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wood flour composites
Advanced Industrial and Engineering Polymer Research 2 (2019) 136e142
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Fatigue analysis and fatigue reliability of polypropylene/wood flour composites Md Minhaz-Ul Haque a, b, *, Koichi Goda a, Shinji Ogoe c, Yuta Sunaga d a
Department of Mechanical Engineering, Yamaguchi University, Ube, Yamaguchi, 755-8611, Japan Department of Applied Chemistry and Chemical Engineering, Islamic University, Kushtia, 7003, Bangladesh c Technology Development Centre, TOCLAS Co., Shizuoka, 432-8001, Japan d Kayaku Akzo Co., Sanyoonoda-City, Yamaguchi, 757-0002, Japan b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 March 2019 Received in revised form 22 April 2019 Accepted 4 July 2019
Fatigue analysis and fatigue reliability of polypropylene (PP)/wood flour (WF) composites were studied. The composites were prepared by incorporating wood flour with and without maleic anhydride and peroxide (MAPO) into cellulose nanofibres impregnated polypropylene. The prepared composites were then characterized by tensile and fatigue analyses. The tensile strength and fatigue life of MAPO mixed composites were lower compared with the composites without MAPO. It was also found that the fatigue experimental data of the composites were widely scattered regardless of the type of composites. Hence, in this study, a fatigue reliability of the composites was sought. Based on the fatigue experimental data, 95% confidence band were created. Since 95% confidence lower band ensure 95% survivability and only 5% failure of the composites, hence, from this study, it is suggested that the fatigue life obtained from 95% confidence lower band can be used as a material reliability index for safe fatigue design of the composites. © 2019 Kingfa SCI. & TECH. CO., LTD. Production and Hosting by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).
Keywords: Melt-viscosity Wood plastic composite (WPC) Fatigue behavior Wood flour Polypropylene
1. Introduction Application and demand of wood-plastic composite (WPC) in different sectors such as automotive industries, office appliances, housewares, furniture, outdoor deck floors, etc. [1e5] are increasing each year due its cheapness and eco-friendly character. The major components of WPC are wood flour and mainly thermoplastic polymers such as polyethylene, polypropylene, polyvinyl chloride etc. Wood flour as an inexpensive filler can reduce the cost of WPC largely [1]. Presence of wood flour into polymer matrices can also impart its physical and mechanical properties such as high strength and stiffness to the final composite materials. Polypropylene (PP)/wood flour (WF) composite, a typical WPC, is basically made of polypropylene matrix and wood flour. In the fabrication of PP/WF composite, another important ingredient, namely, maleic anhydride grafted polypropylene (MAPP), is usually used as a compatibilizer [6e11]. PP/WF compositess have drawn
* Corresponding author. Department of Mechanical Engineering, Yamaguchi University, 2-16-1 Tokiwadai, Ube, Yamaguchi, 755-8611, Japan. E-mail address: [email protected] (M.M.-U. Haque).
the attention of WPC companies because of the free flowing nature of WF compared with the natural fibres [12] as well as scope of fabrication from recycled wood and recycled polypropylene [13e17]. In this study, PP/WF composites were fabricated by incorporating wood flour with and without maleic anhydride and peroxide (MAPO) into cellulose nanofibres (CNF) impregnated polypropylene. Cellulose nanofibres was impregnated into PP matrix to increase the interfacial interaction among composite components. The advantages of addition of CNF into PP polymer had already been reported by Suzuki et al. [18,19]. In our previous study, a higher fatigue life of wet pulverized WF reinforced PP composite was reported [20,21]. Indeed, fibrils were generated on wood flour particles surfaces by the wet pulverization. An improved fatigue life of PP/WF composite was also claimed due to a higher crack deflection behavior of fibrillated WF. The improved fracture toughness and fatigue life of various nanoparticles incorporated polymer systems had already been reported by several researchers. As for examples, improved fatigue life and fracture toughness of epoxy composites were found by incorporating carbon nanotube [22], multi-walled carbon nanotube [23,24], graphene nanoparticles [25,26] etc.
https://doi.org/10.1016/j.aiepr.2019.07.001 2542-5048/© 2019 Kingfa SCI. & TECH. CO., LTD. Production and Hosting by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
M.M.-U. Haque et al. / Advanced Industrial and Engineering Polymer Research 2 (2019) 136e142
Although the demand and application of PP/WF are increasing each year, use of PP/WF in structural applications faces a great challenge because of their fatigue reliability under different load conditions. Structural materials are susceptible to fatigue failure below the tensile strength of that material. Hence, for safe fatigue design, the study of fatigue behavior of the composites is very important, particularly if PP/WF composites are subjected to use in long-term load bearing applications. Since materials deformation mechanism under short-term and long-term mechanical loading is not always the same, and only the tensile test is not sufficient to do a prediction of their long term load bearing application [27]. Generally, fatigue strength depends on the tensile strength of composite materials and composite materials with higher tensile strength exhibited higher fatigue strength [28,29], but this was not always happen, particularly at low cyclic stress. Liang et al. [30] and Shivakumar et al. [31] reported that the higher tensile strength of composites became less significant when cyclic stress was decreased i.e. at low cyclic stress some composite materials were less sensitive to the fatigue damage. Hence, the study of fatigue analysis and fatigue reliability of PP/WF is utmost necessary for safe fatigue design. Prediction of fatigue life of composite materials is also a critical issue as fatigue experimental data of composites are widely scattered. Crack propagation rate for a uniform material was predicted by a linear elastic fracture mechanics of a single through-crack. However, in composite material, fatigue occurs by fibre breakage, matrix cracking, crazing, matrix-fibre debonding, delamination etc. Moreover, in a composite, due to its heterogeneity, cracks were spread to the entire area and some cracks could not propagate over large distance as they were deflected/arrested by hard filler [32]. Thus, due to the heteroginity of composites the fatigue experimental data of composites are widely scattered. The main objective of this study was to find out a fatigue reliability of PP/WF composites. In this study, effect of MAPO mixing on the fatigue life of PP/WF composites was also investigated. Maleic anhydride has two functional groups: anhydride group and double bond. So, each molecule of maleic anhydride has possibility to be bonded with PP radical as generated by peroxide through double bond [33,34] as well as to be bonded with WF surfaces through anhydride group [35,36]. Based on this reaction probability, it was expected that the resultant composite system would exhibit an improved fatigue performance due to a higher components compatibilization. Concerning the fatigue reliability of the composites, since fatigue experimental data are widely scattered, it is necessary to find out absolution that can be employed to calculate the safe fatigue design of the composites. In this study, the experimental data were fitted with a logarithmic linear line by a regression technique. Based on the fatigue testing data, 95% confidence band were also created. Since 95% confidence lower band ensure 95% survivability and only 5% failure of the composites, hence, in this study, fatigue lives of composites at different fatigue strength, based on 95% confidence band, were calculated that can be used as a material reliability index for safe fatigue design of PP/WF composites. 2. Experimental 2.1. Materials Polypropylene (PP) pellet, trade name PPJ107G, melt flow index 30 g/10 min at 230 C/2.16 kg, density 0.9 g/cm3, melting point 150 C was received from Prime Polymer Co., Ltd., Tokyo, Japan. Maleic anhydride grafted polypropylene (MAPP) powder, trade name Kayabrid 006PP, maleic anhydride content 2 wt%, was received from Kayaku Akzo Co., Japan.
