EP3684866A1

EP3684866A1 - A method for superficially coating polymeric foams in order to improve their flame reaction and the related superficially coated flame resistant polymeric foams

Info

Publication number: EP3684866A1
Application number: EP18738380.7A
Authority: EP
Inventors: Alberto FINA; Federico CAROSIO; Guido Saracco
Original assignee: Politecnico di Torino
Current assignee: Politecnico di Torino
Priority date: 2017-09-21
Filing date: 2018-06-21
Publication date: 2020-07-29
Also published as: WO2019058186A1; IT201700105762A1

Abstract

The invention relates to a method for coating superficially a porous substrate in order to improve its flame reaction which, in addition to a treatment in electrolyte solutions, the substrate is treated in an aqueous colloidal suspension comprising at least inorganic and/ or hybrid nanoparticles and a water-soluble polymer. Moreover, the invention relates to a superficially coated porous substrate having an improved flame retardancy obtained by means of such method. Moreover, the invention relates to a flame-resistant panel comprising a superficially coated porous substrate having an improved flame resistance according to the method described above.

Description

"A method for superficially coating polymeric foams in order to improve their flame reaction and the related superficially coated flame resistant polymeric foams"

DESCRIPTION

TECHNICAL FIELD

The present invention relates to the field of flame retardant materials with a low density, in particular to materials comprising a porous substrate preferably formed by a polymeric foam.

More precisely, the present invention relates to a method for coating a superficially porous substrate in order to improve its flame reaction, comprising the following steps:

- exposing the substrate to a solution of a positive electrolyte;

- exposing the substrate to a solution of a negative electrolyte;

- drying the substrate treated in the previous steps;

- exposing the treated and dried substrate to a colloidal aqueous suspension comprising at least inorganic and/ or hybrid nanoparticles and a water-soluble polymer, wherein said nanoparticles have a concentration ranging between 0.01 and 50% by weight with respect to the water concentration, wherein said water-soluble polymer has a concentration ranging between 0.01 and 50% by weight with respect to the water concentration and wherein the total concentration of said nanoparticles and said water-soluble polymer has a value of up to 100% by weight with respect to the water concentration;

- removing the water present in the colloidal aqueous suspension by evaporation, in order to allow the deposition of a surface coating comprising at least said nanoparticles and said water-soluble polymer on the inner walls of the porous substrate, wherein at least one water- soluble polymer acts as a stabilizer of the colloidal aqueous suspension and/ or as a binder among the deposited nanoparticles. Moreover, the present invention relates to a superficially coated porous substrate having an improved flame resistance obtained according to the above method.

Moreover, the present invention relates to a flame-resistant panel comprising a superficially coated porous substrate having an improved flame resistance as provided for in the previous method.

The present invention finds a preferred and advantageous application in the fields using polymeric materials foams, such as thermal insulation of housings, apartments and other buildings; such as the realization of upholstered furniture; such as seats for means of transport such as aircraft, public transport and the like.

STATE OF THE ART

The polymeric foams are usually highly flammable, and therefore pose serious safety problems in all their applications, if subject to exposure to a flame.

It is known adding additives (flame retardants) that, however, may entail risks for environment and health, as some have been recognized as being or suspected toxic; furthermore, the use of flame retardants acting in gaseous phase generally causes a substantial increase in the optical density of the combustion fumes.

For example, in porous materials based on polymeric foams, it is known the use of halogenated additives that act in the gas phase during the combustion of the porous material; many of these halogenated additives have been recognized as being or suspected toxic and/ or having negative impacts on the environment.

It is also known, in the field of upholstered furniture, making flame-retarding fabrics or finishing materials. These finishing materials may, however, be damaged or subject to wear during the service life of the product, with a consequent reduction or complete removal of the flame-retardant function during use and/ or ageing of the product. The application of thin coatings by deposition of polyelectrolytes on polymeric foams is also known; in particular, EP 2226364 Bl concerns a method for depositing multiple layers of polyelectrolytes on a polymeric foam; this document exemplifies the methodology for the realization of thin coatings by alternate deposition of cationic and anionic electrolytes, also defined "layer by layer" deposition.

The "layer by layer" methods require the deposition of many successive layers, typically in a number greater than 20; this, apparently, results industrially complex and costly, in terms of both required plants and time.

In addition, WO 2016/123295 A2 refers to a further method for depositing electrolytes layers; this document exemplifies the methodology of deposition of complexes of anionic and cationic polyelectrolytes, also defined "one-pot" deposition.

The "one-pot" methods have been proved for the deposition of polyelectrolytes for the preparation of so-called nano-intumescent coatings; in some "one-pot" methods (for example, which described in ACS Appl. Mater. Interfaces 2015, 7, 6082-6092) borates are used, species whose toxicity is known.

In summary, therefore, up to the present time, to the knowledge of the Applicant, there are not any known solutions which allow obtaining superficially coated porous substrates showing an improved flame resistance compared with the prior art.

Therefore, the Applicant, with the method, porous substrate and flame-retardant panel according to the present invention, intends remedy this lack.

OBJECTS AND SUMMARY OF THE INVENTION

The object of the present invention is to overcome the drawbacks of the prior art related to the flame reaction.

