- Email: [email protected]
polyurethane foam composite
Materials Science and Engineering A 459 (2007) 111–116
Effect of ultrasound sonication in carbon nanofibers/polyurethane foam composite Md. E. Kabir a,∗ , M.C. Saha b , S. Jeelani c a
b
School of Aeronautics and Astronautics, Purdue University, West Lafayette, IN 47907, USA School of Aerospace and Mechanical Engineering, The University of Oklahoma, Norman, OK 73019, USA c Tuskegee University’s Center for Advanced Materials (T-CAM), Tuskegee, AL 36088, USA Received 20 November 2006; received in revised form 18 December 2006; accepted 11 January 2007
Abstract Doping of nanoparticles into the polymer can tailor its mechanical properties. Mixing of the nanoparticles with the polymer is the most critical issue there. Better mixing between these two can provide higher strength and stiffness whereas poor mixing is seen to decrease those properties. Ultrasound sonication is one of the promising approaches to disperse the nanoparticles into the base material thoroughly. But process parameters and base materials properties affect this mixing process. In this study the effects of different process parameters of sonication technique for the doping of carbon nanofibers (CNFs) into rigid polyurethane (PU) foam have been investigated. Quasi-static compression tests were performed on nanophased PU foam that has been manufactured in different ways and compressive yield strength is taken as the comparative parameter. It is observed that the favorable sonication is achieved for part A of the foam. Sonication has an optimum time limit which varies with sonicator power, wt% of nanoparticles and foam amount. © 2007 Elsevier B.V. All rights reserved. Keywords: Ultrasound sonication; Nanocomposite; Compressive yield strength
1. Introduction Over the last few years, a large number of chemists and engineers working in synthesis and processing have drawn interest in sonochemistry. There are a large number of industrial processes that employ sonication technique as an energy source for the generation of the fine emulsions and dispersions. One of the earliest devices that were developed for this purpose was the socalled liquid whistle and this continues to be used widely. Typical examples of the uses of such whistles include the preparation of emulsion bases for soups, sauces or gravies. Development of nanocomposites—a new class of material which includes more than one solid phase where at least one phase has dimension in the nanometer range, has enriched the scope of sonication technique. Nanocomposites have drawn much interest in recent years as they can lead to new and improved properties when compared to their micro- and macro-composite counterparts. Experimental
∗
Corresponding author. Tel.: +1 765 409 3237; fax: +1 765 494 0307. E-mail address: [email protected] (Md.E. Kabir).
0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.01.031
work on these materials has generally shown that nanoparticles have increased modulus and strength [1–3], decreased permeability [4], decreased shrinkage [5,6], increased heat resistance and decreased flammability [7] of the composite material. However, due to high surface energy, the nanoparticles tend to agglomerate and in most cases it is very difficult to disperse these nanoparticles into the polymer matrix. Agglomerated nanoparticles act as defects and can have detrimental effect on polymer performances. The improved properties are mainly depending on the fine dispersion of nanoparticles inside the matrix. High-intensity ultrasonic waves may be useful in this context as they generate some important nonlinear effects in the liquids, namely transient cavitation and acoustic streaming [8–11]. Liquid medium is necessary because sonochemistry is driven by acoustic cavitation that only occurs in liquids. Acoustic cavitation involves the formation, growth, pulsating and collapsing of tiny bubbles, producing transient (in the order of microseconds) micro-hot spots that can reach temperatures of about 5000 ◦ C, pressures of about 1000 atm, and heating and cooling rates above 1010 K/s [12]. The strong impact coupling with local high temperatures can also enhance the wettability between polymer and nanoparticles; thus can break the agglomerating bodies by
