Multilayer heat−sealable packaging material, a food package made thereof and a method of packaging

The subject of the invention is a multi-layer heat-sealed packaging material, a food packaging method, food packaging, the use of a multi-layer packaging material. More specifically, the packaging material is based on a middle layer of fibrous material such as cardboard and further comprises one or more protective layers to extend the durability of the packaged product, and a welded polymer on the surface of the material to close the package. Moreover, the invention relates to a method based on the use of this packaging material, a food package closed by heat sealing, and the use of the packaging material for packaging food, e.g. milk and juices in heat sealed cartons.

Attempts have been made to make packages for perishable liquid food products such as dairy products and juices to extend their shelf life by protecting against oxygen and odors. The traditional procedure is to use a fibrous packing material with a gray aluminum foil that provides sufficient protection against the ingress of oxygen in the air and the escape of odors from the packaged product. However, the use of aluminum foil, which has been used for a long time, is now declining due to its high cost, environmental risk and material recycling regulations. Aluminum does not break down in landfills and it is difficult to recycle aluminum-lined packaging cardboard.

Aluminum used as a barrier to oxygen and odors in food packaging is increasingly being replaced by polymers. The most important polymers are ethylene vinyl alcohol (EVOH), polyamide (PA) and polyethylene terephthalate (PET). By combining these polymers with adhesives and heat sealing polymers, a multi-ply cardboard has been produced with which sealing properties almost comparable to aluminum are achieved.

Another trend, also dependent on material costs and increasingly stringent environmental protection regulations, is the reduction of the amount of polymer used for the coating of packaging cardboard. One known food packaging board coated with a polymer, which can simultaneously achieve good oxygen and odor barrier and small amounts of material in polymeric heat seal and bond layers, is disclosed in printout of publication FI-96752. The advantage of the packaging board according to this publication is essentially the low sealing temperature of around 250 ° C, at which it is possible to avoid the formation of holes in the polymer layer in the closing step and the subsequent weakening of the gas barrier by this. In addition, the risk of deterioration of the taste and smell of the product is reduced because the sealing temperature is as low as possible.

According to the publication FI-96752, it is possible to introduce mica into the heat-sealed EVOH layer in order to improve the adhesion of this layer to the cardboard. It also turned out that mica influences the gas barrier and reduces the transmittance of ultraviolet radiation. Patent application FI-980086 further describes a multilayer packaging cardboard to which a significant amount of talc has been added to the polymer in the gas barrier layer. This application also describes the protection against ultraviolet radiation created by the heat sealing layer according to this application by adding to this layer, in addition to talc, also a pigment constituting at most 5 wt.%. this layer.

If the packaging cardboard and its polymeric coating layers are thick, they provide relatively good protection against the transmission of both ultraviolet radiation and visible light. The fact that the ultraviolet radiation used to weld the polymer has a weakening effect on the polyethylene is generally an advantage as it aids the decomposition of the material in landfills. However, as there is a desire to use ever thinner packaging material, visible light transmission is increased as a result. This is especially the case when bleached sulphate pulp is used in the material, which is best suited for food packaging due to its organoleptic properties. Unbleached pulp is effective in absorbing light, but its use in food packaging is avoided due to possible deleterious effects on aroma and taste. The lack of transmission of light or ultraviolet radiation has been found in traditional packaging materials containing aluminum foil.

The present invention is partially based on the observation that the multilayer packaging board according to publication FI-96752 and the corresponding modern packaging board can transmit up to 10% of visible light, and partially on the observation that, like oxygen, this light can have a detrimental effect on durability and quality. packaged food. The measurements carried out show that the light transmitted through the packaging material breaks down ascorbic acid in the juice, reducing it to about one third of its original amount when stored for five weeks. In an otherwise similar preservation study, in which the effects of light were removed, approximately 75% ascorbic acid remained at the end of the study.

