Flame-retardant and fire-resistant polymer compositions made from lime having a high specific surface

Abstract

A flame-retardant polymer composition comprising a mineral filler and a polymer, said mineral filler comprising a calcium compound, characterized in that the calcium compound is a fire-resistant additive in the form of calcium hydroxide, having a specific surface calculated according to the BET method greater than 25 m.sup.2/g, preferably greater than 30 m2/g, more preferably greater than 35 m2/g and advantageously greater than 40 m2/g, uses of same and the combustion residue obtained.

Claims

1. A flame-retardant polymer composition comprising a mineral filler and a polymer, said mineral filler comprising a calcium compound, characterized in that the calcium compound is a fire-resistant additive in the form of calcium hydroxide having a specific surface area computed according to the BET method greater than 25 m.sup.2/g and less than 60 m.sup.2/g, the mineral filler being incorporated into the flame-retardant polymer composition in an amount from 40 to 75% by weight, based upon the total weight of the flame-retardant polymer composition.

2. The flame-retardant polymer composition according to claim 1, wherein said mineral filler comprising a calcium compound has a porous volume comprised between 0.10 and 0.30 cm.sup.3/g.

3. The flame-retardant polymer composition according to claim 2, wherein said mineral filler further comprises at least one magnesium compound, in the form of a magnesium hydroxide, as a flame-retardant additive.

4. The flame-retardant polymer composition according to claim 3, wherein the calcium compound and the magnesium compound of said mineral filler are two separate compounds in a mixture.

5. The flame-retardant polymer composition according to claim 3, wherein the calcium compound and magnesium compound of said mineral filler are intimately bound stemming from slaking of quicklime with a suspension of magnesium hydroxide.

6. The flame-retardant polymer composition according to claim 3, wherein the mineral filler comprising a calcium compound and a magnesium compound has a specific surface area greater than 20 m.sup.2/g.

7. The composition according to claim 3, wherein said mineral filler comprises a calcium compound and a magnesium compound has a porous volume greater than 0.10 cm.sup.3/g.

8. The flame-retardant polymer composition according to claim 1, wherein said polymer is an organic polymer in particular a thermoplastic, thermosetting or elastomeric organic polymer, of natural or synthetic origin.

9. The flame-retardant polymer composition according to claim 1, wherein said organic polymer is selected from the group consisting of polyethylene, polypropylene, polystyrene, ethylene and propylene copolymer (EPR), ethylene-propylenediene terpolymer (EPDM), ethylene and vinyl acetate copolymer (EVA) having an acetate content below 28% by weight of the copolymer, ethylene and methyl acrylate copolymer (EMA) having an acrylate content below 28% by weight of the polymer, ethylene and ethyl acrylate copolymer (EEA) having an acrylate content below 28% by weight of the polymer, ethylene and butyl acrylate copolymer (EBA) having an acrylate content below 28% by weight of the polymer, ethylene and octene copolymer, a polymer based on ethylene, a polymer based on polypropylene, a polymer based on polystyrene, a halogenated polymer, a silicone and any mixture of these compounds.

10. The flame-retardant polymer composition according to claim 1, wherein the particles of mineral filler have a particle size d.sub.90 of less than 80 m.

11. The flame-retardant polymer composition, according to claim 1, wherein the particles of mineral filler have a particle size d.sub.97 of less than 200 m.

12. A combustion residue of a flame-retardant composition according to claim 1, characterized in that the residue is a cohesive residue having an average mechanical compressive strength at break which is greater than 3 kPa.

13. The combustion residue according to claim 12, wherein said combustion residue is only crossed by a very limited number of cracks, less than or equal to 3 after combustion according to the standardized method of the cone calorimeter ISO 5660-1 or ASTM E 1354.

14. The combustion residue according to claim 12 having, after combustion according to the standardized method of the cone calorimeter ISO 5660-1 or ASTM E 1354, a maximum size of combustion residue, a section of which is assimilated to a square which may be sampled without being broken in said obtained residue, greater than or equal to 10 mm, from a sample before combustion with a square section of 100 mm.

Description

(1) FIG. 1 is a schematic illustration of the device for measuring the mechanical compressive strength of the combustion residues.

(2) FIGS. 2A and 2B are photographs of the combustion residue of the flame-retardant polymeric composition according to Example 1.

(3) FIG. 3 is a graph illustrating inter alia the fire test results with a cone calorimeter of a flame-retardant polymeric composition according to Example 1.

(4) FIGS. 4A and 4B are photographs of the combustion residue of the flame-retardant polymeric composition according to Example 2.

(5) FIG. 5 is a graph illustrating the fire test results with a cone calorimeter of a flame-retardant polymeric composition according to Example 2.

(6) FIGS. 6A and 6B are photographs of the combustion residue of the flame-retardant polymeric composition according to Example 3. For this same composition, the fire test results with a cone calorimeter are illustrated in FIG. 3.

(7) FIGS. 7A and 7B are photographs of the combustion residue of the flame-retardant polymeric composition according to Example 4. For this same composition, the fire test results with a cone calorimeter are illustrated in FIG. 3.

(8) FIGS. 8A and 8B are photographs of the combustion residue of the flame-retardant polymeric composition according to Example 5.

(9) FIG. 9 is a graph illustrating the fire test results with a cone calorimeter of a flame-retardant polymeric composition according to Examples 5 and 6.

