Saint-Gobain CHR 2925-7 is a pressure-sensitive tape constructed with a glass-foil backing and a silicone pressure-sensitive adhesive. The model designation places the product in the high-temperature electrical insulation and process masking category. The suffix -7 indicates a nominal backing thickness class of 0.178 mm (0.007 in); total finished tape thickness, including adhesive layer and release liner, is measured according to ASTM D3652/D3652M. The glass-foil backing provides low elongation, cut-through resistance, and dielectric stability at elevated temperatures. The silicone adhesive is selected for continuous operating temperatures from approximately -73 °C to 260 °C, with short-term excursions above this range only when validated on the actual substrate and profile. The product is supplied in slit rolls and master logs for precision converting; standard slit widths range from 6.4 mm to 1016 mm. The release liner is selected for clean removal during rotary die cutting and lamination; film liners are preferred where particulate control is required in electrical assembly areas.

Within the 2925 family, the -7 variant is an intermediate thickness class between the thinner -5 and thicker -10 constructions. The choice of a glass-foil backing rather than polyester, polyimide, or aluminum foil changes both electrical and mechanical behavior. Polyester-backed silicone tapes are more conformable but have lower cut-through resistance and a lower continuous thermal ceiling. Polyimide-backed silicone tapes provide high dielectric strength at low caliper but are more costly and less resistant to abrasion. Aluminum foil tapes are electrically conductive and used for shielding or heat reflection, whereas the glass-foil backing in CHR 2925-7 is electrically insulating and can be used as phase-to-phase or layer-to-layer insulation in motor and transformer applications. Compared with acrylic or rubber adhesive tapes, the silicone adhesive maintains shear strength at temperatures where acrylic systems soften or depolymerize, but its room-temperature peel adhesion to stainless steel may be lower and its cost per square metre is typically higher. The product is therefore specified when thermal endurance, low-surface-energy bonding, or silicone-to-silicone compatibility governs the application; it is not generally selected for low-cost general-purpose packaging.

How Does the Silicone Adhesive in CHR 2925-7 Differ from Acrylic and Rubber PSAs?

Silicone pressure-sensitive adhesives are typically formulated from silanol-functional polydimethylsiloxane polymers condensed with silicate resins. The crosslinked network provides a broad service temperature range and low glass transition temperature, but room-temperature tack and peel response are lower than those of solvent acrylic PSAs. In comparative testing under ASTM D3330/D3330M on stainless steel, acrylic tapes often exhibit higher initial peel values, but their performance falls sharply above 150 °C. Rubber PSAs offer high initial grab and low material cost, but they are unsuitable for long-term exposure to ozone, oxidation, or temperatures above 105 °C. CHR 2925-7 is therefore specified for high-temperature electrical insulation or composite masking where thermal aging is the primary failure driver, not for high-speed packaging lines requiring immediate wet-out on corrugated surfaces. The silicone adhesive also has a distinct failure mode in thermal aging: it tends to crosslink further and become firmer, eventually losing tack rather than forming low-molecular-weight residues. This behavior is an advantage in coil varnish applications because adhesive transfer to magnet wire is reduced after the cure cycle.

The applicable test methods are ASTM D3330/D3330M for peel adhesion, ASTM D3654/D3654M for shear holding power, and ASTM D3759/D3759M for backing tensile strength and elongation. Thickness is measured under ASTM D3652/D3652M. Dielectric strength is evaluated with ASTM D149, while electrical tape performance is often characterized under ASTM D1000 and UL 510. Adhesive coat weight is controlled by the manufacturer to balance adhesion and dielectric penetration resistance; a heavier adhesive layer may improve wet-out but can increase the risk of adhesive oozing from the roll edge during high-temperature storage.

The silicone adhesive used in CHR 2925-7 belongs to the class of pressure-sensitive adhesives in which the elastomeric phase is dimethylsiloxane and the tackifying phase is a silicate resin. The resin-to-polymer ratio determines glass transition temperature and the balance between tack and shear. Peroxide-cured silicone PSAs are common in this product class; addition-cured systems are sometimes used where low volatile content is required. Crosslink density controls high-temperature shear and adhesive transfer. If the adhesive is undercured, it may leave residue on magnet wire during varnish cure; if overcured, the tape may lose tack on low-surface-energy substrates. For this reason the manufacturer’s lot-to-lot cure state is controlled within narrow limits, and incoming inspection often includes Fourier-transform infrared spectroscopy for silanol content and differential scanning calorimetry for residual cure exotherm.

The glass-foil backing is produced from continuous glass filaments formed into a thin sheet and saturated with a high-temperature binder. The binder contributes to dimensional stability and prevents glass filament fibrillation during slitting. Backing dielectric strength is thickness- and binder-dependent; moisture absorption can reduce surface resistivity and increase leakage current. In humid environments above 60% RH, the tape should be dried before application in sealed electrical devices because absorbed moisture can lower insulation resistance under ASTM D257. The glass-foil construction is not a hermetic barrier; coatings or varnish impregnation are required where moisture ingress is critical.

