Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B05—SPRAYING OR ATOMISING IN GENERAL; APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
  • B05D—PROCESSES FOR APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
  • B05D7/00—Processes, other than flocking, specially adapted for applying liquids or other fluent materials to particular surfaces or for applying particular liquids or other fluent materials
  • B05D7/14—Processes, other than flocking, specially adapted for applying liquids or other fluent materials to particular surfaces or for applying particular liquids or other fluent materials to metal, e.g. car bodies
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B05—SPRAYING OR ATOMISING IN GENERAL; APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
  • B05D—PROCESSES FOR APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
  • B05D2350/00—Pretreatment of the substrate
  • B05D2350/60—Adding a layer before coating
  • B05D2350/65—Adding a layer before coating metal layer
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B05—SPRAYING OR ATOMISING IN GENERAL; APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
  • B05D—PROCESSES FOR APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
  • B05D2401/00—Form of the coating product, e.g. solution, water dispersion, powders or the like
  • B05D2401/30—Form of the coating product, e.g. solution, water dispersion, powders or the like the coating being applied in other forms than involving eliminable solvent, diluent or dispersant
  • B05D2401/32—Form of the coating product, e.g. solution, water dispersion, powders or the like the coating being applied in other forms than involving eliminable solvent, diluent or dispersant applied as powders
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B05—SPRAYING OR ATOMISING IN GENERAL; APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
  • B05D—PROCESSES FOR APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
  • B05D3/00—Pretreatment of surfaces to which liquids or other fluent materials are to be applied; After-treatment of applied coatings, e.g. intermediate treating of an applied coating preparatory to subsequent applications of liquids or other fluent materials
  • B05D3/10—Pretreatment of surfaces to which liquids or other fluent materials are to be applied; After-treatment of applied coatings, e.g. intermediate treating of an applied coating preparatory to subsequent applications of liquids or other fluent materials by other chemical means
  • B05D3/102—Pretreatment of metallic substrates

