Treated board having improved mechanical properties at high humidity

Abstract

A method for preparing a containerboard or a paperboard is provided. The method comprises the steps: e) providing a containerboard or a paperboard substrate; f) providing a treatment composition comprising 45-75 wt.% glyoxal, 5-30 wt.% polyvinyl alcohol and 15-40 wt.% citric acid based on the total amount of glyoxal, polyvinyl alcohol and citric acid in the treatment composition; g) applying the treatment composition to the containerboard or the paperboard substrate to obtain a treated containerboard or a treated paperboard; and h) curing the treated containerboard or the treated paperboard.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to the field of treated containerboard or paperboard with improved mechanical properties at high humidity.BACKGROUND

[0002] Containerboard or paperboard is commonly used as packaging material due to its suitable mechanical properties and its versatility. However, a drawback of using containerboard or paperboard in packaging applications is its sensitivity towards moisture. In humid environments, a deterioration of the paperboards mechanical properties is observed. This may be observed as paperboard packages failing to maintain their structure and thereby lose their function. Humid environments are a major factor to consider during e.g. cold-chain logistics such as transport.SUMMARY

[0003] The present inventors have realized that there is a need to improve the mechanical properties, such as compressive strength, of containerboards and paperboards in humid environments.

[0004] According to a first aspect of the present disclosure, a method for preparing a containerboard or a paperboard is provided. The method comprises: a) providing a containerboard or paperboard substrate; b) providing a treatment composition comprising 45-75 wt.% glyoxal, 5-30 wt.% polyvinyl alcohol (PVOH) and 15-40 wt.% citric acid based on the total amount of glyoxal, polyvinyl alcohol and citric acid in the treatment composition; c) applying the treatment composition to the containerboard or paperboard substrate to obtain a treated containerboard or treated paperboard; and d) curing the treated containerboard or the treated paperboard.

[0005] Containerboard and paperboard will henceforth be referred to as board and containerboard or paperboard substrate will be referred to as board substrate.

[0006] The containerboard of the first aspect may for example be a linerboard, such as a kraftliner, e.g. a white top kraftliner. Alternatively, the containerboard may be intended to be used as a fluting.

[0007] Improvement of the compressive strength (short-span compression test, SCT) was observed when treating a substrate with a treatment composition comprising 45-75 wt.% glyoxal, 5-30 wt.% PVOH and 15-40 wt.% citric acid based on the total amount of glyoxal, PVOH and citric acid in the treatment composition and curing said treated board. The treatment improved the SCT of the treated board at 50 % relative humidity (RH) as well as at 90 % RH (see Table 1 in the Examples section). The improved mechanical properties at high humidity (e.g. 90 % RH) enables the board to better maintain its structure and function during use.

[0008] Whishing not to be bound by any theory, the improved mechanical properties are assumed to be due to the unique treatment composition creating cross-links between hydroxyl-functionalized polymer chains i.e. cellulose and PVOH, at a suitable density in order to improve the mechanical properties while minimally affecting the strain at break which usually decreases significantly as a result of cross-linking.

[0009] In one embodiment, the treatment composition may comprise 45-65 wt. % such as 50-60 wt.% glyoxal based on the total amount of glyoxal, PVOH and citric acid in the treatment composition.

[0010] In another embodiment, the treatment composition comprises 5-25 wt.%, such as 5-20 wt.%, such as 5-15 wt.% PVOH based on the total amount of glyoxal, PVOH and citric acid in the treatment composition. The proportional amount of PVOH may even be as low as 5-10 wt.% based on the total amount of glyoxal, PVOH and citric acid in the treatment composition.

[0011] In one embodiment, the treatment composition comprises 20-40 wt.% such as 25-40 wt.% citric acid based on the total amount of glyoxal, PVOH and citric acid in the treatment composition. The embodiments relating to the amounts of the different components can be combined in any way so that the amount of the three components is 100 wt.% based on the total amount of glyoxal, PVOH and citric acid.

[0012] In a preferred embodiment, the amount of glyoxal is 50-60 wt. %, the amount of PVOH is 5-15 wt. % and the amount of citric acid is 25-40 wt. % based on the total amount of glyoxal, PVOH and citric acid in the treatment composition.

[0013] The curing may comprise subjecting the treated board to IR drying and / or to air having a temperature of at least 150 °C, such as at least 160 °C, such as at least 170 °C. During this stage preferably both drying and curing occurs.

[0014] The curing may comprise subjecting the treated board to a temperature of at least 30 °C, such as 30-90 °C, such as 40-80 °C for at least 1 min. Preferably, the treated board is subjected to the elevated temperature during the roll-up into tambours and / or during the storage of the tambours.

[0015] In a preferred embodiment, the treated board is first subjected to IR drying and / or heated air followed by subjecting the treated board to an elevated temperature (at least 30 °C) during a prolonged time.