In coil winding and wire harness fabrication, the tape is applied directly over varnished copper or aluminium magnet wire. Winding tension must be limited to prevent cutting through the backing; field data from transformer line setups indicate that edge-tension concentrations above 40 N per linear metre can initiate glass-filament breakage when the tape passes over a 3 mm radius. The glass-foil backing resists cut-through failures observed with polyester-backed tapes on square-wire edges. During varnish cure cycles up to 180 °C to 200 °C, the silicone adhesive releases without leaving conductive residues. For motor slot liners and layer insulation, the tape is sometimes applied as a narrow slit and then pressed into the slot with automated insertion tooling. The low elongation of the glass-foil backing maintains dimensional registration during insertion, which is critical for uniform creepage and clearance distances under IEC 60664-1.

Automated taping machines for coil winding often apply tapes at speeds above 10 m/min. Unwind force of silicone PSA tapes is generally lower than that of rubber PSAs; machine settings developed for rubber tapes can cause slack between the reel and the coil, resulting in wrinkles and inconsistent layer spacing. Tension settings must be revalidated for the glass-foil backing because the backing does not stretch to absorb speed variation. Batch-to-batch variation in liner release can also affect high-speed unwind; converters frequently specify a release value range from 20 g/25 mm to 60 g/25 mm for clean release without liner drop on the unwind spindle. Unwind force itself is evaluated under ASTM D3811/D3811M.

In composite cure masking, the tape is used to define vacuum bag edges, mask tool edges, or protect mating surfaces from resin bleed. Autoclave cycles at 180 °C and 0.69 MPa are within the typical envelope for silicone-backed tape, but local pressure at tooling pins can force the adhesive into surface porosity. On epoxy tooling, release films and liquid release agents should be checked for amine compatibility because low-molecular-weight amines can attack the silicone adhesive and degrade anchorage at the backing interface. Published data for this specific configuration is limited when the tape is exposed to amine-cured epoxy tooling in repeated autoclave cycles; qualification on production-representative coupons is therefore required.

When Dielectric Strength and Flame Resistance Become Selection Criteria in Electrical Insulation

For electrical insulation, the critical parameters are dielectric breakdown voltage, tracking resistance, flame propagation, and thermal endurance. Glass-foil backed silicone tapes are tested under ASTM D149 for dielectric strength; measured values depend on electrode geometry, rate of voltage rise, and sample thickness. CHR 2925-7 is not a substitute for a listed insulation system by itself. It may be used as a component within a system evaluated under UL 510 or IEC 60454-3-1; the end-use electrical equipment must still satisfy the relevant insulation coordination standard. Flame resistance is typically reported by the tape manufacturer through UL 510 or an equivalent vertical burn test; the glass-foil backing contributes to char integrity and does not melt into a conductive bead. Silicone adhesive ash content is normally low after combustion, but the exact residue profile should be confirmed from the manufacturer’s data sheet before use in sealed relay or switchgear environments.

Partial discharge and corona endurance are more severe in high-voltage designs above 1 kV. The glass-foil backing contains microvoids that can ionize under sustained electrical stress; impregnation or vacuum-pressure impregnation with varnish fills these voids and improves corona resistance. In untreated form, the tape may be suitable for layer insulation only where the electric field is below the partial-discharge inception voltage of the host insulation system. This is not a product-specific deficiency of CHR 2925-7 but a characteristic of porous glass-fibre backings; polyimide and polyester backings have different void structures and partial-discharge behavior.

Slitting of glass-foil/silicone tape requires shear-knife or crush-cut tooling with controlled wrap angles. Dull blades produce edge nicks that propagate as web breaks at tension below the backing tensile limit. Laser cutting is possible but generates silica fume and can oxidize the silicone adhesive at the cut edge if assist gas is not adjusted. Rotary die cutting is the preferred method for high-volume parts; die stations should be maintained below 40 °C because liner release coatings can soften and transfer to the adhesive. Adhesive squeeze-out at the roll edge after prolonged storage at temperatures above 35 °C can cause blocking and poor unwind on automated applicators. Rolls should be stored at 21 °C ± 3 °C and 45–55% RH in original polyethylene packaging. If a roll has been cold-soaked, it should be conditioned at the converting room temperature for at least 24 h before slitting to prevent condensation on the liner and subsequent adhesive transfer.

Chemical incompatibility includes prolonged contact with flexible PVC containing high phthalate plasticizer levels; plasticizer migration can reduce peel adhesion and soften the silicone network. Exposure to methyl ethyl ketone, toluene, or chlorinated solvents swells the silicone adhesive and can cause liner distortion. The tape is not recommended for use on untreated fluoropolymer films, silicone elastomers with free silicone oil bleed, or surfaces contaminated with amine-based mould releases unless adhesion is verified under ASTM D3330/D3330M after the intended cure cycle. For FDA-regulated contact applications, a specific food-contact compliance statement under FDA 21 CFR 175.105 should be requested from the manufacturer; a generic silicone adhesive does not automatically confer food-contact suitability.