METHOD FOR ETCHING AT LEAST ONE SURFACE OF A PLASTIC SUBSTRATE

Abstract

The present invention relates to a method for etching at least one surface of a plastic substrate, the method comprising steps (A) to (C), wherein step (B) comprises a contacting with a pre-treatment composition comprising one or more than one fluorine-free surface active compound, and step (C) comprises a contacting with an etching composition comprising chromic acid, wherein after step (B) and prior to step (C) no rinsing is applied, and the etching composition is substantially free of fluorine-containing surface active compounds.

Description

FIELD OF THE INVENTION

The present invention relates to a method for etching at least one surface of a plastic substrate, the method comprising the steps (A) to (C), wherein step (B) comprises a contacting with a pre-treatment composition comprising one or more than one fluorine-free surface active compound, and step (C) comprises a contacting with an etching composition comprising chromic acid, wherein after step (B) and prior to step (C) no rinsing is applied, and the etching composition is substantially free of fluorine-containing surface active compounds.

BACKGROUND OF THE INVENTION

Metallizing non-metallic substrates such as plastic substrates has a long history in modern technology. Typical applications are found in automotive industry as well as for sanitary articles.

However, making a plastic substrate receptive for a metal layer is demanding. Typically, a respective method starts with a surface modification of the substrate's surface, typically known as etching. Usually, a sensitive balance is required in order to ensure a sufficient surface roughening without causing too strong defects.

A variety of methods and etching compositions are known, including very effective etching compositions comprising chromic acid. However, in order to obtain an equally distributed etching pattern all over a surface of a substrate, sufficient wettability is required. This becomes even more demanding if sophisticated surface geometries are present, for example holes and/or bucklings (i.e., kinks) forming angles ranging from 60° to 130°. Under such conditions often insufficient etching is observed in particular in such areas; often even upon utilization of surface active compounds.

Furthermore, chromic acid is highly corrosive and a strong oxidation agent, thereby subjecting organic compounds to strong decomposition. Although the chemical stability of surface-active compounds can be increased by fluorination, waste water treatment and environmental sustainability becomes an issue in return. Therefore, there is a still ongoing demand to further improve existing chromic acid etching processes.

Objective of the Present Invention

It is therefore the objective of the present invention to provide an etching process that eliminates the use of environmentally questionable surface active compounds without compromising the etching quality.

It is furthermore an objective to even improve the etching and process quality compared to commonly used etching compositions comprising fluorinated surface active compounds.

SUMMARY OF THE INVENTION

Above mentioned objectives are solved by a method for etching at least one surface of a plastic substrate, the method comprising the steps

• • (A) providing the substrate,
  • (B) contacting the substrate with an aqueous pre-treatment composition such that a pre-treated substrate is obtained, the pre-treatment composition comprising
    • (B-a) one or more than one fluorine-free surface active compound, and
  • (C) contacting the pre-treated substrate with an etching composition in an etching compartment such that an etched substrate is obtained, the etching composition comprising
    • (C-a) chromic acid, and
    • (C-b) trivalent chromium ions,
  • characterized in that
    • the pre-treated substrate is not rinsed after step (B) and prior to step (C), and
    • the etching composition is substantially free of, preferably does not comprise, fluorine-containing surface active compounds.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is primarily based on a specific combination of steps, namely consecutive steps (B) and (C) as defined above without a rinsing step between both steps (i.e. the pre-treated substrate is not rinsed after step (B) and prior to step (C)). Own experiments have shown that the surface of the plastic substrate remains wetted with the one or more than one fluorine-free surface active compound contained in the pre-treatment composition. Thus, preferred is a method of the present invention, wherein after step (B) at least a portion of (B-a) remains on the substrate while the substrate is transferred to step (C).

The fluorine-free surface active compounds remaining on the substrate are present in a sufficient amount to allow chromic acid to excellently reach even more sophisticated geometries. It appears that it is of great benefit if the surface is already covered with a surface active compound prior to contacting with chromic acid for etching. It was in particular surprising that under such conditions no surface active compounds are additionally needed in the etching composition at all.

