NZ532985A - Improvements relating to paperboard - Google Patents

Abstract

Disclosed is a method for reducing heat induced stickiness of coated paperboard during the production of corrugated board, comprising applying to the coated paperboard, prior to heating, one or more layers of a pigment coating, said pigment coating comprising a bonding agent and a plate-like mineral filler, wherein the amount of plate-like mineral filler in said pigment coating is between about 75% to about 99.5% by weight.

Description

New Zealand Paient Spedficaiion for Paient Number 532985 53 2$ 8 5 NEW ZEALAND PATENTS ACT, 1953 COMPLETE SPECIFICATION After Provisional Nos 532985 dated 18 May 2004 532987 dated 18 May 2004 533091 dated 21 May 2004 IMPROVEMENTS RELATING TO PAPERBOARD We, NEW ZEALAND FOREST RESEARCH INSTITUTE LIMITED, of Sala Street, Rotorua, New Zealand, do hereby declare this invention to be described in the following statement: i^llectual property i or iy.z 1 8 MAY 2005 -Be C E i \/ p p office] IMPROVEMENTS RELATING TO PAPERBOARD This invention relates to coating paper, paperboard or other wood-fibre products (herein referred to as paperboard) for the purpose of controlling the runnability of coated paperboard such as in the corrugating process. This invention also provides a barrier against such substances as water vapour. The invention also relates to heat treatment of coated paperboard for the purpose of further increasing the said barrier properties.

Paperboard is used as a component for various packaging applications. All of those applications require the packaging to maintain its structural integrity and to protect the contents inside the packaging during use. In some cases this is not possible without adding a protective barrier to the paperboard.

Traditionally, extrusion coating with polyethlyene (PE), and other polymer resins, has been used to form barriers in or on paperboard which has enabled adequate structural integrity to be maintained. Extrusion coating, however, has several drawbacks: • high manufacturing costs due to limiting operating speeds, compared with online dispersion coating, due to low mechanical strength of the molten polymer • laminated paperboard is not repulpable with conventional equipment or recyclable in normal paper mill recycling systems • consumer/legislative drive for environmentally acceptable packaging materials (sustainable, recyclable, biodegradable) resulting in PE coated paperboard being increasingly regarded as waste, especially in Europe.

Corrugated board is a paperboard structure formed from two or more paperboard facings, called liners or linerboard, and one or more corrugated paperboard medium, or fluting. Corrugated board is used extensively in NZ for manufacturing boxes for the storage and transportation of horticultural produce. iNTEilECn^LPR^^ office of n z- 27 APR 2087 p F C EI intellectual property office of n.z. 2 7 APR 2007 "3" I R EC EIV E D Conditions in the service environment are severe with high and variable relative humidity which significantly reduce the mechanical strength and performance of a corrugated box. Research has shown that box failure is predominantly by compressive creep mechanisms that are exacerbated by elevated and cycling humidity environments.

In the production of corrugated board, heat treatment of the said coated paperboard occurs at hotplates designed to dry the glueline between the single facer and the double backer liner. A typical temperature for the hotplates is approximately 180°C with a typical dwell time in the hot plate section of 1-5 seconds. Heat treatment of the coated paperboard also occurs in the preheating of the single facer liner.

The performance of the hot plate section is critical to the runnability of the corrugating process. Runnability is a measure of how easy it is to operate a papermaking or corrugating or similar line or machine. Runnability means the smoothness of the (corrugating) operation. Runnability is good when there are no or minimal interruptions in the production process and the production capacity can be fully utilised.

In the case of a corrugator, good runnability means that the production speed can be kept high and there are no web breaks, imperfect end products or similar faults. Typically, if the speed difference between the single facer and double backer liner is more than 1 m/min, severe runnability problems occur.

When coated paperboard is used in the making of corrugated board, the high temperatures the coating is subjected to can result in stickiness of the coating and adherence to equipment surfaces. This can cause sudden variations in the double backer liner speed and the runnability of the corrugating process is interrupted.

It is therefore an object of the invention to at least go some way to overcoming the above mentioned disadvantages, or to at least provide the public with a useful choice.

