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 foodstuffs 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.
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 impermeability 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 resources, 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 polyethylene-coated or -extruded paper has been found on the market, the main reasons being that new processes for obtaining water-vapour impermeable paper are complex, time consuming and/or expensive or that a proper balance between water vapour permeability characteristics and recyclability 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 of 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 deposited 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-A-393 451 and EP-A-487 497 can be referred to. Furthermore, AT-A-395 617 discloses a process for imparting vapour barrier and repulpability properties to a cellulosic material by means of poly(vinyl alcohol) with added particulate fillers and curing agents. The process includes several coating steps and the curing temperature is between 120°C and 170°C.
By the present invention it has unexpectedly become possible to utilize a poly(vinyl alcohol)-based composition as a surface coating for paper and similar materials 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 impermeable 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 treatment 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 barrier characteristics while retaining the repulpability properties 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:
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 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-provided material is directly repulpable without any other equipment than those conventional pulpers which are indigenous 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 purposes. 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 preferably within the range of 16 000-200 000, more preferably 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 viscosities 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 alcohols) 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 embodiment 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 possibility of copolymerizing said vinyl alcohol with a minor amount of any monomer that is copolymerizable therewith without substantially altering the favourable barrier properties aimed at.
The term "major water vapour and aroma barrier-forming agent" in connection with the coating agent means that said poly(vinyl alcohol) based coating agent represents 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 particulate 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 transmission 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 preferably 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 (CaCO3), the two latter in a chemically precipitated form (such as precipitated CaCO3).
The cross-linking agent c) includes 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 network 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 covalent 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).
A cross-linking agent of this type for use in connection with the present invention is ammonium-zirconium carbonate.
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.
An example of this type of cross-linking agent is borax, e.g. in the form of a diluted aqueous solution (typically a 4% solution).
Another agent of said second type is boric acid (a typical acid being a 1% aqueous solution thereof) which is added together with a) and b) but for said agent it will generally 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 B) 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 Na2HPO4, or by using a buffer or acid-base system with a volatile acid, such as acetic acid, which is evaporated before step B) is finalized.
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 ingredients 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 solids contents based on the combined weights of a) + b).
The cross-linking agent c) is, as was mentioned above, present in an amount of 0,2 to 7% by weight, also based on the solids contents and calculated on the combined weights of a) + b). On the same basis the percentage of c) is 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 degree can be obtained, which satisfies the water vapour barrier characteristics as well as the repulpability characteristics referred to above.
Step B) of the process according to the present invention is preferably of a short duration, depending on the temperature, and often the duration or contact time is not more than 10 secs, very often at most 5 secs, preferably at most 2 secs and most preferably at most 1 sec.
The above-mentioned duration or contact times are related to a drying or curing temperature that is rather moderate, 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 conventional manner, e.g. by first dissolving the poly(vinyl alcohol) in water to a 20-50% solution and then adding the filler and the cross-linking agent to the formation of a slurry having a solids content of 20-50% by weight.
Since, as was mentioned above, the poly(vinyl alcohol)-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. However, 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 possible 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/m2, 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 of at most 25 g/m2 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, more preferably at most 15, especially 7-15, g/m2 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), and then immediately dried and cured in step B). 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 substrate, such as selected from the group consisting of paper, board, paperboard and similar materials, said material having water vapour and aroma barrier characteristics as well as repulpability properties, which coated material is characterized in that the coating layer(s) thereof has (have) been obtained essentially from a coating agent comprising:
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 coating 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 impart thereto water vapour and aroma barrier characteristics 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 invention 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 connection 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.
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 substrate from which a web 2 of said paper substrate is unwound. 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 further 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 coating 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 cylinders 7, the coated and cured product then being wound onto the rewind roll 8.
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 increased 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 content 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.
The coating agent was the following mixture:
The coating weight was 5.0 g/m2 on one side of a sulfite-based, bleached paper with a grammage of 95 g/m2. The water vapour transmission rate was 8.2 g/m2. 24 h.
Same as Example 1 except a PVA with a molecular weight of 16 000.
The coating weight was 5.2 g/m2.
Water vapour transmission rate: 7.9 g/m2. 24 h.
Coating agent:
Paper: as in Example 1
Coating agent:
Paper: as in Example 1
Coating agent:
Coated papers as produced in Examples 1-5 were tested 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.