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Thermoplastic filament winding with online-impregnation. Part B. Experimental study of processing parameters
Composites: Part A 33 (2002) 1677–1688 www.elsevier.com/locate/compositesa
Thermoplastic filament winding with online-impregnation. Part B. Experimental study of processing parameters F. Henninger, J. Hoffmann, K. Friedrich* Institut fu¨r Verbundwerkstoffe GmbH, Erwin-Schro¨dinger Strasse, Gebaude 58, D-67663 Kaiserslautern, Germany Received 28 January 2002; revised 1 August 2002; accepted 19 September 2002
Abstract In part A of this paper, a novel process technology was presented which allows to combine thermoplastic filament winding with online melt impregnation of fibre bundles. In this part, a comprehensive study of process parameters was conducted. Circular glass fibre (GF) reinforced polypropylene (PP) or polyamide 12 (PA12) tubes were produced as sample component. Processing speeds of up to 15 m/min could be achieved in case of GF/PP before a drop in quality of the parts had to be accepted. The present limitation in winding speed is not attributed to impregnation problems but to an excessive rise in force required to pull the fibre tow off the impregnation device. Measures for process improvement and increase in productivity were proposed. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Thermoplastic resin; E. Extrusion; A. Tape; E. Filament winding
1. Introduction Filament winding is a widely used processing routine for continuous fibre reinforced polymer for axially symmetric components. Initially developed for thermoset resins, it was successfully adopted for thermoplastics as well. However, an intermediate product form, in which the thermoplastic matrix is already in close contact to the reinforcing fibres (e.g. commingled yarn; powder impregnated bundles; preimpregnated tapes) must usually be used. A novel process technology was described in part A of this paper; it allows combining thermoplastic filament winding with online melt impregnation of fibre bundles. For the sake of completeness, the process principle, which was explained in Part A of this paper, is repeated in Fig. 1. For reasons of clarity, a hot air gun for heating the nip-point and an IRthermometer to control the nip-point-temperature are not displayed in Fig. 1. The location of these devices can be seen in Fig. 2. From the qualitative study of process parameters in part A, which found consideration in the design of the process, it was deducted that it is of high importance to experimentally * Corresponding author. Tel.: þ49-631-2017-201; fax: þ 49-631-2017198. E-mail address: [email protected] (K. Friedrich).
investigate the influence of the following parameters in more detail: (1) impregnation temperature, (2) roving pretension (brake force), (3) process speed, (4) nip-point temperature, and (5) fibre volume content. Fig. 2 shows the principle of the measurement of the force required to the fibre bundle off the impregnation unit. The pulling force generated by the impregnation process was determined in an extra set of experiments without characterising the wound parts.
2. Experimental 2.1. Materials For the parameter study conducted, glass fibre (GF) bundles were impregnated either with polypropylene (PP) or polyamide 12 (PA12). A PP-homopolymer Type Novolenw 1100 VC by TARGOR GmbH, Germany, had been recommended by the manufacturer for this application. It possesses a high melt flow rate and has been developed for thin wall injection moulding [1]. The viscosity at low shear rates and a melt temperature of 230 8C is below 100 Pa s (Fig. 3). The PA12 was a low viscosity, heat and light stabilised Vestamidw 1670 by Degussa-Hu¨ls AG, Germany, which is
1359-835X/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 8 3 5 X ( 0 2 ) 0 0 1 3 6 - 7
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Fig. 1. Schematic of the process combination.
Fig. 2. Measurement of pulling force.
usually used for wire insulations, coils or secondary coating of optical fibres [2]. Fig. 4 shows the viscosity profile as a function of shear rate for different temperatures. All viscosities were measured with a capillary rheometer. Some properties of both matrices are listed in Table 1. For all experiments of filament winding and online fibre bundle impregnation, E-glass fibres were used. A fibre bundle consists of many filaments, and its strength is quantified by the weight per kilometre running length, the so-called TEX-number. The Advantexw GF R43S Type 30w roving of Owens Corning, USA, with 2400 TEX has a silane based sizing designed for the use of these fibres in thermoplastic matrices. The roving can perform particularly well in polyamide, but can also be used in a wide range of thermoplastic polymers such as PP, PC and PPS (Table 2) [3].
After starting the process, a total of 10 m tow was wound separately onto the mandrel as waste material to allow constant conditions to establish. 2.3. Microscopy and void content The volumetric void content was determined by relating the measured density rmeas to the theoretical density ðrth Þ of a void-free composite with the same fibre content: XV ¼
rth 2 rmeas r ¼ 1 2 meas rth rth
ð1Þ
2.2. Processing In the scope of this study exclusively tubular components with an inner diameter of 70 mm were built from four layers. The length of the tubes was 600 mm. The feed rate of the impregnated tow was set to 5 mm per revolution which amounted to a total length of 105 m per tube produced.
Fig. 3. Viscosity of PP (Novolenw 1100 VC) as a function of shear rate.
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Table 2 Characteristic values of Advantexw GF R43S Type 30w rovings [3]
Fig. 4. Viscosity of PA12 (Vestamidw 1670) as a function of shear rate.
The density was measured according to the Archimedes’ principle by weighing the samples in air and a liquid of known density. The theoretical density was calculated using the rule of mixtures of fibre and matrix density. To a large extent the strength and stiffness of a fibrous composite depend on the relative composition of the material. For evaluation of the material’s mechanical potential and theoretical density, the volumetric fibre content must be specified. This was done by weighing the samples before and after combustion of the polymeric matrix. From some wound tubes samples were prepared for light microscopy. Quantitative studies of the impregnation degree by image analysis were discarded. While the gravimetric determination of void content delivers a global value over the sample’s volume, a picture of the crosssection gives information about void size and distribution, as well as fibre – matrix distribution. 2.4. Mechanical characterisation
Property
Value
Tensile strength Elongation at break Tensile modulus Fibre diameter Density
3450 Mpa 4.76% 76 GPa 17 mm 2.60 g/cm3
Criteria for the selection of test methods are their significance with respect to the meaning of the deducted properties for the quality of the composite, and their applicability to curved specimens from tubes [4]. In fibre reinforced composite materials, the interface between fibre and matrix is of decisive importance for the mechanical behaviour, because it serves the stress transfer [5]. In composites with unidirectional reinforcement, the static strength and modulus in fibre direction are strongly dominated by the properties and content of the fibres. Transversely, matrix properties, details in the composite’s mesostructure (e.g. voids, fibre misalignments, bundling phenomena, etc.), and the quality of the fibre – matrixinterface are more decisive [6]. If the mechanical properties of a composite of the same composition vary between different samples, this can be traced back to [7]: † voids, i.e. entrapped air; † poor impregnation of fibres, and hence bad load transfer; † mechanical damages of fibres and matrix cracks. 2.5. Tensile strength of filament wound ring specimens
The mechanical characterisation of specimens contributes, similarly as the optical examination and the determination of density and void content, to a quantification of the influence of processing parameters on the quality of filament wound components.
A tensile test for rings according to ASTM D 2290-92 allows a simple mechanical characterisation, because from the wound tubes no flat specimens can be taken [8]. Fig. 5 shows the test set-up.
