Technical field The present invention is within the field of paper or board or similar cellulosic materials used as wrapping and packaging materials. More specifically the invention relates to a novel process for the treatment of such a cellulosic material to impart thereto water vapour barrier characteristics to make said material useful as a wrapping and packaging material for moisture sensitive articles, e.g. copying or printing quality papers, certain food¬ stuffs or similar. At the same time, however, the treated material shows repulpability properties of such a nature that it can be recycled as a waste material to a paper or board production line without prior screening or sorting being necessary.

The invention also relates to the material treated by said process as well as to the coating agent utilized in said treatment process.

Background of the invention

Although the present invention is generally useful in many applications it will in the following be described with reference to the wrapping of fine quality papers.

Fine paper qualities have to be wrapped in a wrapping material having such a moisture or water vapour imperme¬ ability that the wrapped papers remain more or less at a constant relative humidity for lengthy periods of time. Today the dominating wrapping material for this purpose is a composite material where polyethylene has been extruded onto paper.

However, with the advent of regulations or laws being passed on the subject of preservation of our natural re- sources, highlighting especially on recycling of paper and similar materials, polyethylene-extruded paper is said to be far from an ideal material. Thus, polyethylene-extruded

paper can be rendered recyclable only after difficult, time and money consuming screening processes. A direct recycling of said wrapping material is, thus, impossible as the use thereof in a paper manufacturing process will indispensably give a paper quality that is deficient or said use may even cause a production failure.

Therefore, recently new wrapping materials have been suggested, but up to now no real alternative to polyethy¬ lene-coated or -extruded paper has been found on the mar- ket, the main reasons being that new processes for obtai¬ ning water-vapour impermeable paper are complex, time con¬ suming and/or expensive or that a proper balance between water vapour permeability characteristics and recyclabi- lity properties has not been possible to reach. As will be described below, the present invention is based on the use of a poly(vinyl alcohol)-based material as a coating on paper inspite pf the fact that poly(vinyl alcohol) is known to be a water sensitive material. In this context it should be noted that although poly(vinyl alcohol) has been suggested for use in connection with coatings of the type referred to said poly(vinyl alcohol) has been utilized merely as a primer coating or similar, the proper water vapour barrier characteristics being accomplished by means of additional coating materials de- posited on top of said poly(vinyl alcohol). As prior art relating to poly(vinyl alcohol) as minor ingredients only of surface coating materials for paper materials within this technical field EP Al 393 451 and EP Al 487 497 can be referred to.

Description of the invention

By the present invention it has unexpectedly become possible to utilize a poly(vinyl alcohol)-based compo¬ sition as a surface coating for paper and similar mate- rials to obtain a wrapping of the type referred to above, which wrapping has a unique combination of water vapour barrier and recyclability properties. Thus, inspite of the

fact that poly(vinyl alcohol) is per se a water sensitive substance, the paper treated with said poly(vinyl alcohol) coating composition has been made moisture vapour imper¬ meable to such a degree that it matches polyethylene- coated paper as to water vapour permeability and can still be recycled to a conventional pulper of a paper production plant for direct use therein. This is indeed non-obvious as these two properties are of a contradictory nature.

More specifically, according to a first aspect of the invention there is provided a new process for the treat¬ ment of a cellulosic material, such as selected from the group consisting of paper, board, paperboard and similar materials, to impart thereto water vapour and aroma barri¬ er characteristics while retaining the repulpability pro- perties thereof so as to enable a direct recycling to a pulper of a paper, board or paperboard production plant or line, said process being characterized by:

A) coating said material, on one or both sides thereof, with a coating agent, preferably in the form of an aqueous slurry, comprising a) 30-70% by weight of poly(vinyl alcohol), and b) 70-30% by weight of a non-spherical particulate filler, preferably having an aspect ratio, i.e. a ratio of average particle diameter to thickness, of at least 2:1, said percentages being calculated as solids contents based on the combined weights of a) + b),

B) coating said material, on one or both sides there¬ of, with c) at most 7% by weight of at least one cross-linking agent for said poly(vinyl alcohol), said percen¬ tage also being calculated as solids content based on the combined weights of a) + b), said cross- linking agent being used separate from the coating agent, in a pre- or post-coating step, or alterna¬ tively as an ingredient of said coating agent, and

C) subjecting the coated material to a drying and cu¬ ring operation so as to form a cross-linked barrier layer onto said material, said barrier layer being the sole or major water vapour and aroma barrier-forming layer on the cellulosic material.

