COATING AND METHOD OF PRODUCTION THEREOF

Abstract: A coating and a method of producing it are disclosed. The coating comprises a polymer layer (9), which is extrusion-coated on a substrate (1), the coating being characterised in that it comprises particles (11) of an essentially hydrophobic aluminium silicate zeolite located between the substrate (1) and the polymer layer (9) extrusion-coated thereon, and that the zeolite has a pore diameter of at least 5.5 ANGSTROM , a molar ratio Si/Al in the crystal lattice of at least 35, and a sorption capacity for water at 25 DEG C and 4.6 torr below 10 % by weight. When producing the coating (13), particles (11) of the above zeolite are introduced between the polymer layer (9) and the substrate (1) before the polymer layer (9) is extrusion-coated on the substrate (1). Preferably, the zeolite particles (11) are sprinkled on the substrate surface just before this is brought into contact with the extruded polymer layer (9).

Description: COATING AND METHOD OF PRODUCTION THEREOF The present invention relates to a coating comprising a polymer layer which is extrusion-coated on a substrate, as well as a method of producing such a coating. To be more specific, the invention concerns a coating and a method of production enabling the elimination of substances producing an undesirable odour and/or taste. In many contexts, substances having an undesirable odour and/or taste are produced as a result of the chemical decomposition or conversion of plastic in connec- tion with manufacture or processing, for instance compounding. These substances can be of highly diverse chemical natures, and may constitute a health hazard or merely a source of irritation. Since human beings normally are extremely sensitive to smells and tastes, these substances need not be present in particularly high concentrations in order to be noticed, and as low concentrations as one or a few ppm are mostly sufficient to render a taste or smell quite palpable. As an example of an area in which this problem with undesirable odour- and/or taste-producing substances is of importance, mention may be made of plastic tubes, in particular plastic water pipes, where the fact that plastic material gives off odour- and/or taste-producing substances to the drinking water flowing through the pipe often constitutes a serious problem that is difficult to solve. This problem is also encountered in connection with plastic films and foils as well as containers, such as bottles, made of blow-moulded or moulded plastic and intended for the packaging of food, beverages, or medicine. In an effort to remedy such problems associated with an undesirable odour and/or taste, US 5,436,282, for instance, teaches the homogeneous admixture to the plastic material, while processed in a melt, of less than 0.5% by weight of an essentially hydrophobic aluminium silicate molecular sieve. The aluminium silicate mole- cular sieve (zeolite) is of the type disclosed in US 4,795,482 and consists of an essentially hydrophobic aluminium silicate molecular sieve having a pore diameter of at least 5.5 A, a molar ratio Si/Al in the crystal lattice of at least 35, and a sorption capacity for water at 25 °C and 4.6 torr below 10% by weight. It should here be observed that the above-mentioned molar ratio Si/Al of the molecular sieve only applies to the oxide units of the crystal lattice. The molar ratio Si/Al of the zeolite determined by conventional wet analysis may be much lower as a result of a contamination with aluminium-containing impurities formed in the so- called dealuminising reaction frequently being a substep in the production of the zeolite. The above zeolitic molecular sieves have also been used to get rid of undesirable smell in connection with sanitary products, such as napkins and incontinence towels (see EP 0 348 978). The elimination of an undesirable odour and/or taste in plastic materials in accordance with the above US Patent 5,436,282, achieved by admixing the special zeolite to the plastic material, functions in satisfactory manner in the areas of application indicated above. It has, however, been found that the elimination of an unde- sirable odour and/or taste need be further improved in one particular area, namely the extrusion coating of plastic on a substrate, such as paper and paperboard. The extrusion coating of plastic on a substrate is a technique primarily used for coating paper or paperboard with ethylene plastic. In general terms, extrusion coating is carried out as follows. Thus, the plastic to be extrusion-coated is heated to a high temperature of above about 250°C, such as about 250-350°C, preferably about 270-330°C, and is extruded through a slit die in the form of a film having the high temperature mentioned above. The polymer film is fed downwards towards a roll nip located between a cooling roll and a press roll applied against the cooling roll. Simultaneously, a substrate consisting of a weblike material, such as paper, paper- board, metal foil, plastic film or plastic foil, is fed towards the roll nip. The weblike substrate and the extruded polymer film are brought together in the roll nip, and the polymer film is laminated under pressure onto the substrate. The substrate thus provided with an extrusion coating is then discharged at the other side of the nip and