EP3144414B1

EP3144414B1 - Enzyme treatment of polyamide objects for metallization purposes

Info

Publication number: EP3144414B1
Application number: EP15186070.7A
Authority: EP
Inventors: Adrien MURILLON; Sonja Nixon; Thiago MEDEIROS ARAUJO
Original assignee: Airbus Defence and Space GmbH
Current assignee: Airbus Defence and Space GmbH
Priority date: 2015-09-21
Filing date: 2015-09-21
Publication date: 2022-11-23
Anticipated expiration: 2035-09-21
Also published as: EP3144414A1; WO2017050728A1

Description

FIELD OF THE INVENTION

The present invention relates to an enzyme treatment of polyamide surfaces as a pretreatment for purposes of metallization, and especially for metallization with copper using a Pd/Sn activator.

BACKGROUND OF THE INVENTION

Metallization of polyamide (PA) surfaces is commonly applied for manufacturing of interior and exterior automotive parts such as door handles, covers, operating elements and decorative parts. It is also applied in electronic industry for contact elements and for microcircuit fabrication, and for household goods such as special sanitary elements. The basic requirement for coating any metal onto a polyamide surface is that the non-conducting polyamide is coated with a conducting layer which adheres to the surface during subsequent metallization. Such conducting layer may be made of any metal, for example Cu, Ag, Au, Rh, or Ni. Usually, said conducting layer is a copper layer. A general description of processes for creating such copper layer and the use of said layers is provided in N. Kanani et al., "Kupferschichten - Abscheidung, Eigenschaften, Anwendungen", 1. ed., 2000, Eugen G. Leuze Verlag, and in "Kunststoff-Metallisierung - Handbuch für Theorie und Praxis", 1991, Eugen G. Leuze Verlag. Such copper layer is typically created in two steps: a pretreatment step to prepare the polyamide surface for the subsequent metallization, and a metallization step. The metallization step is typically performed using a palladium catalyst. Usually, said palladium catalyst is used in a form called Pd/Sn activator. The best known metallization process using a Pd/Sn activator is the Crimson Process developed by the Shipley Company (US 4,895,739 , also described in the two textbooks recited above). In said metallization process, a polymer object is provided with a surface having areas of a catalytic metal chalcogenide conversion coating. Said metal chalcogenide conversion coating is formed by treating the object with a combination of Pd/Sn (the "Pd/Sn activator") and subsequently treating the object with a sodium sulfide solution. The resulting conversion coating allows the object to be directly plated with copper by immersion in a solution containing copper ions. The conversion of the copper ions to metallic copper is catalyzed by the Pd/Sn particles adhered to the surface of the polyamide.
Polyamide objects used for metallization are generally manufactured by additive layer manufacturing (ALM), selective laser sintering (SLS) or injection molding. The surfaces of the resulting objects have to be pretreated before they can be metalized. This pretreatment is necessary to create a surface which offers sufficient adhesive properties for metallic coatings and which is hydrophilic. Such hydrophilic surface moreover provides good wetting properties and thus allows a good contact to Pd/Sn activators in water solution as opposed to organometallic activators which are usually applied in a hydrophobic organic solvent.
The most common method of pretreatment of polyamide surfaces to prepare them for metallization is the treatment with chromic acid (H₂CrO₄). Other oxidizing treatments, like treatments in acids, permanganates or halogenides, are also used. Chromic acid for the treatment of polyamide surfaces is usually prepared by adding sulphuric acid to a dichromate solution, and the resulting solution will therefore be designated as "chromic sulphuric acid" in the following. The chromic sulphuric acid oxidizes the polyamide surface and increases its roughness. However, chromic acid and other hexavalent chromium compounds (including chromium trioxide, chromates, chlorochromates) are toxic and carcinogenic. For this reason, chromic acid oxidation is generally not used on an industrial scale except, for example, in the automotive industry. For this reason, chromic acid is also contained in Annex XIV of the European Union regulation 1907/2006 on Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH) which entered into force on 1 June 2007. Thus, it cannot be placed on the market or used after a given date, unless an authorisation is granted for its specific use.
Chromic sulphuric acid etching as pretreatment for metallization of polymers, e.g., polyamide, is state of the art and still applied in the supply chain for e.g. the automotive industry. Currently, permanganate etching solutions for pretreatments are appearing on the commercial market. But, as already indicated above, compounds containing hexavalent chromium (Cr^VI) are targeted by REACH. Permanganate etching solutions, on the other hand, are instable and therefore challenging to maintain.
When using such conventional etching solutions, there is moreover a risk that etching solution remains in pores of the polyamide object and causes cross contamination of subsequent treatments or degradation of the polyamide object or of objects in contact with the polyamide object later on in service. EP1942207 A1 describes a process for the metallization of a polyamide surface comprising a pretreatment of the polyamide surface with an aqueous solution containing a palladium salt, hydrochloric acid, phosphoric acid and an organic solvent, followed by the metallization of the pretreated polyamide surface by electroless deposition of a metal layer.
Treatment by enzymes (single or mixture) is reported in the literature for polyamide fibers and fabrics for enhancement of wettability and dyeing processes (Ah Reum Song, Hye Rim Kim, "Effectiveness of Flavourzyme Treatment on Polyamide Fabric", Fibers and Polymers 2013, Vol.14, No.12, 2212-2220; El-Ola et al., Indian Journal of Fibre & Textile Research 2014, 39:65-71; US 2008/0289120 A1 ).
However, the treatment described in this prior art is not performed for metallization purposes, but for subsequent dyeing of the fibers or fabrics with, e.g., cationic dyes or acid dyes.
Thus, there is a need for a pretreatment of polyamide surfaces for metallization which does not present the problems of the conventional pretreatment processes for metallization indicated above, is safe in handling and results in a pretreated surface suitable for subsequent metallization.

