Laminated glazing with low consumption of materials and optimized impact resistance

Abstract

The invention relates to laminated glazing in which the lamination interlayer successively comprises a first layer, a second layer comprising a material selected from polycarbonates, glass, thermoplastic polyurethane (TPU), ionomer resins, polyethylene terephthalate (PET) and mixtures thereof and a third layer in which the total thickness eTOT of the lamination interlayer is less than or equal to 0.73 mm and eTOT = 0.75 e1,3 + 0.19; eTOT = 0.76 – 3 e2 where e1,3 is the sum of the thicknesses of the first and third layers and e2 is the thickness of the second layer. The invention also relates to an insulating glazing comprising such a laminated glazing and to a method for obtaining this laminated glazing.

Description

Description Title of the invention: Laminated glazing with low material consumption and optimized impact resistance. Prior art

[0001] The present invention belongs to the field of glazing, in particular laminated glazing and insulating glazing, having low material consumption and optimized impact resistance.

[0002] Insulating glazing is typically made up of an assembly of several parallel panes of glass, separated by a cavity containing a layer of gas, often insulating gas.

[0003] In the construction sector in particular, in which these insulating glazings are widely used for sound insulation and thermal insulation, there is an increasing desire to increase their resistance to impacts and perforation, with the aim of giving them an anti-burglary character, of protecting people from broken glass and the risks resulting from a fall against the glazing, or even with the aim of guaranteeing resistance to violent weather (hail), particularly for insulating glazing in roof windows.

[0004] Although the resistance to perforation is better for insulating glazing than for single monolithic glazing, it appears that insulating glazing composed solely of monolithic glazing does not have sufficient resistance to perforation to provide burglar-proof protection or to protect people from broken glass and the risks resulting from falling against the glazing, or to guarantee resistance to violent weather.

[0005] Laminated glazing, made up of two sheets of glass between which an adhesive interlayer called a "lamination interlayer" is laminated, has good impact resistance in some cases.

[0006] The EN 356 standard, for example, defines eight performance classes based on tests representing the ability of glazing to resist objects thrown at it (levels 1 to 5 or P1A to P5A of the EN 356 standard) or attempted break-ins using a sledgehammer or an axe (levels 6 to 8 or P6B to P8B of the EN 356 standard).

[0007] For historical reasons in the field of laminated glazing, the interlayer adhesive layer is usually composed of poly(vinyl butyral) films, each with a thickness of 0.38 mm.

[0008] The thicker the glass sheets in laminated glazing, the better the performance class according to EN 356. Similarly, the thicker the interlayer adhesive layer, the better the performance class according to this standard.

[0009] However, the effect of glass thickness is much smaller than the effect of interlayer thickness, and only a change in glass thickness of several millimeters can significantly improve the performance class according to EN 356, while for the interlayer an increase of only several tens to several hundred micrometers can significantly improve this performance class.

[0010] Thus, commercially available laminated glazing units passing the minimum performance level of the EN 356 standard, i.e. class P1A, are glazing units comprising two 2 mm thick glass sheets and a 0.76 mm thick poly(vinyl butyral) (PVB) interlayer adhesive layer, while glazing units comprising two 4 mm thick glass sheets and a 0.76 mm thick PVB interlayer adhesive layer pass the second level of the EN356 standard, i.e. class P2A. To obtain the P2A performance level with two 2 mm thick panes of glass, document WO 2020 / 152416 proposes increasing the surface mass and therefore the thickness of the interlayer by less than 10%, which represents less than 80 μm for a standard 0.76 mm interlayer.

[0011] It is therefore possible to limit the raw material consumption of the glazing by significantly reducing the glass thickness (from several hundred micrometers to several millimeters), while slightly increasing (from several tens to several hundred micrometers) the thickness of the interlayer. However, when the glass thickness decreases significantly, the resistance to wind, snow and self-weight loads (in the case of a non-vertical window, such as a roof window) also decreases significantly. It is therefore not always possible to reduce the raw material consumption of the glazing by changing the ratio of the glass thickness to the interlayer thickness, for example when the glass of the reference glazing is already very thin.

[0012] There is a class of so-called "structural" interlayers, which are more rigid than standard PVB or ethylene-vinyl acetate (EVA) interlayers, for example based on ionomers or PVB with reduced plasticizer content. These structural interlayers improve the mechanical coupling between the glass sheets and significantly increase resistance to wind, snow and dead weight loads, without increasing the glass thickness or the interlayer thickness. However, these interlayers suffer from poor impact resistance, and therefore do not improve the performance class according to EN 356.

