TECHNICAL FIELD

The present disclosure relates to the field of pigment-coated paperboard. BACKGROUND

In the field of paper packaging, print quality is often a desired property. The surface properties, such as the printing properties, of paper or paperboard may be improved by applying a pigment coating. In addition to the pigment, such a pigment coating often comprises a binder. An alternative or

complementary way of improving surface properties is calendering. SUMMARY

For some applications, the pigment-coated board is covered with a layer of polyethylene (PE) such that a laminate is formed. The purpose of the PE layer is normally to provide a barrier and/or to facilitate heat sealing when a package is formed from the laminate. When the laminate is used, it is important that the PE-layer adheres firmly to the pigment coating, i.e. that delamination is avoided.

It is thus an object of the present disclosure to provide a pigment-coated paperboard having improved PE adhesion and satisfactory printing properties. The inventor has surprisingly found that if a high proportion of the pigment is CaC0 ₃ pigment and, in turn, a high proportion of the CaC0 ₃ pigment has a broad particle-size distribution (i.e. a relatively high amount of relatively small and relatively large particles), the PE adhesion is significantly improved. To obtain satisfactory printing properties, it is however necessary to include relatively small amounts of clay and/or CaC0 ₃ pigment having a more narrow particle-size distribution. The inventor has also found that an increased amount of binder often improves the PE-adhesion, but may impair printability. For cost efficiency, it is also generally desired to keep the amount of binder low.

There is thus provided a coated paperboard comprising a base board having a top side provided with a pigment coating, which pigment coating comprises a pigment mixture, a binder and a rheology modifier, wherein:

the pigment mixture comprises, by dry weight, 0-17 % of clay, 60-90 % of a first CaC0 ₃ pigment and 0-30 % of a second CaC0 ₃ pigment, provided that the amount of clay and second CaC0 ₃ pigment combined is at least 10 %; the d ₅₀ of the first CaC0 ₃ pigment is 0.60-0.80 μιτι and the d ₉ e of the first CaC0 ₃ pigment is at least 2.8 μιτι, such as 2.9-4.0 μιτι, such as 2.9-3.5 um; the d ₅₀ of the second CaC0 ₃ pigment is between 0.53 and 0.73 μιτι μιτι and the d ₇₅ of the second CaC0 ₃ pigment is less than 1.2 μιτι, such as 0.85-1.15 μιτι, such as 0.9-1.1 μιτι;

the binder comprises a styrene-butadiene copolymer or a styrene-acrylate copolymer; and

the dry weight ratio of pigment to binder in the pigment coating is from 100:16 to 100:20.

BRIEF DESCRIPTION OF THE FIGURE

Figure 1 shows the (narrow) particle size distribution for Covercarb 75 (CC75) from OMYA and the (broad) particle size distribution for Hydrocarb 90 from OMYA.

Figure 2 illustrates the set-up for measuring PE adhesion used in Trial 2. DESCRIPTION

The present disclosure provides a coated paperboard comprising a base board. The base board has a top side that is provided with a pigment coating,

In one embodiment, the base board comprises at least two paper layers (typically 2-5 layers), such as at least three paper layers (typically 3-5 layers), wherein a top paper layer of the base board is provided with the pigment coating. The top paper layer of the base board is typically bleached. It may comprise titanium dioxide and/or calcined clay and/or calcium carbonate as filler. The top layer is preferably formed from Kraft pulp, such as bleached Kraft pulp. The grammage according to ISO 536 of the base board may for example be between 30 and 400 g/m ² , e.g. between 120 and 300 g/m ² , such as between 125 and 260 g/m ² .

The base board is preferably hydrophobized from a sizing agent treatment, such as treatment with alkyl ketene dimer (AKD), alkenyl succinic anhydride (ASA) or a combination thereof.

The pigment coating comprises:

a pigment mixture;

a binder; and

a rheology modifier. The pigment mixture comprises, by dry weight:

0-17 %, such as 7-17 %, preferably 9-14 %, of clay;

60-90 %, preferably 65-85 %, more preferably 65-75 %, of a first CaCO ₃ pigment; and

0-30 %, preferably 5-25 % or 10-30 %, more preferably 12-25 % of a second CaCO ₃ pigment.

