METHOD OF PRODUCING AN IMPREGNATED PAPER

Description

TECHNICAL FIELD

[0001] The present disclosure relates to the field of paper-based materials to be used in applications requiring barrier properties.

BACKGROUND

[0002] In the field of packaging, there is a desire to replace plastics with materials of lower carbon footprint, such as paper-based materials. However, it is often difficult for paper-based materials to compete with plastics when it comes to flexibility and barrier properties

SUMMARY

[0003] An objective of the present disclosure is to provide a non-brittle fluorochemical-free paper that provides a grease barrier and/or enables formation of an efficient barrier coating thereon.

[0004] Accordingly, the present disclosure provides a method of producing an impregnated paper, comprising the steps of:

a) forming a paper web from a furnish, which furnish preferably comprises a mixture of hardwood fibers and softwood fibers;

b) dewatering and drying the paper web to a moisture content of 10-15 %; and

c) impregnating the paper web having the moisture content of 10-15 % by applying a water-based composition comprising starch to at least one side of it, wherein the dry matter content of the water-based composition is 8-18 %, such as 10-15 %.

DETAILED DESCRIPTION

[0005] The present disclosure provides a method of producing an impregnated paper. The paper produced by this method obtains its beneficial properties without the addition of fluorochemicals. Hence the impregnated paper of the present disclosure is preferably fluorochemical-free.

[0006] The method comprising the steps of:

a) forming a paper web from a furnish;

b) dewatering and drying the paper web to a moisture content of 10-15 %; and

c) impregnating the paper web having the moisture content of 10-15 % by applying a water-based composition comprising starch to at least one side of it, wherein the dry matter content of the water-based composition is 8-18 %, such as 10-15 %.

[0007] In full-scale machine trials, the present inventors found that when the moisture content was lower, i.e. 8.5%, in step c), the paper web was not impregnated to the same extent, which resulted in a more brittle paper product that broke during wind-up.

[0008] Similarly, a dry matter content of the water-based composition outside the range of 8-18 % results in less efficient impregnation. Preferably, the dry matter content of the water-based composition is 10-15 %.

[0009] Preferably, a film press is used for the impregnation of the paper web in step c).

[0010] In a preferred embodiment, a Yankee cylinder is used in step b). According to this embodiment, the impregnated paper is an impregnated machine glazed (MG) paper. The Bendtsen surface roughness (measured according to ISO 8791-2:2013) of the glazed side of the impregnated paper is preferably below 200 ml/min, such as below 150 ml/min, such as below 120 ml/min. The lower limit for the Bendtsen surface roughness (ISO 8791-2:2013) of the glazed side may for example be 50 ml/min.

[0011] The glazed side of an MG paper is particularly suitable for printing also without a pigmented coating thereon. Hence, in one embodiment, the impregnated paper is an impregnated MG paper and the glazed side thereof is not pigment-coated, but printed. In such an embodiment, the non-glazed side may be provided with a barrier coating. The impregnated paper of this embodiment is preferably used for at least one wall of a package, wherein the glazed (and printed) side is facing outwards and the non-glazed (and barrier-coated) side is facing inwards.

[0012] To obtain sufficient strength, sufficient surface properties and sufficient refinability at the same time, the furnish preferably comprises a mixture of hardwood fibres and softwood fibres. As an example, the dry weight ratio of hardwood fibers to softwood fibers is between 1:1 and 1:3.

[0013] In one embodiment, the dry weight ratio of hardwood fibers to softwood fibers is between 1:1.2 and 1:2.0, such as between 1:1.3 and 1:1.8. Such a relatively high proportion of softwood fibres is particularly preferred when the impregnated paper has an ash content of at least 3%, such as at least 5%.

[0014] The starch of the water-based composition preferably comprises oxidized starch or dextrin, such as maltodextrin. In a particularly preferred embodiment water-based composition preferably comprises oxidized starch and dextrin, such as maltodextrin.