(10) FIGS. 10A and 10B are photographs of the combustion residue of the flame-retardant polymeric composition according to Example 6.

(11) FIGS. 11A and 11B are photographs of the combustion residue of the flame-retardant polymeric composition according to Example 7.

(12) FIG. 12 is a graph illustrating the fire test results with a cone calorimeter of a flame-retardant polymeric composition according to Example 7.

(13) FIGS. 13A and 13B are photographs of the combustion residue of the flame-retardant polymeric composition according to Example 8.

(14) FIG. 14 illustrates a curve showing the carbonation kinetics of lime with a high specific surface area and of standard lime.

(15) FIGS. 15A and 15B are photographs of the combustion residue of the flame-retardant polymeric composition according to the comparative Example 1.

(16) FIGS. 16A and 16B are photographs of the combustion residue of the flame-retardant polymeric composition according to the comparative Example 2.

(17) FIGS. 17A and 17B are photographs of the combustion residue of the flame-retardant polymeric composition according to the comparative Example 3.

(18) FIGS. 18A and 18B are photographs of the combustion residue of the flame-retardant polymeric composition according to the comparative Example 4.

(19) In the figures, identical or similar elements bear the same references.

(20) The present invention therefore relates to a flame-resistant and fire-resistant polymeric composition comprising a polymer and a mineral filler which comprises at least one calcium compound as a calcium hydroxide for which the BET specific surface area is greater than 25 m.sup.2/g, preferably greater than 30 m.sup.2/g, even more preferentially greater than 35 m.sup.2/g and most preferentially greater than 40 m.sup.2/g.

(21) When the filled polymeric composition is raised in temperature, like during a fire for example, different phenomena occur, either independent or not, at various temperatures. There is notably a phenomenon which may be related to intumescence, since the polymeric composition swells and becomes porous. There is also the phenomenon of carbonation of the mineral filler based on Ca(OH).sub.2, a faster and promoted phenomenon when the specific surface area of the mineral filler based on Ca(OH).sub.2 is high, (as illustrated in FIG. 14) and which leads, at the end of the combustion to a cohesive residue having a certain hardness/compressive strength.

(22) FIG. 14, as mentioned earlier, illustrates a curve of the re-carbonation kinetics of lime with a high specific surface area and of standard lime. FIG. 14 shows that the re-carbonation of lime with a high specific surface area is faster than that of standard lime. This measurement was conducted by tracking the mass of a standard hydrated lime or with a high specific surface area during a rising temperature from room temperature to 950 C. at 20 C./min in a N.sub.2/CO.sub.2 atmosphere containing about 18% by volume of CO.sub.2 (N.sub.2 flow rate: 112 ml/min, CO.sub.2 flow rate: 24 ml/min). This measurement was conducted on a thermogravimetric Netzsch STA 449F3 apparatus.

(23) Both of these phenomena, i.e. the phenomena which may be related to intumescence and the re-carbonation phenomenon are advantageous with the purpose of improving the fire behavior of polymers, in the sense that the intumescence phenomenon, which limits heat transfers and diffusion of the combustible gases and of oxygen, slows down the combustion of the polymer (flame-retardant), while the carbonation phenomenon of the filler leads to a cohesive residue (fire-resistant).

(24) Nevertheless, a compromise between both of these phenomena generally has to be found in order to optimize the properties of the polymeric compositions. Indeed, if the intumescence phenomenon is too significant or too fast, the combustion residue is highly porous and the size of its pores is large, which leads to poor cohesion and to low compressive strength.

(25) As the intumescence phenomena are related to interactions between the mineral fillers and the polymeric matrices, they are more or less pronounced in different polymeric matrices or in the presence of various fillers.

(26) The mineral filler according to the present invention may also comprise a magnesium compound. This magnesium compound may be added as a mixture or be intimately bound to the calcium compound by carrying out slaking of quicklime with a suspension of magnesium hydroxide.

(27) When these mineral fillers are incorporated into polymers, for example in an amount from 1 to 80% by weight based on the total weight of the composition, an improved flame retarding effect of the filler is obtained as compared with the use of mineral fillers only comprising a calcium compound as described in the invention, the fire-resistant effect being on the other hand more or less reduced according to the proportion of Mg(OH).sub.2 in the filler, since the cohesion of the combustion residue is related to the calcium compound, to its proportion in the composition and to its properties.

(28) When the magnesium compound is present, the specific surface area of the mineral filler slightly decreases but in any case will be greater than 20 m.sup.2/g.

(29) These flame-retardant polymeric compositions have the extremely advantageous feature of providing at the end of the combustion, a cohesive residue which in addition to a flame-retardant effect gives the filler a fire-resistant effect sometimes also called a ceramizing effect.

(30) No standard test is available at the present time for measuring the cohesion of combustion residues of polymeric compositions. In a first phase, the cohesion of the residue may be qualitatively evaluated by a simple observation of the obtained residue at the end of the cone calorimeter test. During this observation, a few quantitative data may however be estimated: i) the number of cracks crossing the residue (transverse cracks); ii) the depth of these cracks (i.e. if the cracks are only present at the surface or whether they cross the whole thickness of the combustion residue); iii) the cohesion may also be represented by the maximum size of the sample (a sample having the whole thickness of the residue and for which the section is assimilated to a square) which may be sampled without being broken in the residue obtained at the end of the cone calorimeter test (which has a square section with a side of 100 mm).