Table 1 Properties of the selected matrices in comparison [1,2] Property
Novolen 1100VC
Physical and mechanical properties 0.91 Density (g/cm3) Young’s modulus (MPa) 1550 Yield stress (MPa) 35 Elongation at break (%) – Dimensional stability (8C) 90 Coefficient of linear thermal 135 expansion (K21 p 1026) Processing and thermal properties Melting point (8C) 172 Melting range (8C) 159 –185 Specific heat capacity (J/g) 89
Vestamid L1670
1.01 1400 46 .50 120 150
178 165–200 59.4
Fig. 5. Schematic of the testing device for composite rings.
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The test is easy to conduct (preparation of specimen, support in the device, and analysis) but the inhomogeneous stress distribution along the perimeter of the ring and the unsteadiness in the area of the gap between the split disks is a disadvantage. In particular, the occurring bending momentum contributes to a stress concentration in that region. To reduce the influence of undesired bending rather thin walled rings should be tested, and the corner radii as well as the gap height should be minimised. Due to the inhomogeneous elongation of the specimen, modulus cannot be determined [4]. As a consequence of the stress concentration the measured strength is strongly reduced compared to standard tensile testing with flat samples. The tubular tensile strength is calculated from the maximum force Fmax divided by the cross-section A1 þ A2 in the region of the gap between the split disks:
sring ¼
Fmax Fmax ¼ A 1 þ A2 ðw1 d1 Þ þ ðw2 d2 Þ
Fig. 7. Typical Load-deformation curves for interlaminar shear tests.
The formula for calculation of the interlaminar shear strength is entered by the lever of the device, the applied force, and the shear area of the sample:
ð2Þ
The tubular tensile strength represents the average of not less than three tests. The samples’ width was w ¼ 25 mm. 2.6. Interlaminar shear strength It is well known that the shear strength is sensitive to variations in the process parameters [4,9 – 11]. If there is a weak interface between fibres and matrix or if fibre agglomerations are not properly impregnated the shear strength will drop dramatically because load transfer cannot occur to full extent. The same detrimental effect can be caused by voids, which reduce the shear area and in addition act as stress raisers. Parameters which influence shear strength are the properties of fibres and matrix themselves, the fibre – matrix interfacial adhesion, therefore the impregnation quality, and void content. In the present case, the interlaminar shear test designed by Lauke [12,13] was used. The loading direction coincides with the orientation of the fibres in the hoop wound specimen segments (Fig. 6).
t¼
Fp ðl =l ÞF ¼ k s wh Ashear
ð3Þ
where F p is the effective shear force, Ashear ; the shear area, lk ; ls ; the levers of the test device, F, the maximum applied test load, and w; h are the width and height of the sample, respectively. The shear strength values determined represent the average of at least five samples of one parameter set. This test principle is simple, but it requires care regarding set-up and preparation of the specimen. The samples are 12 mm wide and 9 mm high, so that the lever is perpendicular to the direction of loading. Ideally, the specimen fails abruptly due to shear. The other failure mode, in which plastic deformation occurs, leading to a peeling of layers, is not valid (Fig. 7). In general it should be mentioned that the Lauke test usually gives clear lower values of ‘shear strength’ than the short beam shear test performed with flexure bars (usually not well suitable for thermoplastic composites due to their more ductile matrices) or the torsion test carried out with tube like specimens (without any edge effects, but difficult specimen preparation) [14]. This is due to Mode I-crack opening effects that can take place in the vicinity of the sharp edges of shear blades. However, the test is rather simple to perform, especially on specimens taken from real wound parts, and it leads to a reliable ranking of the part quality as a function of processing conditions.
3. Results 3.1. Experiments with GF/PP
Fig. 6. Schematic of interlaminar shear test after Lauke.
The starting point for the parameter study was a standard set of parameters gathered in preliminary trials (Table 3). The impregnation temperature is the temperature set for the melt, the heated mounting of the impregnation wheels,
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Table 3 Standard set of parameters for production of GF/PP-tubes (Impregnation) temperature, T Roving brake force, Fvor Fibre volume content, FF Winding speed, v Consolidation pressure, proller Nip-point-temperature, Tnip
210 8C 8N 25% 9 m/min 2 bar 200 8C
and the chamber. The nip-point is heated by a hot air gun and the temperature controlled by an infrared thermometer.
3.1.1. Effect of impregnation temperature
Fig. 10. Mechanical properties of GF/PP as a function of winding speed.
Figs. 8 and 9 show the effect of the impregnation temperature on the properties of the wound components. Because of possible thermal degradation of the polymer, the temperature was not raised further. Increasing the impregnation temperature comes along with a rise in the mechanical properties, and a corresponding reduction in void content. This is due to the fact that higher temperatures result in a lower melt viscosity, which in turn, favours the impregnation. In addition, the incoming tow has a higher temperature at the nip-point, which is good for the final consolidation process of the wound tubes. However,
the required pulling force increases with the melt temperature. This behaviour has been modelled by Lutz [6].
Fig. 8. Mechanical properties of GF/PP as a function of impregnation temperature.
Fig. 9. Void content and pulling force of GF/PP over impregnation temperature.
3.1.2. Effect of winding speed From an economical point of view, the processing speed is a very important factor for all production routines. In the present case, the mechanical properties remained almost unchanged up to a processing speed of 15 m/min (Fig. 10). Generally, increasing the speed reduces the time available for both fibre bundle impregnation and part consolidation, while there is a linear rise in the required force to pull the fibre bundle off the impregnation device (Fig. 11). The degree of impregnation is still acceptable up to a speed of 15 m/min. However, above this value breakage of single filaments could be observed, which was attributed to the excessive pulling forces. 3.1.3. Effect of roving pretension In spite of the problems resulting from a high pulling force, a roving brake force provided by the magnetic coil brakes is necessary. The fibre pretension is required to help the pins prior to the impregnation wheel to break-up the fibre sizing, to supply the bundle evenly levelled onto the tool, and to guarantee intimate contact of porous material and fibre bundle over the full contact length. Fig. 12 shows that there obviously exists an optimum fibre bundle pretension. For a given parameter combination,
Fig. 11. Void content and pulling force of GF/PP over winding speed.
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Fig. 12. Mechanical properties of GF/PP as a function of roving pretension.
this value is the result of the mentioned benefits and a critical upper limit for the resulting pulling force. Pulling forces not only causes fibre damage, but also compression of the fibre bundle and hence reduction in permeability for the polymer melts. While the mechanical properties have a maximum, there is a minimum void content simultaneously (Fig. 13). 3.1.4. Effect of consolidation pressure Even though the required pulling forces exert pressure in radial direction on the mandrel, no satisfying consolidation between the layers can be achieved without additional measures. By increasing the consolidation pressure of the roller, there is an increase in mechanical properties, due to a drop in the void content (Figs. 14 and 15). However, no further enhancement of the properties can be achieved beyond 4 bar pressure. In addition, only by the use of a compaction roller, a smooth, optically appealing surface of the wound tubes is obtained. 3.2. Experiments with GF/PA12 The starting conditions for the first operation with GF/PA12 were based on the following standard set of parameters (Table 4). From the knowledge gathered with GF/PP, the consolidation pressure was set to proller ¼ 4 bar: For the applied
Fig. 14. Mechanical properties of GF/PP as a function of consolidation pressure.