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damaging the Coulomb and van der Waals forces between the particles and make them disperse homogeneously in the liquid medium. With the intention to modify the matrix properties, the effect of nanoparticles infusion on epoxy matrix has been performed by Zheng and Ning [13] and Wetzel et al. [14]. In their work, Zheng and Ning employed high frequency ultrasonic and mechanical method to disperse spherical SiO2 in epoxy resin. The nanocomposites were characterized by tensile and impact testing as well as TEM studies. They found that with the addition of 3 wt% of SiO2 in epoxy, the tensile strength, stiffness and the impact strength of the nanocomposites were improved by 114%, 13% and 56%, respectively. Wetzel et al. [14] incorporated various amount of nano-sized Al2 O3 particles into the epoxy polymer matrix and investigated the influence of these particles on the impact energy, flexural strength, dynamic mechanical thermal properties and block-on-ring wear behavior. They concluded that the newly developed nanocomposites were capable of exhibiting superior performance to the neat polymer. However, a systematic study of the dispersion of nanoparticles using the ultrasound process has not yet been studied elsewhere. In this paper various process parameters for CNFs dispersion into PU foam such as the effect of temperature control for sonicating materials, choice of foam part to sonicate, sonication time, power amplitude, amount of nanoparticles and foam amount have been studied under tip sonication process. 2. Test materials The base material of this study is two-part PU foam with 240 kg/m3 density and mixing ratio of 52:48 by weight. PU foam precursors in the form of liquid were supplied by Utah Foam Products Inc. [15]. The main ingredients for part A, are 4-4 -diphenylmethane diisocyanate (50–75%); and modified MDIs and other oligomers (25–50%) whereas Part-B is mainly polyol (mixed with blowing and curing agents) with ingredients of polyether resin (50–95%) and polyester resin (0–20%). Rod shaped nanoparticles; CNFs were added to form CNFs/PU nanocomposites. CNFs have purity of 95% and density of 1.95 g/cm3 , average outside diameter of 240–500 nm, core diameter of 0.5–10 nm and length of 5–10 nm were supplied by Nanostructured and Amorphous Materials, Inc. [16]. 3. Manufacturing Manufacturing of nanophased PU foam for this study is a four-step process which is shown in Fig. 1. In step 1, specific wt% of nanoparticles is mixed with preferred PU part. The mixture is then irradiated in step 2 under ultrasonic cavitation technique with a high-intensity ultrasonic horn (Ti-horn, 20 kHz, and 100 W/cm2 ) supplied by Sonics and Materials Inc. [17] for a specific period of time and at a specific power amplitude to produce a homogeneous dispersion. External cooling against the mixing beaker is employed in order to avoid the temperature rise during sonication by submerging the container in a thermostatic bath at around ±1 ◦ C for the entire period of the ultrasound irradiation. In step 3, nanoparticles modified part is mixed with
Fig. 1. Schematic of manufacturing of nanophased PU foam: (a) step 1; (b) step 2; (c) step 3; (d) step 4.
counter foam part at a ratio of A:B = 52:48 using a mechanical stirrer at 2500 rpm for about 30 s. In step 4, the mixture is poured in an open plastic container and is kept inside the oven at 37.8 ◦ C for 24 h for curing. The open plastic container is used in this study in order to reduce the over pack caused by the foam mold. Finally, nanophased foam is taken out from the plastic container for sample preparation. 4. Experimental investigation The Quasi-static compression test was conducted to all samples and yield strength was taken as a comparative parameter. Compression tests were performed on prismatic bar specimens with dimension of 25.4 mm × 25.4 mm × 12.7 mm using a servo-hydraulic MTS testing system according to the ASTM C365-00 with a cross-head speed of 0.127 mm/s. The tests were terminated when applied load was reached about densification level of 60%. Specimen displacements were recorded from the cross-head movement and data acquisition systems, Testware-SX were used to record both the applied load and cross-head displacement during the experiment. The intersection point between the initial slope and the plateau slope is used to calculate the compressive yield strength. The decomposition temperature of the PU foam at 0.5 wt% CNFs content was measured using TGA (Thermogravimetric Analysis) with heating of 5–10 g samples from 30 to 800 ◦ C at 10 ◦ C temperature steps. To observe the dispersion of the nanoparticles into the PU foam; the untested foam samples (perpendicular to the foam rise direction) were examined under a Field Emission Scanning Electron Microscope (FE-SEM). 5. Results and discussion 5.1. Effect of foam parts In order to understand the favorable mixing part more precisely, the nanoparticles were added in both parts A and B separately. To compare the effect of sonication, nanoparticles were also added with PU foam without any sonication. In this particular case only the mechanical mixing is performed for about 30 s to mix all, i.e. part A, part B and CNFs. Compressive stress–strain curves for the neat and nanophased PU foams are shown in Fig. 2. The curves show three stages of deformation;
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Fig. 3. Yield strength plot for different times of sonication. Fig. 2. Compression test plot for CNFs/PU foam with different parts sonication.
