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, an adhesive, and sometimes a liner / carrier, not to mention the technology that goes into making these various components function well together.Some Useful PSA Tapes for Logging Tool Applications
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 (PVC) 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.
Vinyl electrical tape is great stuff if you are an electrician, but have mercy on your lowly logging 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. If hydrocarbons leak into a tool where vinyl electrical tape has been used, the gooey mess increases dramatically. We do find black vinyl electrical tape useful as a temporary light block when testing scintillation detectors on the workbench, but we never use it in a downhole tool. Premium vinyl electrical tape (like 3M Scotch Brand 33+) is typically considered a 105° C class insulation.
Polyester (PET) Film (Mylar) Tape is the least expensive decent tape option for use in downhole logging tools. Half inch Mylar is the stuff to get if you intend to purchase only one type / width of tape. It can be used to tape wires or chips in place inside tools, and half inch is just the right width to conform to irregular surfaces when taping CCL coils and the like. Polyester / Mylar tape works well for assembling scintillation detectors (taping photomultiplier tubes (PMTs) to scintillation crystals) because it has good tensile strength combined with near ideal stretch characteristics. Mylar tape often has a thermosetting rubber adhesive system which exhibits superior initial tackiness (stickiness), but is also made with slightly less tacky acrylic and silicone thermosetting adhesive systems. Mylar is a 130° C (266° F) class tape in all available adhesive systems, usable for days to weeks up to 155° C (311° F), and for a few hours up to 175° C (347° F). Polyester / Mylar tape is usually yellow, but is also available in a range of other colors, and a variety of film thicknesses down to half mil, with one and two mil being the most common. In the rubber adhesive system, the yellow colored version usually exhibits the best adhesion (3M 56 and Saint-Gobain M64 are both specified at 50 oz./in. adhesion to steel, which is quite good for a high performance electronic tape). While the rubber adhesive exhibits good initial adhesion, it may dry out faster than other adhesives; acrylic adhesive may be a better choice if longevity is an issue. For maximum adhesion and solvent resistance with these thermosetting adhesives, heat curing is required, but is not usually necessary (see below). One mil polyester / Mylar film has a typical breaking strength of 25 lbs./in. and an elongation of around 100% at breaking. In Mylar tape, we like yellow 3M Scotch Brand Number 56 with a one mil thick film (2.3 mil overall with rubber thermosetting adhesive); a roll of 1/2 inch 56 should be around seven bucks, a good investment. Black 3M 1318-1 with acrylic adhesive is ideal for a final light-blocking layer when taping up scintillation detectors (our old favorite black 3M 1169 with rubber adhesive has been discontinued).Thermal Class Notes
Polytetrafluoroethylene (PTFE) Film (Teflon) 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 (only one other known solid substance is more slippery than Teflon, diamond-like carbon). Beware of Teflon tape in the surplus market, it is often "seconds" material with defective adhesive (we have many rolls of surplus Teflon tape that are essentially unusable). 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). For maximum adhesion and solvent resistance with these thermosetting adhesives, heat curing is required, but is not usually necessary (see below). 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. Two mil Teflon film has a typical breaking strength of 20 lbs./in. and an elongation of 150-300% at breaking depending on film type (skived, extruded, etc.). In Teflon tape, we like 3M Scotch Brand Number 62 with a two mil thick film (3.5 mil with silicone thermosetting 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 silicone thermosetting adhesive) that is easier to use if you do not need the extra bit of self-stickiness, or a printable back. Fluorinated ethylene propylene (FEP) film tape is a member of the Teflon family, but is optically clear and a bit softer (more conformable) than PTFE tape.
Polyimide Film (Kapton) Tape is increasingly commonly used 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 bucks or so per roll. It has an attractive clear amber or gold color (rarely seen in water clear), and is available in a variety of film thicknesses with one and two mil being the most common. It is also available in antistatic versions (see discussion below). Polyimide / Kapton tape is sometimes used as an insulating wrap around metal capacitor bodies and other electronic components (it is used as an outer wrap on our high temperature
Codatron high voltage regulators). Polyimide / Kapton 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). For maximum adhesion and solvent resistance with these thermosetting adhesives, heat curing is required, but is not usually necessary (see below). One mil polyimide / Kapton film has a typical breaking strength of 30 lbs./in. and an elongation of around 50% at breaking. In Kapton tape, we like 3M Scotch Brand Number 92 with a one mil thick film (2.8 mil overall with silicone thermosetting adhesive).
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, as in where the Teflon insulated lead attaches to the magnet wire on a CCL coil. It is available with rubber, acrylic, and silicone 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). The rubber version 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) For maximum adhesion and solvent resistance with these thermosetting adhesives, heat curing is required, but is not usually necessary (see below). Glass cloth tape is usually available in white or off-white, and 4.5 mil backing is usually the thinnest available and the most common. 4.5 mil glass cloth has a typical breaking strength of 150 lbs./in. and an elongation of around 5% at breaking. In glass cloth tape, we like the 3M Scotch Brand Number 69, a white seven mil overall thickness tape with silicone thermosetting adhesive; a roll of 1/2 inch 69 should be around fifteen bucks in small quantities.
There is considerable confusion and debate in the marketplace regarding the thermal performance of insulation materials, with many terms used to describe thermal characteristics. Trade organizations and associations as well as the various safety testing entities have all gotten into the act. Thermal classification or thermal class is a term now reserved for insulating systems as used in specific equipment, and is no longer recognized as a description of the temperature capability of individual insulating materials. However, individual dielectric materials continue to be commonly referred to as being class 90, class 105, etc. The thermal class is based on the thermal index, a number which permits comparison of the temperature / time characteristics of an electrical insulation material, or a simple combination of materials, based on the temperature in degrees Celsius which is obtained by extrapolating the Arrhenius plot of life versus temperature to a specified time, usually 20,000 hours.Thermosetting Adhesive Notes
The thermal class is the temperature at which an insulation material can be used continuously Since logging tools do not normally stay at high temperatures for prolonged periods of time, there is obviously a little wiggle room on thermal class. Three temperature classes cover all the high performance tapes discussed above. The balance of this discussion is limited to those three classes for the sake of brevity, but there are also class 90, class 105 (A), class 120 (E), class 200 (N), class 220 (R), and class 240 insulation products.
Class 130 a/k/a Class B includes any thermal index in the range of 130-154.9° C. Class 130 / B tapes are usable for days to weeks up to 155° C (311° F), and for a few hours up to 175° C (347° F).
Class 155 a/k/a Class F includes any thermal index in the range of 155-179.9° C. Class 155 / F tapes are usable for days to weeks up to 175° C (347° F), and for a few hours up to 200° C (392° F).
Class 180 a/k/a Class H includes any thermal index in the range of 180-199.9° C. Class 180 / H tapes are usable for days to weeks up to 250° C (482° F), and for a few hours up to 300° C (572° F).
As indicated above, 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.Antistatic PSA Tape
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), or 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.
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.|
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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.
Also see how to tape a blasting cap in Owen on Exposed Detonator (Cap) Sealing (pdf file).
FTC Disclosure: Neither AnaLog Services, Inc. nor the author has an economic interest in any of the companies or products discussed above, and no monetary compensation was received. Free samples were received from various manufacturers. None of the manufacturers / distributors was aware this page would be written.
08-99Part of AnaLog's Tape Overstock