Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
  • B41M—PRINTING, DUPLICATING, MARKING, OR COPYING PROCESSES; COLOUR PRINTING
  • B41M3/00—Printing processes to produce particular kinds of printed work, e.g. patterns
  • B41M3/10—Watermarks
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
  • Y10T428/00—Stock material or miscellaneous articles
  • Y10T428/24—Structurally defined web or sheet [e.g., overall dimension, etc.]
  • Y10T428/24802—Discontinuous or differential coating, impregnation or bond [e.g., artwork, printing, retouched photograph, etc.]
  • Y10T428/24934—Discontinuous or differential coating, impregnation or bond [e.g., artwork, printing, retouched photograph, etc.] including paper layer

Definitions

• This invention relates to a paper sheet product with a chemical watermark and means for making same. More in particular this invention relates to means for forming a chemical watermark in otherwise finished paper sheet products which simulates well known watermarked paper sheet products.
• the present practice of watermarking is usually performed during the paper-making process by providing rolls with raised designs interposed usually at a point before the wet paper Web enters the dryers.
• the fibers in the web are displaced in horizontal position and the paper is thus rendered thinner and consequently more transparent in the area of the watermark when viewed under transmitted light.
• the embossed rolls form the configuration or design of the mark in the damp or wet paper which upon drying remains permanent.
• Another known method of forming a watermark involves the employment of a band fitted about a roll and carrying the design or configuration of the water mark in relief.
• the design of the mark in relief on the band is brought into contact with the wet paper web at the wet presses beyond the Fourdrinier or wet end. At that point the paper is still plastic and compresses readily.
• the resulting compression of fibres, reducing entrapped air and light refraction, increases the transparency in the area conforming to the configuration of the watermark.
• Watermarked paper made as above described has several disadvantages.
• a customer desiring paper bearing a private watermark such as a trademark must purchase a large amount of each kind of paper needed.
• This of course limits such privately watermarked papers to few but relatively large industrial users or governmental departments as it is obvious that smaller enterprises could not economically purchase and stock a supply sufficient for several years.
• econd the cost of the embossed or banded rolls for the paper making machine is obviously extremely high and could therefore not be afforded by smaller industrial users.
• Third, the cost of setting up the embossed or banded rolls involves both labor as Well as loss of production on the paper making machinery which must be taken into account in the net cost to the purchaser.
• the present invention contemplates the elimination of the above mentioned disadvantages. It is therefore one object of this invention to convert blank paper sheet into a paper sheet product having a uniformly positioned chemical watermark therein conforming to a selected design.
• Another object of this invention is to form in a blank cut sheet paper product a visible chemical watermark confroming to a selected design.
• a further object of this invention is to form a chemical watermark by impregnating blank paper in the area of the configuration of the mark with a substantially colorless curable thermosetting resin which may or may not be followed by curing the resin.
• a still further object of the present invention is to form a chemical watermark according to the preceding objects, wherein the cured resin forming the mark is insoluble in water and ordinary organic solvents.
• a yet further object of this invention is to provide blank paper intended for use as paper sheet products with a chemical watermark, according to the preceding objects, wherein the mark may conveniently be positioned uniformly on all sheets.
• Another object of this invention is to provide such paper with a chemical watermark, according to the preceding objects, in small quantities at low cost.
• Another object of this invention is to provide a method of forming a chemical watermark in paper involving the adequate displacement of entrained air within and be tween the fibers of the paper.
• Another object of the invention is to provide a method of forming a chemical watermark by replacing air entrained in a paper sheet product with a material having an index of refraction reasonably close to that of cellulose.
• Another object of the invention is to provide a method of forming a chemical watermarked paper sheet product by impregnation thereof within the area of configuration of said mark to replace entrapped air in the area with a substantially colorless thermosetting material curable to insoluble resin at room temperature.
• compositions employed may chemically react to form substantially colorless cured resins which are essentially insoluble in ordinary or common solvents and thus the mark is resistant to solvent action.
• impregnating compositions arsoess C which are chemically reactive to form insoluble resins instead of chemically unreactive compositions for use in providing blank paper with a chemical watermark.
