METHOD OF RECOVERING IMPREGNATING AGENT COMPOSITIONS

Abstract

The present invention relates to processes for recovering impregnating compositions, the impregnated metal part being brought into contact with a salt-containing cleaning composition which comprises at least 0.7% by weight of at least one salt. In addition, the present invention describes processes for impregnating metal parts, which have at least one step for recovering impregnating compositions.

Description

FIELD OF THE INVENTION

The present invention relates to processes for recovering polymerizable impregnating compositions. Furthermore, the present invention describes processes for impregnating metal parts.

Various metal parts or metal components, such as, for example, engine blocks, cylinder heads, gear housings, etc., are produced by casting of various metals or metal alloys. In general, the metal parts thus produced have pores which may lead to leaks. Great value is increasingly being placed on a low weight of the metal parts. In addition, the production of thin-walled metal parts leads to a saving of material. Accordingly, the necessity of sealing the pores formed by the casting is growing.

PRIOR ART

Impregnating compositions which comprise mixtures of esters of (meth)acrylic acid have long been used for this purpose. However, only a small part of the impregnating compositions used is actually used for sealing the pores. A major part is removed from the metal surface by washwater before the impregnating composition present in the pores is hardened. The impregnating composition present in the washwater must then be disposed of in a complicated manner.

In order to solve this problem, the prior art proposes using specific impregnating compositions which have a density of more than 1.0. This is described, for example, in the publications U.S. Pat. No. 6,761,775 and U.S. Pat. No. 6,712,910. A disadvantage of this procedure is, however, that only very specific impregnating compositions can be used. As a result, the quality of the metal parts may be adversely affected since the properties of the pore seal, for example the resistance thereof to chemicals, is dependent to a high degree on the impregnating composition.

In addition, the publication U.S. Pat No. 5,518,632 discloses a process for recovering impregnating composition, in which the washwater is treated by addition of an additive which increases the density of the water. As a result, water-soluble constituents of the impregnating compositions are separated from the washwater. However, a disadvantage of this process is that large amounts of washwater are required, which have to be purified in a complicated manner. In addition, large amounts of additive, for example sodium chloride, are required. Furthermore, the recovery of the impregnating agent by the process described in U.S. Pat No. 5,518, 632 cannot be operated continuously.

Furthermore, the publications DE-A-44 10 193 and DE-A-101 13 278 disclose processes in which the impregnating agent is separated off from the washwater via reverse osmosis methods. A disadvantage here, however, is that the process is very energy-consumptive, the plants giving rise to a very high maintenance effort.

In addition, the publications JP-A-2004204201, JP-A-2005076014 and JP-A-2005132971 disclose impregnating compositions which can easily be separated off from the washwater. The wash compositions may contain a very specific surfactant. However, a disadvantage is that this teaching leads to satisfactory separation only with very specific compositions, so that the statements made above also apply to these publications.

OBJECT AND ACHIEVEMENT

In view of the prior art, it was now an object of the present invention to provide processes for recovering impregnating compositions which can be used for recovering a multiplicity of different impregnating compositions.

A further object of the invention was to provide a process in which the recovery of the impregnating agent can be carried out continuously.

In addition, it was an object of the present invention to provide processes for recovering impregnating compositions which can be carried out easily and economically. The process should be capable of being carried out thereby with a low water consumption, it being intended to avoid the use of particularly large amounts of additive.

Furthermore, the provision of processes for impregnating metal parts was an object of the present invention. In particular, this process should be capable of being carried out with a multiplicity of different impregnating compositions, in order thus to ensure an optimum quality of the impregnated metal parts. This process should thereby consume as small amounts of impregnating agent as possible.

These objects and further objects which are not explicitly mentioned but can be directly derived or concluded from the contexts discussed in this introduction are achieved by a process having all features of Patent Claim 1. Expedient modifications of the process according to the invention are protected in the dependent claims relating back to Claim 1. With regard to the process for impregnating metal parts, Claim 17 provides an achievement of the underlying object.

The present invention accordingly relates to a process for recovering impregnating compositions, which is characterized in that the impregnated metal part is brought into contact with a salt-containing cleaning composition which comprises at least 0.7% by weight of at least one salt.

