Material for heat and electrical insulation with a capacity of the selective absorption of electromagnetic radiation spectrum and vibration, its production process and use

Technical field of Invention

The invention pertains to the field of technology for the production of heat and electrical insulation materials, resistant to high temperatures up to 1100 °C, based on calcium hydrosilicates belonging to the following classes of the International Patent Classification: C 04B 35/22 and C 01B 33/24. More specifically, the present invention pertains to a material according to the preamble of claim 1. Such a material is for example known from YU-P-553/85.

Technical problem

The technical problem solved by the invention is to provide a new material as well as its production process; said new material being resistant to temperatures up to 1100 °C, with a heat insulation capacity, having a mechanical strength of up to 32 kg/cm², the capability of selective absorption of electromagnetic waves, radiation and vibrations, good electrical resistance, water resistance, resistance to acids and frost, thermal shocks and various industrial combustion gases.

State-of-the-Art

Heat insulation materials based on calcium hydrosilicates and fibrous raw materials are already known. The commercial titles under which they are offered to the market are: Marilite 1, Monolux 40, Monolux 500, Monolite, .... Vidasil.

Furthermore it is known from WO86/05918 to obtain calcium hydrosilicate having the tobermorite structure by treating a water-based composition of line and quartz under hydro thermal conditions.

All known materials based on calcium hydrosilicates may be divided into two categories. Materials belonging to the first category are those having technical characteristics ranging in the following intervals of values:

volume density

600 - 850 kg/cm³

thermal conductivity

0.2 - 0.35 W/m °C

maximum working temperature

150 - 850 °C

modulus of rapture

5 - 17 MN/m²

Neither thermal shock resistance, nor resistance to water, vibration etc., have been registered for this first category of known substances.

The heat insulating material based on a calcium hydrosilicate manufactured under the production process described in YU patent application no. P-553/85, falls into the second category. This heat insulating material represents a new quality among products based on calcium hydrosilicates, both for its homogeneous mineral composition and for its technical properties which are, judging by their number and value, a novelty in the area of calcium hydrosilicate based materials. Its properties have extended the application of calcium hydrosilicates to fields in which, prior to its appearance, the use of such a kind of material was not thought possible.

The mechanical properties of the calcium hydrosilicates produced in accordance with YU pat. application no. P-553/85, as well as numerous other properties, such as fire resistance, have become challenging. Thus, this material has been fitted into the most diverse places and under the most diverse conditions, offering a satisfactory solution for numerous technical problems.

Applying this heat insulating material, many consumers have noted that an improvement of its mechanical strength would provide greater application possibilities. With more frequent use, stricter requirements had been set before it, in an effort to find a material that would, possibly, have the same scope of characteristics, but with increased values.

The technical characteristics of the calcium hydrosilicate material containing tobermorite having an A-axis of 1.13 nm (11.3 Å) and well-known in the present state-of-the-art which is produced in accordance with the process described in YU pat. application no. P-553/85 are as follows:

non-combustibility /JUS U1.040 and JUS U.J1.060/, volume density /ASTM C 303/ from 220 to 317.5 kg/m³ -10 compression strength at 5% deformation /ASTM C 165/:

20°C

600°C

900°C

1.60 MPa

1.40 MPa

1.22 MPa

bending strength /ASTM C 203/ 0.91 MPa

thermal conductivity /JUS U.J5. 040/ depending on the temperatures:

mean temperature °C

W/m K

50

0,079

149

0,107

427

0,157

linear shrinkage at a temperature of 860°C /ASTM C 356/

shrinkage per length

2.0%

shrinkage per width

2-0%

shrinkage per thickness

2.15%

weight loss

9.1%

strength of bolt support /ASTM D-1037, ASTM C-656/:

transverse force of bolt arrest is 25 kg

resistance to rapid temperature changes /DIN 51068/:

resists temperature shocks 20⁺

Description of solution

The proposed material according to the invention is made by use of a new process for obtaining a calcium hydrosilicate material, resistant to high temperatures, having universal insulation qualities and a good mechanical strength, as well as resistance to thermal and other shocks.

The proposed process differs from that disclosed in YU pat. application no. 553/85. The material according to the invention represents a new member in the family of materials of this kind, with significant differences in the values of the technical characteristics.

