What process is used to manufacture low-emissivity coated glass?
Low-emissivity coated glass, commonly referred to as Low-E glass, is produced by applying a thin film with low emissivity properties onto the surface of the glass through physical or chemical processes. In industry, it is called Low Emissivity Coating, hence the term ‘Low-E glass’. At present, the mature commercial manufacturing techniques for Low-E coated glass include vacuum magnetron sputtering (a physical method) and chemical vapor deposition (a chemical method)
What is online Low-E glass, and what are its main features?
Online Low-E glass refers to Low-E coated glass produced directly on the float glass manufacturing line, where a Low-E film is applied in the high-temperature zone during glass formation using chemical vapor deposition (CVD) technology. Because the coating process is completed as part of the flat glass production line, this type of low-emissivity glass is called on-line Low-E glass. Its low-emissivity functional layer consists of semiconductor compounds.
The advantages of on-line Low-E coatings include strong adhesion between the coating and the glass surface due to the high-temperature application, as well as excellent scratch resistance, allowing the glass to be used as single glazing—hence the term “hard coating.” However, its drawbacks include lower precision in controlling film thickness, making it impossible to create multilayer interference coatings that selectively control the solar spectrum transmitted through the glass. Additionally, the coating’s reflected color is limited, and its emissivity is relatively high (greater than 0.15).
What is offline Low-E glass, and what are its key features?
Offline Low-E glass is produced on a vacuum magnetron sputtering coating line. The principle involves depositing a thin film onto the glass surface in a vacuum environment, where a negative high voltage and working gas generate plasma to transfer solid material (the target) onto the glass. When the working gas is argon, the deposited film has the same composition as the target material; when the working gas is a reactive gas such as oxygen or nitrogen, the deposited film is a compound formed by the reaction between the target material and the gas.
Most coating lines are equipped with multiple targets of different materials, enabling the continuous deposition of individual layers that can be stacked into multilayer composite coatings.
Because the coating process is completed on a separate coating line, this type is called “off-line Low-E glass,” with the low-emissivity functional layer typically consisting of a silver metal layer. Its advantages include precise control over film thickness, the ability to produce multilayer optical interference coatings to selectively control the solar spectrum transmitted through the glass, a wide range of adjustable reflective colors, and a low emissivity (less than 0.15).
Its disadvantages include lower film hardness and poor corrosion resistance, hence the term “soft coating.” It should be noted that the latest off-line silver-free Low-E coating technology offers enhanced wear and corrosion resistance, allowing the coated surface to face indoors. This type is commonly referred to as “silver-free Low-E” or “interior Low-E coating.”
Comparison of On-line and Off-line Low-E Glass
| Item | On-line Low-E Glass | Off-line Low-E Glass |
|---|---|---|
| Definition | Produced on the float glass production line by applying a Low-E coating in the high-temperature forming zone using Chemical Vapor Deposition (CVD). | Produced on a separate vacuum magnetron sputtering coating line under vacuum conditions. |
| Coating Process | CVD in the hot zone of float glass manufacturing. | Magnetron sputtering deposition in a vacuum chamber. |
| Functional Layer Material | Semiconductor compounds. | Typically a silver (Ag) layer; advanced technology also allows silver-free coatings. |
| Adhesion & Durability | Strong bond with glass surface; highly scratch-resistant; also called hard coating; can be used as single glazing. | Lower hardness and poorer corrosion resistance; also called soft coating; requires protective glazing, except for new silver-free coatings which can face indoors. |
| Thickness Control | Lower precision; cannot produce complex multilayer interference coatings. | High precision; can produce multilayer optical interference coatings. |
| Optical Control | Limited control over solar spectrum; single reflective color. | Flexible solar spectrum control; wide range of adjustable reflective colors. |
| Emissivity | Relatively high (>0.15). | Low (<0.15). |