Therefore, commonly used fluorine-containing surface active compounds are not necessarily needed in combination with chromic acid etching compositions in order to achieve excellent surface wetting and thus, excellent etching results. Own experiments particularly show that (i) not only environmentally questionable surface active compounds can be avoided (such as fluorinated surface active compounds) but (ii) even an improved wettability and therefore etching quality is achieved if the method of the present invention is carried out compared to commonly utilized chromic acid etching compositions. As a result, the number of defective goods can be even further reduced. Further details are given in the Examples below in the text, wherein details given therein most preferably also apply in general to the method of the present invention as defined throughout the present description.

Step (A)

In step (A) of the method of the present invention, the substrate is provided, which is a plastic substrate.

Preferred is a method of the present invention, wherein in step (A) the substrate comprises a thermoplastic substrate, preferably an amorphous thermoplastic substrate and/or a semi-crystalline thermoplastic substrate.

More preferred is a method of the present invention, wherein in step (A) the substrate comprises butadiene moieties, preferably polybutadiene.

Also preferred is a method of the present invention, wherein in step (A) the substrate comprises nitrile moieties.

Also preferred is a method of the present invention, wherein in step (A) the substrate comprises acryl moieties.

Very preferred is a method of the present invention, wherein in step (A) the substrate comprises polymerized styrene.

Most preferred is a method of the present invention, wherein in step (A) the substrate comprises acrylonitrile butadiene styrene (ABS), acrylonitrile butadiene styrene—polycarbonate (ABS-PC), polypropylene (PP), polyamide (PA), polyetherimide (PEI), a polyetherketone (PEK), or mixtures thereof, preferably acrylonitrile butadiene styrene (ABS) and/or acrylonitrile butadiene styrene—polycarbonate (ABS-PC). Such plastic substrates are typically used in decorative applications such as automotive parts, in particular ABS and ABS-PC.

Preferred is a method of the present invention, wherein the polyetherketone (PEK) comprises polyaryletherketone (PAEK), poly ether ether ketone (PEEK), poly ether ether ether ketone (PEEEK), poly ether ether ketone ketone (PEEKK), poly ether ketone ether ketone ketone (PEKEKK), poly ether ketone ketone (PEKK), and/or mixtures thereof, preferably poly ether ether ketone (PEEK), polyaryletherketone (PAEK), and/or mixtures thereof.

As mentioned already above, in particular substrates with a sophisticated geometry are in is particular challenging for metallization. In many technical applications substrates have an irregular surface including caverns, holes, reliefs, recesses, slits, kinks, etc., forming a number of difficult to access areas on the substrate especially for viscous compositions such as etching compositions comprising chromic acid. However, the method of the present invention in particular addresses such “problematic” plastic substrates. Therefore, preferred is a method of the present invention, wherein in step (A) the substrate comprises at least one hole, cavern, recess, slit, and/or at least one buckling (in the sense of a kink), wherein at least one thereof constitutes at least two subareas relative to each other with an angle not being 180°, preferably having an angle ranging from 60° to 160°, more preferably from 65° to 140°, more preferably from 70° to 110°, most preferably from 80° to 100°. Although considered as problematic/sophisticated geometries, excellent results were obtained with the method of the present invention in particular for such substrates.

Step (B)

In step (B) of the method of the present invention the substrate is contacted with the aqueous pre-treatment composition such that a pre-treated substrate is obtained. The aqueous pre-treatment composition comprises (B-a) one or more than one fluorine-free surface active compound.

In the context of the present invention, the one or more than one fluorine-free surface active compound is a compound, which reduces the surface tension of the aqueous pre-treatment composition compared to the surface tension of water.

Preferred is a method of the present invention, wherein said one or more than one fluorine-free surface active compound is an organic fluorine-free surface active compound.

More preferred is a method of the present invention, wherein said one or more than one fluorine-free surface active compound is a fluorine-free surfactant.

Most preferred is a method of the present invention, wherein said one or more than one fluorine-free surface active compound is an organic fluorine-free surfactant.

Preferred is a method of the present invention, wherein said one or more than one fluorine-free surface active compound is not an organic solvent. Respectively, a method of the present invention is preferred, wherein the aqueous pre-treatment composition is substantially free of, preferably does not comprise, an organic solvent.

More preferably, the aqueous pre-treatment composition is substantially free of, preferably does not comprise, formamide, dimethylformamide, ethylene glycol, or diethylene glycol.