[0015] Oxidized starch and/or dextrin may for example constitute at least 50% of the dry weight of the water-based composition, such as at least 75% of the dry weight of the water-based composition. In one embodiment, oxidized starch and dextrin constitute at least 85% of the dry weight of the water-based composition

[0016] In one embodiment, the water-based composition further comprises a crosslinker. As understood by the skilled person, this crosslinker is capable of crosslinking the starch. An example of such a crosslinker is a PAE resin.

[0017] The amount of the crosslinker in the water-based composition may for example be 1-9 %, such as 3-7 %, based on the dry weight of starch.

[0018] In one embodiment, the dry amount of water-based composition applied in step c) is 0.4-3.0 g/m², such as 0.6-3.0 g/m². If a higher amount is applied, the risk of obtaining a brittle final product is greater (and brittleness may result in breakage during winding or converting). If the applied amount is too low, the oil barrier properties may be insufficient and/or it may be more difficult to form a continuous film on the surface of the paper product.

[0019] The water-based composition is preferably applied to both sides of the paper web having the moisture content of 10-15 % in step c). In such case, the dry amount of water-based composition applied to each side is preferably 0.4-1.5 g/m², such as 0.6-1.4 g/m².

[0020] In an embodiment, the paper web having the moisture content of 10-15 % (i.e. the paper web that is produced in step b) and used in step c)) has a Gurley value when measured according to ISO 536:2019 of at least 75 s, such as at least 80 s, such as at least 85 s. The upper limit is typically 150 s. In one embodiment, the upper limit is 120 s. Such an embodiment maybe preferred since papermaking runnability problems may occur between 120 s and 150 s. As understood by the skilled person, the measurement according to ISO 536:2019 is carried out on a conditioned sample meaning that the moisture content is no longer 10-15 % when the Gurley value is determined. The above-mentioned Gurley values may be achieved by relatively extensive refining of the fibres prior to step a).

[0021] The method of the present disclosure typically further comprises the step of drying the impregnated paper web from step c) to obtain the impregnated paper.

[0022] The impregnation step typically results in a paper of very high air resistance, which reflects high resistance to oil breakthrough and/or low absorption of a barrier coating composition applied thereon. Hence, the impregnated paper preferably has a Gurley value (measured according to ISO 5636-5:2013) of at least 400 s, such as at least 500 s. The Bendtsen porosity (measured according to ISO 5636-3:2013) of the impregnated paper may for example be in the range of 10-50 ml/min.

[0023] The basis weight of the impregnated paper may be 30-65 g/m², such as 35-60 g/m², preferably 35-55 g/m². Higher basis weights are not needed (neither economically motivated) for the intended application. In the present application, basis weight is measured according to ISO 536:2019.

[0024] Despite its strength-reducing effect, it is often desired to have a relatively high ash content.

[0025] In an embodiment, the ash content of the impregnated paper is 3-9 %, such as 5-9 %, such as 5-8 %. Typically, such a relatively high ash content is the consequence of addition of clay. When opacity is of interest, the higher ash contents (i.e. 5-9 %) are particularly preferred in case of lower grammages (i.e. 35-45 g/m², such as 35-40 g/m²).

[0026] A reason for adding clay may be to increase opacity. However, clay reduces strength, which impairs runnability. Softwood pulp and cationic starch is typically included in the furnish to compensate for this strength reduction. In one embodiment, the opacity of the impregnated paper is at least 47%, such as at least 53%. Here, opacity is measured according to ISO 2471:2008. Strength values are discussed below.

[0027] In one embodiment, the burst strength of the impregnated paper is at least 100 kPa and/or the burst strength index is at least 3.3 kN/g, such as 3.3-5.0 kN/g. Here, the burst strength is measured according to ISO 2758:2014.

[0028] In one embodiment, the tear strength in the machine direction (MD) of the impregnated paper is at least 150 mN and/or the tear strength index in the MD of the impregnated paper is at least 4.0 mNm²/g, such as 4.0-6.5 mNm²/g.

[0029] In one embodiment, the tear strength in the cross direction (CD) of the impregnated paper is at least 155 mN and/or the tear strength index in the CD of the impregnated paper is at least 4.3 mNm²/g, such as 4.3-6.8 mNm²/g.

