Resin-treated compressed wood laminates (compregs) - Specification, Part 1: For electrical purposes
1989 Edition

The standard specifies six grain orientation categories for electrical-grade compregs: Type I features grain tangential to the board edge with staggered joints, suitable for large circular rings; Type II has over 75% grain aligned along the principal stress direction with every fourth veneer at 90°, enhancing tensile strength and splitting resistance; Type III has evenly distributed grains radially with angular veneer placement, ideal for gears and wheels; Type IV orients grain approximately 45° to the loading axis, used in components under high voltage stress with limited clearance; Type V aligns grain along the load axis, optimized for tensile stress elements such as rods; and Type VI offers equal grain distribution in perpendicular axes, providing high compressive strength for sheets and blocks.

For Type V compregs, the minimum mechanical strengths include tensile strength of 175 MPa, static bending strength of 195 MPa, compressive strength parallel to laminae of 170 MPa, perpendicular compressive strength of 95 MPa, shear strength parallel to grain of 14 MPa, and perpendicular shear strength of 60 MPa. Type VI compregs require minimum tensile strength of 90 MPa, bending strength of 95 MPa, compressive strength parallel to laminae of 120 MPa, perpendicular compressive strength of 185 MPa, shear strength parallel to grain of 20 MPa, and perpendicular shear strength of 45 MPa. Both types must have minimum Rockwell M hardness of 70 and specified impact strengths. These values ensure suitability for tensile and compressive load applications respectively.

The recommended resin is natural phenol formaldehyde dissolved in an alcohol solution, with a minimum resin content of 45% on an oven-dry basis within the veneer. Vacuum impregnation, optionally combined with pressure, is the preferred method. Curing should be carried out at temperatures between 135°C and 150°C under pressures ranging from 7 to 15.5 MPa. The solvent used should be denatured spirit complying with IS 324:1959 or an equivalent conforming Indian Standard. Additionally, insulating oils and varnishes must comply with IS 10026 (Part 3/ Sections 1-7):1983, and high-viscosity insulating varnishes may be applied as required.

Compregs intended for high voltage applications must have insulation resistance of at least 10 Megohms after immersion in water at 27±2°C, volume and surface resistivity between 2 to 5 × 10^10 ohm-cm, flatwise electric strength in oil at 90±1°C not less than 4 kV/mm for a 6 mm thick sample, and edgewise electric strength in oil at 90°C of at least 25 kV for a 25 mm wide specimen. The power factor (tan δ) is typically about 0.019 at 50 Hz and 20°C. Varnished components should exhibit a Comparative Tracking Index (CTI) of at least 100. These parameters ensure reliable electrical insulation under operational conditions.

Standard compreg board thicknesses include: 3 mm; 4-5 mm; 6 mm; 8 mm; 12 mm; 16 mm; 20 mm; 25 mm; 32 mm; 40 mm; 50 mm; 60 mm; and 70 mm. Length tolerances are +6 mm and -0 mm, width tolerances are +3 mm and -0 mm. Thickness tolerances are ±10% for thicknesses under 6 mm and ±5% for thicknesses 6 mm or greater. For rods, standard lengths and diameters vary: 1500 mm length with diameters from 8 to 50 mm; 1200 mm length with diameters 3 to 50 mm; 900 mm length for diameters 50 to 80 mm; and 600 mm length for diameters 6 to 40 mm. Diameter tolerances are +0 mm and -0.20 mm for diameters up to 40 mm and +0 mm for diameters 50 mm and above. These specifications ensure dimensional accuracy for assembly and performance.