MOWIFLEX

MOWIFLEX is a flexible mica paper laminate manufactured from uncalcined muscovite or calcined phlogopite mica paper impregnated with a methyl-silicone resin binder and reinforced on one or both sides with electrical-grade glass cloth. The product is supplied in thicknesses from 0.15 mm to 0.80 mm and in roll widths up to 1200 mm; sheet stock is available at 1000 mm × 2000 mm. Typical resin content is 15 wt% to 25 wt%, and volatile content after 2 h at 150 °C is specified as ≤ 1.0 wt% when tested in accordance with IEC 60371-3-1. The cold flexibility limit is below 10 °C, at which the product can be wrapped around a mandrel of 10× nominal thickness without cracking. The product is intended for use as conductor insulation, slot cell liners, and form-wound coil groundwall reinforcement in rotating electrical machines rated up to 15 kV class. Because the silicone binder remains thermoplastic above 40 °C, preheating to 35 °C to 45 °C is recommended before wrapping to reduce springback; however, overheating above 60 °C causes binder tack transfer to winding tooling and increases wrinkle formation on short-radius bends.

The reinforcement architecture distinguishes MOWIFLEX from films and papers. The glass cloth is a plain-weave fabric with areal weight 23 g/m² to 100 g/m² and is bonded to the mica paper by the same silicone resin that impregnates the mica paper. In comparison with polyester film laminates, MOWIFLEX has lower cut-through resistance at room temperature but higher thermal endurance; in comparison with aramid paper laminates, MOWIFLEX has lower tensile strength after resin impregnation but lower partial discharge erosion when assessed by erosion depth under IEC 60270 for 100 h. Selection between these products is determined by the slot fill factor and the voltage stress distribution, not by single-property ranking.

What limits continuous operating temperature in MOWIFLEX silicone-bonded mica paper?

The continuous operating temperature is governed by the thermal endurance classification of the mica paper and the methyl-silicone binder system. MOWIFLEX grades based on muscovite mica paper with high-resin silicone binder are assigned Class F 155 °C under IEC 60085, while phlogopite mica paper grades with elevated silicone resin content are assigned Class H 180 °C. The limiting mechanism is oxidative scission of the silicone network at the mica-paper interfaces, not dielectric failure of the mica platelets themselves. Long-term thermal ageing data generated on similar silicone-bonded mica laminates show tensile strength retention of 50 % after 20 000 h at 180 °C when the glass cloth reinforcement is present; unreinforced mica paper does not maintain mechanical integrity because resin embrittlement progresses inward from exposed edges. Published data for MOWIFLEX-specific ageing under combined thermal and partial discharge stress is limited, so accelerated ageing should follow IEC 60216-1 and IEC 60216-5 with end-point criteria based on insulation resistance and dielectric strength, not visual colour change alone.

Processing windows during vacuum-pressure impregnation require control of the resin tank temperature within ± 5 °C of the resin manufacturer’s nominal viscosity point. For epoxy-anhydride systems with initial mixed viscosity 75 mPa·s to 125 mPa·s at 23 °C, MOWIFLEX absorbs resin primarily through capillary channels between mica platelets. If tank temperature drifts below 18 °C, resin viscosity rises above 200 mPa·s and interstitial filling becomes incomplete, leaving voids that increase partial discharge inception voltage scatter by up to 30 %. If tank temperature exceeds 28 °C for more than 4 h, the silicone binder migrates into the resin bath and changes the bath’s gel time from 60 min to 20 min. Production-scale VPI equipment with continuous resin filtration and 100 µm bag filters is specified because mica dust released during wrapping can accumulate in the tank and reduce the electrical strength of the impregnating resin. After impregnation, curing in a nitrogen-pressurised autoclave at 0.3 MPa to 0.6 MPa and 140 °C to 160 °C for 6 h to 10 h is required to reach a dissipation factor below 0.05 at 1 kV when tested under ASTM D150.

Batch-to-batch variance in MOWIFLEX is controlled by the mica paper basis weight and the resin coating line speed. On production-scale coating lines with web widths of 1000 mm and speeds of 8 m/min to 15 m/min, variations in oven temperature of ± 3 °C at the 120 °C to 150 °C drying zones cause resin surface skinning that increases volatile content and reduces interlayer adhesion after lamination. The product should therefore be inspected for thickness profile across the web; edge-to-centre thickness deviation greater than 10 % of nominal thickness can lead to uneven compression during coil pressing and create low-density zones at the slot wall.

Partial discharge resistance and electrical withstand in high-voltage slot sections

Electrical withstand of MOWIFLEX is determined by mica platelet orientation, resin impregnation level, and the presence of glass cloth. Short-time dielectric strength on 0.25 mm sheet tested in oil conforming to ASTM D149 is typically 22 kV/mm to 25 kV/mm for muscovite grades and 20 kV/mm to 23 kV/mm for phlogopite grades. Volume resistivity measured at 500 V DC per ASTM D257 exceeds 1 × 10¹⁴ Ω·cm at 23 °C and 50 % relative humidity. Partial discharge resistance measured under IEC 60270 on a wrapped electrode configuration is superior to aramid-fibre laminates of equivalent thickness because mica platelets form a high-aspect-ratio barrier layer with tortuous path length. The glass cloth reinforcement is not the primary discharge barrier; it functions as a handling and bridging layer, and its presence lowers the discharge inception voltage by 5 % to 10 % when compared with unreinforced mica paper. Therefore, coil designers place the glass side away from the slot wall and preserve the mica-rich face toward the copper conductor.

