WO2009082088A1 - Coating solution for forming insulating film, method for forming insulating film of non-oriented electrical steel sheet and non-oriented electrical steel sheet by using it - Google Patents

Coating solution for forming insulating film, method for forming insulating film of non-oriented electrical steel sheet and non-oriented electrical steel sheet by using it Download PDF

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Publication number
WO2009082088A1
WO2009082088A1 PCT/KR2008/006620 KR2008006620W WO2009082088A1 WO 2009082088 A1 WO2009082088 A1 WO 2009082088A1 KR 2008006620 W KR2008006620 W KR 2008006620W WO 2009082088 A1 WO2009082088 A1 WO 2009082088A1
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WO
WIPO (PCT)
Prior art keywords
phosphate
coating composition
film
steel sheet
insulating film
Prior art date
Application number
PCT/KR2008/006620
Other languages
French (fr)
Inventor
Min-Soo Han
Jung-Woo Kim
Chan-Hee Han
Soon-Bok Park
Jae-Kwan Kim
Original Assignee
Posco
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Filing date
Publication date
Application filed by Posco filed Critical Posco
Priority to CN2008801224055A priority Critical patent/CN101910464B/en
Priority to JP2010540555A priority patent/JP5568479B2/en
Publication of WO2009082088A1 publication Critical patent/WO2009082088A1/en

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    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C22/00—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C22/73—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals characterised by the process
    • C23C22/74—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals characterised by the process for obtaining burned-in conversion coatings
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C22/00—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C22/05—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions
    • C23C22/06—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous acidic solutions with pH less than 6
    • C23C22/07—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous acidic solutions with pH less than 6 containing phosphates
    • C—CHEMISTRY; METALLURGY
    • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
    • C09D—COATING COMPOSITIONS, e.g. PAINTS, VARNISHES OR LACQUERS; FILLING PASTES; CHEMICAL PAINT OR INK REMOVERS; INKS; CORRECTING FLUIDS; WOODSTAINS; PASTES OR SOLIDS FOR COLOURING OR PRINTING; USE OF MATERIALS THEREFOR
    • C09D5/00—Coating compositions, e.g. paints, varnishes or lacquers, characterised by their physical nature or the effects produced; Filling pastes
    • C09D5/08—Anti-corrosive paints
    • C09D5/082—Anti-corrosive paints characterised by the anti-corrosive pigment
    • C09D5/084—Inorganic compounds
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C22/00—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C22/05—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions
    • C23C22/06—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous acidic solutions with pH less than 6
    • C23C22/07—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous acidic solutions with pH less than 6 containing phosphates
    • C23C22/08—Orthophosphates
    • C23C22/20—Orthophosphates containing aluminium cations
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C22/00—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C22/73—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals characterised by the process
    • C23C22/76—Applying the liquid by spraying
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C22/00—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C22/73—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals characterised by the process
    • C23C22/77—Controlling or regulating of the coating process

Definitions

  • the present invention relates to a coating composition for forming an insulating film and a method of forming an insulating film of a non-oriented electrical steel sheet.
  • the present invention relates to a coating composition for a film of a non- oriented electrical steel sheet, which contains no chromium (Cr) and exhibits superior compatibility of components, very high surface gloss in the manufacture of a non- oriented electrical steel sheet having a thick film, superior corrosion resistance and high film adhesion after stress relief annealing (SRA), and to a method of forming the film.
  • Cr chromium
  • the present invention has been made keeping in mind the problems of reduction of compatibility and surface gloss in formation of a thick film, occurring as a result of the addition of phosphate and synthetic resin in the related art, and the present invention provides a coating composition for forming an insulating film, having superior compatibility and surface gloss, and also provides a method of forming an insulating film.
  • a coating composition for forming an insulating film includes, based on 100 g of a mixture solution of monoaluminum phosphate and monozinc phosphate, 0.5-5 g of a mixture solution of cobalt hydroxide and strontium hydroxide, 150-300 g of a polyester emulsion resin, 3-10 g of aluminum silicate, and 0.5-6.0 g of a chelating agent, in which the polyester emulsion resin has a weight average molecular weight of 2,000-15,000, a solid content of 15-25% and a viscosity of 100-500 cp.
  • a method of forming an insulating film includes applying the above coating composition on one surface of a steel sheet at a rate of 2.0-6.0 g/nf, and then heating the applied composition at 350 ⁇ 700°C for 10-50 sec, and a non-oriented electrical steel sheet has high surface gloss while having an insulating film as a thick film having a thickness of 2 /M or more manufactured through the above method.
  • a coating composition composed of a polyester emulsion resin and so on having high compatibility with an inorganic composition is applied on a non-oriented electrical steel sheet
  • a chromium-free insulating film of the non-oriented electrical steel sheet which can exhibit superior compatibility, insulating properties, corrosion resistance and film adhesion after SRA and which is provided in the form of a thick film having a thickness of 2 ⁇ m or more but does not reduce surface gloss, can be obtained.
  • FIG. 1 is a graph showing an increase in storage modulus (G') depending on the molecular weight in a low frequency region (A). Best Mode for Carrying out the Invention
  • An aspect of the present invention provides a coating composition for forming an insulating film, having superior compatibility and surface gloss, which includes, based on 100 g of a mixture solution of monoaluminum phosphate and monozinc phosphate, 0.5-5 g of a mixture solution of cobalt hydroxide and strontium hydroxide, 150-300 g of a polyester emulsion resin, 3-10 g of aluminum silicate, and 0.5-6.0 g of a chelating agent, in which the polyester emulsion resin has a weight average molecular weight of 2,000-15,000, a solid content of 15-25% and a viscosity of 100-500 cp.
