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Effect of fiber morphology on rheological properties of plant fiber reinforced poly(butylene succinate) composites
Composites: Part B 44 (2013) 193–199
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Composites: Part B journal homepage: www.elsevier.com/locate/compositesb
Effect of fiber morphology on rheological properties of plant fiber reinforced poly(butylene succinate) composites Feng Yan-Hong a,⇑, Li Yi-Jie a, Xu Bai-Ping b, Zhang Da-Wei a, Qu Jin-Ping a, He He-Zhi a a National Engineering Research Center of Novel Equipment for Polymer Processing, The Key Laboratory of Polymer Processing Engineering of the Ministry of Education, South China University of Technology, Guangzhou 510641, PR China b Technology Development Center for Polymer Processing Engineering, Guangdong Industry Technology College, Guangzhou 510300, PR China
a r t i c l e
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Article history: Received 7 December 2011 Received in revised form 26 April 2012 Accepted 31 May 2012 Available online 9 June 2012 Keywords: A. Polymer–matrix composites A. Fibers B. Rheological properties Torque rheometer
a b s t r a c t Sisal fibers (SFs), steam exploded sisal fibers (SESFs) and steam exploded bagasse fibers (SEBFs) which have different fiber morphologies, were mixed with poly(butylene succinate) (PBS) using a torque rheometer. The rheological properties of these plant fiber-reinforced PBS composites were evaluated. Results show that the fiber morphology has a large effect on rheological behavior. At the same fiber content (e.g., 10 wt% and 30 wt%), the non-Newtonian index n of composites reinforced by flexible fibers with a higher aspect ratio and larger contact area with the matrix is smaller. In general, n decreases with increasing fiber content but when the fiber content is too high (e.g., 50 wt%), the aggregation of fibers is too extensive so that the actual contact area between fibers and matrix becomes much lower, n increase instead. At the same fiber content (e.g., 10 wt% and 30 wt%), the consistency indices of fibrous filler-reinforced composites are larger than those of powder-filled composites; the larger the actual contact area between the matrix and the fibers, the greater the consistency index of the composite. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction With recent changes in people’s attitudes toward environmental issues, biodegradable polymer materials have received increasing attention from both industrial communities and academic circles for many years [1–4]. As a type of biodegradable polymer material, poly(butylene succinate) (PBS) has better thermal and processing properties than other biodegradable polymer materials [5,6], but its production cost is high. An attractive approach to overcome this is to blend PBS with plant fibers, because they can not only reduce the material cost but also reinforce the matrix [7–12]. It is necessary to accurately measure the rheological properties of such plant fiber reinforced plastic composites for the design of the material composition and the structure of processing equipment, and also for the optimization of a number of process parameters. At present, the capillary rheometer is one of the most commonly used instruments for evaluating rheological properties of wood plastic composite, in which the fillers are mostly powder [13,14]. However, for composites reinforced by fibers of high aspect ratio, these fibers are too large to pass through the die of the capillary (commonly of diameter 1 mm). In addition, for simple premixed composites, plant fibers easily become entangled with each other and separate from the polymer when passing through ⇑ Corresponding author. E-mail address: [email protected] (Y.-H. Feng). 1359-8368/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compositesb.2012.05.051
the cylinder of the capillary, so with a poor dispersion of these fibers, the blends are inhomogeneous, which makes the result of rheological measurement inaccurate. Some researchers premixed fibers and polymer into composites, cut them into granules, and then measured the rheological properties with a capillary rheometer. Kalaprasad et al. prepared granules of sisal-reinforced lowdensity polyethylene (LDPE) composites, glass-reinforced LDPE composites and intimately mixed glass/sisal reinforced LDPE composites with an extruder and then measured the rheological properties of those composites using a capillary rheometer [15]. The results showed that all of those three kinds of composites presented pseudoplastic behaviors. With the same fiber volume fraction (20%), the non-Newtonian index of glass fiber reinforced composite was smaller than that of the sisal fiber reinforced composite, and for the glass/sisal reinforced composites, the nonNewtonian index decreased with the increase of glass fibers while the viscosity increased. Those phenomena indicated that different fiber morphologies of sisal fiber and glass fiber had a large effect on rheological behavior. Smita Mohanty et al. prepared sisal/ high-density polyethylene composites with twin-screw extruder and then studied the steady state rheological properties of the fiber-reinforced composites with a capillary rheometer attached to the twin-screw extruder [16]. The result showed that all the composites exhibited pseudoplastic characteristics which can be represented by power law equation, and the non-Newtonian index decreased linearly with the increase in fiber loading from 10 wt% to
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30 wt% while the consistency index of the composites increased. However, capillary rheometer is usually used to measure the rheological properties of the fiber reinforced composites with low fiber content (less than 40 wt%), Furthermore, during the premix and cut processing, the fibers are snipped and torn, and degradation of the matrix can occur, all of which affect the validity of the rheological results measured by capillary rheometer. To a certain extent, torque rheometer can simulate the real mixing process and reflect real changes in fiber configuration under different mixing conditions and the effect of changes to the fiber configuration on the rheological properties of complex systems. Therefore, some researchers have attempted to use torque rheometer to measure the rheological parameters of polymers. Goodrich and Porter [17] firstly characterized rheological properties of polymer melts with a torque rheometer. In this work, the measuring head of the torque rheometer was considered as two adjacent sets of coaxial cylinders, and the torque, M, was linearly related to angular velocity, S. Blyler and Daane [18] carried out further research, finding that M and S were related to the non-Newtonian index, n, and the consistency index, m, by a power law determined as follows:
M ¼ CðnÞmSn
2. Experimental 2.1. Materials PBS 1020 supplied by Showa Highpolymer Co., Ltd., Tokyo, Japan, was chosen as the matrix. Prior to mixing, it was dried at 85 °C (358 K) for 4 h. Sisal fibers (SFs) were supplied by Dongfang Sisal Group Co., Ltd., Guangdong Province of China. These fibers had an average diameter of about 0.5 mm. Before the experiment, SFs were cut into 4 mm lengths and dried at 105 °C (378 K) for 6 h. Steam exploded sisal fibers (SESFs) and steam exploded bagasse fibers (SEBFs) were produced with a custom-built continuous steam explosion equipment.
ð1Þ
where C(n) is a function involving several parameters, which cannot be obtained in a straightforward manner. Lee and Purdon [19] further explored the relationship and obtained an expression for C(n) and two factors affecting it: Re (an equivalent inner radius defined as the radius of the inner cylinder that produces the same torque as the roller) and a constant parameter, a, which is affected by the properties of the material. However, the authors did not deduce the correction function for a. Based on the theory of Lee, Marquez et al. [20] developed an expression for C(n) containing only one instrument parameter, a (the ratio of Re to the outside radius of the chamber, R0) which simplified the experimental model. Furthermore, the authors verified the accuracy of the model through numerous experiments. This method was also used to evaluate the rheological properties of PVC–plant fiber suspensions [21,22]. The result showed that the non-Newtonian index increased with the concentration of fibers at diluted concentrations, especially in suspensions with flexible fibers, but diminished at more elevated concentrations. And the consistency index presented a monotonous increment with the concentration of fibers. However, usually only the starting temperature is set, and the temperature, during processing, is seldom controlled, so the actual temperature varies with different formulas and processing conditions while mixing, and departs from original set temperature. This will affect the rheological properties of the materials. Thus, the influence of temperature should not be ignored. To take the effect of temperature into consideration, Cheng et al. [23] derived a new expression on the basis of the Arrhenius equation as follows:
lnM ¼ ln½CðnÞk þ DE=ðRTÞ þ nlnS þ blnf
time, the effects of the fiber content and initial length of the fiber on the rheological properties of the composites were also studied [25]. In this paper, three types of fibers differing in morphologies were used to produce different fiber reinforced composites and the effects of fiber morphology on the rheological properties of the fiber reinforced composites are presented.