Due to these surprising findings, it is an object of the invention to provide a solution by which harmful transmission of light through a packaging material can be avoided. The invention thus comprises a heat-sealable packaging material which comprises, in addition to the middle layer of fibrous material, at least one polymer sealing layer protecting the packed product and at least one polymer sealing layer as the surface layer of this material. The invention is characterized in that the heat sealing layer contains a light-absorbing pigment to protect the product from visible light.

The invention therefore requires that a pigment mixed with a heat sealable polymer be used that absorbs visible light in the wavelength range 400-700 nm. The most effective pigments are black pigments absorbing this entire wavelength range, such as soot, which is non-toxic and therefore suitable for food packaging. Carbon black also provides effective protection against ultraviolet radiation penetrating through the packaging.

The invention relates to a multi-layer heat-sealable packaging material, in particular for food packaging, comprising a support layer of fibrous material, at least one gas-tight polymer layer protecting the packed product and at least one heat-sealing polymer layer as a surface layer of the material, characterized in that the material comprises welded layer containing black and white pigments combined with welded polymer, the light absorbing black pigment protects the product from visible light, and the black and white pigments together impart a gray color to the sealed layer.

Preferably the black pigment is carbon black and the white pigment is titanium dioxide.

Preferably, the heat sealing layer comprises 0.05-0.5%, preferably 0.10-0.30%, and most preferably 0.10-0.20% black pigment based on the amount of polymer.

Preferably, the heat sealing layer comprises 5-25%, preferably 10-20% and most preferably 10-15% white pigment based on the amount of polymer.

Preferably, the polymer in the heat sealing layer is a polyolefin, such as low density polyethylene (LDPE).

Preferably, the material comprises polymeric heat sealing layers on both sides of the material, and black and white pigments are provided in the layer on the inner surface of the package.

Preferably, the polymer in the gas-tight layer is ethylene-vinyl alcohol (EVOH) or polyamide (PA) or a mixture thereof.

Preferably, the supporting layer is cardboard made of bleached sulphate pulp.

A further object of the invention is a multi-layer heat sealable packaging material, especially for food, comprising a backing layer made of fibrous material and at least one gray backing layer protecting the packed product, characterized in that the protective layer comprises a polymer in which the gray is achieved by mixing a black pigment therewith. light-absorbing, such as soot, and a white pigment, such as titanium dioxide, yes that the black pigment provides the product with protection against visible light, and the gray-colored polymer protective layer simultaneously acts as a heat seal layer.

Another object of the invention is a method of food packaging in which the box is bent from a packaging material comprising a middle cardboard layer, at least one gas-tight polymer layer and at least one heat seal polymer layer in which the food is sealed by heat sealing, characterized by that black and white pigments are added to the heat sealing polymer to protect the food from visible light and to give the heat sealing polymer layer a gray color similar to aluminum foil.

Preferably, the heat sealing layer is colored gray by mixing with the heat sealing polymer, 0.05-0.5%, preferably 0.10-0.20% black light-absorbing pigment, such as soot, and 5-25%, preferably 10 -15% white pigment such as titanium dioxide.

The box is preferably closed by heat sealing at a temperature of at most about 250 ° C.

Another object of the invention is a food package that contains perishable food, enclosed in a box made by folding and heat sealing a packaging material, comprising a cardboard support layer, at least one gas-tight polymer layer and at least one heat-seal polymer layer, characterized in that the material the packaging includes a sealed layer containing black and white pigments combined with a sealed polymer,

Wherein the black light absorbing pigment protects the packaged food from visible light, and the black pigment and white collectively impart a gray color to the sealed layer.

Preferably, the heat sealing layer is colored gray by mixing with the heat sealing polymer, 0.05-0.5%, preferably 0.10-0.20% black light-absorbing pigment, such as soot, and 5-25%, preferably 10 -15% white pigment such as titanium dioxide.

Preferably the heat sealable polymer is a polyolefin, preferably low density polyethylene (LDPE). Preferably, the barrier layer and the pigmented heat-seal layer are provided inside the support layer, and a colorless polymer heat-seal layer is provided on the package surface outside the support layer.

