JPH03174478A - Method for maintaining coating effect of self-depositing composition - Google Patents

Abstract

PURPOSE:To coat effectively a plurality of metal surfaces with high efficiency by immersing metal surfaces in a self-depositing composition obtained by dispers ing an organic film-forming solid in water, withdrawing a part of a water-solu ble liquid, and centrifuging the liquid to separate into a plurality of liquids. CONSTITUTION:Metal surfaces are immersed in a self-depositing composition obtained by dispersing 1-10% organic film-forming solid which comprises a copolymer of 55-99wt.% vinylidene chloride, 0.5-30wt.% hydrophilic ethylenically unsaturated monomer and 0.1-5wt.% copolymerizable ionic material in water for 30sec to 3min. When a plurality of materials which affect adversely the coating effect of the composition accumulate in the dissolved form in the compo sition, a part of the aqueous solution is withdrawn, flowed into a bowl 12 via an inlet pipe 14 of a centrifugal separator 10 which rotates at 7,200-7,500rpm. A dispersing means 16 is flowed downwardly to spread between multi-layered separating discs 18, thereby separating a resin solid to accumulate in a solid space 20. On the other hand, the liquid is flowed upwardly via a center part of the discs 18 and discharged from an outlet pipe 22 and the resin solid which has accumulated in the space 20 is recovered.

Description

Detailed Description of the Invention [Industrial Application Field] The present invention relates to a method for coating metal surfaces using an autodeposited m-oxide, and in particular to a method that can efficiently and effectively coat a plurality of metal products. .

Autodeposition compositions are aqueous coating compositions that contain as essential components organic film-forming solids dispersed in water and active agents in solution. The organic film-forming solids are, for example, resin solids such as those forming latex.

An activator consists of one or more types of components that dissolve a sufficient amount of metal ions from the metal surface to be coated and directly or indirectly remove the film-forming solids near the metal surface. It has the function of continuously depositing on metal surfaces regardless of the type of substance.

Commonly used activators include ferric fluoride and hydrofluoric acid.

Autodeposition compositions are characterized by their ability to form a film on an immersed metal surface that increases in thickness or weight the longer the metal surface is immersed therein. This coating method is often referred to as "autodeposition" and is similar to electrodeposition (electrophoresis), but does not require the use of external electrical current in operation. The autodeposition coating has the function of protecting the underlying metal surface from corrosion and/or imparting a good appearance to the coated product.

It is well known that continuous use of autodepositing compositions tends to cause the composition to lose its potency, even if the components consumed with use of the composition are replenished. be. Loss of effectiveness manifests itself in a variety of ways. For example, the adhesion strength of a freshly formed wet film on a metal surface tends to decrease if &Il or oxide is used continuously. After a coating bath has been used for an extended period of time, the coating formed in the bath tends to flake off. Furthermore, when using such a composition, the rate of film formation tends to gradually decrease. Furthermore, continued use of the composition may result in loss of stability of the composition.

These phenomena can be seen from the fact that the dispersed solids initially coagulate, but as a result of the coating operation, they gradually coagulate or the entire composition tends to gel. In other words, if the composition is used for a long period of time, even if sufficient replenishment is performed, the composition becomes unusable.

If this composition becomes unusable, not only that, but it may also become impossible to regenerate, so the entire amount must be discarded. This is a very severe problem in industrial operations where baths containing as much as 10,000 to 20,000 kg of autodeposition composition may be used.

What we have learned so far is that there are several factors that cause the aforementioned problems. One such factor is the accumulation of polyvalent metal ions in the composition. These polyvalent metal ions are eluted from the metal surface during coating. Such ions tend to cause the undesirable phenomenon of flocculation of dispersed solids. Other factors include -
This includes accumulation of valence ions (eg, sodium, potassium ions, etc.) in the composition. These -valent ions act as counterions for the negatively charged polymers that make up the film-forming solids in the bath.

The accumulation of counterions in the composition has the effect of increasing the stability of the dispersed film-forming solids present at the metal interface, which increases the rate of precipitation of the film-forming solids on the metal surface and increases the stability of the newly formed solids. The results of reducing the adhesive properties of the film are presented. Sources of such -valent cations include additives that contribute to the components making up the composition, such as buffers, surfactants, and latex used as a source of film-forming resin solids. Examples include those contained in existing polymerization initiators.

Additionally, -valent cations can be used to pre-treat metal surfaces, e.g.
They may also be introduced into the composition as a result of treatment with chemical cleaning compositions.

The present invention relates to an improved means of maintaining the efficacy of autodeposition compositions by removing materials from the composition that have an adverse effect on the composition.