[0013] It is therefore relevant to seek a solution that allows, without changing the glass thickness, to increase the performance level according to the EN 356 standard without consuming more raw materials, or equivalently to maintain the EN 356 performance level while consuming less raw materials. More generally, it is relevant to define, for fixed glass thicknesses, interlayers whose characteristics ensure a given impact performance level for the laminated glazing. In the case of an interlayer composed of a single material, these characteristics can be expressed by the product of the tear resistance times the thickness of the interlayer, a product greater than a threshold value, as formulated in EP 2421705.In the case of an interlayer composed of several materials, such as underlayers superimposed on each other, to the knowledge of the inventors, such characterization of the products has not been identified, in particular with regard to the thickness of these underlayers.

[0014] Compliance with the P2A level of EN 356 is currently measured by a method known as the ball drop test (or hard body drop test), which involves successively dropping three steel balls, each 10 cm in diameter and weighing 4.1 kg, onto the glass from a certain height. To achieve the P2A level, three samples of a glass must withstand three successive ball drops from a height of 3 m.

[0015] From a statistical point of view, it is very difficult to draw clear conclusions about a glass's performance due to the limited number of samples (determination of a failure probability at 3 m with only three samples).

[0016] A more robust method for assessing glass performance according to EN 356 has been developed by the Applicant Company and is the mean break height with three balls (MBH3). This method consists of dropping three balls successively from a certain height onto a glass sample. If the sample passes the test without being punctured by all three balls, then another glass of the same type is tested, dropping the three balls from a greater height corresponding to the height of the previous test plus a fixed increment value. If the sample does not pass the test, then another glass of the same type is tested, dropping the three balls from a lower height corresponding to the height of the previous test minus a fixed increment value.By repeating this test, we will converge and then naturally oscillate around the average perforation height of the glazing by three balls, in other words the height at which half of the samples are perforated and the other half are not. We will preferably choose the starting height close to the average perforation height expected for the glazing tested, the fixed increment value (plus or minus) being preferably close to the standard deviation of the probability distribution examined by the test (probability of failure in the three-ball test as a function of the drop height. balls). For a P2A performance test with laminated glazing, a starting height of 3.6 m and an increment value of 0.3 m will be chosen. A statistical treatment of this method shows that the mean perforation height, as well as the associated standard deviation and the 95% confidence interval on the value of the mean perforation height, can be defined and calculated (see Dixon W.J. Mood A., "A method for obtaining and analyzing sensitivity data", Journal of the American Statistical Association, 43, 1948). Once the mean perforation height and the associated standard deviation are obtained, it can then be estimated whether the difference between the mean perforation height and the target height (e.g., 3 m for P2A level glazing) is sufficiently large compared to the standard deviation of the distribution to ensure that the probability of failure at the target height is negligible.For example, if the target height is 3 m, the mean perforation height is 3.6 m and the standard deviation is 0.3 m, this means that the difference of 0.6 m is twice the standard deviation, and the probability of failure at 3 m is then 2.3% for a normal (Gaussian) distribution.

[0017] To solve the problem of finding a glazing unit that allows, without changing the glass thickness, to maintain the EN 356 performance level while consuming less raw materials, the Applicant Company has developed an insulating glazing unit comprising at least one laminated glazing unit at least of level P2A of the EN 356 standard, verified according to the standard method but also according to the more statistically rigorous method of average perforation height as described above.

[0018] To this end, and according to a first aspect, the invention relates to a laminated glazing comprising two sheets of glass separated by a lamination interlayer, in which the lamination interlayer successively comprises: - a first layer comprising a polymer chosen from poly(vinyl butyral) (PVB), ethylene vinyl acetate (EVA), ionomer resins and their mixtures, - a second layer comprising a material selected from polycarbonates, glass, ionomers, thermoplastic polyurethane (TPU), ionomer resins, polyethylene terephthalate (PET) and mixtures thereof, and - a third layer comprising a polymer chosen from poly(vinyl butyral) (PVB), ethylene vinyl acetate (EVA), ionomer resins and mixtures thereof, in which the total CTOT thickness of the lamination interlayer is less than or equal to 0.73 mm and CTOT = 0.75 61.3 + 0.19 6TOT= 0.76 — 3 62 where ei,3 is the sum of the thicknesses of the first and third layers and e2 is the thickness of the second layer.

