IN-SITU-PULTRUSION STRUCTURAL THERMOPLASTIC FRP-PARTS Stefan Epple, Institut für Kunststofftechnik, University of Stuttgart, Germany Christian Bonten, Institut für Kunststofftechnik, University of Stuttgart, Germany Abstract Fiber reinforced plastics, produced by In-Situ- Pultrusion are supposed to be used as local reinforcement of injection molded plastic parts. For this purpose, the pultruded parts are overmolded after being inserted into the injection mold. From process technologies reasons, the parts made by In-Situ-Pultrusion consist of cast Polyamide 6 (PA6). This cast PA6 differs from standard injection molding PA6. In previous studies, it was shown that a good bonding between the partners can be reached. In this work, a new bending girder made of PA6GF30 and In-Situ-pultruded PA6 is presented. Some mechanical properties were tested and compared with an equal bending girder, made of pure PA6GF30. Introduction In traditional fiber reinforced plastics, a thermosetting resin is used as matrix. These fiber reinforced thermosets have disadvantages, especially in batch production because e. g. the cycle times are too long, they are not weldable and poorly recyclable. In batch productions it is common to use fiber reinforced thermoplastics. Often, short fibers are used as reinforcement. By using glass fibers, mechanical properties increase, however, highly-stressed components needs continuous fiber reinforced plastics. The two major advantages of thermoplastic composites are that they have a higher impact resistance to comparable thermoset composites and can be welded and recycled. One objective of the current research is to produce a pultruded thermoplastic composite that could be heated and remolded to have complex shapes. This is not possible with thermosetting resins. It will also be possible to recycle the thermoplastic composites in contrast to thermoset composites. Thermosetting resins are of low viscosity and thereby can easily impregnate the reinforcing fibers before they react. In contrast, thermoplastics are usually of high viscosity, even in the molten state. It is not easy to impregnate reinforcing fibers. To impregnate reinforcing fibers with a thermoplastic matrix, plastic films and woven fabrics must be laminated. High pressure and high temperature are required. These fiber-reinforced thermoplastic laminates are called organo-sheets. Fiber mat or long glass fiber reinforced thermoplastics are also used to produce rugged parts. The problem of these parts is their limited shape variety. In-Situ-pultrusion is one way to produce thermoplastic FRP. Like in conventional pultrusion with a thermoset matrix, continuous fibers are impregnated with the monomeric precursor of the matrix material, which then reacts and forms the polymer. When using ε-caprolactam as a monomer, cast PA6 will be formed and act as the matrix [1]. Different to conventional pultrusion, the fibers cannot get impregnated by pulling them through a resin bath, as the surrounding humidity would disturb the chemical reaction. Thus, the reaction has to take place in an inert gas atmosphere inside the pultrusion die. The products from in-situ-pultrusion can e. g. be used as reinforcements for injection molded parts. One example is the addition of rip structures to bending beams composed of two FRP profiles via back injection molding. For this purpose, the profiles are inserted into an injection mold and overmolded with a thermoplastic. Thus, cycle times like in conventional injection molding processes are possible, combined with excellent mechanical properties and high component complexity. [2] State of the Art To bring the advantages of continuous fiber reinforced plastics also into parts with complex geometries, the Institut für Kunststofftechnik (IKT) put into effect a process based on the pultrusion process combined with reaction injection molding (RIM). In reaction injection molding, two components are mixed together. The mixture is then injected into the mold, where it reacts to the plastic. The RIM process offers the advantage of a very low viscosity of the mixture before the reaction starts. In this state, the two component mixture can impregnate the fibers very well. It was the objective of the development to produce thermoplastic parts with a high amount of continuous glass fibers in various shapes and thicknesses. Adhesion between fibers and matrix and fiber coating were optimized [3]. Basic principles of the In-situ pultrusion process Two principles are coupled in the In-Sizu-pultrusion process. First, the two reactive components are mixed together. Then the mixture is injected into a mold. This process is often used to produce parts made of polyurethane. In pultrusion processes, fibers are pulled through a bath of thermosetting resin. Afterwards, the impregnated fibers are pulled through a heated die, where the resin reacts to the formed composite. SPE ANTEC Indianapolis 2016 / 433