BRIEF SUMMARY OF THE INVENTION

In view of the foregoing, the present invention provides a process for metalizing a polyamide surface, the process comprising the steps:

(i) pretreating a polyamide surface, the process comprising the steps:
1. (a) providing an object having a polyamide surface;
2. (b) treating the polyamide surface with a protease;
  and
3. (c) removing the protease from the treated polyamide surface, thus providing an object having a protease treated polyamide surface; and
(ii) metalizing the resulting pretreated polyamide surface using a palladium catalyst, wherein the metallization step (ii) is an electroless metallization.

The protease is preferably Flavourzyme^®, in particular Flavourzyme^® L, or a protease which is comprised in Flavourzyme^® L.
The polyamide is preferably polyamide 12 (PA 12).
The metallization step (ii) is a metallization using a palladium catalyst as activator. In one embodiment, said catalyst is a Pd/Sn activator. In this metallization, the metal ions used for metallization may be any metal ions which can be converted by electroless or electrolytic metallization using a noble metal catalyst into their corresponding elemental metal. Preferably, the metal ions are selected from the group consisting of copper, silver, gold, ruthenium, and nickel ions, and they can be converted to their corresponding elemental metal using a palladium catalyst. More preferably, the metal ions are copper ions, and the metal layer resulting from the metallization is a copper layer. Most preferred is an electroless copper metallization using a palladium catalyst, especially a Pd/Sn activator.
An object comprising a metalized polyamide surface resulting from the metallization process according to the present invention is also disclosed.
The enzymatic treatment of a polyamide surface with a protease hydrolyzes amide bonds of the polyamide. Thus, functional groups (-COOH and-NH₂) are created on the polyamide surface. These functional groups are necessary in the present invention for good wettability and contact with the metallization reagents. According to chapter 2.3.4 in "Kunststoff-Metallisierung", Eugen G. Leuze Verlag, 1991, good wettability is directly linked to improved adhesion properties of the palladium catalyst (typically a Pd/Sn activator) which is conventionally used for metallization. Moreover, the -NH₂ groups created by the enzymatic hydrolysis of amide bonds bind to metal ions, e.g. the palladium and copper ions used in a preferred embodiment of the present invention ("Fortschritte bei der Kunststoffmetallisierung und Oberflächenbeschichtung von Glas, Keramik und Silizium", Eugen G. Leuze Verlag, 2002). Because of this, in the present invention a protease treatment of a polyamide surface is used as pretreatment for metallization of said polyamide surface. Said metallization is a metallization using metal ions, in particular copper ions, which are converted to elemental metal by a palladium catalyst, in particular the commonly used Pd/Sn activator. Said catalysts are typically used in the form of ionogenic metal salt solutions or colloidal suspensions. The protease treatment according to the present invention enables the polyamide surface to anchor these catalysts.
The good anchorage of the catalyst (in particular of a Pd/Sn activator) and/or of the metal ions which are the source of the metal layer created by the metallization (in particular of copper ions), which is provided by the enzymatic pretreatment assures an excellent resistance of the resulting metal (in particular, copper) layer to separation from the polyamide substrate after metallization.
The protease pretreatment process according to the present invention is REACH compliant and does not bear the risk that remaining pretreatment solution might contribute to degradation of the polyamide object later on in service. A cross contamination of subsequent treatments will also have less severe consequences than a cross contamination with hexavalent chromium or an instable permanganate solution.

