1.1.9 Stress in Glass

3.When rotating a stopcock that is free-standing (i.e., one end of the stopcock is attached to glassware and the other is neither attached to another part of the same apparatus nor supported by a clamp), eliminate the lever arm by holding the stopcock with one hand and rotating the plug with the other hand. If the plug requires grease, be sure to clean and change the stopcock grease at regular intervals. Old grease is harder to rotate and creates more torque when rotated, and silicon grease needs to be changed every couple of months. Be sure to use stopcock grease and not joint grease: Greases made for stopcocks have more lateral "slip" than do joint greases, and therefore they develop less torque when rotated (see Sec. 3.3.3).

4.In assembling and disassembling distillation apparatus (typically composed of sections of varying lengths, curved pieces, and pieces of different shapes connected by joints with no lateral movement), tremendous torque can be created when the piece is secured by support clamps. Use caution and common sense when attaching and tightening support clamps so as not to torque pieces toward or away from each other. An additional problem involves joints, or stopcocks, which have become frozen because either distillation solvents have stripped them of grease, or improper cleaning has left dirt on joint or stopcock members. Tremendous torque can be created when trying to disassemble stuck joints and stopcocks on a distillation assembly. Prevention of stuck joints and stopcocks is achieved by selecting greases that will not be affected by the solvents used and/or by using Teflon sleeves (these sleeves are excellent because they cannot be affected by most solvents). Always be sure that joint members and stopcocks are clean and dust-free. Wipe both members with acetone on a Kimwipe before applying grease or Teflon sleeves and assembling the joint or stopcock. (See Sec. 3.3.7 for more suggestions on how to free stuck stopcocks and joints.)

In summary, limit physical abrasion to glassware, reduce the lever arm, and provide adequate lubrication to moving items. Your foresight will reduce glass breakage.

Fracturing glass at the site of a flaw is based entirely on the amount of tensile stress (or deformation) at a specific location, rather than on the amount of stress the glass was put under to achieve that level of tensile stress. More stress must be placed on thick glass than on thin glass to achieve a given amount of flexing (and resultant tensile stress on the surface). In other words, thick glass is physically stronger than thin glass because it is harder to flex. Unfortunately, the risk of broken glass in the lab cannot be resolved by making all glass thick because of thermal stresses.

Thermal stresses are more difficult to imagine than physical stresses, but we can observe what causes them anytime we look at a liquid thermometer. When materi-

als get hot, they expand. When glassware becomes uniformly hot, it expands uniformly. When glassware becomes nonuniformly hot (above its strain point), there is uneven expansion of the glass. The regions between the uneven expansion will develop strain; and the strain will remain if the glass was heated past the strain point, even after the glass has cooled. This strain can be great enough to cause fracture if there is a flaw in the region of the strain. We've all seen the effects of thermal stress when we've poured a hot liquid (i.e., freshly brewed tea) on ice cubes. The warmer exterior of the cube expands faster than the cooler interior, and trapped air bubbles provide the flaw that causes the ice to fracture.

Different materials expand at different rates when heated. Different types of the same materials can have different rates of expansion as well. Thus, different types of glass have radically different expansion properties. The measure of the rate of expansion of materials is the thermal coefficient of expansion stated in Acm/cm/ °C. Because the thermal coefficient of expansion of any material varies as the temperature varies, the thermal coefficient of expansion that is attributed to any particular glass is based on an average of the expansion figures compiled from a 0-300°C range. Table 1.2 includes the thermal coefficients of expansion for a variety of glasses.

The larger the thermal coefficient of expansion number, the less radical a temperature change the glass can withstand and vice versa. However, a thin piece of glass with a high thermal coefficient of expansion will be able to withstand a more radical temperature change than a very thick piece of glass with the same or lower thermal coefficient of expansion.

We see the effects of a high thermal coefficient of expansion when we pour boiling water in a cold drinking glass: the glass breaks. We see the effects of a low thermal coefficient of expansion when we take quartz glass at melting temperatures and plunge it into ice water with no problems. Telescope mirrors are made out of materials such as quartz glass because they do not distort with changes of temperature to the degree that glasses of higher thermal coefficients of expansion will.

1.1.10 Managing Thermal and Physical Stress in the

Laboratory

Glassware manufacturers must determine whether an item is more likely to be used in a physically stressing environment or in a thermally stressing environment. For use in a more physically stressing situation, glass must be made thicker to achieve physical strength, and for use in a more thermally stressing situation, glass must be made thinner to achieve thermal strength. It is not possible to make glass optimally strong for both physically and thermally stressful environments: A compromise must always be struck.

We see very thick glass used for kitchenware such as measuring cups and bakeware. Kitchenware receives minimal heat stress (baking is a slow heating process and therefore is not a thermally stressing activity) and is more likely to receive

physical stress as it is banged around in cupboards and drawers or nested within other kitchenware. A glass coffee pot, however, is expected to withstand rapid heat changes (and is not generally stacked or banged against other objects) and is therefore made out of thinner glass. Kitchenware and coffee pots are made from exactly the same type of glass (common laboratory borosilicate glass), but are designed to withstand radically different types of stress.

We also see thick and thin glass in the laboratory. Because their concave bottoms could not otherwise withstand the force of a vacuum, filter flasks are made of thick glass. However, do not place a filter flask on a heating plate—it cannot tolerate the (heat) stress. The standard Erlenmeyer, by comparison, is thin-walled, designed to withstand thermal stress. However, a standard Erlenmeyer flask cannot tolerate the physical stresses of a vacuum: The flask's concave bottom will flex (stress) and is likely to implode in regions of flaws.

The shape of glassware can be a clue as to how and/or where it can be used. The more rounded its corners, the better it can diffuse thermal stress. This idea is similar in concept to the sharpness of flaws and can be compared to Eq. (1.1), the stress concentration factor. Although that equation is intended to be used for surface flaws on glass, the principle is the same.

The three most frequently used laboratory containers are the beaker, the Erlenmeyer, and the round-bottom flasks. Their shapes, radii of curvature of their bases (see Fig. 1.6), and functions are all different. If you measure the radii of curvature at the bases of 250-ml beakers, Erlenmeyer flasks, and round-bottom flasks, you will find that they are approximately 6 mm, 12 mm, and 42 mm, respectively. Because the bottom of a beaker is essentially a right angle, as the bottom expands, when subjected to heat, the walls receive stress at right angles. There is little, if any, means to diffuse the stress. Because the Erlenmeyer has a larger curvature on its base corners, it can diffuse thermal stress, but only to a limited degree. Because the round-bottom flask is all curvature, it is best suited for diffusing thermal stress. If you are using a Bunsen burner flame as a heat source, the safest heating vessel to use is clearly a round-bottom flask. If you must use an Erlenmeyer flask or beaker, you can diffuse the intensity of the flame by placing a wire square

Radius of

curvature

Fig. 1.6 The radius of curvature on the bottom of a container is smallest on the beaker, greater on the Erlenmeyer, and largest on the round-bottom flask.