CN110746799A

CN110746799A - Heat radiation type heat insulation coating and preparation method thereof

Info

Publication number: CN110746799A
Application number: CN201911129800.2A
Authority: CN
Inventors: 黄祖炜; 张瑜瑜; 陈越; 李金钟
Original assignee: ASIA PAINT (SHANGHAI) Co Ltd
Current assignee: ASIA PAINT (SHANGHAI) Co Ltd
Priority date: 2019-11-18
Filing date: 2019-11-18
Publication date: 2020-02-04

Abstract

The invention relates to a heat radiation type heat preservation coating and a preparation method thereof, wherein the heat radiation type heat preservation coating comprises metal oxide, rare earth compound, inorganic composite aerogel, reflective heat insulation titanium dioxide, inorganic film forming matter, emulsion, pigment and filler, auxiliary agent and water. The metal oxide, the rare earth compound, the inorganic composite aerogel and the reflective heat-insulation titanium dioxide are synergistic, so that the coating has excellent heat-insulation performance under the conditions of alternate cold and heat in winter and summer and temperature difference between day and night, and the stability is high, and the fireproof performance, weather resistance, ageing resistance and stability of the coating are further improved by adding the inorganic film forming material.

Description

Heat radiation type heat insulation coating and preparation method thereof

Technical Field

The invention relates to the technical field of coatings, in particular to a heat radiation type heat insulation coating and a preparation method thereof.

Background

At present, most buildings are high-energy-consumption non-energy-saving buildings, and the buildings consume a large amount of energy in the aspects of heating, air conditioning, ventilation, hot water supply and the like during use, and the energy accounts for about 30-40% of the total energy consumption of human beings.

In order to maintain the indoor temperature of a building and reduce the energy consumption of an air conditioner so as to respond to the energy-saving requirement of the building, the conventional energy-saving scheme of the building on the market at present is an external thermal insulation system or an internal thermal insulation system of an external wall, and the core thermal insulation material is a flame-retardant molded polystyrene board for thermal insulation. Although the material has good heat insulation performance, the fire-proof grade of the material can only reach B1 grade (flame retardant), and the material cannot meet the fire-proof grade requirement of building materials in many occasions; the rock wool as a substitute material of the same kind has a fire-proof grade up to A (non-combustible), but the heat-insulating property is slightly inferior to that of a molded polystyrene board, and due to large volume, high water absorption and poor tensile strength, the rock wool is easy to fall off in practical application, and the potential safety hazard is extremely high.

For the heat-insulating coating system, related products on the market at present are mainly a reflective heat-insulating coating series. The main effect of the product is to achieve a certain heat insulation effect by reflecting infrared light in sunlight, but the product is only suitable for summer and daytime periods, and cannot achieve the heat insulation effect for night periods and winter periods. Considering that the heating area in the north in winter is large, the energy consumption is high, if the coating system can play the heat preservation effect of a part of molded polystyrene boards, the energy utilization rate of the building can be greatly improved.

For buildings, heat transfer from building walls to inside or outside of a room is the result of a combination of thermal, conductive and convective heat transfer. The heat transfer mode needs the existence of temperature difference, and the larger the temperature difference is, the more heat is transferred. In winter or summer, the inner and outer surfaces of the wall body are subjected to heat radiation heat transfer and convection heat transfer preferentially, and finally, the heat transfer is conducted.

Thermal radiation is divided into projection radiation (reflection) and emission radiation (spontaneous and stimulated). Although any paint or white cement and the like have the function of projecting radiation, the function of emitting radiation is weak, and the heat insulation effect is not obvious; the building heat-insulating paint has the main functions of emitting radiation and exchanging heat.

In summary, the prior art has the following disadvantages: (1) the conventional heat insulation board, such as a molded polystyrene board and a rock wool board, has poor fire resistance and cannot meet the requirement of higher fire resistance grade of building materials; the latter has a poor heat preservation effect, requires a larger thickness to meet the same energy-saving requirement, but has large volume weight, high water absorption rate, poor tensile strength, is easy to fall off, and has extremely high potential safety hazard in the practical application of high-rise buildings. (2) The conventional reflective heat-insulating coating has the main effect of achieving a certain heat-insulating effect by reflecting infrared light in sunlight, but is only suitable for summer and daytime periods, and cannot achieve the heat-insulating effect for night periods and winter periods.