DETAILED DESCRIPTION

In the context of the present invention, the term "about" followed by a numerical value means a range of ±10% of said value, in a preferred embodiment ±5% of said value.
In the context of the present invention, the terms "a" and "an" shall encompass one or more entities unless indicated otherwise. In one embodiment, they shall mean one entity.
In the context of the present invention, the term "comprising" shall include "consisting of', and in preferred embodiments shall mean "consisting of'.
In the context of the present invention, the term "aqueous" means comprising water, preferably more than 50 vol.% water, more preferably more than 80 vol.% water, even more preferably more than 90 vol.% water. Even more preferably, this term means that water is the sole solvent used in the composition or mixture designated as "aqueous".
The present invention pertains to the pretreatment of polyamide surfaces. The polyamide (abbreviation: "PA") which is treated may be any polyamide, in particular any polyamide which is currently commercially available. In a preferred embodiment, the polyamide is selected from the group consisting of aliphatic polyamides. More preferably, the polyamide is selected from the group consisting of polyamide 6 (CAS number 25038-54-4), polyamide 6.6 (CAS number 32131-17-2), polyamide 6.10 (CAS number 9011-52-3 or 9008-66-6), polyamide 6.12 (CAS number 26098-55-5), polyamide 10 (CAS number 25266-58-4), polyamide 11 (CAS number 25035-04-5), polyamide 12 (CAS number 24937-16-4), polyamide 12.12 (CAS number 36348-71-7), and combinations thereof. Even more preferably, the polyamide is selected from the group consisting of polyamide 6 (CAS number 25038-54-4), polyamide 6.6 (CAS number 32131-17-2), polyamide 6.10 (CAS number 9011-52-3 or 9008-66-6), polyamide 10 (CAS number 25266-58-4), polyamide 11 (CAS number 25035-04-5), polyamide 12 (CAS number 24937-16-4), and combinations thereof. Polyamide 12 (CAS number 24937-16-4) is the most preferred polyamide for performing the present invention.
The polyamide object which is subject to the protease treatment according to the present invention may have been manufactured by any conventional manufacturing process, including milling, molding, ALM and SLS. The protease treatment according to the present invention is particularly suitable for preparing polyamide parts manufactured by SLS for metallization. In general, the protease treatment according to the present invention is advantageous for polyamide parts having a rough surface and/or being porous because of the good adhesion of the resulting metal coating even to such rough or porous surface. Typically, the polyamide surface to be treated with protease according to the present invention has a porosity of from 0 to about 5% (as determined by performing a 2D computer tomography scan with a cross section of the sample and calculating the area% of the pores in relation to the total cross section area) and/or a surface roughness of from about 1 to about 15 µm (arithmetic mean roughness Ra according to DIN EN ISO 4287:2010-07). It is possible to use the present invention on a polyamide surface having a porosity of from about 2 to about 5% and/or a surface roughness of from about 5 to about 15 µm, and even on a polyamide surface having a porosity of from about 3 to about 5% and/or a surface roughness of from about 10 to about 15 µm.
The polyamide surface pretreated for metallization according to the present invention may be any polyamide surface, either in total or in part. Said surface may be the surface of an object completely consisting of polyamide, or the surface of a polyamide part of an object which also contains parts made from other material. Such object containing parts made from other material may in one embodiment be a glass fiber reinforced polyamide or a carbon fiber reinforced polyamide. In one preferred embodiment, the polyamide surface is the surface or part of the surface of an object completely consisting of polyamide.
The polyamide object or polyamide part comprising the polyamide surface consists preferably of a nonfibrous (synonym: "compact") polyamide. However, polyamide fibers or fabrics may also be pretreated for metallization using the process of the present invention.
The present invention provides a process for pretreating a polyamide surface with a protease. A protease is an enzyme that is able to split an amide bond, thus creating a free -COOH and a free -NH₂ group. In the context of the present invention, the term "protease" shall encompass any enzyme able to split an amide bond, i.e. proteases (e.g., with MDL number MFCD00132092), peptidases and complex mixtures ("complex proteases") containing one or more proteases and/or peptidases (e.g., Flavourzyme^®). In a broader context, other enzymes which are able to split an amide bond in PA are also contemplated for use but are not part of the invention, for example cutinases and polyamidases (Song et al., Fibers and Polymers 2013, Vol.14, No.12, 2212-2220; El-Ola et al., Indian Journal of Fibre & Textile Research 2014, 39:65-71).
According to the present invention, a protease is used which is able to split one or more amide bonds in a polyamide. Preferred proteases for use in the present invention are selected from the group consisting of aminopeptidases (EC 3.4.11.) (in particular, leucyl aminopepdidases), serine proteases (EC 3.4.21.), dipeptidyl peptidases, exopeptidases, endopeptidases, aspartic proteases, cysteine proteases, metallo-proteases, and mixtures of two or more thereof. Such proteases useful for the present invention are for example the proteases described in Ah Reum Song, Hye Rim Kim, "Effectiveness of Flavourzyme Treatment on Polyamide Fabric", Fibers and Polymers 2013, Vol.14, No.12, 2212-2220; El-Ola et al., Indian Journal of Fibre & Textile Research 2014, 39:65-71; US 2008/0289120 A1 .
