SUBSTRATE COATING AND APPARATUS FOR APPLYING THE SAME

Abstract: Apparatus for applying a coating to a substrate (20) comprising a first cylinder (12) onto which a liquid emulsion of the coating is introduced, a scraper (14) for removing excess coating from the first cylinder (12) and a second cylinder (16) in contact with the first cylinder (12) and arranged so as to transfer the coating from the first cylinder (12) to the substrate (20), wherein the scraper (14) comprises two mutually opposed scraper blades (28, 30) and a coating reservoir (32).

Description: SUBSTRATE COATING AND APPARATUS FOR APPLYING THE SAME Background of the Invention This invention relates to a substrate coating. More particularly, the invention relates to a paperboard coating for food packaging. The invention also relates to an apparatus and method of applying the coating to a substrate. When paperboard cartons are used for packaging food items, for example ready-meals or meat which contain liquid, a barrier in the form of a separate layer of material must be applied to the paperboard to ensure that the paperboard itself remains dry as far as possible and therefore retains its mechanical strength. The barrier also ensures that the liquid does not seep through the packaging. Preferably, the barrier should, additionally, be relatively impermeable to air in order to substantially prevent meat contained within the packaging from becoming discoloured, be insoluble when in contact with a wide range of solvents, be colour free, have substantially no odour, a high coefficient of friction and have hot-seal properties. Most commonly, a plastics film layer is extruded or glued onto paperboard to make water or greaseproof packaging and can be used to package a wide range of foodstuffs including liquids. The extrusion process offers the fastest of the two coating methods and is the cheapest option for large production volumes, but requires high capital expenditure. The method is only viable if the process is to be carried out continuously. The gluing process has a lower capital outlay, but can only be carried out at lower speed and is therefore not cost effective for large production volumes. For both methods, the specific weight of the film that is applied is generally in the region of 20g/m 2 . The film used is non-biodegradable, and is difficult to recycle because once the film has been adhered to the paperboard, it is problematic to separate the two again. Additionally, a number of layers applied as a liquid have been proposed for use in food contact applications. US 5 776 619 to Shanton discloses a paperboard layer for use in forming rigid food containers. The layer comprises a base coat comprising a styrene acrylic polymer latex and pigment that is applied directly to the paperboard, and a top coat comprising a styrene acrylic polymer latex and pigment that is applied to the base coat. The layer seeks to improve grease, oil and cut resistance and enhance printing qualities. US 4 097 297 to Keene discloses a nitrocellulose lacquer varnish that may be applied to either a clay coated or non-clay coated side of paperboard. To improve water and oil repellency, a fluorochemical surface tension modifier is added to the varnish. Summary of the Invention The present invention seeks to meet the above criteria and overcome or at least mitigate the problems of the prior art. One aspect of the invention provides an apparatus for applying a coating to a substrate comprising a first cylinder onto which a liquid emulsion of the coating is introduced, a scraper for removing excess coating from the first cylinder and a second cylinder in contact with the first cylinder and arranged so as to transfer the coating from the first cylinder to the substrate, wherein the scraper comprises two mutually opposed scraper blades and a coating reservoir. Preferably, pressure may be applied to the substrate so as to compress the substrate by approximately 0.1 mm at working speeds. According to an optional feature of this aspect of the invention the apparatus may be flexographic apparatus. According to another optional feature of this aspect of the invention the second cylinder is resilient. Preferably, the second cylinder may be manufactured from EPDM or a photopolymeric flexographic sheet. According to another optional feature of this aspect of the invention the first cylinder maybe a honeycomb ceramic cylinder. A second aspect of the invention provides a method of applying a coating to a substrate comprising the steps of: i) applying a liquid emulsion of the coating to a first cylinder of a coating apparatus from a coating reservoir. ii) Removing excess coating from the first cylinder using a scraper blade associated with the coating reservoir, iii) Transferring the coating to a second cylinder in contact with the first, iv) Transferring the coating to a substrate in contact with the second cylinder. Preferably, there may further comprise the step of drying the coating using a heating system. A third aspect of the invention provides an acrylic coating for application to a substrate comprising resin