The invention in question is therefore able to be used for the manufacture of glass panes or panels which are at the same time transparent and heat or fire-resistant.

More particularly, the present method and the apparatus are advantageously intended to be used in fire-breaker doors.

Background art The current tendency in the fire prevention sector is to contain the outbreak of fires within closed spaces in order to prevent the entry of air which fuels combustion.

For this purpose all the door and window fittings providing communication between the rooms to be isolated are made of heat and fire-resistant material.

Preferably the fittings have transparent surfaces able themselves to withstand heat without collapsing or shattering.

Advantageously, according to the known art, these transparent surfaces consist of stratified elements comprising several superimposed sheets of transparent material, for example glass, with, arranged in between, a material-also transparent and with intumescent characteristics-intended to join together the individual sheets.

"Intumescence"is understood as meaning the property of a material to form a large-volume insulating foam when it is subjected to high temperatures. This foam, retaining gas inside it, increases the resistance to heat transmission as a result of both conduction and irradiation and moreover retains the parts of a pane which may be broken.

At present, in accordance with the art known hitherto, numerous methods for preparing heat-resistant, transparent, stratified elements are known.

For example, heat-resistant, transparent, stratified elements are known where the space between two sheets is filled with an intumescent organic gel in aqueous solution containing a high quantity of water.

The high water content increases the fluidity of the material, facilitates filling of the cavities and increases the intumescent properties.

This constructional solution is not without drawbacks. In fact, the intermediate layer remaining liquid, is able to escape in the event of breakage or solely accidental cracking of one of the sheets or may with time seep through the lateral seals provided. These drawbacks are particularly serious when the stratified elements are used for the manufacture of fire-breaker doors and occur in a particularly obvious manner in the event of impacts. Owing to undesirable leakages, unaesthetic bubbles or even zones without intumescent properties and heat resistance may form inside the stratified element.

In the event of particularly extreme temperatures it may happen that the solution containing the gel solidifies, increasing its volume until it causes breakage of the stratified element.

In order to avoid these drawbacks, more recently compositions have been developed, resulting in the formation of a gel able to solidify during production directly between the sheets of the stratified element. It is also known to use

additives in order to lower the freezing point of the gel or increase the adherence thereof to the surfaces of the sheets. It is also known that it is possible to degassify the solution before introducing it into the chamber defined by the parallel sheets in order to limit the formation of bubbles.

In accordance with this latter known embodiment, the solid layer inside the chamber, acting as an element joining together the glass sheets, increases the mechanical strength of the stratified transparent element, allowing also machining thereof by means of cutting.

For example, EP-A-1531 describes a method for the manufacture of fire-resistant glass panes consisting of at least two sheets of glass which are parallel to each other and kept spaced from each other so as to form a sealed chamber for liquids intended to be filled with an aqueous solution-containing at least 65% of water- of intumescent gel based on carbon bonds.

Heat-resistant stratified elements, comprising a solution of hydrated alkali polysilicate as the intumescent element, are also known. In the event of fire, owing to the action of the heat, the water evaporates and the alkali polysilicate is converted into a solid foam thus becoming a good heat insulator and retaining, at the same time, the pieces of the glass panes which may have shattered.

The intumescent properties of alkali polysilicates have been known for some time and for example are described in the text written by J. G. Vail entitled :"Soluble silicates, their properties and uses", Vol. 1 and 2, NY, published in 1952.

Alkali polysilicate has a fairly complex structure which is normally represented in the form xSi02 yMe20 zH20 in which xSi02 indicates the silica component, yMe20 indicates the component of the alkali metal oxide, generally lithium, sodium or potassium, and zH20 the hydration water. The type of polysilicate is defined by means of the molar or weight ratio, Si02/Me20, of the silica content and alkali metal oxide content.

In the remainder of the present patent, the term"alkali polysilicate"refers to a compound of the type indicated above having a molar ratio SiO2/Me20 of between 1.5 to 25.

