HK1161285B - Quaternary ammonium salts as a conversion coating or as anticorrosive additive in paints - Google Patents

Description

Quaternary ammonium salts as conversion coatings or as anticorrosion additives in paints

The patent application of the invention is a divisional application of the invention patent application with the international application date of 9/12/2005, the international application number of PCT/EP2005/013191, the application number of 200580046500.8 entering the Chinese national stage and the invention name of quaternary ammonium salt as a conversion coating or an anti-corrosion additive in paint.

Technical Field

The invention relates to the use of quaternary amines containing non-halogen anions such as carbonate, bicarbonate, phosphate and glycolate as active ingredients for conversion coatings and as additives for paints and coatings.

Background

There is always a risk of corrosion during the process in which the metal surface will come into contact with water, whether liquid water or moist air. This problem is particularly acute when the metal itself has a tendency to corrode and is not coated.

Examples of metals that have a tendency to corrode are found in stamped metal automotive parts made from ferrous alloys, abrasive surfaces such as machined steel parts, and machine components made from cast iron.

Conversion coatings are used to inhibit corrosion and ensure good adhesion of the paint to the metal surface (especially to steel). Bare steel does not form a good bond with organic coatings. To improve the adhesion of metal/organic coatings, chromate or phosphate conversion coatings chemically react with steel to form a rough surface structure with a thin metal-phosphate layer that provides mechanical and chemical interlocking (adhesion) of the organic polymer to the metal surface. See, e.g., Watson, j. "a refresher: underwantangprediction ", Powder Coating 1996, 7(3) and Ferguson, d.; monzyk, B., "Nonpoluting replacement for chromatography conversion and zinc phosphor Powder coatings", Powder Coating 2001, 12 (7). Here and hereinafter, conversion coating is understood to mean a protective barrier film or layer on the surface of a metal substrate which has both corrosion inhibition and paint adhesion capabilities and comprises: (i) an organic thin film, (ii) an inorganic oxide obtained from a metal etching and redeposition process, (iii) an inorganic oxide obtained from a true deposition process, or (iv) any mixture of inorganic and organic materials from (i), (ii) and (iii). The most widely used conversion coatings are chromating, phosphochromating and zinc phosphate treatment. It is believed that chromate treatment produces better corrosion inhibition, while phosphochromate treatment produces better paint adhesion. Prior to applying the conversion coating to the metal surface, the metal surface must be cleaned to remove any grease on the metal surface. The conversion coating may then be applied by dip coating or spray coating. Chromate and phosphochromate conversion coatings are the most common chemical conversion coatings, but other types of conversion coatings are needed for health, safety and environmental considerations.

Summary of The Invention

It has been found that quaternary ammonium salts containing non-halogen anions such as bicarbonate, carbonate, phosphate and glycolate, more specifically dialkyl dimethyl ammonium salts containing bicarbonate, carbonate, phosphate and glycolate as the anion, and various combinations thereof, have applicability as conversion coatings for metal surfaces, particularly steel surfaces. In addition, it has been found that the quaternary ammonium salts are capable of producing the desired surface transformations, such as the desired surface oxides (i.e., magnetite, Fe)₃O₄) Growth on a steel substrate. The formation of this oxide produces distinct microcrystalline regions of adhesion to the metal substrate to which coatings such as paints are expected to exhibit improved adhesion. In addition, the quaternary ammonium salts can be added directly to paints to provide better adhesion of the paint to the metal surface, thereby improving corrosion resistance.

The corrosion inhibiting properties of quaternary ammonium carbonates and bicarbonates have been disclosed in co-pending U.S. patent application No. 10/810279 (US 2005/0003978a 1). In addition, it has been found that the desired surface oxides (i.e., magnetite, Fe) grow on the protected steel surface under the inhibitor film₃O₄). The formation of this oxide creates areas for which organic coatings such as paints are expected to exhibit improved adhesion.

Detailed Description

It has surprisingly been found that certain quaternary ammonium salts containing non-halogen anions, such as di-n-decyldimethylammonium carbonate, when applied to the surface of iron and iron alloys such as steel, have a tendency to produce the known desired transformations on the surface of iron and iron alloys such as steel under much milder temperature conditions than previously known. While it is known in the art that heat-induced transformation of steel surfaces at > 110 ℃, the high temperatures required to effect this transformation have heretofore limited their commercial exploitation. However, the above quaternary ammonium salts appear to be capable of producing similar transformations of the steel surface at temperatures as low as room temperature.