[0016] The total amount of glyoxal, PVOH and citric acid in the treatment composition may be 1-10 wt.%, such as 2-7 wt.%, such as 3-6 wt.%.

[0017] In one embodiment, the treatment composition may be an aqueous composition of glyoxal, PVOH and citric acid. The aqueous composition may comprise glyoxal, PVOH, citric acid and water in an amount of at least 95 wt. %, such as at least 98 wt.%. The treatment composition according to the first aspect may further comprise additives such as defoamers.

[0018] The board substrate may have a grammage of at least at least 60 g / m 2 , such as 100-350 g / m 2< , such as 115-300 g / m 2< . The upper limit may be 440 g / m 2< .

[0019] According to a second aspect, a board produced according to the first aspect is provided. It follows from the above that the board of the second aspect may be a linerboard, such as a kraftliner, e.g. a white top kraftliner, or an NSSC liner. Alternatively, the board of the second aspect may be a containerboard that is intended to be used as a fluting (i.e. intended to be corrugated) or that has already been corrugated such that it is a fluting.

[0020] The board of the second aspect may have a compressive strength index (short-span compression test, SCT index) in cross direction of at least 14 Nm / g at 50 % RH and at least 6 Nm / g at 90 % RH as determined by ISO 9895:2008.

[0021] The board of the second aspect may have a tensile strength index in machine direction of at least 40 Nm / g at 50 % RH and at least 20 Nm / g at 90 % RH as determined by ISO 1924-3:2005.

[0022] The board of the second aspect may further have a tensile stiffness index in machine direction of at least 5 kNm / g at 50 % RH and at least 2 kNm / g at 90 % RH as determined by ISO 1924-3:2005.

[0023] The ISO standards used to determine the SCT index, tensile strength index and tensile stiffness index disclose the determination of these properties at 50 % RH. The properties at 90 % RH were measured according to the respective standards with the exception that conditioning of the samples was performed at 90 % RH rather than at 50 % RH. The determination of the mechanical properties at 90 % RH was, hence, performed by following the procedures of the respective ISO standard but conditioning the samples at 90 % RH instead of 50 % RH.

[0024] According to a third aspect, there is provided a corrugated board in which a board according to the second aspect forms a layer. As know to the skilled person, a corrugated board is a material comprising at least one liner and at least one fluting.BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Aspects and embodiments are now described, by way of example, with reference to the accompanying drawings, in which: Fig 1-3 shows the SCT in cross direction at 50 % RH for the treated boards in Example 1 and their references. Fig 4-6 shows the SCT in cross direction at 90 % RH for the treated boards in Example 1 and their references. Fig 7-8 shows the tensile strength in machine direction at 50% and 90 % RH for boards 6-8 in Example 1 and their reference. Fig 9-10 shows the tensile stiffness in machine direction at 50% and 90 % RH for boards 6-8 in Example 1 and their reference. DETAILED DESCRIPTION

[0026] The moisture sensitivity of a board may cause issues such as loss of mechanical properties when exposed to higher humidity during e.g. cold chain logistics. By cross-linking the board, i.e., cross-linking of the cellulose chains, the moisture sensitivity may be reduced and the mechanical properties such as compressive strength (SCT) may be improved at increased humidity.

[0027] The present inventors have realized that treating a board substrate with a treatment composition comprising glyoxal, PVOH and citric acid and curing said treated board according to the first aspect, an improvement in compressive strength (SCT) can be obtained at both 50 % and 90 % RH.

[0028] The treatment composition according to the first aspect comprises a cross-linking agent in the form of a difunctional aldehyde, i.e., glyoxal. Glyoxal reacts with hydroxyl-groups and forms stable acetal-bonds. An acidic environment, the presence of proton donors, is necessary in order for the reaction between glyoxal and hydroxyl-groups to be driven fully to the stable acetal-bond. If the cross-linking composition is lacking proton donor, the reaction will lead to the formation of hemi-acetal bonds which are more susceptible to cleavage and hence less stable. Citric acid is chosen as the proton donor as it is biobased, readily available, and environmentally friendly. In addition to glyoxal and citric acid, PVOH is added to the cross-linking composition. PVOH will also have the possibility to react with glyoxal to form cross-links. The addition of PVOH may enable longer cross-links as well as promote interfibrillar cross-links which in the end will make the treated paperboard less brittle compared to a treated paperboard wherein the treatment composition did not comprise PVOH.

[0029] The treatment composition comprises 45-75 wt. %, preferably 45-60 wt. %, more preferably 50-55 wt.% glyoxal, 5-30 wt.%, preferably 5-25 wt.%, more preferably 10-15 wt.% PVOH and 15-40 wt.%, preferably 5-25 wt.%, more preferably 10-15 wt.% citric acid based on the total amount of glyoxal, PVOH and citric acid in the treatment composition. This ratio between the three active components enables using a lower total amount of active components while still obtaining an improvement in compressive strength. Reducing the amount of chemicals may reduce the environmental impact of the treatment and the cost of production of the treated board. The treatment composition may further have a minimal negative effect on the strain at break.

