Patent GB-637433-A (GB)
Title: Treatment of fusible and reactive materials in high frequency electric fields
IPC: B23K13/02
Filed: 1944-10-12
Abstract: 637,433. Di-electric heating. ARNOLD, O. M. Oct. 12, 1944, No. 19648. Convention date, Sept. 9, 1943. [Class 39 (iii)] In methods of treating bodies with alternating or intermittent high frequency electrostatic fields, the frequency of the applied field is chosen to be within the anomalous dispersion range under the conditions of environment of molecular units which it is desired to agitate and nearer the centre of that range than that of other molecular units of the body under their conditions of environment. The invention may be applied to methods of welding, fusing, diffusing, cutting, vaporizing and distilling, a maximum energy absorption being found to occur at the centre of the anomalous dispersion range. This range is sensitive to the presence of other molecules, and to the temperature and viscosity of the material. The invention permits selective application of the energy to the surface or to the interior of a mass or body and/or to particular substances in a mass or body or group of bodies. Thus, the surface molecules may be heated without appreciably affecting the interior molecules, or vice-versa, or in a mixture of substances, selected constituents only may be heated. Heating, diffusion or chemical changes resulting from the treatment may afford automatic control of the operation by taking the material out of the range of maximum energy absorption. Where a continued action is required this may be accomplished either 'by suitable adjustment of the frequency, or by using multiple or complex frequencies, the various components of which become effective as the environmental conditions alter. Thus in bonding of plastics a plasticizer may be first diffused into the contacting surfaces and a second component of the frequency may be used to soften the surfaces sufficiently to cause adhesion. In some cases the frequency which causes the initial action may also cause the second action under the changed conditions. Frequencies between 10<SP>7</SP> and 10<SP>9</SP> cycles per second with voltages of 10 to 10<SP>2</SP> are stated to be suitable, and in the case of chemical reactions it is preferable to work at a pressure of 10 to 10<SP>2</SP> atmospheres. Where high voltages are used breakdown between the electrodes is prevented by insulating them with a refractory insulator e.g. ceramic material, beryllia, mica or mica substitutes. Where the treatment takes place at atmospheric temperature it may be advantageous to cool the electrodes and expose them directly to the material. The circuit should be tuned to resonance at the particular frequency selected, so that the electrodes themselves cannot be used as pressure platens or rollers unless the circuit is automatically maintained in resonance. This may be done by means of a lamp placed in the field, which will be lighted at full brilliancy when the circuit is in resonance. By means of a tuning motor controlled by a photo-electric relay the circuit may be maintained in resonance and the efficiency at a maximum. The necessity for retuning is preferably avoided however by using insulating platens or rollers as in Fig. 1 wherein two sheets 30 are welded together or seamed by passing between toroidal rollers 10, 12 of highly refractory ceramic material pressed together and driven by concave rubber rollers 14. The sheets 30 may be of cellulose acetate coated with a plasticizer such as triacetin, the range of anomolous dispersion lying between 2 and 20 megacycles for the acetate plastic and between 100 and 500 megacycles for the triacetin, the exact frequency being determined by experiment. The electrodes proper, 16 and 18, are disposed within the rollers 10, 12 and accurately spaced apart, conductors 22 and 24 being led in through insulating tubes and connected at points distributed over the treating area of the electrode, as at 28. In Fig. 3 a wire lead 35 provided with a glass sealing flange 36 secured thereto by intermediate bonding glass 37 is sealed into a glass envelope 38. The field is applied between the wire 35 and a ring electrode 40 placed around the periphery of the flange 36, the frequency being regulated to soften the glass in the flange 36 and in the edge of the glass 38. In a modification, Fig. 4 (not shown), the envelope is of metal, the field being applied between the wire and the envelope and the frequency chosen near the centre of the anomalous dispersion range for the glass washer. The invention is also applied to fabrics composed at least in part of thermoplastic fibre by passing the fabric through the field to fuse together the individual fibres where they come into contact. Knitted fabrics may be treated thus to prevent " ladders ", and batts of thermoplastic fibres e.g. cellulose acetate, fibre glass or rock wool, or non-softenable fibres coated with a material which becomes tacky on heating, can be strengthened and made more resistant to packing under the influence of gravity and vibration. Fabrics may be made by merely overlaying warp on woof and bonding at the junctures. Stretched fibres or films may be manufactured by passing through rolls between electrodes (as in Fig. 1) and then through a second pair of rolls operating at higher speed. Where an intermittent uni-directional field is used its tendency to orient the molecules may be utilized to produce the orientation desired in the finished fibre or film, the agitation of the molecules becoming less and less as orientation is established thus giving automatic termination of the treatment. In the manufacture of so-called shatterproof " glass the bonding plastic is softened and rendered adherent by a frequency near the centre of its anomalous dispersion range without heating either of the two sheets of glass. Where materials are spot-welded, strip-welded or cut without pressure ultra-high frequencies (10<SP>8</SP> or more c.p.s.) are used, with parabolic reflectors to direct the field to the area of treatment. In the moulding of plastic and plastic coated substances the degree of density or porosity may be controlled. Granules, powders, or fibres of the plastic material or of filler material coated with suitable plastic may be placed in a mould and compressed to the desired extent and subjected to a high frequency field which softens the binder and thus bonds the particles together. Similarly the porosity of ceramic articles may be controlled, the frequency being chosen for the interior of the particles if denser articles are required, and the mass being pressed while it is being subjected to the alternating field. The Specification as open to inspection under Sect. 91 comprises also the use of frequencies of the order of 1/second. This subject-matter does not appear in the Specification as accepted.