Method for producing a polymer coated copper-zinc alloy structure with improved adherence

Abstract

 The present invention relates to a method of coating polymers on Cu-Zn alloy substrates with improved adhesion. According to first aspect of the invention the alloy is treated in a plasma reactor with a two-step procedure in order to remove the impurities from the surface and also to improve the surface oxide characteristics. The first step is carried out under vacuum with an inert gas plasma wherein organic and inorganic contamination becomes volatile and is eventually removed. The second step of the method involves treatment of the alloy with oxygen plasma for modifying and selectively oxidizing its surface. By virtue of the superior adhesion performance of maleic anhydride grafted polymers with the above treated alloy, the method of the invention further comprises the step of coating maleic anhydride grafted polymers onto the surface of so treated alloy. Satisfactory and good results were obtained when said polymer is a maleic anhydride grafted polypropylene or polyethylene.

Description

Technical Field of the Invention

[0001] The present invention relates to a method for applying a polymeric material onto a substrate consisting substantially of copper and zinc, more particularly the invention relates to a method for applying polymeric materials onto a Cu-Zn alloy surface with improved interfacial adhesion between Cu-Zn alloy surface and said polymer, said polymer being grafted with maleic anhydride.

Background of the Invention / Prior Art

[0002] Degree of adhesion inbetween dissimilar materials such as metals and polymers has been one of the focus areas of those skilled in the art. It is therefore an objective of the skilled person to obtain better metal-plastic bond so that an improved coated structure may be obtained. Forces that occur in the contact areas are generally obtained by chemical, mechanical, dispersive, diffusive or electrostatic means. The resulting structure may advantageously prevent corrosion of the metals and alloys.

[0003] Treating of a metal surface with chemical agents in order to improve adherence quality is a well known way of increasing adhesion forces between metals and polymers as disclosed, for instance in GB-A-1161097. Accordingly, it was reported that treating a metal surface with a chromium trioxide solution may improve adherence of an organic thermoplastic material upon the treated metal surface.

[0004] Further techniques involving for instance an electrochemical treatment generally includes forming of the plastic layer inside said reactor and is not flexible for separately forming the plastic layer outside the reactor with easier techniques. EP-A1-580944 discloses a method for producing metal/plastic composite structures, especially dental prostheses, which involves forming a mediator layer such as a silane adhesive layer for improving metal-plastic bonding structure by way of an electrochemical process.

[0005] In the prior art techniques as exemplified above, removal of impurities by wet treatment and rinsing treatment leads to considerable amount of chemical consumption and necessity of discharging waste water which may be detrimental in many respects. Furthermore, drying should be processed in a controlled atmosphere, otherwise occurred moisture on the surface may negatively affect the adhesion quality. Ultimately, this practice comprises some disadvantages such as waste of time, environmental contamination and/or high energy cost.

[0006] To solve the aforementioned problems, a further technique is proposed by JP-A-2005007710 which involves treating a metal surface by dry plasma for degreasing and surface reforming on an aluminum substrate and forming a corrosion resistant and adhesion-promoted layer on the aluminum surface.

[0007] The prior art as identified above, however fails to propose a specific technique for laminating polymers onto a substrate consisting substantially of a Cu-Zn alloy. Accordingly, there is considerable need for improving surface adhesion characteristics of the Cu-Zn alloys, such as brass, for various polymers which may be useful in various industrial fields.

[0008] Therefore, it is one of the objectives of the invention to provide a method for producing a polymer coated Cu-Zn alloy structures which eliminate the expensive and harmful steps of treating the metal surface with chemicals and forming mediator layers for improving adhesiveness. Further objectives of the invention include providing a novel method which is applicable for coating of a Cu-Zn alloy substrate wherein oxidation of Zn is selectively promoted for better adhesion.

Summary of the Invention

[0009] The present invention relates to a novel method for coating polymers on Cu-Zn alloy substrates with improved adhesion. According to first aspect of the invention the alloy is treated in a plasma reactor with a two-step procedure in order to remove the impurities from the alloy surface and also to improve the surface oxide characteristics. The first step is carried out under vacuum with an inert gas plasma wherein organic and inorganic contamination becomes volatile and is eventually removed. The second step of the method involves treatment of the alloy with oxygen plasma for modifying and selectively oxidizing its surface. By virtue of the findings according to the present invention as regards superior adhesion performance of maleic anhydride grafted polymers with the above treated alloy, the invention further comprises the step of coating maleic anhydride grafted polymers onto the surface of so treated alloy. Satisfactory and good results were obtained when said polymer is a maleic anhydride grafted polypropylene or polyethylene.

Brief Description of the Figures

[0010] 

Figure 1 is a simplified representation of a Cu-Zn alloy system having an oxide layer before treatment with the method of the present invention.

Figure 2 is a simplified representation of a Cu-Zn alloy system having an improved oxide layer after treatment with the method of the present invention.