As a result, it was possible in an unforeseeable manner to provide a process for recovering impregnating compositions, which process can be used for recovering a multiplicity of different impregnating compositions.

Furthermore, the process according to the invention makes it possible to carry out the recovery of the impregnating agent continuously.

In addition, the process for recovering impregnating compositions can be carried out easily and economically. The water consumption can be kept low thereby, it also being possible to avoid the use of particularly large amounts of additive.

Furthermore, the process according to the invention can be carried out as a part-step in a process for impregnating metal parts. Surprisingly, it is thus possible to provide a process for impregnating metal parts which can be carried out with a multiplicity of different impregnating compositions. By the easy and economical recovery of excess impregnating composition, this process provides a very environmentally friendly variant for impregnating metal parts, these advantages also being achieved with the use of monomer compositions which are hazardous to health.

The process according to the invention serves for recovering impregnating compositions. In the case of the customary processes for impregnating metal parts, only a small part of the impregnating composition is used for filling the pores. A greater part, in general more than 90%, is removed from the metal surface by cleaning compositions, for example washwater. The present process now serves for recovering the impregnating composition used in excess. According to the invention, for this purpose, the impregnated metal part is brought into contact with a salt-containing cleaning composition. Here, the expression “impregnated metal part” means that the pores of the metal part are filled with impregnating composition, the metal part generally being brought into contact prior to hardening of the impregnating composition with the salt-containing cleaning composition in order to remove excess impregnating composition from the metal surface.

According to the invention, the cleaning composition comprises at least one salt. The expression “salt” is furthermore known among those skilled in the art and designates an ionic compound which may be of an organic and/or inorganic nature, inorganic salts being preferred. In contrast to inorganic salts, organic salts have hydrocarbon groups. Preferably, the salt should be environmentally compatible and economical. Expedient salts have only a slight emulsifying effect, particularly preferably no emulsifying effect. In addition, the salt should as far as possible not lead to corrosion of the metal part. The preferred salts include in particular halides, for example chlorides, bromides and fluorides, sulphates, sulphites and salts of phosphorus, for example phosphates, phosphonates or phosphinates. Preferred salts may comprise in particular alkali metals, for example sodium, lithium or potassium, and alkaline earth metals, for example magnesium or calcium, as a cation. For example, sodium chloride (NaCl), potassium chloride (KCl), sodium sulphate (Na₂SO₄) and/or potassium sulphate (K₂SO₄) can be used. Particularly preferably, the cleaning compositions of the present invention comprise in particular phosphates of alkali metals, such as, for example, sodium phosphate or potassium phosphate, it being possible for these salts, depending on pH, also to be present as sodium dihydrogen phosphate NaH₂PO₄, disodium hydrogen phosphate Na₂HPO₄, potassium dihydrogen phosphate KH₂PO₄ and/or dipotassium hydrogen phosphate K₂HPO₄. These salts can also be used as a mixture.

The salt content of the cleaning composition is preferably in a range from 4 to 20% by weight, particularly preferably in a range from 8 to 15% by weight, the salt being present in this amount in solution in the cleaning composition. The cleaning composition preferably comprises water as the main constituent.

Of particular interest are especially cleaning compositions which preferably have a density in the range from 1.005 to 1.20 g/l, particularly preferably in the range from 1.04 to 1.16 g/l.

Impregnating compositions for impregnating metal parts have long been known, it being possible for each of these impregnating compositions in principle to be recovered by the present process. Especially impregnating compositions which comprise (meth)acrylates are particularly expedient.

The expression (meth)acrylates includes methacrylates and acrylates and mixtures of the two. The (meth)acrylates present in the impregnating compositions have long been known, these monomers being described, inter alia, in the publications U.S. Pat. No. 6,761,775 and U.S. Pat No. 6,712,910.

Preferred impregnating compositions may comprise, inter alia, hydroxyalkyl (meth)acrylates. These include, inter alia, 2-hydroxyethyl (meth)acrylate (HEMA), 2-hydroxypropyl (meth)acrylate (HPMA), 3-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 3,4-dihydroxybutyl (meth)acrylate.