and a dispersant, the coating being substantially water resistant, grease resistant and having low migration characteristics. Preferably, the coating may be an oil in water emulsion. More preferably, it may comprise epoxide PE wax approved for use by the DGCCRF for direct food contact in the European Union. Even more preferably, the epoxide PE wax may be included at a concentration of between 0.01 and 5%. Preferably, the epoxide PE wax maybe included in a concentration of 5%. Alternatively the coating may be a water based solution so as to be absorbed, partially by the substrate. According to another optional feature of the third aspect of the invention there may further comprise an anti-foaming agent. According to another optional feature of the third aspect of the invention the coating may be approved by the DGCCRF. According to another optional feature of the third aspect of the invention the coating may be substantially colour free. According to another optional feature of the third aspect of the invention the coating may be substantially odour free. According to another optional feature of the third aspect of the invention the coating be used in heat sealing processes. According to another optional feature of the third aspect of the invention there may further comprise a surface tension modifier. Preferably, the surface tension modifier may lower the surface to less than 36 dynes/cm. Alternatively, the surface tension may be 36 to 40 dynes/cm. Brief Description of the Drawings The present invention is further described by way of example only, with reference to the accompanying drawings in which: FIGURE 1 is a schematic side view of the apparatus for applying the coating to paperboard illustrating the reservoir; FIGURE 2 is a side elevation of the scraper and scraper blades; FIGURE 3 is a perspective view of one end of the scraper; and FIGURE 4 is a view of the scraper illustrating the inlet and outlet pipes for the liquid coating. Detailed Description of the Preferred Embodiments In general, prior art coatings that can be used in contact with moist foodstuff such as meat and ready-meals have been insufficiently resistant to water, leading to the foodstuff adhering to the paperboard, resulting in tearing of the board and likely unsightly discolouration of the board. Through extensive trials, a coating has been formulated that has been approved for direct contact with food and has achieved ISEGA certificate of conformity: recommendation XXXN BgNN for contact with moist, dry and greasy foodstuffs. Specifically, French DGCCRF (Direction Generale de la Consommation, de la Concurrence et de la Repression des Fraudes) approval has been obtained the formulation complies with European Council Resolution (96) 5. The DGCCRF in turn complies with EU directives 82/711, 93-8, 90-128, 92-39, 85-572 and the modifications to the 82/711 by 97/48/CE implemented on 29 July 1997. Furthermore, all compounds comprising the coating are on current EU non-toxic "positive lists", specifically resolution AP96/5 "Surface coatings intended to come into contact with foodstuffs" of the European Council and adopted by the Council of Ministers on 2 October 1996. As part of the approval process, migration testing of the coating into foodstuffs (as simulated by a number of solvents e.g. distilled water, non-polar isooctane and polar ethanol) was carried out. The maximum migration permissible for each solvent is 10 mg/dm 2 or 60mg/kg of foodstuff over a seven day contact period. The results of the testing of trays made from coated paperboard are summarised in table 1 below: Table 1 The next stage of the procedure for testing coating formulation procedure was the conducting of sensorial tests to ensure that odours from the coating do not taint foodstuffs with which they come into contact. The method followed is in accordance with French standard AFNOR XPN 09-0009 (September 1995) entitled "Method of evaluating foreign bodies migrating from packaging into foodstuffs" and ΝF N 09-013 (July 1983) "Sensorial analysis-triangular methodology test" (ISO 4120 equivalent). The test is conducted as follows: butter (which is used due to the ease with which it becomes tainted and retains flavours) is placed in contact with the coated board for a set period of 48 hours at 10°C. Two samples of exposed butter and an additional control sample are submitted to a taste panel and if more than one of the three panel members find the control butter, when assessed over a statistically significant number of tests, the test is failed. A coating formulation has been found that passes this test. A voluntary final stage of the procedure is the specific migration test in which specific monomers and catalysts used in the coating are subjected to a migration test using a gas chromatograph. This test was developed to test migration into meat particularly. The formulation process has resulted in an acrylic coating formulation, one example of which is an oil in water emulsion. The emulsion comprises a liquid disperse phase and a continuous liquid phase. The liquid disperse phase comprises epoxide PE wax