initial linear behavior, linear plateau region, and finally, densification. Three samples are tested in each category and the average is shown in Table 1. It is observed that only the compressive yield strength for nanophased foam prepared from sonication of CNFs and the part A of PU is higher than neat foam. Sonicating CNFs and part B of the foam gives lower yield strength than the samples with no sonication but both have the same wt% content of CNFs. This is because the sonication technique mixes the nanoparticles better than hand mixing and part A is less reactive and has low viscosity which makes it easier to mix thoroughly with the nanoparticles. 5.2. Effect of sonication time Sonication cannot be done for an indefinite period of time. There should be an optimum period for a particular wt% of nanoparticles. For this study a total of 100 g (includes 51.74 g part A, 47.76 g part B and 0.5 g of CNFs) was chosen. A series of nanophased PU foams were prepared with different sonication time such as 10, 20, 30 and 40 min. Samples are prepared from these four batches and compressive strength is plotted in Fig. 3. A best fit polynomial curve is plotted and the maximum compressive yield strength is found at about 22 min of sonicating time. Further increase in sonication time causes gradually decrease in yield strength from 3.27 MPa (20 min) to 3.02 MPa (40 min). From the trend of the compressive yield strength value in the CNFs/PU nanocomposite samples, with different sonicating times, it is clear that the time for the ultrasound sonication must be adequately controlled in order to achieve the maximum
mechanical performance of the composites. At any sonicating time lower or higher than the optimum value, the yield strength will be adversely affected and may be worse than the original PU foam sample. Decomposition temperature was also measured for 0.5 wt% CNFs/PU foam samples with different sonication time such as 0, 10, 20, 30 and 40 min of sonication using TGA with heating of 5–10 g samples from 30 to 800 ◦ C at 10 ◦ C temperature steps. Three samples are taken from each kind and the average along with the error bar is shown in Fig. 4. As shown from Fig. 4 the average value is almost similar in all the cases. But the trend of error bar is like a parabola where minimum breadth is found between 20 and 30 min sonication samples. The breadth becomes higher again after that region. To understand the effect of sonication time SEM was performed on 20 and 40 min samples. Samples were cooled to freeze in liquid nitrogen which were cut with a sharp razor blade and were placed on a sample holder with a silver paint. Then the samples were coated with gold palladium to prevent charge build-up by the electrons to be absorbed by the specimen. Sputtering was done using Hummer 6.2 to coat the samples. Resulting samples
Table 1 Compressive yield strength for different types of foam part sonication Type of foam
Yield strength (MPa)
Percent enhancement
Neat no sonication 0.5 wt% CNFs—no sonication 0.5 wt% CNFs—part A sonication 0.5 wt% CNFs—part B sonication
2.96 2.52 3.27 2.28
– −14.5 10.5 −23.0
Fig. 4. Decomposition temperature plot for different times of sonication.
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Fig. 5. SEM images for CNFs and 0.5 wt% CNFs/PU foam with different times of sonication: (a) CNFs with inset of a single fiber; (b) 0.5 wt% CNFs/PU—no sonication; (c) 0.5 wt% CNFs/PU—20 min sonication; (d) 0.5 wt% CNFs/PU—40 min sonication.
were analyzed under a FE-SEM (Hitachi S-900) JEOL JSM 5800. Images were obtained using a 30 kV accelerating voltage. SEM micrographs of CNFs are shown in Fig. 5a. It is observed that nanoparticles are seen to form agglomeration due to high surface energy. Since CNFs have length in the order of micron, it is possible to observe individual CNF (inset of Fig. 5a). The diameter of CNF is measured to be about 200 nm, which agrees with the data sheet [16]. As the size of nanoparticles is small, specific surface area is large, and consequently also the surface energy. Thus the adhesive force between the nanoparticles is strong and the particles easily agglomerate. Relatively large agglomerating bodies with a number of weak joining interfaces are thus formed which can be seen from this figure. All foam pictures (Fig. 5b–d) are taken perpendicular to the foam rise direction for different sonication times. CNFs were only found in the cell edges. It can be seen from Fig. 5 that the CNFs are agglomerated for no sonication, they becomes homogenously dispersed for 20 min sonication (Fig. 5c) whereas in the 40 min sample (Fig. 5d) a chunk of CNFs is found again. This suggests that the best property is only obtained at optimum sonication time whereas higher or lower time than optimum may make the properties lower by worsening the mixing between nanoparticles and base material. Ultrasound sonication is a form of vibration that provides energy for the agglomerated CNFs to escape from the surrounding resisting force. If there is not enough energy given to the CNFs/PU mixture, the CNFs agglomerate cannot escape the resisting force within the CNFs
clusters; thus the aid for dispersion is limited. On the other hand, if too much energy is given to the CNFs cluster to move around, then the frequency of collision between each single CNF will be increased. The chance for each single CNF to tangle up and react to form a larger CNFs cluster would be increased. Hence the dispersion mechanism may be adversely affected with too much energy given to the CNFs. Therefore, an optimum sonicating time must be achieved in order to the maximum dispersion ability. 5.3. Effect of sonicator power amplitude The sonicator used in this study for the manufacturing of nanophased foam, has an intensity of 100 W/cm2 and a frequency of 20 kHz. It can operate at different powers by changing the amplitude (with respect to its limit value). In order to understand the effect of sonication, the amplitude was varied in different percentage (40, 50 and 60). In each case four different sonication times 10–40 min (at an interval of 10 min) were used. From each power amplitude percentage, maximum yield strength and time to reach maximum yield strength were obtained by plotting a cubic polynomial curve. These values are plotted against power amplitude percentage which is shown in Fig. 6. As can be seen from Fig. 6 the yield strength in all the cases is almost same. But the time to reach the maximum yield strength lowers as the amplitude percentage is increased. This is because the amount of energy input to the liquid media is
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Fig. 6. Effect of power amplitude on sonication.
