EP0652283B1 - Cleaning of plastic - Google Patents

Description

The invention relates to a method and means for cleaning plastics in unprocessed and processed form, in particular plastics that come into contact with chemically highly pure liquids. During production, use and especially during storage, contaminants can get into the liquids from the container surfaces, so that they become noticeably contaminated, especially after prolonged contact and especially at higher temperatures, even if only special methods such as neutron activation are used in the container walls Impurities are detectable.

High-purity liquids for analytical, diagnostic and therapeutic purposes as well as in semiconductor manufacturing are increasingly being processed and stored in plastic containers and conveyed in plastic lines, with some of the valuable high-purity liquid generally being used to rinse out the line or the container and then cleaned or discarded again must become. This method is therefore very unsatisfactory.

It has been found that high-purity liquids stored in extruded plastic containers contain traces of heavy metals which were not previously detectable in the liquid and are only detectable in slight traces in the plastic surface. Precautionary cleaning of the plastic surfaces with an alkanolamine, as described in "Metallfläche" 38 (1984) 4, pages 163 to 168, especially page 164, described, brought no improvement.

From US-A 3 908 680 a method for cleaning plastic contact lenses by treatment with a basic aqueous solution of an active oxygen-releasing per compound is known, which is characterized in that the contact lenses are additionally treated with an acidic aqueous solution of an active oxygen releasing per compound treated and after removal from the second aqueous solution treated with a nonionic detergent and then rinsed with water. Preferably, additional chelating agents can be added to the first and / or second solution, namely preferably aminocarboxylic acid compounds, and the acidic groups can also be used in the form of their water-soluble salts. Other chelating agents mentioned are citric acid or citrates and polyphosphates. But even with this known method, the problem underlying the invention could not be solved satisfactorily.

From DE-A-30 22 767 an aqueous-liquid, builder-containing detergent is known which contains at least one synthetic, detergent-active compound, an alkali metal fatty acid soap, mono-, di- or triethanol- or isopropanolamine, a hydrotrope and, as a builder, sodium tripolyphosphate and tetrapotassium pyrophosphate and optionally contains hydrogen peroxide. This detergent is stable for at least one month at both 0 ° C and 52 ° C. As a washing bleach, it is comparable to a high-performance powder with 17% perborate content.

Surprisingly, it has now been found that a single-stage process for cleaning plastics gives good results, which is characterized in that the plastic is brought into contact with an alkaline aqueous solution which contains an inorganic peroxide compound and a complexing agent without acid groups in free or salt form and, if necessary, contains a base.

• a) einem anorganischen Peroxid,
• b) einem Komplexbildner ohne saure Gruppen in freier oder Salzform und, falls erforderlich,
• c) einer Base.

The invention further relates to an aqueous, alkaline builder-free cleaning solution, which is characterized by a content of

• a) an inorganic peroxide,
• b) a complexing agent without acid groups in free or salt form and, if necessary,
• c) a base.

In a preferred embodiment of the invention, the complexing agent is an alkanolamine, although this - as mentioned above - by itself does not produce a satisfactory effect. This is because it was also found that the extrusion tool is a possible source of the contaminants introduced. The presence of an oxidizing agent is therefore required to bring the traces of heavy metals, in particular iron, into ionic form so that the complexing agent can develop its effect. Iron is treated here and in the following as a "leading impurity" because it is by far the highest concentration compared to other heavy metals and is also difficult to remove. Experiments have shown that removal of the iron usually also removes the other heavy metal contaminants.

For the complexation of iron, in the case of the preferred complexing agents, the alkanolamines, the hydroxyl groups appear more important than the nitrogen atom (which appears to be more important for other heavy metals such as chromium). For good complex formation, two hydroxyl groups on the same nitrogen atom which are bound via "spacer" groups are required. The "spacer" can be an alkylene group with up to 6 carbon atoms, which can be interrupted by oxygen atoms and nitrogen functions, which should be understood to mean, for example, compounds of the type of ethylenediamine tetra (hydroxyalkyl) compounds.

Easily accessible alkanolamines such as the adducts of ethylene and propylene oxide with ammonia, primary alkylamines with 1 to 4 carbon atoms and ethylenediamine are preferred. Compounds which can be easily obtained, for example by sublimation or distillation, are particularly preferred for higher purification requirements.

The alkanolamines have the further advantage that they not only do not adhere to the organic substrates, but also detach inorganic and organic impurities adhering to them. In addition, their "dirt-carrying capacity" prevents recontamination of the surfaces that have already been cleaned. The preferred alkanolamine is triethanolamine. It is a mild alkali (a 0.1 N aqueous solution shows a pH of 10.5) and does not cause any skin irritation.

