1888
Ind. Eng. Chem. Res. 1989, 28, 1888-1892
High-Quality Polyamide/Metal Compositest Kirkor Sirinyan* and Gerhard-Dieter Wolf Zentralbereich Zentrale Forschung, Bayer AG, 0-5090 Leuerkusen 1, Bayerwerk, West Germany
Conventional wet-chemical metallization methods consisting of 10-15 steps can produce firmly adhering metal films only on two-phase and multicomponent systems such as ABS (acrylonitrilebutadiene-styrene) graft copolymers and high-impact polystyrenes. With the help of a newly developed process, it is now possible to cover the surface of polyamides, an important class of engineering plastics, with a metallic layer. The peeling strength of the metailic layer is much greater than has been realized with conventional ABS metallization methods. The adhesion between the polymer surface and the metal layer is improved by pretreatment of the polyamide injection moldings with a swelling solution containing a special Pd complex. The mechanical properties of the bulk polymer are not affected by the surface treatment. The metallized polyamide injection moldings exhibit good electrical conductivity and shield electronic equipment from electromagnetic interference and electronic eaves-dropping. The shielding effectiveness of the metallic layer is greater than 40 dB. 1. Introduction Heat- and solvent-resistant thermoplastics, such as polyamides (PA), polyimides (PI), polysulfones (PES), poly(pheny1ene sulfides) (PPS) and poly(ether ketones) (PEK), are opening up entirely new opportunities in product design. These materials, in contrast to metals, especially enable parts of the complex geometrical shape to be produced inexpensively (Wolf, 1987). For technical or decorative reasons, injection moldings consisting of the plastics just mentioned are often subjected to an additional finishing process, such as chemical metallization (Weiner, 1977; Goldie, 1968). According to conventional processes, the surface of the polymer is (i) etched at a high temperature, i.e., chemically decomposed at the surface, with oxidizing agents, such as chromosulfuric acid, and washed; (ii) detoxified in dilute sodium sulfite solution and washed; (iii) activated in aqueous colloidal or ionogenic Pd solution and washed; (iv) sensitized in a reducing medium, possibly after the Pd nuclei have been fixed on the surface of the substrate; and (v) covered in a wet-chemical metallization bath with an electrically conductive metal coating with a thickness of 0.1-1.0 ym (Goldie, 1968; Maguire and Kadison, 1970; Hartsing et al., 1986). The thickness of the metal coating deposited in the last step can be increased by normal electrolytic methods. Conventional metallization baths contain not only the metal cation to be deposited, e.g., Ni2+,but also reducing agents, such as sodium hypophosphite, and complexing agents, such as citric acid and boric acid. The reductive reaction of the metal cation is inhibited by the complexing agents. Selective metal deposition cannot occur until the surface of the plastic to be metallized has been provided with an activator. Activation is therefore the most important step. Apart from consisting of several operations, the conventional metallization methods unfortunately have the added disadvantage that owing to the oxidation or etching process they are only able to produce firmly adhering metal films on two-phase and multicomponent systems, such as ABS graft copolymers and high-impact polystyrenes. This restricts their general applicability, e.g., in the metallization of technical plastics, such as polyamides, polyimides, and similar materials. Dedicated t o Professor H. Rudolph, Bayer AG, D-5090 Leverkusen 1, on the occasion of his 60th birthday.