[Prior Art and Problems] A large number of methods have been proposed for treating autodeposition compositions to maintain their efficacy during continuous use. Such proposals fall into two categories.

In other words, there are "chemical treatment" and "non-chemical treatment" of the autodeposition composition.

Methods involving chemical treatments are far more numerous than non-chemical treatments. i.e. U.S. Patent No. 3,791,431
According to the specification, excess metal ions eluting from the metal surface being coated and accumulating in the composition (e.g.
It is disclosed that it should be removed from the composition (by precipitating it) or converted into a harmless form (eg, by encapsulating it in the composition using a chelating agent). Phosphoric acid and ethylenediaminetetraacetic acid have been shown as precipitants and chelating agents, respectively, that can be used when coating steel. Canadian Patent No. 10
No. 75,982 discloses the use of masking agents to counteract the deleterious effects likely to be caused by dissolved metals accumulating in autodeposited compositions. The use of potassium iodide to mask the deleterious effects that tend to be caused by the accumulation of copper ions is given as an example. The use of ion exchange agents to remove unwanted metal ions from autodeposition compositions is described in U.S. Pat. No. 3,839.
, 097. Further, regarding ion exchange agents used to remove unnecessary cations and/or unnecessary anions from acidic aqueous coating compositions containing dispersed resin solids, U.S. Pat.
See specification No. 743. U.S. Patent No. 4.186
, No. 219 discloses that an oxidizing agent having the function of oxidizing metal ions dissolved from the metal surface being coated to a higher valence state is selectively added to the autodeposition composition. is disclosed. In a preferred embodiment for coating steel surfaces, the autodeposition composition containing an activator based on FeFs/HF is supplemented with superposition for the purpose of oxidizing ferrous ions to ferric ions. Hydrogen oxide is added in the required amount. This patent teaches that excess ferrous ion is a troublesome and harmful ion, so when oxidized with peroxide to ferric ion, it combines with the fluoride component of the composition. Forms harmless complexes.

According to U.S. Pat. No. 4,414,350, certain types of carboxylic acids are used to complex excess ferrous ions in autodeposition compositions used to coat ferrous surfaces. Discloses that it will be used.

U.S. Patent Nos. 4,229,492 and 4,357,3
No. 72 discloses that when autodeposition compositions are used, the surfactants used with the dispersed solids can be chemically modified during the coating process and become harmful. There is. Nos. 4.229.492 and 4.357 above.
No. 372 discloses that modified surfactants can be removed from the bath by solvent extraction or sequestered by soluteization within surfactant micelles or yocell substitutes. ing. Special methods of adding supplementary ingredients into the compositions are also disclosed in these above-mentioned patents.

There are also drawbacks to the chemical treatment methods described above. Precipitation of excess metal ions creates sludge in the autodeposition composition bath, which must be removed from the bath in a timely manner. Also, the use of ion exchange agents to remove metal ions is relatively costly and impractical, and the use of oxidizing agents to oxidize dissolved metal ions to such high valences that complexes are obtained can result in excess amount of activator (ferric fluoride) is formed, which in turn results in an excessive increase in the rate of dissolution of the metal surface. Therefore, if the composition is used continuously, for example, a portion of the composition may be discarded,
It becomes necessary to replenish components that do not contain ferric ions.

The use of ferric ion complexing agents, as well as the selective use of oxidizing agents, are completely ineffective in addressing the problems that arise associated with the accumulation of monovalent counterions in the composition. . Some improved surfactant approaches remove such counterions or excessive amounts of troublesome polyvalent cations.
l It is completely ineffective in eliminating deleterious substances.

Regarding the use of other processing methods to address the aforementioned problems, see U.S. Pat. The specification of No. 1 is cited as a reference. These publications discuss the use of electrodeposition and ultrafiltration using microporous membranes. The deposition includes dipping a negatively charged electrode and a positively charged electrode into the composition. Dissolved monovalent and polyvalent cations can be removed from the liquid phase of the composition by being attracted to and precipitating on the negatively charged electrode. However, the resins most widely used in autodeposition are also negatively charged, which poses problems for electrodeposition. This is because such a resin is undesirable because it is attracted to positively charged electrodes. The use of separation membranes to protect positively charged electrodes from dispersed resin solids has been ineffective until now because the membranes become clogged relatively quickly by the resin that accumulates on them. There wasn't. Blockage is also a problem encountered when using ultrafiltration to separate dissolved and dispersed resin solids in autodeposition compositions.

