Process for preparing heat-insulating structural elements and products obtained thereby

Abstract: A process for preparing heat-insulating structural elements is described, which comprises the following steps: (a) applying to the convex portion of a first half-shell a layer of beads of semi-foamed or foamed thermoplastic polymer, so as to form a layer of substantially uniform thickness; (b) sintering the beads; and (d) applying to the composite thus obtained a second half-­ shell which matches the shape of the sintered layer.

Description: The present invention relates to a process for preparing heat-insulating structural elements. More particularly, the present invention relates to a process for preparing heat-insulating structural elements and to the products obtained thereby. Still more particularly, the present invention relates to a process for preparing heat-insulating structural elements, according to which foamed materials are obtained by means of the use of foaming agents not belonging to the class of chlorofluoroalkanes. The term "heat insulating structural element", as used herein, covers any rigid, circular or polygonal structural element for use in the fields of transportation, electrical home appliances, building industry, car industry, telecommunications, business machines, etc., such as doors, covers, cases, in particular for refrigerators or freezers, panels, containers (for example, for portable heat-insulated bags), etc. According to the prior art, articles of the above types are generally obtained by starting from two half-shells which are mechanically bound together by soldering or adhesive-bonding, the hollow space defined between the two half-shells being subsequently filled with foamed polyurethane. According to another technique, disclosed in IT-A-21 815 A/87, a hollow case of thermoplastic polymer is formed by blow-moulding, and said hollow case is then filled with a reactive polyurethane mixture, which is then foamed. Foamed polyurethane, used as the heat-insulating material in both of the above methods, is obtained by starting from a formulation which is composed of an organic diisocyanate, a polyol, a silicone surfactant, a polymerization catalyst and a foaming agent belonging to the class of chlorofluoroalkanes, such as Freon (R) . At present, the use of foamed polyurethane as heat-insulating material causes environmental problems in that the foaming agents of the chlorofluoroalkane type such as Freon (R) are regarded to be one of the main causes of change and destruction of the ozone layer in the stratosphere. Unfortunately, the problem of replacing polyurethane with an equivalent material is not easy to be solved in that this polymer, by being capable of being foamed in situ according to the well-known R.I.M. (reaction injection moulding) technique, makes it possible to obtain rigid, self-supporting structural elements even if the outer case is made of thermoplastic polymer, i.e., is not in the form of a metal sheet. This is due to the fact that polyurethane, by reacting in situ adheres perfectly to the inner walls of the case, thus giving rise to the formation of a single structural body. US-A-3,650,871, on which the preamble of present claim 1 is based, discloses a method for producing insulated panels of a honeycomb structure surrounded by a frame, in which the panels are continuously fed below a discharging device which fills the honeycomb with polystyrene, granular or ground waste material having insulating properties. Before or after filling, the honeycomb structure may be closed by a subjacent cover board or sheet, and an upper cover board or sheet is applied after filling, the upper and lower cover boards or sheets being adhesively bonded to the honeycomb. GB-A-964203 is directed to a method of producing a heat-insulated casing, for example a refrigerator casing, which consists of a self-supporting casing wall provided at the inner side or at the outer side with a layer of heat-insulating material, wherein the said self-supporting casing wall consists of a synthetic plastics material and is shaped to the desired form by means of an injection molding process, wherein the heat-insulating material consists of a synthetic plastics material in the form of a foam which is applied to the inner side of the molded casing wall or to the outer side thereof by casting a liquid synthetic plastics material mixed with a foaming agent and with an agent suited to dissolve slightly the surface of the casing wall so that the foam layer, after its setting, is firmly bonded to the casing wall to which it has been applied. EP-A-27477, on which the preamble of present claim 10 is based, discloses a brick with cavities which are open at at least one side, said cavities being filled with sintered particles of a thermoplastic polymer, e.g., foamed polystyrene. US-A-4,524,037 describes a process for forming a foam plastic article using a mold with a mold cavity and a pair of electrodes across which an RF field may be impressed. The foam material that is to be processed is inserted into the mold and is then compressed. An RF heating field is applied while maintaining the foam material compressed and after the heat is terminated, the foam material is permitted to expand to conform to a mold shape as the material cools. A process has now, surprisingly, been found which allows heat-insulating structural elements to be obtained by using foamed thermoplastic materials which do not require the use of chlorofluorocarbons as foaming agents and which, compared with the products of the prior art, show the same or even better insulating characteristics at the same thickness. Simultaneously, said process makes it possible to keep the relevant technologies of production at the industrial level nearly unchanged. The subject matter of the present invention is, therefore, a process for preparing heat-insulating structural elements, which process comprises the following steps: (a) applying to the convex portion of a first half-shell a layer of beads made of semi-foamed or foamed thermoplastic polymer, so as to form a layer of substantially uniform thickness; (b) sintering the beads; (c) optionally, drying the sintered layer; and (d) applying to the composite thus obtained a second half-shell which matches the shape of the sintered layer. The sintering of the beads can, for example, be carried out by means of high-temperature steam, hot air and/or radio frequency. According to the present invention, the beads of thermoplastic material preferably have a substantially spherical shape with an average diameter of from 0.1 to 2 mm. Any thermoplastic material capable of yielding foamed or semi-foamed beads can be used in the instant process, although polystyrene and impact-resistant polystyrene are preferred. Examples of alternative materials are polystyrene modified with polar monomers such as acrylonitrile, polyolefins such as polyethylene, polypropylene, poly(vinyl chloride), etc., acrylic or methacrylic resins such as poly(methyl methacrylate), etc., or mixtures thereof. The beads of thermoplastic materials in foamed or semi-foamed form, are products known in the art and available on the market, for example, under the trade name EXTIR (R) (Montedison S.p.A.) or can be produced according to the process disclosed in US-A-2 983 692. The sintering of the beads is preferably carried out with steam at a temperature of from 100 to 200°C. The sintered layer has a substantially uniform thickness of generally less than 10 cm, preferably ranging from 2 to 5 cm. Therefore, this layer reproduces the shape of the convex portion of the first half-shell. At the end of the sintering step, the resulting composite can be conveyed to a drying station in order to eliminate any traces of moisture contained therein. The drying operation, which is preferably carried out when the sintering has been effected with steam, is generally conducted under atmospheric pressure and at a temperature of from 50 to 70°C, optionally in the presence of air circulation. Preferably, the first half-shell is composed of a single piece, has a thickness within the range of from 1 to 5 mm, and may be made of a plate of thermoplastic polymer shaped according to the design of the desired heat-insulating structural element. Examples of thermoplastic polymers which can be used in order to prepare the first half-shell are: polystyrene, impact-resistant polystyrene, polystyrene modified with polar monomers such as acrylonitrile, ABS resins, poly(vinyl chloride), high-, medium- and low-density polyethylene, polypropylene, acrylic or methacrylic resins such as pole(methyl methacrylate), polyphenylethers, polyester resins such as PET and PBT etc., or mixtures thereof. Inasmuch as it does not perform a supporting action but only has to serve as containment and protection means for the heat-insulating layer, the second half shell can be composed of a single piece, or it can comprise a plurality of pieces, also made of different materials such as, for example, a metal plate or a metal grid, which pieces are connected to one another. For example, when the heat-insulating structural element is a refrigerator cabinet, the second half-shell may consist of three pieces to be combined with one another. The first two pieces, made of a metal sheet, are used in order to form the side (external) walls of the cabinet, and the third piece, made of a metal grid, is used as bottom wall onto which the coil for the heat exchange of the refrigerator fluid is to be applied. On the other hands if the heat insulating structural element represents, for example, a refrigerator door a second half-shell, made of a single piece shaped so as to match the sintered layer, is preferably used. The heat-insulating structural elements according to the present invention hence comprise a first half-shell, a heat-insulating layer formed by a layer of beads of foamed or half-foamed thermoplastic polymer sintered onto said first half-shell in a manner such as to reproduce the shape thereof, and a second half-shell acting as a sealing element and matching the shape of the sintered layer. Generally, the heat-insulating structural elements according to the present invention have a heat conductivity of less than 0.030 kcal/mh°C, as a function of their volumetric mass, as measured according to UNI 7819/7891/7745, and a compression strength, according to UNI 6350, of more than 1 kg/cm², usually of from 1.8 to 2.9 kg/cm².

