US 3057792 A
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Oct. 9, 1962 H. FROHLICH 3,057,792
METHOD FOR IMPROVING THE IMPRINTABEELITY OF SYNTHETIC MATERIAL Filed Dec. 22, 1958 5 Sheets-Sheet l Fig.1 1
Oct. 9, 1962 H. FROHLICH 3,05 7,792.
METHOD FOR IMPROVING THE IMPRINTABILITY OF SYNTHETIC MATERIAL Filed Dec. 22, 1958 3 Sheets-Sheet 2 Fig.2 1
H. FRUHLICH Oct. 9, 1962 METHOD FOR IMPROVING THE IMPRINTABILITY OF SYNTHETIC MATERIAL Filed Dec. 22, 1958 3 Sheets-Sheet 3 3,057,792 Patented Oct. 9, 1962 Free 31,857,792 METHUD FQR IMPROVENG THE EMPRINTABILETY F SYNTHETKI MATERIAL Heinz Frtthlieh, Erlangen, Germany, assignor to Siemens- Schnckertwerke Atrtiengeseiisehaft, Berlin-Siemensstadt, Germany, a corporation of Germany Filed Dec. 22, 1958, Ser. No. 782,125 Claims priority, application Germany Dec. 21, 1957 4 Claims. (Cl. 204-165) My invention relates to a method for improving the imprintability of synthetic plastics by modifying their surface texture for better adherence of printing dyes.
Generally, the adherence of such dyes to the surfaces of synthetic materials is rather poor, some of these materials, such as polyethylene, being not readily imprintable unless the surface is first subjected to special processing. According to one of the known processes, the synthetic material, for example in form of a foil, is subjected to a high voltage spray field in atmospheric pressure. Such a field can be produced by applying a low or high frequency alternating voltage in the order of several kv. to the electrodes of an air-gap capacitor with the synthetic material to be processed located between the electrodes.
The improvement of the dye-holding ability thus obtainable is due to the action of ozone ions and probably also due to the ions of certain oxygen-nitrogen compounds evolving from the electric discharge. The duration of the processing period required depends upon the obtainable ion concentration, an increase in degree of ionization in the discharge gap having the effect of more rapidly producing the desired result.
The degree of ionization for a given electrode geometry is determined by the magnitude of the alternating voltage and the current intensity in the spray discharge gap. Since the voltage cannot be increased indefinitely and the current intensity is limited by the capacitance of the device, the known processing devices, operating with low-frequency alternating voltage, required relatively long discharge distances for attaining practically applicable proc essing periods, or it was necessary to operate with highfrequency voltage thus incurring a considerable expenditure.
Furthermore, the known method and devices are successfully applicable only with certain synthetic materials that respond to ions producible within normal air, and the processing of large-size bodies of synthetic material is extremely difficult because of the existing technological and economical limitations as to size and shape of the capacitor electrodes.
In addition to such limitation in applicability, the known devices have considerable disadvantages with respect to operational requirements. Since the processing of the synthetic materials is effected in air, a detrimental influence of variations in the humidity content of the air upon the desired effect is virtually inevitable, particularly in view of the fact that a continuous current of air must be passed through the apparatus for avoiding injurious effects of the ozone upon the health of the personnel. Furthermore, the concentration of neutral ozone in the spraytype electric discharge is relatively large because at atmospheric pressure the ions possess a great recombination probability. It cannot be avoided, therefore, that the surface of the synthetic material becomes charged with neutral ozone gas which is inactive as regards the desired result and has merely the effect of impairing the smell of the synthetic material.
It is an object of my invention to improve the imprintability of synthetic materials by a method and means suitable for use in industrial large-scale manufacture and free of the above-mentioned limitations and disadvantages.
According to a feature of my invention, the high ion concentration required for the processing can be produced in a considerably simpler and more effective manner by utilizing the plasma of a low-pressure electric gas discharge. Despite the slight gas pressure, the ion concentration in such discharges can be increased by increasing the current density of the discharge up to any desired multiple in comparison with the ion concentration attainable in high-voltage discharges at atmospheric pressure.
According to a more specific feature of my invention therefore, the synthetic material to be processed for better dye-holding ability is placed into a vacuum, and the surface of the material is subjected to the plasma of an electric low-pressure gas discharge. The vacuum may have a pressure in the decimal order of magnitude of 1 mm. Hg. The composition of the gases in the discharge may be controlled or kept constant by supplying the vacuum or negative-pressure chamber with given dosages of desired kinds of gas.