Further, the amount of clay and second CaCO ₃ pigment combined has to be at least 10 %.

The clay is preferably kaolin clay, such as delaminated kaolin clay.

In one embodiment, the pigment mixture comprises, by dry weight, 7-17 % of clay, 65-85 % of the first CaCO ₃ pigment and 5-25 % of the second CaCO ₃ pigment, which means that the amount of clay and second CaCO ₃ pigment combined is at least 12 % In another embodiment, the pigment mixture comprises, by dry weight, 65- 85 % of the first CaCo3 pigment and i5%-35 % of the second CaC0 ₃ pigment, but no clay. The first and the second CaC0 ₃ pigment have different particle size distributions. The distribution of the first CaC0 ₃ pigment is broader than the distribution of the second CaC0 ₃ pigment (which is exemplified in Figure 1).

The d ₅₀ of the first CaC0 ₃ pigment is 0.60-0.80 μιτι, preferably 0.65-0.75 μιτι. The d ₉ 8 of the first CaC0 ₃ pigment is at least 2.8 μιτι, such as 2.9-4.0 μιτι, such as 2.9-3.5 um or 3.0-3.4 μιτι.

The d ₅₀ of the second CaC0 ₃ pigment is 0.63 μιτι ± ο.ιο μιτι, preferably 0.63 μιτι ± 0.05 um. The d ₇₅ of the second CaC0 ₃ pigment is less than 1.2 μιτι, such as 0.9-1.1 μιτι. The d ₉ e of the second CaC0 ₃ pigment is preferably below 3 μιτι, such as 2.3 ± 0.30 μιτι, such as 2.3 ± 0.20 μιτι. The skilled person working with calcium carbonate pigments is familiar with d values. For determining the d values, a Sedigraph 5100 or 5120 device from the company Micromeritics, USA, can be used. The particle size distributions given herein were determined using a Micromeritics Sedigraph III with software version V.1.04. Preferably, the first and the second CaC0 ₃ pigment are both ground calcium carbonate (GCC).

The binder comprises a styrene-butadiene copolymer or a styrene-acrylate copolymer. The styrene-acrylate copolymer binder is preferred due to lesser health and environmental concerns. The dry weight ratio of pigment to binder in the pigment coating is in the range from 100:16 to 100:20, preferably from 100:16 to 100:19, roughly corresponding to a dry weight % within the range of about 13-17 wt% of binder in the pigment-binder mixture. When the binder comprises a styrene- acrylate copolymer, the dry weight ratio is preferably in the range from 100:16 to 100:20, preferably from 100:16 to 100:19.

The glass transition temperature (Tg) of the binder is preferably between 3°C and 25 °C, e.g. between 15 and 25°C, such as between 20 and 22°C.

Accordingly, pigment preferably amounts to 83-87 %, such as 84.5-86.5 % of the dry weight of the pigment coating. The binder preferably amounts to 13.8-16.7 %, such as 14.0-15.3 % of the dry weight of the pigment coating.

The rheology modifier typically amounts to 0.05-2.0 %, such as 0.1-1.0 %, of the dry weight of the pigment coating. The rheology modifier may for example be CMC or an acrylic co-polymer, such as an alkali-swellable emulsion (ASE) or a hydrophobically modified, alkali-swellable emulsion (HASE).

The coat weight of the pigment coating on the top side is preferably 5-30 g/m ² , such as 6-25 m ² , such as 7-22 m ² , such as 8-20 g/m ² .

A pre-coating layer comprising pigment and a binder may be provided between the top side of the base board and the pigment coating.

A reverse side of the base board may be provided with a reverse side pigment coating, which optionally has the same composition as the top side pigment coating. The coat weight of the reverse side pigment coating may for example be 1-30 g/m ² , such as 5-25 m ² , such as 7-22 m ² , such as 8-20 g/m ² .

There is also provided a method of producing a coated paperboard, comprising applying a pigment coating composition on a top side of a base board, which top side is optionally coated with a pre-coating, characterized in that the pigment coating composition comprises the components defined above. There is also provided a use of a coated paperboard according to the above for producing a package, such as a package for food or liquids.