• chemical watermark in paper products as herein employed is intended to mean a mark of selected design wherein the area within the configuration of the mark is impregnated with a chemical composition accord ing to this invention resulting in a mark on the paper which substantially meets all of the requirements possessed by a watermark made by the well-known watermarking process.
• the chemical watermark of this invention is indistinguishable from the well known watermark.
• chemical watermark is used to distinguish the marks obtained according to this invention from the marks obtained by means of the known watermarking methods.
• blank paper i.e., having no watermark
• a chemical watermark by impregnating the area of configuration with a novel liquid composition of this invention whereby after curing the liquid resin of the composition to solid, transparent or translucent areas are created in the paper sheet conforming to the desired configuration of the mark.
• the resulting chemical watermark possesses for all practical purposes the same general physical characteristics in use as a watermark formed by the known processes previously described.
• a chemical composition suitable for forming a chemical Watermark as above mentioned must have the ability to mark the paper in a manner to render it more translucent or substantially transparent in the impregnated area. Further the chemical composition employed to form the chemical watermark should not alter the surface of the paper adversely. For example it must not render the surface glossy in the impregnated area and must not alter the erasability characteristic of the paper. In addition the chemical composition must withstand aging without substantial discoloration and must not become indistinct through slow migration of the chemicals or otherwise. And of course, the area within the chemical Watermark must accept typing, pencilling, printing and writing inks without adverse effects such as feathering or skipping. Certain chemical compositions of this invention have been successfully employed in the formation of chemical watermarks which conform to the above mentioned requirements as hereinafter described.
• a conventional watermark from a fundamental viewpoint, possesses a translucency of higher degree in the area of the mark than that possessed by the paper in the unmarked areas.
• the most common type of conventional watermark appears more transparent than the surrounding unmarked paper when viewed under transmitted light.
• a substantially colorless solution of a curable liquid resin in an organic solvent wherein the resin is curable to a substantially insoluble colorless state rapidly by hardening agents (curing promoters) which may be added to the resin solution provided that the cured resins obtained with the aid of hardening agents remains substantially colorless.
• hardening agents curing promoters
• Commercial sources of some liquid resins including trade names and manufacturers which were found satisfactory for this invention are shown in Table I below.
• thermosetting type All of the resins in Table I are of the thermosetting type. However, it is understood that the two Paraplex resins are of the polyester type; the Epiphen resin is understood to be of the phenol formaldehyde modified epoxy type; the two Crestoset resins are understood to be of the modified urea formaldehyde type and the remaining resins are understood to be of the epoxy class.
• One method for manufacturing an epoxy type resin suitable for use in this invention is the soluble resinous product of the reaction between Bisphenol A (4,4' isopropylidine diphenol) and epichlorohydrin as indicated below.
• the epichlorohydrin and Bisphenol A are placed in a distilling flask, provided with a stirrer and temperature control means, and the flask placed in an inert gas atmosphere such as nitrogen.
• a reaction period of about 120150 hours at about 30-50" C. is maintained after which period the temperature is raised to about 80-100 C. and the pressure reduced for the purpose of removing excess epichlorohydrin.
• the residual intermediate product thus formed is then dissolved in an organic solvent, such as toluene, and an aqueous solution of an inorganic hydroxide (e.g., NaOH) is added slowly with agitation.
• an organic solvent such as toluene
• an aqueous solution of an inorganic hydroxide e.g., NaOH
• the amount of inorganic hydroxide added should be slightly in excess of the stoichiometric equivalent to the chlorine present.
• the resulting mixture is stirred until the reaction is substantially complete.
• the resinous mass thus obtained is washed with distilled water to remove the inorganic chloride salt formed and excess inorganic hydroxide.
• the water is then removed by distillation and the residue is a colorless, clear epoxy type liquid resin which is diglycidyl ether of Bisphenol A.
• the resin thus obtained is a mixture of component compounds having an average molecular weight of about 340-350 may be employed to form chemical watermarks acc0rdand conforming to the following formula:
• Such resins might be cured more where 11 is zero to about 20 or more.
