Understanding Glass – Types, Properties and Fabrication

According to Thomas Net:

From heat resistant Pyrex to PVB laminated safety windshields, glass is available in many forms. Some of the materials commonly referred to as glasses are actually plastic or plastic-glass mixes, although the term glass does not explicitly indicate a particular chemical composition. The term can describe any number of hard, amorphous, inorganic, and uniform solids produced when fused molten materials are cooled fast enough to prevent crystallization. However, the typical substances used in glass making are silicates, borates, and phosphates.

Glass is an inflexible material that is formed by heating a mixture of dry solid materials until it reaches a semi-solid state, then cooling the mixture quickly to prevent it from forming the crystalline structure that most solid materials have. As the glass cools, the atoms become locked in a disordered state similar to that of a liquid before they can form the crystalline state of a solid. As glass is neither a liquid nor a solid, but instead has the qualities of both, glass exist as a separate type of matter.

Because of its strength and versatility, glass has near limitless applications. It is used extensively in construction, providing facing for most modern buildings and regular architectural glass features for most other habitable structures. It finds diverse uses in the home, whether as cookware, television screens, or light bulbs. It is likely the single most important material in astronomy, which was originally made possible by the use of different glass lenses. In addition to its obvious uses in biological, chemical and medical laboratories (test tubes, beakers, microscopes), glass also provides part of the casing for most instrumentation. Although polycarbonate lenses have largely replaced glass in eyeglasses, glass lenses were, historically, the only means to improve vision. Even art owes a great deal to glass, as stained glass and many decorative glass objects from antiquity have survived and provided inspiration to artists for more than a thousand years.

In modern industry, glass serves many biomedical and optic-related functions. It is also a necessary component in numerous aerospace and avionic devices, as well as a useful substance in semiconductor technology and electronics. Due to its unique properties, certain types of glass are used in integrated circuits. It also provides a reinforcing material for laminated plastics. Glass beads are used in sandblasting, and glass sheets are a prerequisite of most mirror manufacturers.

This article gives an understanding of the unique properties of glass, explaining the different types of glass and their makeups. Additionally, the way glass if fabricated is explored, along with some of the various ways glass can be treated or finished after fabrication.

While there are many different types of glass, and their properties vary with their chemical compositions, there are a few characteristics, most kinds of glass have in common. Despite its fragile reputation, glass is mechanically strong. Surface imperfections weaken glass, but there are processes to minimize flaws and strengthen it. Glass is a hard material that resists scratches and abrasions to some level. Glass is generally chemically resistant against most industrial and food acids, and its resistance to other chemicals varies. It is elastic and yields under stress before bending back to its original shape. Of course, glass has a breaking point that varies by type.

Glass is thermal-shock resistant, meaning it can withstand sudden temperature changes well, and is able to endure intense heat and cold, to various degrees. It is heat-absorbent, retaining heat instead of conducting it, and absorbs heat better than metal. Glass can reflect, bend, transmit, and absorb light with great accuracy, and is highly valued for its optical properties. It strongly resists electrical current and stores electricity well.

Types of Glass
Glass is divided into type based on its chemical composition. These are some of the more common types.

Soda-lime glass
Soda-lime glass, also known as soda-lime-silica glass or window glass, is the most common and least expensive type of glass. It contains about 70% silica, along with soda, lime, and small amounts of other compounds. The soda lowers the temperature at which the silica melts, while the lime stabilizes the silica. About 90% of glass manufactured is soda-lime. It’s chemically stable, often inexpensive, and is very workable because it can be re-softened multiple times during the fabrication of a product. It is a softer glass, which is an asset for fabrication via cutting, but this does mean it is less scratch resistant than other types of glass, such as borosilicate and fused quartz.

Soda-lime glass is often chemically strengthened to increase its strength, or it can be tempered to increase its thermal shock resistance and strength. As its nickname suggests, it is commonly used in windows. It‘s also used for household glass containers. Soda-lime has a wide range of applications.

Lead glass
Lead glass, also called lead-oxide glass or lead crystal, is at least 20% lead oxide. It has also been called flint glass since the original formula from the 1600s used calcined flint as a source of silica, but now flint is no longer used in its creation. It is a softer glass, making it easier to cut into designs that show off its high refractive index. It cannot withstand high temperatures or sudden changes in temperature.