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Cellulose nanofibres (CNF) suspension (solid content 10 wt%), BiNFi-s WFo-10010, fibres diameter 10e50 nm, specific surface area 120 m2/g were supplied from Sugino, Japan. Wood flour (WF), trade name ARBOCEL®C100, bulk density 140e180 g/l, mesh number 100, particle size 70e152 mm was purchased from Rettenmaier & Sohne GmbH Co., Ltd., Rosenberg, Germany. Maleic anhydride, purity 99.5%, solidifying point 52 C, was purchased from Nippon Shokubai Co. Ltd., Japan. Organic peroxide, t-Butyl peroxy 3,5,5-trimethylhexanoate, purity 97 wt%, half life at 120, 140, 160, and 180 C, are about 1 h, 10 min, 1 min, and 15 s, respectively were purchased from, Kayaku Akzo Co., Japan. 2.2. Processing of the composites The PP/WF composites were prepared in three step processes. In the 1st step, cellulose nanofibres were dispersed into polypropylene matrix by mixing of CNF suspension (solid content 10 wt %) with MAPP and neat PP at 190 C in a mixer. The mixing ratio of PP, MAPP, and CNF was 90:5:5. In the 2nd step, mixture of PP/CNF (20 wt%) and wood flour (WF) (80 wt%) with and without maleic anhydride (2 phr) and peroxide (0.5 phr) was obtained by mixing them in a mixer at 120e180 C for 5e20 mins. In the 3rd step, the ground mixture were processed in a twinscrew extruder (AS30, Nakatani, machinery, Co., Ltd.) by the addition of required amount of neat PP so that the components composition wt% in each final composite become PP45/MAPP2.5/ CNF2.5/WF50. During processing of the composites, the temperature at different zones in the extruder were maintained as followings: (1) 165 C, (2) 200 C, (3) 215 C, (4) 200 C, (5) 190 C, and (6) 190 C. The screw speed of extruder was 85 rpm and total throughput was 9e12 kg/h. The processed composites based on different conditions were coded as reported in the Table 1. 2.3. Characterization techniques Tensile test of dumbbell shaped specimens, obtained by injection molding, of PP/WFcomposites was carried out by a Material Testing Machine, EHF-F1, Shimadzu, Japan. A strain gauge (Kyowa strain gauge, KFGS-2N-120-C1-11, Japan) of 2 mm with a gauge factor of 2.14 ± 1.0% was inserted on the surface (at the middle point) of each specimen. The specimens were put in a humidity control chamber at 25 C with 40% relative humidity (RH) for 5 days. The conditioned specimens were then tested using a load cell of 1 kN, gauge length of 18 mm and cross-head speed of 10 mm/min at 25 C and 40% RH. The average maximum strength of composites was calculated by performing the test on five specimens of each material. Fatigue tests of the composite specimens were carried out by the same instrument which was used for the tensile test. The fatigue test of the composites was also conducted at conditions of 25 C and 40% RH. The average maximum tensile strength value, obtained by tensile test, was used as a reference maximum stress level in estimation of the applied stress. The following test conditions such as tension-tension loading mode, stress ratio 0.1, frequency 3.5 Hz and applied stress: 90, 80, 70 and 60% level of ultimate tensile strength were applied in fatigue test. At each load, at least two specimens of the composites were tested. Measurements of rheological properties were performed using a capillary rheometer, LCR 7000, Dynixco, Japan. The diameter and length of the capillary were 2 mm and 8 mm, respectively. Tests were carried out at 180 C and at shear rates 100, 149, 223, 334 and
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Table 1 List of composite samples with their code. Sample code
A B C D E F G H
Mixing conditions Temperature ( C)
Time (min)
Maleic anhydride (phr)
Peroxide (phr)
140 140 120 140 160 180 180 180
10 10 10 20 10 10 5 20
2 2 2 2 2 2 2
0.5 0.5 0.5 0.5 0.5 0.5 0.5
500 s1. From this test melt-viscosity of PP/WF composites were measured. 3. Results and discussion 3.1. Tensile properties of the composites To determine the cyclic stress level of fatigue test, each composite sample was firstly subjected to the tensile test. Fig. 1 represents a stress-strain curves with an over imposed column diagram of tensile strength of neat PP and composites. The average tensile strength values and average toughness values calculated from the stress-strain curves of different PP/WF composites are also reported in Table 2. In Fig. 1, it is noticed that the MAPO containing composites BeH exhibited lower tensile strength compared with composite A. The lower tensile strength of MAPO containing composites BeH was probably due to the molecular degradation of PP polymer by MAPO [37]. Although a little influence of MAPO mixing temperature and time on the tensile strength of MAPO containing composites was noticed, MAPO mixing temperature and time changes did not show a higher value of tensile strength of the composites compared with the composite without MAPO. 3.2. Fatigue behavior and fatigue reliability of the composites Generally, composite materials display higher fatigue performance than that of its matrix material. Rigid filler particles have ability to reduce the degree of matrix deformation in front of the crack tip that propagates in soft matrices [32,38]. Our previous study [39] on fatigue analysis of PP/WF composites also showed a good agreement with the literature results. In this study, we
attempted to find a solution for fatigue reliability of PP/WF composites. To study the fatigue performance of the composites, maximum stress versus number of cycles to fracture (SeN) curves of the composites were plotted using the fatigue experimental data. The data were then fitted in a logarithmic linear regression equation as shown below:
smax ¼ b,logðNÞ þ c
(1)