It is acknowledged that the term 'comprise' may, under varying jurisdictions, be attributed with either an exclusive or an inclusive meaning. For the purpose of this specification, and unless otherwise noted, the term 'comprise' shall have an inclusive meaning - i.e. that it will be taken to mean an inclusion of not only the listed components it directly references, but also other non-specified components or elements. This rationale will also be used when the term 'comprised' or 'comprising' is used in relation to one or more steps in a method or process.

We have also now found that the water vapour transmission rate of paperboard can be reduced by coating the paperboard with a coating material which comprises an effective amount of kaolin or similar hydrous phyllosilicate such as other clays. The immediate benefit of using kaolin or a similar hydrous phyllosilicate is that the coating can be applied by the use of dispersion coating technology which is much less costly and cumbersome than the extrusion/laminating technique employed with polyethylene.

We have further found that paperboard can be acceptably improved in strength against damage caused by compressive creep mechanisms by dispersion coating the paperboard with a coating material which includes an effective amount of kaolin or similar hydrous phyllosilicate such as other clays to reduce the water vapour transmission rate (WVTR).

Dispersion coatings, in accordance with the invention are latex-containing dispersions, usually in water and which can contain filler particles and other additives, and can be applied to the paperboard surface to provide a solid, uniform and nonporous film with certain barrier properties.

Dispersion coating technology has the following advantages over extrusion coating dispersion coatings are usually water-based formulations that can be applied using existing coating equipment (pigment coating lines) and readily available materials - hence, no major capital investment in new equipment required manufacturing costs are lower than extrusion coated paperboard since machine speeds are up to 10 times greater with no major issues with runnability • dispersion coated paperboard is repulpable and recyclable. fi 2 7 APR 2007 received: Dispersion coatings can also be used to protect the packaging contents from the ingress of water vapour and liquid water such as in the case of dried food ingredients and powders, washing powders and detergents, and copy paper.

According to one aspect the invention provides a method for reducing heat induced stickiness of coated paperboard during the production of corrugated board, comprising applying to the coated paperboard, prior to heating, one or more layers of a pigment coating, said pigment coating comprising a bonding agent and a plate-like mineral filler, wherein the amount of plate-like mineral filler in said pigment coating is between about 75% to about 99.5% by weight. Preferably the pigment coating is applied prior to heating of the coated paperboard during the adhesive fixing stage of the production of corrugated board.

In one embodiment the coated paperboard comprises a water-vapour barrier coating. Accordingly, there is also provided a method of reducing the water vapour transmission rate (WVTR) of paperboard comprising applying a coating to the paperboard of a mineral filler such as kaolin or a similar hydrous phyllosilicate together with a bonding agent.

In a further aspect of the invention the water vapour transmission rate is reduced to 50 gsm/day or below.

In preferred embodiments the water-vapour barrier coating comprises one or more layers of a dispersion coating.

In a further aspect of the invention there is provided a method of reducing the damage to the structural integrity of paperboard, using the coating method of the invention.

In a further aspect of the invention there is provided a method of reducing the damage to the structural integrity of paperboard caused by compressive creep, using the coating method of the invention.

The invention further provides a coated paperboard produced by such a method.

The paperboard when coated can be made printable and as well the final coated paperboard will generally be recyclable. Laboratory compression creep testing has shown that these coatings have the potential to allow the down-weighting of paperboard packaging by up to 40% (by weight). nntilectuarp^EBTNl 1 1 office of n.^ I 2 7 APR 2007 received] In preferred embodiments this invention provides water-based dispersion coating for paperboard consisting of a filler containing kaolin or a similar hydrous phyllosilicate together with an effective amount of bonding agent and optionally other additives.

The method and coating can provide protection of the structural integrity of the paperboard in high humidity environments. The amount of kaolin or a similar hydrous phyllosilicate can range up to 80% of the total weight of the coating solids. These materials preferably have a plate-like structure, such that the aspect ratio is between about 5-150, preferably greater than 30 and an average particle size of preferably about 0.5 to 5 microns or less, more preferably about 2 microns or less.

