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JWF, Glass Types

Glass Types

As discussed in the History of Glass section, the amount of heat used in the glass manufacturing process has a direct effect on the type of glass produced. Basically, the more heat used during the glass production process the stronger the final glass product will be. The following provides a breakdown of different types of glass available starting with the lowest amount of heat used (Annealed) and working up to the hottest method (Tempered).

Annealed Glass – This is the most common type of glass used today. Produced using the Float method, molten glass is poured continuously onto a bed of molten tin. Because the melting point of tin is less than glass, the glass solidifies as it cools on top of the tin. Increasing or decreasing the rate of molten glass flow onto the tin can control thickness levels. Once the glass becomes solid, it is then cooled in an annealing oven to remove any residual stress. Annealed glass is the most fragile type of manufactured glass because of the relatively low amount of surface heat compression. When glass breakage occurs, it does so in many small and irregularly shaped pieces.

Heat-Treated Glass – Heat-Treated Glass is twice as strong as annealed glass. This process takes annealed glass and heats it to its softening point (1200 degrees F) where it is controlled and cooled using water. Water cools the surface of the glass quicker than the interior glass, thus providing an extra degree of surface compression. An increase in the rate of cooling will result in a stronger surface compression. Heat Treated glass is stronger than standard annealed glass, and is more resistant to stress caused by heat, wind and flying objects (but not as strong as safety glazing products).

Tempered Glass
Thermally Tempered Glass – When the heat-treatment method is increased to provide the highest amount of surface compression, the result is Thermally Tempered Glass (TTG). This type of glass is four times as strong as annealed glass and is very resistant to thermal temperature changes that cause cracking. When tempered glass is broken, it breaks into smaller pieces (shards) that are less dangerous than larger fragments created by annealed glass. Tempered glass is beneficial to areas where glass breakage is most common such as car windshields and commercial storefronts. An etching label is normally on the corner of most tempered glass. This is to ensure that the glass is fully tempered.

Types of Glass Construction
Basically, there are 3 types of glass construction:

1. Monolithic Glass – Monolithic glass is
the most basic of glass forms. It is simply a single piece of glass constructed using one glass thickness. Monolithic glass is produced using either the annealed, heat-treated or tempered glass float method.

2. Laminated Glass – Laminated glass is constructed by combining two panes of glass fused together with a middle layer of Polyvinyl Butylenes Film (PVB). PVB acts like a bonding agent to hold broken glass together. This feature creates an effective barrier against entry and reduces the chances of flying shards of glass. Automotive windshields and buildings/homes in hurricane areas (e.g. Florida) use laminated glass.

Diagram A: Shows the basic Monolithic Glass.
Diagram B: Shows Laminated Glass with the Interlayer of Polyvinyl in the middle.

IG Unit illustration
3. Insulated Glass – Insulated glass is a term used to describe two pieces of glass separated by airspace. Airspace is created using spacers or edge seals located at the top and bottom of an Insulated Glass (IG) unit. Additionally, a sealant is incorporated in the edge seals to absorb excess water vapor that travels across the seal. The airspace between the glasses minimizes heat transfer through conduction and convection, providing heavy, inert gases such as Argon and
Krypton are sometimes placed in the airspace to slow down convection and reduce the amount of heat transfer. Window Film dealers should be well aware that IG units are highly susceptible to thermal stress fracture when films with high heat absorption (e.g. dyed films) are applied.

I.G. Unit close-up
Other types of glass construction include wired, textured
and patterned glass. However, the manufacturing process associated with these types of glass usually create surface and edge flaws increasing the chance of thermal stress. Bullet resistant glass is thick and is made using multiple laminates of glass and polycarbonates. These multiple layers increase the chance of thermal stress fracture since they will heat and cool at varying temperatures. Therefore, solar control films are not recommended for bulletproof glass. Sometimes a clear film will be applied to the interior surface to hold glass shards together at impact.

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