In addition, the unique properties of the quaternary ammonium salts described above (neat or in a formulation) allow for a one-step metal surface treatment process prior to the final coating application. It has been found that exposure of metals to their solutions will accomplish cleaning, corrosion inhibition and primer coating of the metal surface in preparation for the final coating application.

This is clearly superior to current processes which perform the following independent functions in a step-wise manner and which produce large quantities of industrial effluents which must be disposed of:

a) the raw metal is passivated with oil to prevent rapid rusting (flash rust).

b) Removing and discarding passivation oil

c) Prior to application of the final coating, the metal surface is treated with a conversion coating solution (acidic, heavy metal containing chemicals) and cured.

It has also been found that steel surfaces treated according to the invention are resistant to rapid rusting. Rapid rusting occurs when the newly formed bare steel surface is exposed to water and reacts to form a red iron hydroxide rust layer.

Coil steel (coil steel) treated in accordance with the present invention is resistant to edge contamination that occurs when moisture condenses on the coil sheet (coils) edges. At present, steel plates are sprayed with light oil before being stored or transported. The oil must be removed prior to other surface treatments or coatings.

The oxide film formed according to the invention has the potential to act as an under-paint conversion coating or primer for steel. Bare steel does not form good adhesion with organic coatings. To improve the adhesion of metal/organic coatings, chromate or phosphate conversion coatings chemically react with steel to form a rough surface structure with metal-phosphate plates that provide mechanical and chemical interlocking (adhesion) of the organic polymer to the metal surface.

It has also been found that the conversion coatings and corrosion protection paints of the invention exhibit some self-healing properties when scratched, which means that even scratched "bare" metal surfaces exhibit some passivation, possibly due to migration of quaternary ammonium salts from the surrounding coating. In some cases, a permanent oxide transition is also observed, resulting in a long-term passivation of the scratch area. This is an effect similar to the self-healing observed on chromium coated metal surfaces.

In particular, the present invention relates to a method of applying a conversion coating to a surface of a metal substrate, the method comprising the step of contacting the substrate with a composition comprising:

(a) at least one quaternary ammonium salt containing a non-halogen anion, and

(b) optionally, a solvent.

Preferably, the quaternary ammonium salt has the formula:

wherein R is¹Is optionally aryl-substituted C_1-20Alkyl radical, R²Is optionally aryl-substituted C_1-20Alkyl radical, R³And R⁴Independently of one another is C_1-4Alkyl radical, X^n-Is an anion selected from the group consisting of: hydroxyl ion, carbonate ion, bicarbonate ion, phosphate ion, phosphite ion, hypophosphite ion, nitrate ion, sulfate ion, borate ion, saturated and unsaturated acyclic C_1-20Anionic, saturated and unsaturated C of monocarboxylic acids_2-20Anions of dicarboxylic acids and anions of hydroxy-substituted carboxylic acids, n representing a suitable number of negative charges of said anions.

Here and hereinafter, C_1-20Alkyl is a straight or branched chain alkyl group having 1 to 20 carbon atoms, including, but not limited to, methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, tert-butyl, pentyl, isopentyl, hexyl, heptyl, octyl, nonyl, isononyl, decyl, dodecyl, tetradecyl, hexadecyl, octadecyl, and eicosyl (icosyl). Aryl substituted C_1-20The alkyl group is any of the above groups having an aryl group (particularly a phenyl group) as a substituent. Preferred aryl substituted C_1-20Examples of alkyl groups are benzyl, phenethyl and phenylpropyl.

It will be appreciated that the term "phosphate ion" includes the acid and neutral salts of phosphoric acid, i.e., dihydrogen phosphate ion (H)₂PO₄ ^2-) Hydrogen phosphate ion (HPO)₄ ^2-) And phosphate ion (PO)₄ ^3-) And salts of oligo-and polyphosphoric acids, such as diphosphates (pyrophosphates) and triphosphates.

The phosphite ion being H containing an anion₂PO₃ ^-And/or HPO₃ ^2-A salt of phosphorous acid of (a).

The sulfate ion is hydrogen sulfate ion (HSO)₄ ^-) And sulfate ion (SO)₄ ^2-) And a disulfate ion (S)₂O₇ ^2-) And related anions.

The borate ion is any ion derived from boric acid (H)₃BO₃) And anions of various polyboronic acids.

