Plasmatreat Shanghai, a subsidiary of the German Plasmatreat GmbH, will show exciting and new highlights about its Openair-Plasma technology for surface treatment at Chinaplas in Shanghai from April 13 to 16.

Medical Application: When treating catheters with plasma to clean and activate the surface, the use of plasma replaces, for example, chemical treatments such as sodium etching. Therefore, the plasma treatment is more environmentally friendly than other treatment methods

16:9

Plasmabehandlung

Oberflächentechnik

Reinigen und Vorbehandeln

Oberflächenveredelung

Dienstleistungen

Lohnfertigung

Plasmatreat at Chinaplas 2021

The use of recyclates confronts companies with new requirements in the processing, bonding, printing and labeling of materials and components. Atmospheric plasma technology, in which Plasmatreat GmbH has specialized, has established itself as a key technology: it often makes the use of recycled plastics possible in the first place – and also proves to be the means of choice on the way to more environmentally friendly, more resource-efficient production processes.

When using Openair-Plasma, the plasma jet works in a location-selective manner and follows the component geometry with millimeter precision. During the plasma treatment, very little heat is generated, so that the treated components remain dimensionally stable.

Atmospheric Plasma Technology as the Key to More Resource Conservation

Bedrucken

Chemisches Bearbeiten (Auftragen)

Additive Fertigung

Kunststoff-Rezyklat

Kunststoff Werkstoffe

Kreislaufwirtschaft

Improved Processing of Recyclates

The Plasmatreat Academy gives an overview of reliable and sustainable manufacturing solutions concerning plasma surface treatment.

In the use of Openair-Plasma (left), the plasma beam operates selectively and follows the part geometry. Only very little heat is produced, so that the treated parts remain dimensionally stable. In the case of PlasmaPlus coating (right), specific additives are introduced into the plasma via a special nozzle head. They are excited by the plasma significantly increasing their reactivity.

The Plasmatreat Academy Builds Bridges between Research and Industry

Beschichten von Kunststoffen

Weiterbildung, allgemeine

Seminare, Schulungen, Kongresse

Not Plucked out of the Air

The exhibitors at K 2022 showed ways and means of achieving special surfaces by injection molding. A small selection.

The material mix of the “rear-end cover” allows backlit elements to be generated with selective light barriers – with no undesirable light scattering.

State-of-the-Art Injection Molding Solutions at K 2022, Part 4

Spritzgießen

Kunststoffverarbeitung: Verfahren

Doriat, Clemens, Dr.

Temperiertechnik

Künstliche Intelligenz für die Qualitätssicherung

Industrie 4.0

Industrie 4.0 für die Qualitätssicherung

Mehrkomponenten-Spritzgießen

Folientechnik (IMD, IML, FIM, u.a.)

Spritzprägen

Dynamische / variotherme Temperierung

Polycarbonat

Thermoplaste

Impressive Results

Manufacturers of high-quality film nowadays control their entire production using film inspection systems, which flag any errors detected and immediately alert the operator. The process, the raw material and the end product are all monitored. 

Inspection Systems for Process Analysis and Yield Improvement 

Flachfolien- und Plattenextrusion

Oberflächeninspektion

Qualitätsnachweise

Film Inspection and Process Control  

What defects can the systems detect? Can the measurement data be used for process control? What is the technical standard and how are the inspection systems used?

Film inspection system in a blown film line: the image processing systems detect surface defects such as gels, black specks, fish eyes, coating cracks, streaks, flow lines or insects.

Process Optimization and Cost Reduction by Standards

Bildverarbeitungs-Software

Fertigungsautomation

Bildverarbeitung

Bildverarbeitungssysteme

Flachfolienanlagen

Kunststoffverarbeitung: Maschinen und Komponenten

Extrusionsanlagen

Messtechnik

Form- / Oberflächenmesstechnik

Folien

Halbzeuge und Zwischenprodukte

Kunststoff-Halbzeuge

Film Inspection for  Medical Applications

Increasing demands on component functionality and the need for efficient manufacturing processes are keeping multi-component injection molding squarely in the focus of plastics processing. Not all materials can be processed reliably in the desired combination. The new InMould-Plasma process, which is fully integrated into the injection molding process, facilitates more-efficient production and paves the way for completely new material combinations.