Thus, by using poly(vinyl alcohol) at a percentage of at least 30% by weight and a non-spherical filler at a percentage of at least 30% by weight as well as a cross- linking agent at a percentage of at most 7% by weight and by curing the coated material so as to cross-link said poly(vinyl alcohol), generally at a moderate temperature, a coated material is obtained where said coating agent is the sole or major water vapour barrier-forming agent and outstanding water vapour barrier properties are still accomplished. In addition thereto, the new barrier-pro¬ vided material is directly repulpable without any other equipment than those conventional pulpers which are indi¬ genous to the paper or board industry.

The cellulosic base material to be barrier coated in accordance with the present invention can be any type of material suitable as a wrapping or packaging material. For instance it can be any type of bleached or unbleached kraft or sulphite paper which has been refined in such a way that it achieves high tensile, tearing and bursting strengths.

As concerns the major ingredients of the coating agent, i.e. ingredients a) and b), the following details can be given.

The poly(vinyl alcohol) can generally be selected among such qualities which are suitable for coating pur¬ poses. Furthermore, it should be highly hydrolyzed and have a low to medium molecular weight. Preferably this means a degree of hydrolysis within the range of 95-100%, more preferably 97-100%, for instance 98% or around 98%, expressed as mole %. The molecular weight or mass is pre¬ ferably within the range of 16 000-200000, more prefer¬ ably 16 000-75 000, most preferably 16 000-47 000, g/mole,

the molecular weights being determined according to the GPC (Gel Permeation Chromatography) method.

A molecular weight of at most 47 000 is preferable for instance for the reason that it enables low viscosi- ties of the coating agent to be attained (e.g. below 5 000 cps/Brookfield, 100 rpm), which in turn makes it possible to carry out the coating process with a great precision.

It is also possible to mix two or more poly(vinyl al¬ cohols) together, such as one with a molecular weight close to the lower limit (e.g. 16 000-30 000) and the other with a molecular weight more close to the upper limit (e.g. 130000-200 000), for instance in a ratio of from 8:2 to 6:4, depending on the desired viscosity of the coating agent. One specific example of a preferable embo- diment in this respect is one with a molecular weight of about 16 000 and a degree of hydrolysis of about 98%, and the other with a molecular weight of about 145 000 and a degree of hydrolysis of about 99%.

Although generally a homopolymer of vinyl alcohol is referred to, the invention does not exclude the possibi¬ lity of copolymerizing said vinyl alcohol with a minor amount of any monomer that is copolymerizable therewith without substantially altering the favourable barrier pro¬ perties aimed at. The term "major water vapour and aroma barrier-for¬ ming agent" in connection with the coating agent means that said poly(vinyl alcohol) based coating agent repre¬ sents more than 50% by weight, preferably more than 75% by weight and more preferably more than 85% by weight of the total weight of the coating materials used to cover the cellulosic material.

The particulate filler for the poly(vinyl alcohol) can be of any type or nature that is generally used in connection with poly(vinyl alcohol) or any other particu- late filler normally used for pigment coating. A major feature of the filler to be used in the present invention is, however, that it is non-spherical, i.e. it has to be

elongated, flat, flaky or similar, as it seems that some kind of mutual over-lapping arrangement of the filler is needed for obtaining the required moisture vapour trans¬ mission rate. In other words the aspect ratio, which is defined as the ratio of average particle diameter to thickness of the filler, should preferably be at least 2:1. The upper limit of said aspect ratio should not be critical to the invention, but generally said ratio is within the range of 2:1-300:1. According to a preferable embodiment thereof said ratio is 3:1-300:1, more prefer¬ ably 5:1-300:1, and most preferably 5:1-200:1.