More specifically, it is an object of the present invention to overcome the drawbacks of the prior art regarding the relatively poor or anyway improvable resistance of current porous materials based on polymeric foams with relevant additives.

In particular, the present invention aims to solve the problem of improving the adhesion of the flame-retardant particles deposited on a porous material substrate. Still more in particular, the present invention aims to solve the problem of improving the quality/ quantity of the particle deposition.

These objectives are achieved with the method according to the present invention that, advantageously and thanks to the presence of a water-soluble - preferably organic - polymer, allows the dual effect of stabilizing the suspension and promoting an adhesive action among the particles after their deposition.

If salts are added, an adjustment of the ionic strength of the solution is obtained, with the effect of improving the quality/ quantity of the deposition. In addition, using phosphates, additional effects in the flame reaction may be obtained.

Specifically, the above and other objects and advantages of the invention, as will appear from the following description, are achieved with a method as that according to claim 1.

Moreover, the above and other objects and advantages of the invention are achieved with a porous substrate as that according to claim 15.

Moreover, the above and other objects and advantages of the invention are achieved with a flame-retardant panel as that according to claim 26.

Preferred embodiments and variations of the process, method and porous substrate according to the present invention form the object of the dependent claims; in particular, in a preferred and advantageous embodiment, the method is a method for coating superficially polymeric foams in order to improve their flame reaction, and the porous substrate is a coated superficially polymeric foam, that results having a better flame reaction, such embodiment representing the optimal solution to the problem faced.

Another aspect of the present invention relates to a flame-retardant panel comprising a superficially coated porous substrate having an improved flame resistance.

It is understood that all the appended claims form an integral part of the present description and that each of the technical features claimed therein is possibly independent and can be used autonomously with respect to the other aspects of the invention.

It will be immediately apparent that countless modifications could be made to what described (for example, related to shape, sizes, arrangements and parts with equivalent functionalities) without departing from the scope of protection of the invention as claimed in the appended claims.

Advantageously, the technical solution according to the present invention, which provides a method for coating superficially a porous substrate and the related superficially coated porous flame-retardant substrates, allows to:

- improve the flame reaction;

- increase the adhesion of the particles deposited on the porous material based on polymeric foam;

- improve the quality/ quantity of particle deposition by means of the adjustment of the ionic strength of the solution to which a porous substrate is exposed to obtain the porous material according to the present invention;

- realize the coating with a limited number of depositions (preferably < 5);

- adjust the thickness and relative mass of the coating by varying the volume and concentration of the colloidal suspension used;

- adjust the coating adhesion and the uniformity by means of the properties of the first cationic-anionic bilayer (or anionic-cationic);

- produce polymeric foams with the combination of properties of resistance to flame, forced combustion and flame penetration; and

- avoid the use of halogenated molecules, organo-phosphates, borates or other products of known toxicity.

The method and the porous material thus obtained can also be applied to porous materials not derived from polymeric foams, and this extends the potential application range of the present invention.

Further advantageous characteristics will become more apparent from the following description of preferred, but not exclusive, embodiments, provided purely by way of example and not of limitation.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described hereinafter by means of some preferred embodiments, provided by way of example and not of limitation, with reference to the accompanying drawings. These drawings illustrate different aspects and examples of the present invention and, where appropriate, similar structures, components, materials and/ or elements in different figures are denoted by similar reference numerals.

FIG. 1 is a flowchart showing the steps of the method for coating superficially polymeric foams in order to improve their flame reaction according to the present invention;

FIG. 2 is a schematic representation of the method of FIG. 1; FIG. 3A shows, to a first magnification, a cell of a polymeric foam, specifically polyurethane;

FIG. 3B shows, to a second magnification higher than the first magnification, the cell of FIG. 3A;

FIG. 4A shows the cell of figure 3A after application of the surface coating with the method according to the present invention;

FIG. 4B shows the cell of figure 4A after application of the surface coating with the method according to the present invention;

FIG. 5 shows a diagram that summarizes in a comparative way the results of flame penetration tests on the three samples in polymeric foam, specifically polyurethane, superficially coated with the method according to the present invention;

FIG. 6A shows a sample of polyurethane foam as such at the end of the flame penetration test;

FIG. 6B shows a sample of a material according to a first embodiment of the present invention at the end of the flame penetration test;

FIG. 6C shows a sample of a material according to a second embodiment of the present invention at the end of the flame penetration test;

FIG. 6D shows a sample of a material according to a third embodiment of the present invention at the end of the flame penetration test; and

FIG. 7 shows a diagram that summarizes in a comparative way the results of flame penetration tests on two more samples in polymeric foam, specifically polyurethane, superficially coated with the method according to the present invention on a material obtained according to a further embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

While the invention is susceptible to various modifications and alternative realizations, some preferred embodiments are shown in the drawings and will be described in detail hereinbelow.

It should be understood, however, that there is no intention to limit the invention to the specific embodiments illustrated, but, on the contrary, the invention is intended to cover all modifications, alternative realizations, and equivalents which fall within the scope of the invention as defined in the claims.

In the following description, therefore, the use of "for example", "etc.", "or/either" indicates not exclusive alternatives without any limitation, unless otherwise indicated; the use of "also" means "including, but not limited to" unless otherwise indicated; the use of "includes/comprises" means "includes/comprises, but not limited to" unless otherwise indicated.