0888-5885/89/2628-1888$01.50/0
Table I. Durethan Grades Most Suitable for the Process Described Polyamide 6 (Glass-Filled/Elastomer-Modified) Durethan BKV 115 (with 15% glass fibers) Durethan BKV 130 (with 30% glass fibers) injection moldings with very high flexural strength and heat deflection temperatures, but their surfaces have a slightly uneven appearance Polyamide 6 (Mineral-Filled) Durethan BM 240 (with 40% mineral filler) Durethan KU 2-2412 (with 30% mineral filler) injection moldings with smooth surfaces, high flexural strength, and high heat deflection temperatures Polyamide 6 (Unreinforced/Elastomer-Modified) Durethan BC 402 very tough injection moldings with smooth surfaces Polvamide 66 (SDecial Flame-Retarded Grade) KU 2-2227 high-quality injection moldings, used mainly in the electrical and automotive industries
Metallized PA injection moldings are being used increasingly where high performance is demanded, as in the electronics sector and automotive and aircraft industries. This article will deal with a new process for the chemical metallization of PA injection moldings, which requires no oxidizing agents (and therefore places little burden on the environment), and with the properties of the resulting polymer/metal composite. 2. Experimental Section 2.1. Test Materials. The metallization tests were performed on injection moldings consisting of conventional types of PA 6 and PA 66. Table I lists a selection of Durethan (Nydur) materials that are particularly suitable for this purpose. Durethan is a registered trademark of Bayer AG, 5090 Leverkusen 1, W. Germany. Injection moldings of optimal quality are produced at a mass temperature of 290-300 "C and mold temperature of 40-60
"C. 2.2. Wet-Chemical Metal Deposition. (i) Activation of the PA injection moldings occurs in a selective pretreatment bath containing, in addition to the Pd complex, mainly Ca and A1 salts (Sirinyan et al., 1985; Wolf and Sirinyan, 1984). The exact composition of the activation bath can be found in, e.g., US Patent 4.554.183, example 7 (Sirinyan et al., 1985). For reasons of industrial hygiene, the solvent used is technical ethanol consisting of 96 wt 0 1989 American Chemical Society
Ind. Eng. Chem. Res., Vol. 28, No. 12, 1989 1889 ?& ethanol, 3.7 wt 7% toluene, and 0.3 wt 7% water. In the interest of product reliability, the parts are activated for 5 min at 20-25 "C. (ii) Possibly-depending on the type of polyamide or processing conditions-additional chemical roughening of the activated surface occurs in a bath having the same composition as the selective pretreatment bath except that Pd is not present. (iii) Sensitization of the pretreated PA surface occurs in a bath consisting of alkaline DMAB ((dimethylamino)borane). This bath consists of 12 g of DMAB, 2 g of NaOH, and loo0 g of distilled water. The sensitization is carried out at 20-25 "C and lasts 5 min. From the tests we have performed so far, it is evident that the polyamide injection moldings can absorb 0.2-1.0 wt ?& water during the sensitizing operation. (iv) Deposition of the first electrically conductive Ni film at a thickness of 0.2-1.0 pm occurs in a chemical Ni bath of normal composition; this film already has good electrical conductivity. All the baths are used generally at room temperature. The times of residence are 5-10 min in the heat activation and sensitization baths and 20-30 min in the chemical Ni baths. 2.3. Determination of the Physical and Chemical Properties of the PA Surface. (i) X-ray wide-angle scattering measurements were used to investigate the physical nature of the PA surface. For this purpose, a diffractometer with filtered Cu K a radiation was used. The results were evaluated by computer measurement of the peak intensities and half-widths of the principle reflections at 28 = 20". In addition, the surface topographies of the treated and untreated samples were investigated with a SEM (Scanning Electron Microscope). (ii) The chemical nature of the sample surface was investigated with a LAMMA (Laser Micro Mass Analyzer). 2.4. Determination of the Mechanical Properties of the PA Injection Moldings. The purpose of this investigation was to find out whether and, if so, to what extent the original properties of the plastic parts are affected by the treatment in the metallization baths. After the various treatments, the impact strength data of small standard test pieces were determined according to DIN 53 453 and compared with the values of the untreated samples. 2.5. Properties of the Metal Film. Thermocycling Test. The thermocycling test was performed according to DIN 59 496. The severest conditions specified in this standard (the conditions of class A) are provided by alternating exposure to +80 and -40 "C.