[0019] The inventors have discovered that, in the structure of a lamination interlayer, the substitution of a thicker slice of a polymer usual in laminated glazings, by a thin intermediate layer, corresponding to the second layer of the lamination interlayer of a glazing according to the present invention, comprising a material with better tear resistance properties, makes it possible, for a lower consumption of raw materials, to obtain laminated glazings having at least equivalent impact resistance performances. The insertion of such a material with better tear resistance properties between two layers of a polymer usual in laminated glazings also makes it possible to keep unchanged the glass / polymer interfaces, critical for the ease of manufacture of the laminated glazing, its resistance to aging as well as its capacity to stop an impactor without perforation of the glazing.

[0020] Thus, in order to obtain a level of performance at least equivalent to a 0.76 mm PVB lamination interlayer, and for a total thickness CTOT of the lamination interlayer of the glazing according to the invention less than or equal to 0.73 mm, the thickness of the second intermediate layer 62 is (0.76 - CTOT) / 3. While the sum of the thicknesses of the first and third layers ei,3, arranged on either side of the second layer, is (CTOT- 0.19) / 0.75.

[0021] The material included in the second layer is selected from polycarbonates, glass, thermoplastic polyurethane (TPU), ionomer resins, polyethylene terephthalate (PET) and their mixtures.

[0022] By “common polymers in laminated glazing” we mean PVB, LEVA, ionomer resins and their mixtures.

[0023] Thus, a laminated glazing according to the present invention has a performance level according to standard EN 356 at least equivalent to a laminated glazing comprising a 0.76 mm thick PVB interlayer, i.e. a P2A level according to said standard, while consuming less raw material.

[0024] The glazing according to the invention therefore belongs to class P2A according to standard NF EN 356.

[0025] In particular, in a glazing according to the invention, the value of the adhesion between the glass and the lamination interlayer, measured by the TCT method at 33 mm. s 1 and 20°C is included by 4 kJ / m 2 + 8 kJ / m 3 x CTOT (mm) at 14 kJ / m 2 + 8 kJ / m 3 x CTOT (mm), and the interlayer adhesive layer has a tear opening and propagation resistance greater than 30 kJ / m 2 .

[0026] The TCT method (acronym for Through-Cracked-Tensile (TCT) test) is a method for measuring the energy absorbed per unit area created between the crack lips of broken glass, a surface created due to delamination at the glass-interlayer interface and due to the deformation of the latter. This method is notably described in the document "Mechanical behavior in tension of cracked glass bridged by an elastomeric ligament", S. Muralidhar, A. Jagota, SJ Bennison, S. Saigal, Acta Materialia, Volume 48, numbers 18-19, December 1, 2000, pages 4577-4588, a document which also highlights the importance for energy dissipation (and therefore for the absorption of the kinetic energy of an object impacting the glazing) of these delamination mechanisms at the glass-interlayer interface and of deformation of the latter.The value of glass-interlayer adhesive adhesion measured by the TCT method at 33 mm. s. 1 and 20 °C included 4 kJ / m 2 + 8 kJ / m 3x CTOT (mm) and 14 kJ / m2 + 8 kJ / m3 x CTOT (mm) corresponds substantially, at the thickness values ​​of the interlayer adhesive layer used, to values ​​between 4 and 7 on the Pummel scale, not too low to guarantee good retention of the glass fragments, to prevent them from detaching, but not too high to allow delamination during an impact, thus allowing substantial energy absorption, without the interlayer tearing. In this application, any reference to TCT tests also refers to the document “Adhesion rupture in laminated glass: influence of adhesion on the energy dissipation mechanisms”, P. Fourton, K. Piroird, M. Ciccotti and E. Barthel, Glass Structures & Engineering, vol. 5, pp. 397-410, 2020.