[0030] The treatment composition is preferably an aqueous composition. The total amount of glyoxal, PVOH and citric acid in treatment composition may be 1-10 wt.%, such as 2-7 wt.%, such as 3-6 wt.% of the total treatment composition. This concentration of active components (i.e. glyoxal, PVOH and citric acid) in the treatment composition enables improvement in SCT while minimizing the amount of chemicals used.

[0031] The treatment composition may be coated onto the board substrate by a coating means. The treatment composition may slightly impregnate the board substrate. It is advantageous to apply the aqueous treatment composition to the board substrate in an amount of at least 10 g / m 2< such as at least 12 g / m 2< , such as at least 14 g / m 2< . An upper limit of the applied amount of the aqueous treatment composition may be 22 g / m 2< . This application amount may lead to a particularly good improvement in compressive strength.

[0032] The board substrate may comprise at least one ply, such as at least two plies, such as at least three plies. The board substrate may further comprise conventional additives such as sizing agents, fillers and colouring agents, this is however optional. In a further embodiment, the board substrate may have a grammage of at least 60 g / m 2< , such as 100-350 g / m 2< , such as 115-300 g / m 2< . The upper limit may be 440 g / m 2< .

[0033] After the treatment composition has been applied to the board substrate, the treated paperboard is cured. The curing may comprise subjecting the treated board to IR drying and / or to air having a temperature of at least 150 °C, such as at least 160 °C, such as at least 170 °C. During this stage preferably both drying and curing occurs.

[0034] The curing may comprise subjecting the treated board to a temperature of at least 30 °C, such as 30-90 °C, such as 40-80 °C for at least 1 min. Preferably, the treated board is subjected to the elevated temperature during the roll-up into tambours and / or during the storage of the tambours. This step of the curing may last until the glyoxal and the hydroxyl-groups have reacted fully, e.g. up to 20 min. However, no adverse effect is obtained if this step is prolonged, i.e., longer than 20 min.

[0035] The treated board may be first subjected to IR drying and / or heated air followed by subjecting the treated board to an elevated temperature (at least 30 °C) during a prolonged time.

[0036] The treated board that is produced by using the inventive treatment composition has improved compressive strength compared to non-treated board.

[0037] The board may have: a. a compressive strength index (short-span compression test, SCT index) in cross-direction of at least 14 Nm / g at 50 % RH and at least 6 Nm / g at 90 % RH. The upper limit of SCT index in cross-direction may be 32 Nm / g at 50 % RH and 18 Nm / g in 90 % RH. b. a tensile strength index in machine direction of at least 40 Nm / g at 50 % RH and at least 20 Nm / g at 90 % RH. The upper limit of tensile strength index in machine direction may be 170 Nm / g at 50 % RH and 100 Nm / g at 90 % RH. c. a tensile stiffness index in machine direction of at least 5 kNm / g at 50 % RH and at least 2 kNm / g at 90 % RH. The upper limit of tensile stiffness index in machine direction is 14 kNm / g at 50 % RH and 10 kNm / g at 90 % RH.

[0038] The SCT index was determined according to ISO 9895:2008 and tensile strength and stiffness indices were determined according to ISO 1924-3:2005. These ISO standards disclose the determination of said properties at 50 % RH. The properties at 90 % RH were determined according to the respective standards with the exception that conditioning of the samples was performed at 90 % RH rather than at 50 % RH.EXAMPLES Example 1

[0039] A pilot trial was performed wherein the effect of the different components in the treatment composition was evaluated. Eight different treatment compositions were evaluated wherein the compositions comprised only glyoxal (compositions 1-2), glyoxal and PVOH (compositions 3-5) or glyoxal, PVOH and citric acid (compositions 6-8), see Table 1.

[0040] The PVOH used is Kuraray Poval 28 / 99 from Kuraray and has a degree of hydrolysis of 99.0-99.8.

[0041] For the pilot trial, Pure Performance ™< , a white top kraftliner having a grammage of 135 gsm was used as the substrate. The aqueous treatment compositions were coated onto the substrate using a pilot coater so that approximately 16 g / m 2< of the aqueous treatment composition is added to the substrate. The treated substrates were subjected to IR drying and heated air having a temperature between 180-235 °C. The treated substrates were rolled up into tambours and stored. The temperature during the roll-up and storage was 50-70 °C. The curing continued during the roll-up and storage.

[0042] Table 1. Different treatment compositions used in the pilot trial. "wt.% in aq." means the percentage of the component in the aqueous treatment composition. "wt. % of comp." means percentage of the total amount of components, i.e. the total amount of glyoxal, polyvinyl alcohol and citric acid. CompositionGlyoxalPVOHCitric acidwt. % in aq.wt. % of comp.wt. % in aq.wt. % of comp.wt. % in aq.wt. % of comp.11100----22100----32890.2411--42810.4819--52680.9532--62560.2471.33772530.48131.33482470.95221.331

[0043] The SCT of the treated substrates and their references were determined according to ISO 9895:2008 and can be seen in Table 2 and Fig. 1-6. The standard discloses the determination of SCT at 50 % RH, the measurement at 90 % RH were performed according to the standard with the exception that the conditioning was performed at 90 % RH rather than at 50 % RH.

