METHOD FOR PRODUCING LAMINATED ARTICLES, PARTICULARLY FOR PACKAGING PERISHABLE AND/OR CHEMICALLY AGGRESSIVE SUBSTANCES, AND CONTAINERS PRODUCED WITH SAID METHOD

Abstract: A method for producing laminated items, particularly for packaging and storing perishable and/or chemically aggressive substances, includes the following steps: a) preparing a mold that is shaped complementarily to the item to be produced; b) depositing, at least two superimposed layers of thermoplastic materials having chemical and physical characteristics that provide impermeableness to fluids, mechanical strength, resistance to chemicals on different levels, wherein one of the characteristics is predominant with respect to the others in one of the layers; c) welding the superimposed layers, on the surfaces that are mutually in contact, by simultaneous heating and compression; d) rapid cooling of the resulting item. Welding occurs at relatively low temperatures and pressures, so as to avoid substantially altering the chemical and physical properties of the basic materials of the individual layers.

Description: METHOD FOR PRODUCING LAMINATED ARTICLES, PARTICULARLY FOR PACKAGING PERISHABLE AND/OR CHEMICALLY AGGRESSIVE SUBSTANCES, AND CONTAINERS PRODUCED WITH SAID METHOD The present invention relates to a method for producing laminated items, for example containers for packaging and storing perishable and/or chemically aggressive substances, such as foods, beverages, drugs, cosmetics, and the like. A second aspect of the invention relates to laminated or multilayer containers and to the corresponding lids produced with the method. A container that is ideal for the above mentioned purpose must not contaminate or alter the substances contained therein, must not release or absorb odors and/or tastes, and must also withstand the various operating conditions, be impermeable to liquids and gases, and must be biodegradable or recyclable, aesthetically pleasant, and cheap. In order to extend the shelf life of perishable substances, the oxygen contained in the air must be prevented from entering the sealed container by diffusion through the walls. Likewise, for perishable substances packaged in a controlled atmosphere, this atmosphere must be prevented from flowing outward. In general, known single-layer containers made monolithically of plastics and similar materials by injection-molding, thermoforming, blow-molding, extrusion, et cetera, do not have all the characteristics of the ideal container. For example, most of them are not highly impermeable to oxygen, which by penetrating through the walls of the container can oxidize the contents. In order to improve conventional packages, known production methods have been perfected for multilayer or laminated containers that are constituted by two or more materials having different and mutually complementary physical and chemical properties. In particular, a conventional method provides for the coupling of the various materials in layers, by virtue of combined injection-molding, blow molding, or coextrusion. Another conventional method provides for the mixing of two or more materials, for example polyamide (PA) and polyethylene (PET), to form containers for example by thermoforming. A drawback of conventional container production methods is that the processes that are used can cause a partial loss of some physical and chemical properties of the initial materials, for example. Therefore, the resulting containers may have poor mechanical strength and rigidity and relatively high permeability to gases. Another drawback of conventional methods is the difficulty in applying decorations and/or labels during forming (so- called "in-mold labeling"). The decoration or the label is in fact often damaged by the water and/or by the high process temperature. Another drawback is the high cost of the conventional methods for producing containers with relatively thick walls (275 μm to 500 μm) ensuring sufficient impermeability to gases as well. The aim of the invention is to overcome the above drawbacks by providing a method for producing items, for example disposable laminated containers for packaging and storing perishable substances, having perfect impermeableness to fluids in the liquid and gaseous state, low cost, strength, recyclability and/or biodegradability. Another object of the invention is to provide a method for producing containers with intact, functional, and clearly visible labels and decorations. This aim, this object, and others that will become apparent hereinafter are achieved by a method for producing laminated articles, particularly for packaging and storing perishable and/or chemically aggressive substances, as claimed in the appended claims. The superimposed layers are welded at a relatively low temperature that can vary between 50°C and 250°C and at a pressure that can vary between 5xl0 5 Pa and 2xl0 7 Pa. In this manner, one achieves strong adhesion of the superimposed layers while preserving the chemical and physical properties of the individual materials. Preferably the layers are at least four: a first outer layer, made of a plastic material that is impermeable to liquids and gases and is optically transparent; a second pigmented layer, which forms a decoration and/or a label; a third layer of laminated thermoplastic material; and a fourth layer of granular thermoplastic material. It is also possible to provide, ar the innermost face of the article, an additional layer of laminated aluminum, joined to the fourth layer of granular thermoplastic material by a sixth layer of laminated thermoplastic material. The aluminum layer forms a first unidirectional barrier from the inside toward the outside. The method according to the invention allows to provide items such as multilayer containers with characteristics that significantly increase, with respect to conventional containers, the shelf life of the products contained in the provided containers. A second aspect of the invention provides for the production