The method, porous substrate and panel of the present invention are based on the innovative concept of depositing a continuous, uniform and compact layer of inorganic and/ or hybrid particles on a porous substrate to obtain a structure capable of resisting the flame and keeping the thermal insulation properties of the original foam.

Actually, the inventors have surprisingly and unexpectedly found the method according to the present invention that allows obtaining polymer foams capable of resisting the flame penetration, while it was - and still is - commonly believed that these foams could not/ can not resist such stress, due to their organic nature.

As anticipated, by means of the deposition of a continuous, uniform and compact layer of inorganic particles, instead a structure capable of resisting the flame and keeping the thermal insulation properties of the original foam has been obtained: it was not, and is not, obvious that such a uniform coating could/can be obtained by sagging only the suspension and evaporating the water; actually, the uniformity of deposition and its adhesion appear to be closely related to the deposition of the first double layer that acts as a "primer" and controls the deposition of the coating.

Also, it was not and it is not obvious that the foam could/can keep its deformability; however, some stiffening was noticed.

In summary, the present invention provides an innovative method for the production of thin coatings (indicatively defined between 10 nm 100 microns) to confer flame- retardant properties to organic porous polymeric substrates, without influencing substantially their deformability properties.

In order to ensure the application and industrial sustainability of the technology, it is necessary that the coating is made with a limited number of depositions (preferably < 5).

Substrates are considered having at least partially open porosity and densities in the range of 0.1-500 g/dm³ (as a non-limiting example, polyurethanes, polyesters, polyolefins, polyamides, phenolic resins are mentioned).

In detail, flame-retardant properties are defined as conferring one or more of the specific properties to the coated substrate:

- non-flammability as a result of the exposure to a small flame (as per ASTM D 4986),

- non-ignition under forced combustion conditions with an imposed heat flow representing a fire at its early stages of development (for example 35 kW/ m² according to ISO 5660 test), and

- resistance to the penetration of a small localized flame, incident on the coated substrate surface, maintaining the thermal insulation properties of the foam.

The product structural properties include:

- continuous and compact coating of all the internal walls of the porous structure, without having clogged pores, with a layer of inorganic and/ or hybrid nanoparticles, linked by the effect of an organic polymer retained among the nanoparticles,

- coating adhesion, thanks to the first cationic-anionic (or anionic -cationic) bilayer, which acts as a surface activator and primer, and

- maintaining the deformability of the porous substrate.

In this description, the term "to surface coat/ surface coating" means the application by means of sagging, spreading, spraying a substance that, as a result of the application, settles on every surface available of the substrate and preferably, but not necessarily, covers its surface evenly and continuously.

In the present description, the term "porous substrate" means a material whose cells are least partially open.

In the present description, the term "polymeric foam" means a porous polymeric material, having an at least partially open porosity, with a density ranging between 0,1-500 g/dm³.

In the present description, the term "nanoparticles" means the portions of inorganic or hybrid organic/ inorganic substance, the dimensions of which are less than 100 nm in at least one of the directions.

In the present description, the term "colloidal suspension" means a water-based mixture in which nanoparticles are dispersed.

In this description, the term "stabilizer" means any additive suitable for stabilizing the colloidal suspension over time.

In this description, the term "binder" means an additive, preferably polymeric, suitable to consolidate and stabilize the deposition of nanoparticles over time.

Referring to FIG. 1, the method for superficially coating a porous substrate 1 in order to improve its flame reaction, according to the present invention comprises the steps of:

- exposing the substrate to a solution of a positive electrolyte 2 (step 100);

- exposing the substrate to a solution of a negative electrolyte 4 (step 101);

- drying the substrate treated in the previous steps (step 102);

- exposing the treated and dried substrate to a colloidal aqueous suspension 6 comprising at least inorganic and/ or hybrid nanoparticles

60 and a water-soluble polymer 61, wherein said nanoparticles have a concentration ranging between 0.01 and 50% by weight with respect to the water concentration, wherein said water-soluble polymer has a concentration ranging between 0.01 and 50% by weight with respect to the water concentration and wherein the total concentration of said nanoparticles and said water-soluble polymer has a value of up to 100% by weight with respect to the water concentration (step 103);

- removing the water present in the colloidal aqueous suspension 6 by evaporation, in order to allow the deposition of a surface coating 3 comprising at least said nanoparticles 60 and said water-soluble polymer

61 on the inner walls 10 of the porous substrate 1, wherein said at least one water-soluble polymer 61 acts as a stabilizer of the colloidal aqueous suspension and/ or as a binder among the deposited nanoparticles (step 104).

In particular, the quantities of water, nanoparticles and water-soluble polymer are in a variable ratio between 2:1:1 and 10,000:1:1 by weight.

In an alternative embodiment, the method according to the present invention provides for that the order of said steps 100 and 101 is reversed, namely that step 101 takes place before step 100.

Optionally, the method according to the present invention further comprises the following steps:

- washing between the steps of exposing to the solutions of a positive (or negative) and a negative (or positive) electrolyte; and/ or

- washing before the drying step.

