Thermal and/or acoustic insulation materials shaped from silica

Abstract

Thermal and/or acoustic insulation materials based on dried precipitated silica, having a total pore volume of from 1 to 5 cm3/g and optionally containing reinforcing fillers and/or opacifying agents, are prepared by: (A) filtering an aqueous dispersion D containing precipitated silica particles in a filter press, whereby a compacted filter cake is obtained; and then (B) drying the filter cake in the compacted state as obtained after step (A).

Claims

1. A method of preparing a thermal and/or acoustic insulation material based on dried precipitated silica, comprising: (A) filtering an aqueous dispersion D containing precipitated silica particles in a filter press, whereby a compacted filter cake is obtained by a compacting operation performed at a pressure of about 7 bar or less; and (B) drying the filter cake in the compacted state as obtained after step (A) wherein the thermal and/or acoustic insulation material has a content of dried precipitated silica comprising at least 50% by weight of the insulation material and wherein the thermal and/or acoustic insulation material has a total pore volume of between 1 cm.sup.3/g and 5 cm.sup.3/g, at least 40% of the total pore volume consists of pores smaller than 1,000 nm and at least 50% of the pore volume of pores smaller than 1,000 nm consists of pores smaller than 100 nm.

2. The method of claim 1, wherein the compacting operation is performed at a pressure of between about 2 and about 7 bar.

3. The method of claim 1, wherein step (A) comprises: (A1) a filtration operation at a pressure of about 0.5 to about 2 bar; and (A2) the compacting operation carried out on the filter cake obtained at a pressure of between about 2 and about 7 bar.

4. The method of claim 1, wherein the compacted filter cake obtained after step (A) has a solids content of between 10 and 35% by weight.

5. The method of claim 1, wherein the aqueous dispersion D used in step (A) contains a precipitated silica which, once dried, has a BET specific surface area of between 80 and 400 m.sup.2/g and a CTAB specific surface area of between 80 and 350 m.sup.2/g.

6. The method of claim 1, wherein the aqueous dispersion D used in step (A) further contains a reinforcing filler.

7. The method of claim 6, wherein said reinforcing filler comprises reinforcing fibers selected from aluminum silicate fibers, alumina fibers, mineral wool fibers, glass fibers, quartz fibers, ceramic fibers, polymer fibers and cellulose fibers.

8. The method of claim 6, wherein the (silica/reinforcing filler) mass ratio within the aqueous dispersion D is between 75/25 and 99/1 by weight.

9. The method of claim 1, wherein the aqueous dispersion D used in step (A) further contains an opacifying agent capable of reflecting, absorbing and/or dispersing at least part of the infrared radiation.

10. The method of claim 9, wherein the opacifying agent is selected from the group consisting of chromium oxide, zirconium oxide, iron oxide, titanium dioxide, manganese dioxide, ilmenite, quartz powder, silicon carbide, boron carbide, tantalum carbide, carbon black and graphite.

11. The method of claim 9, wherein the (silica/opacifying agent) mass ratio is between 50/50 and 99/1 within the aqueous dispersion D.

12. The method of claim 1, wherein step (B) is carried out by allowing the compacted filter cake obtained after step (A) to dry at a temperature of between 10 and 30 C.

13. The method of claim 1, wherein step (B) is carried out by subjecting the compacted filter cake obtained after step (A) to a progressive temperature rise from room temperature up to a temperature of at least 100 C., at a rate of temperature rise of less than 2 C. per minute, optionally with the temperature being held at one, two or more intermediate temperature levels.

14. The method of claim 1, wherein the pressure in step (A) is less than about 6 bar.

15. The method of claim 1, wherein the insulation material comprises at least 75% by weight of the dried filter cake.

16. The method of claim 1, wherein the insulation material has a total pore volume of at least 2.0 cm.sup.3/g, and wherein at least 70% of the total pore volume consists of pores smaller than 1,000 nm.

17. The method of claim 1, wherein at least 70% of the pore volume of pores smaller than 1,000 nm consists of pores smaller than 100 nm.

Description

EXAMPLE 1: MANUFACTURE OF AN INSULATING SILICA PANEL

(1) Introduced into a 25 liter stainless steel reactor were 6.7 l of an aqueous sodium silicate solution having an SiO.sub.2/Na.sub.2O mass ratio (R.sub.w) of 3.48 and an SiO.sub.2 concentration of 5 g/l. The solution was then stirred and heated to 80 C. While keeping the temperature at 80 C., an aqueous sulfuric acid solution with a concentration of 80 g/l was added until the pH of the mixture reached a value of 4.

(2) While still keeping the mixture at 80 C., the following were introduced simultaneously into the reactor: an aqueous sodium silicate solution (S1) having a concentration of 230 g/l and an R.sub.w of 3.48, with a flow rate of 50 g/minute; and an aqueous sulfuric acid solution (S2) of 80 g/l concentration, with a regulated flow rate so as to keep the pH reaction mixture at a value of 4 over the entire duration of simultaneous addition of the sodium silicate and the sulfuric acid.

(3) The simultaneous addition of the solutions (S1) and (S2) under the aforementioned conditions was carried out over 80 minutes.