A suitable bonding agent is a water-based latex of styrene/butadiene or styrene/acrylate copolymers, but the invention is not limited to these latexes. It is preferably a styrene/butadiene latex having a glass transition point of 0-35 °C.

Other additives can include, but not be limited to, dispersants, colouring agents, crosslinkers, slipping agents, defoamers, pH control agent, plasticizers, thickeners and water-retention aids.

The kaolin or a similar hydrous phyllosilicate is normally dispersed in a water to form a homogeneous dispersion. The latex and additives are added to this dispersion and mixed to form a homogenous coating.

The invention is based on the application of the coating to paperboard for the purpose of providing a barrier layer to paperboard, for improving the water vapour barrier of the paperboard. The coating can be applied to paperboard by using coating techniques known in the art, including but not limited to, air-knife, blade, rod, spray, curtain coating, surface sizing, film coating, and flexographic and gravure printing. The same paperboard can be surface sized prior to application of said coating formulation. The coating can be applied as a single or multiple coating. Subsequent coating formulations, such as pigment coatings, can also be applied to impart other properties.

The coating is dried at elevated temperatures to induce film formation in the coating and to evaporate excessive water from the coating and the substrate.

The coated paperboard can be converted into various packaging materials, including corrugated board packaging used in the horticultural industry. It will also find uses where paperboard is subjected to stress for a significant period of time. The afore-mentioned coating reduces the water vapour transmission rate of the paperboard, which increases the mechanical strength of the packaging material in high and varying humidity conditions. In particular, the paperboard compression creep response is reduced, especially in a high and/or varying humidity, low temperature environments.

The invention also envisages barrier lamination of two or more coatings using the aforementioned coating materials and method.

The invention also provides the application of at least one dispersion coating and at least one pigment coating to paperboard for the purpose of providing a water vapour barrier on said paperboard and to increase runnability of coated paperboard especially in the production of corrugated board.

Further in accordance with other aspects of the invention there is provided a method for improving the water vapour barrier properties of a paperboard coated with one or more layers of a dispersion coating such as a coating containing kaolin or a similar hydrous phyllosilicate comprising applying as an external surface one or more layers of a pigment coating. The said coated paperboard can be exposed to heat to further increase the barrier properties.

Further in accordance with the invention there is provided a coated paperboard having one or more layers of a dispersion coating and as an external surface one or more layers of a pigment coating.

The invention provides combinations of a water-based dispersion coating and a water-based pigment coating that may contain bonding agent, filler and additives.

A suitable dispersion coating consists of about 20-100% latex, such as but not limited to styrene-butadiene and styrene acrylate copolymers, about 0-80% (such as 30 to 70%) of mineral filler such as but not limited to talc and kaolin, and additives such as but not limited to waxes, dispersing agents and pH adjusting agents. All percentages throughout the specification are expressed by weight.

A suitable pigment coating consists of about 75-99.5% mineral pigment, comprising but not limited to hydrous phyllosilicates such as kaolin, talc, calcium carbonate, gypsum, titanium dioxide, organic water-soluble fillers such as modified starch, chitin and polystyrene, about 0.5-25% bonding agent or a combination of bonding agents, such as but not limited to latex, starch, CMC (carboxy methyl cellulose) and PVOH (polyvinyl alcohol), and additives, such as but not limited to dispersing agents, pH adjusting agents, slipping agents and cross-linkers.

The processes to produce dispersion and pigment coating of the invention can utilise techniques known in the art.

The dispersion and pigment coating processes of the invention can utilize coating techniques known in the art including but not limited to air-knife, blade, rod, spray, curtain coating, surface sizing, film coating, and flexographic and gravure printing.

The paperboard can be surface sized prior to application of said coating formulations.

The dispersion coating structure can consist of one or more base coating layers and a topcoating layer or multiple topcoating layers. The base coating layer or layers can consist of a water-based dispersion coating and the topcoating layer or layers can consist of water-based pigment coating. The base coating(s) are applied on paperboard prior to the top coating or top coatings. Each coating layer is usually at least partly dried before a subsequent coating layer is applied. The coated sheet so produced is herein referred to as 'dispersion and pigment coated paperboard'.