In a preferred embodiment, the method according to the present invention provides for that steps 103 and 104 are cyclically repeated in a number equal to or less than 5. In a preferred embodiment, the method in accordance with the present invention provides for that the superficially coated porous substrate 1 is an organic polymeric foam substrate, having at least a partially open porosity and a density ranging between 0.1-500 g/dm³.

Preferably, the substrate in organic polymeric foam is polyurethane, polyester, polyolefin, polyamide, phenolic resin.

Preferably, the inorganic and/ or hybrid nanoparticles 60 are electrolytes or non- electrolytes.

Preferably, the inorganic and/ or hybrid nanoparticles 60 are lamellar or needle- shaped or isodimensional, and more preferably are selected from graphene, graphene oxide, nanographite, boron nitride, natural or synthetic clays, zirconium phosphate and derivatives, molybdenum disulphide or other dichalcogenides, sepiolites, carbon nanotubes, halloysites, carbon or silica or alumina nanofibers, metal oxides, polyhedral oligomeric silsesquioxane (POSS), metal-organic clusters and the like. Preferably, the water-soluble polymer 61 is an ionic or non-ionic surfactant; more preferably, the water-soluble polymer 61 is an organic compound.

Preferably, the polymer 61 is natural or synthetic; more preferably, the polymer 61 is selected from alginates, carboxymethyl cellulose and other cellulose derivatives, chitosan, gelatin, DNA, lignin, polyacrylic acid, poly diallyl dimethyl ammonium chloride, branched polyethylene imine, ammonium polyphosphate, polyphosphoric acid and the like.

Optionally, the method according to the present invention provides for that the colloidal aqueous suspension 6 further comprises at least one salt 62, wherein said at least one salt has a concentration ranging between 0.01 and 50% by weight with respect to the water concentration and wherein the total concentration of the nanoparticles, water-soluble polymer and salt has a value of up to 150% by weight with respect to the water concentration.

In particular, the quantities of water, nanoparticles, water-soluble polymer and salts are in a variable ratio between 2:1:1:1 and 10,000:1:1:1 by weight.

Preferably, the at least one salt 62 is a phosphorus, nitrogen, sulphur or boron salt.

Preferably, the residual water after removal by evaporation is less than 10% by weight with respect to the water present before removal by evaporation.

Preferably, the surface coating 3 deposited on the inner walls 10 of the porous substrate

1 has a thickness ranging from 10 nm to 100 microns.

Preferably, the flame retardancy is defined as having at least one of the following characteristics:

a. non-flammability as a result of the exposure of the coated substrate to a small flame (i.e. 2 cm, equivalent to 50 W) according to the ASTM D 4986 standard;

b. non-ignition under forced combustion conditions with an imposed heat flow representing a fire at its early stages of development (i.e. equal to or greater than 35 kW/ m²) according to ISO 5660 standard;

c. resistance to the penetration of a small flame (i.e. 150 W), focused and perpendicularly incident to the surface of the coated substrate, maintaining the thermal insulation characteristics (i.e. temperature on the back of the insulating layer during the flame application at < 200 °C for at least 3 minutes).

The method according to the present invention is described below in greater detail with reference to the following Examples, which have been developed on the basis of experimental data and which are meant to be illustrative, but not limitating, of the present invention. Example 1

Polyurethane foams used to make the porous substrate 1 are a commercial product with a density equal to 15 g/ dm³.

Graphene oxide (GO) was provided by the company Avanzare (Spain) as a 1% by weight suspension in water. Sodium montmorillonite (MMT) was purchased from Southern Clays (USA). Sepiolite (SEP) was purchased from Tolsa (Spain). Polyacrylic acid (PAA, Mw ~ 100,000, 35% by weight in H₂O), poly diallyl dimethyl ammonium chloride (PDAC, 20% by weight in H₂O), sodium alginate (Al) and sodium hexametaphosphate (SHMP) were purchased from Sigma-Aldrich (USA). PAA and PDAC have been used to prepare aqueous solutions at 1% by weight. The used water (18.2 ΜΩ) was taken from a Q20 water purification system by Millipore (Italy).

GO, MMT, SEP, alginate and SHMP were used to prepare three types of suspensions with the following compositions:

As a result steps 103 and 104, the water-soluble polymer and the nanoparticles penetrate the porous substrate 1 and coat the individual cells of the latter with a continuous, uniform coating that does not alter the open cell structure of substrate 1, as shown in FIG.s 4A-4B (FIG.s 3A-3B illustrate some of the cells before steps 102, 104). Flammability Test

The reaction to a flame exposure was assessed by means of a flammability test in a horizontal configuration. A sample of porous material obtained according to the method of the present invention having a size of 5x15x1.8 cm³ is placed on a metal grid, the 5 cm side is exposed to a 2 cm-high natural gas blue flame for 3 seconds.

Below the grid, cotton wool is placed to assess, during combustion, the formation of incandescent polymer drops. During the test, the sample behaviour is assessed according to the formation or absence of incandescent drops and the capability of the material to prevent the flame propagation (self-extinguishing). Samples that showed self-extinguishing in a horizontal configuration have been tested in a vertical configuration. In this configuration, the sample is held vertically by a clamp that snaps over the short side, the time of application and the size of the flame are the same adopted for the horizontal configuration. For each formulation, 3 samples were tested.

The samples were conditioned in a climate chamber (23 °C, 50% R.H.) for 24 hours before the test.