The present invention provides an improved means of separating dissolved solids and dispersed resin solids of an autodeposition composition, wherein the dispersed solids content is reduced as can be achieved during ultrafiltration. is circulated into the autodepositing composition bath, most or all of the problems encountered when using ultrafiltration are avoided.

[Means for Solving the Problems] The method of the present invention will be described below. An autodeposition film is formed on the metal surface by immersing the metal surface in a bath of an aqueous autodeposition composition in which organic film-forming solids are dispersed, and the coating effectiveness of the composition increases while the bath is in use. a bath solution of an aqueous solution of said composition containing a predetermined ratio of dispersed organic film-forming solids to the dissolved substance(s) when one or more substances that have an adverse effect accumulate in a dissolved state; In the method of extracting a part of the bath liquid from the bath, treating it and returning it again to the bath, the following improvements can be made, namely, centrifuging the extracted part of the bath liquid to divide it into a plurality of parts, and at least one divided part. An improved method is to return the dispersed solids to the dissolved material to the bath at a ratio greater than a predetermined ratio.

More preferred embodiments of the present invention will be described below. First, a part of the autodeposition composition bath supplied to the centrifugal separator is separated into a liquid containing almost no dispersed solids and a liquid in which the dispersed solids are highly concentrated, and the former liquid is discharged from the system. The method of the present invention is used to maintain the coating efficacy of coating compositions containing ferric ions and hydrofluoric acid as activators, a pigment dispersion, and resin solids. The most preferred resins are those containing copolymers containing high proportions of vinylidene chloride, as described below.

The compositions used in the present invention are then used to coat iron surfaces, and the ratio of ferric to ferrous ions in the composition is as described in the aforementioned US Pat. No. 4,186,219. The process of the invention also includes a continuous process, for example by adding hydrogen peroxide to the composition to convert ferrous ions to ferric ions using the means described in the specification. ,
In this method, a portion of the autodeposition composition bath is continuously fed to a continuously operating centrifuge to separate the dispersed solids from the bath liquid and return them to the bath liquid. The ratio of dispersed solids/dissolved substances is continuously maintained at a predetermined value.

The advantages obtained by using the present invention are enormous. That is, the present invention makes it possible to recycle the dispersed solid content, particularly the resin solid content, which is the most expensive liquid component among the components of the autodeposition composition, and to remove dissolved substances that cause coating problems from the composition. The coating effect of the bath liquid can be maintained by appropriately separating and extracting it and discharging it outside the system.

Now, the autodepositing m-acid used in the present invention is an aqueous composition containing an organic film-forming solid content dispersed in the composition and dissolving metal from the metal substrate to be coated. Functional dispersed organic film-forming solids are typically resin solids, but may also include other materials, such as the waxes described in U.S. Pat. No. 3,776,848. Film-forming lubricants may also be included. Examples of the various types of resins that can be used to formulate autodeposition compositions are provided in U.S. Pat.
No. 85.084, No. 3.709.743, No. 4.19
No. 1.676, No. 4.347,172, and No. 4.6
No. 57.788. Autodeposition compositions include latexes, ie, solid resins prepared by, for example, emulsion polymerization, dispersed in water.

For use in the present invention, the specific gravity of the resin solids should be greater than 1, such as at least about 1.05, and more preferably at least about 1.5.

If the specific gravity of the resin solids is not sufficiently greater than the water of the liquid medium in which the resin solids are dispersed, the solids will not be separated satisfactorily from the liquid medium.

The resin solids typically have ionically charged groups, which are believed to be the source of the unique ability to form autodeposition films. See, eg, US Pat. No. 4,191,676). The ionic-charged groups can be part of the structure of the surfactant molecules adsorbed to the surface of the resin particle (externally stabilized resin) or part of the chemical structure of the resin particle (internally stabilized resin). Sometimes there are. This group typically carries a negative charge.

A preferred type of acrylic resin used for autodeposition is that described in US Pat. No. 4,31,861. A particularly preferred type of resin used for autodeposition is one based on vinylidene chloride, as described in US Pat. No. 4,562,098. This resin is
It may be an externally stable vinylidene chloride copolymer containing mostly vinylidene chloride or an internally stable resinized vinylidene copolymer. Among these, internally stable copolymers are particularly preferred.

Most preferred are those capable of forming essentially crystalline resin films. For this purpose, the resin should be used in a relatively high proportion, for example at least 80% by weight.
It is made from vinylidene chloride. Autodeposition films prepared from vinylidene chloride copolymers have very high corrosion resistance, so there is no need for pretreatment with an aqueous solution of chromium compounds, and there is no need for post-treatment to improve the corrosion resistance of the autodeposition film. Nor. Furthermore, such coatings can be cured at relatively low temperatures, have extremely high hardness, excellent solvent resistance, and have very high adhesion and bonding properties even immediately after coating formation.