(31) In the sense of the invention, a transverse crack designates a crack which crosses right through the combustion residue obtained at the end of the cone calorimeter test and which appears as a sample with a square section with a side of 100 mm. In order to guarantee the cohesion of the residue, in addition to be present in a limited number, these transverse cracks have not to be very deep, i.e. they should not be present over the whole thickness of the residue. Any residue comprising 1 to 10 deep cracks is considered as pretty cohesive. Any residue comprising more than 10 deep cracks is considered as not being cohesive.

(32) Now by considering the maximum size of the sample (the side of the sample having the whole thickness of the residue and for which the section is assimilated to a square) which may be sampled without being broken in the obtained residue at the end of the cone calorimeter test (which itself has a square section with a side of 100 mm), the residue is said to be cohesive if this size is greater than or equal to 10 mm, preferably greater than or equal to 30 mm, preferably greater than or equal to 50 mm.

(33) Moreover, a quantitative method was developed within the scope of this invention in order to determine the mechanical strength of the combustion residues. This method is only applicable for combustion residues in which it is possible to take a sample, the section of which relates to a square with a side of at least 10 mm, i.e., the section of which is at least as large as the surface of the movable plate used for this measurement.

(34) It consists of conducting a compressive strength measurement by means of a texturometer Chatillon (model DFGS50) on three samples of a combustion residue. These three samples are taken at different locations in the residue with a square section having a side measuring 100 mm, obtained at the end of the cone calorimeter measurements. The principle of this method is illustrated in FIG. 1. Each sample 2 is placed at the centre of a fixed rectangular metal plate with a measured size of about 100200 mm. Manually, a second metal plate 3, much smaller this time and circular (with a diameter of 12 mm), is brought, by means of a lever 6 allowing the movable plate to be moved downwards manually, into contact with the sample 2. As this second plate 3 is connected to a force gauge 4, the force applied on the sample by the movable plate at the moment of the total failure of the sample may be determined and is indicated by an arrow 5. The useful value is the average of the breaking forces measured for the three samples of a combustion residue. The measured force is expressed in Newtons (N), but may be standardized by the surface area of the circular movable plate in order to determine the mechanical compressive strength of the residue in Pascals (Pa). Of course, the samples used for this measurement should have a section at least as large as the surface area of the circular movable plate so that the force is applied over the whole surface of this plate.

(35) This method does not allow determination of a single criterion and its result depends on antagonistic effects. Indeed, the more the residue is porous, as a result for example of phenomena assimilated to intumescence, the lower is its compressive strength, although its cohesion may be very good. Conversely, a not very porous residue may have poor cohesion and a high cracking rate while each sample of this residue may have a high strength.

EXAMPLES

Example 1

(36) Use as a flame-retardant filler, of a hydrated lime with a high specific surface area no. 1 in a MDPE polymer composition.

(37) A hydrated lime with high specific surface area no. 1 was obtained industrially by calcination of natural limestone, and then by hydration (slaking) with an excess of water, in an industrial hydrator, of the quicklime obtained after calcination. The produced slaked lime thus has a humidity of 15-30% by mass at the outlet of the hydrator. It is then dried, de-agglomerated and then subjected to an industrial step for grain size separation giving the possibility of removing the coarsest particles. At the end of this industrial manufacturing process, the slaked lime is once again separated with different separation steps in air, at a laboratory scale this time, in order to obtain a fine grain size grade. The properties of this hydrated lime with a high specific surface area no. 1 are grouped in Table 1.

(38) This filler is incorporated in an amount of 50% by mass into a medium density polyethylene matrix (MDPE 3802, a grade for a cable produced by Total) by using a mixer of the Brabender type. Plates, prepared with a hydraulic press, with a size of 1001004 mm.sup.3 are then subject to the fire test with a cone calorimeter. The residues obtained at the end of the cone calorimeter test (combustion residues) are in a first phase observed and photographed in order to estimate their cohesion degree, and then their mechanical compressive strength is characterized by following the method described earlier in the text.

(39) The results of the fire tests with a cone calorimeter are illustrated in FIG. 3. They are compared with those obtained for the non-filled host polymer (MDPE) and for the same polymer, filled under the same conditions and with the same proportions of ATH (Albemarle Martinal OL 107 LEO, described in Table 2) and of MDH (Albemarle Magnifin H10, described in Table 2).

(40) The cone calorimeter tests conducted for this composition indicate that if the hydrated lime with high specific surface area no. 1 is a less efficient flame-retardant in MDPE than ATH or MDH generally used, mainly because of the higher decomposition temperature of Ca(OH).sub.2 as compared with Al(OH).sub.3 and Mg(OH).sub.2, its flame-retardant role is quite obvious when the curve of the composition of this example is compared with the curve of non-filled MDPE.

(41) The observation of the combustion residues shown in FIGS. 2A and 2B and characterized in Table 5 allows the conclusion to be drawn that at the end of its combustion, the composition of this example has led to the formation of a cohesive residue: this residue consists of a single non-cracked layer and not of a powder, of an ash or of any other divided material. This residue therefore has a totally different aspect from that of the residues obtained for the compositions based on ATH or MDH which appear as a powder or ash (FIGS. 15A and 15B in the case of ATH and 16A and 16B in the case of MDH). The layer of the residue of this example is swollen and hollow (FIG. 2A). The measurement of the mechanical compressive strength of the residue leads to an average value (over 3 measurements conducted with the piece of equipment shown in FIG. 1 on samples taken at different locations in the combustion residue) of 110 kPa, the three measured values being 71 kPa, 120 kPa and 140 kPa (Table 5).