According to a preferable embodiment of the invention the filler is a mineral pigment, especially preferred pigments being kaolin, talcum, chalk and marble (CaC0«), the two latter in a chemically precipitated form (such as precipitated CaC0 ₃ ).

As to the cross-linking agent c) it should be noted that the term cross-linking agent is used in a broad sense in connection with the present invention. Thus, it inclu- des an agent that can create covalent bonds between the linear chains of the poly(vinyl alcohol) to form a network structure as well as an agent that can create such a net¬ work by means of ionic bonds. Thus, said network formation can also be referred to as gelling or similar. The first group of cross-linking agent creating cova¬ lent bonds is generally selected among agents which are previously known per se as cross-linking agents for poly- (vinyl alcohol), especially as thermal cross-linking agents which do not react to any appreciable extent until the temperature is raised above room-temperature. This means that generally they are added together or admixed with the coating agent a) + b). In other words this often means that step B) is not any separate step but coincides with step A). A preferable group of cross-linking agents of this type for use in connection with the present invention is the group consisting of ammonium-zirconium carbonate,

glyoxal, melamine-formaldehyde resins, urea-formaldehyde resins, dimers of alkylketone, epichlorohydrin-modified polyamides, and chromium compounds.

The other type of cross-linking agent is, thus, an agent selected from the group consisting of salt or ionic bond-forming substances which immobilize or gel the poly- (vinyl alcohol) coating agent. Depending on their reacti¬ vities they can be added together or admixed with the coa¬ ting agent (i.e. without any separate step B)) or in a se- parate step B).

A preferable example of this type of cross-linking agent is borax, e.g. in the form of a diluted aqueous so¬ lution (typically a 4% solution), which is generally used as a pre-coating, i.e. where step B) is performed before step A). In fact it has been found that borax used as a pre-coating and as the only or major cross-linking agent imparts outstanding properties to the cellulosic material referred to.

Another agent of said second type is boric acid (a typical acid being a 1% aqueous solution thereof) which can be added together with a) and b), as well as in pre- and post-coating steps, but for said agent it will gene¬ rally be necessary to adjust the pH of the coating agent to an alkaline or basic one, e.g. at least pH 8, before the curing step C) is performed, to initiate the curing or gelling reaction. Such a pH adjustment can e.g. be made by the addition of a base, such as Na ₂ HP0 ₄ , or by using a buffer or acid-base system with a volatile acid, such as acetic acid, which is evaporated before step C) is fina- lized.

A combination of the two types of cross-linking agents is of course also possible.

As has been described above the mutual composition of the coating agent concerning poly(vinyl alcohol) to filler is essential to the invention, which means that said in¬ gredients a) and b) are present in a weight ratio of from 3:7 to 7:3. A more preferred composition as to said two

ingredients is when a) is present at 40-60, especially 42- 58, % by weight and b) is present at 60-40, especially 58- 42, % by weight, calculated as defined above, i.e. as so¬ lids contents based on the combined weights of a) + b). The cross-linking agent c) is, as was mentioned above, present in an amount of not more than 7% by weight, also based on the solids contents and calculated on the combined weights of a) + b). On the same basis the per¬ centage of c) is generally within the range of 0.2-7% by weight, preferably 0,5-6% by weight and most preferably 1- 5% by weight.

Thus, by using the cross-linking agent at the above- defined percentages a proper curing or cross-linking de¬ gree can be obtained, which satisfies the water vapour barrier characteristics as well as the repulpability cha¬ racteristics referred to above.

Step C) of the process according to the present in¬ vention is preferably of a short duration, depending on the temperature, and often the duration or contact time is not more than 10 sees, very often at most 5 sees, prefer¬ ably at most 2 sees and most preferably at most 1 sec.

The above-mentioned duration or contact times are re¬ lated to a drying or curing temperature that is rather mo¬ derate, preferably at most 120°C, more preferably at most 105°C. By temperature is hereby meant the temperature which the coated material reaches during said drying and curing step. Preferably said temperature is within the range of about 70-120°C, more preferably 70-105°C, such as 75-100°C. The coating agent to be used in step A) is generally prepared in the form of an aqueous slurry in a conventio¬ nal manner, e.g. by first dissolving the pol (vinyl alco¬ hol) in water to a 20-50% solution and then adding the filler and optionally the cross-linking agent to the for- mation of a slurry having a solids content of 20-50% by weight.