The bonding agent of the invention forms a continuous film during drying of the coating layer. It also binds the coating into the paperboard and forms bonds between the filler particles. The bonding agent can be any suitable latex, which is capable of film formation. iNTEuiCTG^PROPERTY office of n.z. 2 7 APR 2807 received.

For cost reasons, latexes as used in pigment coating, will normally be chosen. Latexes that form a suitable water vapour barrier are styrene-butadiene (SB) and styrene-acrylate latexes. There are a number of different chemical companies supplying SB latexes. The preferred latexes are DL966 and DL925 from Dow Chemicals. A suitable latex for use in the invention are those providing a WVTR of less than 50 gsm/day at a coat weight of 11 gsm, at 23°C and 50% rh.

The preferred fillers of this invention to reduce the WVTR are kaolin or similar hydrous phyllosilicate such as other clays. Mined kaolin is normally processed into many grades some of which are suitable grades for use in the invention, as detailed above. This invention also contemplates the use of other plate-like fillers having characteristics similar to those of the preferred form of kaolin, as discussed below, and which will equally be useful in this invention.

The particle size distribution and particle shape of the fillers are of significance. An average particle size of about 0.5 to 5 microns, based on Sedigraph measurements, is preferred. Particle shape or aspect ratio, which is the ratio between particle length and height is of major importance. A high aspect ratio between about 5-150 is desirable, with a particle size preferably less than about 2 microns, for fillers for use in the current invention.

Generally the filler needs to have a narrow particle size distribution, a plate-like structure and an average particle size of about one third of the thickness of the coating film to achieve the preferred barrier effect. Selection of the best filler for use in the current invention is based on the source of the filler, the milling process, and resultant average particle size and particle size distribution, all balanced against cost.

Further mineral fillers can be added having characteristics of the fillers described above but also including hydrophobic plate-like substances such as talc.

The filler (kaolin plus other mineral fillers if desired) is dispersed in water using a high shear mixer to break the agglomerates down into individual particles. Three stages are involved in dispersion: wetting of the filler, disintegration of the agglomerate and stabilisation of the ImiilicT^LjTOPER^ OFFICE OF N.Z. 2 7 APR 2007 RECEIVER. dispersion. Initially, a dispersant is added to water in a mixing vessel. The pH can be adjusted with NaOH, if required. The filler is gradually added to the water/dispersant mixture, more rapidly at first and more slowly as the solids content and viscosity increases. Once all the filler has been added, mixing should be continued for 15-30 minutes. The latex and additives are then added to the dispersed filler and mixed until a homogeneous coating results.

A suitable dispersant for use in the invention is a sodium salt of a polyacrylic acid such as Dispex N400 from Ciba Specialty Chemicals, Switzerland.

The coating materials are generally pre-mixed before being applied to the paperboard. The aim is to achieve the minimum WVTR with a minimum amount of coating material to reduce costs. Coat thickness variations should be kept as low as possible. The coating materials can be applied in one or more layers, with the first coating being normally dried before the second coating is applied. Drying of the coating does have a significant impact on the WVTR, with drying at higher temperatures providing a significant improvement in the WVTR of the dried coat. It is clearly within the scope of the invention for drying conditions to be chosen to enable evaporation of all the moisture to achieve an acceptable barrier control. Hence temperatures between 50-250°C can be used. Coat weights will typically be between 0.5-30 gsm, but preferably 5-15 gsm. Multiple layers provide optimum barrier properties. Multiple coatings can be used to incorporate different formulations in each layer. For example, a cheap base coat can be applied to reduce porosity and increase the smoothness of the base paper, a second layer to provide the effective barrier properties and a third layer to improve printability, glueability and runnability in subsequent operations. Normal pigment coating lines for paperboard can be used.

The effectiveness of a coating to increase the strength of paperboard in certain environments is gauged in accordance with the invention by the water vapour transmission rate (WVTR). WVTR measurements in this specification are made by the gravimetric cup method according to Australian Standard method AS 1301.419s-89. Conditions used this specification are 23°C, 50% rh and 3.5°C, 95% rh. The coated side of the samples is exposed to humid, controlled atmosphere. Anhydrous calcium chloride is used as a desiccant.