Preferred is a method of the present invention, wherein said one or more than one fluorine-free surface active compound is saturated. More preferred, the one or more than one is fluorine-free surface active compound consists of carbon atoms, hydrogen atoms, and oxygen atoms.

Preferred is a method of the present invention, wherein in (B-a) the one or more than one fluorine-free surface active compound comprises a non-ionic surfactant, preferably a non-ionic alkyl polyethylene glycol ether, more preferably a non-ionic alcohol alkoxylate, most preferably a non-ionic fatty alcohol ethoxylate.

More preferred is a method of the present invention, wherein in the non-ionic alcohol alkoxylate the alcohol comprises 6 to 20 carbon atoms, preferably 7 to 18, more preferably 8 to 16, even more preferably 9 to 14, most preferably 10 to 12. This preferably applies likewise to the very preferred non-ionic fatty alcohol ethoxylate.

Preferred is a method of the present invention, wherein in the non-ionic alcohol alkoxylate the alkoxylate comprises 10 to 30 carbon atoms, preferably 12 to 28, more preferably 14 to 26, even more preferably 16 to 24, most preferably 18 to 22. This preferably applies likewise to the very preferred non-ionic fatty alcohol ethoxylate.

Preferred is a method of the present invention, wherein the non-ionic alcohol alkoxylate comprises a total of 20 to 40 carbon atoms, preferably 22 to 38, more preferably 24 to 36, even more preferably 26 to 34, most preferably 28 to 32. This preferably applies likewise to the very preferred non-ionic fatty alcohol ethoxylate.

Very preferred is a method of the present invention, wherein the non-ionic alcohol alkoxylate comprises a total of 28 to 34 carbon atoms, wherein thereof the alcohol comprises 10 to 12 carbon atoms and the alkoxylate 18 to 22 carbon atoms. This preferably applies likewise to the very preferred non-ionic fatty alcohol ethoxylate.

Most preferred is a method of the present invention, wherein said one or more than one fluorine-free surface active compound is the only surface active compound utilized throughout steps (B) and (C).

Preferred is a method of the present invention, wherein in the pre-treatment composition (B-a) has a total concentration ranging from 0.1 wt.-% to 40 wt.-%, based on the total weight of the pre-treatment composition, preferably from 0.2 wt.-% to 20 wt.-%, even more preferably from 0.3 wt.-% to 10 wt.-%, most preferably from 0.4 wt.-% to 5 wt.-%. Most preferably these total concentrations apply and no further surface active compounds are present in the pre-treatment composition.

Preferred is a method of the present invention, wherein in step (B) the pre-treatment is composition has a pH ranging from 1 to 9, preferably from 1.5 to 7.5 more preferably from 2 to 6.5, even more preferably from 2.5 to 5.5, most preferably from 3 to 5. It is generally preferred that the pH is acidic.

The pre-treatment composition utilized in the method of the present invention is aqueous. Thus, the primary (i.e. more than 50 wt.-%) solvent is preferably water, preferably the pre-treatment composition comprises 60 wt.-% or more water, based on the total weight of the pre-treatment composition, more preferably 70 wt.-% or more, even more preferably 80 wt.-% or more, yet even more preferably 90 wt.-% or more, most preferably 95 wt.-% or more. Most preferably water is the only solvent.

Preferred is a method of the present invention, wherein in the pre-treatment composition water and (B-a) together form 98 wt.-% or more of the total weight of the pre-treatment composition, preferably 98.5 wt.-% or more, more preferably 99 wt.-% or more, even more preferably 99.5 wt.-% or more, yet even more preferably 99.9 wt.-% or more, most preferably 99.99 wt.-% or more.

Preferred is a method of the present invention, wherein step (B) is not a swelling step.

Preferred is a method of the present invention, wherein in step (B) the contacting is carried out for a time ranging from 20 seconds to 15 minutes, preferably from 1 minute to 10 minutes, most preferably from 2 minutes to 6 minutes.

Preferred is a method of the present invention, wherein in step (B) the contacting is carried out at a temperature ranging from 20° C. to 70° C., preferably from 22° C. to 60° C., more preferably from 25° C. to 50° C., most preferably from 30° C. to 40° C.