In the polysilicates used as transparent stratified elements this ratio may vary from 3to6.

On the basis of that described in the books"Liquid glass"by P. N. Grigoriev, M. A.

Matveev, Moscow, 1956;"The Chemistry of silica", by R. K. Iler, 1982 ;"Soluble and liquid glass"by V. I. Korneev, V. V. Danilov, Leningrad, 1996, and other publications, it is known that when water is present in the system xSi02 yMe20-zH20 in) arge quantities, it also acts as a solvent and the alkali polysilicate remains in solution form, while when it is present in small quantities it behaves as a copolymerizer and the solution solidifies. In fact, when water is removed from an aqueous solution of alkali polysilicate, for example by means of natural evaporation or by means of heat, the hydrated alkali polysilicate passes from the liquid transparent form of an aqueous solution to the solid transparent form, retaining a considerable quantity of hydration water.

US-A-4,190,698 describes a method for the manufacture of stratified elements consisting of a plurality of glass sheets with an alkali polysilicate arranged in between. The polysilicate is prepared by means of mixing of an aqueous solution of alkali silicate having a silica/metal oxide ratio of about 3.3 with a dispersion of colloidal silica in water containing about 30% by weight of silica.

In accordance with the method described in the abovementioned US-A-4,190,698, the polysilicate layer is first spread on one side of a first sheet of glass and is then hardened by means of drying in a heated environment. At this point, a second sheet is arranged with pressure on the first sheet. These steps are repeated until the desired thickness or number of final sheets is reached. The hydrated alkali polysilicate obtained has metal silica/metal oxide ratio of between 3.3 and 5.

Drawbacks of this known method consist in the difficulties of regulating the parameters which govern the drying process, the difficulties of ensuring a uniform distribution of the solution on the sheet, the difficulties of achieving a constant thickness of the layers arranged in between and the difficulties of combining the sheets in a sandwich structure without trapping undesirable air bubbles inside.

US-A-5,565,273 describes a method for the manufacture of transparent stratified elements by means of pouring, into the spaces between the sheets, an aqueous solution containing alkali polysilicate and colloidal silica with an overall molar ratio SiO2/Me20 greater than 4: 1 and with a water content of between 44 and 65% by weight. Without drying, the solution hardens, incorporating all the water present since there is no removal thereof.

Methods are also known where the aqueous solution of hydrated alkali polysilicate is first inserted in liquid form inside the chamber defined by the two sheets and is then made to solidify by means of the action of hardening reagents and/or accelerators and/or retardants.

A drawback of these known methods is the need to add these components in order to induce or prevent polymerization and control the reaction speed.

Moreover, these same added components may reduce the transparency or modify the refraction index of the stratified element or also not be stable to UV radiation, becoming less transparent and opaque over time.

Moreover, a high ratio Si02/Me20 reduces the transparency of the polysilicate and therefore its popularity on the market.

Summary of the invention The main object of the present invention is to eliminate the abovementioned

drawbacks by providing a method and an apparatus for the manufacture of heat- resistant, transparent, stratified elements, which is simple and low-cost to achieve and allows stratified elements with optimum fire-resistant properties to be obtained.

A particular object of the present invention is to provide a method for the manufacture of transparent stratified elements which allows the transparency and the refractive index to be controlled.

A further object of the invention is to obtain a transparent stratified element which has a high resistance to UV radiation and which does not vary its colour with time.

Another object of the present invention is to obtain a transparent stratified element which has a high mechanical strength in particular with regard to impacts.

Another object of the present invention is that of providing a method which allows complete filling of the chambers located between the sheets which form part of the transparent stratified elements to be controlled in a simple manner.

Another object of the present invention is that of providing a method for the manufacture of transparent stratified elements which uses good-quality raw materials available on the market and at a low cost.