Alkali peroxides can be used as inorganic peroxide compounds, but - as with the alkanolamines - the addition of a base is not necessary. However, hydrogen peroxide is advantageously used. The easiest way to determine the amount of hydrogen peroxide is to carry out preliminary tests under the conditions of use, since this compound is known to be temperature sensitive. However, it has been shown that the alkanolamines used preferably as complexing agents have a stabilizing effect.

At temperatures up to about 80 ° C., concentrations of at least about 5% by weight, based on the finished cleaning solution, are expedient. Lower concentrations often require a long exposure time, higher concentrations are generally only required if the substrate is heavily contaminated.

The alkaline cleaning solution advantageously has a pH in the range from 7.5 to 12, preferably 8 to 10.

Since alkanolamines react alkaline, the addition of another base is not always necessary when they are used. However, it is often useful to add aqueous ammonia solution, and the amount of the more expensive alkanolamine can be reduced. The cheapest in individual cases The combination can be determined by simple preliminary tests.

A preferred embodiment of the invention is that the plastic is used in finely divided form so that the contaminants are extracted by the cleaning solution according to the invention.

The cleaning method according to the invention can be used over a wide temperature range. When cleaning molded articles, the temperature sensitivity of this molded article will be used and, for example, working in a range from room temperature to approximately 120 ° C., preferably approximately 50 ° C. If a finely divided plastic is extracted, it is advantageous to choose a higher temperature, suitably about 80 to 120 ° C. In general, shorter treatment times are sufficient at higher temperatures. Here, however, the increased decomposition of the peroxide compound must be taken into account, that is, if necessary, metered in or used in sufficient concentration right from the start.

The cleaning method according to the invention is generally applicable to all plastics. It also has an excellent effect on hydrophobic plastics that are used in the packaging sector, such as polyethylenes, polypropylenes, polyvinyl chlorides and polyesters. However, it can also be used with advantage for the so-called high-performance plastics such as polyacetals, polyphenylene sulfides, polyether ketones and, above all, fluoropolymers.

As mentioned at the beginning, in contrast to the process known from DE-C 24 43 147, a one-stage treatment is generally sufficient. Pretreatment or, if necessary, post-treatment with acidic solutions should not be ruled out. So come For example, a hydrofluoric acid aftertreatment may be considered, it being possible to add peroxides and optionally auxiliaries such as surfactants to the HF solution. A mixture of the composition (in% by weight) has proven itself:
89.5% water
10.0% H₂O₂, 30%
0.5% HF, 50%
Such an aftertreatment can, if necessary, remove residual iron impurities.

In the process according to the invention, further additives, such as further complexing agents, surfactants, buffers or the like, can also be added to the alkaline solution. Nonionic surfactants such as adducts of ethylene and / or propylene oxide with long-chain alcohols, alkylphenols and the like are particularly suitable as surfactants. Amounts up to 400 ppm, preferably 100 to 300 ppm, especially about 200 ppm, based on the finished cleaning solution, are expedient. This accelerates the wetting of the surfaces without causing disruptive foaming.

According to the invention, multiple treatment of the substrates can also be considered if significant migration of contaminants from the depth of the plastic can be determined, for example after the substrate has been heated.

The addition of organic solvents is also possible, although generally not advantageous, since the recovery is complex.

Particularly preferred embodiments of the invention are explained in more detail in the following examples.

In the examples, high purity chemicals were used to ensure that these were not heavy metals be introduced, which falsify the measured results. These examples are therefore of a model nature, since in practice the technical qualities of the chemicals used can be used because of the dirt-carrying capacity of the alkanolamines used.

All tests are carried out taking into account the usual rules of trace analysis work. Thus, double-distilled water and triethanolamine distilled under high vacuum were used; Hydrogen peroxide and ammonia solution were commercially available "VLSI" qualities (Merck, Darmstadt). The iron was determined by atomic absorption spectrometry (atomic absorption spectrometer from Varian Spectraa 400 and graphite furnace and sampler unit GTA 96) at 248.3 nm, slit width 0.2 nm, background correction: deuterium lamp, injection volume 20 µl, detection limit 0.3 ng / ml, linear calibration range 0 to 13 ng / ml.

A copolymer of 96% by weight tetrafluoroethylene and 4% by weight perfluoro-n-propyl vinyl ether with an MFI 2 g / 10 minutes, determined according to ASTM D 1238-57 T (at 372 ° C. and 5 kg load), served as the substrate. with an iron content of 500 to 550 ng / g copolymer. As can be seen from the following experiments, this iron content is largely in the interior of the plastic, since repeated treatment - without intermediate heating of the plastic - only resulted in relatively small amounts of extracted iron.

Examples 1 to 7

50 g of plastic granules with an average particle size of 16.9 mm 3 are left for 30 or 60 minutes with 25 ml of cleaning solution of the composition given in Table 1. The results are shown in Table 1 below.

Table 1 contains comparative examples V1 to V3 in which one of the components of the cleaning solution according to the invention is missing.