  • the phosphate may have a solid content of 55-60 wt% and a viscosity of 30-70 cp.
  • the mixture solution of cobalt hydroxide and strontium hydroxide may have a solid content of 55-60 wt% based on the total weight of the phosphate.
  • the polyester emulsion resin may have a glass transition temperature (Tg) of
  • the chelating agent may be one or more selected from among titanium-based compounds including triethanolamine titanate, titanium 2,2,2-nitrilotrisethanolate, and mixtures of organic titanate and inorganic porous compounds.
  • another aspect of the present invention provides a method of forming an insulating film of a non-oriented electrical steel sheet, including applying the above coating composition on one surface of a steel sheet at a rate of 2.0-6.0 g/nf, and then heating the applied composition at 350 ⁇ 700°C for 10-50 sec, thus forming an insulating film.
  • a further aspect of the present invention provides a non-oriented electrical steel sheet having the insulating film as a thick film having a thickness of 2 ⁇ m or more manufactured through the above method and exhibiting superior compatibility and surface gloss.
  • the non-oriented electrical steel sheet is blanked according to a product standard, thus obtaining blanked pieces, after which the blanked pieces are stacked and then welded or bonded, thus forming an iron core.
  • SRA SRA
  • adhesion, insulating properties and corrosion resistance may change after annealing.
  • the properties after SRA are regarded as important.
  • chromium is contained in the coating composition for a film of a non-oriented electrical steel sheet, it is very helpful in improving the properties of the film after SRA.
  • phosphate may be used in lieu of chromium, but may cause problems of stickiness due to free phosphoric acid present in a very small amount in the film or of adhesivity in annealing. Further, it is impossible to obtain the compactness of the film based on the filling effect of a film with a chromium compound, and thus limitations are imposed on preventing the reduction of properties of the film.
  • the phosphate After drying of the film, the phosphate generates free phosphoric acid and the phosphoric acid thus generated is hygroscopic, and thus, adhesion and corrosion resistance of the steel sheet coated with the film are reduced. Also, the degree of reduction of film adhesion after SRA may vary depending on the compatibility of the emulsion organic resin and the phosphate which are added to the coating composition. Such compatibility may be solved through a relation between rheological vis- coelasticity depending on the molecular weight of the emulsion resin and surface gloss after the application of the composition.
  • the phosphate mixture of monoaluminum phosphate (A1(H2PO4)3) and monozinc phosphate (Zn(H2PO4)2) is added the mixture of cobalt hydroxide and strontium hydroxide.
  • the mixture of cobalt hydroxide and strontium hydroxide may have a solid content of 55-60 wt% based on the total weight of the phosphate.
  • the coating composition Based on 100 g of the phosphate mixture, 0.5-5 g of the mixture of cobalt hydroxide and strontium hydroxide, 150-300 g of the polyester emulsion resin having a weight average molecular weight of 2,000-15,000 and a solid content of 20-25 wt%, 3-10 g of aluminum silicate, and 0.5-6.0 g of the chelating agent are added, thus preparing the coating composition.
  • the composition thus prepared is applied on one surface of the non-oriented electrical steel sheet at a rate of 2.0-6.0 g/ m 2 , and then heated at 350 ⁇ 700°C for 10-30 sec, thus forming the insulating film.
  • the phosphate is the phosphate mixture of monoaluminum phosphate (A1(H2PO4)3) and monozinc phosphate (Zn(H2PO4)2).
  • the mixing ratio of monoaluminum phosphate (A1(H2PO4)3) and monozinc phosphate (Zn(H2PO4)2) has a great influence on viscosity as related to adhesion, and this is shown well in Table 1 below.
  • the emulsion resin is the polyester emulsion resin having a weight average molecular weight of 2,000-15,000 and a solid content of 20-25 wt%.
  • the reduction of surface gloss upon formation of a thick film was revealed to be greatly affected by the properties of the emulsion resin itself and the compatibility with phosphate. Specifically, when the emulsion resin having a specific molecular weight is mixed with the phosphate, the reduction of glass can be prevented upon formation of a thick film, thereby enabling the preparation of the coating composition for a thick film having a good external appearance.
  • the compatibility between the phosphate and the emulsion resin affects film adhesion after SRA. If the compatibility between the two components is poor, a microscopic phase separation phenomenon or a coagulation phenomenon, which is not observed with the naked eye, may oxur immediately after the coating composition is prepared. When the film is formed using the coating composition having poor compatibility and is then subjected to SRA at 75O 0 C for about 2 hours, impurities such as black ash may remain on the surface of the steel sheet. So, in the case where such a steel sheet is used to fabricate a motor or a compressor, problems in which a pipe for supplying cooling oil is clogged or the lifespan of products is shortened may occur. With the goal of solving such problems, in the present invention, the compatibility with phosphate and the adhesion may be improved through control of the molecular weight of the emulsion resin. [27] Below, the present invention is described in detail.
  • the phosphate mixture of monoaluminum phosphate (A1(H2PO4)3) and monozinc phosphate (Zn(H2PO4)2) is used.
  • the phosphate mixture affects the viscosity related to corrosion resistance and adhesion depending on the mixing ratio of components thereof.
  • the use of the phosphate may cause problems of stickiness due to free phosphoric acid or precipitation of powder as mentioned above.
  • the mixture of cobalt hydoroxide and strontium hydroxide was confirmed to inhibit the precipitation of free phosphoric acid and to improve the compactness of the film.
  • cobalt hydoroxide and strontium hydroxide are appropriately mixed, stickiness and powder precipitation may be prevented and corrosion resistance may be improved.
