3M 398FRP Printed Version Glass Cloth Tape is a flame-resistant, glass cloth-backed pressure-sensitive adhesive tape supplied with a customer-specified printed legend on the white topcoat. The product is used primarily for direct identification of wire harnesses and electrical cable bundles in aerospace, rail, and industrial equipment where the marking must survive elevated temperature, mechanical abrasion, and fluid exposure. The printed version integrates the legend into the tape structure rather than requiring a separate adhesive label or heat-shrink marker. The woven glass cloth provides high tensile strength and cut-through resistance; the silicone adhesive maintains peel and shear holding at temperatures above the limits of common acrylic and rubber pressure-sensitive adhesives. Representative thickness is 0.18 mm (7.0 mil), with backing and adhesive layers contributing approximately 0.13 mm and 0.05 mm, respectively. These dimensional values are production averages and should be confirmed against the current 3M technical data sheet before acceptance testing.

Backing Construction and Silicone Adhesive Architecture

The backing consists of a woven glass cloth saturated with a white printable coating. The weave structure influences the tape’s resistance to edge fraying, tear initiation, and conformability around small wire bundles. Continuous-filament glass yarns offer high modulus and low elongation; the printed topcoat provides a controlled surface energy for ink transfer and legend retention. The pressure-sensitive adhesive is a silicone system with low glass transition temperature, allowing application at low temperature and continuous service at elevated temperatures. Silicone adhesives typically build adhesion through viscoelastic wet-out rather than instantaneous high peel; bond strength increases during the first 24 h to 72 h after application at 20 °C to 25 °C. The product is self-wound on a release liner or supplied in roll form. Surface preparation of the wire bundle with isopropanol or methyl ethyl ketone removes processing oils and improves adhesion to epoxy-insulated wire and polyimide insulation. Silicone pressure-sensitive adhesives are also sensitive to sulfur-containing vulcanization residues and amine-based corrosion inhibitors; those contaminants can inhibit wet-out and reduce tack.

Roll storage before application should follow the manufacturer’s shelf-life guidance, commonly 24 months from date of manufacture when stored in original packaging at 21 °C ± 2 °C and 40–50% relative humidity. Exposure to high humidity before application can reduce initial wet-out on braided fiberglass sleeving and coarse wire insulation. The tape should be applied with sufficient tension to remove air pockets between overlapping layers; excessive tension on thin-wall wire insulation can cause cold flow or conductor deformation. The recommended minimum application temperature is 10 °C, below which the silicone adhesive stiffens and surface wet-out is incomplete. Published data for adhesion to low-surface-energy substrates such as sintered PTFE and silicone rubber is limited; these substrates require plasma or corona treatment before bonding. In automated tape-wrapping stations, the glass cloth backing’s low elongation keeps printed legend spacing more stable under winding tension than polyester films with higher elongation, but edge damage during splicing can initiate fraying. Butt splices with thin polyimide film backing are used in some production lines to maintain uniform thickness through the wrapping head.

What Test Methods Bound the Thermal and Dielectric Performance?

Flame-retardant classification for the glass cloth product is commonly assessed by vertical burn testing under FAR 25.853(a); aerospace wire-harness applications may also require compliance with airframer-specific burn length and afterflame limits. The silicone adhesive system is formulated for continuous thermal exposure up to approximately 180 °C, with short-term excursions to 200 °C possible when the tape is not under high peel load. Dielectric breakdown measured under ASTM D1000 is a single-layer laboratory value and is not a direct indication of insulated-wire performance under partial discharge. The presence of voids, wrinkles, and butt joints in wrapped layers reduces the withstand voltage of a finished harness assembly. The glass cloth backing is dimensionally stable under thermal cycling but has low elongation; it should not be used across tight flex points where repeated bending concentrates stress at the edge of the tape. At temperatures above the silicone adhesive’s thermal degradation threshold, outgassing of low-molecular-weight siloxanes can occur; published outgassing data for this specific configuration is limited and should be generated for vacuum or sealed enclosure applications. In vertical burn testing, the sample configuration matters: the tape should be mounted on a representative wire bundle or aluminum panel because unsupported film may shrink and alter the burn path.