3. Results and Discussion An important part of the new metallization process is a new activation principle according to which new activators, preferably with specific affinity for the substrate in question, are used in selective swelling agents. The activator consists in principle of (i) a group capable of attaching itself to the polymer surface, (ii) an organic nucleus containing the group capable of complexing a noble metal atom, and (iii) the noble metal, preferably Pd (Sirinyan et al., 1983, 1985). It is thus possible for the activator to be applied to the surface of the polyamide-ideally as a unimolecular film-in such a way that it adheres very well. The activator on the PA surface is detectable by surface analysis with the LAMMA instrument (see Figure 1). As mentioned in the Experimental Section, the Pd complex is applied to the parts by dipping them in the selectively
100 Of0
50
C 10
20
30
40
50
60
70
80
90 100 110 120 mass [amu] +
Figure 1. LAMMA surface analysis of an activated injection molded PA sheet.
swelling alcholic solution containing the Pd complex at a concentration of 0.5-0.8 g/L. After being activated in this way, the surface of the PA is sensitized by immersing the part for 5-10 min in an aqueous solution of DMAB ((dimethy1amino)borane)at room temperature, This step reduces the divalent Pd complex to metallic palladium (eq 1). 3L2PdC12 + (CHJ2NHBH3 + 3H20 3Pd0 + 6L + H3B03+ (CH3),H2NC1 5HC1 (1) +
+
L = complex ligand The fundamental reactions of the wet-chemical metal deposition process, as exemplified by chemical copper plating or nickel plating, are represented by eq 2a and 2b. NiSOl
+ - +
+ NaHZPO2+ HzO
activator
NiO
Cu2++ H2C0 + 30H-
activator
NaH2P03+ H2S04 (2a) Cuo
HCOO-
+ 2H20 (2b)
Our investigations have shown that chemical metallization baths containing DMAB, instead of hypophosphite, as the reducing agent make it unnecessary to sensitize the activated sample. We were able to demonstrate that in these baths the activator, a divalent Pd complex, is reduced by DMAB to metallic Pd in the first reaction step (eq 1). The metallic palladium enables Ni to be deposited directly (eq 3). Hence, this highly simplified wet-chemical metal activator
2(CH3)2NHBH3+ 6NiS04 + 6H20 pdo 6Ni0 + 2[H2N(CH3)l2SO4 + 2H3B03+ 4H2S04 (3) deposition process consists of the following two steps: (i) activation, followed by (ii) wet-chemical metal deposition. A sheet activated with the new activator can even be metallized without difficulty after having been stored for several months. The use of an appropriate solvent or solvent mixture for the activation bath enables the adhesion of the metal film
1890 Ind. Eng. Chem. Res., Vol. 28, No. 12, 1989
Figure 2. Injection molded PA 6 sheet; (a, top) untreated, (b, bottom) activated. Table 11. Peel Strength According to DIN 53 494 for Selected Durethan Grades Durethan grade Dee1 strength, N/25 mm BKV 115 60 BKV 130 60 B M 240 50 K U 2-2412 50 BC 402 55
deposited in the final step to be improved. If the physical and chemical nature of the solvent in which the activator is dissolved is such that the polymer coils on the surface of the substrate are partly swollen, expanded, or dissolved out, the metal ions are able to diffuse into the intergranular spaces thus rendered and are deposited as metal. This gives an additional physical anchorage. Attention is drawn in this connection to the extreme uniformity of the microroughening of the polyamide surfaces thus activated (see Figure 2). The crystalline formations have a depth of 0.2 pm and are considerably smaller than those given by conventional etching. It should be noted that in many cases metal films deposited on PA by the new metallization process cannot be removed without destruction of the metal matrix (see Figure 3a). The same structures are also recognizable on the nickelplated side of the metal film (see Figure 3b). This characteristic is seen only in ideal polymer/metal composites. The peel strengths (150 N/in.) of the metal films on PA composites are considerably higher than those of corresponding films on ABS (15-25 N/in.). The peel strength values are shown in Table 11. The polymer/metal composites have outstanding thermal properties. They pass the thermocycling test and
Figure 3. (a, top) Injection molded PA 6 sheet, after removal of the metal film. (b, bottom) Polymer side of the metal film after removal from a metallized PA 6 sheet.