  • the film when it is provided in the form of a thin film having a thickness of 1 ⁇ m or less, products having good surface gloss may be obtained. However, when the film is provided in the form of a thick film having a thickness of 2 ⁇ m or more, surface gloss is drastically reduced and film adhesion after SRA may become bad.
  • the major reason of the above problems may be considered to be due to the molecular weight of the emulsion resin.
  • a polymer solution including the emulsion resin and so on shows viscoelastic (non-Newtonian) behavior unlike a viscous (Newtonian) fluid such as water.
  • the viscoelastic behavior of the fluid is a property regarding flow and deformation of a material.
  • the elasticity of the fluid increases proportionally to an increase in the molecular weight of the polymer.
  • the viscoelastic properties of a material are measured using a rheometer, and the vis- coelastidty of the emulsion resin depending on the molecular weight thereof may be evaluated by storage modulus (G') obtained from a measurement system.
  • G' storage modulus
  • FIG. 1 shows the storage modulus of the emulsion resin depending on the molecular weight thereof. As the molecular weight of the emulsion resin is increased, the storage modulus can be seen to increase in the region A of FIG. 1, namely, the low frequency region. This phenomenon may be theoretically estimated even using an emulsion model simulating the behavior of a fluid including elastic particles.
  • the polyester resin having a molecular weight in the range in which viscoelastidty is not drastically increased is used to improve the compatibility with phosphate, resulting in superior surface gloss upon formation of a thick film and improved film adhesion after SRA.
  • the titanium-based chelating agent is introduced.
  • the titanium-based chelating agent may be one or more selected from among triethanolamine titanate, titanium 2,2,2-nitrilotrisethanolate, and mixtures of organic titanate and inorganic porous compounds.
  • the mixture of cobalt hydroxide and strontium hydroxide is added to the phosphate mixture of monoaluminum phosphate (A1(H2PO4)3) and monozinc phosphate (Zn(H2PO4)2) sirh that the solid content thereof is 55-60 wt% based on the total weight of the phosphate.
  • the polyester emulsion resin having a molecular weight adequate for preventing strong elastic properties from being exhibited 150-300 g of the polyester emulsion resin having a molecular weight adequate for preventing strong elastic properties from being exhibited, 3-10 g of aluminum silicate, and 0.5-6.0 g of the chelating agent are added based on 100 g of the phosphate, thus preparing the coating composition, after which the coating composition is applied on one surface of the steel sheet at a rate of 2.0-6.0 g/m 2 , and then heated at 350 ⁇ 700°C for 10-30 sec, thus forming the insulating film.
  • polyester emulsion resin useful is a material in an emulsion state having a weight average molecular weight of 2,000-15,000, a glass transition temperature (Tg) of 40 ⁇ 50°C and a solid content of 15-25 wt%. If the molecular weight of the polyester emulsion resin is below 2,000, the viscosity becomes too low, and thus the resulting coating composition is difficult to be applied as a thick film. In contrast, if the molecular weight thereof is above 15,000, viscoelastic properties are strong and thus upon mixing with phosphate, compatibility is reduced, and also, upon formation of a thick film, the gloss of the product is lowered.
  • Tg glass transition temperature
  • the emulsion resin having a molecular weight of 2,000-15,000 is used in an amount below 30 g based on 100 g of the phosphate, the proportion of the phosphate is relatively increased, undesirably causing the stickiness of phosphate and the precipitation of powder thereof.
  • the emulsion resin is used in an amount above 100 g, corrosion resistance and compatibility are drastically reduced.
  • silica may be aqueous alumina colloidal silica the amount of which is 3-10 g in solid weight. If the silica is used in an amount below 3 g, film formability and corrosion resistance are reduced. On the other hand, if the silica is used in an amount above 10 g, phase separation of the coating composition is accelerated, thus reducing the adhesion before and after SRA.
  • the coating composition when the coating composition is applied on one surface of the non-oriented electrical steel sheet at a rate of 2.0-6.0 g/m 2 and then heated at 350 ⁇ 700°C for 10-30 sec, an insulating film having superior adhesion, corrosion resistance and compatibility and high surface gloss even upon formation of a thick film can be formed on the non-oriented electrical steel sheet.
  • a non-oriented electrical steel sheet (an area of 120 mm x 60 mm) including 0.1 wt% of
  • Si and having a thickness of 0.50 mm was used as a test sample, and various treatment liquids were applied on the test sample in the range of 2.0-6.0 g/m 2 using a coating bar. Subsequently, the test sample treated in this way was dried at a temperature of 65O 0 C for several seconds and was then air-cooled.
  • test sample was evaluated through the following methods.
  • the SRA of the test sample was evaluated through heat treatment at 75O 0 C for 2 hours in a dry 100% N2 gas atmosphere, and the insulating properties thereof were evaluated by measuring the current value when voltage of 0.5 V and current of 1.0 A were applied under pressure of 300 PSI.
  • the adhesion thereof was evaluated by measuring the minimum diameter of the arc on which there was no peeled film when the test sample before and after SRA was bent at an angle of 180° while leaning against respective arcs having diameters of 10, 20, and 30-100 mm ⁇ .
  • the external appearance of a film was evaluated by observing striping, gloss and the like with the naked eye.
  • the corrosion resistance of the test sample was evaluated by measuring the area of rust generated on the surface of the test sample when immersing the test sample in a 5% NaCl solution at 35 0 C for 8 hours.
  • the case in which the area of generated rust was below 5% was evaluated to be excellent
  • the case in which the area of generated rust was below 20% was evaluated to be good
  • the case in which the area of generated rust was 20-50% was evaluated to be poor
  • the case in which the area of generated rust was above 50% was evaluated to be bad.
