Field of the Invention

This invention relates to a water-based method for cleaning metal surfac¬ es such as, most importantly, the surfaces of iron, zinc, aluminum, and the like. More particularly, with the goal of implementing environmental measures such as protection of the ozone layer, this invention relates to a method for cleaning metal surfaces that can replace the use of solvents, such as Freon™, trichloro- ethane, and the like. Statement of Related Art

Organic solvents, mainly Freon™, trichloroethane, and the like, have here- tofore been used in the metal article cleaning operations that are frequently im¬ plemented as a pre-treatmentto metal working, painting, heat treatment, conver¬ sion treatment, and the like. This practice is followed because it offers a number of advantages, such as: (1 ) strong ability to dissolve oily contaminants, (2) easy drying,

(3) no risk of corrosion because there is no contact with water,

(4) simple cleaning equipment that does not require a large amount of space, and

(5) no requirement for waste water treatment facilities. However, because the release of these solvents into the atmosphere is related to destruction of the earth's ozone layer, in the last few years there has been a widening global movement to control their production and use. This has led to the current strong and urgent demand for cleaning procedures that do not employ the aforesaid solvents. Nevertheless, water-based cleaners suffer from several major drawbacks, such as requiring long drying times and having a pro¬ nounced tendency to promote corrosion in the case of metals.

Cleaning with water-based cleaners typically requires the following steps: (1) cleaning, (2) a water rinse, and (3) drying. Among these steps, rusting is most readily produced in the drying step (3) because it is here that the workpiece

is exposed to the highest temperatures. As a result, a supplementary process is often inserted between the water rinse and drying steps with the goal of inhibi¬ ting rust development. This supplementary process consists of application to the metal substrate of a corrosion-preventive agent. Due to the low vapor pressure of water at ambient temperature and the long evaporation times necessitated thereby, the drying step in water-based cleaning operations generally employs techniques such as immersion in hot water prior to drying, hot-air blow drying using a stream of hot air at a temperature of at least 100° C, and vacuum drying (optionally with a heater) wherein drying is promoted by reducing the pressure in order to reduce the boiling point of the water. One example of a drying method that does not require these types of drying devices is disclosed in Japanese Pat¬ ent Application Laid Open [Kokai or Unexamined] Number Hei 4-76381 [76,381/ 1992]. In this method, the workpiece, after an upstream plating operation, is dried by exposure to a current of pressurized steam. Because large amounts of water remain on the surface after a water rinse as a result of the much higher surface tension of water as compared to organic solvents, these already known water-based cleaning/drying methods must invest large amounts of thermal energy in order to evaporate and dry the water. This has resulted in a requirement for expensive high-capacity drying equipment and has also necessitated longer drying times than in the case of organic solvents. When an organic solvent-based cleaning facility is converted into a water-based cleaning operation, these factors cause a loss in operating efficiency and produc¬ tivity and raise costs due to the requirement for purchasing new drying equip¬ ment. In addition, simply blowing steam onto the workpiece by itself still leaves large amounts of water on the surface. The evaporation of this water removes large quantities of heat, with the result that this method does not necessarily yield the same drying performance as for the use of organic solvent. Problems to Be Solved bv the Invention

The present invention takes as its object the introduction of a method for cleaning metal surfaces which uses water-based cleaners, does not require spe¬ cial drying equipment, is free of problems such as the development of rust during and after drying, and provides an excellent drying performance, i.e., makes

possible drying in as short a time period as for the use of organic solvent — a performance level hereinafter referred to simply as a high drying performance. Summary of the Invention

It was discovered that the problems described above for the prior art could be solved by cleaning with a water-based cleaning composition (usually hereinaf¬ ter called a "bath" for brevity even though it may be used by any method of con¬ tact and is not restricted to use by immersion) that contains an ester and/or a salt (which for these acids may also be called a "soap") of a C ₈ to C ₂₆ fatty acid and thereafter contacting with steam. The present invention was achieved as a result of this discovery.

Detailed Description of the Invention and Preferred Embodiments Thereof

In specific terms, the present invention introduces a high drying perform¬ ance method for cleaning metal surfaces that characteristically comprises clean¬ ing the metal surface with a water-based cleaning bath that preferably contains 0.01 to 5, or more preferably 0.02 to 3.0, weight % of ester and/or salt of C ₈ to C _x fatty acid, and thereafter bringing the metal surface into contact, preferably for at least 5 seconds, with steam having a temperature of at least 80° C.