CN106010005B discloses a heat-reflecting heat-insulating coating, which is prepared from the following raw materials: 10-20 parts of water, 10-20 parts of acrylic emulsion, 15-25 parts of hollow glass beads, 5-15 parts of pigment, 15-25 parts of titanium dioxide, 15-25 parts of aluminum silicate, 20-30 parts of calcium carbonate, 3-9 parts of sodium polyacrylate, 10-20 parts of heat insulation synergist and 0.5-5 parts of antifreeze. The heat-reflecting heat-insulating coating provided by the invention can effectively prevent the heat of the sun from accumulating on the surface of an object to be heated, can automatically radiate heat to cool, and has the advantages of excellent antibacterial property and wear resistance, convenience in construction and high efficiency, but has no good heat-insulating effect in winter and at night.

CN108117812A discloses a heat-radiation-blocking type heat-insulating coating for buildings, which comprises the following raw material components in percentage by weight: 30-40 parts of synthetic emulsion, 15-20 parts of water, 1-2.5 parts of propylene glycol, 0.5-1.5 parts of ethylene glycol, 0.5-1.5 parts of alcohol ester dodeca, 10-20 parts of titanium dioxide, 2-3 parts of kaolin, 0.5-1 part of flatting agent, 1-2 parts of thickening agent, 0.1-1 part of wetting agent, 0.1-1 part of pH regulator, 1-3 parts of inorganic composite aerogel, 1-2 parts of mica powder, 1-3 parts of barium sulfate, 5-7 parts of zinc oxide, 5-8 parts of aluminum hydroxide and 5-8 parts of magnesium chloride, and the composite material is prepared by stirring, grinding and filtering. The construction method is simple, environment-friendly and pollution-free, and has good heat radiation blocking effect even if the color of the coating is added due to the addition of various metal and nonmetal powders, so that the coating not only can effectively block solar light and near infrared light, but also can effectively block mid-infrared light and far-infrared light to achieve good heat insulation effect, but also has poor weather resistance and fire resistance.

Therefore, the development of a heat-insulating coating with excellent heat-insulating property and fireproof property and good weather resistance, stability, aging resistance and the like is urgently needed in the field.

Disclosure of Invention

One of the purposes of the invention is to provide a heat radiation type heat insulation coating which has excellent heat insulation performance and fire resistance performance and simultaneously has good weather resistance, stability and aging resistance.

In order to achieve the purpose, the invention adopts the following technical scheme:

the invention provides a heat radiation type heat preservation coating which comprises metal oxide, rare earth compound, inorganic composite aerogel, reflective heat insulation titanium dioxide, inorganic film forming matter, emulsion, pigment filler, auxiliary agent and water.

The invention provides a novel heat radiation type heat insulation coating, which uses metal oxide and rare earth compound, plays a role of radiation in a system, greatly improves the stimulated radiation at one end of heating in winter, locks heat in a room and plays a role of heat insulation. At night in summer, the spontaneous radiation of the outer wall body continuously radiates the heat, and meanwhile, the external heat is blocked to enter, so that the indoor cooling effect is achieved.

In addition, inorganic composite aerogel is added, the heat conduction effect of metal elements in metal oxides and rare earth compounds can be reduced by adding the inorganic composite aerogel, the reflective heat-insulation titanium dioxide is added, and the four components have synergistic effect, so that the coating has excellent heat-insulation performance under the conditions of cold and hot alternation in winter and summer and temperature difference between day and night, and the stability of the coating is ensured. The 2mm coating of the coating has the same heat insulation effect as a 3cm polystyrene board, and the adhesive force of the coating is far greater than that of a board on the wall surface.

Meanwhile, the inorganic film forming material is used in the invention to act together with the four substances, so that the fire-proof grade of the coating can reach the requirement of A grade, and the weather resistance, the aging resistance and the stability of the coating can be further improved.