  • the film strength of the test sample after SRA was evaluated using an image processing method by quantifying, as a percentage, the degree of attachment of the peeled film to an adhesive tape and the contamination of the adhesive tape occurring when a piece of adhesive tape having a predetermined size was attached to the film and separated from the film after SRA.
  • R>r example when the film strength of the sample is indicated as 0, it means that the peeled film is not formed on the surface of the film, and when the film strength thereof is indicated as 100, it means that the total area of the adhesive tape is contaminated by the peeled film. Therefore, as the number indicating the film strength thereof increases, this means that the film strength is not good.
  • Table 1 shows the corrosion resistance of the coating composition depending on the kind of phosphate and metal oxide.
  • the weight ratio of phosphate and polyester emulsion resin (weight average MW: 14,000) enabling the thick film coating was obtained through a pre-test. As a result, addition of 150-300 g of the polyester emulsion resin per 100 g of the phosphate was the most preferable.
  • corrosion resistance and adhesion should be ensured through effective combination of phosphates, and metal oxide should be able to inhibit the stickiness of phosphate and the precipitation of powder thereof.
  • the phosphate used in the present invention when phosphates respectively prepared through adjustment of the molar ratio of aluminum or zinc oxide and phosphoric acid are mixed, the phosphate combination having a viscosity varying on the component ratio may result.

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Abstract

Disclosed is a coating composition having superior compatibility and surface gloss for forming an insulating film and a method of forming the film. The coating composition includes, based on 100 g of a mixture solution of monoaluminum phosphate and monozinc phosphate, 0.5-5 g of a mixture solution of cobalt hydroxide and strontium hydroxide, 150-300 g of a polyester emulsion resin, 3-10 g of aluminum silicate, and 0.5-6.0 g of a chelating agent, in which the polyester emulsion resin has a weight average molecular weight of 2,000-15,000, a solid content of 15-25% and a viscosity of 100-500 cp. Even when the coating composition which is chromium-free is applied in the form of a thick film on a non-oriented electrical steel sheet, superior insulating properties, corrosion resistance, film adhesion, compatibility and surface gloss can be exhibited.

Description

Description
COATING SOLUTION FOR FORMING INSULATING FILM, METHOD FOR FORMING INSULATING FILM OF NON-ORIENTED ELECTRICAL STEEL SHEET AND NON-ORIENTED ELECTRICAL STEEL SHEET BY
USING IT Technical Field
[1] The present invention relates to a coating composition for forming an insulating film and a method of forming an insulating film of a non-oriented electrical steel sheet. Particularly, the present invention relates to a coating composition for a film of a non- oriented electrical steel sheet, which contains no chromium (Cr) and exhibits superior compatibility of components, very high surface gloss in the manufacture of a non- oriented electrical steel sheet having a thick film, superior corrosion resistance and high film adhesion after stress relief annealing (SRA), and to a method of forming the film. Background Art
[2] Recently, as non-oriented electrical steel sheets have improved magnetic properties, they are produced to be high quality and are employed in large wind or steam power generators. Accordingly, the demand for high insulating properties between the steel sheets is increasing.
[3] Conventional methods of increasing insulating resistance include use of chromium oxide, but chromium may cause environmental problems, and thus, in the current situation, the use thereof is stringently restricted in individual countries.
[4] Thorough research into preparation of a chromium-free coating composition for a film of an electrical steel sheet is being conducted these days, and includes methods of introducing phosphate so that corrosion resistance and adhesion which are weakened due to the absence of chromium oxide are strengthened, and of introducing colloidal silica to induce a barrier effect. Although phosphate may improve adhesion and corrosion resistance, the case where phosphate is used suffers from that free phosphoric add present in phosphate causes the film to be sticky. In order to prevent the stickiness due to the free phosphoric add, techniques for adding an organic add and so on have been proposed.
[5] However, in the coating composition for a film composed mainly of phosphate or colloidal silica, phosphate has a sticky problem and colloidal silica is limited in improving corrosion resistance, and thus commercialization of perfect alternative techniques to chromium oxide has not yet been realized.
[6] Also, methods of adding synthetic resin have been proposed to solve the sticky problem and to improve corrosion resistance and insulating properties. In cases where synthetic resin is added, insulating properties are improved but compatibility with inorganic components such as phosphate and so on is decreased. In particular, the case of the non-oriented electrical steel sheet coated with a thick film is problematic in that gloss is drastically lowered. Disclosure of Invention Technical Problem
[7] Accordingly, the present invention has been made keeping in mind the problems of reduction of compatibility and surface gloss in formation of a thick film, occurring as a result of the addition of phosphate and synthetic resin in the related art, and the present invention provides a coating composition for forming an insulating film, having superior compatibility and surface gloss, and also provides a method of forming an insulating film. Technical Solution
[8] According to the present invention, a coating composition for forming an insulating film includes, based on 100 g of a mixture solution of monoaluminum phosphate and monozinc phosphate, 0.5-5 g of a mixture solution of cobalt hydroxide and strontium hydroxide, 150-300 g of a polyester emulsion resin, 3-10 g of aluminum silicate, and 0.5-6.0 g of a chelating agent, in which the polyester emulsion resin has a weight average molecular weight of 2,000-15,000, a solid content of 15-25% and a viscosity of 100-500 cp.
[9] In addition, according to the present invention, a method of forming an insulating film includes applying the above coating composition on one surface of a steel sheet at a rate of 2.0-6.0 g/nf, and then heating the applied composition at 350~700°C for 10-50 sec, and a non-oriented electrical steel sheet has high surface gloss while having an insulating film as a thick film having a thickness of 2 /M or more manufactured through the above method.
