In dry-type transformer interlayer insulation, the 0.051 mm FEP backing of CHR 2355-2 is applied to copper magnet wire and rectangular bus bar before vacuum-pressure impregnation with epoxy or unsaturated polyester varnish. The 0.038 mm silicone pressure-sensitive adhesive layer maintains adhesion to bare copper at winding temperatures from 20 °C to 80 °C without oven pre-cure. The FEP backing supplies a nonporous dielectric barrier; manufacturer-reported dielectric strength of the 0.089 mm tape is 7 kV per ASTM D149. Continuous operating temperature of 204 °C supports Class H dry-type designs per IEC 60085, provided the insulation system as a whole passes thermal evaluation. During vacuum-pressure impregnation, the FEP surface resists styrene and methyl methacrylate varnish attack, but adhesive squeeze-out at slit edges must be controlled because uncured varnish can extract low-molecular-weight silicone species from the pressure-sensitive adhesive and reduce interlayer adhesion. Servo-driven taping heads on rectangular conductor lines require closed-loop unwind tension; excessive tension above the yield point of 0.051 mm FEP causes necking and loss of dielectric coverage at corner radii.

What Limits Siloxane Outgassing in Engine-Zone Wire Harness Overwrap?

On aircraft engine pylons and auxiliary power unit harnesses, FEP-backed silicone PSA tape is overwrapped at 50% to 60% overlap on nickel-plated and tin-plated copper conductors. The 204 °C continuous rating of the backing accommodates radiant heat soak from engine cases, but the silicone adhesive is the application boundary in sealed compartments. Low-molecular-weight siloxane fractions migrate from unreticulated silicone PSA above 150 °C; when the harness exits into an avionics bay or optical sensor housing, ASTM E595 total mass loss and collected volatile condensable material screening should be performed. Typical non-post-cured silicone pressure-sensitive adhesives may exceed the 0.1% CVCM limit used for spacecraft materials; therefore CHR 2355-2 is not a direct substitute for skived PTFE-backed, low-outgassing acrylic tapes in those enclosures. The tape does prevent chafing against titanium clamp edges and reduces foreign object debris from braided sleeving. On high-vibration lines, spiral wrapping rather than longitudinal lay-up is preferred because FEP has a low dynamic coefficient of friction and does not lock into convoluted conduit bends. Field failure analysis on engine-harness trials has documented edge lifting at clamp points when the adhesive side is contaminated with turbine oil; solvent wiping with isopropyl alcohol before wrap is required at 60% to 70% relative humidity or below to avoid condensation at the adhesive interface.

When Heat Seal Jaw Temperatures Oscillate Near 204 °C

Rotary heat seal jaws on form-fill-seal machines are wrapped with CHR 2355-2 to prevent molten polyethylene or polypropylene from building up on serrated and knurled surfaces. The FEP backing retains release behavior at jaw temperature setpoints of 120 °C to 140 °C for LLDPE, 150 °C to 180 °C for cast PP, and 160 °C to 190 °C for PET/PE laminate structures. The silicone adhesive transfers heat from the jaw surface while maintaining tack at the 204 °C maximum continuous limit. Temperature oscillation above 204 °C, even for 2 s to 3 s impulse spikes, can embrittle the adhesive at the exposed edge and create a release-liner-like residue on the seal area. A jaw-surface thermocouple is recommended because embedded cartridge heater setpoints often overshoot by 15 °C to 20 °C at startup. Dynamic coefficient of friction of FEP against 25 µm LLDPE film is below 0.2, which permits film slip and prevents dragging across cross seals. On reciprocating jaw machines, the tape is die-cut to cover only the sealing face; untrimmed edges can catch film at the package cut-off knife and deposit adhesive transfer.

Autoclave cure of epoxy prepreg laminates at 177 °C and 0.69 MPa requires reusable nonporous release on aluminum or steel caul plates. CHR 2355-2 is bonded to the tool side of the caul plate with the silicone adhesive, while the FEP side contacts the bleeder or vent layer. The 0.051 mm FEP backing does not pin through peel ply, so the release side does not disturb the resin front during caul compaction. Because the adhesive is silicone, edge masking must be set back from the part trim line by at least 3 mm; contact between uncured epoxy and silicone PSA leaves a non-polar contamination layer that reduces secondary bonding lap shear. On caul plates for co-cured stringer caps, the tape is replaced after 10 to 15 autoclave cycles because repeated thermal expansion of the aluminum plate produces adhesive microcracking at the plate edges. FEP has a lower thermal expansion coefficient than aluminum but remains conformable at room temperature; removal in a refrigerated tool at 4 °C increases adhesive cohesive failure and protects the plate surface from residue. For cure cycles above 190 °C with BMI resins, published data for this specific adhesive configuration is limited and qualification should include water-break-free surface testing on subsequent bonded assemblies.

Powder Coat Masking Line Speed and Adhesive Residue Thresholds

Masking threaded bosses, bearing seats, and electrical contact pads before electrostatic polyester or epoxy powder coating uses CHR 2355-2 at cure temperatures from 180 °C to 200 °C for 10 min to 20 min. The FEP surface does not allow powder to fuse, so the tape strips cleanly after the oven cycle. The 2.7 N/cm adhesion to steel permits high line density in pre-coat masking cells without spontaneous flagging, but powder may enter under edges at sharp thread transitions if the tape is not pressed with conductive silicone rollers. Adhesive residue on aluminum wheels or machined bearing journals after the cure cycle is controlled by peel removal when the part temperature drops below 60 °C; hot removal above 60 °C increases cohesive splitting and leaves a visible silicone film. On parts with phosphate conversion coatings, adhesion is reduced by powder-free area; a wipe with 70% isopropanol and 30% deionized water is required before tape application. Masking line speed for threaded studs with M8 to M12 diameters is governed by die-cut tape geometry rather than backing temperature resistance; pre-die-cut pads reduce placement time compared with manual strip cutting.

Titanium Chemical Milling Uses CHR 2355-2 as an Auxiliary Mask, Not a Primary Maskant

Titanium airframe skins are chemically milled in mixed hydrofluoric acid and nitric acid baths at solution temperatures from 35 °C to 50 °C. CHR 2355-2 is used as supplemental mask over conventional elastomeric maskants at cutouts, step transitions, and sensor-mounting pads. The FEP backing resists attack from 20% to 30% nitric acid and 2% to 5% hydrofluoric acid for short exposures up to 4 h, but the silicone adhesive is not resistant to continuous hydrofluoric acid underfilm attack. Cut edges must be overlaid with a 12 mm minimum strip of perfluoroelastomer or lead tape to prevent acid wicking and titanium pitting along the tape boundary. After milling, the tape is removed from the unmasked surface, and the underlying titanium is inspected for pitting indicators per ASTM E1417. Published data for this specific tape in hydrofluoric acid immersion is limited; qualification should include edge-wicking coupons before production masking. FEP masking does not provide the etch depth tolerance of scribed elastomeric maskant; it is an auxiliary, not a primary, maskant.