Demonstration mold for the production of a housing cover by the InMould plasma process

More Material Combinations,  Lower Costs

Thermoplastische Elastomere

Spritzgießwerkzeuge

Werkzeuge für die Kunststoffverarbeitung

Dichtungsschäumen

Schäumen / Reaktive Verarbeitung

InMould-Plasma Streamlines the 2-Component Process and Expands  the Material Spectrum

Magnus and Lukas Buske join the management team of the family owned business headquartered in Germany.

Plasmatreat founder Christian Buske (middle) with his sons Magnus Buske (left) and Lukas Buske.

Personen

kunststoffe.de

Plasmatreat to Expand Management Board

Openair-Plama surface pretreatment technology enables users to pre-treat the surface of various materials prior to printing, painting, bonding or sealing.

When treating large-area components, such as a dashboard, the use of flame treatment can be dispensed with Openair-Plasma technology.

Messe Fakuma

kunststoffe-international.com

Eco-Friendly Pretreatment Technology for the Plastics Industry

Coating companies, who paint interior and exterior parts made of plastic for the automotive industry, have to meet high quality and efficiency requirements. To equip a paint line for surface treatment with a fully automated dry ice blasting unit, including a robot and dry ice granulate production, can increase the profitability.  

Fully Automated Cleaning of Scooping Components before Painting 

Automatisierungstechnik

Automatisierungskomponenten

Teilereinigung

Dry Ice Shower for  Complex Components 

The disciplines in surface technology are just as varied as the resulting properties. The applications in plastics processing show an equally broad array of facets. Finishing processes often arise which induce the established techniques to reach even higher performances. 

Surface Treatment: Would You Like a Bit More? 

In previous years, innovative technology and functions have been a major factor in making the decision to purchase a product. With the increasing similarity of technical capabilities, interest has begun to focus more and more on design. This means that a product must not only function as expected, but also satisfy the increasing customer expectations regarding form, surface finish and feel. 

Backmolding on Film: Decoration and Function Combined 

Metallic sheen, specular coatings and shiny decorative articles make users’ hearts jump for joy. However, it only takes the tiniest of dust and dirt grains to turn such products into production scrap. A cleaning method that utilizes sword brushes solves this problem and makes for particle-free surfaces. 

Cleaning: No Chance for Dirt Particles 

In today’s industry, atmospheric pressure plasma systems are used primarily for cleaning and activating surfaces. Their integrational ability is increased, if the transformer is fitted in the plasma head. Tests have shown that plasma pretreatment significantly increases the surface energy of plastics and metals. 

Plasma System with Variable Output for Pretreating Bonding Surfaces 

Thoroughly Cleaned and Activated 

Dry ice cleaning from Cold Jet GmbH, Weinsheim, Germany, finds particular application in the automotive, packaging and medical sectors. 

 PCS 60 for flexible and mobile dry ice cleaning (© Cold Jet) 

Cold Jet at K 2019 

Reinigung

Dry Ice for Cleaning Different Surfaces  

Polymer surface functionalization using a new µ-wave driven atmospheric pressure plasma-jet device

A newly developed compact µ-wave plasma source was utilized for activating the surfaces of several polymers. A remarkable improvement of the surface adhesion was achieved.

A newly developed compact µ-wave plasma source was utilized for activating the surfaces of several polymers. A remarkable improvement of the surface adhesion was achieved. The plasma-treated surfaces were investigated by AFM, XPS and surface free energy measurements. Also, actual adhesion improvements were evaluated by pull-off testing of paint-coatings applied after the plasma treatment. The plasma itself was characterized by optical emission spectroscopy, gas temperature determination as well as waste gas measurements.

Nowadays, plasma processes, especially under ambient pressure conditions, have become an indispensable part of a variety of manufacturing processes for the activation of polymer materials. The basic effect behind this is that highly reactive plasma species in combination with high-energy UV radiation can make non-polar plastic surfaces accessible for subsequent bonding or printing by creating polar functional groups. On the one hand, this can be achieved by purely activating plasma processes with helpful reactive gases, in which in particular oxygen- or nitrogen-containing functional groups are created on the surface. On the other hand, layer-forming precursors or primer systems can be added into the plasma process, which leads to the formation of new thin adhesion promotion layers on the surface as well.