Pre-sizing of the paperboard with a sizing agent can in some circumstances reduce the WVTR. Polyvinyl alcohol (PVOH) is a well known sizing material. At high relative humidities (eg 95%) pre-sizing with PVOH has limited effect. However, at lower relative humidities (for example 50%) and a temperature of 23°C pre-sizing with PVOH can reduce WVTR of a later applied dispersion coating. Elvanol 90-50, from DuPont, is a fully hydrolyzed PVOH that can be used.

Compression creep is a phenomenon that occurs in paperboard, and boxes made there-from, which are subjected to compressive stresses. It is accelerated by water vapour transmission into the paperboard when exposed to changes in humidity, such as in a cool store environment. Such changes even to a small extent accelerate creep. It has been found that reduction of WVTR to 50 gsm/day is a useful guide to determining an effective dispersion coating. The lower the value of the WVTR the better. The phenomenon of creep and the mechanism for its measurement is described in a paper by D G Morgan "A Mechanistic Creep Model and Test Procedure", which is herein incorporated by reference, (paper presented at the 57th Appita Annual Conference, Melbourne, Australia, 5-7 May 2003).

The dispersion coating can be used as a glue to laminate paperboard sheets together. The coating formulation is applied as a single layer to the base sheet, a second sheet is applied on top of the coating and the sandwich is pressed and dried. The resulting paperboard laminate can have superior water vapour barrier properties to a traditional top-coated product, depending on the selection of the top ply.

Dispersion coating formulations are comparatively similar to pigment coating formulations; both are water-based dispersions containing filler particles, latexes, or bonding agents, and other ingredients in various amounts. Coating viscosities are in the same range in both cases.

Dispersion and pigment coatings differ in the proportion of components. The mineral filler content of pigment coatings is typically 80% to 90% of the dry formulation weight. The purpose of the bonding agent latex in the pigment coating is to cement the pigment particles firmly to the paper surface and to each other. Thus a pigment coated surface is a porous structure of pigment particles with a high degree of light scattering producing a surface with improved gloss, colour, printing detail and brilliance.

Dispersion coatings have a much lower mineral filler content - typically 30-50% of the dry formulation weight- and a much higher proportion of latex. The purpose of the high proportion of bonding agent latex employed is to form a continuous, nonporous barrier film on the paper surface. This provides the impervious barrier to, for example, water vapour and/or other gases. The same latexes designed for pigment coating can be successfully used in dispersion coating.

Corrugated board is a paperboard structure formed from two or more paperboard facings, called liners or linerboard, and one or more corrugated paperboard medium, or fluting. Corrugated board is used for making corrugated boxes and other packaging.

As stated, the formulations of the invention comprise a latex, such as styrene butadiene (SB), usually with a filler, such as kaolin clay or talc, and additives, such as dispersants and pH control agents dispersed in water.

For example, a typical dispersion coating composition comprises 20-99.5% styrene butadiene (SB) latex, 0.5-80% kaolin clay or talc, 0.005-0.5% dispersant and 0.005-1% pH control agent. Preferably, the SB latex content is 30-75%, most preferably 35-70%. The kaolin clay or talc content is preferably 25-70%, and most preferably 30-65%. All percentages are based on the total weight of solids in the coating. The balance of the coating comprises water in such an amount that the final coating solid content is 45-70%, by weight.

The preferred latex is a styrene butadiene latex such as DL966 from Dow Chemicals, Australia. The preferred is kaolin clay is Eckalite 2 from Imerys Minerals, Australia.

Further aspects and advantages will become apparent by the following Examples, which are not intended to limit the scope of the invention. iNTBleCTUAr^ROPE^ office of m.2 27 APR 2007 nriticn Experimental Example 1 Kaolin dispersion lg of 10% sodium hydroxide solution and 0.75g Dispex N400 was added to 50.54 g water in a mixing vessel. The contents were mixed at 1500-3000 rpm for 2 minutes, after which filler was added. 101.32g of Eckalite 2 clay was slowly added to the vessel over a period of 15 minutes whilst mixing continued. The result was a kaolin slurry of about 65% solids content.

