Process for manufacturing xerogels

Abstract

The present invention is related to a process for manufacturing xerogels optionally containing a fibrous reinforcement material, to an insulating, self-supporting single-layer composite panel of thickness between 30 mm and 70 mm of xerogel comprising a fibrous reinforcement material comprising a nonwoven fibrous batting obtainable by this process and to the use thereof for the manufacture of building materials and thermal insulations.

Claims

1. A process for manufacturing xerogel having a thermal conductivity of between 5 and 25 mW/m.K measured using the guarded hot plate method of standard NF EN 12667 at 20 C. and at atmospheric pressure, comprising the successive steps of: a) pouring a sol with alcohol as solvent into a reactor in which a fibrous reinforcement material has previously been placed, b) gelling the sol to an alcogel, c) ageing the alcogel, d) hydrophobization treatment of the alcogel after which a hydrophobized alcogel is obtained, e) pre-drying of the alcogel under subcritical conditions at a temperature equal to or lower than 80 C., and f) drying the alcogel under subcritical conditions, said drying being a dielectric or convective drying, so that the xerogel obtained has a residual quantity of alcohol by panel weight of 3% or lower as per standard EN/ISO 3251, provided that at least steps a), b), c), d) and e) are implemented in the same reactor, a smallest characteristic distance of said reactor between at least two inner walls being between 30 mm and 70 mm.

2. The process according to claim 1, wherein steps a), b), c) d) and e) are conducted in a first reactor, the condensed alcogel is then released from the first reactor and transferred to a convective or dielectric drier in which step f) is performed.

3. The process according to claim 1, wherein the sol used at step a) is selected from the group consisting of silica, titanium oxide, manganese oxide, calcium oxide, calcium carbonate, zirconium oxide sols, and mixtures thereof.

4. The process according to claim 1, wherein the alcogel obtained at step b) comprises 70 to 90% by weight of alcohol, of alcohol relative to the weight of the starting sol.

5. The process according to claim 1, wherein step d) comprises the contacting of the alcogel obtained at step c) with a hydrophobizing agent in an acid medium of pH between 1 and 3.

6. The process according to claim 5, wherein the hydrophobizing agent used is selected from the group consisting of organosiloxanes, organochlorosilanes and organoalkoxysilanes.

7. The process according to claim 1, wherein an additive is added to the sol at step a).

8. The process according to claim 1, wherein step f) is a drying step of convective drying performed at a temperature higher than 100 C.

9. The process according to claim 1, wherein step f) is a microwave dielectric drying step under a pressure between 10 mbar and 1 bar.

10. The process of claim 6, wherein the hydrophobizing agent used is selected from the group consisting of hexamethyldisiloxane (HMDSO), trimethylchlorosilane and trimethylethoxysilane.

11. The process of claim 7, wherein the additive contains an opacifier.

12. The process according to claim 8, wherein the convective drying is performed at a temperature of between 120 C. and 180 C.

Description

FIGURE

(1) FIG. 1: Trend in the thickness of the self-supporting, insulating, single-layer composite panel as a function of drying time, under the operating conditions of Example 5.

(2) The following examples are intended to illustrate the present invention in more detail but are in no way limiting.

EXAMPLES

Example 1

Preparation of Silica Xerogels in a Cylindrical Chamber with Various Thicknesses

(3) 1) Preparation of Hydrophobic Silica Alcogels

(4) A silica sol having the following composition: 36.2% polyethoxydisiloxane in 20% solution in ethanol and obtained by partial hydrolysis of tetraethoxysilane (TEOS) in the presence of hydrochloric acid, 54.3% ethanol, 8.9% permuted water, 0.6% ammonia was placed before gelling in a closed cylindrical chamber of variable dimensions (cylindrical discs of diameter 100 mm and thickness of 30, 50, 70 or 100 mm). This silica sol was gelled after placing in the chamber to form a silica alcogel.

(5) After an ageing phase of the gel in ethanol for 19 h30 at 70 C., the mixture of hydrochloric acid (3 weight %) and hexamethyldisiloxane (97 weight %) was placed in the reactor so as to cover the alcogel entirely. The reaction medium was heated and held at 70 C. for 6 h. The reaction medium was then separated from the hydrophobized alcogel by percolation.