Electrical insulation in a wrapped harness depends on the number of tape layers, overlap geometry, and the presence of potting or overjacket materials. A single spiral wrap provides limited dielectric isolation; a half-lapped double-layer wrap is often specified for voltages above 600 V in maintenance drawings. The tape’s glass cloth backing is more abrasion-resistant than polyester or polyimide film tapes of similar thickness, but its woven surface produces higher friction during pull-through of harness bundles. Installers report that the edge of the tape can fray when cut with dull blades; shear cutting is preferred over razor-tooth cutting to maintain a clean edge. The printed legend repeat length is fixed by the customer’s print file; in spiral wrapping, the legend may rotate out of view if the wrap pitch changes. For linear flag identification, the tape is applied as a longitudinal flag strip around the bundle rather than spirally wrapped. The glass cloth backing permits the printed legend to remain visible after abrasion from clamps and adjacent wires, but repeated scraping with metal edges removes the topcoat and degrades legibility.

When Flame-Retardant Marking Becomes a Certification Requirement

The printed version is specified where engineering drawings require wire numbers, part numbers, or harness identification codes to remain legible after installation and after exposure to thermal cycling. Because the legend is printed on the tape topcoat before wrapping, the harness does not carry an additional label flag that can become detached during wire routing or service vibration. Printed legends on the 398FRP tape are used in high-temperature compartments where polyester label stock would shrink or degrade. The printed topcoat is designed for legend retention under dry heat; however, published data for ink adhesion after immersion in hydraulic fluids, turbine oils, and deicing fluids is limited. Qualification programs therefore include legibility checks after immersion panels under the specific airframe or rail operator’s material specification. The base tape cannot be re-printed in the field using standard thermal-transfer printers unless the specific topcoat and ribbon combination have been validated; the product is normally supplied as a pre-printed roll.

Application on wire harnesses is typically performed by hand or by semi-automatic spiral wrapping machines. The low elongation of the glass cloth backing, approximately 7% under ASTM D3759, prevents major print distortion during moderate winding tension, but tension settings must be verified on the actual wrapping head because the backing does not stretch to correct drift. The product is not recommended for use as a primary insulation over live conductors unless the installation is evaluated for voltage class, environmental sealing, and mechanical protection. Flame-retardant certification of the tape alone does not automatically confer flame-retardant certification on the completed harness assembly; the final assembly may require additional testing under the applicable aerospace or rail fire-performance specification.

Contrasting the Printed Version with 398FR, 3M 69, and Polyimide Film Tapes

3M 398FRP shares the woven glass cloth and silicone adhesive architecture of the unprinted 3M 398FR. The principal difference is the printed topcoat and the availability of customer-specific legends. 3M 398FR is selected when permanent marking is not required or when a secondary marker will be used. 3M 69 glass cloth tape also uses a silicone adhesive and is commonly used for high-temperature electrical insulation; its thermal rating is often cited at 200 °C, but it is not configured as a printed-legend tape. Polyimide film tape provides a thinner profile, typically 0.07 mm (2.7 mil) for 1 mil film with adhesive, and offers higher dielectric strength per thickness, but it lacks the heavy woven glass cloth’s cut-through and abrasion resistance. The choice between 398FRP and polyimide film depends on whether the harness requires high mechanical protection or low added thickness. Compared with polyester printable tapes, 398FRP has higher temperature resistance and flame-retardant classification, but its woven backing is thicker and may increase bundle diameter more than a film tape.

Compatibility with wire insulation systems should be verified because the silicone adhesive can extract plasticizer from some PVC compounds, causing the insulation to become stiff or the tape bond to soften. The tape is not recommended for direct application to uncured silicone rubber or to surfaces coated with silicone mold release. When removal is required after heat aging, the adhesive may leave residue on porous surfaces; mechanical cleaning or solvent wiping with xylene or methyl ethyl ketone may be necessary. Published data for residue behavior after long-term thermal exposure is specific to the substrate and should be evaluated on production-representative harness samples. The product should not be used in oxygen-rich environments or in direct contact with liquid oxygen systems without additional compatibility testing. For printed legends, qualification should be performed with the actual ink system, wire insulation, and harness overjacket because the printed topcoat’s fluid resistance cannot be extrapolated from the unprinted backing alone.