Looking at the multitude of different plasma systems, one first finds a great variety of large-area systems, mostly based on dielectric barrier discharges (DBD), e.g. [1, 2]. These systems are particularly suitable for the treatment of flat materials, e.g. textile woven and non-woven, certain sheet materials or plastic foils. Secondly, so-called jet plasma systems have increasingly entered the market in recent years, for example, by Plasmatreat, e.g. [3], Tigres, e.g. [4], neoplas, e.g. [5] or relyon [6] (all in Germany), vito, e.g. [7] (Belgium) or Tantec, e.g. [8] (Denmark). These systems are especially suited for local and selective treatments as well as for the treatment of three-dimensional materials. A comprehensive overview of different types of discharges as well as jet plasma devices and their relevance to adhesion can be found in [9].

This paper aims to introduce a new plasma system using microwave excitation and to present corresponding results in the field of plasma-based polymer surface activation. A major advantage of this system is its compact form factor. A controller unit integrated with the plasma source eliminates the need for additional external supply modules and resulting supply lines. Furthermore, this µ-wave plasma source can be more advantageous than other conventional plasma systems in terms of power consumption and feasibility. In this paper, we investigate the influence of plasma treatment on the surface properties of pre-treated polymers. These results are supported by adhesion and optical emission spectroscopy measurements to determine the reactive species in the plasma and by gas temperature measurements. Finally, results on waste gas production generated while µ-wave plasma ignition will be presented.

Vivek Faldu, Oliver Beier, Andreas Pfuch, Kerstin Horn; INNOVENT e.V., Jena

Johannes Zender, Denis Wolf, Thomas Filler, Wolfgang Heinrich, Ulrike Winterwerber, Neysha Lobo Ploch; Ferdinand-Braun-Institut gGmbH, Leibniz-Institut für Höchstfrequenztechnik, Berlin

Oberflächenaktivierung von Polymeren mit Hilfe einer neuen Mikrowellen-Atmosphärendruck-Plasmaquelle

Eine neu entwickelte kompakte Plasmaquelle mit Mikrowellenanregung wurde hinsichtlich ihrer Eignung zur Vorbehandlung von Oberflächen untersucht. Dabei konnten deutliche Steigerungen der Haftfestigkeit von Lackierungen auf den vorbehandelten Oberflächen ermittelt werden. Als Analysemethoden kamen AFM- und XPS-Messungen an den vorbehandelten Bauteiloberflächen und Bestimmungen der freien Oberflächenenergie zum Einsatz. Ergänzt wurden die Untersuchungen durch Stempelabrisstests zur Bestimmung der Haftung von nachfolgend aufgebrachten Lackierungen. Das Plasma selbst wurde durch optische Emissionsspektroskopie, Gastemperaturbestimmungen sowie Abgasuntersuchungen charakterisiert.

Polymer surface functionalization using a new ?-wave driven atmospheric pressure plasma-jet device

V. Faldu, O. Beier, A. Pfuch., K. Horn, T. Filler, W. Heinrich, U. Winterwerber, N.L. Ploch, J. Zender, D. Wolf

A new atmospheric pressure µ-wave plasma source based on the plasma jet principle was developed by the Ferdinand-Braun-Institut (FBH), fig. 1. The device is a highly integrated low-temperature atmospheric plasma source that comprises a microwave power oscillator, a resonator, which excites the plasma, and the control circuit, all integrated into a single, miniaturized package, fig. 2.

Figure 1: Compact µ-wave atmospheric pressure plasma device, developed by the Ferdinand-Braun-Institut, the plasma jet is visible as a small plasma flame at the front-side of the source

The microwave power oscillator which operates in the 2.45 GHz ISM band uses a gallium-nitride high electron mobility transistor (GaN HEMT) [10]. The high-frequency power generated by this transistor is coupled into a special ?/4-coaxial resonator which is responsible for both ignition and continuous stimulation of the plasma. With an external gas feed, the plasma is blown out at the front of the device. To maximize functionality and ease of use, the system has a built-in controller unit for easy plug-and-play applications and can be operated stand-alone or with an external PC. In addition, all supply voltages are provided by integrated DC/DC converters, requiring only a single external power supply of 48 V DC.