Example 2

(42) Use as a flame-retardant filler, of a hydrated lime of high specific surface area no. 1 in an ethylene and vinyl acetate co-polymer composition.

(43) The mineral filler of Example 2 is the same as the one used in Example 1, but the polymeric matrix is different in that the MDPE is replaced with an ethylene and vinyl acetate copolymer containing 14% of vinyl acetate (EVA 714, Escorene, Ultra FL00014, produced by ExxonMobil Chemical).

(44) The results of the cone calorimeter fire tests recorded for this composition are illustrated in FIG. 5. They are compared therein with the results obtained for the non-filled host polymer (EVA 714) and again indicate a clear flame-retardant effect of the hydrated lime with high specific surface area no. 1.

(45) The residues obtained at the end of the cone calorimeter test (combustion residues), were characterized in the same way as in Example 1. The observation of these residues shown in FIGS. 4A and 4B and characterized in Table 5 allows the conclusion to be drawn that at the end of its combustion, the composition of this example has led to the formation of a cohesive residue: this residue consists of a non-cracked single layer, the surface of which is smooth and clean and not of a powder or divided material. The measurement of the mechanical strength of the residue leads to an average value (over 3 measurements conducted on samples taken at different locations in the combustion residue) of 65 kPa, the three measured values being 43 kPa, 59 kPa and 91 kPa.

Example 3

(46) The use as a flame-retardant filler, of a hydrated lime with high specific surface area no. 2 in a MDPE polymer composition.

(47) The hydrated lime used in Example 1 is replaced with a hydrated lime with high specific surface area no. 2, obtained industrially, under the same conditions as the hydrated lime no. 1, but from a different limestone. The properties of the hydrated lime with a high specific surface area no. 2 are grouped in Table 1.

(48) At the outlet of the hydrator, the hydrated lime with a high specific surface area no. 2 is dried, de-agglomerated and roughly separated industrially. Unlike the hydrated lime with a high specific surface area no. 1, no additional grain size separation step is carried out for this hydrated lime with high specific surface area no. 2 after the industrial separation steps. Grain size control is consequently much less extensive than in Example 1, which explains the coarser grain size of the hydrated lime with high specific surface area no. 2.

(49) If the hydrated lime with a high specific surface area no. 2 has a high specific surface area as compared with standard hydrated limes, it is on the other hand substantially lower than that of the hydrated lime no. 1, probably from the fact of more substantial carbonation during drying, de-agglomeration, separation and handling steps, as well as from a more substantial presence of impurities (chemical impurities, unfired substances . . . ) related to a clearly less fine grain size separation than in the case of the lime with a high specific surface area no. 1 of Example 1.

(50) Samples of the composition containing 50% by weight of the hydrated lime with high specific surface area no. 2 and the remainder of MDPE were prepared and characterized (fire test) in the same way as in Example 1. Again, the combustion residues obtained after the cone calorimeter test of the samples of the composition of this example were characterized.

(51) The results recorded during the measurements with the cone calorimeter are compared with those obtained for the non-filled host polymer (MDPE) and for the composition of Example 1 in FIG. 3.

(52) The differences between the hydrated limes with a high specific surface area no. 1 and no. 2, mentioned above, are without any effect on the results of the cone calorimeter tests characterizing the flame-retardant role of the mineral fillers. The curves corresponding to the composition based on both of these hydrated limes are quite similar.

(53) The observation of the combustion residue shown in FIGS. 6A and 6B and characterized in Table 5 allows the conclusion to be drawn that at the end of its combustion, the composition of this example has led to the formation of a cohesive residue: This residue consists of a relatively cohesive layer as compared with residues obtained for compositions based on ATH or MDH, the residues of which are similar to a powder or an ash (FIGS. 15A and 15B in the case of ATH and 16A and 16B in the case of MDH), even if this layer is cracked in several locations. The residue comprises two transverse cracks and these cracks are not very deep cracks which do not cross the thickness of the residue. In spite of this cracking, a sample with a section similar to a square with a side of 30 mm may be taken on the whole of the thickness of this residue. Moreover, the measurement of the mechanical strength of the residue leads to an average value (over 3 measurements conducted on samples taken at different locations in the combustion residue) of 20 kPa, the three measured values being 14 kPa, 16 kPa and 31 kPa. The mechanical strength of this residue is lower than that of the combustion residue of Example 1, perhaps because of less good distribution of the filler in the polymeric matrix, which may be related to the clearly coarser grain size of the hydrated lime no. 2 as compared with that of the hydrated lime no. 1 (Table 1).

Example 4

(54) Use as a flame-retardant filler, of a mineral filler consisting of Ca(OH).sub.2 and Mg(OH).sub.2 intimately bound in an MOPE polymer composition.