Expedient embodiments comprise in particular 2-hydroxyethyl methacrylate and/or 2-hydroxypropyl methacrylate.

An impregnating composition can preferably comprise 5% by weight to 95% by weight, particularly preferably 25 to 60% by weight, based on the weight of the impregnating composition, of hydroxyalkyl (meth)acrylates.

Of particular interest are furthermore impregnating compositions which contain crosslinking (meth)acrylates. These include in particular (meth)acrylates having two double bonds, such as, for example, (meth)acrylates which are derived from unsaturated alcohols, such as, for example, 2-propynyl (meth)acrylate, allyl (meth)acrylate, vinyl (meth)acrylate, and (meth)acrylates which are derived from diols or alcohols having a higher hydricity, such as, for example, glycol di(meth)acrylates, such as ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, tetra- and polyethylene glycol di(meth)acrylate, in particular polydiol-400 di(meth)acrylate, 1,3-butanediol (meth)acrylate, 1,4-butanediol (meth)acrylate, neopentyl glycol di(meth)acrylates, 1,6-hexanediol di(meth)acrylate, glyceryl di(meth)acrylate and diurethane dimethacrylate; (meth)acrylates having three or more double bonds, such as, for example, glyceryl tri(meth)acrylate, trimethylolpropane tri(meth)acrylate, pentaerythrityl tetra(meth)acrylate and dipentaerythrityl penta(meth)acrylate.

Preferred modifications comprise, for example, 5% by weight to 75% by weight, particularly preferably 20% by weight to 50% by weight, of crosslinking (meth)-acrylates, based on the weight of the impregnating composition.

Furthermore, preferred impregnating compositions may comprise 5% by weight to 60% by weight, particularly preferably 20% by weight to 40% by weight, of comonomers, based on the weight of the impregnating composition. Comonomers are monomers which can be copolymerized with the abovementioned (meth)acrylates. These include, for example, (meth)acrylates which are derived from saturated alcohols. These monomers comprise in particular (meth)acrylates having 1 to 6 carbon atoms in the alcohol radical, such as, for example, methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, tert-butyl (meth)acrylate and pentyl (meth)acrylate, hexyl (meth)acrylate; (meth)acrylates having 7 to 15 carbon atoms in the alcohol radical, such as, for example, 2-ethylhexyl (meth)acrylate, heptyl (meth)acrylate, 2-tert-butylheptyl (meth)acrylate, octyl (meth)acrylate, 3-isopropylheptyl (meth)acrylate, nonyl (meth)acrylate, decyl (meth)acrylate, undecyl (meth)acrylate, 5-methylundecyl (meth)acrylate, dodecyl (meth)acrylate, 2-methyldodecyl (meth)acrylate, tridecyl (meth)acrylate, 5-methyltridecyl (meth)acrylate, tetradecyl (meth)acrylate, pentadecyl (meth)acrylate; (meth)acrylates having 16 to 30 carbon atoms in the alcohol radical, such as, for example, hexadecyl (meth)acrylate, 2-methyl-hexadecyl (meth)acrylate, heptadecyl (meth)acrylate, 5-isopropylheptadecyl (meth)acrylate, 4-tert-butyloctadecyl (meth)acrylate, 5-ethyloctadecyl (meth)-acrylate, 3-isopropyloctadecyl (meth)acrylate, octadecyl (meth)acrylate, nonadecyl (meth)acrylate, eicosyl (meth)acrylate, cetyleicosyl (meth)acrylate, stearyleicosyl (meth)acrylate, docosyl (meth)acrylate and/or eicosyltetratriacontyl (meth)acrylate.