where, smax is the applied maximum stress and N is a number of cycles to fracture. The values of b and c are constant and depend on the type of materials. For each and every composite material the values of correlation coefficient and R2 were found to be about 0.83 and >0.98, respectively. The value of correlation coefficient, 0.83 indicated a strong downhill (negative) linear relationship. The above regression equation (1) is similar to the fatigue model equation developed by Mandell [40,41]:
smax ¼ b,logðNÞ þ suts
(2)
where, suts is ultimate tensile strength. Fatigue strengths of different PP/WF composites at two different number of cycles were also calculated based on the equation (1) and the values are reported in Table 2. Fig. 2a shows SeN curves of neat PP and PP/WF composites with and without MAPO. As shown in Fig. 2, composites exhibited higher fatigue life compared with the neat PP. In Fig. 2a, it can also be noticed that composites without MAPO i.e. composite A exhibited higher fatigue life compared with MAPO containing composites B. In fibre reinforced composite, debonding of fibre/ matrix, fibre breakage, delamination and matrix cracking are the major damage mechanisms [38]. Maleic anhydride has two functional groups: anhydride group and double bond. A molecule of maleic anhydride has a possibility to be bonded with PP radical generated by peroxide through double bond as well as to be bonded withWF surfaces through the anhydride group. Consequently, the resultant composite system would exhibit an improved fatigue
Table 2 Tensile strength and fatigue strength of CNF impregnated PP/WF composites. Wood flour
PP A B C D E F G a
Fig. 1. Tensile strength of neat PP and PP/WF composites.
b
Toughness (MJ/M3)b
8.7 4.6 4.4 3.9 3.8 3.9 4.4 4.4
Tensile strength (MPa)
33.0 ± 0.4 46.9 ± 1.0 45.5 ± 0.4 45.3 ± 0.9 44.2 ± 0.2 44.5 ± 0.5 46.1 ± 0.5 45.1 ± 0.7
Fatigue strength (MPa) at different number of cycle (N)a 103 (N)
106 (N)
25.5 40.9 40.3 41.0 39.0 39.1 40.6 38.1
17.9 29.2 27.1 26.9 24.9 26.0 27.1 25.6
Calculated fatigue strengths based on equation (1). Toughness from the area of stress-strain curves.
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Fig. 2. SeN curves of neat PP and composites: (a) effect of MAPO, (b) effect of MAPO mixing temperature (c) effect of MAPO mixing time at 140 C, (d) effect of MAPO mixing time at 180 C.
performance due to the components compatibilization. However, the fatigue performance of the MAPO composites were lower compared with the composites without MAPO. Since the main components (PP, CNF and WF) and processing conditions were the same both in MAPO mixed as well as in non-mixed composites and maleic anhydride might also be grafted onto PP [34] that should have a positive effect on the fatigue life of composites, but, the fatigue performance of composites was not improved. Thus, this behaviour indicated that MAPO had played a serious role on the deterioration of fatigue performance of the composites. It had been reported that fatigue life increased with increasing molecular weight of polymer [42]. Since melt-processing of PP with MAPO causes chain scission of polypropylene [43,44], hence this lower fatigue life of MAPO containing composites compared with the composite A can be attributed to degradation of PP chain that increased the rate of matrix cracking. MAPO mixing temperature and mixing time on the fatigue life of composites were also investigated. Fig. 2b displays the SeN curves of composites B, C, E and F with MAPO mixing temperature 140, 120, 160 and 180 C, respectively. Composite E with MAPO mixing temperature 160 C exhibited the lowest fatigue performance as did tensile strength. Fig. 2c and d represent the effect of MAPO mixing time on the fatigue life of MAPO mixed composites at two different temperature 140 C and 180 C, respectively. In Fig. 2c, it can be noticed that composite B with MAPO mixing time 10 min displayed higher fatigue life compared with the composite D with MAPO mixing time 20 min. Whereas, at 180 C temperature composite F with MAPO mixing time 10 min displayed higher fatigue life compared with the composite G with MAPO mixing time 5 min. Although MAPO mixing temperature and mixing time had influenced on the fatigue performances of the MAPO mixed composites that indicated some interaction among maleic anhydride, PP and WF. However, variation of MAPO mixing temperature or time did
not show a higher fatigue life of the MAPO mixed composites compared with the non-mixed composite A. Fig. 3 represents the 95% confidence band of fatigue experimental data of neat PP, composite A, and composite B as examples. Since fatigue experimental data were widely scattered, therefore, based on the fatigue testing data, 95% confidence band of neat PP and composites were created using a SigmaPlot software. 95% confidence upper band indicated 5% survivability and 95% failure of the sample specimens, whereas 95% confidence lower band indicated 95% survivability and 5% failure of the sample specimens. From the 95% confidence band the fatigue strengths at different frequency were also calculated and the values are reported in Table 3. Plot of 95% confidence lower band of neat PP and PP/WF composites were created as shown in Fig. 4. Since 95% confidence lower band ensure 95% survivability and only 5% failure of samples [45]. Hence, from this study it can be suggested that the calculated data reported in Table 3 or the calculated fatigue life from 95% confidence lower band in Fig. 4 can be used as material reliability index with 95% of certainty for safe fatigue design. 3.3. Melt-viscosity of the composites To investigate the effect of MAPO on the fatigue performances of PP/WF composites the melt-viscosity of the composites were measured. Fig. 5 represents the melt-viscosity of PP/WF composites as a function of shear rates of 100e500 1/s. The melt-viscosity experimental data of the composites were fitted to the classical power law expression [46].