[0030] Here, tear strength is measured according to ISO 1974:2012.

[0031] In one embodiment, at least one side of the impregnated paper has a Cobb 60 s value of less than 20 g/m², such as less than 19 g/m², such as less than 18.5 g/m². In one embodiment, both sides of the impregnated paper have a Cobb 60 s value below 21 g/m², such as below 20 g/m². Here, the Cobb 60 s value is measured according to ISO 535:2014.

[0032] In one embodiment, the Bendtsen surface roughness (ISO 8791-2:2013) of a side of the impregnated paper is in the range of 200-500 ml/min. In case of an MG paper, this side is typically the non-glazed side.

[0033] The tensile strength in the MD of the impregnated paper is preferably above 3.5 kN/m. Further, the tensile index in the MD of the impregnated paper is preferably in the range of 90-145 Nm/g.

[0034] The tensile strength in the CD of the impregnated paper is preferably above 1.8 kN/m. Further, the tensile index in the CD of the impregnated paper is preferably in the range of 50-100 Nm/g, such as 50-85 Nm/g.

[0035] Here, tensile strength is measured according to ISO 1924-3:2005.

[0036] In one embodiment, the palm kernel oil breakthrough (measured according to ISO 16532-1) of at least one side of the impregnated paper is above 15 min, preferably above 18 min, such as above 20 min.

[0037] In one embodiment, the palm kernel oil breakthrough (measured according to ISO 16532-1) of both sides of the impregnated paper is above 15 min, preferably above 18 min, such as above 20 min.

EXAMPLES

[0038] A furnish comprising softwood kraft fibres and hardwood kraft fibres in a 60:40 dry weight ratio was provided. The furnish contained clay in such an amount that the final ash content of the paper was 6.8%. Further, the furnish contained 3.7 kg/tonne dry fibre of cationic starch, 2.5 kg/tonne dry fibre of rosin size and retention polymer. In addition, NaOH and alum was added to adjust the pH to 5.6. The furnish was refined to such an extent that the paper (before impregnation) had a Gurley value of 87 s.

[0039] An MG paper was formed from the furnish on a full-scale paper machine equipped with a Yankee cylinder. The degree of drying was such that the paper web leaving the Yankee cylinder had a moisture content of 12%.

[0040] This paper web having a moisture content of 12% was impregnated on both sides in a (full-scale) film press.

[0041] In more detail, the upper side of the paper web was impregnated with a first water-based composition comprising:

13 wt.% of a starch mixture of oxidized starch and maltodextrin (Perfectafilm B4085 from Avebe);

5 pph (based on the amount of starch mixture) of PAE resin (Kymene GHP20 from Solenis, a crosslinker); and

3 pph of AKD (Solenis Aquapel F220).

[0042] The wire side (glazed side) of the paper web was impregnated with a second water-based composition comprising:

13 wt.% of a starch mixture of oxidized starch and maltodextrin (Perfectafilm B4085 from Avebe); and

5 pph (based on the amount of starch mixture) of PAE resin (Kymene GHP20 from Solenis, a crosslinker).

[0043] The applied dry amount of the first water-based composition was 0.96 g/m². The applied dry amount of the second water-based composition was 0.86 g/m².

[0044] The impregnated paper was then dried such that it had 6.2% moisture when rolled up. The basis weight at roll-up was 38 g/m². Further properties are provided in table 1 below.

[0045] Table 1. Properties of the impregnated paper. "MD" means machine direction. "CD" means cross direction. "US" means upper side. "WS" means wire side. Oil breakthrough was measured using palm kernel oil.