in a 5% concentration to provide the grease barrier properties. The formulation further comprises a wax dispersant, and preferably a surface tension modifier to aid the transfer of the coating from the cylinders and onto the paper by lowering the surface tension to less than 36-40 dynes/cm (the surface tension of the board) and an antifoaming agent. In this example, the formulation has a surface tension of 32 dynes/cm. A second example dispenses with the epoxide PE wax to form an entirely water based solution of the coating without the PE wax. The coating has excellent water resistance (Cobb test: <=lg/m 2 passing after one minute and <=5g/m 2 passing after 30 minutes) and grease resistance with a low permeability and low overall migration characteristics. A further advantage of the coating is that it is an oxygen barrier to increase the shelf life of the foodstuff held by the package. Furthermore, the coating is substantially non-adhesive and colour free, is insoluble over a wide range of different simulating solvents, has a low odour and is non-delaminating but does have some relative adhesion for foodstuffs such as turkey. This may be achieved by the addition of a blocking agent to the formulation. Some relative adhesion is advantageous as this permits cartons made from the coated paperboard and containing foodstuffs to be placed on shelving angled toward purchasers of the foodstuffs whilst maintaining the foodstuffs in a desired position within the carton. This in turn enhances the presentation and marketability of the foodstuffs. The coating is also advantageously heat sealable. A coating to coating seal can be achieved at a temperature of 150°C and a pressure 2 bars for 2 seconds. For a coating to board seal, a temperature of 160°C and a pressure of 3 bars should be maintained for 5 seconds. The fact that any attempt to tear board coated using the above coating results in the delamination of the board indicates that a good seal is achieved. In one class of embodiments, the coating is absorbed partially by the substrate, for example the coated board. Turning to Figure 1 there is shown apparatus 10 for applying a coating to a substrate such as paperboard 20 using a flexography process. A liquid emulsion or solution of the coating is applied to the surface of an anilox cylinder 12. In a preferred embodiment, a ceramic honeycomb cylinder permits an accurate proportion of the coating to be transferred to the blanket cylinder 16. The honeycombs are preferably 35 microns in depth, with there being 90 cells/linear cm giving an effective capacity of 11-14 cm 3 /m 2 . For the preferred coating, 6g/m 2 is transferred in one pass (40% of the theoretical volume) with a 60° engraving angle. This type of anilox cylinder 12 is used as this enables an increased amount of the coating to be carried by the cylinder. A scraper 14 removes any excess coating before the coating is transferred from the anilox cylinder 12 to the blanket cylinder 16. The cylinder 16 is advantageously made of resilient material such as butyl, EPDM (ethylene propylene diene methylene), nitrile or silicone rubber. Preferably however, EPDM or a Cyrel R ™ type photopolymeric flexographic sheet having a hardness of approximately 50-80° shore is used. Referring in particular to Figures 2, 3 and 4 the scraper is illustrated in more detail. It can be seen from Figure 2 that the scraper 14 comprises upper and lower scraper blades 28 and 30 mounted on a coating reservoir portion 32. The scraper 14 is, in this embodiment, adjustably mounted in relation to the anilox cylinder 12 by a pair of locating holes 40a and 40b that engage corresponding fingers (not shown) provided on a frame that supports the cylinders. Adjustment means for adjusting the position of the scraper in relation to the cylinder 12 is provided. In this embodiment, a pair of pressure adjusting screws 36a and 36b is provided . Increasing the pressure between the scraper blades 28, 30 and the anilox cylinder 12 reduces the coating thickness applied on the anilox cylinder 12 and hence the coating weight ultimately applied to the paperboard 20. The screws may be used in conjunction with pressure gauges that are calibrated such that screw adjustments may be directly related to the thickness of coating to be applied. In a particularly preferred embodiment, the method of applying pressure is automated as part of an overall control system for the apparatus (not shown). It can also be seen from Figures 2 and 3 that suitable sealing means, for example resilient seals 34a and 34b, formed from rubber or similar materials, are proved at each end of the reservoir 32 to prevent, or at least substantially minimise coating leakage. Referring now to Figure 4 which shows a rear view of the scraper 14, the infeed apertures 38a and 38b and outfeed aperture 42 for the liquid coating can be seen. In this embodiment, pipes (not shown) feed the liquid coating into the reservoir 32 at each end thereof preferably from a peristaltic pump. Any excess coating then exits the reservoir at the centre through outfeed aperture 42 and is recycled. A closed