Step (C)

In step (C) of the method of the present invention the pre-treated substrate obtained after step (B) is contacted with an etching composition in an etching compartment such that an etched substrate is obtained. The etching composition comprises (C-a) chromic acid, which is the active etching species.

Preferred is a method of the present invention, wherein in the etching composition (C-a) has a total concentration ranging from 100 g/L to 450 g/L, based on the total volume of the etching composition and referenced to CrO₃, preferably from 200 g/L to 430 g/L, even more preferably from 300 g/L to 410 g/L, most preferably from 350 g/L to 400 g/L.

More preferred is a method of the present invention, wherein the etching composition in step (C) is a chromic acid/sulfuric acid etching composition.

Preferred is a method of the present invention, wherein the etching composition comprises sulfuric acid, preferably in a total concentration ranging from 250 g/L to 450 g/L, based on the total volume of the etching composition, preferably from 280 g/L to 430 g/L, more preferably from 300 g/L to 410 g/L, even more preferably from 330 g/L to 400 g/L, most preferably from 350 g/L to 390 g/L.

The etching composition also comprises (C-b) trivalent chromium ions. Trivalent chromium ions are typically a result of the etching process itself. While hexavalent chromium in chromic acid is reduced to trivalent chromium ions, chemical compounds in the plastic substrate are oxidized and decomposed such that an etching pattern is obtained on the substrate's surface.

In the method of the present invention additional trivalent chromium ions are formed due to the dragged in one or more than one fluorine-free surface active compound utilized in the pre-treatment composition. They are also decomposed by chromic acid, typically after a comparatively short time. Thus, preferred is a method of the present invention, wherein the one or more than one fluorine-free surface active compound in the pre-treatment composition is at least partly dragged out from the pre-treatment composition and dragged in into the etching composition.

More preferred is a method of the present invention, wherein any total amount of fluorine-free surface active compounds in the etching composition originates from the pre-treatment composition.

However, this additional formation of trivalent chromium ions in the method of the present invention is desired. It preferably regulates the concentration of surface active compounds in the etching composition. Furthermore, it ensures that constantly only fresh surface active compounds are provided in the etching composition, which are present directly on the substrate's surface, where they are needed. This is an optimal concentration control within the etching composition.

Preferred is a method of the present invention, wherein in the etching composition (C-b) has a total concentration of 35 g/L or below, preferably of 33 g/L or below, more preferably of 30 g/L or below, even more preferably of 27 g/L or below, yet even more preferably of 25 g/L or below, most preferably of 21 g/L or below. Typically, the etching composition can tolerate a moderate amount of trivalent chromium ions, preferably up to 35 g/L. If this concentration is significantly exceeded, the etching capabilities are getting impaired. Most preferably, a maximum total concentration of 50 g/L should not be exceeded, which preferably also applies in general to the following preferred ranges as maximum is concentration.

More preferred is a method of the present invention, wherein in the etching composition (C-b) has a total concentration ranging from 1 g/L to 35 g/L, based on the total volume of the etching composition, preferably from 5 g/L to 33 g/L, more preferably from 8 g/L to 30 g/L, even more preferably from 10 g/L to 27 g/L, yet even more preferably from 11 g/L to 25 g/L, most preferably from 12 g/L to 21 g/L.

Preferred is a method of the present invention, wherein in the etching composition the chromic acid (based on CrO₃) and the trivalent chromium ions form a molar ratio which is 5 or more, preferably is 7 or more, more preferably is 8 or more, even more preferably is 10 or more, yet even more preferably is 13 or more, most preferably is 18 or more.

Particularly in long-term applications of the method of the present invention, additional measures are needed in order to maintain the concentration of trivalent chromium ions at 35 g/L or below (at least not exceeding 50 g/L). Therefore, preferred is a method of the present invention, wherein at least a portion of the trivalent chromium ions in the etching composition is oxidized through an electrical current such that chromium with the oxidation number (VI) is obtained, preferably in a recycling compartment, most preferably in a recycling compartment different from the etching compartment.