Fig. 15. Void content and pulling force of GF/PP over consolidation pressure.
polymer processing conditions for PA12, the impregnation temperature was set to an allowable upper limit of 240 8C and not further varied. 3.2.1. Effect of winding speed In comparison to GF/PP, a drop in tensile and shear strength is already observed from lower processing speeds (Fig. 16). Fig. 17 displays the diagrams of void content and pulling force over winding speed. In comparison to GF/PP (Fig. 11), the void content is slightly lower in this system, whereas the pulling forces exceed those measured for GF/PP within the same range of winding speeds. In spite of lower void content and a similar impregnation degree the drop in the mechanical properties for lower processing speeds can be attributed to excessive pulling forces. Broken fibres were already visible during processing. A final explanation for this discrepancy cannot be given at this point, since also Table 4 Standard set of parameters for production of GF/PA12-tubes
Fig. 13. Void content and pulling force of GF/PP over roving brake force.
(Impregnation) temperature, T Roving brake force, Fvor Fibre volume content, FF Winding speed, v Consolidation pressure, proller Nip-point-temperature, Tnip
240 8C 8N 25% 9 m/min 4 bar 200 8C
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Fig. 16. Mechanical properties of GF/PA12 as a function of winding speed.
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Fig. 19. Void content and pulling force of GF/PA12 over roving brake force.
As with processing speeds, this is the result of the considerably higher pulling forces (Fig. 19). Broken filaments were observed while processing.
Fig. 17. Void content and pulling force of GF/PA12 over winding speed.
3.2.3. Effect of nip-point-temperature A parameter which has not been considered in the case of GF/PP winding was the nip-point-temperature. Compared to the temperature of the standard parameter set, only minor improvements of the property profile were achieved. However, the significance of this parameter is particularly emphasised when considering the relatively low shear strength and slightly higher void content measured at the lower nip-point-temperature of Tnip ¼ 180 8C (Fig. 20).
the measured viscosity of PA12 was higher than PP at the corresponding processing conditions.
4. Variation of fibre volume content in radial direction
3.2.2. Effect of roving pretension Like in the case of GF/PP, the mechanical properties have a distinct maximum in case of a variation of roving brake force. However, for GF/PA12 the maximum is already achieved at 5 N, compared to 8 N in the case of GF/PP (Fig. 18).
When processing semi-finished composite materials by filament winding, the fibre volume content of the component is predetermined by the prepreg material used. In some cases, however, a gradual change in fibre volume fraction over the wall thickness of the part might be desirable. This can be achieved by filament winding with online-impregnation. To realise this, two routes can be followed: (1) adaptation of melt flow with constant processing speed, or (2) running
Fig. 18. Mechanical properties of GF/PA12 as a function of roving brake force.
Fig. 20. Shear strength and void content of GF/PA12 over nip-pointtemperature.
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For both GF/PP and GF/PA12, the values are in the range of what was expected from the overall fibre volume content in the part. However, this is only directly true for the tensile strength. In case of the shear strength, it must be considered that the shear plane is in the middle of the sample. Therefore the locally measured data here have to be compared to those measured for the non-graded specimens with the same fibre loading. Nevertheless the data are in good agreement with each other.
5. Discussion Fig. 21. Mechanical properties for GF/PP as a function of fibre volume content.
5.1. General comments
the process with constant melt flow and modulation of processing speed according to the desired fibre volume content. The adaptation of melt flow takes a few seconds to build up a constant melt pressure for the fibre content desired. However, a variation of processing speed is not workable because for economic reasons the process should run with the highest possible speed for the quality demanded. Here, the wound tubes were build up of four layers, and the fibre volume content was varied from layer to layer by a change in matrix supply. For the first layer on the mandrel and for the surface layer of the tube, a low fibre volume content was selected in order to achieve smoothly, homogeneously looking surfaces. This resulted in a fibre volume sequence selected per layer as 15, 25, 35, and 15% in the first set of trials, and 15, 35, 45, and 15% in the second set of trials. Combustion of samples showed that average fibre volume content of 21.8% resp. 26.8% were reached in case of GF/PP, whereas GF/PA12 gave values of 21.1% resp. 26.9%. The tolerance from the calculative fibre volume contents of 22.5 and 27.5% is in the range of the same scatter as measured for samples prepared under constant melt flow. The measured characteristic values of samples with a gradual variation in fibre content were compared to that of the experiments with constant content over the crosssection. Figs. 21 and 22 show the results for GF/PP.
The shear strengths achieved with the GF/PA12 system were significantly above that of GF/PP, while fibre dominated tensile strengths of tubes were found to be on the same level. This is not surprising, since the shear properties of the PA12-matrix are clearly higher than those of PP. However, to what extent load transfer within a fibrous composite can take place is additionally dependent on the fibre –matrix adhesion. A high impregnation level and a good consolidation alone do not guarantee a high mechanical property profile. The glass fibres employed in the study were treated with a sizing especially designed for performing particularly well with PA12. Although the manufacturer pointed out that this sizing could also be used with other thermoplastics, including PP, in case of the present study no good fibre – matrix adhesion could be achieved with PP. Fig. 23 shows the comparative scanning electron microscopy pictures of shear planes of GF/PP and GF/ PA12. While in the case of GF/PP, bare fibres indicate a poor fibre –matrix adhesion, for GF/PA12 residual matrix still remained on the fibres, indicating a good adhesion between the fibres and the polymer matrix. For a given material system, quality problems, indicated by a decline in mechanical properties, are reflected in void content, as shown for GF/PP in Fig. 24. Shear strength reacts more sensible to variations in void content than the strongly fibre dominated tensile strength, because voids directly reduce the effective shear area. A high void content is not necessarily related to insufficient impregnation because if no good consolidation between layers is achieved, large interlaminar voids must be expected. Insufficient impregnation, on the other hand, means that the gaps between filaments are not completely filled with polymer, and hence this contributes to a higher void content. With increasing fibre volume content shear strength values were decreasing (Fig. 21). In fact the latter property is strongly dependent (in a negative sense) on the fibre – matrix interface which, in turn, is growing with increasing fibre content. Wetting of fibres with a viscous thermoplastic
Fig. 22. Void content and pulling Force of GF/PP over fibre volume content.
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Fig. 25. Influence of pulling force on shear strength.
Fig. 23. SEM pictures of shear planes of GF/PP (top) and GF/PA12 (bottom).
melt is aggravated, which can lead to higher void content with a lower impregnation degree [15,16]. In the process studied, there is a strong relation between the forces required to pull the fibre tow off the impregnation wheels and the mechanical properties tested, as shown in Fig. 25 for shear strengths. While a certain fibre pretension (by applying roving brake force) is necessary to provide that the fibre bundles arrive in a suitable condition at the impregnation wheels, this can result in excessive pulling forces and damage of filaments at the other end of the device.
Fig. 24. Influence of void content on mechanical properties of GF/PP.