Dispersion coating preparation and application The contents were then mixed for a further 10 minutes. 198.4g of styrene-butadiene latex DL 966 from Dow Chemicals, Australia, was added to the kaolin slurry and mixed for 15 minutes.

The resulting coating was applied to a kraft linerboard of 160 gsm using a laboratory rod coating unit from RK Print Coat. The coating rod used was a wire-wound Meyer rod having a theoretical wet film deposit of 10 microns. The coated sheet was dried in a laboratory air-circulated oven at 150°C for 1 minute. The dried sheet was removed from the oven, cooled to ambient temperature for 5 minutes, and aforementioned sequence was repeated on the coated surface, resulting in a double-coated paperboard sheet.

The coated paperboard sheet was conditioned at 23°C, 50% rh for 24 hours. The coatweight was determined by the weight gain per unit area of the original material. The dry coatweight was 13-17 gsm.

The aforementioned dispersion coating can also be used as a glue to laminate paperboard sheets together. The coating formulation is applied as a single layer to the base sheet, a second sheet is applied on top of the coating and the sandwich is pressed and dried.

Example 2 Coated paperboard testing Water vapour transmission rates (WVTR) of the coated paperboard of Example 1 were measured according to Australian Standard method AS 1301.419s-89 in 3.5°C, 95% rh and 23°C, 50% rh. Typical WVTR rates were 10-30 gsm/day. Corrugated board panels formed from the same coated paperboard were tested in compression creep, the procedure for which is described in a paper by D G Morgan "A mechanistic creep model and test procedure" (paper presented at the 57th Appita Annual Conference, Melbourne, Australia, 5-7 May 2003). The coated paperboard was glued to fluting so that the coated paperboard surface formed the outer surface of the corrugated panel. The compression creep performance of the said corrugated panel was superior to that of an uncoated corrugated board of the same materials and construction, and equal to that of an uncoated corrugated board of similar materials of 67% higher grammage than the coated corrugated board, based on fibre only. This infers that a coated corrugated board can be reduced in grammage by 40% and maintain the same compression creep performance.

Hence, this invention provides a method of coating a paperboard that not only improves the strength of coated paperboard but improves its resistance to compression creep. This has potential to reduce the amount of paperboard needed in boxes. The external surface of the box can be printable and the whole box can be repulpable.

Example 3 Talc dispersion .4g of water was added in a mixing vessel and the mixing was started. 60 g of predispersed talc (90.4% solids), from Luzenac, France was slowly added to the mixing vessel while mixing at 1500-3000rpm. After all the talc had been added, mixing was continued for a further 10 minutes to obtain a homogenous talc dispersion.

Dispersion coating containing talc: 90g of the said talc dispersion was added into a mixing vessel and mixing was started. 162.3g of styrene-butadiene latex DL 966 from Dow Chemicals was added into the vessel and mixing was continued for 10 minutes to obtain a homogenous dispersion.

The above dispersion was then applied to the kraft linerboard following the method of Example 1.

A pigment coating containing kaolin can be applied to the dispersion coated kraft linerboard by a similar method as described in Example 4 below.

Example 4 Filler dispersion (kaolin slimy) 2 g of 10% sodium hydroxide and 1.5 g of Dispex N400 (a dispersing agent which is a sodium salt of a polyacrylic acid) from Ciba Specialty Chemicals, Switzerland, was added to 101.08 g of water in a mixing vessel. The contents were mixed at 1500-3000 rpm for 2 minutes, after which addition of filler was started. 202.64 g of Eckalite 2 kaolin from Imerys Minerals, Australia was slowly added to the vessel over a period of 15 minutes whilst mixing was continued. After the addition of filler was completed, the contents were mixed for a further 10 minutes. The result was a kaolin slurry of about 65% solids content.

Dispersion coating preparation 101.3 g of said kaolin slurry was added into mixing vessel and the mixing was started at 1500-3000 rpm. 198.4 g of styrene-butadiene latex DL 966 from Dow Chemicals, Australia, was added to said kaolin slurry and mixing was continued for a further 15 minutes.

