EP0305827A1 - Process for the ultrasonic cleaning of hard parts - Google Patents

Abstract

The invention relates to a process for the cleaning of hard material surfaces by treatment with ultrasound in wetting aqueous surfactant-containing baths. An important point here is that, to speed up the detachment of soiling and/or to eliminate contaminants which cannot be detached, or only incompletely, at least before a final stage of acoustic irradiation, the surfaces to be cleaned are wetted so intensively with a surfactant-containing liquid phase that the residual air adhering in microdispersed distribution to the micro-structure of the surfaces and the soiled areas is dispelled, at least to a great extent.

Description

The invention relates to improvements in the known ultrasonic cleaning technology for detaching stubborn dirt of various origins from the surface of hard materials by sonication in liquid baths.

A summary of the current state of ultrasonic cleaning technology can be found, for example, in "Metall" Internationale Zeitschrift für Technik und Wirtschaft, 8 (1981), 763 ff. According to current practice, washing or cleaning liquids are, on the one hand, aqueous media, but to a large extent liquid halogenated hydrocarbons - the CKW (chlorinated hydrocarbons) and FKW (fluorocarbons) - used. Aqueous media are used to remove primarily water-soluble substances, such as hardness salts, water-based lapping, polishing and grinding pastes, complex soiling of all kinds, as well as to completely wash off pigment-like dust-like residues from parts that have been pre-degreased using CHC. CHCs are primarily used for degreasing and de-oiling machined parts, for washing off easily soluble polishing pastes and other contaminants that are soluble in CHCs. Finally, HFCs are used to a large extent for washing printed circuit boards after soldering, for washing off complex soiling, in particular in the form of emulsions from HFC and water, and for other purposes.

Because of their broad applicability to a wide variety of substrates, aqueous media are particularly interesting tools for the area concerned because the use of halogenated hydrocarbons of the type concerned here is becoming increasingly common in industry and technology. An extensive exchange of such organic cleaning or treatment baths for aqueous media could become of considerable importance in the future. A serious obstacle, however, is the often limited effectiveness of the cleaning effect triggered by the action of ultrasound in the aqueous bath. To date, the use of halogenated hydrocarbons appears to be indispensable for a large number of soils. In addition, there are a large number of cleaning problems which, using the knowledge available hitherto, cannot be solved by any of the known techniques in the ultrasonic cleaning process.

The invention is based on the object of substantially improving processes of the type mentioned, which work in particular with aqueous surfactant baths as the liquid phase. On the one hand, the invention thus intends to open up new areas for the technique of ultrasonic cleaning known per se, on the other hand, it is a sub-task of the invention to improve the effectiveness of ultrasonic cleaning in aqueous-surfactant liquor in such a way that the halogenated hydrocarbon liquors can be replaced by aqueous treatment media .

The technical solution of the process according to the invention is based on the knowledge that a preparatory partial aspect of the overall process is of crucial importance for the cleaning result to be finally set, especially when working with aqueous surfactant liquors. This involves wetting the surface microstructure to be cleaned and thereby especially around the wetting of the microstructure of corresponding contaminated areas with surfactant-containing aqueous liquid phase. It is crucial for the subsequent cleaning result under the influence of ultrasound that the wetting succeeds, at least largely, in displacing the residual air adhering to and in the solid surface or in the dirt in a microdispersed manner by this liquid phase.

The most general version of the invention accordingly relates to a method for cleaning hard material surfaces by treating them with ultrasound in aqueous surfactant baths. The new method is characterized in that the surfaces to be cleaned are so intensively wetted with a surfactant-containing liquid phase at least before a final sonication stage that the residual air adhering to the surface microstructure and the contaminated areas is at least largely displaced. The method according to the invention is particularly suitable for accelerating the detachment of dirt and / or for removing dirt which otherwise cannot be removed or can only be removed incompletely under the action of ultrasound.

Scanning electron micrographs of metal surfaces in the clean and especially in the dirty state show that the surface which appears to be macroscopically smooth actually has a deeply jagged structure. When the workpiece is immersed, especially in the dry state in aqueous phases, such a surface structure is only apparently completely wetted. In fact, however, considerable proportions of microdisperse residual air are trapped in the fissured metal surface and / or in the correspondingly designed contaminated area and immobilized here by being coated with a liquid film. Greasy dirt can substantially increase this effect. This is particularly pronounced when working with water Fleets, while CHCs and CFCs pose fewer problems as better solvents and wetting agents right from the start.

