Chemically strengthened glass

Chemically strengthened glass is a type of glass that has increased strength as a result of a post-production chemical process. Chemical strengthening is the name given to glass products that have been strengthened by means of an ion-exchange process. It is a surface treatment which occurs at a temperature lower than glass melting temperature. The process is particularly useful for thin glass, tiny glass and shape glass which cannot be tempered by ordinary physical tempering.

Chemically strengthened glass is typically six to eight times the strength of float glass. In the case of breakage, chemically strengthened glass breaks into bigger pieces which are not as sharp as those of non-toughened glass. The surface compression condition which is higher in the case of a chemically strengthened glass also involves an increase of flexion resistance, which is one of the main characteristics of chemically strengthened glass.

Chemical strengthening results in a strengthening similar to toughened glass. Chemically strengthened glass has little or no bow or warp, optical distortion or strain pattern. This differs from toughened glass, in which slender pieces can be significantly bowed.

Chemically strengthened glass may be cut after strengthening, but loses its added strength within the region of approximately 20 mm of the cut. Similarly, when the surface of chemically strengthened glass is deeply scratched, this area loses its additional strength. Chemically strengthened glass retains its colour and light transmission properties after treatment.

Chemically strengthened glass offers an improved scratching, impact and bending strength, as well as an increased temperature stability.

Manufacturing process

The glass is chemically strengthened by a surface finishing process. The glass to be treated is dipped into a bath of dissolved potassium salts at a temperature about 380oC for duration from 4 to 30 hours, producing an ionic exchange between the superficial sodium ions in the glass and potassium ions inside the bath. The cycle time would be greatly reduced if the glass is made of certain elements such as lithium or magnesium because ion mobility between potassium and these elements is a lot faster. The process parameters such as ion exchanging time and temperature would be modified according to the type of glass to be treated and the required strengthen specification.

The introduction of potassium ions which are larger in size than the sodium ions results in the establishment of a system of residual stress characterized by compression stretches on the surface counterbalanced by traction stretches within the glass

Sodium ions and thus, creates stress on glass surface. During cooling, the potassium on surface shrinks little while the sodium in inner shrinks larger. Hence, stress is induced between glass surface and inside and consequently, the glass is strengthened.

Advanced process

There also exists a more advanced two-stage process for making chemically strengthened glass, in which the glass article is first immersed in a sodium nitrate bath at 450 °C, which enriches the surface with sodium ions. This leaves more sodium ions on the glass for the immersion in potassium nitrate to replace with potassium ions. In this way, the use of a sodium nitrate bath increases the potential for surface compression in the finished article.

Classifications

Chemical strengthened glass is classified by two strength components: surface compression and depth of layer (DOL). Surface compression values relate to flexural (bending) strength (MOR), impact strength, hardness penetration (scratching) and thermal shock resistance. Depth of layer values relate primarily to the amount of sustained abrasion resistance and the impact resistance of the surface compression layer.

Applications

Chemically strengthened glass was used for the aircraft canopy of some fighter aircraft. The chemically treated glass boasts a transparency range from the UV through the visible and into the infrared. This permits weapons systems designers to operate guidance devices whether they are CCD, radio frequency, infrared or laser based. The material's proponents stress that chemically treated glass is not just for use in military applications.

It can be used in numerous applications that demand toughness and optical clarity. The material is also useful for viewports, protective covers, and front surface optics in hostile environments whose elements may include high temperature, high pressure and vacuum conditions. Less demanding applications include point of sale scanner windows used in grocery store and retail scanners.

Annealed Glass

Annealed glass is glass produced without internal stresses imparted by heat treatment, i.e., rapid cooling, or by toughening or heat strengthening. Glass becomes annealed if it is heated above a transition point then allowed to cool slowly, without being quenched. Glass is treated with heat in order to change its properties by the annealing process. Annealed glass is the most common glass used in windows. Annealed glass is also known as a standard sheet of float glass.

Annealing is actually a process of slowly cooling glass to relieve internal stresses after it is formed. The glass, formerly annealed on shelves in a melting furnace, is now usually carried on rollers through temperature-controlled kiln known as a Lehr (annealing ovens). The shaped glass is annealed to relieve stresses caused by manipulation, then is slowly cooled.

Glass which has not been annealed is liable to crack or shatter when subjected to a relatively small temperature change or mechanical shock. Annealing glass is critical to its durability. If glass is not annealed, it will retain many of the thermal stresses caused by quenching and significantly decrease the overall strength of the glass.

Annealing Process



The glass is heated until the temperature reaches a stress-relief point, that is, the annealing temperature (also called annealing point) at a viscosity, η, of 1013 Poise = 1012 Pa•s, at which the glass is still too hard to deform, but soft enough for the stresses to relax. The piece is then allowed to heat-soak until its temperature is even throughout.