Definitions

• the surface must be thoroughly cleaned of all dirt, oil, oxidation products and any other foreign matter.
• sites to which the powder can bond must be available on the surface. These are generally provided by depositing certain crystals on the surface during the preparation operations.
• the coating must be a functional type, which, unlike decorative coatings, is especially formulated to impart corrosion resistant properties to the steel when applied in thicknesses generally less than 0.010 inches (0.25 mm).
• a major advantage of the use of galvanized steel over mild steel as the substrate is realized where the powder coating becomes physically damaged, as can occur, for example, in shipping, rigging, or installing of industrial equipment.
• the corrosion resistance is determined only by the substrate material and the zinc cladding of the galvanized steel provides significantly greater corrosion protection than is possible through the use of a passivating rinse, as typically used on mild steel.
• the state of the art can be for example illustrated by GB-A-1 281 767.
• the teaching of this reference is to heat the galvanized metal to a very high temperature in order to promote oxidation.
• the coating step is carried out on the essentially zinc oxide free galvanized metal.
• the present invention is concerned with a process for coating galvanized metal of the type comprising the following steps:
• the steps of cleaning and etching are carried out simultaneously, that the heating step is performed so as to dry the galvanized metal, and that the coating step is performed on the essentially zinc oxide free galvanized metal.
• the present invention is also concerned with a powder coated galvanized metal, characterized in that the powder coating is applied on galvanized metal pretreated according to the process defined above.
• the present invention which is an improved four-step pretreatment process will then be described.
• the eight-stage or eight-step pretreatment process is as follows:
• Step 1 - The first step cleans the galvanized metal zinc substrate surface of any grease or dirt that is present.
• the grease and/or dirt is removed using an alkaline-type cleaning solution with its pH maintained so that it will not attack the zinc.
• Step 2 - A water rinse is applied to remove the alkaline cleaner from the substrate. Due to carry-over of the alkaline cleaner from step 1, this rinse is a mild alkaline rinse.
• Step 3 A second water rinse is utilized to remove any of the mild alkaline residues remaining on the surface following step 2. Thorough removal of all alkaline residues is important because the fourth step requires a delicate acid balance for the zinc phosphate solution. If any alkalinity is left on the substrate, it will affect the acid balance of the zinc phosphate solution.
• Step 4 Substrate passivation, through the spraying of a zinc phosphate solution on the substrate, is accomplished in this step, which is the key to the eight-step system. Reaction of this acidic solution with the zinc substrate results in the formation and deposition of water insoluble zinc phosphate crystals on the surface. It is important that this zinc phosphate solution be maintained at a pH near 3 or a powdery precipitate will be deposited on the substrate. This precipitate is undesirable, as it will significantly reduce coating adhesion.
• the purpose of the zinc phosphate crystals is to passivate the substrate and give irregular molecular sites to which the powder coating can mechanically bond.
• Step 5- is a water rinse which is necessary to remove excess zinc phosphate and any water soluble salts (chlorides, sulfates, or nitrates) that may be on the surface of the substrate. These water soluble salts have to be removed from the surface or they will reduce adhesion of the coating.
• water soluble salts chlorides, sulfates, or nitrates
• Step 6 - The sixth step is an acidified rinse using chromium compounds such as chronic acid.
• the purpose of this rinse is to remove the less soluble salts remaining from the water rinse in step 5.
• the chromium compound is used to deposit an additional barrier coat and to give the substrate some added corrosion protection.
• the chrome also fills some of the pores which exist in the zinc phosphate crystal film, thereby enhancing the passivation of the metal while providing additional molecular bonding sites for the powder coating.
• Step 7 - The seventh step of the pretreatment process is a water rinse whose purpose is to remove any foreign salts or minerals.
• Step 8 - The last step involves thoroughly drying the galvanized metal by the application of heat.
• This invention comprises a pretreatment process, particularly a four-step pretreatment process for galvanized metal prior to its coating with an organic powder.
• This pretreatment process was specifically developed for application with a galvanized metal substrate, which does not require passivation to assure protection of the steel from corrosion.
• the first step in the preparation of the galvanized metal for coating involves cleaning and acid etching to assure a clean and oxide-free surface. This is accomplished through the use of a typical biodegradable solution (liquid acid solution) containing phosphoric acid, solvents and surfactants and which is obtainable commercially in various formulations from, for example, Oakite Products, Inc., in Berkeley Heights, New Jersey. It is formulated for removing light grease, shop dirt, welding fluxes, oxides, and mill scale from the galvanized metal and in general microscopically etches the metal prior to organic powder coating.
• the cleansing and etching agent contains phosphoric acid, a small amount of zinc phosphate crystals, resulting from the chemical reaction of the phosphoric acid with the zinc, are randomly distributed over the surface substrate of the metal.
• a 3-20% solution by volume of this acid compound at 110-150 0 is used.
• the exact temperature and concentration must be adjusted to achieve thorough cleaning with minimal attack of the zinc.
• the solution is usually contained in a vat which is large enough so that the entire piece of galvanized metal can be dipped therein. It has been found that the minimum dipping time in this solution is 11 ⁇ 2 to 3) minutes with more time needed if the galvanized metal is heavily contaminated with dirt or oil or has been treated with special rinses by the metal vendor. Sometimes, instead of dipping, the cleaning and etching solution is sprayed onto the galvanized metal.
• the second step involves a water rinse which is required to remove excess cleaning agent from step 1 plus any salts, such as chlorides, nitrates or sulfates which may be on the surface of the substrate.
• any salts such as chlorides, nitrates or sulfates which may be on the surface of the substrate.
• tap water is used, but in areas where it contains high mineral levels, deionized water may be more suitable.
• Water used in this step becomes slightly contaminated with the acidic cleansing agent used in step 3 as a result of carry-over of the cleansing agent on the surface of the galvanized steel.
• Step 3 also involves a water rinse. As stated previously, this water rinse is required to assure that all of the dilute cleaning agent, plus any salts and minerals remaining on the surface following step 2, are removed.
• step 4 the treated galvanized metal must be dried prior to its coating with the organic powder. This drying is usually done by inserting the pretreated and rinsed galvanized metal into an oven at 130°-400°F (54°-204°C) for approximately 2 to 10 minutes so that it thoroughly dries. A preferred typical condition would be inserting the pretreated and galvanized metal in the oven at 250°F (121°C) for approximately 5 minutes.
• the galvanized metal Upon completion of the four-step pretreatment process, the galvanized metal must be powder coated within a short period of time or at least before any amount of zinc oxide develops on the galvanized metal as a result of exposure to the atmosphere of other oxidizing agents. This is so because unlike conventional pretreatment systems, no passivation coating is deposited in the instant invention. Ideally, then the galvanized metal should be powder coated immediately after it has been dried. As a practical matter, however, due to equipment limitations and location, manpower requirements, moving needs, and the like, it has been found that the time lag between drying (step 4) and powder coating of the galvanized metal can range from about 5 minutes to 6 hours with an average time lag of about 10 minutes.
• the coating to be used is a typical organic powder such as, for example, epoxy, polyester, acrylics, or hybrids which are mixtures of polyester and epoxy powders.
• a typical organic powder such as, for example, epoxy, polyester, acrylics, or hybrids which are mixtures of polyester and epoxy powders.
• epoxy coating powders such as Pulvalure @ , Scotchkote @ , Corvel O , or Vedoc O , which are manufactured by and available from various suppliers.
• These typical epoxy coating powders are hemogenous, melt-mixed, 100% solids designed for application to metals.
• the organic powder coating is applied typically by electrostatic spray although it can also be applied by dipping the treated, rinsed, and dried galvanized metal into a fluidized bed of this powder.
• electrostatic spray the powder issues from a gun which imparts it with an electrically positive charge. Since the galvanized metal is negatively charged, the powder is attracted and caused to adhere to the metal.
• the particular thickness of the coating is not critical, although a typical thickness which has been found to be ideal is .004 inches (0.1) millimeters ⁇ .001 inches (.0254 millimeters). This is the approximate thickness which is deposited on the pretreated, rinsed, and dried galvanized metal before putting it in an oven for curing. Proper curing involves baking the powder coated metal in an oven at approximately 300°-550°F (149°-288°C) for about 1-20 minutes. Temperatures and curing times are usually given by the manufacturer of the organic powder used.