Detailed Description of the Invention

[0011] In the adhesion of polymers on metallic surfaces, it is known that carboxylic groups (-COOH) of the polymers behave as Lewis acid whereas the oxide layer on the metal behaves as Lewis base. By transfer of electrons to carboxylic group covalent bonding is constituted between polymers and metal surfaces. Therefore, formation of an oxide layer on the metal surface on which the polymer is to be coated is highly desirable.

[0012] Surface structure of Cu-Zn alloy substrates, when carries out an oxidation, may contain a layer consisting mainly of oxides of zinc and copper. However, oxidation of copper results in a biphasic oxide layer consisting of Cu₂O and CuO. On the other hand, oxidation of zinc produces mainly ZnO. Despite the fact that all these oxides may facilitate adhesion as mentioned above, each of them provides different affinities for polymer materials. CuO is known as an oxidation state of copper which has higher stability as compared to Cu₂O. Therefore, Cu₂O may be regarded as more preferable for adhesion purposes (Popova TV, Aksenova NV, Complexes of copper in unstable oxidation states, 2003, Russian Journal of Coordination Chemistry, Vol 29-11, 743-765).

[0013] Besides these facts, the inventors surprisingly found that ZnO provides the at most affinity for adhesion to maleic anhydride grafted polymeric materials such as the MAH grafted thermoplastics. Thus, one of the objectives of the present invention appears in that providing a method for coating polymers onto a Cu-Zn alloy surface wherein said surface is treated with a selective oxidation for increasing the ZnO proportion as well as reducing the amount of CuO as much as possible.

[0014] In the course of developing such a method for achieving the above objects, the inventors surprisingly found that exposing the Cu-Zn alloy to a two-step plasma treatment provided rapid cleaning of the alloy surface and formation of the oxide layers CuO, Cu₂O and ZnO. The inventors further noted that the two-step plasma treatment as envisaged in the scope of the invention permits selective oxidation of Cu and Zn causing concentration of ZnO to increase relatively more than CuO and Cu₂O. It was even observable that Cu content in the uppermost surface of the alloy underwent an oxidation state with Cu₂O relatively more than CuO. This clearly implicate one of the advantages of the invention with excellent selectivity in providing an oxide layer predominantly comprising ZnO which has crucial benefits in terms of the adherence properties as stated supra.

[0015] The plasma treatment equipment that can be used in the present invention may include a compartment for establishing a sealed low pressure medium and electrodes positioned around said compartment. The equipment may further comprise a reactor, power supplier and a vacuum pump as conventionally used in many applications. The power supplier may be in the form of a RF, LF or MW supply as well known by the specialists in the field.

[0016] According to the first method step, metallic samples (such as brass) are placed into the plasma reactor and interior pressure of the reactor is decreased by applying vacuum. According to a preferred embodiment, said pressure is reduced to a value between 0.05 and 0.30 mbar, more preferably to a value between 0.10 and 0.12 mbar. Then the internal pressure of the reactor is increased to a value between 0.10 and 0.50 mbar, more preferably to 0.20-0.30 mbar by injecting an inert gas into the reactor. Argon is the preferred inert gas for use in the present invention. The inert gas, preferably argon plasma is applied onto the samples for at least 5 minutes, more preferably at least 10 minutes, and most preferably for 15 minutes. It is to be appreciated that the duration of the plasma treatment may be more or less than the above stated treatment times depending on the amount of impurities and contamination on the surface. Moreover, the power applied in this term may be in the range of 50-2000 W depending on the volume of the reactor. In the second method step, the inert gas is disconnected and synchronously, oxygen is started to be injected into the reactor, and the oxygen plasma is applied onto the samples for at least 10 minutes, more preferably for 20 minutes by applying a power approximately in the range of 50-2000 W depending on the volume of the reactor. In a preferred application, the medium pressure during the oxygen treatment is adjusted to a value between 0.10 and 0.20 mbar. Later on, the internal pressure of the reactor is increased to the atmospheric pressure and the samples whose surfaces are covered with a layer substantially of zinc oxide are obtained as ready for a coating process.

[0017] The first step of the disclosed method according to the present invention that is carried out under vacuum with exposure to inert gas provides effective removal of the impurities and surface activation of the copper-zinc alloy. With the effect of the inert plasma physical ablation of the impurities on the sample surface is achieved in a rapid and efficient manner. The in situ surface cleaning continues during oxygen plasma treatment since the impurities are forced to undergo an oxidation reaction which results in an additional cleansing effect. Therefore, the invention is advantageous especially in the event that excessive amount of contamination exist on the sample surface.

