Methods for manufacturing packaging board

Details Methods for manufacturing packaging board Methods for manufacturing packaging board

1999-07-14

2001-03-13

Cameron, Erma (Department: 1762)

Coating processes

Direct application of electrical, magnetic, wave, or...

Polymerization of coating utilizing direct application of...

C427S387000, C427S391000, C427S393400, C428S448000, C428S452000, C428S537500

Reexamination Certificate

active

06200644

ABSTRACT:

The object of the invention is a method for manufacturing packaging board, in which a board base of paperboard or cardboard is provided with at least one silicon-based, liquid-tight and gas-tight layer of coating. Another object of the invention is a method based on the coating of paper or a board base to manufacture liquid-tight and gas-tight packages, and products provided by using the methods, including foodstuff packages and trays.
In order to be useful, for packages of liquid and other wet foodstuffs or foodstuffs which spoil easily the board or the paper must be provided with a liquid-tight and gas-tight coating. The coating prevents the oxygen in the air from penetrating the package and spoiling the product, and it also prevents the package from getting wet and the aromas of the product escaping from the package. Corresponding gas tightness can be required from medicine, cosmetics, and detergent packages.
An effective way to render liquid packages, such as juice containers, liquid-tight and gas-tight is to provide the board of the container with a thin aluminium foil. Aluminium as such has also been used for peelable covers of yoghurt and curdled milk cups and butter and margarine boxes. However, aluminium foil has disadvantages: high manufacturing costs, it is not biologically decomposable, there are difficulties in regenerating the packaging material, and the package cannot be heated in a microwave oven. Another problem with detachable aluminium covers is that they tear and burst easily.
An alternative solution for tightening the board or the paper used for packages is to provide it with one or more layers of polymeric coating. The number of layers and the material used depend on the requirements set by the packaged product. The best coating materials have essentially reached a tightness corresponding to that of aluminium foil and, as substitutive materials, they have eliminated the above-mentioned disadvantages connected with aluminium. However, it has been necessary to combine various polymeric materials in these substitutive solutions so that they comprise, for example, an oxygen-tight, water vapor-tight, and aroma-tight barrier layer, heat-seal layers on both sides of the paper or the board, and one or more layers of binding material to bind the polymers to the paper or the board and to each other. Therefore, the structure of the packaging paper or board becomes complex and the consumption of polymeric material is extensive.
Examples of packages tightened according to the above description include containers which are intended to be used as packages of milk, cream, sour milk, juice or other similar, liquid foodstuffs and which are entirely made of board provided with layers of polymeric coating. In these containers, the board is typically provided with four or even five layers of polymeric coating so that, for example, the board comprises an oxygen-tight and aroma-tight barrier of, e.g., polyamide, a layer of binding material on top of that, and, at the very top, a heat-seal layer of polyethylene, for example, and another heat-seal layer of polyethylene is provided on the opposite side of the board. Another typical package application is a portion package of, for example, milk, curdled milk, yoghurt, water, juice, desserts or ice-cream, in which the package is in the form of a small cup or a container, is typically made of plastic or coated paperboard, and is provided with a heat-sealed, peelable cover. The cover material is paper which is coated with an oxygen-tight and aroma-tight barrier consisting of, for example, polyamide, ethylene vinyl alcohol copolymer (EVOH) or polyethylene terephtalate (PET), with a layer of binding material, and with a heat-seal layer which is against the mouth of the container or the cup and which consists of, for example, styrene-modified copolymer of ethylene and methacrylic acid, making the product both heat-sealable and easy to be peeled off. Cosmetic products and pharmaceutical pills have been packed in a similar manner, using plastic or glass containers provided with a peelable paper cover which is sealed with a polymeric coating.
Patent publication U.S. Pat. No. 5,340,620 describes paperboard provided with a silicon-based polymeric coating, in which the polymer serves as an oxygen-tight barrier. According to the publication, the coating is provided by polymerizing organosilane by using UV irradiation, whereby, in addition to an inorganic polymeric backbone, organic bonds are formed in the coating when the organic groups of the silane react with each other. However, the portion of the inorganic polymeric backbone is prevalent in the coating, which is why it can be too fragile to withstand, for example, the creasing which is part of the manufacture of paperboard or cardboard containers; furthermore, there is no mention of the water vapor-tightness of the coating. It is obvious that the coating material of the embodiment of the publication cannot provide paperboard or cardboard suitable for liquid packages. Moreover, organosilanes are an expensive raw material for the coating.
Silicon-based coatings have also been described e.g. in the published applications DE 4 020 316 and 4 025 215, which cite paper as one possible substrate of the coating but which describe in detail the coating of plastic or metal only, and according to the publications, the purpose of the coating is to provide resistance to wear so that the film-like substrate still maintains its flexibility. Therefore, the publications are not concerned with packaging technology which is the object of the present invention.
Another use of tightened packaging board are foodstuff underlayers, such as ovenable microwave or conventional oven trays which can be part of consumer packages of foodstuffs, such as casserole foods intended to be heated, or which can be sold as separate products. Such underlayers must be impermeable to water and grease; and in addition to this, sufficient heat-resistance is required from ovenable trays. Polyester coated paperboard has been used in oven trays; however, its disadvantages include the thickness of the required polymeric layer and the fact that it is very difficult for the polymeric coating to withstand typical oven temperatures of more than 200° C. Polypropylene has been used as the polymer coating in microwave ovenable trays.
The purpose of the invention is to present a new solution to provide a board base of paperboard or cardboard intended to be used as packaging material with a polymeric layer of coating which renders the package liquid-tight and gas-tight. The purpose is particularly to provide a simple structure of coated board and savings in coating material, while at the same time making the coating tough enough to withstand the creasing required of paperboard or cardboard containers without breaking. The invention is characterized by the steps of providing a polymerizing reaction mixture containing at least one silicon compound to form an inorganic, chain-like or crosslinked polymeric backbone containing alternating silicon and oxygen atoms, and at least one reactive organic compound to form organic side chains and/or crosslinks to the polymeric backbone, spreading said mixture on the board base, and curing said mixture to form a layer of coating.
The process according to the invention can be implemented, starting from a silicon compound, such as silane, an organic compound reacting with it, water, and a possible catalyst, whereby the hydrolyzed groups of the silicon compound are first partly condensed, forming colloidal particles in the solution. With the sol ageing and/or with a catalyst being added, the reaction continues with the particles growing and being combined, resulting in a chained or crosslinked gel covering the surface of the board, the gel being finally dried and cured by heating or irradiating it using UV, IR, laser or microwave radiation to form a thin, tight coating on the board. Depending on the circumstances, the curing time may vary from fractions of a second to several hours. The coating thus obtained simult

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