Sheet absorbing product for packaging liquid and gas releasing food articles

Abstract: Sheet absorbing product, comprising 10-50% by weight of a polyolefine based polymeric grid and 90-50% by weight of an inorganic absorbing grid, which can be used, in particular for packaging food articles or products, and contacting liquid and gas releasing food articles comprising sugar solutions, salt solutions, serum, blood, oily liquids, water steam and organic substance vapours.

Description: The present invention relates to a sheet absorbing product for packaging liquid and gas releasing food articles. The invention also relates to a method for making said absorbing sheet product. There are at present available on the market, for such an application, absorbing sheets comprising several layers of cellulose, and which can be coupled to a microperforated plastic material film. This film is provided for contacting the packaged goods and operates to prevent the cellulose material impregnated by liquid from disgregating and sticking to the food articles and being also provided for preventing stains due to the absorbed liquids, the microperforations allowing the released liquid to be conveyed on the cellulose material. While the above mentioned prior absorbing sheets are rather satisfactory in operation, they, however, are affected by limitations and drawbacks. They absorb the liquids with a high rate in a horizontal direction with respect to the sheet laying plane, and generate, in the case of colored liquids, halos about the packaged food article; as they are pressed, they operate as a sponge and release a great portion of the absorbed liquid; notwithstanding the provision of the plastic material film, the outer edges of the cellulose layers tend to flake off, with the consequence that small parts of the cellulose material stick to the packaged product; because of the multi-layered construction, moreover, they have a comparatively high thickness and volume and, accordingly, require comparatively large storing spaces; the provision of two different materials, that is cellulose and plastic materials, finally, originates recycling problems. The document EP-A-0 400 460 discloses an absorbing product which can be formed in sheets having substantially the features of the preamble of the main Claim. SUMMARY OF THE INVENTION The aim of the present invention is to overcome the above mentioned disadvantages and drawbacks of prior absorbing sheets used for packaging food articles. It is achieved with a sheet product according to claim 1, which is prepared by a method according to claim 15. Within the scope of the above mentioned aim, a main object of the present invention is to provide a sheet product, adapted for packaging food articles, and to irreversibly absorb and hold, even if it is subjected to a pressure, liquids and gas substances released by the food articles, such as salt solutions, sugar solutions, serum, blood, oily liquids, water steam, organic substance vapours and so on, and which is constituted by a reduced thickness single layer so as to greatly reduce the space required for storing it, which does not flake off as it is impregnated by liquids, and which allows to provide packages in which the food articles are held in a perfectly dry condition, without halos and stains on the food article contacting regions. Another object of the present invention is to provide such a flexible sheet or foil product, which has a reduced compressibility and high absorbing properties for the above mentioned liquid and gas substances, and which is essentially constituted by two tri-dimensional grids which are mutually interengaged, one formed by particles of materials having a high affinity with respect to said liquid and gas substances, and the other formed by polymeric chains forming the connective tissue. The product according to the present invention is made, as it will be disclosed in a more detailed manner hereinafter, by subjecting the feed stock of absorbing inorganic material and polyolefines, with the desired weight proportions and in the presence of a suitable plasticizing oil and process aids, to a mixing and homogenizing operation at a sufficiently high temperature, generally 220-280°C, and for a sufficiently long time, to provide a mixture which is then extruded through a flat-head die, calendered, finished and formed to the proper size for the final use. The polymeric grid comprises polyethylene, polypropylene, both of high or very high molecular weight and related mixtures. The determination of the molecular weight is performed based on the intrinsic viscosity, calculated according to the ASTM D4020-81 method; optimum values are from 2 to 35 dl/g for polyethylene and from 6 to 18 dl/g for polypropylene. This amount constitutes from 10 to 50% by weight and, advantageously, from 20 to 40% by weight of the finished product. The absorbing material grid comprises silica, silica gel, silica gel and alumina, clay earths such as bentonite and kaoline, diatomite, talc, aluminium silicate and sodium silicate, aluminium silicate, calcium silicate or the related mixtures. Advantageously, there are used the above mentioned silica and clays and, more specifically, there is used that silica obtained by precipitating with inorganic acids, in particular sulphuric acid, from water soluble silicates, such as sodium silicate. Together with the