Saturated and unsaturated acyclic C_1-20Monocarboxylic acids are in particular alkanoic acids, such as formic acid, acetic acid, propionic acid, butyric acid, valeric acid, hexanoic acid (hecanoic acid), octanoic acid, decanoic acid, dodecanoic acid, tetradecanoic acid, hexadecanoic acid, octadecanoic acid and eicosanoic acid; or olefinic acids, such as acrylic acid, methacrylic acid, oleic acid, and linolenic acid.

Saturated and unsaturated acyclic C_2-20Dicarboxylic acid utensilAlkane diacids such as oxalic acid, malonic acid, succinic acid, glutaric acid, and adipic acid; or an enedioic acid, such as fumaric acid or maleic acid.

A hydroxy-substituted carboxylic acid is any carboxylic acid having at least one hydroxyl group in addition to a carboxylic acid group, such as glycolic acid, malic acid, citric acid, or salicylic acid.

More preferred quaternary ammonium salts exhibiting the above effects are quaternary ammonium carbonates, quaternary ammonium bicarbonates, quaternary ammonium phosphates and quaternary ammonium glycolates.

More preferably, R in the quaternary ammonium of formula I³And R⁴Is methyl.

In a preferred embodiment, R¹Is methyl.

In another preferred embodiment, R²Is benzyl or phenethyl.

In another preferred embodiment, R¹And R²Is the same C_1-20An alkyl group.

In a preferred embodiment, R¹And R²Is C₁₀An alkyl group.

In a more preferred embodiment, R¹And R²Is n-C₁₀An alkyl group.

In a particularly preferred embodiment, the quaternary ammonium salt is di-n-decyldimethylammonium carbonate and/or di-n-decyldimethylammonium bicarbonate.

Preferably, the metal substrate is steel.

More preferably, the steel is a sheet coil type steel.

Another object of the present invention is an article comprising a metal substrate treated according to the above method or any preferred embodiment thereof.

Again, the preferred metal substrate is steel.

Another object of the invention is an anticorrosive paint comprising:

(a) at least one quaternary ammonium salt containing non-halogen anions,

(b) an adhesive agent is added to the mixture of the components,

(c) optionally, a solvent and

(d) optionally, a pigment.

As in the above-described method, the quaternary ammonium salt in the anticorrosive paint preferably has the following general formula:

wherein R is¹Is optionally aryl-substituted C_1-20Alkyl radical, R²Is optionally aryl-substituted C_1-20Alkyl radical, R³And R⁴Independently of one another is C_1-4Alkyl radical, X^n-Is an anion selected from the group consisting of: hydroxyl ion, carbonate ion, bicarbonate ion, phosphate ion, nitrate ion, sulfate ion, borate ion, saturated and unsaturated acyclic C_1-20Anionic, saturated and unsaturated C of monocarboxylic acids_2-12Anions of dicarboxylic acids and anions of hydroxy-substituted carboxylic acids, n representing a suitable number of negative charges of said anions.

The more preferred embodiments of the quaternary ammonium salts in the above process are similarly applicable to anticorrosion paints.

Preferably, the binder is selected from the group consisting of: acrylic resins, casein (milk protein), vinyl resins, latex paint resins, acetate resins, epoxy resins and mixtures thereof.

In a preferred embodiment, the corrosion protection paint is a latex paint.

In another preferred embodiment, the corrosion protection paint is a porcelain spray paint.

In a more preferred embodiment, the corrosion inhibiting paint is an E-coat paint.

In a more preferred embodiment, the corrosion protection paint is an acrylic paint.

Another object of the invention is an article comprising a metal substrate coated with the above-described anticorrosion paint. All preferred embodiments regarding the nature of the anti-corrosion paint, the ammonium salt contained in the paint and the metal substrate apply equally to the article of manufacture.

Another object of the present invention is the use of quaternary ammonium salts having the general formula:

wherein R is¹Is optionally aryl-substituted C_1-20Alkyl radical, R²Is optionally aryl-substituted C_1-20Alkyl radical, R³And R⁴Independently of one another is C_1-4Alkyl radical, X^n-Is an anion selected from the group consisting of: hydroxyl ion, carbonate ion, bicarbonate ion, phosphate ion, phosphite ion, hypophosphite ion, nitrate ion, sulfate ion, borate ion, saturated and unsaturated acyclic C_1-20Anionic, saturated and unsaturated acyclic C of monocarboxylic acids_2-20Anions of dicarboxylic acids and anions of hydroxy-substituted carboxylic acids, n representing a suitable number of negative charges of said anions as anti-corrosion additives in paints or coatings.