Nominal values represent manufacturer datasheet ranges and are verified by IEC 60371-3-1 thickness and resin content methods; lot-to-lot variation should be confirmed by incoming inspection.

MOWIFLEX 200 and 300 differ primarily in mica type and thermal class. The phlogopite paper in MOWIFLEX 300 has lower dielectric strength than muscovite paper but withstands higher temperature and has better flexibility; the mica paper basis weight in the 300 series is increased to 120 g/m² to 200 g/m² to compensate for the lower platelet aspect ratio. MOWIFLEX 400 incorporates a polyester film layer for improved moisture barrier and handling stiffness, but the film layer limits continuous operating temperature to Class F 155 °C and reduces resin uptake by approximately 10 % to 15 % compared with the glass-only grades.

Thermal conductivity of mica paper laminates is anisotropic. In-plane thermal conductivity measured by laser flash in accordance with ASTM E1461 is typically 0.25 W/m·K to 0.35 W/m·K, while through-thickness conductivity is 0.15 W/m·K to 0.20 W/m·K. This difference should be used in thermal network models; using a single isotropic value overestimates heat transfer from the copper conductor to the stator core and can underpredict hot-spot temperature by 10 °C to 15 °C in machines above 1 MW.

When MOWIFLEX replaces glass-backed mica tape in resin-rich coil consolidation

In resin-rich coil manufacturing, a switch from mica tape to MOWIFLEX sheet is not a direct thickness-for-thickness substitution because the sheet form eliminates overlap regions but increases bending stiffness at conductor corners. Hot pressing of MOWIFLEX at 160 °C with a ramp rate of 2 K/min and a consolidation pressure of 4 MPa to 6 MPa is required to compress the mica paper and expel air from the glass interstices. When press temperature deviates more than ± 5 °C from the target, the silicone binder either does not flow sufficiently to fill the mica paper voids or overcures at the surface and traps volatiles in the core. The resulting insulation can show a two-layer structure with dense outer skins and porous centre, detected by ultrasonic C-scan attenuation increases above 6 dB. In contrast, mica tape processes tolerate a temperature window of ± 10 °C because the open wrap structure allows gas escape during initial heating. For this reason, MOWIFLEX is used in flat slot portions and overlapped only at the end-winding knuckles where mechanical stress is lower, while mica tape remains preferred for continuously transposed conductor stacks that require conformable taping under 15 N to 25 N wrap tension.

Slitting of MOWIFLEX is performed with rotary shear knives set to a gap of 5 % of sheet thickness; dull blades produce edge delamination and release mica dust that contaminates winding halls. A precision guillotine with a clearance of 0.02 mm is specified for sheet cutting. Edge delamination greater than 2 mm from the cut line is a rejection criterion for parts used above 6.6 kV because exposed mica paper edges create a direct path for resin starvation and later partial discharge.

Compared with rigid micanite, MOWIFLEX retains conformability after full cure of the silicone binder because the resin remains thermoplastic and the glass reinforcement limits crack propagation. Rigid micanite products, which use shellac or epoxy hardening binders, exhibit brittle fracture below 5 % flexural strain and require preforming to final shape. MOWIFLEX can be bent to an inner radius of 5× thickness at 23 °C without visible cracking, whereas rigid micanite typically requires heating to 80 °C to 120 °C before forming. However, creep resistance of MOWIFLEX under continuous compressive load is lower than that of rigid micanite; designers should limit continuous slot pressure to 2 MPa to avoid thickness reduction above 5 % after thermal cycling.

Storage before use must avoid relative humidity above 60 %; mica paper adsorbs moisture at the resin-paper interface and can reduce interlayer adhesion during hot pressing. If exposure to high humidity exceeds 24 h, the product should be pre-dried at 80 °C to 90 °C for 2 h to 4 h in a ventilated oven and then conditioned to 35 °C before wrapping. MOWIFLEX should not be installed with amine-based liquid resins that have amine hydrogen equivalent weights below 60 g/eq, because the silicone binder can catalyse early exothermic crosslinking at the laminate surface and produce a low-molecular-weight boundary layer with reduced shear strength.

For applications requiring flame resistance, MOWIFLEX is evaluated by limiting oxygen index under ASTM D2863 and vertical burn classification under UL 94. Typical silicone-bonded mica paper laminates with glass cloth achieve oxygen index above 35 % and do not sustain flame after the ignition source is removed. The mica platelets create a char barrier that reduces smoke release compared with organic polyester film laminates. However, the silicone resin decomposes above 250 °C and releases silicon-containing volatiles; exhaust extraction is required during hot pressing or soldering operations.

In applications where direct liquid cooling is used, the product should be tested for hydrolysis stability under the specific coolant chemistry; published data for ester-based cooling fluids in contact with MOWIFLEX is limited, and extended compatibility testing according to IEC 60464-2 should be performed by the end-user. The glass cloth reinforcement can wick coolant along fibre bundles if edge sealing is incomplete, so cut edges must be sealed with a compatible insulating varnish or overlapping tape before exposure to coolant flow.