Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
1999-11-29
2003-04-08
Cain, Edward J. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C524S013000, C428S496000, C428S507000
Reexamination Certificate
active
06545079
ABSTRACT:
Method of forming a transparent and gas-permeability decreasing coating to a paper or board web and a coating formulation for the method
The present invention relates to a method for forming a coating to a paper or board web. It is an object of the coating method according to the invention to provide paper or board with a barrier resisting the transmission of gases and vapours such as oxygen and water vapour as is required in the art of packaging, for instance.
It is a further object of the invention to provide a coating with the above-mentioned qualities that is also transparent.
It is still another object of the invention to provide a coating with the above-mentioned qualities that also can reduce the adherence risk of the web surfaces in a winded paper of board roll.
The invention also concerns a coating formulation capable of providing the above-described qualities when applied to a paper web.
As known, the gas barrier property required in the packaging industry is conventionally achieved by means of coating the paper or board web with a polymer film. Desirably, the thin-film coatings used herein are capable of preventing penetration of liquids, such as water, and oxygen, water vapour and aroma components as well as oils and greases through them.
Handling of scrapped packaging materials coated with a functional polymer film often involves a high cost due to the poor degrading and recycling properties of these materials.
On the other hand, it is known in the art that a moisture barrier property can be given to a paper web by means of coating the web with a polymer latex having a wax emulsion added thereto. It is also known that these coating components are selected from the group of conventional synthetic polymer lateses such as styrene butadiene, acrylate, styrene acrylate and polyvinyl acetate lateses. Generally, the fraction of polymer lateses in the solids of the coating mixture is very high. Examples of the above-described technology can be found in patent publications. For instance, GB Pat. No. 1,593,331 (filed by J. Vase, Kemi Oy) and FI Pat. Appl. No. 901,928 (filed by Neusiedler A G) teach the methods of said technology.
According to another published reference, the amount of wax dispersion that may conventionally be added to a latex polymer is advantageously maximally about 10 wt-%, while also significantly higher amounts are possible. The waxes most commonly used include paraffin wax, microcrystalline wax, polyethylene wax and a mixture of one of these waxes with at least one other type of wax. A coating formulation thus prepared gives an extremely hydrophobic coating. Of publication closely related to the art, reference can be made to the patent publication WO-A1-9605054 (International Paper Company, USA), in which the above techniques are discussed.
The techniques taught in cited publications are capable of rendering desired barrier properties to the web. However, as the drying of the web leaving the coating section of the paper machine elevates the temperature of the coating applied to the paper web, the adherence risk of the coating in the winding of the paper web becomes so high as to exclude the use of above-mentioned conventional coating formulations when the goal is to achieve a uniform and defect-free coating. A further disadvangae is, that the high fraction of polymer latex in the coating formulation complicates the reuse of the coated paper in pulping, because the amount of anionic contaminants rises excessively high.
It must also be noted that the use of wax emulsions in applications related to the invention lowers the surface energy level of the coating applied to the paper web. The reduced level of surface energy in the coating complicates and, in many cases, even makes it impossible to perform surface sizing with, e.g., starches. The use of waxes also deteriorates the printability qualities of the coating, particularly for water-based inks. The use of wax coatings offers the benefit that it lowers the adherence risk of the coating, the improvement obtained herein is frequently, however, insufficient unless some mineral filler or pigment additive is used.
From the prior art is further known that polymer chains, particularly starch, of natural origin may be added to the latex polymer during its preparation. Paper products which are coated with a polymer latex thus prepared are easier to repulp than paper grades coated with pure polymer latexes. An example of this technique can be found in the patent literature. The technique is taught in publication WO-A-93/11300 (Raision Tehtaat Oy Ab). While the technique can aid the pulping of the recycled paper product, it simultaneously compromises heavily the water-vapour barrier property of the product due to the hydrophilic character of the added starch.
On the other hand the prior art teaches that talc particles can be used in a coating formulations intended for coating a paper or board web. Paper grades parti-cularly suited for gravure printing are coated using a coating formulation containing a significant fraction of talc particles.
Talc particles are also used as a constituent of coating mixtures developed for the coating of release paper grades and of coating mixtures applied to corrugated board grades suitable for flexo printing. Examples of this latter type of technique can be found in the patent literature. For instance, patent publications JP 62-64038 (filed by Oji Paper Co.) and JP 52-118016 (filed by Toyo Ink Mfg. KK) teach the above-described type of technique.
The technology according to these prior-art methods results in good release properties and improved resistance against water vapour transmission. However, due to the high proportion of talc particles with respect to the proportion of polymer latex in the coating mixture, the water vapour barrier property required in foodstuff packages, for instance, cannot be attained.
It is an object of the present invention to overcome the above-described disadvantages and to provide a paper product with sufficient barrier properties, yet suitable for reuse in papermaking and, moreover, permitting dumping by composting.
The goal of the invention is achieved by virtue of applying a coating containing at least one polymer dispersion on a sheet of paper, board or similar cellulosic fiber based web in at least one coating step, wherein the polymer dispersion used contains extremely pure talc particles.
In the context of the present invention, the term extremely pure talc particles is used when reference is made to particles having a degree of purity of about 90-100%. Advantageously, about 90% of the particles are smaller than 40 &mgr;m. Talc having said properties minimizes the need of dispersants in dispersing the talc for the coating paste. High dispersant levels in coating formulations intended to have barrier properties are not favoured.
Herein, the term polymer latex refers to a polymer dispersion having advantageously a 30-60% solids content and advantageously the glass transition point in the range of −20-+70° C. for use in applications according to the invention. For use according to the invention, suitable polymer lateses are selected from the group of synthetic polymer lateses including styrene butadiene, acrylate, styrene acrylate and polyvinyl acetate lateses and polymer dispersions made from a biologically degrading polymer, as well as mixtures of said polymer dispersions.
Accordingly, the polymer dispersion can be a product made starting from a mixture of monomers containing vinyl acetate and at least one ester formed from acryl acid and/or methacryl acid with a lower alcohol (that is, methyl, ethyl, propyl or butyl alcohol) as the main components.
Alternatively, the method can be implemented using a product made starting from a mixture of monomers containing styrene and at least one ester formed from acryl acid and/or methacryl acid with a lower alcohol (that is, a methyl, ethyl, propyl or butyl alcohol) as the main components.
Alternatively, the polymer dispersion can be a product starting from a mixture containing at least one es
Carne Tom
Nurmi Kari
Santamäki Kimmo
Cain Edward J.
Connolly Bove & Lodge & Hutz LLP
Raisio Chemicals Oy
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