proportional to the amplitude. The higher the input energy the lower is the time for optimum mixing. 5.4. Effect of amount of nanoparticles Different weight percentages of CNFs (0.5–1.5%) were infused in the base material to observe its effect on yield strength. In this particular case a 50% power amplitude of the sonicator was used. Four different sonication times were used in each case at an interval of 10 min: such as for 0.5 wt% 10–40 min, for 1.0 wt% 30–60 min and for 1.5 wt% 40–70 min. From each wt%, maximum yield strength and time to reach maximum yield strength were obtained by plotting a cubic polynomial curve. These values are plotted against weight percentage which is shown in Fig. 7. As can be seen from the figure the yield strength is increased for doping of CNFs up to a wt% percentage of 1.0. For 1.5 wt% CNFs the yield strength is reduced again. A highest value of 3.55 MPa was obtained for 1.0 wt% (20% higher than neat PU). Time to reach maximum yield strength also increases with the increase of CNFs wt%. 5.5. Effect of amount of foam Optimum time for sonication is dependent on the amount of base material. In order to get the optimum time to reach maximum yield strength, three different amounts of PU foam
Fig. 8. Effect of amount of foam on sonication.
were taken such as: a total of 100, 200 and 300 g PU including CNFs. In all the cases 0.5 wt% CNFs was used. Four different sonication times at an interval of 10 min were used in each case such as for 100 and 200 g, 10–40 min and for 300 g, 20–50 min. From each type maximum yield strength and time to reach maximum yield strength were obtained by plotting of a cubic polynomial curve. These values are plotted against amount of foam which is shown in Fig. 8. As seen from Fig. 8 the yield strength in all the cases is almost same. But the time to reach the maximum yield strength goes higher as the amount of base material is increased. This is because for higher amount of nanophased foam (including PU and the nanoparticles), it requires more time to reach maximum yield strength. 6. Summary The following are the summary of the investigation: 1. Nanophased polyurethane foam was manufactured with different process conditions using sonication technique. Samples were tested in compression configuration. 2. Part A was found to be best suited for sonication. 3. Sonication time has an optimum value for a specific amount of base material and a specific wt% of nanoparticles. 4. Sonication time varies inversely with the power amplitude of the sonicator. 5. Higher sonication time is required for higher wt% percentage of nanoparticles keeping the total amount of material fixed. 6. For higher amount of base material, higher sonication time is required for sonicator to generate optimum mixing level, even though PU has a specific wt% of nanoparticles. Acknowledgement
Fig. 7. Effect amount of nanoparticles on sonication.
The authors acknowledge with appreciation the National Science Foundation (NSF) under CREST program for supporting this work.
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References [1] H. Mahfuz, S. Islam, V.K. Rangari, M.C. Saha, S. Jeelani, Composites: Part B 35 (2004) 543. [2] C. Zilg, R. Thomann, R. Muelhaupt, Finter, J. Adv. Mater. 11 (1999) 49. [3] Y. Kojima, A. Usuki, M. Kawasumi, A. Okada, Y. Fukushima, T. Kurauchi, O. Kamigaito, J. Mater. Res. 8 (1993) 1185. [4] P.B. Messersmith, E.P. Giannelis, J. Polym. Sci. A: Polym. Chem. 33 (7) (1995) 1047. [5] Y. Kojima, K. Fukumori, A. Usuki, A. Okada, T. Kurauchi, J. Mater. Sci. Lett. 12 (1993) p889. [6] P. Kelly, A. Akelah, S. Qutbuddin, A. Moet, J. Mater. Sci. 29 (1994) 2274. [7] S.D. Burnside, E.P. Giannelis, Chem. Mater. 7 (9) (1995) 1597. [8] W.L.M. Nyborg, Acoustic Streaming, in: W.P. Mason (Ed.), Physical Acoustics, IIB, Academic Press, New York, 1965, p. 355.