h ¼ ARh uðnRh Þ
(3)
where h represents the melt-viscosity of the composites and u represents the shear rate. In equation (3) ARh is a pre-exponential
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Fig. 3. SeN curves with 95% confidence band of fatigue experimental data of neat PP, composite A, and composite B. Table 3 Fatigue strength of the composites calculated from 95% confidence band (lower line - upper line values). Sample
PP A B C D E F G
Fatigue strength (MPa) at different frequency (N) 103
104
105
106
107
108
109
25.4e27.3 39.7e42.0 39.5e41.0 39.9e42.1 38.1e40.0 37.6e40.6 39.2e42.0 37.4e38.7
22.9e24.7 35.9e38.1 35.2e36.6 35.2e37.4 33.4e35.2 33.4e36.0 34.8e37.4 33.3e34.5
20.5e22.2 32.0e34.3 30.8e32.2 30.5e32.7 28.9e30.4 29.1e31.5 30.5e32.7 29.1e30.4
18.0e19.7 28.1e30.4 26.4e27.8 25.8e27.9 24.2e25.6 24.9e27.0 26.1e28.1 25.0e26.2
15.5e17.1 24.2e26.5 22.0e23.4 21.0e23.2 19.6e20.8 20.7e22.5 21.7e23.4 20.8e22.0
13.0e14.6 20.3e22.6 17.6e18.9 16.3e18.5 15.0e16.1 16.4e18.0 17.4e18.8 16.7e17.9
10.5e12.0 16.4e18.7 13.3e14.5 11.6e13.8 10.4e11.3 12.2e13.5 13.0e14.1 12.5e13.7
factor or consistency index and nRh is a shear thinning exponent or flow behavior index whose values were found to be 0.5e0.6. For each and every composite material, the values of correlation coefficient and R2 were found to be about 0.93 and >0.99, respectively (see Table 4). The value of correlation coefficient, 0.93 indicated a very strong downhill (negative) linear relationship between shear rate and melt-viscosity. In Fig. 5, it is obvious that the melt-viscosity of composites decreased with increasing shear rates indicating a pseudoplastic or shear-thinning characteristic of the composite materials. The pseudo-plastics or shear-thinning behavior of PP/WF composites had also been reported by Li et al. [47]. In Fig. 5a, it can also be noticed that the melt-viscosity of MAPO mixed composite B was lower compared the composite without MAPO, A. Although, a higher melt-viscosity of composite B was expected considering a higher interfacial interaction between PP matrix phase and WF particles in presence of MAPO. The lower values of melt-viscosity of MAPO containing composite B compared with the composite A indicated the molecular degradation of PP polymer in MAPO mixed composites. Melt-viscosity of PP polymer is depended on the molecular weight of PP polymer [48] as well as in the presence of peroxide molecular degradation of PP by chain scission has already been investigated by Azizi et al. [44].
Fig. 4. 95% confidence lower band of neat PP and PP/WF composites.
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composites compared with the composites without MAPO. This behaviour suggested that the effect of molecular degradation of PP (as indicated by rheological measurement) on the fatigue performance of the composites was more prominent compared with the compatibilizing effect of MAPO. Since fatigue experimental data were widely scattered, hence in this study a fatigue reliability was sought. Based on the fatigue experimental data 95% confidence band of neat PP and PP/WF composites were created. Since 95% confidence lower band ensure 95% survivability and only 5% failure of the composites, hence, from this study, it is suggested that the calculated fatigue life from 95% confidence lower band can be used as a material reliability index for safe fatigue design of PP/WF composites. Conflicts of interest The authors declare no conflict of interest. Fig. 5. Melt-viscosity of the composites against shear rates: (a) effect of MAPO, (b) effect of MAPO mixing temperature, (c) effect of MAPO mixing time.
Table 4 Obtained values of power law parameters and correlation coefficient for PP/WF composites. Sample
ARh
nRh
(R2)a
Correlation coefficient
A B C D E F G H
20045 16002 10949 9236 9374 11701 10783 11589
0.552 0.549 0.505 0.501 0.499 0.531 0.516 0.529
0.9999 0.9991 0.9974 0.9995 0.9975 0.9999 0.9991 0.9995
0.94 0.93 0.95 0.94 0.93 0.94 0.94 0.95
a
R2: correlation factor squared.