Microdispersed residual air adheres to or in the dirt and the solid surface leads to a substantial impairment of the cleaning result when exposed to ultrasound. The portions not wetted with the liquid phase adhere firmly to the surface of the material. In addition, the ultrasound wave practically completely breaks off at the liquid / gaseous interface, so that these areas are strong insulators against a deeper-reaching ultrasound effect.

If, on the other hand, the surface to be cleaned is intensively wetted with a surfactant-containing liquid phase prior to sonication or at least before a final sonication step, with displacement of the residual air adhering to the microdisperse, cleaning tasks can be solved with aqueous media which have so far not appeared accessible to any technology of ultrasonic cleaning. This can be seen from the example described below.

In the autophoretic coating of, for example, steel sheets with corrosion-resistant lacquer layers, the metal parts are coated by film-forming polymers from the aqueous phase, see, for example, GB-PS 15 38 911, 11 30 687, 15 59 118 and 14 67 151. For good adhesion of the coating the purity of the metal surface is of great importance. The removal of carbon from steel surfaces poses particular problems. The carbon is deposited on the surface during the heat treatment of the steel parts, for example in the case of leaf springs in the automotive industry. Since the coating does not adhere to the carbon-containing surfaces, the carbon must be removed. Attempts to clean with and without the use of ultrasound technology, but also the non-sonic cleaning with alkaline cleaners and oxidizing agents have so far not led to the desired success. Only The deposits could be removed by the action of strong mechanical forces - for example by treatment with steel brushes. However, such cleaning is not suitable for practical use.

Using the principles of the method according to the invention described in detail below, it is now possible to completely remove the carbon deposits in aqueous surfactant baths by the action of ultrasound. Metal parts of the subsequent autophoretic coating which have previously been regarded as unsuitable for this purpose are thus also accessible.

The displacement of the residual air contained in the microstructure of the solid surface and / or in the applied impurity is of decisive importance in the process according to the invention, so that there the concentration of gas-filled microvoids is at least substantially reduced. To solve this subtask, a multitude of effective wetting aids are available to the person skilled in the art, which can be classified in the class of surfactants, emulsifiers and / or detergency or cleaning enhancers, as used in conventional technical cleaning processes or in textile washing or -cleaning are used. From the wide range of relevant wetting aids given to the person skilled in the art, suitable means can be easily determined in simple preliminary tests in coordination with the other conditions of the cleaning process. In this context, the following further findings on which the teaching according to the invention is based are of importance.

The effect of ultrasound of the technical frequencies customary today does not have to make any, or no, significant relief in the removal of microdisperse residual air in the problem areas mean. The sustained and, in particular, continuous action of ultrasound on the workpiece to be cleaned can accordingly not be cleaning-enhancing, but rather self-inhibiting.

The displacement of the microdispersed residual air required according to the invention takes place through the suitably selected play of forces of the wetting process known per se, which can thus even become the time-determining step of the cleaning process under the influence of ultrasound. The effect of ultrasound can influence this wetting process, but not necessarily accelerate it. Areas of dirt contaminated with water and tenside are evidently removed almost immediately when exposed to sound. Then, however, it is necessary for the wetting liquid phase to penetrate further into the deep structure of the dirt to be removed and for the air, which is held here microdispersed, to be displaced before further cleaning results can become visible through the action of sound. The technical solution to the problem on which the invention is based lies in the correct combination of the forces, which can be subsumed on the one hand by the concept of wetting in the conventional sense and on the other hand by the concept of surface cleaning by the action of ultrasound, in particular using the cavitation forces caused thereby. In the preferred embodiment of the method according to the invention, the wetting process for displacing this residual air is carried out at least in part with the exclusion of ultrasound.

In a preferred embodiment of the invention, the cycle of nets and subsequent sonication can be repeated one or more times to remove stubborn stains. The stages of wetting and sonication can take place under the same process conditions - in particular with the same surfactant-containing liquid phase at a fixed process temperature - but it can also be done according to the invention be preferred to carry out these stages of wetting and sonication under different conditions which are optimized for the respective process purpose.