[9] B.E. Noltingk, E.A. Neppiras, Proc. Phys. Soc. 63B (1950) 674. [10] K.S. Suslick (Ed.), Ultrasound: It’s Chemical, Physical, and Biological Effects, VCH, New York, 1988. [11] O. Abramov, Ultrasound in Liquid and Solid Metals, CRC Press, Boca Raton, FL, 1994, p. 289. [12] K.S. Suslick, Y. Didenko, M.M. Fang, T. Hyeon, K.J. Kolbeck, W.B. Mcnamara, M.M. Midleleni, M. Wong, Phil. Trans. R. Soc. Lond. A 357 (1999) 335. [13] Y. Zheng, R. Ning, Mater. Lett. 57 (2003) 2940. [14] B. Wetzel, F. Haupert, M.Q. Zhang, Compos. Sci. Technol. 63 (2003) 1. [15] Material data sheet: PUR240, Utah Foam, 3609 South 700 west, Salt Lake City, UT 84119, USA. [16] Material data sheet: CNFs, Nanostructured and Amorphous Materials, Inc., USA. [17] Sonics Vibra Cell, Sonics and Materials Inc., USA.
Effect of ultrasound sonication in carbon nanofibers/polyurethane foam composite Md. E. Kabir a,∗ , M.C. Saha b , S. Jeelani c a
b
School of Aeronautics and Astronautics, Purdue University, West Lafayette, IN 47907, USA School of Aerospace and Mechanical Engineering, The University of Oklahoma, Norman, OK 73019, USA c Tuskegee University’s Center for Advanced Materials (T-CAM), Tuskegee, AL 36088, USA Received 20 November 2006; received in revised form 18 December 2006; accepted 11 January 2007
Abstract Doping of nanoparticles into the polymer can tailor its mechanical properties. Mixing of the nanoparticles with the polymer is the most critical issue there. Better mixing between these two can provide higher strength and stiffness whereas poor mixing is seen to decrease those properties. Ultrasound sonication is one of the promising approaches to disperse the nanoparticles into the base material thoroughly. But process parameters and base materials properties affect this mixing process. In this study the effects of different process parameters of sonication technique for the doping of carbon nanofibers (CNFs) into rigid polyurethane (PU) foam have been investigated. Quasi-static compression tests were performed on nanophased PU foam that has been manufactured in different ways and compressive yield strength is taken as the comparative parameter. It is observed that the favorable sonication is achieved for part A of the foam. Sonication has an optimum time limit which varies with sonicator power, wt% of nanoparticles and foam amount. © 2007 Elsevier B.V. All rights reserved. Keywords: Ultrasound sonication; Nanocomposite; Compressive yield strength
1. Introduction Over the last few years, a large number of chemists and engineers working in synthesis and processing have drawn interest in sonochemistry. There are a large number of industrial processes that employ sonication technique as an energy source for the generation of the fine emulsions and dispersions. One of the earliest devices that were developed for this purpose was the socalled liquid whistle and this continues to be used widely. Typical examples of the uses of such whistles include the preparation of emulsion bases for soups, sauces or gravies. Development of nanocomposites—a new class of material which includes more than one solid phase where at least one phase has dimension in the nanometer range, has enriched the scope of sonication technique. Nanocomposites have drawn much interest in recent years as they can lead to new and improved properties when compared to their micro- and macro-composite counterparts. Experimental
∗
Corresponding author. Tel.: +1 765 409 3237; fax: +1 765 494 0307. E-mail address: [email protected] (Md.E. Kabir).