With respect to spontaneous emission: the background temperature of the space is 4K and since there is a thick atmospheric layer around it, objects on the ground do not directly exchange radiative heat with such a low space environment. However, in the wave band of 8-13 μm, the absorption capacity of water vapor and carbon dioxide is weak, and the heat radiation energy of the normal temperature object is exactly concentrated in the wave band. Most of the radiation wave bands of the paint provided by the invention are concentrated at 8-13 mu m, so that spontaneous radiation at night generates a cooling effect, which is called radiation refrigeration.

Preferably, the metal oxide comprises Fe₂O₃、MnO₂、Co₂O₃CuO and Al₂O₃Any one or a combination of at least two of them, preferably Fe₂O₃And/or Al₂O₃。

Preferably, the rare earth compound comprises a rare earth oxide.

Preferably, the rare earth compound comprises Y₂O₃、CeO₂、Y(PO₃)₃And La (PO)₃)₃Any one or a combination of at least two thereof, preferably Y₂O₃And/or CeO₂。

Preferably, the inorganic composite aerogel comprises a silica aerogel.

Preferably, the thermal conductivity of the inorganic composite aerogel is 0.01-0.02W (m.K)^-1For example 0.011W (m.K)^-1、0.012W·(m·K)^-1、0.013W·(m·K)^-1、0.014W·(m·K)^-1、0.015W·(m·K)^-1、0.016W·(m·K)^-1、0.017W·(m·K)^-1、0.018W·(m·K)^-1、0.019W·(m·K)^-1Etc., preferably 0.015W (m. K)^-1。

Preferably, the inorganic film-forming material comprises a potassium silicate solution and/or a silica sol, preferably a potassium silicate solution.

The invention preferably selects the potassium silicate solution as a film forming substance, and the addition of the potassium silicate solution can not only improve the fire resistance of the coating, but also further improve the weather resistance and the aging resistance of the coating.

Preferably, the modulus of the potassium silicate solution is 3.0-4.4, preferably 3.5.

"modulus" means in particular silicon dioxide (SiO)₂) With potassium oxide (K)₂O) is referred to as the modulus of the water glass. The stability is best when the modulus of the potassium silicate solution is 3.5.

Preferably, the metal oxide comprises Fe₂O₃And/or Al₂O₃Said rare earth compound comprises Y₂O₃And/or CeO₂The inorganic film-forming material comprises a potassium silicate solution.

Preferred Fe in the present invention₂O₃And/or Al₂O₃、Y₂O₃And/or CeO₂And the potassium silicate solution is used together, so that a nano microstructure can be formed in the process of combining silicate ions in the potassium silicate with Al, Fe, Y and Ce, and precipitation cannot occur, so that the stability of the coating is further improved.

Preferably, the thermal radiation type heat-insulating coating comprises the following components in percentage by mass:

the preferable thermal radiation type heat-insulating coating has the formula, the content of organic film-forming substances (emulsion) is reduced by using inorganic film-forming substances, the fire-retardant grade of the coating is improved, the fire-retardant grade of the coating can reach A1 grade, and in addition, the components in the formula are combined according to a specific ratio, so that the coating can obtain the optimal heat-insulating property, weather resistance and ageing resistance.

The content of the metal oxide is 15 to 25%, for example, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, etc.

The content of the rare earth compound is 3-5%, for example, 3.2%, 3.5%, 3.8%, 4%, 4.1%, 4.2%, 4.3%, 4.5%, 4.7%, 4.9%, etc.

The content of the reflective heat-insulation titanium dioxide is 5-10%, such as 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.5% and the like.

The content of the inorganic composite aerogel is 5-10%, such as 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.5%, and the like.

The content of the inorganic film-forming material is 12 to 25%, for example, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, etc.

The content of the emulsion is 5 to 10%, for example, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.5%, etc.

The pigment and filler content is 3 to 5%, for example, 3.1%, 3.2%, 3.3%, 3.4%, 3.5%, 3.6%, 3.7%, 3.8%, 3.9%, 4%, 4.1%, 4.2%, 4.3%, 4.4%, 4.5%, 4.6%, 4.7%, 4.8%, etc.

The content of the auxiliary is 2.1 to 6%, for example, 2.2%, 2.3%, 2.5%, 2.7%, 2.9%, 3%, 3.2%, 3.5%, 3.8%, 4%, 4.2%, 4.5%, 4.8%, 5%, 5.3%, 5.5%, 3.8%, etc.

