Method for improving the heat sealability of packaging material and method for manufacturing heat-sealed container or package

Abstract

A method for improving the heat sealability of a packaging material and a method for manufacturing a heat-sealed container or package are described. The material can be polymer-coated packaging paper or cardboard, or a polymeric packaging film. The material includes a polymer layer that contains polyester, particularly polylactide, the heat sealability of which is improved by ultraviolet radiation. Polylactide is useful as such or when blended, for example, with other biodegradable polyester. The containers and packages thus manufactured include disposable drinking cups and cardboard carton and box packages.

Claims

1. A method of improving the heat sealability of a packaging material, comprising: extruding onto a web of fibrous substrate selected from paper or cardboard a coating layer containing polylactide, moving the coated web of fibrous substrate at a speed of 5-100 m/min, and improving heat-sealability by directing ultraviolet radiation from an electrically powered UV radiator to said coating layer while said coated web is being moved, to obtain a UV radiated web of packaging material.

2. The method according to claim 1, wherein the polylactide and polybutylene adipate terephthalate are blended.

3. The method according to claim 1, wherein an inner polymeric adhesion layer and an outer polylactide-containing layer are co-extruded onto the fibrous substrate.

4. The method according to claim 1, wherein a wavelength of the UV radiation is 100-400 nm.

5. A method of manufacturing a heat-sealed container or package, comprising: providing a fibrous substrate selected from paper or cardboard which is provided with a polymer coating that contains polylactide; UV-radiating the coating from an electrically powered UV radiator, while a web of the coated substrate is being moved at a speed of 5-100 m/min, to obtain a UV radiated web of packaging material; and sealing the container or package by heat-sealing the UV-radiated coating polymer.

6. The method according to claim 5, wherein the container is a cardboard cup, the vertical seam of its jacket being formed by heat-sealing a polymer coating of the inner surface of the cup.

7. The method according to claim 6, wherein the polymer-coated inner surface of the cup is heat-sealed to uncoated outer surface of the cup.

8. The method according to claim 6, wherein the fibrous substrate is provided on both sides with a polymer coating and that, in the heat sealing of the container or package, the polymer coatings of its inner and outer surfaces are sealed to each other.

9. The method according to claim 5, wherein the package is a cardboard box package, its polymer-coated outer surface being heat-sealed to uncoated inner surface of the package.

10. The method according to claim 5, wherein the fibrous substrate is provided on both sides with a polymer coating and that, in the heat sealing of the container or package, the polymer coatings of inner and outer surfaces of the container or package are sealed to each other.

11. The method according to claim 5, wherein an inner polymeric adhesion layer and an outer polylactide-containing layer are co-extruded onto the fibrous substrate.

12. The method according to claim 5, wherein the heat sealing is carried out with hot air.

13. The method according to claim 5, wherein heat sealing is carried out by heated sealing jaws.

14. A method of sealing polylactide coated packaging material, comprising: directing ultraviolet radiation from an electrically powered UV radiator to a surface of a moving paper or cardboard web moving at a speed of 5-100 m/min, said surface comprising polylactide or a polymer mixture that contains polylactide, to obtain a UV radiated web of packaging material, whereafter the radiated surface is heat-sealed to a counter surface.

15. The method according to claim 14, wherein a packaging film comprising polylactide or a polymer mixture that contains polylactide, or a surface layer of the film is heat-sealed.

Description

BRIEF DESCRIPTION OF THE DRAWING FIGURES

(1) FIG. 1 shows sealing results for example 1.

(2) FIG. 2 shows sealing results for example 2.

(3) FIG. 3 shows sealing results for example 3.

(4) FIG. 4 shows sealing results for example 4.