This invention also introduces a high drying performance method for cleaning a metal surface that characteristically comprises cleaning the metal surface with a water-based cleaning bath that preferably contains 0.01 to 5 weight % of ester and/or salt of C ₈ to C ₂₆ fatty acid, then rinsing with water, and thereafter bringing the metal surface into contact, preferably for at least 5 sec¬ onds, with steam having a temperature of at least 80° C.

No narrow limitations apply to metals which may be subjected to the pres- ent invention, but the main target metals will be iron, zinc, aluminum, and the like.

A cleaning method according to the present invention comprises at least two process steps. In the first step, a water-repellent surface is generated by cleaning with a water-based cleaning bath or by cleaning with a water-based cleaning bath and thereafter additionally rinsing with a water rinse. The second necessary process step consists of drying by contact with high-temperature steam. An effective method for obtaining a water-repellent surface, after clean¬ ing by the water-based cleaning bath, consists of treatment with a water-based

cleaning bath that contains fatty acid alkali salt and/or fatty acid ester. This forms a water-repellent film through the adsorption of fatty acid anions and/or fatty acid ester on the metal surface that has been cleaned by the water-based cleaning bath. One or more C ₈ to C ₂₆ fatty acid salts, of which suitable and preferred ex¬ amples are the sodium and potassium salts of fatty acids such as oleic acid, lauric acid, palmitic acid, stearic acid, and the like, may be used as the fatty acid salt in the water-based cleaning bath.

Suitable fatty acid esters are, for example, the esters of C ₈ to C ₂₆ fatty acids, preferably relatively long chain acids such as oleic acid, lauric acid, palmit¬ ic acid, stearic acid, and the like. Particularly preferred are esters between these fatty acids and polyhydric alcohols, as specifically exemplified by the sorbitan es¬ ters such as sorbitan trioleate, sorbitan monooleate, and sorbitan monolaurate. The reasons for specifying a range of C ₈ to C ₂₆ for the fatty acids are as follows: An inadequate water repellency is normally generated at below C ₈ , while exceeding C ₂₆ leads to such a large add-on that a clean surface can not readily be obtained. A range of 12 - 18 for the number of carbons per fatty acid mole¬ cule for at least some of the fatty acid salts and/or esters is more preferred, because the salts and esters of C _B . fatty acids may not form a satisfactorily hy- drophobic layer in some cases when used alone.

The concentration of fatty acid salt and fatty acid ester in the treatment bath preferably should be from 0.01 to 5 weight %. An adequate water repellen¬ cy usually can not be developed at below 0.01 weight %, while the large amount adsorbed at above 5 weight % prevents a clean surface from being readily ob- tained. More preferably, the total concentration of salts and esters of C ₈ to C ₂₆ fatty acids in a water-based cleaner to be used according to this invention should be from 0.02 to 3 %.

Suitable long-chain fatty acid esters for the invention are by themselves poorly soluble or dispersible, and they are therefore preferably dispersed or solu- bilized through the use of anionic surfactant, such as the salts of the sulfate esters of higher alcohols, alkyl ether carboxylate salts, and of course the alkali salts of fatty acids.

When the water-repellency treatment and degreasing are to be simultane¬ ously carried out in the water-based cleaning bath according to the present in¬ vention, an excellent cleaning effect and water repellency can be simultaneously obtained through the presence in the water-based cleaning bath of 0.01 to 5 weight % of nonionic surfactant with a hydrophile-lipophile balance value (herein¬ after usually abbreviated as "HLB") in the range from 9 to 16 in addition to the fatty acid salt or fatty acid ester. Suitable nonionic surfactants are exemplified by polyoxyethylene surfactants such as polyoxyethylene alkyl ethers and poly- oxyethylene alkylphenyl ethers. If desired, the cleaning effect can also some- times be improved by the addition of an alkali builder such as sodium carbonate, sodium phosphate, sodium hydroxide, and the like.