The water content is 15-20%, for example, 16%, 17%, 18%, 19%, etc.

Preferably, the pigment filler comprises kaolin and/or mica powder.

Preferably, the auxiliary agent comprises any one or at least two of an antifreezing agent, a film forming auxiliary agent, a thickening agent and a stabilizing agent.

Preferably, the antifreeze agent comprises any one or a combination of at least two of propylene glycol, polyethylene glycol, ethylene glycol and polypropylene glycol.

Preferably, the coalescent includes an alcohol ester twelve.

Preferably, the emulsion comprises any one or at least two of pure acrylic emulsion, styrene acrylic emulsion and silicone acrylic emulsion.

another object of the present invention is to provide a method for preparing the heat radiation type heat insulating coating material according to the first object, the method comprising: mixing metal oxide, rare earth compound, inorganic composite aerogel, reflective heat insulation titanium dioxide, inorganic film forming matter, emulsion, pigment filler, auxiliary agent and water, and stirring to obtain the heat radiation type heat insulation coating.

Preferably, the preparation method specifically comprises the following steps:

(1) mixing water, a stabilizer, reflective heat-insulation titanium dioxide, kaolin, a rare earth compound, a metal oxide, an inorganic film former, an antifreezing agent, inorganic composite aerogel, mica powder and a film forming auxiliary agent according to the formula ratio, and then stirring for the first time;

(2) adding the emulsion and the thickening agent, and stirring for the second time to obtain the heat radiation type heat insulation coating.

Preferably, in step (1), the first stirring rate is 1200-1600 r/min, such as 1250r/min, 1300r/min, 1350r/min, 1400r/min, 1450r/min, 1500r/min, 1550r/min, etc., preferably 1500 r/min.

Preferably, in the step (1), the time for the first stirring is 20-40 min, such as 22min, 25min, 28min, 30min, 32min, 35min, 38min, and the like, and preferably 30 min.

Preferably, in the step (2), the second stirring speed is 600-900 r/min, such as 650r/min, 700r/min, 750r/min, 800r/min, 850r/min and the like, and is preferably 800 r/min.

Preferably, in the step (2), the time for the second stirring is 8-12 min, such as 8.5min, 9min, 9.5min, 10min, 10.5min, 11min, 11.5min, etc., preferably 10 min.

Compared with the prior art, the invention has the following beneficial effects:

according to the invention, the metal oxide and the rare earth compound are added into the formula of the heat radiation type heat insulation coating, so that the heat radiation type heat insulation coating plays a role in radiation in a system, the stimulated radiation is greatly improved at the heating end in winter, the heat is locked in a room, and the heat insulation effect is played. At night in summer, the spontaneous radiation of the outer wall body continuously radiates the heat, and meanwhile, the external heat is blocked to enter, so that the indoor cooling effect is achieved.

In addition, the inorganic composite aerogel is added, the heat conduction effect of metal elements in metal oxides and rare earth compounds can be reduced by adding the inorganic composite aerogel, the reflective heat insulation titanium dioxide is added, and the four components have synergistic effect, so that the radiation type heat insulation coating disclosed by the invention can enable the coating to have excellent heat insulation performance under the conditions of cold and hot alternation in winter and summer and temperature difference between day and night, and the stability of the coating is ensured.

Detailed Description

The technical solution of the present invention is further explained by the following embodiments. It should be understood by those skilled in the art that the examples are only for the understanding of the present invention and should not be construed as the specific limitations of the present invention.

Examples 1 to 7 respectively provide a radiation type heat-insulating coating, and the specific formula is detailed in table 1.

TABLE 1

In Table 1, the emulsion type is Pasteur RS-5939, the antifreeze agent is propylene glycol, the film-forming aid is alcohol ester twelve, the thickener type is TT-60, the stabilizer type is CR-51, the reflective heat-insulating titanium dioxide is reflective heat-insulating titanium white W400 of hensmy, and the silica aerogel is AG-D aerogel powder particles of Shenzhen Zhongji technology (thermal conductivity is 0.015W (m.K))^-1)。

Example 8

The difference from example 1 is that Al is added₂O₃Replaced by MnO of equal mass fraction₂Of Fe₂O₃Replaced by Co of equal mass fraction₂O₃。

Example 9

The difference from example 1 is that Y is₂O₃Replacing with La (PO) with equal mass fraction₃)₃。

Example 10

The difference from example 1 is that the potassium silicate solution was replaced with an equal mass fraction of diatomaceous earth.

