Table 1.6 Comparison of Flexible Tubing Characteristics (continued)

1.3Corks, Rubber Stoppers, and Enclosures

1.3.1Corks

Cork, a thick, lightweight product from a Mediterranean oak, has been used in the laboratory for years in many ways. Typically, it is used as seals for glassware, as rings for supporting round-bottom flasks, and as sheets to protect surfaces from impact shock. Despite the incredible variety of plastics and other elastomers available, cork is still the material of choice for many operations in the laboratory.

Corks (stoppers made from cork) are still widely used to cap many materials within glassware. They are essential when storing organic solvents or other materials that could react with rubber stoppers. Each cork typically fits several different size tubes. An examination of various cork sizes is displayed in Table 1.7.

Corks are graded into five levels of quality: X, XX, XXX, XXXX, and Select. The grades are an assessment of the number and degree of irregular cavities and cracks on the walls of the cork. Grade X is the lowest quality and Select is the highest. As would be expected, the better the quality, the more expensive the cork. Regardless of the stated quality, always examine the integrity of the cork before assuming that it will contain your solution. Cork quality can vary significantly, even in better grades.

ments and meet FDA requirements. They are resistant to high-aniline point oils and chlorinated di-phenyls but are not recommended for most petroleum fluids, ketones, or water and steam.

1.3.3 Preholed Stoppers

Stoppers can be ordered with none, one, or two holes. As can be seen in Table 1.8, holes are sized according to the size of the stopper. One brand of white stoppers (Twistit®, made by the J.P. Stevens Co.) has three "pre-holes." You simply twist/ tear off small nipples on the bottom of the stopper to make the holes as needed.

Obviously, a hole of 3 to 5 mm cannot take a very large tube. In general, it is safe to insert a tube which is about 1 to 2 mm in diameter larger than the original hole size. As can be seen in Table 1.8, you can only insert 6- to 8-mm-diameter tubes or rods into the holes of size 2 or greater stoppers (about the diameter of the average glass thermometer).

If you find that the holes in your "preholed" stopper are the wrong size or in the wrong location, you can take a solid stopper and custom drill holes which meet your specific requirements. There are several techniques for drilling holes in stoppers, but regardless of the technique used, always start with a solid stopper. Because rubber distorts with pressure, it is more difficult to alter a holed stopper than to drill a new hole. In addition, because elastomers distort and compress, drill hole(s) the same size (rather than slightly smaller) as the tubing you wish to insert. This practice will allow easier insertion of the tube into the hole, yet will still provide a satisfactory leakproof seal. When the stopper is placed into its final location, compression from the container neck will further improve the seal.

Freezing and Drilling. Because of the flexibility and friction of rubber, it is essentially impossible to use a standard drill bit on a rubber stopper. However, if you freeze the rubber stopper in liquid nitrogen or pack it up in crushed dry ice, you can then drill a hole with a standard drill bit and drill press (or a machinist's lathe). Never hold the frozen rubber stopper with your hands while drilling. The stopper is extremely cold, and you will injure your fingers just by holding the rubber stopper. Furthermore, the rubber stopper may start to defrost during the drilling (the drilling friction will cause heat), or the rubber could grab the drill bit and spin out of your hand. It is equally recommended to use a drill press or machinist's lathe and not a hand-held drill. You cannot drill straight with a hand drill, and the potential for slipping is far too great. Be sure the drill bit is new or newly sharpened. Dull bits cause more friction, causing the stopper to heat up faster. Drilling speed should be at the same slow speed used for plastics. Let the bit cut its way into the stopper by itself, and do not force the bit into the stopper while drilling.

When drilling rubber stoppers with a drill press, do not use a rag to hold the frozen rubber stopper. If the rubber grabs the drill bit, the rag could catch rings,

Table 1.7 Comparisons of Cork and Rubber Stopper Sizes

Cutting edge

I.D. O.D.

Fig. 1.12 A hand cork borer.

there is constant demand for custom-sized rubber stopper holes in your laboratory, it can be worth the expense. When selecting the proper bit with which to drill, use the cutting edge (inside diameter), not the outside diameter, as your guide for size.

Motorized cork boring requires a lubricant. There are two choices: glycerin, which is also used with a hand cork borer, and beeswax (or paraffin). To use beeswax, or paraffin, you must to momentarily press the cutting edge of the bit against the stopper so that the friction heats up the bit. Then, press a block of wax against the edge of the turning bit, allowing the melted wax to coat the drill. Now you can begin drilling the hole using an even, light to moderate pressure. After the beeswax cools and hardens, it can be easily picked off. Although it may seem like a bit more work to use paraffin than glycerin, it may be preferable because there is less to clean up afterward, the cleanup is easier, and the wax is less messy than glycerin. Glycerin can be wiped off with a rag, although some soap and water may be necessary for a more complete cleaning.

Never force a bit through a stopper, but rather let a bit cut into a stopper on its own. It may be necessary to add more beeswax onto a bit while you drill. This addition can be made by extracting the bit from the stopper, pressing the block of beeswax against the bit, then continuing the drilling process. If you force the bit

Motor (in back)

Replaceable drill bit

Drilling stand

Lever arm

Fig. 1.13 Motorized cork borer.