Causes and prevention of thermal cracking of coated glass

Coated glass will not break under regular use. Most of the cracks of coated glass are caused by thermal stress. Under strong sunlight, glass absorbs the radiation energy in the sunlight. It converts it into heat energy in the glass body, which makes the temperature of the irradiated part of the glass relatively high and in a state of thermal expansion. However, the edge area of ​​the glass inside the aluminum frame structure cannot be subjected to the same solar radiation or shadow, and the heat dissipation is uneven, which leads to uneven temperature distribution of the glass as a whole, resulting in internal thermal stress. The thermal expansion of the middle area of ​​the glass produces tensile stress on the edge area of ​​the glass. This tensile stress exceeds the tensile strength of the edge area, which will cause the glass to break. This phenomenon is called thermal stress cracking of glass. Since coated glass absorbs heat more significantly, the difference in absorbing solar radiation energy causes a more significant temperature difference in the glass, and thermal stress cracking is more common. This phenomenon occurs more frequently in coated glass used for windows and exposed frame curtain walls than in hidden frame curtain walls. The following further discusses the characteristics, causes, and prevention of coated glass thermal strain cracking.

Glass is a poor conductor of heat. The temperature difference caused by uneven heat absorption and dissipation makes it difficult to achieve balance independently. Therefore, when installing glass, use elastic materials with low thermal conductivity, such as padding, as much as possible to isolate the glass from aluminum frames with good thermal conductivity, cold walls, etc., to reduce the formation of uneven temperature differences. Generally, hidden frame curtain walls rarely explode. A big reason for this is that the glass periphery is in soft contact with the aluminum frame using elastic materials with poor thermal conductivity, so there is no heat exchange between the glass and the aluminum frame, reducing the temperature difference in various parts of the glass caused by heat dissipation of the glass; on the contrary, for coated glass used in exposed frames, fixed windows, sliding windows, floor-to-ceiling glass, etc., the middle part absorbs heat and causes temperature rise. In contrast,
the periphery is clamped and cannot absorb heat energy, forming a low-temperature zone. Suppose the glass is in contact with the aluminum frame. In that case, the glass will conduct heat to the aluminum frame, which not only keeps the glass inside the aluminum frame but also keeps the glass around the aluminum frame at a low temperature.
In many cases, the temperature in the middle of the glass reaches 60-70℃, while the edge is only 25-30℃, a difference of dozens of degrees, which can be felt even with your hands. Therefore, significant thermal stress is generated inside the glass, which can easily cause the glass to break. The most common thermal stress cracking of this type of coated glass is currently thermal stress cracking. To avoid the above problems, we must try to make the glass and the aluminum frame thermally insulated. When installing the glass, two rubber blocks are placed on the lower balance pad to ensure elastic contact between the glass and the lower part, avoid local stress, and make them thermally insulated. Sufficient gaps are maintained between the glass and aluminum frame’s sides and top. Rubber strips are sandwiched between the gaps between the sides of the glass and the aluminum frame before glass glue can be applied. The purpose is to ensure their absolute separation, elastic contact, and insulation.

For coated glass, the thicker the glass is, the greater the absorption of solar radiation energy is, which increases the thermal stress of the glass and makes it easier for the glass to break. Similarly, the larger the area of ​​the glass is, the easier it is to break. The greater the ratio of the long and short sides of the glass, the easier it is to form bending stress, increasing the probability of thermal stress fracture. In addition, the more irregular the shape of the glass, the more unbalanced the force in different parts and directions, which will also increase the probability of thermal stress fracture. Reasonable design is a crucial factor in reducing the impact in this regard.