ð2Þ
where k is the prefactor, R is the gas constant, DE is activation energy, b is an undetermined parameter, and f is the filling degree (the ratio of the volume of the filled material to the volume of the mixing chamber). Further, the value of the activation energy DE can be fitted with Eq. (2). The validity of this method was proven by comparing the rheological parameters of high density polyethylene, polystyrene, and polymethylmethacrylate derived from both the torque rheometer and the capillary rheometer. However, k must be calculated using the data derived from capillary rheometer. Because there are limitations in the studies of Marquez and Cheng, a new method combining these two models was presented in our previous work [24] to evaluate the rheological parameters of plant fiber reinforced composites with a torque rheometer. Significant effort was put into testing the validity of the method and, at the same
2.2. Measurement of the fiber aspect ratio Selected mixed samples were soaked in dichloromethane to dissolve the PBS, and the isolated fibers were then filtered and dried. At least 100 fibers for each sample were randomly chosen, and their lengths and widths measured using a stereomicroscope (Stemi2000) manufactured by ZEISS, Germany. SF, SESF and SEBF average aspect ratios were then evaluated.
2.3. Rheological measurement A Brabender Plasticorder torque rheometer, type W50EHT, manufactured by the Brabender Corporation was used, for which R0 is 19.75 mm, L is 47.3 mm, and the ratio between the two roller velocities is 2:3. The fiber content ranged from 0 to 50 wt%. According to the densities of PBS and SF (1.26 gcm3 and 1.30 gcm3, respectively) and the fiber content of each group, the masses of PBS and SF for each group were calculated, with the filling degree fixed at 85%. The experiments were performed at an initial temperature of 130 °C, with angular velocities, S, ranging between 1.05 rad s1 (10 rpm) and 5.25 rad s1 (50 rpm). Measurements were performed as follows: the temperature was set 130 °C and the roller angular velocity was held constant; PBS was poured into the measuring head; after the PBS melted, the fiber was gradually added and the head then closed; the torque and temperature were recorded 9 min after the measuring head was closed; the sample was removed from the measuring head and cooled to room temperature. Each group of tests was repeated three times, with the average torque and temperature used to evaluate the rheological parameters.
2.4. Observation of fiber distribution Samples of plant fiber reinforced PBS composites with different fiber contents were selected and hot pressed into thin translucent pieces. The original samples that selected randomly were small and thin enough so as to maintain the original fiber distribution as far as possible after hot pressing. The hot pressed flakes were observed with an optical image measuring instrument (VMC250S) manufactured by Shenzhen Chi Tai Precision Instrument Co., Ltd., China. And images of fiber distribution were taken.
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3.2. Experimental data for rheological measurements using the torque rheometer
3. Results and discussion 3.1. Analysis of plant fiber morphologies Sisal is a type of perennial leaf fiber crop. The SFs used here were fiber bundles, which contain many fiber cells glued together, and the fibers are rigid. The morphology of the sisal fiber is shown in Fig. 1a. Bagasse is a by-product of sugar cane, which is a type of perennial graminaceous plant. Bagasse fiber cell is the longest and widest fiber cell of all herbaceous plants. However, there are many other types of cells, mainly including parenchyma cells and pith cells that are spongy and of no fixed shapes. The morphology of the bagasse fiber is shown in Fig. 1c. The mechanism of steam explosion begins with water vapor that infiltrates hemicellulose, lignin, and the amorphous area of cellulose by virtue of high temperature and pressure. The fiber bundles are thus swelled and hemicellulose is degraded into soluble sugars. The lignin within the intercellular layer is plasticized and some is degraded, resulting in a weakening of the cohesion between fiber cells. After remaining under pressure for a certain period of time, the reactor pressure is rapidly reduced, allowing a portion of the water in the fiber bundles to vaporize and do work through expansion. The energy released can break hydrogen bonds and even covalent bonds in the fiber bundles and facilitates the degradation of amorphous hemicellulose and lignin. Thus, the tissue structure of the original fiber bundles is destroyed and fiber bundles dissociate from the intercellular layer into individual fiber cells, thereby completing preparation of plant fibers with a large aspect ratio [26,27]. Compared with untreated SFs, SESFs are slender, flexible and easily deformed, as shown in Fig. 1b. The average fiber width was 22 lm and the average aspect ratio was approximately 80. The specific surface area of SESFs increased substantially. For bagasse, the fiber cells were also separated from fiber bundles by steam explosion but the average aspect ratio of SEBFs was about 48, which was smaller than that of SESFs. At the same time, cells other than fiber cells that have thinner cell walls were destroyed into fine powders, which can be seen in Fig. 1d.
The torques and temperatures reached at the end of the measurements for PBS and composites reinforced by three kinds of fibers with different fiber contents and at different angular velocities are shown in Table 1. It can be seen that both end torques and temperatures of all the composites increased with fiber content when the angular velocity was constant, which is consistent with the phenomena described in our previous papers [24,25]. Also, both end torques and temperatures of SESF/PBS composites were much higher than those of SF/PBS and SEBF/PBS composites with the same fiber content and at the same angular velocity. All of these changes were related to the different fiber morphologies of these three plant fibers which caused different fiber–fiber and fiber–matrix interactions. With the experimental data of temperature, torque and rotation speed, the non-Newtonian index and consistency index were fitted and the reasons for the changes of the non-Newtonian index and consistency index were analyzed in the following parts.
3.3. Analysis of non-Newtonian index, n Fig. 2 shows the relationship between n and the fiber content calculated on the basis of the data in Table 1. It can be seen that the n values of the composites reinforced by three different kinds of fibers were all smaller than that of pure PBS, which was 0.84. That means both the PBS and the fiber reinforced composites show non-Newtonian behaviors. The smaller the value of n is, the stronger the non-Newtonian characteristic of the material is and the more obvious the shear thinning behavior becomes. This occurs because during the mixing process the mutual tangled fibers are disentangled and cut short by shear stress so that the fibers gradually disperse into the PBS melt and orient along the shear direction. Therefore, the shear thinning behaviors of the composites reinforced by the three types of fiber are much more obvious than that of pure PBS, and furthermore, their shear thinning behaviors were affected by the morphologies of the fibers. For the SF/PBS and SESF/PBS composites, the non-Newtonian index n initially decreased and reached a minimum when the fiber content was 30 wt%. The value of n subsequently increased, in
Fig. 1. Morphology comparison of different fiber materials (a) SF; (b) SESF; (c) Bagasse; (d) SEBF.