Figure 2: The plasma source can be divided into three main sections: control unit (A), microwave power oscillator (B) and resonator (C)

The µ-wave plasma source is protected by a patent [11] and offers a number of advantages compared to more conventional sources. Due to the high frequency of the microwave power oscillator, the printed circuit board and the electrical components are very small, resulting in extremely compact device dimensions of 124 mm x 46 mm x 24.5 mm. Thanks to its small size and only three inputs for the 48 V DC power supply, water cooling and gas supply, it is easy to handle either as a stand-alone plasma tool or as part of a larger industrial system. The cooling water and the gas feed can be applied at the one-touch fittings mounted at the back of the device. Furthermore, since all structures needed to generate the plasma are integrated into the compact housing, this device has neither high voltage nor high frequency supply line which are known to be hard to handle and may radiate into the environment. Although water-cooling of the system is recommended to extract the heat generated during operation, the system can be adapted for active air cooling to enable easy integration into production lines. Another advantage of the compact µ-wave plasma source is that the generated plasma is very homogeneous and has a low temperature which is very important for the treatment of temperature sensitive materials. This can be attributed to the high electron density and reduced ion mobility of microwave driven plasmas. Finally, the plasma source can be operated using different gases such as air, argon, oxygen or nitrogen without any hardware modifications. With different gases and flow rates, the temperature of the exhausted gas as well as the generated species of NOx and ozone will be influenced. An optimum flow rate of 2 l/min with the use of air has been determined.

Various polymer materials with dimensions of 50 x 50 mm2 and substrate thicknesses of 2 to 3 mm (Rocholl GmbH) were used for the investigations. Polyamide nature (PA6, manufacture Sustaplast #26013), polypropylene nature (PP, manufacture Simona DWST #47846), polyethylene nature (LDPE, manufacture Simona PEW #68874 and HDPE, manufacture Simona HWST #68873), polycarbonate (PC, manufacture Makroform 099, #29678) and polymethyl methacrylate (PMMA, Röhm Plexiglas XT20070FF, #37191) were chosen as test polymers. Before treatment, all samples were pre-cleaned by hand with kimwipe tissues and isopropanol to remove any residues of release agents from the production process, after that the samples were sufficiently aired.

For plasma treatment, the samples were fixed on an x-y stage unit (Pasim Direktantriebe GmbH) and treated with different stage speeds (varying between 20 and 100 mm/sec) and different torch-substrate distances (varying between 2 and 10 mm), fig. 3. The samples were passed under the plasma source in a meandering manner with an offset of 1 mm, with only one system run. The used working gas was air, which flowed through the plasma system at 2 l/min and was adjusted by a variable area flowmeter of type Uniflux (VAF Fluid-Technik GmbH). The overall power consumption of the plasma source was about 90 W with at least 20 W plasma output.

Figure 3: Image of the experimental setup depicting the µ-wave plasma system, test sample and x-y stage

Immediately after the plasma treatments, the samples were painted with a polyurethane-based paint system (Mankiewicz H2O ALEXIT, decorative paint 341–34 920 C satin matt and hardener 345–24) that was applied with a spray gun. The lacquer was dried for 30 min at 80 °C and then for 24 h at room temperature. For the purpose of artificial aging of the applied lacquer, the samples treated in this way were stored in deionised water at 80 °C for 96 h, followed by drying of the samples for one day at room temperature. Afterwards, steel stamps (Erichsen GmbH & Co. KG) of 20 mm diameter were glued onto the lacquer surfaces that were sandblasted, treated with Pyrosil® and spray coated by a primer SuraLink21 (both Sura Instruments GmbH). The adhesive used was a ScotchWeld DP460 (2K epoxy from 3M Deutschland GmbH) which was cured for 3 h at 65 °C. With regard to the adhesion of lacquer – polymer, the adhesive bonding strength was quantitatively investigated by carrying out face pull-off tests. Ten bonding pairs were fabricated for each individual plasma parameter set for pull-off tests, and three samples each were reserved for analytical purposes. The pull-off tests were carried out using an inspect table 50 kN tensile testing machine (Hegewald & Peschke, Meß- und Prüftechnik GmbH) where the painted test specimen were fixed and the bonded steel stamps were pulled vertically upwards with a test speed of 5 mm/min.