More preferred embodiments of the quaternary ammonium salts in the above process are similarly suitable for use as corrosion inhibiting additives.

Drawings

FIG. 1 is an SEM micrograph of the steel surface after immersion for two weeks in 3.5% aqueous NaCl solution containing 0.1% di-n-decyldimethylammonium carbonate. Note the formation of hexagonal plates on the surface.

FIG. 2 is an SEM micrograph of the steel surface after immersion for two months in 3.5% NaCl aqueous solution containing 0.1% di-n-decyldimethylammonium carbonate. The surface is full of plate bodies and spherical particles. One of these phases is ferromagnetic (magnetite, Fe)₃O₄)。

FIG. 3 is an SEM micrograph of a steel surface after immersion in 0.1% aqueous 0.1% solution of di-n-decyldimethylammonium glycolate in tap water at 70 ℃ for 48 hours.

FIG. 4 is an SEM micrograph of a steel surface after 1 hour immersion in tap water at 70 ℃ (control experiment).

Figure 5 depicts the results of paint peel tests after salt spray exposure for several E-coated steel samples.

FIG. 6 depicts the results of a salt spray test for a latex paint coated steel sample with corrosion protection by the addition of a quaternary ammonium salt.

Fig. 7 depicts the results of the salt spray test for steel samples coated with the gloss protection spray enamel containing various amounts of quaternary ammonium salts.

The following examples illustrate the invention, but do not limit it. All parts and percentages are given by weight unless otherwise indicated.

Example 1

To prepare steel plates (S-46, available from Q-Panel Lab Products, Cleveland, Ohio), use1000 (di-n-decyldimethylammonium bicarbonate), "Phosphoquat" (di-n-decyldimethylammonium phosphate) and various related controls were subjected to conversion coating studies (ref.: NB5751-001 to 018).

Plastic tanks with a diameter of 27.9 cm (11 ") and a height of 27.9 cm (11") were used to treat these steel plates. The plastic tank was filled with 9,600 grams of the test solution. A 10.16 cm x15.24 cm x1.59 mm (4 "x 6" x1/16 ") steel (S-46) plate with a hole 6.35 mm (1/4") in diameter from the uppermost edge 6.35 mm (1/4 ") was processed by passing a plastic straw through the hole in the plate. Skew (skew) up to three plates using the same pipette. The plates are made to stand upright with only the plate bottom touching the tank bottom. The cell was then placed in an oven at 70 ℃ for one hour. After one hour, the plate was removed from the trough, dried with a paper towel and then packaged in a plastic package, labeled. The test solutions prepared can be found in the following table. All concentrations are given in weight percent. The above-mentionedAndis the true concentration of the active compound:

test solutions

Deionized water

Tap water (Allendale, NJ)

5% saline (NaCl) water

0.1% in deionized water1000*)

0.1% in tap water (Allendale, N.J.)1000

0.1% in 5% saline (NaCl) water1000

0.1% Phosphoquat in deionized water (pH 2.5)

0.1% Phosphoquat (pH 4.0) in deionized water

0.1% Phosphoquat (pH 7.5) in deionized water

0.1% Glycoquat in deionized water (pH 7.5)

*)1000 is a 50% strength aqueous solution of di-n-decyldimethylammonium carbonate (90 mole% bicarbonate, 10 mole% carbonate).

Phosphoquat is a di-n-decyldimethylammonium phosphate solution prepared by adding an aqueous 85% solution of phosphoric acid in an amount necessary to achieve a particular pH1000 to yield the solution.

Glycoquat is a di-n-decyldimethylammonium glycolate prepared by adding the amount of glycolic acid required to achieve a particular pH to a solution of glycolic acid1000 to yield the solution.

After these steel plates were treated with the test solutions, they were painted using three different painting methods. Two steel plates were painted by various painting methods. In one of the painting methods, use is made ofPaint spraying of professional high efficiency porcelain (Hunter Green) steel plates. Four steel panels were laid end to end and the paint applied to the upper surface of the steel panels with about 18 grams of paint spray painted and allowed to dry overnight. The next day the plate bottom was treated in a similar way. Use of’s Painters(Hunter Green) emulsion paint the second set of steel panels. The paint was applied to the steel panels with a foam roller. A total of two paint coatings were applied to each side of the steel sheet. A third group of steel panels was painted using a commercial painting method known as E-Coating (E-Coating). These samples were painted using a proprietary method at Royal E-Coat (Costa Mesa, California). It can be added at 0.1%, 0.5% or 1.0%1000 of’s Painters(Hunter Green) latex paint A fourth set of steel panels (untreated). Also used are compounds added in the indicated amounts1000. Of phosphoquats or glycoquats' s gloss protection spray enamel (Hunter Green) paint the fifth set of steel panels (untreated).