Advantageous Effects
[10] According to the present invention, when a coating composition composed of a polyester emulsion resin and so on having high compatibility with an inorganic composition is applied on a non-oriented electrical steel sheet, a chromium-free insulating film of the non-oriented electrical steel sheet, which can exhibit superior compatibility, insulating properties, corrosion resistance and film adhesion after SRA and which is provided in the form of a thick film having a thickness of 2 μm or more but does not reduce surface gloss, can be obtained. Brief Description of Drawings
[11] FIG. 1 is a graph showing an increase in storage modulus (G') depending on the molecular weight in a low frequency region (A). Best Mode for Carrying out the Invention
[12] An aspect of the present invention provides a coating composition for forming an insulating film, having superior compatibility and surface gloss, which includes, based on 100 g of a mixture solution of monoaluminum phosphate and monozinc phosphate, 0.5-5 g of a mixture solution of cobalt hydroxide and strontium hydroxide, 150-300 g of a polyester emulsion resin, 3-10 g of aluminum silicate, and 0.5-6.0 g of a chelating agent, in which the polyester emulsion resin has a weight average molecular weight of 2,000-15,000, a solid content of 15-25% and a viscosity of 100-500 cp.
[13] The phosphate may have a solid content of 55-60 wt% and a viscosity of 30-70 cp.
[14] The mixture solution of cobalt hydroxide and strontium hydroxide may have a solid content of 55-60 wt% based on the total weight of the phosphate.
[15] The polyester emulsion resin may have a glass transition temperature (Tg) of
40~50°C and an acidity (pH) of 4-5.
[16] The chelating agent may be one or more selected from among titanium-based compounds including triethanolamine titanate, titanium 2,2,2-nitrilotrisethanolate, and mixtures of organic titanate and inorganic porous compounds.
[17] In addition, another aspect of the present invention provides a method of forming an insulating film of a non-oriented electrical steel sheet, including applying the above coating composition on one surface of a steel sheet at a rate of 2.0-6.0 g/nf, and then heating the applied composition at 350~700°C for 10-50 sec, thus forming an insulating film.
[18] In addition, a further aspect of the present invention provides a non-oriented electrical steel sheet having the insulating film as a thick film having a thickness of 2 μm or more manufactured through the above method and exhibiting superior compatibility and surface gloss.
[19] Typically, in the case where a non-oriented electrical steel sheet is used for an iron core of a motor or a transformer, the non-oriented electrical steel sheet is blanked according to a product standard, thus obtaining blanked pieces, after which the blanked pieces are stacked and then welded or bonded, thus forming an iron core. In this procedure, if SRA is performed, in particular, adhesion, insulating properties and corrosion resistance may change after annealing. Hence, the properties after SRA are regarded as important. When chromium is contained in the coating composition for a film of a non-oriented electrical steel sheet, it is very helpful in improving the properties of the film after SRA. On the other hand, in a chromium-free coating composition, phosphate may be used in lieu of chromium, but may cause problems of stickiness due to free phosphoric acid present in a very small amount in the film or of adhesivity in annealing. Further, it is impossible to obtain the compactness of the film based on the filling effect of a film with a chromium compound, and thus limitations are imposed on preventing the reduction of properties of the film.
[20] After drying of the film, the phosphate generates free phosphoric acid and the phosphoric acid thus generated is hygroscopic, and thus, adhesion and corrosion resistance of the steel sheet coated with the film are reduced. Also, the degree of reduction of film adhesion after SRA may vary depending on the compatibility of the emulsion organic resin and the phosphate which are added to the coating composition. Such compatibility may be solved through a relation between rheological vis- coelasticity depending on the molecular weight of the emulsion resin and surface gloss after the application of the composition.
[21] Hence, in the present invention, in order to overcome problems of a reduction of corrosion resistance and adhesion occurring as a result of excluding chromium and to improve film adhesion after SRA, to the phosphate mixture of monoaluminum phosphate (A1(H2PO4)3) and monozinc phosphate (Zn(H2PO4)2) is added the mixture of cobalt hydroxide and strontium hydroxide. The mixture of cobalt hydroxide and strontium hydroxide may have a solid content of 55-60 wt% based on the total weight of the phosphate. Based on 100 g of the phosphate mixture, 0.5-5 g of the mixture of cobalt hydroxide and strontium hydroxide, 150-300 g of the polyester emulsion resin having a weight average molecular weight of 2,000-15,000 and a solid content of 20-25 wt%, 3-10 g of aluminum silicate, and 0.5-6.0 g of the chelating agent are added, thus preparing the coating composition. The composition thus prepared is applied on one surface of the non-oriented electrical steel sheet at a rate of 2.0-6.0 g/ m2, and then heated at 350~700°C for 10-30 sec, thus forming the insulating film.
[22] Even when the amount of composition to be applied is increased, the prevention of reduction of surface gloss and the improvement in film adhesion after SRA may be realized through the following method.
[23] When the phosphate mixture of monoaluminum phosphate (A1(H2PO4)3) and monozinc phosphate (Zn(H2PO4)2) containing the mixture of cobalt hydroxide and strontium hydroxide is mixed with the polyester emulsion resin having a weight average molecular weight of 2,000-15,000 and a solid content of 15-25 wt%, gloss can be ensured upon formation of a thick film. The phosphate which functions as a binder between the organic-inorganic mixed composition and the steel sheet serving as a base substrate plays an important role in improving not only adhesion but also heat resistance.
[24] In the present invention, the phosphate is the phosphate mixture of monoaluminum phosphate (A1(H2PO4)3) and monozinc phosphate (Zn(H2PO4)2). The mixing ratio of monoaluminum phosphate (A1(H2PO4)3) and monozinc phosphate (Zn(H2PO4)2) has a great influence on viscosity as related to adhesion, and this is shown well in Table 1 below.