The new process allows the production of a new material, the thermo-physical properties of which are twice as good as those pertaining to the calcium hydrosilicate produced in the process designed by the same author, and filed under YU patent application no. P-553/85. The values of other technical characteristics are also increased. Some of them that are hardly present in the calcium hydrosilicate tobermorite having an A-axis of 1.13 nm (11.3 Å) produced in accordance with YU Pat. application no. 553/85, are increased in proportion to other properties, giving the material new values in use. The difference lies in the ratio of the constituents of the process CaO:SiO₂, as well as in the altered concentration of additives, and in the relationship between the liquid and solid phase, in the change of temperature, i.e. of pressure at which the reaction between CaO and SiO₂ takes place, and in the duration of this reaction, as well as in the change of the fiber length and quantity of water used. By increasing the temperature and finding an optimum quantity of liquid in relation to the solid phase, better conditions have been obtained for a synthesis reaction of calcium hydrosilicate tobermorite having an A-axis of 1.13 nm (11.3 Å). It is interesting to note that the form of tobermorite mineral crystals in the material according to the invention is similar to that of sea corals. This crystalline form in the newly created tobermorite phase, as well as the total degree of crystallization of this phase, considerably effect the technical characteristics. An even distribution and other properties of the fibrous components should also be added to this.

Apart from these significant positive effects on the quality, the proposed process is more economical in the sense that it provides a greater degree of nominal equipment capacity exploitation, as well as of energy saving, because the pulp contains a smaller quantity of liquid phase which, besides being a part of the product, also serves as a medium for the development of the chemical synthesis of tobermorite. It is certain that these findings are not ultimate, and that each knowledge requires a verification and affirmation of its role in respect to the quality of the material.

Through the new process, a material is obtained, the technical properties of which are strictly reproductional, and are within a tolerance range equal to the scope of incoming parameters.

The process according to the invention is conducted in the following manner:

A natrium carboxymethyl cellulose solution is prepared, and is successively dosed by Ca/OH/₂ and quartz powder. Then, a suspension of water and fibers is added to this suspension, and mixed. The obtained pulp is poured into dies which are then subject to treatment with saturated water vapor of 11.0 - 12.5 bars, for a period of 14 to 28 hours. The material is removed from the cooled dies and is air-dried at temperatures up to 250 °C.

The process of pulp hardening until reaching the strength of the new material is the result of the following chemical reaction: $${\text{5 Ca/OH/}}_{\text{2}}{\text{+ 6 SiO}}_{\text{2}}{\text{+ 5.5 H}}_{\text{2}}{\text{O q/J/ → 5 CaO + 6 SiO}}_{\text{2}}{\text{+ 10.5 H}}_{\text{2}}\text{O /tobermorite/}$$

The technical properties of the material depend on the degree of the transition of CaO and SiO₂ into the new mineral form, but apart from the quantity of the newly formed tobermorite phase, the properties of the material are also influenced by the form and size of the crystals in the new phase, its distribution in the product, as well as the overall microstructure of the obtained material. In order to achieve continuity of the chemical reaction for the production of calcium hydrosilicate out of the solution, it is necessary to have a liquid phase provided with sufficient quantities of Ca and SiO₂ ions, at all times, as well as with such quantities of energy necessary for creating the new mineral phase.

As the mechanism of the reaction for the formation of calcium hydrosilicate, tobermorite lacks clarity from a theoretical point of view. However, one thing is certain, namely, that it is possible to prove by this process that the formation of tobermorite runs directly, and that the state of the formed phase may range from amorphous to exceptionally fine crystalline. It has also been shown that the micro shape of the crystal may be in the form of sheets or fibers arranged in a radial or uneven crossed pattern. The mobility of the liquid phase in the synthesis period seems to be very important, as well as the fact that it always contains Ca and SiO₂ ions, capable of bonding and remaining bonded. The role and quantity of the liquid phase becomes crucial, and the texture of the material is in function of the microstructure of the new phase, of pore size and distribution. These are only the basic parameters effecting the establishment of technical properties of the material, and it is probable that in this process, not all the inter-relation between the process parameters and the quality of the synthesis of the new material have been detected.

The organic fibers that may be used in the process are of cellulose composition, approximately 5 mm long.

Among the inorganic fibers chrisotile asbestos with 6 mm long fibers, may be used. Furthermore, medium class vermiculite may be used and all other mineral fibers showing resistance in an alkaline medium at a temperature of 200 °C.

The material according to the invention has a wide range of applications, which proves that it incorporates a number of necessary properties, which make it fit to be built into various places in which calcium hydrosilicate material had never before been used:

• As the surface layer of industrial plant hearths in which light fuel oil, propane-butane gas and natural gas are burnt.
• As a protective coating of steel constructions.