Effect of MAPO mixing temperature on the melt-viscosity was also evaluated as shown in Fig. 5b. It was found that the composite B with MAPO mixing temperature 140 C displayed the highest melt-viscosity among the MAPO containing composites B, C, E, and F with MAPO mixing temperatures 140, 120, 160, and 180 C, respectively. At 140 and 180 C, the effect of MAPO mixing time on the melt-viscosity of the composites was also evaluated as reported in Fig. 5c and d respectively. It was found that the melt-viscosity of the composites was significantly decreased with increasing MAPO mixing time at 140 C, but no significant change was observed at 180 C. This behaviour also support the molecular degradation of PP in presence of MAPO as half-life of peroxide at 180 C is 15 s only. 4. Conclusion Fatigue analysis and fatigue reliability of PP/WF composites with and without MAPO have been reported. In this study, we also investigated the effects of MAPO mixing temperature and time on the properties of composites. The fatigue testing results revealed that the composites containing MAPO exhibited lower fatigue performance compared with the composite without MAPO. Although it was expected that presence of MAPO in the composite would improve the physical and mechanical properties by enhancing the interfacial interaction among PP and WF. However, incorporation of MAPO into PP/WF composites did not improve the fatigue performance of the composites. In spite of some effect of MAPO mixing temperature and time on the fatigue performance of the composites, neither mixing temperature nor time variation could show a higher fatigue performance of the MAPO containing
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Advanced Industrial and Engineering Polymer Research journal homepage: http://www.keaipublishing.com/aiepr
Fatigue analysis and fatigue reliability of polypropylene/wood flour composites Md Minhaz-Ul Haque a, b, *, Koichi Goda a, Shinji Ogoe c, Yuta Sunaga d a
Department of Mechanical Engineering, Yamaguchi University, Ube, Yamaguchi, 755-8611, Japan Department of Applied Chemistry and Chemical Engineering, Islamic University, Kushtia, 7003, Bangladesh c Technology Development Centre, TOCLAS Co., Shizuoka, 432-8001, Japan d Kayaku Akzo Co., Sanyoonoda-City, Yamaguchi, 757-0002, Japan b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 March 2019 Received in revised form 22 April 2019 Accepted 4 July 2019
Fatigue analysis and fatigue reliability of polypropylene (PP)/wood flour (WF) composites were studied. The composites were prepared by incorporating wood flour with and without maleic anhydride and peroxide (MAPO) into cellulose nanofibres impregnated polypropylene. The prepared composites were then characterized by tensile and fatigue analyses. The tensile strength and fatigue life of MAPO mixed composites were lower compared with the composites without MAPO. It was also found that the fatigue experimental data of the composites were widely scattered regardless of the type of composites. Hence, in this study, a fatigue reliability of the composites was sought. Based on the fatigue experimental data, 95% confidence band were created. Since 95% confidence lower band ensure 95% survivability and only 5% failure of the composites, hence, from this study, it is suggested that the fatigue life obtained from 95% confidence lower band can be used as a material reliability index for safe fatigue design of the composites. © 2019 Kingfa SCI. & TECH. CO., LTD. Production and Hosting by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).
Keywords: Melt-viscosity Wood plastic composite (WPC) Fatigue behavior Wood flour Polypropylene
1. Introduction Application and demand of wood-plastic composite (WPC) in different sectors such as automotive industries, office appliances, housewares, furniture, outdoor deck floors, etc. [1e5] are increasing each year due its cheapness and eco-friendly character. The major components of WPC are wood flour and mainly thermoplastic polymers such as polyethylene, polypropylene, polyvinyl chloride etc. Wood flour as an inexpensive filler can reduce the cost of WPC largely [1]. Presence of wood flour into polymer matrices can also impart its physical and mechanical properties such as high strength and stiffness to the final composite materials. Polypropylene (PP)/wood flour (WF) composite, a typical WPC, is basically made of polypropylene matrix and wood flour. In the fabrication of PP/WF composite, another important ingredient, namely, maleic anhydride grafted polypropylene (MAPP), is usually used as a compatibilizer [6e11]. PP/WF compositess have drawn
* Corresponding author. Department of Mechanical Engineering, Yamaguchi University, 2-16-1 Tokiwadai, Ube, Yamaguchi, 755-8611, Japan. E-mail address: [email protected] (M.M.-U. Haque).
the attention of WPC companies because of the free flowing nature of WF compared with the natural fibres [12] as well as scope of fabrication from recycled wood and recycled polypropylene [13e17]. In this study, PP/WF composites were fabricated by incorporating wood flour with and without maleic anhydride and peroxide (MAPO) into cellulose nanofibres (CNF) impregnated polypropylene. Cellulose nanofibres was impregnated into PP matrix to increase the interfacial interaction among composite components. The advantages of addition of CNF into PP polymer had already been reported by Suzuki et al. [18,19]. In our previous study, a higher fatigue life of wet pulverized WF reinforced PP composite was reported [20,21]. Indeed, fibrils were generated on wood flour particles surfaces by the wet pulverization. An improved fatigue life of PP/WF composite was also claimed due to a higher crack deflection behavior of fibrillated WF. The improved fracture toughness and fatigue life of various nanoparticles incorporated polymer systems had already been reported by several researchers. As for examples, improved fatigue life and fracture toughness of epoxy composites were found by incorporating carbon nanotube [22], multi-walled carbon nanotube [23,24], graphene nanoparticles [25,26] etc.
https://doi.org/10.1016/j.aiepr.2019.07.001 2542-5048/© 2019 Kingfa SCI. & TECH. CO., LTD. Production and Hosting by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
M.M.-U. Haque et al. / Advanced Industrial and Engineering Polymer Research 2 (2019) 136e142
Although the demand and application of PP/WF are increasing each year, use of PP/WF in structural applications faces a great challenge because of their fatigue reliability under different load conditions. Structural materials are susceptible to fatigue failure below the tensile strength of that material. Hence, for safe fatigue design, the study of fatigue behavior of the composites is very important, particularly if PP/WF composites are subjected to use in long-term load bearing applications. Since materials deformation mechanism under short-term and long-term mechanical loading is not always the same, and only the tensile test is not sufficient to do a prediction of their long term load bearing application [27]. Generally, fatigue strength depends on the tensile strength of composite materials and composite materials with higher tensile strength exhibited higher fatigue strength [28,29], but this was not always happen, particularly at low cyclic stress. Liang et al. [30] and Shivakumar et al. [31] reported that the higher tensile strength of composites became less significant when cyclic stress was decreased i.e. at low cyclic stress some composite materials were less sensitive to the fatigue damage. Hence, the study of fatigue analysis and fatigue reliability of PP/WF is utmost necessary for safe fatigue design. Prediction of fatigue life of composite materials is also a critical issue as fatigue experimental data of composites are widely scattered. Crack propagation rate for a uniform material was predicted by a linear elastic fracture mechanics of a single through-crack. However, in composite material, fatigue occurs by fibre breakage, matrix cracking, crazing, matrix-fibre debonding, delamination etc. Moreover, in a composite, due to its heterogeneity, cracks were spread to the entire area and some cracks could not propagate over large distance as they were deflected/arrested by hard filler [32]. Thus, due to the heteroginity of composites the fatigue experimental data of composites are widely scattered. The main objective of this study was to find out a fatigue reliability of PP/WF composites. In this study, effect of MAPO mixing on the fatigue life of PP/WF composites was also investigated. Maleic anhydride has two functional groups: anhydride group and double bond. So, each molecule of maleic anhydride has possibility to be bonded with PP radical as generated by peroxide through double bond [33,34] as well as to be bonded with WF surfaces through anhydride group [35,36]. Based on this reaction probability, it was expected that the resultant composite system would exhibit an improved fatigue performance due to a higher components compatibilization. Concerning the fatigue reliability of the composites, since fatigue experimental data are widely scattered, it is necessary to find out absolution that can be employed to calculate the safe fatigue design of the composites. In this study, the experimental data were fitted with a logarithmic linear line by a regression technique. Based on the fatigue testing data, 95% confidence band were also created. Since 95% confidence lower band ensure 95% survivability and only 5% failure of the composites, hence, in this study, fatigue lives of composites at different fatigue strength, based on 95% confidence band, were calculated that can be used as a material reliability index for safe fatigue design of PP/WF composites. 2. Experimental 2.1. Materials Polypropylene (PP) pellet, trade name PPJ107G, melt flow index 30 g/10 min at 230 C/2.16 kg, density 0.9 g/cm3, melting point 150 C was received from Prime Polymer Co., Ltd., Tokyo, Japan. Maleic anhydride grafted polypropylene (MAPP) powder, trade name Kayabrid 006PP, maleic anhydride content 2 wt%, was received from Kayaku Akzo Co., Japan.
