METHOD OF USE OF POLYLACTIDE AND METHOD OF MANUFACTURING A HEAT-SEALED PAPER OR BOARD CONTAINER OR PACKAGE

Abstract

The invention relates to use of polylactide (PLA) as an extruded polymer coating on paper or board intended for the production of containers and packages, which are heated in a stove or microwave oven. According to the invention a polyfunctional cross-linking agent, such as trialkyl isocyanureate (TAIC), is blended with PLA, and the extruded coating layer is subjected to cross-linking electron beam (EB) radiation. PLA may be used as such or blended with another biodegradable polyester such as polybutylene succinate (PBS). EB radiation has been found to improve adhesion of the coating to the paper or board substrate, heat-scalability of the coating, and heat-resistance of the finished containers and packages.

Claims

1. A method for forming an extruded polymer coating on a fibrous substrate, comprising blending a polyfunctional cross-linking agent with polylactide, extruding the blend as a coating layer on the substrate, and subjecting the extruded coating layer to cross-linking electron beam (EB) radiation.

2. The method of claim 1, wherein the polymer-coated substrate is turned by heat-sealing into a container or package, which is configured to be heated in a stove or microwave oven.

3. The method of claim 1, wherein the coating layer is a monolayer coating layer.

4. The method of claim 1, wherein the polyfunctional cross-linking agent is triallyl isocyanureate (TRIC).

5. The method of claim 1, wherein the share of the polyfunctional cross-linking agent in the blend with polylactide is 1 to 5 wt %.

6. The method of claim 1, wherein the polylactide blended with the cross-linking agent is EB irradiated to improve adhesion to the paper or board substrate.

7. The method of claim 1, wherein the EB radiation is applied in a dosage in the range of 20 to 200 kGy.

8. The method of claim 1, wherein the EB radiation is carried out in an inert atmosphere, or in vacuum.

9. The method of claim 1, wherein another biodegradable polymer is used as a blend with the polylactide in the extruded coating.

10. The method of claim 1, wherein fibers or inorganic filler particles are included in the coating.

11. The method of claim 1, wherein the substrate comprises a coated board that is turned into a disposable drinking cup or food tray.

12. The method of claim 1, wherein the substrate comprises a coated board that is turned into a closed carton package.

13. The method of claim 1, wherein the substrate is a coated board that is turned into a container closed with a heat-sealed lid.

14. A method of manufacturing a container or package from polylactide-coated paper or board, comprising (i) blending polylactide with a polyfunctional cross-linking agent, (ii) extruding the blend as a coating layer onto the paper or board, (iii) subjecting the coating layer to cross-linking electron beam (EB) radiation, and (iv) turning the coated paper or board into the container or package by heat-sealing of the coating layer.

15. A method of manufacturing a container or package from polylactide-coated paper or board, comprising (i) blending polylactide with a polyfunctional cross-linking agent, (ii) extruding the blend as a coating layer onto the paper or board, (iii) turning the coated paper or board into the container or package, and (iv) subjecting the coating layer to cross-linking electron beam (EB) radiation.

16. A method according to claim 1, wherein the share of the polyfunctional cross-linking agent in the blend with polylactide is 2 to 3 wt-%.

17. A method according to claim 1, wherein EB radiation is applied in a dosage in the range of 50 to 100 kGy.

18. A method according to claim 8, wherein the inert atmosphere is nitrogen.

19. A method according to claim 9, wherein said another biodegradable polymer is polybutylene succinate (PBS).

Description

EXAMPLES

[0028] In the following, the invention is described in more detail by means of application examples and tests conducted.

[0029] An example of the preferred implementations of the invention is to extrude, onto paper or board made of kraft, CTMP or mechanical pulps, the weight of which is 40-500 g/m.sup.2, a polymer coating that substantially consists of PLA, or of a blend of 40-95 weight-% of PLA and 5-60 weight-% of PBS, and includes 1-5 weight-% of TAIC, and has a weight of 5-20 g/m.sup.2. The other side of the paper or cardboard can be left uncoated. The polymer-coated web is conveyed past an EB radiator, with its coated side towards the device, at a velocity of 5-600 m/m in, preferably 200-600 m/min. The EB-irradiated web is cut into blanks, which are heat-sealed into containers, such as paperboard trays, or packages, such as packing boxes or cartons. The sealing can be performed with hot air, whereby the air temperature can be about 420-470 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 145-160 C.; also in this case, the lowest for materials that are radiated the most.

[0030] Instead of a moving web, the EB 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 PLA-coated baking cardboard may be cited as an example.