[0044] Table 2. The difference in SCT at 50 % and 90 % RH between the treated substrates and their respective reference. Board 1 is treated with composition 1 and so on. BoardSCT 50 % RHSCT 90 % RH10 %4 %20 %4 %32 %8 %40 %8 %55 %3 %66 %14 %74 %14 %88 %12 %

[0045] As can be seen in Table 2 and Fig. 1-6, treating a substrate with only glyoxal gave rise to no improvement in SCT at 50 % RH and only a slight improvement at 90 % RH. Adding PVOH to the treatment composition resulted, in some cases, in a slightly larger increase in SCT at 50 % RH and 90% RH. Finally, the treatment composition comprising all three components, i.e. glyoxal, PVOH and citric acid, gave rise to the largest increase in SCT at both 50 % and 90 % RH.

[0046] The effect of the treatment on the SCT was most noticeable at 90 % RH.

[0047] Using different amounts of PVOH in the treatment compositions did not have a major impact on the SCT. This shows that a relatively low amount of PVOH can be used in the treatment composition, which leads to a cost reduction as well as a lowering of the environmental impact of the treatment composition.

[0048] Tensile strength and tensile stiffness were also determined for the substrates treated with the compositions 6-8 and their references. The treated substrates exhibited a slight increase in tensile strength and tensile stiffness compared to their references, see Fig. 7-10. The tensile strength and tensile stiffness were determined according to ISO 1924-3:2005. The standard discloses the determination of tensile properties at 50 % RH, the measurement at 90 % RH were performed according to the standard with the exception that the conditioning was performed at 90 % RH rather than at 50 % RH.

[0049] In compositions 6-8 above, the concentration of citric acid is 1.3 wt. % based on the total aqueous composition. Previous experiments have shown that this concentration can be reduced to at least 0.65 wt. % without a major loss of efficiency. These experiments motivate that the amount of citric acid in the composition can be as low as 15 wt.% based on the total amount of glyoxal, polyvinyl alcohol and citric acid. However, an amount 25-40 wt.% is preferred.

[0050] In conclusion, treating the substrates with compositions comprising 45-75 wt.% glyoxal, 5-30 wt.% polyvinyl alcohol and 15-40 wt.% citric acid based on the total amount of glyoxal, polyvinyl alcohol and citric acid in the treatment composition such as compositions 6-8 gave rise to a significant increase in SCT especially at 90 % RH.

Examples

example 1

Example 1

[0039]A pilot trial was performed wherein the effect of the different components in the treatment composition was evaluated. Eight different treatment compositions were evaluated wherein the compositions comprised only glyoxal (compositions 1-2), glyoxal and PVOH (compositions 3-5) or glyoxal, PVOH and citric acid (compositions 6-8), see Table 1.

[0040]The PVOH used is Kuraray Poval 28 / 99 from Kuraray and has a degree of hydrolysis of 99.0-99.8.

[0041]For the pilot trial, Pure Performance ™< , a white top kraftliner having a grammage of 135 gsm was used as the substrate. The aqueous treatment compositions were coated onto the substrate using a pilot coater so that approximately 16 g / m 2< of the aqueous treatment composition is added to the substrate. The treated substrates were subjected to IR drying and heated air having a temperature between 180-235 °C. The treated substrates were rolled up into tambours and stored. The temperature during the roll-up and storage was 50-70 ...

TECHNICAL FIELD

[0001] The present disclosure relates to the field of treated containerboard or paperboard with improved mechanical properties at high humidity.BACKGROUND

[0002] Containerboard or paperboard is commonly used as packaging material due to its suitable mechanical properties and its versatility. However, a drawback of using containerboard or paperboard in packaging applications is its sensitivity towards moisture. In humid environments, a deterioration of the paperboards mechanical properties is observed. This may be observed as paperboard packages failing to maintain their structure and thereby lose their function. Humid environments are a major factor to consider during e.g. cold-chain logistics such as transport.SUMMARY

[0003] The present inventors have realized that there is a need to improve the mechanical properties, such as compressive strength, of containerboards and paperboards in humid environments.

[0004] According to a first aspect of the present disclosure, a method for preparing a containerboard or a paperboard is provided. The method comprises: a) providing a containerboard or paperboard substrate; b) providing a treatment composition comprising 45-75 wt.% glyoxal, 5-30 wt.% polyvinyl alcohol (PVOH) and 15-40 wt.% citric acid based on the total amount of glyoxal, polyvinyl alcohol and citric acid in the treatment composition; c) applying the treatment composition to the containerboard or paperboard substrate to obtain a treated containerboard or treated paperboard; and d) curing the treated containerboard or the treated paperboard.