DETAILED DESCRIPTION OF THE INVENTION

(5) One example of the preferred implementations of the invention is to co-extrude, onto paper or cardboard made of kraft, CTMP or mechanical pulps, the weight of which is 40-350 g/m.sup.2, a two-layer coating that has an inner adhesion layer of a weight of 5-20 g/m.sup.2 that consists of biodegradable polyester (other than polylactide), such as PBAT, or a mixture of polylactide (50-95 weight-%) and other biodegradable polyester (5-50 weight-%), such as PBAT, and an outer heat-sealing layer of a weight of 5-20 g/m.sup.2 that consists of polylactide or a mixture of polylactide (40-80 weight-%) and other biodegradable polyester (20-60 weight-%), such as PBAT. The other side of the paper or cardboard can be left uncoated. The polymer-coated web is conveyed past an ultraviolet lamp, the wavelength of which is 100-400 nm, with its coated side towards the lamp, at a velocity of 5-100 m/min, preferably 5-20 m/min. The UV-radiated web is cut into blanks, which are heat-sealed into containers, such as cardboard drinking cups, or packages, such as packing boxes or cartons. The sealing can be performed with hot air, whereby the air temperature can be about 310-400 C. For materials that are radiated more intensively, that is, at a slower web velocity, the air temperature required for a complete sealing is lower than for materials that receive less radiation. Instead of hot air, sealing jaws can be used, the temperature of which can be about 130-160 C.; also in this case, the lowest for materials that are radiated the most.

(6) Instead of a moving web, the UV radiation can also be directed to the sealing lines of a web or blank that is stationary with respect to the radiator, which lines thus receive a larger portion of radiation, while the other parts of the polymer surface are not exposed to radiation. Tray blanks consisting of PET-coated baking cardboard should be cited as an example.

(7) In the following, the invention is described in more detail by means of application examples and tests conducted.

Example 1

(8) Onto packing board weighing 280 g/m.sup.2, an inner adhesion layer was co-extruded, consisting of polybutylene adipate terephthalate (PBAT) and weighing 10 g/m.sup.2, and an outer heat-sealing layer that consisted of technical polylactide and weighed 15 g/m.sup.2. The extruded two-layer coating was allowed to cool. Thereafter, the coated cardboard web was conveyed at a velocity of 10 m/min to an UV radiator (UV-technik, model UVH-lamp), the electric power of which was 120 W/cm and its energy spectrum in different wavelength ranges was about 15% UV-C, about 8% UV-B, about 7% UV-A, about 15% visible light, and about 55% IR. A blank that was cut from the radiated cardboard was bent and heat-sealed into the conically widening jacket of a drinking cup, so that the coated and radiated inner surface of the cup was sealed to the uncoated outer surface of the cup that was not radiated. The sealing was carried out with hot air of 340 C., and by hot sealing jaws, the temperature of which was 130 C. In each case, a perfect sealing was accomplished, during the opening of which, a 100% tear occurred in the fibre layer. In other words, no opening of the seal by peeling off could be observed.

Example 2

(9) Onto the packing board used in example 1, a single-layer coating was extruded, its composition being 45% polylactide and 55% PBAT, and weighing 24 g/m.sup.2. After the coating had cooled, the coated web was radiated by the UV radiator used in example 1, the velocity of the web being 10 m/min. A blank that was cut from the coated and radiated cardboard was bent and heat-sealed into the jacket of a drinking cup, similarly to example 1. The sealing was carried out with hot air of 340 C. A perfect sealing was accomplished, during the opening of which, a 100% tear occurred in the fibre layer.

Example 3

(10) Onto the packing board according to example 1, a single-layer coating was extruded, its composition being 95% polylactide and 5% ethylene butyl acrylate glycidyl methacrylate terpolymer, and weighing 25 g/m.sup.2. After cooling, the web was UV-radiated and a blank that was cut from the web was bent and heat-sealed into the jacket of a drinking cup, similarly to example 2. A perfect sealing was accomplished by sealing with hot air of 340 C., during the opening of which, a 100% tear occurred in the fibre layer.