0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.01.031
work on these materials has generally shown that nanoparticles have increased modulus and strength [1–3], decreased permeability [4], decreased shrinkage [5,6], increased heat resistance and decreased flammability [7] of the composite material. However, due to high surface energy, the nanoparticles tend to agglomerate and in most cases it is very difficult to disperse these nanoparticles into the polymer matrix. Agglomerated nanoparticles act as defects and can have detrimental effect on polymer performances. The improved properties are mainly depending on the fine dispersion of nanoparticles inside the matrix. High-intensity ultrasonic waves may be useful in this context as they generate some important nonlinear effects in the liquids, namely transient cavitation and acoustic streaming [8–11]. Liquid medium is necessary because sonochemistry is driven by acoustic cavitation that only occurs in liquids. Acoustic cavitation involves the formation, growth, pulsating and collapsing of tiny bubbles, producing transient (in the order of microseconds) micro-hot spots that can reach temperatures of about 5000 ◦ C, pressures of about 1000 atm, and heating and cooling rates above 1010 K/s [12]. The strong impact coupling with local high temperatures can also enhance the wettability between polymer and nanoparticles; thus can break the agglomerating bodies by
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damaging the Coulomb and van der Waals forces between the particles and make them disperse homogeneously in the liquid medium. With the intention to modify the matrix properties, the effect of nanoparticles infusion on epoxy matrix has been performed by Zheng and Ning [13] and Wetzel et al. [14]. In their work, Zheng and Ning employed high frequency ultrasonic and mechanical method to disperse spherical SiO2 in epoxy resin. The nanocomposites were characterized by tensile and impact testing as well as TEM studies. They found that with the addition of 3 wt% of SiO2 in epoxy, the tensile strength, stiffness and the impact strength of the nanocomposites were improved by 114%, 13% and 56%, respectively. Wetzel et al. [14] incorporated various amount of nano-sized Al2 O3 particles into the epoxy polymer matrix and investigated the influence of these particles on the impact energy, flexural strength, dynamic mechanical thermal properties and block-on-ring wear behavior. They concluded that the newly developed nanocomposites were capable of exhibiting superior performance to the neat polymer. However, a systematic study of the dispersion of nanoparticles using the ultrasound process has not yet been studied elsewhere. In this paper various process parameters for CNFs dispersion into PU foam such as the effect of temperature control for sonicating materials, choice of foam part to sonicate, sonication time, power amplitude, amount of nanoparticles and foam amount have been studied under tip sonication process. 2. Test materials The base material of this study is two-part PU foam with 240 kg/m3 density and mixing ratio of 52:48 by weight. PU foam precursors in the form of liquid were supplied by Utah Foam Products Inc. [15]. The main ingredients for part A, are 4-4 -diphenylmethane diisocyanate (50–75%); and modified MDIs and other oligomers (25–50%) whereas Part-B is mainly polyol (mixed with blowing and curing agents) with ingredients of polyether resin (50–95%) and polyester resin (0–20%). Rod shaped nanoparticles; CNFs were added to form CNFs/PU nanocomposites. CNFs have purity of 95% and density of 1.95 g/cm3 , average outside diameter of 240–500 nm, core diameter of 0.5–10 nm and length of 5–10 nm were supplied by Nanostructured and Amorphous Materials, Inc. [16]. 3. Manufacturing Manufacturing of nanophased PU foam for this study is a four-step process which is shown in Fig. 1. In step 1, specific wt% of nanoparticles is mixed with preferred PU part. The mixture is then irradiated in step 2 under ultrasonic cavitation technique with a high-intensity ultrasonic horn (Ti-horn, 20 kHz, and 100 W/cm2 ) supplied by Sonics and Materials Inc. [17] for a specific period of time and at a specific power amplitude to produce a homogeneous dispersion. External cooling against the mixing beaker is employed in order to avoid the temperature rise during sonication by submerging the container in a thermostatic bath at around ±1 ◦ C for the entire period of the ultrasound irradiation. In step 3, nanoparticles modified part is mixed with
Fig. 1. Schematic of manufacturing of nanophased PU foam: (a) step 1; (b) step 2; (c) step 3; (d) step 4.
counter foam part at a ratio of A:B = 52:48 using a mechanical stirrer at 2500 rpm for about 30 s. In step 4, the mixture is poured in an open plastic container and is kept inside the oven at 37.8 ◦ C for 24 h for curing. The open plastic container is used in this study in order to reduce the over pack caused by the foam mold. Finally, nanophased foam is taken out from the plastic container for sample preparation. 4. Experimental investigation The Quasi-static compression test was conducted to all samples and yield strength was taken as a comparative parameter. Compression tests were performed on prismatic bar specimens with dimension of 25.4 mm × 25.4 mm × 12.7 mm using a servo-hydraulic MTS testing system according to the ASTM C365-00 with a cross-head speed of 0.127 mm/s. The tests were terminated when applied load was reached about densification level of 60%. Specimen displacements were recorded from the cross-head movement and data acquisition systems, Testware-SX were used to record both the applied load and cross-head displacement during the experiment. The intersection point between the initial slope and the plateau slope is used to calculate the compressive yield strength. The decomposition temperature of the PU foam at 0.5 wt% CNFs content was measured using TGA (Thermogravimetric Analysis) with heating of 5–10 g samples from 30 to 800 ◦ C at 10 ◦ C temperature steps. To observe the dispersion of the nanoparticles into the PU foam; the untested foam samples (perpendicular to the foam rise direction) were examined under a Field Emission Scanning Electron Microscope (FE-SEM). 5. Results and discussion 5.1. Effect of foam parts In order to understand the favorable mixing part more precisely, the nanoparticles were added in both parts A and B separately. To compare the effect of sonication, nanoparticles were also added with PU foam without any sonication. In this particular case only the mechanical mixing is performed for about 30 s to mix all, i.e. part A, part B and CNFs. Compressive stress–strain curves for the neat and nanophased PU foams are shown in Fig. 2. The curves show three stages of deformation;
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Fig. 3. Yield strength plot for different times of sonication. Fig. 2. Compression test plot for CNFs/PU foam with different parts sonication.