The composition of the cleaning solution is defined as follows: "parts" are parts by weight, the hydrogen peroxide being used as a 30% aqueous solution and the ammonia as a 25% aqueous solution. The concentration of triethanolamine (TEA) is given in ppm. The extraction result is given in ng iron / g plastic. Table 1 example Parts TEA [ppm] Fe [ng / g] H₂O H₂O₂ NH₃ 30 min 60 min V1 4th - 1 500 0.56 0.69 V2 4th - 1 1000 0.49 0.62 V3 3rd 1 1 --- 0.82 1.07 1 3rd 1 1 500 1.04 1.29 2nd 3rd 1 1 1000 1.10 1.41 3rd 3rd 1 0.5 1000 1.02 1.39 4th 4th 1 1 1000 0.91 1.32 5 4th 1 1 10,000 1.58 2.01 6 5 1 1 1000 0.82 1.21 7 6 1 1 1000 0.92 0.96

Comparative examples V1 and V2 show that an ammoniacal TEA solution shows only a relatively low extraction effect. Comparative example C3 shows that an ammoniacal hydrogen peroxide solution produces significantly poorer extraction results than the cleaning solutions according to the invention.

The examples according to the invention show that - as expected - the cleaning effect is concentration-dependent. This applies above all to the TEA, but with the existing measuring device, it is not possible to provide reliably reproducible information at even higher concentrations. However, higher concentrations of TEA are advantageous not only with regard to the cleaning effect, but also with regard to the stabilization of the hydrogen peroxide.

Example 8

The procedure of Example 3 is followed, but extraction is carried out for 75 minutes, a final concentration of extracted iron of 1.5 ng / g of plastic being achieved. By re-using a fresh solution, 0.47 ng / g is extracted after 60 minutes.

Example 9

Example 4 is repeated, but the sample is shaken at about 100 Hz during the extraction. After 30 minutes, 3.59 ng Fe / g plastic are extracted.

Examples 10 to 12

Vials with a capacity of 50 ml formed from the copolymer are filled with the cleaning solution used in Example 4 and stored at different temperatures for 30 or 60 minutes. Table 2 shows the extraction results. Table 2 example Temperature [° C] Fe [ng / g] 30 min 60 min 10th Room temperature 0.73 1.47 11 50 1.30 1.81 12th 80 1.83 2.35

Example 13

Example 11 is repeated, but the vial is immersed at 3/4 its height in an ultrasonic bath. After 30 minutes 1.58 and after 60 minutes 2.31 ng Fe / g plastic are extracted.

Examples 14 to 16

Example 3 is repeated, but increasing amounts of a nonionic surfactant (commercial product ®TRITON X 100) are added. The results are shown in Table 3. Table 3 example Surfactant [ppm] Fe [ng / g] 30 min 60 min 3rd 0 1.02 1.39 14 100 1.51 1.59 15 200 1.40 1.67 16 400 1.83 2.42

With increasing addition of surfactant, foaming increases. In addition, a faster decomposition of the hydrogen peroxide is observed in Example 16.

Examples 17 to 20

The vials used in Examples 10 to 12 are filled at room temperature with 25 ml of the cleaning solution shown in Table 4 and stored for 30 or 60 minutes. Table 4 shows the extracted chromium content [ng (Cr)], based on the amount of plastic used [g]. V4 is a comparative example with water. Table 4 example Parts TEA [ppm] Cr [ng / g] H₂O H₂O₂ NH₃ 30 min 60 min 17th 3rd 1 1 1000 0.70 0.95 18th 3rd 1 0.25 1000 0.40 0.59 19th 4th 1 1 1000 0.54 0.72 20th 5 1 1 1000 0.45 0.65 V4 1 - - --- 0.031 ---

The same definitions apply for table 4 as for table 1.

Claims (8)

1. A process for the purification of plastics, which comprises bringing the plastic into contact with an alkaline aqueous solution which contains an inorganic peroxide compound and a complexing agent without acidic groups in free or salt form and, if required, a base.
2. The process as claimed in claim 1, wherein the plastic is used in the form of a molding, which is purified on the surface.
3. The process as claimed in claim 1, wherein the plastic is used in finely divided form and the impurities are extracted.
4. An aqueous, alkaline purification solution, which is free of builder, containing

   a) an inorganic peroxide,

   b) a complexing agent without acidic groups in free or salt form and, if required,

   c) a base.
5. The solution as claimed in claim 4, wherein the complexing agent is an alkanolamine.
6. The solution as claimed in claim 4 or 5, wherein the complexing agent is triethanolamine.
7. The solution as claimed in one or more of claims 4 to 6, wherein the peroxide is hydrogen peroxide.
8. The solution as claimed in one or more of claims 4 to 7, wherein the solution contains a surfactant.