Combustion Test

An oxygen-consumption cone calorimeter is used to perform a test according to ISO

5660 standard, using samples having a size of 5x5x1.8 cm³. The heat flow used for the tests was 35 kW/ m². The assessed parameters are the following: ignition time (TTI, s), velocity peak of heat release (pkHRR, kW/ m²), total produced heat (THR, MI/ m²) and final residue (%). For each formulation, 4 samples were tested. The samples were conditioned in a climate chamber (23 °C, 50% R.H.) for 24 hours before the test.

Flame Penetration Test

Flame penetration tests were conducted by placing the sample (5x5x1.8 cm³) into a vertical ceramic structure and exposing one side of the sample to the flame of a torch powered by butane (150 W), placed 10 cm apart. During the test, the temperatures of the side exposed to the flame and the opposite side were monitored using 1 mm armoured thermocouples of type K. The test was performed twice for each of the six formulations. The samples were conditioned in a climate chamber (23 °C, 50% R.H.) for 24 hours before the test.

Results of the Flammability Tests

Sample % Mass Dripping Horizontal Vertical test Residue

Added Self- by extinguishin

Depositi g Test

on

PU Yes No Burns out 13% completely

GO/ alginate 71% No Yes The flame 90% reaches the clamp

GO/ alginate/ S 166% No Yes Self- 99% HMP extinguishing

MMT/ alginate 52% No No Not tested 95%

MMT/ alginate 114% No Yes The flame 99% /SHMP reaches the

clamp

SEP/ alginate 67% No Yes The flame 99% reaches the clamp

SEP/alginate/S 116% No Yes Self- 99% HMP extinguishing

The non-modified PU polyurethane immediately ignites, and the flame propagates to the whole sample causing a conspicuous dripping that propagates the flames also to the cotton wool. The sample burns out completely leaving a 13% residue. Regarding the coated porous materials, after application of the flame, a self-extinguishing behaviour is observed: the flame remains confined within the first 2-3 cm of the sample and is extinguished after a few seconds. Only in the case of MMT/ alginate sample, a small flame remains confined and propagates only on the edge of the sample, reaching the side not exposed to methane flame. All tested coatings were able to stop the dripping phenomenon. The samples with self-extinguishing behaviour were tested, together with the non-modified polyurethane, in a vertical configuration. The non- modified polyurethane burns out completely and with vigorous flames reaching the clamp that holds the sample, causing it to fall, and the flames propagate to the cotton wool. With regard to the materials treated, only GO/ alginate/ SHMP and SEP/ alginate/ SHMP formulations were able to stop the flame propagation showing a self-extinguishing behaviour also in a vertical configuration. It is important to note that for the formulations that failed the vertical test anyway showed the absence of vertical dripping or detachment of foam pieces during combustion. Actually, the flames enveloped the sample, but they remained confined to its surface without causing any dimensional change of the sample.

If some literature data are considered (see table below "Results of the Flammability Tests for Materials reported in the Literature") for "layer by layer" depositions that have self-extinguishing properties (for example, those set forth in Macromol. Mater. Eng. 2016, 301, 665-673; in RSC Adv., 2014, 4, 16674-16680 and RSC Adv., 2014, 4, 46164), it is clear that in order to obtain self-extinguishment with "layer by layer" depositions, a very high number of depositions is required, much more than those made with the method according to the present invention.

Results of the Flammability Tests for Materials reported in the Literature

Flammability in

Number of Flammability in Vertical

Horizontal Reference Depositions Configuration

Configuration

6 BL (12 steps of Self-extinguishment achieved

deposition, 25 total steps only with 9 BL (18 deposition Macromol. Mater. considering the washing Self-extinguishment steps, 37 total steps considering Eng. 2016, 301, after each deposition and the washing after each deposition 665-673 drying) and drying)

4 QL (16 steps of

deposition, 33 total steps

RSC Adv., 2014, 4, considering the washing Self-extinguishment Not assessed

16674-16680 after each deposition and

drying)

5 BL (11 steps of RSC Adv., 2014, 4, deposition, 23 total steps Self-extinguishment Not assessed

46164 considering the washing

Forced Combustion Test Results (Cone Calorimeter Tests)

The cone calorimeter allows assessing the exact behaviour of materials when exposed to a heat flow typical of fires under development. Normally, when exposed to heat flow, the material melts and then begins to release products of decomposition; when the volatile products exceed the limit of flammability, ignition and combustion of the material occur. During the combustion of the sample, the cone calorimeter measures the oxygen consumption and calculates the rate of heat release. The non-modified polyurethane ignites after just 4 seconds. During the combustion, the foam collapses losing its original shape, giving rise to a mass of molten polymer that burns out completely reaching a pkHRR of 304 kW/ m². Unlike the reference, modified samples do not collapse, but retain the original shape of the foam; the ignition is delayed on average of a few seconds and the combustion takes place with flames of smaller size than those observed for the reference. All formulations showed to be capable of reducing the pkHRR of at least 50% . The best performances were obtained with GO/ alginate/ SHMP and SEP/ alginate/ SHMP samples. Unlike other samples, the set of GO/ alginate foams did not ignite during the test; consequently, the degradation products did not reach the limit of flammability. The instrument has measured in any case an HRR signal which can be traced back to the sample oxidation without flame. This behaviour is to be considered exceptional and leads to significantly lower pkHRR values (due to the absence of combustion) of the reference. It should be pointed out that, to the best knowledge of the inventors, such behaviour has never been reported in the literature for modified foams with "layer by layer" depositions. In particular, it results from the literature that to get reductions on HRR peak equal to or lower than those obtained according to the present invention, larger % masses added through the deposition are needed.