Since, as was mentioned above, the poly(vinyl alco¬ hol)-based coating used in the process claimed is the major or only water vapour and aroma barrier, the coating operation per se can be performed in a simple way and with a non-complex equipment. This generally means that it may well be performed in one single operation only where a web or sheet of the substrate material is surface coated with one single layer of the coating agent and then dried and cured in the described way, the cross-linking agent being added to the coating agent or in a pre- or post-step. How¬ ever, if said coating operation will not be sufficient for certain intended purposes, it may well be repeated one or more additional times with the same or similar coating agent. Within the scope of the invention it is also poss- ible to coat one or both sides of the substrate material. At all events, the coating or combined coatings of step A) are preferably performed to a dry surface weight of 2-30 g/m 2, especially 3-15 g/m2, per side of said coating agent. Generally this means that a coated end product is manufactured, which has a water vapour transmission rate

2 of at most 25 g/m and 24 h, according to the test method referred to below. Preferably a product is manufactured having a water vapour transmission rate of at most 20,

2 more preferably at most 15, especially 7-15, g/m and 24 h, according to the same test method.

The test method used for determining the water vapour transmission rate is TAPPI Standard T 448 m-49, "Water

Vapour Permeability of Paper and Paper Board". The process according to the present invention is preferably performed as a continuous operation where a running web of said material is surface coated in step A), optionally also with a separate step B), and then immedia¬ tely dried and cured in step C). This operation can be performed in a step separate from the manufacture of the paper web or in direct connection therewith.

According to a second aspect of the invention a novel coated material obtainable by the above-defined process is provided.

More specifically the coated material according to the present invention comprises a cellulosic based sub¬ strate, such as selected from the group consisting of paper, board, paperboard and similar materials, said ma¬ terial having water vapour and aroma barrier characteris¬ tics as well as repulpability properties, which coated material is characterized in that the coating layer(s) thereof has (have) been obtained essentially from a coa¬ ting agent comprising: a) poly(vinyl alcohol) b) a non-spherical particulate filler, preferably having an aspect ratio, i.e. a ratio of average particle diameter to thickness, of at least 2:1, and c) a cross-linking agent for said poly(vinyl alcohol), a) and b) being present in a weight percentage of 30- 70% and 70-30%, respectively, and c) being present in a weight percentage of at most 7%, all percentages being calculated as solids contents based on the combined weights of a) + b), said coating agent forming a cross- linked layer having water vapour and aroma barrier cha¬ racteristics as well as repulpability properties.

As concerns preferable embodiments of said coated material according to the invention reference is made to the preferable embodiments of the process according to the invention, which are also applicable to said novel coated material.

A third aspect of the invention relates to the coa¬ ting agent per se, i.e. a coating agent for a cellulosic based material, such as selected from the group consisting of paper, board, paperboard and similar materials to im¬ part thereto water vapour and aroma barrier characteris¬ tics while retaining the repulpability properties thereof,

the characterizing features of said coating agent being the same as have been defined above in connection with the process aspect of the invention.

Finally, according to a fourth aspect of the inven- tion a novel use is provided, viz. the use of a coated material whenever prepared as defined in connection with the process aspect of the invention or as defined in con¬ nection with the coated material aspect of the invention, as a waste paper to be recycled directly to a pulper of a paper, board or paperboard production line.

In this context the term "directly" generally means that there is no separation or other pretreatment step needed before the coated material can be utilized in a pulper of such a line.

Figure

A schematic illustration of a suitable equipment for performing the process according to the invention, and which was in fact used in the working examples described below, is shown in the accompanying drawing.

Said figure shows a supply roll 1 for a paper sub¬ strate from which a web 2 of said paper substrate is un¬ wound. Said runnig web 2 then passes via guide rolls 3 through a coating station 4, which is a common flexography coater with three coating rolls 5 and a backing roll 6 and over three separate heated drying cylinders 7 and via fur¬ ther guide rolls 3 onto a rewind roll 8. Close to the first one of the three heated cylinders 7 there is also an IR drier 9. Thus, the coating process comprises applying the coa¬ ting agent from the coating station 4 onto the running web 2 and curing the coated substrate by means of the IR drier 9 and post-drying the same by means of the drying cylin¬ ders 7, the coated and cured product then being wound onto the rewind roll 8.