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Cellulose nanofibres (CNF) suspension (solid content 10 wt%), BiNFi-s WFo-10010, fibres diameter 10e50 nm, specific surface area 120 m2/g were supplied from Sugino, Japan. Wood flour (WF), trade name ARBOCEL®C100, bulk density 140e180 g/l, mesh number 100, particle size 70e152 mm was purchased from Rettenmaier & Sohne GmbH Co., Ltd., Rosenberg, Germany. Maleic anhydride, purity 99.5%, solidifying point 52 C, was purchased from Nippon Shokubai Co. Ltd., Japan. Organic peroxide, t-Butyl peroxy 3,5,5-trimethylhexanoate, purity 97 wt%, half life at 120, 140, 160, and 180 C, are about 1 h, 10 min, 1 min, and 15 s, respectively were purchased from, Kayaku Akzo Co., Japan. 2.2. Processing of the composites The PP/WF composites were prepared in three step processes. In the 1st step, cellulose nanofibres were dispersed into polypropylene matrix by mixing of CNF suspension (solid content 10 wt %) with MAPP and neat PP at 190 C in a mixer. The mixing ratio of PP, MAPP, and CNF was 90:5:5. In the 2nd step, mixture of PP/CNF (20 wt%) and wood flour (WF) (80 wt%) with and without maleic anhydride (2 phr) and peroxide (0.5 phr) was obtained by mixing them in a mixer at 120e180 C for 5e20 mins. In the 3rd step, the ground mixture were processed in a twinscrew extruder (AS30, Nakatani, machinery, Co., Ltd.) by the addition of required amount of neat PP so that the components composition wt% in each final composite become PP45/MAPP2.5/ CNF2.5/WF50. During processing of the composites, the temperature at different zones in the extruder were maintained as followings: (1) 165 C, (2) 200 C, (3) 215 C, (4) 200 C, (5) 190 C, and (6) 190 C. The screw speed of extruder was 85 rpm and total throughput was 9e12 kg/h. The processed composites based on different conditions were coded as reported in the Table 1. 2.3. Characterization techniques Tensile test of dumbbell shaped specimens, obtained by injection molding, of PP/WFcomposites was carried out by a Material Testing Machine, EHF-F1, Shimadzu, Japan. A strain gauge (Kyowa strain gauge, KFGS-2N-120-C1-11, Japan) of 2 mm with a gauge factor of 2.14 ± 1.0% was inserted on the surface (at the middle point) of each specimen. The specimens were put in a humidity control chamber at 25 C with 40% relative humidity (RH) for 5 days. The conditioned specimens were then tested using a load cell of 1 kN, gauge length of 18 mm and cross-head speed of 10 mm/min at 25 C and 40% RH. The average maximum strength of composites was calculated by performing the test on five specimens of each material. Fatigue tests of the composite specimens were carried out by the same instrument which was used for the tensile test. The fatigue test of the composites was also conducted at conditions of 25 C and 40% RH. The average maximum tensile strength value, obtained by tensile test, was used as a reference maximum stress level in estimation of the applied stress. The following test conditions such as tension-tension loading mode, stress ratio 0.1, frequency 3.5 Hz and applied stress: 90, 80, 70 and 60% level of ultimate tensile strength were applied in fatigue test. At each load, at least two specimens of the composites were tested. Measurements of rheological properties were performed using a capillary rheometer, LCR 7000, Dynixco, Japan. The diameter and length of the capillary were 2 mm and 8 mm, respectively. Tests were carried out at 180 C and at shear rates 100, 149, 223, 334 and
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Table 1 List of composite samples with their code. Sample code
A B C D E F G H
Mixing conditions Temperature ( C)
Time (min)
Maleic anhydride (phr)
Peroxide (phr)
140 140 120 140 160 180 180 180
10 10 10 20 10 10 5 20
2 2 2 2 2 2 2
0.5 0.5 0.5 0.5 0.5 0.5 0.5
500 s1. From this test melt-viscosity of PP/WF composites were measured. 3. Results and discussion 3.1. Tensile properties of the composites To determine the cyclic stress level of fatigue test, each composite sample was firstly subjected to the tensile test. Fig. 1 represents a stress-strain curves with an over imposed column diagram of tensile strength of neat PP and composites. The average tensile strength values and average toughness values calculated from the stress-strain curves of different PP/WF composites are also reported in Table 2. In Fig. 1, it is noticed that the MAPO containing composites BeH exhibited lower tensile strength compared with composite A. The lower tensile strength of MAPO containing composites BeH was probably due to the molecular degradation of PP polymer by MAPO [37]. Although a little influence of MAPO mixing temperature and time on the tensile strength of MAPO containing composites was noticed, MAPO mixing temperature and time changes did not show a higher value of tensile strength of the composites compared with the composite without MAPO. 3.2. Fatigue behavior and fatigue reliability of the composites Generally, composite materials display higher fatigue performance than that of its matrix material. Rigid filler particles have ability to reduce the degree of matrix deformation in front of the crack tip that propagates in soft matrices [32,38]. Our previous study [39] on fatigue analysis of PP/WF composites also showed a good agreement with the literature results. In this study, we
attempted to find a solution for fatigue reliability of PP/WF composites. To study the fatigue performance of the composites, maximum stress versus number of cycles to fracture (SeN) curves of the composites were plotted using the fatigue experimental data. The data were then fitted in a logarithmic linear regression equation as shown below:
smax ¼ b,logðNÞ þ c
(1)