[0027] The resistance to tear opening and propagation is measured as follows. Twenty 5 x 10 cm samples 2 of interleaf are cut before lamination. Two slits are cut from two opposite edges in the middle of each sample, so as to separate the rectangle into two 5 x 5 cm squares 2each. The separation is incomplete, however, because an intact ligament remains between the two cuts. Each sample has a different ligament length 1. All samples have the same thickness b (e.g., 0.76 mm). A tensile test is carried out at 20 °C and 100 mm / min until complete rupture on each sample. For each sample, the work W until rupture is measured. The diagram of W / 1b as a function of 1 is a straight line that is extrapolated for 1 = 0. The extrapolated value, in J / m2, is the intrinsic resistance of the interlayer to opening and tear propagation, independent of the geometry of the sample. A resistance to opening and tear propagation of at most 30 kJ / m 2 manifests itself by a failure of the pendulum test for interlayers whose glass - interlayer adhesive layer adhesion value measured by the TCT method at 33 mm. 1 and 20 °C is included by 4 kJ / m2 + 8 kJ / m 3 x CTOT (mm) at 14 kJ / m 2 + 8 kJ / m 3 x CTOT (mm) and whose thickness is between 0.7 and 0.8 mm.

[0028] The first and third layers of the lamination interlayer of a laminated glazing unit according to the invention comprise a polymer chosen from poly(vinyl butyral) (PVB), ethylene vinyl acetate (EVA), ionomer resins and mixtures thereof.

[0029] In particular, the first layer may comprise a polymer selected from PVB and LEVA, preferably PVB. The first layer may consist of a polymer selected from PVB and LEVA, preferably PVB

[0030] In particular, the third layer may comprise a polymer selected from PVB and LEVA, preferably PVB. The third layer may consist of a polymer selected from PVB and LEVA, preferably PVB.

[0031] According to a particular embodiment, the first and third layers may comprise a polymer chosen from PVB and LEVA, preferably PVB.

[0032] According to an even more particular embodiment, the first and third layers may consist of a polymer chosen from PVB and LEVA, preferably PVB.

[0033] Thus, in a laminated glazing according to the invention, at least one of the first and third layers may comprise PVB, in particular the first and third layers comprise PVB and preferably, the first and third layers are made of PVB.

[0034] A PVB suitable for the present invention is, for example, chosen from the PVBs marketed by the company Eastman under the references Saflex RB 11 and RB41 or by the company Kuraray under the reference Trosifol B200 Clear.

[0035] The second layer of the lamination interlayer of a laminated glazing according to the invention comprises a material chosen from polycarbonates, glass, thermoplastic polyurethane (TPU), ionomer resins, polyethylene terephthalate (PET) and mixtures thereof.

[0036] In a particular embodiment of the invention, said second layer comprises PET, preferably it is made of PET.

[0037] A PET suitable for the present invention is, for example, the PET marketed by the company Eastman under the reference Saflex XIR 75.

[0038] In the laminated glazing according to the invention, CTOT can be from 0.25 mm to 0.73 mm, in particular CTOT is from 0.40 mm to 0.70 mm and preferably CTOT is from 0.50 mm to 0.61 mm.

[0039] When CTOT is less than 0.25 mm, at the glass / PVB interfaces, adhesion and energy absorption phenomena, as described in “Adhesion rupture in laminated glass: influence of adhesion on the energy dissipation mechanisms”, P. Fourton, et al., Glass Structures & Engineering, vol. 5, pp. 397-410, 2020, are insufficient to achieve a P2A performance level according to EN 356.

[0040] The two glass sheets of the laminated glazing according to the invention may have identical or different thicknesses, ranging from 1.0 mm to 25.0 mm, in particular from 1.4 mm to 6.0 mm and preferably from 1.6 mm to 4.0 mm.

[0041] Said glass sheets may be made of mineral glass such as float, soda-lime, aluminosilicate, borosilicate, possibly thermally toughened or chemically strengthened. The glass is colorless or tinted.

[0042] They can also advantageously carry at least one transparent functional layer or stack of layers such as obtained by magnetron-assisted sputtering, by chemical vapor deposition (CVD), by liquid means such as sol-gel, consisting of a thermal control, anti-solar, low-emissivity, anti-reflective layer or stack, a surface tension modification layer, hydrophobic, hydrophilic, photocatalytic self-cleaning, an electrically conductive layer connected to a source of electric current, anti-frost, anti-fog heating.

[0043] According to a second aspect, the invention relates to an insulating glazing unit comprising an assembly of parallel glazing units, two consecutive glazing units in the assembly being separated by a cavity containing a gas layer, said insulating glazing unit comprising at least one laminated glazing unit according to the invention.