(55) In this example, the polymeric matrix is MOPE as this was already the case in Examples 1 and 3. On the other hand, unlike the previous examples, the mineral filler was synthesized in the laboratory. The synthesis is achieved in a pilot hydrator, in which quicklime is hydrated in the presence of Mg(OH).sub.2 of commercial origin appearing as an aqueous suspension, or a suspension containing 53% by mass of Mg(OH).sub.2. The quicklime and Mg(OH).sub.2 suspension flow rates are computed so as to obtain, at the outlet of the hydrator, a product containing about 13% by mass of Mg(OH).sub.2 and the remainder of hydrated lime (Ca(OH).sub.2), of impurities and unfired substances. The Mg(OH).sub.2 suspension is diluted, the water brought by the suspension into the hydrator being adjusted so as to have during the hydration reaction a humidity of the product at the outlet of the hydrator of the order of 16 to 22%. Hydration is carried out continuously. As soon as it exits from the hydrator, the product is dried and de-agglomerated. It then passes through the same grain size separation steps as those to which the hydrated lime with a high specific surface area no. 1 shown in Example 1 was subject, in order to obtain a fine grain size grade adapted to the preparation of compositions as described in the invention.

(56) During the synthesis, Mg(OH).sub.2 is not subject to any modification, because of the high humidity of the product at the outlet of the hydrator (excess of hydration water), the calcium portion of the product obtained at the end of the synthesis appears as a Ca(OH).sub.2, therefore a hydrated lime with high specific surface area and porous volume. The thereby synthesized product is therefore an intimate mixture between an Mg(OH).sub.2 with standard properties and a Ca(OH).sub.2 with high specific surface area and porous volume. The mixed product accordingly has also high specific surface area and porous volume, which depend on the mass proportions of Mg(OH).sub.2 and of Ca(OH).sub.2 in the mixture and on their respective specific surface areas and porous volumes. The properties of this mixed product, subsequently called a laboratory filler with high specific surface area no. 1, are grouped in Table 3.

(57) In the same way as in Examples 1 and 3, this filler is incorporated into MDPE in an amount of 50% by weight and the thereby prepared compositions are characterized, just like the combustion residues.

(58) The results of the fire tests with a cone calorimeter are illustrated in FIG. 3. The results recorded for the composition of this example are compared with those obtained for the non-filled host polymer (MDPE) and for the same polymer, filled under the same conditions and with the hydrated limes no. 1 and no. 2 described in Table 1.

(59) Once again, the cone calorimeter test indicates that this filler has a very good flame-retardant effect on MDPE. This effect is better than that of hydrated limes with high specific surface area no. 1 and no. 2 and is close to the effect of MDH, because of the presence in the filler of this example of both a Ca(OH).sub.2 with high specific surface area and a magnesium hydroxide.

(60) By observing the combustion residues shown in FIGS. 7A and 7B and characterized in Table 5, it is possible to state that at the end of its combustion, the composition of this example has led to the formation of a cohesive residue as compared with the residues obtained for composition based on ATH or MDH, the residues of which are similar to a powder or an ash (FIGS. 15A and 15B in the case of ATH and 16A and 16B in the case of MDH). If the residue of the composition of this example comprises a few cracks, these cracks are on the other hand not transverse and are not very deep. In spite of this surface cracking, a sample with a section similar to a square with a side of 30 mm approximately may be taken on the whole thickness of this residue. Moreover, the measurement of the mechanical strength of the residue leads to an average value (over 3 measurements conducted on samples taken at different locations in the combustion residue) of 38 kPa, the three measured values being 28 kPa, 42 kPa and 43 kPa.

Example 5

(61) Use as a flame-retardant filler, of a mineral filler consisting of Ca(OH).sub.2 and of ATH mixed in a MDPE polymer composition.

(62) The composition of this example is a composition based on MDPE in which the mineral filler is a mixture of two powders. Like in Examples 1, 3 and 4, the mineral filler is incorporated in an amount 50% of the total weight of the composition, but this filler is a mixture of 30% by weight of ATH (Albemarle Martinal OL 107 LEO described in Table 2) and of 20% by weight of hydrated lime with high specific surface area no. 1 as used in Examples 1 and 2 above and the properties of which are repeated in Table 1, these percentages being expressed, based on the total weight of the composition. The mixture of both of these powders is achieved manually, before introduction into the gravimetric metering device which allows control of the mineral filler level in the composition upon preparing the composition.

(63) Like in Examples 1, 3 and 4, this mixture of fillers is incorporated into the MDPE and the thereby prepared compositions are characterized, just like the combustion residues.

(64) The results are compared with those obtained for the non-filled host polymer (MDPE) and for the same polymer, filled under the same conditions and with 50% of ATH. The results of the fire tests with a cone calorimeter are illustrated in FIG. 9.

(65) The results obtained with the cone calorimeter for this composition indicate that the mixture of 30% of ATH+20% of hydrated lime with high specific surface area no. 1 is a flame-retardant almost as efficient as ATH alone. The HRR curve versus time is slightly shifted towards higher values as compared with the composition based on ATH alone, but the general appearance of the curves is comparable.

(66) By observing the combustion residue shown in FIGS. 8A and 8B and characterized in Table 5, it is possible to draw the conclusion that at the end of its combustion, the composition of this example has led to the formation of a cohesive residue: this residue is black and covered with white film (most probably CaCO.sub.3), it consists of a non-cracked single layer and not of a powder, an ash or any other divided material. This residue therefore has a totally different aspect from that of residues obtained for compositions based on ATH alone, which appear as a divided material in a multitude of small fragments (FIGS. 15A and 15B). The measurement of the mechanical strength of the residue leads to an average value (over 3 measurements conducted on samples taken at different locations in the combustion residue) of 10 kPa, the 3 measured values being 4 kPa, 7 kPa and 19 kPa.

