Laboratory Uses for Polyethylene

[lugh]

Some ideas for using various polyethylene and polytetrafluoroethylene products in the laboratory, some require varying degrees of mechanical skills and equipment:

Labeling Plastic Bottles
The difficulty of making paper labels ad­here to the smooth sides of polyethylene and similar plastic bottles used to contain reagents in the laboratory may be over­come by marking the boule itself with a sharp instrument such as a heavy needle or carborundum marking pencil. Engrave the lettering directly into the side of the bottle and fill with a colored wax pencil or graduate filler. Wipe off the excess filler to produce a legible, permanent label.

Storing Reagent Botties in Polyethylene Bags

The release of corrosive or obnoxious fumes inro the laboratory by the "breath­ing" of bottles containing volatile or viti­able reagents can be prevented by placing each bottle in a polyethylene bag together with an absorbent and sealing the mouth by folding it over several times and then clamping with a large spring paper clasp.
Acidic fumes and halogens released in the bag can be absorbed by calcium hydroxide and basic fumes by sodium bisulfate; a reagent can be protected from water vapor by calcium chloride. Combinations of these or other absorbents may be re­quired in certain instances. Typical rea­gents which can be stored in open shelves by application of this technique are the strong acids. bromine, acid chlorides, thi­onyl chloride, phosphorus trichloride, and anhydrous aluminun chloride.

Suggested Use for Polyethylene Bottles

The chemist is frequently called on to analyze volatile or fuming liquids which must be weighed out in sealed glass bulbs or ampoules. The scaled bulb is then broken under the surface of water or other absorbing liquid depending upon the analytical proedure. This is ordinarily accomplished by vigorously shaking the bulb in a flask with the liquid. OccasionaIly glass beads or broken glass tubing may be added to fadliute breaking the bulb. Unless the bulb has extremely thin walls it is very often quite resistant to breakage and there is always the danger that vigorous shaking niay crack the flask.

It has been found expedient to place the bulb and absorbing liquid in a polyethylene bottle. The bottle is cornpressed to displace most of the air and it is then tighdy sealed. By manipulating thc bulb it niay be brought between the flexible walls of the container and crushed by hand or, if necessary, by a light tap of a wooden mallet. It is desirable to select a bottle of such size that it will be less than half filled with the absorbing liquid so that the walls may be collapsed sufficiently to break the sample bulb. The 16-ounce size will satisfy most requirements. These boules are available at most lab­oratory supply bouses. Very often titrations inay be carried out directly in the polyethylene bottle, although this may not be satisfactory in all cases owing to the translucent nature of polyethylene.
 
Freezing Liquids Safely in Polyethylene Bottles

When water samples or aqueous solutions in glass bottles must be shipped or stored outdoors during cold weather, there is danger that the water will freeze and burst the bottles. This may cause the loss of valuable samples. Corrosive liquids may later cause damage when they thaw out and leak from their broken containers. Further, it is sometimes desirable to store biological solutions for example biochemical preparations, bacteriological cultures and putrescible samples at or below freezing.
Difficulty from breakage can be avoided by using polyethylene rather than glass containers. Because ice occupies about ten percent more volume than the water from which it is made, a cylindrical bottle need only about three percent in diameter to accomodate the added volume on freezing. It has been found that molded polyethylene bottles can readilly expand by this amount without suffering permanent distortion.
Polyethlene bottles of 1, 4, 16 and 32 ounce capacity were tested by completely filling them with water, capping them tightly, and placing them in the freezer eompartment of a refrigeraror. All with­stood the strain of expansion, gently buIging at the sides and bottom to provide the additional required volume.
The bottles of ice were dropped from a height of thrce foot without damage, the only effect being that the ice developed cracks. On thawing, the bottles returned to their original sizes and shapes. Before all the liquid is frozen, pressure tends to cause leakage around the cap, especially if plastic caps with waxed cardboard liners are used. This difficulty can be avoided if the caps are made of, or lined with, polyethylene.
Polyethylene vessels of larger capacity are sometimes fabricated by heat-welding together standard sections such as cylin­ders, cones, discs, etc. Such containers have not been tested to sce if they will withstand the strain of expansion when their contents are frozen.