(6) 2) Obtaining Silica Xerogels

(7) The gels obtained had a thickness of 3 cm, 5 cm, 7 cm and 10 cm respectively and were dried in a ventilated oven at 160 C. for 1 h30.

(8) All the beds of silica xerogel particles obtained after drying exhibited a bulk density of between 0.05 g.Math.cm.sup.3 and 0.1 g.Math.cm.sup.3, irrespective of their thickness. The bulk densities measured on the beds of xerogel particles obtained were 50 kg/m.sup.3, 70 kg/m.sup.3, 85 kg/m.sup.3 and 100 kg/m.sup.3 for gel thicknesses of 3, 5, 7 and 10 cm, respectively.

(9) The thermal conductivity values measured on the samples obtained, using the guarded hot plate method of standard NF EN 12667 at 20 C. and at atmospheric pressure, were 21.6 mW/m.Math.K, 21.8 mW/m.Math.K, 23.5 mW/m.Math.K and 25.7 mW/m.Math.K for gel thicknesses of 3, 5, 7 and 10 cm respectively.

(10) This example clearly shows the threshold effect related to the size of the reaction chamber. When the chamber has a thickness of 70 mm or less, the thermal conductivity of the silica xerogel obtained is lower than 25 mW/m.Math.K.

Example 2

Preparation of Silica Xerogels with Various Drying Conditions

(11) 1) Preparation of a Hydrophobic Silica Alcogel

(12) A silica sol obtained in the same conditions as example 1 by hydrolysis of alkoxysilane in the presence of hydrochloric acid is gelled in the presence of ammonia. After an ageing phase of 4 h under ethanol refluxing, hydrochloric acid and hexamethyldisiloxane (3:97) (hydrophobizing agent) were added to the reactor so as to fully cover the silica alcogel. The reaction medium was heated and held under reflux for 4 h. The reaction medium was then separated from the hydrophobic silica alcogel by percolation.

(13) The hydrophobic silica alcogel (250 g) thus obtained was then divided in pieces having a size comprised between 1 and 20 mm and introduced in a crystallizing dish.

(14) 2) Preparation of a Condensed Hydrophobic Silica Alcogel

(15) The crystallizing dish containing the hydrophobic silica alcogel (250 g) divided in pieces was placed in a ventilated oven and the sample was dried at 80 C. until it has lost about 50% of its initial weight.

(16) 3a) Obtaining a Hydrophobic Silica Xerogel by Convective Drying in a Ventilated Oven

(17) The condensed hydrophobic silica alcogel previously obtained was dried in a ventilated oven at 160 C. for 60 min. The bed of hydrophobic silica xerogel granules obtained exhibited a bulk density of 0.06 g.Math.cm.sup.3, and the xerogel granules obtained had dimensions comprised between about 0.1 and 10 mm. The thermal conductivity value measured on the granules having a size comprised between 1 mm and 1.2 mm, using the guarded hot plate method of standard NF EN 12667 at 20 C. and at atmospheric pressure, was 19.8 mW/m.Math.K.

(18) 3b) Obtaining a Hydrophobic Silica Xerogel by Dielectric Drying Under Reduced Pressure

(19) The condensed hydrophobic silica alcogel previously obtained was dried in a microwave drier under vacuum (40-60 mbar), under gentle stirring, and by applying an incident power of 0.5 kW/kg of alcogel. After 20 min of drying, the power reflected by the system was above 160 W.Math.h/kg of alcogel initially introduced. At this stage, the incident power was adjusted to 0.3 kW/kg of alcogel initially introduced and samples were collected at various drying times (33 min, 39 min, 46 min, 51 min and 55 min) corresponding to a total power absorbed by the system of 0.225 Wh, 0.233 Wh, 0.234 Wh, 0.236 Wh and 0.238 Wh per kg of alcogel initially introduced in the drier. The surface temperatures recorded during the drying of the various samples were between room temperature (recorded during the first 20 min of drying) to 78 C. (recorded at the end of the drying). The contents in volatile components of these samples were respectively 16%, 2.5%, 1.6%, 1.0% and 0.9%. The bulky densities of these samples were respectively 0.249 g/cm.sup.3, 0.086 g/cm.sup.3, 0.078 g/cm.sup.3, 0.078 g/cm.sup.3 and 0.078 g/cm.sup.3. The thermal conductivity values measured on granules having a size of between 1 mm and 1.2 mm, using the guarded hot plate method of standard NF EN 12667 at 20 C. and at atmospheric pressure, were respectively 45.8 mW/mK, 19.7 mW/mK, 17.6 mW/mK, 18.0 mW/mK and 18.1 mW/mK.

