GB1572264A - Reactive glass compositions - Google Patents

Description

PATENT SPECIFICATION ( 11) 1 572 264

t ( 21) Application No 31480/77 ( 22) Filed 1 Mar1977 ( 19) 1 N ( 62) Divided out of No 1572263 R@ 2 ( 31) Convention Application No 670945 ( 32) Filed 26 Mar1976 in i J ( 33) United States of America (US) n ( 44) Complete Specification Published 30 Jul1980 1 ( 51) INT CL 3 CO 3 C 3/14 MO$ O ( 52) Index at Acceptance C 1 M 107 108 116 119 129 133 140 141 144 150 155 157 159 171 175 178 179 213 AG ( 54) IMPROVEMENTS IN OR RELATING TO REACTIVE GLASS COMPOSITIONS ( 71) We, EAGLE-PICHER INDUSTRIES, INC, of 580 Walnut Street, Cincinnati, Ohio 45201, United States of America, a corporation organised and existing under the laws of the State of Ohio, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed,

to be particularly described in and by the following statement: 5

This invention relates to the application of porcelain enamel coatings and is concerned, in particular, with glass compositions which can be provided between the surface of the ferrous substrate and an overlying porcelain enamel coating Our copending Application No 8586/77 (Serial No 1572263) is directed to a method of applying a porcelain enamel coating to a ferrous workpiece having a clean, unpickled, nickel-free surface, which com 10 prises electrostatically depositing on the workpiece surface a first coating comprising a particulate low-melting fritted glass composition, such composition being capable of reacting with the underlying workpiece surface at a firing temperature so as to etch the surface, electrostatically depositing a second coating comprising a porcelain enamel frit upon the first coating and firing the thus-coated workpiece at a temperature sufficient at least to 15 cause the glass composition to react with and so etch the underlying workpiece surface and thereafter at a temperature sufficient to fuse the procelain enamel frit into an enamel coating.

Ever since sheet steel has been provided with porcelain enamel coatings, it has been considered necessary to subject the steel to a varied series of plating and preparation steps 20 in order to improve the adhesion and the appearance of the porcelain enamel The only exception to this rule has been in connection with very thin hollow ware of a very inexpensive nature, where surface preparation was kept at a minimum In any quality work, however, it was always considered necessary to prepare the surface by chemical and mechanical procedures before the porcelain enamel frit was applied A typical series of treatment steps 25 for a steel sheet included first repeatedly dipping the sheet in an alkaline cleaner at a temperature ranging from 1400 to 2120 F ( 600 to 1000 C) This action was followed by a warm rinse with water at 1200 to 140 'F ( 490 to 600 F) Following the warm rinse, there was usually a cold rinse with water at room temperature Then came a pickling step with which the washed material was treated with a 5 % to 10 % sulphuric acid solution at a temperature 30 ranging from about 1500 to 160 OF ( 660 to 71 'C) Following the pickling step, the material was again rinsed, at room temperature, with water containing some small amount of sulphuric acid Following the cold rinsing step, a flash nickel deposition took place at a temperature of 1300 to 1800 F ( 600 to 820 C), so as to deposit 0 02 to 0 6 grams of nickel per square foot Following the nickel deposition, the steel was again coldrinsed with water and 35 a small amount of sulphuric acid to prevent formation of ferric iron Finally, the surface was neutralized with agents such as sodium carbonate and borax at a temperature of 1200 to 1300 F ( 490 to 540 C) The equipment required for this extensive heat treatment necessitated substantial capital investment and the time and labour involved were responsible for a substantial portion of the cost of the enamelled product 40 There have been a few disclosures in the prior art which sought to avoid the extensive and time-consuming pickling and nickel coating steps in enamelling steel, but to our knowledge they had little or no success commercially One such disclosure appears in the expired

Zimmerman U S Patent No 2 828 218, which describes a frit which was applied as a ground coat to a metallic surface at an application weight of 1/8 to 3/8 ounce per square 45 1,572,264 foot dry weight The frit composition included materials such as flint, feldspar, dehydrated borax, soda ash, sodium nitrate, fluorspar, calcspar and red iron oxide A conventional porcelain enamel cover coat was applied over the fired ground coat.