• epoxide equivalent weight means the weight tween the indexes of refraction results in a corresponding in grams of resin containing one gram equivalent of greater amount of diffusion of light. From this it is apepoxide. It is usually (but not necessarily) about 0neparent that if the difference in the indexes of refraction half the average molecular weight as most of these resins 10 is large the resulting chemical watermark is faint and are diepoxides. may be perceived by the eye with difiiculty which of course In the case of Dow D.E.R.
• epoxy type resins 0 CH3 0 generally do not possess any part cularly obnoxious odors g] as compared with some other resins.
• OH -OI-Io- Accelerated curing of resins of the type indicated in km Table I can be effected conveniently by compounds known
• the Epoxide 201 resin is believed to be a diepoxide in the trade as hardeners SL161) that the resinswill cure rived from peracetic acid, known as 3,4-epoxy-6-methylat room temperatilres (7 90 m tune"
• the cure time for these carboxylate of the following structural formula.
• Tesins may be as long as two i sun qmte satisfactory. Thus short cure time is not necessarily re- H H2 H H H quired in this invention.
• the hard- O0C eners may be classed as curing agents but not neces- 0 0 sarily catalyst-s. Strictly speaking a true catalyst is not 3 3 7 chemically combined with the cured resin as distinct from H H H H H2 H a compound which accelerates curing but also becomes Mamglas 655 is believed to be an epoxy resin made by chemically combined with the cured resin.
• curing agents becau se they Th Epiphen ER 823 resin is believed to be a phenolaccelerate the curing rate.
• Diepoxide AG-13E is understood to be bis-epoxydito be satisfactory are shown in Table II by their comcyclopentyl ether of ethylene glycol. Epo l 741 i b briefly available trade names and manufacturers. lieved to be epoxidized soybean oil further described in TABLE II US. Patent No. 2,919,283.
• Oxiron 2 00 2 is understood to be an epoxidized poly- T d N M I t olefin prepared in accordance with US. Patents Nos. m 6 mm 1 auu ac um 2,326,556; 2,833,747; 2,329,131 and versamid 115 General M1115 Corporation Aside from the fact that the resin employed must be B32: 3-52% %D- g g: gggggg-g DOWDEIIQmiCaICOIHpaHY- Substantially Colorless f i g p -W rm rk shenH 21 f; sheuc'hemicaloompwy Rohm and Haas Co.
• Shell Chemical Co. Harshaw Chemical Comaccording to this invention it must also posses an index DMP"30 5 Maraset H655. of refraction within critical limits of 1.4 to 1.7 inclusive lgg ag s t lisss and the preferred range of limits is 1.45 to 1.65 as T I measured at 25 by conventlonal the N-Aminoethyl Piperazine J'efiersdn Chemical Comwell known Abbe refractometer. Th1s is predicated on pany,Ine.
• Hardener 951 CIBA Company, Inc. fluids measured by an Abbe refractometer, and it is de- Hammer EC Bergen Chemical sidable to employ a resin having an index of refraction g gg Q ggl i e cg gll efi csl Corp. reasonably close to that of the cellulose of the paper to H xahydro hthane Auhydride Allied Chemical Corp.
• the Dow D.E.R. Exp. Resin X-2654.4 is believed to be a polyamine.
• BF -4OO is understood to be a complex of boron fluoride with monoethylamine.
• Eporal is understood to be 4,4'-diaminodiphenyl sulfone.
• Maraset H655 is characterized as a tertiary amine.
• the DMP- is understood to be 2,4,6- tri(dimethyl aminoethyl) phenol.
• curing agents for epoxy resins found to be satisfactory include diethylene triamine, pyromellitic dianhydride, N-arnino ethyl piperazine, methyl nadic anhydride, trimellitic anhydride, dibutyl tin maleate, nadic anhydride, diethylene triamine, diphenyl phosphite, N-acetylethanolamine and dipropylene triamine.
• the amount of hardener may range from as little as 0.2 percent to as high as about 60 percent by weight Where a solvent is not employed and as low as 0.2 percent to as high as about 30 percent by Weight when a solvent is employed.
• Curing agents found satisfactory for the urea formaldehyde and melamine formaldehyde resins of Table l include maleic anhydride, citric acid, n-butyl acid phosphate, chloroacetic acid, dichloroacetic acid, oxalic acid, monon-butyl orthophosphate, sulfamic acid and glacial acetic acid.