Different combinations of networks and ultrasound treatment can lead to the desired success. It is within the scope of the action according to the invention, in particular in the wetting stage, to use process conditions which promote the desired penetrating wetting while displacing the residual air captured. In this context, the following parameters, which can be used individually or in combination with one another, should be mentioned without claiming to be complete.

To promote the wetting process, at least in these stages of the process, work can be carried out at elevated temperatures. While it is known that the cleaning forces triggered by cavitation under the influence of ultrasound increase with decreasing temperatures, the wetting process can be favored by increasing temperatures, especially when working with aqueous surfactant solutions. In this stage of wetting, temperatures in the range up to 90 ° C., preferably in the range from about 35 to 70 ° C., can be used, whereby it is often sufficient to use temperatures in the range from about 35 to 50 ° C. When wetting and ultrasound treatment are separated into two different process steps, each process step, particularly in this regard, can be optimally adapted to the intended process purpose.

Another way of increasing the wetting process in the direction of displacing unwanted residual air is to increase the surfactant concentration in the liquid phase during the wetting stage. The invention sees here in particular working with different baths in the stages of wetting and Ultrasound treatment before. The wetting process can take place in a comparatively high-surfactant bath. The wetted article is then transferred to a low-surfactant or even surfactant-free aqueous bath, where it is exposed to the action of ultrasound.

To reinforce the wetting, it can also be provided according to the invention to expose the piece of material to be cleaned to laminar and / or preferably turbulent flow in the wetting stage, so that in particular the liquid film touching the solid surface is additionally exposed to mechanical forces.

The preferred embodiment of the action according to the invention provides that the duration of the respective sonication phases is limited. In this preferred embodiment, the uninterrupted action of ultrasound on the workpiece to be cleaned and immersed in the liquid phase lasts at most about 10 minutes, but is preferably for much shorter periods, e.g. in the range of about 0.2 to 5 minutes. The reason for this is the discovery that wetted dirt particles are almost immediately removed under the influence of ultrasound. If cleaning is not yet sufficient, it is more correct to continue wetting in the absence of ultrasound than to extend the ultrasound treatment.

Accordingly, the duration of an individual sonication period within the overall process can be comparatively short. Frequently, time spans in the seconds range, for example 5-60 seconds, are sufficient for such a sound reinforcement period. In general, a sonication period will not be longer than about 5 minutes. Preferred values for the duration of each sonication phase are in the range from approximately 2 to 200 seconds and in particular in the range from approximately 3 to 120 seconds.

The duration of the network stages that follow the respective public address phase in the case of multiple sound systems are determined by the parameters used in the network and thus the network intensity. The duration of the network stages can be chosen to be shorter overall, approximately the same or longer than the sum of the public address stages. In important embodiments of the action according to the invention, the duration of the network stages used overall corresponds at least approximately to the duration of the sound reinforcement stages, the period for wetting also being able to make up several times the total period spent for the sound reinforcement. Using the example mentioned at the beginning of the removal of carbon deposits on hardened steel sheets, it will be shown in the following that the "rocking" into the cleaning process with short-term phases of networks and sonication can be considerably more effective than comparatively longer phases, in particular ultrasound treatment.

In the preferred embodiment, wetting and sonication are carried out with aqueous baths which can have the same or, as indicated, different compositions. For example, in addition to the use of surfactant components in a network bath, the use of other auxiliaries for displacing the microdisperse residual air can be expedient. Here, for example, the admixture of water-soluble organic liquid phases and / or the use of other washing or cleaning power boosters are considered, as are known from the relevant literature in the field of metal cleaning and / or textile washing. An important aid in this sense is the use of soluble electrolyte salts, for example sodium sulfate. The wetting effect of a given aqueous surfactant liquor and thus the displacement of the microdisperse residual air can be increased significantly by adding considerable amounts of such soluble electrolyte salts in the wetting stage.

For example, amounts of the electrolyte salts of at least 2, preferably at least 10 grams per liter are suitable. The upper limit is the solubility of the respective electrolyte salt, usually in amounts of about 80 grams per liter, preferably in amounts of about 50 grams per liter.