An improvement is disclosed in U S Patent No 2 786 782, Zimmerman et al, which comprises adding black (magnetic) iron oxide and zinc oxide to frit compositions in order to 5 improve their adherence This material was applied in the conventional manner with mill additions as a ground coat and, after firing of the ground coat, a conventional porcelain enamel frit was applied and fired Typically, the ground coat contained 5 % to 13 % magnetite, 2 % to 7 % zinc oxide, 35 % to 42 % silica, 20 % to 25 % boric oxide, 11 % to 22 % of one 10 Sor more alkali metal oxides, 4 % to 6 a% calcium oxide and 3 % to 5 % alumina sf U.S Patent No 2 864 721, King et al, also refers to the possibility of eliminating surface preparation, such as etching or metal plating, and deals with a ground coat produced by milling a slip directly from raw batch enamelling materials, without fritting, and applying the slip to the metallic article at a dry weight of 8 to 17 grams per square foot The preferred 15 ground coat includes 1 % to 15 % magnetite, O % to 9 % zinc oxide, 30 % to 45 % silica, 17 % to 28 % boric oxide, 11 %to 22 %of an alkali metal oxide, 3 %to 8 %calcium oxide and % to 7 % alumina.

In recent times, the technique of applying a ground coat and a cover coat of porcelain enamel by means of electrostatic deposition has become more popular Particular emphasis has been placed upon providing a two coat-one fire system for steels such as cole-rolled 20 steel Examples of such current practice are found in publications such as "Rationalization in the Enamelling Industry With Electrostatic Dry Procedures, a paper given at the International Congress in Vitreous Enamelling, in October, 1975; an article in "Iron Age" for December 15, 1975 entitled "Enamelers Place Hopes in Dry Frit Spraying", pages 49 to 52; and the article entitled "Powder: A Shot in the Arm for Porcelain Enamelling" appear 25 in in ndustrial Finishing, January,1976,pages 29 to 31 Ineach ofthesearticles, however, the two coat-one fire system was applied to regularly pickled workpieces.

Our co-pending Application No 8586/77 (Serial No 1572263) relates to an improved method for applying high quality porcelain enamel to a ferrous workpiece, particularly to workpieces composed of steels such as cold-rolled steel which have heretofore been enamel 30 led only with great difficulty The method of the invention of our aforesaid application (as can be seen from the reference to it in the opening paragraph above) makes it unnecessary to employ either a pickling step of a flash nickel coating or surface roughening in preparing the metal surface for enamelling Is firs ing out the method disclosed in our aforesaid application, a reactive coating is first depositied on the surface of the workpiece This 35 reactive coat is typically classified as a "soft" glass, that is, it is highly fluid, and so has a low viscosity at temperatures below the firing temperature for the cover coat Typically, the soft glass used as the first coating in carrying out the method of the aforesaid application has such a low viscosity at a temperature of 1,200 W h ( 648 c C) or even lower This fluidity at relatively low temperatures, coupled with the chemical nature of the glass, permits the 40 reactive coat to actually etch and react with the underlying ferrous surface before the overlying porcelain enamel cover coat melts The present invention relates to the reactive aco ito prseglass compositionsperse.

According to the present invention, a fritted reactive glass composition is provided, which forms a fluid composition tat t aure not exceeding 1200 ( 648 C) and which is 45 capable in the molten state of etching a ferrous surface and which comprises a matrix of the following melted composition, by weight Si O 2 16 %-45 % F 2 1 7 % 12 1 % 50 Na 2 O or K 20 or both Na 2 Oand K 20 10 % -25 % B 203 10 %-26 % 55 Ca O or Ba O or both Ca O and Ba O 2 %-20 % the composition also containing 1 % to 12 % by weight of Cu O or O 5 % to 1 6 % by weight of 60 Ni O or both Cuo and Ni O in the specified amounts.

According to a preferred embodiment of the glass composition of this invention, the matrix has the following melted composition, by weight:

Si O 2 19 %-39 % 6.5 1,572,264 F 2 3 4 % 8 6 % Na 2 O or K 20 or both Na 2 O and K 20 17 %-23 % 5 B 203 17 %-26 % Ca O or Ba O or both Ca O and Ba O 11 %-17 % 10 Preferably, the particles are encapsulated to provide an electrical resistivity in the range from 1012 to 106 ohm centimetres.