Broken filaments, in turn, are not any more available for load transfer in the tensile mode, and in case of shear load, broken fibres produce discontinuities in the shear plane. In both cases, reductions in the properties have to be expected. Besides the difficulties due to fibre breakage, the forces to pull the fibre tow off the impregnation wheels provide a radial pressure which draws the fibres within the still molten polymer towards the mandrel. Although this is not sufficient for a good consolidation between the individual layers, it results in an inhomogeneous fibre distribution within the cross-section of the component, leading in particular to matrix rich intermediate layers. Fig. 26 shows the typical structure of fibre aggregation and matrix rich areas, but also the high impregnation quality that is achieved. By controlling nip-point-temperature and applying a consolidation roller with appropriate consolidation pressure good consolidation between the individual layers is achieved. 5.2. Pulling forces Due to their significance, the influence of processing parameters on the required pulling forces was studied in a separate experimental series, also without matrix supply. Fig. 27 shows the influence of the roving brake force set by magnetic brake (in the fibre spool supply) on the force required to pull the impregnated tow out off the impregnation wheels. Without matrix and a pulling speed of 9 m/min, a roving brake force of 16 N ðFpull < 120 NÞ is sufficient to cause noticeable fibre breakage events. A further raise in pretension leads to rupture of the whole bundle. Fig. 28 shows obviously that a linear correlation exists between pulling force and processing speed. If the processing speed is raised with a roving brake force of 8 N, numerous filament breakages can be observed above 21 m/min, and rupture of the tow occurs at 27 m/min. Interestingly the susceptibility for fibre breakage is clearly below the forces which cause a similar degree of fibre damage at lower speeds but higher roving brake force.
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Fig. 28. Influence of fibre volume content on pulling force.
Fig. 26. Influence of fibre bundle pretension on pulling force.
Obviously at larger relative speeds between the fibre bundle and the pins, respectively, the impregnation wheels, the stress distribution among the single filaments is worse than at lower speeds, so that locally filament strength values are exceeded more easily. Overall it can be recorded that in the present process rovings are prone to fibre breakage at average stresses, calculated by relating the pulling force to the overall cross-section area of the bundle, which are 30 times lower than the theoretical tensile strength of the tow, given by the manufacturer.
Fig. 27. Influence of processing speed on pulling force.
The required pulling forces are significantly increasing if matrix is present on the surface of the impregnation wheels. With PA12 matrix, the forces are noticeably above the ones for PP. However, the course of pulling force over the variation of different parameters is qualitatively similar, also without melt flow (Figs. 27 and 28). While there is a linear correlation with the processing speed, for the roving brake force there is a slightly progressive rise in pulling force. Lowering the viscosity of the melt by higher temperature leads to increasing pulling forces, as can be seen in Fig. 9. The latter were, on the other hand decreasing with increasing fibre volume content, as it was already expected from Figs. 27 and 28 (Fig. 29). The pulling forces which cause visible fibre damage lie at about 160 N if the polymer matrix is present. This is significantly higher than observed for fibre bundles without a polymer matrix. It can therefore be concluded that a thermoplastic melt in the presence of a fibre bundle favours the stress distribution between the filaments and protects them from an excessive frictional failure due to the contact with the porous metal impregnation wheel.
Fig. 29. Microscopy of samples produced under optimum processing conditions.
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Fig. 30. Filament wound tubes with online impregnation.
6. Summary and outlook
Acknowledgements
It could be shown that with the process technology developed it is possible to realise thermoplastic filament winding with online impregnation of fibre bundles, so as to realise fibre reinforced tubes at competitive processing speeds. Fig. 30 shows examples of the tubes which have been produced. At present, however, technological restraints regarding the processing speed have to be accepted compared to thermoplastic tape winding. In spite of this, the assessment of economic efficiency (in part A of this paper) has shown that reasonable speeds at the limit for profitableness were achieved [17]. In particular, a process combination is advantageous if no preimpregnated tapes are commercially available. The present restrictions of winding speeds have to be understood with respect to both the difficulties related to the of pulling forces, and the early stage of development of this technology. This dilemma arises from the conflict between the required roving pretension and the generated pulling force increasing with processing speed. For future applications, this problem can be evaded by an altered construction, using driven impregnation wheels, as it was demonstrated by Lutz [6] in the present state of development of the impregnation wheel technology for production of flat thermoplastic tapes and circular shaped rods.
The authors wish to thank the German Science Foundation (Deutsche Forschungsgemeinschaft) for the financial support granted to conduct this study (DFG FR 675/23-3). Further we also appreciate the help of Alena Korzikava, who held a DAAD fellowship, for preliminary experiments.
References [1] [2] [3] [4]
[5] [6]
[7] [8]
[9]
NN. Product information. Mainz, Germany: Targor GmbH. NN. Product information. Marl, Germany: Degussa-Huels AG. NN. Product information. OH, USA: Owens Corning. Voigtla¨nder R. Herstellung und Charakterisierung von gekru¨mmten Thermoplastwickelverbunden unter dem Aspekt der Werkstoffoptimierung. PhD thesis. Technical University, Dresden; 1997. Michaeli W, Wegener M. Einfu¨hrung in die Technologie der Faserverbundwerkstoffe. Munich: Hanser; 1990. Lutz A. Beitrag zur Entwicklung innovativer Fertigungstechniken fu¨r die Verarbeitung thermoplastischer Faserverbundwerkstoffe. Berlin: Mensch und Buch; 1999. Simult.: PhD thesis, Kaiserslautern; 1999. Friedrich K. Mesoscopic aspects in polymer composites: processing, structure, properties. J Mater Sci 1998;33:5535 –56. ASTM D2290-92. Standard test method for apparent tensile strength of ring or tubular plastics and reinforced plastics by split disk method; 1992. Skop-Cardarella K, Ma¨der E, Za¨h W, Mayer K. Textile Fla¨chengebilde fu¨r low-cost Thermoplastverbunde. Proceedings of Verbundwerkstoffe und Werkstoffverbunde, Kaiserslautern; 1997. p. 367– 71.
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F. Henninger et al. / Composites: Part A 33 (2002) 1677–1688
[10] Bechthold G. Pultrusion von geflochtenen und axial verstrkten Thermoplast-Halbzeugen und deren zersto¨rungsfreiePorengehaltsbestimmung. Kaiserslautern: Institut fu¨r Verbundwerkstoffe GmbH; 2000. Simult.: PhD thesis, Kaiserslautern; 2000. [11] Kerbiriou V, Friedrich K. Heizpressen und Pultrusion von Thermoplastpulverimpra¨gnierten und-umhu¨llten Glasfaserbu¨ndeln. Proceedings of Polymerwerkstoffe 96, Merseburg; 1996. p. 184– 7. [12] Lauke B, Schneider K, Friedrich K. Interlaminar shear strength measurement of thin composite rings fabricated by filament winding. Proceedings of the ECCM-5, Bordeaux; 1992. p. 313–8. [13] Lauke B, Scho¨ne A, Friedrich K. In: Chandra T, Dhingra AK, editors. High performance thermoplastic composites fabricated by filament winding. Proceedings of International Conference on Advanced Composites, Wollongong, Australia; 1993. p. 883 –9.
[14] Hoecker F. Grenzfla¨ cheneffekte in Hochleistungsfaserverbundwerkstoffen mit polymeren matrices. Du¨ sseldorf: VDI; 1996. [15] Hampe A, Hennecke M, Mielke W. Pru¨fmethoden fu¨r Polymerwerkstoffe. Materialpru¨fung 1992;34(7/8):228–31. [16] Ma CM, Yn M. Poly(1-caprolactam)-poly(butadiene-co-acrylonitrile) block copolymers. II. Processability and properties of pultuded glass-fiber-reinforced composites. Mater Chem Phys 1995;42:17–24. [17] Henninger F, Friedrich K. Thermoplastic filament winding with online-impregnation. Part A.:Process technology and operating efficiency. Composites, Part A 2002; 31/11: 1479–86.