(20) 3c) Obtaining a Hydrophobic Silica Xerogel by Dielectric Drying at Atmospheric Pressure

(21) The condensed hydrophobic silica alcogel previously obtained was dried in a microwave drier at atmospheric pressure, in a stream of nitrogen having a flow of 1.0 L/min, under gentle stirring, and by applying an incident power of 6.7 kW/kg of alcogel. After 3.5 min of drying, the power reflected by the system was 2.7 kW/kg of alcogel initially introduced. At this stage, the incident power was adjusted to 1.85 kW/kg of alcogel initially introduced for 45 min, until the reflected power of the system was 2 kW/kg of alcogel. During the drying, the surface temperature of the sample was between room temperature and 50 C. for the first 3.5 min of the drying and between 50 and 78 C. during the last 45 min of this drying. The silica xerogels obtained were in the form of translucent granules having dimensions comprised between about 0.1 and 10.0 mm. The bulky density of the bed of granules thus obtained was 0.076 g/cm.sup.3. The thermal conductivity value measured on granules having a size of between 1 mm and 1.2 mm, using the guarded hot plate method of standard NF EN 12667 at 20 C. and at atmospheric pressure, was 18.0 mW/mK.

Example 3

Preparation of a Self-supporting, Single-layer, Composite Panel of 25 Mm Thickness with No Pre-drying Step

(22) 1) Preparation of a Composite Silica Alcogel

(23) A silica sol obtained in the same conditions as example 1 by hydrolysis of alkoxysilane in the presence of hydrochloric acid followed by the addition of ammonia, was poured before gelling onto a nonwoven fibrous batting in sheep wool (85% wool and 15% PET) of size 11010030 mm.sup.3 previously placed in a closed chamber of size 12012070 mm.sup.3. After gelling, the reinforced alcogel was aged for 20 h at 60 C. The solvent released during the ripening step (ageing) was then removed by decantation. Hydrochloric acid and hexamethyldisiloxane (hydrophobizing agent) in a weight ratio of 3:97 were then added to the reactor so as to fully cover the composite alcogel. The reaction medium was heated and held at 60 C. for 20 h. The reaction medium was then separated from the reinforced, hydrophobic silica alcogel by percolation.

(24) 2) Obtaining a Panel of Hydrophobic Composite Silica Xerogel.

(25) The alcogel reinforced with the nonwoven fibrous batting was dried directly in a ventilated oven at 140 C. for 2 hours. The xerogel panel obtained measured 25 mm in thickness and displayed a thermal conductivity of 32.2 mW/m.Math.K, measured using the guarded hot plate method of standard NF EN 12667 at 20 C. and at atmospheric pressure.

(26) This example clearly shows that the pre-drying step is necessary to obtain a thermal conductivity value equal to or less than 25 mW/m.Math.K for a self-supporting, single-layer, composite xerogel panel.

Example 4

Preparation of a Silica Xerogel According to the Invention

(27) 1) Preparation of a Hydrophobic Silica Alcogel

(28) A silica sol obtained in the same conditions as example 1 by hydrolysis of alkoxysilane in the presence of hydrochloric acid was gelled with ammonia after being placed in a cylindrical reactor equipped with a candle casting mould and having a characteristic distance of 6 cm. After an ageing phase of 4 h under ethanol refluxing, hydrochloric acid and hexamethyldisiloxane (3:97) (hydrophobizing agent) were added to the reactor so that they fully covered the silica alcogel. The reaction medium was heated and held under reflux for 4 h. The reaction medium was then separated from the hydrophobic silica alcogel by percolation.

