Saint-Gobain COHRlastic 440S is a solid silicone rubber tape carrying a silicone pressure-sensitive adhesive on one face. The designation identifies an unsupported cured silicone backing rather than a silicone-coated fabric or an adhesive-free fusion tape. The product is used in electrical insulation, high-voltage splice overwrap, silicone rubber splicing, thermal-spray masking, powder-coating rack protection, and masking operations where the backing must survive elevated temperature without charring or embrittlement. The construction contains no woven glass, polyester film, or metal foil carrier; the load-bearing element is the cured silicone elastomer. The tape is supplied in log rolls and slit to specified widths for production use.

Because the carrier is unsupported, COHRlastic 440S exhibits high elongation at break and elastic recovery. The silicone PSA provides immediate wet-out on clean silicone rubber, aluminium, stainless steel, glass, and selected ceramics. No secondary cure is required. Bond strength develops after pressure application and dwell at room temperature. This immediate pressure-sensitive behaviour separates the product from self-fusing silicone tapes that require sustained tension and time for cohesive fusion.

The 440S model designation separates it from glass cloth and self-fusing silicone tape formats within the product family. The user should not interchange 440S with an unsupported silicone sheet that lacks adhesive or with a silicone-coated glass cloth designated for high-tension wrapping; the installation process and failure characteristics are different.

Material Certification and Standardized Test Data

Qualification data are normally reported under the test methods listed in Table 1. The values represent a single unsupported layer at standard laboratory conditions; they are not design allowables for a complete insulated joint.

The values in Table 1 are representative and not design allowables for a complete insulated joint. Users must obtain lot-specific test certificates because silicone compound formulations and surface treatments can shift measured values. The test methods are standard, but the test fixture, electrode geometry, and cure state of the adhesive affect dielectric and mechanical results.

The unsupported construction does not share the tensile and tear behaviour of a glass-cloth laminate. Under winding tension the backing elongates before failure; excessive tension can neck the tape and reduce wall thickness below nominal. The tape is therefore positioned and consolidated with a roller or semi-automated wrap head rather than being drawn under high linear force. A cured silicone rubber substrate does not require abrasion, but surface contamination by silicone oil, mould release, or plasticizer must be removed before application. A solvent wipe with isopropyl alcohol or a specified aliphatic solvent is used, and the solvent must evaporate fully before the tape is laid down to avoid entrapped solvent at the adhesive interface.

How Does Unsupported Solid Silicone Differ From Reinforced Silicone-Coated Fabrics?

Where glass cloth-reinforced tape is chosen for resistance to tearing and dimensional stability, COHRlastic 440S is chosen when the wrap must follow a compound radius or restore a continuous silicone jacket. A glass cloth tape may lift at the inner radius of a tight bend because the woven carrier cannot compress in the plane of the wrap. The unsupported silicone tape absorbs that compression by local thickening. That difference is measurable as a variation in elongation-at-break: 450% for the 440S backing versus below 5% for many woven glass cloth products. The trade-off is that the glass cloth tape can be applied with higher mechanical tension and retains a more uniform thickness on flat sections. A production line that currently uses a woven glass cloth tape on straight busbars may find the unsupported product less stable if the winding tension is not reduced.

Compared with adhesive-free self-fusing silicone tapes, 440S does not require tension and time for molecular fusion to develop. Self-fusing tapes are wrapped under tension and rely on intimate contact of uncured or partially cured silicone to form a cohesive bond. 440S bonds immediately through its pressure-sensitive adhesive. This difference is important during repair operations on a vertical cable riser or a machine that cannot be rotated: an adhesive-equipped tape can be placed and pressed into position without maintaining sustained tensile load. The limitation is that 440S leaves a discrete adhesive layer, whereas a fully fused self-fusing tape forms a more homogeneous silicone wall after cure.

Against skived PTFE tapes, COHRlastic 440S offers higher elastic recovery and better anchorage on silicone rubber. PTFE films have broad chemical resistance but low elongation and a low-surface-energy backing. A silicone adhesive on PTFE may fail under peel because the tape does not conform and recover. The solid silicone tape is therefore used for wrapping silicone rubber parts, while PTFE is usually reserved for chemical release, abrasion resistance, or non-stick masking where mechanical stiffness is acceptable.

When Thermal Spray Masking Requires High Elongation Without Fiber Contamination

HVOF and plasma-spray guns generate particle temperatures above 2000 °C, but the mask line is subjected to radiant heat and overspray rather than the full plume temperature. The solid silicone backing remains elastic up to its rated continuous temperature of 260 °C; short excursions above this threshold can occur at the mask edge when the gun dwells inadvertently. Published data for the specific particle impact energy or maximum peak temperature that causes the silicone PSA to deposit residue on the protected surface are limited. Validation should be performed on the actual gun, standoff distance, and traverse speed. The general processing advantage over glass cloth is that no cut fiber ends are available to shed into the coating booth, and the silicone backing leaves no fibrous witness mark after demasking.