Another object of the present invention is that of providing a method which allows the manufacture of transparent stratified elements of the desired size without requiring complex and costly cutting operations.

The abovementioned objects, together with others, are all achieved by a method for the manufacture of transparent stratified elements in question, which comprises the following operating steps: a step involving the preparation of at least two sheets of transparent material which are arranged substantially facing each other and are able to define between them at least one receiving chamber ; a step

involving mixing, in a container, an aqueous solution of alkali polysilicate with colloidal silica dispersed in water, with consequent formation of an intumescent liquid mixture; a step involving a first irradiation of the intumescent liquid mixture by means of electromagnet radiation at a frequency less than 30 GHz; a step involving degassing of the intumescent liquid mixture with extraction of at least a part of water in the vapour phase; a step involving filling of the receiving chamber with the intumescent liquid mixture; a step involving hardening of the intumescent liquid mixture inside the receiving chamber. The electromagnetic radiation is in the microwave range or in the radiofrequency range.

Advantageously the aqueous solution of alkali polysilicate and the colloidal silica dispersion, which compose the intumescent liquid mixture, are mixed thoroughly with each other with the aid of ultrasound.

Moreover, the application of a vacuum and ultrasound during the degassing step causes expulsion of dissolved gases and the evaporation of solvent water.

As a result of the present method and apparatus it is possible to produce easily transparent stratified elements with excellent optical properties which are heat- resistant and composed of several transparent elements, for example, made of glass, with a solid transparent intumescent material arranged in between.

The novel solution according to the present invention may be advantageously used in the construction of heat-resistant glass walls or door and window fittings, for example fire-breaker doors, or in fire refuge shelters or premises which must be able to withstand high temperatures.

Brief description of the drawings Further features and advantages of the invention will be more clearly understood from the detailed description of a preferred, but not exclusive, embodiment of a apparatus for the manufacture of heat-resistant, transparent, stratified elements,

which follows, furnished by way of a non-limiting example with reference to attached drawings in which: - FIG. 1 shows a heat-resistant transparent stratified element ; - FIG. 2 shows a functional diagram of an embodiment implementing the method for manufacturing heat-resistant stratified elements according to the present invention.

Detailed description of a preferred embodiment The accompanying Fig. 1 denotes overall with A a heat-resistant, transparent, stratified element provided in accordance with the present invention and essentially formed, as will be described more fully below, by two or more superimposed transparent sheets V, made of glass or other transparent material, such as polycarbonate for example, between which layers S of hardened alkali polysilicate are arranged, joining together rigidly the said sheets.

With reference to the drawing shown in the accompanying Fig. 2,1 denotes overall a functional diagram of an apparatus for the manufacture of heat-resistant stratified elements according to the invention. It comprises retaining means 2 intended to support a plurality of transparent sheets V in a preferably vertical position, with their surfaces arranged facing each other and parallel, being spaced from each other by a distance d of between 0.1 mm and 20 mm, preferably between 0.1 mm and 3 mm, so as to define a corresponding plurality of receiving chambers C.

In the case of sheets with a limited surface area, this distance d may be between 2 cm and 9 cm.

Each of the chambers C is sealed peripherally by the retaining means 2 with the aid of seals (not shown in detail) so as to make them suitable for sealably containing any fluid introduced into them under pressure or under vacuum.

The apparatus 1 also envisages a container 3 which is intended to receive an aqueous solution of alkali polysilicate and colloidal silica dispersed in water and with which heating means 4 for heating up the solution contained therein, stirring means 5 and at least one ultrasound source 19 for mixing inside it the abovementioned components are operationally associated, resulting in the formation of an intumescent liquid mixture M.

The apparatus also comprises circulation means 6 for transferring the intumescent liquid mixture M from the container 3 to the retaining means 2 and therefore to the chambers C to which the latter are connected.