Autodeposition compositions containing said vinylidene chloride copolymers are described in US Pat. No. 4,562,098, the contents of which are incorporated by reference regarding such copolymers. One of the internally stable resins includes the following polymeric components, and the components are the following three types.

(1) about 55% to about 99% by weight of vinylidene chloride monomer (based on the total weight of monomers used); (2) about 0.5% to about 99% by weight of a second relatively hydrophilic ethylenically unsaturated monomer; about 30% by weight (based on the total weight of materials (1) and (2)), the monomeric material having a solubility in both water and oil of the polymer latex of at least 1% by weight at the polymerization temperature; .

(3) 0.1 to about 5% by weight (based on the total weight of other monomers) of a material that is polymerizable with (2) above, is ionic, and has high solubility in water. This material has the formula: %Formula %) (wherein R is selected from the group consisting of vinyl and substituted vinyl, such as alkyl-substituted vinyl, and rZJ represents the double bond in the vinyl or substituted vinyl group. indicates a trifunctional bonding group to be activated, 'QJ is a divalent hydrocarbon,
The bonds are attached to different carbon atoms,
and "M"" indicates a cation), and is selected from sulfonic acids and salts thereof.

Examples of preferred hydrophilic monomers for the above substance (2), particularly when used together with vinylidene chloride monomer, are methacrylic acid and methyl methacrylate. Other monomers which may be advantageously employed include hydroxyethyl and propyl acrylates, hydroxyethyl methacrylate, ethylhexyl acrylate, acrylic acid, acrylonitrile, methacrylatrile, acrylamide,
and lower alkyl and dialkylamides, acrolein, methyl vinyl ketone, and vinyl acetate.

"Z" which activates a double bond present in a vinyl or substituted vinyl group includes structures such as C--C-0-, -0-C--C-N-, and the like. Preferably, the alkyl group is alkyl of 1 to about 8 carbon atoms, especially methyl, ethyl, propyl. Examples of divalent hydrocarbon 'QJ' in which the bonds are attached to different carbon atoms include alkylene and arylene divalent hydrocarbon radicals. The maximum number of alkylene (CH2) groups is approximately 20
carbon atoms, but generally from 1 to about 8 carbon atoms.

The solubility of the copolymerizable ionic materials described herein is:
Strongly affected by cation M+. Examples of cations are free acids, alkali metal salts, ammonium and amine salts, and sulfonium and tertiary ammonium salts. Preference is given to free acids, alkali metal salts, especially sodium and potassium salts, and ammonium salts.

Sodium sulfoalkyl methacrylate of the formula: % where n is 2 is a highly preferred copolymerizable ionic material for use in the preparation of the above copolymers. Sodium sulfoethyl methacrylate is particularly effective when used in the amounts and manner described above with vinylidene chloride monomer and the relatively hydrophilic monomer methyl methacrylate or methacrylic acid. .

The amount of resin that makes up the autodeposition composition can vary within a wide range. For many applications, about 25 to
It is possible to obtain good results using resin solids contents of about 100 g/f.

Examples of activators that can be used in formulating autodeposition compositions include U.S. Pat.
No. 103,049 and No. 4.554.305. Although the aforementioned patents disclose autodeposition compositions that are acidic, autodeposition compositions that are alkaline, as described, for example, in U.S. Pat. It is also known to formulate autodeposition compositions.

Preferred activators for use in formulating autodeposition compositions include:
Examples include ferric ion and hydrofluoric acid, as described in U.S. Pat. No. 4,347,172. Briefly, this patent discloses that the ferric compound and the hydrofluoric acid are combined, for example, from about 0.025 to about 3.5 g of ferric ion/
i, most preferably from about 0.3 to about 1.
6g/j! and an amount of hydrofluoric acid sufficient to provide the composition with a po in the range of about 1.6 to about 5. be. Examples of the aforementioned ferric compounds include ferric nitrate, ferric chloride, ferric phosphate, ferric acid, and ferric fluoride, with ferric fluoride being particularly preferred. .

Other substances can also be selectively added to the composition as desired. For example, the range of use of the present invention can be widened most by forming an autodeposition composition containing a pigment. For this purpose, for example, pigments in water-dispersed form are applied.

The specific gravity of the pigment may be greater or less than 1 and is therefore not a factor in practicing the invention.

When it is greater than 1, the pigment will separate from the liquid medium, similar to the separation of resin solids. When the specific gravity of the pigment is less than 1, its separation does not occur, but this does not have serious consequences. Since the amount of pigment lost should be small, replenishment can be done at low cost.