Claims: Process for preparing a heat-insulating structural element by applying to one side of a first outer element a granular thermoplastic polymer, heating said granulate and covering it with a second outer element, characterized in that it comprises the steps of: (a) applying to the convex portion of a first half-shell a layer of beads of semi-foamed or foamed thermoplastic polymer, so as to form a layer of substantially uniform thickness; (b) sintering the beads; (c) optionally, drying the sintered layer; and (d) applying to the composite thus obtained a second half-shell which matches the shape of the sintered layer. Process according to claim 1, wherein the beads of thermoplastic material have a substantially spherical shape and an average diameter of from 0.1 to 2 mm. Process according to any one of claims 1 and 2, wherein the thermoplastic polymer is polystyrene or impact-resistant polystyrene. Process according to any one of the preceding claims, wherein the sintering of the beads is carried out with steam at a temperature of from 100 to 200°C and the sintered layer is dried. Process according to any one of claims 1 to 3, wherein the sintering of the beads is carried out with hot air and/or by means of radio frequency. Process according to any one of the preceding claims, wherein the sintered layer has an essentially uniform thickness of less than 10 cm, particularly of from 2 to 5 cm. Process according to any one of the preceding claims, wherein the drying of the sintered layer is carried out under atmospheric pressure, at a temperature of from 50 to 70°C, optionally in the presence of air circulation. Process according to any one of the preceding claims, wherein the first half-shell is composed of a single piece and is made from a plate of thermoplastic polymer. Process according to any one of the preceding claims, wherein the second half-shell is composed of a single piece or of a plurality of pieces to be linked to one another. Heat-insulating structural element comprising a hollow structure the void space whereof is filled with sintered beads of foamed or semi-foamed thermoplastic material, characterized in that it comprises a first half-shell, a heat-insulating layer formed by a layer of beads of foamed or semi-foamed thermoplastic polymer sintered onto said first half-shell in a manner such as to reproduce the shape of the latter and a second half-shell acting as a sealing element and matching the shape of the sintered layer.

Inventors: ADDEO ANTONIO, BONVINI ALBERTO, REGGIANI ROMANO, VEZZOLI ANNIBALE, ADDEO, ANTONIO, BONVINI, ALBERTO, REGGIANI, ROMANO, VEZZOLI, ANNIBALE
Applicants: SVILUPPO SETTORI IMPIEGO SRL, CENTRO SVILUPPO SETTORI IMPIEGO S.R.L.
Application year: 1990
Main classification: B29C44
IPC codes: B65D81, B29C44, C08J9, F16L59