In use, after application of the reactive coat, i e the fritted glass composition, to the ferrous substrate, application of a dry porcelain enamel cover coat is effected Preferably, 15 this cover coat also is encapsulated to provide an electrical resistivity for the particles in the aforementioned range Then, the thus-coated workpiece is fired at a temperature which is at least sufficient to cause the first coating, i e soft glass or active coating, to react with the underlying workpiece surface by etching it The firing is then continued at a temperature sufficient to cause the porcelain enamel frit to fuse into and so become the desired cover coat Typical firing temperatures range from 13000 to 1600 'F ( 7040 to 871 'C) and prefer 20 ably from 14000 to 1500 i F ( 7600 to 816 'C), as explained in our aforesaid copending application The latter also includes a detailed description, given in conjunction with the single figure of the drawing thereof, of a form of continuous coating apparatus which may conveniently be employed, in applying a reactive glass composition of this invention.

The nature of the reactive coating has a great deal to do with the improved results 25 obtained Preferably, the reactive coating is of the type characterized as a very soft glass, i e.

having a low viscosity (in comparison with ordinary glasses) at a relatively low temperature, e.g 1200 'F or less The chemical nature of the soft glass also, we believe, renders it very reactive toward the ferrous surface so that it actually chews or attacks the metal, creating a strong anchoring bond to which the fused porcelain enamel cover coat can readily adhere 30 The chemistry of the reactive coating can vary widely as long as it has the low viscotity and chemical nature required to react with the underlying metal surface The reactive coating comprises a borosilicate glass matrix having, in its melted form, a composition comprising the components indicated above The oxides and other components form a base composition which gives a glassy frit to hold the bond promoter and other oxides, i e the 35 cupric oxide (Cu O) and nickel oxide (Ni G).

Certain other oxides are desirably added, but are not essential in the sense that the bond-promoting oxides can work without the added oxides These added oxides are materials such as cobalt oxide (Co O) in amounts of from 0 to 1 % and preferably from 0 5 % to 0 9 % of the melted composition Another oxide which can be added is manganese oxide 40 (Mn O) in amounts of 0 % to 5 % by weight of the melted composition, preferably from 0 % to 1 % by weight.

Still other oxides can be added for various purposes, including improvement of bond or adjustment of the flow rate These miscellaneous oxides are given in the following table:

Oxide Broad Range Preferred Range 45 Zn O 0-5 % by weight 2-4 % by weight A 1203 0-6 % by weight 0-2 % by weight P 205 0-5 % by weight 0-2 % by weight Ti O 2 0-5 %by weight 0-1 %byweight 50 Li 2 O 0-2 2 %byweight 0-1 5 %byweight While many different oxides can be used in the reactive coating, as noted above, there are several oxides which should not be used because they tend to destroy the bond when added to the reactive coating These oxides are iron oxide, antimony oxide and molybdenum 55 oxide The effect of iron oxide is just the contrary to its effect in the compositions described in U S Patents Nos 1 786 782, 2 828 218 and 2 864 721, which disclose the use of iron oxide as an adherence metal oxide in conventional ground coat frits If used in the compositions of the present invention, iron oxides would give no bond and provide pits in the coating 60 The reactive coating can be smelted in the conventional manner for the frits Typically, we use an 1800 F ( 982 C) rotary smelter with a 30-minute residence time.

Specific reactive coating compositions have been made up from the following raw batch compositions:

6 ( 5 RAW MATERIAL Silica Feldspar Fluorspar Soda Ash Sodium Nitrate Cryolite Zinc Oxide Calcium Carbonate Barium Carbonate Potassium Carbonate Sodium Silicofluoride Anhydrous Borax Cobalt Oxide Manganese Dioxide Lithium Carbonate Sodium Phosphate Nickel Oxide Anhydrous Boric Acid Titanium Dioxide (Rutlic) Copper Oxide Raw Batch Compositions A B C D E F G H I J K L M N % % %% % % % % % % % % % % 20.7 20 7 16 0 14 6 20 9 25 7 20 2 20 4 16 0 15 9 20 0 17 8 20 9 15 6 21 1 3.1 3 1 3 1 11 0 5 9 10 9 10 9 11 0 7 5 3 1 3 1 3 1 3 1 3 1 0.5 3 1 2 2 3 9 1 2 6 7 4 4 4 7 13 9 2 3 5 1 2 3 5 1 0.8 0 8 0 8 0 9 0 9 0 9 0 9 0 9 1 0 0 8 0 8 0 9 0 8 0 8 6.6 6 8 6 8 6 8 6 6 2.9 2 9 2 9 3 0 3 1 3 0 3 0 3 0 2 9 2 9 2 9 2 9 2 9 9.5 9 4 9 4 2 4 6.9 6 9 6 9 7 1 9.4 9 5 9 5 9 4 9 5 7.1 7 1 7 1 9 6 6 9 6 9 6 9 6 9 6 9 4.9 4 8 4 9 5 0 5 1 5 0 5 0 5 0 4.9 4 2 4 9 4 9 4 9 7.7 13 4 13 4 6 8 11 8 0 8 0 8 0 8 0 5 13 4 7 6 13 6 13 4 13 5 26.0 25 6 25 7 33 4 34 3 22 7 26 8 26 8 24 0 25 7 26 1 22 9 25 8 30 4 0.7 0 7 0 7 0 7 0 7 0 7 0 7 0 7 0 5 0 7 0 7 0 7 0 7 0 7 0.8 0 8 0 8 0 8 0 8 0 8 4 4 0 8 1 0 0 8 0 8 0 8 3.2 3 2 3 2 3 4 3 4 3 3 3 3 3 4 3 2 0.8 3.2 3 2 3 2 1.0 1 0 1 0 1 0 1 0 1 0 1 0 5 9 1 0 1 0 1.1 1 1 1 1 1 2 1 2 1 0 1 0 1 2 1 3 1 1 1 1 1 1 1 1 1 7 2.2 1.4 2 7 3.6 3 5 7 9 8 2 8 3 3 6 3 7 3 7 3 6 7 9 3 6 3 6 3 6 3 6 O 000 O 000 000 100 0 100 0 100 0 100 0 100 0 100 0 100 0 100 0 100 010001000 1000 1000 CD Melted compositions OXIDE A B C D E F G H I J K L M N % % % % % % % % % %%%% % %% % % % % % % % % % 8 Si O 2 26 9 29 4 23 8 19 4 27 4 30 0 24 0 24 0 38 2 28 8 26 0 25 8 27 6 23 8 F 2 7 3 7 3 7 3 7 3 7 3 7 3 7 3 7 3 3 4 7 3 7 3 7 3 7 3 7 3 o Na 2 O 16 6 16 6 16 6 16 6 16 6 16 6 16 6 16 6 10 7 16 6 19 6 16 6 16 6 16 6 Zn O 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 33 33 3 3 3 3 3 3 Ca O 8 6 8 5 8 5 8 5 4 5 8 5 8 5 8 5 6 3 8 5 8 5 8 5 8 5 8 5 Ba O 6 0 6 0 6 0 6 0 2 0 6 0 6 0 6 0 8 8 6 0 6 0 6 0 6 0 6 0 K 20 3 7 3 7 3 7 3 7 3 7 3 7 3 7 3 7 2 7 3 7 3 2 3 7 3 7 3 7 A 1203 1 8 1 8 1 8 1 8 4 4 1 8 1 8 Co O 0 7 0 7 0 7 0 7 0 7 0 7 0 7 0 7 0 6 0 7 0 7 0 7 0 7 0 7 Mn O 0 6 0 6 0 6 0 6 0 6 0 6 3 6 0 6 0 9 0 6 0 6 0 6 0 6 O Li 2 O 1 5 1 5 1 5 1 5 1 5 1 5 1 5 1 5 1 5 1 5 1 5 1 5 P 205 0 6 0 6 0 6 0 6 0 6 0 6 0 6 3 6 O 6 O 6 Ni O 13 1 3 1 3 1 3 1 3 1 3 1 3 1 3 1 6 1 3 1 3 1 3 1 3 1 3 B 203 20 1 20 1 20 1 25 1 25 1 17 1 20 1 20 1 19 7 20 1 20 1 20 1 20 1 20 1 Cu O 4 1 41 9 1 9 1 9 1 4 1 4 1 4 1 4 1 4 1 4 1 4 1 4 1 9 1 Ti O 3 0 103 1103 1103 1103 1 103 1RW 103 1 4 031 103 O 1 031 103 1 103110141031 1031 1031 1031 1031 Minus 0 for F 2 3 1 3 1 3 1 3 1 3 1 3 1 3 1 3 1 1 4 3 1 3 1 3 1 3 1 3 1 o 0 100 1 0 1 00 0 100 0 100 0 100 0 1 00 0 1 00 0 1 00 O 100 O 1000 10 100 O 1000 1,572,264 One of the features of the reactive coating of the present invention which distinguishes it from the materials heretofore used in ground coats is the hydrophilic nature of the glass.