Another object of the invention is the use of a multilayer packaging material comprising a support layer of fibrous material, at least one gas-tight polymer layer, and at least one light-absorbing polymer sealing layer, comprising black and white pigment combined with a heat-sealable polymer, in a heat-sealed light-sensitive food package.

Preferably, multi-layer packaging material is used in a package for liquid food such as a milk or juice carton.

The incorporation of pigment into the heat seal layer is primarily due to the fact that the heat seal layer is substantially thicker than the other polymer coating layers in the multi-layer cardboard. The pigment is evenly distributed throughout the polymer layer without streaks disturbing the appearance of the layer. Secondly, the sealing properties of the polymers in the oxygen-retaining layer often depend on their purity, and the intention is to avoid the addition of foreign components thereto. The pigments mix well with the polyolefins normally used as the heat seal polymer and, at the concentrations used, they do not impair the extrudability or heat sealability of the polymer.

Particularly preferably, the heat seal layer of the food packaging material of the present invention has been colored gray by mixing a black pigment with a white pigment, respectively. The white pigment reflects light from the surface of the material and therefore reduces the light transmission through the material. However, the most important advantage is the appearance of the gray-colored material. The gray surface layer looks like the aluminum foil traditionally used in food packaging, to which consumers have become accustomed over time. It is very important that the material is approved on the market. The black pigment itself would result in an indescribably dark, aesthetically questionable surface of the material that could be returned by the markets, although the material is technically functional in all respects.

In practice, almost zero light transmission in the visible wavelength range can be achieved by adding a rather small amount of black pigment to the heat seal polymer. The amount of pigment in the heat sealing layer is, for example, 0.05-0.5% by weight, preferably 0.10-0.30% by weight, and most preferably 0.10-0.20% by weight. The limit is 0.2 wt.% And the higher content does not improve the light absorption in practice, but may be exceeded in order to achieve the appropriate level of gray. Given the appropriate level of gray, the amount of white pigment needs to be significantly greater than the amount of black pigment, e.g. about 5-25%, preferably 10-20% and most preferably 10-15 wt%. welded layer. By mixing 0.15 wt. % carbon black and 12 wt. titanium dioxide with low-density polyethylene PE-LD), the optimal concentration is obtained in the welded layer, which on cardboard resembles aluminum foil.

The density of the low-density polyethylene in the welding layers is, for example, 912-935 kg / m3³preferably 915-930 kg / m3³and the melt viscosity (MFR2) of 0.5-20 g / 10 min, preferably 3-10 g / 10 min. In addition to the pigments, at most 0.5% of a molecular sieve, such as sodium aluminosilicate, is introduced into the sealing layer as a deodorizing agent.

The heat-sealable package material normally has a polymer heat seal layer on both sides. Only the heat seal layer remaining inside the packages is then mixed with the pigment according to the invention. The welded layer on the outer surface of the package is left colorless so that it does not obscure the prints on the cardboard.

Regarding the oxygen-retaining layer of the packaging material according to the invention, attention should be paid in particular to the printout of publication FI-96752. In this publication, the oxygen-scavenging polymer may be EVOH, PET or PA, the latter mixed with EVOH, and also PA itself; separate EVOH and PA layers glued together. In addition, mixed sealing polymers are also possible

PL 204 684 B1 with minerals such as talc. In all cases, the oxygen and aroma barrier and the protection against visible light are combined in the same packaging material according to the invention.

The invention further comprises a heat-sealable packaging material which, in addition to the middle layer of fibrous material, comprises at least one polymer-based protective layer colored gray by admixing thereto with a light-absorbing black pigment, such as carbon black, and a white pigment, such as titanium dioxide, such that the black pigment provides the packed product with protection against visible light, and said gray protective layer simultaneously functions as a heat sealing layer of the material. The proportions of the mixture of pigments in the protective layer may be as set out above. The material is suitable for use with the packaged product, which must, above all, be protected from light during storage.

