Ultrasonic sealing of packaging films - influencing material properties

Transcription

1 Ultrasonic sealing of packaging films - influencing material properties Karsten Thürling a, Sascha Bach b a Fraunhofer Application Centre for Processing Machinery and Packaging Technology, Dresden, Germany (karsten.thuerling@avv.fraunhofer.de) b Technische Universität Dresden, Faculty of Mechanical Engineering, Institute of Processing Machines and Mobile Machines, Dresden, Germany (sascha.bach@tu-dresden.de) ABSTRACT Ultrasonic sealing of polyolefin-films is an upcoming alternative sealing technology to the commonly used heat contact sealing. The main advantages are the extreme short seal times, better reliability in sealing of contaminated seams and less heat treatment of the product due to the unheated tools. The films get joined with the help of a high frequently oscillating tool and by dissipating kinetic energy of the movement into thermal energy. Different polymeric materials have quite varying ultrasonic sealing behaviour, which was the reason for a research project to detect the material properties, most influencing the ultrasonic seal ability. Selected polyolefin sealing materials were analysed regarding their mechanical, thermal and rheological properties (e.g. Young s modulus, loss modulus, storage modulus, heat capacity and dynamic viscosity). For characterizing the properties of the ultrasonic sealed seams strength tests have been performed. The flow of the molten material has been analysed by microscopic analysis of microtome cuts. Using these results it was possible to detect the behaviour of the molten material and the process of melting itself as most influencing material property onto the ultrasonic seal ability. The results disclose an interrelation between sealing behaviour and the declension of the dynamic viscosity during the melting of the material. This property seems to be a good indicator for good or poor ultrasonic seal ability. Keywords: ultrasonic; seal; join; film; material properties INTRODUCTION Ultrasonic sealing is an upcoming joining technique in the field of packaging machines to join polymer films. One of its advantages is the very short sealing time (down to 20 ms) compared to the commonly used heat contact sealing (around 500 ms). Another aspect are the unheated tools, which are good for heat sensitive products, such as frozen food or chocolate. Ultrasonically sealed seams are very small in consequence of the shaped tools, which helps saving packaging material, energy and costs. Even seams, which are contaminated with product, are less problematic, because the vibrating tools in connection with the small and contoured tool are pushing out the contamination of the seam during the sealing process. Figure 1: configuration of an ultrasonic sealing unit for films Figure 2: cross sectional view of the fusion zone The ultrasonic sealing tools (Figure 1) are consisting of a so called horn, which applies the ultrasonic vibration and an anvil as a passive counterpart. The horn is oscillating in a longitudinal resonant frequency mode within the ultrasonic frequency range. Typical frequencies for ultrasonic sealing applications are in

2 arrange between 20 and 40 khz. The oscillation of the horn is generated by the converter, which is a piezoelectric actor. It converts a high frequently oscillating electrical field into a mechanical oscillation. Additionally a booster can be inserted between horn and converter to amplify the amplitude of the oscillation. The horn also amplifies the oscillation, which leads to an amplitude at the horn surface of 15 up to 50 µm [1]. The anvil, which is the counterpart of the horn is reverberant and has a small contour at the film located surface, the energy director (Figure 2). The energy director is needed for a defined dissipation of energy in the film pair. To join two films both tools were pressed together with a specific sealing force and the horn starts to oscillate. Caused by the viscoelastic losses and the surface friction between the polymer films the temperature of the films rises up to the melting point and enables fusion processes. After a specific sealing time the ultrasonic impulse stops, the solidification of the material begins and the films are bonded [2]. The ultrasonic seal ability of different packaging materials is highly varying, even for films, which have almost identical behaviour at conductive heat sealing. The objective of this research was to identify the material properties, which are most influencing the ultrasonic seal ability of packaging films with a seal layer out of polyethylene and polypropylene. MATERIALS & METHODS Eight different types of polyethylene and polypropylene films were selected for the investigations, shown in Table 1. The Table 1: investigated materials and its properties Young's modulus density melt flow rate melt peak temperature material [N/mm²] [g/m³] [g/10mim] [ C] PE-LLD C6_ , PE-LLD C6_ ,920 0,9 127 PE-LD 366 0,923 0, PE-LLD C ,921 1,1 123 PP-RB_ ,95 1,9 230 PP-TD_ , PP-RD_ , PP-BD_ , ultrasonic sealing process can be divided in two main procedures: (i) the heating of the film up to the specific melting point of each material and (ii) the flow of the molten material. Concerning the heating behaviour (i) the loss modulus of each material has been analysed using dynamic mechanical analysis (DMA), which characterizes the dissipative properties of a material. Higher loss moduli indicate a better ability to dissipate kinetic energy into heat forced by oscillating deformation. Furthermore IR-thermography imaging was used to measure the temperature distribution in the fusion zone by looking into the cross section of the films during the sealing process. Also surface roughness and surface friction have been measured in this part but it was found out that for the selected materials the heat generation caused by surface friction is poor compared to the heat generation inside the films [3]. The detailed procedure and results of this analysis have been described in [4]. The rheological behaviour which influences the flow of the molten material (ii) during the ultrasonic sealing process was investigated using a rotary plate-plate rheometer (HAAKE-MARS II by Thermo-Fisher ). The tests were performed temperature depending, to describe the flow behaviour at different temperatures. For analysing ultrasonic sealing behaviour of a material, ultrasonic sealing tests have been performed with varying sealing times and a constant sealing force for materials with almost similar Young s modulus to assure similar strain conditions. The experiments have been done, using a laboratory ultrasonic sealing machine (DIALOG TOUCH by Herrmann Ultraschalltechnik ) with a frequency of 20 khz, an amplitude of 44 µm and a tool shape radius of 2,5 mm. The sealed seams strengths have been tested, according to DIN 55529, using a tensile testing machine (Z005 by Zwick ).