where, smax is the applied maximum stress and N is a number of cycles to fracture. The values of b and c are constant and depend on the type of materials. For each and every composite material the values of correlation coefficient and R2 were found to be about 0.83 and >0.98, respectively. The value of correlation coefficient, 0.83 indicated a strong downhill (negative) linear relationship. The above regression equation (1) is similar to the fatigue model equation developed by Mandell [40,41]:
smax ¼ b,logðNÞ þ suts
(2)
where, suts is ultimate tensile strength. Fatigue strengths of different PP/WF composites at two different number of cycles were also calculated based on the equation (1) and the values are reported in Table 2. Fig. 2a shows SeN curves of neat PP and PP/WF composites with and without MAPO. As shown in Fig. 2, composites exhibited higher fatigue life compared with the neat PP. In Fig. 2a, it can also be noticed that composites without MAPO i.e. composite A exhibited higher fatigue life compared with MAPO containing composites B. In fibre reinforced composite, debonding of fibre/ matrix, fibre breakage, delamination and matrix cracking are the major damage mechanisms [38]. Maleic anhydride has two functional groups: anhydride group and double bond. A molecule of maleic anhydride has a possibility to be bonded with PP radical generated by peroxide through double bond as well as to be bonded withWF surfaces through the anhydride group. Consequently, the resultant composite system would exhibit an improved fatigue
Table 2 Tensile strength and fatigue strength of CNF impregnated PP/WF composites. Wood flour
PP A B C D E F G a
Fig. 1. Tensile strength of neat PP and PP/WF composites.
b
Toughness (MJ/M3)b
8.7 4.6 4.4 3.9 3.8 3.9 4.4 4.4
Tensile strength (MPa)
33.0 ± 0.4 46.9 ± 1.0 45.5 ± 0.4 45.3 ± 0.9 44.2 ± 0.2 44.5 ± 0.5 46.1 ± 0.5 45.1 ± 0.7
Fatigue strength (MPa) at different number of cycle (N)a 103 (N)
106 (N)
25.5 40.9 40.3 41.0 39.0 39.1 40.6 38.1
17.9 29.2 27.1 26.9 24.9 26.0 27.1 25.6
Calculated fatigue strengths based on equation (1). Toughness from the area of stress-strain curves.
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Fig. 2. SeN curves of neat PP and composites: (a) effect of MAPO, (b) effect of MAPO mixing temperature (c) effect of MAPO mixing time at 140 C, (d) effect of MAPO mixing time at 180 C.
performance due to the components compatibilization. However, the fatigue performance of the MAPO composites were lower compared with the composites without MAPO. Since the main components (PP, CNF and WF) and processing conditions were the same both in MAPO mixed as well as in non-mixed composites and maleic anhydride might also be grafted onto PP [34] that should have a positive effect on the fatigue life of composites, but, the fatigue performance of composites was not improved. Thus, this behaviour indicated that MAPO had played a serious role on the deterioration of fatigue performance of the composites. It had been reported that fatigue life increased with increasing molecular weight of polymer [42]. Since melt-processing of PP with MAPO causes chain scission of polypropylene [43,44], hence this lower fatigue life of MAPO containing composites compared with the composite A can be attributed to degradation of PP chain that increased the rate of matrix cracking. MAPO mixing temperature and mixing time on the fatigue life of composites were also investigated. Fig. 2b displays the SeN curves of composites B, C, E and F with MAPO mixing temperature 140, 120, 160 and 180 C, respectively. Composite E with MAPO mixing temperature 160 C exhibited the lowest fatigue performance as did tensile strength. Fig. 2c and d represent the effect of MAPO mixing time on the fatigue life of MAPO mixed composites at two different temperature 140 C and 180 C, respectively. In Fig. 2c, it can be noticed that composite B with MAPO mixing time 10 min displayed higher fatigue life compared with the composite D with MAPO mixing time 20 min. Whereas, at 180 C temperature composite F with MAPO mixing time 10 min displayed higher fatigue life compared with the composite G with MAPO mixing time 5 min. Although MAPO mixing temperature and mixing time had influenced on the fatigue performances of the MAPO mixed composites that indicated some interaction among maleic anhydride, PP and WF. However, variation of MAPO mixing temperature or time did
not show a higher fatigue life of the MAPO mixed composites compared with the non-mixed composite A. Fig. 3 represents the 95% confidence band of fatigue experimental data of neat PP, composite A, and composite B as examples. Since fatigue experimental data were widely scattered, therefore, based on the fatigue testing data, 95% confidence band of neat PP and composites were created using a SigmaPlot software. 95% confidence upper band indicated 5% survivability and 95% failure of the sample specimens, whereas 95% confidence lower band indicated 95% survivability and 5% failure of the sample specimens. From the 95% confidence band the fatigue strengths at different frequency were also calculated and the values are reported in Table 3. Plot of 95% confidence lower band of neat PP and PP/WF composites were created as shown in Fig. 4. Since 95% confidence lower band ensure 95% survivability and only 5% failure of samples [45]. Hence, from this study it can be suggested that the calculated data reported in Table 3 or the calculated fatigue life from 95% confidence lower band in Fig. 4 can be used as material reliability index with 95% of certainty for safe fatigue design. 3.3. Melt-viscosity of the composites To investigate the effect of MAPO on the fatigue performances of PP/WF composites the melt-viscosity of the composites were measured. Fig. 5 represents the melt-viscosity of PP/WF composites as a function of shear rates of 100e500 1/s. The melt-viscosity experimental data of the composites were fitted to the classical power law expression [46].