Results of Tests of Flame Penetration

Foams treated with particle/ alginate/ SHMP formulations were subjected to flame penetration tests as described above. FIG. 5 shows temperature diagrams of both the surface exposed to the flame and the opposite surface. FIG.s 6B to 6D show sample images at the end of the test.

For comparison, a reference material was also tested in the polyurethane PU (FIG. 6A) that, due to the low density and the high flammability of the foam, is penetrated immediately by the flame without therefore showing any relevant performance of resistance to the flame penetration. As regards the materials according to the present invention, as shown by the images of the residues at the end of the test (FIG.s 6B-6D), all tested formulations had been able to resist the flame penetration for more than three minutes, while maintaining the original size and isolating the side not exposed to the flame. The degree of insulation obtained is similar to the formulations containing GO and MMT, for which it has obtained a temperature gradient of about 600 °C through a thickness of 18 mm. The best results were obtained with the formulation SEP/ alginate/ SHMP with a temperature gradient of 650 °C. It is important to emphasize that in the literature no data is available for comparison since, for a technical prejudice, it has never been deemed possible to obtain data about the resistance to flame penetration with surface modified polyurethane foams. On the other hand, the present invention demonstrates that it is possible to obtain excellent results with a very reduced number of depositions.

Example 2

The flame penetration test was also performed on a substrate of open cell polyurethane, with activation of 1 BL PAA/PDAC, followed by sagging and evaporation of a colloidal suspension containing nanoparticles of graphene (GNP), and carboxymethyl cellulose (CMC) with the optional addition of SHMP, as described in Example 1.

The porous material of this example was subjected to the flame penetration test as described above, and the results are illustrated in FIG. 7. As noted above, polyurethane modified foams with the formulation that contains the three components (particle, polymer and phosphate salt) have been able to resist the flame penetration for more than 5 minutes. The GNP/carboxymethyl cellulose/ SHMP formulation scored the best performance of all tested samples (including those of Example 1) reaching a temperature gradient of 685 °C.

Furthermore, a superficially coated porous substrate 11 having an improved flame resistance represents an independent aspect that can be used autonomously with respect to other aspects of the invention and comprises:

- a porous substrate 1 exposed to a solution of a positive electrolyte 2 and to a solution of a negative electrolyte 4, and subsequently dried; and

- a surface coating 3,

wherein said surface coating 3 has at least one layer and is deposited in a continuous manner and with a homogeneous orientation in the direction parallel to the inner walls 10 of the porous substrate 1 by exposure to a colloidal aqueous suspension 6 comprising at least inorganic and/ or hybrid nanoparticles 60 and a water-soluble polymer 61, wherein said nanoparticles have a concentration ranging between 0.01 and 50% by weight with respect to the water concentration, wherein said water-soluble polymer has a concentration ranging between 0.01 and 50% by weight with respect to the water concentration and wherein the total concentration of said nanoparticles and said water-soluble polymer has a value of up to 100% by weight with respect to the water concentration. In a preferred embodiment, the surface coating 3 has a number of layers equal to or lower than 5.

In a preferred embodiment, the superficially coated porous substrate 1 is an organic polymeric foam substrate, having at least a partially open porosity and a density ranging between 0.1-500 g/ dm³.

Preferably, the nanoparticles 60 are electrolytes or non-electrolytes.

Preferably, the inorganic or hybrid nanoparticles 60 are lamellar or needle-shaped or isodimensional, and more preferably are selected from graphene, graphene oxide, nanographite, boron nitride, natural or synthetic clays, zirconium phosphate and derivatives, molybdenum disulphide or other dichalcogenides, sepiolites, carbon nanotubes, halloysites, carbon or silica or alumina nanofibers, metal oxides, polyhedral oligomeric silsesquioxane (POSS), metal-organic clusters and the like. Preferably, the water-soluble polymer 61 is an ionic or non-ionic surfactant; more preferably, the water-soluble polymer 61 is an organic compound.

Preferably, the water-soluble polymer 61 is natural or synthetic; more preferably, the polymer 61 is selected from alginates, carboxymethyl cellulose and other cellulose derivatives, chitosan, gelatin, DNA, lignin, polyacrylic acid, poly diallyl dimethyl ammonium chloride, branched polyethylene imine, ammonium polyphosphate, polyphosphoric acid and the like.

Optionally, the superficially coated porous substrate 11 having an improved flame resistance according to the present invention provides that the colloidal aqueous suspension 6 further comprises at least one salt 62, wherein said at least one salt has a concentration ranging between 0.01 and 50% by weight with respect to the water concentration and wherein the total concentration of the nanoparticles, water-soluble polymer and salt has a value of up to 150% by weight with respect to the water concentration.

Preferably, the at least one salt 62 is a phosphorus, nitrogen, sulphur or boron salt. Preferably, the surface coating 3 deposited on the inner walls 10 of the porous substrate I has a thickness ranging from 10 nm to 100 microns.