absorbing inorganic material there is provided inert inorganic material such as carbon, graphite, titanium bioxide, iron oxide, zinc oxide, aluminium oxide, magnesium oxide, barium sulphate, calcium sulphate, calcium carbonate, magnesium carbonate or mixtures thereof. The optimum size of the primary particles constituting the agglomerates of the absorbing inorganic materials and inert materials, generally corresponds to a particle size less than 0.03µm. Collectively this phase constitutes 90-50% and, preferably, 80-60% by weight of the finished product, with a contents of absorbing material not less than 70% by weight of the grid itself. The components included in the finished product also comprise small amounts of a plasticizing oil (0-1% by weight), which constitutes a residue of the amounts used for the starting mixture. As plasticizing oil there are used mineral oils water-processed up to a pharmacopeia degree, petrolatum oil, synthetic white oils, such as polyisobutylene, natural oils (cotton oil, linseed oil, castor oil and derivatives thereof, epoxydated soybean oil) and the silicone oils. Advantageously, there are used petrolatum oil, water-processed mineral oils and the synthetic poly-isobutylene oil, having viscosity from 60 to 180 CST (mm 2 /s) at 40°C and, preferably, from 80 to 140 CST at 40°C. In the finished product there are moreover present small amounts of process aids (0.5-3% by weight), which have a specific function in the processing, such as an anti-oxidizing function for the oil, the polyethylene and polypropylene (for example sterically hindered phenols), a stabilizing agent for polyethylene and polypropylene (such as long chain esters of the thio propionic acid) and an outer lubricating agent (such as wax materials, calcium stearate, magnesium stearate). A modified embodiment provides to include in said polymeric grid other polymeric materials different from polyethylene and polypropylene of high and very high molecular weight and, in particular, in an amount up to 30% of said grid, of low density polyethylene, high density polyethylene, low density linear polyethylene, an ethylene-propylene copolymer, an ethylene-acrylic acid and/or methacrylic acid copolymer, either not salified or partially or fully salified by alkaline metals or earth-alkaline metals, ethylene-vinyl acetate copolymer or mixtures thereof. A further modified embodiment provides to include in the finished product amounts of a plasticizing oil greater than 1% and up to a maximum limit of 15% by weight, in order to provide the finished product with particular characteristics, such, for example, a very high duration to the atmospheric agents or a very great softness. The several above mentioned variations are efficiently hereinbelow resumed, with respect to the final composition of the product: POLYMERIC GRID: 10-59% by weight, preferably, 20-40% by weight of polyethylene, polypropylene, both of high or very high molecular weight, or mixtures thereof; polymeric materials different from polyethylene and propylene of high or very high molecular weight = 0-30% by weight of the grid itself. ABSORBING GRID INORGANIC : 90-50% by weight and, preferably, 80-60% by weight, with a contents of absorbing material not less than 70% of said grid. PROCESSING AID: 0.5-3% by weight PLASTICIZING OIL: 0-15% by weight The product according to the present invention has a water-wettable porosity from 30 to 85% of the full volume. This characteristics is determined by calculating the weight Wo of the dried sample, the weight Ww of the sample at environment temperature and as saturated by demineralized water to boiling for 0.5 hours and the weight Wa of that same water saturated sample and immersed in water at environment temperature and using the following formula, which is analogous to that suggested for determining the porosity of battery separators by BCI (Battery Council International) of Chicago, Illinois (USA) Another characteristic which is typical of the above mentioned product is a very high aqueous liquid absorbing rate in a vertical direction with respect to the sheet laying plane and a practically negligible absorbing rate of said liquids along sections parallel to the sheet laying plane. The mentioned absorbing rate, with respect to water, varies from 3 to 20 seconds to pass through a thickness of 0.5 mm and, for different thicknesses these limits vary proportionately. The control is performed by putting a water drop on the surface of the sheet and by measuring the time taken by the drop for exiting the opposite face of the sheet. This absorbing phenomenon along the vertical axis, with respect to the sheet laying plane, is due to viscous creeping effects occurring during the making of the web sheet, which cause an anisotropic performance during the absorbing step so as to increase the density of the absorbing particles along the axis perpendicular to the sheet laying plane. Such a vertical type of absorbing originates a particular phenomenon of an apparent sticking between the wet bodies and the absorbing surface. It is actually a physical phenomenon in which there is formed an air free region between the wet body itself and the absorbed moisture layer. These characteristics are very important with respect to the present invention since they prevent colored halos from forming about the packaged food articles which allows, in prior