Furthermore, the impregnating compositions may comprise (meth)acrylates of ether alcohols, such as tetrahydrofurfuryl (meth)acrylate, vinyloxyethoxyethyl (meth)acrylate; amides and nitriles of (meth)acrylic acid, such as N-(3-dimethyl-aminopropyl) meth)acrylamide, N-(diethylphosphono)(meth)acrylamide, (1-methacryloylamido-2-methyl-2-propanol; cycloalkyl (meth)acrylates, such as 3-vinylcyclohexyl (meth)acrylate, bornyl (meth)acrylate; aryl (meth)acrylates, such as benzyl methacrylate or phenyl methacrylate, it being possible for the aryl radicals in each case to be unsubstituted or up to tetrasubstituted; carbonyl-containing methacrylates, such as 2-carboxyethyl methacrylate, carboxymethyl methacrylate, oxazolidinylethyl methacrylate, N-(methacryloyloxy)formamide; acetonyl methacrylate, N-methacryloylmorpholine, N-methacryloyl-2-pyrrolidinone, N-(2-methacryloyloxyethyl)-2-pyrrolidinone, N-(3-methacryloyloxypropyl) -2-pyrrolidinone, N-(2-methacryloyloxypentadecyl)-2-pyrrolidinone, N-(3-methacryloyloxyheptadecyl)-2-pyrrolidinone; glycidyl methacrylate; phosphorus, boron and/or silicon-containing methacrylates, such as 2-(dimethylphosphato)propyl methacrylate, 2-(ethylenephosphito)propyl methacrylate, dimethylphosphinomethyl methacrylate, dimethylphosphonoethyl methacrylate, diethyl methacryloyl phosphonate, dipropyl methacryloyl phosphate, 2-(dibutylphosphono)ethyl methacrylate, 2,3-butylenemethacryloylethyl borate, methyldiethoxymethacryloylethoxysilane, diethylphosphatoethyl methacrylate; vinyl halides, such as, for example, vinyl chloride, vinyl fluoride, vinylidene chloride and vinylidene fluoride; heterocyclic (meth)acrylates, such as 2-(1-imidazolyl)ethyl (meth)acrylate, 2-(4-morpholinyl)ethyl (meth)acrylate and 1-(2-methacryloyloxyethyl)-2-pyrrolidone; vinyl esters, such as vinyl acetate; styrene, substituted styrenes having an alkyl substituent in the side chain, such as, for example, α-methyl-styrene and α-ethylstyrene, substituted styrenes having an alkyl substituent on the ring, such as vinyl toluene and p-methylstyrene, halogenated styrenes, such as, for example, monochlorostyrenes, dichlorostyrenes, tribromostyrenes and tetrabromostyrenes; heterocyclic vinyl compounds, such as 2-vinylpyridine, 3-vinylpyridine, 2-methyl-5-vinylpyridine, 3-ethyl-4-vinylpyridine, 2,3-dimethyl-5-vinylpyridine, vinylpyrimidine, vinylpiperidine, 9-vinylcarbazole, 3-vinylcarbazole, 4-vinylcarbazole, 1-vinylimidazole, 2-methyl-1-vinylimidazole, N-vinylpyrrolidone, 2-vinylpyrrolidone, N-vinylpyrrolidine, 3-vinylpyrrolidine, N-vinylcaprolactam, N-vinylbutyrolactam, vinyloxolane, vinylfuran, vinylthiophene, vinylthiolane, vinylthiazoles and hydrogenated vinylthiazoles, vinyloxazoles and hydrogenated vinyloxazoles; vinyl ethers and isoprenyl ethers.

Preferred comonomers are, inter alia, (meth)acrylates which are derived from saturated alcohols having 7 to 15 carbon atoms or 16 to 30 carbon atoms.

The abovementioned hydroxyalkyl (meth)acrylates, crosslinking (meth)acrylates and comonomers can be used individually or as a mixture.

Preferred impregnating compositions comprise, for example, 5% by weight to 95% by weight, preferably 25 to 60% by weight, of hydroxyalkyl (meth)acrylates, in particular 2-hydroxyethyl methacrylate and/or 2-hydroxypropyl methacrylate; 5% by weight to 75% by weight, preferably 20% by weight to 50% by weight, of crosslinking (meth)acrylates, in particular ethylene glycol dimethacrylate, 1,6-hexanediol dimethacrylate triethylene glycol dimethacrylate, tetraethylene glycol dimethacrylate and/or trimethylolpropane trimethacrylate; and 5% by weight to 50% by weight, preferably 10% by weight to 40% by weight, based in each case on the weight of the impregnating composition, of comonomers, in particular (meth)acrylates having 7 to 15 carbon atoms in the alcohol radical.