Aqueous dispersions of pigments, such as aqueous dispersions of resin solids, include surfactants or dispersants to keep the particles dispersed. When using such dispersions to formulate autodepositing compositions, the surfactant concentration in the composition bath is preferably below the critical micelle concentration, preferably within the range of surface tension and surfactant concentration in the composition. The surfactant concentration is preferably below the inflection point in the graph of the logarithm of
(Please refer to the specification).

Other additives that can be used in the autodeposition composition used in the present invention include those commonly applied to paints, such as UV stabilizers and viscosity modifiers.

The coating process of the present invention may for the most part be similar to prior art processes. For example, the cleaning of the metal surface prior to coating and the subsequent water rinsing step may be performed in accordance with the teachings of the aforementioned U.S. Pat. No. 4,191,676. As noted above, such a cleaning step introduces dissolved substances into the coating composition that do not essentially participate in film formation, and during subsequent use of the composition, such substances may be introduced into the coating composition. It accumulates. These sources of essentially unrelated substances may be due to carry-overs from inadequate cleaning methods or from the final wash water if it was not deionized. This is due to the fact that they were brought in.

Examples of these substances include sodium, potassium, and ammonium salts of carbonic acid, phosphoric acid, etc. dissolved in water.

With respect to contacting the metal surface with the autodeposition composition, the desired coating thickness is typically obtained by immersing the metal surface in the composition for a period of time ranging from about 30 seconds to about 3 minutes or less. Good results can be consistently obtained using preferred autodeposition compositions containing from about 1 to about 10 weight percent resin solids and soaking times from about 60 seconds to about 180 seconds.

When using a bath of an autodeposition composition, the amount of film-forming solids and pigment (if used) that are dispersed decreases. In continuous coating operations it is necessary to replenish the bath with such materials. On the other hand, such replenishment can be a source of accumulation of dissolved substances that are not consumed during the coating process and are not carried away with the coated parts, which can adversely affect the coating process. For example, latex is commonly used to replenish the film-forming solid dispersion that is consumed once the film is formed. The latex contains additives such as buffers, remnants of the initiator used in the polymerization of the resin, and surfactants. Examples of such additives are trisodium phosphate, ammonium citrate, potassium persulfate, sodium alkylbenzenesulfonates.

What needs to be understood here is that with replenishment, such dissolved substances can increase in concentration by a factor of 2 to 10 between two consecutive runs unless removed from the bath. . As mentioned above, monovalent cations such as natrium and potassium form counterions that inhibit film formation and are factors that adversely affect the adhesion of newly formed films to metal substrates. . Also, since the pigments added to the bath are usually applied in the form of an aqueous dispersion, this can also be a source of dissolved substances.

To maintain the effectiveness of the coating composition in a continuous process, it is important to include an activator at a preferred concentration; Need to pay attention. When using such compositions to coat iron surfaces, hydrofluoric acid is consumed and must be replenished. HF
The concentration of HF to be added can be conveniently measured using an IJNEGtlARD meter 101.
The amount of O can be added either intermittently or continuously as required.

On the other hand, ferric fluoride components do not necessarily need to be supplemented.
As disclosed in the aforementioned US Pat. No. 4,186,219, ferrous iron is dissolved from the iron surface by the chemical action of the composition on the surface. Unless steps are taken to control the accumulation of excess ferrous iron in the composition, coating operations will be adversely affected by the presence of ferrous iron. No. 4,186,219, cited above, discloses that ferrous iron can be oxidized to ferric iron by optionally adding an oxidizing agent, such as hydrogen peroxide, to the composition. It is disclosed that it is possible to control the amount.

The proportion of ferrous iron in the composition can be monitored by recording the redox potential of the composition. Redox potential is a measure of the ratio of ferrous to ferric iron. measuring the redox potential of a satisfactorily operated composition, adding an oxidizing agent such as hydrogen peroxide to the composition as needed;
Prevent the redox potential from decreasing. however,
If ferric iron is generated in the bath by the above addition, the amount of ferric iron will be excessive, and this will increase the dissolution rate of the iron surface, making it necessary to add more oxidizing agent. More ferric fluoride will be produced. To break this vicious cycle, it is necessary to remove ferric fluoride from the bath. However, since ferric fluoride is a component in the coating operation, care must be taken to avoid excessive removal of this component. This consists in measuring the total amount of iron in a satisfactorily operating bath and removing the excess amount in accordance with the present invention to maintain the total iron concentration at a predetermined level, while at the same time keeping the total iron concentration at a predetermined level. This can be achieved by maintaining the desired redox potential of .