may, for instance, be used for producing packages for various products, such as food and medicine. Since, for health and safety reasons, extremely high purity requirements are placed on food and medicine packages, thereby to prevent any foreign substances from contaminating the food or medicine, it is extremely i - portant that the plastic film or foil used for such products does not give off any undesirable or contaminating substances. The specific problem regarding the generation of an undesirable odour and/or taste in extrusion coating of plastic on a substrate seems to be a consequence of the high temperature prevailing in the extrusion coating. Normally, temperatures in the order of 100-200 °C are used in the production and processing of plastic, whereas the temperature used in extrusion coating is much higher and in the order of about 250 C C or above, such as about 250-350 °C. It is assumed that it is this high temperature in combination with the presence of oxygen ( air) that causes the special problems with an undesirable odour and/or taste occurring in the extrusion coating of plastic. According to the present invention, it has now surprisingly been found that the disadvantages of the generation of undesirable odour- and/or taste-producing substances in extrusion coating of plastic on a substrate can be obviated by sprinkling zeolite particles of the above type on the substrate surface before this is brought into contact with the extruded polymer layer. Instead of being homogeneously distributed inside the extrusion-coated polymer layer, as is the case in the prior art according to US Patent 5,436,282, the zeolite particles according to the invention will be provided between the substrate surface and the polymer layer extrusion-coated thereon. Thus, the coating on the substrate comprises an extrusion-coated polymer layer as well as a layer of zeolite particles. In this context, it must be regarded as unexpected that the inventive technique serves to eliminate the giv- ing-off of odorous and/or tasting substances from the free surface of the extrusion-coated polymer layer, in spite of the fact that the zeolite particles are enclosed in the interface between the polymer layer and the sub- strate, at a distance from the free surface of the polymer layer . The present invention thus provides a coating comprising a polymer layer, which is extrusion-coated on a substrate, said coating being characterised in that it comprises particles of an essentially hydrophobic aluminium silicate zeolite located between the substrate and the polymer layer extrusion-coated thereon, and that the zeolite has a pore diameter of at least 5.5 A, a molar ratio Si/Al in the crystal lattice of at least 35, and a sorption capacity for water at 25 °C and 4.6 torr below 10% by weight. The invention further provides a method of producing a coating on a substrate, said coating comprising a polymer layer which is extrusion-coated on the substrate, said method being characterised in that, before the polymer layer is extrusion-coated on the substrate, zeolite particles are introduced between the polymer layer and the substrate, the zeolite being an essentially hydrophobic aluminium silicate zeolite having a pore diameter of at least 5.5 A, a molar ratio Si/Al in the crystal lattice of at least 35, and a sorption capacity for water at 25°C and 4.6 torr below 10% by weight. Further characteristics and advantages of the invention will appear from the following description and the appended claims. Preferred embodiments of the invention will now be described with reference to the accompanying drawing, in which Fig. 1 schematically illustrates a device for extrusion coating; and Fig. 2 is an enlarged sectional view of the extru- sion-coated material according to A in Fig. 1. In extrusion coating, a two-dimensional, i.e. a sheetlike or weblike, substrate is provided with a coating by extrusion of melted plastic at a high temperature. The weblike material being coated in accordance with the invention may consist of any of a great number of different materials, and is preferably selected from cel- lulosic materials, metallic materials and polymeric materials. Among cellulosic materials, mention may especially be made of paper and paperboard, whereas metal foils, such as aluminium foil, are preferred metallic materials. As regards polymeric materials, mention may be made of plastic films and plastic foils, for instance of polyethylene terephthalate, polyamide, biaxially-oriented polypropylene and cellophane. Also woven fabrics and non- woven materials, whose fibres consist of e.g. polypropylene or polyethylene, may constitute the substrate employed . The polymer that is extrusion-coated on the substrate may be any of a number of different polymers, but preferably is an ethylene plastic, i.e. plastic based on polyethylene or copolymers of ethylene, in which the ethylene monomer makes up most of the mass, or propylene plastic, i.e. plastic based on polypropylene or copolymers of propylene, in which the propylene monomer makes up most of the mass. Special examples of ethylene plastic are LD polyethylene, ethylene/(meth)aerylate copolymers and ethylene/vinyl acetate copolymers. The temperature in the extrusion coating depends on the particular polymer that is being extrusion-coated. For exemplifying purposes, it