US2364790A - Treatment of organic dielectric materials with high-frequency electric current - Google Patents

Description

Patented Dec. 12, 1944 TREATMENT OF ORGANIC DIELECTRIC l VIA- TERIALS" WITH HIGH-FREQUENCY ELEC- TRIC CURRENT I Charles B. Hemming, Parlln, N. J assignor to E. I. du Pont de Nemours & Company, Wilmington,

Del., a corporation of Delaware llo Drawing. Application September 13, 1941, Serial No. 410,785

7 Claims.

This invention relates to the application of high frequency electrostatic fields to organic substantially non-conducting materials and more particularly to a means for increasing the electrical losses of such materials when exposed to high frequency electrostatic fields.

external heat sources.

tions.

objects.

mercial practice.

absorbed by metallic conductors.

stants of 20 or less at 60 cycles.

times the power factor. The larger the product of these constants, the greater is the energy absorption from a radio frequency field. For example, polystyrene with a dielectric constant of 2.5 and a power factor of 0.0001 (at 60 cycles) Methyl A further object is the The heat- The high dielectric conhas as its product a value of 0.00025.

A unique method for joining a plurality of methacrylate withadielectric constant of 3.5 and parts by exposing a thermo-sensitive adhesive a power factor of 0.06 has as its product the value having relatively high hysteresis characteristics 0.21 or one over 800 times as great as styrene. to high frequency electrostatic field is described 10 In an electrostatic field having a frequ y of in U. S. Patent 2,087,480. It was found that 30 megacycles per second and animpressed voltsufilcient heating occurs to soften or fuse or a e f 2 00. m t yl t y t heats readily otherwise affect the adhesive to cause bonding. wh l p y tyr s practically unaffected- It Previously heat had been applied through cons i po ant s a r ul f t fact to develop duction by hot pressure plates, ovens or oth means for modifying the more perfect dielectrics Dependent upon the heat so that they will heat readily and quickly in the conductivity of the parts to be joined, the time radio frequency fields. necessary to secure satisfactory joints was often It is an object of this invention to P o ide a long and seriously delayed manufacturing operameans of controlling the absorption of electrical As suggested in the patent, it is characen y y organic p m ri ma perticw teristic of the action of high frequency fields that y thermoplastic. Organic resinous dielectrics uniform and rapid heating of the materials ext radio f q posed occurs, that is, absorption of electrical pr vision f a m n of in re s he energy energy occurs throughout the materials, limited absorpti y the more p f t d tr A y by the hysteresis characteristics of the still further object is the provision of modifiers terials exposed. Excessive surface heating is, for organic po y c materials which (10 not therefore, avoided, and the time required to heat t y alter h physical properties of the thick objects is substantially the same as for thin po i io O r objects w l app h r I have found that titanium dioxide in its rutile It has been found advantageous to use elecand br k e rystalline forms has electrical trical frequencies in the more commonly used p p t Such t although t p ave y radio frequency range of 100,000 cycles to 100 cflicient dielectric alone at radio frequencies, it megacycles per second. The oltage applied may acts in an unexpected manner when incorporated be varied over wide ranges but in general an in substantially non-conducting organic c p input voltage of at least 1000 is necessary in comi io and giv ed mp n f greater hysteresis characteristics than represented by the The process is particularly adaptable to the titanium d e f ate cementing of the component parts of shoes, for Titanium dioxide is known in three crystalline the manufacture of plywood, for cementing cork forms: rutile, brookite, and anatase. granules and many other cementing operations. 40 .ing of brookite and anatase above 820 0. tends The process is particularly advantageous where toconvertthem totherutileform. Allthreeforms the materials to be joined are non-metallic and possess high dielectric constants, with those of are poor conductors of heat. It is a characterrutile and brookite having the very abnormal istic of the electrostatic field that in the radio dielectric constants of approximately 114 and 78, frequency range very little electrical energy is respectively. The anatase form shows a dielec- The system is, tric constant of 3|. therefore, adapted to organic polymeric matestants are coupled with very low power factors, rials which are ordinarily considered to be good so that the product of the dielectric constant dielectrics for direct or low frequency alternating times the power factor is low. The tendency of currents, that is, materials having dielectric conthe rutile to absorb electrical energy from a radio frequency field is slight, and the material can be It is generally assumed that the extent of the used as an insulator in these fields. hysteresis loss of a material can be expressed Unexpectedly, however, when .finely divided comparative to those of other materials by com rutile or brookite is incorporated into a polyparing the products of the dielectric constant meric organic material, the hysteresis losses of obtained. In many cases adhesives which cannot be made to heat satisfactorily in the available electrostatic fields can be modified with the rutile or brookite to produce materials which can be activated.

The following examples are given by way of iilustration and no limitations are intended thereby except as indicated in the present invention:

Example 1 Percent by 1 weight Polyvinyl butyral resin; 1.8.7 Dibutyl phthalate g 9.4 Denatured ethyl alcohol 36.3 Tnlunl 35.6

This example was-comparedin heating characteristics in a radio frequency field of 2000 Volts at megacycles per second, with the same composition in which 34% by weight of the total solids content of finely divided rutile had been incorporated by rapid mixing. Both compositions were applied by brushing to the rough side of 1 x 6 inch strips of shoe sole leather and allowed to dry hour at 60 C.

A second coat was then applied and dried for ,5 hour at 60 C. following which they were allowed to stand for 4 hours in a room at 245 C.