Thermoplastic filament winding with online-impregnation. Part B. Experimental study of processing parameters F. Henninger, J. Hoffmann, K. Friedrich* Institut fu¨r Verbundwerkstoffe GmbH, Erwin-Schro¨dinger Strasse, Gebaude 58, D-67663 Kaiserslautern, Germany Received 28 January 2002; revised 1 August 2002; accepted 19 September 2002
Abstract In part A of this paper, a novel process technology was presented which allows to combine thermoplastic filament winding with online melt impregnation of fibre bundles. In this part, a comprehensive study of process parameters was conducted. Circular glass fibre (GF) reinforced polypropylene (PP) or polyamide 12 (PA12) tubes were produced as sample component. Processing speeds of up to 15 m/min could be achieved in case of GF/PP before a drop in quality of the parts had to be accepted. The present limitation in winding speed is not attributed to impregnation problems but to an excessive rise in force required to pull the fibre tow off the impregnation device. Measures for process improvement and increase in productivity were proposed. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Thermoplastic resin; E. Extrusion; A. Tape; E. Filament winding
1. Introduction Filament winding is a widely used processing routine for continuous fibre reinforced polymer for axially symmetric components. Initially developed for thermoset resins, it was successfully adopted for thermoplastics as well. However, an intermediate product form, in which the thermoplastic matrix is already in close contact to the reinforcing fibres (e.g. commingled yarn; powder impregnated bundles; preimpregnated tapes) must usually be used. A novel process technology was described in part A of this paper; it allows combining thermoplastic filament winding with online melt impregnation of fibre bundles. For the sake of completeness, the process principle, which was explained in Part A of this paper, is repeated in Fig. 1. For reasons of clarity, a hot air gun for heating the nip-point and an IRthermometer to control the nip-point-temperature are not displayed in Fig. 1. The location of these devices can be seen in Fig. 2. From the qualitative study of process parameters in part A, which found consideration in the design of the process, it was deducted that it is of high importance to experimentally * Corresponding author. Tel.: þ49-631-2017-201; fax: þ 49-631-2017198. E-mail address: [email protected] (K. Friedrich).
investigate the influence of the following parameters in more detail: (1) impregnation temperature, (2) roving pretension (brake force), (3) process speed, (4) nip-point temperature, and (5) fibre volume content. Fig. 2 shows the principle of the measurement of the force required to the fibre bundle off the impregnation unit. The pulling force generated by the impregnation process was determined in an extra set of experiments without characterising the wound parts.
2. Experimental 2.1. Materials For the parameter study conducted, glass fibre (GF) bundles were impregnated either with polypropylene (PP) or polyamide 12 (PA12). A PP-homopolymer Type Novolenw 1100 VC by TARGOR GmbH, Germany, had been recommended by the manufacturer for this application. It possesses a high melt flow rate and has been developed for thin wall injection moulding [1]. The viscosity at low shear rates and a melt temperature of 230 8C is below 100 Pa s (Fig. 3). The PA12 was a low viscosity, heat and light stabilised Vestamidw 1670 by Degussa-Hu¨ls AG, Germany, which is
1359-835X/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 8 3 5 X ( 0 2 ) 0 0 1 3 6 - 7
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Fig. 1. Schematic of the process combination.
Fig. 2. Measurement of pulling force.
usually used for wire insulations, coils or secondary coating of optical fibres [2]. Fig. 4 shows the viscosity profile as a function of shear rate for different temperatures. All viscosities were measured with a capillary rheometer. Some properties of both matrices are listed in Table 1. For all experiments of filament winding and online fibre bundle impregnation, E-glass fibres were used. A fibre bundle consists of many filaments, and its strength is quantified by the weight per kilometre running length, the so-called TEX-number. The Advantexw GF R43S Type 30w roving of Owens Corning, USA, with 2400 TEX has a silane based sizing designed for the use of these fibres in thermoplastic matrices. The roving can perform particularly well in polyamide, but can also be used in a wide range of thermoplastic polymers such as PP, PC and PPS (Table 2) [3].
After starting the process, a total of 10 m tow was wound separately onto the mandrel as waste material to allow constant conditions to establish. 2.3. Microscopy and void content The volumetric void content was determined by relating the measured density rmeas to the theoretical density ðrth Þ of a void-free composite with the same fibre content: XV ¼
rth 2 rmeas r ¼ 1 2 meas rth rth
ð1Þ
2.2. Processing In the scope of this study exclusively tubular components with an inner diameter of 70 mm were built from four layers. The length of the tubes was 600 mm. The feed rate of the impregnated tow was set to 5 mm per revolution which amounted to a total length of 105 m per tube produced.
Fig. 3. Viscosity of PP (Novolenw 1100 VC) as a function of shear rate.
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Table 2 Characteristic values of Advantexw GF R43S Type 30w rovings [3]
Fig. 4. Viscosity of PA12 (Vestamidw 1670) as a function of shear rate.
The density was measured according to the Archimedes’ principle by weighing the samples in air and a liquid of known density. The theoretical density was calculated using the rule of mixtures of fibre and matrix density. To a large extent the strength and stiffness of a fibrous composite depend on the relative composition of the material. For evaluation of the material’s mechanical potential and theoretical density, the volumetric fibre content must be specified. This was done by weighing the samples before and after combustion of the polymeric matrix. From some wound tubes samples were prepared for light microscopy. Quantitative studies of the impregnation degree by image analysis were discarded. While the gravimetric determination of void content delivers a global value over the sample’s volume, a picture of the crosssection gives information about void size and distribution, as well as fibre – matrix distribution. 2.4. Mechanical characterisation
Property
Value
Tensile strength Elongation at break Tensile modulus Fibre diameter Density
3450 Mpa 4.76% 76 GPa 17 mm 2.60 g/cm3
Criteria for the selection of test methods are their significance with respect to the meaning of the deducted properties for the quality of the composite, and their applicability to curved specimens from tubes [4]. In fibre reinforced composite materials, the interface between fibre and matrix is of decisive importance for the mechanical behaviour, because it serves the stress transfer [5]. In composites with unidirectional reinforcement, the static strength and modulus in fibre direction are strongly dominated by the properties and content of the fibres. Transversely, matrix properties, details in the composite’s mesostructure (e.g. voids, fibre misalignments, bundling phenomena, etc.), and the quality of the fibre – matrixinterface are more decisive [6]. If the mechanical properties of a composite of the same composition vary between different samples, this can be traced back to [7]: † voids, i.e. entrapped air; † poor impregnation of fibres, and hence bad load transfer; † mechanical damages of fibres and matrix cracks. 2.5. Tensile strength of filament wound ring specimens
The mechanical characterisation of specimens contributes, similarly as the optical examination and the determination of density and void content, to a quantification of the influence of processing parameters on the quality of filament wound components.
A tensile test for rings according to ASTM D 2290-92 allows a simple mechanical characterisation, because from the wound tubes no flat specimens can be taken [8]. Fig. 5 shows the test set-up.