may be mentioned that a temperature of about 250-270°C is used in extrusion coat- ing of ethylene/vinyl acetate copolymer comprising 9% by weight of vinyl acetate; a temperature of about 270-300°C is used in extrusion coating of ethylene/methyl acrylate copolymer having a methyl acrylate content of 20% by weight; and a temperature of about 300-330°C is used in extrusion coating of LD polyethylene. In extrusion coating of polypropylene, a temperature of about 280-315 C C is typically employed. What these temperatures all have in common is the fact that they are elevated and above about 250°C. The amount of plastic being extrusion-coated on the substrate may vary within wide limits, but preferably is in a range of about 5-60 g/m^ . Also the rate at which the plastic is extrusion-coated on the substrate may vary within wide limits, but preferably is in the range of about 100-700 m/min. The zeolite employed in accordance with the invention is of the type defined above, i.e. it consists of an essentially hydrophobic aluminium silicate zeolite having a pore diameter of at least 5.5 A, a molar ratio Si/Al in the crystal lattice of at least 35, and a sorption capacity for water at 25 °C and 4.6 torr below 10% by weight. The water sorption of the zeolite, which is an indication of its hydrophobic (oleophilic) properties, preferably is below 6% by weight at 25°C and 4.6 torr. Furthermore, its molar ratio Si/Al preferably is 200-500. The pore diameter of the zeolite preferably is at least 6.2 A. The zeolite, which generally is in the form of a powder, should have such a small particle size that it does not adversely affect the appearance or other proper- ties of the plastic material. Thus, the zeolite should have an average particle size not exceeding about 10 μm, such as 0.1-7 μm, preferably not exceding about 5 μm. The amount of zeolite used in the coating according to the invention preferably amounts to about 0.05-5% by weight, based on the weight of the polymer layer. More preferably, this amount is about 0.05-0.5% by weight, most preferred about 0.05-0.3% by weight, based on the weight of the polymer layer. In addition to the materials described above, the coating according to the invention may include conventional additives, such as colouring agents, pigments and stabilisers. Following this description of the materials forming part of the coating according to the invention, an example of a preferred production of the coating will, for illustrative purposes, be given below with reference to the accompanying drawing. As appears from Fig. 1, a weblike substrate 1, such as paperboard, is fed from a storage roller (not shown) towards a roll nip 2, which is formed between a cooling roll 3 of metal and a press roll 4 which is applied against the cooling roll and preferably is made of rubber. Above the roll nip 2, there is provided an extruder 5, which comprises a feed hopper 6 for plastic raw material, a screw cylinder 7 for feeding and heating the plastic raw material, and a die 8 for discharging the melted plastic in the form of a film 9 towards the roll nip 2. In conventional extrusion coating, the film or polymer layer 9 is brought together with the substrate 1 in the roll nip 2, and the polymer layer 9 is laminated onto the substrate 1 under the action of the press force of the press roll 4 applied against the cooling roll 3. The press roll 4 and the cooling roll 3 rotate in the directions indicated by the arrows in Fig. 1, and the polymer layer 9 sets, under the action of the cooling roll 3, on the substrate so as to form a laminate structure, which then is fed, via a detaching roll 10, to a storage roller ( not shown ) . As is evident from the above description, a zeolite of a certain type is introduced between the substrate and the polymer layer before the latter is extrusion-coated on the substrate. The introduction of the zeolite between the substrate and the polymer layer can be performed by applying the zeolite either on the polymer layer 9 or on the substrate 1 before these are brought together. For reasons of practicality, however, it is preferred that the particulate zeolite 11 is applied on the substrate 1, for instance by being sprinkled from a zeolite supply 12 onto the substrate 1 close to the roll nip 2. The laminate structure obtained in the production of the coating according to the present invention is schematically illustrated in Fig. 2, which is an enlarged sec- tional view of the encircled area A in Fig. 1. As appears from Fig. 2, the laminate structure comprises the substrate 1 and the coating 13, which in turn is made up of the polymer layer 9 and the zeolite particles 11. The embodiment described above and illustrated in Figs 1 and 2 bears upon the case where a one-layered substrate 1 is provided with a coating 13 consisting of a polymer layer 9 and zeolite particles 11. It will be appreciated that the substrate 1 may be made up of more than one layer, such as two or three layers, which may be identical or different from one another. Also, it is to be understood that the coating 13 on the substrate 1 may consist of two or more polymer layers 9. Such a multi- layered coating may, for instance, be obtained by coex- trusion with the aid of a die having several slits or with the aid of a plurality of roll nips with associated extruders arranged