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Table 1 Temperatures and torques reached at the end of the measurements for the pure PBS and fiber reinforced composites. Speed (rads1)
PBS 10 wt%SF 10 wt%SESF 10 wt%SEBF 30 wt%SF 30 wt%SESF 30 wt%SEBF 50 wt%SF 50 wt%SESF 50 wt%SEBF
1.05
2.09
3.14
4.19
5.24
M (Nm)
T (K)
M (Nm)
T (K)
M (Nm)
T (K)
M (Nm)
T (K)
M (Nm)
T (K)
1.10 3.15 5.35 2.50 8.25 18.00 8.25 20.60 33.50 21.65
405 405 405.5 405 406 408 406 409 412 409
1.80 4.25 6.95 4.05 10.60 21.5 10.05 24.50 43.50 22.05
405 405.5 407 405 408 413 408.5 414.5 421 413
2.70 5.05 8.45 5.05 11.20 23.15 11.55 26.80 47.70 22.05
406 407 409 407 411 418 411 421 431 419
3.30 5.85 9.50 5.30 11.95 25.10 13.10 25.25 48.70 22.40
407.15 408.15 411.15 408.15 413.65 424.48 415.15 426.65 439.15 424.65
3.90 6.25 10.00 6.60 12.50 24.80 13.55 24.20 48.75 21.90
408 410 413.5 410 417 430 418 432 448.5 429.5
Fig. 2. Relationship between n and the fiber content.
contrast to that of the SEBF/PBS composite, for which n decreased monotonically. When the fiber content was relatively low (e.g., 10 wt%), n for the SESF/PBS composite was a minimum because SESFs are flexible and are easily entangled into aggregated states. An increase in the rotation speed results in an increased force applied on the fiber aggregates. The degree of disentanglement and orientation of the fibers was therefore much more obvious than for the other two composites, which resulted in a rapid reduction in the apparent viscosity with increasing rotation speed. That is to say, the apparent viscosity is more sensitive to a change in velocity, and n is small. Untreated SFs are rigid and do not readily disentangle and orient under weak shear stress, so the flow of the melt is difficult. Therefore, the apparent viscosity of the SF/PBS composite is less sensitive to changes in shear rate than the SESF/PBS composite and thus n was higher than that of the SESF/PBS composite. SEBFs are much shorter than SESFs and there are many tiny fragments of cells other than fiber cells, so when the fiber content is relatively low, SEBFs can readily disperse in the melt. As a result, the reduction of n is much lower from that of the pure PBS compared with the other two composites. When the fiber content increases to 30 wt%, the n values of the three types of composite are further reduced. This is because the higher the fiber content, the higher the force applied on the fibers and the more disentangled and cut fibers become. Therefore, the composites show greater shear-thinning behavior when the fiber content increases and the n values for the three types of composites decrease to a similar level.
When the fiber content further increases to 50 wt%, the n values of SF/PBS and SESF/PBS composites increase, which differs from the trend for the SEBF/PBS composite, in which n continues to decrease. This is because the viscosity of the PBS used is very low so the increase in shear stress with shear rate is not large enough to disperse the high content fibers and the amount of the fiber aggregates increased. Thus, the actual contact area between fibers and matrix became smaller and the viscosity of the composite is less sensitive to changes in shear rate than that of composites with lower fiber content. Additionally, the slender and flexible SESFs deformed easily so they tangled to a greater extent than SFs and the n value of the SESF/PBS composite was higher than that of the SF/PBS composite. This meant the shear thinning behavior of SESF/PBS composite was not as strong as that of the SF/PBS composite with the same fiber content. With regard to the SEBF reinforced composite, because there are many tiny fragments of cells other than fiber cells and the average aspect ratio of BF fiber cells is much smaller than that of the SF fiber cell, despite the small applied stress, the shear stress became large enough to disentangle and orient the SEBFs so that the fiber could be more evenly dispersed in the PBS matrix with increasing shear rate. Thus, the SEBF/PBS composite had the lowest value of n.
3.4. Analysis of consistency index, m Fig. 3 shows the relationship between the consistency index, m (the viscosity at unit shear rate, i.e., at c_ ¼ 1 s1 ), and the fiber content of the three types of composite reinforced by fibers of different morphologies. It can be seen from Fig. 3 that the m values of the plant fiber reinforced composites are all higher than that of the pure PBS. This can be explained as follows. First, fibers with a large aspect ratio are easily entangled, resulting in an increased force between the fibers when the fibers move relative to other fibers. Second, the tangled fibers block the flow of the matrix. Therefore, the addition of the fibers increases the consistency index over that of the PBS. For the SF/PBS and SESF/PBS composites, their consistency indices initially increased then decreased, while the m value of the SEBF/PBS composite increased with increasing fiber content. This special phenomenon is related to the variation in the difference between fiber impregnation degrees of the three types of composite, caused by changes in fiber content. When the fiber content is relatively low (10 wt%, 30 wt%), the SESF/PBS composite had the highest value of m. This is because the slender SESFs are flexible and deform easily, so the fibers in the mixture readily entangle into fiber aggregates. When the shear rate is very low (such as unit shear rate), it is difficult to impregnate, move and separate these aggregated fibers, making the consistency index of the SESFs reinforced composite much larger than those of PBS and the other two types of composite. Compared with SESFs, SFs are rigid, easily broken, and the average aspect ratio
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Fig. 3. Relationship between m and fiber content.
of these fibers is smaller, so when the fiber content is not high, the degree of the entanglement is low, which gives a smaller m value than that of the SESF composite. With regard to the SEBF/PBS composite, BF fiber cells are much shorter than SF fiber cells and there are fine fragments of cells other than fiber cells in SEBFs. When the fiber content is relatively low, SEBFs do not readily entangle and can be easily dispersed in the matrix, so the m value of the SEBF composite is much smaller than that of the SESF composite. When the fiber content increases (from 10 wt% to 30 wt%), the melt content is reduced correspondingly, so impregnation, movement and separation of entangled fibers becomes more difficult, resulting in an increased consistency index. As the fiber content increases to 50 wt%, one may expect m to increase, which is a trend that have been reported in our previous works [24,25]. However, in the present work, the m value of SEBF/ PBS composites continues to increase while, in contrast, those of the SESF/PBS and SF/PBS composites decrease. This is due to the presence of cell fragments in the SEBFs as described above and the fact that BF fiber cells are much shorter than SF fiber cells. Thus, SEBFs can be dispersed more easily than SFs and SESFs and there is very contact area between SFBFs and the matrix. Because the fibers hinder the flow of the matrix, the larger the contact area between fibers and matrix, the more difficult it is for the matrix to flow. At low shear rates, it is difficult to move the SEBF/PBS composite so the m value of the SEBF/PBS composite is very large.