Preferred is a method of the present invention, wherein the etching compartment and the recycling compartment are fluidically connected, preferably by means of at least one pipe, more preferably by means of at least one flow-in and at least one flow-out pipe. Preferably the at least one flow-in and at least one flow-out pipe are fluidically connected with the etching compartment. Fluidically connected denotes that liquids can flow or migrate (preferably circulate) between the etching compartment and the recycling compartment, preferably by at least one pump; preferably either continually or dis-continually.

Preferred is a method of the present invention, wherein the recycling compartment comprises at least one anode, at least one cathode, and at least one semi-permeable separator.

Preferred is a method of the present invention, wherein in the recycling compartment the at least one anode is selected from the group consisting of a lead anode, a lead alloy anode, a precious metal anode, and a mixed metal oxide anode. A preferred lead alloy anode is a lead tin anode. A preferred precious metal anode comprises platinum, more preferably is a platinized titanium anode.

Preferred is a method of the present invention, wherein in the recycling compartment the semi-permeable separator is a membrane.

Most preferred is a method of the present invention, wherein in the recycling compartment is the semi-permeable separator is a ceramic diaphragm.

Preferred is a method of the present invention, wherein in the recycling compartment the at least one cathode comprises lead or steel.

Most preferably the at least one anode and the at least one cathode comprises lead and/or a lead alloy.

Preferred is a method of the present invention, wherein in the recycling compartment the at least one anode forms a total anode surface and the at least one cathode forms a total cathode surface, wherein the total anode surface is larger than the total cathode surface. More preferred is a method of the present invention, wherein the ratio of the total cathode surface to the total anode surface ranges from 1:1.5 to 1:100. In some cases a method of the present invention is preferred, wherein said ratio ranges from 1:1.5 to 1:10, preferably from 1:1.7 to 1:8, more preferably from 1:1.9 to 1:6, most preferably from 1:2 to 1:4. In other cases a method of the present invention is preferred, wherein said ratio ranges 1:10 to 1:100, preferably from 1:11 to 1:95, more preferably from 1:20 to 1:90, even more preferably from 1:25 to 1:85, most preferably from 1:30 to 1:80.

Preferred is a method of the present invention, wherein in the recycling compartment the at least one anode and the separator form an anolyte sub-compartment comprising an anolyte.

Preferred is a method of the present invention, wherein in the recycling compartment the electrical current ranges from 100 A to 400 A, preferably from 200 A to 300 A. Most preferably only in the recycling compartment an electrical current is applied.

Preferred is a method of the present invention, wherein in the recycling compartment the anolyte comprises water and sulfuric acid, preferably 40 wt.-% to 80 wt.-% sulfuric acid, based on the total weight of the anolyte, more preferably 50 wt.-% to 70 wt.-%.

Preferred is a method of the present invention, wherein the etching composition in step (C) has a density ranging from 1.2 g/ml to 1.9 g/ml, preferably from 1.4 g/ml to 1.7 g/ml, most preferably from 1.5 g/ml to 1.6 g/ml.

In some cases a method of the present invention is preferred, wherein the etching composition comprises palladium ions. Palladium ions in the etching composition affect the etching quality insofar that in a subsequent palladium activation step a comparatively low concentration of palladium is required for efficient and sufficient activation, compared to an etching composition not comprising palladium ions. In addition, the etching quality itself directly after the etching is also improved if palladium ions are present.

Preferred is a method of the present invention, wherein in step (C) in the etching composition fluorine-free surface active compounds have an average total concentration is ranging from 0.01 wt.-% to 1 wt.-%, based on the total weight of the etching composition, preferably from 0.02 wt.-% to 0.5 wt.-%, more preferably from 0.03 wt.-% to 0.2 wt.-%, most preferably from 0.04 wt.-% to 0.1 wt.-%. Average denotes during normal continuous operation such that continually surface active compounds are dragged in into the etching composition.

Preferred is a method of the present invention, wherein in step (C) the etching composition has a surface tension of 69 mN/m or below, preferably of 68 mN/m or below, more preferably of 66 mN/m or below, even more preferably of 63 mN/m or below, yet even more preferably of 59 mN/m or below, almost most preferably of 55 mN/m or below, most preferably of 50 mN/m or below. Thus, the one or more than one fluorine-free surface active compound dragged in into the etching composition has a positive effect on the etching composition. In the context of the present invention this means that the surface tension is reduced (instead of increased) compared to an identical etching composition but being free of said fluorine-free surface active compound.