Example 6

(67) Use as a flame-retardant filler, of a mineral filler consisting of Ca(OH).sub.2 and of ATH mixed in a MDPE polymer composition.

(68) This example is similar to Example 5 above, the mineral filler incorporated into the MDPE being always a mixture of ATH and of hydrated lime no. 1. Nevertheless, the proportions of ATH and of hydrated lime with high specific surface area no. 1 are different since this time the composition contains 40% by weight of ATH and only 10% by weight of hydrated lime with high specific surface area no. 1, the percentages being always expressed based on the total weight of the composition.

(69) Like in Example 5, this mixture of fillers is incorporated into the MDPE and the thereby prepared compositions are characterized, just like the combustion residues.

(70) FIG. 9 shows that this composition has a flame-retardant effect similar to the composition of Example 5, a clearly improved behavior as compared with the non-filled polymer and quasi comparable with that of the compositions only containing as a flame-retardant filler, ATH.

(71) By observing the combustion residues shown in FIGS. 10A and 10B and characterized in Table 5 it is possible to draw the conclusion that at the end of its combustion, the composition of this example has led to the formation of a cohesive residue very comparable to the residue obtained with the composition of Example 5. The measurement of the mechanical strength of the residue leads to an average value (over 3 measurements conducted on samples taken at different locations in the combustion residue) of 4 kPa, the three measured values being 3 kPa, 4 kPa and 4 kPa. The presence of a smaller amount of hydrated lime of high specific surface area in this example as compared with Example 5 may explain the reduction in the mechanical strength of the combustion residue as compared with the mechanical strength of the residue of Example 5.

Example 7

(72) Use as a flame-retardant filer, of a hydrated lime with high specific surface area no. 1 in a polystyrene polymer composition.

(73) Example 7 is similar to examples 1 and 2 but the polymer matrix is different in that MDPE is replaced with polystyrene (PS, Polystyrol VPT0013 GR2).

(74) The results of the fire tests with a cone calorimeter recorded for this composition are illustrated in FIG. 12. They are compared therein with the results obtained for the non-filled host polymer (PS) and indicate, once again, a highly significant flame-retardant effect of the hydrated lime of high specific surface area no. 1. The residues obtained at the end of the cone calorimeter test (combustion residues) were characterized in the same way as in Example 1.

(75) By observing the combustion residues shown in FIGS. 11A and 11B and characterized in Table 5, it is possible to state that at the end of its combustion, the composition of this example has led to the formation of a moderately cohesive residue: This residue consists of a single layer and not of a powder or divided material, even if this layer is cracked in several locations. The residue comprises two transverse cracks but which are surface cracks which do not cross the thickness of the residue. In spite of this cracking, a sample with a section similar to a square with a side of about 20 mm may be taken over the whole thickness of this residue. Moreover, the measurement of the mechanical strength of the residue leads to an average value (over 3 measurements conducted on samples taken at different locations in the combustion residue) of 124 kPa, the three measured values being 109 kPa, 128 kPa and 134 kPa. If the cohesion of the combustion residue is not as good for this composition as for the compositions of Examples 1 and 2 above consisting of the same mineral filler but of other polymer matrices, the mechanical strength of the residue is on the other hand very good for the composition of this example.

Example 8

(76) Use as a flame-retardant filler, of a hydrated lime with high specific surface area treated at the surface with calcium stearate in a MDPE polymer composition.

(77) The flame-retardant filler used in the composition of this example is obtained by treating the surface, with calcium stearate, of a hydrated lime with high specific surface area. For this, a hydrated lime with high specific surface area quite comparable with the one used in Example 1 is selected and then 2 kg of this lime are placed in a horizontal plow mixer of the Lodiger brand with a total capacity of 20 dm.sup.3 (model M20), heated beforehand to 60 C. Calcium stearate is then added into this mixture in an amount of 4% of the mass of the hydrated lime with high specific surface area (i.e. 80 g of calcium stearate). Stirring in the mixer is started and then the mixture is brought to 200 C. (about 17 minutes are required for heating the mixture from 60 to 200 C.). When this temperature of 200 C. is reached, the mixing continues for 10 minutes at 200 C., before being stopped, and then the product is left at rest until it completely cools down.

(78) This filler treated at the surface is incorporated in an amount of 50% by mass in a matrix of medium density polyethylene (MDPE 3802) by following the same procedure as the one described in Example 1. Plates identical with those of Example 1 are then prepared and used for the cone calorimeter measurements.

(79) If the surface treatment of the hydrated lime with high specific surface area with calcium stearate does not influence the flame-retardant properties of the filler (cone calorimeter results not shown here), this treatment on the other hand seems to promote dispersion of the filler into the polymeric matrix and the contact between the filler and the polymer, thus leading to the formation of a cohesive residue at the end of the combustion. This combustion residue is shown in FIGS. 13A and 13B. When comparing these figures to FIGS. 2A and 2B which illustrate the combustion residue of the composition of Example 1, it is found that the residue from the combustion containing the hydrated lime of high specific surface area treated with calcium stearate is less swollen (less intumescence).

