Metallic layers added by plasma on polyethylene

Metallic layers added by plasma on polyethylene

Progress in Organic Coatings 64 (2009) 225–229 Contents lists available at ScienceDirect Progress in Organic Coatings journal homepage: www.elsevier...

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Progress in Organic Coatings 64 (2009) 225–229

Contents lists available at ScienceDirect

Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat

Metallic layers added by plasma on polyethylene ˜ G.J. Cruz ∗ , M.G. Olayo, E. Colin, J.C. Palacios, R. López, E. Granda, A. Munoz, R. Valencia Departamento de Física, Instituto Nacional de Investigaciones Nucleares, Apdo. Postal 18-1027, D.F., CP. 11801, Mexico

a r t i c l e

i n f o

Article history: Received 3 June 2008 Received in revised form 22 July 2008 Accepted 12 August 2008 Keywords: Plasma Polyethylene Stainless steel Titanium Aluminum

a b s t r a c t This work presents a study of the addition of layers composed by different metals on thin films of lowdensity polyethylene (PE) in order to combine the properties of both components. The process involves the immersion of PE in ionic atmospheres of O, N, Fe, Cr, Ni, and Ti to form layers and agglomerates on the surface of the polymer. As this process requires a great exchange of energy, many experimental conditions were studied in order to preserve the structural integrity of the organic compounds. The results indicate that the surface of PE eroded and reacted with the particular reactivity of the plasma gases in the first step, and after that, the metallic particles were adhered forming thin layers. The combination of N and O in different proportions in the plasma produced in PE combinations of fibered and parallel-tracked morphologies that influenced in the polymer–metal interface. © 2008 Elsevier B.V. All rights reserved.

1. Introduction The electrical and mechanical properties in polymers can be greatly enhanced with the inclusion of metallic particles in their structure. This process may involve the physical mixture of components, up to the simultaneous polymerization of monomers and ablation of metals. There are many studies about the mechanical combination of polyethylene (PE) and metals in literature [1–3]. In many of them, the studies correlate the concentration of the metallic fraction and the final mechanical and electrical properties. Another technique to combine polymers and metals is the simultaneous polymerization and metallic doping by plasma. However, this technique has not been thoroughly studied, but the few works about it indicate that the metallic fraction bonds chemically in some points with the polymeric chains producing a strong interface between them and an enhanced conductivity [4,5]. In the formation of alternate layers of polymers and metals, the characteristics of the interfaces are very important, because one material should permeate the other. This process can be carried out by the sequential polymerization of monomers and ablation of metals. However, the main restriction of this kind of systems is the linkage between phases, because their physical and chemical differences difficult their union [4,5]. Plasma immersion ion implantation has been tested in this case with good results in antimicrobial medical applications of Cu–PE

∗ Corresponding author. E-mail address: [email protected] (G.J. Cruz). 0300-9440/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2008.08.025

composites [6]. The most common plasma gases used in these processes are Ar, N, O and mixtures of them, which results in different erosion patterns in the polymers and in the releasing of metals. In this work we study the addition of metallic layers of stainless steel (SS) and Ti on low-density PE films. The process involves the superficial modification of PE in ionic atmospheres of N, O, Fe, Cr, Ni, and Ti; the erosion of metals and the subsequent condensation of metallic layers. These processes involve a great transference of energy, which usually goes beyond the degradation temperature of most polymers.

2. Experimental In order to release metallic atoms that later could be deposited on PE, electrical discharges between electrodes of each metal of interest were used. A combination of gases is added to the system in order to produce ions that accelerate and collision against the surfaces of PE and electrodes. This effect release metallic and polymeric particles that, depending on the intensity of the electrical field, spread over the nearest surfaces. If the chemical reactivity of the particles and surfaces is sufficiently high, a competitive effect of erosion and chemical combination may occur producing that the metallic and polymeric fragments adhere on the surfaces forming layers and agglomerates. The metallic ablation of this work was performed in a vacuum cylindrical chamber made of SS with 0.3 m diameter and 0.6 m height, with several access ports for diagnostic probes, gas inlets, electrodes, etc., see Fig. 1(a). The electric discharges were set between electrodes at 4 Pa, direct current (dc) in the 400–600 V and 0.5–3 A intervals and temperature between 300 ◦ C and 600 ◦ C. The

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Fig. 1. Experimental set-up. (a) Reactor for metallic ablation. (b) Reactor for modification of PE.

