Ovenable food tray and its manufacturing method

Details Ovenable food tray and its manufacturing method Ovenable food tray and its manufacturing method

1999-07-19

2001-10-23

Hoang, Tu Ba (Department: 3742)

Electric heating

Microwave heating

Cookware

C099SDIG014, C525S507000

Reexamination Certificate

active

06307192

ABSTRACT:

This application is the national phase under 35 U.S.C. §371 of PCT International Application No. PCT/FI97/00701 which has an International filing date of Nov. 17, 1997 which designated the United States of America.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The object of the invention is an ovenable food tray consisting of a board base of paperboard or cardboard provided with at least one heat resistant polymeric coating layer. Another object of the invention is a manufacturing method of such a food tray.
2. Description of Related Art
Ovenable food trays, such as microwave oven or conventional oven trays, are used as parts of consumer packages of foodstuffs, such as casserole foods intended to be heated, and they are also 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. Up till now, polyester-coated paperboard has been used in ovenable trays. 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. The microwave oven trays intended to be heated in microwave ovens have been provided with a polymeric coating of polypropylene but its heat-resistance is also limited.
In the EP application 0 245 005 there is described an ovenable food tray which consists of a laminate of paper and paperboard layers and has a coating of a food grade resin such as polyethylene terephtalate (PET) on its food contacting side and a nonburning coating of silicone polymer on the reverse side, covering the paper layer of the laminate. While the silicone coating possesses an increased resistance to heat the use of polyethylene terephtalate still limits the ability of the tray to withstand high oven temperatures.
SUMMARY OF THE INVENTION
The purpose of the invention is to provide a paperboard or cardboard food tray, such as a microwave oven or conventional oven tray, with improved properties, specifically improved heat-resistance together with reduced weight, as compared with known board trays.
The tray according to the invention is characterized in that the polymeric coating of the tray is lying at least on the side of the tray coming into contact with the food and contains a polymerized crosslink structure consisting of an inorganic, chained or crosslinked polymeric backbone which contains alternating silicon and oxygen atoms and which also comprises side chains and/or crosslinks formed by organic groups or chains.
In the food tray according to the invention the use of purely organic coatings has been avoided. There is instead a silicon-based coating layer with a superior heat resistance based on the partially inorganic nature of the coating material. The coating is lying at least on the food contact side of the tray and preferably on both sides of the tray.
The coated paperboard or cardboard used in the tray according to the invention can be manufactured, starting from silane, an organic compound reacting with is water, and a possible catalyst, whereby the silane is hydrolyzed and condensed, forming colloidal particles and reacts with the organic compound so that the silane produces a polymeric backbone mainly consisting of silicon and oxygen, and the organic compound works as a crosslinker. When organosilane containing reactive, organic groups is used, it may be unnecessary to use a separate organic compound. This results in a sol consisting of colloidal particles in which the reaction continues with the particles growing and being combined so that a chained or crosslinked gel is obtained, covering the surface of the board, the gel being finally 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 drying/curing time may vary from fractions of a second to several hours. The coating thus obtained simultaneously features typical characteristics of both an inorganic and an organic substance, and the properties of the coating can be adjusted by selecting components that react in a proper way.
DETAILED DESCRIPTION OF THE INVENTION
The water- and grease-proof coating layer of the food tray according to the invention which is tough, withstands creasing, and does not break when bent, can be rendered very thin without creating small, visually unperceivable pin holes in the coating, during the forming stage or later when heated or jointed, which constitute a problem in known coating materials made of organic polymers and because of which the layers of coatings had to be made relatively thick. On the basis of preliminary tests, a tight layer of coating can be provided on a smooth paperboard base by as low amount of coating as 1 g/m
2
, and, in practice, a preferred amount of coating is in the range of about 2 to 6 g/m
2
. Therefore, the invention provides essential savings in material and a decrease in the weight of the board as compared with those known from before. Another advantage of the invention is that the spreading of the coating mixture is easy to accomplish using the methods generally used in paper and board industry, such as rod coating or blade coating techniques or by spraying. The spreading of the coating may thus be effected in the board machine by using the “on-line” principle as part of the manufacturing process of the board, by using the same types of spreading devices that are used for application of normal coating mixes. The coating can also be spread on premoulded tray blanks or in connection with the moulding of the tray. When needed, fillers can be added, the most preferable materials including scale- or slatelike mineral fillers, such as talc, mica or glass flakes which settle in the direction of the coating and contribute to its properties of impermeability. It is also possible to dye the coating by adding pigments or organic colouring agents to the mixture, or to add organic and/or inorganic fibres or particles to the formulation, the fastening of which to the coating can be improved by the use of coupling agents. Furthermore, it is possible to include, in the mixture, an organic, polymerizing agent which forms a separate polymeric structure with respect to the inorganic chain or crosslinked structure according to the invention and which intermeshes with it. In addition to the board machine, the spreading of the coating can be carried out, in connection with a printing process, for example, on a finished board base which does not necessarily have to be dried first. In this case, the board can be precoated with any kind of coating commonly used in paper and board industry.
The good heat-resistance of the coating is a special advantage of the food tray according to the invention. The board can be moulded into a tray by pressing at a high temperature and the trays easily withstand the normal temperatures of kitchen stoves and microwave ovens, and even temperatures exceeding 300° C. at which the board base will begin to char. At the same time, the layers of coating protect the board from the softening effect of steam coming from the food when heated so that the tray maintains its form. When baked, the food does not stick to the coating according to the invention. The tray provided in accordance with the invention can be part of the consumer package of prepared food, for example, whereby the food is intended to be heated in the tray after opening the package, or the trays can be sold to consumers as such.
The chain or crosslinked structure of the polymeric coating provided according to the invention can consist of silicon or metal atoms and oxygen atoms which alternate with them. The structure preferably consists of mainly silicon and oxygen, and small numbers of metal atoms may be combined with the same backbone as substitutes for silicon. The metals can preferably include Ti, Zr, and Al, for example. Organic groups combined with the polymeric structure can mainly include substituted or unsubstituted alkyl and aryl groups.

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