[25] The emulsion resin is the polyester emulsion resin having a weight average molecular weight of 2,000-15,000 and a solid content of 20-25 wt%. The reduction of surface gloss upon formation of a thick film was revealed to be greatly affected by the properties of the emulsion resin itself and the compatibility with phosphate. Specifically, when the emulsion resin having a specific molecular weight is mixed with the phosphate, the reduction of glass can be prevented upon formation of a thick film, thereby enabling the preparation of the coating composition for a thick film having a good external appearance.
[26] The compatibility between the phosphate and the emulsion resin affects film adhesion after SRA. If the compatibility between the two components is poor, a microscopic phase separation phenomenon or a coagulation phenomenon, which is not observed with the naked eye, may oxur immediately after the coating composition is prepared. When the film is formed using the coating composition having poor compatibility and is then subjected to SRA at 75O0C for about 2 hours, impurities such as black ash may remain on the surface of the steel sheet. So, in the case where such a steel sheet is used to fabricate a motor or a compressor, problems in which a pipe for supplying cooling oil is clogged or the lifespan of products is shortened may occur. With the goal of solving such problems, in the present invention, the compatibility with phosphate and the adhesion may be improved through control of the molecular weight of the emulsion resin. [27] Below, the present invention is described in detail.
[28] One of representative properties as requirements of the coating composition for the insulating film of the non-oriented electrical steel sheet is corrosion resistance. Conventionally, corrosion resistance has been improved upon using chromium oxide. However, because chromium has a bad influence on the human body upon contact with the skin and causes environmental problems upon discharge of wastewater, there is a current tendency to exclude chromium.
[29] In order to prevent the reduction of corrosion resistance and film compactness due to the exclusion of chromium oxide, in the present invention, the phosphate mixture of monoaluminum phosphate (A1(H2PO4)3) and monozinc phosphate (Zn(H2PO4)2) is used. The phosphate mixture affects the viscosity related to corrosion resistance and adhesion depending on the mixing ratio of components thereof. As a result of preparation of combinations of various phosphates including monoaluminum phosphate (A1(H2PO4)3), monozinc phosphate (Zn(H2PO4)2), monomagnesium phosphate (Mg(H2PO4)2) and so on, and of addition of the polyester emulsion resin thereto, particularly useful was the mixture of monoaluminum phosphate (A1(H2PO4)3) and monozinc phosphate (Zn(H2PO4)2) having a solid content of 55-60 wt% and a viscosity of 30-70 cp, in terms of corrosion resistance (Table 1). In the case where the proportion of monoaluminum phosphate (A1(H2PO4)3) is high, an appropriate viscosity cannot be maintained, and also, after drying, a sticky phenomenon oxurs. If the proportion of monozinc phosphate (Zn(H2PO4)2) is high, adequate corrosion resistance cannot be obtained.
[30] However, the use of the phosphate may cause problems of stickiness due to free phosphoric acid or precipitation of powder as mentioned above. Hence, in the present invention, as a result of experiment using various metal oxides or hydroxides in order to solve the above problems, the mixture of cobalt hydoroxide and strontium hydroxide was confirmed to inhibit the precipitation of free phosphoric acid and to improve the compactness of the film. In particular, when cobalt hydoroxide and strontium hydroxide are appropriately mixed, stickiness and powder precipitation may be prevented and corrosion resistance may be improved.
[31] Recently, since non-oriented electrical steel sheet products having superior magnetic properties have appeared and are thus used for large wind or steam power generators, high insulating properties between the steel sheets are required. However, when the coating composition is applied into a thick film, surface gloss is remarkably reduced and the product thus coated may have severe surface defects upon treatment of SRA. This is considered to be due to the compatibility between the phosphate and the emulsion resin. The compatibility between the two components is closely related to film adhesion after SRA. Phenomenologically, if the compatibility between the two components is poor, a fine phase separation phenomenon or a coagulation phenomenon of two components, which is not observed with the naked eye, may occur immediately after the preparation of the coating composition. In the case where the film is formed using the composition thus prepared, when it is provided in the form of a thin film having a thickness of 1 μm or less, products having good surface gloss may be obtained. However, when the film is provided in the form of a thick film having a thickness of 2 μm or more, surface gloss is drastically reduced and film adhesion after SRA may become bad.
[32] The major reason of the above problems may be considered to be due to the molecular weight of the emulsion resin. Generally, a polymer solution including the emulsion resin and so on shows viscoelastic (non-Newtonian) behavior unlike a viscous (Newtonian) fluid such as water. The viscoelastic behavior of the fluid is a property regarding flow and deformation of a material. The elasticity of the fluid increases proportionally to an increase in the molecular weight of the polymer. The viscoelastic properties of a material are measured using a rheometer, and the vis- coelastidty of the emulsion resin depending on the molecular weight thereof may be evaluated by storage modulus (G') obtained from a measurement system. FIG. 1 shows the storage modulus of the emulsion resin depending on the molecular weight thereof. As the molecular weight of the emulsion resin is increased, the storage modulus can be seen to increase in the region A of FIG. 1, namely, the low frequency region. This phenomenon may be theoretically estimated even using an emulsion model simulating the behavior of a fluid including elastic particles.
[33] Through the above experiment, as the molecular weight of the emulsion resin becomes higher, the elastic properties of the fluid increase, and accordingly, the compatibility with phosphate is reduced. This is specifically described in the example which will be mentioned later. Thus, in the present invention, the polyester resin having a molecular weight in the range in which viscoelastidty is not drastically increased is used to improve the compatibility with phosphate, resulting in superior surface gloss upon formation of a thick film and improved film adhesion after SRA.