Soaking is a process of subjecting glass to a steady temperature. The higher the temperature the glass is soaked at, the shorter the period the glass needs to be exposed to such a temperature. Of course, glass exposed to very high temperatures requires longer time to cool down.
Caution should be taken to not expose the glass to a temperature that can adversely affect its structure. On the contrary, when glass is annealed at lower temperatures, it takes longer soaking time but requires commensurately less cooling time. The type of soak a glass should be subjected to depends on the type of glass.

The time necessary for soaking varies depending on the type of glass and its maximum thickness. The glass is then slowly cooled at a predetermined rate until its temperature is below the strain point (η = 1014.5 Poise). Following this, the temperature can safely be dropped to room temperature at a rate limited by the heat capacity, thickness, thermal conductivity, and thermal expansion coefficient of the glass. After the annealing process the material can be cut to size, drilled or polished.

At the annealing point (η = 1013 Poise) stresses relax within several minutes, while at the strain point (η = 1014.5 Poise) stresses relax within several hours.[2] Stresses that are still present below the strain point are permanent.

Float glass is annealed during the process of manufacture. However, most toughened glass is made from float glass that has been specially heat-treated. Annealed glass breaks into large, jagged shards that can cause serious injury, thus considered a hazard in architectural applications.

Care should be taken when choosing locations to install annealed glass. Building codes in many parts of the world restrict the use of annealed glass in areas where there is a high risk of breakage and injury, for example in bathrooms, indoor panels, fire exits and at low heights in schools or domestic houses.

Annealed glass has the surface strength that provides the wind-load performance and thermal-stress resistance needed in most architectural applications. In areas of high wind loads, or in conditions where higher-than-normal thermal stresses occur, heat-treated glass may be required.

Curved Annealed Glass

Curved annealed glass is used in applications that do not require the use of safety glass. This includes shop windows and display counters. Curved annealed glass can be produced for custom designs and has the advantage of being able to be cut and processed after it has been bent to its desired shape/form.

Acid-Etched Glass

Acid etching is a process that uses a strong acid to cut into another substance. It is used for both industrial and artistic purposes. For example, etching can be used to prepare flooring like cement for painting or refinishing, while artists use it to create detailed pictures on metal or glass.

Acid-etched glass has a distinctive, uniformly smooth and satin-like appearance. Acid-etched glass admits light while providing softening and vision control.

Origin of Acid-etched glass

During the middle ages, acid glass-etching was somehow clouded with controversy since its acid medium, hydrofluoric acid, caused too much of a health risk to the artisans. In fact the acid was so potent that users were found to have been poisoned even by its mere fumes. Accidents most often happened where a skin contact with the acid dissolved into the tissues, which later resulted in mutilation or loss of the artisan’s fingers. As a result, acid etched glass craftsmanship lacked refinement and thus lost its luster as an art collection.

Now, there are etching tools such as swivel knives, pick knives, adhesive masks aside from the squeegee which makes it possible for an ordinary person to work on acid glass-etching.

Acid-Etching Glass production

Acid etched glass is produced by acid etching one side of float glass. Etched glass is created by cutting a design stencil that is made of an abrasive resistant material, such as vinyl or rubber. The resulting stencil is called a resist. The resist is then secured onto the glass to be etched. A blaster gun, powered by an air compressor, is used to bombard the glass with the abrasive. Every part of the glass that is not covered by the resist will take the frosted effect while the parts protected by the resist will remain clear, thus producing a piece of etched glass.

Etching glass - Hydrofluoric acid

Glass is etched by hydrofluoric acid, or by hydrofluoric acid gas. The gaseous acid has the property of producing a surface which resembles ground glass in its appearance; the liquid acid produces clear etching. Etching glass, therefore, consists of 2 distinct branches. First, the production of a dull image on a clear surface (when the gas is used) and second, the production of a clear image on a surface previously ground or dulled by means of the liquid acid.

The glass plate to be etched is cleaned and gently warmed until hot enough to melt wax. The surface is then covered with an equable layer of white wax, by rubbing the wax over it. When cold, the design is cut out of the wax with a graver. A shallow leaden trough, about the size of the plate (but a trifle smaller) is obtained, into which is placed a small quantity of finely - powdered fluorspar. This must be weighed and then gently sifted over the bottom of the trough. To every 2 parts by weight of fluorspar add 3 of good oil of vitriol. Stir quickly with a wooden stick, and place on the hob or other warm place. Vapour will soon rise.

Now the trough is removed and covered over with the waxed and graved plate, wax side downwards. In a very short time, the acid will have etched the bare portions of the glass. When sufficiently etched, remove the wax by melting. To prepare the liquid acid for clear etching, place 2 parts fluorspar and 3 of sulphuric acid in a leaden retort, the tube of which must dip into a leaden bottle half - filled with water.