Preferred is a method of the present invention, wherein in step (C) no electrical current is applied to etch the substrate. This means that the process of etching itself is purely chemical based and no electrical current is involved. This does not exclude the electrical current utilized to re-oxidize trivalent chromium ions to chromic acid in the recycling compartment.

Preferred is a method of the present invention, wherein in step (C) the contacting is carried out for a time ranging from 1 minute to 120 minutes, preferably from 2 minutes to 80 minutes, more preferably from 3 minutes to 60 minutes, even more preferably from 5 minutes to 45 minutes, yet even more preferably from 6 minutes to 30 minutes, most preferably from 8 minutes to 15 minutes. A contacting ranging from 8 minutes to 15 minutes is most preferred for substrates comprising ABS and/or ABS-PC. A longer contacting is typically preferred if the substrate comprises a polyetherketone (PEK), preferably about 60 minutes or less.

Preferred is a method of the present invention, wherein in step (C) the contacting is carried at a temperature ranging from 50° C. to 80° C., preferably from 60° C. to 75° C., most preferably from 65° C. to 72° C.

Preferred is a method of the present invention, wherein in step (C) the etching composition is substantially free of, preferably does not comprise, manganese species. This means, that manganese species are preferably not intentionally/purposely added to the etching composition.

Furthermore, preferred is a method of the present invention, wherein the substrate provided in step (A) is not contacted with any composition comprising a manganese species, most is preferably throughout the entire method of the present invention.

As already mentioned above, in the method of the present invention commonly used fluorinated surface active compounds are not used and can be completely prevented. Thus, preferred is a method of the present invention, wherein the pre-treatment composition and the etching composition are substantially free of, preferably do not comprise, fluorinated surface active compounds.

More preferred is a method of the present invention, wherein the pre-treatment composition and the etching composition comprise identical fluorine-free surface active compounds. However, they differ in its concentration significantly.

Preferred is a method of the present invention comprising after step (C) the step

• • (D) contacting the etched substrate with an activation composition such that an activated substrate is obtained.

In this way the method of the present invention is not only a method for etching the at least one surface of the plastic substrate but is furthermore a method for activating said surface.

In many cases a rinsing between steps (C) and (D) is preferred, more preferably with water. Most preferably, after step (C) a contacting with a reducing-agent containing composition is carried out in order to chemically reduce residual amounts of chromic acid on the substrate to trivalent chromium. Preferably, the rising is carried out afterwards to preferably remove any chromium residues.

In the method of the present invention, preferably step (C) is a step separated and independent from step (D). In other words, the etching composition utilized in step (C) is not the activation composition utilized in step (D). This applies mutatis mutandis to steps (B) and (C); i.e. the etching composition utilized in step (C) is not the pre-treatment composition utilized in step (B).

Preferred is a method of the present invention, wherein in step (D) the activation composition comprises palladium, preferably dissolved palladium ions or colloidal palladium, most preferably colloidal palladium. Preferably, the colloidal palladium comprises tin.

Preferred is a method of the present invention, wherein in step (D) the activation composition comprises palladium in a total concentration ranging from 5 mg/L to 200 mg/L, based on the total volume of the activation composition, preferably ranging from 10 mg/L to 150 mg/L, more preferably from 15 mg/L to 80 mg/L, even more preferably from 17 mg/L to 50 mg/L, most preferably from 20 mg/L to 40 mg/L. Preferably, this total concentration is applies to both dissolved palladium ions and colloidal palladium. Above concentrations are based on the element palladium.

Preferred is a method of the present invention, wherein in step (D) the activation composition has a temperature ranging from 25° C. to 70° C., preferably from 28° C. to 60° C., more preferably from 30° C. to 55° C., even more preferably from 32° C. to 50° C., most preferably from 35° C. to 46° C.

Preferred is a method of the present invention, wherein in step (D) the contacting is carried out for a time ranging from 1 minute to 15 minutes, preferably from 2 minutes to 12 minutes, more preferably from 2.5 minutes to 9 minutes, most preferably from 3 minutes to 7 minutes.

