GB2040861A - Coated packages - Google Patents

Abstract

A food package is made by electron irradiating a polyethylene coating on paper board and then forming and heat-sealing. The electron irradiation enhances the heat resistance of the polyethylene, but also allows package seams to be heat sealed. The electrons impinge on the coated board surface at a glancing incidence to provide a uniform dose of radiation. <IMAGE>

Description

SPECIFICATION Packages This invention relates to packages made wholly or in part from coated paper board. It relates more specifically to food packages that are suitable for being exposed to heat, for example in an ordinary domestic oven, for the purpose of cooking the contents of the package.

It is well known that packaging materials made of or coated with thermoplastic polyethylene can be formed to shape and/or enclosed by heatsealing, that is by pressing appropriate parts together under heat and pressure until a bond is formed. Such materials are, however, quite unsuitable for applications involving heating to temperatures above the softening-point of the polyethylene.

It is also well known that the heat resistance of polyethylene can be increased by subjecting it to high energy irradiation, but this has the effect of making the material infusible, and it has hitherto been assumed that such irradiated materials could not be satisfactorily heat-sealed.

Surprisingly, we have now discovered that electron-irradiated polyethylene-coated paper board can be heat-sealed by pressing the parts together and heating to a temperature above the softening point of the polyethylene prior to cross-linking, and moreover that the bond so formed is surprisingly strong even at temperatures higher than that at which the bond was formed.

In accordance with the invention, a heat-sealed food package capable of exposure to a radiant oven at 200"C for 39 minutes without leakage is made wholly or in part from paper board thinly coated on at least one side with polyethylene and irradiated with electrons, after coating but before heat sealing, to give a degree of cross-linking that results or would result from irradiation of polyethylene without any sensitiser to a dose, averaged over the thickness of the coating, in the range 5-50 (preferably 15-30) Mrad.

The heat-sealed seam or seams may close the package in a hermetically sealed fashion, or they may simply form the material of the package to the required shape, e.g. by securing a corner or an open topped tray. Preferably the (or each) coating has a weight not greater than 100 g/m2.

'Preferably the polyethylene is free of any additives, but radiation sensitisers and other additives may be present in the polyethylene provided they are suitable for use in contact with food.

Preferably the electrons impinging on the surface ofthe laminate have no more energy than is necessary (bearing in mind the angle of incidence) to given an adequately uniform dose through the thickness of the coating, and a dose that is half as great as the polyethylene/board interface as at the surface can be considered adequate.

The use of high-energy electrons impinging on the laminate at glancing incidence is attractive as it offers the prospect of a high dose rate and consequently a high line speed; a particularly convenient arrangement is to inject a diverging electron beam into the open end of a parallel-sided or narrow V-shaped loop of the laminate, to give high-energy electrons at glancing incidence close to the source and low-energy scattered electrons at a larger angle of incidence further away from the source.

In the accompanying drawings Figures 1 and 2 are mutually perpendicular views of irradiation apparatus of this kind, showing in broken lines an alternative form.

In the basic arrangement, paper board 1 with a coating of polyethylene on its upper surface is formed into a loop by guide rolls 2,3 and 4, and a diverging beam of electrons 5 emerging from a linear curtain electron beam accelerator 6 is passed into the open end of the loop, and thereby falls on a large area of coating at a glancing incidence angle (which falls somewhat along the length of the loop).

In an alternative arrangement, two additional guide rolls 7 and 8 are used with the effect of making the angle of incidence larger but more uniform over the area of exposure.

Electrocurtain and scanning types of electron accelerator may be used, and the irradiation process may be in-line with the coating of the polyethylene on the board, or with the shaping of the food package, or with both of them.

Example A paper board with a substance weight of 325g/m2 coated on one face with 23g/m2 of a commercial coating grade of polyethylene was cross-linked by electron irradiation to a dose of 20 Mrad for some samples and 30 Mrad for others. Comparison tests were made with the same material not exposed to radiation.

Samples cut from the respective materials were subjectto a shear test, a peel test and to a practical cooking test.