temperature shock test without difficulty and have good long-term heat stability. Their heat-deflection temperatures are -200 “C. The metal plating, which has excellent thermal conductivity, prevents the formation of hot spots on the surface of the material. It is well-known that PA 6 and PA 66 are materials of high crystallinity. The high crystallinity of PA is explained by the intermolecular hydrogen bridge bonds, which are detectable by IR and NMR spectroscopies, for example (Votteler and Hoffmann, 1984; Simon and Argay, 1978). The crystallinity of conventional PA 6 is stated in the literature as being approximately 66%. The crystalline zones of the P A matrix consist of (i) CY modifications representative of two-dimensional “net planes” and (ii) y modifications consisting of three-dimensional regular or static networks. The additional modifications frequently referred to can be explained as mixed forms of these two basic modifications. Some authors, for example, use the term p modification for a strictly alternating CY and y lattice arrangement, which, however, has not yet been observed in the pure form. In the case of the y modification, as already mentioned, the hydrogen bridges lie between parallel oriented chains, which is only possible if the binding angles and distances are distorted or the intermolecular bridge bonds weakened (see Figure 4). PA 6 parts with high y content are well
Ind. Eng. Chem. Res., Vol. 28, No. 12, 1989 1891 Table 111. Effect of the Pretreatment and Metal Coating on Impact Strength According to DIN 53 453 impact strength, kJ/m2
Durethan grade BKV 115 BKV 130 BKV 30 H BM 240
activated for adhesion 57 69 no failure no failure
untreated 58 69 60 no failure
after chem Ni plating 58 69 60 no failure
after chem and electrolytic Ni plating 56 67 no failure no failure
after chem and electrolytic Ni plating and electrolytic Cr plating 45 59 54 35
Polyamide 6 Equatorialprincipal reflections
a-modlflcallon
a (anti-parallel)
y (parallel)
y-modifmtmn
monoclinic a= 9.56A b= 8OlA C= 1724A V= 112.5'
momxlinlc
Figure 5. PA housing, shielded from electromagnetic waves, of the electronic antiknock regulator. I
I
1
30'
25'
20'
1 15O
1 30°
I
I
25"
20°
I 15O
1
30°
I
1
25'
20'
I 15'
Figure 4. X-ray measurements on PA 6, principal reflections of the a and y modifications and of the a + y mixed form. I
suited for metallization. Here it appears very likely that the activation first removes the amorphous PA phase from the surface and that the finely roughened surface is formed by the remaining y modification (see Figure 2). In subsequent investigations, we found that the modification, be it CY or y, depends very much on the temperature of the mold. Injection moldings of very high y content which are well suited for wet-chemical metallization can be produced a t low mold temperatures, particularly in the range 40-60 "C. X-ray investigations of samples produced a t mold temperatures of -80-100 "C show clearly that these consist mainly of CY modification. Later investigations showed them to be less suitable for the application of firmly adhering metal coatings. Experiments with the samples produced a t 40-60 "C revealed that heat treatment a t 180 "C causes a high proportion of the y modification to be transformed into the a modification. After 5 min, this process comes to an end, further heat treatment for about 10 or 20 min leading to no increase in the proportion of CY modification. Thus, postmolding heat treatment has an adverse effect. The suitability of polyamide for metallization is not significantly affected by such other parameters as macroscopic roughness, the weld lines attributable to the rheology of the melt, and changes in surface polarity caused by mold release agents and fillers. Hence, no further reference will be made to them in this article. We have been able to demonstrate that the mechanical properties of the polyamide, such as its impact strength, are not adversely affected by the activation and nickelplating processes (see Table 111).
I 1
Figure 6. Electromagnetically shielded PA coupling half-shells.