cellulose based absorbing systems are caused by horizontal migrations of the absorbed liquids; moreover, the above mentioned characteristics provide a substantial immobilization of the absorbing material with respect to the food product. The mechanical properties of the subject product are mainly determined by the compactness and by the extension of the polymeric grid. These properties can be controlled by a dynamometer, by measuring the ultimate tensile stress on test pieces or specimens having a width of 1 cm, anchored at a starting distance of 5 cm, with an elongation rate of 50 mm/minute. The tests are carried out on specimens which are cut in parallel and perpendicularly with respect to the making direction. For the application provided according to the invention, the minimum ultimate tensile stress is of 2.5 N/mm 2 for specimens cut in a parallel direction, and 1N/mm 2 for specimens or test pieces cut in a perpendicular direction. With respect to the surfaces, the product according to the present invention can be considered as of three main types: with both the faces smooth; with a face smooth and the other ribbed; with both the faces ribbed. The ribs, which are obtained by a thermoforming operation by using, for example, suitable embossing cylinders, in a calendering machine, may have different directions; in particular, each rib may have any desired inclinations with respect to the other ribs provided on the same face and with respect to possible ribs provided on the opposite face. The thickness of the mentioned ribs is usually included between 0.1 mm and 3 mm, whereas the thickness of the smooth base is included between 0.15 and 2 mm. As they are parallel, their distance is from 0.15 to 15 mm. The provision of the above mentioned ribs provides a geometric contour which allows, in addition to adjusting in a desired manner the stiffness of the finished product, to provide, in actual practice, a larger contact surface contacting the food product and a greater absorbing capability for unit of actual surface. Other types of embossed patterns, of any suitable configurations, will allow to obtain good optical effects on the product. The sheet product according to the present invention, which can be made in coil-form, can be used in the following configurations: a) as a small sheet of suitable shape and size to be introduced into the available food product trays, usually made of polystyrene, foamed polystyrene, PVC etc., and being affixed on the bottom of said trays by any known means (for example by an adhesive); b) for making trays provided for replacing the above mentioned known trays or to be associated therewith, so as to provide the advantage, in both cases, of providing an absorbing surface extended to the overall package, which can be accordingly exhibited or arranged in a slanted condition, without causing an accumulation of leakage liquid in regions which are not protected by the absorbing sheet. The trays of this type will be made by a batch-wise thermoforming operation starting from pre-cut foils or starting from a coil being continuously unwound; c) as a sheet element for directly wrapping the food products. In addition to the above mentioned advantages, the product according to the present invention also provides the further following advantages: it can be sealed as well as glued; with respect to prior cellulose absorbing sheets, it can be applied automatically to the holding trays; it can be printed on according to any prior methods, and by using inks suitable for food products, thereby constituting an advertising means. The method for making the product according to the present invention comprises several steps, according to the following sequence: MIXING OF THE COMPONENTS: in a batch-wise manner, in high speed mixers or in a continuous manner directly in twin-screw extruders, provided with different feeders for each main component; PLASTICIZING AND HOMOGENIZING in a twin-screw extruder, in a BUSS piston extruder or in BAMBURY mixers and roller mixers and so on; EXTRUSION TO A FOIL, by means of a flat head die directly supplied by an extruder or a gear pump; CALENDERING AND COOLING, by means of specifically designed tools; EXTRACTION OF THE OIL, by means of a solvent, in counter current on the calendered sheet and with a subsequent recovery of the two components in a specifically designed apparatus; there are preferred chlorinated solvents and light hydrocarbons such n-hexane and n-heptane, of any suitable type for producing packages to be contacted with food articles. The recovered oil amount will vary from 100% to 85% of the amount used during the processing; DRYING, by hot air and/or steam solvents, adapted to recover the solvent substance used for extracting oil and present on the sheet; the residue contents, in the latter, will be usually less than 0.1 ppm; WINDING in coils; CUTTING into sheets of a size suitable for the provided application. A technologic sequence, which is critical for making the product according to the present invention, is based on the use of co-rotating twin-screw extruders, in which the material is brought to a plasticizing temperature and homogenized in mixing sections having a specifically designed contour. Downstream of the extruder, a gear pump will convey the material being processed to the subsequent above mentioned finishing steps. The raw materials can be supplied