Polyethylene Stoppers   

Rubber and cork stoppers are olten unsatislactory to use in test tubes or flasks as they may contaminate the contents. A metal-foil covering is also unsatisfactory because the foil often ruptures or is attacked hy corrosivc chemi­cals. Polyeethylene stoppers have wide application in the lahoratory because polyethylene is not dissolved bv strong acids, alkalies or common laboratory solvents at room temperature.
These stoppers may be fabriicated easily ou a lathe from solid polyethylene. The hollow center adds flexibility and thus provides an effective seal. When mixiug liqnids. the stoppers should bc pressed firmly into the test tube to form a tight seal.

Polyethylene stoppers are especialy useful for diluting chromatographic fractions collected in test tube prior to ultra-violet spetrophotometric analysis, and also for rinsing glassvvare with organic sohvents without fear of introducing a contaminant. The stoppers should also be useful in mixing liqiids in color reactions, light-adsorption measurements, polarography, or for any operation in which contamination must be avoided.

Simple All-Plastic Valve

The need for a simple and rugged fluoro­plastic valve having no auxiliary operating parts (springs, pistons, diaphragms) and yet better flow control than a glass stopcock was met by the illustrated device. The valving mechanism is essentially that of an ordinary stopcock except that a slot, E, is used because a hole through the rotating plastic plug, D, would tend to weak­en it too much. No lubrication of this all ­plastic valve is required. The valve, as shown, is intended as a burette stopcock. Tubing may be press-fitted into holes A and F, or the top portion, B, which is turned down, may be pushed inside a burette barrel that has been cut off square.

This all-plastic valve may also be substituted for glass stopcocks in low pressure systems. Further, it could be used at moderate pres­sures with the addition of more rugged con­nections, such as longer tubes with compres­sion clamps or threaded ends for flange-type connections. A C-clamp arrangement to hold the plug in place will increase the effective pressure range. The valve may be used in vacuum applications since there is no stick­ing in the closed position as with diaphragm or needle types. Valves of this design have given excellent service for over one year.
 
Construction. The slot E should be V-shaped and is eut with a sharp knife; it gives better flow control than a square-bottomed machined cut. To assure that the valve is leakproof, the hole for   the plug should be drilled with a sharp drill so that the walis are smooth and the plug should be machined smooth about 0.002 inch larger than the hole. In order to provide a tight seal with glass tubing, holes A and F should be about 0.002 inch smaller than the outside diameter. of the tubing ro used and B should be 0.01 in. larger than the in­side diameter of the barrel of the burette. For vacuum applications, a 0.005-inch oversize machined ring about 0.02 inch wide may be placed on plug D at point G to improve the vacuum seal. Handle C may be fashioned from steel or plastic as desired.

Simple Gas-Tight Siirrer Bearing

The problem of operating mechanical stir­rers in closed vessels is commonly solved by the use of clumsy mercury seals or of relatively expensive stirring glands. A most inexpensive and satisfactory solution is to employ a standard-taper, hollow polyethylene stopper (now available from laboratory supply houses). Such a stopper is drilled to secure a tight fit with the stirrer shaft. (A hole made with a No. 3 cork borer is suitable for a 1/4-in.stainless steel rod.) The stopper is then inserted, as illustrated, into the ground glass joint of the reaction vessel (a No. 6 stopper lits a 29/42 joint).

The bearing is self-lubricating and will with­stand long use if the stirrer shaft is accurately positioned and runs true. When appropriate, the stopper may be filled with a suitable liquid as a further seal. One such assembly was filled with water and run a 1500 rpm for three days without leakage.

Some Laboratory Aids of Polyethylene


Powder Funnel. When polyethylene bottles are cut off near the shoulder to make beakers or centri­fuge cups, as previously suggested, the neck por­tion remaining may be used as a powder funnel.

Scoops of various sizes can be sawed from poly­ethylene bottles, as illus­trated, or test cubes or centrifuge tubes. The neck may be closed with a polyethylene stopper or cap; two scoops are chus obtained at one cutting.

Sample Storage & Mailing Tubes. Small bore poly­ethylene tubing (e.g., inside diam. 0.07-016 in.) is useful for the transport or storage of samples. The tubing is heat-sealed, at one end by warming over a small flame until the transluscence just fades; then the tube is pinched closed with tweezers that have been dipped in water. Liquid samples are introduced by a syringe or a drawn­out dropping pipette, and solid samples in the manner in which capillary melting point tubes are filled. Then the tube is sealed in the described manner. For mailing or storage, the tube may be fastened by pressure-sensitive tape to a 3 by 5-in. card on which the sample description is recorded.