[0044] The insulating glazing of the invention is in particular double glazing (one gas blade), or triple glazing (two gas blades).

[0045] Preferably, the insulating glazing consists of double glazing with a gas blade having a thickness of between 10 and 20 mm.

[0046] Preferably, the insulating glazing consists of double glazing with a thickness of between 18 and 30 mm.

[0047] In the insulating glazing according to the invention, the glazings which are not the at least one laminated glazing according to the invention may be monolithic glasses, in particular chosen from structural polymers, preferably poly(methyl methacrylate), polycarbonate, structural polyurethane; soda-lime glasses; aluminosilicate glasses; borosilicate glasses; optionally thermally toughened or chemically toughened.

[0048] According to a particular embodiment, the monolithic glasses can have a thickness of between 1 mm and 25 mm.

[0049] In particular, in the insulating glazing according to the invention, each gas blade has a thickness of between 4 mm and 30 mm.

[0050] In particular, in the insulating glazing according to the invention, the gas is an insulating gas chosen from air, argon, krypton, xenon and their mixtures.

[0051] Finally, according to a third aspect, the invention relates to a method for obtaining laminated glazing according to the invention, comprising a step of laminating the lamination interlayer between two sheets of glass.

[0052] In particular, the rolling step may include: - a heating step at a temperature of from 40°C to 100°C, in particular from 45°C to 80°C and preferably from 50°C to 70°C, - a calendering step under a pressure of 2 to 10 bars, in particular 3 to 8 bars and preferably 4 to 6 bars, and - an autoclaving step lasting from 1 to 10 hours, in particular from 2 to 8 hours and preferably from 3 to 6 hours, at a temperature of from 80°C to 200°C, in particular from 100°C to 180°C, and preferably from 120°C to 160°C and under a pressure of from 10 to 17 bars, in particular from 11 to 16 bars, preferably from 12 to 15 bars. Examples

[0053] The laminated glazing in the examples described below is made by bonding two 4 mm thick sheets of glass marketed by Saint-Gobain Glass under the reference Planiclear using the lamination interlayers described in Table 1 below. [Table 1]

[0054] Laminated glazing is obtained by a rolling process comprising successive heating at 60°C, calendering at 5 bars and autoclaving under 13 bars at 140°C for 5 hours.

[0055] The said glazings were evaluated by the MBH3 method as described above, using at least 30 glazings of 1100x900 mm 2 by composition (examples 1 and 2 according to the invention and comparative examples 1a, 1b, 2a and 2b). It appears initially that the laminated glazings according to the invention satisfy level P2A of standard EN 356.

[0056] For the same quantity of PVB consumed, a laminated glazing according to the invention comprising an interlayer with a thin layer of PET represents a significant increase in the impact resistance performance of said glazing. Indeed, example 1 according to the invention represents an increase of 2.48 m of MBH3 or 134% compared to comparative example 1a comprising the same quantity of PVB, while example 1 comprises a central layer of 70 μm of PET. Similarly, example 2 according to the invention, comprising a central layer of 30 μm of PET, represents an increase of 91.6% compared to comparative example 2a comprising the same thickness of interlayer, consisting solely of PVB.

[0057] It thus appears that the laminated glazings according to the invention make it possible, for an equivalent lamination interlayer thickness, to significantly improve the level of performance in terms of impact resistance compared to conventional laminated glazings, i.e. comprising a lamination interlayer made of PVB.

[0058] Table 2 below describes Example 3 according to the invention and Comparative Example 3, carried out according to the same procedure as the examples previously presented. [Table 2]

[0059] The comparison of example 3 according to the invention below and comparative example 3 demonstrates that, for laminated glazings according to the invention, a level of impact resistance performance at least similar to a conventional laminated glazing is obtained for an interlayer thickness of approximately 20% lower.

[0060] Furthermore, since it is demonstrated that, for the same total thickness of interlayer, the interlayers in a laminated glazing according to the invention allow a significant increase in the impact resistance performance of said glazing, it follows that, for the same impact performance, the interlayers in a laminated glazing according to the invention allow a significant reduction in the thickness of the interlayer, consuming less material for the constitution of laminated glazing, in particular intended to be used in insulating glazing.