of a throwaway multilayer container of the type described in the preamble, including a tray and a lid, characterized in that it is produced according to the above described method. Conveniently, the choice of the materials of the layers allows to perfectly adapt the characteristics of the container to the substances that it is meant to contain, in order to achieve optimum storage thereof. Further characteristics and advantages will become apparent from the following detailed description of the method and of the container according to the invention, illustrated only by way of non-limitative example in the accompanying drawings, wherein: Figure 1 is a perspective view of a multilayer container produced with the method according to the invention; Figure 2 is an enlarged-scale sectional view of a portion of the container of Figure 1. With reference to the above figures, the method according to the invention comprises the preparation of a mold that is shaped complementarily with respect to the item to be produced and on which at least two superimposed layers of thermoplastic materials are deposited which have chemical and physical characteristics that provide impermeableness to fluids, mechanical strength, resistance to chemicals, and differentiated transparency to light. In particular, the materials of the layers can be chosen for example among polyamides (PA), polyolefins (HDPE, LDPE, PP, PE), linear polyester (PET), polyvinyl chloride (PVC), styrene polymers (PS), cellulose esters, polycarbonates (PC), fluoridized polymers, ethylene vinyl acetate. The preliminarily superimposed layers are then mutually welded along the surfaces that are in contact and by simultaneous heating and compression. Finally, the resulting item is rapidly cooled by means of a low-temperature fluid which, for example, passes through a cooling circuit that lies inside the mold. In particular, in each layer, at least one of the above mentioned characteristics of the individual materials is predominant with respect to the others, so as to maximize in the resulting multilayer item, as a whole, all the characteristics of the individual layers. The welding or adhesion of the superimposed layers is substantially of the autogenous type produced by softening and pressure, and is performed at a relatively low temperature that can generally vary between 40°C and 250°C, with maximum temperatures of approximately 300°C, depending on the polymer that is used, and at pressures that can vary between 3x10 Pa and 3xl0 7 Pa, preferably comprised between 5xl0 5 and 2xl0 7 Pa. This type of welding produces strong adhesion of the superimposed layers while keeping the chemical and physical properties of the materials of each individual layer substantially intact. More specifically, the method according to the invention, in one embodiment, may include the superimposition of at least four layers of material: a first outermost layer of a plastic material that is impermeable to liquids and gaseε and is optically transparent; a second pigmented layer that forms a decoration and/or a label; a third layer of laminated thermoplastic material; and a fourth layer of granular thermoplastic material. The fourth layer tends to melt during welding, permanently assuming the shape of the mold. Subsequent rapid cooling causes considerable hardening of all the thermoplastic layers, which mutually cooperate to increase the overall mechanical strength of the container. It is also possible to superimpose a further fifth layer on the fourth layer. The fifth layer is constituted by a sheet of laminated aluminum that is arranged on the innermost face of the container and is made to adhere to the fourth layer by means of a sixth layer of thermoplastic material. The aluminum sheet constitutes an effective barrier against any escape of fluids from the inside of the container toward the outside. A second aspect of the invention relates to a container produced with the above described method. The container, generally designated by the reference numeral 1, includes a tray 2 and a lid 3. In the embodiment shown in the figures, the container has a rectangular base and has a reduced size , but the shape and the dimensions may be any ac cording to the spec i f i c requirements without thereby abandoning the scope o f the invention . For example, the resulting container can have walls having a total thickness s of approximately 870 μm and can be constituted by: a first outer layer 4 made of transparent nylon ^ ', with a thickness of approximately 15 μm and a density of approximately 1.3 kg/cm ; a second layer 5 of an ink or pigmented product that is suitable to form a label, a decoration or an ornament and is approximately 5 μm thick; a third layer 6 of polypropylene that is approximately 75 μm thick and has a density of approximately 0.92 kg/cm 3 ; a fourth layer 7 of polypropylene that is approximately 750 μm thick and has the same density as the second layer; a fifth layer 8, which is constituted by a sheet of laminated aluminum 5 to 20 μm thick and suitable to form a unidirectional barrier from the inside toward the outside of the container; and a sixth layer 9 of thermoplastic material that is approximately 20 μm thick and is suitable to fix the aluminum layer 8 to the underlying layers. The resulting container is very strong and rigid, has very low bidirectional permeability to fluids, does not contaminate the contents in any way, and is ecological and recyclable. All these characteristics are obtained with lower costs than those of conventional containers having the same characteristics. The layer of pigmented material that forms the label or a decoration of the container introduced in the mold is protected by the outer layer and is not damaged in any way during the production process and during subsequent use; therefore both the inscriptions and the decorations are sharp and clearly visible from outside. Experimental tests conducted at the laboratories of the University of Salerno, Department