Table 1 Properties of the selected matrices in comparison [1,2] Property
Novolen 1100VC
Physical and mechanical properties 0.91 Density (g/cm3) Young’s modulus (MPa) 1550 Yield stress (MPa) 35 Elongation at break (%) – Dimensional stability (8C) 90 Coefficient of linear thermal 135 expansion (K21 p 1026) Processing and thermal properties Melting point (8C) 172 Melting range (8C) 159 –185 Specific heat capacity (J/g) 89
Vestamid L1670
1.01 1400 46 .50 120 150
178 165–200 59.4
Fig. 5. Schematic of the testing device for composite rings.
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The test is easy to conduct (preparation of specimen, support in the device, and analysis) but the inhomogeneous stress distribution along the perimeter of the ring and the unsteadiness in the area of the gap between the split disks is a disadvantage. In particular, the occurring bending momentum contributes to a stress concentration in that region. To reduce the influence of undesired bending rather thin walled rings should be tested, and the corner radii as well as the gap height should be minimised. Due to the inhomogeneous elongation of the specimen, modulus cannot be determined [4]. As a consequence of the stress concentration the measured strength is strongly reduced compared to standard tensile testing with flat samples. The tubular tensile strength is calculated from the maximum force Fmax divided by the cross-section A1 þ A2 in the region of the gap between the split disks:
sring ¼
Fmax Fmax ¼ A 1 þ A2 ðw1 d1 Þ þ ðw2 d2 Þ
Fig. 7. Typical Load-deformation curves for interlaminar shear tests.
The formula for calculation of the interlaminar shear strength is entered by the lever of the device, the applied force, and the shear area of the sample:
ð2Þ
The tubular tensile strength represents the average of not less than three tests. The samples’ width was w ¼ 25 mm. 2.6. Interlaminar shear strength It is well known that the shear strength is sensitive to variations in the process parameters [4,9 – 11]. If there is a weak interface between fibres and matrix or if fibre agglomerations are not properly impregnated the shear strength will drop dramatically because load transfer cannot occur to full extent. The same detrimental effect can be caused by voids, which reduce the shear area and in addition act as stress raisers. Parameters which influence shear strength are the properties of fibres and matrix themselves, the fibre – matrix interfacial adhesion, therefore the impregnation quality, and void content. In the present case, the interlaminar shear test designed by Lauke [12,13] was used. The loading direction coincides with the orientation of the fibres in the hoop wound specimen segments (Fig. 6).
t¼
Fp ðl =l ÞF ¼ k s wh Ashear
ð3Þ
where F p is the effective shear force, Ashear ; the shear area, lk ; ls ; the levers of the test device, F, the maximum applied test load, and w; h are the width and height of the sample, respectively. The shear strength values determined represent the average of at least five samples of one parameter set. This test principle is simple, but it requires care regarding set-up and preparation of the specimen. The samples are 12 mm wide and 9 mm high, so that the lever is perpendicular to the direction of loading. Ideally, the specimen fails abruptly due to shear. The other failure mode, in which plastic deformation occurs, leading to a peeling of layers, is not valid (Fig. 7). In general it should be mentioned that the Lauke test usually gives clear lower values of ‘shear strength’ than the short beam shear test performed with flexure bars (usually not well suitable for thermoplastic composites due to their more ductile matrices) or the torsion test carried out with tube like specimens (without any edge effects, but difficult specimen preparation) [14]. This is due to Mode I-crack opening effects that can take place in the vicinity of the sharp edges of shear blades. However, the test is rather simple to perform, especially on specimens taken from real wound parts, and it leads to a reliable ranking of the part quality as a function of processing conditions.
3. Results 3.1. Experiments with GF/PP
Fig. 6. Schematic of interlaminar shear test after Lauke.
The starting point for the parameter study was a standard set of parameters gathered in preliminary trials (Table 3). The impregnation temperature is the temperature set for the melt, the heated mounting of the impregnation wheels,
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Table 3 Standard set of parameters for production of GF/PP-tubes (Impregnation) temperature, T Roving brake force, Fvor Fibre volume content, FF Winding speed, v Consolidation pressure, proller Nip-point-temperature, Tnip
210 8C 8N 25% 9 m/min 2 bar 200 8C
and the chamber. The nip-point is heated by a hot air gun and the temperature controlled by an infrared thermometer.
3.1.1. Effect of impregnation temperature
Fig. 10. Mechanical properties of GF/PP as a function of winding speed.
Figs. 8 and 9 show the effect of the impregnation temperature on the properties of the wound components. Because of possible thermal degradation of the polymer, the temperature was not raised further. Increasing the impregnation temperature comes along with a rise in the mechanical properties, and a corresponding reduction in void content. This is due to the fact that higher temperatures result in a lower melt viscosity, which in turn, favours the impregnation. In addition, the incoming tow has a higher temperature at the nip-point, which is good for the final consolidation process of the wound tubes. However,
the required pulling force increases with the melt temperature. This behaviour has been modelled by Lutz [6].
Fig. 8. Mechanical properties of GF/PP as a function of impregnation temperature.
Fig. 9. Void content and pulling force of GF/PP over impregnation temperature.
3.1.2. Effect of winding speed From an economical point of view, the processing speed is a very important factor for all production routines. In the present case, the mechanical properties remained almost unchanged up to a processing speed of 15 m/min (Fig. 10). Generally, increasing the speed reduces the time available for both fibre bundle impregnation and part consolidation, while there is a linear rise in the required force to pull the fibre bundle off the impregnation device (Fig. 11). The degree of impregnation is still acceptable up to a speed of 15 m/min. However, above this value breakage of single filaments could be observed, which was attributed to the excessive pulling forces. 3.1.3. Effect of roving pretension In spite of the problems resulting from a high pulling force, a roving brake force provided by the magnetic coil brakes is necessary. The fibre pretension is required to help the pins prior to the impregnation wheel to break-up the fibre sizing, to supply the bundle evenly levelled onto the tool, and to guarantee intimate contact of porous material and fibre bundle over the full contact length. Fig. 12 shows that there obviously exists an optimum fibre bundle pretension. For a given parameter combination,
Fig. 11. Void content and pulling force of GF/PP over winding speed.
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Fig. 12. Mechanical properties of GF/PP as a function of roving pretension.
this value is the result of the mentioned benefits and a critical upper limit for the resulting pulling force. Pulling forces not only causes fibre damage, but also compression of the fibre bundle and hence reduction in permeability for the polymer melts. While the mechanical properties have a maximum, there is a minimum void content simultaneously (Fig. 13). 3.1.4. Effect of consolidation pressure Even though the required pulling forces exert pressure in radial direction on the mandrel, no satisfying consolidation between the layers can be achieved without additional measures. By increasing the consolidation pressure of the roller, there is an increase in mechanical properties, due to a drop in the void content (Figs. 14 and 15). However, no further enhancement of the properties can be achieved beyond 4 bar pressure. In addition, only by the use of a compaction roller, a smooth, optically appealing surface of the wound tubes is obtained. 3.2. Experiments with GF/PA12 The starting conditions for the first operation with GF/PA12 were based on the following standard set of parameters (Table 4). From the knowledge gathered with GF/PP, the consolidation pressure was set to proller ¼ 4 bar: For the applied
Fig. 14. Mechanical properties of GF/PP as a function of consolidation pressure.