In order to carry out the invention in a preferred form, the bath liquid partially removed from the autodeposition composition is continuously separated into two sections: a dispersed solid content and a liquid containing substantially no dispersed solid content. Use a capable centrifuge. Examples of centrifuges capable of performing such functions are illustrated in FIGS. 1 and 2. A commercially available centrifuge of this type is manufactured by Alfa-Laval.
Inc.), and is known as a multiple separation disc type centrifuge for two-phase clarification separation.

In operation, the portion taken out from the autodeposition composition bath liquid is pumped into a centrifuge 1 in the form of a flow from the composition bath.
0, the flow enters the bowl portion 12 of the centrifuge through the inlet tube 14. The composition flows downwardly into the dispersing means 16, towards the multiple separation discs, and spreads between the discs 18. The centrifugal force generated by the rotation of the bowl 12 pulls the solids, such as resin solids, in which &lI compound acid is dispersed. The solids separated in this way accumulate in the solids space 20 of the bowl in loosely packed form (sludge form). The liquid, from which substantially all of the solids have been separated, migrates to the central portion of the multi-disk and flows upwardly into the outlet tube 22, from where it exits the centrifuge. The ratio of dissolved material to dispersed solids in such an effluent is much greater than the ratio in the bath liquid, eg, at least about twice as large.

The centrifugal force required to effect the separation described above will vary depending on a number of factors, such as the difference in specific gravity between the liquid medium and the dispersed solids and the degree of stabilizing effect of the surfactant on the dispersed solids. Approximately 7200~750Or with this type of centrifugal M machine
The autodeposition composition liquid can be effectively treated within the pm range. In this case, the autodeposition composition isotherm has a specific gravity of about 1.
05 to about 1.75, and up to about 0.5 weight percent of pigment, having a specific gravity of about 1 to about 1.8. With such a rotation speed, approximately 1
A force of 1,600 to 12,600 g is generated.

Loosely packed solids accumulate in the solids space 20 of the bowl and are then removed therefrom without interfering with the flow of the autodeposition composition liquid to the centrifuge. Removing accumulated solids from the solids chamber 20 is known in the art as "chuting from the bowl." Referring to FIG. 2, this is accomplished by releasing the hydraulic pressure that normally seals the bowl bottom 30 to the top of the bowl. When this is done, centrifugal force forces the loosely packed solids (sludgy solids) and some of the liquid
The bowl 12 is drained circumferentially and outwardly through openings 32 at the periphery of the bowl 12. Shooting from the bowl can occur in a very short time, eg, a fraction of a second to two seconds, after which the bottom of the bowl is resealed to the top of the bowl.

A shot from the bowl can be a partial shot or a complete shot.

In the case of a partial chute, only part of the contents is emptied.

This is done by resealing the bowl bottom before the time has come when all of the contents of the bowl have exited the bowl. Centrifuges of the type shown in Figures 1 and 2 are designed to have a large opening 32 so that the entire contents of the bowl can be emptied (full chute) in the same amount of time as is used in the partial chute. It is possible to modify it to

The liquid fraction leaving the centrifuge via outlet tube 22 can be discharged as waste water or treated as desired. The slurry dispersed solids removed from the centrifuge opening 32 are present in a much higher proportion than the proportion of solids originally present in the bath of autodeposition composition.

For example, the dispersed solids content, when removed from a centrifuge, can be more than one to about twice the solids originally present in the bath of autodeposition composition.

The ratio of dispersed solids to dissolved solids is also much higher than the ratio that was present in the bath, eg, more than about 1 times and even up to about 2 times.

The dispersed solids are circulated to the bath. Water is added to the dispersed moisture that has been made highly concentrated using a centrifuge, and the solid content is
For example, it can be returned to the bath liquid by bringing the solids concentration approximately close to that of the bath. Alternatively, this highly concentrated solids can be circulated undiluted to the bath in a concentrated state. Filtration is necessary in the circulating system to remove relatively large particle size solids, sludge, aggregates/impurities such as pigments and resins, etc.

In a highly preferred version B, the separation/circulation operations described above are carried out in continuous mode. That is, a portion of the composition isothermal is continuously supplied to the centrifuge.

In such operation, the dispersed solids will be periodically recycled to the bath as they are periodically discharged from the centrifuge during the bowl chute.