All paint samples were sent to Assured Testing Services (224)

River Rd., Ridge-way, PA 15853) was subjected to the salt spray test (ASTM B117). One set of steel plates is scored with an "X" in the center of the steel plate. The amount of paint pull back (in mm) of the scribed X was measured at 5 different positions. These measurements are then averaged. The lower the average, the better the handling. During the course of the test, samples were measured at different time intervals. The edges of the second set of samples were coated with wax to prevent edge effects. The undisturbed steel sheet surface was periodically evaluated for total visible rust and graded from 0 to 100%. Conversion coating/paint/salt spray test results:

the painted (porcelain) steel plate showed the greatest improvement in paint adhesion relative to the control. For most of the test samples evaluated, the latex paint samples showed no difference between the test samples and the corresponding control samples. The E-Coat sample showed minimal paint pull back after 144 hours. The results relating to the differences existing between the test samples and the control samples are summarized in tables 1 and 2 below.

TABLE 1

^*) Average paint peel (mm)

This data clearly shows when it is used1000 and Phoquat materials when the test panels were pretreated, the paint peel was significantly improved in the case of commercial spray enamels.

Table 2: summary of average paint peel (peel-back) data for E-coated samples.

PQ＝Phosphoquat；DI-deionized water.

The results listed in table 2 are also depicted in fig. 5.

Replicate sets of unscored paint panels were prepared and subjected to salt spray exposure testing. Table 3 below is a summary of the main results of the differences that exist between the test samples and the control samples.

Table 3: summary of salt spray test on major undisturbed test panel surface

Overall, this data demonstratesThe 1000 and phosphoquat type "conversion coating" pretreatments significantly reduced the total rust of commercial latex paint and porcelain paint treatments in the salt spray test.

Table 4: incorporating brush latex paints1000 peel (peel-back) data

This data is displayed when1000 was incorporated directly into commercial paint formulations, the properties in terms of paint stripping were significantly improved.

In another experiment, various amounts of the active ingredient were incorporatedThe amount of visible rust (%) was determined on 1000(CS) enhanced latex paint painted undisturbed test panel surfaces after 24 to 456 hours of salt spray exposure. The data are summarized in table 5 below and depicted in fig. 6.

Table 5: by incorporation1000 parts of a latex paint-reinforced undisturbed test panel surface.

The samples were prepared by brushing.

In another experiment, the composition contains various amounts1000(CS), Phoquat and Glycoquat gloss protection spray painted undisturbed test panel surfaces the amount (%) of visible rust was measured after 24 to 168 hours of salt spray exposure. The data are summarized in table 6 below and depicted in fig. 7.

Table 6: by incorporation1000(C S), Phoquat (PQ) and Glycoquat (GQ) enhanced gloss protected spray-on-enamel (GPE) painted undisturbed panel surface

Where applicable, all samples were prepared by brushing for spike strengthening.

TABLE 5&6 when the data is displayed1000 or related phosphoquat or glycoquat materials are significantly improved in corrosion resistance when incorporated directly into commercial paint formulations.

Example 2& comparative example 1

Two latex paints were formulated according to the amounts of ingredients and the order of processing steps listed in Table 7. A formulation (example 2) comprising1000, and the other (comparative example 1) contains an anionic surfactant (X-100) and conventional corrosion inhibitors (327) Combinations of (a) and (b).

Table 7: summary of compounding ingredients

250HBR is water-soluble hydroxyethylcellulose from Hercules, Wilmington, Delaware, USA.

850 is a sodium polyacrylate available from Rohm and Haas, Philadelphia, Pa.

X-100 is octylphenol ethoxylate of Dow Chemical Company (Dow Chemical Company).

L-493 is an antifoaming agent from Chemicals Company for Boonton Asia Shilan, N.J. (Ashland specialty Chemical Company).

327 is a base of Enqi chemical Company (ANGS chemical Company), a subsidiary of the Dow chemical CompanyCorrosion inhibitors for oxazolidines.