Fig. 15. Void content and pulling force of GF/PP over consolidation pressure.
polymer processing conditions for PA12, the impregnation temperature was set to an allowable upper limit of 240 8C and not further varied. 3.2.1. Effect of winding speed In comparison to GF/PP, a drop in tensile and shear strength is already observed from lower processing speeds (Fig. 16). Fig. 17 displays the diagrams of void content and pulling force over winding speed. In comparison to GF/PP (Fig. 11), the void content is slightly lower in this system, whereas the pulling forces exceed those measured for GF/PP within the same range of winding speeds. In spite of lower void content and a similar impregnation degree the drop in the mechanical properties for lower processing speeds can be attributed to excessive pulling forces. Broken fibres were already visible during processing. A final explanation for this discrepancy cannot be given at this point, since also Table 4 Standard set of parameters for production of GF/PA12-tubes
Fig. 13. Void content and pulling force of GF/PP over roving brake force.
(Impregnation) temperature, T Roving brake force, Fvor Fibre volume content, FF Winding speed, v Consolidation pressure, proller Nip-point-temperature, Tnip
240 8C 8N 25% 9 m/min 4 bar 200 8C
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Fig. 16. Mechanical properties of GF/PA12 as a function of winding speed.
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Fig. 19. Void content and pulling force of GF/PA12 over roving brake force.
As with processing speeds, this is the result of the considerably higher pulling forces (Fig. 19). Broken filaments were observed while processing.
Fig. 17. Void content and pulling force of GF/PA12 over winding speed.
3.2.3. Effect of nip-point-temperature A parameter which has not been considered in the case of GF/PP winding was the nip-point-temperature. Compared to the temperature of the standard parameter set, only minor improvements of the property profile were achieved. However, the significance of this parameter is particularly emphasised when considering the relatively low shear strength and slightly higher void content measured at the lower nip-point-temperature of Tnip ¼ 180 8C (Fig. 20).
the measured viscosity of PA12 was higher than PP at the corresponding processing conditions.
4. Variation of fibre volume content in radial direction
3.2.2. Effect of roving pretension Like in the case of GF/PP, the mechanical properties have a distinct maximum in case of a variation of roving brake force. However, for GF/PA12 the maximum is already achieved at 5 N, compared to 8 N in the case of GF/PP (Fig. 18).
When processing semi-finished composite materials by filament winding, the fibre volume content of the component is predetermined by the prepreg material used. In some cases, however, a gradual change in fibre volume fraction over the wall thickness of the part might be desirable. This can be achieved by filament winding with online-impregnation. To realise this, two routes can be followed: (1) adaptation of melt flow with constant processing speed, or (2) running
Fig. 18. Mechanical properties of GF/PA12 as a function of roving brake force.
Fig. 20. Shear strength and void content of GF/PA12 over nip-pointtemperature.
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For both GF/PP and GF/PA12, the values are in the range of what was expected from the overall fibre volume content in the part. However, this is only directly true for the tensile strength. In case of the shear strength, it must be considered that the shear plane is in the middle of the sample. Therefore the locally measured data here have to be compared to those measured for the non-graded specimens with the same fibre loading. Nevertheless the data are in good agreement with each other.
5. Discussion Fig. 21. Mechanical properties for GF/PP as a function of fibre volume content.
5.1. General comments
the process with constant melt flow and modulation of processing speed according to the desired fibre volume content. The adaptation of melt flow takes a few seconds to build up a constant melt pressure for the fibre content desired. However, a variation of processing speed is not workable because for economic reasons the process should run with the highest possible speed for the quality demanded. Here, the wound tubes were build up of four layers, and the fibre volume content was varied from layer to layer by a change in matrix supply. For the first layer on the mandrel and for the surface layer of the tube, a low fibre volume content was selected in order to achieve smoothly, homogeneously looking surfaces. This resulted in a fibre volume sequence selected per layer as 15, 25, 35, and 15% in the first set of trials, and 15, 35, 45, and 15% in the second set of trials. Combustion of samples showed that average fibre volume content of 21.8% resp. 26.8% were reached in case of GF/PP, whereas GF/PA12 gave values of 21.1% resp. 26.9%. The tolerance from the calculative fibre volume contents of 22.5 and 27.5% is in the range of the same scatter as measured for samples prepared under constant melt flow. The measured characteristic values of samples with a gradual variation in fibre content were compared to that of the experiments with constant content over the crosssection. Figs. 21 and 22 show the results for GF/PP.
The shear strengths achieved with the GF/PA12 system were significantly above that of GF/PP, while fibre dominated tensile strengths of tubes were found to be on the same level. This is not surprising, since the shear properties of the PA12-matrix are clearly higher than those of PP. However, to what extent load transfer within a fibrous composite can take place is additionally dependent on the fibre –matrix adhesion. A high impregnation level and a good consolidation alone do not guarantee a high mechanical property profile. The glass fibres employed in the study were treated with a sizing especially designed for performing particularly well with PA12. Although the manufacturer pointed out that this sizing could also be used with other thermoplastics, including PP, in case of the present study no good fibre – matrix adhesion could be achieved with PP. Fig. 23 shows the comparative scanning electron microscopy pictures of shear planes of GF/PP and GF/ PA12. While in the case of GF/PP, bare fibres indicate a poor fibre –matrix adhesion, for GF/PA12 residual matrix still remained on the fibres, indicating a good adhesion between the fibres and the polymer matrix. For a given material system, quality problems, indicated by a decline in mechanical properties, are reflected in void content, as shown for GF/PP in Fig. 24. Shear strength reacts more sensible to variations in void content than the strongly fibre dominated tensile strength, because voids directly reduce the effective shear area. A high void content is not necessarily related to insufficient impregnation because if no good consolidation between layers is achieved, large interlaminar voids must be expected. Insufficient impregnation, on the other hand, means that the gaps between filaments are not completely filled with polymer, and hence this contributes to a higher void content. With increasing fibre volume content shear strength values were decreasing (Fig. 21). In fact the latter property is strongly dependent (in a negative sense) on the fibre – matrix interface which, in turn, is growing with increasing fibre content. Wetting of fibres with a viscous thermoplastic
Fig. 22. Void content and pulling Force of GF/PP over fibre volume content.
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Fig. 25. Influence of pulling force on shear strength.
Fig. 23. SEM pictures of shear planes of GF/PP (top) and GF/PA12 (bottom).
melt is aggravated, which can lead to higher void content with a lower impregnation degree [15,16]. In the process studied, there is a strong relation between the forces required to pull the fibre tow off the impregnation wheels and the mechanical properties tested, as shown in Fig. 25 for shear strengths. While a certain fibre pretension (by applying roving brake force) is necessary to provide that the fibre bundles arrive in a suitable condition at the impregnation wheels, this can result in excessive pulling forces and damage of filaments at the other end of the device.
Fig. 24. Influence of void content on mechanical properties of GF/PP.
