Effect of injections on the orientation of short fibre composites. An optical microscopic analysis

Polymer Tesring 15 (1996) 467475 0 1996 Ekvier Science Ltd Printed in Great Britain. All rights reserved 0142-9418/96/$15.00

SO142-9418(96)00007-4 ELSEVIER

MATERIAL CHARACTERISATION Effect of Injections on the Orientation of Short Fibre Composites. An Optical Microscopic Analysis R. A. CorrEa,G* R. C. R. Nune9

& W. Z. Franc0 Filhob

“Institute de MacromolCculas Professora Eloisa Mano, Universidade Federal do Rio de Janeiro, PO Box 68525, 21946-970, Rio de Janeiro, Brazil “COFADE - Sociedade Fabricadora de ElastBmeros Ltda, S%o Paula, Brazil (Received

3 November

1995; accepted 22 January 1996)

ABSTRACT Short jibre reinforced thermoplastic composites were studied using polyurethane as the matrix and aromatic polyamide, and carbon fibre or regenerated cellulose as the structural component. For each type offibre, the amount was varied and mechanical properties (tensile and abrasion resistance) and optical microscopy were investigated from injected sheets. The tensile resistance results were in accordance observed

with the surface

light reflected photomicrographs.

with those

Arithmetic average

values of tensile resistance for samples taken at different directions of stress were close to the values observed for injected pieces in those regions where the fibrillar arrangement in the matrix was random. In general, the abrasion resistance was also improved. Copyright 01996

Elsevier Science Ltd

1 INTRODUCTION Short-fibre reinforced thermoplastic composites have gained importance because of their advantages in processing, improvement of anisotropic properties and ease of dispersion.’ The mechanical parameters of these materials are difficult to foresee principally when moulding processes like extrusion and injection are used, which may cause orientation effects in the fibrillate composite. It has been noted that the type of mould and injection point are both responsible for technical properties including anisotropy.’ *To whom correspondence

should be addressed.

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Mathematical models have also been reported in order to study the influence of fibre orientation on mechanical properties3.4 as well as the influence of the fibre variables like type, content and aspect ratio.5-7 The effect of milling parameters on the fibre orientation has also been reported.8 In this paper, three types of fibres were studied, using thermoplastic polyurethane (PU) as the matrix. For each type, the fibre amount was varied and the properties were compared with results for the pure PU. The composites were submitted to tensile and abrasion tests and to optical microscopy to investigate the aspects of the fibres at the surface. Anisotropy analysis was carried out at different positions of the injected specimen.

2 EXPERIMENTAL 2.1 Processing The fibres used were: aromatic polyamide, AF (Twaron, Akzo Nobel Ltda, Sao Paulo, Brazil); carbon fibre, CF (FC140/90)-R33, Fractual Technology Ltda, Sao Paulo, Brazil); and regenerated cellulose, RC (Rhodia Indtistrias Quimicas e Texteis, Sao Paulo, Brazil). The ester based thermoplastic polyurethane, Elastollan, was provided by Cofade, Sociedade Fabricadora de Elastomeros Ltda (Sao Paulo, Brazil). In this work 10, 20 or 30 parts of the different fibres were incorporated into the PU matrix by dry blending at the beginning, followed by extrusion in a Brabender plasticorder type GNF 10612, using a single screw extruder (L/D=25) at 30 r-pm. The processing was performed at temperatures of 130°C at zone 1, 140°C at zone 2, 150°C at zone 3 and 160°C at the circular die of 3.1 mm diameter. The extrudate was chopped into 3 mm pieces and dried in a hot air oven at 100°C for 2 h before use. Sheets were injection moulded using a conventional machine (Ferbate, BSKF 80-100). The processing conditions were: injection pressures, 4900 GPa (PU and CF), 5.884 GPa (AF and RC); holding pressure, 2.492 GPa; back pressure, 1.961 GPa; injection time, 50 s, and cooling time, 20 s. The processing temperatures were: 165°C at zones 1, 2 and 3 (except for composites with regenerated cellulose, namely RClO, RC20 and RC30; for these composites, temperatures were 170°C at zones 1 and 2, and 175°C) and approximately 35°C in the mould. All the injected materials were crosslinked in a hot air oven at 100°C for 20 h.