[34] Further, in order to thoroughly prevent the generation of black ash on the surface of the steel sheet after SRA of thick film-coated products, the titanium-based chelating agent is introduced. The titanium-based chelating agent may be one or more selected from among triethanolamine titanate, titanium 2,2,2-nitrilotrisethanolate, and mixtures of organic titanate and inorganic porous compounds.
[35] Thus, in order to overcome problems of corrosion resistance and adhesion which may oxur as a result of excluding chromium and to improve film adhesion after SRA, the mixture of cobalt hydroxide and strontium hydroxide is added to the phosphate mixture of monoaluminum phosphate (A1(H2PO4)3) and monozinc phosphate (Zn(H2PO4)2) sirh that the solid content thereof is 55-60 wt% based on the total weight of the phosphate. Also, in order to exhibit superior surface gloss and insulating properties which are not realized in a conventional thin film type chromium-free coating composition, 150-300 g of the polyester emulsion resin having a molecular weight adequate for preventing strong elastic properties from being exhibited, 3-10 g of aluminum silicate, and 0.5-6.0 g of the chelating agent are added based on 100 g of the phosphate, thus preparing the coating composition, after which the coating composition is applied on one surface of the steel sheet at a rate of 2.0-6.0 g/m2, and then heated at 350~700°C for 10-30 sec, thus forming the insulating film.
[36] As the polyester emulsion resin, useful is a material in an emulsion state having a weight average molecular weight of 2,000-15,000, a glass transition temperature (Tg) of 40~50°C and a solid content of 15-25 wt%. If the molecular weight of the polyester emulsion resin is below 2,000, the viscosity becomes too low, and thus the resulting coating composition is difficult to be applied as a thick film. In contrast, if the molecular weight thereof is above 15,000, viscoelastic properties are strong and thus upon mixing with phosphate, compatibility is reduced, and also, upon formation of a thick film, the gloss of the product is lowered. When the emulsion resin having a molecular weight of 2,000-15,000 is used in an amount below 30 g based on 100 g of the phosphate, the proportion of the phosphate is relatively increased, undesirably causing the stickiness of phosphate and the precipitation of powder thereof. In contrast, when the emulsion resin is used in an amount above 100 g, corrosion resistance and compatibility are drastically reduced.
[37] Further, silica may be aqueous alumina colloidal silica the amount of which is 3-10 g in solid weight. If the silica is used in an amount below 3 g, film formability and corrosion resistance are reduced. On the other hand, if the silica is used in an amount above 10 g, phase separation of the coating composition is accelerated, thus reducing the adhesion before and after SRA.
[38] In the present invention, when the coating composition is applied on one surface of the non-oriented electrical steel sheet at a rate of 2.0-6.0 g/m2 and then heated at 350~700°C for 10-30 sec, an insulating film having superior adhesion, corrosion resistance and compatibility and high surface gloss even upon formation of a thick film can be formed on the non-oriented electrical steel sheet.
[39] (Example)
[40] A non-oriented electrical steel sheet (an area of 120 mm x 60 mm) including 0.1 wt% of
Si and having a thickness of 0.50 mm was used as a test sample, and various treatment liquids were applied on the test sample in the range of 2.0-6.0 g/m2 using a coating bar. Subsequently, the test sample treated in this way was dried at a temperature of 65O0C for several seconds and was then air-cooled.
[41] The test sample was evaluated through the following methods.
[42] The SRA of the test sample was evaluated through heat treatment at 75O0C for 2 hours in a dry 100% N2 gas atmosphere, and the insulating properties thereof were evaluated by measuring the current value when voltage of 0.5 V and current of 1.0 A were applied under pressure of 300 PSI. The adhesion thereof was evaluated by measuring the minimum diameter of the arc on which there was no peeled film when the test sample before and after SRA was bent at an angle of 180° while leaning against respective arcs having diameters of 10, 20, and 30-100 mmΦ. The external appearance of a film was evaluated by observing striping, gloss and the like with the naked eye. The corrosion resistance of the test sample was evaluated by measuring the area of rust generated on the surface of the test sample when immersing the test sample in a 5% NaCl solution at 350C for 8 hours. In the test, the case in which the area of generated rust was below 5% was evaluated to be excellent, the case in which the area of generated rust was below 20% was evaluated to be good, the case in which the area of generated rust was 20-50% was evaluated to be poor, and the case in which the area of generated rust was above 50% was evaluated to be bad. Further, the film strength of the test sample after SRA was evaluated using an image processing method by quantifying, as a percentage, the degree of attachment of the peeled film to an adhesive tape and the contamination of the adhesive tape occurring when a piece of adhesive tape having a predetermined size was attached to the film and separated from the film after SRA. R>r example, when the film strength of the sample is indicated as 0, it means that the peeled film is not formed on the surface of the film, and when the film strength thereof is indicated as 100, it means that the total area of the adhesive tape is contaminated by the peeled film. Therefore, as the number indicating the film strength thereof increases, this means that the film strength is not good.
[43] Table 1 shows the corrosion resistance of the coating composition depending on the kind of phosphate and metal oxide. The weight ratio of phosphate and polyester emulsion resin (weight average MW: 14,000) enabling the thick film coating was obtained through a pre-test. As a result, addition of 150-300 g of the polyester emulsion resin per 100 g of the phosphate was the most preferable. In order to prepare the chromium-free coating composition, corrosion resistance and adhesion should be ensured through effective combination of phosphates, and metal oxide should be able to inhibit the stickiness of phosphate and the precipitation of powder thereof. In the phosphate used in the present invention, when phosphates respectively prepared through adjustment of the molar ratio of aluminum or zinc oxide and phosphoric acid are mixed, the phosphate combination having a viscosity varying on the component ratio may result.
