RU2513809C2 - Production of vacuum expanded glass - Google Patents

Abstract

FIELD: process engineering.

SUBSTANCE: invention relates to production of heat-insulation foamed materials. Glass powder is heated in helium medium at 0.2-0.5 MPa to 800°C. Molten glass mass is held in heating zone to saturation with helium. Then, gas pressure is decreased to 0.01-0.1 kPa to allows foaming of glass medium by dissolved gas. Rarefaction in gas is maintained to cool down said glass mass to vitrification stage.

EFFECT: low heat conductivity foamed glass with rarefied helium in its bubbles.

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Description

The invention relates to the field of manufacturing heat-insulating foam materials that can be used in energy-saving construction, cryogenic technology, for thermal insulation of refrigerated trucks and in other industries related to thermal protection. The method describes the production of foam glass, in the pores of which there is gas at a pressure below atmospheric, which ensures low thermal conductivity of the material.

Known methods in which finely divided silicate glass (particles 2-10 μm) are mixed with a blowing agent (for example carbon), the resulting homogeneous mechanical mixture (charge) is fed in molds or on a conveyor belt to a tunnel kiln. As a result of heating to 800-900 ° C, the glass particles soften to a viscous-liquid state, and carbon is oxidized with the release of gaseous CO ₂ and CO, which froth the viscous-liquid glass mass and form foam glass [1].

Known methods do not allow to obtain materials with thermal conductivity below 0.04-0.05 W / (m · K).

Known methods for producing vacuum foamed materials.

A known method of obtaining a vacuum insulating panel of a solid porous material (Pat. US 5989371, 1999; Pat. US 6266941, 2001). The porous base of the panel is formed from a mixture containing polyurethane foam powder and a filler in the form of a powder of thermoplastic resin. The mixture is placed in a mold and subjected to compression compression in a vacuum environment at a temperature not lower than the glass transition temperature of the polyurethane and not lower than the melting temperature of the thermoplastic resin, after which it is sealed in a packaging material, inside which the vacuum is preserved, and the porous base maintains the given panel shape. As a filler, mica flakes, plastic films can be used. As a thermoplastic resin, styrene resin can be used. During molding, vibration may be used. A similar method for producing a vacuum heat insulating material, but without compression, is described in US Pat. No. 5,888,067.

According to US Pat. No. 7,166,348, the pasty material is extruded into the low pressure region to cause pore formation, and a porous material with open porosity is obtained, the surface of which is then coated with a packaging barrier film at a pressure of about 10 Pa.

It is known that heat transfer through a porous material is carried out both through a connected array of a solid phase and through a disconnected or weakly connected gas phase filling gas bubbles. The thermal conductivity of the gas phase depends on the mean free path of the gas molecules, i.e. from gas pressure. The open porosity used in patent patents is characterized by the presence of open pores consisting of a network of capillaries, channels and cracks communicating with each other and with the surface of the material, i.e. the gas phase forms a cohesive region.

A disadvantage of the known methods for producing vacuum panels is that maintaining a reduced gas pressure in a material with open porosity is very problematic. The heat-shielding properties of the material are easily violated if the packaging barrier is damaged.

The closest in technical essence to the present invention is a method for the manufacture of heat-insulating foam materials according to the patent for invention RU No. 2332364 according to application No. 2006101087, based on the passage of superheated steam through a glass melt. Overheated steam is passed through a glass melt at a temperature of 950-1200 ° С, obtained from substances that can be adsorbed upon cooling on the walls of closed bubbles, the foam is extruded into molds, cooled with a decrease in vapor pressure in the bubbles below atmospheric due to their condensation, removed from the molds and annealed by smooth cooling in a thermal insulation cabinet. Taken as a prototype.

The disadvantages of this method is the technological complexity of producing foam glass, in particular bubbler melt superheated steam. The porosity of the foam glass, achieved in a known manner, is low, as is the degree of rarefaction of the gas in the pores, which prevents the achievement of low thermal conductivity.

To solve this problem, it is necessary to obtain a material with closed porosity with gas pressure in the gas bubbles of foam glass below atmospheric.

To do this, in a known method for producing vacuum foam glass, including heating the glass powder and foaming the molten glass melt, the glass powder is heated to a melting point in helium medium at a gas pressure of 0.2-0.5 MPa, the molten glass melt is kept in the heating zone, saturating the glass melt with gas due to adsorption. Then the excess gas pressure is released and the remaining gas is pumped out with a vacuum pump to a pressure of 10-100 Pa, due to which the glass melt foams with dissolved helium. Maintaining the achieved vacuum in a gaseous medium, cool the foamed glass to the glass transition stage. After cooling to room temperature, a block of foam glass with incoherent porosity is obtained, which can be mechanically processed.

The signs of the method, common with analogues and prototype:

a) particles of ground glass are heated to the stage of melting;

b) a substance is introduced into the molten glass that can form nuclei of disconnected gas bubbles (pore formation centers). In the method of the invention, dissolved helium serves as such pore formation centers;

c) the expanded molten glass is cooled to the glass transition stage;

g) receive a final gas pressure in the pores below atmospheric.