In the method of the present invention, a centrifugal separator is used which is capable of dividing a part of the autodeposition composition bath into a plurality of liquids having different degrees of solid content separation. In this case, at least one of the partitioned liquids has solids dispersed therein at a higher proportion than the solids dispersed in a uniform temperature. An example of a type of centrifuge that can be used to accomplish this is shown in FIG. This type of centrifuge is commercially available from Alfa Laval and is known as a Sanwa separator. The structure and basic operation of this type of centrifugal separator are similar to the centrifugal separators shown in Figures 1 and 2, but the difference is that the structure in Figure 3 is based on autodeposition composition. It has the ability to separate the liquid into two parts and one with a high solids content. This type of centrifugal separator requires a higher bath liquid supply rate than the centrifugal separators shown in FIGS. 1 and 2 for boat fishing.

See Figure 3. The autodeposited composition enters the bowl 42 of the centrifuge 40 from the port 44 which leads to the dispersion means 46. The autodeposition & Il compound oxides flow downward into the separation discs and spread between the discs 48.

The centrifugal force generated by the rotation of the bowl 42 pulls the dispersed solids outward and clogs the solids chamber 50 . The dispersed liquid containing almost no solids passes between the discs, enters the inside, moves to the center of the multiple separation disc,
It flows out of the centrifuge through outlet tube 52. The centrifugal force generated by the rotation of the bowl forms a slurry containing a relatively high concentration of dispersed solids. Such liquid flows around the top disk 48 and exits upwardly through the space between the top disk 48 and the bowl skin 54 into the outlet tube 56. The centrifugal separator shown in Figure 3 is equipped with the same means as the centrifugal separators shown in Figures 1 and 2 for removing solid matter clogged in the form of mud; This means is not shown in the figure.

The liquid discharged via outlet tube 52 is substantially free of dispersed solids, but can be discharged as waste water or otherwise treated as desired. The high solids exiting the centrifuge through outlet tube 56 and/or the solids removed from solids space 50 are recycled to the bath of autodeposition composition.

Upon chute from the bowl, the flow rate of material exiting the centrifuge is much greater than the feed flow rate into the centrifuge. The centrifuge bowl is therefore only partially filled or completely empty after the chute. It is not particularly practical to increase the flow rate of the feed pump, so it will take some time to fill the bowl.
For example, it may take about 130 seconds to fill a bowl approximately 1.1 gallons in size at a feed rate of approximately 0.5 gallons/minute. This is a disadvantage because the actual processing time per cycle is this much less.

If a centrifugal separator with a high raw material feed rate is used to compensate for the above-mentioned drawbacks, the solid separation efficiency will decrease. Therefore, after each bowl chute, it is preferable to add water to the bowl at high speed using a high speed suction pump.

Over time, solids may become stuck in the solids space of the centrifuge and cannot be removed using the bowl chute. If the volume of clogged solids increases and operation becomes difficult, it is necessary to stop the operation of the centrifuge and cleanly remove the clogged solids.

A significant advantage provided by the practice of the present invention is the ability to circulate a high proportion of resin solids into the bath of the composition, while at the same time ensuring adequate removal of dissolved material to maintain coating functionality. It is to be. By way of example, at least about 90%, and sometimes more than 95%, of the resin solids present in the composition liquid removed from the bath can be separated, recycled, and at least about 20% of the dissolved material
It is possible to remove more than % by weight.

EXAMPLES The following describes practical implementations of the present invention, in particular the use of a centrifuge to separate dispersed autodeposition composition solids.

The autodeposition composition is as follows.

-Quantity Vinylidene monochloride resin latex, 88 g Solids 54% by weight Black pigment dispersion, Solids 35.6% by weight 3.8 g
Hydrofluoric acid 10 g Difluoric acid 3 g Deionized water
The remainder of the total amount 11 The structure of the above-mentioned vinylidene chloride resin includes a sulfone base salt for the purpose of stabilization. The resin has a specific gravity of approximately 1.7. The specific gravity of the pigment is approximately 1.
It is 25.

Experience with coating steel products with this type of composition indicates that the redox potential of the composition is approximately 350 mV + 100 mV.
mV (platinum electrode and silver-saturated/silver chloride electrode).

This redox potential is maintained within the desired range by adding hydrogen peroxide to the composition in the required amount. The total iron content of an effective composition of this type should be approximately 1-3 g/l. This type of autodeposition composition 1000-50.0
Experience in coating steel products in baths containing 0.00 gallons indicates that approximately 7 to 10 gallons per square foot of metal surface being coated is required to maintain the total iron concentration at the desired operating level. It has been found that it is necessary to remove the coating composition from the bath Mi acid at a rate of .