[44] As is apparent from Table 1, superior corrosion resistance was exhibited when using the mixture of monoaluminum phosphate (A1(H2PO4)3) and monozinc phosphate (Zn(H2PO4)2) at an appropriate mixing ratio. Also, when cobalt hydoroxide and strontium hydroxide were mixed at an appropriate mixing ratio, the stickiness of phosphate and the precipitation of powder thereof could be prevented and corrosion resistance could be improved.
[45]
[46] Table 1
[Table 1]
Figure imgf000013_0001
[47] [48]
Figure imgf000014_0001
Note: Evaluation of Properties ': excellent, O: good, Δ: fair, V: poor, x: bad
[49] [50] 100 g of the phosphate mixture (solid content: 60 wt%) of monoaluminum phosphate (A1(H2PO4)3) and monozinc phosphate (Zn(H2PO4)2) obtained from Table 1 was mixed with 200 g of each of polyester emulsion resins having different molecular weights, thus preparing a coating composition, which was then formed into a thick film (2.5 μm). The surface gloss and film adhesion after SRA were measured. The results are shown in Table 2 below.
[51] Table 2
[Table 2]
Figure imgf000015_0001
Note: Evaluation of Properties - excellent, O : good, Δ: fair, V: poor, x: bad
[52] [53] As is apparent from Table 2, when the weight average molecular weight of the emulsion resin was 1,500, surface gloss and adhesion after SRA were good but workability was very poor attributable to unsatisfactory viscosity of the resin. When the weight average molecular weight of the emulsion resin was 2,000-15,000, the film in which surface gloss was not reduced upon formation of the thick film and also adhesion after SRA was relatively good could be obtained. In particular, when the weight average molecular weight of the emulsion resin was 5,100, the film having the greatest surface gloss could result. In the coating compositions prepared using the emulsion resins having a weight average molecular weight above 15,000, as the weight average molecular weight of the emulsion resin was increased, products in which the surface thereof was opaque and the surface gloss was very poor upon formation of a thick film were obtained. As mentioned above, in the polyester emulsion resin having a weight average molecular weight above 15,000, while the viscoelastic properties of the emulsion resin increased (region A of FIG. 1) proportionally to an increase in the weight average molecular weight thereof, compatibility with phosphate was reduced, thereby causing phase separation. In this way, when the coating composition having reduced compatibility was formed into a thick film, surface gloss and adhesion after SRA were considerably deteriorated. [54] However, as shown in Table 1, in order to exhibit properties equivalent to those of the chromium-containing coating composition, an improvement in film adhesion after SRA was ultimately required. Thus, as shown in Table 2, the above problem was solved by using a chelating agent and colloidal silicate. In Table 3 below, to 200 g of the polyester emulsion resin having a weight average molecular weight of 5,100 per 100 g of the phosphate, aluminum silicate and chelating agent were added in varying amounts, thus preparing coating compositions. Table 4 below shows the properties of thick films (2.5 μm) formed using the coating compositions thus prepared.
[55] Table 3 [Table 3]
Figure imgf000016_0001
[56] Table 4 [Table 4]
Figure imgf000017_0001
Note : Evaluation of Properties - excellent, O : good, Δ : fair, V : poor, x : bad
[57] [58] As is apparent from Table 4, as in the test sample Nos. 15-3-8 and 15-3-9, when 1.0-3.0 g of aluminum silicate and 0.1-0.5 g of the chelating agent were added based on 100 g of the phosphate mixture of monoaluminum phosphate (A1(H2PO4)3) and monozinc phosphate (Zn(H2PO4)2) containing the mixture of cobalt hydroxide and strontium hydroxide, corrosion resistance and film adhesion could be seen to be superior. This is evaluated to be equivalent to or higher than those of a conventional chromium-containing coating composition.

Claims

Claims
[1] A coating composition having superior compatibility and surface gloss for forming an insulating film, comprising, based on 100 g of a mixture solution of monoaluminum phosphate and monozinc phosphate, 0.5-5 g of a mixture solution of cobalt hydroxide and strontium hydroxide, 150-300 g of a polyester emulsion resin, 3-10 g of aluminum silicate, and 0.5-6.0 g of a chelating agent, wherein the polyester emulsion resin has a weight average molecular weight of 2,000-15,000, a solid content of 15-25% and a viscosity of 100-500 cp.
[2] The coating composition according to claim 1, wherein the mixture solution of monoaluminum phosphate and monozinc phosphate has a solid content of 55-60 wt% and a viscosity of 30-70 cp.
[3] The coating composition according to claim 1, wherein the mixture solution of cobalt hydroxide and strontium hydroxide has a solid content of 55-60 wt% based on total weight of the phosphate.
[4] The coating composition according to claim 1, wherein the polyester emulsion resin has a glass transition temperature (Tg) of 40~50°C and an acidity (pH) of 4-5.
[5] The coating composition according to claim 1, wherein the chelating agent is one or more selected from among titanium-based compounds including tri- ethanolamine titanate, titanium 2,2,2-nitrilotrisethanolate, and mixtures of organic titanate and inorganic porous compounds.
[6] A method of forming an insulating film having superior corrosion resistance and surface gloss on a non-oriented electrical steel sheet, comprising applying the coating composition of any one of claims 1 to 5 on one surface of a steel sheet at a rate of 2.0-6.0 g/nf, and then heating the applied composition at 350~700°C for 10-50 sec, thus forming an insulating film.
[7] A non-oriented electrical steel sheet including an insulating film as a thick film having a thickness of 2 /M or more manufactured through the method of claim 6 and having superior corrosion resistance and surface gloss.
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