Acid, neutral or alkaline treatment baths can be used both in the wetting stage and in the sonication. For the intensive wetting of dirt on metal surfaces, wetting in weakly acidic to neutral baths can be particularly useful. Here it can be preferred to set corresponding pH ranges of the bath from about 3-7, in particular from about 4-7 and preferably from about 5-6.5. The use of non-corrosive aids to adjust the pH is preferred. Acidic salts and / or weak acids, in particular organic acids, are particularly suitable for this purpose. A suitable means for setting weakly acidic pH values in the bath are, for example, polyfunctional lower carboxylic acids of the type of oxalic acid, citric acid, maleic or fumaric acid and the like.

Suitable surfactants, emulsifiers, detergent boosters and / or other auxiliaries for improved wetting are selected in coordination with the selected conditions of the network stage. Again, the use of cationic surfactants has proven to be particularly effective for wetting metal surfaces. In addition to or instead of the cationic surfactants, nonionic surfactant components or detergency boosters are of particular importance in the context of the teaching according to the invention. As is well known, the chemistry of detergent-active surfactants has found particular development in the context of textile washing. The relevant literature provides extensive information on suitable surfactant components for aqueous surfactant liquors and in particular also on the class of the cationic and / or nonionic, preferably water-soluble, surfactant compounds.

In this context, reference is made, for example, to Ullmann "Encyclopedia of Industrial Chemistry", 4th edition, volume 24, detergents, in particular subchapter 3.1 "surfactants", loc. Cit. Pages 81 to 91. In order to support the surfactant effect, the builder effect known from textile detergent chemistry can also be used in the process according to the invention to use suitable additives which enhance the washing power. The builder substances include, in particular, certain alkaline components, such as sodium carbonate, sodium silicate, sodium diphosphate and / or sodium triphosphate and the like. Such builder substances, which are also suitable in the process according to the invention for intensifying the wetting process, are referred to the above-mentioned literature Ullmann cited above. Subchapter 3.2 Builder, pages 91 to 97.

Suitable surfactant contents for the wetting process step are, for example, in the range from about 0.5 g of active substance (AS) / l to 10 g AS / l. However, even higher surfactant concentrations can be used if this proves to be helpful for penetrating ventilation in individual cases. Typical surfactant levels can range from about 0.5 g ai / l to about 5 g ai / l. The surfactant levels during the sonication step can be in the same ranges, although they are far less critical here. For example, the piece of water which has been wetted with surfactants and is still wet can be introduced into a per se surfactant-free aqueous liquor and sonicated there, so that ultimately a surfactant content builds up only by surfactant transfer in the sonication stage.

As the frequency range for the implementation of the method according to the invention, the range known and used today comes into consideration. Preferred frequencies in the sound system are thus in the range up to approximately 100 kHz, the range from approximately 20 to 60 kHz and in particular the range from approximately 20 to 40 kHz can be particularly suitable. The service entry or the In the preferred embodiment, the power density in the sonicated bath volume is also the values customary today, for example the values up to about 25 W / l and in particular in the range up to about 15 W / l.

The method according to the invention is also particularly and particularly suitable for the removal of water-insoluble or water-difficultly soluble impurities under the action of ultrasound in aqueous surfactant liquors. This covers both large areas of grease or oil contamination up to insoluble solid contamination, which are apparently not detachably firmly attached to the surface of the solid material. The carbon impurities that have escaped from the corresponding metal surfaces during tempering are an example of this. Other examples are firmly adhering residues from metal processing or processing, such as firmly adhering residues from polishing pastes, drawing media or any complex soiling.

The method of the invention is not limited to the cleaning of metal parts, it is generally suitable for cleaning hard materials, in addition to metals, in particular for molded parts made of plastic, glass, ceramics and the like. As is well known, unusually stuck soiling from the use of the molded plastic part can be present on plastic parts in particular, which cannot be completely removed in the previous practice of ultrasonic cleaning. Thanks to the cycle of netting and sonication according to the invention, which can be repeated as often as required and, in particular, cuts the cycles of the sonication stages to a minimum in terms of their time, satisfactory cleaning results can be obtained here with reduced expenditure of energy and time.

example

Truck leaf springs from technical production with a surface contaminated with carbon deposits are used as the basis for the investigations.

In each test, the steel sample was first wetted by immersing it in the surfactant solution for the period specified and then treated with ultrasound. For easier assessment of the cleaning success, only half of the steel sheets were immersed in the cleaning solutions.