According to a further expedient modification, the impregnating compositions according to the invention can preferably comprise 40% by weight to 80% by weight, preferably 50 to 70% by weight, of crosslinking (meth)acrylates, in particular ethylene glycol dimethacrylate, 1,6-hexanediol dimethacrylate, triethylene glycol dimethacrylate, tetraethylene glycol dimethacrylate and/or trimethylolpropane trimethacrylate; and 10% by weight to 50% by weight, preferably 20 to 40% by weight, based in each case on the weight of the impregnating composition, of comonomers, in particular (meth)acrylates having 7 to 15 carbon atoms in the alcohol radical.

Of particular interest furthermore are impregnating compositions which comprise

30% by weight to 90% by weight, preferably 60 to 85% by weight, of hydroxyalkyl (meth)acrylates, in particular 2-hydroxyethyl methacrylate and/or 2-hydroxypropyl methacrylate; and 1% by weight to 70% by weight, preferably 5% by weight to 50% by weight, based in each case on the weight of the impregnating composition, of crosslinking (meth)acrylates, in particular ethylene glycol dimethacrylate, 1,6-hexanediol dimethacrylate, triethylene glycol dimethacrylate, tetraethylene glycol dimethacrylate and/or trimethylolpropane trimethacrylate.

For hardening, the impregnating composition preferably has at least one initiator. Suitable initiators are known to those skilled in the art. These include, inter alia, azo compounds, peroxy compounds, persulphate compounds or azoamidines. Non-limiting examples are dibenzoyl peroxide, dicumene peroxide, cumene hydroperoxide, diisopropyl peroxydicarbonate, bis(4-tert-butylcyclohexyl)peroxydicarbonate,dipotassium persulphate, ammonium peroxydisulphate, 2,2′-azobis(2-methylpropionitrile) (AIBN), 2,2′-azobix(isobutyramidine) hydrochloride, benzpinacol, dibenzyl derivatives, methyl ethylene ketone peroxide, 1,1-azobiscyclohexanecarbonitrile, methyl ethyl ketone peroxide, acetylacetone peroxide, dilauryl peroxide, didecanoyl peroxide, tert-butyl per-2-ethylhexanoate, ketone peroxide, methyl isobutyl ketone peroxide, cyclohexanone peroxide, dibenzoyl peroxide, tert-butyl peroxybenzoate, tert-butyl peroxyisopropylcarbonate, 2,5-bis(2-ethyl-hexanoylperoxy) -2,5-dimethylhexane, tert-butyl peroxy-2-ethylhexanoate, tert-butyl peroxy-3,5,5-trimethylhexanoate, tert-butyl peroxyisobutyrate, tert-butyl peroxyacetate, dicumyl peroxide, 1,1-bis(tert-butylperoxy)cyclohexane, 1,1-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane, cumyl hydroperoxide, tert-butyl hydroperoxide, bis(4-tert-butylcyclohexyl) peroxydicarbonate and the free radical formers obtainable from DuPont under the name ®Vazo, for example ®Vazo V50 and ®Vazo WS.

The initiators are usually used in an amount in the range from 0.01 to 10% by weight, preferably from 0.05 to 5% by weight and very particularly preferably 0.1 to 1% by weight, based on the weight of the impregnating composition.

In addition, initiator systems which are used for cold hardening are also known. In general, these initiators have a redox system comprising an accelerator and one or more of the peroxidic catalysts or initiators described above. The amounts used here are frequently calculated such that the impregnating composition hardens within 5 to 120 minutes at temperatures in the range from 5 to 50° C.

The accelerator is usually used in an amount of 0.01 to 5% by weight, preferably 0.5 to 1.5% by weight, based on the weight of the impregnating composition. The compounds particular suitable as accelerators include, inter alia, amines and sulphimides, such as, for example, tributylamine and benzoic acid sulphimide (saccharine).

Furthermore, organic metal salts which are usually used in the range from 0.0001 to 0.5% by weight, based on the weight of the impregnating composition, can serve as accelerators. These include, inter alia, copper octanoate.