When a surface treated by the subject method is rinsed with a small quantity of water, the contact angle observed for the water will be at least 70°, which is indicative of a water repellency sufficient for practical applications. Much less water will remain on the surface in this case than in the case of a hydrophilic surface, and it is often possible, and is preferred, to obtain even less than about one-tenth of that adhering on a hydrophilic surface. When the amount of water retained on the surface of the metal workpiece is one-tenth as much as before, only one-tenth as much heat is required to evaporate the water, with the result that even the low heat capacity metal workpiece can be dried without external heating. Since application of the method according to the present invention causes a substantial reduction in the amount of water retained on the surface after water rinsing, the metal workpiece will dry spontaneously in a short period of time, after the input of heat by a brief contact with high-temperature steam.

In contrast to this, large amounts of water remain on the surface after a conventional water-based cleaning. As a result, due to the very large latent heat of evaporation required for the evaporation of the surface-retained water, even pre-drying thermal inputs to the metal workpiece by, for example, a hot water rinse or steam, cannot prevent a drop in the surface temperature of the metal workpiece during drying — with the resulting requirement for long drying times. When, after the water-repellency treatment described above, the cleaned article

is contacted for at least 5 seconds with steam having a temperature of at least 80° C, the steam simultaneously heats the article and efficiently washes off the surface by condensing on the metal workpiece. The net result is a substantial reduction in the time required for drying. Steam contact times below 5 seconds normally result in long drying times and thereby increase the risk of rust development. Steam contact times of at least 10 seconds are more preferable. Because the atmospheric oxygen and oxygen dissolved in the retained water, which are the causes of rusting, reach high levels in the case of steam below 80° C, the development of rust can there- fore be inhibited by the use of steam at or above 80° C. Generally it will be more convenient, and therefore preferable, to utilize steam at a temperature at or above 100° C, which will be stable under normal ambient atmospheric pressure. When the condensate from the steam by itself provides an inadequate water wash, cleaner surfaces can be obtained by implementing a short water rinse im- mediately prior to contact with the steam. This water rinse, by temporarily drop¬ ping the temperature of the metal surface being treated, increases the amount of steam that subsequently condenses and thereby increases the cleaning effect. The decision whether to implement a water rinse should be based on the re¬ quired level of surface cleanliness. The water-repellent film formed on the metal surface is very thin and rep¬ resents only a small add-on. As a result, it usually presents no particular prob¬ lem to the execution of heat treatment or painting after the drying process. How¬ ever, it can be completely evaporated or decomposed if desired by raising the drying temperature. The present invention will be explained in greater detail below with refer¬ ence to several examples and comparative examples of actual cleaning process¬ es.

Specimen substrates

Rectangular coupons 130 x 62 millimeters (hereinafter usually abbreviated "mm") in size and 0.6 mm thick of cold-rolled steel (Japanese Industrial Standard type SPCC-SD) were coated with a rust-preventive oil (Nox-Rust™ 881 from Par¬ ker Kosan Kabushiki Kaisha).

Water-based cleaning baths (with identifying letters used in later tables)

A: 1.5 grams per liter (hereinafter "g/L") of polyoxyethylene alkyl ether with an HLB = 11 + 5 g/L of sodium carbonate + 3.5 g/L of sodium oleate (C ₁₈ ) + water to make up the balance of the bath. B: 1.5 g/L of the same polyoxyethylene-type nonionic surfactant as in bath

A + 5 g/L of sodium carbonate + water to make up the balance of the bath. C: 30 g/L of potassium oleate + 1.0 g/L of sorbitan trioleate (C ₁₈ ) + water to make up the balance of the bath. D: 0.2 g/L of sodium stearate (C ₁₈ ) + water to make up the balance of the bath. E: 1.5 g/L of sorbitan monopalmitate (C ₁₆ ) + sodium hexadecyl sulfate + water to make up the balance of the bath. F: 2.0 g/L of polyoxyethylene nonylphenyl ether + 2 g/L of sodium laurate (C ₁₂ ) + water to make up the balance of the bath.

G: 1.0 g/L of sorbitan monooleate (C ₁₈ ) + 2.0 g/L of sodium caprylate (C ₈ ) + water to make up the balance of the bath. Note: The expressions in parentheses refers to the number of carbons in the fatty acid. Performance evaluations

Drying time: The time in seconds required after the start of drying for the surface of the specimen to become completely dry as visually judged was measured.

Rustin ^g : The development of rusting on the surface of the specimen after drying was visually evaluated.