Burst strength (ISO 2758)	144 kPa
Burst strength index (ISO 2758)	3.8 kN/g
Tear strength, MD (ISO 1974)	186 mN
Tear strength index, MD (ISO 1974)	4.9 mNm2/g
Tear strength, CD (ISO 1974)	197 mN
Tear strength index, CD (ISO 1974)	5.2 mNm2/g
Cobb 60 s, US (ISO 535)	17.7 g/m2
Cobb 60 s, WS (ISO 535)	19.0 g/m2
Opacity (ISO 2471)	53%
Surface roughness, US (ISO 8791-2)	361 ml/min
Surface roughness, WS (ISO 8791-2)	91 ml/min
Porosity Bendtsen (ISO 5636-3)	23 ml/min
Air resistance (Gurley) (ISO 5636-5)	535 s
Tensile strength, MD (ISO 1924-3)	4.5 kN/m
Tensile index, MD (ISO 1924-3)	118 Nm/g
Tensile strength, CD (ISO 1924-3)	2.3 kN/m
Tensile index, CD (ISO 1924-3)	61 Nm/g
Oil breakthrough, US, left (ISO 16532-1)	21 min
Oil breakthrough, US, middle (ISO 16532-1)	23 min
Oil breakthrough, US, right (ISO 16532-1)	21 min
Oil breakthrough, WS, left (ISO 16532-1)	27 min
Oil breakthrough, WS, middle (ISO 16532-1)	22 min
Oil breakthrough, WS, right (ISO 16532-1)	27 min

[0046] Bags was formed from the paper of table 1. In the bags, the upper side was facing inwards. Freshly fried hash browns were placed in the bags. After containing the hash browns, the paper of the bags was evaluated and found to have a satisfactory grease barrier, comparatively low sogginess and no tack issues.

[0047] If the upper side of the impregnated paper is intended to be provided with a barrier coating, e.g. in an off-line coating operation, it may be preferred to omit the wax from the first water-based composition since it may otherwise be difficult to wet and hence efficiently coat the upper side with a water-based barrier coating composition.

[0048] The wire side (glazed side) of the impregnated paper has low surface roughness and is thus particularly suitable for printing.

Claims

1. A method of producing an impregnated paper, comprising the steps of:

a) forming a paper web from a furnish, which furnish preferably comprises a mixture of hardwood fibers and softwood fibers;

b) dewatering and drying the paper web to a moisture content of 10-15 %; and

c) impregnating the paper web having the moisture content of 10-15 % by applying a water-based composition comprising starch to at least one side of it, wherein the dry matter content of the water-based composition is 8-18 %, such as 10-15 %.

2. The method of claim 1, wherein a Yankee cylinder is used in step b).

3. The method of claim 1 or 2, wherein the furnish comprises a mixture of hardwood fibres and softwood fibres in which the ratio of hardwood fibers to softwood fibers is between 1:1 and 1:3.

4. The method of claim 3, wherein the dry weight ratio of hardwood fibers to softwood fibers is between 1:1.2 and 1:2.0, such as between 1:1.3 and 1:1.8.

5. The method of any one of the preceding claims, wherein the water-based composition comprises oxidized starch and dextrin, such as maltodextrin.

6. The method of any one of the preceding claims, wherein the water-based composition further comprises a crosslinker, such as a PAE resin.

7. The method of claim 6, wherein the amount of the crosslinker in the water-based composition is 1-9 %, such as 3-7 %, based on the dry weight of starch.

8. The method of any one of the preceding claims, wherein the dry amount of water-based composition applied in step c) is 0.4-3.0 g/m², such as 0.6-3.0 g/m².

9. The method of any one of the preceding claims, wherein the water-based composition is applied to both sides of the paper web having the moisture content of 10-15 % in step c).

10. The method of claim 9, wherein the dry amount of water-based composition applied to each side is 0.4-1.5 g/m², such as 0.6-1.4 g/m².

11. The method of any one of the preceding claims, wherein paper web having the moisture content of 10-15 % has a Gurley value measured according to ISO 536:2019 of at least 75 s, such as 80-150 s, such as 85-150 s.

12. The method of any one of the preceding claims, wherein the ash content of the impregnated paper is 3-9 %, such as 5-9 %, such as 5-8 %.

13. The method of any one of the preceding claims, wherein the basis weight measured according to ISO 536:2019 of the impregnated paper is 30-65 g/m², such as 35-55 g/m².

14. The method of any one of the preceding claims, wherein the Gurley value measured according to ISO 5636-5:2013 of the impregnated paper is at least 400 s, such as at least 500 s.

15. The method of any one of the preceding claims, wherein a film press is used in step c).