Furthermore, a flame-resistant panel including a superficially coated porous substrate

II having improved flame retardancy represents an independent aspect that can be used autonomously with respect to other aspects of the invention, as described above. As it can be deduced from the foregoing, the innovative technical solution described here has the following advantageous features:

- it offers better performances in case of fire than polymeric foams available today;

- it can be applied to a generic porous material, and therefore is very flexible;

- it improves the flame reaction;

- it increases the adhesion of the particles deposited on the porous material based on polymeric foam;

- it improves the quality/ quantity of particle deposition by means of the adjustment of the ionic strength of the solution to which a porous substrate is exposed to obtain the porous material according to the present invention;

- it allows to realize the coating with a limited number of depositions (preferably <

5);

- it allows to adjust the thickness and relative mass of the coating by varying the volume and concentration of the colloidal suspension used;

- it allows to adjust the coating adhesion and uniformity by means of the properties of the first cationic-anionic (or anionic-cationic) bilayer;

- it allows to produce polymeric foams with the combination of properties of resistance to flame, forced combustion and flame penetration; and

- it avoids the use of halogenated, organo-phosphate, borate flame retardants additives or other products of known toxicity.

From the description above, it is therefore apparent how the method for coating superficially a porous substrate, specifically a polymeric foam, to improve its flame reaction, and how the related superficially coated flame retardant porous substrates according to the present invention allow achieving the proposed objects.

Therefore, it is also apparent to a person skilled in the art that it is possible to make modifications and further variants to the solution described with reference to the accompanying figures, without departing from the teaching of the present invention and from the scope of protection as defined by the appended claims.

Claims

1. A method for superficially coating a porous substrate (1) in order to improve its flame reaction, comprising the following steps:

- exposing the substrate to a solution of a positive electrolyte (2) (step 100);

- exposing the substrate to a solution of a negative electrolyte (4) (step 101);

characterised in that it comprises the following steps:

- drying the substrate treated in the previous steps (step 102);

- exposing the treated and dried substrate to a colloidal aqueous suspension (6) comprising at least inorganic and/ or hybrid nanoparticles (60) and a water-soluble polymer (61), wherein said nanoparticles have a concentration ranging between 0.01 and 50% by weight with respect to the water concentration, wherein said water- soluble polymer has a concentration ranging between 0.01 and 50% by weight with respect to the water concentration and wherein the total concentration of said nanoparticles and said water-soluble polymer has a value of up to 100% by weight with respect to the water concentration (step 103);

- removing the water present in the colloidal aqueous suspension (6) by evaporation, in order to allow the deposition of a surface coating (3) comprising at least said nanoparticles (60) and said water-soluble polymer (61) on the inner walls (10) of the porous substrate (1), wherein said at least one water-soluble polymer (61) acts as a stabilizer of the colloidal aqueous suspension and/ or as a binder among the deposited nanoparticles (step 104).

2. The method according to claim 1, wherein the order of said steps 100 and 101 is inverted, i.e. wherein said step 101 occurs before said step 100.

3. The method according to claim 1 or 2, further comprising the following steps:

- washing between the steps of exposing to the solutions of a positive (or negative) and a negative (or positive) electrolyte; and/ or - washing before the drying step.

4. The method according to claim 1 or 2 or 3, wherein steps 103 and 104 are cyclically repeated in a number equal to or lower than 5.

5. The method according to any of the preceding claims, wherein said porous substrate (1) is an organic polymeric foam substrate, having at least a partially open porosity and a density ranging between 0.1-500 g/ dm³.

6. The method according to claim 5, wherein said organic polymeric foam substrate is polyurethane, polyester, polyolefin, polyamide, phenolic resin.

7. The method according to any of the preceding claims, wherein said inorganic and/ or hybrid nanoparticles (60) are electrolytes or non-electrolytes.

8. The method according to claim 7, wherein said inorganic and/or hybrid nanoparticles (60) are lamellar or needle-shaped or isodimensional and preferably are selected from graphene, graphene oxide, nanographite, boron nitride, natural or synthetic clays, zirconium phosphate and derivatives, molybdenum disulphide or other dichalcogenides, sepiolites, carbon nanotubes, halloysites, carbon or silica or alumina nanofibers, metal oxides, polyhedral oligomeric silsesquioxane (POSS), metal-organic clusters and the like.

9. The method according to any of the preceding claims, wherein said water-soluble polymer (61) is an ionic or non-ionic surfactant, preferably organic.

10. The method according to claim 9, wherein said polymer (61) is natural or synthetic, preferably selected from alginates, carboxymethyl cellulose and other cellulose derivatives, chitosan, gelatin, DNA, lignin, polyacrylic acid, poly diallyl dimethyl ammonium chloride, branched polyethylene imine, ammonium polyphosphate, polyphosphoric acid and the like.

11. The method according to any of the preceding claims, wherein said colloidal aqueous suspension (6) further comprises at least one salt (62), preferably a phosphorus or nitrogen or sulphur or boron salt, wherein said at least one salt has a concentration ranging between 0.01 and 50% by weight with respect to the water concentration and wherein the total concentration of said nanoparticles, said water- soluble polymer and said salt has a value of up to 150% by weight with respect to the water concentration.

