Method for producing partially expanded bodies with a PVC-based cellular structure and mould for their production

Abstract: The present invention relates to a method for producing partially expanded bodies with a PVC-based embryo cellular structure comprising the steps of filling a plurality of moulds (1) with a paste containing PVC, loading these moulds (1) onto a press provided with heating and cooling means designed to subject the paste to the combined action of an increase in pressure and variation in temperature, expansion of the paste inside the moulds (1) and consequent forming of partially expanded bodies with an embryo cellular structure. In accordance with the present invention, during the expansion step, a step involving transfer of a first excess amount of paste from a main compartment (4) of the mould (1) to a secondary compartment (5) of the mould (1) takes place. This results in the formation, during the expansion and consequent forming step inside the main compartment (4), of a partially expanded main body with an embryo cellular structure and, inside the secondary compartment (5), of a partially expanded secondary body with an embryo cellular structure. The containing area (3) of the mould (1) is divided by a separation baffle (6) into two compartments, i.e. a main compartment (4) and a secondary compartment (5), communicating with each other by means of connection channels (8) formed on the separation baffle (6) itself.

Description: The present invention relates to a method for producing partially expanded bodies with a PVC-based embryo cellular structure and to a mould for the production of these bodies. More precisely, the method and the mould which form the subject of the present invention are advantageously used for the production of partially expanded bodies (called "embryo cellular bodies" in the specific technical sector) which, as a result of subsequent chemical-physical treatment, can be used to obtain expanded products, mainly in the form of panels, which can be used as thermal/acoustic insulators in the building sector or for applications in the naval or aeronautic sector or also for imparting floating properties to boats, etc. As is known, the production process which results in the manufacture of the panels envisages initially the formation of a paste consisting of a mixture of powders (PVC and other compounds) and liquid substances (in particular isocyanates). Subsequently the paste is subjected to pressure, heating and subsequent cooling inside suitable moulds from where the so-called "embryo cellular bodies" are then extracted. The latter, following further heat treatment in water or steam ovens, are further expanded as a result of the hydrolysis reaction of the isocyanate groups present in the material, giving rise to the formation of the panels. At present, the methods for the production of "embryo cellular bodies" envisage filling the moulds with quantities of pastes greater than that required (with an excess amount, in terms of weight, equal to about 8% of the product leaving the mould) so as to allow gaseous inclusions to escape from the moulds during the heating step. In other words, the excess paste introduced into the moulds allows, at the end of processing, panels with a perfectly regular shape, devoid of gaseous forms or cavities, to be obtained. In accordance with a first known embodiment of a method and apparatus for the production of "embryo cellular bodies", the excess paste equivalent to 8% by weight emerges from the top edge of the mould. Generally this occurs after about 5-6 minutes from the start of heating of the moulds following the increase in pressure which occurs inside them owing to the chemical reactions which develop in the pastes. Consequently there is a non-recoverable wastage of material (since at high temperature the PVC gelatinizes and the expanding substances deteriorate). At the same time the drawing off of the harmful vapours is necessary, with a consequent obvious increase in the production costs which have a negative effect on the cost of the final product. More recently, in order to overcome at least partly these drawbacks, a second embodiment of a method and apparatus for the production of "embryo cellular bodies" has been devised, by means of which it is possible to recover and therefore re-use the excess paste introduced initially into the moulds, thereby avoiding the wastage of material and the danger of environmental pollution caused by the evaporation of the iscyanates. This second embodiment, which is described in the patent No. IT 1,242,873 and envisages the extraction of the excess paste during a step of the process involving heating of the paste in the moulds, has in practice proved to have certain drawbacks. In fact, it uses means for extraction of the excess paste which are costly, impractical, pose considerable maintenance problems and do not allow subsequently basically an optimum re-use of the excess paste. A third embodiment of a method and apparatus for the production of "embryo cellular bodies" is disclosed in the patent US 2,525,880. This embodiment envisages the use of a main mold compartment connected to a secondary mold compartment through a passageway. During the expansion of the paste the excess paste flows form the main mold compartment to the secondary compartment through the passageway. In such way an expanded article can be produced both in the main and in the secondary compartment. The main object of the present invention is therefore that of improving the quality of the article produced in the secondary compartment of a mould for the production of partially expanded bodies with a