Because lead glass is refractive and more expensive than soda-lime glass, it used to be commonly used for decorative glass dishware. However, since the dangers of ingesting lead are now well-known, today lead glass is used mainly for electrical applications because of its electrical insulating properties and lower melting temperature. The low melting temperature is desirable for heat-sealing so it can be sealed around electronics sensitive to higher temperatures. It is also used for shielding applications to protect against x-rays and gamma rays in medical, technical, and research work, and for optical glasses because of its refractive index.

Borosilicate glass is composed of at least 5% boric oxide. Durable and heat resistant, borosilicate glass is the material of choice for a wide range of applications, from cookware to laboratory use.

Creating borosilicate glass requires higher temperatures than those necessary for the production of regular glass, although this also accounts for its higher heat resistance. It also faces far less material stress than regular glass due to its lower thermal expansion coefficient, which also adds to its exceptional performance at high temperatures. Additionally, borosilicate glass is far more durable than traditional glass and can withstand accidents that would break other glassware. Even when it does crack, it typically performs better, as it rarely shatters.

Borosilicate glass is often used for scientific and medical laboratories since it offers excellent chemical resistance in addition to its other useful qualities. Everything from test tubes, rods, and beakers to graduated cylinders, pipettes, and stopper attachments are produced from borosilicate and used in laboratories around the world. Although borosilicate glass offers exceptional acid resistance, it is less resistant to a range of alkalis, and occasionally other materials should be considered. Borosilicate glass is also used in certain optics such as mirrors because it retains shape well throughout changes in temperature. Other uses include the strengthening of various plastic compounds and in various gages and protective glass surfaces.

The main distinction of borosilicate glass from traditional glass is the substitution of boron oxide for soda and lime in the manufacturing process. Borosilicate glass must contain at least five percent boron oxide, which helps bind the silicate and aluminum oxide and sodium oxide.

Aluminosilicate glass contains aluminum oxide. It varies in composition but usually has between 20% and 40% aluminum oxide. It has comparable properties to borosilicate glass but is more heat resistant, tolerating temperatures up to 800o Celsius, and has a better chemical resistance. Because of this heat resistance, it’s much more difficult to melt and therefore to fabricate than borosilicate glass. The two main kinds of aluminosilicate glass are alkaline earth aluminosilicate glass and alkali aluminosilicate glass. Alkaline earth aluminosilicate glasses have a very high softening point and are typically used for glass bulbs for halogen lamps, high-temperature thermometers, and can be coated in an electrically conductive film and used for resistors in electronic circuitry. The high alkali content of alkali aluminosilicate glasses improves their surface compressive strength. They are also very hard and scratch resistant. They are commonly used for touch displays, such as smartphone screens, and for solar cells cover glass and laminated safety glass.

High Silica Glass
High silica glass is created by melting glass down to remove almost all of the non-silicate elements from it. This process can result in a makeup of 95 to 99% silica. Because of the lack of fluxing agents, high silica glass is extremely hard to melt, with a deformation temperature as high as 1,700ºC, meaning it can be used at such high temperatures as around 1,000o C. High silica glass has a very low thermal expansion, very good chemical durability, optical properties, and mechanical properties, but the extremely high processing temperatures is a limiting factor in the production and application on a larger scale.

As technology improves, the ability to reach a greater purity of high silica glass has improved, making it possible to fabricate higher and higher qualities of glass. High silica glass is often used in the semiconductor industry since silica doesn’t contaminate silicon wafers, and for fiber optics, UV-transmissive lamp tubes, precision optics, refractory tubes, and as a fiber reinforcer in composites.

Fused Quartz
Fused quartz glass, also called fused-silica glass or vitreous-silica glass, is fabricated by purifying and melting down naturally occurring crystalline silica, found in sand or rock crystal, either with electrical or flame fusion. The resulting glass is highly transparent, even to ultraviolet and infrared light, and weather and shock resistant. It is very difficult to fabricate, as fusion occurs at approximately 1650o C, so it is very expensive. However, this high fusion temperature also means it can handle temperatures of up to 1400° C for short periods, making it able to withstand the highest temperatures of any glass. Because of this resistance, fused quartz glass is often used for aerospace applications, specifically the windows of manned spacecraft.