In greater detail, the circulation means 6 comprise a first duct 7 which, in accordance with the diagram shown in Fig. 2, connects the container 3 to the retaining means 2 by means of an opening 8 formed at the bottom of the said retaining means 2. In accordance with this configuration, as will be clarified more fully below, it is therefore possible to convey, by means of a pressure difference and/or a pump 9, the intumescent liquid mixture M from the container 3 to the chambers C, causing the gradual filling thereof from below. With this solution it is possible to limit the formation of bubbles.

According to another configuration, the intumescent liquid mixture M is introduced laterally into the chambers C.

In accordance with a further aspect of the present invention, the apparatus 1 is provided with degassing means 10 for performing extraction of gases dissolved in the intumescent liquid mixture M and, as will be specified further below, for evacuation of evaporated water part. These means 10 advantageously consist of a second duct 11, which connects the top part of the container 3 to the top part of the retaining means 2, the valve 18 for regulating the vacuum inside the container 3, located on the duct 11, and suction means 12 associated with the second duct 11 for subjecting the container 3 and the retaining means 2 to the desired degree of negative pressure or vacuum.

According to the invention, the apparatus is provided with at least one source 13 or 14 of electromagnetic radiation having a frequency of less than 30 GHz and at least one ultrasound source 19 or 30, which are intended to irradiate the intumescent liquid mixture M contained in the container 3 and/or contained inside the receiving chambers C.

Advantageously, a first electromagnetic radiation source 13 and a first ultrasound source 19 and the heating means 4 mentioned above, all associated with the container 3 and operational acting on the mixture M, are incorporated in a first oven 15 indicated in Fig. 2.

Preferably, the heating means 4, the first electromagnetic radiation source 13 and the first ultrasound source 19 combine to cause evaporation of a water part of the mixture M intended to be evacuated by the degassing means 10.

For this purpose, the container 3 inside which mixing of the solutions of alkali polysilicate and colloidal silica is performed using the stirring means 5 and/or ultrasound is made of material which is transparent to electromagnetic radiation.

Similarly, the retaining means 2 supporting the sheets V may be inserted in a second oven 16 provided with a second electromagnetic radiation source 14, with a second ultrasound source 20 and with conventional heating means not shown in the Figures.

The ovens 15,16 are obviously especially screened with respect to the exterior by means of walls able to absorb the electromagnetic radiation, as is moreover required by the standards governing safety at the place of work.

Suitably the oven 16 may comprise a tank 21 able to receive liquids, for example water, inside which the retaining means 2 supporting the sheets V are immersed.

In turn, the two ovens 15,16 may also be incorporated in a single structure for the sake of constructional simplicity.

From a functional point of view, the apparatus 1 described hitherto from a mainly structural point of view operates as is described here below.

Initially the desired number of transparent sheets V is arranged between the retaining means 2 which keep them vertical and at the desired distance d.

The retaining means 2 with the sheets V are placed in the tank 21 containing water so as to perform washing of the surfaces of the said sheets.

Suitably, this operation is facilitated by the presence, in the water, of compounds with an ion exchange function, pH correctors and surfactants and by the simultaneous action of ultrasound generated by the source 20. After this operation, the sheets V are sealed peripheral.

For this purpose, the said retaining means 2 seal, by means of seals, the sheets V peripherally so as to define the receiving chambers C.

The container 3 is then filled to the desired level with the solution of alkali polysilicate and the aqueous dispersion of colloidal silica, which are then mixed together by the stirring means 5 and/or the action of ultrasound.

At this point, the intumescent liquid mixture M is subjected to irradiation by means of electromagnetic radiation, which completes the mixing thereof, mixing together the components thoroughly and inducing within them the conditions for subsequent solidification.

Advantageously, the mixture M may at the same time be heated to the desired temperature by the heating means 4.

At the same time as irradiation and heating or, alternatively, during subsequent stages, the degassing means 10 perform, via the suction means 12, extraction through the second duct 11 of a certain quantity of water evaporated as a result of the effect of heating and/or the electromagnetic radiation and/or the ultrasound.