Heating tests were then carried out using a treater input voltage of 2100 at a frequency of 30 megacycles. between two adjustable metal plates, between which the leather samples were placed in the same plane. leather strips were placed between the treater plates for seconds. The temperature of the strips at the end of the treatment was obtained by inserting a quick-acting metallic thermometer of the Weston type between the strips before any appreciable heat could be dissipated through the leather. The strips coated with the unmodifled adhesive composition were tested under the same electrical conditions and finally the strips coated with the adhesive containing the rutile were similarly tested. The following averaged results were obtained:

1 Rise above m room temperature 36', C. Leather stri -no adhesivm 65 40 Leather ad esive 71 46 Leather adhesive-Hume-.. 76 50 Longer periods of treatment gave higher temperature rises but 'of a comparable order of difference. Excellent bonds were secured with the adhesive containing the rutile modifier.

The electrostatic field was set upv As the initial test, two uncoatedsame 2 Percent by weight Ethyl cellulose 18.7 Dibutyl phthalate 9.4 Ethyl alcohol (denatured) 38.2 Toluol- 35.7

66.2 parts by weight of this composition were mixed with 33.8 parts by weight of rutile and the mixture applied by brushing two coats on four 1'' x 6" sole leather strips. Similarly, two coats of the unmodified composition were applied to four other leather strips. All samples were dried at C. for /2 hour between coats and subjected to a similar drying period after the second coat and then cooled for 3 hours at a temperature of 24.5 C. and a relative humidity of 31%.

Pairs of uncoated and coated leather strips with the coated surfaces held together under moderate pressure with flexible rubber bands were suspended in a high frequency electrostatic field of 30 megacycles per second and an applied input voltage to the oscillator of 2100 volts for a period of 45 seconds and the rise in temperature determined as in Example 1.

The following averaged results were obtained:

Further illustrative of the novel and desirable effects afforded by the use of rutile for inducing heat in organic dielectric materials are the power absorption results obtained by testing sheets of polymerized methyl methacrylate. Two sheets of this material 1 x 3" and /a" in thickness and held together with flexible rubber bands were suspended in a high frequency field of 30.6 megacycles per second and an impressed input voltage to the oscillator of 2100 volts. Power absorption was found to be 21 watts. This test was repeated with a layer of rutile amounting to about 50% of the weight of the sheets uniformly interposed between the two sheets of polymerized methyl methacrylate resin and in this case the power absorption was 63 watts.

-As indicated above, titanium dioxide in the rutile or brookite form is preferred in the operation of the present invention because of their unusually highdielectric constants. The anatase form with its relatively low dielectric constant fails to provide the advantages of the rutile and brookite forms and, therefore, is of little use as a means for increasing the hysteresis losses in the polymeric organic materials herein described.

It has been found that an amount of rutile or brookite of between about 20 to 50% by weight of the total solids of the composition to be treated gives improved results with about 30 to 40% by weight of the total solids giving greatest improvement.

The invention is of general utility for inducing heat in organic polymeric materials when such materials are exposed to energized electrostatic fields of radio frequencies, preferably frequencies between about 100,000 cycles and megacycles and especially at frequencies of merized state including phenolic, urea aldehyde.

acrylic acid resins, alkyd and styrene type resins promotes the heating of such materials when exposed to high frequency electrostatic fields thereby converting them to the polymeric state. The agents are also useful in cellulose derivative compositions containing cellulose nitrate, cellulose acetate, cellulose ethers and mixed cellulose esters such as cellulose acetobutyrate and cellulose acetopropionate.

- Of particular advantage is the increase in hysteresis losses at high frequencies occasioned by the addition of the rutile or brookite forms of titanium dioxide to organic polymeric dielectric materials. In some instances, such materials have such low losses at radio frequencies that no heating results are experienced when they are placed in an electrostatic field of such fre quencies. However, the incorporation of the suggested modifying agents, even though they are excellent dielectric materials themselves. causes a change in the electrical properties of the organic material so that the hysteresis losses are increased to a point that heating results at such frequencies. The invention presents desirable advantages even with organic polymeric materials which normally respond to electrostatic fields of frequencies between 100,000 cycles and 100 megacycles in that the rate and amount of heat so induced is markedly increased.

It is apparent that many widely different embodiments of the invention may be made without departing from the spirit and scope thereof and, therefore, it is not intended to be limited except as indicated in the appended claims.

'I claim:

1. The process of inducing heat in organic materials of high dielectric value which coniprises dispersing titanium dioxide selected from the group consisting of rutile and brookite in said organic material and placing the so modified material in an energized electrostatic field of radio frequency.

2. Process of claim 1 in which the frequency of the electrostatic field is between 100,000 cycles per second and 100 megacycles per second.

3. Process of claim 1 in which the frequency of the electrostatic field is about 30 megacycles per second.

4. In the process of heating organic monomeric and polymeric materials of high dielectric value by exposing said materials to an energized high frequency electrostatic field, the step of incorporating titanium dioxide selected from the group consisting of rutile and brookite in said organic material and thereafter placing the so modified material in an energized high frequency field, said frequency being from 100,000 cycles per second to 100 megacycles per second.

5. Process of claim 4 in which the titanium dioxide is present in an amount between about 20 and by weight of the total solids.

6. Process. for promoting the polymerization of unpolymerized and monomeric organic materials of high dielectric value which comprises dispersing titanium dioxide selected from the group consisting of rutile and brookite in said organic material and exposing the so modified material to an energized electrostatic high frequency field until the material is polymerized. said frequency being from 100,000 cycles per second to megacycles per second.

7. Process of claim 6 in which the monomeric material is monomeric styrene resin.

CHARLES B. HEMMING.