Contact angle measurements and determination of the surface free energy on the plasma pre-treated polymer surfaces were carried out using an OCA15+ contact angle measuring device and the corresponding SCA 20 software (dataphysics GmbH), with each contact angle measurement value being averaged from ten measurements. As test liquids for contact angle measurements water, diiodomethane, ethylene glycol and thiodiglycol were used, and the drop volume was at 1 µl. All contact angle measurements were performed in a static regime at room temperature. The determination of the surface energy was performed according to the Owens, Wendt, Rabel and Kaelble (OWRK) model considering diiodomethane, ethylene glycol and thiodiglycol as test liquids. The surface roughness of the specimens was determined using an MFP 3D-Classic atomic force microscope (Asylum Research Oxford Instruments Inc.) over 10 x 10 µm2 measuring areas. Here, each sample was analysed before and after plasma treatment at three different measuring points. The corresponding roughness values Sa were determined according to EN ISO 25178 (software MarSurf MfM Premium, version: 7.4.8737). X-ray photoelectron spectroscopy (XPS) measurements for element-specific analysis of the polymer surfaces were performed using an Axis Ultra DLD (Kratos Analytical Ltd.) with a monochromatic X-ray source (Al K?: 1468.6 eV).

To characterise the plasma source itself, plasma gas temperature measurements were done with air as the process gas and a flow of 2 l/min using a type K thermocouple and an YC-747U data logger (TC Mess- und Regelungstechnik GmbH). The species excited in the effluent were detected by optical emission spectroscopy measurements in the wavelength range between 200 and 1100 nm using an EMICON MC system (Plasus GmbH). For this, the photosensor was aligned at a distance of 65 mm with a „view“ directly into the plasma source.

Gas sensory measurements served as a further characterisation method to detect the number of certain reaction products of the plasma source. In a closed test chamber with a volume of approx. 3.3 m³ and under constant gas circulation conditions, the plasma jet was operated with identical parameters as mentioned before. At a distance of about 45 cm from the plasma source, three gas sensing devices of the type Pac 8000 (Dräger Safety AG & Co. KGaA) were positioned, with sensitive characteristics to ozone (O3, 0 – 10 ppm), nitrogen dioxide (NO2, 0 – 50 ppm) and nitrogen monoxide (NO, 0 – 200 ppm). The measurement time was 720 seconds. It has to be emphasized that the cross sensitivity of the O3 gas sensor towards NO2 is about 55%. For the O3 gas sensor towards NO, the NO gas sensor towards O3, NO2 and the NO2 gas sensor towards O3, NO no cross sensitivities are stated.

For all plasma-treated polymers, a significant increase in surface energy was observed. The values of the total surface energy of all plasma-treated samples were in the region of about 50 mN/m, see table 1. Whereas the dispersed part of the surface energy changed only within small margins, the polar component of the surface energy for all polymers increased substantially. Fig. 4 (left) compares the polar component of the surface energies determined on the basis of contact angle measurements and was carried out for both untreated and plasma-treated polymer samples. The increase in surface energy depends on the plasma parameters themselves and hence in the graph, the highest surface energy values for each polymer material are shown which correspond to the optimized plasma parameters stated in table 1. As can be seen, the polar component of the surface energy increased after plasma in most cases significantly and reached an average of approximately 10 mN/m. For PA6 we observed only a moderate increase due to the higher initial value for the untreated material. Furthermore, in the case of PA6 and PMMA, the spread of the measured surface energy values decreases substantially compared to the non-plasma-treated samples. In analogy, fig. 4 (right) shows the averaged adhesion values of the applied coating after aging, also for the process parameters where the highest increases could be measured. Significant improvements in the wettability of the surfaces as well as in the adhesion strength of the applied coatings have been observed for all plasma-treated polymer samples.

Figure 4: Polar component of the surface energy (left) and adhesion strength values (right) for plasma pre-treated polymer substrates, the area for contact angle measurements was about 50 x 50 mm2, the glued surface area had a diameter of 20 mm, the number of measurements per each polymer was 10

The increase of the surface energy corresponds well with literature data based on atmospheric plasma treatments (certain jet plasma systems as well as DBD systems) on different polymers like PP [12 – 14, 3], PA6 [15 – 17, 3], LDPE [18 – 21], HDPE [22, 23, 17, 3], PC [24 – 26] and PMMA [27 – 30]. Furthermore, the measured tensile strength values are comparable with previous results that have been observed by using the same test equipment and with samples treated by certain atmospheric pressure plasma as well as flame procedures [31].