Although SFs and SESFs tangle and aggregate more easily than SEBFs when the fiber content increases to 50 wt%, the aggregation of SFs and SESFs is more extensive, so the actual contact areas between SFs or SESFs and the matrix become much lower than that between SFBFs and the matrix, and the m values for the SF/PBS and SESF/PBS composites are lower than that of the SEBF/PBS composite. In addition, at low shear rate (such as unit shear rate), as the degree of aggregation became more extensive in the SF/PBS and SESF/PBS composites with increased fiber content from 30 to 50 wt%, the actual contact areas between fibers and matrix reduced, as illustrated by Fig. 4, so their m values decreased compared with the corresponding composites at 30 wt% fiber content. 3.5. Analysis of average shear viscosity, average shear stress and the average shear rate The average shear viscosity, average shear stress and the average shear rate can be determined by methods presented by Marquez et al. [20], and their values can be solved on the basis of the n and m values calculated above. It can be seen from Fig. 5 that the average shear viscosities of all composites with the same fiber content differ because of the different morphologies of the fibers and also that the average shear viscosities of these composites are higher than that of pure PBS. The flexible and slender SESFs, which have the largest surface areas,
Fig. 4. Fibers dispersion of composites of different fiber content under low shear rate (a) low fiber content and (b) high fiber content.
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Fig. 5. Relationship between the average shear viscosity and average shear rate.
Fig. 6. Relationship between the average shear stress and average shear rate.
can be deformed easily, so when the fiber content is fixed, the average g values of the SESF/PBS composite are usually higher than those of the other two composites at the same shear rate. Although the SEBFs have a high surface area, the average aspect ratio is much smaller than that of SESFs and there are many fragments of cells other than BF fiber cells, so the average g values of the SEBF/PBS composite are usually the smallest. With regard to SFs, which have a smaller surface area than the other two types of fiber, the fibers are rigid and during mixing they can produce a high flow resistance for the matrix, so the g values are higher than that of the SEBF/PBS composite but lower than that of the SESF/PBS composite. It is shown in Fig. 6 that the average shear stress of the three types of composite increases with average shear rate. For composites with a fiber content of 50 wt%, when the rotation speed reaches 50 rpm, the average shear stress of the SEBF/PBS composite is the smallest, at about 132 kPa, the shear stress of the SF/PBS composite is about 374 kPa, and the shear stress of the SESF/PBS composite is the highest, at about 639 kPa, which is about 4.8
times higher than that of the SEBF/PBS composites and 19 times higher than that of pure PBS. Therefore, the morphology of the reinforcement fibers in the composites has a marked effect on the processing power.
4. Conclusions The rheological properties of three different types of plant fiber reinforced PBS composites were evaluated using a torque rheometer. The results show that even with the same fiber content, the rheological properties of the composites reinforced by fibers of different morphologies differed. The non-Newtonian index n is related to the degree of change of the orientation, disentanglement and fracture of fibers caused by a change in rotation speed. When the fiber content is relatively low, the fibers can be uniformly dispersed in the matrix and the fibers become more oriented with the increase in rotation speed. Therefore, compared with the n value of PBS, the decrements in the n values of the SF/
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PBS and SESF/PBS composites (in which the reinforcements are fibrous) are larger than that of the SEBF/PBS composite, which includes many small fragments of cells other than BF fiber cells. At medium fiber content, the degree of fiber entanglement is more extensive, increasing the interaction among these fibers. With an increase in rotation speed, an increased degree of fiber disentanglement, orientation and fracture occurs because of the increase in stress applied to the fibers, therefore causing a further reduction in the values of n. When the fiber content is relatively high, there is a high tendency for the fibrous SFs and SESFs to tangle into aggregates within the matrix. In addition, the viscosity of the matrix is too low to generate sufficient stress to evenly disperse the fibers in the matrix so these still exist in the matrix even at increased shear rate. As a result, the actual contact area between matrix and fibers decreases, causing the shear viscosities in these two types of composites to become less sensitive to changes in shear rate. Therefore, the n values of these two composites are higher than that of the SEBF/PBS composite, in which there are not as many aggregates because of the shorter average aspect ratio of SEBFs and the presence of cell fragments other than BF fiber cells. The consistency index of the composite is also closely related to fiber morphology. Generally speaking, at the same fiber content, the consistency indices of fibrous filler-reinforced composites are larger than those of powder-filled composites. The larger the fibrous filler surface area, the higher the consistency index of the composite. The larger the actual contact area between the matrix and the fibers, the greater the consistency index of the composite. Acknowledgments The authors acknowledge the National Nature Science Foundation of China (Nos. 50903033, 50973035 and 51073061), the National Key Technology R&D Program of China (Nos. 2009BAI84B05 and 2009BAI84B06), Program for New Century Excellent Talents in University (No. NCET-11-0152), Pearl River Talent Fund for Young Sci-Tech Researchers of Guangzhou City (No. 2011J2200058), the Fundamental Research Funds for the Central Universities (No. 2011ZZ0011) and Opening Project of Technology Development Center for Polymer Processing Engineering of Guangdong Industry Technology College (No. 2010001) for financial support. References [1] Frederick TT, David GC, Milan M. Biodegradation of a synthetic co-polyester by aerobic mesophilic microorganisms. Polym Degrad Stab 2008;93(8):1479–85. [2] Nagahama H, New N, Jayakumar R. Novel biodegradable chitin membranes for tissue engineering applications. Carbohyd Polym 2008;73(2):295–302. [3] Kim HS, Lee BH, Lee S. Enhanced interfacial adhesion, mechanical, and thermal properties of natural flour-filled biodegradable polymer bio-composites. J Therm Anal Calorim 2011;104(1):331–8.
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[4] Yee TW, Rahman W, Sin LT. Properties and morphology of poly(vinyl alcohol) blends with sago pith bio-filler as biodegradable composites. J Vinyl Addit Technol 2011;17(3):184–9. [5] Gao M, Wang XF. Progress of study on PBS-based biodegradable materials. Polym Bull 2004;10(5):51–5. [6] Fujimaki T. Processability and properties of aliphatic polyesters, ‘BIONOLLE’, synthesized by polycondensation reaction. Polym Degrad Stab 1998;59(1– 3):209–14. [7] Thirmizir MZA, Ishak ZAM, Taib RM. Kenaf-Bast-fiber-filled biodegradable poly(butylene succinate) composites: effects of fiber loading, fiber length, and maleated poly(butylene succinate) on the flexural and impact properties. J Appl Polym Sci 2011;122(5):3055–63. [8] Liang ZC, Pan PJ. Mechanical and thermal properties of poly(butylene succinate)/plant fiber biodegradable composite. J Appl Polym Sci 2010; 115(6):3559–67. [9] Bao L, Chen YW, Zhou WH. Bamboo fibers/poly(ethylene glycol)-reinforced poly(butylene succinate) biocomposites. J Appl Polym Sci 2011; 122(4):2456–66. [10] Dash BN, Nakamura M. Mechanical properties of hemp reinforced poly(butylene succinate) biocomposites. J Biobased Mater Bio 2008;2(3): 273–81. [11] Shinji O. Development of high strength biodegradable composites using Manila hemp fiber and starch-based biodegradable resin. Composites A 2006;37(11):1879–83. [12] Tran HN, Shinji O. Effect of alkali treatment on interfacial and mechanical properties of coir fiber reinforced poly(butylene succinate) biodegradable composites. Composites B 2011;42(6):1648–56. [13] Maiti SN, Subbarao R, Nordin Ibrahim Mohd. Effect of wood fibers on the rheological properties of i-PP/wood fiber composites. J Appl Polym Sci 2004;91(1):644–50. [14] Carrino L, Ciliberto S. Effect of filler content and temperature on steady-state shear flow of wood/high density polyethylene composites. Polym Compos 2011;32(2):796–809. [15] Kalaprasad G et al. Melt rheological behavior of intimately mixed short sisalglass hybrid fiber-reinforced low-density polyethylene composites. I: Untreated fibers. J Appl Polym Sci 2003;89(2):432–42. [16] Smita Mohanty, Nayak