(29) The hydrophobic silica alcogel (250 g) thus obtained was transferred to a crystallizing dish for drying according to the invention.

(30) 2) Preparation of a Condensed Silica Alcogel

(31) The crystallizing dish containing the 250 g of hydrophobic silica alcogel was placed in a ventilated oven and the sample was dried at 80 C. until it had lost about 50% of its initial weight.

(32) 3) Obtaining a Silica Xerogel

(33) The condensed hydrophobic silica alcogel previously obtained was dried in a ventilated oven at 160 C. for 60 minutes. The bed of hydrophobic silica xerogels obtained had a bulk density of 0.06 g/cm.sup.3 and the size of the xerogel granules obtained was between about 0.1 and 10 mm. The thermal conductivity value, measured on granules having a size of between 1 mm et 1.2 mm using the guarded hot plate method of standard NF EN 12667 at 20 C. and at atmospheric pressure, was 19.8 mW/m.Math.K.

Example 5

Preparation of a Self-Supporting, Insulating, Single-Layer Composite Panel of 40 Mm Thickness According to the Invention

(34) 1) Preparation of a Composite Hydrophobic Silica Alcogel

(35) A silica sol obtained in the same conditions as example 1 by hydrolysis of alkoxysilane in the presence of hydrochloric acid followed by the addition of ammonia, was poured before gelling onto a nonwoven fibrous batting in polyethylene terephthalate (PET) of size 10010040 mm.sup.3 in a closed chamber of size 12012070 mm. After gelling, the reinforced alcogel was aged for 4 h under ethanol refluxing. Hydrochloric acid and hexamethyldisiloxane (3:97) (hydrophobizing agent) were then placed in the chamber so as to fully cover the composite alcogel. The reaction medium was heated and held under reflux for 4 h. The reaction medium was separated from the hydrophobic silica alcogel by percolation.

(36) 2) Preparation of a Reinforced Condensed Alcogel

(37) The reinforced hydrophobic silica alcogel was placed in a ventilated oven and dried at 80 C. for 1 h20 until it lost about 50% of its initial weight.

(38) 3) Obtaining a Panel of Hydrophobic Composite Silica Xerogel

(39) The condensed alcogel reinforced with the nonwoven fibrous batting was dried in a ventilated oven at 160 C. for 2 h15. The xerogel panel obtained measured 40 mm thick and displayed a thermal conductivity of 15 mW/m.Math.K, measured using the guarded hot plate method of standard NF EN 12667 at 20 C. and at atmospheric pressure.

(40) The trend in thickness of the composite throughout drying is illustrated in FIG. 1. A so-called spring effect is observed.

Example 6

Preparation of a Self-supporting, Insulating, Single-layer Composite Panel of 25 Mm Thickness According to the Invention

(41) 1) Preparation of a Composite Hydrophobic Silica Alcogel

(42) As for example 6, a silica sol obtained by mixture of a solution of polyethoxydisiloxane in ethanol and ammonia, was poured before gelling onto a nonwoven fibrous batting in PET of size 30026025 mm.sup.3 in a closed chamber. After gelling, the reinforced alcogel was aged for 4 h under ethanol refluxing. Hydrochloric acid and hexamethyldisiloxane (3:97) (hydrophobizing agent) were then placed in the chamber so as to fully cover the composite alcogel. The reaction medium was heated and held under reflux for 4 h. The reaction medium was separated from the hydrophobic silica alcogel by percolation.

(43) 2) Preparation of a Reinforced Condensed Alcogel

(44) The reinforced hydrophobic silica alcogel was placed in a ventilated oven and dried at 80 C. for 1 h20 until it lost 48% of its initial weight.

(45) 3) Obtaining a Panel of Hydrophobic Composite Silica Xerogel

(46) The condensed alcogel reinforced with the nonwoven fibrous batting was dried in a ventilated oven at 160 C. for 2 h. The xerogel panel obtained measured 25 mm thick and displayed a thermal conductivity of 14 mW/m.Math.K, measured using the guarded hot plate method of standard NF EN 12667 at 20 C. and at atmospheric pressure.