or open loop coating supply system may be employed. It is preferred that a closed system is used as this substantially prevents contact between the coating and the atmosphere. Turning back to Figure 1, anilox cylinder 12 is arranged to be in contact with blanket cylinder 16, the rotation thereof transferring the coating from the anilox cylinder 12 to the blanket cylinder 16. The thickness of the coating is adjusted by changing the velocity of one of the anilox cylinder or blanket cylinder relative the other. A controller is used to control the rotational velocities of each cylinder. The linear velocity of the anilox cylinder 12 is preferably 1.05 to 1.10 times greater than the linear velocity of the blanket cylinder 16. This enables a thick layer of coating to be more evenly applied to the paperboard 20. Paperboard, either in the form of a continuous sheet or in discrete pieces 20 as shown, is fed into the apparatus by a first conveyor belt 22 running at the same linear velocity as the blanket cylinder 16. A force is applied by the apparatus to the anilox cylinder 12, in a direction X, the load being transferred via the blanket cylinder 16 such that the paperboard 20 is typically compressed by approximately 0.05 - 0.1 millimetres at normal running speeds and the coating is transferred to the paperboard 20 and in greater amounts than would be possible using an off-set process. The above described arrangement ensures that an even layer of the liquid emulsion is applied to the paperboard 20. An impression roller 18 is provided underneath the paperboard 20 so as to guide the paperboard past the blanket cylinder. Where it is necessary to blank areas of paperboard from printing, portions of the blanchet cylinder 16 may be recessed so as not to pick up the coating from the anilox cylinder 12. The coated paperboard is then preferably transferred by a second conveyor 24 to a heater 26 in direction Y. This ensures that the coating is dry before the paperboard 20 is processed further, or is stored. A particularly preferable heating system comprises a hot air flow at a temperature of approximately 180°C used in addition to infrared radiation for at least two seconds. It is preferable that the temperature should reach at least 85°C on the support conveyor 24. hi a particularly preferred arrangement, four 6kW infrared lamps (not shown) are used that emit radiation in a spectrum of 500 nm to 2000 nm wavelength to give a good coating surface whilst avoiding "orange skin" effects. A long drying station may be used to avoid off-setting as this provides sufficient time for the coating to dry before the paperboard 20 is stacked or rolled up. The coating is partially absorbed into the paperboard, with any remainder forming a layer on the surface thereof. The degree of absorption depends upon, amongst other factors, the characteristics of the paperboard and the interval between applying the coating and the paperboard entering the heater 26. To ensure that a sufficiently thick and even layer of the coating is provided on a surface of the paperboard 20 in some classes of embodiment, a second coating is advantageously applied to the paperboard by a similar apparatus as described above. Thus, a thicker coating for example 9g/m 2 can be obtained. In certain classes of embodiment, the second layer has different properties from the first, and the two layers have complementary properties so as to achieve the desired result. For example, the first layer may include a pigment. For the coated board to be considered biodegradable "mono" material according to German standards (Topfer regulation and the Nerpackungs-Nerordnung), the coating must comprise less than 5% of the board weight. Therefore, for a standard 300 g/m 2 board, the coating weight must be lower than 15 g/m 2 . Furthermore, in order for the finished board to be considered recyclable it is necessary for the coating weight to be less than 10 g/m 2 . However, the preferred total coating weight applied by the above apparatus is in the range of 6-7 g/m 2 dry. In other classes of embodiment, dependent upon the actual set-up of the apparatus and the properties of the coating, a single pass with a longer drying time may be sufficient. The chances of achieving a coating weight of 6-7 g/m 2 in a single pass are enhanced if the ceramic "honeycomb" anilox cylinder 12 is used. Thus, there is provided a recyclable and repulpable coated substrate. The coating is in the range of 3 to lOg/m 2 and comprises one or more coatings, which formed a pinhole free continuous coating resistant to water. The above described apparatus is generally capable of coating 130-150 metres of paperboard per minute by the abovementioned continuous application process. Following coating, the paperboard is cut, creased and glued as necessary. If aseptic packaging is required, the paperboard can then be treated with a hydrogen peroxide solution or other suitable antimicrobial agent. It should be understood that numerous changes may be made within the scope of the invention. For example, alternative substrates such as linerboard or paper may be used.