Furthermore, the impregnating composition may comprise at least one emulsifier. The total amount of emulsifier is preferably 0.1 to 15% by weight, in particular 2 to 10% by weight, based on the total weight of the impregnating composition. Particularly suitable emulsifiers are non-ionic emulsifiers or mixtures thereof, in particular alkylpolyglycol ethers, preferably having 8 to 20 carbon atoms in the alkyl radical and 8 to 40 ethylene oxide units; alkylarylpolyglycol ethers, preferably having 8 to 20 carbon atoms in the alkyl or alkylaryl radical and 8 to 40 ethylene oxide units; ethylene oxide/propylene oxide copolymers, preferably block copolymers, for example having 8 to 40 ethylene oxide or propylene oxide units.

Of particular interest are especially impregnating compositions whose solubility in the salt-containing cleaning composition is not more than 6% by weight, particularly preferably not more than 4% by weight. Expediently, the solubility of hydroxypropyl methacrylate in the salt-containing cleaning composition is preferably not more than 5% by weight, particularly preferably not more than 4% by weight. The solubility of the impregnating composition in the cleaning composition can be adjusted, inter alia, via the salt content of the cleaning composition.

Of particular interest are especially impregnating compositions which preferably have a density in the range from 0.9 g/l to 1.2 g/l, particularly preferably in the range from 0.95 g/l to 1.1 g/l.

According to an expedient modification of the process according to the invention, the cleaning composition may have a density which differs substantially from the density of the impregnating composition. The density difference of cleaning composition and impregnating composition is preferably at least 0.01 g/l, particularly preferably at least 0.05 g/l and very particularly preferably at least 0.1 g/l.

By the measures according to the invention, the process can advantageously be carried out continuously. In the context of the invention, the expression “continuously” means that the cleaning composition can be separated continuously from excess impregnating composition and reused.

Expediently, the cleaning composition can be circulated, the cleaning composition first being brought into contact with the impregnated metal surface. As a result, excess impregnating composition is taken up by the cleaning composition and removed from the metal part.

The cleaning composition thus obtained can then be separated from the impregnating composition. For this purpose, after being brought into contact with the metal surface, the cleaning composition can advantageously be passed into known apparatuses, such as, for example, oil separators, which separate off the impregnating composition from the surface of the cleaning composition, or internals which collect impregnating composition at the bottom of the cleaning composition. This separation takes place particularly easily at a low solubility of the impregnating composition in the cleaning composition.

The cleaning composition thus depleted with respect to the impregnating composition can then be used again for recovering excess impregnating composition.

The impregnating composition recovered by the recovery step can be used directly for impregnating further metal parts, it being possible, if appropriate, to replenish monomers which remain in the cleaning composition owing to a solubility. In the case of preferred processes, however, only very small amounts are required for this purpose since the solubility of these monomers can be kept low owing to the salt content.

The process according to the invention for recovering impregnating compositions can be used in particular in processes for impregnating metal parts. Accordingly, the present invention also relates to processes for impregnating metal parts.

Processes for impregnating metal parts have been carried out for decades, these being described in detail, inter alia, in the publications U.S. Pat. No. 6,712,910, U.S. Pat. No. 6,761,775, U.S. Pat. No. 5,518,632, DE 27 18 770, EP -A-0 014 062 and GB-1 547 801.

In general, the metal parts to be impregnated are first cleaned. This can be effected, for example, by steam or by alkaline solutions, metal parts then being dried.

After the cleaning, the metal parts are preferably first exposed to a dry vacuum. As a result of this, the air is removed from the pores. The metal part is then impregnated with the impregnating composition. For this purpose, the metal part can be immersed in the impregnating composition in vacuo. The vacuum may be, for example, 0.1 mbar to 100 mbar, preferably 1 mbar to 20 mbar. The steps described above generally require about 1 to 10 minutes, without it being intended to impose a limitation hereby.

Thereafter, the vacuum can be eliminated and the metal part immersed in the impregnating composition can be stored at atmospheric pressure for in general about 2 to 20 minutes, preferably 5 to 10 minutes. As a result of the atmospheric pressure, the pores are completely filled with impregnating composition.

After the impregnation of the metal part, a major part of the impregnating composition can be separated from the metal part by centrifuging or similar methods.