A slight loss of impact strength is observed after the metal film has been reinforced electrolytically (see Table 111). This results, in all probability, from the additional notching effect of the chromium metal coating. As the composites combine the desirable properties of the metal with those of the polyamide, the applications of this plastic are growing in both the decorative and functional sectors, as mentioned already. Because the metal adheres to the polyamide firmly, parts consisting of this material can now be shielded from electromagnetic waves. In the past, this protection could only be conferred on certain ABS and polycarbonate grades (Bledzkiand Stankowska, 1984; Ebneth and Fitzky, 1982). Injection moldings with an -0.5 pm thick metal coating are well shielded from electromagnetic waves in the range 1-70 GHz. They have a shielding effectiveness of 240 dB. Injection moldings with a metal coating of 23 pm have a very good shielding effect toward electromagnetic waves both in the frequency range 1.0-300 MHz and in that of 1-70 GHz. These parts have a shielding effec-
1892 Ind.
Eng. Chem. Res., Vol. 28, No. 12, 1989 exploit the good dynamic stability and high heat distortion temperatures offered by polyamides. The Velour-nickel-plated holder half-shell shown in Figure 6 provides another example of electromagnetic shielding. This part belongs to the coupling of a shielded cable. It is important that a plastic used in the vicinity of conductors should have a sufficiently high heat deflection temperature. Figure 7 shows a black chromium-plated filter cap belonging to an analytical air tester. Durethan is used in this case because the filter must have particularly good resistance to chemical air pollutants. Still under development is a reflective housing for the reading lamps of the Airbus (Figure 8). Polyamides have been chosen because resistance to high temperatures is needed. In order to have the required reflective capacity, the parts must be suitable for decorative chromium plating. According to the test results referred to in this article, the parts may be expected to function reliably in continuous use for many years. The new process is now at the market introduction stage. Registry No. DMAB, 1838-13-7; Ni, 7440-02-0; Cu, 7440-50-8; P A 6, 25038-54-4; P A 66, 32131-17-2; Durethan BKV 115,
Figure 7. Black chromium-plated PA filter cap.
113833-79-7; Durethan BKV 130,113833-80-0; Durethan BC 402, 96827-40-6; chromium, 7440-47-3.
Literature Cited
Figure 8. PA reflective housing for the reading lamps of the Airbus.
tiveness of 20 to 280 dB in all frequency ranges. Figure 5 shows the shielded housing of an automobile’s electronic antiknock regulator. As the regulator is close to the engine compartment, it is important to be able to
Bledzki, A.; Stankowska, D. Kunststoffe 1984, 74, 89-92. Ebneth, H.; Fitzky, H. G. Plastverarbeiter 1982,33, 760-764. Goldie, W. Metallic Coating of Plastics; Electrochemical Publications Ltd.; Middlesex, Great Britain, 1968; Vol. 1. Hartsing, T. F.; Sauers, M. E.; Robenson, L. M. European Patent Application 0.180.042, 1986. Maguire, E. G.; Kadison, L. P. German Patent Application 2.046.689, 1970. Simon, P.; Argay, G. J. Polym. Sci., Polym. Phys. Ed. 1978, 16, 935-937. Sirinyan, K.; Gieseke, H.; Wolf, G. D.; Ebneth, H.; Merten, R. European Patent Application 0.081.129, 1983. Sirinyan, K.; Wolf, G. D.; Merten, R.; v. Gizycki, U. U.S. Patent 4.554.183, 1985. Votteler, Ch.; Hoffmann, V. Makromol. Chem. 1984,185,1953-1977. Weiner, R. Electroplating of Plastics; Finishing Publications Ltd.: Twickenham, Middlesex, Great Britain, 1977; Chapters 4 and 5. Wolf, M. Kunststoffe 1987, 77, 613-616. Wolf, G. D.; Sirinyan, K. Galvanotechnik 1984, 75,977-982.
Received for review February 28, 1989 Accepted September 28, 1989