Broken filaments, in turn, are not any more available for load transfer in the tensile mode, and in case of shear load, broken fibres produce discontinuities in the shear plane. In both cases, reductions in the properties have to be expected. Besides the difficulties due to fibre breakage, the forces to pull the fibre tow off the impregnation wheels provide a radial pressure which draws the fibres within the still molten polymer towards the mandrel. Although this is not sufficient for a good consolidation between the individual layers, it results in an inhomogeneous fibre distribution within the cross-section of the component, leading in particular to matrix rich intermediate layers. Fig. 26 shows the typical structure of fibre aggregation and matrix rich areas, but also the high impregnation quality that is achieved. By controlling nip-point-temperature and applying a consolidation roller with appropriate consolidation pressure good consolidation between the individual layers is achieved. 5.2. Pulling forces Due to their significance, the influence of processing parameters on the required pulling forces was studied in a separate experimental series, also without matrix supply. Fig. 27 shows the influence of the roving brake force set by magnetic brake (in the fibre spool supply) on the force required to pull the impregnated tow out off the impregnation wheels. Without matrix and a pulling speed of 9 m/min, a roving brake force of 16 N ðFpull < 120 NÞ is sufficient to cause noticeable fibre breakage events. A further raise in pretension leads to rupture of the whole bundle. Fig. 28 shows obviously that a linear correlation exists between pulling force and processing speed. If the processing speed is raised with a roving brake force of 8 N, numerous filament breakages can be observed above 21 m/min, and rupture of the tow occurs at 27 m/min. Interestingly the susceptibility for fibre breakage is clearly below the forces which cause a similar degree of fibre damage at lower speeds but higher roving brake force.
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Fig. 28. Influence of fibre volume content on pulling force.
Fig. 26. Influence of fibre bundle pretension on pulling force.
Obviously at larger relative speeds between the fibre bundle and the pins, respectively, the impregnation wheels, the stress distribution among the single filaments is worse than at lower speeds, so that locally filament strength values are exceeded more easily. Overall it can be recorded that in the present process rovings are prone to fibre breakage at average stresses, calculated by relating the pulling force to the overall cross-section area of the bundle, which are 30 times lower than the theoretical tensile strength of the tow, given by the manufacturer.
Fig. 27. Influence of processing speed on pulling force.
The required pulling forces are significantly increasing if matrix is present on the surface of the impregnation wheels. With PA12 matrix, the forces are noticeably above the ones for PP. However, the course of pulling force over the variation of different parameters is qualitatively similar, also without melt flow (Figs. 27 and 28). While there is a linear correlation with the processing speed, for the roving brake force there is a slightly progressive rise in pulling force. Lowering the viscosity of the melt by higher temperature leads to increasing pulling forces, as can be seen in Fig. 9. The latter were, on the other hand decreasing with increasing fibre volume content, as it was already expected from Figs. 27 and 28 (Fig. 29). The pulling forces which cause visible fibre damage lie at about 160 N if the polymer matrix is present. This is significantly higher than observed for fibre bundles without a polymer matrix. It can therefore be concluded that a thermoplastic melt in the presence of a fibre bundle favours the stress distribution between the filaments and protects them from an excessive frictional failure due to the contact with the porous metal impregnation wheel.
Fig. 29. Microscopy of samples produced under optimum processing conditions.
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Fig. 30. Filament wound tubes with online impregnation.
6. Summary and outlook
Acknowledgements
It could be shown that with the process technology developed it is possible to realise thermoplastic filament winding with online impregnation of fibre bundles, so as to realise fibre reinforced tubes at competitive processing speeds. Fig. 30 shows examples of the tubes which have been produced. At present, however, technological restraints regarding the processing speed have to be accepted compared to thermoplastic tape winding. In spite of this, the assessment of economic efficiency (in part A of this paper) has shown that reasonable speeds at the limit for profitableness were achieved [17]. In particular, a process combination is advantageous if no preimpregnated tapes are commercially available. The present restrictions of winding speeds have to be understood with respect to both the difficulties related to the of pulling forces, and the early stage of development of this technology. This dilemma arises from the conflict between the required roving pretension and the generated pulling force increasing with processing speed. For future applications, this problem can be evaded by an altered construction, using driven impregnation wheels, as it was demonstrated by Lutz [6] in the present state of development of the impregnation wheel technology for production of flat thermoplastic tapes and circular shaped rods.
The authors wish to thank the German Science Foundation (Deutsche Forschungsgemeinschaft) for the financial support granted to conduct this study (DFG FR 675/23-3). Further we also appreciate the help of Alena Korzikava, who held a DAAD fellowship, for preliminary experiments.
References [1] [2] [3] [4]
[5] [6]
[7] [8]
[9]
NN. Product information. Mainz, Germany: Targor GmbH. NN. Product information. Marl, Germany: Degussa-Huels AG. NN. Product information. OH, USA: Owens Corning. Voigtla¨nder R. Herstellung und Charakterisierung von gekru¨mmten Thermoplastwickelverbunden unter dem Aspekt der Werkstoffoptimierung. PhD thesis. Technical University, Dresden; 1997. Michaeli W, Wegener M. Einfu¨hrung in die Technologie der Faserverbundwerkstoffe. Munich: Hanser; 1990. Lutz A. Beitrag zur Entwicklung innovativer Fertigungstechniken fu¨r die Verarbeitung thermoplastischer Faserverbundwerkstoffe. Berlin: Mensch und Buch; 1999. Simult.: PhD thesis, Kaiserslautern; 1999. Friedrich K. Mesoscopic aspects in polymer composites: processing, structure, properties. J Mater Sci 1998;33:5535 –56. ASTM D2290-92. Standard test method for apparent tensile strength of ring or tubular plastics and reinforced plastics by split disk method; 1992. Skop-Cardarella K, Ma¨der E, Za¨h W, Mayer K. Textile Fla¨chengebilde fu¨r low-cost Thermoplastverbunde. Proceedings of Verbundwerkstoffe und Werkstoffverbunde, Kaiserslautern; 1997. p. 367– 71.
1688
F. Henninger et al. / Composites: Part A 33 (2002) 1677–1688
[10] Bechthold G. Pultrusion von geflochtenen und axial verstrkten Thermoplast-Halbzeugen und deren zersto¨rungsfreiePorengehaltsbestimmung. Kaiserslautern: Institut fu¨r Verbundwerkstoffe GmbH; 2000. Simult.: PhD thesis, Kaiserslautern; 2000. [11] Kerbiriou V, Friedrich K. Heizpressen und Pultrusion von Thermoplastpulverimpra¨gnierten und-umhu¨llten Glasfaserbu¨ndeln. Proceedings of Polymerwerkstoffe 96, Merseburg; 1996. p. 184– 7. [12] Lauke B, Schneider K, Friedrich K. Interlaminar shear strength measurement of thin composite rings fabricated by filament winding. Proceedings of the ECCM-5, Bordeaux; 1992. p. 313–8. [13] Lauke B, Scho¨ne A, Friedrich K. In: Chandra T, Dhingra AK, editors. High performance thermoplastic composites fabricated by filament winding. Proceedings of International Conference on Advanced Composites, Wollongong, Australia; 1993. p. 883 –9.
[14] Hoecker F. Grenzfla¨ cheneffekte in Hochleistungsfaserverbundwerkstoffen mit polymeren matrices. Du¨ sseldorf: VDI; 1996. [15] Hampe A, Hennecke M, Mielke W. Pru¨fmethoden fu¨r Polymerwerkstoffe. Materialpru¨fung 1992;34(7/8):228–31. [16] Ma CM, Yn M. Poly(1-caprolactam)-poly(butadiene-co-acrylonitrile) block copolymers. II. Processability and properties of pultuded glass-fiber-reinforced composites. Mater Chem Phys 1995;42:17–24. [17] Henninger F, Friedrich K. Thermoplastic filament winding with online-impregnation. Part A.:Process technology and operating efficiency. Composites, Part A 2002; 31/11: 1479–86.