The impregnated metal part is then brought into contact, according to the invention, with a salt-containing cleaning composition. For this purpose, for example, the cleaning solution can be sprayed onto the metal part. In addition, the metal part can be immersed in the cleaning composition. An additional cleaning effect can be achieved by passing the cleaning composition under pressure over the surface of the metal part.

Thereafter, excess cleaning composition can be removed from the metal part by centrifuging or similar methods.

In a further step, remaining residues of the cleaning composition and of the impregnating composition can be eliminated by washing with washwater. The washwater can likewise be separated from the metal part by centrifuging. Advantageously, the washwater can be used several times, since any small residues of impregnating composition are removed from the metal surface by this step. In addition, owing to the low content of impregnating composition, the washwater can be biologically purified.

After the excess impregnating composition has been removed from the metal part, the impregnating composition present in the metal part is hardened. Depending on the initiator system, this can be effected at room temperature or by heating. Expediently, the metal part can be heated for example to a temperature in the range from 40 to 120° C., preferably 60 to 90° C., in order to harden the impregnating composition. Furthermore, the hardening can be effected at a temperature in the range from 5° C. to 50° C.

Plants for carrying out the process described above correspond substantially to the plants which are described in the prior art cited above, in particular in the publications U.S. Pat. No. 6,712,910, U.S. Pat. No. 6,761,775, U.S. Pat. No. 5,518,632, DE 27 18 770, EP-A-0 014 062 and GB 1 547 801. These plants generally comprise a plurality of containers in order to carry out the individual steps.

Claims

1. A process for recovering impregnating compositions, wherein an impregnated metal part is brought into contact with a salt-containing cleaning composition which comprises at least 0.7% by weight of at least one salt.

2. The process according to claim 1, wherein the salt is an inorganic salt.

3. The process according to claim 1, wherein the salt is a phosphate.

4. The process according to claim 1, wherein the cleaning composition is circulated.

5. The process according to claim 1, wherein the cleaning composition comprises 4 to 20% by weight of salt.

6. The process according to claim 1, wherein the impregnating composition comprises (meth)acrylates.

7. The process according to claim 6, wherein the impregnating composition comprises 5% by weight to 95% by weight of hydroxyalkyl (meth)acrylates.

8. The process according to claim 6, wherein the impregnating composition comprises 5% by weight to 75% by weight of crosslinking (meth)acrylates.

9. The process according to claim 6, wherein the impregnating composition comprises 5% by weight to 60% by weight of comonomers.

10. The process according to claim 1, wherein the impregnating composition comprises at least one initiator.

11. The process according to claim 10, wherein the impregnating composition comprises from 0.01 to 10% by weight of initiator.

12. The process according to claim 1, wherein the impregnating composition comprises at least one emulsifier.

13. The process according to claim 1, wherein the solubility of the impregnating composition in the salt-containing cleaning composition is not more than 6% by weight.

14. The process according to claim 7, wherein the solubility of hydroxypropyl methacrylate in the salt-containing cleaning composition is not more than 4% by weight.

15. The process according to claim 1, wherein the density of the impregnating composition is in the range from 0.9 g/l to 1.2 g/l.

16. The process according to claim 1, wherein the density difference between the impregnating composition and cleaning composition is at least 0.05 g/l.

17. A process for impregnating metal parts, comprising at least one step for recovering impregnating compositions according to the process of claim 1.

18. The process according to claim 17, wherein the metal part is first impregnated with an impregnating composition, the impregnated metal part is then brought into contact with a salt-containing cleaning composition, the metal part is then cleaned with water and the impregnating composition present in the metal part is hardened.

19. The process according to claim 17, wherein the impregnating composition is brought into contact with the metal part at a pressure in the range from 0.1 mbar to 100 mbar.

20. The process according to claim 17, wherein the metal part is exposed to a vacuum before the impregnation.

21. The process according to claim 17, wherein excess impregnating composition is removed by centrifuging after the impregnation of the metal part.

22. The process according to claim 18, wherein the impregnating composition present in the metal part is hardened at a temperature in the range from 60 to 120° C.

23. The process according to claim 18, wherein the impregnating composition present in the metal part is hardened at a temperature in the range from 5 to 50° C.