Pigment coating preparation 72.0 g of said kaolin slurry was added into a mixing vessel and the mixing was started at 1500-3000 rpm. 12.82 g of DL 966 was added to said kaolin slurry and mixing was continued for a further 15 minutes.

Application of coatings Said dispersion coating was applied to a kraft linerboard of 160 gsm using a laboratory rod coating unit from RK Print Coat, England. The coating rod used was a wire-wound Meyer rod having a theoretical wet film deposit of 10 (am. The coated sheet was dried in a laboratory air-circulation oven at 150°C for 1 minute. The dried sheet was removed from the oven, let to cool to ambient temperature, and aforementioned sequence was repeated on the coated surface, resulting in a double-coated linerboard sheet. The said pigment coating was then applied to the coated side of said coated sheet using a wire-wound Meyer rod having a theoretical wet film deposit of 10 fim. The coated sheet was dried in a laboratory air-circulation oven at 150°C for 1 minute, resulting in a single-side, triple-coated linerboard sheet.

Heat treatment of the coated paperboard Said triple-coated linerboard sheet was placed in a laboratory hotplate dryer set to 180°C for 3 seconds.

Testing of coated paperboard Water vapour transmission rates (WVTR) were measured according to Australian Standard method AS 1301.419s-89 in 3.5°C, 95% RH. Typical WVTR rates were 25-30 g/m2/24h for the dispersion coated paperboard, 20-30 g/m2/24h for the dispersion and pigment coated paperboard without heat treatment and 10-20 g/m2/24h for the heat treated dispersion and pigment coated paperboard.

The liquid water barrier of coated paperboard was tested using 30 minute Cobb test according to the Australian Standard method AS 1301.41 ls-89. Typical Cobb results were 22-25 g/m2 for the dispersion coated paperboard, 10-12 g/m2 for the dispersion and pigment coated paperboard without heat treatment and 5-7 g/m for the heat-treated dispersion and pigment coated paperboard.

The blocking tendency of the coated paperboard was tested using a laboratory hotplate set to various temperatures. The blocking tendency was evaluated qualitatively as the difficulty in separating the coating layer from the hot plate surface or opening the samples that were folded coated side together. A slight blocking produces a faint cracking noise while severe blocking is similar to sealing. The single-side coated paperboard sheet was placed between the hotplates as unfolded, coated side folded together and uncoated side folded together and was kept between the hotplates for one minute. The dispersion coated paperboard showed slight blocking at 75°C and severe blocking at 90°C for the samples folded coated side together. The dispersion coated paperboard samples that were unfolded or folded uncoated side together showed slight blocking between the coating and hotplate surface at 110°C and more severe blocking at 180°C.

The dispersion and pigment coated paperboard, with or without heat treatment, showed no blocking at temperatures of 180°C and below.

The invention can reduce the stickiness of a dispersion coating layer when subjected to high temperatures, such as in manufacturing corrugated board, to acceptable levels. Furthermore, the invention provides a barrier coating structure, which can achieve markedly increased barrier properties when subjected to heat.

While the invention has been described with reference to preferred embodiments, it is not to be construed as limited thereto. Moreover, where specific materials and steps in the processes are described and a known equivalent exists thereto, such equivalents are incorporated herein as if specifically set forth, without departing from the scope as defined in the appended claims.

Claims (34)

WHAT WE CLAIM IS:

1. A method for reducing heat induced stickiness of coated paperboard during the production of corrugated board, comprising applying to the coated paperboard, prior to heating, one or more layers of a pigment coating, said pigment coating comprising a bonding agent and a plate-like mineral filler, wherein the amount of plate-like mineral filler in said pigment coating is between about 75% to about 99.5% by weight.

2. A method as claimed in claim 1 wherein the pigment coating is applied prior to heating of the coated paperboard during the adhesive fixing stage of the production of corrugated board.

3. A method as claimed in claim 1 or claim 2 wherein the plate-like mineral filler comprises one or more of hydrous phyllosilicates including but not limited to kaolin, talc, calcium carbonate, gypsum, titanium dioxide, organic water-insoluble fillers including but not limited to modified starch, chitin and polystyrene.