Sanjay K. Rheological characterization of HDPE/sisal fiber composites. Polym Eng Sci 2007;47(10):1634–42. [17] Goodrich JE, Porter RS. A rheological interpretation of torque-rheometer data. Polym Eng Sci 1967;7(1):45–7. [18] Blyler LL, Daane JH. An analysis of brabender torque rheometer data. Polym Eng Sci 1967;7(3):178–81. [19] Lee GCN, Purdon JR. Brabender viscometry. I: Conversion of brabender curves to instron flow curves. Polym Eng Sci 1969;9(5):360–7. [20] Marquez A, Quijano J, Gaulin M. A calibration technique to evaluate the powerlaw parameters of polymer melts using a torque-rheometer. Polym Eng Sci 1996;36(20):2556–63. [21] Ayora M, Rios R, Quijano J, Marquez A. Evaluation by torque rheometer of suspensions of semi-rigid and flexible plant fibers in a matrix of poly(vinyl chloride). Polym Compos 1997;18(4):549–60. [22] Marquez A, Quijano J, Rios R, Ayora-Camara MH. Study of the flow behavior of polymer–plant fiber suspensions in the power law validity range. Polym Compos 1999;20(2):279–92. [23] Cheng BJ, Zhou CX, Yu W, Sun XY. Evaluation of rheological parameters of polymer melts in torque rheometers. Polym Test 2001;20(7):811–8. [24] Feng YH, Zhang DW, Qu J. Rheological properties of sisal fiber/poly(butylene succinate) composites. Polym Test 2011;30(1):124–30. [25] Zhang DW, Li YJ, Feng YH. Effect of initial fiber length on the rheological properties of sisal fiber/polylactic acid composites. Polym Compos 2011;32(8):1218–24. [26] Ramos LP. The chemistry involved in the steam pretreatment of lignocellulosic materials. Quim Nova 2003;26(6):863–71. [27] Biswas BA, Kentaro U, Yang WH. Change of pyrolysis characteristics and structure of woody biomass due to steam explosion pretreatment. Fuel Process Technol 2011;92(10):1849–54.
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Composites: Part B journal homepage: www.elsevier.com/locate/compositesb
Effect of fiber morphology on rheological properties of plant fiber reinforced poly(butylene succinate) composites Feng Yan-Hong a,⇑, Li Yi-Jie a, Xu Bai-Ping b, Zhang Da-Wei a, Qu Jin-Ping a, He He-Zhi a a National Engineering Research Center of Novel Equipment for Polymer Processing, The Key Laboratory of Polymer Processing Engineering of the Ministry of Education, South China University of Technology, Guangzhou 510641, PR China b Technology Development Center for Polymer Processing Engineering, Guangdong Industry Technology College, Guangzhou 510300, PR China
a r t i c l e
i n f o
Article history: Received 7 December 2011 Received in revised form 26 April 2012 Accepted 31 May 2012 Available online 9 June 2012 Keywords: A. Polymer–matrix composites A. Fibers B. Rheological properties Torque rheometer
a b s t r a c t Sisal fibers (SFs), steam exploded sisal fibers (SESFs) and steam exploded bagasse fibers (SEBFs) which have different fiber morphologies, were mixed with poly(butylene succinate) (PBS) using a torque rheometer. The rheological properties of these plant fiber-reinforced PBS composites were evaluated. Results show that the fiber morphology has a large effect on rheological behavior. At the same fiber content (e.g., 10 wt% and 30 wt%), the non-Newtonian index n of composites reinforced by flexible fibers with a higher aspect ratio and larger contact area with the matrix is smaller. In general, n decreases with increasing fiber content but when the fiber content is too high (e.g., 50 wt%), the aggregation of fibers is too extensive so that the actual contact area between fibers and matrix becomes much lower, n increase instead. At the same fiber content (e.g., 10 wt% and 30 wt%), the consistency indices of fibrous filler-reinforced composites are larger than those of powder-filled composites; the larger the actual contact area between the matrix and the fibers, the greater the consistency index of the composite. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction With recent changes in people’s attitudes toward environmental issues, biodegradable polymer materials have received increasing attention from both industrial communities and academic circles for many years [1–4]. As a type of biodegradable polymer material, poly(butylene succinate) (PBS) has better thermal and processing properties than other biodegradable polymer materials [5,6], but its production cost is high. An attractive approach to overcome this is to blend PBS with plant fibers, because they can not only reduce the material cost but also reinforce the matrix [7–12]. It is necessary to accurately measure the rheological properties of such plant fiber reinforced plastic composites for the design of the material composition and the structure of processing equipment, and also for the optimization of a number of process parameters. At present, the capillary rheometer is one of the most commonly used instruments for evaluating rheological properties of wood plastic composite, in which the fillers are mostly powder [13,14]. However, for composites reinforced by fibers of high aspect ratio, these fibers are too large to pass through the die of the capillary (commonly of diameter 1 mm). In addition, for simple premixed composites, plant fibers easily become entangled with each other and separate from the polymer when passing through ⇑ Corresponding author. E-mail address: [email protected] (Y.-H. Feng). 1359-8368/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compositesb.2012.05.051
the cylinder of the capillary, so with a poor dispersion of these fibers, the blends are inhomogeneous, which makes the result of rheological measurement inaccurate. Some researchers premixed fibers and polymer into composites, cut them into granules, and then measured the rheological properties with a capillary rheometer. Kalaprasad et al. prepared granules of sisal-reinforced lowdensity polyethylene (LDPE) composites, glass-reinforced LDPE composites and intimately mixed glass/sisal reinforced LDPE composites with an extruder and then measured the rheological properties of those composites using a capillary rheometer [15]. The results showed that all of those three kinds of composites presented pseudoplastic behaviors. With the same fiber volume fraction (20%), the non-Newtonian index of glass fiber reinforced composite was smaller than that of the sisal fiber reinforced composite, and for the glass/sisal reinforced composites, the nonNewtonian index decreased with the increase of glass fibers while the viscosity increased. Those phenomena indicated that different fiber morphologies of sisal fiber and glass fiber had a large effect on rheological behavior. Smita Mohanty et al. prepared sisal/ high-density polyethylene composites with twin-screw extruder and then studied the steady state rheological properties of the fiber-reinforced composites with a capillary rheometer attached to the twin-screw extruder [16]. The result showed that all the composites exhibited pseudoplastic characteristics which can be represented by power law equation, and the non-Newtonian index decreased linearly with the increase in fiber loading from 10 wt% to
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30 wt% while the consistency index of the composites increased. However, capillary rheometer is usually used to measure the rheological properties of the fiber reinforced composites with low fiber content (less than 40 wt%), Furthermore, during the premix and cut processing, the fibers are snipped and torn, and degradation of the matrix can occur, all of which affect the validity of the rheological results measured by capillary rheometer. To a certain extent, torque rheometer can simulate the real mixing process and reflect real changes in fiber configuration under different mixing conditions and the effect of changes to the fiber configuration on the rheological properties of complex systems. Therefore, some researchers have attempted to use torque rheometer to measure the rheological parameters of polymers. Goodrich and Porter [17] firstly characterized rheological properties of polymer melts with a torque rheometer. In this work, the measuring head of the torque rheometer was considered as two adjacent sets of coaxial cylinders, and the torque, M, was linearly related to angular velocity, S. Blyler and Daane [18] carried out further research, finding that M and S were related to the non-Newtonian index, n, and the consistency index, m, by a power law determined as follows:
M ¼ CðnÞmSn
2. Experimental 2.1. Materials PBS 1020 supplied by Showa Highpolymer Co., Ltd., Tokyo, Japan, was chosen as the matrix. Prior to mixing, it was dried at 85 °C (358 K) for 4 h. Sisal fibers (SFs) were supplied by Dongfang Sisal Group Co., Ltd., Guangdong Province of China. These fibers had an average diameter of about 0.5 mm. Before the experiment, SFs were cut into 4 mm lengths and dried at 105 °C (378 K) for 6 h. Steam exploded sisal fibers (SESFs) and steam exploded bagasse fibers (SEBFs) were produced with a custom-built continuous steam explosion equipment.
