Sesame-seed-sized molecular sieves hold the secret to the durability of insulating glass used in home renovations.
Insulating Glass and Molecular Sieves
Insulating glass (standard configuration) consists of raw glass panes, aluminum spacers, molecular sieves, a primary butyl sealant, and a secondary sealant. To ensure a compliant product capable of achieving a reasonable service life (15 years), high-quality raw materials are essential, followed by proper production practices (covering the environment, machinery, processes, and operational standards).
This construction gives insulating glass its energy-saving properties:
Glass is a good conductor of heat, with a thermal conductivity of 0.90 W/(m·K) and low thermal resistance; air is a poor conductor of heat, with a thermal conductivity of 0.024 W/(m·K) and high thermal resistance. A single 6 mm thick pane of glass has a heat transfer coefficient (K-value) of approximately 5.8 W/(m·K), whereas an insulating glass unit with a 5 mm + 12 A + 5 mm configuration (two 5 mm panes with a 12 mm air gap) has a K-value of approximately 2.72 W/(m·K). The K-value is a primary parameter for measuring the energy efficiency of glass, reflecting its heat transfer performance under temperature differences; a lower K-value indicates better performance.
An insulating glass unit with a 6 mm + 12 A + 6 mm configuration has a K-value of approximately 2.70 W/(m·K), while a 10 mm + 12 A + 10 mm unit has a K-value of approximately 2.65 W/(m·K). The K-values differ very little despite the variations in glass thickness, demonstrating that the energy-saving capability of insulating glass is not determined by increasing the glass thickness itself, but rather by the presence of the insulating air gap, which significantly lowers the K-value.
The key to the energy-saving performance of insulating glass lies in ensuring that the gas within the insulating gap remains sealed and dry.
According to relevant studies, the relative humidity of the air inside the gap of an insulating glass unit under "normal use" is around 0.5% RH. This is far lower than the relative humidity levels we typically experience in daily life-ranging from 45% RH to 75% RH (with 60%–70% being the most comfortable range for human perception). Clearly, while the primary and secondary sealants of an insulating glass unit (IGU) play a crucial role in sealing out air and moisture, the sealant itself cannot directly lower the humidity of the air within the cavity.
Even if the IGU is manufactured in an extremely dry workshop, the sealant does not provide a "perfect" or absolute seal; air-carrying moisture-possesses a certain degree of permeability. Over time, the sealant effectively allows for gas exchange, causing the humidity level within the cavity to rise.
Therefore, the production of insulating glass requires a "key ingredient": molecular sieves.
Molecular sieves are a type of desiccant; their function is to rapidly absorb moisture from the air within the cavity after the IGU is assembled, quickly lowering the relative humidity to approximately 0.5% RH.
Furthermore, throughout the unit's long service life-spanning over a decade or more-they adsorb the moisture that inevitably permeates through the sealant, thereby delaying the onset of condensation and preventing the IGU from failing.
Without molecular sieves to absorb moisture from the air within the cavity, two main issues arise.
First, regarding performance: high humidity or condensation (occurring when the dew point is reached) within the cavity increases the air's heat transfer coefficient (the coefficient for dry air is 0.021 kcal/(m²·h·℃), whereas for water it is 0.5 kcal/(m²·h·℃)). An increase in the air's heat transfer coefficient reduces thermal resistance, thereby diminishing the unit's thermal insulation performance.
The presence of condensation within the cavity also indicates that the IGU has reached the end of its effective service life and no longer provides the intended thermal insulation.
Second, prolonged exposure to moisture or condensation can lead to issues such as mold growth, alkali leaching, white spotting, and corrosion of the spacer bar on the inner glass surfaces, or even glass breakage.