Indeed, the glass is sufficiently soluble in water so that it could not be effectively used in conjunction with mill additions in forming an aqueous slip for application to a metal surface 5 as a conventional ground coat.

The reactive coating is preferably applied in fritted form Since the composition is highly susceptible to water, it is recommended that the frit be produced by roll quenching instead of by quenching under water.

The dry particles of frit making up the reactive coating are applied by electrostatic 10 deposition This is a technique which has received considerable attention in recent years.

Electrostatic spraying techniques are explained in the article by M L Pouilly in the October, 1953 issue of "Finish" magazine, pages 71 to 73 This article refers to the use, in the electrostatic spraying of porcelain enamel, of high-voltage packs which are capable of producing 80,000 to 140,000 volts and currents from 1 to 10 milliamperes 15 It is also highly desirable that both the reactive coating and the porcelain enamel should be finely divided for deposition by electrostatic coating Generally, the frit in each case is ground in a ball mill to a retention of 0 2 % to 2 % on a 200 mesh screen ( 3 to 12 % on a 325 mesh screen).

is Since the reactive coat and the cover coat are both applied by electrostatic deposition, it 20 is important that the resistivity of the porcelain enamel powder be controlled for spraying efficiency and also for adherence to the metal Consequently, it is highly desirable for the particles to be electrostatically sprayed to be encapsulated e g in a synthetic resin in order to increase their resistivity to the range of 1012 to 1016 ohm centimetres To accomplish this, the techniques described in U S Patent No 3 930 062 are preferably used These techni 25 ques, briefly, are based upon a method of pretreating borosilicate glass powders to reduce their caking tendencies and to bring their resistivities up to where they can be successfully sprayed by electrostatic deposition techniques The anti-caking capabilities of the compositions are improved by reacting the frit in particulate form with an alkoxysilane having the formula: 30 3 u Rn Si-(OCH 3)4 _.

where R is a methyl or phenyl group, or both, and N is 1 or 2.

These materials are combined with the frit in an amount of 0 2 %-0 8 % by weight of the mixture Apparently, the alkoxysilane reacts with water present to form a silanole which then presumably reacts with the hydroxyl groups in the glass to improve the electrical 35 resistivity.

The dry adherence of the frit to the substrate can be improved by treating the frit with an adhesion promoter consiting of silazane having the following formula:

R 3-Si-NH-Si-R 3 where R is hydrogen, an alkyl radical, an aryl radical, or a combination of alkyl and aryl 40 radicals.

These materials are preferably added in amounts of 0 05 % to 0 5 % by weight.

Additional benefits are derived in terms of improving the deposition rate, when the mixture being sprayed also includes a chlorosilane having the formula:

Rn-Si-C 14 45 where R is an alkyl or aryl radical or both, and N is 1, 2 or 3.

The reactive coating is desirably applied to the ferrous surface to a thickness in the range of 0 5 to 2 mils ( 12 to 51 microns) Preferably, the coating thickness is on the order of 1 to 1.5 mils ( 25 to 38 microns).

After application of the reactive coating, a conventional porcelain enamel frit can be 50 applied dry over the precoated surface, also by means of electrostatic deposition In order to increase the electrical resistivity of the porcelain enamel frit to increase its adhesion tendencies, this frit is also treated by encapsulation with a synthetic resin such as polyethylene, or with the silane compounds referred to in the aforementioned Nedeljkovic patent In any event, the electrical resistivity of the porcelain enamel frit is modified until it is in the range of 1012 to 1016 ohm centimetre.

Following application of the cover coat, the coated article is then fired in the normal way at temperatures ranging from about 1300 to 16000 F ( 704 to 871 'C) and preferably from 1400 to 1500 OF ( 7600 to 8160 C).