Table 1 compares the surface energy values and the adhesion strength values with the corresponding process parameters. With a view especially on the process parameters given in table 1 and considering previously obtained results [31, 32], it has to be mentioned that there is not necessarily a strong correlation between high surface energy and high adhesion strength. Instead, it is confirmed that adhesion tests rather than surface energy measurements should be used to assess the effectiveness of a pre-treatment procedure.

Table 1:  Results of surface energy and adhesion strength measurements, the area for contact angle measurements was about 50 x 50 mm2, the glued surface area had a diameter of 20 mm, the number of measurements per each polymer was 10

The observed improvements in adhesion strength can be attributed to the creation of new functional groups on the polymer surface. To verify this, XPS measurements were carried out for all plasma-treated polymers. Fig. 5 represents the results of such measurements exemplary for the PA6 material. In this matter especially, the measurement of XPS detail spectra of the carbon peak for treated material and its comparison to the untreated material is of interest due to the slight shift in binding energy for the carbon signal. Namely, whereas for the pure C-C single bond a binding energy of 284,6 eV will be observed, for the C-N bond, the C-O bond, the O=C-N bond and the C=O bond 285,6 eV, 286,4 eV, 287,5 eV and 288,5 eV will be expected, respectively. Fig. 5 (left) shows such a detailed spectrum for plasma-treated PA6 material together with the results of the fitting procedure for the C-C bond and these new bonding states. Thus, the investigations of the bonding states of carbon allow concluding new functional groups that lead to stronger physical bonds with the subsequently applied adhesive system.

Figure 5:  XPS-detail spectrum of the carbon C1s signal (left) and amount (in at. %) of functional groups obtained on a plasma-treated PA6 polymer surface compared to pure C-C bonds (right)

It can be seen from fig. 5 (right) that the relative amount of single carbon bonds on the surface decreases significantly due to the plasma treatment, while the amount of carbon-nitrogen and carbon-oxygen single bonds increases significantly. Similarly, the relative proportion of carboxylic acid, amide groups and carbon-oxygen double bonds increases. Considering the total elemental concentrations of the PA6 reference and after plasma treatment, a decrease of carbon C1s from 77.4 at.-% to 66.1 at.-% and an increase of oxygen O1s from 11.8 at.-% to 20.7 at.-% as well as of nitrogen N1s from 10.2 at.-% to 13.1 at.-% were measured.

In the context of adhesion processes, mechanical bond types also play an important role. In this respect, the consideration of roughness is an interesting point. Looking at the measured surface topographies of the plasma-treated samples, a different picture emerges. Fig. 6 shows two AFM topography measurements, measured on PA6 samples. In this case, a significant roughening of the surface on the nanometer scale is observed.

Figure 6:  AFM topography measurements on a PA6 surface, reference (left), plasma-treated (with 20 mm/s velocity, 2 mm distance) (right), the measurement area was 10 x 10 µm2

Fig. 7 (left) illustrates the change in the surface roughness of all investigated polymers due to the µ-wave plasma treatment. Each polymer was treated using its optimal process parameters (see table 1, adhesion strength measurements), i.e. the parameter set which obtained the highest surface adhesion.

Figure 7: Roughness values Sa of un-treated and plasma-treated polymer surfaces (left), the measurement area for determination of the roughness was 10 x 10 µm2, the number of roughness measurements per each polymer was 3, the plasma treatment was applied in accordance with the parameters given in table 1 for the best adhesion strength values, the change of the surface roughness in dependence on typical glass transition temperatures [33, 34] is shown on the right side

It was observed that the roughness increases for the PA6, PC and PMMA surface after plasma treatment but tends to decrease for PP, LDPE and HDPE. Taking into account the measured adhesion values for the investigated polymers it has to be mentioned that we did not find a strong correlation between the surface roughness and the adhesion values obtained. Nevertheless, a possible explanation for the changes in roughness in both directions, both as roughening and levelling, could be the different glass transition temperatures of the used polymers. Fig. 7 (right) shows the dependence of the change in roughness on typical glass temperature values [33, 34] of the used polymers indicating this possible correlation.