h ¼ ARh uðnRh Þ
(3)
where h represents the melt-viscosity of the composites and u represents the shear rate. In equation (3) ARh is a pre-exponential
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Fig. 3. SeN curves with 95% confidence band of fatigue experimental data of neat PP, composite A, and composite B. Table 3 Fatigue strength of the composites calculated from 95% confidence band (lower line - upper line values). Sample
PP A B C D E F G
Fatigue strength (MPa) at different frequency (N) 103
104
105
106
107
108
109
25.4e27.3 39.7e42.0 39.5e41.0 39.9e42.1 38.1e40.0 37.6e40.6 39.2e42.0 37.4e38.7
22.9e24.7 35.9e38.1 35.2e36.6 35.2e37.4 33.4e35.2 33.4e36.0 34.8e37.4 33.3e34.5
20.5e22.2 32.0e34.3 30.8e32.2 30.5e32.7 28.9e30.4 29.1e31.5 30.5e32.7 29.1e30.4
18.0e19.7 28.1e30.4 26.4e27.8 25.8e27.9 24.2e25.6 24.9e27.0 26.1e28.1 25.0e26.2
15.5e17.1 24.2e26.5 22.0e23.4 21.0e23.2 19.6e20.8 20.7e22.5 21.7e23.4 20.8e22.0
13.0e14.6 20.3e22.6 17.6e18.9 16.3e18.5 15.0e16.1 16.4e18.0 17.4e18.8 16.7e17.9
10.5e12.0 16.4e18.7 13.3e14.5 11.6e13.8 10.4e11.3 12.2e13.5 13.0e14.1 12.5e13.7
factor or consistency index and nRh is a shear thinning exponent or flow behavior index whose values were found to be 0.5e0.6. For each and every composite material, the values of correlation coefficient and R2 were found to be about 0.93 and >0.99, respectively (see Table 4). The value of correlation coefficient, 0.93 indicated a very strong downhill (negative) linear relationship between shear rate and melt-viscosity. In Fig. 5, it is obvious that the melt-viscosity of composites decreased with increasing shear rates indicating a pseudoplastic or shear-thinning characteristic of the composite materials. The pseudo-plastics or shear-thinning behavior of PP/WF composites had also been reported by Li et al. [47]. In Fig. 5a, it can also be noticed that the melt-viscosity of MAPO mixed composite B was lower compared the composite without MAPO, A. Although, a higher melt-viscosity of composite B was expected considering a higher interfacial interaction between PP matrix phase and WF particles in presence of MAPO. The lower values of melt-viscosity of MAPO containing composite B compared with the composite A indicated the molecular degradation of PP polymer in MAPO mixed composites. Melt-viscosity of PP polymer is depended on the molecular weight of PP polymer [48] as well as in the presence of peroxide molecular degradation of PP by chain scission has already been investigated by Azizi et al. [44].
Fig. 4. 95% confidence lower band of neat PP and PP/WF composites.
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composites compared with the composites without MAPO. This behaviour suggested that the effect of molecular degradation of PP (as indicated by rheological measurement) on the fatigue performance of the composites was more prominent compared with the compatibilizing effect of MAPO. Since fatigue experimental data were widely scattered, hence in this study a fatigue reliability was sought. Based on the fatigue experimental data 95% confidence band of neat PP and PP/WF composites were created. Since 95% confidence lower band ensure 95% survivability and only 5% failure of the composites, hence, from this study, it is suggested that the calculated fatigue life from 95% confidence lower band can be used as a material reliability index for safe fatigue design of PP/WF composites. Conflicts of interest The authors declare no conflict of interest. Fig. 5. Melt-viscosity of the composites against shear rates: (a) effect of MAPO, (b) effect of MAPO mixing temperature, (c) effect of MAPO mixing time.
Table 4 Obtained values of power law parameters and correlation coefficient for PP/WF composites. Sample
ARh
nRh
(R2)a
Correlation coefficient
A B C D E F G H
20045 16002 10949 9236 9374 11701 10783 11589
0.552 0.549 0.505 0.501 0.499 0.531 0.516 0.529
0.9999 0.9991 0.9974 0.9995 0.9975 0.9999 0.9991 0.9995
0.94 0.93 0.95 0.94 0.93 0.94 0.94 0.95
a
R2: correlation factor squared.
Effect of MAPO mixing temperature on the melt-viscosity was also evaluated as shown in Fig. 5b. It was found that the composite B with MAPO mixing temperature 140 C displayed the highest melt-viscosity among the MAPO containing composites B, C, E, and F with MAPO mixing temperatures 140, 120, 160, and 180 C, respectively. At 140 and 180 C, the effect of MAPO mixing time on the melt-viscosity of the composites was also evaluated as reported in Fig. 5c and d respectively. It was found that the melt-viscosity of the composites was significantly decreased with increasing MAPO mixing time at 140 C, but no significant change was observed at 180 C. This behaviour also support the molecular degradation of PP in presence of MAPO as half-life of peroxide at 180 C is 15 s only. 4. Conclusion Fatigue analysis and fatigue reliability of PP/WF composites with and without MAPO have been reported. In this study, we also investigated the effects of MAPO mixing temperature and time on the properties of composites. The fatigue testing results revealed that the composites containing MAPO exhibited lower fatigue performance compared with the composite without MAPO. Although it was expected that presence of MAPO in the composite would improve the physical and mechanical properties by enhancing the interfacial interaction among PP and WF. However, incorporation of MAPO into PP/WF composites did not improve the fatigue performance of the composites. In spite of some effect of MAPO mixing temperature and time on the fatigue performance of the composites, neither mixing temperature nor time variation could show a higher fatigue performance of the MAPO containing
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