in succession. In such an embodiment, there is provided at least one station for feeding zeolite onto the substrate at the first roll nip, and zeolite-feeding stations are preferably arranged at each roll nip. Thus, the zeolite particles will be provided in the boundary surfaces between the original substrate 1 and the first polymer layer extrusion-coated thereon, as well as in the boundary surface between the first polymer layer ( then constituting a substrate ) and the second polymer layer extrusion-coated thereon, and so forth. In order to further elucidate the invention, here follows a few illustrative Examples. Example 1 In this Example, two different substrates were used: a polyester film for odour testing and an aluminium laminate for taste testing. The Al laminate comprised an Al foil laminated to paperboard. The Al laminate was extrusion-coated with the coating polymer on top of the Al foil. The coating polymer was an LD polyethylene (LDPE) with a melt flow rate (MFR) of 7.5 g/10 min and a density of 0.918 g/cm 3 . The LDPE did not contain any additives. The LDPE was extrusion-coated on the polyester film and the Al laminate using a method and an apparatus corresponding to those described above in connection with Fig. 1. The melt temperature at the extrusion-coating was 315°C and the coating weight of the LDPE on the substrate was 40 g/m 2 . The running speed of the apparatus was 100 m/min. The odour testing was carried out in the following way: After the extrusion-coating, the LDPE coating was separated from the polyester film and the LDPE coating was cut into samples with the dimension 105 mm x 148 mm. Every sample was placed inside a 250 ml Erlenmeyer glass flask, which was then sealed with a glass stopper and placed in an oven at 40°C for one hour. Thereafter the flasks were removed from the oven and left at room tempe- rature for 30 minutes. A panel of eight persons then evaluated the samples by removing the stoppers and smelling if any odour was given off from the LDPE samples. The evaluation was made according to a rating from 0 to 3, where 0 = no odour 1 = weak, barely noticeable odour 2 = clear odour 3 = very strong odour. The results of the odour testing are given in Table 1. The taste testing was carried out in the following way: After the extrusion-coating of the LDPE on the Al foil, 210 mm x 296 mm pouches were made of the material by sealing together two 210 mm x 296 mm sheets of the material with the LDPE layers facing each other. Each of the pouches was filled with 150 ml pure water, and as a reference pure water in glass flasks was used. The pouches and the reference flasks were placed in an oven at 40°C for two hours, and then removed from the oven and evaluated for taste by a panel of eight persons . Each person evaluated about 40 ml of water from the pouches. The evaluation was made according to a rating from 0 to 3 , where 0 = no taste 1 = weak, barely noticeable taste 2 = clear taste 3 = very strong taste. The results of the taste testing are given in Table 1. Example 2 The procedure of Example 1 was followed, with the exception that 0.15 % by weight of a zeolite according to the invention (Abscent 3000, from UOP Molecular Sieves, 25 East Algonquin Road, Des Plaines, 60017-5017 Illinois, USA) was dry-blended in a drum mixer with the polymer before it was extrusion-coated. The results of the odour testing and the taste testing are given in Table 1. Example 3 The procedure of Example 1 was followed, with the exception that 0.5-1.0 g/m 2 of zeolite (Abscent 3000) was spread evenly with the aid of a 60 mesh metal screen on the substrate just before the coating station/roll nip as is shown and described in connection with Fig. 1. The results of the odour testing and the taste test- ing are given in Table 1. Table 1 Odour testing Taste testing Example 1 2 3 1 2 3 Rating 1.2 1.3 0.5 1.9 1.7 0.1 As is evident from Table 1, the present invention (Example 3) gives superior results with regard to elimination of odour as well as taste. Example 4 The procedure of Example 1 was followed, with the exception that an ethylene-butylacrylate copolymer with 17 % by weight of butylacrylate, a density of 0.924 g/cm 3 and an MFR of 7 g/10 min was substituted for the LDPE of Example 1 as the extrusion-coating polymer. In addition the melt temperature was 270°C instead of 315°C. All other conditions were the same as in Example 1. The results of the odour testing and the taste testing are given in Table 2. Example 5 The procedure of Example 4 was followed, with the exception that 0.3 % by weight of zeolite ( Abscent 3000 ) was dry-blended in a drum mixer with the polymer before the polymer was extrusion-coated. The results of the odour testing and the taste testing are given in Table 2. Example 6 The procedure of Example 4 was followed, with the exception that 1-2 g/m of zeolite (Abscent 3000) was spread evenly with the aid of a 60 mesh metal screen on the substrate just before the coating station/roll nip as is shown and described in connection with Fig. 1. The results of the odour testing and the taste testing are given in Table 2. Table 2 Odour testing Taste testing Example 4 5 6 4 5 6 Rating 1.5 1.2 0.6 2.3 2.0 0.8 As is evident from Table 2, the present invention (Example 6) gives superior results with regard to elimination of odour as well as taste.