Injections on the orientation

of short fibre composites

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2.2 Optical microscopy The photomicrographs were obtained on an optical microscope (Zeiss Axioplan) using a reflected light technique and, for the composite with carbon fibres, (CF) polarized light was used. All specimens were examined through a magnification of 100X. The specimens were cut from different positions, A-G in Fig. l(a), the injection point being indicated by I. The anisotropy developed in the moulded sheet due to the preferred orientation of the fibres was investigated as indicated in Fig. l(b) by measuring the angle between the fibre preferential orientation and the sheet longitudinal axis. 2.3 Tensile test All tests were conducted in a universal testing machine, according to DIN 53504. Following industrial procedures, which are a modification of the DIN test, the samples were cut from the moulded positions l-4, as shown in Fig. 2 which also shows the external dimensions of the piece in mm. 2.4 Abrasion test The abrasion measurements followed the DIN 53516 procedure. The specimens were taken from positions 5 and 6 in the moulded piece; their dimensions are shown in Fig. 2.

(a)

0))

Fig. 1. Injected piece showing: (a) the different positions from where the specimens were obtained for optical microscopy. I is the injection point. (b) The angle between the fibre preferential orientation and the longitudinal axis.

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radius a

\ thick 2

Fig. 2. Injected piece showing different positions from where the specimens were obtained for mechanical properties analysis. (Dimensions given in mm.)

3 DISCUSSION The best images from optical microscopy were obtained for those composites with carbon fibre (CF) due to the contrast of the components. The photomicrographs of CF20, Figs 3(a-g), show that there is an influence, depending on the position in the moulded sheet from where the sample was cut, according to Fig. l(a). Figure 4 represents the measured angle between each different direction used for microscopic analysis and the longitudinal axis of the moulded for the composites CFlO, CF20 and CF30. The values in parentheses are the standard deviation and the positions asterisked show random disposition of the fibres. The optical microscopic analysis allows us to ensure that fibre alignment varies depending on the position inside the mould, influenced by the flow induced at the injection point. Figures 3(a) and (b) are photomicrographs of material from positions A and B, respectively, taken according to Fig. l(a). In these figures an alignment parallel to the considered longitudinal axis is seen. Figures 3(c) and (d) correspond to the positions F and G in Fig. l(a). The photomicrographs show the oblique arrangement of fibres, close to a symmetry, with respect to the considered axis. With the objective to investigate the effect of injection flow in the injected specimen, Figs 3(e-g), corresponding to intermediate positions, respectively C, E and D, was looked at. From these figures, it is possible to confirm that for the divergent flow, responsible for the arrangement of the fibres and emphasized at the position D, Fig. 3(g), a random distribution prevails.* It was shown earlier that the increase in the fibre content causes a predominance of parallel arrangement.9 This hypothesis is supported by the decrease in the

Injectionson the orientation

of short /ibre composites

Fig. 3. Photomicrographs of material from positions (a-g) according to Fig. l(a). (a) position A, (b) position B, (c) position F, (d) position G, (e) position C, (f) position E, (g) position D.

R. A. Corrt!a et al.

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Fig. 4. Measured angle between different directions. Standard deviations are in parentheses and the asterisk refers to the points with random distribution.

standard deviation results (Fig. 4) which are smaller as the filler content increases, indicating a small dispersion in the values of the angles measured by optical method. The effects brought by such a variety of alignments assumed by the fibres can be visualized through tensile tests, as shown in Fig. 5. Experimental data for the different specimens are plotted as a function of the position, according

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I

I

I I

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positions seen in fig 2 Fig. 5. Tensile strength as function of the positions from where the specimens were cut.