ð1Þ
where C(n) is a function involving several parameters, which cannot be obtained in a straightforward manner. Lee and Purdon [19] further explored the relationship and obtained an expression for C(n) and two factors affecting it: Re (an equivalent inner radius defined as the radius of the inner cylinder that produces the same torque as the roller) and a constant parameter, a, which is affected by the properties of the material. However, the authors did not deduce the correction function for a. Based on the theory of Lee, Marquez et al. [20] developed an expression for C(n) containing only one instrument parameter, a (the ratio of Re to the outside radius of the chamber, R0) which simplified the experimental model. Furthermore, the authors verified the accuracy of the model through numerous experiments. This method was also used to evaluate the rheological properties of PVC–plant fiber suspensions [21,22]. The result showed that the non-Newtonian index increased with the concentration of fibers at diluted concentrations, especially in suspensions with flexible fibers, but diminished at more elevated concentrations. And the consistency index presented a monotonous increment with the concentration of fibers. However, usually only the starting temperature is set, and the temperature, during processing, is seldom controlled, so the actual temperature varies with different formulas and processing conditions while mixing, and departs from original set temperature. This will affect the rheological properties of the materials. Thus, the influence of temperature should not be ignored. To take the effect of temperature into consideration, Cheng et al. [23] derived a new expression on the basis of the Arrhenius equation as follows:
lnM ¼ ln½CðnÞk þ DE=ðRTÞ þ nlnS þ blnf
time, the effects of the fiber content and initial length of the fiber on the rheological properties of the composites were also studied [25]. In this paper, three types of fibers differing in morphologies were used to produce different fiber reinforced composites and the effects of fiber morphology on the rheological properties of the fiber reinforced composites are presented.
ð2Þ
where k is the prefactor, R is the gas constant, DE is activation energy, b is an undetermined parameter, and f is the filling degree (the ratio of the volume of the filled material to the volume of the mixing chamber). Further, the value of the activation energy DE can be fitted with Eq. (2). The validity of this method was proven by comparing the rheological parameters of high density polyethylene, polystyrene, and polymethylmethacrylate derived from both the torque rheometer and the capillary rheometer. However, k must be calculated using the data derived from capillary rheometer. Because there are limitations in the studies of Marquez and Cheng, a new method combining these two models was presented in our previous work [24] to evaluate the rheological parameters of plant fiber reinforced composites with a torque rheometer. Significant effort was put into testing the validity of the method and, at the same
2.2. Measurement of the fiber aspect ratio Selected mixed samples were soaked in dichloromethane to dissolve the PBS, and the isolated fibers were then filtered and dried. At least 100 fibers for each sample were randomly chosen, and their lengths and widths measured using a stereomicroscope (Stemi2000) manufactured by ZEISS, Germany. SF, SESF and SEBF average aspect ratios were then evaluated.
2.3. Rheological measurement A Brabender Plasticorder torque rheometer, type W50EHT, manufactured by the Brabender Corporation was used, for which R0 is 19.75 mm, L is 47.3 mm, and the ratio between the two roller velocities is 2:3. The fiber content ranged from 0 to 50 wt%. According to the densities of PBS and SF (1.26 gcm3 and 1.30 gcm3, respectively) and the fiber content of each group, the masses of PBS and SF for each group were calculated, with the filling degree fixed at 85%. The experiments were performed at an initial temperature of 130 °C, with angular velocities, S, ranging between 1.05 rad s1 (10 rpm) and 5.25 rad s1 (50 rpm). Measurements were performed as follows: the temperature was set 130 °C and the roller angular velocity was held constant; PBS was poured into the measuring head; after the PBS melted, the fiber was gradually added and the head then closed; the torque and temperature were recorded 9 min after the measuring head was closed; the sample was removed from the measuring head and cooled to room temperature. Each group of tests was repeated three times, with the average torque and temperature used to evaluate the rheological parameters.
2.4. Observation of fiber distribution Samples of plant fiber reinforced PBS composites with different fiber contents were selected and hot pressed into thin translucent pieces. The original samples that selected randomly were small and thin enough so as to maintain the original fiber distribution as far as possible after hot pressing. The hot pressed flakes were observed with an optical image measuring instrument (VMC250S) manufactured by Shenzhen Chi Tai Precision Instrument Co., Ltd., China. And images of fiber distribution were taken.
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3.2. Experimental data for rheological measurements using the torque rheometer
3. Results and discussion 3.1. Analysis of plant fiber morphologies Sisal is a type of perennial leaf fiber crop. The SFs used here were fiber bundles, which contain many fiber cells glued together, and the fibers are rigid. The morphology of the sisal fiber is shown in Fig. 1a. Bagasse is a by-product of sugar cane, which is a type of perennial graminaceous plant. Bagasse fiber cell is the longest and widest fiber cell of all herbaceous plants. However, there are many other types of cells, mainly including parenchyma cells and pith cells that are spongy and of no fixed shapes. The morphology of the bagasse fiber is shown in Fig. 1c. The mechanism of steam explosion begins with water vapor that infiltrates hemicellulose, lignin, and the amorphous area of cellulose by virtue of high temperature and pressure. The fiber bundles are thus swelled and hemicellulose is degraded into soluble sugars. The lignin within the intercellular layer is plasticized and some is degraded, resulting in a weakening of the cohesion between fiber cells. After remaining under pressure for a certain period of time, the reactor pressure is rapidly reduced, allowing a portion of the water in the fiber bundles to vaporize and do work through expansion. The energy released can break hydrogen bonds and even covalent bonds in the fiber bundles and facilitates the degradation of amorphous hemicellulose and lignin. Thus, the tissue structure of the original fiber bundles is destroyed and fiber bundles dissociate from the intercellular layer into individual fiber cells, thereby completing preparation of plant fibers with a large aspect ratio [26,27]. Compared with untreated SFs, SESFs are slender, flexible and easily deformed, as shown in Fig. 1b. The average fiber width was 22 lm and the average aspect ratio was approximately 80. The specific surface area of SESFs increased substantially. For bagasse, the fiber cells were also separated from fiber bundles by steam explosion but the average aspect ratio of SEBFs was about 48, which was smaller than that of SESFs. At the same time, cells other than fiber cells that have thinner cell walls were destroyed into fine powders, which can be seen in Fig. 1d.