4. A method as claimed in any one of claims 1-3 wherein the plate-like mineral filler has an aspect ratio of between about 5 to about 150.

5. A method as claimed in any one of claims 1-4 wherein the plate-like mineral filler has an aspect ratio of between about 30 and about 150.

6. A method as claimed in any one of claims 1-5 wherein the plate-like mineral filler has a particle size of about 5 jam or below.

7. A method a claimed in any one of claims 1-6 wherein the plate-like mineral filler has a particle size of about 2(j,m or below.

8. A method as claimed in any one of claims 1-7 wherein the bonding agent is a water-based latex.

9. A method as claimed in claim 8 wherein the latex comprises a styrene/t 873982 1.DOC 2 7 APR 2007 received. -19-

10. A method as claimed in any one of claims 1-9 wherein the amount of plate-like mineral filler in the pigment coating is between about 80% to about 90% by weight.

11. A method as claimed in any one of claims 1-10 wherein the amount of bonding agent in the pigment coating is between about 0.5% to about 25% by weight.

12. A method as claimed in any one of claims 1-11 wherein the amount of bonding agent in the pigment coating is between about 0.5% to about 20% by weight.

13. A method as claimed in any one of claims 1-12 wherein the pigment coating increases the runnability of paperboard.

14. A method as claimed in any one of claims 1-13 wherein the pigment coating decreases the blocking of paperboard.

15. A method as claimed in any one of claims 1-14 wherein the coated paperboard comprises a water-vapour barrier coating.

16. A method as claimed in claim 15 wherein the water vapour transmission rate of the coated paperboard is below 50 gsm/day.

17. A method as claimed in claim 15 or claim 16 wherein the moisture-vapour barrier coating reduces the damage to the structural integrity of paperboard.

18. A method as claimed in claim 17 wherein the moisture-vapour barrier coating reduces the damage to the structural integrity of paperboard caused by compressive creep.

19. A method as claimed in any one of claims 15-18 wherein the moisture-vapour barrier coating comprises one or more layers of a dispersion coating.

20. A method as claimed in claim 19 wherein the dispersion coating comprises a plate-like mineral filler and a bonding agent. 873982 1.DOC intellectual property office of n.z. 2 7 APR 2007 BFCEIVED

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31.

32, 21X -20- A method as claimed in claim 20 wherein the amount of plate-like mineral filler in the dispersion coating is between about 0% and about 80% by weight. A method as claimed in claim 20 or claim 21 wherein the amount of plate-like mineral filler in the dispersion coating is between about 25% to about 70% by weight. A method as claimed in any one of claims 20-22 wherein the amount of plate-like mineral filler in the dispersion coating is between about 30% to about 50% by weight. A method as claimed in any one of claims 20-23 wherein the dispersion coating plate-like mineral filler comprises a hydrous phyllosilicate including but not limited to kaolin. A method as claimed in any one of claims 20-24 wherein the plate-like mineral filler has an aspect ratio of between about 5 to about 150. A method as claimed in any one of claims 20-25 wherein the plate-like mineral filler has an aspect ratio of between about 30 and about 150. A method as claimed in any one of claims 20-26 wherein the plate-like mineral filler has a particle size of about 5 |am or below. A method a claimed in any one of claims 20-27 wherein the plate-like mineral filler has a particle size of about 2pm or below. A method as claimed in any one of claims 20-28 wherein the dispersion coating bonding agent is a water-based latex. A method as claimed in claim 29 wherein the latex comprises a styrene/butadiene copolymer. A method as claimed in any one of claims 19-30 wherein the paperboard is dried at an elevated temperature following application of the dispersion coating. Coated paperboard produced by a method as claimed in any one of claims 1-31. ——> OF N.Z. 1 \ 27 APR 2007 \ \RECEi^^ -21 -

33. A method as defined in claim 1 substantially as herein described with reference to any example thereof.

34. Coated paperboard as defined in claim 30 substantially as herein described with reference to any example thereof.