power in these conditions varied from 200 W to 1800 W. The plasma was formed with nitrogen and oxygen in different combinations. The polymers were thin films of low-density PE, 10 mm × 10 mm × 1 mm. In the zone between the electrodes, the temperature reaches up to 600 ◦ C. However, to avoid the degradation of the polymer, the samples were positioned in zones of the reactor with temperature less than 300 ◦ C. Due to this restriction, the interaction of the metallic particles with the polymer was indirect. The metallic layers studied were composed of a 304 stainless steel alloy, and titanium. The morphology of PE after the plasma treatments was studied with a Philips XL-30 electron microscope. The superficial elemental analysis was performed by means of energy dispersive spectrometry with an EDAX DX4i sapphire probe coupled to the microscope mentioned before.

face of PE with 30 min of air plasma is roughed with spherical and cylindrical agglomerates with diameters in the range of 100 nm; see Fig. 2(d). According with the chemical interaction of O plasmas discussed before, in this micrograph, the O reactivity prevailed on PE. However, after 60 min of air plasma, the surface of PE changed to a tracked and uneven morphology, see Fig. 2(e), which indicates that the content of N in the plasma gained in the competitive process with the oxygen. In the same trend, Fig. 2(f and g) shows a surface with parallel tracks with an approximate width of 1 ␮m and a relatively smooth surface in each track. In this case, the higher content of Nitrogen prevailed completely on the evolution of PE [7], however, the oxygen content influences the width of the tracks which is approximately 20 times smaller than in the case of PE treated with nitrogen plasmas. 3.2. PE with SS and Ti

3. Results and discussion 3.1. PE The evolution of PE with the plasma gases used in this work was studied to follow the interaction of the reactive species of N and O, in different combinations, with the surface without the influence of metals. The study was done by means of dc glow discharges with SS electrodes of 70 mm diameter and a 100-mm gap between them; see Fig. 1(b). A MDX 1 K Advanced Energy dc source was used for the discharges at 30 W and 3 × 10−1 mbar. Fig. 2 shows different aspects of the surface of PE after the treatment with N, O and air plasmas. The surface of PE without treatment is smooth with small voids and protuberances; see Fig. 2(a). It has an atomic percentage of C and O of 92% and 8%, respectively. This elemental analysis does not include hydrogen atoms. However, it shows that the initial surface was oxidized, probably due to the interaction of PE with the atmosphere. With these numbers, the O/C initial atomic ratio was calculated, yielding O/C = 0.087. The exposure of PE to plasmas of oxygen produces selective chemical and physical erosion that resulted in a surface covered by networks of fibers with an approximate diameter and length of 80 nm and 500 nm, respectively; see Fig. 2(b). On the other hand, the reactivity of nitrogen resulted in morphology of parallel tracks with an approximated track width of 20 ␮m; see Fig. 2(c). The surface of the tracks shows layered erosion, but preserves an appearance without great protuberances. The combination of the chemical and physical interaction of both gases in air plasmas (N/O = 79/21 in %vol) can be seen in Fig. 2(d–g) during exposure times, from 30 min to 180 min. The sur-

In the case of PE with layers of Ti and SS (PE–SS–Ti), N and O were used with the following volumetric N/O ratio: 0 (only oxygen), 2.3 (70/30) and 4 (80/20). As a point of comparison, N/O of air is 3.76. These combinations of gases were used to study the interface between the polymers and metals through the evolution of morphology. The micrographs in Fig. 3 present different perspectives of the PE–SS–Ti composites. They indicate that the first metallic layers grow following closely the surface of the polymer. However, as the thickness grows, the morphology evolves differently. Thus, at the interface, both phases have patterns as those described above in the interaction of PE with O plasma atmospheres; the roughness is high, when the physicochemical activity of O predominates in the plasma. This characteristic can be seen in Fig. 3(a), which shows the morphology of PE–SS–Ti formed with N/O = 0. The gray layer is the metallic fraction and the dark color represents the polymeric support. The thickness of the metallic layer varies from 192 nm to 222 nm with an approximate growing rate of 0.77 nm/min. The edge of the layer suggests that the face of the metals in contact with the polymer has roughness similar to that of the PE treated with O plasmas. The height of the peaks is approximately one third of the entire layer thickness. There are two micrographs showing different cases of PE–SS–Ti formed with N/O = 2.3. In Fig. 3(b), the gray layer belongs to the metallic component with an average thickness of 350 nm and a growing rate of 1.3 nm/min and the dark surface represents the PE substrate. The micrograph shows that the metallic fraction was formed with agglomerates over a polymeric surface with rough-

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Fig. 2. Surface of PE after the treatment with different plasma gases. (a) PE without treatment. (b) PE with 180 min of O plasmas. (c) PE with 180 min of N plasmas. (d) PE with 30 min of air plasmas. (e) PE with 60 min of air plasmas. (f) PE with 120 min of air plasmas. (g) PE with 180 min of air plasmas.

