Low-E (low emissivity) coating is a microscopically thin, virtually invisible layer applied to the surface of window glass to enhance its thermal properties.
Low-E coatings reflect infrared (and sometimes ultraviolet) radiation, helping to regulate the interior temperature of a building by reflecting unwanted thermal radiation (heat) and allowing visible light to pass through.
The process of making Low-E glass involves depositing multiple layers of metallic or metal oxide materials onto the glass. Here's an overview of how Low-E coatings are typically applied:
1. Pyrolytic (or "Hard Coat"):
Process:
This method involves depositing the coating directly onto the glass ribbon while it's still slightly molten, in the float glass production process.
A tin bath (in the float process) is where the raw materials are turned into a ribbon of glass. The coating is applied at high temperatures, allowing it to fuse to the hot glass surface.
Characteristics:
The resulting layer is very durable, often referred to as a "hard coat."
Hard-coated Low-E glasses can be used in single-glazing applications and can be tempered after the coating process.
2. Magnetron Sputtering (or "Soft Coat"):
Process:
This is a more complex process that takes place in a vacuum chamber.
The glass is introduced into a vacuum chamber where inert gases, such as argon, are introduced.
Using a magnetron (a type of vacuum tube), a voltage is applied which ionizes the gas, turning it into plasma.
Multiple layers might be deposited, depending on the desired properties of the coating.
Characteristics:
Soft-coated Low-E glasses typically have better thermal performance than hard-coated ones.
Because these coatings are more delicate (hence the term "soft coat"), they're often used in insulated glass units (IGUs) where they're protected from potential damage by being sandwiched between two panes of glass.
Soft-coated Low-E glasses can't be exposed to air and moisture for extended periods, as they can corrode. Thus, they're usually either immediately used in IGUs or temporarily protected with a special sealant until they can be incorporated into a window unit.
After the coating process, the Low-E glass is then cut to size, heat-treated if necessary, and fabricated into window units.
The choice of whether to use a hard coat or soft coat method depends on various factors like the desired thermal properties, the specific application, durability needs, and production capabilities.