EXAMPLES

Trial l Paperboard was coated with different pigment coating compositions (see table l).

Components of the coating compositions were:

Kaolux HS ("Clay"), kaolin clay from Thiele having a brightness (GE) of 90, a particle size of 84% <2 μιτι, 69% <ΐμιτι and median (dso) 0.75 μιτι, and an aspect ratio of 14;

Hydrocarb 90 ("HC90"), CaC0 ₃ pigment from Omya: broad particle size distribution, d ₅₀ = 0.7 μιτι, d ₇₅ = 1.2 μιτι and d ₉ e = 3.2 μιτι;

Covercarb 75 ("CC75") , CaC0 ₃ pigment from Omya: narrow particle size distribution, d ₅₀ = 0,63 μιτι, d ₇₅ = 1 μιτι and d ₉ e = 2.30 μιτι; - Styrene-acrylate latex ("Latex")

The paperboard was then printed and the STFI mottle (for cyan) and the Printing density were measured (see table 1).

Table 1. The components of the respective compositions are given as parts by dry weight. All compositions also included the same rheology modifier.

Sample Clay HC90 CC75 Latex STFI Printing

(parts) (parts) (parts) (parts) mottle density

1 20 40 40 17 1.00 1.41

2 10 60 30 17 1.11 1.41

3 10 70 20 17 1.04 1.42

4 10 80 10 17 1.12 1-39

5 10 90 0 17 1.17 1-37

6 10 70 20 15 1.01 1.40 7 10 8o 10 15 1.00 1.41

8 10 90 0 15 i.o8 1.40

9 10 90 0 13 0-93 1.41

10 10 70 20 19 1-15 1.40

11 30 6o 10 19 1.65 1.40

Sample 1 was included as a reference sample known to provide satisfactory printing properties. For the STFI mottle, a general rule of thumb is that a difference of 0.03 is detectable by an experienced eye, while a difference of 0.05 is detectable by an average consumer.

The inventor has found that the use of a relatively high proportion of CaC0 ₃ pigment (and thus a relatively low proportion of clay pigment) in the pigment mixture improves the PE adhesion. The inventor has also found that CaC0 ₃ pigment having a broad particle size distribution, such as HC90, results in a particular improvement of the PE adhesion. However, Table 1 shows that if only HC90 is used as the CaC0 ₃ pigment (sample 5), a relatively high STFI mottle is obtained. If ten parts of HC90 is replaced by CC75 (sample 4), the STFI print mottle is decreased. If another ten parts of HC90 is replaced by CC75 (sample 3), such that a 70:20 weight ratio is obtained between the two types of CaC0 ₃ pigment, the STFI print mottle is further decreased.

Surprisingly, the STFI mottle increases again if yet another ten parts of HC90 is replaced by CC75 (sample 2).

The inventor has also found that a higher amount of binder generally results in better PE adhesion. More than 15 parts of binder is necessary for a satisfactory result. However, a comparison of samples 10 and 3 in table 1 shows that it may be preferred to have less than 19 parts of binder, at least when the binder is a styrene-acrylate copolymer, when STFI mottle is considered.

Trial 2 Paperboard was coated with a pre-coating and various top coatings. Components of the coating compositions were:

Kaolux G ("Clay"), kaolin clay from Thiele having a brightness (GE) of 90 and a particle size of 80-85 % <2 μιτι;

Covercarb 75 ("CC75") , CaC0 ₃ pigment from Omya: narrow particle size distribution, d ₅₀ = 0,63 μιτι, d ₇₅ = 1 μιτι and d ₉ e = 2.30 μιτι;

Hydrocarb 90 ("HC90"), CaC0 ₃ pigment from Omya: broad particle size distribution, d ₅₀ = 0.7 μιτι, d ₇₅ = 1.2 μιτι and d ₉ e = 3.2 μιτι;

Styrene-acrylate latex ("Latex"), Styron SA 95085.01;

Rheology modifier ("Rheo"), Coatex Rheocoat 66 (an acidic emulsion of acrylic copolymer)

The paperboard was then coated with polyethylene (PE) and Fmax and Favg (the maximum force and the average force determined from a peel test), which represent PE adhesion, was measured. The method of measuring Fmax and Favg is described in more detail below. The top coating compositions and the resulting F values are presented in Table 2.