Preferred is a method of the present invention, wherein step (D) comprises step

• • (D-1) contacting the activated substrate with an accelerator composition to modify the activated substrate, the accelerator composition comprising
    • no reducing agent but at least one complexing agent for tin ions, if in step (D) the activation composition comprises colloidal palladium, or
    • a reducing agent for reducing palladium ions to metallic palladium, if in step (D) the activation composition comprises palladium ions but no colloidal palladium.

Preferred is a method of the present invention, wherein in step (D-1) the accelerator composition comprises no reducing agent but at least one complexing agent for tin ions and is acidic, preferably comprising in addition sulfuric acid.

Preferred is a method of the present invention comprising after step (D) the step

• • (E) contacting the activated substrate with at least one metallization composition such that a metallized substrate is obtained.

In this way the method of the present invention is not only a method for etching and activating the at least one surface of the plastic substrate but is furthermore a method for metallizing said surface.

The invention will now be illustrated by reference to the following non-limiting example.

Examples

In the following examples ABS substrates (ca. 9×6 cm) were used having sharp edges, sharp bucklings (i.e. having 90° kinks) and relief structures in order to simulate a sophisticated surface geometry. The etching composition has a total volume of 200 L, wherein the pre-treatment composition of 70 L.

Example 1 (Comparative)

Pre-treatment composition: n/a

Pre-treatment parameters: n/a

Etching composition: ca. 380 g/L chromic acis (as CrO₃),

• • ca. 380 g/L sulfuric acid, and
  • no surface active compounds and no palladium ions

Etching parameters: 10 minutes,

• • 68° C.

After etching, the etched substrate was treated with a reducing composition in order to reduce chromic acid. Afterwards, the reduced substrate was rinsed with water. Subsequently, the rinsed substrate was treated with an activator composition comprising colloidal palladium (ca. 3 to 5 minutes, 40° C.) for subsequent nickel and copper plating.

The copper plated substrate was visually inspected for skip plating, in particular in areas of sharp edges and sharp bucklings (i.e. kinks). The results are summarized in Table 1.

Example 2 (Comparative)

Example 1 was repeated with the difference that the etching composition contained ca. 0.1 g/L of a fluorine-containing surface active compound (partly fluorinated C6 to C10 alkylsulfonic acid).

Example 3 (According to the Invention)

Pre-treatment composition: ca. 1.5 wt.-% of a non-ionic alcohol alkoxylate, the alcohol

• • having 10 to 12 carbon atoms, as fluorine-free surface active compound
  • besides water, no further compounds

Pre-treatment parameters: 5 minutes,

• • 40° C.

Etching composition: ca. 380 g/L chromic acid (as CrO₃),

• • ca. 380 g/L sulfuric acid, and
  • no surface active compounds besides drag in from the pre-treatment composition, no palladium ions

Etching parameters: 10 minutes,

• • 68° C.

The pre-treated substrate was not rinsed after the pre-treatment and directly transferred into the etching composition.

The mentioned non-ionic alcohol alkoxylate was the only surface active compound utilized throughout the entire process.

All further steps were carried out as described in Example 1.

Although palladium ions are typically contained in the etching composition, for better emphasis of etching results based on the method of the present invention it was omitted in the present examples. However, further examples including palladium ions showed an improved etched result insofar that the amount of palladium in the activator composition can be reduced compared to examples utilizing no palladium ions (data not shown) and giving an even improved etching pattern.

Furthermore, in an alternative experimental setup a portion of the trivalent chromium ions is transferred into a recycling compartment, comprising a ceramic diaphragm, a lead-containing anode as well as a lead-containing cathode. An electrical current was applied to oxidize trivalent chromium ions back to chromic acid in order to keep the concentration of trivalent chromium ions in the etching composition permanently below ca. 30 g/L (specific data not shown). In this alternative setup the entire process was maintained over several weeks resulting in stable and uniform etching results.