More preferably, the protease for use in the present invention is selected from the group consisting of Flavourzyme^® (EC number 232-752-2; MDL number MFCD00132092; D. Spellmann et al., Int. Dairy J. 2003, 13:447), in particular Flavourzyme^® L, more particularly Flavourzyme^® 500L (e.g. from Sigma Aldrich, Germany, product number P6110) or Flavourzyme^® 1000L; bromelain, papain; alcalase; Corolase N; and mixtures of two or more thereof. Even more preferably, the protease for use in the present invention is Flavourzyme^® or an enzyme selected from the group of aminopeptidases, dipeptidyl peptidases and endopeptidases identified as key Flavourzyme^® enzymes by Merz et al., J. Agric. Food Chem. 2015 63:5682-93. Flavourzyme^® 500L or 1000L (as described in Ah Reum Song, Hye Rim Kim, "Effectiveness of Flavourzyme Treatment on Polyamide Fabric", Fibers and Polymers 2013, Vol.14, No.12, 2212-2220, are especially preferred. Most preferred is Flavourzyme^® 500L (e.g. from Sigma Aldrich, Germany, product number P6110) which was used in Example 1 herein. Flavourzyme^® is a complex protease consisting of several enzymes including aminopeptidases, dipeptidyl peptidases, and endopeptidases (Merz et al., J. Agric. Food Chem. 2015 63:5682-93) fom A. oryzae which is widely used commercially and commercially available from Sigma Aldrich (Germany) and Novozymes (Denmark). It possesses endopeptidase and exopeptidase activity on proteins. However, it also hydrolyzes amide bonds in polyamide. This effect is described in Ah Reum Song, Hye Rim Kim, "Effectiveness of Flavourzyme Treatment on Polyamide Fabric", Fibers and Polymers 2013, Vol.14, No.12, 2212-2220, describing Flavourzyme^® L, its hydrolytic properties and appropriate reaction conditions for this enzyme.
The protease is applied to the polyamide surface as ingredient of a pretreatment mixture, which is typically a liquid mixture, i.e. a solution or suspension of the enzyme. The amount of enzyme used in the pretreatment mixture according to the present invention is preferably calculated in relation to the surface area (in m²) treated with the enzyme. It is preferably within the range of from about 0.05 to about 100 g/m², more preferably from about 1 to about 50 g/m², even more preferably from about 3 to about 25 g/m², and even more preferably from about 5 to about 15 g/m² enzyme/surface area. A range from about 8 to about 14 g/m² is especially preferred, and one particularly preferred amount is about 11.5 g/m² enzyme/surface area (for Flavourzyme^® 500L, see Example 1). Typical advantageous protease activities in the pretreatment mixture are within the range of from about 0.5 U/L to about 200 U/L, preferably from about 5 U/L to about 100 U/L, even more preferably from about 20 U/L to about 50 U/L. E.g., in Example 1 0.07 g/L of Flavourzyme^® 500L are used, which would mean an activity of 35 U/L or more. Excess enzyme may lead to a decrease in the pretreatment effect. Such decrease might be due to enzyme aggregation and can be addressed by using a lower enzyme concentration. A deficit in the amount of enzyme may lead to insufficient adhesion properties of the resulting pretreated surface. However, this can be addressed by using a higher enzyme concentration. Variation of the enzyme concentration is within the professional reach of a person experienced with handling enzymes.
The pretreatment mixture has to be compatible with the use of an enzyme, i.e. its other ingredients besides the enzyme should not inhibit the enzyme.
For example, it is well known that the structure of proteins is dependent on the pH of the material which surrounds them. Because the structure of the protein has a decisive influence on the activity of the enzyme, the pH of the material surrounding the enzyme must be set in an appropriate manner to achieve a high enzyme activity.
The pH value of the pretreatment mixture according to the present invention is typically the pH optimum ± 20%, preferably ± 10%, more preferably ± 5% and even more preferably ± 0% of the protease which is used. For commercially available proteases, the pH optimum indicated by the provider should be used. In one embodiment, the pH value of the pretreatment mixture according to the present invention is in a range from pH 3 to pH 10, preferably in a range from pH 6 to pH 8, more preferably in a range from pH 6.5 to pH 7.5. The preferred proteinase Flavourzyme^® is preferably used at a pH in a range from 6.5 to 8.0, more preferably at a pH of about 7. Song et al. (see above) found that Flavourzyme^® works well in a pH range from 6.5 to 8.0, and that the pH optimum of Flavourzyme^® is pH 7.0 at 40°C. Flavourzyme^® contains endoproteases and exoproteases. Whilst endoproteases work at neutral and acid pH (pH 5.5 to 8.0), exoproteases are active only at a pH of about 7.0. The examples of the present invention used Flavourzyme^® at pH 7.0 and 40°C. Thus, pH values from 6.5 to 8.0 ± 10%, more preferably ± 5% or more preferably ± 0% are preferred for Flavourzyme^®, and pH values of 7.0 ± 10%, more preferably ± 5% or more preferably ± 0% are particularly preferred for Flavourzyme^®. However, if the activity of one of the specific enzymes comprised in Flavourzyme^® which are described by Merz et al., J. Agric. Food Chem. 2015 63:5682-93 shall be favoured, the pH value of the pretreatment mixture is preferably the pH optimum ± 20%, preferably ± 10%, more preferably ± 5% and even more preferably ± 0% of said specific enzyme as described by Merz et al..
To establish the pH value of the pretreatment mixture, the use of a buffer is preferred. Said buffer may be any buffer which is conventional for buffering enzyme solutions and suitable to establish the desired pH within its buffering range. Typically, the buffer has a pKa within ± 20%, preferably ± 10%, more preferably ± 5% of the pH optimum of the enzyme used in the pretreatment mixture. Sodium phosphate (pK_a 7.2) is preferred as buffer for Flavourzyme^®. The typical buffer concentration is conventional or as prescribed by the enzyme provider. For example, a sodium phosphate concentration of about 50 mM may be used for Flavourzyme^®.