• a resin of Table I with or without a curing agent could be satisfactorily used directly to make a chemical watermark according to this invention.
• a suitable organic solvent was used with the resin.
• the solvent employed must, in addition to being a solvent for the resin used, be essentially colorless, non-corrosive to paper, possess a vapor pressure at room temperatures high enough to evaporate with a sutficent rapidity to prevent migration of the resin in paper, possess a vapor pressure low enough to carry the resin into the paper (penetration) prior to evaporation, and, lastly, chemically inert under the conditions employed in this invention.
• the solvent preferably should be of the polar type as it appeared that wetting of the paper fibers was better than in the case of non-polar solvents although nonpolar solvents can be used as will be seen later.
• the solvent also must be compatible with the resin seiected. If the viscosity of the solution is too low the penetration into the paper is too rapid causing the chemical watermark to be blurred or indistinct. On the other hand if the viscosity of the solution is too high the penetration into the paper is poor and a substantial portion of the resin is deposited on the surface of the paper sheet resulting in a chemical watermark which may have a glossy surface. This is not only undesirable in appearance but may be adversely sensitive to inks.
• the hand impregnation was accomplished by depositing a small metered amount (e.g., l to 2 grams) of the composition on the surface of a foot-square piece of onequarter inch thick plate glass.
• the liquid composition was spread uniformly over the glass surface with a conventional handproofer consisting of an engraved roll bearing 75 depressions per inch and fitted with a handle.
• a die or hand stamp of rubber-like material having inscribed in relief on the face thereof a design or configuration conforming to a typical Watermark was pressed against the wet film of the composition and then stamped on the paper sheet.
• the chemical watermarked paper sheet was allowed to dry and cure at ambient room temperatures (7090 F.) and then examined.
• ambient room temperatures 7090 F.
• the two week curing time requirement is generally not objectionable because packing and shipping operations for paper stationery together with upacking and use by the customer usually is at least this long.
• the curing rate of the resin can be greatly accelerated by heating the chemical watermarked paper to 7090 C. for a period of about one hour without discoloring the paper.
• Example 1 Dow D.E.R. 332 60 grams, 33%. Dow Hardener X2654.4 12 grams, 7%. Ethyleneglycol monomethyl ether 108 grams, 60%.
• Ethyleneglycol monomethyl ether Diethyleneglycol monoethyl ether
• Example 6 Dow D.E.R. 332 60 grams, 33%.
• Dow Hardener X2654.4 Diethyleneglycol monobutyl ether
• Example 10 Dow D.E.R. 332 60 grams, 33%.
• Dow Hardener X-2654.4 12 grams, 7%.
• Ethyleneglycol monoethylether acetate 108 grams, 60%.
• Example 12 Dow D.E.R. 332 60 grams, 33%. Dow Hardener X2654.4 12 grams, 7%. Methyl isobutyl ketone 108 grams, 60%.
• Example 13 Dow D.E.R. 332 100 grams, 58%. Dow Hardener X2654.4 20 grams, 12%. Methyl isobutyl ketone 51.4 grams, 30%.
• Example 14 Dow D.E.R. 332 60 grams, 33%. Dow Hardener X2654.4 12 grams, 7%. Dimethyl formarnide 108 grams, 60%.
• Example 15 Dow D.E.R. 332 100 grams, 58%. Dow Hardener X2654.4 grams, 12%. Dimethyl formamide 51.4 grams, 30%.
• Example 16 Dow D.E.R. 332 60 grams, 33%. Dow Hardener X2654.4 12 grams, 7%. Dimethyl sulfoxide 108 grams, 60%.
• Example 17 Dow D.E.R. 332 100 grams, 58%. Dow Hardener X2654.4 20 grams, 12%. Dimethyl sulfoxide 51.4 grams, 30%.
• Example 18 Dow D.E.R. 332 60 grams, 33%. Dow Hardener X2654.4 12 grams, 7%. Ethyleneglycol monoethyl ether '108 grams, 60%.
• Example 19 Dow D.E.R. 332 100 grams, 58%. Dow Hardener X2654.4 20 grams, 12%. Ethyleneglycol monoethyl ether 51.4 grams, 30%.