In powder-coating operations, a rack or hanger wrapped with 440S can survive cure cycles at 200 °C to 230 °C without charring. The adhesive ages when the same rack is cycled repeatedly and the coating film builds up on the tape surface. The replacement interval is governed by coating line speed, hook surface area, and part geometry; a universal cycle count does not apply. Racks that are blown down with compressed air after coating can extend the interval, but the tape should not be subjected to burn-off ovens because pyrolysis of silicone polymer produces silica ash and adhesive residue.

Peel, Dwell, and Solvent Wiping Define the Adhesion Boundary

Silicone pressure-sensitive adhesives exhibit lower initial peel than acrylic PSA systems. Initial tack is sufficient to hold a wrap in place, but the full bond develops only after the adhesive has flowed into the substrate surface. Dwell time and temperature are therefore process variables. A bond made at room temperature may show higher peel after a dwell period than immediately after wrapping. If the part is placed into a hot zone directly after wrapping, the adhesive softens before it has developed residency on the substrate, and edge lift can occur on curved surfaces. Moderate pre-warming of the substrate can accelerate wet-out on silicone rubber and metal, but the tape should not be heated above its service limit before the bond has stabilized.

Substrate incompatibilities include low-surface-energy thermoplastics. Untreated polypropylene, polyethylene, and fluoropolymers do not provide adequate wet-out and may require plasma or corona priming. The tape bonds well to clean glass, anodized aluminium, stainless steel, and cured silicone rubber. Copper and silver contact surfaces present a specific risk: volatile siloxanes from the adhesive can migrate under arcing and form insulating silicon dioxide deposits. The product is not recommended for use inside sealed relay enclosures or on arcing contacts without verifying a low-volatile silicone grade and conducting contact-resistance testing under the expected switching voltage.

Exposure to hydrocarbon oils, toluene, or methanol can swell the silicone backing and reduce tensile strength during immersion. The bonded area resists casual splash, but continuous immersion in aggressive solvents is an incompatible service condition. Strong alkaline solutions above pH 10 and concentrated sulphuric acid are similarly outside the long-term chemical resistance envelope of silicone elastomers.

Volatile low-molecular-weight siloxanes are released from silicone PSA during elevated-temperature service. In open industrial equipment the ventilation typically disperses these species. In sealed optical modules, secondary optics adjacent to LED sources, or hermetically sealed sensors, condensed siloxane films can attenuate light transmission or shift surface energy. Parts that incorporate 440S should be screened under a controlled outgassing method such as ASTM E595 when the service environment is enclosed and sensitive to condensable species. The absence of a solvent-based adhesive does not eliminate this behaviour.

Processing Behavior on Cylindrical Mandrels and High-Voltage Splices

Wrapping tension is controlled to avoid visible width reduction. Measurement of slit width before and after application is used to establish the limit on a given mandrel diameter and substrate hardness. The tape is applied with a silicone or metal roller to press the adhesive into the substrate. Air pockets are excluded because they become partial-discharge initiation sites in high-voltage wraps. A void-free bondline is more critical than the nominal dielectric strength of the backing alone.

On a high-voltage cable splice, the tape is used as an outer insulation layer or as a compatible silicone jacket repair. The overlap should be at least 50% of the tape width, and successive layers are staggered so that seams do not stack. A stacked seam creates a continuous air path and reduces creepage distance along the splice. The cut edge should be pressed down with a roller; an exposed edge can lift over thermal cycles because silicone rubber has a higher coefficient of thermal expansion than copper or aluminium. On metal busbars, differential expansion between the tape and the conductor must be accommodated by selecting a width that allows the backing to expand without buckling. The 450% elongation provides strain capacity, but repeated thermal cycling may produce progressive PSA creep at the overlap.

For busbar insulation where a second layer is applied, the outer layer should be registered so that no seam aligns with the underlying seam. A butt joint or an aligned overlap seam creates a low-density path between electrodes and can reduce the surface creepage distance. The tape should be applied without air bubbles; a consolidation roller is used after each wrap. In high-voltage service, partial discharge testing under IEC 60270 or an equivalent partial discharge method is used to verify the absence of discharges at the required operating voltage. The pass criterion is defined by the complete insulation system, including terminations and stress-control elements, not by the tape alone.

For a 0.51 mm unsupported layer, the minimum practical bend radius depends on substrate temperature and backing hardness. Below the elastic compression limit, the solid silicone may display visible whitening or microcracking on the inside radius. On sharp edges, a pre-formed silicone fillet or a high-elongation mastic is used beneath the tape to prevent the edge from cutting the backing during thermal expansion.