A device for performing the method according to the invention may operate, for example, with a low-voltage glow discharge of approximately 400 to 500 volts between the electrodes, or the plasma may also be produced by thermionic cathodes operating with an arc voltage of approximately volts. The high current densities of such discharges permit reducing the processing time down to fractions of those previously required. For example, when processing polyethylene in an oxygen atmosphere of about 1 mm. Hg pressure a processing period of only One second at a current density of 2 ma./cm. is sufiicient. With higher current densities, a further reduction in processing time is obtainable. The power consumption in the example just given amounts to approximately one-fifth of the consumption of a high-voltage spray discharge device H of the same through-put per time unit operating at utilityline frequency (50 or 60 c.p.s.).
Since the processing takes place in vacuum, the charging of the surface with neutral ozone is eliminated, thus also avoiding bad smell of the product.
The production of the plasma may be effected with direct current or alternating current. It has been found that a high-frequency alternating field of 1 to 1000 megacycles per second is particularly favorable. Producing such a field involves a relatively small expenditure in material and space because of the low arc voltages required. As a rule, the synthetic material is available in form of foils, sheets, tapes or other webs and hence must be passed continuously through the negative-pressure chamber with the aid of vacuum locks. This can readily be done with synthetic foils of any thickness or width. Applicable for this purpose, for example, is a vacuum lock having at least one pro-vacuum and one post-vacuum chamber provided with slots for the passage of the material, the gaps remaining between the inserted synthetic material and the edges of the slots being smaller than the mean free paths of the gas molecules in the spaces that communicate with each other through the slots. When using such slot-type vacuum locks any sealing means can be dispensed with.
The method according to the invention is also suitable for the processing of bodies of synthetic materials having any desired shape, because the spacial geometry of the discharge can be chosen at will by giving the electrodes a corresponding shape.
The foregoing and other objects and features of my invention, the novel features being set forth with particularity in the claims annexed hereto, will be apparent from, and will be further explained in, the following with reference to the drawings in which:
FIG. 1 illustrates schematically in longitudinal section a processing device according to the invention in conjunction with its electrical accessories.
FIG. 2 shows in a similar manner a modified form of processing equipment.
FIG. 3 shows a longitudinal section through another embodiment of a device according to the invention suitable for the processing of three-dimensional bodies, such as bottles or tubes; and
FIG. 4 is a cross-section along the line IV--IV indicated in FIG. 3.
The apparatus illustrated in FIG. 1 operates with a selfsustaining low-pressure glow discharge for the processing of synthetic foils in form of a sheet or tape. The foil material 1 passes from the right through a narrow entrance slot 2 whose cross-section and axial length are so chosen that the flow resistance in the slot suffices for producing in the pre-vacuum chamber 3 of the device a desired negative pressure by means of pumps (not illustrated) to be connected to nipples 4 located on both sides respectively of the foil 1. If desired, two or more such pre vacuum chambers may be provided one behind the other.
Another slot 2a, designed in the same manner as the slot 2, connects the chamber 3 with the processing chamber 5 proper, the foil passing horizontally through the center of the chamber. Located on both sides of the foil 1 in chamber 5 are two pairs 6 and '7 of electrodes. The two electrodes of each pair are connected to respective leads 8 and 9 which pass through air-tight seals in the chamber wall to the outside where they are connected, in series with respective adjustable resistors it) and 11, with the secondary winding of a transformer 22 whose primary winding is energized from an alternating current source of any desired frequency, such as about 50 or 60 cycles per second. A double-pole switch 13 permits disconnecting the electrode pair 7 in cases where only one side of the foil is to be treated. The processing chamber 5 is con nected With vacuum pumps (not shown) that communicate with the upper and lower portion of the chamber through respective nipples 14. By means of these pumps, the chamber 5 is evacuated down to the vacuum, preferably 0.5 to 1 mm. Hg, required for producing a glow discharge between the electrodes of each pair. The voltage required for such glow discharge depends upon the negative pressure in chamber 5 and may amount to a few hundred volts, for example.
The device is preferably provided with gas supply nipples 15 located on both sides of the foil 1 and on both sides respectively of the processing chamber 5. The nipples 15 permit supplying the device with the gas or gas mixture that is to be active in the processing chamber 5. A supply of oxygen is suitable for the processing of polyethylene, but non-oxidizing gases are also applicable, chlorine gas being suitable for most synthetic materials other than polyethylene. The nipples 15 are closed when no such additional gas is needed. The chamber 5 is connected through another slot 21;, corresponding to those described above, with a post-vacuum chamber 16. The chamber is preferably provided with nipples 17, which permit adjusting the negative pressure in the chamber to any desired value by means of a pump.
By maintaining in the gas supply nipples 5 a gas pressure higher than the air pressure existing at these respective locations of slots 2a and 2b, the ingress of atmospheric air into the processing chamber 5 can be substantially or fully prevented if this is required for best processing results. In this case, and in the absence of oxygen, it is also possible to use, instead of the alternatingcurrent energized, self-sustaining low-pressure glow discharge, a low-pressure arc discharge as is illustrated in the modified device shown in FIG. 2.