In short, it can therefore be stated that the combined action of vacuum, heat, electromagnetic radiation, and ultrasound causes complete degassing of the liquid mass contained in the container 3, with homogeneous mixing of its components, alkali polysilicate, colloidal silica and water, which result in a mixture M having a high degree of transparency.

Once these operations have been terminated, the mixture M is conveyed by means of the circulation means 6 into the receiving chambers C, passing through the opening 8 formed in the retaining means 2. For this purpose, opening of at least one valve 17 provided on the first duct 7 is actuated.

At the same time, by means of the second duct 11 and the regulating valve 18, a predefined degree of vacuum is generated in order to favour the complete filling of the chambers C and improved wetting of the sheets V, preventing the formation of gaseous bubbles.

Advantageously filling is performed with simultaneous ultrasound irradiation of the mixture M. In this way it is possible to fill the chambers C easily and rapidly with a very viscous liquid mixture also in the case of very small spaces d between the sheets V, of less than 5 millimetres.

The back-pressure produced externally by the liquid present in the tank 21 counterbalances the internal pressure produced by the polysilicate solution, preventing breakage thereof.

At this point, the entire stratified element A may be subjected to a second stage of irradiation with electromagnetic radiation generated by the second source 14 in

order to accelerate the process of solidification of the mixture M and form the layers S of hardened polysilicate.

The present invention also comprises a method for manufacture of heat-resistant, transparent, stratified elements A, which may be advantageously implemented by means of the apparatus described above, to which reference will be made by way of example in the description which follows.

The method according to the invention comprises the following operating steps: a) a step involving preparation of the sheets V of transparent material, between the retaining means 2 so as to define the corresponding receiving chambers C; b) a step involving mixing, in the container 3, by means of ultrasound waves with an intensity of between 2x10-3 W/cm3 and 0.2 W/cm3 and with a frequency of up to 1 MHz, of the aqueous solution of alkali polysilicate with the colloidal silica dispersed in the water, resulting in the formation of the intumescent liquid mixture M preferably with a molar ratio Si02/Me20 of between 3 and 5 and with a density greater than 1300 Kg/m3 ; c) a first stage of irradiation with electromagnetic radiation of the intumescent liquid mixture M thus obtained ; d) a step of degassing of the mixture M, subject to the effect of ultrasound with a frequency of up to 1 MHz with extraction of at least a part of water in the vapour phase; e) a step involving filling, subject to the effect of ultrasound having a frequency of up to 1 MHz, of the receiving chambers C with the mixture M; f) a step involving hardening of the mixture M inside the intumescent chambers C.

The sheets V made of transparent material are located at a distance d of between 0.1 mm and 20 mm, preferably at a distance of between 0.1 and 3 mm.

Transparent sheets V with a limited surface area may be arranged at a distance of between 20 and 90 mm, preferably between 70 mm and 80 mm.

The liquid alkali silicate in the form of an aqueous solution and the colloidal silica solution used in the manufacturing method in question can be easily found commercially.

In fact, on the market solutions of sodium and/or potassium silicate in water with concentrations of between 25% and 50% by weight and Si02/Me20 ratios of between 1.5 and 4 are available.

The colloidal silica consists of sub-microscopic particles-with a diameter of about 10 nm-of amorphous silica dispersed in water. In commerce, solutions with a silica content variable from 15 to 50% by weight, typically used for forming ceramic bonds in fibres or refractory materials are available.

Both the components are therefore known, can be easily found and are extremely low-cost.

In accordance with the method in question, the mixing of these two components is preferably performed in two steps, i. e. a first step using conventional stirring means 5 and/or ultrasound produced by the source 19 and a second step, preferably after the first step, with a source of electromagnetic radiation 13 able to irradiate the mixture M, causing thorough mixing of its components.