Claims: CLAIMS 1. Apparatus for applying a coating to a substrate comprising a first cylinder onto which a liquid emulsion of the coating is introduced, a scraper for removing excess coating from the cylinder and a second cylinder in contact with the first cylinder and arranged so as to transfer the coating from the first cylinder to the substrate, wherein the scraper comprises two mutually opposed scraper blades and a coating reservoir. 2. Apparatus according to claim 1 wherein pressure is applied to the substrate so as to compress the substrate by approximately 0.05 - 0.1 mm at working speeds. 3. Apparatus according to claim 1 or claim 2 which is flexographic apparatus. 4. Apparatus according to any one of claims 1 to 3 wherein the second cylinder is resilient. 5. Apparatus according to claim 4 wherein the second cylinder is manufactured from EPDM or a photopolymeric flexographic sheet. 6. Apparatus according to any one of claims 1 to 5 wherein the first cylinder is a honeycomb ceramic cylinder. 7. A method of applying a coating to a substrate comprising the steps of: i) applying a liquid emulsion of the coating to a first cylinder of a coating apparatus from a coating reservoir, ii) removing excess coating from the first cylinder using a scraper blade associated with the coating reservoir, iii) transferring the coating to a second cylinder in contact with the first cylinder, and iv) transferring the coating to a substrate in contact with the second cylinder. 8. A method according to claim 7 further comprising the step of drying the coating using a heating system. 9. An acrylic coating for application to a substrate comprising resin and a dispersant, the coating being substantially water resistant, grease resistant and having low migration characteristics. 10. A coating according to claim 9 which is an oil in water emulsion. 11. A coating according to claim 10 further comprising epoxide PE wax approved for use by the DGCCRF for direct food contact in the European Union. 12. A coating according to claim 11 wherein the epoxide PE wax is included at a concentration of between 0.01% and 5% 13. A coating according to claim 12 wherein the epoxide PE wax is included in a concentration of 5% 14. A coating according to claim 9 which is a water based solution. 15. A coating according to any one of claims 9 to 14 further comprising an anti-foaming agent. 16. A coating according to any one of claims 9 to 15 which is substantially colour free. 17. A coating according to any one of claims 9 to 16 which is substantially odour free. 18. A coating according to any one of claims 9 to 17 which may be used in heat sealing processes. 19. A coating according to any one of claims 9 to 18 further comprising a surface tension modifier. 20. A coating according to claim 19 wherein the surface tension modifier lowers the surface tension to less than 36 dynes/cm. 21. A coating according to any one of claims 9 to 18 wherein the surface tension is 36 to 40 dynes/cm. 22. A substrate to which a coating according to any one of claims 9 to 20 has been applied. 23. A substrate according to claim 21 which is paperboard.

Inventors: LEBRAS PHILIPPE, TIMBAL OLIVIER, LEBRAS, PHILIPPE, TIMBAL, OLIVIER
Applicants: LEBRAS PHILIPPE, MEAD CORP, TIMBAL OLIVIER, LEBRAS, PHILIPPE, THE MEAD CORPORATION, TIMBAL, OLIVIER
Application year: 2001
Main classification: B05C
IPC codes: B05C1