[0031] In order to determine the effect of EB irradiation to adhesion of an extruded coating layer to a fibrous substrate a test series was performed with a monolayer of 35 g/m.sup.2 of PLA blended with 2 wt-% of TAIC extruded onto one side of a web of paperboard. The extruded coating layer was subjected to varying dosages of EB radiation. Adhesion to the surface of the board web, through ease of peeling off of the coating, was gauged on a scale of

[0032] 0=no adhesion

[0033] 1=slight sticking to the web

[0034] 2=sticking to the web

[0035] 3=firmly sticking to the web

[0036] 4=firmly sticking to the web, tearing some fibres

[0037] 5=firmly sticking to the web, tearing a lot of fibres

[0038] The EB radiation dosages were 0 (reference), 25 kGy, 50 kGy, 100 kGy and 200 kGy, and the levels of adhesion on the above scale were 3, 4, 5, 5 and 5, respectively. In other words, a dosage of 50 kGy turned out to improve adhesion from adequate to excellent, as the PLA coating layer no longer peeled off from the fibrous surface along the borderline between the board and the coating, but an attempted peeling caused tearing of the structure within the board. This is the standard requirement for perfect adhesion.

[0039] As a comparison similar adhesion tests were carried out for 35 g/m.sup.2 of PLA on paperboard without added TAIC. The levels of adhesion with EB radiation dosages of 0, 25, 50, 100 and 200 kGy were 2, 3, 3, 5 and 5, respectively. In other words, a dosage of 100 kGy was needed to achieve perfect adhesion.

[0040] The same EB irradiated samples of a monolayer of 35 g/m.sup.2 of PLA blended with 2 wt-% of TAIC on paperboard were then used for determining the effect of EB to heat-sealability. For each sample the initiation heat-seal temperature was measured, as the temperature of hot sealing air at an electrically heated air nozzle before hitting the surface of the coat layer. At the temperatures indicated the polymer had melted sufficiently for perfect sealing with the uncoated reverse side of the fibrous board. As in case of adhesion, the requirement is that attempted opening of the seal results in tearing within the structure of the board.

[0041] FIG. 1 is a diagram showing the heat-seal temperatures ( C.) for the different dosages of EB radiation measured as kGy. It is seen that the EB treatment markedly improves heat-sealability by gradually decreasing the heat-sealing temperature, from initial 500 C. down to 430 C., as the radiation dosage is increased from zero (ref=no treatment) up to 200 kGy.

[0042] As a comparison, FIG. 2 comprises results from a test series corresponding to that of FIG. 1 but for 35 g/m.sup.2 of PLA on paperboard without added TAIC. In this case the gradually increased EB radiation dosage brought the heat-sealing temperature from initial 500 C. down to 420 C.

[0043] FIG. 3 shows graphs plotting measured shear viscosities to shear rates from extruded polymer films, which have been remelted at 240 C. for the measurements. Graph 1 represents as a reference a film of mere PLA untreated with EB radiation, graphs 2 and 3 represent films of PLA blended with 3 wt-% of TAIC, which have been EB treated before remelting with EB radiation dosages of 100 kGy and 200 kGy, respectively, and graph 4 represents a film of PLA blended with 5 wt-% of TAIC, which has been EB treated before remelting with EB radiation dosage of 200 kGy. The conditions in heat-sealing are estimated to correspond to shear rates of about 5 to 50 1/s. It will be seen that within this range the use of TAIC in PLA and EB irradiation have clearly decreased the shear viscosity of the melt in comparison with the reference, which is an indication of improved heat-sealability, i.e. lower hot air temperatures required for heat-sealing.

[0044] Another important finding is that at low shear rates the use of TAIC and EB irradiation have markedly increased the shear viscosity of the melt, which shows that at static conditions the EB treated PLA blends have a superior thermal stability and heat resistance in comparison with untreated PLA. The increased viscosity, i.e. restricted macromolecular long range motion, can be interpreted as resulting from cross-linking between polymer chains, lending improved ovenability to the coated paper or board and to products made thereof. The heat resistance increased by means of cross-linking EB, as such known from the prior art, is thus preserved in spite of the detected improved heat-sealability.

[0045] FIG. 4 shows graphs plotting measured oscillation viscosities to angular frequencies for the same melted materials 1 to 4 as in FIG. 3. The results are very much in line with those from shear viscosities, confirming the improved heat-sealability and thermal stability achieved by use of TAIC as a cross-linking agent for PLA in EB irradiation.

[0046] PBS was in general found to have higher melt viscosities than PLA, both with EB irradiation treatment and without. It may be concluded that blends of PBS and PLA would yield still better heat resistance than PLA alone, but the heat-sealability could be impaired. Finding an optimal composition for such a blend to meet specific requirements for an ovenable product would be within the skills of an artisan.