18 TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS HEATING OF POLYMER-POLYMER COMPOSITES BY INDUCTIVE MEANS T. Bayerl*, P. Mitschang Institut für Verbundwerkstoffe GmbH, Kaiserslautern, Germany * Corresponding author (thomas.bayerl@ivw.uni-kl.de) Keywords : Inductive heating, ferromagnetic particle, HDPE, selective melting applications [4-6]. It was found, that nickel, as a 1 Introduction promoter material, showed a dependency on particle Polymer-polymer composites consist of a polymer size and oxidation state as well as on frequency, and matrix and polymer reinforcement. When they are magnetic field strength. The presented study manufactured exclusively of components from the proceeds with this approach and adapts it to SRP same polymer family, they are called self-reinforced material, which demands accurate temperature polymers (SRP). Polymer-polymer composites offer control. the possibility of designing extremely light-weight components with excellent impact behavior and 2 Experimental increased tensile properties [1]. Additionally, SRP offer increased sustainability potential, due to the For induction tests, particulate doped high density application of similar molecular structures, which do polyethylene (HDPE) sheets with the dimensions of not have to be disposed separately. 60 x 60 x 2 mm³ were exposed to a magnetic field in The processing of a SRP semi-finished material is a distance of 2 mm to the coil with a medium challenging, since a too high process temperature frequency of 450 kHz. The field was provided by a may damage and possibly destroy the polymer pancake coil and driven by a Hüttinger TruHeat reinforcement, even when matrix and fiber do not 5010 MF (Trumpf Hüttinger, Germany) generator. melt at the same temperature. Nevertheless, recent HDPE was chosen due to its low melting industrially realized approaches accept fiber melting temperature and served as the matrix material for the during processing to a certain extent, which later manufactured self-reinforced polyolefin SRP unintentionally weakens the nominal strength with polypropylene fibers. potential of the material. Amongst the incorporated heating promoters, which A new approach for heating polymer-polymer have been added individually in various fractions, composites by inductive means is presented in this ferromagnetic (cast iron, magnetite, and nickel) and paper. The induction based melting of non-polymer electrically conductive materials (carbon black, fiber reinforced thermoplastic composite materials carbon nano tubes) were tested. A detailed has recently been enhanced in the field of welding description of the experimental work has been applications [2,3]. The electrical properties of the published elsewhere [7]. reinforcement are mainly used in this case for matrix The obtained temperature profile was analyzed and melting. compared according to the linear heating rate of Since SRP consist exclusively of non-conductive each promoter material. The linear heating rate is polymers, which do not offer the possibility to be characterized by its linear rise from the experimental heated by an alternating magnetic field on their own, start to the onset of crystallite melting. A typical ferromagnetic heating promoters are added. The temperature evolution of a particle doped sample is promoters have a particulate shape and are illustrated in Fig 1. The temperature rise decelerates exclusively used in the matrix phase to limit the in the region close to the melting temperature heating of the reinforcement and avoid its melting. because the energy is consumed to melt the Particulate heating of polymers has been recently crystallites instead of further heating the sample. In developed for polysulfone tape welding the literature, this energy is referred to as latent heat
and is commonly described within a heat capacity verified for all tested iron sample series with high graph, which is unique for each material. The accordance (Fig. 3). At currents higher than 20 A, temperature curve accelerates again when most the heating rate of 20 wt-% iron doped HDPE lies crystallites have become molten. approximately four times higher than the one of low Regarding the linear heating rate, the influence of doped materials. By reducing the generator current particle material, generator power, and coupling to 10 A and lower, the linear heat rate drops for all distance has been examined with neat HDPE cases close to zero. Consequently, the effect of the material in an experimental series for the later use filler fraction is negligible. The provided energy to with polymer-polymer material. The influence of the particles by the electromagnetic field at low other parameters, like frequency and semi-finished currents is so small that it is hardly enough to material shape, has been already investigated in a compensate the heat loss to the surrounding preliminary test series [8]. environment. The matrix degradation, which is related to a shift of An expected exponential relationship between linear the melting temperature due to molecular chain heating rate and coupling distance, which describes changes, has been investigated by means of the distance between sample and coil, was also differential scanning calorimetry (DSC). Therefore, confirmed (Fig. 4). It could be shown that at the melting temperature change of thermally increasing distance the difference between different damaged infrared radiated material was compared to fillers decreases. At 12 mm coupling distance, the induction heated material. filler fraction of magnetite in the HDPE sample (5 wt-% vs. 10 wt-%) has only a subordinate influence on the heating rate. 3 Results and Discussion For an industrial application, the effect of coupling distance should be precisely regarded according to 3.1 Induction with Neat Material the magnetic field distribution. For this study, the The comparison of filler degree influence on the minimum coupling distance was 2 mm and mainly heating rate shows an identical behavior for different limited due to lack of space when mounting the particle materials. It was found that all curves follow samples on the coil. an exponential relation (Fig. 2). The highest heating rates were observed in the ferromagnetic material, 3.2 Induction with Polymer-Polymer Material from the highest, cast iron powder and magnetite, to the lowest in this case, nickel. The electrically As a result from the preliminary test series, the conductive carbon black shows an insufficient maximum generator current at 450 kHz was used for heating rate for the use as a SRP heating promoter. the study with particle doped SRP material. The This behavior was also found for 1 wt-% carbon coupling distance remained at its minimum of 2 mm. nano tube doped material. The domination of As polymer-polymer composite, a polypropylene ferromagnetic properties on the particulate induction (PP) material with polyester (PET) fibers was used, heating behavior, which has been previously stated which provides a high melting temperature gap by other research groups, was hereby confirmed between matrix and reinforcement. Consequently, [5,9]. Due to this effect, the iron particles, which the heating process is easily controllable. As heating have higher magnetic properties, show a significant promoter, iron powder, which showed the highest higher linear heating rate than materials with lower heating rates, was applied. Two filler fractions were magnetic properties. Additionally, the incorporation tested: 5 wt-% in the matrix, which is regarded as of more particles into the matrix consequently leads the minimum content for an industrial process, and to higher heating rates. In this context, even with 20 wt-%, which provides a linear heating rate close filler fractions as low as 5 wt-%, heating rates of to 10 K/s. 1 K/s can be reached. The obtained results show a very quick heating of Regarding the influence of the magnetic field the highly doped material, which exceeds the strength and in this case the related current provided melting temperature of the reinforcement already by the generator, a quadratic influence on the after only 25 seconds (Fig. 5). Although the melting heating rate was expected [10]. This could be temperature gap is approx. 80 °C, the quick heating
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