12. The method according to any of the preceding claims, wherein the residual water after removal by evaporation is less than 10% by weight with respect to the water present before removal by evaporation.

13. The method according to any of the preceding claims, wherein said surface coating (3) deposited on the inner walls (10) of the porous substrate (1) has a thickness ranging from 10 nm to 100 microns.

14. The method according to any of the preceding claims, wherein the flame resistance of the superficially coated porous substrate (1) is defined as having at least one of the following characteristics:

b. non-ignition under forced combustion conditions with a compulsory heat flow representing a fire at its early stages of development (i.e. equal to or greater than 35 kW/m²) according to ISO 5660 standard;

c. resistance to the penetration of a small flame (i.e. 150 W), focused and perpendicularly incident to the surface of the coated substrate, with the maintenance of the thermal insulation characteristics (i.e. temperature on the back of the insulating layer during the flame application < 200 °C for at least 3 minutes).

15. A superficially coated porous substrate (11) having an improved flame resistance, comprising:

- a porous substrate (1) exposed to a solution of a positive electrolyte (2) and to a solution of a negative electrolyte (4), and subsequently dried; and

- a surface coating (3),

wherein said surface coating (3) has at least one layer and is deposited in a continuous manner and with a homogeneous orientation in the direction parallel to the inner walls (10) of the porous substrate (1) by exposure to a colloidal aqueous suspension (6) comprising at least inorganic and/ or hybrid nanoparticles (60) and a water-soluble polymer (61), wherein said nanoparticles have a concentration ranging between 0.01 and 50% by weight with respect to the water concentration, wherein said water-soluble polymer has a concentration ranging between 0.01 and 50% by weight with respect to the water concentration and wherein the total concentration of said nanoparticles and said water-soluble polymer has a value of up to 100% by weight with respect to the water concentration.

16. The superficially coated porous substrate (11) having an improved flame resistance according to claim 15, wherein said surface coating (3) has a number of layers equal to or lower than 5.

17. The superficially coated porous substrate (11) having an improved flame resistance according to claim 15 or 16, wherein said porous substrate (1) is an organic polymeric foam substrate, having at least a partially open porosity and a density ranging between 0.1-500 g/ dm³.

18. The superficially coated porous substrate (11) having an improved flame resistance according to claim 17, wherein said organic polymeric foam substrate is polyurethane, polyester, polyolefin, polyamide, phenolic resin.

19. The superficially coated porous substrate (11) having an improved flame resistance according to any claims 15 to 18, wherein said inorganic and/ or hybrid nanoparticles (60) are electrolytes or non-electrolytes.

20. The superficially coated porous substrate (11) having an improved flame resistance according to claim 19, wherein said inorganic and/ or hybrid nanoparticles (60) are lamellar or needle-shaped or isodimensional and preferably are selected from graphene, graphene oxide, nanographite, boron nitride, natural or synthetic clays, zirconium phosphate and derivatives, molybdenum disulphide or other dichalcogenides, sepiolites, carbon nanotubes, halloysites, carbon or silica or alumina nanofibers, metal oxides, polyhedral oligomeric silsesquioxane (POSS), metal-organic clusters and the like.

21. The superficially coated porous substrate (11) having an improved flame resistance according to any claims 15 to 20, wherein said water-soluble polymer (61) is an ionic or non-ionic surfactant, preferably organic.

22. The superficially coated porous substrate (11) having an improved flame resistance according to claim 21, wherein said polymer (61) is natural or synthetic, preferably selected from alginates, carboxymethyl cellulose and other cellulose derivatives, chitosan, gelatin, DNA, lignin, polyacrylic acid, poly diallyl dimethyl ammonium chloride, branched polyethylene imine, ammonium polyphosphate, polyphosphoric acid and the like.

23. The superficially coated porous substrate (11) having an improved flame resistance according to any claims 15 to 22, wherein said colloidal aqueous suspension (6) further comprises at least one salt (62), preferably a phosphorus or nitrogen or sulphur or boron salt, wherein said at least one salt has a concentration ranging between 0.01 and 50% by weight with respect to the water concentration and wherein the total concentration of said nanoparticles, said water-soluble polymer and said salt has a value of up to 150% by weight with respect to the water concentration.

24. The superficially coated porous substrate (11) having an improved flame resistance according to any claims 15 to 23, wherein said surface coating (3) deposited on the inner walls (10) of the porous substrate (1) has a thickness ranging from 10 nm to 100 microns.

25. The superficially coated porous substrate (11) having an improved flame resistance according to any claims 15 to 24, wherein the flame resistance of the superficially coated porous substrate (1) is defined as having at least one of the following characteristics:

b. non-ignition under forced combustion conditions with a compulsory heat flow representing a fire at its early stages of development (i.e. equal to or greater than 35 kW/m²) according to ISO 5660 standard; c. resistance to the penetration of a small flame (i.e. 150 W), focused and perpendicularly incident to the surface of the coated substrate, with the maintenance of the thermal insulation characteristics (i.e. temperature on the back of the insulating layer during the flame application < 200 °C for at least 3 minutes).

26. A flame-resistant panel comprising a superficially coated porous substrate (11) having an improved flame resistance according to any of the claims 15 to 25.