Abstract: Coated glass is a deep-processed glass product made by coating one or both surfaces of the glass with a thin film of metal, metal oxide, etc. using physical or chemical methods. Different coating colors and different reflectivities of light make buildings decorated with coated glass crystal clear and brilliant. Low-E coated glass can control the incidence of sunlight and reduce air-conditioning energy consumption, while low-e coated glass can limit indoor heat from radiating outward, which has significant energy-saving effects in cold areas.
Keywords: coating, low-E, shading coefficient, relative heat gain
Coated glass (Reflective glass) is also called reflective glass. Coated glass is to coat one or more layers of metal, alloy or metal compound films on the surface of the glass to change the optical properties of the glass and meet certain specific requirements. Coated glass can be divided into the following categories according to the different characteristics of the product: heat reflective glass, low-emissivity glass (Low-E), conductive film glass, etc.
In construction, heat reflective glass is most commonly used.
Low-E glass is generally coated with one or more thin films composed of metals such as chromium, titanium or stainless steel or their compounds on the surface of the glass to make the product rich in color, with appropriate transmittance for visible light and high infrared rays. It has a high reflectivity and high absorption rate for ultraviolet rays. In addition, it also has good transmittance. Therefore, it is also called sunlight control glass.
1: Preparation principle of low-E glass
Two Current Low-E Glass Production Methods
(1) Online high-temperature pyrolysis deposition method: Online high-temperature pyrolysis deposition method "Low-E" glass has products from many companies in the United States. Such as PPG's Surgate200, Ford's Sunglas H. R"P". These products are produced during the float glass cooling process. Liquid metal or metal powder is sprayed directly onto the hot glass surface. As the glass cools, the metal film becomes part of the glass. As a result, the film is hard and durable. The "Low-E" glass produced by this method has many advantages: it can be hot-bent, tempered, does not have to be used in a hollow state, and can be stored for a long time. Its disadvantage is that its thermal properties are relatively poor. Unless the coating is very thick, its "u" value is only half that of sputtered "Low-E" coated glass. If you want to improve its thermal properties by increasing the film thickness, its transparency will be very poor.
(2) Offline vacuum sputtering method: Offline method is used to produce Low-E glass. Vacuum magnetron sputtering coating technology is currently commonly used in the world. Unlike the high-temperature pyrolytic deposition method, the sputtering method is offline. And depending on the glass transmission position, it can be divided into horizontal and vertical. The sputtering process to produce "Low-E" glass requires a layer of pure silver film as a functional film. The pure silver film is between two metal oxide films. The metal oxide film protects the pure silver film and acts as an intermediate layer between the film layers to increase color purity and light transmittance. In the vertical production process, the glass is placed vertically on the shelf and sent into a vacuum environment of the order of 10-1 Pa. An appropriate amount of process gas (inert gas Ar or reactive gas O2, N2) is introduced, and the vacuum degree is kept stable. The target material Ag, Si, etc. is embedded in the cathode, and a magnetic field is placed in the horizontal direction perpendicular to the cathode to form a magnetron target. With the magnetron target as the cathode and a DC or AC power supply, the process gas is ionized under the action of high voltage to form plasma. Among them, under the combined action of electric field and magnetic field, electrons perform high-speed spiral motion, collide with gas molecules, and generate more positive ions and electrons; under the action of electric field, positive ions reach a certain energy and then hit the cathode target and are sputtered. The ejected target material is deposited on the glass substrate to form a thin film. To form a uniform film, the cathode target moves back and forth close to the glass surface. In order to obtain a multi-layer film, multiple cathodes must be used. Each cathode moves back and forth on the glass surface to form a certain film thickness. The horizontal method is largely similar to the vertical method. The main difference is the placement of the glass. The glass is transported by horizontally arranged wheels through the cathode. After the glass passes through a series of pinned valves, the degree of vacuum also changes. When the glass reaches the main sputtering chamber, the coating pressure is reached, the metal cathode target is fixed, and the glass moves. As the glass passes through the cathode, a film forms.
At present, the target products of domestic and most imported magnetron sputtering coating production lines are solar control film glass, which is mainly coated with elemental films and metal films. The process of this type of product is relatively simple and requires low equipment. Therefore, these production lines cannot meet the requirements for coating LOW-E glass.
The sputtering method to produce "Low-E" glass has the following characteristics: Since there are a variety of metal target materials to choose from and a variety of metal target combinations, the sputtering method to produce "Low-E" glass can have a variety of configurations. In terms of color and purity, sputter plating is also better than thermal spraying. Moreover, because it is an offline method, it is also more flexible in new product development. The main advantage is that the "u" value of "Low-E" insulating glass produced by sputtering is better than that of pyrolysis products, but its disadvantage is that the silver oxide film layer is very fragile, so it is impossible Use like ordinary glass. It must be made into insulating glass, and it is not suitable for long-distance transportation until it is made into a hollow product.
2: Application principles of low-E glass in construction
Shading Coefficient: Abbreviated as SC, called shading coefficient (abbreviated as Se) in GB/T2680. It is an important restriction index for glass in the building energy-saving design standards. It refers to the ratio of the amount of solar radiation energy transmitted through the window glass to the amount transmitted through the same area of 3mm transparent glass. SC is calculated by dividing the total solar transmittance of the sample glass by the total solar transmittance of standard 3mm white glass (the theoretical value in GB/T2680 is 0.889, and the international standard is 0.87). SC=SHGC÷0.87 (or 0.889). The smaller the shading coefficient, the better the performance in blocking sunlight heat from radiating indoors. However, only in hot climate areas and large window-to-wall ratios, glass with a low shading coefficient is beneficial to energy conservation. In cold areas and when the window-to-wall ratio is small, glass with a high shading coefficient is more conducive to utilizing solar heat to reduce heating energy consumption. Energy saving.
