Multi-layer product.

Multilayer product especially for the manufacture of packaging

The present invention relates to a multilayer product in particular for the production of packages, which multilayer product comprises a layer of fibrous packaging material having a gas-tight multilayer plastic coating on one surface comprising a barrier plastic layer, a layer of binder and a layers closest to the fiber-based layer of packaging material.

Multilayer products of the type described above are now well known, for example, in connection with the packaging of citrus juices. The fiber-based material is often cardboard. An example of a previously known solution is the solution disclosed in EP-A-0 293 098.

In connection with previously known solutions, e.g. the solution according to EP-Jul20 0 293 098, problems have been observed in the manufacturing stage of packages, in particular when heat-sealing plastic surfaces together, whereby holes may form in the multilayer plastic coating. The formation of holes is particularly problematic if they form on the inner surface of the package. The good gas-tightness properties of the plastic coating are then partially lost, because gas flows can occur even through small micro-holes, even if the coating is liquid-tight despite the existence of the micro-holes.

The perforation of the plastic coating is due to e.g. that the high heat applied to the heat seal coating softens and partially melts the plastic layers. The reason for the holes is also that the board is also heated from the sealing area and the moisture in the board tends to pass through the softened and partially melted plastic layers under vapor pressure, creating holes in the coating. The situation is that the higher the heat sealing temperature and the higher the moisture content in the board, the more susceptible to the formation of holes. The vapor pressure of the board tends to split the plastic layer against the board in small places. The cracked site acts as an initiator of hole formation.

The object of the invention is to provide a multilayer product by means of which the disadvantages of the prior art solution, e.g. the solution according to EP 0 293 098, can be eliminated. In connection with the invention, it has been found that by selecting a heat-sealable plastic grade of a multilayer structure so that heat-sealing can take place at a low temperature, perforation can be reduced and even completely prevented. Em. the choice of the plastic layer has the effect that the plastic layers do not soften or lose their strength as strongly as in previous solutions, making it difficult to form holes. On the other hand, the vapor pressure generated by the moisture of the board decreases. In addition, the formation of holes can be resisted by choosing a plastic grade that remains tough and strong at the heat-sealing temperature as the plastic layer against the board. The formation of holes can be further influenced by selecting a plastic type as one of the layers, which remains tough in the heat seal. The multilayer product according to the invention is characterized in that the layer thickness of the barrier plastic layer is in the range from 1 to 4 g / m²and the layer thickness of the binder layer is in the range of 1 to 4 g / m 2².

The advantage of the invention compared to the prior art is, above all, that the multilayer product is provided with higher strength and elongation than before, and the above-mentioned properties are also maintained at high temperatures, such as heat sealing. Due to the above-mentioned fact, the invention is achieved with coating thicknesses up to 15-30% lower than before. In addition, it is essential that in heat sealing the so-called hot tack and seam strength are better than before, so that in heat sealing the line speed can be increased by 10 - 20% using the same temperatures and amounts of heat. Em. this contributes to reducing perforation in the plastic coating. A further advantage of the multilayer product according to the invention is that the Environmental Stress Crack Resistance ESCR is much better than before. This feature is significant in the so-called. in the packaging of hard to hold products. For example, in liquid detergent packages, a good ESCR allows the total thickness of the plastic coating to be reduced by about 30% if a shelf life of about one year is desired. A further advantage of the invention is good wear resistance. As a result, no plastic dust is generated on the fast packaging production lines and the blanks run more evenly on the lines. As an example in this connection, it can be mentioned that when using, for example, linear low-density polyethylene, i.e. PE-LLD material, as the surface layer of the multilayer plastic coating, the wear resistance is about twice as good as with previously known products. The other advantages presented above have also been described mainly with the PE-LLD material being the surface layer material of the multilayer plastic coating.

The invention will now be explained in more detail by means of the preferred embodiments described in the accompanying drawing, in which Figures 1-6 of the drawing show some preferred embodiments of the multilayer product according to the invention.

Figure 1 shows a first preferred embodiment of a multilayer product according to the invention. Reference numeral 1 in Fig. 1 denotes a fiber-based packaging material, which may be, for example, cardboard. In this embodiment, a layer 2 of low density polyethylene, PE-LD, is arranged on the second surface of the board. The layer 2 is intended to form the outer surface of the finished package. PE-LD layer 2 is not necessary, as the outer surface of the finished package can be formed anyway. Examples are a lacquer surface or a surface otherwise treated to repel water, or in some cases even an untreated board surface.

In this application, a gas-tight multilayer plastic coating 3 is applied to the second surface of the carton 1, i.e. to the surface intended to form the inner surface of the finished package.

In the example of Figure 1, the barrier plastic layer 4a is formed of 32% ethylene vinyl alcohol copolymer material and the surface layer 6 forming the inner surface of the package is in turn made of linear low density polyethylene, i.e. PE-LLD. The binder layer 5 may be, for example, acid-modified PE-LLD. In the example of Figure 1, the layer thicknesses of the different layers of the multilayer plastic coating are as follows, barrier plastic layer 4a 1-4 g / m², binder layer 5 1-4 gm² and the thickness of the surface layer 6 is selected so as to be sufficient for liquid-tight heat sealing. The thickness of layer 6 may vary depending on the product to be packaged, for example with juices of less than 20 g / m 2² and aggressive substances such as detergents, for example more than 30 g / m 2². The thickness of the PE-LD layer 2 on the other side of the board can be, for example, 20 g / m 2².