to the extruder in the form of a preformed mixture, or they can be supplied by a plurality of separated feeding units the number of which will correspond to that of the main raw materials, the powder additives being usually supplied with a polymer. In the second case, the inner construction of the extruder will form the mixture in particular regions before the plasticizing operation proper. The basic condition in order to provide an even and smooth plasticizing of the polymer and its subsequent homogenization at a stable rheologic regimen, is provided by an even distribution of the oil on the inorganic material used for making the absorbing grid. This operation is performed by using the available co-rotating twin-screw extruders, which will provide different mixing arrangements as well as different plasticizing patterns depending on the material being used. In the case of preparing the product according to the present invention, this is carried out under the following conditions: the inorganic material is supplied at the start by the screw, by gravimetric metering devices adapted to provide an even and accurate metering; at a distance not greater than 4 diameters, corresponding to a 1 zone according to the standards of the makers of these extruders, there is supplied the oil, as pre-heated usually to about 100°C, by gear continuous metering pumps; the amount of supplied oil will usually corresponds to 150-250% of the inorganic material loaded amount; immediately downstream and for at least 2 zones (8 diameters), the inorganic material and oil are mixed with a mixing pattern suitable to increase the contact time, instead of providing high shear efforts on the material, thereby low transport capability patterns are preferred. Moreover, the provision at one end of a reversed step element will increase the filling-in coefficient. In order to reduce the viscosity of the oil and favour the mixing operation, it is necessary a temperature not less than 200°C and preferably not less than 150°C; after at least 4 zones (16 diameters) from the inorganic material supplying point and, preferably, after 6 zones (24 diameters), there is introduced the polymeric material mixed with the provided additives and, also in this case, will be used gravimetric metering devices adapted to provide an even and accurate metering. In order to provide a progressive plasticizing and homogenizing by means of the oil operating as an inner lubricating agent of the polymeric chains of which it will reduce the viscosity, in this region the temperature must be held from 200 to 300°C and, preferably, from 220 to 280°C. With respect to the thermoforming of the product sheets, excessive times and temperatures, while improving the geometric pattern of the produced article of manufacture, can involve a very high reduction of the porosity; the optimum working conditions will provide, accordingly: a temperature from 140 to 250°C, and, preferably, from 160 to 200°C; total heating and molding times of 20 seconds max for thicknesses greater than 0.7 mm; 15 seconds max for thicknesses between 0.4 and 0.7 mm; 8 seconds max for thicknesses less than 0.4 mm. Typical values are, at a temperature of 200°C: 15 seconds for thicknesses of 0.65 mm; 10 seconds for thicknesses of 0.5 mm; 6 seconds for thickness of 0.25 mm.

Claims: A liquid and gas absorbing sheet product, having a reduced compressibility and a high absorption for gas and liquid substances, said sheet product being constituted by two tri-dimensional grids one being an inorganic grid formed by particles of materials having a high affinity with respect to said liquid and gas substances and the other being a polymeric grid formed by polymeric chains forming a connective tissue of said sheet product, characterized in that said polymeric grid forms 10-15% of the total weight of said sheet product and includes polyethylene, polypropylene or mixtures thereof having prior to the preparation of the sheet product an intrinsic viscosity, according to ASTM D4020-81, of 2-35 dl/g and 6-18 dl/g, and that said inorganic grid is highly porous inorganic grid having a high absorbing capability of said liquid and gas substances and constitutes 90-50% of said total weight of said sheet product and includes, for a level not less than 70% of said inorganic grid, silica, silica gel, silica gel and aluminium, clay earths, diatomite, talc, aluminium and sodium silicate, aluminium silicate, calcium silicate or mixtures thereof and, for the remaining part to 100% of said inorganic grid, carbon, graphite, titanium bioxide, iron oxide, zinc oxide, aluminium oxide, magnesium oxide, calcium sulphate, calcium carbonate, magnesium carbonate and mixtures thereof said sheet product further comprising from 0.5 to 3% of said total weight of processing acids including an anti-oxidizing agent, a stabilizing agent, and an outer lubricating agents, said sheet product having a water porosity, determined by a BCI method, from 30 to 85% of said total volume and a very water absorbing rate in a vertical direction with respect to a laying plane of said sheet product and a comparatively negligible absorbing rate in a direction parallel to said laying plane of said sheet product. An absorbing sheet product according to Claim 1, wherein said polymeric grid constitutes from 20 to 40% of said total weight and said inorganic grid constitutes 80-60% of said