Adjustable Curve. An adjustable drafting curve may be improvised by inserting wire fore, hard solder (95% tin, 5% lead) into a length of 3/16-in, bore, 1/16-in.wall polyethylene tubing about 3/4 in. longer than the wire solder. The ends are heat-sealed in the manner described above.

Polyethylene Ball and Socket Joints and Connecting Lines

Polyethylene bail and socket joints and con­necting lines have been fabricated and evaluated in the assembly of radiochemical apparatus in contained and shielded facilities. Such joints meet ail the requirements of conventional joints, and in addition are resistant to radiation, sufficiendy, flexible to facilitate remove manipulation, leaktight without the use of lubricants, and resistant to breakage.The ball joints are molded with guides which facilitate remote assembly by acting as a temporary support while the joint clamp is brought into place. Capillary-type hall joints, without guides, can be used as valves and flow regulators by tipping the bail in the ungreased socket.
FIGURE 1. CONSTRUCTION OF 12/3 POLYETHYLENE BALL AND SOCKET JOINTS.
  
Fabrication. Polyethylene bail and socket joints
(Figure 1) were molded to fit standard ground glass joints. Molds of 2024 aluminum (Figure 2) were made in the machine shop. The inside moid surfaces should have a roughness of no more than 32 microinches. The molds were heated to 150°, suffcient polyethylene was iuserted to fill the mold at room temperature, and the assembly was compressed in a screw press. The molds were al­lowed to cool slowly to room temperature. In all moldi­ing operations a mold release (Silicone Spray Moid Release) was used to facilitate the removal of the molded part. Compression molding was satisfactory for making joints on a laboratory scale, but joints made by injection molding are inexpensive and can be made by various injection molding manufacturers.
FIGURE 2. CONSTRUCTION OF 12/3 POLYETHYLENE BALL AND SOCKET MOLDS. (Third guide rod omitted for clarity.)

Attachment of Joints. Joints are attached to poly­ethylene tubing by heating the two in intimate contact while they are contained in a short length of glass tubing. A polished metal mandrel is used to maintain the internal dimensions during heat­ing. Both the glass tubing mold and mandrel should be coated with a silicone mold release for easy removal. The polyethylene is heated with a Bunsen humer and becomes transparent when the fusing temperature is reached. The plastic parts are pressed together manually while they cool slowly. Rapid cooling causes voids to form in the polyethylene. This saure procedure can be used in sealing tubular polyethylene parts to glass by fusing the polyethylene over and around the glass part. As the polyethylene cools, it shrinks on the glass and forms a hermetic seal. If the glass tubing moid cannot be slipped from the joint, it can be removed by crushing it cau­tiously in a vise. Alternatively, 2-part molds may be used. Although they are difficult to construct, they are easier to remove from the cast plastic and can be reused.

Evaluation. Leak rates of polyethylene joints were compared with other standard joints and results are given in Table I. All joints were selected at random and were put together dry and the leak rates were tested in a static system as an absolute pressure of 0.010 mm Hg. Polyethylene ball and socket joints showed no noticeable loss of vacuum during a 15-min. test period and less than 0.1 mm. in 16 hours.

[Organikum]

First: Thanks lugh for this marvelous piece of information!

I want to allow myself to add some tidbits - tried and true:

- pieces of the same diameter can be connectec by simply wrapping thick PE-foil around the two pieces to fix the foil with some wire wrapping alufoil around and to put it into the oven at about 130°C for 10 minutes. The prestretched PE will shrink in the heat and fix and seal the pieces gas and liquidproof. If not needed anymore, just cut open with a knife. A hot-air gun can be used instead of the oven cure in special if bigger parts are to be connected.

- often no machined aluminium mold is necessary, a simple gypsum mold will do it. Alufoil prevents sticking but oiling the gypsum with saladoil will do it also and better in most cases. (oiling the alufoil slightly is a good idea anyways)

- last not least:
PE is a good reducing agent for metaloxides for example. Dont waste your waste - use it!


oRg  

[sYnThOmAtIc]

You can precipitate say, copper metal with PE and CuO? I never knew PE was a reducing agent.. Wouldn't the PE need to be dissolved? You couldn't just chop it up into chunks or shave with a rasp to use it, could you?

[Organikum]

and heat it together with the metaloxide.

Thats it.

[roger2003]

Resistance to chemicals and other media:

http://www.basell.com/pdfs/476.pdf