Claims

Claims

1. Laminated glazing comprising two sheets of glass separated by a lamination interlayer, in which the lamination interlayer successively comprises: a first layer comprising a polymer chosen from poly(vinyl butyral) (PVB), ethylene vinyl acetate (EVA), ionomer resins and mixtures thereof, a second layer comprising a material chosen from polycarbonates, glass, thermoplastic polyurethane (TPU), ionomer resins, polyethylene terephthalate (PET) and mixtures thereof, and a third layer comprising a polymer chosen from poly(vinyl butyral) (PVB), ethylene vinyl acetate (EVA), ionomer resins and mixtures thereof, in which the total thickness CTOT of the lamination interlayer is less than or equal to 0.73 mm and CTOT = 0.75 61.3 + 0.19 CTOT =0.76 — 3 62 where 61.3 is the sum of the thicknesses of the first and third layers and 62 is the thickness of the second layer.

2. Laminated glazing according to the preceding claim, in which the value of the adhesion between the glass and the lamination interlayer, measured by the TCT method at 33 mm. s 1 and 20°C is included by 4 kJ / m 2 + 8 kJ / m 3 x CTOT (mm) at 14 kJ / m 2 + 8 kJ / m 3 x CTOT (mm), and the interlayer adhesive layer has a tear opening and propagation resistance greater than 30 kJ / m 2 .

3. Laminated glazing according to any one of the preceding claims, wherein at least one of the first and third layers comprises PVB, in particular the first and third layers comprise PVB and preferably the first and third layers are made of PVB.

4. Laminated glazing according to any one of the preceding claims, wherein the second layer comprises PET, in particular the second layer is made of PET.

5. Laminated glazing according to any one of the preceding claims, said glazing belonging to class P2A according to standard NF EN 356.

6. Laminated glazing according to any one of the preceding claims, in which CTOT is from 0.25 mm to 0.73 mm, in particular CTOT is from 0.40 mm to 0.70 mm and preferably CTOT is from 0.50 mm to 0.61 mm.

7. Laminated glazing according to any one of the preceding claims, in which the glass sheets are made of mineral glass such as float, soda-lime, aluminosilicate, borosilicate, optionally thermally toughened or chemically reinforced.

8. Laminated glazing according to any one of the preceding claims, in which at least one glass sheet carries at least one transparent functional layer or stack of layers as obtained by magnetron-assisted sputtering, by chemical vapor deposition (CVD), by liquid means such as sol-gel, consisting of a thermal control, anti-solar, low-emissivity, anti-reflective layer or stack, a surface tension modification layer, hydrophobic, hydrophilic, photocatalytic self-cleaning, an electrically conductive layer connected to a source of electric current, anti-frost heating, anti-fog.

9. Insulating glazing comprising an assembly of parallel glazings, two consecutive glazings in the assembly being separated by a cavity containing a gas layer, said insulating glazing comprising at least one laminated glazing according to any one of the preceding claims.

10. Insulating glazing according to the preceding claim, in which the glazings which are not the at least one laminated glazing according to any one of claims 1 to 8, are monolithic glasses, in particular chosen from structural polymers, preferably poly(methyl methacrylate), polycarbonate, structural polyurethane; soda-lime glasses; aluminosilicate glasses; borosilicate glasses; optionally thermally toughened or chemically toughened.

11. Insulating glazing according to the preceding claim, in which the monolithic glasses have a thickness of from 1 mm to 25 mm.

12. Insulating glazing according to any one of claims 9 to 11, in which each gas blade has a thickness of from 4 mm to 30 mm.

13. Insulating glazing according to any one of claims 9 to 12, in which the gas is an insulating gas chosen from air, argon, krypton, xenon and mixtures thereof.

14. Method for obtaining laminated glazing according to any one of claims 1 to 8, comprising a step of laminating the lamination interlayer between two sheets of glass.

15. A method according to the preceding claim, wherein the rolling step comprises: - a heating step at a temperature of from 40°C to 100°C, in particular from 45°C to 80°C and preferably from 50°C to 70°C, - a calendering step under a pressure of 2 to 10 bars, in particular 3 to 8 bars and preferably 4 to 6 bars, and - an autoclaving step lasting from 1 to 10 hours, in particular from 2 to 8 hours and preferably from 3 to 6 hours, at a temperature of from 80°C to 200°C, in particular from 100°C to 180°C, and preferably from 120°C to 160°C and under a pressure of from 10 to 17 bars, in particular from 11 to 16 bars, preferably from 12 to 15 bars.

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