3 The seam strength of the ultrasonic sealed seams where referenced to conductive sealed seams which have been done, using a laboratory heat sealing machine (by Kopp ) with heat impulse sealing jaws (by Ropex). For heat contact sealing a material specific plateau appears in the seam strength curves, which was taken as maximum reachable strength reference for each material. With this procedure it was possible to normalize the strength of ultrasonic sealed seams of different materials considering their different tear strength, which was already described in [2]. RESULTS & DISCUSSION The test results points out different materials reaching different values of seam strength at ultrasonic sealing. The seam strength results vs. seal time of the mentioned materials are shown in Figure 3. Figure 3: ultrasonic seam strength vs. seal time The normalized seam strengths are shown in Figure 4. Some of the materials reaches 100% of the heat contact seal strength plateau, but other materials only reaches 30%. Figure 4: normalized seam strength vs. seal time [5] had been shown that the heating mechanisms at polyolefin materials are almost identical also at different materials. At similar strain conditions the heat propagation in different materials is almost the same. Keeping this in mind and looking at the different reachable seam strengths, shown in Figure 4 the different material

4 behaviour at ultrasonic sealing may have it reasons in the melting and melt flow procedures (ii) during the process. Looking at the measured dynamic viscosity curves for the selected materials, which are shown in Figure 5, it is obvious that with rising temperature the dynamic viscosity decreases for each material. It can also be pointed out, that the level of dynamic viscosity before and after the melting range is different for each material. But also the slope of the decreasing viscosity is different. Thereby polypropylene types reaching a lower viscosity level. Figure 5: dynamic viscosity vs. temperature Some materials have a quite sharp decreasing of viscosity during their melting range; others are melting in a wider temperature range and therefore have a smoother decay of viscosity. By comparing the normalized seam strengths in Figure 4 with the decreasing dynamic viscosities in Figure 5, it can be seen that materials with a smooth decay of the dynamic viscosity vs. temperature are reaching a higher level of the normalized seam strengths. This correlation is shown in Figure 6. Figure 6: normalized seam strength vs. decay of dynamic viscosity The maximum decay of the dynamic viscosity was calculated using the 1 st derivative of the dynamic viscosity with respect to the temperature. Figure 6 shows that a smooth decay of the dynamic viscosity,

5 which is a lower maximum in the 1 st derivative of the dynamic viscosity, leads to higher reachable seam strengths of a sealing material. These results can be explained by the interaction of the material and the shaped tool, which is however necessary for the process. Thereby materials having a steeply decay of the dynamic viscosity are pushed out of the seam zone very easily and abruptly. Therefore the molten material is not able to connect with each other properly. Materials with a soft decay of viscosity have the ability to be partly molten and therefore are able to connect with each other. At the same time parts of the material are not molten, having a higher viscosity and therefore can t be pushed out so easily. These materials are better suited to connect with each other, which results in higher seam strengths. Also the seams of these materials are not as narrowed down as for materials with a strong decay of viscosity. CONCLUSION The demonstrated varying behaviour of polymer packaging films during the ultrasonic sealing process is mainly caused by the flow behaviour of the molten material. The heating up to the melting temperature is less important for the seam quality and almost the same for different polyolefin materials under comparable conditions. But also other factors are influencing the resulting seam properties, such as the cooling of the molten material and seam front propagation. These influences will be topic in a follow-up research project. ACKNOWLEDGMENTS This project (IGF-project BG15809) of the industry-association IVLV e.v. was founded by the AiF within the scope of a program to advance industrial community research and development of the German Federal Ministry of Economics and Technology based on an enactment of the German Parliament (Bundestag). REFERENCES [1] Wodora, J.: Ultraschallfügen und -trennen, DVS-Verlag, Düsseldorf, 2004 [2] Akutin, A.S., Sorokin, V.A., Osipcik, V.S.: Die chemische Umwandlung beim Ultraschallschweißen von Thermoplasten, Plasticeskie massy, Vol. 12, S , 1968 [3] Ritter, J.: Untersuchungen zur Energiewandlung und zum Schwingverhalten des Systems Sonotrode, Fügeteil und Amboss beim Ultraschallschweißen ausgewählter Thermoplaste, Dissertation, TU München, 1986 [4] Bach S., Thürling K. & Majschak J.-P., 2010, Analysis and modelling of the heating and sealing behaviour of polymer films during Ultrasonic sealing, 14th International Scientific Conference on Polymeric Materials, Halle (Saale) Germany, September, 2010, Proceedings p.180, ISBN [5] Thürling K., Bach S. & Majschak J.-P., 2010, Influence of material parameters on ultrasonic sealing behaviour of polymer films for packaging, 14th International Scientific Conference on Polymeric Materials, Halle (Saale) Germany, September, 2010, Proceedings p. 274, ISBN