injectionson the orientationof short fibre composites

473

to Fig. 2, occupied by each specimen in the moulded piece. TO approach a representative tensile strength value, arithmetic averages were calculated taking into account the contribution for the tensile strength of all specimens in each different position. These averages also shown in Fig. 5, are close to the values obtained for the samples from positions 2 and 3 and, due to this fact, they are plotted between these positions, where the fibrillar arrangement in the PU matrix is random. As seen before, the composites with regenerated cellulose or polyamide fibres did not have sufficient contrast to be analysed by optical microscopy. Nevertheless, the tensile strength results for these materials are similar to those for compositions with carbon fibre. The best results shown in Fig. 5 are for the sample from position 1 (Fig. 2), where fibres are aligned according to the direction of the applied tension. In this case the smallest value corresponds to position 4, thus corroborating the influence of the fibres direction on the properties. Comparing all the average results for the composites to those for the matrix, it is possible to say that three composites, AR20, AR30, and CF30, show reinforcing behaviour. The graphic analysis (Fig. 5) shows that, as expected, the tensile strength decreases as the angle between the fibre alignment and the direction of the applied load increases. Figure 6 shows the volume loss obtained in the abrasion test, for each amount of incorporated fibre. It can be seen that fibres are arranged in plans parallel to the wear promoted by the abrasion test. By increasing the fibre content, the abrasion resistance is improved (the loss volume is smaller). The results show the fragility of the CFIO, CF20 and CF30, characteristic of the breakable behaviour of carbon fibres in opposition to the AF composites. The scanning electron microscopy technique allows us to say that this type of polyamide splits apart, causing interaction of the fibre with the matrix, resulting in better resistance. lo Compositions RClO, RC20 and RC30 show intermediate behaviour and can be considered an alternative solution. 4 CONCLUSIONS l

l

l

The tensile strength test suggests that the results should be observed with caution due to the anisotropy of the injected piece. The arithmetic averages are close to the values obtained for the samples from positions 2 and 3, where the fibrillar arrangement in the PU matrix is random. The abrasion resistance, typical of polyurethanes, has improved values due to the incorporation of the studied fibres. It was seen that the injection point is fundamental to short fibres composites.

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R. A. Corrc?aet al. 300.00

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Fig. 6. Abrasion resistance-volume

l

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loss as function of fibre concentration.

The optical microscopy shows the existence of anisotropy in the injected piece which corroborates the mechanical properties information.

ACKNOWLEDGEMENTS The authors wish to thank Dr Regina Sandra S.V. Nascimento, the technicians Narciso and Ana Paula for assistance with the optical microscopy, and to CAPES, CNPq and CEPG/UFRJ for financial support.

REFERENCES 1. Goettler, L. A., Short fiber composites. In Handbook of Elastomers, Eds H. K. Bromwich & H. L. Stephen, Marcel Dekker Inc., New York, 1988. 2. Milewsky, J. V., Whiskers and short fiber technology. Polymer Composites, 13 (1992) 223-236.

3. Chin, W. K.;Liu, H. T. & Lee, Y. D., Effects of fiber length and orientation distribution on the elastic modulus of short fiber reinforced thermoplastics. Polymer Composites, 9 (1988) 27-35. 4. Kocsis, J. K., Microstructural aspects of fracture and fatigue behavior in short

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5. 6. 7.

8.

9.

10.

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fiber reinforced injection molded PPS-PEEK and PEN-composites. Polymer Buletin, 27 (1991) 23-29. Bay, R. S. & Tucker III, C. L., Fiber orientation in simple injection moldings. Part I: theory and numerical methods. Polymer Composites, 13 (1992) 3 17-33 1. Bay, R. S. & Tucker III, C. L., Fiber orientation in simple injection moldings. Part II: experimental results. Polymer Composites, 13 (1992) 332-341. Matsuoka, T., Takabatake, J.., Inoue, Y. & Takahashi, H., Prediction of fiber orientation in injection molded parts of short fiber reinforced thermoplastics. Polymer Engineering and Science, 30 (1990) 957-966. Berflisson, H., Franze, B., Klason, C., Kubat, J. & Kitano, T., The influence of processing on fiber orientation and creep in short carbon fiber reinforced low density polyethylene and polycarbonate. Polymer Composites, 13 (1992) 121131. Corrsa, R.A., Composites de poliuretano elastomerico corn fibras curtas, MSc. Thesis, Institnto de Macromoleculas Professora Eloisa Mano, Universidade Federal do Rio de Janeiro, Brazil, 1994. O’Connor, J.E., Short fiber elastomer composites. Rubber Chemistry and Technology, 50 (1977) 945-958.