A method for packaging food according to the invention in which a box is bent from a packaging material comprising a central cardboard layer, at least one gas-tight sealing polymer layer and at least one sealing polymer layer in which the food is sealed by heat sealing, characterized in that the sealing layer is mixed with the light-absorbing pigment to protect the food from visible light. This heat sealing layer is preferably colored gray by using the pigments described above. The carbon black and titanium dioxide pigments used according to the invention do not interfere with the sealing of the package, so that approximately 250 ° C. is sufficient for sealing. The technique known from the publication FI-96752 can be used according to the invention without losing the advantages mentioned in the said publication.

The invention is further described with reference to the drawing, in which Figures 1-5 are schematic illustrations of five different laminated packaging materials according to the invention, and Figures 6 and 7 are light transmittance versus wavelength light measured for some packaging materials according to the invention and some packaging materials. according to the state of the art.

Figures 1-5 show examples of multi-layer heat sealable packaging materials according to the invention, generally for food packaging. The packaging material 1 consists in both cases of a heat-sealable surface layer 2 of low-density polyethylene (PELD), which forms the outer surface of the finished closed package. This is followed by a middle layer 3 of fibrous material, which may be, for example, bleached kraft cardboard. Further in fig. 1-3 is followed by an oxygen and perfume tight polymer layer 4 which is made of vinyl alcohol ethylene copolymer (EVOH), polyethylene terephthalate (PET) or a mixture of vinyl alcohol ethylene copolymer (EVOH) and polyamide (PA). The packaging materials of Figures 4 and 5 include two sealing layers 4, 5 bonded to each other and made of EVOH and PA. In Fig. 4, PA is placed against cardboard and EVOH is bonded to PA, and in Fig. 5 EVOH is placed against the cardboard and PA is bonded to EVOH. The sealing layer 4 or layers 4, 5 are followed by an adhesive layer 6, which is, for example, low-density polyethylene modified with maleic anhydride, and the purpose of this layer is to bond the sealing layer to the welded layer 7 of low-density polyethylene (PE-LD), which forms the inner the surface of the packaging.

In the packaging materials 1 shown, one or more pigments are used in the heat-sealable layer 7, forming the inner surface of the package according to the invention, so that visible radiation cannot penetrate into the closed package, as this radiation could damage the packed product. Preferably, the pigment used according to the invention is carbon black, which already at low concentrations provides almost perfect protection against light. According to the invention, a white pigment, such as titanium dioxide, can also be added to the heat-sealed layer 7, which has a certain effect on protection against visible radiation, but above all gives the sealing layer an aesthetic, pleasant appearance resembling aluminum foil.

In the packaging materials 1 shown, the grammage of the middle cardboard layer 3 is at least 170 g / m2²preferably 200-400 g / m 2². The superimposed polymer layers 4-7 can be placed on the cardboard 3 in one step by co-extrusion. The amount of material for the sealing layers 4, 5 and the adhesive layer 6 is 1-10 g / m2² in one layer, preferably 2-5 g / m2². The amount of material of both welded layers 1.7 is 5-60 g / m²preferably 20-50 g / m2²most preferably 30-40 g / m2². The carbon black content of the welded layer 7 forming the inner surface of the package is 0.05-0.5 wt.%, Preferably 0.15 wt.%, And the titanium dioxide content is 5-25 wt.%, Preferably approximately 12 wt.%. .

PL 204 684 B1

The packaging material of the invention, produced in the form of a continuous web, can be die cut into blanks, which in turn can be folded and heat sealed to form closed food packages. The sealing temperature is at most 250 ° C. The packaged products may in particular be food liquids, e.g. juices and dairy products such as milk, cream, sour milk, yoghurt and ice cream. Likewise, crate packages for dry foodstuffs such as flour, powder, flake, cereal and animal food packages are possible. Furthermore, closed ready-meal plates may be used, where both the plate and its lid are made of light-tight packaging material according to the invention.

