Method of welding thermoplastic substrates with microwave frequencies

A method for continuously welding fiber-reinforced substrates of thermoplastic synthetic resinous material, comprising,

• (a) continuously overlapping sections of substrates to provide an interface at which a continuous weld is to be made;
• (b) interposing at said interface, in contact with each substrate, in a weld-zone in which said weld is to be made, a strip having an average thickness in the range from about 5 to about 50 mils comprising a thermoplastic carrier polymer miscible with said material, said carrier having dispersed therein from about 1% to 20% by weight of submicron electrically conductive carbon black particles which are absorptive to microwave frequencies in the range from about 0.5 GHz to about 10 GHz, have a surface resistivity less than 100 ohm.multidot.cm/cm, and a BET surface area of at least 100 m.sup.2 /g, said substrates and strip forming an assembly;
• (c) continuously moving said assembly while subjecting a portion thereof to an electromagnetic field having sufficient power in said frequency range to heat said assembly non-uniformly, the temperature in said weld-zone diminishing from above the glass transition temperature T.sub.g or melting point T.sub.m of said material near said interface, to below said T.sub.g or T.sub.m outside said weld-zone;
• (d) terminating exposure of said assembly to said field when said carrier vanishes by forming a single phase with the thermoplastic material in said weld-zone;
• and,
• (e) cooling said weld-zone to form a composite having essentially the same dimensions as said assembly.

A shaped article suitable for use as a structural vehicle component and consisting essentially of at least two sections of a fiber-reinforced material welded together by microwave energy in the frequency range from about 0.5 GHz to about 10 GHz, said article being formed by interposing between and in contact with said sections in a weld-zone, a strip having an average thickness in the range from about 5 to about 50 mils comprising a thermoplastic carrier polymer miscible with said material, said carrier having homogeneously dispersed therein from about 1% to 20% by weight of submicron electrically conductive carbon black particles which are absorptive to microwave frequencies in said range, which particles have a surface resistivity less than 100 ohm.multidot.cm/cm, a BET surface area of at least 100 m.sup.2 /g, and an average primary particle size smaller than about 100 nanometers, said strip functioning to position said carbon black particles in said weld-zone;

• subjecting said weld-zone to an electromagnetic field having sufficient power in said frequency range to heat said assembly non-uniformly, the temperature in said weld-zone diminishing from above the glass transition temperature T.sub.g or melting point T.sub.m of said material near said interface, to below said T.sub.g or T.sub.m outside said weld-zone;
• terminating exposure of said assembly to said field when said carrier vanishes by forming a single phase with the thermoplastic material in said weld-zone;
• and, cooling said weld-zone to form said article having essentially the same dimensions as that of said assembly.