(4) After 80 minutes of simultaneous addition, the introduction of the solution S2 was stopped and the introduction of the solution (S1) was continued until the pH of the mixture reached a value of 8.

(5) Once the pH reached 8, a further combined addition of the solutions (S1) and (S2) was carried out over 20 minutes, again with a flow rate of the solution S1 of 50 g/minute and a flow rate of the solution (S2) regulated so as to keep the pH of the reaction mixture constantly at a value of 8 throughout the duration of this second simultaneous addition.

(6) After this second simultaneous addition for 20 minutes, the addition of the solution (S1) was stopped and the reaction mixture brought to a pH of 4 by the addition of sulfuric acid solution (S2). During all these steps, the mixture was kept at a temperature of 80 C.

(7) After these various reactions, a reaction slurry was obtained, from which 250 ml samples were taken.

(8) Added to these 250 ml reaction slurry samples, with stirring, were 2 ml of an FA10 solution (polyoxyethylene with a molecular weight of 510.sup.6 mol/g) with a concentration of 1% by weight. The mixture obtained was introduced into a filter press with an inside diameter of 7 cm. A pressure of 1 bar was applied so as to carry out a first operation to remove the water contained in the reaction slurry. A cake was thus obtained which was then washed twice with 150 ml of demineralized water with a pressure exerted during the washing of 1 bar. After these washing steps (carried out in the filter press), the pressure of the filter press was increased to 5 bar and this pressure was maintained for 2 minutes. A silica cake characterized by a solids content of 22% by weight was thus obtained. This concentrated cake was demolded from the filter press and left to dry. The cake thus formed was left to dry at room temperature and demolded.

(9) After 15 days, a dried cake having a solids content of 95% was obtained, this cake constituting a silica panel having the following characteristics: density: 0.27; total pore volume: 3.63 cm.sup.3/g with the following pore distribution: pores with a size of greater than 100 nm: 31% pores with a size of between 100 and 1000 nm: 29% pores with a size of less than 100 nm: 40%.

EXAMPLE 2: PREPARATION OF AN INSULATING SILICA PANEL

(10) A silica cake was produced according to the method described in example 1 of EP 520 862. The cake, as obtained after the filtration step in a filter press, which had a solids content of 21%, was diluted with water so as to obtain a silica suspension or slurry having a solids content of 13.5% by weight, and the pH of this silica suspension was increased to 5 by the addition of sulfuric acid (80 g/1 aqueous solution).

(11) A dispersion of 1 g of polyamide fibers, with a mean length of 4 mm and a mean diameter of 200 microns, was added, with stirring, to 100 g of the silica suspension prepared beforehand (with a solids content of 13.5% by weight and a pH of 5).

(12) The mixture obtained was then filtered in the filter press used in Example 1 above (filter press with an inside diameter of 7 cm). A compacting pressure of 5 bar was applied, after which a compacted cake having a solids content of 27% by weight was obtained. The formed cake thus obtained was demolded and left to dry at room temperature as in the previous example. After 10 days, a dried cake with a solids content of 95% was obtained. Thus, a silica panel having a good mechanical strength and a density of 0.4 was obtained.

(13) The silica panel obtained had a total pore volume of 3.3 ml/g, with a pore volume of pores smaller in size than 100 nm of 1.2 ml/g.

EXAMPLE 3: THERMAL CONDUCTIVITY MEASUREMENTS

(14) Thermal conductivity measurements were carried out using a TC-meter on the two silica panels of Examples 1 and 2. The principle used was that called the thermal shock probe, which is similar to the method used according to the ISO 8814-1 standard.

(15) More precisely, the thermal conductivity coefficient of the silica panels of Examples 1 and 2 was measured at various pressures in the following manner: a thermal probe was placed within the material to be characterized.

(16) After having thermally stabilized the mixture to room temperature (20 C.), the thermal equilibrium of the material was disturbed by generating a heat flux by means of the heating element of the probe.

(17) Using a temperature sensor integrated into the probe introduced into the material, the temperature behavior of the material was recorded. A temperature behavior corresponding to an equation of the type T(t)=Q/(4)(ln(t)+A) was observed, in which: T denotes the temperature in C. recorded by the temperature sensor; t represents the time in seconds; Q represents the heat flux in W/m; denotes the thermal conductivity of the material in W/m/K; and A represents a constant.

(18) By linear regression, the thermal conductivity of the material was determined at various pressures.

(19) The measurements were carried out on each of the materials at increasing pressure, with 5 different pressures ranging from 0.03 mbar (3 Pa) up to atmospheric pressure.

(20) The results obtained are given in Tables 1 and 2 below.

(21) TABLE-US-00001 TABLE 1 Measurement of the thermal conductivity of the material of Example 1 Pressure (Pa) Thermal conductivity (mW/m/K) 3 14.6 100 16.0 1010 1.1 20600 32.3 99600 38.8

(22) TABLE-US-00002 TABLE 2 Measurement of the thermal conductivity of the material of Example 2 Pressure (Pa) Thermal conductivity (mW/m/K) 3 12.7 100 22.3 1000 31.5 23200 39.4 99500 45.4