The hydrolytic activity of the protease in one preferred embodiment is enhanced by adding mild reducing agents as "activators" to the pretreatment mixture. Such reducing agents may be selected from the group consisting of cysteine, sulfide, sulfite, cyanide and salts thereof. The use of such activators has been described in the prior art (see references cited by Song et al., Fibers and Polymers 2013, Vol.14, No.12, 2212-2220). The use of cysteine is preferred, and most preferred is the use of L-cysteine, as described for Flavourzyme^®, e.g., by Song et al., Fibers and Polymers 2013, Vol.14, No.12, 2212-2220. The concentration of activator in the pretreatment mixture may be any concentration which is increasing the enzyme activation. Generally, the higher the enzyme concentration and the substrate concentration, the higher the concentration of activator. Typically, the activator concentration may be from 0 (no activator) to 200 mM, preferably from 5 mM to 100 mM, more preferably from 10 mM to 30 mM. The activator concentration/enzyme concentration ratio is described by Song et al.
The pretreatment mixture typically contains water as solvent. In addition to or instead of water, if necessary, other solvents can also be used, provided they are hydrophilic and mix with water, for example, a C₁-C₄ alcohol, etc., and provided they do not interfere with the enzyme activity. But water is preferred as single solvent.
Some of the features of the pretreatment mixture, in particular the ion concentration, solvent composition and pH, can be set to be suitable by the addition of water, organic and/or inorganic acids and/or bases and/or salts and/or buffer mixtures in the usual professional manner.
The pretreatment mixture is preferably applied by immersion or by means of other generally known procedures for the application of solutions or suspensions on surfaces, like spraying. To achieve sufficient activity of the protease and/or contact of the polyamide surface with the pretreatment mixture, shaking (e.g. on an orbital shaker) may be advantageous.
It is useful if the pretreatment mixture remains on the treated polyamide surface for a time period from 5 min to 500 min, preferably from 5 min to 300 min, more preferably from 10 min to 240 min, even more preferably from 20 min to 200 min, and even more preferably from 30 min to 180 min. A typical example is a time period of about 120 min. However, shorter time periods for the protease treatment, e.g. about 30 min, might also be sufficient pretreatment times (as described, e.g., in El-Ola et al.).
The incubation temperature for treating the polyamide surface with the pretreatment mixture according to the present invention is typically the temperature optimum ± 20%, preferably ± 10%, more preferably ± 5% and even more preferably ± 0% of the protease which is used as ingredient of the pretreatment mixture. For commercially available proteases, the temperature optimum indicated by the provider should be used. A typical incubation temperature is within the range of from about 20°C to about 50°C, preferably from about 25°C to about 45°C, and more preferably from about 30°C to about 40°C. For Flavourzyme^®, an incubation temperature of from about 30°C to about 50°C, preferably of about 40°C is preferred. However, if the activity of one of the specific enzymes comprised in Flavourzyme^® which are described by Merz et al., J. Agric. Food Chem. 2015 63:5682-93 shall be favoured, the incubation temperature is preferably the temperature optimum ± 20%, preferably ± 10%, more preferably ± 5% and even more preferably ± 0% of said specific enzyme as described by Merz et al..
Any combination of the reaction conditions and pretreatment mixture compositions listed above may be used in the pretreatment process of the present invention. However, it is preferred that a combination of said reaction conditions and pretreatment mixture compositions is used which is a combination of the preferred embodiments listed for the single reaction conditions (in particular, the temperature and incubation time) and pretreatment mixture compositions (in particular, the pH and enzyme concentration) above. In an especially preferred embodiment, the reaction conditions are the optimum working conditions for the protease. In the preferred embodiment using Flavourzyme^® 500L, the preferred pretreatment reaction conditions are about 120 min incubation time at about 40°C and pH about 7.0, with about 11.5 g/m² and/or about 70 mg/L enzyme contained in the pretreatment mixture. For other specific enzymes, other conditions may apply, like an incubation time of about 30 min at about 30°C and pH about 8 with about 0.05 mg/ml enzyme, as described for the proteases used by El-Ola et al..
Before the treatment with enzyme, one or more cleaning and/or processing steps may be performed. For example, degreasing with nonionic surfactant or any other degreasing agent and rinsing with water or deionized water can be performed one or more times. After the treatment with enzyme, the object is preferably treated at an elevated temperature for deactivation of enzyme, either directly in the pretreatment mixture containing the enzyme in solution or suspension (the elevated temperature will deactivate the enzyme), or after transfer into water or an aqueous solution of, e.g., a buffer or salt. The object may, in one particular embodiment, additionally or instead of treatment at elevated temperature be treated with aqueous sodium carbonate for enzyme deactivation. After enzyme deactivation, the object is preferably washed with an aqueous nonionic detergent (e.g. Triton X114) solution and then rinsed with water. An exemplary and preferred pretreatment process comprising cleaning, enzyme treatment and subsequent processing steps comprises the following steps in the following order:

(1) Washing;
(2) Rinsing;
(3) Enzyme treatment;
(4) Inactivation (which may encompass one or more inactivation treatments);
(5) Rinsing;
(6) Washing; and
(7) Rinsing. See also Figure 1.

A specific and preferred pretreatment process showing cleaning, enzyme treatment and subsequent processing steps is shown in Figure 2 and described in Example 1.
The amount of free -COOH and free -NH₂ groups on the polyamide surface which are created by the pretreatment process according to the present invention may be determined using the methods for characterizing the effectiveness of Flavourzyme^® treatment described by Ah Reum Song and Hye Rim Kim in Fibers and Polymers 2013, Vol.14, No.12, 2212-2220.
Before the metallization process is performed, the pretreated surface is preferably dried. However, it can also be used without drying in a wet in wet process.
The metallization process is performed as conventionally known in the art and as described, e.g., in "Kunststoff-Metallisierung - Handbuch für Theorie und Praxis", Eugen G. Leuze Verlag, 1991, and in N. Kanani et al., "Kupferschichten - Abscheidung, Eigenschaften, Anwendungen", 1. ed., Eugen G. Leuze Verlag, 2000. The metallization is electroless. The catalyst used for the metallization is palladium, in particular in its form as the conventionally used Pd/Sn activator. The metal ions used for metallization may be any metal ions which can be converted by electroless or electrolytic metallization using a noble metal catalyst into their corresponding elemental metal. Preferably, the metal ions are selected from the group consisting of copper, silver, gold, ruthenium, and nickel ions. More preferably, the metal ions are copper ions, and the metal layer resulting from the metallization is a copper layer. In the present invention, an electroless copper metallization, preferably an electroless copper metallization using a palladium catalyst, especially a Pd/Sn activator, is preferred. Such metallization processes which result in a copper coating are common general knowledge in the art, as illustrated by the textbooks cited in this paragraph. A particularly preferred metallization process is demonstrated in Example 4 herein.