Claims: CLAIMS 1. A coating comprising a polymer layer ( 9 ) , which is extrusion-coated on a substrate (1), cha r a c t e r i s e d in that it comprises particles ( 11 ) of an essentially hydrophobic aluminium silicate zeolite located between the substrate ( 1 ) and the polymer layer ( 9 ) extrusion-coated thereon, and that the zeolite has a pore diameter of at least 5.5 A, a molar ratio Si/Al in the crystal lattice of at least 35, and a sorption capacity for water at 25°C and 4.6 torr below 10% by weight. 2. A coating as claimed in claim 1, c h a r a c - t e r i s e d in that the polymer layer (9) consists of polyethylene. 3. A coating as claimed in claim 1 or 2, ch a r a c t e r i s e d in that the substrate ( 1 ) is selected from cellulosic materials, metallic materials and poly- meric materials. 4. A coating as claimed in claim 3, ch a r a c t e r i s e d in that the substrate ( 1 ) is selected from paper, paperboard, aluminium foil, plastic films, plastic foils, woven textile materials and non-woven materials. 5. A coating as claimed in any one of claims 1-4, ch a r a c t e r i s e d in that it comprises 0.05-5% by weight of zeolite, based on the weight of the polymer layer. 6. A method of producing a coating on a substrate, said coating comprising a polymer layer which is extrusion-coated on the substrate, ch a r ac t e r i s e d in that, before the polymer layer is extrusion-coated on the substrate, zeolite particles are introduced between the polymer layer and the substrate, the zeolite being an essentially hydrophobic aluminium silicate zeolite having a pore diameter of at least 5.5 A, a molar ratio Si/Al in the crystal lattice of at least 35, and a sorption capacity for water at 25°C and 4.6 torr below 10% by weight. 7. A method as claimed in claim 6, ch a r a c t e r i s e d in that 0.05-5% by weight of zeolite, based on the weight of the polymer layer, is introduced between the polymer layer and the substrate. 8. A method as claimed in claim 6 or 7, c h a r a c t e r i s e d in that the polymer layer consists of polyethylene. 9. A method as claimed in any one of claims 6-8, c h a r a c te r i s e d in that the zeolite is provided on the substrate, which is selected from paper, paper- board, aluminium foil, plastic films, plastic foils, woven textile materials and non-woven materials. 10. A method as claimed in any one of claims 6-9, ch a rac t e r i s e d in that the polymer layer and the substrate are in the form of webs which are fed towards a roll nip, before which the zeolite particles are sprinkled on the substrate surface that is to be brought together with the polymer layer.

Inventors: GUSTAFSSON BILL, LAIHO ERKKI, MAEKINEN TUOMO, SAINIO MARKKU, GUSTAFSSON, BILL, LAIHO, ERKKI, MAEKINEN, TUOMO, SAINIO, MARKKU
Applicants: BOREALIS AS, GUSTAFSSON BILL, LAIHO ERKKI, MAEKINEN TUOMO, SAINIO MARKKU, BOREALIS A/S, GUSTAFSSON, BILL, LAIHO, ERKKI, MAEKINEN, TUOMO, SAINIO, MARKKU
Application year: 1997
Main classification: B32B27
IPC codes: B32B27, B32B37