Influence of Plasma Processes on the Molecular Orientation of Polymers

Transcription

1 Influence of Plasma Processes on the Molecular Orientation of Polymers W. Michaeli 1, Ch. Hopmann 1, H. Behm 1, F. von Fragstein 1, K. Bahroun 1, W. Dorscheid 1 1 Institute of Plastics Processing (IKV) at RWTH Aachen University, Aachen, Germany Abstract: Plasma processes constantly gain importance in the field of plastics processing. They are influenced by process parameters on the one hand and by the substrate itself on the other hand. The properties of polymers vary considerably depending on their processing conditions, their history and outer influences. To define the influence of a plasma treatment on the process-induced inner properties, sheet-shaped polypropylene samples are injection moulded. Parts are produced while varying mass temperature, and mould temperature and injection speed. As a result the inner properties vary due to the different cooling conditions. Samples are extracted close to and. The samples are treated in a nitrogen plasma. In order to generate a profile of the molecular orientation 20 µm-thin-sections are cut from the sample sheet and analysed by means of FT-IR spectroscopy. The findings indicate a decrease in molecular orientation as a result of the plasma treatment. This effect gets more distinctive the more cooling conditions vary from equilibrium conditions. Those orientations are more likely to relax during a plasma process, because of a temperature rise of the polymer. This effect is not limited only to the exposed to the plasma, but also influences the bulk material. Keywords: Polymer, Plasma-Substrate-Interaction, Molecular Orientation, Treatment 1. Introduction In plastics processing plasmas are often used to modify properties of thermoplastic parts. There is a wide range of modification possibilities. Next to cleaning, etching and activation processes the deposition of highly functional polymer films is possible. Cleaning and activation processes are used e.g. to enhance the printability or wettability [1, 2]. Etching can be used to implement micro structures or reduce adhesion for example of bulk goods. Deposition processes are used in order to create scratch resistant s, reduce friction or to enhance permeation barriers [3-5]. The used plasma processes are rather complex and influenced by a great number of parameters. Next to the boundary conditions, the properties of the polymer itself are important in order to gain satisfying results. The polymer properties vary considerably depending on their processing conditions, their history and outer influences. Therefore the influence of plasma processes, for instance molecular orientation and crystallinity, is exemplarily investigated for semicrystalline thermoplastics. This research is embedded within the collaborated research project SFB-TR 87 Pulsed high power plasmas for the synthesis of nanostructured functional layers aiming at fundamentally describing the process chain from the production of a thermoplastic part to the finished barrier coated polymer. 2. Theoretical Background The injection moulding process is a discontinuous process. Polymer pallets are fed into a plasticising unit in which they are melted by heat transfer and friction. The conditioned melt is injected into a temperature controlled moulding cavity within a few seconds. Next to the geometry of the part, the inner properties such as molecular orientation, residual stress, crystallinity and morphology are influenced by the process parameters like mass temperature, mould temperature and injection speed [6]. Furthermore the inner properties are highly anisotropic and so are the properties. The backbone of most polymers is a chain of carbon atoms. These polymer chains are usually not straight but form an entangled bulk. With increasing temperature, chain mobility rises. Polypropylene is a semi-crystalline material. While cooling from the melt, a partial alignment of molecule chains can take place. With the parallel

2 alignment and/or folding of the molecules crystallites arise [7]. Because of the material s flow and solidification processes a molecular orientation takes place during the mould filling phase. By the melt front, a biaxially stretched membrane of polymer material with high viscosity is deposited at the mould wall; there it solidifies instantly. The molecular orientation is a result of the velocity profile and the resulting shear rate. An orientation profile forms over the cross section as well as the length of the thermoplastic part with an orientation maximum at the. The overall molecular orientation decreases over the material s flow path. A second maximum near the is the result of shear effects between frozen layer and flowing melt (Figure 1) [6]. gate orientation in flow direction Figure 1. Orientation pattern along the flow path (schematically) [3] 3. Experimental Details In the present case, sheet-shaped samples with the dimensions 118 x 118 x 4 mm³ are injection moulded. The used material is a polypropylene (PP 505 P by Sabic, Sittard, Netherlands). injection moulding parameter mass temp. [ C] mould temp. [ C] injection speed [cm/s] C (cold) M (moderate) W (warm) Table 1. Injection moulding parameters for the fabrication of polypropylene sheet-shaped samples To apprehend the influence of a plasma process on the substrate, samples are injection moulded with three different process parameter combinations varying mass temperature, mould temperature and injection speed according to Table 1. For the subsequent treatment a microwave driven low pressure plasma process is used. The reactor is shown in Figure 2. ventilation valve vacuum pumps M spectrometer pressure transducer substrat shutter with sample microwave generator process gases Figure 2. Low pressure plasma reactor V plasma plasma reactor massflow controller The microwaves are generated with a frequency of 2.45 GHz and form an electromagnetic field inside the rectangular waveguide. A quartz-glass-tube with a diameter of 35 mm is incorporated in the waveguide. The process gas nitrogen (N 2, purity: 5.0, Linde AG, Wiesbaden, Germany) is released into the glass-tube and excited to a plasma. Using a downstream setup, the excited plasma is transported due to the gas flow to the substrate. The temperature of the substrate during the plasma treatment is about 80 C. The substrate is placed inside a substrate shutter. The shutter is closed at the beginning of the process and opened 20 seconds after plasma ignition. The used process parameters are listed in Table 2. process parameter value treatment time 60 s process pressure 20 Pa gas flow of N 2 50 sccm microwave power 600 W Table 2. Process parameters for the plasma treatment of polypropylene In order to identify the profile of molecular orientation over the substrates thickness, 20 µm thinsections are cut out from the sample sheets with the help of a microtome and analysed. Since the properties vary over the flow path of the polymer during the process, samples are taken