[0018] To show the effect of the two-step plasma treatment on a copper-zinc structure, Electron Spectroscopy for Chemical Analysis (ESCA) was sequentially performed on a sample before and after plasma treatment. In the analysis, carbon, oxygen, copper and zinc contents were investigated by using Mg-K alpha source. Depth analyses are carried out for all of the samples by bombarding with argon ions. In mentioned ion bombardment treatment parameters of energy (1000eV) and time (3 min.) are stabilized for each cycle. According to the results as shown in Figures 1 and 2 excellent oxidation of the alloy is attained in a selective manner. Accordingly, relative amount of the zinc oxide is considerably increased while the copper oxides undergo dramatic decrease by virtue of the plasma conditions. The analysis confirms the envisaged advantages with respect to selective oxidation (promoting ZnO) and cleansing of the surface without any need of additional cleaning agents. The inventors report that effect of the plasma treatment is improved when the sample is closer to the electrodes.

[0019] The subsequent step of the method according to the present invention involves coating of a polymer onto the zinc-copper alloy as treated in the above procedure. The polymer as referred herein is preferably of a thermoplastic nature, and is more preferably polypropylene, polyethylene or a blend thereof. In the course of optimizing and selecting the appropriate polymer materials to be coated on a Cu-Zn alloy sample, the inventors noted that when the said polymer is grafted with maleic anhydride, the objective adhesion performance is further increased. Excellent results were obtained when maleic anhydride grafted polypropylene (PP-g-MAH) or maleic anhydride grafted polyethylene (PE-g-MAH) is applied on the surface of a Zn-Cu alloy. The coating procedure may include, but not limited to, lamination, electrostatic powder coating, dip coating and fluidized bed powder coating.

[0020] Normally, use of polypropylene and polyethylene with polar polymers is restricted because of its hydrophobic properties. On the other hand, adhesive properties of them restrict their use with additives for improvement of properties such as wettability and hydrophilic properties. Conventional methods for the modification of polypropylene and polyethylene to overcome these disadvantages include flame treatment, chromo sulphuric acid treatment, chromyl chloride treatment, electric discharge plasma treatment and UV sensitizer treatment.

[0021] To provide a polymer structure which acquires better adhesion with Cu-Zn substrates, it is noted that modifying the polymers with maleic anhydride gives surprising results with plasma treated alloy of the present invention. This feature is attributed to the better interaction of MA with the alloy having improved ZnO content. Non-polar main chain and grafted polar chains are thermodynamically incompatible. As a consequence, polar branched chains are situated actively on the surface of polymer which is thought to contribute the desired adhesion effect.

[0022] Maleic anhydrite (C₄H₂O₃) is known per se and is generally used in the production of polymers, agricultural chemicals, pharmaceutical products, surfactants and plasticizers. The inventors attribute the good performance of maleic anhydride grafted polymers, especially of MA grafted polypropylene and MA grafted polyethylene, in the method of the present invention further to the fact that MA undergoes hydrogenation to succinic anhydride which has higher affinity to ZnO than the other copper oxides.

[0023] MA grafted polymers may be coated on Zn-Cu alloys by any conventional method such as thermal coating and electrolytic coating. Therefore, the method of the present invention eliminates the necessities for additional chemical treatment steps for cleaning and adherence purposes while providing excellent adhesion performance inbetween polymers and Cu-Zn alloys. The method is applicable with any kind of alloy system having predominant Cu-Zn content such as brass which may find large application areas, for instance boilers, piping systems, electronics, bearings and ornamentals.

Claims

1. A method for coating polymers on a Cu-Zn alloy substrate comprising the steps of:

treating the Cu-Zn alloy in a plasma reactor under vacuum and supply of an inert gas for conditioning and cleaning of the alloy surface,

disconnecting the inert gas supply, injecting oxygen into the reactor chamber and treating the alloy under oxygen plasma for modifiying and selectively oxidizing its surface,

increasing the internal pressure of the reactor to the atmospheric pressure and obtaining the treated alloy, and

coating maleic anhydride grafted polymers onto the surface of the treated alloy.

2. A method according to claim 1, wherein the inert gas is argon.

3. A method according to claim 1, wherein the coating is carried out by way of a method selected from the group consisting of lamination, electrostatic powder coating, dip coating and fluidized bed powder coating.

4. A method according to claim 1, wherein the polymers are selected from thermoplastics.

5. A method according to claim 4, wherein the thermoplastic is polypropylene, polyethylene or a blend thereof.

6. A method according to claim 1, wherein the treatment with inert gas plasma under vacuum is carried out under a pressure between 0.05 and 0.30 mbar.

7. A method according to claim 6, wherein the treatment with inert gas plasma under vacuum is carried out under a pressure between 0.10 and 0.12 mbar.

8. A method according to claim 1, wherein the power supplied to reactor chamber in the course of inert gas plasma treatment ranges from 50 to 2000 W.

9. A method according to claim 1, wherein the oxygen plasma is applied for at least 20 minutes.

10. A method according to claim 1, wherein the medium pressure during the oxygen plasma treatment is adjusted to a value between 0.10 and 0.20 mbar.

11. A method according to claim 1, wherein the power supplied to reactor chamber in the course of oxygen plasma treatment ranges from 50 to 2000 W.

12. A method according to claim 1, wherein the alloy is brass.

Drawing