Apply heat to the retort as long as the water will absorb the fames generated. If a ground glass be prepared with wax, as above, and a ledge of wax or putty be made round it, on pouring the liquid acid on the plate, clear lines on the dull ground will result; or a "flashed" colored glass may, by the same means, a colorless picture on a colored ground can be done. The sheets of clear glass may themselves be dulled by exposing them, without previously waxing, to the fumes of the acid gas.

Applications

Acid etched glass is perfect for both interior and exterior applications. Architecture and construction, like in houses, restaurants, hotels, commercial buildings, etc. They are found in many residential applications such as home decoration like furniture components. Some of the suggested applications are:

• Interior partitions
• Railings
• Shelves
• Shower and bath enclosures
• Doors and windows
• Glass walls
• Kitchens
• Interior and exterior doors

Body-tinted Glass

Body-tinted glass is normal float glass into whose melt colorants are added for tinting and solar-radiation absorption properties. This tinted glass saves energy and reduces heat penetration into buildings and gives a striking visual effect. Coloured glass is an important architectural element for the exterior appearance of façades.

Tinted glass refers to any glass that has been treated with a material such as a film or coating, which reduces its ability to transmit light. Glass can be tinted with various types of coating, which block and/or reflect different amounts and types of light, according to the needs and preferences of the consumer. Glare reduction is another important property of tinted glass. Glare

The production process of body-tinted glass is similar to that of float glass. The only variation is in the colorants mixed at the beginning with the standard raw materials. Body-tinted glass is produced when colorants and iron are introduced during the glass manufacturing process. Different additives may produce differently coloured glasses. Bronze, dark grey and green are the commonly used tints.

The end product does not affect the basic structure of the glass itself, but does enhance its performance in relation to the (solar) electromagnetic spectrum. The colour is homogenous throughout the thickness of the glass. The solar energy transmission, shading coefficient and visible light passing through the tinted glass will vary according to the colour selected.

During the float glass melt process, chemical colorants can be added which tint the colour and increase absorption from the sun. This helps minimize the solar radiation that enters a building, keeping it cool from the inside and protecting furniture from fading. As an example of the colorants used - to create a purple exterior, manganese is added, while pinks and reds can be produced from selenium.

Colorants and colors

Some of the most-used colorants and the colours they produce are listed below:

Iron – Green, brown, blue
Manganese – Purple
Chromium – Green, yellow, pink
Vanadium – Green, blue, grey
Copper – blue, green, red
Cobalt – blue, green, pink
Nickel – yellow, purple
Titanium – purple, brown
Cerium – yellow
Selenium – pink, red
Gold – Red
Cadmium-Sulphide – yellow
Carbon & Sulphur – amber, brown

Double-Glazed with High-Performance Tinted Glass

Tinted Glass is intended for universal application. Either as single or double glazing for a basic level of solar control, and even in furniture, interior design, partitions, etc. It is also the base glass for many high performance comfort glasses.

Doubly-glazed tinted glass reduces solar heat gain to below that of bronze or gray tint but has a visible transmittance closer to clear glass. High-performance or spectrally selective tinted glass products are typically light green or light blue. The tint has no effect on the U-factor but reduces solar heat gain. Doubly glazed tinted glass allows 51 percent of solar heat gain and 69 percent transmission of visible light.

Advantages

• Saves energy, controls solar heat and gives a striking visual effect
• Meets the increasing demands for light in workplaces, creates attractive interiors and gives a feeling of spaciousness
• Offers a practical, stylish alternative to traditional materials when used in screens, partitions and furniture at home or in the office
• Gives designers the freedom to create attractive modern environments that are also economical and easy to maintain
  

Body tinted glass gives the added benefit of making a building look unique and contemporary, creating a lasting impression for business HQs.
Applications

The range of available thicknesses enable glass to be used where superior strength, greater spans, reduced deflection, higher daylight transmission and enhanced noise suppression are required.

Automobiles

One of the most common applications of tinted glass is in automobile windows. Almost all cars come with tinting at the top of the windshield to reduce solar glare when the sun is low in the sky. Apart from this, the windows of several cars are tinted either at the factory or as an aftermarket add-on by the consumer, to provide privacy to the car’s occupants, as also to reduce the build-up of heat in a car while it is parked outdoors.

Dwellings

Another popular use of tinted glass is in windows of homes and commercial buildings. Residential glass tinting is much easier to do than automotive tinting. It can even be done by the homeowner himself, with some practice. Tinted glass in homes serves many practical purposes, such as limiting ultraviolet light transmission through windows, and reducing overall heat gain inside the home by reflecting solar heat energy, thereby saving the homeowner money on air-conditioning.

Commercial Buildings

Tinted glass is also used in commercial buildings. Apart from keeping the interiors cooler, it gives the outside of a building a more uniform, aesthetically pleasing appearance. Depending on the creative use of different colours of tinted glass, the building can also take on a unique and interesting appearance while being insulated from the sun at the same time.