Relative heat gain: refers to the sum of heat gained and lost through glass, taking into account the impact of temperature difference heat transfer and solar radiation on the room. Relative heat gain = (outdoor temperature - indoor temperature) X heat transfer coefficient K + solar radiation intensity X shading coefficient SCX0.87. When it is greater than 0, it means that more and more heat is gained indoors; when it is less than 0, it means that more and more heat is lost outside the room. When the weather is hot and the outdoor temperature is high, the first term of the formula is positive and heat is transferred indoors. At this time, the smaller the K value and SC, the smaller the relative heat gain of the glass, which is beneficial to reducing cooling energy consumption. When the weather is cold and the outdoor temperature is low, the first term of the formula is negative and heat is transferred to the outdoors. The second term of the solar radiation transfers heat to the indoors. The larger the SC, the more heat the solar radiation enters to make up for the heat lost to the outdoors. Therefore, in cold climates, the higher the SC value of glass, the more it can reduce heating energy consumption.
Heat transfer coefficient: referred to as K value or u value (for glass, the two are just different abbreviations). It is an important limiting value for glass in building energy-saving design standards. It refers to the heat transfer through 1 square meter of glass per unit time when the air temperature difference on both sides of the glass is 1 degree under stable heat transfer conditions, expressed in W/(m2k) or W/(m2℃) expressed. Foreign U-values are expressed in British units as Btu/hr/ft2/F. The U-value in British units is multiplied by the conversion factor of 5.678 to obtain the U-value in metric units. The lower the heat transfer coefficient, the better the thermal insulation performance of the glass. The heat transfer coefficient of a single piece of ordinary glass is about 5.8 W/(m2K), that of a single piece of Yaohua Low-E is about 3.6 W/(m2K); that of ordinary 6+12+6 insulating glass is about 2.9 W/(m2K) , the Low-E hollow heat transfer coefficient with the same configuration is below 1.9W/(m2k).
Solar radiation transfers heat directly through heat transfer and convection conduction. The total thermal power Q transmitted through glass per square meter can be expressed by the following formula: Q=630Sc+U (inside T-outside T)
In the formula, 630 is the solar intensity through 3mm transparent glass, (T inside - T outside) is the air temperature on both sides of the glass, both are parameters related to the environment. Sc and U are the inherent parameters of the glass itself, and their meanings are as follows: Sc —The shading coefficient of glass, ranging from 0 to 1, reflects the shielding effect of glass on direct solar radiation. U—The heat transfer coefficient of glass, which reflects the ability of glass to conduct heat.
3: low-E glass has the following characteristics:
(1) Excellent thermal performance: Heat loss from exterior door and window glass is the main part of building energy consumption, accounting for more than 50% of building energy consumption. Relevant research data shows that the heat transfer on the inner surface of glass is dominated by radiation, accounting for 58%. This means that the performance of the glass must be changed to reduce the loss of heat energy. The most effective method is to suppress the radiation on its inner surface. The emissivity of ordinary float glass is as high as 0.84. When coated with a silver-based low-emissivity film, its emissivity can be reduced to less than 0.15. Therefore, using Low-E glass to make building doors and windows can greatly reduce the transfer of indoor heat energy to the outdoors due to radiation, achieving ideal energy-saving effects. Another significant benefit brought about by reduced indoor heat loss is environmental protection. In the cold season, the emission of harmful gases such as CO2 and SO2 caused by building heating is an important source of pollution. If Low-E glass is used, due to the reduction of heat loss, the fuel consumed for heating can be greatly reduced, thereby reducing the emission of harmful gases.
The heat transmitted through the glass is two-way, that is, the heat can be transferred from indoor to outdoor, and vice versa, and it is done at the same time. It is just a matter of poor heat transfer. In winter, the indoor temperature is higher than the outdoor temperature, so insulation is required. In summer, the indoor temperature is lower than indoors, so the glass is required to be insulated, that is, the outdoor heat is transferred into the room as little as possible. LOW-E glass can meet the requirements of winter and summer. It can both keep heat and insulate, and has an environmentally friendly and low-carbon effect. (2) Good optical properties: The visible light transmittance of Low-E glass ranges from 0% to 95% in theory (6mm white glass is difficult to achieve). The visible light transmittance represents indoor lighting. Outdoor reflectivity ranges from about 10% to 30%. Outdoor reflectivity is visible light reflectance, which represents the intensity or dazzling degree of reflection. So far, China requires that the visible light reflectance of curtain walls should not exceed 30%.
The above characteristics of Low-E glass make it increasingly widely used in developed countries. my country is a relatively energy-poor country, with per capita energy consumption being very low, and building energy consumption already accounts for about 27.5% of the country's total energy consumption. Therefore, vigorously developing the production technology of Low-E glass and promoting its application fields will definitely bring significant social and economic benefits.
In the United States and Europe, low-emissivity (Low-E) coated glass has received great attention due to its superior performance. In particular, Germany's Wschvo regulations have led to rapid development of Low-E glass.
European manufacturers began laboratory research on "Low-E" in the late 1960s. In 1978, Interqane in the United States successfully applied "Low-E" glass to buildings.
The superiority of "Low-E" is unquestionable. Since 1990, the usage of "Low-E" has increased at an annual rate of 5% in the United States. It is not known whether "Low-E" will become the dominant form of window glass in the future, but owners and door and window companies attach great importance to energy-saving doors and windows.
References:
1: "Flat Glass Deep Processing", Wuhan University of Technology. :
2: "Glass Surface and Surface Treatment", Wang Chengyu, Tao Ying, China Building Materials Industry Press.
3: "Energy-Saving Effect of Low-E Glass in Buildings", Tong Fei and Zhang Zhigang.