[0005] Containerboard and paperboard will henceforth be referred to as board and containerboard or paperboard substrate will be referred to as board substrate.

[0006] The containerboard of the first aspect may for example be a linerboard, such as a kraftliner, e.g. a white top kraftliner. Alternatively, the containerboard may be intended to be used as a fluting.

[0007] Improvement of the compressive strength (short-span compression test, SCT) was observed when treating a substrate with a treatment composition comprising 45-75 wt.% glyoxal, 5-30 wt.% PVOH and 15-40 wt.% citric acid based on the total amount of glyoxal, PVOH and citric acid in the treatment composition and curing said treated board. The treatment improved the SCT of the treated board at 50 % relative humidity (RH) as well as at 90 % RH (see Table 1 in the Examples section). The improved mechanical properties at high humidity (e.g. 90 % RH) enables the board to better maintain its structure and function during use.

[0008] Whishing not to be bound by any theory, the improved mechanical properties are assumed to be due to the unique treatment composition creating cross-links between hydroxyl-functionalized polymer chains i.e. cellulose and PVOH, at a suitable density in order to improve the mechanical properties while minimally affecting the strain at break which usually decreases significantly as a result of cross-linking.

[0009] In one embodiment, the treatment composition may comprise 45-65 wt. % such as 50-60 wt.% glyoxal based on the total amount of glyoxal, PVOH and citric acid in the treatment composition.

[0010] In another embodiment, the treatment composition comprises 5-25 wt.%, such as 5-20 wt.%, such as 5-15 wt.% PVOH based on the total amount of glyoxal, PVOH and citric acid in the treatment composition. The proportional amount of PVOH may even be as low as 5-10 wt.% based on the total amount of glyoxal, PVOH and citric acid in the treatment composition.

[0011] In one embodiment, the treatment composition comprises 20-40 wt.% such as 25-40 wt.% citric acid based on the total amount of glyoxal, PVOH and citric acid in the treatment composition. The embodiments relating to the amounts of the different components can be combined in any way so that the amount of the three components is 100 wt.% based on the total amount of glyoxal, PVOH and citric acid.

[0012] In a preferred embodiment, the amount of glyoxal is 50-60 wt. %, the amount of PVOH is 5-15 wt. % and the amount of citric acid is 25-40 wt. % based on the total amount of glyoxal, PVOH and citric acid in the treatment composition.

[0013] The curing may comprise subjecting the treated board to IR drying and / or to air having a temperature of at least 150 °C, such as at least 160 °C, such as at least 170 °C. During this stage preferably both drying and curing occurs.

[0014] The curing may comprise subjecting the treated board to a temperature of at least 30 °C, such as 30-90 °C, such as 40-80 °C for at least 1 min. Preferably, the treated board is subjected to the elevated temperature during the roll-up into tambours and / or during the storage of the tambours.

[0015] In a preferred embodiment, the treated board is first subjected to IR drying and / or heated air followed by subjecting the treated board to an elevated temperature (at least 30 °C) during a prolonged time.

[0016] The total amount of glyoxal, PVOH and citric acid in the treatment composition may be 1-10 wt.%, such as 2-7 wt.%, such as 3-6 wt.%.

[0017] In one embodiment, the treatment composition may be an aqueous composition of glyoxal, PVOH and citric acid. The aqueous composition may comprise glyoxal, PVOH, citric acid and water in an amount of at least 95 wt. %, such as at least 98 wt.%. The treatment composition according to the first aspect may further comprise additives such as defoamers.

[0018] The board substrate may have a grammage of at least at least 60 g / m 2 , such as 100-350 g / m 2< , such as 115-300 g / m 2< . The upper limit may be 440 g / m 2< .

[0019] According to a second aspect, a board produced according to the first aspect is provided. It follows from the above that the board of the second aspect may be a linerboard, such as a kraftliner, e.g. a white top kraftliner, or an NSSC liner. Alternatively, the board of the second aspect may be a containerboard that is intended to be used as a fluting (i.e. intended to be corrugated) or that has already been corrugated such that it is a fluting.

[0020] The board of the second aspect may have a compressive strength index (short-span compression test, SCT index) in cross direction of at least 14 Nm / g at 50 % RH and at least 6 Nm / g at 90 % RH as determined by ISO 9895:2008.

[0021] The board of the second aspect may have a tensile strength index in machine direction of at least 40 Nm / g at 50 % RH and at least 20 Nm / g at 90 % RH as determined by ISO 1924-3:2005.

[0022] The board of the second aspect may further have a tensile stiffness index in machine direction of at least 5 kNm / g at 50 % RH and at least 2 kNm / g at 90 % RH as determined by ISO 1924-3:2005.