initial linear behavior, linear plateau region, and finally, densification. Three samples are tested in each category and the average is shown in Table 1. It is observed that only the compressive yield strength for nanophased foam prepared from sonication of CNFs and the part A of PU is higher than neat foam. Sonicating CNFs and part B of the foam gives lower yield strength than the samples with no sonication but both have the same wt% content of CNFs. This is because the sonication technique mixes the nanoparticles better than hand mixing and part A is less reactive and has low viscosity which makes it easier to mix thoroughly with the nanoparticles. 5.2. Effect of sonication time Sonication cannot be done for an indefinite period of time. There should be an optimum period for a particular wt% of nanoparticles. For this study a total of 100 g (includes 51.74 g part A, 47.76 g part B and 0.5 g of CNFs) was chosen. A series of nanophased PU foams were prepared with different sonication time such as 10, 20, 30 and 40 min. Samples are prepared from these four batches and compressive strength is plotted in Fig. 3. A best fit polynomial curve is plotted and the maximum compressive yield strength is found at about 22 min of sonicating time. Further increase in sonication time causes gradually decrease in yield strength from 3.27 MPa (20 min) to 3.02 MPa (40 min). From the trend of the compressive yield strength value in the CNFs/PU nanocomposite samples, with different sonicating times, it is clear that the time for the ultrasound sonication must be adequately controlled in order to achieve the maximum
mechanical performance of the composites. At any sonicating time lower or higher than the optimum value, the yield strength will be adversely affected and may be worse than the original PU foam sample. Decomposition temperature was also measured for 0.5 wt% CNFs/PU foam samples with different sonication time such as 0, 10, 20, 30 and 40 min of sonication using TGA with heating of 5–10 g samples from 30 to 800 ◦ C at 10 ◦ C temperature steps. Three samples are taken from each kind and the average along with the error bar is shown in Fig. 4. As shown from Fig. 4 the average value is almost similar in all the cases. But the trend of error bar is like a parabola where minimum breadth is found between 20 and 30 min sonication samples. The breadth becomes higher again after that region. To understand the effect of sonication time SEM was performed on 20 and 40 min samples. Samples were cooled to freeze in liquid nitrogen which were cut with a sharp razor blade and were placed on a sample holder with a silver paint. Then the samples were coated with gold palladium to prevent charge build-up by the electrons to be absorbed by the specimen. Sputtering was done using Hummer 6.2 to coat the samples. Resulting samples
Table 1 Compressive yield strength for different types of foam part sonication Type of foam
Yield strength (MPa)
Percent enhancement
Neat no sonication 0.5 wt% CNFs—no sonication 0.5 wt% CNFs—part A sonication 0.5 wt% CNFs—part B sonication
2.96 2.52 3.27 2.28
– −14.5 10.5 −23.0
Fig. 4. Decomposition temperature plot for different times of sonication.
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Fig. 5. SEM images for CNFs and 0.5 wt% CNFs/PU foam with different times of sonication: (a) CNFs with inset of a single fiber; (b) 0.5 wt% CNFs/PU—no sonication; (c) 0.5 wt% CNFs/PU—20 min sonication; (d) 0.5 wt% CNFs/PU—40 min sonication.