The electromagnetic radiation may be in the range of microwaves with a frequency of between 1 GHz and 30 GHz or the radiofrequency range with a frequency of between 5 KHz and 50 KHz.

Preferably the microwaves have a frequency of between 1 and 3 GHz. The irradiation with microwaves is performed for a few minutes, resulting in a total power absorption of the mixture M preferably of between 0.5 and 4 W/cm3.

The action of the microwaves on the polar molecules, such as water, allow the transfer of energy directly to the molecules without intermediate steps, resulting in

efficient activation thereof. Therefore the microwaves are particularly suitable both as heating means and as means for accelerating the chemical reactions. More particularly, according to the invention, the effect of the microwaves produces intense mixing of the components of the mixture M, thus resulting in efficient polymerization of the alkali polysilicate with consequent hardening without the addition of other reagents.

Advantageously the reaction is facilitated by the simultaneous heating due to the action of the said microwaves on the polar molecules present.

Surprisingly this effect results in improved transparency and an improved refractive index of the stratified element A thus obtained.

Preferably, in accordance with the existing regulations in this area, the electromagnetic radiation in the radiofrequency range has a frequency close to 27.12 MHz.

Radiofrequency technology also allows energy transfer selectively to the water molecules present without intermediate steps.

However, the intumescent liquid mixture M may also be heated by the abovementioned heating means 4, so as to favour mixing of its components and help evaporation of a part of water.

In accordance with a further aspect of the method, in order to improve further the polymerization of the components of the mixture M, a second irradiation step for irradiation of the mixture M inside the receiving chamber C may be envisaged.

By means of the degassing step, the gases present in the mixture M and incorporated or dissolved in the preceding treatments or present in the starting solutions as acquired from the supplier, are extracted together with the part of

water evaporated by the action of the ultrasound, the electromagnetic radiation and/or the heating, allowing polymerization of the polysilicate.

Degassing is performed by subjecting the intumescent liquid mixture M to a vacuum preferably in the range of between 20 and 500 millibars.

Advantageously, this operation is performed with simultaneous application of ultrasound.

In accordance with operation of the plant shown by way of example in Fig. 2, the step of filling of the receiving chamber C with the mixture M is performed from below using the same chamber in vacuum conditions following activation of the suction means 12 and owing to the difference in pressure regulated by the valve 18 between the container 3 and the receiving chamber C.

According to another configuration, the filling step is performed by introducing laterally the intumescent liquid mixture M into the receiving chambers C.

Operationally speaking, this filling operation is facilitated by the application of ultrasound for a duration of between 10 and 50 minutes.

Once the hardening step has terminated, when the polysilicate layer which has filled the chambers C has hardened, the stratified element A is removed from the second oven 16, releasing it from the retaining means 2.

Table 1 shows some non-limiting examples of compositions of alkali polysilicates solutions (of sodium, of potassium, of lithium or mixtures thereof) and colloidal silica.

Table 1 Composition (% by weight) Sodium polysilicate Na20 27.8 Si02 8.3 Molar weight Si02/Na20 3.45 Potassium polysilicate K20 23. 7 Si02 10. 5 Molar weight Si02/K20 3.54 Colloidalsilica 30 Na20 0. 3 Si02 30 Colloidal silica 40 Na20 0. 4 Si02 40 Table 2 shows some non-exhaustive examples of preferred formulations for the preparation of the mixture M.