For all samples, the pre-coating composition consisted of 20 parts of Clay, 80 parts of HC60, 14 parts of Latex and 0.26 parts of Rheo. The viscosity of the pre-coating was about 1400 mPas and its dry matter content was about 65 %. The coat weight of the pre-coating was 8.5 g/m ² . The viscosity of the top coatings was about 1300 mPas and their dry matter content was about 63 %. The coat weight of the top coatings was 11.5 g/m ² .

Table 2.

Sample 1 2 3 4 5 6 7

Clay 20 10 10 10 10 10

(parts)

CC75 40 20 20 20 20 20 25

(parts)

HC90 40 70 70 70 70 70 75

(parts)

avg 1.21 1-43 1.72 2.01 1.24 0. 4 n a

Table 2 continued.

* Dry weight ratio of pigment to binder

^§ No mean value (Favg) is reported for sample 7. For this sample, the force was very high and uneven across the 15 mm length that was measured, and there was no way to produce a meaningful mean value according to the method employed. The maximum value (Favg) is however relevant and shows that the force needed to separate the PE film from the coated paperboard surface was very high.

Table 2 shows that decreasing the amount of latex from 21 parts (sample 2) to 19 parts (sample 3), increased Fmax from a relatively low level (1.53) to a satisfactory level (1.87). A further decrease of the amount of latex to 17 parts (sample 4) resulted in a significant increase of the Fmax to a particularly high level (2.13). At 15 parts (sample 5), Fmax was again below a satisfactory level and at 13 parts (sample 6), Fmax was even lower than at 15 parts. Trial 2 thus supports that the amount of latex should be from 16-20 parts, that 16-19 parts of latex is a preferred range and that 16-18 parts of latex is an even more preferred range.

Table 2 also shows that the samples having 60-90 parts of HC90 and less than 20 parts of clay resulted in satisfactory PE adhesion properties when the proportion of binder (Latex) was 17 or 19 parts. In contrast, the samples having less than 60 parts of HC90 and/or 20 or more parts of clay resulted in inferior PE adhesion properties.

Method of measuring PE adhesion

Poor adhesion of extruded PE to a pigment-coated paper board is almost always caused rupture in the pigment-coating layer, at least for weaker coatings. Hence, "coating layer cohesion" may be a more correct terminology than "PE adhesion". The method below is designed to determine how well a final product, such as a liquid package, performs when subjected to mechanical stress by measuring the cohesion of the coating layer. The instrument used for the method is made by Instron, but any quality tensile tester will suffice.

1. One extrusion coated sample, 50-100 cm x web width is sampled from production.

2. Gently make a cut in the PE film in the cross direction (CD), relative to the production direction (MD).

3. Separate by hand the PE film from the paperboard along the cut.

4. Cut samples I5mm*i5cm so that you get the delaminated film in the one end (15mm CD and 15 cm MD)

5. Place the samples one at the time onto the wheel, as depicted in figure 2, with double adhesive tape 3M 410 and release the film to the clamp. The wheel moves during the measurement to always secure 90 ⁰ angle. In case of very high values for delamination the film may stretch or rupture. This was the case here and so the Scotch Tape 371 PP clear was used on top of the PE to make it stronger and to only measure the adhesion force between the paper coating and the PE film and not stretch of the approximately 20 micron thick PE film. Start pulling upwards with the Instron and record the force in N and extension in mm. Measure for 15mm length, 4 times in MD and 4 times in anti MD. Report the data as both a maximum within an error free area and a mean value of N/m between two points on the curve where the force is constant (Fmax and Favg, respectively). In this case, all data between

1.5 and 13.5mm was considered, which means that the data from the first 10 % and the last 10 % of were ignored. Report the average of 8 samples whereof 4 is in MD and 4 in anti MD.