• Example 20 Dow D.E.R. 332 60 grams, 33%. Dow Hardener X2654.4 12 grams, 7%. Ethyleneglycol monomethyl ether 108 grams, 60%.
• Example 21 Dow D.E.R. 332 100 grams, 58%. Dow Hardener X2654.4 20 grams, 12%. Ethyleneglycol monomethyl ether 51.4 grams, 30%.
• Example 22 Dow D.E.R. 332 60 grams, 33%. Dow Hardener X2654.4 12 grams, 7%. Ethyleneglycol monobutyl ether 108 grams, 60%.
• Example 65 Dow D.E.R. 332 (resin) 2 50 Methyl nadic anhydride 50 I
• Example 66 Dow D.E.R. 332 (resin) 41.6 Methyl nadic anhydride 58.4
• Example 71 Maraglas 655 (resin) 77 'Maraset Hardener H555 23
• Example 72 Maraglas 655 (resin) 50 Maraset Hardener H555 50
• Example 73 Dow D.E.R. 332 (resin) 47- Pyrornellitic anhydride 4.8 Diethyleneglycol monomethyl ether 47.6
• Example 74 Dow D.E.R. 332 (resin) 4O Pyromellitic anhydride 20 Diethyleneglycol monomethyl ether 40
• Example 75 Dow D.E.R. 332 (resin) 56 BP -400 4 Diethyleneglycol monomethyl ether 4O
• Example 76 Percent Dow D.E.R.
• Example 81 Dow D.E.R. 332 (resin) 40 Eporal (hardener) 20 Diethyleneglycol monomethyl ether 40
• Example 82 Dow D.E.R. 332 (resin) 58.6
• the BF -Piperidine Complex is commercially available but may be prepared by bubbling boron trifluoride in piperidine at room temperature.
• Example 93 Dow D.E.R. 332 (resin) 33 Crestoset M (resin) 33 BF -Phenol Complex (hardener) 1 Diethyleneglycolmonomethyl ether 33
• Example 94 Dow D.E.R. 661 (resin) 29.6 Dow D.E.R. X2673.2 (hardener) 7.4 N-Aminoethyl Piperazine 3.0 Diethyleneglycol monomethyl ether 60.0
• Example 97 Crestoset M (resin) 50 Oxalic acid (hardener) 5 Diethyleneglycol monomethyl ether
• Example 98 Crestoset M (resin) 45 Oxalic acid (hardener) 5 Diethyleneglycol monobutyl ether 45
• Example 99 Crestoset M (resin) 50 50 Oxalic acid (hardener) 5 Diacetin 45
• Example 100 Crestoset M (resin) 50 Oxalic acid (hardener) 5 Monacetin 45
• Example 101 Crestoset M (resin) 50 Oxalic acid (hardener) 5 Dipropyleneglycol 45
• Example 102 Crestoset M (resin) 50 Citric acid (hardener) 5 Diethyleneglycol monomethyl ether 45
• Example 103 Crestoset M (resin) 50 Citric acid (hardener) 5 Diethyleneglycol monobutyl ether 45
• Example 104 Crestoset M (resin) 50 Citric acid (hardener) 5 Dia
• a curing agent hardener
• a solvent is not always required.
• Some of the resins require the presence of a solvent and the amount of solvent employed is governed to an appreciable extent by the impregnating characteristics of the paper to be chemical watermarked in accordance with this invention.
• Example 141 Crestoset M (resin) 55 Dimethylacetamide 45
• Example 142 Dow D.E.R. 667 (resin) 60 Diethyleneglycol monomethyl ether 40
• Example 143 Crestoset M (resin) 55 Diacetin 45
• Example 144 Crestoset M (resin) 55 Diethyleneglycol monomethyl ether 45
• Example 145 Crestoset M (resin) 55 Monacetin 45
• the proportional amount may range from zero to as high as 80% although the range of 10 to 70% is more preferable.
• some of these compositions are currently used for chemical watermarking paper commercially using conventional printing machinery.
• the blank paper product employed in applying the compositions to form chemical watermarks according to this invention was of the bond grade consisting of 25% cotton and 75% woodpulp.
• the apparent density of the blank paper stock was about 5.0 and its thickness about 4 mils.