Table 2 Composition (% by weight) Formulation 1 Sodium polysilicate 91 Si02 28 Colloidal silica 30 9 Na20 7.6 Water 64.4 Si02/Na20 (molar) 3.8 Formulation 2 Sodium polysilicate 87 Si02 28 Colloidal silica 30 13 Na20 7.2 Water 64.8 Si02/Na20 (molar) 4 Formulation 3 Sodium polysilicate 90 Si02 29 Colloidal silica 40 10 Na20 7.5 Water 63. 5 Si02/Na20 (molar) 4 Formulation 4 Sodium polysilicate 81 Si02 28 Colloidal silica 30 19 Na20 7 Water 65 Si02/Na20 (molar) 4.1 Formulation 5 Sodium polysilicate 75.3 Si02 30.7 Colloidal silica 40 24.7 Na2O 6.3 17 Water 63 Si02/Na20 (molar) 5 Formulation 6 Potassium polysilicate 94 Si02 24 Colloidal silica 30 6 K20 10 Water 66 Si02/K20 (molar) 3.8 Formulation 7 Potassium polysilicate 90 Si02 24 Colloidal silica 30 10 K20 9.5 Water 66.5 Si02/K20 (molar) 4 Formulation 8 Potassium polysilicate 92.5 Si02 25 Colloidal silica 40 7.5 K20 10 Water 65 Si02/K20 (molar) 4 Formulation 9 Potassium polysilicate 85 Si02 25 Colloidal silica 30 15 K2O 9 Water 66 Si02/K20 (molar) 4.3 Formulation 10 Potassium polysilicate 80 Si02 27 Colloidal silica 40 20 K20 8 Water 65 Si02/K20 (molar) 5.3

Example 1 Using Formulation 1 in Table 2, stratified elements A were constructed, said elements being composed of four sheets V of glass with a thickness 4.5 mm kept parallel at a distance of 1 mm so as to form at the end of the process transparent stratified elements A with a final thickness A of 21 mm.

The initial solution indicated in the first solution of Table 2, after mixing the polysilicate and colloidal silica solutions for 10 minutes with ultrasound at an

intensity of 2x10-2 W/cm3 and frequency of up to 1 MHz, is irradiated for a few minutes with microwaves at an intensity of 0.8 W/cm3 and then introduced, from below with the aid of a vacuum and subject to the effect of ultrasound at an intensity of 5x10-2 W/cm3 and frequency of up to 1 MHz, into the chambers C between the sheets V at a speed of between 0.1 and 2x10-5 m3/sec.

The coefficient of refraction of the stratified elements A thus obtained is between 1.45 and 1.5. The coefficient of transparency, defined as the ratio between the quantity of light transmitted and the total quantity of incident light, is 0.95 for radiation emitted by filament lamps, 0.85 for mercury vapour lamps and 0.4 for gamma radiation. After undergoing accelerated ageing tests and after heating to 85°C for 5 hours and irradiation with UV radiation at an intensity of 8 105 lum/m2, there were no visible changes in the properties.

Example 2 Stratified elements A as described in Example 1 were constructed using Formulation 6 in Table 2 and irradiated for a few minutes with microwaves at an intensity of 1.2 W/cm3. The stratified elements A thus obtained have the same optical properties obtained in the preceding example.

Example 3 Stratified elements A as described in Example 1 were constructed using Formulation 10 in Table 2 and irradiated for a few minutes with microwaves at an intensity of 1 W/cm3. The stratified elements A thus obtained have the same optical properties as in Example 1.

An advantage of the method according to the present invention consists in the fact of obtaining a mixture M which is very fluid, easy to insert into the chambers C between the sheets V and which hardens quickly owing to the effect of microwave radiation and removal of water.

Operationally speaking, control of the filling speed is performed easily by regulating the vacuum.

Advantageously the optical properties are regulated simply by the intensity of the electromagnetic radiation and by the quantity of water removed.

However, it will be possible to introduce into the liquid mixture M a percentage of additive in order to modify the mechanical properties and the intumescence of the protective layer, for example up to 10% of silane and/or up to 10% of saccharides, without thereby departing from the scope of protection of the present invention.

The invention thus conceived therefore achieves the intended objects.

As a matter of fact, it may assume, in its practical embodiment, other forms and configurations different from those illustrated above without, departing from the present scope of protection.

Moreover all the details may be replaced by technically equivalent elements, and the dimensions and forms used may be any depending on the requirements.