Glass Fabrication and Finishing
While some fabrication variations exist in creating different types of glass, the following outlines the basic process used to create the more common types of glass, such as soda-lime.

The ingredients that make up glass vary depending on the type of glass. The main component of glass, called former, must be heated to a very high temperature to become viscous. The most common former is silicon dioxide, found in sand. The former is mixed with a flux, which helps it to melt at a lower temperature. Common fluxes are soda ash and potash. A stabilizer is also used to keep the glass from dissolving or forming unwanted crystal impurities. A common stabilizer is calcium oxide, from limestone. These dry ingredients are mixed together in a batch. A furnace melts the batch to form a liquid compound. Cullet, which is made up of broken glass, is added to the batch to help it melt.

If colored glass is being fabricated, a metallic oxide is added to the batch. Iron colors glass green, copper turns it light blue, cobalt a dark blue, gold a deep red. Low-iron glass is recommended when coloring glass any color besides green. In small amounts, manganese dioxide is used to decolorize glass, but in large amounts, it colors glass purple, or with a higher amount, black.

After it is melted, the viscous glass is then poured into a bath of molten tin, then formed into a ribbon and cooled. The slow and even cooling process is called annealing. Glass must be cooled evenly, because if one area stays hotter longer it becomes thicker, and the different levels of thickness results in stress on the piece of glass. An improperly-annealed piece of glass is more likely to crack.

Next, annealed glass is cut down to the desired dimensions. This is usually done with Computer Numerical Control machines, or CNC machines, which are capable of extremely precise operations. CNC machines operate according to specific CAM and CAD software programs, which enable them to machine any number of workpieces with identical precision. They are also able to perform a wide range of machining tasks normally accomplished by specialized equipment: they can cut curves and straight lines, drill holes, and grind grooves. The CNC machines used in glass fabricating use distinctive tooling, including diamond abrasive tooling, diamond points, and carbide wheels, to achieve better accuracy and glass working capabilities.

After the glass is cut and shaped, manufacturers usually perform some glass polishing, laminating, and other finishing services. Polished mirrors and lenses make up a significant part of the glass fabrication market. These items generally demand extreme precision, and surface tolerances must be exact in order for components to function as desired. These precision components are used in telescopes, prisms, laser lenses and mirrors, and other optics, all of which are greatly affected by imperfections and inaccuracies.

Tempering is a heat treatment that strengthens glass to about four times the strength of non-tempered glass. If tempered glass does break, it fractures into small, rounded pieces instead of jagged shards.

The tempering process begins with cut and washed glass traveling through a tempering oven, either in a batch or a continuous feed. The glass is heated to more than 600o Celsius before it goes through quenching, the cooling process. During quenching, high-pressure air blasts the glass from nozzles in many different positions. The outer surface of the glass cools much more quickly than the center, which leads to the center of the glass trying to pull back from the outer surface. The outcome is that the center remains in tension, and the outside goes into compression, giving tempered glass its strength.

Glass can also be chemically tempered. The glass is submerged in a molten potassium salt bath that causes the sodium ions in the glass to be replaced with larger potassion ions. The larger potassium ions fill the gaps left by the sodium ions, which creates a state of compression in the outer surface of the glass. This method is more expensive than using a tempering oven, so it is not as widely used.

A disadvantage of tempered glass is that it cannot be re-worked once tempered, so it must be shaped completely before the process. Also, because of the balanced stress of tempered glass, if any part of it is damaged the whole piece of glass is likely to shatter.

Tempered glass is a type of safety glass, used for car windows, entrance doors, and other applications where glass shattering may endanger humans.

Laminating is another way to create safety glass. Laminating involves strengthening the glass with a plastic material interlayer. The interlayer not only strengthens the glass but also holds glass pieces together if broken, preventing it from shattering.

There are a few laminating procedures. Two or more pieces of glass are bonded, using heat and pressure, between one or more layers of adhesives, usually polyvinyl butyral (PVB) or ethylene-vinyl acetate (EVA). Another method is to bond two or more pieces of glass with an aliphatic urethane or EVA interlayer, using heat and pressure. Glass can also be interlaid with a cured resin or with EVA.

Laminated glass is difficult to cut because of its plastic layers, but not impossible. When laminated glass is damaged, it usually cracks in a spider web pattern instead of shattering into multiple dangerous pieces.

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