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Fig. 3. Interface of PE–Ti–SS, which shows the influence of O in the plasma. (a) Gas ratio N/O = 0. (b) Gas ratio N/O = 2.3, prevailing O reactivity. (c) Gas ratio N/O = 2.3, prevailing N reactivity. (d) Gas ratio N/O = 4.

ness similar to that of the metallic layer synthesized with a gas ratio N/O = 0. In Fig. 3(c), the surface of PE shows the predominant character of the nitrogen chemistry with a parallel-tracked morphology. The crack in the micrograph shows the surface of PE below the metallic layer. The morphology of PE–SS–Ti deposited with N/O = 4 is shown in Fig. 3(d). The upper layer is metallic with a great variation in thickness and an average of 172 nm associated to a growing rate of 1.36 nm/min, approximately 50% lower than that of the N/O = 2.3 gas ratio explained above. This micrograph shows that the separation between both phases is in the order of few nanometers. However, if we consider that the peaks belonging to both phases partially share the space between them, the gap can be considered almost negligible. This interpenetration of peaks helps in the physical adherence of layers, limiting their displacement and separation. The erosion in the electrodes and PE, because of the oxidative atmosphere, makes that both phases have the contribution of all elements involved in the process. The analysis of the components in both phases provides a good indicator of the composite evolution. In this work, this analysis was done through the average atomic ratios calculated in PE, electrodes and in the PE–SS–Ti layers. These ratios are shown in Table 1. The O/C ratio suggests oxidation between five and seven times greater in PE–Ti–SS than in untreated PE, most surely due to the

oxidative character of the plasmas used in the processes. The O/Fe ratio indicates also a great oxidation with some dispersion in the numbers. Before the experiments, the O content was almost negligible in the SS electrodes. The participation of O in the plasmas affected also the proportion of Cr/Fe, which increased almost twice when N/O = 0 than in the electrodes or in the other layers with different gas ratio. On the other hand, the Ni/Fe ratio had only little changes with the different combination of gases. The highest ratio was found in the electrodes, and the lowest in PE–SS–Ti with N/O = 2.3. The contribution of the Ti electrodes, represented by the Ti/Fe ratio, was low in all cases, but decreased as the proportion of N increased in the plasma.

Table 1 Elemental analysis of the electrodes and PE–SS–Ti. Atomic ratio

O/C O/Fe Cr/Fe Ni/Fe Ti/Fe

PE

SS

0.087 0.304 0.187

PE–SS–Ti N/O = 0

N/O = 2.3

N/O = 4

0.49 18.5 0.62 0.15 0.04

0.59 2.59 0.33 0.13 0.023

0.47 13.68 0.38 0.16 0.01

Fig. 4. X-ray diffraction of PE–SS–Ti.

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This evolution indicates that the releasing of metallic particles from the electrodes and the condensation of agglomerates and layers on the surface of PE can be modified, not only by the electric variables, but also by the physicochemical activity of the plasma gases. The diffraction spectra of PE and PE–SS–Ti are shown in Fig. 4. The diffraction of PE and PE–SS–Ti with N/O = 0 are similar, as both are oxidized, without the influence of the nitrogen species. The main peaks of the orthorhombic arrangement of PE at 21.6◦ , 23.9◦ and 36.2◦ are preserved. However, the other peaks at 29.5◦ , 39◦ , 43.3◦ and 48.7◦ disappeared with the metallic layers. There is a small displacement of 0.4◦ in the peak at 21.6◦ in the composite with N/O = 2.5. The apparent hardness of untreated PE was approximately 2.5 Vickers; however, once the metallic layers were added, it increased up to 50 Vickers, as a function of the thickness and characteristics of the metals.

tracked morphologies in PE, respectively, that influenced the polymer–metal interface. The influence of O in the plasma resulted in a combination of roughed surfaces with peaks in both phases that share the interface in both sides. This interpenetration of peaks reduced the separation between phases and helps in the physical adherence of layers, limiting their displacement and separation. The results presented in this work allow preparing metallic layers of hard metals on the surface of PE without degradation of the polymer. The layers can be combinations of different metals, which can be handled as a unique phase, reaching growing rates of 1.5 nm/min.

4. Conclusions

References

The addition of layers composed by SS and Ti on thin films of low-density PE was studied in this work to combine the properties of both components. The process used electric discharges in O and N to release the metallic particles and to prepare the surface of PE without degrading the polymer. The results indicated that PE reacted physically and chemically in a first stage with the particular reactivity of the plasma gases and later, the metallic particles formed agglomerates and layers on the eroded surface. The combination of O and N in different proportions in the plasma produced combinations of fibered and parallel-

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Acknowledgement The authors would like to thank Conacyt for the partial financial support to this work under the project 47467.