[0023] The ISO standards used to determine the SCT index, tensile strength index and tensile stiffness index disclose the determination of these properties at 50 % RH. The properties at 90 % RH were measured according to the respective standards with the exception that conditioning of the samples was performed at 90 % RH rather than at 50 % RH. The determination of the mechanical properties at 90 % RH was, hence, performed by following the procedures of the respective ISO standard but conditioning the samples at 90 % RH instead of 50 % RH.

[0024] According to a third aspect, there is provided a corrugated board in which a board according to the second aspect forms a layer. As know to the skilled person, a corrugated board is a material comprising at least one liner and at least one fluting.BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Aspects and embodiments are now described, by way of example, with reference to the accompanying drawings, in which: Fig 1-3 shows the SCT in cross direction at 50 % RH for the treated boards in Example 1 and their references. Fig 4-6 shows the SCT in cross direction at 90 % RH for the treated boards in Example 1 and their references. Fig 7-8 shows the tensile strength in machine direction at 50% and 90 % RH for boards 6-8 in Example 1 and their reference. Fig 9-10 shows the tensile stiffness in machine direction at 50% and 90 % RH for boards 6-8 in Example 1 and their reference. DETAILED DESCRIPTION

[0026] The moisture sensitivity of a board may cause issues such as loss of mechanical properties when exposed to higher humidity during e.g. cold chain logistics. By cross-linking the board, i.e., cross-linking of the cellulose chains, the moisture sensitivity may be reduced and the mechanical properties such as compressive strength (SCT) may be improved at increased humidity.

[0027] The present inventors have realized that treating a board substrate with a treatment composition comprising glyoxal, PVOH and citric acid and curing said treated board according to the first aspect, an improvement in compressive strength (SCT) can be obtained at both 50 % and 90 % RH.

[0028] The treatment composition according to the first aspect comprises a cross-linking agent in the form of a difunctional aldehyde, i.e., glyoxal. Glyoxal reacts with hydroxyl-groups and forms stable acetal-bonds. An acidic environment, the presence of proton donors, is necessary in order for the reaction between glyoxal and hydroxyl-groups to be driven fully to the stable acetal-bond. If the cross-linking composition is lacking proton donor, the reaction will lead to the formation of hemi-acetal bonds which are more susceptible to cleavage and hence less stable. Citric acid is chosen as the proton donor as it is biobased, readily available, and environmentally friendly. In addition to glyoxal and citric acid, PVOH is added to the cross-linking composition. PVOH will also have the possibility to react with glyoxal to form cross-links. The addition of PVOH may enable longer cross-links as well as promote interfibrillar cross-links which in the end will make the treated paperboard less brittle compared to a treated paperboard wherein the treatment composition did not comprise PVOH.

[0029] The treatment composition comprises 45-75 wt. %, preferably 45-60 wt. %, more preferably 50-55 wt.% glyoxal, 5-30 wt.%, preferably 5-25 wt.%, more preferably 10-15 wt.% PVOH and 15-40 wt.%, preferably 5-25 wt.%, more preferably 10-15 wt.% citric acid based on the total amount of glyoxal, PVOH and citric acid in the treatment composition. This ratio between the three active components enables using a lower total amount of active components while still obtaining an improvement in compressive strength. Reducing the amount of chemicals may reduce the environmental impact of the treatment and the cost of production of the treated board. The treatment composition may further have a minimal negative effect on the strain at break.

[0030] The treatment composition is preferably an aqueous composition. The total amount of glyoxal, PVOH and citric acid in treatment composition may be 1-10 wt.%, such as 2-7 wt.%, such as 3-6 wt.% of the total treatment composition. This concentration of active components (i.e. glyoxal, PVOH and citric acid) in the treatment composition enables improvement in SCT while minimizing the amount of chemicals used.

[0031] The treatment composition may be coated onto the board substrate by a coating means. The treatment composition may slightly impregnate the board substrate. It is advantageous to apply the aqueous treatment composition to the board substrate in an amount of at least 10 g / m 2< such as at least 12 g / m 2< , such as at least 14 g / m 2< . An upper limit of the applied amount of the aqueous treatment composition may be 22 g / m 2< . This application amount may lead to a particularly good improvement in compressive strength.

[0032] The board substrate may comprise at least one ply, such as at least two plies, such as at least three plies. The board substrate may further comprise conventional additives such as sizing agents, fillers and colouring agents, this is however optional. In a further embodiment, the board substrate may have a grammage of at least 60 g / m 2< , such as 100-350 g / m 2< , such as 115-300 g / m 2< . The upper limit may be 440 g / m 2< .

[0033] After the treatment composition has been applied to the board substrate, the treated paperboard is cured. The curing may comprise subjecting the treated board to IR drying and / or to air having a temperature of at least 150 °C, such as at least 160 °C, such as at least 170 °C. During this stage preferably both drying and curing occurs.

