle. The drying of the coated board comprises both an IR drying step and an air-drying step. The present inventors realized that the operation of the IR drying step has a large influence on the print mottle and reaching a high maximum temperature of the coated surface during the IR drying step, will give rise to a coated board having a high print mottle. By instead optimizing the drying of the coated board so that a maximum temperature of 60-75 °C of the coated surface is obtained during the IR drying and then air drying the pre-dried coated board to its final desired moisture content, is associated with a reduction in print mottle while efficiently drying the coated board. The present disclosure is further associated with increased brightness and whiteness as well as a smoother surface of the dried coated board.
The dried coated board, obtained after air drying, may have a moisture content of 6-9 %, such as 7-8 %.
The maximum temperature of the coated surface during the IR drying step will, henceforth, be referred to as maximum surface temperature.
The pre-dried coated board may reach a maximum surface temperature of 60-70 °C, which may be associated with a particularly good print mottle, brightness and surface smoothness. Preferably, the maximum surface temperature obtained in the IR drying step is 62-68 °C.
The coat weight of the coating composition may be 2-25 g/m², such as 5-25 g/m², such as 5-20 g/m².
The board substrate may be a precoated board substrate. Hence, the board substrate is coated with a pre-coating prior to being coated with the coating composition. Using a precoated board substrate may improve the formation of the coating composition once applied and/or may result in improved surface properties of the final board.
The precoated board substrate may be obtained by coating an uncoated board substrate with a precoating composition comprising binder and pigment. The precoated board is, preferably, dried prior to coating the precoated board substrate with the coating composition.
The binder in the coating composition may comprise a synthetic binder such as a styrene-based binder, preferably the synthetic binder is a styrene-acrylate binder or a styrene-butadiene binder. A styrene-acrylate binder is preferred as it is a more environmentally friendly alternative.
The binder in the coating composition may comprise a biobased binder such as a starch-based binder.
The coating composition may further comprise other additives such as co-binders and/or rheology modifiers. The co-binder may be PVOH and/or CMC.
The pigment in the coating composition may comprise calcium carbonate, clay and/or talc.
The dry weight ratio of pigment to binder in the coating composition may be from 100:10 to 100:25.
The method may further comprise coating the opposite side of the coated board with a second coating composition.
The air used during air drying may have a temperature of 50-175 °C, such as 50-160 °C.
The board substrate may have a grammage of 120-500 g/m², such as 180-480 g/m².
The IR drying is performed by using one or more IR dryers arranged in series, preferable at least two IR dryers arranged in series. This is preferred in order to minimize the risk of over-drying the coated board.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig 1 shows pictures showing the print mottle of the dried coated paperboards for A) trial point 1 (maximum surface temperature of 95 °C), B) trial point 2 (maximum surface temperature of 80 °C) and C) trial point 3 (maximum surface temperature of 65 °C).
Fig 2 shows the optical properties of trial point 1 (maximum surface temperature of 95 °C) and 3 (maximum surface temperature of 65 °C). A) shows the brightness, B) shows the whiteness, C) show the L* and D) shows the gloss.
Fig 3 shows the Parker Print surf roughness of trial points 1 (maximum surface temperature of 95 °C) and 3 (maximum surface temperature of 65 °C).