Unless otherwise stated, the wetting times and the duration of the ultrasound exposure were each 1 minute.

The tests are carried out in an ultrasonic bath from Bandelin electronic, Berlin, bath volume 2.5 l, frequency 35 kHz.

Attempts to clean the metal surface with alkaline cleaners and oxidizing agents in conventional processes had no success.

Preliminary tests for surface cleaning under the influence of ultrasound had shown that the best results can be expected with cationic surfactants in the acidic pH range. In addition, certain successes were also found using nonionic surfactants. In no case, however, was it possible to remove the carbon deposits to the extent required by conventional methods. Immersing the non-wetted leaf springs directly into the ultrasonic bath leads to significantly worse results than working step-by-step with a previous wetting step in the absence of ultrasound exposure and only after subsequent ultrasound treatment.

The removal of the carbon deposits was assessed visually by comparing the immersed part with the untreated piece of metal and rated on a scale from 0 to 6. The value "0" is assigned to an untreated metal part, while "6" means the complete removal of the carbon deposit.

Lauryltrimethylammonium chloride (Dehyquart LT) and laurylpyridinium bisulfate (Dehyquart D) from the class of the cationic surfactants were used. Nonylphenoloctaglycolether (NP 8) was used as the nonionic surfactant.

Citric acid is used to adjust the weakly acidic pH in the bath. The cleaning performance of the acid compared to the carbon deposits is only slight and increases only slightly with an increased citric acid concentration. When trying to work with baths containing sulfuric acid, increased corrosion tendency is observed.

To test the cleaning performance of the cationic surfactants, the use of 1 g / l citric acid and 1 g / l surfactant is selected as the standard conditions. It can be seen that, under these conditions, the desired cleaning effect does not occur with a single cycle of nets and sonication for one minute each. Increasing the surfactant concentration removes the carbon deposits from the metal surfaces more. In these experiments, the metal parts are first pretreated with the surfactant solutions without ultrasound and then treated with ultrasound in a surfactant-free bath containing citric acid. The results of the tests are summarized in Table 1 below. Table 1 cleaning Surfactant c (g / l): 1 5 aqueous concentrated solution Dehyquart LT 1 1 4th Dehyquart D 1 2nd 3rd c _{Citric acid} = 1 g / l

To improve the wetting and thus to improve the carbon removal from the metal surface, the bath temperature is increased from 25 ° C to 40 ° C. This improves cleaning performance. A further increase in temperature to 60 ° C did not lead to a significant improvement in the already good carbon removal. However, an increased tendency for corrosion of the metal parts is observed. All other attempts to optimize carbon removal are therefore carried out at 40 ° C. The temperature dependence of the cleaning performance is summarized in Table 2 below. Table 2 cleaning Surfactant Temperature: 25 ° C 40 ° C 60 ° C Dehyquart D 3rd 5 5 c _{citric acid} = 3 g / l; c _{Dehyquart D} = 5 g / l

The dependence of the carbon removal on the acid and cationic surfactant concentration at 40 ° C is shown in Table 3 below. The cleaning effect of the ultrasonic bath can be significantly increased by increasing the concentration of citric acid and the cationic surfactant Dehyquart D in the mesh stage. A further increase in the concentrations above the values given in Table 3 below did not result in an improved cleaning effect. Table 3 Additives (g / l) cleaning Citric acid Dehyquart D 1 1 1 1 5 2nd 3rd - 3rd 3rd 5 5 40 ° C

The use of mixtures of cationic surfactants (Dehyquart D) and nonionic surfactants (NP8) means that the carbon removal can no longer be significantly increased compared to the sole use of the cationic surfactant, but corresponding studies show that the duration of the ultrasound treatment required for cleaning is reduced can be.