7 1,572,264 7 EXAMPLE

Pieces of cold-rolled steel having thicknesses between 18 and 22 gauge ( 0 079 to 0 127 cm) were coated with a reactive fritted glass composition having the following melted analysis:

analsis Si O 2 26 8 % by weight 5 F 2 7 3 % by weight Na 2 O 16 6 % by weight Zn O 3 3 % by weight Ca O 8 5 % by weight 10 Ba O 6 2 % by weight K 20 3 7 % by weight A 1203 1 8 % by weight Co O 0 7 % by weight 15 Mn O 0 6 % by weight Li 20 1 5 % by weight P 205 0 6 % by weight Ni O 1 3 % by weight 20 B 203 20 1 % byweight Cu O 4 1 % by weight 103 1 Minus O for F 2 3 1 25 0 The coating was applied to a thickness of about 1 mil ( 25 microns) by means of a De Vilbiss electrostatic spray gun used for the application of porcelain enamel coatings The surface of the cold rolled steel had merely been washed, rinsed and dried prior to applica 30 tion of the coating Next, a commercial cover coat (Chi-Vit 14350) was milled to a fineness of 0 5 retention on a 200 mesh screen and it was then screened using a 60 mesh screen.

Approximately 0 5 % of methyl trimethoxy silane and 0 2 % of hexamethyl disilizane were added to the powder This material was electrostatically sprayed over the precoated part and then fired at 1450 F ( 778 C) for 3-1/2 minutes The resulting porcelain enamel 35 coating exhibited good adherence to the substrate and was of good qualtiy.

Claims (9)

WHAT WE CLAIM IS:-

1 A fritted reactive glass composition forming a fluid composition at a temperature not exceeding 1200 F ( 648 C) and being capable in the molten state of etching a ferrous surface, such composition having a matrix of the following melted composition, by weight: 40 Si O 2 16 %-45 % F 2 1 7 % 12 1 % Na 2 O or K 20 or both Na 20 and K 20 10 %-25 % B 203 10 %-26 % Ca O or Ba O or both Ca O and Ba O 2 %-20 % 50 the composition also containing 1 % to 12 % by weight of Cu O or 0 5 % to 1 6 % by weight of Ni O or both Cu O and Ni O in the specified amounts.

2 A composition according to claim 1, in which the matrix has the following melted 55 composition, by weight:

Si O 2 19 %-39 % F 2

3

4 % 8 6 % Na 2 O or K 20 60 or both Na 20 and K 20 17 %-23 % B 203 17 %-26 % 1,572,264 Ca O or Ba O or both Ca O and Ba O 11 % 17 % 3 A composition according to claim 1 or 2,which includes 2 % to 9 % by weight of Cu O of O 9 % to 1 3 % byweightof Ni O or both Cu O and Ni O in the specified amounts

5 4 A composition according to any preceding claim, which also includes, by weight of the melted composition, either or both of:

Co O O to 1 % Mn O O to 5 % 10 A composition according to claim 4, in which O 5 % to O 9 % of Co O and/or O to 1 % of Mn O, by weight of the melted composition, are present.

6 A composition according to any preceding claim, which also includes by weight, one or more of the following oxides: 15 Zn O 0-5 % A 1203 0-6 % P 205 0-5 % 20 Ti O 2 O 5 % Li 20 0-2 2 % 25

7 A composition according to any preceding claim, which comprises dry fritted particles.

8 A composition according to claim 7, in which the particles are encapsulated to provide an electrical resistivity in the range from 1012 to 10 16 ohm centimetres.

9 A composition according to claim 8, in which the particles are encapsulated in an 30 alkoxysilane.

A fritted reactive glass composition according to claim 1, substantially as herein described.

POLLAK MERCER & TENCH Chartered Patent Agents, 35 Eastcheap House, Central Approach, Letchworth, Hertfordshire, SG 6 3 DS -andHigh Holborn House, 52-54 High Holborn, London, WC 1 V 6 RY Agents for the Applicants Printed for Her Majesty's Stationery Office, by Croydon Printing Company Limited, Croydon Surrey, 1979 Published by the Patent Office, 25 Southampton Buildings, London, WC 2 A l AY, from which copies may be obtained.