AMP-95^TMIs 2-amino-2-methyl-1-propanol containing 5% water, available from enziss chemical company.

R-931 is a rutile titanium dioxide pigment from DuPont titanium technology (Wilmington, Delaware).

A-15 is microcrystalline silica from Unimin minerals (Tamms, Illinois).

123 is a high solids styrene-acrylic binder of the UCAR latex system (Cary, North Carolina).

Is 2, 2, 4-trimethyl-1, 3-pentanediol monoisobutyrate from Istman Chemical Company (Eastman Chemical Company, Kingsport, Tennessee).

RM-825 is a rheology modifier based on acrylate copolymers from BASF AG (Ludwigshafen, Germany).

The formulations of table 7 were tested for storage stability and viscosity as well as scrub resistance and hiding power of the paint film on the metal surface.

The formulation of example 2 was stable for several days, even better than comparative example 1, with less separation. The formulation is also "thicker" (more desirable) than the comparative formulation. This observation confirmed the determination1000 may be compatible with latex paint systems.

Hiding power is the ability of the coating to hide stains, 96.8% in the comparative example, and 97.2% in the example according to the invention. Thus showing1000 action is superior to surfactantX-100. Hiding power was measured by using the contrast ratio of birdbar draw down (76.2 microns-3 mils) on a standard Leneta (type 2C) card and expressed as the ratio of the black and white L-values (experimental LAB color system).

The scrub resistance of the paint film prepared from the formulation of example 2 was 480 cycles to failure, while the scrub resistance of the paint film prepared from the formulation of comparative example 1 was only 370 cycles to failure. The improvement is about 30 percent obviously. Scrub resistance was measured using standard ASTM D2486 methodology.

Gloss measurements showed no significant difference between the coatings prepared from the two formulations.

The Statoma (Stormer) viscosity of the formulation of example 2 was significantly higher than that of comparative example 1, i.e., 83KU vs. 73 KU. This can reduce the amount of the relevant thickener added to the formulation.

Claims (8)

1. An anti-corrosion paint comprising:

(a) at least one quaternary ammonium salt containing non-halogen anions,

(b) an adhesive agent is added to the mixture of the components,

(c) optionally, a solvent, and

(d) optionally, a pigment

Wherein the quaternary ammonium salt has the general formula:

wherein R is¹Is optionally phenyl-substituted C_1-20Alkyl radical, R²Is optionally phenyl-substituted C_1-20Alkyl radical, R³And R⁴Independently of one another is C_1-4Alkyl radical, X^n-Is an anion selected from the group consisting of: hydroxyl ion, carbonate ion, bicarbonate ion, phosphate ion, phosphite ion, hypophosphite ion, nitrate ion, sulfate ion, borate ion, saturated and unsaturated acyclic C_1-20Anionic, saturated and unsaturated acyclic C of monocarboxylic acids_2-20Anions of dicarboxylic acids and anions of hydroxy-substituted carboxylic acids, n representing a suitable number of negative charges of said anions.

2. The anti-corrosion paint according to claim 1, wherein said binder is selected from the group consisting of: acrylic resins, casein, vinyl resins, latex resins, acetate resins, epoxy resins, and mixtures thereof.

3. The anti-corrosion paint according to claim 1, which is a latex paint.

4. The anti-corrosion paint according to claim 1, which is a porcelain spray paint.

5. The anti-corrosion paint according to claim 1, which is an e-coat paint.

6. The anti-corrosion paint according to claim 1, which is an acrylic paint.

7. An article of manufacture comprising a metal substrate coated with the corrosion protection paint of any one of claims 1 to 6.

8. Use of a quaternary ammonium salt having the general formula:

wherein R is¹Is optionally phenyl-substituted C_1-20Alkyl radical, R²Is optionally phenyl-substituted C_1-20Alkyl radical, R³And R⁴Independently of one another is C_1-4Alkyl radical, X^n-Is an anion selected from the group consisting of: hydroxyl ion, carbonate ion, bicarbonate ion, phosphate ion, phosphite ion, hypophosphite ion, nitrate ion, sulfate ion, borate ion, saturated and unsaturated acyclic C_1-20Anionic, saturated and unsaturated acyclic C of monocarboxylic acids_2-20Anions of dicarboxylic acids and anions of hydroxy-substituted carboxylic acids, n representing a suitable number of negative charges of said anions.