(11) Testing

(12) The reference material was packing board weighing 210 g/m.sup.2, which had an extruded low density polyethylene (LDPE) single-layer coating on it, weighing 25 g/m.sup.2. The test material was packing board weighing 210 g/m.sup.2, which had a polylactide (PLA) single-layer coating extruded onto it, weighing 27 g/m.sup.2. A test series was created from this by conveying the PLA-coated cardboard past the UV radiator mentioned above in example 1 at five different velocities, which were 5 m/min, 10 m/min, 20 m/min, 40 m/min, and 80 m/min. Additionally, the test series included a PLA-coated cardboard that was not radiated.

(13) The materials described above were heat-sealed, in each case, by sealing the polymer-coated side of the cardboard to the uncoated counter surface of the cardboard. The sealing was carried out with hot air or hot sealing jaws at different temperatures to find the lowest temperature, at which the perfect sealing could be achieved. The criterion then was for the seal not to open by peeling off, but by tearing off 100% from the cardboard layer.

(14) The results of the sealing tests are presented graphically in the appended drawing 1. It can be observed that the PLA-coated cardboard that was not UV-radiated required a considerably higher sealing temperature than the LDPE-coated cardboard that was used as reference material. The UV radiation clearly decreased the sealing temperature of the PLA-coated cardboard at all web velocities used; the more, the slower the web velocity was. For the PLA-coated cardboard that was radiated at a velocity of 10 m/min, the sealability was at least as good as that of the reference material, the LDPE-coated cardboard that was not radiated, and at a velocity of 5 m/min, even clearly better. The result was essentially the same in the sealing carried out both with hot air and the hot sealing jaws.

(15) The tests were continued by the heat sealing of the cardboard obtained according to the example 2 above and coated with the mixture of PLA and PBAT, the cardboard obtained according to example 3 and coated with the mixture of PLA and ethylene butyl acrylate glycidyl methacrylate terpolymer, and the PET-coated (25 g/m.sup.2) cardboard (280 g/m.sup.2). The sealing processes were carried out with hot air only. In addition to the UV-radiated test materials, the reference materials comprised the same coated cardboards without the UV radiation. The results are shown in the appended drawing 2. It can be seen that for both coating compositions containing PLA, the UV radiation clearly decreases the temperature required for a perfect heat sealing. For PET, the decrease in sealing temperature was minor and yet adequate to render the UV radiation reasonable, in practice.

(16) In a third test series, the sealing of a PLA coating to itself on the cardboard was studied. PLA-coated cardboard, which was of the same grade as in the first test series, that is, a 27 g/m.sup.2 PLA coating layer on cardboard weighing 210 g/m.sup.2, was radiated by the UV radiator according to the above at web velocities of 10-80 m/min. The reference material comprised PLA-coated cardboard that was not radiated. The sealing was carried out by bending the cardboard and sealing its PLA coating against itself. The results are shown graphically in the appended drawing 3. By comparing with the first test series, it can be observed that it is clearly easier for the PLA layer to seal to itself than to an uncoated cardboard surface. Also in this case, however, the UV radiation of the PLA coating clearly improves the sealability.

(17) In a fourth test series, PLA-coated cardboard according to the above was radiated by the above-mentioned UV radiator at web velocities of 20 m/min and 50 m/min, and the sealing was carried out by hot sealing jaws. The reference material comprised PLA-coated cardboard that was not radiated. The sealing temperature of the jaws was gradually raised to find the lowest temperature, at which the sealing would be perfect. The results are presented graphically in the appended drawing 4, where the vertical axis describes the sealing on the scale: (1) no sealing, (2) poor sealing; the seam rustles when opening, (3) poor sealing; the cardboard tears on less than 50% of the seam's surface area, (4) reasonable sealing; the cardboard tears on over 50% of the seam's surface area, and (5) perfect sealing; the cardboard tears throughout the seam's surface area. It can be observed that the UV radiation of the PLA coating also improves the sealing carried out by the jaws; in particular, at a slower web velocity of 20 m/min, the difference to the reference material not radiated is obvious.