Figures 6 and 7 show light transmission curves in the wavelength range 400-700 nm for visible radiation for certain multilayer packaging materials according to the invention and the prior art. In Fig. 6, the curve 8 was obtained for cardboard made of bleached cellulose pulp with a basis weight of 240 g / m 2.² and with a coating of 20 g / m² made of transparent polyethylene (PE-ED), without pigment. Curve 9 was obtained for a similarly coated cardboard made of bleached cellulose pulp with a basis weight of 300 g / m². The curve 10 was obtained for a similarly coated cardboard from unbleached cellulose pulp with a basis weight of 239 g / m²and the curve 11, illustrating the invention, was obtained for a cardboard made of bleached pulp with a grammage of 240 g / m 2² with 20 g / m coating² polyethylene (PE-LD) which has been colored gray by mixing with it 0.12% carbon black and 7.5% titanium dioxide.

Comparing the curves 8 and 9 in Fig. 6, it can be seen an increase in light transmission due to the reduction of the thickness of the cardboard made of bleached mass. The curve 10 further shows that if the board is made of unbleached pulp, then there is still a serious problem of light transmission. A substantially analogous light transmission of curve 11 is achieved by the layer of the polymer coating according to the invention dyed gray with white and black pigments.

Fig. 7 contains the light transmission curves measured as in Fig. 6 illustrating the effect of the amounts of titanium dioxide and soot on light absorption. The prior art curve 9 and the curve 11 illustrating the present invention are similar to the curves of Figure 6. Curve 12 was obtained on a 300 gsm bleached pulp board.² and coated with 20 gsm² polyethylene (PELD) containing 7.5% titanium dioxide. The curve 13 was obtained with the same cardboard that was coated with 17 g / m2² of said bleached polyethylene mixed with 3 g / m²of said gray-colored polyethylene. When comparing the curves in Fig. 7, it can be seen that titanium dioxide has a relatively little light transmission reduction effect, and that soot, even in small amounts of 0.018%, again reduces the material's light transmission to less than a third of that without adding a pigment.

The invention was further tested by following the change in ascorbic acid concentration in packaged apple juice over a five-week test at a storage temperature of 9-23 ° C. The packages were cardboard sealed by welding, in which the packaging cardboard according to the invention with a grammage of 240 g / m2 was used.²which was coated with 5 g / m2² vinyl alcohol-ethylene copolymer, 6 g / m² polymeric binder and on top 45 g / m² polyethylene (PE-LD) mixed with 0.12% carbon black and 7.5% titanium dioxide, which turned this layer gray. The reference material was packaging cardboard similarly coated with polymer, but without pigments added to the uppermost sealed layer.

The concentration of ascorbic acid in the juices was measured at the time of packaging and after a two-week and five-week storage period. The results are shown in the table below.

Table

Change in the concentration of ascorbic acid (mg / l) in apple juice

<td>Storage time</td><td> 0</td><td>2 weeks</td><td>5 weeks</td>

<td>According to the invention, 9 ° C</td><td> 450</td><td> 395</td><td> 355</td>

<td>According to the invention, 23 ° C</td><td> 450</td><td> 355</td><td> 340</td>

<td>State of the art, 9 ° C</td><td> 450</td><td> 375</td><td> 155</td>

<td>State of the art, 23 ° C</td><td> 450</td><td> 275</td><td> 145</td>

These results demonstrate the significant improvement in the stability of ascorbic acid in packaged juice achieved by the invention.

PL 204 684 B1

It will be appreciated by those skilled in the art that the various embodiments of the invention are not limited to the above examples, but may vary within the scope of the appended claims. Instead of cardboard, it is also possible to use paper as the middle layer of the packaging materials, and is suitable for dry packaging. Moreover, the polymer sealing layer can only be provided on one side of the packaging material. On the other side of this material, especially on the outer side of the dry packages, a heat-sealable varnish may be used.