total weight. An absorbing sheet product according to Claims 1 and 2, wherein said inorganic grid comprises silica prepared from sodium silicate by a precipitation with inorganic acids and/or clay earths of the bentonite type and kaoline. An absorbing sheet product according to Claims 1 and 3, wherein said polymeric grid comprises, up to 30% of the weight thereof, low density polyethylene, high density polyethylene, low density linear polyethylene, and ethylene-propylene copolymer, and ethylene-acrylic and/or methacrilic acid copolymer not salified or either partially or fully salified by alkaline metals or alkaline-earth metals, ethylene-vynil acetate copolymers or mixtures thereof. An absorbing sheet according to Claims 1 to 4, wherein said anti-oxidizing agent comprises sterically hindered phenols, said stabilizing agent comprises long chain esters of thiopropionic acid, and said outer lubricating agent comprises wax materials, calcium stearate and magnesium stearate. An absorbing sheet product according to Claims 1 to 5, wherein said absorbing products also contain a plasticizing oil comprising hydroprocessed mineral oils, petrolatum oil, synthetic white oil of the polyisobutylene type, cotton oil, linseed oil, castor oil, or derivatives thereof, epoxydized soyben oil, siliconic oils or mixtures thereof. An absorbing sheet product according to Claim 6, wherein said hydroprocessed mineral oils, said petrolatum oil and said white polyisobuthylene oil have at 40°C a viscosity of 60-180 CST, preferably from 80 to 140 CST (mm 2 /s). An absorbing product according to Claims 1 to 7, wherein the primary particles of said inorganic grid have a particle size less than 0.03µM. An absorbing sheet product according to Claims 1 to 8, wherein it is formed as a sheet to be introduced into food article trays, or as a sheet for wrapping the food products. An absorbing sheet product according to Claim 1, wherein said water vertical absorbing rate is defined by a time from 3 to 20 seconds to allow water to pass through a thickness of 0.5 mm of said sheet product. An absorbing sheet product according to Claims 1 to 10, wherein said sheet product has a minimum ultimate tensile stress on specimens cut in parallel and perpendicularly to the making direction of said sheet product of respectively 2.5 and 1 N/mm 2 . An absorbing sheet product according to Claims 1 to 11, wherein the two faces of said sheet product are smooth or both said faces or a single face thereof are provided with ribs. An absorbing sheet product according to Claim 12, wherein each said rib has any desired inclination with respect to the other ribs provided on the same face or to the possible ribs provided on the opposite face. An absorbing sheet product according to Claims 12 and 13, wherein said ribs have a thickness from 0.1 to 3 mm, the smooth base thereof has a thickness of 0.15-2 mm, and the distance of parallel ribs is of 0.5-15 mm. A method for making an absorbing sheet product according to the preceding claims, wherein said method comprises the steps of: mixing of the components for making said sheet product; plasticizing and homogenizing said mixed sheet product components; extruding said plasticized and homogenized components to a foil; calendering said foil to a sheet and coiling said sheet; extracting plasticizing oil by a solvent and recovering it; drying and recovering said solvent; winding said sheet to provide a sheet coil; and cutting said sheet oil to size into individual sheets for forming trays or directly wrapping food articles or thermoforming into trays. A method according to Claim 15, wherein said mixing plasticizing and homogenizing steps are performed in a co-rotating twin-screw extruder having a number of separated feeding devices corresponding to a number of the main raw materials, inorganic absorbing material, plasticizing oil and polyolefine polymer. A method according to Claims 15 and 16, wherein said inorganic material is supplied at the start of the extruding screw, said plasticizing oil, in an amount corresponding to 125-250% of said inorganic material amount and pre-heated to about 100°C is supplied after at maximum 4 diameters, said mixing step is continued for at least 8 diameters at a temperature not less than 150°C, and said polyolefinic polymers and the additives thereof are supplied after at least 16 diameters from the absorbing product introducing point and said homogenizing step is continued for at least other 8 said diameters at a temperature of 200-300°C. A method according to Claim 17, wherein said polyolefinic polymers and additives thereof are supplied after 24 diameters from the absorbing product introducing point, the homogenizing temperature being held at from 220 to 280°C. A method thermoforming a product according to Claims 1 to 14, wherein said method provides a molding temperature from 160 to 220°C, and a maximum molding time of 20 seconds for thicknesses of said sheet greater than 0.7 mm, a maximum molding time of 15 seconds for thicknesses of said sheet from, 0.4 to 7 mm, and a maximum molding time of 8 seconds for thicknesses of said sheet less than 0.4 mm.

Inventors: CRESPI PAOLO, QUINTAVALLE UMBERTO, CRESPI, PAOLO, QUINTAVALLE, UMBERTO, CRESPI, PAOLO, P.I.L. S.R.L., QUINTAVALLE, UMBERTO, P.I.L. S.R.L.
Applicants: DARAMIC INC, DARAMIC, INC.
Application year: 1995
Main classification: B01J20
IPC codes: B01J20