Landscapes

• Life Sciences & Earth Sciences (AREA)
• Engineering & Computer Science (AREA)
• Wood Science & Technology (AREA)
• Chemical Treatment Of Metals (AREA)
• Other Surface Treatments For Metallic Materials (AREA)
• Superconductors And Manufacturing Methods Therefor (AREA)
• Electrical Discharge Machining, Electrochemical Machining, And Combined Machining (AREA)
• Laminated Bodies (AREA)
• Cleaning And De-Greasing Of Metallic Materials By Chemical Methods (AREA)
• ing And Chemical Polishing (AREA)
• Coating With Molten Metal (AREA)
• Application Of Or Painting With Fluid Materials (AREA)

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Applications Claiming Priority (2)

Publications (3)

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• 1981
  • 1981-03-31 IE IE735/81A patent/IE51072B1/en unknown
  • 1981-03-31 AT AT81400514T patent/ATE15775T1/de active
  • 1981-03-31 EP EP81400514A patent/EP0057334B1/en not_active Expired
  • 1981-03-31 DE DE8181400514T patent/DE3172403D1/de not_active Expired
  • 1981-03-31 BR BR8101935A patent/BR8101935A/pt unknown
  • 1981-03-31 ES ES500918A patent/ES8205884A1/es not_active Expired
  • 1981-03-31 DK DK145181A patent/DK156445C/da not_active IP Right Cessation
  • 1981-04-01 GR GR64548A patent/GR75616B/el unknown
  • 1981-04-01 MX MX186672A patent/MX155753A/es unknown
  • 1981-04-01 PT PT72784A patent/PT72784B/pt unknown
  • 1981-04-01 JP JP56047326A patent/JPS57116777A/ja active Granted
  • 1981-04-01 AU AU68987/81A patent/AU548696B2/en not_active Ceased
  • 1981-04-01 ZA ZA00812177A patent/ZA812177B/xx unknown
  • 1981-04-03 CA CA000374660A patent/CA1185841A/en not_active Expired

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