The autodeposition composition as described above is pumped into a centrifuge of the type shown in FIGS. 1 and 2 at an average rate of about 2.4 pounds per minute over a period of about 2 hours. This centrifuge is commercially available from Alfa Laval with product identification number C) IPX-407. The capacity of the centrifuge bowl is 1.1 gallons. The total solids content of the feed composition to the centrifuge was about 4.9% by weight, with about 4.2% by weight of dispersed solids and about 0.7% by weight of dissolved solids. Weight%. The centrifuge was operated at a bowl speed of 730Q rps+ to separate most of the dispersed solids from the liquid, leaving the solids loosely packed in the form of a slurry around the bowl of the centrifuge. accumulate.

After every 4 minutes of operation, the bowl complete chute removes the accumulated solids from the bowl. Ejection takes place within 5 seconds. Because the volume of liquid leaving the bowl in less than 5 seconds is much larger than the volume of autodeposited composition introduced into the bowl by the feed pump, additional fluid is added over 14 seconds at a rate of approximately 4.7 gallons per minute. The autodeposition composition is charged into a bowl.

During a 2 hour run period, the total solids concentration of the liquid fraction exiting the centrifuge is measured at approximately 10-15 minute intervals. The average concentration of total solids in liquid rain is approximately 0.9% by weight.
The solid content is approximately 0.7% by weight dissolved. It can therefore be seen that approximately 95% of the solids dispersed in the feed liquid can be separated from the liquid and recycled to the bath of autodeposition composition. The run continues for 8 hours. At the end of this period, no solids buildup inside the bowl is observed. The liquid/solid samples obtained from the three experiments above were adjusted to a dispersed solid concentration of 5% by weight, and the total iron content was titrated. Values ranging from 0.57 to 0.83 g/fL are measured. This is the total amount of iron present in the composition, 1.
48g/j! It is compared to

Each of the aforementioned samples is used as an autodeposition composition by adding Fed and HF to the samples so that the concentrations of these respective components are about 3 g/l and about 10 g/l.

Steel panels are each coated with these compositions and then evaluated to determine the effect of centrifugation on coating performance. A fourth composition sample is prepared with fresh, "virgin" latex. This is used to coat steel panels as a reference standard. Each film on the panel is about 0
．． It has a thickness of 6 to 0.7 mil. Samples of each coated panel were subjected to a 2-hour water immersion test, a 96-hour wet test, and a 500-hour water immersion test.
Subject to time salt spray test. It is clear from these test results that there was no deterioration in coating performance due to centrifugation, and that the performance of all 4° coated samples, including the control sample, was
They are virtually identical in this series of tests.

In summary, the present invention provides an improved means for maintaining the efficacy of autodeposition compositions.

[Brief explanation of the drawing]

FIG. 1 is a cutaway elevational view of a centrifugal separator of an actual usable type according to the present invention. FIG. 2 is similar to FIG. 1, but shows the centrifuge of FIG. 1 in the state of discharging the bowl. FIG. 3 is similar to FIG. 1, but shows a different type of centrifuge that can be used in practice according to the invention. 10.40... Centrifugal separator, 12.42... Bowl, 14.44... Inlet pipe, 16.46... Dispersion means, 18.48... Separation disk, 20.50. ...Solid content chamber, 22.52.56... Outlet pipe, 30... Bowl bottom, 32... Opening, 54... Bowl outer plate.

Claims (1)

[Scope of Claims] 1. During the formation of an autodeposition film by immersing a metal surface in a bath of an autodeposition composition containing organic film-forming solids dispersed in an aqueous phase, the coating effectiveness of the composition is reduced. If one or more substances that have an adverse effect accumulate in the composition in a dissolved state, the use of said composition containing a predetermined ratio of dispersed organic film-forming solids to said dissolved substance(s). A method in which a part of the aqueous solution is extracted from the bath, treated, and returned to the bath, wherein the extracted bath liquid is centrifuged and divided into a plurality of liquids, and at least one of the divided liquids is dissolved. A method of maintaining the coating efficacy of an autodeposition composition in which the ratio of organic film-forming dispersed solids to the substance is higher than the predetermined ratio, and a liquid containing this high proportion of dispersed solids is returned to the bath liquid. . 2. The autodeposition composition bath contains a vinylidene chloride resin copolymer as a dispersed solid content, ferric iron and HF as activators, and the metal to be coated is iron. The method described in 1. 3. the composition bath dissolves iron surfaces to produce ferrous iron;
After adding an oxidizing agent to oxidize the ferrous iron to ferric iron, a portion of the bath liquid is separated using a centrifuge until the bath liquid contains almost no dispersed solids. 3. The method of claim 2, wherein the ferric iron concentration in the composition is maintained at a predetermined level by discarding the bath solution.