Comparative Example 1

(80) Use as a flame-retardant filler, of ATH in a MDPE polymer composition.

(81) In this comparative example, the polymeric matrix is again MDPE. This time, the filler is no longer a calcium filler with a high specific surface area, but ATH of commercial origin (Albemarle Martinal OL 107 LEO) the main properties of which are indicated in Table 2.

(82) The ATH is incorporated into the polymer matrix in an amount of 50% by weight of the composition by following the procedure described in Example 1 above. As previously, samples of 1001004 mm.sup.3 are prepared and used for the fire tests with a cone calorimeter (FIG. 3) and the combustion residues collected at the end of the cone calorimeter test are characterized as explained above.

(83) The observation of the combustion residue shown in FIGS. 15A and 15B and characterized in Table 5 indicates that the composition of this example does not lead at the end of the combustion to the formation of a cohesive residue. This residue actually appears as a divided material with a multitude of small fragments (ash or powder). The measurement of the mechanical strength is impossible to carry out on this residue since it is not possible to take samples in this residue which have a section at least as large as the surface area of the circular movable plate used for the measurement (FIG. 1).

Comparative Example 2

(84) Use as a flame-retardant filler, of MDH in an MDPE polymer composition.

(85) This Comparative Example 2 corresponds to the Comparative Example 1 in which ATH is replaced with MDH of commercial origin (Albemarle Magnifin H10) for which the main characteristics are grouped in Table 2.

(86) MDH is incorporated into MDPE in an amount of 50% by weight of the composition by following the procedure described in Example 1 above. As earlier, samples of 1001004 mm.sup.3 are prepared and tested for the fire tests with a cone calorimeter (FIG. 3) and the combustion residues collected at the end of the cone calorimeter test are characterized as explained above.

(87) The observation of the combustion residues shown in FIGS. 16A and 16B and characterized in Table 5 indicates that the composition of this example does not lead, at the end of the combustion, to the formation of a cohesive residue. This residue actually appears as a divided material which may be considered as a powder. For the same reasons as in Comparative Example 1, the measurement of the mechanical strength is impossible to conduct on this residue.

Comparative Example 3

(88) Use as a flame-retardant filler, of a mineral filler consisting of Ca(OH).sub.2 and ATH mixed in a MDPE polymer composition.

(89) The composition of this example is similar to that of Example 5 above insofar that the mineral filler is a mixture of 30% by weight of ATH (Albemarle Martinal OL 107 LEO described in Table 2) and of 20% by weight of hydrated lime, these percentages being expressed, based on the total weight of the composition. Nevertheless, in this comparative example, the hydrated lime is a standard hydrated lime and not a hydrated lime of high specific surface area as this is the case in Example 5. If the hydrated limes with high specific surface area are obtained industrially by hydrating quicklime with an excess of water before being dried and then de-agglomerated, standard hydrated limes are obtained by hydrating quicklime via a dry route and leave the hydrator with sufficiently low humidity so as not to require a drying step (humidity <2% by weight). The main properties of the standard hydrated lime according to this example are repeated in Table 4.

(90) As in Example 5, this mixture of fillers is incorporated into the MDPE and the thereby prepared compositions are characterized, just like the combustion residues.

(91) The observation of the combustion residues shown in FIGS. 17A and 17B indicate that the composition of this example leads, at the end of the combustion, to the formation of a non-cohesive residue, unlike the residue from the combustion of the composition of Example 5 shown in FIGS. 8A and 8B. This observation confirms the positive impact of a high specific surface area of the filler on the cohesion of the combustion residue.

Comparative Example 4

(92) Use as a flame-retardant filler, of a hydrated lime with low specific surface area in a MDPE polymer composition.

(93) This comparative example is similar to Comparative Example 3, except that the standard hydrated lime of Comparative Example 3 was replaced with hydrated lime with an even lower specific surface area. This hydrated lime of low specific surface area is industrially synthesized with a method similar to the one used for producing a standard hydrated lime, but by adding during the synthesis, an additive which accelerates the hydration kinetics of quicklime, leads to closing of the porosity and prevents the development of the specific surface area of the hydrated lime. The main characteristics of the hydrated lime of this example are grouped in Table 4.

(94) The observation of the combustion residues shown in FIGS. 18A and 18B and characterized in Table 5 indicates that the composition of this example leads, at the end of the combustion, to the formation of a cohesive residue, but this residue is clearly less cohesive than the residues obtained with the composition based on hydrated lime with high specific surface area in Examples 1 and 3. It is even less cohesive than the combustion residue of the composition of Comparative Example 3. This residue actually has 8 to 10 transverse cracks and these cracks are deep. With this cracking, a sample with a section similar to a square with a side of about 10 mm may only be taken over the whole thickness of this residue. Moreover, the measurement of the mechanical strength of the residue leads to an average value (over 3 measurements conducted on samples taken at different locations in the combustion residue) of 8 kPa, the three measured values being 4 kPa, 7 kPa and 14 kPa.

(95) It is quite understood that the present invention is by no means limited to the embodiments described above and that many modifications may be made thereto without departing from the scope of the appended claims.