of Chemical and Food Engineering, to measure permeability to oxygen, nitrogen, and carbon dioxide gases have shown that the multilayer material of the containers obtained with the method according to the invention has, under equal test conditions, a permeability that is distinctly lower than that of the materials commonly used to produce similar containers. These tests were conducted with a GPM 200 permeability gauge made by Lyssy, Zollkon (Switzerland) on uniform samples, according to the ASTM D 1434 testing specification (25°C - 0% RH). The samples of material were constituted by strips approximately 0.8 mm thick and of a size that was adequate for the permeability measurements; these measurements were performed in a circular cell measuring 100 mm in diameter and the strips were obtained from food packaging trays. The specimens to be subjected to the permeability test were cut from the strips. Only one specimen was obtained for each material due to the presence, in the strip, of the residue of the molding sprue, which as a matter of precaution was excluded from the specimens to ensure the absence of microfissures. Each specimen was conditioned for eight hours in a vacuum oven at 40°C before each test. The permeability values listed hereafter were averaged on the basis of at least three findings. The results of the above tests are listed in the following Table 1. Table I Permeability in/ml (s-cm 2 «Pa) •10~ 13 Samples ""— — — _ B co 2 °2 N 2 1 3.2 1.2 0. 51 2 4.5 1.4 0.85 3 1.1 0.44 0.25 4 1.0 0.40 0.15 In particular, samples 1 and 2 are taken from commonly commercially available containers made of molded plastic. In particular, sample 1 is formed by a multilayer item composed of two coupled layers of PP, with a decorative ink interposed, and of a third layer of granular PP. Sample 2 is constituted by a single layer of PP without coverings that is simply molded. Samples 3 and 4 are obtained from containers produced with the method according to the invention. The method according to the invention and the items obtained with the method are susceptible of numerous modifications and variations, all of which are within the scope of the inventive concept expressed by the accompanying claims. A ll the details may be replaced with technical equivalents, which are understood to be equally protected. The materials, - li ¬ the shapes, and the dimensions of the containers can vary according to the requirements.

Claims: CLAIMS 1. A method for producing laminated articles, particularly for packaging and storing perishable and/or chemically aggressive substances, characterized by the following steps: a) preparing a mold that is shaped complementarily to the item to be produced; b ) depos it ing , at l east two supe rimposed l ayers o f thermoplastic materials , in said mold, having chemical and physical characteristics that provide impermeableness to fluids, mechanical strength, resistance to chemicals , and different transparency to light , wherein at least one of said characteristics is predominant with respect to the others in each one of said layers; c ) welding said superimposed layers, on the surfaces that are mutual ly in contact , by simultaneous heating and compression; d) rapid cooling of the resulting article. 2. The method according to claim 1, characterized in that it comprises at least four layers: a first outer layer, made of a thermoplastic material that is impermeable to liquids and gases and is optically transparent; a second pigmented layer, which forms a decoration and/or a label; a third layer of laminated thermoplastic material; and a fourth layer of granular thermoplastic material. 3. The method according to claim 2, characterized in that it comprises a fifth layer, on the innermost face of the item, said fifth layer being of laminar aluminum and being joined to said fourth layer of thermoplastic material by a sixth layer of laminated thermoplastic material, wherein said aluminum layer forms a unidirectional barrier from the inside toward the outside. 4. The method according to one or more of the preceding claims, characterized in that at least one of said layers of thermoplastic materials is distinctly thicker than the others in order to provide a supporting structure of said container. 5. The method according to claim 1, characterized in that said welding of said step c) occurs at a relatively low temperature between 40°C and 300°C and at a pressure that can vary between 3xl0 5 Pa and 3x10 7 Pa. 6. The method according to claim 1, characterized in that said rapid cooling in said step d) is performed by circulating in the mold a cooling medium at low temperature with a holding time of approximately 4 seconds. 7. The method according to claim 1, characterized in that said rapid cooling in said step d) occurs by a cooling means that acts on said mold. 8. The method according to claim 1, characterized in that the heating of said superimposed layers during said step c) occurs indirectly by conduction through said mold or directly by means of an adapted pre-heater. 9. Method according to claim 1, characterized in that said compression of step c) is performed by a mechanical and/or hydraulic and/or pneumatic means according to a preset criterion. 10. Method according to one or more of the preceding claims, characterized in that said layers of thermoplastic material are chosen among polyamides (PA), polyolefins (HDPE, LDPE, PP, PE), linear polyester (PET), polyvinyl chloride (PVC), styrene polymers (PS), cellulose esters, polycarbonates (PC), fluoridized polymers, ethylene vinyl acetate. 11. Disposable laminated container, particularly for packaging and storing perishable and/or chemically aggressive substances, comprising a tray (2) and a lid (3), characterized in that it is produced according to the method described in the preceding claims.

Inventors: RIGLIETTI ANTONIO, RIGLIETTI, ANTONIO
Applicants: RIGLIETTI ANTONIO, RIGLIETTI, ANTONIO
Application year: 1996
Main classification: B29C51
IPC codes: B29C51, B65D65