• the resulting chemical watermarks possessed a high degree of transparency.
• Other types of paper sheet products such as commercial mimeograph, tracing and even newsprint were successfully chemical watermarked with many of these compositions according to the invention.
• opaque and semiopaque can refer to varying levels of opacity, depending on the specific type of paper under discussion. For purposes herein, more definite meanings can be given these terms by the following explanation.
• semi-opaque is generally accepted by the trade as a standard weight sheet of bond paper having a TAPPI opacity somewhere between 87% and 65%.
• TAPPI opacity less than 65 are often referred to as transparent. It will suffice to say that the lower the TAPPI opacity, the more often the trade refers to transparency, and the closer the opacity is to 65%, the more often the trade uses the the term translucent. Translucent papers usually retain the more common characteristics of bond papers, such as formation and brightness, whereas transparent sheets most often have a more uniformly translucent fibre mass with considerably less brightness. This holds true whether the paper under discussion is made entirely of chemical wood pulp, a blend of chemical wood pulp and cotton or linen fibres.
• the term translucent usually implies that a ray of light passing through a material is diffused while the term transparent implies that a ray of light passing through is not diffused.
• a translucent object may vary from opaque to transparent according to the magnitude of diffusion of light passing therethrough.
• transparent is used to denote that the magnitude of diffusion of light passing through the paper is not greater than that found in commercial tracing paper.
• TAPPI opacity of less than 65% it is termed herein as transparent and conversely if 65 or greater it is termed herein as translucent.
• the amount of curable impregnating composition of this invention in terms of percent by weight, may vary due to the density of the paper and its impregnating characteristics. Therefore a sufficient amount effective to reduce the TAPPI opacity to below 65% is employed when working with translucent paper.
• an impregnation in the amount of about or less by weight of the resin based on the weight of the paper within the area impregnated gave satisfactory results.
• a method of imparting a chemical watermark of selected design to a finished sheet of paper which comprises the steps of printing the design on a portion of the surface of said paper sheet with a liquid composition consisting essentially of a synthetic, thermosetting, curable resin and an organic solvent, and removing said solvent to obtain a substantially colorless chemical watermark of said deposited resin having an index of refraction of about 1.4 to about 1.7, and said chemical watermark being substantially free from gloss, having ink receptivity and resisting discoloration with aging substantially in the same way as the paper.
• a method of imparting a chemical watermark of selected design to a finished sheet of paper which comprises the steps of printing the design on a portion of the surface of said paper sheet with a liquid composition consisting essentially of synthetic, thermosetting curable resin and up to about 60% by weight of a curing agent for said resin, and hardening the composition by curing the resin to obtain a substantially colorless, chemical watermark having an index of refraction of about 1.4 to about 1.7, and said chemical watermark being substantially free from gloss, having ink receptivity, and resisting discoloration with aging in substantially the same way as the paper.
• a paper article with a chemical watermark which includes, in combination, a paper sheet, a chemical watermark composition applied on said sheet in the configuration of a selected design, the chemical watermarking composition consisting essentially of a hardened, synthetic, thermosetting resin having an index of refraction of about 1.4 to about 1.7, and said chemical watermark being substantially free from gloss, having ink receptivity and resisting discoloration with aging substantially in the same way as the paper.
• An article as in claim 3 further characterized in that the resin is an epoxy resin.
• An article as in claim 3 further characterized in that the resin is a melamine formaldehyde resin.
• An article as in claim 3 further characterized in that the resin is a urea-formaldehyde resin.
• An article as in claim 3 further characterized in that the resin is a polyester resin.
• a paper article with a chemical watermark which includes, in combination, a paper sheet, a chemical watermark composition printed on said sheet in the configuration of a selected design, the chemical Watermarking composition consisting essentially of a hardened, synthetic, thermosetting resin having an index of refraction of about 1.4 to about 1.7, and said chemical watermark being substantially free from gloss, having ink receptivity and resisting discoloration with aging substantially in the same way as the paper.

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Cited By (16)

Citations (12)

• 0
  • BE BE656943D patent/BE656943A/xx unknown
• 1960
  • 1960-12-27 US US78228A patent/US3140959A/en not_active Expired - Lifetime

Patent Citations (12)

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