Cleanin ^g effect: The oil remaining on the surface of the specimen after drying was measured using a surface carbon analyzer (from the Leco Company). The cleaning effect for the surface of the specimen was re¬ ported according to the following scale: A: less than 6 milligrams per square meter (hereinafter usually abbreviated as "mg/m ² ") of carbon on the surface.

B: at least 6 mg/m ² but less than 10 mg/m ² of carbon on the surface. C: at least 10 mg/m ² but less than 20 mg/m ² of carbon on the surface. D: at least 20 mg/m ² of carbon on the surface. Treatment process steps • Treatment process for Table 1

Spray with the water-based cleaning bath at 60° C for 3 minutes.

Exposure to steam at 100°C for the time reported in Table 1.

Jj Spontaneous drying in the atmosphere at 25° C and 40 % relative humidity.

• Treatment process for Table 2

Spray with the water-based cleaning bath at 60° C for 3 minutes.

4 Water spray rinse at 30° C for 10 seconds with tap water.

4 Exposure to steam at 100°C for the time reported in Table 1.

4 Spontaneous drying in the atmosphere at 25° C and 40 % relative humidity. Examples 1 to 6

Water-based cleaning baths A, C, D, and F were used. Treatment was carried out without a water rinse using different steam contact times. The drying times, cleaning effects, and rusting were evaluated, and these results are report¬ ed in Table 1. Comparative Examples 1 to 3

Water-based cleaning bath B was used. Treatment was carried out with¬ out a water rinse using different steam contact times. The drying times, cleaning effects, and rusting were evaluated, and these results are reported in Table 1.

Table 1 TEST RESULTS FROM THE EXAMPLES AND COMPARATIVE EXAMPLES

(WITHOUT WATER RINSE)

Identifier Cleaning Steam Contact Cleaning Drying Rust De¬ Bath Time, Seconds Effect Time, velop¬ Rating Seconds ment?

Example 1 A 10 C 80 No

Example 2 A 120 B 35 No

Example 3 C 60 B 60 No

Example 4 D 180 B 15 No

Example 5 E 60 B 60 No

Example 6 F 240 B 13 No

CE 1 B 120 C 90 Yes

CE 2 B 60 C 240 Yes

CE 3 B 10 D 380 Yes

Abbreviation for Table 1 : "CE" = Comparative Example

Table 2 TEST RESULTS FROM THE EXAMPLES AND COMPARATIVE EXAMPLES

(WITH WATER RINSE)

Identifier Cleaning Steam Contact Cleaning Drying Rust De¬ Bath Time, Seconds Effect Time, velop¬ Rating Seconds ment?

Example 7 A 10 B 80 No

Example 8 A 120 A 35 No

Example 9 C 60 A 60 No

Example 10 D 180 A 15 No

Example 11 E 60 A 60 No

Example 12 G 240 A 13 No

CE 4 B 120 B 90 Yes

CE 5 B 60 B 240 Yes

CE 6 B 10 B 380 Yes

Abbreviation for Table 2: "CE" = Comparative Example

Examples 7 to 12

Water-based cleaning baths A, C, D, E, and G were used. Treatment was carried out with a water rinse using different steam contact times. The drying times, cleaning effects, and rusting were evaluated, and these results are report- ed in Table 2.

Comparative Examples 4 to 6

Water-based cleaning bath B was used. Treatment was carried out with a water rinse using different steam contact times. The drying times, cleaning ef¬ fects, and rusting were evaluated, and these results are reported in Table 2. The results obtained in Examples 1 to 12 and Comparative Examples 1 to 6 support the following conclusions.

1. In Examples 1 to 12, which employed water-based cleaning baths A, C, D, E, F, and G containing C ₈ to C ₂₆ fatty acid salt or ester, the specimens dried rapidly, there was no rusting, and excellent cleaning effects were obtained.

2. In Comparative Examples 1 to 6, which employed water-based cleaning bath B containing only builder and surfactant that was not according to the invention, long drying times were required, rusting occurred, and poor cleaning effects were obtained. Benefits of the Invention

Viewed from the perspective of drying performance, which is a problemat¬ ic point for conventional water-based cleaning operations, the water-based clean¬ ing method according to the present invention affords drying times as short as in organic solvent-based cleaning (e.g., with trichloroethane, Freon™, and the like) even without the use of special drying facilities such as hot-air dryers or vac¬ uum dryers. Moreover, the invention method is a very practical cleaning method relative to such issues as equipment cost reduction and space savings.