Understanding the Physical Properties of Glass
Different types of glass possess different qualities depending upon their chemical makeup and how they have been produced. Choosing the right type of glass for a particular application also means understanding the different physical properties each different type of glass possesses.
There are 5 main properties of glass to be considered:
- Thermal Properties
- Optical Properties
- Chemical Properties
- Electrical Properties
- Mechanical Properties
Glass is measured in a variety of factors which greatly affect your choice of glass. The Coefficient of Thermal Expansion (CTE) is the expansion measurement of glass as temperature is raised. This is an important factor to consider when placing glass in a frame since glass expands much less than most metals and plastics, and may cause breakage upon cooling. The thermal conductivity is the ability to conduct heat through the glass or away from the heat/light source. This is important when considering glass as a view port exposed to high temperatures or for high infrared applications. Each type of glass has a maximum operating temperature and thermal shock rating. These will guide the choice of glass depending on the amount of heat the glass will withstand, and how it cools after the glass is subjected to a rapid change in temperature. Glass may be strengthened to change these thermal properties by heat strengthening, heat tempering, or chemically strengthening. Click here to learn more about glass strengthening.
There are several important measurements when determining the amount of light passing through glass. The refractive index determines how much a light wave is “bent” when entering or leaving the surface of the glass. This is important in producing certain optical devices or effects, such as lenses. The dispersion measures the separation of light into its component colors, such as a prism dispersing white light into a color band or a rainbow effect. The transmission measures the amount of light passing through the glass material, and its opposite, reflectivity which measures the return of light from the surface. The absorption property is the amount of light energy converted to heat within the glass that is not transmitted nor reflected. Tinted materials will absorb more light than clear materials.
All soda lime type glasses and some borosilicate glasses contain sodium or alkali metal ions. Prolonged exposure to liquids or vapor, such as water, will cause the sodium/alkali ions to migrate to the surface of the glass called sodium or alkali leaching. This can cause cloudiness or haze on the surface of the glass. Porous coatings may also incur this phenomenon, causing a disruption of the bond between the coating and the glass surface. In high humidity or critical surface applications, this must be considered when specifying the material. Placing a “barrier” coating, such as silicon dioxide, on the glass will limit the amount of reaction. The acid resistance and alkali resistance measure the time it takes to remove a layer of specified thickness for each test.
When choosing a glass for electrical or electronic applications, there are several characteristics to consider. The volume resistivity is the resistance in ohms between opposite faces of a centimeter cube of the glass tested. This is important when glass is used as an electrical insulator. The dielectric constant of a glass is the ratio of energy stored in a condenser with the glass as the dielectric, compared with the energy stored in the same condenser with air as the dielectric. This measures the ability of a glass to store electrical energy, and varies with the frequency of the voltage applied to the condenser. This is important when the glass is used as a substrate for electrical or electronic devices. Surface resistivity is the ratio of the potential gradient parallel to the current along its surface, to the current per unit width of the surface. This method is used to measure the conductivity of coated glass.
The mechanical properties of glass determine the amount of stress a glass can withstand. Stress is defined as the perpendicular force per unit area applied to an object, in a way that compresses (compressive stress) or stretches (tensile stress) the object. Strength of the ability of glass to withstand these stresses. Non-strengthened glass materials have relatively low tensile strength yet high compressive strength. Therefore, most glass breakage is due to tensile stress failure. Mechanical properties are measured in a variety of ways: Modulus of Rupture (MOR) test measures the bending or flexural strength; shear modulus measures the amount of shearing or twisting forces a glass can withstand; Knoop Hardness Number (KHN) measures the hardness of glass; density is the mass value per unit of volume specific gravity is the ratio of the density of the glass to the density of water.
Abrisa Technologies is a recognized global supplier of high quality, fabricated glass components, optical thin film coatings, and custom glass solutions for a wide variety of industries. From our US based Abrisa Industrial Glass fabrication facility in Santa Paula, CA and our ZC&R Coatings for Optics division in Torrance, CA we serve diverse industries such as microelectronics and displays, semiconductor, military, automotive, aerospace, medical, biomedical and scientific R&D. We provide custom specialty flat glass and coating products for applications such as: flat panel display, touch and gesture recognition; visible to IR imaging and surveillance; entertainment, indoor and outdoor lighting; advanced instrumentation; and photonics.