(96) TABLE-US-00001 TABLE 1 Hydrated lime of Hydrated lime of high specific high specific surface area surface area no. 1 (Examples 1, 2, no. 2 5, 6 and 7) (Example 3) Ca(OH).sub.2 (mass %) 94.6 86.2 CaCO.sub.3 (mass %) 4.7 11.7 Sum of impurities.sup.(1) (mass 0.99 2.54 %) Fe.sub.2O.sub.3 (mass %) 0.09 0.26 Humidity at 150 C. (mass %) 0.86 1.33 BET surface area (m.sup.2/g) 41.3 36.3 Porous volume (cm.sup.3/g) 0.231 0.206 Grain size d.sub.3 (m) 0.9 1.1 d.sub.10 (m) 1.3 1.8 d.sub.50 (m) 3.5 8.9 d.sub.90 (m) 9.8 76.4 d.sub.97 (m) 22.7 161.2 d.sub.100 (m) 43.7 449.7 Ca(OH).sub.2 decomposition T 410-590 400-600 ( C.) 95% T ( C.) 482 490 .sup.(1)MgO + SiO.sub.2 + Al.sub.2O.sub.3 + Fe.sub.2O.sub.3 + MnO + P.sub.2O.sub.5 + K.sub.2O + SO.sub.3

(97) TABLE-US-00002 TABLE 2 ATH (Examples 5, 6 and Comparative MTH (Comparative Example 1) Example 2) Al(OH).sub.3 (mass %) 99.8 Mg(OH).sub.2 (mass %) 99.9 Sum of impurities.sup.(1) 0.19 0.03 (mass %) BET surface area (m.sup.2/g) 5.3 9.5 Porous volume (cm.sup.3/g) 0.017 0.029 Grain size d.sub.3 (m) 0.6 0.5 d.sub.10 (m) 0.9 0.7 d.sub.50 (m) 1.8 1.7 d.sub.90 (m) 3.5 25.0 d.sub.97 (m) 4.4 52.6 d.sub.100 (m) 7.4 83.9 Hydroxide decomposition 210-550 340-550 T ( C.) 95% T ( C.) 301 413 .sup.(1)SiO.sub.2 + CaO + Fe.sub.2O.sub.3 + Na.sub.2O + SO.sub.3 + Cl

(98) TABLE-US-00003 TABLE 3 Laboratory filler of high specific surface area no. 1 (Example 4) Ca(OH).sub.2 (mass %) 76.0 Mg(OH).sub.2 (mass %) 13.6 CaCO.sub.3 (mass %) 8.8 Sum of impurities.sup.(1) 0.98 (mass %) Fe.sub.2O.sub.3 (mass %) 0.24 Humidity at 150 C. (mass 1.4 %) BET surface area (m.sup.2/g) 29.5 Porous volume (cm.sup.3/g) 0.151 Grain size d.sub.3 (m) 0.7 d.sub.10 (m) 1.0 d.sub.50 (m) 2.7 d.sub.90 (m) 6.8 d.sub.97 (m) 8.6 d.sub.100 (m) 14.3 Mg(OH).sub.2 decomposition 250-430 T ( C.) Ca(OH).sub.2 decomposition T 430-580 ( C.) 95% T ( C.) 400 .sup.(1)MgO + SiO.sub.2 + Al.sub.2O.sub.3 + Fe.sub.2O.sub.3 + MnO + P.sub.2O.sub.5 + K.sub.2O + SO.sub.3

(99) TABLE-US-00004 TABLE 4 Hydrated lime of low Standard hydrated specific surface lime (Comparative area (Comparative Example 3) Example) Ca(OH).sub.2 (mass %) 92.8 96.4 CaCO.sub.3 (mass %) 5.9 1.5 Sum of Impurities.sup.(1) 1.26 1.68 (mass %) Fe.sub.2O.sub.3 (mass %) 0.13 0.20 Humidity at 150 C. (mass 0.7 6.5 %) BET surface area (m.sup.2/g) 15.8 7.5 Porous volume (cm.sup.3/g) 0.073 0.033 Grain size d.sub.3 (m) 0.8 0.8 d.sub.10 (m) 1.2 1.7 d.sub.50 (m) 2.9 7.4 d.sub.90 (m) 6.2 101.1 d.sub.97 (m) 33.0 161.2 d.sub.100 (m) 309.6 309.6 Ca(OH).sub.2 decomposition T 400-560 400-610 ( C.) 95% T ( C.) 455 477 .sup.(1)MgO + SiO.sub.2 + Al.sub.2O.sub.3 + Fe.sub.2O.sub.3 + MnO + P.sub.2O.sub.5 + K.sub.2O + SO.sub.3

(100) TABLE-US-00005 TABLE 5 Examples Comparative Examples 1 2 3 4 5 6 7 1 2 3 4 Number of transverse cracks 0 0 2 1 0 0 2 >10 >10 >10 8-10 Deep cracks No No No Yes Yes Yes Yes Maximum sample size (mm) .sup.(1) 30 30 20 <10 <10 <10 10 Average mechanical strength (kPa) 110 65 20 38 9 4 124 8 Minimum mechanical strength (kPa) 71 43 14 28 4 3 109 4 Maximum mechanical strength (kPa) 140 91 31 43 19 4 134 14 .sup.(1) maximum sample size (sample representing the whole thickness of the residue and the section of which is assimilated to a square) which may be taken without being broken in the residue obtained at the end of the cone calorimeter test which has a measured side of 100 mm.