Example 11

The difference from example 1 is that Al₂O₃Content of (B) 5%, Y₂O₃The content of (A) is 10%, the content of the reflective heat-insulation titanium dioxide is 15%, and the content of the silicon dioxide aerogel is 13%.

Example 12

The difference from example 1 is that the potassium silicate solution content is 10% and the emulsion content is 18%.

The preparation method of the radiation type heat-insulating coating provided by the embodiment 1-12 comprises the following steps:

(1) mixing water, a stabilizer, reflective heat-insulation titanium dioxide, kaolin, a rare earth compound, a metal oxide, an inorganic film former, an antifreezing agent, inorganic composite aerogel, mica powder and a film forming auxiliary agent according to the formula ratio, and stirring at the speed of 1500r/min for 30 min;

(2) adding the emulsion and the thickening agent, and stirring at the speed of 800r/min for 10min to obtain the heat radiation type heat insulation coating.

Comparative example 1

The difference from example 1 is that Al is not added₂O₃And Fe₂O₃And mixing Al₂O₃And Fe₂O₃Is distributed to Y according to the proportion of 5:10:8₂O₃The reflective heat-insulation titanium dioxide and the silicon dioxide aerogel.

Comparative example 2

The difference from the embodiment 1 is that,without addition of Y₂O₃And the mass fraction is distributed to Al according to the proportion of 20:5:10:8₂O₃、Fe₂O₃The reflective heat-insulation titanium dioxide and the silicon dioxide aerogel.

Comparative example 3

The difference from the embodiment 1 is that the reflective heat-insulation titanium dioxide is replaced by equal amount of non-reflective heat-insulation titanium dioxide (Bailey Union R699).

Comparative example 4

The difference from example 1 is that no silica aerogel is added and the mass fraction is distributed to Al in a ratio of 20:5:5:10₂O₃、Fe₂O₃、Y₂O₃And reflective heat-insulating titanium dioxide.

Comparative example 5

The difference from example 1 is that the content of the emulsion was 28% without adding the potassium silicate solution.

And (3) performance testing:

(1) test of Heat insulating Property

Respectively coating the coatings obtained in the embodiment and the comparative example on a wall body, respectively building a closed space of the wall body coated with the coatings, placing the closed space in a test laboratory, simulating summer illumination and heating an environment outside the wall body to a high temperature, and testing the temperature difference between the inner wall and the outer wall of the closed space by using an infrared temperature gun after 4 hours; the test result is the temperature difference between the inner wall and the outer wall;

(2) fire rating test

Method for testing fire resistance of GB/T9978.1-2008 building elements part 1: general requirements;

(3) aging resistance test

GBT 9755-;

(4) stability test

GBT 9755-;

the results of the performance tests are shown in table 2.

TABLE 2

As can be seen from Table 2, the heat radiation type heat insulation coating provided by the invention has excellent heat insulation, fire resistance, stability and aging resistance, the temperature difference between the inside and the outside of a wall body is 16-22 ℃, the fire resistance level is A1 or A2, the aging resistance time is 400-800 h, and the coating does not have serious layering.

Compared with the example 1, the metal oxide, the rare earth compound, the reflective heat-insulating titanium dioxide and the inorganic composite aerogel are not added in the comparative examples 1 to 4 respectively, so that the comprehensive performance of the obtained coating is reduced, specifically, the stability, the heat preservation performance, the fire resistance and the ageing resistance of the comparative example 1 are obviously reduced, and the heat preservation performance of the comparative examples 2 to 4 is obviously reduced compared with the example 1, so that the fact that the coating has excellent heat preservation performance, fire resistance, stability and ageing resistance can be guaranteed only through the synergistic effect of the four substances is proved.

Compared with the example 1, the inorganic film forming material is not added, the content of the emulsion is increased, the fire resistance level is reduced, the ageing resistance and the stability are obviously reduced, and the heat preservation performance of the coating is reduced due to the layering of the solution.