The torques and temperatures reached at the end of the measurements for PBS and composites reinforced by three kinds of fibers with different fiber contents and at different angular velocities are shown in Table 1. It can be seen that both end torques and temperatures of all the composites increased with fiber content when the angular velocity was constant, which is consistent with the phenomena described in our previous papers [24,25]. Also, both end torques and temperatures of SESF/PBS composites were much higher than those of SF/PBS and SEBF/PBS composites with the same fiber content and at the same angular velocity. All of these changes were related to the different fiber morphologies of these three plant fibers which caused different fiber–fiber and fiber–matrix interactions. With the experimental data of temperature, torque and rotation speed, the non-Newtonian index and consistency index were fitted and the reasons for the changes of the non-Newtonian index and consistency index were analyzed in the following parts.
3.3. Analysis of non-Newtonian index, n Fig. 2 shows the relationship between n and the fiber content calculated on the basis of the data in Table 1. It can be seen that the n values of the composites reinforced by three different kinds of fibers were all smaller than that of pure PBS, which was 0.84. That means both the PBS and the fiber reinforced composites show non-Newtonian behaviors. The smaller the value of n is, the stronger the non-Newtonian characteristic of the material is and the more obvious the shear thinning behavior becomes. This occurs because during the mixing process the mutual tangled fibers are disentangled and cut short by shear stress so that the fibers gradually disperse into the PBS melt and orient along the shear direction. Therefore, the shear thinning behaviors of the composites reinforced by the three types of fiber are much more obvious than that of pure PBS, and furthermore, their shear thinning behaviors were affected by the morphologies of the fibers. For the SF/PBS and SESF/PBS composites, the non-Newtonian index n initially decreased and reached a minimum when the fiber content was 30 wt%. The value of n subsequently increased, in
Fig. 1. Morphology comparison of different fiber materials (a) SF; (b) SESF; (c) Bagasse; (d) SEBF.
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Table 1 Temperatures and torques reached at the end of the measurements for the pure PBS and fiber reinforced composites. Speed (rads1)
PBS 10 wt%SF 10 wt%SESF 10 wt%SEBF 30 wt%SF 30 wt%SESF 30 wt%SEBF 50 wt%SF 50 wt%SESF 50 wt%SEBF
1.05
2.09
3.14
4.19
5.24
M (Nm)
T (K)
M (Nm)
T (K)
M (Nm)
T (K)
M (Nm)
T (K)
M (Nm)
T (K)
1.10 3.15 5.35 2.50 8.25 18.00 8.25 20.60 33.50 21.65
405 405 405.5 405 406 408 406 409 412 409
1.80 4.25 6.95 4.05 10.60 21.5 10.05 24.50 43.50 22.05
405 405.5 407 405 408 413 408.5 414.5 421 413
2.70 5.05 8.45 5.05 11.20 23.15 11.55 26.80 47.70 22.05
406 407 409 407 411 418 411 421 431 419
3.30 5.85 9.50 5.30 11.95 25.10 13.10 25.25 48.70 22.40
407.15 408.15 411.15 408.15 413.65 424.48 415.15 426.65 439.15 424.65
3.90 6.25 10.00 6.60 12.50 24.80 13.55 24.20 48.75 21.90
408 410 413.5 410 417 430 418 432 448.5 429.5
Fig. 2. Relationship between n and the fiber content.
contrast to that of the SEBF/PBS composite, for which n decreased monotonically. When the fiber content was relatively low (e.g., 10 wt%), n for the SESF/PBS composite was a minimum because SESFs are flexible and are easily entangled into aggregated states. An increase in the rotation speed results in an increased force applied on the fiber aggregates. The degree of disentanglement and orientation of the fibers was therefore much more obvious than for the other two composites, which resulted in a rapid reduction in the apparent viscosity with increasing rotation speed. That is to say, the apparent viscosity is more sensitive to a change in velocity, and n is small. Untreated SFs are rigid and do not readily disentangle and orient under weak shear stress, so the flow of the melt is difficult. Therefore, the apparent viscosity of the SF/PBS composite is less sensitive to changes in shear rate than the SESF/PBS composite and thus n was higher than that of the SESF/PBS composite. SEBFs are much shorter than SESFs and there are many tiny fragments of cells other than fiber cells, so when the fiber content is relatively low, SEBFs can readily disperse in the melt. As a result, the reduction of n is much lower from that of the pure PBS compared with the other two composites. When the fiber content increases to 30 wt%, the n values of the three types of composite are further reduced. This is because the higher the fiber content, the higher the force applied on the fibers and the more disentangled and cut fibers become. Therefore, the composites show greater shear-thinning behavior when the fiber content increases and the n values for the three types of composites decrease to a similar level.
When the fiber content further increases to 50 wt%, the n values of SF/PBS and SESF/PBS composites increase, which differs from the trend for the SEBF/PBS composite, in which n continues to decrease. This is because the viscosity of the PBS used is very low so the increase in shear stress with shear rate is not large enough to disperse the high content fibers and the amount of the fiber aggregates increased. Thus, the actual contact area between fibers and matrix became smaller and the viscosity of the composite is less sensitive to changes in shear rate than that of composites with lower fiber content. Additionally, the slender and flexible SESFs deformed easily so they tangled to a greater extent than SFs and the n value of the SESF/PBS composite was higher than that of the SF/PBS composite. This meant the shear thinning behavior of SESF/PBS composite was not as strong as that of the SF/PBS composite with the same fiber content. With regard to the SEBF reinforced composite, because there are many tiny fragments of cells other than fiber cells and the average aspect ratio of BF fiber cells is much smaller than that of the SF fiber cell, despite the small applied stress, the shear stress became large enough to disentangle and orient the SEBFs so that the fiber could be more evenly dispersed in the PBS matrix with increasing shear rate. Thus, the SEBF/PBS composite had the lowest value of n.
3.4. Analysis of consistency index, m Fig. 3 shows the relationship between the consistency index, m (the viscosity at unit shear rate, i.e., at c_ ¼ 1 s1 ), and the fiber content of the three types of composite reinforced by fibers of different morphologies. It can be seen from Fig. 3 that the m values of the plant fiber reinforced composites are all higher than that of the pure PBS. This can be explained as follows. First, fibers with a large aspect ratio are easily entangled, resulting in an increased force between the fibers when the fibers move relative to other fibers. Second, the tangled fibers block the flow of the matrix. Therefore, the addition of the fibers increases the consistency index over that of the PBS. For the SF/PBS and SESF/PBS composites, their consistency indices initially increased then decreased, while the m value of the SEBF/PBS composite increased with increasing fiber content. This special phenomenon is related to the variation in the difference between fiber impregnation degrees of the three types of composite, caused by changes in fiber content. When the fiber content is relatively low (10 wt%, 30 wt%), the SESF/PBS composite had the highest value of m. This is because the slender SESFs are flexible and deform easily, so the fibers in the mixture readily entangle into fiber aggregates. When the shear rate is very low (such as unit shear rate), it is difficult to impregnate, move and separate these aggregated fibers, making the consistency index of the SESFs reinforced composite much larger than those of PBS and the other two types of composite. Compared with SESFs, SFs are rigid, easily broken, and the average aspect ratio
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Fig. 3. Relationship between m and fiber content.
