1.1.11 Tempered Glass

Car rear window

\7

Cooling marks from air jet

Fig. 1.8 The telltale signs of "cooling"marks indicating tempered glass.

regions cool too fast, preventing the compression and tension forces to develop. In addition, tempered glass cannot be used in high thermal environments because the constant heating and cooling cycles would eventually untemper the glass.

Tempering and heat-strengthening glass decrease its flexibility as well as increase its ability to resist impact abrasion. Heat-tempered or heat-strengthened glass cannot be cut once it is tempered. However, both types of glass-strengthen- ing processes can be reversed by simply annealing the glass. Once the strengthening has been reversed, the glass can be cut in any normal fashion.

If you have a pair of polarized sunglasses, you are likely to have seen the effects of tempered glass on rear and side windows of cars.* The telltale white or gray spot array pattern is an indication of where the cold air jets were located (see Fig. 1.8).

There are other techniques used to temper glass. One technique is called ion exchange. In this process, glass is "boiled," or soaked, in liquid potassium, during which the surface sodium ions exchange places with the potassium ions. The potassium atom is larger than the sodium atom, so during ion exchange the potassium must "squeeze" itself into positions that formerly held the sodium. This exchange produces the same compression effect on the glass surface as rapid cooling. Although it is a more expensive process, ion exchange tempering produces a more uniform compressed surface than the air jet process. In addition, the ion exchange process can temper much thinner pieces of glass than using a heat approach. Ion-exchanged tempered glass cannot be untempered by annealing the glass as with heat-tempered glass. When glass catalogs mention that all (or part) of a piece of glassware is strengthened (like the tip of a burette), it is typically done by blowing cold air on hot glass, and not by ion exchange.

*The front windshield of a car is not tempered. Rather, there is a piece of plastic sandwiched by two pieces of glass. This prevent, pieces of glass from flying into the driver's face, and it maintains visibility because broken tempered glass is difficult to see through.

Although both heat and ion tempering both successfully strengthen glass, heatstrengthening and ion tempering do not satisfy either ANSI or federal specifications for safety glazing.

A third technique to produce tempered ware uses two glasses (or ceramics) formed together (laminated), each with different thermal coefficient of expansion. This is used to make Correll® dishes by Corning. For this tableware, a pyroceramic* material with one type of thermal coefficient of expansion is covered with another pyroceramic with a greater thermal coefficient of expansion and is then baked until the outer layer melts uniformly. Materials with greater thermal coefficients of expansion will expand more when heated and will contract more when cooled. The greater contraction (once cooled) of the outside material causes compression.

1.1.12 Glass and Internal Pressure

Because tubing walls1^ on a vacuum system are in compression, vacuum systems do not require heavy walled glass. On the other hand, pressure on the inside of a tube creates tension on the walls of the tube. If the tension is great enough, the tube will explode from internal pressure. The strength of tubing to withstand a given amount of pressure is called its bursting strength. To increase the bursting strength and withstand internal pressure requires heavy-wall or small-diameter tubing. This requirement is based on the ratio of the diameter of the tubing to the wall thickness. For example, 2-mm tubing has an inside diameter of 1.0 mm. For 100-mm tubing to have the same percentage wall thickness, it would need an internal diameter of 50 mm. Standard 100-mm wall tubing has an inside diameter of 95.2 mm.

Wall thickness is not the only factor that determines the potential bursting strength of tubing. Glass strength is also based on surface quality and preexistent strains. Either of these properties can significantly jeopardize the theoretical bursting strength of tubing.

The quality of the surface is based on the amount and degree of surface flaws. Obviously, abusive handling of glassware can cause flaws, but so can normal glass handling, such as laying a piece of glass on a table or against other glass. Any visible surface markings can significantly decrease the potential strength of a glass, but not all flaws are readily visible.

A preexistent strain is any stress within the tubing prior to receiving internal pressure. These flaws can come from poor-quality annealing (thermal strain) or torquing the glass during assembly (physical strain). The extent to which flaws and inherent strain will affect a glass's bursting strength cannot be predetermined.

*A pyroceramic material is a special type of glass that has been devitrified by controlled nucleation of crystals. It has certain properties that are like glass and other properties that are like ceramics.

Standard round tubing, not square, flat, or distorted walls.

34 Materials in the Lab

Therefore, any calculations determining bursting strength must have built-in safety tolerances to account for the unknown.

The glass industry assumes that new tubing, with no observable flaws, can withstand stresses of up to 4800 psi. It has been calculated that the maximum amount of stress applied to lightly used glassware should not be greater than 1920 psi. However, typical laboratory glass receives a great amount of surface abuse, so an even greater safety factor should be considered. Unstrained laboratory glassware should not receive stress greater than 960 psi. If preexisting stress in the glassware is considered as a safety factor, the tolerable stress should again be lowered to 750 psi.

Despite such a large safety factor, any pressurization of glassware should be done behind a safety shield. In addition, glassware should be wrapped in a threaded tape such as cloth surgical tape or fibered packaging tape. Both of these tapes lose their adhesive ability and strength as they age, so periodically check the quality of the wrap. The tape serves an additional function by protecting the surface from abrasion which could weaken it. Masking tape is not satisfactory for either purpose and should not be used.

To calculate the tangential or hoop* strength, Shand27 offers Eq. (1.2). When stress created by internal pressure is greater than the strength of the tubing (based on the location of a potentially susceptible flaw), the tubing will fail (break).

p = pressure

d = inside diameter

D = outside diameter.

Unfortunately, this simple equation is only good when a tubing's wall thickness is 5% or less of the total outside diameter, thus omitting most of the tubing found in a laboratory. Shand offers the somewhat more complicated equation for calculating the hoop strength for all sizes of laboratory tubing:

W\2

5

The average stress (da) is the amount of stress that the system is receiving with any given tubing dimensions and under the pressure loads provided. The internal

"The area around the circumference of the tube is called the hoop, similar in concept to the hoop surrounding a wooden wine barrel.

stress (a,) is the amount of stress placed on the internal wall by any pressure on the inside of the glass.

If Eq. (1.4) is solved for pressure and we substitute for c; the allowable bursting pressure of 750 psi, we can calculate the acceptable pressure limits for commercially available tubing.

P =

> • *

l~D

As an example of safe pressure limits of a standard commercial tubing, consider one-half-inch tubing and let O = 750 psi. Standard wall tubing* can withstand a pressure of 173.1 psi, one-half-inch medium wall tubing can withstand pressures of 268.3 psi, and one-half-inch heavy wall tubing can withstand pressures of 420.4 psi.

Shand28 also provides Eq. (1.5) for flat plates.

This equation can also be used on rectangular plates by substituting the narrow dimension of a rectangle for d.

WARNING: Observation of a flaw on the surface of, or within, a piece of glass that is to be used in any pressure situation warrants discarding that piece and replacing it with a new, flawless piece.

Medium and heavy wall tubing were originally measured in English measurements and now use the metric equivalent. Standard wall tubing has always been measured in metric measurements. Because there is no one-half-inch standard wall tubing, 12-mm tubing was used for this example.