[0034] The curing may comprise subjecting the treated board to a temperature of at least 30 °C, such as 30-90 °C, such as 40-80 °C for at least 1 min. Preferably, the treated board is subjected to the elevated temperature during the roll-up into tambours and / or during the storage of the tambours. This step of the curing may last until the glyoxal and the hydroxyl-groups have reacted fully, e.g. up to 20 min. However, no adverse effect is obtained if this step is prolonged, i.e., longer than 20 min.

[0035] The treated board may be first subjected to IR drying and / or heated air followed by subjecting the treated board to an elevated temperature (at least 30 °C) during a prolonged time.

[0036] The treated board that is produced by using the inventive treatment composition has improved compressive strength compared to non-treated board.

[0037] The board may have: a. a compressive strength index (short-span compression test, SCT index) in cross-direction of at least 14 Nm / g at 50 % RH and at least 6 Nm / g at 90 % RH. The upper limit of SCT index in cross-direction may be 32 Nm / g at 50 % RH and 18 Nm / g in 90 % RH. b. a tensile strength index in machine direction of at least 40 Nm / g at 50 % RH and at least 20 Nm / g at 90 % RH. The upper limit of tensile strength index in machine direction may be 170 Nm / g at 50 % RH and 100 Nm / g at 90 % RH. c. a tensile stiffness index in machine direction of at least 5 kNm / g at 50 % RH and at least 2 kNm / g at 90 % RH. The upper limit of tensile stiffness index in machine direction is 14 kNm / g at 50 % RH and 10 kNm / g at 90 % RH.

[0038] The SCT index was determined according to ISO 9895:2008 and tensile strength and stiffness indices were determined according to ISO 1924-3:2005. These ISO standards disclose the determination of said properties at 50 % RH. The properties at 90 % RH were determined according to the respective standards with the exception that conditioning of the samples was performed at 90 % RH rather than at 50 % RH.EXAMPLES Example 1

[0039] A pilot trial was performed wherein the effect of the different components in the treatment composition was evaluated. Eight different treatment compositions were evaluated wherein the compositions comprised only glyoxal (compositions 1-2), glyoxal and PVOH (compositions 3-5) or glyoxal, PVOH and citric acid (compositions 6-8), see Table 1.

[0040] The PVOH used is Kuraray Poval 28 / 99 from Kuraray and has a degree of hydrolysis of 99.0-99.8.

[0041] For the pilot trial, Pure Performance ™< , a white top kraftliner having a grammage of 135 gsm was used as the substrate. The aqueous treatment compositions were coated onto the substrate using a pilot coater so that approximately 16 g / m 2< of the aqueous treatment composition is added to the substrate. The treated substrates were subjected to IR drying and heated air having a temperature between 180-235 °C. The treated substrates were rolled up into tambours and stored. The temperature during the roll-up and storage was 50-70 °C. The curing continued during the roll-up and storage.

[0042] Table 1. Different treatment compositions used in the pilot trial. "wt.% in aq." means the percentage of the component in the aqueous treatment composition. "wt. % of comp." means percentage of the total amount of components, i.e. the total amount of glyoxal, polyvinyl alcohol and citric acid. CompositionGlyoxalPVOHCitric acidwt. % in aq.wt. % of comp.wt. % in aq.wt. % of comp.wt. % in aq.wt. % of comp.11100----22100----32890.2411--42810.4819--52680.9532--62560.2471.33772530.48131.33482470.95221.331

[0043] The SCT of the treated substrates and their references were determined according to ISO 9895:2008 and can be seen in Table 2 and Fig. 1-6. The standard discloses the determination of SCT at 50 % RH, the measurement at 90 % RH were performed according to the standard with the exception that the conditioning was performed at 90 % RH rather than at 50 % RH.

[0044] Table 2. The difference in SCT at 50 % and 90 % RH between the treated substrates and their respective reference. Board 1 is treated with composition 1 and so on. BoardSCT 50 % RHSCT 90 % RH10 %4 %20 %4 %32 %8 %40 %8 %55 %3 %66 %14 %74 %14 %88 %12 %

[0045] As can be seen in Table 2 and Fig. 1-6, treating a substrate with only glyoxal gave rise to no improvement in SCT at 50 % RH and only a slight improvement at 90 % RH. Adding PVOH to the treatment composition resulted, in some cases, in a slightly larger increase in SCT at 50 % RH and 90% RH. Finally, the treatment composition comprising all three components, i.e. glyoxal, PVOH and citric acid, gave rise to the largest increase in SCT at both 50 % and 90 % RH.

[0046] The effect of the treatment on the SCT was most noticeable at 90 % RH.

[0047] Using different amounts of PVOH in the treatment compositions did not have a major impact on the SCT. This shows that a relatively low amount of PVOH can be used in the treatment composition, which leads to a cost reduction as well as a lowering of the environmental impact of the treatment composition.

[0048] Tensile strength and tensile stiffness were also determined for the substrates treated with the compositions 6-8 and their references. The treated substrates exhibited a slight increase in tensile strength and tensile stiffness compared to their references, see Fig. 7-10. The tensile strength and tensile stiffness were determined according to ISO 1924-3:2005. The standard discloses the determination of tensile properties at 50 % RH, the measurement at 90 % RH were performed according to the standard with the exception that the conditioning was performed at 90 % RH rather than at 50 % RH.

