Analysis of the creep behavior of polypropylene and glass fiber reinforced polypropylene composites Péter Bakonyi 1, a, László Mihály Vas 1,b

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1 Materials Science Forum Vol. 729 (213) pp (213) Trans Tech Publications, Switzerland doi:1.428/ Analysis o the creep behavior o polypropylene and glass iber reinorced polypropylene composites Péter Bakonyi 1, a, László Mihály Vas 1,b 1 Department o Polymer Engineering, Budapest University o Technology and Economics, 1-3 Műegyetem rkp., 1111 Budapest, Hungary a bakonyi@pt.bme.hu, b vas@pt.bme.hu Keywords: Non-linear viscoelastic, creep, polypropylene, composite. Abstract. In this paper tensile and creep tests were perormed on polypropylene (PP) and its glass iber reinorced composites. The tensile tests were carried out on 6 dierent glass iber content reinorced PP composites (, 5, 1, 2, 3 and 4%) while the creep tests were perormed on the unreinorced and 3% and 4% iber reinorced ones o industrial importance. 5 N/s constant orce rate was used until the specimen ailed (tensile test) or the preset load level was reached (creep test). The applied load levels or the creep experiments were determined as given ratios o the average breaking orce. The tensile breaking strain and tensile strength versus iber content relationship were analyzed and described by empirical ormulas based on the correction and averaging procedure developed. Introduction Thermoplastics and its short iber reinorced composites are widely used in various applications. The mechanical properties o these materials depend on their structure and environmental actors such as temperature, humidity and time. That means thermoplastics show considerable creep and stress relaxation even at moderate temperatures which makes the designer's work rather diicult. Furthermore, there are ew data available on long term behavior because these measurements are much more expensive and time-consumed than traditional mechanical tests, while in general, designing parts o long duration requires the knowledge o creep behavior. In the last decades numerous methods have been worked out to describe the creep behavior o thermoplastics. The relationship between stress relaxation, creep and tensile behavior o polymer materials and the eects o environmental parameters such as temperature and humidity have been studied or a long time. Among others or example Urzumtsev and Maksimov summarized the results o their comprehensive investigations o several decades in their book [1], some other theoretical models and experimental results can be ound in Retting s book [2]. As a novel approach Grzywinski and Woodord [3] predicted the creep behavior rom stress relaxation experiments. Vas and Nagy [4] investigated the relationship between the constant strain rate tensile test and stress relaxation behavior o PP. One o the most oten applied methods is based on the linear viscoelastic theory o polymers and the time-temperature equivalence. These methods are based on short or medium term creep measurements at only one load level, but at dierent temperatures, where the long term behavior can be predicted with the master curve method like in the recent papers by Sakai and Somiya [5] or by Dorigato et al. [6]. Ater having studied the creep behavior o the unreinorced PP [7] and the possibilities o predicting that based on short term measurements, this paper presents the results o tensile and creep tests o PP and its glass iber reinorced composites using constant orce rate or loading. During the creep tests the same orce rate was used till reaching the preset load level. These measured curves were corrected and averaged in order to have base curves or an analysis and a special non-linear viscoelastic estimation o the creep curve planned. All rights reserved. No part o contents o this paper may be reproduced or transmitted in any orm or by any means without the written permission o TTP, (ID: /8/12,9:44:3)