The features of the method according to the invention, distinguishing it from the prototype method, in which the glass melt is blown with steam:

a) instead of superheated water vapor, soluble gas - helium is used to form bubbles;

b) the melting stage is carried out under excess gas pressure;

C) in the prototype, the pressure of water vapor in the pores below atmospheric is achieved due to the binding of water with highly hygroscopic molecules in the walls of the bubbles. In the method according to the invention, vacuum in the pores is achieved by glass transition of the melt in vacuum.

The method is carried out in several stages as follows. The glass powder is placed in a gaseous medium under a pressure of several atmospheres, and then subjected to heat. Under the action of heat, the glass melts, and the gas in the pores dissolves in the viscous glass. The solubility of helium in a glass melt is about a hundred times higher than that of argon and other inert gases, which makes it a good foaming agent.

The implementation of the method is illustrated by an example.

Example: To obtain foam glass, finely dispersed glass powder with a particle size of less than 200 microns is used. Glass powder can be obtained, for example, by melting a mixture of the following composition, wt.%:

lead acetate 87.2 boric acid 10.0 quartz sand 2,8

The mixture is maintained at a temperature of approximately 900 ° C for an hour. The resulting molten glass is cooled, ground in a mill and sieved through a 200 micron sieve.

Stage 1. A portion of the glass powder was poured into a form that can withstand high temperature, such as a graphite crucible. The mold was placed in a container that can withstand high pressure, such as an autoclave. A helium medium was provided in the vessel at elevated pressure by supplying gas from a cylinder through a gas reducer.

Stage 2. The glass powder, in the pores of which there is a soluble gas helium at a pressure of from 0.2 to 0.5 MPa (approximately 2 to 5 atm), was placed in an oven, heated to a melting point, which is approximately 600-800 ° C. .

Stage 4. Then, the excess gas pressure was vented through the gas valve, and the remaining gas was pumped out with a foreline pump with a capacity that provided foaming by expanding the dissolved gas contained in the glass mass.

Stage 5. The foamed mass was cooled in vacuum to the glass transition stage.

The process proceeds as follows. At the melting stage, the fine powder is saturated with a gas under pressure. During melting, glass particles become visco-plastic, and helium saturates the melt under pressure. In this case, the particles of the glass powder melt together, part of the gas in the powder forms gas bubbles, due to the developed gas-liquid contact surface, part of the gas is intensively adsorbed by the glass and dissolves in the melt. The method does not require the addition of any foaming agents, as is done in traditional methods for producing foam glass, since the centers of pore formation are helium dissolved in viscous glass melt. The saturation time of viscous glass melt with gas depends on the properties of the glass used to prepare the glass powder, temperature, melt viscosity and is selected empirically. With a subsequent decrease in the pressure of the gaseous medium by venting the gas to atmospheric pressure and pumping it out using a fore-vacuum pump, the dissolved helium expands and remains in the glass mass in the form of discharged gas inclusions. With a rapid depressurization and a sufficiently low melt viscosity, which can be controlled by temperature, the gas molecules dissolved in the glass mass act as pore formation centers, foaming occurs, i.e. in the glass, a lot of incoherent pores are formed. In conditions of low, the so-called technical vacuum with a pressure below 1 mmHg (0.01-0.1 kPa), the thermal conductivity of such pores is lower than the thermal conductivity of pores, for example, with carbon dioxide, the formation of which is often used to foam foam glass in analogous methods. After cooling to the glass transition stage, a strong porous mass is formed, the pores of which contain helium at a pressure below atmospheric.

The technical result is to obtain a thermal insulation material - foamed glass of low thermal conductivity, in the bubbles of which there is rarefied helium. The estimated coefficient of thermal conductivity of the material is approximately 0.02 W / (m · K), which is lower than that of a material with the same porosity obtained by the analogous method.

Information sources

1. Demidovich B.K. Foam glass. - Minsk: Science and Technology, 1975.

2. Pat. US 5989371, 1999; Vacuum heat-insulating panel and method for producing the same.

3. Pat. US 5889067, 1999; Open cell rigid polyurethane foam and method for producing the same and method for making vacuum insulation panel using same.

4. Pat. US 6266941, 2001. Vacuum heat-insulating panel and method for producing the same.

5. Pat. US 7166348, 2007. Core material for vacuum heat insulation material, and vacuum heat insulation material.

6. Patent for invention RU No. 2332364. Publ. 08/27/2008. Prototype.

Claims (2)

1. A method of producing vacuum foam glass, comprising heating the glass powder and foaming the molten glass melt, characterized in that the glass powder is heated in a helium medium at a pressure of 0.2-0.5 MPa to a melting point, preferably up to 800 ° C., the molten glass is held in the heating zone until it is saturated with helium, then the pressure of the gaseous medium is reduced to 0.01-0.1 kPa, providing foaming of the glass melt with dissolved gas, after which the achieved vacuum in the gaseous medium is maintained and the glass melt is cooled to and glass. 2. The method according to claim 1, characterized in that the particle size of the glass powder does not exceed approximately 200 microns.