DETAILED DESCRIPTION

The present inventors realized that optimization of the drying of coated boards can minimize the print mottle of the final dried coated board. The optimization of the drying was obtained by controlling the maximum surface temperature of the coated board to be 60-75°C during the IR drying step, and then air drying the pre-dried coated board, preferably until the final moisture content is obtained. Without being bound by any specific scientific theory, limiting the maximum surface temperature to 60-75 °C during the IR drying step is believed to result in reduced binder migration towards the surface and hence a reduction in print mottle.
The board substrate may comprise at least two plies, such as at least three plies, wherein the top ply of the board substrate is coated with the coating composition comprising binder and pigment and forms the printing layer. The top ply of the board substrate is typically bleached. The board substrate may have a grammage of 120-500 g/m², such as 180-480 g/m².
Each ply of the board substrate may comprise hydrophobic size such as alkenyl succinic anhydride (ASA), alkyl ketene dimer (AKD) and/or rosin size.
The board substrate may further comprise other conventional additives such as fillers and colouring agents. This is however optional.
The board substrate is preferably a precoated board substrate. A precoating composition, preferably comprising binder and pigment is applied to the printing side, i.e., top ply, of an uncoated board substrate and the precoated board is dried. Preferably, the drying of the pre-coated board is performed using IR drying and/or air drying. It is especially preferable that the precoated board is dried using both IR drying and air drying. A maximum surface temperature of 60-75 °C may be obtained during the IR drying. After IR drying, the precoated board substrate is subjected to air drying until the desired moisture content, such as 6-8 %, is reached.
The binder in the precoating composition may be synthetic or biobased. Examples of binders in the precoating may be styrene-butadiene binders, styrene-acrylate binders and starch-based binders. Other binder options may be other acrylate-based binders. Calcium carbonates, clays and/or talc may be used as the pigment in the precoating composition.
The precoating composition may be applied to the board substrate in a coating amount of 2-25 g/m², such as 2-20 g/m², such as 5-20 g/m² based on the dry weight of the precoating composition.
Furthermore, the precoated board may be coated with a second precoating composition. The second precoating composition may also comprise binder and pigment and forms a second precoating on top of the first precoating. The second precoating is, preferably, dried in the same way as the first precoating prior to applying the coating composition.
The optionally precoated board substrate is coated with a coating composition comprising binder and pigment to obtain a coated board. The coating composition is applied to the printing side of the optionally precoated board, i.e., if the board substrate is precoated, the coating composition is applied onto the precoating otherwise the coating composition is applied directly onto the top ply of the board substrate.
The binder in the coating composition may be a synthetic binder and/or a biobased binder. The synthetic binder may be a styrene-based binder such as a styrene-acrylate binder or a styrene-butadiene binder. Styrene-butadiene binders may be less expensive alternatives while styrene-acrylic binders have been found advantageous in applications with taste and odour requirements such as food packaging. A styrene-acrylic copolymer is also preferred (over styrene-butadiene) from an environmental and health perspective. Another option is another acrylate-based binder.
The biobased binder may be a starch-based binder. An example of a starch-based binder is a dispersion of starch particles, preferably starch nanoparticles, i.e., a biolatex. The starch-based binder may comprise cross-linked starch nanoparticles.
The pigment in the coating composition may comprise calcium carbonate, clay and/or talc. Preferably, the pigment is clay and/or calcium carbonate.
The dry weight ratio of pigment to binder in the coating composition may be from 100:10 to 100:25, such as from 100:10 to 100:20. The dry weight ratio of pigment to binder of e.g. 10:100 is understood to mean per 100 parts pigment there is 10 parts binder present.
The coating composition may further comprise other additives such as co-binders and/or rheology modifiers. The co-binder may be PVOH and/or CMC.
The additives present in the coating composition may be present in an amount of 0-10 wt.% based on dry weight. The co-binder may be present in an amount of 0-6 wt.% based on dry weight.
The coating composition may be applied in a dry weight of 2-25 g/m², such as 5-25 g/m², such as 5-20 g/m².
After coating, the coated board is subjected to IR drying until a maximum surface temperature of 60-75 °C is obtained, followed by air-drying until desired moisture content is obtained, usually 6-8 %, and a dried coated board is obtained.
Without wishing to be bound to any theory, it is believed that the method according to the present disclosure results in a decreased migration of the binder in the coating composition towards the surface while still obtaining solidification of the coating during the IR drying. A "tough" IR drying (obtaining a maximum surface temperature >75 °C during the IR drying step) is believed to increase the migration of the binder in the coating towards the surface. This results in a higher concentration of the binder at the surface which leads to uneven ink absorption during the printing process, hence resulting in increased print mottle. By instead drying the coated board to obtain a maximum surface temperature of 60-75 °C during the IR drying step, solidification of the coating can be obtained while binder migration is kept to a minimal. Once solidification of the binder is obtained, the coated board may be completely dried during the air-drying step without obtaining further binder migration.
Obtaining a maximum surface temperature of at least 60 °C during the IR drying results in solidification of the coating composition which then reduces the tackiness of the coated board and improves the processability downstream the IR drying. Furthermore, a maximum surface temperature above 75 °C is disadvantageous for the reason discussed above. Thus, obtaining a maximum surface temperature of 60-75 °C during the IR drying followed by air drying until dry, enables a reduction in print mottle while maintaining good processability. In addition, the present disclosure further increases brightness and whiteness of the dried coated board and reduces the surface roughness.
Preferably, the maximum surface temperature of the coated surface after IR drying is 60-70 °C, more preferably 62-68 °C. This surface temperature may be associated with a particularly low print mottle, brightness and surface smoothness.
The IR drying may be performed using at least one IR dryer, preferably using at least two IR dryers arranged in series.
In order to obtain the desired maximum surface temperature of 60-75 °C during the IR drying, adjustment of the efficiency of the IR dryer/s and/or the velocity of the board through the IR dryers may be performed.
After IR drying, the pre-dried coated board is subjected to air-drying and a dried board is obtained. The air during air drying may have a temperature of 50-100 °C. Preferably, the final dried coated board has a moisture content of 6-8 %, such as 7-8%.
Furthermore, a second coating composition may be applied to the opposite side of the coated board, i.e., the back side opposite the printing side. The second coating composition may comprise binder and pigment.
The dried coated board is preferably used in packaging, such as liquid packaging.