[0049] In compositions 6-8 above, the concentration of citric acid is 1.3 wt. % based on the total aqueous composition. Previous experiments have shown that this concentration can be reduced to at least 0.65 wt. % without a major loss of efficiency. These experiments motivate that the amount of citric acid in the composition can be as low as 15 wt.% based on the total amount of glyoxal, polyvinyl alcohol and citric acid. However, an amount 25-40 wt.% is preferred.

[0050] In conclusion, treating the substrates with compositions comprising 45-75 wt.% glyoxal, 5-30 wt.% polyvinyl alcohol and 15-40 wt.% citric acid based on the total amount of glyoxal, polyvinyl alcohol and citric acid in the treatment composition such as compositions 6-8 gave rise to a significant increase in SCT especially at 90 % RH.

Examples

example 1

Example 1

[0039]A pilot trial was performed wherein the effect of the different components in the treatment composition was evaluated. Eight different treatment compositions were evaluated wherein the compositions comprised only glyoxal (compositions 1-2), glyoxal and PVOH (compositions 3-5) or glyoxal, PVOH and citric acid (compositions 6-8), see Table 1.

[0040]The PVOH used is Kuraray Poval 28 / 99 from Kuraray and has a degree of hydrolysis of 99.0-99.8.

[0041]For the pilot trial, Pure Performance ™< , a white top kraftliner having a grammage of 135 gsm was used as the substrate. The aqueous treatment compositions were coated onto the substrate using a pilot coater so that approximately 16 g / m 2< of the aqueous treatment composition is added to the substrate. The treated substrates were subjected to IR drying and heated air having a temperature between 180-235 °C. The treated substrates were rolled up into tambours and stored. The temperature during the roll-up and storage was 50-70 ...

Claims

1. A method for preparing a containerboard or a paperboard comprising: a) providing a containerboard or a paperboard substrate; b) providing a treatment composition comprising 45-75 wt.% glyoxal, 5-30 wt.% polyvinyl alcohol and 15-40 wt.% citric acid based on the total amount of glyoxal, polyvinyl alcohol and citric acid in the treatment composition; c) applying the treatment composition to the containerboard or the paperboard substrate to obtain a treated containerboard or a treated paperboard; and d) curing the treated containerboard or the treated paperboard.

2. The method according to claim 1, wherein the treatment composition comprises 45-65 wt. % such as 50-60 wt.% glyoxal based on the total amount of glyoxal, polyvinyl alcohol and citric acid in the treatment composition.

3. The method according to any one of the preceding claims, wherein the treatment composition comprises 5-25 wt.% such as 5-20 wt.%, such as 5-15 wt. % polyvinyl alcohol based on the total amount of glyoxal, polyvinyl alcohol and citric acid in the treatment composition.

4. The method according to any one of the preceding claims, wherein the treatment composition comprises 20-40 wt.%, such as 25-40 wt.% citric acid based on the total amount of glyoxal, polyvinyl alcohol and citric acid in the treatment composition.

5. The method according to any one of the preceding claims, wherein the treatment composition is an aqueous composition.

6. The method according to any one of the preceding claims, wherein the total amount of glyoxal, polyvinyl alcohol and citric acid in the treatment composition is 1-10 wt.%, such as 2-7 wt.%, such as 3-6 wt.%.

7. The method according to any one of the preceding claims, wherein the curing comprises subjecting the treated containerboard or paperboard to IR drying and / or heated air having a temperature of at least 150 °C, such as at least 160 °C, such as at least 170 °C.

8. The method according to any one of the preceding claims, wherein the curing comprises subjecting the treated containerboard or paperboard to a temperature of at least 30 °C for at least 1 min.

9. The method according to any one of the preceding claims, wherein the containerboard or paperboard substrate has a grammage of at least 60 g / m2, such as 100-350 g / m2, such as 115-300 g / m2.

10. A containerboard or a paperboard produced according to any one of claims 1-9.

11. The containerboard or a paperboard according to claim 10 having a compressive strength index (short-span compression test, SCT index) in cross-direction of at least 14 Nm / g at 50 % RH and at least 6 Nm / g at 90 % RH as determined by ISO 9895:2008.

12. The containerboard or a paperboard according to any one of claims 10-11 having a tensile strength index in machine direction of at least 40 Nm / g at 50 % RH and at least 20 Nm / g at 90 % RH as determined by ISO 1924-3:2005.

13. The containerboard or a paperboard according to any one of claims 10-12 having a tensile stiffness index in machine direction of at least 5 kNm / g at 50 % RH and at least 2 kNm / g at 90 % RH as determined by ISO 1924-3:2005.

14. A corrugated board in which a containerboard or a paperboard according to any one of claims 10-13 forms a layer.

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