It can be seen by comparing examples 1, 4 and 5 that when the modulus of the potassium silicate solution is 3.5 (example 1), the stability of the coating is better and excellent heat-insulating properties are ensured.

Comparative examples 1, 8 and 9 show that Fe₂O₃And/or Al₂O₃、Y₂O₃And/or CeO₂The use of potassium silicate solution (example 1) further improves the stability of the coating and ensures excellent heat-insulating properties, and replacement of either (examples 8 and 9) results in a decrease in the stability of the coating and a consequent decrease in heat-insulating properties.

Comparing examples 1 and 10, it is seen that the potassium silicate solution (example 1) further improves the durability of the coating material as compared to other types of inorganic film-forming materials (example 10).

It can be seen from comparison of examples 1, 11 and 12 that the combination of the components according to the specific formulation provided by the invention (example 1) allows the best heat retention, fire resistance and ageing resistance of the coating to be obtained, and that the adjustment of the component content outside the formulation (examples 11 and 12) leads to a reduction in the overall properties of the coating.

The present invention is illustrated in detail by the examples described above, but the present invention is not limited to the details described above, i.e., it is not intended that the present invention be implemented by relying on the details described above. It should be understood by those skilled in the art that any modification of the present invention, equivalent substitutions of the raw materials of the product of the present invention, addition of auxiliary components, selection of specific modes, etc., are within the scope and disclosure of the present invention.

Claims

1. The thermal radiation type heat-insulating coating is characterized by comprising metal oxide, rare earth compound, inorganic composite aerogel, reflective heat-insulating titanium dioxide, inorganic film forming matter, emulsion, pigment and filler, auxiliary agent and water.

2. The heat radiation type heat insulating paint according to claim 1, wherein the metal oxide includes Fe₂O₃、MnO₂、Co₂O₃CuO and Al₂O₃Any one or a combination of at least two of them, preferably Fe₂O₃And/or Al₂O₃。

3. The heat radiation type heat insulating paint according to claim 1, wherein the rare earth compound includes a rare earth oxide;

4. The heat radiation type thermal insulation coating material as claimed in claim 1, wherein the inorganic composite aerogel comprises silica aerogel;

preferably, the thermal conductivity of the inorganic composite aerogel is 0.01-0.02W (m.K)^-1Preferably 0.015W (m.K)^-1。

5. The heat radiation type heat-insulating paint according to claim 1, wherein the inorganic film-forming substance comprises a potassium silicate solution and/or a silica sol, preferably a potassium silicate solution;

preferably, the modulus of the potassium silicate solution is 3.0-4.4, preferably 3.5;

6. The heat radiation type heat-insulating paint according to claim 1, wherein the heat radiation type heat-insulating paint comprises the following components in parts by mass:

7. the heat radiation type heat insulating paint according to claim 1, wherein the pigment and filler includes kaolin and/or mica powder;

preferably, the auxiliary agent comprises any one or at least two of an antifreezing agent, a film forming auxiliary agent, a thickening agent and a stabilizing agent;

preferably, the antifreeze comprises any one or at least two of propylene glycol, polyethylene glycol, ethylene glycol and polypropylene glycol;

preferably, the coalescent comprises an alcohol ester twelve;

8. The heat radiation type heat-insulating paint according to claim 1, wherein the heat radiation type heat-insulating paint comprises the following components in parts by mass:

9. a method for producing a heat radiation type heat insulating paint according to any one of claims 1 to 8, characterized by comprising: mixing metal oxide, rare earth compound, inorganic composite aerogel, reflective heat insulation titanium dioxide, inorganic film forming matter, emulsion, pigment filler, auxiliary agent and water, and stirring to obtain the heat radiation type heat insulation coating.

10. The preparation method according to claim 9, characterized in that the preparation method specifically comprises the steps of:

(2) adding the emulsion and the thickening agent, and stirring for the second time to obtain the heat radiation type heat insulation coating;

preferably, in the step (1), the speed of the first stirring is 1200-1600 r/min, preferably 1500 r/min;

preferably, in the step (1), the time for the first stirring is 20-40 min, preferably 30 min;

preferably, in the step (2), the speed of the second stirring is 600-900 r/min, preferably 800 r/min;

preferably, in the step (2), the time for the second stirring is 8-12 min, preferably 10 min.