EXAMPLES

Example 1

A trial was performed in a full-scale paperboard machine on an uncoated 3-ply paperboard having a grammage of 235 g/m² formed in the same machine. The uncoated paperboard substrate had a bleached top ply that constituted the printing side.
Three different trial points were evaluated, wherein a precoated paperboard substrate was coated with a coating composition i.e. top coating composition and the coated paperboard was dried with IR driers until a maximum surface temperature of 95, 80, or 65 °C was obtained, followed by air drying until a final moisture content of 7-8 % was obtained.
The coated paperboards of all three trial points were coated with the same precoating, top coating and back side coating and dried in the same way with the exception that different maximum surface temperatures were obtained during the IR drying. Below follows the general coating and drying procedure for the trial points.
The uncoated paperboard substrate was first coated on the printing side with a precoating comprising an SA-latex as the binder and a mixture of calcium carbonate and clay as the pigment. The dry weight ratio of pigment to binder in the precoating composition was 100:16 and the precoating was applied at a coat weight of 5 g/m². The coating was performed using a rod coater. The precoated paperboard was dried using two consecutive IR driers followed by air-drying with air having a temperature of ~60 °C. A moisture content of 6-9 % was obtained after air drying the precoating.
A coating composition was then coated directly onto the dried precoating forming a top coating. The coating composition comprised an SA-latex as the binder and a mixture of clay and calcium carbonate as the pigment. The dry weight ratio of pigment to binder in the coating composition was 100:16 and the top coating was applied at a coat weight of 9 g/m². The coated paperboard was dried using three consecutive IR driers, the IR effect and the velocity of the paperboard through the IR dryers was adjusted so that the surface of the coated paperboard obtained a maximum temperature of 95, 80 or 65 °C (depending on the trial point) during the IR drying. The coated paperboard was then dried using air having a temperature of 60-70°C until a moisture content of 7-8 % was obtained.
Furthermore, the back layer was coated with a coating composition comprising SA-latex as a binder and calcium carbonate as the pigment and dried using IR drying and air drying.
Print mottle, brightness, whiteness, gloss and surface roughness were evaluated for all three trial points.
The print mottle was assessed using visual assessment and the evaluation can be seen in Fig. 1 and table 1. Table 1 shows the maximum surface temperature of the top coating during IR drying and the print mottle of the dried coated paperboard at the three different trial points.