It can be seen that the polymers with negative glass transition temperature all show a decrease in the roughness after plasma treatment. The heat impact of the plasma might macerate the surface of these polymers leading to a flattening of the surface. On the other hand, PA6, PMMA, and PC all have significantly higher glass transition temperatures, where the roughening may occur due to the direct plasma impact (electrons, ions, excited species and irradiation). With changes in the optimal treatment parameters for the different polymers, the thermal impact of the µ-wave plasma on each polymer surface also changes to some extent. The highest gas temperature, measured with a thermocouple, see fig. 9, is only about 70 °C for the most intense plasma parameter (h = 2 mm; v = 20 mm/s). Hence, it can be expected that the maximum temperature at the polymer surfaces does not exceed this value.

Regarding the creation of new functional groups at the polymer surface, the reactive species in the plasma jet and their interaction with the polymer surface play an essential role. Optical emission spectroscopy was used to determine the reactive species in the plasma by aligning the photo sensor directly along the plasma jet axis (view into the plasma source). Fig. 8 shows three spectra taken at different radial distances from the center of the plasma jet. At first glance, one can see the dramatic loss of intensity when the photo sensor is moved away from the axis of the plasma jet. Moving the sensor 1.1 mm away, the signal intensity of the most prominent emissions decreases already to about 20 %, in comparison to the intensity of the plasma center. Due to this, a grid spacing distance of 1 mm was chosen, to get a uniform plasma treatment over the whole polymer area. Focusing on the species excited in the air plasma center, emissions of nitric oxides (NO), the second positive system (N2 SPS) and first positive system (N2 FPS) of nitrogen molecules, the first negative system of the nitrogen molecular ions (N2+) and atomic oxygen (O) were detected.

Figure 8: Optical emission spectra of the gas excited by the µ-wave plasma

Besides a direct interaction of excited electrons from the plasma with the polymer surfaces and following reactions with O2 and N2 (surrounding air), also the observed oxygen and nitrogen containing reactive species can play an important role with regard to the measured chemical changes on the polymer surface (see XPS results from fig. 5). Especially the excited atomic oxygen (O) states are expected, to increase the concentration of C-O and C=O bondings and the overall oxygen concentration, as observed for the plasma treated PA surfaces. Also the measured increase of nitrogen containing functional groups can be at least partially attributed to interactions with the excited N2, N2+ and NO states.

The observed nitrogen-containing species and emission systems (NO, N2, N2+) are very similar to descriptions from Deng et al. for a direct current (dc) plasma jet at atmospheric pressure [35]. A plasma generated by air discharge and electrical current of 14 mA (stable discharge mode) showed in the active plasma zone the same emission systems. In contrast, lines of atomic oxygen were not present in this type of plasma. Also a commercial plasma jet, the MEF (Tigres GmbH) with a dc pulsed electric barrier discharge and air as process gas emits very similar irradiation, like the µ-wave plasma source. Emissions of NO, N2, N2+ and O could be found and additionally small intensities of OH as well as Cu, which was correlated to the metallic (brass) high voltage electrode [36].

The group of Lommatzsch et al. investigated the relaxing plasma of a pulsed arc plasma jet (Plasmatreat GmbH) with air as a process gas and excitation frequencies between 17 and 22 kHz [37]. Besides NO, N2 SPS and O, like for the µ-wave plasma system, a continuum emission of NO2 molecules in the wavelength range of 450 – 800 nm could be also identified. Likewise, other groups confirmed the observation of NO2 continuum emissions for this kind of arc plasma jet [38, 39].

With a view on air driven atmospheric plasma jets also completely different spectra or ratios of excited species can be found, in comparison to the plasma sources described before and the microwave discharge. In the descriptions of Ahn et al., where a plasma jet from a micro nozzle array is formed, the optical emission spectrum is dominated by excited oxygen atoms at 777 and 845 nm and molecules above 500 nm, which were assigned to N2 SPS and N2 FPS [40].