BRIEF DESCRIPTION OF THE FIGURES

Fig. 1 shows a typical sequence of steps for pretreating a polyamide surface with a protease according to the present invention.
Fig. 2 shows the sequence of steps performed in Example 1 for pretreating a PA 12 surface with Flavourzyme^®.
Fig. 3 shows the result of the copper coating on a PA 12 surface which was pretreated with Flavourzyme^® according to Example 1. Cross cut has been applied for evaluation of resistance of the copper coating to separation from the polyamide surface (cross cut lines at the right bottom corner).
Fig. 4 shows the result of the copper coating on a PA 12 surface which was pretreated with chromic sulphuric acid according to Example 2. Cross cut has been applied for evaluation of resistance of the copper coating to separation from the polyamide surface.
Fig. 5 shows the result of the copper coating on a PA 12 surface which was pretreated with sulphuric acid according to Example 3. In the bottom part, the metallic coating has been removed by tape removal according to DIN EN ISO 2409:2013. The dark coloured surface in the top part is copper, the light coloured surface in the bottom part is polyamide. Cross cut has been applied for evaluation of resistance of the copper coating to separation from the polyamide surface, but the cross cut lines are not visible any more since the copper coating has been completely removed from the bottom part. The cross cut lines are only visible in the copper layer (compare Figures 3 and 4).
Fig. 6 shows a scanning electron microscope image of a cross section of a metalized PA 12 sample which was pretreated with enzyme according to Example 1. The surface is merely micro roughened, the copper coating is completely covering the surface.
Fig. 7 shows a scanning electron microscope image of a cross section of a metalized PA 12 sample which was pretreated with chromic sulphuric acid according to Example 2. The surface is roughened, the copper coating is completely covering the surface.
Fig. 8 shows a scanning electron microscope image of a cross section of a metalized PA 12 sample which was pretreated with sulphuric acid according to Example 3. The PA 12 material is degraded, the copper coating is not completely covering the surface.

The present invention is discussed further on the basis of the non-restrictive examples below, wherein examples 2 and 3 are not according to the invention.

EXAMPLES

Materials:

The following enzymes have been used:
Flavourzyme^® 500L (Sigma Aldrich, Germany, product number P6110)
The following polyamide has been used:
Polyamide 12 (PA 12) (PA2200; EOS GmbH, Germany), selective laser sintered (SLS)
The resistance of the copper coating to separation from the polyamide surface was determined using a cross cut method and a tape removal method according to DIN EN ISO 2409:2013 (version June 2013). The cross cut method was performed using the following motor-driven apparatus: motor-driven Erichsen 430 P-I Scratch Hardness Tester, loading force 12 N (without cutting tip); and using a single blade cutting tip (Erichsen, Germany; order number 0564.01.32) as cutting tool. The cuts were spaced 1 mm apart. For the tape removal method according to DIN EN ISO 2409:2013, which is described in Annex A thereof, the following tape was used: tesa^® 4651.
The metallization layers were further analyzed by scanning electron microscope (SEM) characterization of a cross section of the metalized plates.
All commercially available products and apparatus used in the Examples were used according to the provider's instructions unless indicated otherwise.
The water used herein was deionized ("DI") water unless indicated otherwise.

Example 1: Flavourzyme^® pretreatment of PA12 SLS plates before chemical copper metallization

PA12 SLS plates were treated as shown in Figure 2. The sample size was 50 x 50 x 5 mm. The enzyme used was Flavourzyme^® as described in Ah Reum Song, Hye Rim Kim "Effectiveness of Flavourzyme Treatment on Polyamide Fabric", Fibers and Polymers 2013, Vol.14, No.12, 2212-2220, but the Flavourzyme ^® used in the present Example was 500L, not 1000L as used in Song et al.. Said Flavourzyme^® is the commercial protease FLV (Flavourzyme^® 500L, Sigma Aldrich, Germany, Product Number P6110). It has a specific activity of 500 U/g (one unit is the amount of enzyme which hydrolyzes 1 µmol of L-leucine-p-nitroanilide per minute).
All steps shown in Figure 2 were performed by completely immersing the sample in the solutions in 1 L glass beakers and continuous orbital shaking at 300 rpm.
The resulting pretreated polyamide samples were then dried and subsequently copper metalized as described in Example 4.