Results

Results are summarized in Table 1 below, showing the following:

• • + no skip plating on flat areas; severe skip plating on edges and in the kinks;
  • ++ no skip plating on flat areas; less skip plating on edges and in the kinks as in Example 1 but still observable in some cases;
  • +++ no skip plating on flat areas; no skip plating on edges and sharp bucklings;

TABLE 1
Example Skip plating
1       +
2       ++
3       +++

Furthermore, Example 3 has shown a comparatively high consumption of surface active compounds in the pre-treatment composition. Since no rinsing is applied after the pre-treatment, significant amounts of the fluorine-free surface active compound are dragged out from the pre-treatment composition and dragged in into the etching composition while surprisingly a sufficient amount remains on the surface of the substrate. Due to significantly lower chemical resistance of fluorine-free surface active compounds towards chromic acid, is the fluorine-free surface active compounds are quite quickly decomposed in the etching composition. However, Example 3 shows that the stability of the fluorine-free surface active compound is still totally sufficient to obtain an excellent wetting for a complete etching.

As shown in Table 1 above, a pre-treated substrate as in Example 3 results in an even further improved etching and therefore even less skip plating compared to Example 2. It is believed that chromic acid and surface active compounds compete with each other on sophisticated and less accessible areas, thereby lowering the efficiency of surface active compounds.

In Example 2 it was furthermore observed that even fluorinated surface active compounds are subject to decomposition although at a significantly lower rate. However, decomposition products are also fluorine-containing and therefore stable over a comparatively long time and therefore accumulate, which is not desired. Such a drawback is overcome in Example 3. Furthermore, Example 3 completely relies on bio-degradable surface active compounds because they are fluorine-free.

Claims

1. A method for etching at least one surface of a plastic substrate, the method comprising the steps

(A) providing the substrate,

(B) contacting the substrate with an aqueous pre-treatment composition such that a pre-treated substrate is obtained, the pre-treatment composition comprising (B-a) one or more than one fluorine-free surface active compound, and

(C) contacting the pre-treated substrate with an etching composition in an etching compartment such that an etched substrate is obtained, the etching composition comprising (C-a) chromic acid, and (C-b) trivalent chromium ions,

characterized in that the pre-treated substrate is not rinsed after step (B) and prior to step (C), and the etching composition is substantially free of fluorine-containing surface active compounds.

2. The method of claim 1, wherein in step (A) the substrate comprises acrylonitrile butadiene styrene (ABS), acrylonitrile butadiene styrene—polycarbonate (ABS-PC), polypropylene (PP), polyamide (PA), polyetherimide (PEI), a polyetherketone (PEK), or mixtures thereof.

3. The method of claim 1, wherein in (B-a) the one or more than one fluorine-free surface active compound comprises a non-ionic surfactant.

4. The method of claim 3, wherein the non-ionic surfactant is a non-ionic alcohol alkoxylate, wherein in the non-ionic alcohol alkoxylate the alcohol comprises 6 to 20 carbon atoms.

5. The method of claim 1, wherein in the pre-treatment composition (B-a) has a total concentration ranging from 0.1 wt.-% to 40 wt.-%, based on the total weight of the pre-treatment composition.

6. The method of claim 1, wherein in the pre-treatment composition water and (B-a) together form 98 wt.-% or more of the total weight of the pre-treatment composition.

7. The method of claim 1, wherein in the etching composition (C-a) has a total concentration ranging from 100 g/L to 450 g/L, based on the total volume of the etching composition and referenced to CrO3.

8. The method of claim 1, wherein the one or more than one fluorine-free surface active compound in the pre-treatment composition is at least partly dragged out from the pre-treatment composition and dragged in into the etching composition.

9. The method of claim 1, wherein any total amount of fluorine-free surface active compounds in the etching composition originates from the pre-treatment composition.

10. The method of claim 1, wherein in the etching composition (C-b) has a total concentration of 35 g/L or below.

11. The method of claim 1, wherein at least a portion of the trivalent chromium ions in the etching composition is oxidized through an electrical current such that chromium with the oxidation number (VI) is obtained.

12. The method of claim 1, wherein in step (C) the contacting is carried out for a time ranging from 1 minute to 120 minutes.

13. The method of claim 1, wherein in step (C) the contacting is carried at a temperature ranging from 50° C. to 80° C.

14. The method of claim 1 wherein the pre-treatment composition and the etching composition are substantially free of fluorinated surface active compounds.

15. The method of claim 1, wherein the pre-treatment composition and the etching composition comprise identical fluorine-free surface active compounds.