The simultaneous measurement of the gas concentration with three gas sensor devices (O3, NO, NO2) showed the following behavior for the investigated plasma source. Over the measurement time of 720 seconds, no NO was detected. For O3 and NO2, on the other hand, a continuous increase in gas concentration was detected, with a transition into a saturation state starting at about 600 seconds after switching on the plasma device. The maximum gas concentrations were found to be 0.98 ppm for O3 and 2.20 – 2.26 ppm for NO2. One possible explanation for the absence of NO could be the relatively low temperatures (see next section – thermocouple measurements), compared to other plasma jet sources. Using the same experimental setup with a commercial pulsed arc plasma jet (Openair-Plasma® – Nozzle PFW10, Plasmatreat GmbH), air as a process gas and standard electrical parameters (frequency: 21.0 kHz, voltage: 280 V, current: 9.1 A) the maximum gas concentrations were measured to be 93.5 ppm for NO, 5.85 ppm for O3 and 14.9 ppm for NO2. The group of Malik et al. investigated an atmospheric pressure surface discharge at different temperatures of the electrodes (20 °C to 420 °C) and observed a similar trend regarding the NO gas concentrations [41]. At 20 °C and 120 °C, no NO (detection limit: 100 ppm) and from 220 °C onwards rising NO concentrations were detected. Vice-versa, the O3 concentrations decreased with rising electrode temperatures and were below the detection limit of ? 220 °C. Formed NO could be further oxidized to NO2 or higher oxidation states by interaction with O3 or O, which are also described as fast reactions at low / room temperature. On the other hand, high plasma temperatures (e.g. in arc plasmas) can destroy O3 and hence minimize the NO to NO2 conversion [41].

The optimal process parameters in table 1 are directly related to the temperature of the plasma gas. Keeping this in mind, appropriate thermocouple measurements were done, see fig. 9. Directly at the nozzle outlet, relatively high gas temperatures of about 200 °C were observed in the static regime, at an air gas flow through the plasma source of 2 l/min. At a distance of about 7–8 mm from the nozzle outlet, the gas temperature drops to less than 100 °C. In a dynamic regime, a maximum temperature of approx. 70 °C was measured for the parameter with the highest intensity (h = 2 mm; v = 20 mm/s). With increasing distance as well as with higher velocity, the maximal measured temperatures drop in each case to the range of approx. 50 °C. Based on these investigations, the µ-wave plasma source can be efficiently used for the plasma treatment of thermally sensitive materials.

Figure 9: Temperature measurements in the plasma jet for different distances from the nozzle – statically (left) and dynamically for different treatment velocities between 20 – 100 mm/s (maximal observed temperature values, right)

In summary, a new compact atmospheric pressure µ-wave plasma source based on the plasma-jet principle has been presented for use both as a stand-alone device and as part of an industrial system. It can be clearly stated from the experimental observations that the µ-wave plasma source is very effective and perfectly suitable for improving the surface energies and the adhesive properties of all the examined polymers. Despite a large increase in surface energies of PC and PMMA through the µ-wave plasma treatment, the adhesion strength of these materials increased only moderately, which indicates that the other factors like surface roughness and chemical compositions on the surfaces are important factors supporting the adhesion mechanism. The optical emission spectroscopy revealed the exciting species present inside the plasma and also provided information on the effective area of the plasma jet for the treatment of polymer materials. Precise thermal measurements of the µ-wave plasma jet along the central plasma axis for static and dynamic plasma conditions show that the µ-wave plasma source can be efficiently used for the plasma treatment of thermally sensitive materials. A mild concentration of O3 and NO2 was detected during the µ-plasma ignition, while no indication of NO generation was observed.

Thus, the presented new compact µ-plasma source is excellently suited for the selective and local functionalization of polymer surfaces for adhesion improvement.

01022023_Kunststofftechnik.pdf

01022023_FIN-347 Pfuch (2)

Authors Vivek Faldu, B.Eng., Oliver Beier, M.Sc., Dr. rer. nat. Andreas Pfuch, Dr. rer. nat. Kerstin Horn INNOVENT e.V. Jena

Johannes Zender, M.Sc., Denis Wolf, Dipl.-Ing., Thomas Filler, Dipl.-Ing., Prof. Dr.-Ing. Wolfgang Heinrich, Ulrike Winterwerber, M.Sc., Dr. rer. nat. Neysha Lobo Ploch Ferdinand-Braun-Institut gGmbH Berlin

Published in Zeitschrift Kunststofftechnik / Journal of Plastics Technology 19 (2023) 2, page 52 – 52 © Carl Hanser Verlag GmbH & Co. KG, ISSN 1864 – 2217