The carbon removal can be greatly improved by multiple treatment of the metal parts. If the metal samples are wetted several times in succession for 1 minute each in the surfactant solution and then treated with ultrasound for 1 minute, the cleaning effect sometimes increases significantly. This effect is particularly evident in cleaning solutions that are not sufficient when the metal parts are treated once Showed carbon removal. A multiple treatment with NP8 alone and with a reduced citric acid concentration has already achieved a good cleaning effect. It turned out to be sufficient to reduce the wetting time and duration of the ultrasound treatment to 15 seconds per process step. By repeated repetition of corresponding working cycles, a complete removal of the carbon deposit can be achieved with the mixture of Dehyquart D and NP8 in the presence of 1 g / l citric acid. Table 4 below gives a summary of important process results for such multiple treatment at 40 ° C. Table 4 Additions g / l cleaning Citric acid Dehyquart D NP8 1. 2nd 3rd 4th 1 - 1 2nd 4th - - 1 1 1 2nd 3rd - - 1 5 5 - - - 6 3rd 5 5 5 5 4th - 1. 1 minute wetting + 1 minute ultrasound 2. 2 x 1 minute wetting + 1 minute ultrasound 3. 3 x 1 minute wetting + 1 minute ultrasound 4. 3 x 1 minute wetting + 1 minute ultrasound + 5 x 15 seconds wetting + 15 seconds ultrasound

Claims (16)

1. A method for cleaning hard material surfaces by treating them with ultrasound in wetting aqueous surfactant baths, characterized in that the surfaces to be cleaned are accelerated at least before a final sonication step in order to accelerate the detachment of dirt and / or to remove dirt that cannot be removed or can only be removed incompletely intensively wets with a surfactant-containing liquid phase that at least largely displaces the residual air adhering to the surface microstructure and the soiled areas in a microdispersed manner. 2. The method according to claim 1, characterized in that one carries out the wetting process for the displacement of the microdisperse residual air at least partially with the exclusion of ultrasound. 3. The method according to claim 1 and 2, characterized in that the cycle of networks and subsequent sonication is repeated one or more times, in particular to remove stubborn stains. 4. The method according to claims 1 to 3, characterized in that the stages of wetting and sonication are carried out under the same or different conditions optimized for the respective process purpose. 5. Process according to Claims 1 to 4, characterized in that aqueous surfactant solutions are also used in wetting. 6. The method according to claims 1 to 5, characterized in that to promote the wetting and the displacement of microdisperse adhering residual air at least in one network stage at elevated temperatures. 7. Process according to Claims 1 to 6, characterized in that temperatures in the range up to 90 ° C, preferably in the range from about 30 to 70 ° C, are used in the wetting step. 8. The method according to claims 1 to 7, characterized in that in the wetting step with - compared to the cleaning bath - increased surfactant concentration in the liquid phase. 9. The method according to claims 1 to 8, characterized in that a laminar and / or preferably turbulent flow is formed in the wetting liquid film during the wetting stage. 10. The method according to claims 1 to 9, characterized in that acidic, neutral or alkaline, preferably for cleaning metal surfaces with weakly acidic to neutral treatment baths. 11. The method according to claims 1 to 10, characterized in that at least in the wetting process with weakly acidic to neutral aqueous baths are used, which contain cationic, nonionic and / or amphoteric surfactants, which are preferably readily water-soluble in the working conditions of the wetting process, preferably the Wetting action of the aqueous liquors by using surfactants and / or detergency boosters known in particular from metal cleaning and / or textile washing and / or by The use of soluble salts, in particular corresponding neutral salts, is reinforced. 12. The method according to claims 1 to 11, characterized in that with baths of the pH range from about 3 to 7 with preferred surfactant contents (active substance) in the range from about 0.5 g / l to 10 g / l. 13. The method according to claims 1 to 12, characterized in that one works with sonication phases each lasting up to about 10 minutes, preferably from about 0.2 to 5 minutes, and then, if necessary, wets again in the absence of ultrasound before a further cleaning step with ultrasound is used. 14. The method according to claims 1 to 13, characterized in that ultrasonic frequencies up to about 100 kHz, preferably from 20 to 60 kHz. 15. Application of the method according to claims 1 to 14 for removing insoluble residues, in particular carbon deposits on metal surfaces by ultrasonic cleaning in weakly acidic to neutral baths with the addition of cationic and / or nonionic surfactants, in particular for a subsequent coating with corrosion protection layers in the autophoretic process. 16. Embodiment according to claim 15, characterized in that it is wetted with aqueous solutions of weak organic acids, in particular of the type of citric acid with the addition of cationic surfactants and, if desired, nonionic surfactants and subsequently sonicated in the same or, if desired, also largely surfactant-free bath.