PVC-based embryo cellular structure, which is simple, reliable and does not require high maintenance costs. The technical characteristics of the invention, in accordance with the abovementioned objects, may be clearly understood from the contents of the claims indicated below and the advantages thereof will emerge more clearly from the detailed description which follows, with reference to the accompanying drawings, which show some purely exemplary and non-limiting embodiments in which: Figure 1 shows a schematic plan view of the mould according to the present invention; Figure 2 shows a side view, on a larger scale and in cross-section, of a portion of the mould according to Figure 1; Figure 3 shows a partial schematic plan view of a portion of a variation of embodiment of the mould according to the present invention. In accordance with the figures of the accompanying drawings, 1 denotes, in its entirety, the mould according to the present invention. The mould 1 has a substantially rectangular shape bounded perimetrally by a perimetral edge 2 (with a height of about 3 cm) which defines a containing area 3 of the mould 1. During a step for filling of the mould 1, a paste (in an excess amount equivalent to about 8% by weight) containing PVC, expanding substances, stabilizers and liquids containing isocyanates and, optionally, plasticizers is introduced inside this area 3. Each mould 1 is provided in a conventional manner with a rectangular shaped lid. Then, the mould 1 (together with other moulds) is placed, during a special loading step, onto a press (not shown in the accompanying figures in that of the known type) between heating and cooling plates which are designed to subject the paste to a combined action involving an increase in pressure and variation in temperature. More particularly, when the press is closed, the heating produced by the plates (containing a heat-exchange coil inside) starts. In some cases, the heating may occur before the introduction of the moulds 1 into the press. The step involving expansion of the paste inside the moulds 1 then takes place. In this way, after a few minutes have passed by and the pressure inside each mould 1 has reached a threshold value, the passage, during a special transfer step, of a first excess amount of paste from a main compartment 4 to a secondary compartment 5 of each mould 1 occurs. For this purpose, each mould 1 (see Fig. 1) is provided with a separation baffle 6 which divides the containing area 3 into the two compartments, i.e. main compartment 4 and secondary compartment 5 mentioned above. As can be seen in Figure 2, the upper surface 7 of the separation baffle 6 has, formed in it, transverse grooves 8 which act as channels 8 connecting the main compartment 4 and the secondary compartment 5. It should be noted that the excess paste (equivalent to about 8% by weight with respect to the weight of the product leaving the press at the end of the method), which is introduced inside the mould 1 at the end of the filling step, is necessary for allowing the escape of the gaseous inclusions during the step involving expansion (and heating) of the paste inside the mould 1. In other words, in the moulding methods using these PVC-based materials, this excess paste (i.e. of the order of 7-9%) allows products with a regular form to be obtained at the end of processing. In accordance with the subject of the present invention, the method for producing partially expanded bodies with an embryo cellular structure therefore envisages a step involving expansion of the paste in the main compartment 4 and simultaneous expansion of the excess in the secondary compartment 5. Consequently, a forming step takes place, resulting in the formation of a main body (not shown) inside the main compartment 4 and a secondary body (also not shown) inside the secondary compartment 5. As can be seen from the plan view of the mould 1 illustrated in Figure 1, the secondary compartment 5 has smaller dimensions than the main compartment 4. This is so because the secondary compartment 5 must receive only the first excess amount of paste of the main compartment which is equivalent to about 8% by weight of the paste originally introduced into the mould 1. In turn the secondary compartment 5, in order to produce an embryo cellular secondary body which is regular and devoid of gaseous inclusions, also needs to discharge about 8% of the paste which fills it, i.e. about 8% of the first excess amount of paste. Therefore, it has to discharge externally a second excess amount of paste which therefore amounts to about 0.64% of the paste originally introduced into the mould 1. It is obvious that already with the division of the mould into two compartments a practically negligible wastage of paste is achieved. In particular, as a result of this, it is not necessary to provide costly disposal systems which for example perform long and complex treatment of the production waste using hot water or steam so as to allow complete reaction of the isocyanates. In accordance with the present invention, discharging of the second excess amount of paste takes place during the expansion and subsequent forming step inside a perimetral groove 9 formed on top of the perimetral edge 2 of the mould 1. In accordance with a preferred embodiment of the invention, the groove 9 may advantageously be formed only in the secondary compartment 5. In this case, as can be seen from the accompanying Figure 3, the groove 9 has a section 10 which is arranged parallel to the long side of the secondary compartment 5 and two short sections 11 which are arranged parallel to the two short sides of the secondary compartment 5.