However, with the appearance of the calcium hydrosilicate, produced in accordance with YU pat. application no. P-553/85, the requirements set by consumers regarding their desire to have a calcium hydrosilicate material with improved mechanical properties were defined more clearly then ever.

The new type of calcium hydrosilicate with tobermorite having an A-axis of 1.13 nm (11.3 Å), according to the invention /Vidasil 400/, fulfills all the strictest requirements for fire resistant materials built into housing constructions, public buildings and public transportation means.

The technical properties of this new type of calcium hydrosilicate, Vidasil 400, will help solve problems related to protection against natural electromagnetic radiation. The electrical resistance of this material goes up to 200,000 MΩ for 10 mm thick plates.

Its properties of not conducting heat and not burning, of enduring direct contact with molten metals and hot gases, provide excellent possibilities and widespread applications. This also justifies the belief that through different production processes, raw materials CaO and SiO₂, may serve for the manufacturing of materials with a very wide spectrum of technical possibilities.

Example 1

1.6 kg of hydrated lime containing 72 wt % of CaO and 1.632 kg of quartz powder with 96 wt % of SiO₂, and having a particle size of below 63 microns, were successively added to 9 liters of water containing 12 g of natrium carboxylcellulose. To this suspension, a mixture of 4 liters of water and 161 g of 4 mm long cellulose fibers was added. The mixed components were poured into a die dimensioned 0.6 x 0.2 x 0.2 m. The die containing the pulp was subject to saturated water vapor pressure of 11.0 - 12.5 bars, over a period of 18 hours. After the die had cooled, a block with a moisture content of approximately 67 mass % was removed from the die, and air-dried at a temperature of approximately 100°C.

Example 2

557.6 kg of calcium hydroxide containing 72 wt % of CaO and 565.8 kg of quartz powder containing 98 wt % of SiO₂, were successively added to 2600 liters of water, in which 5.3 kg of natrium carboxylcellulose had been dissolved. Into the obtained suspension, 1560 liters of water containing 56.13 kg of 5 mm long cellulose fibers was added. After thorough mixing, the pulp was poured into dies, which were then exposed to a pressure of 11.5 - 12.5 bars, over a period of 14 hours. After cooling, blocks having a moisture content of approximately 67 wt % were removed from the dies and air-dried at a temperature of 200 °C. Technical properties of the material for heat and electrical insulation according to the invention /Vidasil-400/ are as follow:

SHAPE: Flat plates and pipe profiles

DENSITY: At room temperature 400 kg/m³

BORDER TEMPERATURE OF APPLICATION IN CONDITIONS OF LONG EXPLOITATION IS 1000 °C

RAY DIFFRACTION ANALYSES: 1 Tobermorite 2 Quartz

THERMAL CONDUCTIVITY FOLLOWING THERMAL TREATMENT AT 900 °C

Temperature °C

Thermal Conductivity W/mK

Hot Face

Cold Face

Mean

201

27

114

0.118

400

41

220

0.127

800

91

448

0.166

SPECIFIC HEAT

Mean temperature °C

65,209,320,427,483

Specific heat J/kgK

721,771,866,894,928,947

PRESSURE RESISTANCE

T °C

At 5% deformation MPa

To maximum destruction MPa

room

3.28

4.30

over 900 °C

2.42

4.57

BENDING RESISTANCE

Temperature °C

MPa

PSi

room

1.48

215

over 900

0.61

88

RESISTANCE TO BOLT DRAWING

Temperature °C

Max Load /kg/

Max Load /kg/

room

32.7

0.32

over 900

30.4

0,3

THERMAL SHOCK RESISTANCE

The material when annealed at a temperature of 1100 °C "survived" a quench test in cold water. This test was carried out in order to obtain good quality results.

DETERMINATION OF THERMAL SHOCK RESISTANCE PARAMETER Rst

The Rst parameter value of 44.80 m^1/2 was obtained for material treated at 900 °C

ELECTRICAL RESISTANCE MΩ

Plate width

0.5 mm

20

Plate width

10 mm

200,000

MODULUS OF ELASTICITY

Temperature °C

GPa

PSi

room

1.57

227650

over 900

0,74

107300

SHRINKAGE AFTER 24 HOUR HEATING

Temperature °C

Shrinkage

Length

Width

Thickness

600

1.40

1.97

0.94

800

3.43

2.89

2.55

TOTAL OPEN POROSITY: 80 %

• /figure 1/ on the vibration damping characteristics and
• /figure 2/ on the infrared radiation transmission spectrum are presented