Many types of glass are used for the front sash component of the fume hood, and people often ask which is best. The two most common types of sash glass are tempered safety glass and laminated safety glass. A third option is polycarbonate glazing material, for a few less common applications.
This video compares the endurance of tempered glass vs laminated glass side-by-side:
Tempered safety glass is made by heating high quality annealed glass to approximately 600 degrees Celsius, then cooling it rapidly with a precisely controlled blast of cold air. This process cools the outside while keeping the interior of the glass hot, therefore using compression of the outside surface and tension of the inside surface to create a tougher piece of glass (1). This type of glass can withstand impacts five times stronger than ordinary glass.
If it is broken, it shatters into small, blunt pieces, preventing serious injury from what could be large, very sharp pieces. Tempered glass has high heat shock resistance, meaning it can withstand radical changes in temperature of up to 200 degrees C, compared to 70 degrees C for ordinary float glass (2). When tempered glass does break, the fragments are in the form of rounded chunks that are easily cleaned and safer for the clean-up crew.
Laminated safety glass is comprised of two pieces of float glass with a layer of polyvinyl butyral (PVB) between them (3). Pressure is applied to this “sandwich” and then it is heated to bond the layers together. When this type of glass breaks, the shards tend to adhere to the PVB layer inside, affording protection from flying or falling pieces of glass. Both standard and tempered glass can be laminated. It just depends on what the budget is for the project.
Tempered glass has a much higher thermal resistivity than laminated glass (200 degrees C vs. 70 degrees C) (4). This performance difference is pronounced during routine use of drying ovens, other heat emitting equipment and fires in the fume hood.
Chemical resistance is virtually identical in both tempered and laminated glass. However, due to its superior heat and impact performance, tempered glass is by far the superior choice, particularly if the chemical spatters are combined with a hot temperature or semi-solid matter.
Tempered glass has a cracking threshold five to seven times greater than laminated safety glass, so it will fail at a much higher impact and turn into small, less dangerous cubical pieces. Tempered glass resists impact from blunt objects better than laminated, but will “dice” when penetrated, or crumble (5).
If one of the layers of laminated glass loses its integrity, the plastic layer is therefore exposed and could melt under the right conditions. The laminated glass will crack under less abuse and if it is penetrated, it can create shards of large pieces with jagged edges.
Polycarbonate is a substitute for a glass sash for applications that include working with Hydrofluoric Acid (HF). HF etches glass, therefore, using HF in a standard hood over time will eventually require sash replacement due to clouding. This type of barrier can be installed on most fume hood types. Make sure the liner material is compatible with HF to ensure the longest life out of the fume hood. Labconco’s PVC Acid Digestion hood comes standard with a Polycarbonate sash.
All these types of safety glass are used in the fume hood industry in different situations. Due to its superior strength, Labconco uses tempered safety glass as the standard glazing material on a wide range of hoods. If you have an application that uses Hydrofluoric Acid, a polycarbonate sash needs to be specified to ensure a long lasting sash. Please contact Labconco with any questions regarding a sash recommendation for your application.
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