The device of FIG. 2 is to a large extent similar to that of FIG. 1, the same reference numerals being used in both illustrations for similar components respectively. However, the device shown in FIG. 2 is provided with two thermionic cathodes 13 and cooperating with respective anodes 19 and 21. The cathodes IS and 26 are connected by leads 22 and 23 with a transformer 25 for heating the two cathodes from an alternating voltage source. The leads 22 and 23 pass through gas-tight seals in the walls of the processing chamber 5. The anodes It? and 21 are connected by respective leads 3 and 9 through adjustable resistors it} and ill with the positive pole of a direct-current generator 24 whose negative pole is connected to the cathodes 16 and 26. A switch 13 permits disconnecting the lower arc gap if only one side of the foil l i to be treated. The voltage between each cathode and anode may amount to a few hundred volts depending upon the gas pressure in the processing chamber 5.
The device illustrated in FIGS. 3 and 4 permits the processing of such bodies of synthetic material as bottles, collapsible tubes and the like. The device comprises a housing structure 31 which forms a vacuum chamber in its interior and can be closed and vacuum-tightly sealed by means of a front door 32. Located between the side walls of housing 31 are two insulating plates 34 which are provided with parallel longitudinal grooves 40 for the insertion of metallic electrodes 33, preferably consisting of planar metal plates. The electrodes 33 engage respective contacts 36 mounted on vertical connector strips 35. The contacts 36 supply the electrodes 33 with electric voltage whose polarity changes from plate to plate. The voltage is applied to the contacts 36 through leads 41 passing through vacuum-tight seals in the rear wall of the housing structure 311. During operation a glow discharge is maintained between the electrodes 33.
The electrodes 33 are preferably designed as perforated plates for facilitating the evacuation of the processing chamber. However, the electrodes 33 may also consist of Wire mesh, grid-shaped gratings, or of non-perforate plates or sheets which need not necessarily be planar, but may be given any desired shape corresponding to the shape of the material or bodies to be processed. If necessary, the electrode plates may carry inserts of any desired shape (not illustrated) corresponding to the shape of the bodies to be processed, the inserts being in electrically conducting connection with the electrodes.
The spacing between the electrodes 33 can be adapted to the size of the bodies of synthetic material to be processed, by inserting the electrode plates in properly chosen grooves 4%) of the insulating plates 34. The bodies to be processed are placed upon the electrode plates, or are held by means of inserts or parts placed upon the electrode plates or suspended therefrom. The vacuum pump (not illustrated) for evacuating the processing chamber in housing 31, can be connected to a nipple 37. The device is further provided with a valve 38 for admitting atmospheric air, and with a pressure measuring instrument 39.
If desired, devices according to the invention can readily be automated by providing them with electric and vacuumresponsive sensing means for automatically controlling the pumping operations, the starting of the glow or are discharge in dependence upon the vacuum pressure, and the subsequent ventilating of the processing chamber. It will also be apparent to those skilled in the art, upon a study of this disclosure, that devices according to my invention may be modified in various other respects and hence may be given embodiments other than those particularly illustrated and described herein, Without departing from the essence of the invention and Within the scope of the claims annexed hereto.
1. The method of improving the dye-holding ability of synthetic plastics, which comprises placing the synthetic material in a vacuum and subjecting its surface to the plasma of an electric low-pressure glow discharge of a few hundred volts.
2. The method of improving the dye-holding ability of synthetic plastics, which comprises placing the synthetic material into a vacutun chamber having a pressure in the decimal order of magnitude of 1 mm. Hg, supplying a non-oxidizing gas to the chamber while maintaining said pressure, and subjecting the surface of the material to the plasma of an electric low-pressure glow discharge of a few hundred volts.
3. The method of improving the dye-holding ability of synthetic plastics, which comprises placing the synthetic material into a gaseous atmosphere having a pressure in the decimal order of magnitude of 1 mm. Hg, and subjecting the surface of the material to the plasma of an electric self-supporting glow discharge of a few hundred volts.
4. The method of improving the dye-holding ability of synthetic plastics, Which comprises placing the synthetic material into a gaseous atmosphere having a pressure in the decimal order of magnitude of 1 mm. Hg, and subjecting the surface of the material to the plasma of a thermionic arc discharge of approximately one hundred volts.
References Cited in the file of this patent UNITED STATES PATENTS 1,888,419 Ainsworth Nov. 22, 1932 2,454,757 Smith Nov. 23, 1948 2,468,173 Cotton Apr. 26, 1949 2,551,035 Miller May 1, 1951 2,837,654 Berghaus et al. June 3, 1958 2,864,755 Rothacker Dec. 16, 1958 2,881,470 Berthold et al Apr. 14, 1959