The solution presented above provides a more economical thin coating and better barrier properties. The solution according to Figure 1 still has the advantages of good mechanical wear resistance, advantageous ESCR, good heat sealability, low seal temperature and good post-treatment properties. The application according to Figure 1 is furthermore a preferred so-called hard-to-hold with liquids.

Figure 2 shows another preferred embodiment of the invention. The example of Fig. 2 essentially corresponds to the example of Fig. 1, so in Fig. 2 the same reference numerals as in Fig. 1 are used. The only difference to the example of Fig. 1 is that mica (mica) mixture. Em. the mixture is a substance known per se and is described in more detail eg U.S. Patent 4,818,782.

The mica gives EVOH plastic good adhesion to the board in the extrusion coating and good adhesion contributes to re ducing the bubbling sensitivity. Mica also gives EVOH plastics more strength and this increase in strength is especially important in heat sealing, reducing the sensitivity to bubbling. Mica is also a better thermally conductive material than EVOH plastic, so the heat of heat sealing is not stored as strongly in EVOH plastic.

The mica gives EVOH plastic good gas barrier properties and acts to reduce the transmission of UV light. In terms of recycling, EVOH + mica works better than EVOH alone.

Thanks to the enamel, the heat-sealing temperature can be kept at the same temperature in the blank production and filling machine as when using Al foil board. Similarly, the so-called heat sealing. the operating window in the case of EVOH + mica is as wide as when using Al-laminated board. Without mica, the heat sealing temperature must be reduced by about 30 ° C and the operating window must be reduced by about 20 ° C. However, when mica addition is used, it is best to operate at the lower end of the operating window, where the vapor pressure of the water contained in the board is low and no micro-holes occur at all. Without enamel, the presence of micro-holes would be relatively likely.

Figure 3 shows a third preferred embodiment of the invention. The embodiment of Figure 3 substantially corresponds to the embodiments of Figures 1 and 2. The only difference is that in the embodiment of Figure 3, the barrier plastic layer 4c is formed from a mixture of an ethylene vinyl alcohol copolymer and an ethylene polymer containing CO groups. In Fig. 3, the same reference numerals as in Figs.

As stated above, in the embodiment of Figure 3, the barrier plastic layer 4c is formed from a mixture of an ethylene vinyl alcohol copolymer and an ethylene polymer containing CO groups, i.e., a mixture of EVOH + ECO. The melt strength of ECO-containing EVOH plastic is better than the melt strength of EVOH plastic alone. Good melt strength is an important factor for extrusion coating. The poor melt strength of pure EVOH plastic causes perforation in the EVOH layer already directly in the extrusion coating. This is because the fibers of the board to be coated tend to puncture the weak molten EVHO layer. For example, the addition of ECO at 30% by weight to EVH0 improves the melt strength so substantially that the fibers do not perforate the molten plastic film. This, in turn, allows EVOH + ECO to be coated directly against the board.

Good melt strength prevents EVOH + ECO plastic from cracking during heat sealing and thus prevents the formation of micro-holes. The EVOH + ECO plastic layer generally remains very strong and tough in heat sealing, and thus the bubbling sensitivity is substantially lower than with EVOH plastic alone.

EVOH + ECO is also less sensitive to fluctuations in ambient humidity than EVOH alone. This is reflected in the heat sealing properties so that the vapor pressure from the board does not impair the strength of the mixture as in the case of pure EVOH plastic. On the other hand, the decrease in sensitivity to water is also reflected in a clear decrease in the effect of ambient humidity on gas tightness.

Figure 4 shows a fourth preferred embodiment of the invention. The embodiment of Figure 4 substantially corresponds to the embodiments of Figures 1-3. A significant difference from the previous 96752 silicon examples is that the barrier plastic layer 4d is formed from a mixture of ethylene vinyl alcohol copolymer and polyamide. In Fig. 4, the same reference numerals as in the examples of Figs.

In the embodiment of Figure 4, the barrier layer is EVOH + PA, i.e. a mixture of EVOH plastic and PA. Various mixtures of EVOH and PA are known per se, for example U.S. Patents 4,952,628, 5,110,855 and 5,126,401. PA plastic is a very tough and strong plastic. The melting point of PA is between 230-240 ° C while the melting point of EVOH is 168 ° C. The gas tightness properties of PA are not as good as those of EVOH. By mixing EVOH and PA together, the following advantages are achieved in multilayer solutions. Strength and toughness are improved both in connection with extrusion coating and in heat sealing. Due to this, perforation does not occur during the extrusion coating and, on the other hand, due to the toughness, the EVOH + PA layer also does not crack and does not allow vapor pressure to pass through. The operating window is 30 ° C wider than EVOH alone for heat sealing. Pure PA is too tough plastic for pulping. EVOH + PA in a suitable ratio is a suitable solution for pulping.