were analyzed under a FE-SEM (Hitachi S-900) JEOL JSM 5800. Images were obtained using a 30 kV accelerating voltage. SEM micrographs of CNFs are shown in Fig. 5a. It is observed that nanoparticles are seen to form agglomeration due to high surface energy. Since CNFs have length in the order of micron, it is possible to observe individual CNF (inset of Fig. 5a). The diameter of CNF is measured to be about 200 nm, which agrees with the data sheet [16]. As the size of nanoparticles is small, specific surface area is large, and consequently also the surface energy. Thus the adhesive force between the nanoparticles is strong and the particles easily agglomerate. Relatively large agglomerating bodies with a number of weak joining interfaces are thus formed which can be seen from this figure. All foam pictures (Fig. 5b–d) are taken perpendicular to the foam rise direction for different sonication times. CNFs were only found in the cell edges. It can be seen from Fig. 5 that the CNFs are agglomerated for no sonication, they becomes homogenously dispersed for 20 min sonication (Fig. 5c) whereas in the 40 min sample (Fig. 5d) a chunk of CNFs is found again. This suggests that the best property is only obtained at optimum sonication time whereas higher or lower time than optimum may make the properties lower by worsening the mixing between nanoparticles and base material. Ultrasound sonication is a form of vibration that provides energy for the agglomerated CNFs to escape from the surrounding resisting force. If there is not enough energy given to the CNFs/PU mixture, the CNFs agglomerate cannot escape the resisting force within the CNFs
clusters; thus the aid for dispersion is limited. On the other hand, if too much energy is given to the CNFs cluster to move around, then the frequency of collision between each single CNF will be increased. The chance for each single CNF to tangle up and react to form a larger CNFs cluster would be increased. Hence the dispersion mechanism may be adversely affected with too much energy given to the CNFs. Therefore, an optimum sonicating time must be achieved in order to the maximum dispersion ability. 5.3. Effect of sonicator power amplitude The sonicator used in this study for the manufacturing of nanophased foam, has an intensity of 100 W/cm2 and a frequency of 20 kHz. It can operate at different powers by changing the amplitude (with respect to its limit value). In order to understand the effect of sonication, the amplitude was varied in different percentage (40, 50 and 60). In each case four different sonication times 10–40 min (at an interval of 10 min) were used. From each power amplitude percentage, maximum yield strength and time to reach maximum yield strength were obtained by plotting a cubic polynomial curve. These values are plotted against power amplitude percentage which is shown in Fig. 6. As can be seen from Fig. 6 the yield strength in all the cases is almost same. But the time to reach the maximum yield strength lowers as the amplitude percentage is increased. This is because the amount of energy input to the liquid media is
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Fig. 6. Effect of power amplitude on sonication.
proportional to the amplitude. The higher the input energy the lower is the time for optimum mixing. 5.4. Effect of amount of nanoparticles Different weight percentages of CNFs (0.5–1.5%) were infused in the base material to observe its effect on yield strength. In this particular case a 50% power amplitude of the sonicator was used. Four different sonication times were used in each case at an interval of 10 min: such as for 0.5 wt% 10–40 min, for 1.0 wt% 30–60 min and for 1.5 wt% 40–70 min. From each wt%, maximum yield strength and time to reach maximum yield strength were obtained by plotting a cubic polynomial curve. These values are plotted against weight percentage which is shown in Fig. 7. As can be seen from the figure the yield strength is increased for doping of CNFs up to a wt% percentage of 1.0. For 1.5 wt% CNFs the yield strength is reduced again. A highest value of 3.55 MPa was obtained for 1.0 wt% (20% higher than neat PU). Time to reach maximum yield strength also increases with the increase of CNFs wt%. 5.5. Effect of amount of foam Optimum time for sonication is dependent on the amount of base material. In order to get the optimum time to reach maximum yield strength, three different amounts of PU foam
Fig. 8. Effect of amount of foam on sonication.
were taken such as: a total of 100, 200 and 300 g PU including CNFs. In all the cases 0.5 wt% CNFs was used. Four different sonication times at an interval of 10 min were used in each case such as for 100 and 200 g, 10–40 min and for 300 g, 20–50 min. From each type maximum yield strength and time to reach maximum yield strength were obtained by plotting of a cubic polynomial curve. These values are plotted against amount of foam which is shown in Fig. 8. As seen from Fig. 8 the yield strength in all the cases is almost same. But the time to reach the maximum yield strength goes higher as the amount of base material is increased. This is because for higher amount of nanophased foam (including PU and the nanoparticles), it requires more time to reach maximum yield strength. 6. Summary The following are the summary of the investigation: 1. Nanophased polyurethane foam was manufactured with different process conditions using sonication technique. Samples were tested in compression configuration. 2. Part A was found to be best suited for sonication. 3. Sonication time has an optimum value for a specific amount of base material and a specific wt% of nanoparticles. 4. Sonication time varies inversely with the power amplitude of the sonicator. 5. Higher sonication time is required for higher wt% percentage of nanoparticles keeping the total amount of material fixed. 6. For higher amount of base material, higher sonication time is required for sonicator to generate optimum mixing level, even though PU has a specific wt% of nanoparticles. Acknowledgement
Fig. 7. Effect amount of nanoparticles on sonication.
The authors acknowledge with appreciation the National Science Foundation (NSF) under CREST program for supporting this work.
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