EXAMPLES

The following examples serve to illustrate preferable embodiments of the invention. All examples were carried out using a coating equipment of the type illustrated in the figure.

The production speed was 75 m/min; the IR-drier in¬ creased the web-temperature from 22°C to 75°C; the web temperature after the third contact drier was 103°C; the temperature of the coating agent was 35°C; the solids con¬ tent of the coating agent was 33%; and the water vapour transmission rates were determined according to TAPPI T 448-m-49 (6 hours at 23°C, 50% humidity). All percentages are % by weight.

EXAMPLE 1

The coating agent was the following mixture: 44% of poly(vinyl alcohol) (PVA)

(98% degree of hydrolysis and 27 000 as molecular weight).

56% of kaolin, delaminated

4.5% of ammonium zirconium carbonate (AZC).

2

The coating weight was 5.0 g/m on one side of a sul-

2 fite-based, bleached paper with a grammage of 95 g/m . The

2 water vapour transmission rate was 8.2 g/m . 24 h.

EXAMPLE 2

Same as Example 1 except a PVA with a molecular weight of 16 000.

2 The coating weight was 5.2 g/m .

2 Water vapour transmission rate: 7.9 g/m . 24 h.

EXAMPLE 3

Coating agent: 30% of PVA (98%, 27 000 Mw)

70% of kaolin, delaminated 4.5% of AZC

Paper: as in Example 1

2

Coating weight: 5.0 g/m

2

Water vapour transmission rate: 19.7 g/m . 24 h.

EXAMPLE 4

Paper: as in Example 1 Coating agent: 60% of PVA (98%, 27 000 Mw) 40% of kaolin, delaminated

4.5% of AZC

2 Coating weight: 5.2 g/m

2 Water vapour transmission rate: 16.5 g/m . 24 h

EXAMPLE 5

Paper: as in Example 1 Coating agent: 35% of PVA (98%, 27 000 Mw) 9% of PVA (99%, 145 000 Mw) 56% of kaolin, delaminated

4.5% of AZC

2 Coating weight: 5.1 g/m

2 Water vapour transmission rate: 7.2 g/m . 24 h.

EXAMPLE 6

Paper: as in Example 1

Coating agent: same as in Example 1 except that AZC was replaced by glyoxal.

2 Coating weight: 4.9 g/m

2

Water vapour transmission rate: 8.6 g/m . 24 h.

EXAMPLE 7

Paper: Same base paper as in Example 1 but it had previously been treated with a 4% solution of borax (so¬ dium tetraborate decahydrate).

Coating agent: same as in Example 1

2 Coating weight: 4.5 g/m

2 Water vapour transmission rate: 5.7 g/m . 24 h.

EXAMPLE 8

Paper: same as in Example 1 but after the application of the coating agent and before the curing operation the coated paper was treated with a 4% solution of borax in a separate operation. Coating agent: same as in Example 1

2 Coating weight: 5.0 g/m

2 Water vapour transmission rate: 3.6 g/m . 24 h.

EXAMPLE 9 Same as in Example 1 but without AZC in the coating agent and with pre-treated (borax) paper as in Example 8.

2 Coating weight: 4.5 g/m

2 Water vapour transmission rate: 5.5 g/m . 24 h.

EXAMPLE 10

Paper: as in Example 1.

Coating agent: same as in Example 1 but also with 1% of boric acid added. After the coating operation the coa¬ ted paper was treated with a 20% solution of Na^HPO^ to increase the pH value of the coated paper to approximately

8.5.

2 Coating weight: 5.0 g/m

2

Water vapour transmission rate: 10.5 g/m . 24 h.

Coated papers as produced in Examples 1-10 were tes¬ ted for their repulpability in a laboratory desintegrator. After 30 000 revolutions hand sheets were formed and the fiber uniformity was evaluated visually. All papers proved to be fully repulpable.