Figure 5 shows a fifth preferred embodiment of the invention. This application also substantially corresponds to the previous examples. The difference from the previous examples is that in this example the barrier plastic layer 4e is formed of a thermoplastic polyester or a thermoplastic copolyester. Examples of the above materials include polyethylene terephthalate and polybutylene terephthalate. In Fig. 5, the same reference numerals as in the previous examples are used in the corresponding sections.

In the example of Figure 5, the barrier plastic is polyethylene terephthalate PET. The gas tightness properties of PET are not as good as those of EVOH or PA, but it is significant that the barrier properties of PET do not change under the influence of ambient humidity. PET has good heat resistance properties, melting point 255 ° C. PET also has good strength properties and remains tough at temperatures above 200 ° C. Thus, PET does not microrelink during heat sealing, nor does perforation occur during extrusion coating. PET plastics only have sufficient adhesion to the board when the amount of PET coating is about 40 g / m² or more. However, by using co-extrusion coating, for example to form a 3-layer coating, adhesion can be achieved even with small amounts of PET coating. All the amounts of coating mentioned in connection with Figure 1 are also suitable for the applications of Figures 2-5.

A further feature of PET plastics is that variations in melt viscosity cause even severe instability in the extrusion coating. In three-layer extrusion coating, the instability of PET plastics is compensated by the good stability of the two layers. Good Stability improves basis weight variations and the corrugation of the edges of the plastic coating disappears and these factors together make the use of PET economical for production. As a rigid plastic, PET plastic also improves the rigidity of the package and enables long-term storage without bulging the package. Em. this is particularly important in the case of aseptic packaging.

In all the above applications, the barrier plastic layer is arranged directly against the fiber-based packaging material layer 1. Such an arrangement is particularly advantageous when the surface of the layer of fibrous packaging material is sufficiently smooth. An example of such a material is hot calendered board. If the surface of the board is rough, it is advantageous to place a leveling layer on the surface of the board, which smooths out the unevenness of the board surface, whereby a barrier plastic layer of preferred thickness provides the desired effect. The upright fibers of the uneven board surface easily extend through the barrier plastic layer, impairing the functionality of the product. The purpose of the leveling layer is to achieve the desired functionality without having to increase the thickness of the barrier plastic layer unreasonably.

Figure 6 shows an embodiment based on the embodiment of Figure 1. In the embodiment of Figure 6, the above-mentioned leveling layer 7 is used between the barrier plastic layer 4a and the fiber-based packaging material layer 1. The embodiment of Figure 6 also does not have a PE-LD layer on the second surface of the layer 1, but for example, a lacquer layer is applied to the surface as described above. However, the lacquer layer is not shown in Figure 6. It is clear that the application according to Figure 6 can also be formed from the applications of Figures 2-5. Furthermore, it is clear that even in the case of an application utilizing the leveling layer 7, the PE-LD layer coming to the outer surface of the finished package can be used. The leveling layer 7 can preferably be formed of the same material as the binder layer 5. The thickness of the leveling layer 7 can preferably be of the same order as the thickness of the binder layer, for example less than 3 g / m².

The multilayer structure according to the invention can be manufactured, for example, in the following manner. The preparation is explained in the following by means of the example according to Figure 2, but it is clear that the preparation of all embodiments can take place in a similar way. If necessary, the paperboard web 1 to be coated can be pretreated by flame, corona, primer or plasma methods on both sides. The pretreated board or paper is coated on both sides in one go as follows. The PE-LD coating 2 of the surface forming the outer surface of the finished package is made on a flame-treated board using an extrusion method. The plastic coating 3 on the inner surface of the finished package is made in one run by extruding all the necessary layers at once so as to obtain the 3-layer coating shown in the figure. This allows low extrusion temperatures to be used so that heat-sensitive EVOH does not thermally decompose. EVOH + mica is fed from its own screw, binder polymer from its own screw and PE-LLD from its own screw. The outer surface of the paperboard laminate coated as described above is electro-corona treated to allow the ink to adhere and to improve heat sealing. Other processing that promotes printing, such as PG or Printable Gluable processing, is also possible. The paperboard-plastic laminate thus prepared can now be heat-sealed by conventional methods.

The above invention is particularly advantageous as a material for citrus juices, water and other liquid food packaging, for example in packaging that is folded into square, rectangular or cylindrical packaging.

The above invention is in no way limited to the accompanying application examples, but the invention can be modified completely freely within the scope of the claims. Thus, it is clear that the multilayer product according to the invention or its details do not necessarily have to be exactly as shown in the figures, but other solutions are also possible. For example, layer 6 is not limited to PE-LLD material, but the polyolefin of the layer may be, for example, low density polyethylene, high density polyethylene, polypropylene, polybutene, etc. The polyolefin material of layer 6 may also be a blend, for example a blend of linear low density polyethylene and low density polyethylene. The fiber-based packaging material does not necessarily have to be cardboard, but other materials are possible, for example paper, plastic-coated cardboard, etc.