of these fibers is smaller, so when the fiber content is not high, the degree of the entanglement is low, which gives a smaller m value than that of the SESF composite. With regard to the SEBF/PBS composite, BF fiber cells are much shorter than SF fiber cells and there are fine fragments of cells other than fiber cells in SEBFs. When the fiber content is relatively low, SEBFs do not readily entangle and can be easily dispersed in the matrix, so the m value of the SEBF composite is much smaller than that of the SESF composite. When the fiber content increases (from 10 wt% to 30 wt%), the melt content is reduced correspondingly, so impregnation, movement and separation of entangled fibers becomes more difficult, resulting in an increased consistency index. As the fiber content increases to 50 wt%, one may expect m to increase, which is a trend that have been reported in our previous works [24,25]. However, in the present work, the m value of SEBF/ PBS composites continues to increase while, in contrast, those of the SESF/PBS and SF/PBS composites decrease. This is due to the presence of cell fragments in the SEBFs as described above and the fact that BF fiber cells are much shorter than SF fiber cells. Thus, SEBFs can be dispersed more easily than SFs and SESFs and there is very contact area between SFBFs and the matrix. Because the fibers hinder the flow of the matrix, the larger the contact area between fibers and matrix, the more difficult it is for the matrix to flow. At low shear rates, it is difficult to move the SEBF/PBS composite so the m value of the SEBF/PBS composite is very large.
Although SFs and SESFs tangle and aggregate more easily than SEBFs when the fiber content increases to 50 wt%, the aggregation of SFs and SESFs is more extensive, so the actual contact areas between SFs or SESFs and the matrix become much lower than that between SFBFs and the matrix, and the m values for the SF/PBS and SESF/PBS composites are lower than that of the SEBF/PBS composite. In addition, at low shear rate (such as unit shear rate), as the degree of aggregation became more extensive in the SF/PBS and SESF/PBS composites with increased fiber content from 30 to 50 wt%, the actual contact areas between fibers and matrix reduced, as illustrated by Fig. 4, so their m values decreased compared with the corresponding composites at 30 wt% fiber content. 3.5. Analysis of average shear viscosity, average shear stress and the average shear rate The average shear viscosity, average shear stress and the average shear rate can be determined by methods presented by Marquez et al. [20], and their values can be solved on the basis of the n and m values calculated above. It can be seen from Fig. 5 that the average shear viscosities of all composites with the same fiber content differ because of the different morphologies of the fibers and also that the average shear viscosities of these composites are higher than that of pure PBS. The flexible and slender SESFs, which have the largest surface areas,
Fig. 4. Fibers dispersion of composites of different fiber content under low shear rate (a) low fiber content and (b) high fiber content.
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Fig. 5. Relationship between the average shear viscosity and average shear rate.
Fig. 6. Relationship between the average shear stress and average shear rate.
can be deformed easily, so when the fiber content is fixed, the average g values of the SESF/PBS composite are usually higher than those of the other two composites at the same shear rate. Although the SEBFs have a high surface area, the average aspect ratio is much smaller than that of SESFs and there are many fragments of cells other than BF fiber cells, so the average g values of the SEBF/PBS composite are usually the smallest. With regard to SFs, which have a smaller surface area than the other two types of fiber, the fibers are rigid and during mixing they can produce a high flow resistance for the matrix, so the g values are higher than that of the SEBF/PBS composite but lower than that of the SESF/PBS composite. It is shown in Fig. 6 that the average shear stress of the three types of composite increases with average shear rate. For composites with a fiber content of 50 wt%, when the rotation speed reaches 50 rpm, the average shear stress of the SEBF/PBS composite is the smallest, at about 132 kPa, the shear stress of the SF/PBS composite is about 374 kPa, and the shear stress of the SESF/PBS composite is the highest, at about 639 kPa, which is about 4.8
times higher than that of the SEBF/PBS composites and 19 times higher than that of pure PBS. Therefore, the morphology of the reinforcement fibers in the composites has a marked effect on the processing power.
4. Conclusions The rheological properties of three different types of plant fiber reinforced PBS composites were evaluated using a torque rheometer. The results show that even with the same fiber content, the rheological properties of the composites reinforced by fibers of different morphologies differed. The non-Newtonian index n is related to the degree of change of the orientation, disentanglement and fracture of fibers caused by a change in rotation speed. When the fiber content is relatively low, the fibers can be uniformly dispersed in the matrix and the fibers become more oriented with the increase in rotation speed. Therefore, compared with the n value of PBS, the decrements in the n values of the SF/
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PBS and SESF/PBS composites (in which the reinforcements are fibrous) are larger than that of the SEBF/PBS composite, which includes many small fragments of cells other than BF fiber cells. At medium fiber content, the degree of fiber entanglement is more extensive, increasing the interaction among these fibers. With an increase in rotation speed, an increased degree of fiber disentanglement, orientation and fracture occurs because of the increase in stress applied to the fibers, therefore causing a further reduction in the values of n. When the fiber content is relatively high, there is a high tendency for the fibrous SFs and SESFs to tangle into aggregates within the matrix. In addition, the viscosity of the matrix is too low to generate sufficient stress to evenly disperse the fibers in the matrix so these still exist in the matrix even at increased shear rate. As a result, the actual contact area between matrix and fibers decreases, causing the shear viscosities in these two types of composites to become less sensitive to changes in shear rate. Therefore, the n values of these two composites are higher than that of the SEBF/PBS composite, in which there are not as many aggregates because of the shorter average aspect ratio of SEBFs and the presence of cell fragments other than BF fiber cells. The consistency index of the composite is also closely related to fiber morphology. Generally speaking, at the same fiber content, the consistency indices of fibrous filler-reinforced composites are larger than those of powder-filled composites. The larger the fibrous filler surface area, the higher the consistency index of the composite. The larger the actual contact area between the matrix and the fibers, the greater the consistency index of the composite. Acknowledgments The authors acknowledge the National Nature Science Foundation of China (Nos. 50903033, 50973035 and 51073061), the National Key Technology R&D Program of China (Nos. 2009BAI84B05 and 2009BAI84B06), Program for New Century Excellent Talents in University (No. NCET-11-0152), Pearl River Talent Fund for Young Sci-Tech Researchers of Guangzhou City (No. 2011J2200058), the Fundamental Research Funds for the Central Universities (No. 2011ZZ0011) and Opening Project of Technology Development Center for Polymer Processing Engineering of Guangdong Industry Technology College (No. 2010001) for financial support. References [1] Frederick TT, David GC, Milan M. Biodegradation of a synthetic co-polyester by aerobic mesophilic microorganisms. Polym Degrad Stab 2008;93(8):1479–85. [2] Nagahama H, New N, Jayakumar R. Novel biodegradable chitin membranes for tissue engineering applications. Carbohyd Polym 2008;73(2):295–302. [3] Kim HS, Lee BH, Lee S. Enhanced interfacial adhesion, mechanical, and thermal properties of natural flour-filled biodegradable polymer bio-composites. J Therm Anal Calorim 2011;104(1):331–8.
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