Test procedure were as follows: Shear test An air oven was thermostated at 200"C as indicated by a mercury-in-glass thermometer situated about 10 cm from the roof of the oven. A metal base plate, 450 g weight and loading former of dimensions 15 x 20 mm (used to define the bonding area) were left in contact with the base of the oven to equilibrate.

Section of the coated board 20 mm wide were cut and marked 15 mm from one end.

Two lengths of board, with the polyethylene surface in contact and an overlap of 15 mm were then positioned on the equilibrated base plate, the loading former was positioned and the 450 g weight applied (loading pressure 14.7 kN/m2 (150 g cm2)) for 1 minute.

The load was removed, the sample removed from the oven and left to cool in air.

Subsequent tests indicated that the bonding temperature reached a maximum of 230"C due to contact with the base of the oven.

The oven was set at an air temperature of 200"C. A wire loop was attached to one end of each of the test samples. A sample was then suspended in the oven, near to the bulb of the thermometer, and left for 5 minutes. Test loads in the range 30 to 450 g were then applied to the samples and the time for the bond to fail noted.

Peel test Strips 20 mm wide and 150 mm in length were prepared. The mid position along the length axis was marked together with points 15 mm either side. The strip was folded at the centre point to form a "V" and the two outer marks to define the contact area. Finally the fold at the base of the "V" was cut away.

Samples were prepared by this method at bonding temperatures 135"C and 230"C with the samples held under various loads for 1 minute or 5 minutes as the case may be. The loading procedure was the same as for the shear est.

Cooking test Samples of the board were folded, coated side in, to form an open topped tray measuring 120 mm long by 80 mm wide by 20 mm high and the corners heat-sealed under pressure by holding for 1 minute at 200"C. A quantity of minced beef was placed in each tray and it was heated for 110 minutes in an ordinary radiant oven at a nominal temperature of 200"C.

The results of the shear test and peel test are given in a table at the end of description. In the cooking test, both irradiated samples survived without significant deformation or leaking; the corners of the non-irradiated tray partly collapsed and leakage of the content occurred.

The invention includes a method of cooking food comprising heating it while contained in the packages described to a temperature higher than the softening-point of the polyethylene before cross-linking, and food cooked by the method.

SHEAR AND PEEL TEST RESULTS Test Bonding Radiation Test Method at 14.7kN/m2 Dose Load Fail time S s MRad N s Shear 0 0.65 250 0 1.16 80 230 60 0 1.67 50 20 0.65 > 4200 20 4.4 420-600 0 0.32 135 0 0.49 45 Peel 230 60 0 0.65 3.5 20 0.65 350 20 1.16 165 60 0 0.32 15 300 0 0.32 17 60 0 0.65 < 1 135 300 0 0.65 5 60 20 0.65 130 300 20 0.65 130 60 30 0.65 360 300 30 0.65 300* *After 300s bond had parted over 90 percent of initial bond area. Further loading fractured the coating to-board bond.

Claims (9)

1. A heat-sealed food package, capable of exposure to a radiant oven at 200"C for 30 minutes without leakage, made wholly or in part from paper board thinly coated on at least one side with polyethylene and irradiated with electrons, after coating but before heat-sealing, to give a degree of cross-linking with results or would result from irradiation of polyethylene without any sensitiser to a dose, averaged over the thickness of the coating, in the range 5-50 Mrad.

2. A package in accordance with Claim 1 characterised in that polyethylene is free of any additives.

3. A package as claimed in Claim 1 or Claim 2 characterised in that the radiation dose at the polyethylene/board interface is half as great as that at the surface of the polyethylene.

4. A method of making board for use in the package of Claim 1 comprising subjecting polyethylene coated paper board to high energy electrons impinging on the coated surface of the laminate at glancing incidence.

5. A method of cooking food comprising heating it while contained in a package claimed in any one of Claims 1 - 3 to a temperature higher than the softening-point of the polyethylene before cross-linking.

6. Food cooked by the method claimed in Claim 5.

7. A food package substantially as described with reference to the example.

8. A method of making polyethylene coated paper board for use in the food package of Claim 1 including an irradiation process substantially as described with reference to the drawings.

9. Minced beef cooked by the method described as an example of the invention.