Example 2: Pretreatment in chromic sulphuric acid

A sample of the same PA 12 and with the same dimensions as used in Example 1 was immersed in aqueous chromic sulphuric acid (400 g/L H₂CrO₄, 400 g/L H₂SO₄) at 60 °C for 10 min. The sample was rinsed once for 5 min with water and subsequently immersed in aqueous sodium hydrogen sulphite (10 mL/L of 50 wt.% aqueous sodium hydrogen sulphite), pH 1-2, at 25 °C for 2 min. The sample was rinsed again and then immersed in an aqueous acidic cleaner solution prepared with Securiganth HC-F45 Acid Cleaner (EU) (Atotech, Germany) according to the provider's instructions. Finally, the sample was rinsed again.
The resulting pretreated polyamide samples were then dried and subsequently copper metalized as described in Example 4.

Example 3: Pretreatment in sulphuric acid

A sample of the same PA 12, but with different dimensions (32 x 17 x 3 mm) as used in Examples 1 and 2 was treated like the sample in Example 2 using 50 wt.% aqueous sulphuric acid instead of the aqueous chromic sulphuric acid used in Example 2. The resulting pretreated polyamide samples were then dried and subsequently copper metalized as described in Example 4.

Example 4: Chemical copper metallization of pretreated PA12 SLS plates

The enzyme treated polyamide samples resulting from Example 1 and the acid treated polyamide samples resulting from comparative Examples 2 and 3 were copper metalized by chemical (electroless) copper metallization. First, the Neoganth activator system (Atotech, Germany) was applied according to the provider's instructions for depositing a palladium catalyst on the surface of the polyamide samples. Second, the Printoganth system (Atotech, Germany) was used as acidic copper bath for electroless creation of a copper coating on the Neoganth-treated plates according to the provider's instructions. In said second step, the palladium deposited on the PA plates acted as catalyst for reduction of the copper ions contained in the acidic copper bath.
After the chemical copper plating the samples were rinsed in water at room temperature for 5 minutes. The resulting samples were dried at room temperature and used within 2 days for performing the cross cut and tape removal tests according to DIN EN ISO 2409:2013. The copper coating had a thickness of about 1 µm.
The resulting metalized plates are shown in Figures 3 to 5 (top view of the plates) and Figures 6 to 8 (cross section of the plates). It can be seen that metallic copper coating completely covers the samples in case of Example 1 and 2 (enzyme treatment, chromic sulphuric acid treatment). In case of Example 3 the copper coating does not cover the complete polyamide surface because the copper coating was separated from the bottom part of the plate by the tape removal test according to DIN EN ISO 2409:2013. In the area where a cross cut test has been performed the copper layer adheres well on samples treated according to Example 1 and 2. On the sample treated according to Example 3 the metallic coating delaminated at the location of the cross cut after the tape removal test according to DIN EN ISO 2409:2013 had been applied. The SEM pictures of the cross sections show that the surface of the PA was least roughened when it was pretreated with enzyme according to Example 1. The surface was rougher after treatment with chromic sulphuric acid according to Example 2. When treated with sulphuric acid alone, the PA degraded and the copper did no longer adhere to the complete PA surface.

Claims

A process for metalizing a polyamide surface, the process comprising the steps:
(i) pretreating the polyamide surface, the process comprising the steps:
(a) providing an object having a polyamide surface;

(b) treating the polyamide surface with a protease;
and

(c) removing the protease from the treated polyamide surface, thus providing an object having a protease treated polyamide surface; and

(ii) metalizing the resulting pretreated polyamide surface using a palladium catalyst,
wherein the metallization step (ii) is an electroless metallization.
The process of claim 1, wherein the object having a polyamide surface is an object consisting of polyamide.
The process of claim 1 or 2, wherein the protease is selected from the group consisting of aminopeptidases, serine proteases, dipeptidyl peptidases, exopeptidases, endopeptidases, aspartic proteases, cysteine proteases, metallo-proteases, and mixtures of two or more thereof.
The process of any one of claims 1 to 3, wherein the protease is applied to the object having a polyamide surface as ingredient of a pretreatment mixture.
The process of claim 4, wherein the pretreatment mixture is a buffered aqueous solution or suspension of the protease.
The process of any one of the preceding claims, wherein step (b) is performed at the temperature optimum of the protease.
The process of any one of the preceding claims, wherein the polyamide is PA 12.
The process of any one of the preceding claims, wherein the metal ions used for metallization are copper ions, and the metal layer resulting from the metallization is a copper layer.