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Have mercy on your lowly technician; never, ever use vinyl electrical tape inside a logging tool! Even if you only log shallow cool holes, tools become extremely hot internally laying around on the rig floor on sunny days, and that turns even good grades of vinyl electrical tape into a gooey annoying mess. For trivia buffs, vinyl electrical tape was not patented until 1946 by 3M. Earlier attempts to produce a workable vinyl tape failed because of compatibility problems between the adhesive and the plasticizers added to make vinyl flexible. Before vinyl electrical tape there was friction tape, manufactured from tar-soaked cotton and a potentially corrosive vulcanized rubber adhesive (often wrapped over rubber tape). Interestingly, the original standard vinyl electrical tape was yellow, followed by white, and finally the familiar black, because it performed better in ultraviolet rich sunlight, and electricians liked that it was the same color as the familiar old friction tape. A roll of pressure-sensitive adhesive (PSA) tape looks like a simple contrivance, but the materials science involved is quite complex. A typical tape will consist of a release coating, the backing film, a primer, and an adhesive, not to mention the technology that goes into making these various components function well together.
Some Useful PSA Tapes for Logging Tool Applications
Polyester Film Tape, that (usually) yellow stuff more commonly called Mylar, is the cheapest decent tape option for use in downhole logging tools. Half inch is the size to get if you intend to purchase only one width. It can be used to tape wires or chips in place, and half inch is just the right width to conform to irregular surfaces when taping CCL coils and the like. Mylar is a 130° C (266° F) class tape, usable for days to weeks up to 155° C (311° F), and for a few hours up to 175° C (347° F). Mylar tape often has a thermosetting rubber adhesive system which exhibits superior tackiness (stickiness), but is also made with a slightly less tacky acrylic thermosetting adhesive system. For maximum adhesion and solvent resistance with these thermosetting adhesives, heat curing is required, but is not usually necessary. In Mylar tape, we like the 3M Scotch Brand Number 56 with a one mil thick film (2.3 mil overall with adhesive); a roll of 1/2 inch 56 should be around seven dollars...a good investment. 3M black 1318-1 with acrylic adhesive is ideal for a final light-blocking layer when taping photomultiplier tubes (PMTs) to scintillation crystals (the better back 3M 1169 with rubber adhesive has been discontinued). Polyester / Mylar tape is also available in a range of other colors including clear and white, and a variety of film thicknesses with one and two mil being the most common.
Teflon Film Tape is often used for temperatures that exceed those suitable for Mylar. Teflon tape is expensive, typically twenty to thirty dollars per 1/2 inch wide roll in small quantities. As you might imagine, it is no small engineering feat to get adhesives to perform well and reliably on the ultimate non-stick surface. Beware of Teflon tape in the surplus market, it is often "seconds" material with defective adhesive. Teflon tape usually has a silicone thermosetting adhesive system producing a 180° C (356° F) class tape, usable for days to weeks up to 250° C (482° F), and for a few hours up to 300° C (572° F). A few Teflon tape products have an acrylic thermosetting adhesive system producing a 155° C (311° F) class tape, usable for days to weeks up to 175° C (347° F), and for a few hours up to 200° C (392° F). Because the silicone and acrylic adhesives used are both thermosetting, maximum adhesion and solvent resistance is achieved after heat curing, but curing is not usually necessary. In Teflon tape, we like 3M Scotch Brand Number 62 with a two mil thick film (3.5 mil with adhesive). It has a treated bondable backside which promotes higher adhesion to its own backing (and makes it printable), but the carrier strip is a bit of a pain to peel off. 3M 60 is a conventional two mil thick film tape (3.5 mil with adhesive) that is easier to use if you do not need the extra bit of self-stickiness, or a printable back. Teflon tape is available mostly in an ugly natural color ranging from grayish to tannish, and two mil film is usually the thinnest available with three and five mil film also common.