Trial point 1 Trial point 2 Trial point 3

Surface temp. (°C) 95 80 65

Print mottle* High Medium No

*Based on the pictures in Fig. 1.
It can be clearly observed that the print mottle decreases with decreased maximum surface temperature during the IR drying and that no print mottle was observed when a maximum surface temperature of was 65 °C during IR drying.
The brightness was measured according to ISO 2470-2, the whiteness according to ISO 11475, L* according to ISO 5631-2 and gloss according to ISO 8254-1. An increase in brightness, whiteness, L* and gloss was observed with decreased maximum surface temperature during the IR drying, see Fig. 2A-D. It is assumed that obtaining a lower maximum surface temperature (≤75°C) of the coated paperboard during IR drying results in a more even binder distribution on the surface and throughout the coating which leads to better light scattering and thus improved whiteness, brightness and gloss.
The surface roughness, Parker Print-surf roughness (PPS), was measured according to ISO 8791-4. A lower PPS was observed when the pre-dried paperboard obtained a surface temperature of 65 °C compared to when a maximum surface temperature of 95 °C was obtained.
In conclusion, drying a coated paperboard using IR drying and air drying wherein the coated paperboard obtains a maximum surface temperature of 60-75 °C (trial point 3) during the IR drying, has proven to reduce the print mottle as well as improve the brightness, whiteness, gloss and surface roughness of the final dried coated paperboard.

Claims

Method of producing a coated paperboard or linerboard, comprising the steps of:
- providing a paperboard or linerboard substrate;

- coating the paperboard or linerboard substrate with a coating composition comprising binder and pigment to obtain a coated paperboard or a coated linerboard having a coated surface;

- infrared (IR) drying of the coated paperboard or the coated linerboard to obtain a pre-dried coated paperboard or a pre-dried coated linerboard, wherein the temperature of the coated surface reaches a maximum of 60-75 °C in the IR drying step; and

- air drying of the pre-dried coated paperboard or the pre-dried coated linerboard to obtain a dried coated paperboard or a dried coated linerboard.
The method according to claim 1, wherein the dried coated paperboard or the dried coated linerboard has a moisture content of 6-9 %.
The method according to any one of the preceding claims, wherein the temperature of the coated surface reaches a maximum of 60-70 °C, such as 62-68 °C, in the IR drying step.
The method according to any one of the preceding claims, wherein the coating composition is applied in a dry weight of 2-25 g/m².
The method according to any one of the preceding claims, wherein the paperboard substrate or the linerboard substrate is a precoated paperboard substrate or a precoated linerboard substrate.
The method according to claim 5, wherein the precoated paperboard substrate or the precoated linerboard substrate is obtained by applying a precoating composition comprising binder and pigment.
The method according to any one of claims 5-6, wherein the precoated paperboard substrate or the precoated linerboard substrate is dried prior to coating the precoated paperboard substrate or the precoated linerboard substrate with the coating composition.
The method according to any one of the preceding claims, wherein the dry weight ratio of pigment to binder in the coating composition is from 100:10 to 100:25.
The method according to any one of the preceding claims, wherein the binder in the coating composition comprises a synthetic binder such as a styrene-based binder.
The method according to any one of the preceding claims, wherein the binder in the coating composition comprises a biobased binder such as a starch-based binder.
The method according to any one of the preceding claims, wherein the pigment in the coating composition is calcium carbonate, clay and/or talc.
The method according to any one of the preceding claims, further comprising coating the opposite side of the coated paperboard or the coated linerboard with a second coating composition.
The method according to any one of the preceding claims, wherein the air used during air drying has a temperature of 50-175 °C.
The method according to any one of the preceding claims, wherein the paperboard substrate or linerboard substrate has a grammage of 120-500 g/m², such as 180-480 g/m².
The method according to any one of the preceding claims wherein the IR drying is obtained by using one or more IR dryers arranged in series, preferable at least two IR dryers arranged in series.