2 Materials Science Forum Vol Materials and methods As unreinorced material and as matrix o the glass iber reinorced composites an isotactic polypropylene homopolymer (Tipplen H 949A rom TVK, Hungary) was used. The reinorcement was glass iber treated by silane based sizing (Thermo Flow EC rom Johns Manville, USA) having a nominal ilament diameter o 13 µm and a chopped strand length o 4 mm. Two percent o the iber content maleic anhydride grated polypropylene (Orevac CA1 rom Arkema, France) was used as adhesion promoter o the iber reinorced materials. Unreinorced, 5, 1, 2, 3 and 4% glass iber reinorced granulates were produced on Brabender Plastograph extruder machine. Type 1A dumbbell specimens o length 148 mm, width 1 mm and thickness 4 mm were injection molded according to the ISO on an Arburg Allrounder 32 C-GE 5-17 injection molding machine. A Zwick Z-5 universal testing machine equipped with a 5 kn nominal capacity standard load cell was employed or the tensile and creep tests. The gauge length was 1 mm long so the elongation in millimeter equals the strain in percent numerically. 5 N/s constant orce rate was used until the specimen ailed (tensile test) or the preset load level was reached (creep test). At creep measurements ater the load had reached the preset level by constant rate load it was held constant or 1 hours and using the crosshead signal the length variation o the specimen was recorded. The applied load levels in the creep experiments were 4, 3, 2 and 1 percent o the average breaking orce at each iber content levels. In the Scanning Electron Microscope (SEM) images a good adhesion between the PP matrix and glass ibers can be observed, that was due to the proper connection between the matrix and the glass ibers provided by the maleic anhydride grated PP in the matrix and the silane based sizing material on the surace o the glass ibers (Fig. 1). In Figure 2 a crazing phenomena can be seen on the racture surace which results in voids o about 5-3 µm size. Fig.1 Good adhesion can be observed between Fig.2 Crazing phenomena on the racture surace the polypropylene matrix and glass ibers o the glass iber reinorced PP composite Results and discussion At the start o the tensile or creep test due to the inertia o the drive system o the tester and the time-delay o the data collection a zero-point error can be ound. It can cause serious diiculties in calculating the correct mean curves. In order to obtain correct averaged characteristics we need to shit the curves in the unction o time one by one till the initial tangents o the measured strain-time curves are crossing exactly the origin (Fig. 3) and ater the shiting we need to correct the irst points o the curves at beginning section are to be replaced by those o the initial tangent (Fig.4). The average tensile curves calculated in this way show indeterminate strain values in the breaking range (Fig. 5) that we have to correct and smooth by shiting the end section o the strain-time curve. The measured creep curves were averaged point by point in the time in the above written way.

3 34 Materials Science, Testing and Inormatics VI Strain[mm] Strain (shited) Strain (original) orce (shited) orce (original) Initial tangent orce [N] Strain[mm] Strain (corrected) orce (corrected) orce [N] Test time [s] Fig.3 Shiting the curves by the initial tangents zero-point delay to the origin Test time [s] Fig.4 Corrected beginning points o the constant orce rate tensile and creep curves Fig.5 Indeterminate strain values at the breaking Fig.6 Corrected and averaged tensile curves range o shited curves and breaking points The average tensile curves were calculated rom 3 constant orce rate tensile measurements (Table 1; Fig. 6). In the unction o the iber content the mean breaking strain shows a hyperbolalike decrease, while the test time and the breaking orce proportional to the ormer increase with the iber content. We have taken into account that in the case o iber reinorced composites there are two dierent eects on the breaking elongation because o the two components. We assume that one o them is that o the parts o the strongly extendable matrix material, which can be ound relatively distant rom the ibers hence they can be deormed relatively reely, and this eect decreases with the iber content because the breaking elongation o ibers is much less. The other is that o those parts o matrix material which orm the direct vicinity o the ibers and are bonded to them tightly. The latter eect is weaker on the elongation o the composite and it is supposed to decrease with the iber content, as well. This two eects can be described by a ormula (Eq. 1) ormed by the sum o two exponential unctions ( ε 1 ( ) and ε 2( )) and a constant ( ε )(Fig.7): ε 1 2 ( ) ε + ε ( ) + ε ( ) = ε + ε e + ε e = (1) where [m%] is the iber content, ε = 2.8 %, ε = 6.367%, ε = 2%, % = 2, and 2 =18.65% are constants itted with least squares method. 2

4 Materials Science Forum Vol Table 1 Mean strain and breaking orce at orce controlled tensile tests Fiber content strain [mm] deviation o strain [mm] breaking orce [N] deviation o breaking orce [N] Strain ε +ε 1 +ε 2 strain strain at breaks ε= l Fitted [mm] strain curve: ε(φ ) g1ε 1 (φ ) g2ε 2 (φ ) ε ε ε φ φ 2 Glass iber content Fig.7 strain and its itted curve rom two dierent eects in the unction o iber content In the case o the breaking orce the measured mean values (F s (φ )) versus iber content (φ ) relationship could be described by a so called logistic curve (Eq. 2, Fig.8), as ollows: F s ( ) = + F t a ( t t 1) e 1 (2) where F = 5 N/s is the constant rate load, t, t, and a are itted constants and t = 32.1 s is or the unreinorced PP, t = 93 s is a kind o asymptotic value, while a =.634. Fig.8 d breaking orces and its itted curve in the unction o the glass iber content Fig.9 Bundles o shited and corrected creep curves o the unreinorced PP material The average creep curves were computed rom 5 creep tests perormed at 5 load levels or unreinorced and at 6 load levels or 3% or 4% iber reinorced composites (Fig. 9-11, Table 2.). The averaged and corrected tensile and creep curves are satisactory smooth without breaks or jumps hence they seem to be a good base o predicting the long term creep behavior o the polypropylene and the reinorced composites with dierent iber contents (Fig ) by using regression method and non-linear viscoelastic estimation [7].