Polyimide Film (Kapton) Tape is also sometimes seen in logging tools. Polyimide / Kapton tape is also expensive, typically twenty to thirty dollars per 1/2 inch wide roll in small quantities from premium suppliers like 3M, but it is also marketed by secondary sources for as little as ten dollars or so per roll. It has an attractive clear amber or gold color (rarely seen in water clear) and is often used as an insulating wrap around metal capacitor bodies. Polyimide tape is available with either silicone or acrylic thermosetting adhesive systems. The silicone version is a 180° C (356° F) class tape, usable for days to weeks up to 250° C (482° F), and for a few hours up to 300° C (572° F). The acrylic version is a 155° C (311° F) class tape, usable for days to weeks up to 175° C (347° F), and for a few hours up to 200° C (392° F). Again, because the acrylic and silicone adhesives used are both thermosetting, maximum adhesion and solvent resistance is achieved after heat curing, but curing is not usually necessary. Polyimide / Kapton tape is available in a variety of film thicknesses with one and two mil being the most common, and it is also available in antistatic versions (see below).
Glass Cloth Tape is sometimes used to wrap CCL and other coils in logging tools. Being thick and tough, it is ideal as an insulation bed for splices, like where the Teflon insulated lead attaches to the magnet wire in a CCL coil. It is available with rubber, acrylic, and silicone thermosetting adhesive systems. In glass cloth tape, we like the 3M Scotch Brand Number 69, a white seven mil overall thickness tape. It is a 180° C (356° F) class tape, usable for days to weeks up to 250° C (482° F), and for a few hours up to 300° C (572° F). 3M 69 sells for around fourteen dollars per 1/2 wide roll in small quantities.
Thermosetting Adhesive Notes
As mentioned in the above discussion, most of the tapes we use in downhole logging tools employ a thermosetting adhesive. This simply means that to achieve maximum bond and/or solvent resistance, curing at elevated temperatures is required. Usually these tapes work just fine as applied at room temperature without the need for any additional special curing. There are also non-thermosetting pressure-sensitive adhesive tapes which do not advance in cure with the application of heat.
In order to achieve solvent resistance with thermosetting adhesives, the following cure cycles are recommended: Both Thermosetting Rubber (RT) and Acrylic (A) adhesives require three (3) hours at 120° C (250° F), two (2) hours at 135° C (275° F), or one (1) hour at 150° C (300° F). Thermosetting Silicone (ST) adhesive requires three (3) hours at 260° C (500° F), and for maximum solvent resistance, 24 hours at that same temperature.
Antistatic PSA Tape
Triboelectric effect (from the Greek tribein, to rub) can cause significant levels of static electricity and electrostatic discharge (ESD) events both when PSA tape is unrolled, and when it is subsequently removed from surfaces to which it has been adhered. (If you are old enough to have been taught real science in school, you will recall the classic triboelectric demonstration of wool or rabbit fur rubbed on a plastic rod (traditionally amber), or the old balloon rubbed on the head trick.) This has profound implications in the electronics world since many modern solid state devices are ESD sensitive. Antistatic tapes are therefore popular in the electronics industry, and particularly relevant to this page are the antistatic polyimide / Kapton tapes.
Antistatic tapes are usually made with the addition of something to the film and/or adhesive to make them at least a little conductive. This poses no problem when these antistatic tapes are used to mask portions of printed circuit boards to prevent those areas from being soldered or plated, and other similar applications. However, these tapes may present a problem in certain well logging electronics applications. One of our good friends down in Texas reports using an antistatic polyimide / Kapton tape on a high voltage power supply in a downhole tool, with some rather startling and unpleasant results. It is recommended that antistatic or low ESD polyimide / Kapton tape be avoided for use in downhole well logging tools.
As a side note, Triboluminescence (a/ka mechanoluminescence and fractoluminescence) is an interesting related phenomenon. Certain crystalline materials luminesce when rubbed or crushed, a popular example being wintergreen lifesavers. Likewise, many PSA tapes similarly yield a faint glow, usually bluish, as they are peeled off the roll. Old darkroom denizens will remember seeing this when the tape used to secure the end of a roll of film was removed, sometimes causing a visible fogging of the film directly under the tape.
Happy taping!
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