3M 398FR Glass Cloth Tape is a single-sided pressure-sensitive tape consisting of a woven glass cloth carrier and a silicone adhesive system. The product is self-wound without a release liner and is supplied in converter-slit widths from narrow aerospace harness tapes to wide industrial roll stock. Manufacturer-reported typical total thickness is 0.18 mm (7.0 mil), comprising a glass cloth backing of approximately 0.13 mm (5.0 mil) and a silicone adhesive layer. The FR designation identifies a flame-retardant qualification; it does not indicate halogen-free composition or a specific UL recognition class. The tape is specified for aircraft wire-harness overwrap, high-temperature coil bundling, ceramic fiber insulation facing, and protective wrapping of cables routed through high-temperature equipment compartments. Unlike polyester-backed tapes that shrink above 150°C, the woven glass cloth carrier remains dimensionally stable within the published continuous service range of -54°C to 180°C, with short-term excursions to 200°C reported for silicone adhesive systems of this class.

The silicone adhesive is thermosetting after coating and winding; it does not melt-flow at elevated temperature the way thermoplastic rubber adhesives do. This characteristic supports short-term excursions to 200°C but also limits repositioning after initial contact. Once the adhesive wets a high-energy surface, removal after heat aging may leave silicone residue that must be cleaned with solvent. The woven glass cloth carrier has high cut-through resistance compared with polyester film but can fracture when creased sharply; folds should be avoided during wrap terminations.

The product is differentiated from general-purpose glass cloth tapes by its flame-retardant acceptance and by its silicone adhesive behavior. Silicone adhesives exhibit lower initial tack than acrylic adhesives and require greater contact time on low-energy surfaces, but they maintain peel strength over a wider temperature window and resist oxidative degradation. Reported typical tensile strength is 263 N/100 mm (150 lb/in) with elongation at break of 7% under ASTM D3759/D3759M; peel adhesion to steel is reported at 49 N/100 mm (45 oz/in) under ASTM D3330/D3330M. These data are lot-typical values, not design allowables. When source-controlled aerospace drawings invoke the tape, the active revision of the supplier’s technical data sheet and certificate of conformance remain the controlling documents.

What specification framework governs 398FR performance?

Physical, electrical, and flammability performance is assessed through separate methods. The table below consolidates typical values reported in supplier literature. Because silicone adhesive peel values are substrate- and dwell-time-dependent, verification on the actual substrate is required before production release.

Specimens for physical testing are conditioned at 23±1°C and 50±5% relative humidity for at least 24 h before testing. Tensile strength and elongation are measured on a constant-rate-of-extension tester with a jaw separation speed of 300 mm/min; dielectric strength is measured with a hi-pot tester using a ramp rate of 500 V/s in accordance with ASTM D1000. The dielectric value is significantly affected by relative humidity and backing moisture: glass cloth is hygroscopic, and water absorbed in the woven carrier can reduce dielectric strength. Pre-drying at 60°C for 4 h may be necessary when rolls have been stored at relative humidity above 60% or when dielectric acceptance margins are below 20%. Rolls should be stored in original packaging at 21°C ± 3°C and 50% relative humidity; shelf life for silicone glass cloth tapes of this class is typically 24 months from date of manufacture when unopened. Rolls removed from cold storage must be allowed to equilibrate to room temperature before opening to prevent condensation on the adhesive and backing.

Aerospace overwrap and high-temperature bundling operations

In production-scale aircraft harness shops, 3M 398FR is applied with servo-driven spiral wrapping stations that control tape tension and overlap. The woven glass cloth has low elongation; therefore drag force is typically maintained below 10% of ultimate tensile strength to prevent transverse narrowing. For a 19 mm wide tape, the 10% threshold is approximately 5.0 N; typical drag settings are between 2.0 N and 4.0 N depending on backing stiffness and wrap speed. Excessive tension produces edge curling and can reduce the effective dielectric coverage at the overlap boundary after thermal cycling from -54°C to 85°C. Field-reported processing bottlenecks include occasional backing stiffness variation between lots, requiring trim adjustment of spindle braking torque to maintain consistent 50% overlap. Because the tape is self-wound without a liner, high-speed unwinding through small-radius pulleys below 15 mm can induce glass yarn breakage; idler rollers with low-friction bearings and larger radii reduce break frequency.

Surface cleaning before application uses 70% isopropyl alcohol/water or methyl ethyl ketone, followed by solvent flash-off. Adhesion to cold substrates below 10°C is reduced because the silicone adhesive requires molecular contact; full bond development on low-surface-energy substrates such as PTFE or heavily fluorinated wire jackets may require up to 72 h. The tape is also used as a retention overlayer for ceramic fiber insulation on gas turbine external ducting. In that application, the backing tolerates radiant heat better than polyester, but direct exposure to open flame will consume organic sizing and leave a glass-fiber char. No post-cure is required after application; the adhesive is supplied in a crosslinked state.

Converter slitting tolerance is normally ±0.5 mm for widths up to 50 mm; edge fraying from worn shear-cut blades can produce glass filament debris that can bridge adjacent conductors in avionics harnesses. Incoming inspection should include light-table examination for weave distortion and adhesive voids larger than 1.0 mm in diameter. Pinholes at weave intersections are typical of glass cloth backings and are acceptable only when the tape is not used as primary electrical insulation.

Evaluating dielectric strength and electrical insulation performance

Dielectric strength is reported at a typical value of 4.0 kV for the 0.18 mm composite, but the value is sensitive to backing moisture and pinhole defects at the weave intersections. For electrical insulation use, processors should inspect slit edges for conductive fiber debris and maintain roll cleanliness in the wrapping area. In wire-harness applications, 3M 398FR is not typically used as primary insulation; it serves as mechanical protection and secondary insulation. The dielectric test should be performed in accordance with ASTM D1000 on a dry specimen. Published data for this specific configuration is limited to supplier data sheets; for safety-critical insulation, type-specific qualification with the actual conductor bundle is required.

Production hi-pot testing of wrapped bundles is usually performed at 1500 VDC or 1500 VAC depending on airframer specification; the tape’s dielectric strength is not the only acceptance parameter, because the underlying wire insulation and air gaps define the withstand voltage. For form, fit, and function replacement, the tape must not reduce bundle flexibility below the airframer’s minimum bend radius requirement.

Substitution of 3M 398FR for general-purpose glass cloth tapes such as 3M 69 or 3M 3615 requires verification of flame-retardant acceptance, dielectric strength, adhesive thickness, and lot traceability. The FR designation indicates flame-retardant qualification of the composite, not equivalence of all physical properties. Polyimide tapes offer higher thin-gauge dielectric strength, but the glass cloth product provides higher abrasion and cut-through resistance in bundle routing. Polyester tapes are lower cost but shrink above 150°C and do not provide the same high-temperature bundling envelope. Published data for this specific configuration is limited; direct substitution must be supported by side-by-side quality data from the same supplier and test laboratory.

When the adhesive faces aromatic hydrocarbons or plasticized PVC

Silicone adhesives are not universally chemical-resistant. 3M 398FR is not recommended for continuous immersion in aromatic hydrocarbons, esters, ketones, or chlorinated solvents because solvent uptake swells the silicone network and reduces peel strength. Heavy plasticizer migration from PVC wire jacketing can plasticize the adhesive and reduce interfacial adhesion; the use of an ETFE overwrap or fluoropolymer barrier tape between the PVC and the glass cloth is standard practice in harness designs that must survive 105°C long-term aging. Published chemical compatibility data for this specific product is limited; qualification tests using the actual service fluid, temperature, and exposure duration are required before release.