5 36 Materials Science, Testing and Inormatics VI Fig.1 Bundles o shited and corrected creep curves o the 3% GF reinorced composites Fig.11 Bundles o shited and corrected creep curves o the 4% reinorced composites % o the average breaking load Table 2 Mean strain at the end o the 1 hour test time at various iber contents and loads Nominal initial load [N] Unreinorced 3% iber reinorced 4% iber reinorced strain deviation o strain Nominal initial load [N] strain deviation o strain Nominal initial load [N] strain deviation o strain The average creep-strain curves have shown a progressive growth in the unction o the nominal initial load, and the dierence between creep behavior o the unreinorced and the two reinorced materials can be well observed in Figure 15, where the mean strain measured at 1 hour test time versus nominal initial load is depicted. Fig.12 Corrected and averaged tensile and creep curves o the unreinorced PP material Fig.13 Corrected and averaged tensile and creep curves o 3% GF reinorced composite

6 Materials Science Forum Vol Fig.14 Corrected and averaged tensile and Fig.15 The average strain at 1 hour creep test creep curves o 4% GF reinorced composite time in the unction o the nominal initial load Summary Tensile and 1 hour creep tests were perormed on unreinorced and glass iber (GF) reinorced polypropylene (PP) samples at dierent load levels in a time range o about ten hours. The procedure presented is suitable to generate smooth. The sum o two exponential expressions was itted to describe the hyperbola-like decrease o the breaking strain as a unction o the iber content. The monotonous increase relationship between the mean breaking orce and the iber content was approximated by a logistic curve. The corrected and smoothed mean curves and the itted ormulas both containing exponential expressions orm a good base or the urther estimating o the long term creep behavior. Acknowledgment This work is connected to the scientiic program o the "Development o quality-oriented and harmonized R+D+I strategy and unctional model at BME" project. This project is supported by the New Széchenyi Plan (Project ID:TÁMOP-4.2.1/B-9/1/KMR-21-2). The work reported in this paper has been developed in the ramework o the project "Talent care and cultivation in the scientiic workshops o BME" project. This project is supported by the grant TÁMOP B- 1/ The work was supported by a grant rom the Hungarian Scientiic Research Fund OTKA K68438 and K1949 as well. Special thanks or Laszlo Meiszel and the Qualchem zrt or the indispensable help at acquisition the raw materials. Reerences [1] J.S. Urzumtsev, R.D. Maksimov, Deormation o Plastics, Prognostics on the Basis o Similarity Principles (in Hungarian), Műszaki Könyvkiadó, Budapest, [2] W. Retting, Mechanik der Kunststoe, Hanser Verlag, München, [3] G.G. Grzywinski, D.A. Woodord, Creep analysis o thermoplastics using stress relaxation data, Polymer Engineering and Science 35 (1995) [4] L.M. Vas, P. Nagy, Investigating the time dependent behavior o thermoplastic polymers under tensile load. Macromolecular Symposia, 239 (26) [5] T. Sakai, S. Somiya, Analysis o creep behavior in thermoplastics based on visco-elastic theory, Mechanics o Time-Dependent Materials, Vol. 15 Nr. 3 (211) [6] A. Dorigato, A. Pegoretti, L. Fambri, C. Lonardi, M. Šloul, J. Kolařik, Linear low density polyethylene/cycloolein copolymer blends. express Polymer Letters 5(1) (211) [7] L.M. Vas, P. Bakonyi, P. Nagy, Investigation o the creep behavior o PP and its estimation rom constant orce rate tensile measurements. 6th Conerence on Mechanical Engineering, Budapest, 28.