3 and of the injection moulded samples (Figure 3). bar gate D y z Figure 3. Sample location flow path far from gate x y IR ray 4 mm mapping measurement To characterise the samples, a Fourier transformed infrared spectroscopy (FT-IR microscope, Nexus 870 by Thermo Nicolet, Waltham, MA, USA) is used. With this method it is possible to analyse the samples chemistry, molecular orientation and crystallinity. 4. Results & Discussion Polypropylene primarily contains CH 3 - and CH 2 - groups. Therefore, IR-spectra mainly consist of CH 3 - and CH 2 -vibrations. For qualitative validation of the orientation polarised FT-IR spectra are recorded with the polariser being aligned parallel and perpendicular to the flow path, i.e. the orientation axis, of the polymer. The dichroic ratio is given by the ratio of the particular absorbance measured each wise parallel and perpendicular [8]. The ratio correlates to the molecular orientation. The spectra show an absorbance at 841 cm -1 that is caused by methyl rocking modes and stretching of the carbon atoms from the backbone (Figure 4) [9] CH 2, CH CH wavenumber [cm -1 ] Figure 4. Example of an ATR spectrum 841 cm CH absorbance [-] This region corresponds to the isotactic conformation of the polypropylene and the crystalline parts respectively. It is used to characterise the orientation of crystalline regions [10, 11]. Figure 5 shows the orientation profile. Comparing the three injection moulding parameter sets, it is obvious, that a decreasing injection speed and an increasing melt and mould temperature result in lower dichroic ratios at the. On the left hand side the untreated is shown, the treated is on the right side. C M W Figure 5. Orientation distribution Especially, parameter W has a distinctly smaller dichroic ratio than C. The variation between parameter C and M is not as definite. The parameter variation does not influence the bulk material significantly. The molecular orientation decreases over the flow path of the polymer (Figure 6). The gradient between the ratio in the bulk material and the is less steep. C M W Figure 6. Orientation distribution

4 Figure 7 shows the orientation distribution of an untreated and one treated in the described way. untreated 60 s treated Figure 7. Orientation distribution of treated and untreated samples (process M) The orientation distribution is in good correlation with the schematically displayed orientation in Figure 1. The dichroic ratio is highest near the s and lowest in the middle of the sample. It slightly increases from 0 to 200 µm before decreasing. A plasma treatment of 60 s affects the whole sample and not only the. A decrease in molecular orientation can be seen throughout the sample. The cause of this behaviour is the heat induced due to the plasma process. At sample temperatures of about 80 C chains relax into a more favourable situation. Interestingly the influence of the plasma treatment is different from the area near the gate (Figure 8). untreated 60 s treated Figure 8. Orientation distribution of treated and untreated samples (process M) The orientation at the seems to increase while it decreases a few µm inside the bulk. The centre of the sample shows no mentionable change in the dichroic ratio. The molecular orientation inside the bulk is after the plasma treatment near and almost the same. The orientation values near and at the approach each other. The interesting question is how the plasma treatment influences the polymer part. We can clearly see a change in the part s molecular orientation. Since the orientation is always connected with the part s properties, a plasma treatment might result in a geometry change (warpage) due to the relaxation of the polymer. In order to minimize the changes in inner part properties the temperature should always be as low as possible during the process. 4. Conclusion Plasma treatment influences the molecular orientation of a polypropylene sample. The level of this effect depends on the process parameters. Due to a locally varying cooling behaviour of the polymer the influence of the plasma treatment differs over the flow path of the polymer melt. The area is more sensitive to a plasma treatment. The material freezes far from equilibrium in this area. A subsequent plasma process allows chains to relax into a more favourable situation. Near the gate the whole sample is affected by the treatment, while, only the and near areas are influenced. The relaxation process gets less distinct over the flow path. When treating a polymer part, it is always exposed to particle bombardment, radiation (e.g. UV radiation) and heat. Not investigated was the influence of the UV radiation induced during the plasma process. Further studies are necessary regarding this concern. 5. Acknowledgements The depicted research has been funded by the Deutschen Forschungsgemeinschaft (DFG / German Research Foundation) as part of the Collaborative Research Centre SFB-TR 87. References [1] W. Michaeli, A. Hegenbart, D. Binkowski, F. Von Fragstein, Zeitschrift Kunststofftechnik 3, 2, (2007), p [2] R. H. Hansen, H. Schonhorn, Polymer Letters 4, (1966), p

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