Claims: Method for producing partially expanded bodies with a PVC-based embryo cellular structure, comprising the steps of: filling at least one mould (1) with a paste containing PVC, expanding substances, stabilizers and liquids containing isocyanates; loading said mould (1) onto a press operationally associated with heating and cooling means designed to subject the paste to the combined action of an increase in pressure and variation in temperature; expansion of said paste inside said mould (1); forming a partially expanded body with an embryo cellular structure inside said mould (1); transferring a first excess amount of paste from a main compartment (4) of said mould (1) to a secondary compartment (5) of said mould (1), during said expansion step; said expansion step and said forming step consequently producing inside said main compartment (4) and secondary compartment (5) respectively a partially expanded main body and secondary body with an embryo cellular structure, characterized in that it furthercomprises a second step of transferring a second excess amount of paste from the secondary compartment (5) of said mould (1) to a perimetral groove (9) of said mould (1), during said expansion step. Method according to Claim 1, characterized in that said transfer steps take place during said expansion step when the pressure inside the paste reaches a threshold value designed to allow the passage of the first excess amount of paste through connection channels (8) arranged between said main compartment (4) and said secondary compartment (5). Mould for the production of partially expanded bodies with a PVC-based embryo cellular structure, which can be charged with a paste containing PVC, expanding substances, stabilizers and liquids containing isocyanates and which can be used in a press operationally associated with heating and cooling means which are designed to subject said paste to the combined action of an increase in pressure and variation in temperature, said mould (1) being peripherally bounded by a perimetral edge (2) designed to define a containing area (3) for receiving said paste and produce said partially expanded bodies with an embryo cellular structure, the containing area of said mould (1) being divided by a separation baffle (6) into at least two compartments, i.e. a main compartment (4) and a secondary compartment (5), which communicate with each other by means of connection channels (8) formed on said separation baffle (6), said main compartment (4) discharging via said connection channels (8) a first excess amount of paste into said secondary compartment (5), characterized in that a perimetral groove (9) is formed on top of said perimetral edge (2), said secondary compartment (5) discharging a second excess amount of paste into said perimetral groove (9). Mould according to Claim 3, characterized in that said secondary compartment (5) has dimensions smaller than those of said main compartment (4). Mould according to Claim 4, characterized in that said first excess amount of paste is equivalent to 7-9% by weight of the paste originally introduced inside said mould (1). Mould according to Claim 3, characterized in that said second excess amount of paste is equivalent to 7-9% by weight of the paste originally discharged inside it from said main compartment (4). Mould according to Claim 3, characterized in that said connection channels (8) consist of transverse channels formed on the upper surface (7) of said separation baffle (6). Mould according to Claim 3, characterized in that said groove (9) is formed on the perimetral edge (2) only in the region of said secondary compartment (5).

Inventors: OLIVIER GIACOMA, OLIVIER, GIACOMA
Applicants: OLIVIER GIACOMA, OLIVIER, GIACOMA
Application year: 1999
Main classification: B29C44
IPC codes: B29C44