A puller behind the die is pulling the material through the process at a consistent rate. Behind the puller, the parts are sawn off to the desired length. In current researches, the matrix of the composite was polyamide 6 (PA 6). That is why ε-caprolactam was used in the activated anionic polymerization process from monomer to polymer. The resulting PA 6, also known as cast PA 6, has good properties like dimensional accuracy, low moisture adsorption and a good creep resistance. The RIM pultrusion process used for the production of continuous glass fiber reinforced thermoplastics is schematically shown in. Figure 1. resin tanks mixer die puller saw While it is easier to handle glass fibers with higher tex numbers, the possibility to impregnate the fibers decreases. A roving with the tex number of 2400 was selected for the tests. In Figure 2, the rovings are shown. Fiber diameters can be adjusted to the production process. Typical fiber diameters are 16 respective 20 µm. The mechanical properties of the fiber reinforced products depend largely on the ratio of fiber diameter to fiber length. If profiles are produced continuously, the fiber length in the product is virtually endless. Therefore, the fiber diameter plays a subordinate role in the mechanical properties of the final product. The finer fiber diameter of 16 µm was chosen. fiber rovings Figure 1: Scheme of the new RIM pultrusion process The glass fibers are pulled through a pre-warming and sorting tool before they are entering the die. In the die, the fibers are impregnated with the monomer. In the following zones of the die, the monomer ε-caprolactam reacts to PA 6. The first part of the die is only heated while in the second part of the die, cooled or heated, depending on the process behaviour. This is important because in the process heat can be generated by the reaction which has to be removed from the die. The puller and the saw are also shown in the scheme. Providing the mixtures of ε-caprolactam and catalyst as well as ε-caprolactam and activator in the correct way is very important for the process. Protective gas has to be used to reduce the influence of moisture on the reaction. Basic principles of the activated anionic polymerization of PA 6 The activated anionic polymerization of PA 6 can occur within minutes. In this polymerization, the monomer respectively the growing macromolecule is an anion. An activator to accelerate the catalytic reaction is also used in this system. Especially the low temperatures below the melting temperature of PA 6 which are needed for this reaction (130 to 170 C) allow its use in the present process. Basic principles of glass fibers Usually glass fibers are delivered in wound bundles called rovings. These rovings usually have tex numbers of 300, 600, 1200, 2400, 4800 or 9600. A small beam diameter is characterized by a low tex number. Figure 2: Glass fibers in the sorting tool To ensure the fiber-matrix adhesion, as well as the handling of the fibers, a sizing is applied to the glass fibers during the process. A number of chemical compounds are known for preventing or at least affecting the anionic polymerization. For the first experiments an already available sized fiber was selected. Test facility at IKT At IKT, an In-Situ-pultrusion facility with two component units, puller, saw, die and mixer was installed. In-Situ-pultrusion die To design the In-Situ-pultrusion die (Figure 3), preliminary examinations were done and the reaction behavior of ε-caprolactam was described. It is important for the process that the material does not polymerize in the nozzle. The nozzle opening was therefore separated from the rest of the die and can be cooled. The die length was chosen in a way that in the times known from the preliminary studies, polymerization can take place in the die. It should be noted that the die should not be unnecessarily long. Otherwise the puller reaches its performance limits. On the other hand the die must not be too short because then the material would have too few time to polymerize. SPE ANTEC Indianapolis 2016 / 434
macroscopic scale, first statements about the distribution of fibers in the component can be made. Figure 5 shows the straight areas of the specimen. The fibers which can be identified as black spots are well distributed and surrounded by the PA 6 matrix. Figure 3: RIM pultrusion facility Mixer To mix the two components (ε-caprolactam with activator and ε-caprolactam with catalyst), a mixing head was developed, which has been especially adapted to the requirements of the process. Below about 80 C, the mixture is in a solid state. Above about 120 C it begins to polymerize. This means that the part of the mixer outside the die, in which a static mixer is located, has to be heated, whereas the nozzle through which the monomer is passed into the die must be cooled so that the material does not polymerize prematurely [2]. Experimental For the RIM pultrusion experiments activator concentration of 2.5 % and catalyst concentration of 3.75 % were chosen. The temperature in the tanks and the mixer was set to 100 C. The temperature in the first part of the die was adjusted to 160 C. In the second half of the die, the temperature was regulated to 140 C by a temperature control unit. To achieve a high amount of glass fibers, 114 glass fiber rovings were used. The glass fibers were sorted in a box before entering the die. In this box, the fibers also get dried by dry and hot air, to prevent the influences of moisture to the activated anionic polymerization. It was possible to produce a continuous glass-fiberreinforced PA 6 part with a sufficiently good surface in case of specimens (Figure 4). Figure 4: Pultruded parts To evaluate the quality of the specimens, microsections of the samples were made. Using these images, on Figure 5: Straight area of the specimen To study the adhesion between FRP profiles with a cast PA6 matrix and regular PA6, a specimen shape was used, which allowed to vary the effective adhesion surface without applying a bending moment (as it would occur in a conventional three-point flexural test). Therefore a PA6 overlap on two sides of the profile was realized (Figure 6), which allowed for pull-out tests on a tensile-testing machine. Figure 6: Side view of the test specimen The overlap length can be varied by choosing different lengths of the pultruded parts. The test specimen were produced on an injection molding machine of the type Arburg Allrounder 520S 1600-400. The PA6 used in the tests was a Lanxess Durethan B 30 S. Before injecting the PA6 into the mold, the pultruded profile was heated under defined conditions. For this purpose, the ceramic heater was placed 17 mm in front of the profile. Then the profile surface temperature was measured with the pyrometer to ensure that the profile temperature had reached 230 C. The heating and measuring equipment was then pulled out of the mold and SPE ANTEC Indianapolis 2016 / 435
the injection molding cycle was started. The injection pressure was 900 bar, the dwell pressure was 525 bar and the cooling time was 30 seconds at a mold temperature of 80 C. The test specimen (Figure 7) were sealed within airproof bags to later be tested as-molded [3]. Figure 10: bending girder (PA6GF30/pultruded parts) Figure 7: Test specimen as-molded After the results of these tests have shown, that the bonding strength between the pultruded profile and the PA6 was sufficient, a bending girder has been designed (Figure 8). This bending girder has the pultruded parts in the highest stressed surface areas and is overmolded with short glass fiber reinforced PA6 (PA6GF30). The Tests have shown, that while the Youngs- Modulus and the maximum load are equal between the specimens, the maximal displacement is more than three two times higher with the In-Situ-pultruded reinforcement parts (Figure 11). This is probably caused by the fact that the structure between the reinforcement parts was the same in both specimens. Figure 8: bending girder Results and Discussion The bending girders with pultruded parts (Figure 10) were tested versus PA6GF30 bending girders (Figure 9). Figure 9: bending girder (PA6GF30) Figure 11: mechanical properties Through testing, there was no delamination between the PA6GF30 and the PA6GF30 with pultruded parts. In- Situ-pultruded parts therefore can be used as local reinforcement for injection molded complex parts. Conclusion and Outlook It was shown and already publicized, that continuous glass-fiber-reinforced PA 6 parts can be produced by In- Situ-pultrusion at the Institut für Kunststofftechnik / University of Stuttgart. The method provides new horizons to produce continuous fiber reinforced parts with various geometries and larger thicknesses. It was now shown that it is possible to get a good adhesion between in-situ-pultruded parts with a cast PA6 matrix and regular PA6. It was also shown that structural parts can be made of In-Situ-pultruded parts and SPE ANTEC Indianapolis 2016 / 436
PA6GF30. In further studies, injection molding parameters have to be varied systematically, to get information about their influence, concerning the mechanichal properties of the finished parts. Acknowledgements The authors thank the companies Lanxess Germany, Brüggemann Chemical and Johns Manville for the provided material. References 1. NING, X.; ISHIDA, H.: RIM-Pultrusion of Nylon-6 and Rubber-Toughened Nylon-6 Composites. In: Polymer Engineering and Science, Vol. 31, No. 9 (1991) 2. EPPLE, S.; BONTEN, C.: Production of Continuous Fiber Thermoplastic Composites by In-Situ Pultrusion. In AIP Conference Proceedings (2014) 3. EPPLE, S.; BONTEN, C.: In-Situ-Pultrusion Bonding of FRP-Parts to PA6 (PPS Europe/Africa Regional Conference 2015, Graz, Austria, 21. - 25. September 2015). Graz, 2015 SPE ANTEC Indianapolis 2016 / 437