Лако-красочные материалы — производство

Almost all electrical tapes have thermosetting adhesive characteristics, and a suitable test to measure satisfactory thermosetting has been described in Section II. B. Many electrical tapes are used to wrap coils of various diameters, and there may be a tendency for the end of the tape to lift away after application, known as flagging. This can occur either after the coil is taped and held in storage awaiting the next process, during a thermoset cycle in preparation for a varnish dip, or in the varnish dip process itself. Any flagging test must cover all eventualities. How much challenge the tape receives will depend upon the dia­meter of the coils wrapped; the smaller the diameter the greater the stress and therefore the greater the tendency to flag. One suitable test is to use a 0.5 in. (12mm)-diameter mandrel and wrap the tape around the mandrel with an exact 0.5 in. (12 mm) overlap. These should be set aside and examined 1 week later for any tendency to flag. While the storage con­ditions for this 1-week period would be expected to be ambient, temperatures of as high as 30°C (85°F) and humidities of 80 to 90% can be expected in a coil manufacturer’s working environment, and so must be brought into the evaluation study. The test assemblies, or others set up simultaneously, are then subjected to the manufacturer’s recommended cure cycle for that adhesive, or if unknown, 1 h at 150°C (300°F), and again examined for flagging. The cured assemblies are then dipped for 20 min in the commercial varnish that the adhesive would normally encounter and reexamined for flagging. When sufficient experience has been gained, including comparative studies, this may be modified to a 5-min dip in the solvent system used in the varnish.

In the case of an electrical tape used as a harness wrap, the flagging test can be modified [28] by using a 0.125 in. (3 mm) diameter rod, spiral wrapping the rod with the tape under test, ensuring a 50% overlap, then examining for any tendency to flag after an aging cycle.

Electrical tapes may come into contact with current-carrying fine bare copper wires, a potential condition for electrolytic corrosion. While the tendency for electrolytic corro­sion can be, and is, estimated indirectly from the reciprocal of the insulation resistance of an adhesive tape, insulation resistance is determined using the total cross-section of the tape, whereas electrolytic corrosion is related to the adhesive only. A more reliable esti­mate can be made directly by attaching two 32-gauge fine bare copper wires, 0.25 in. (6 mm) apart, along the adhesive surface of a 6 in. (150°mm)-length of tape, using the standard roll-down application method. Care should be taken to avoid any kinks in the wire. With 250 V DC applied across the wires, this assembly is placed in an enclosure that provides a saturated atmosphere and then aged at 50°C (120°F) for 20 h. After removing the applied voltage and allowing to cool, the wires are carefully removed and the tensile test performed on each wire. The direct electrolytic corrosion is reported as the ratio of the tensile strengths of the two wires, no corrosion giving a factor of 1.

A much simpler version of this test, which can be used as a screening test, is to apply the adhesive tape to a freshly polished soft copper panel using a 100-grit aluminum oxide polishing wheel or 150-grit paper, then heat-age this at 100°C (212°F) for 72 h. After allowing to cool, the tape is removed and the panel inspected for any indication of staining.

Electrical tapes can also encounter various solvents as used in varnishes, or oils as used in transformers, and here a test can be devised that is specific to the end use by immersing a test panel on which the adhesive tape under consideration has been applied, as for an adhesion test, in the liquid in question, the time period being relevant to that which would be encountered in practice. After removal of the panel from the liquid, the excess liquid is removed carefully using an adsorbent, and a standard adhesion test carried out, the result being compared to the original value. In most cases, some adhesive edge transfer is permissible. There are other tests relating to electrical properties such as dielec­tric strength and insulation resistance, but these are more related to the backing and as such will not be discussed here.