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Polyurethane composite foams including CaCO3 fillers for enhanced sound absorption and compression properties
Composites Science and Technology 194 (2020) 108153
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Composites Science and Technology journal homepage: http://www.elsevier.com/locate/compscitech
Polyurethane composite foams including CaCO3 fillers for enhanced sound absorption and compression properties Hyeon Choe , Jae Heon Lee , Jung Hyeun Kim * Department of Chemical Engineering, University of Seoul, 163 Seoulsiripdae-ro, Dongdaemun-gu, Seoul, 02504, South Korea
A R T I C L E I N F O
A B S T R A C T
Keywords: Polyurethane foam Composites Sound absorption Hysteresis loss Compression
Polyurethane (PU) foams are versatile in automotive applications for sound absorption and seat cushioning products due to their easiness of manufacturing process and cost effectiveness. In this study, PU composite foams including CaCO3 fillers after chemical treatments with oleic acid were fabricated to examine the sound ab sorption and compressive properties. The cell and pore sizes in the PU composite foams did not show much differences with filler contents, but the open porosity revealed clear difference depending on the chemical treatments. The open porosity had lower values with chemically treated fillers than with no-treated cases, and it resulted in the high sound absorption coefficient. The compression strengths were also higher with the chemi cally treated fillers than with no-treated filler cases. Stress relaxation and hysteresis loss showed superior results with the chemically treated fillers compared with the cases with no-treated cases, and it is mainly due to the enhanced interfacial contacts and compatibility between PU matrix and CaCO3 filler surfaces. Therefore, it is much important to modify the filler surface characteristics for good interfacial compatibility to achieve improved physical properties of the PU composite foams.
1. Introduction Since polyurethane (PU) foams have many advantages such as light weight, inexpensive cost, and simple manufacturability, they are widely applied in diverse industries for various applications [1–4]. Especially, in the automobile industries, the PU foams are utilized as sound absorbing and seat cushioning materials to achieve comfortable driving conditions. For examples, on the purpose of sound absorptions, the PU foams have excellent performance in absorbing sound waves with fre quencies higher than 1600 Hz, generally sound generated from engine noises. In addition, in case of seat cushion, the PU foams have excellent capability in both static (posture) comfort for supporting the weight of human and dynamic (vibration) comfort for reducing vibration trans mission [5]. This is because porous PU foams allow air molecules to flow in and out and because soft PU chains cause heat dissipation by con verting the received force into kinetic energy. In the PU composite foams, the foam structures can be dependent on the types of filler and the filler surface characteristics because the interfacial compatibility of fillers with the PU matrix plays a crucial role in forming cellular morphology of the foams. Therefore, it is important to consider the type of appropriate fillers to achieve the targeted properties of PU composite
foams. Recently, many studies reported various ways to improve physical properties of the PU foams [6–9]. Basic formulations were investigated in fabricating the PU foams by changing the contents of base materials [10–15]. Also, micro- and nano-scale fillers (e.g., wood fiber [16], rice hull [17], sawdust [18], graphene oxide [19], magnesium hydroxide [20]) were added for the PU composite foams. In those studies, the interfacial compatibility between fillers and PU matrix was reported as a critical factor in improving physical properties of PU composite foams. In addition, few papers [21,22] reported the effects of hydrophilicity and hydrophobicity of fillers on the physical properties of the PU com posite foams. Amongst various fillers, SiO2 is advantageous to apply for compression purposes [23], and graphene is good for sound absorbing target products [24]. In case of the CaCO3 fillers, it greatly affects the changes of the morphology of PU composites foams [25], and thus it would be meaningful to further investigate whether it is suitable filler for the practical applications of PU composite foams. It is also important to examine the effect of CaCO3 filler surface modifications on the physical properties of the PU composite foams. In this study, the CaCO3 fillers were chosen in fabrications of the PU composite foams, and the filler surfaces were chemically treated with
* Corresponding author. E-mail address: [email protected] (J.H. Kim). https://doi.org/10.1016/j.compscitech.2020.108153 Received 5 March 2020; Received in revised form 28 March 2020; Accepted 31 March 2020 Available online 6 April 2020 0266-3538/© 2020 Elsevier Ltd. All rights reserved.
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oleic acid in certain experimental conditions. The surface treatment was confirmed using Fourier transform infrared (FTIR) spectroscopy by detecting vibrations of carbon chains from the chemically treated CaCO3 Fillers. The PU composite foams with chemically treated or no-treated CaCO3 fillers were fabricated to investigate the effects of chemical treatments on the sound absorption and physical properties. Cellular morphology (cavity and pore sizes, open porosity) of the PU composite foams were analyzed using the scanning electron microscopy (SEM) images to interpret the physical properties. For analyzing sound ab sorption property, impedance tube measurements were performed to calculate sound absorption coefficients and acoustic activities. In addi tion, compressive physical properties of PU composite foams were measured using universal testing machine (UTM) for compression strength, stress relaxation, and hysteresis loss.
for 10 min. Subsequently, isocyanate was added to the polyol system, and further mixing was carried out at 6000 rpm for 10 s. Finally, the full mixture was poured into an aluminum mold (200 mm � 200 mm � 50 mm), and the mold was then kept at 60 � C for 20 min to form the PU composite foams. After the complete reaction, the PU foams were removed from the mold, and they were stored for 3 days under room condition with a relative humidity of 50 � 10% for the following experimental measurements. 2.4. Characterizations FTIR spectroscopy (Frontier, PerkinElmer) with attenuated total reflection accessory was used to examine the chemical functionalities on the CaCO3 surfaces after the oleic acid treatments. The FTIR spectra were obtained by averaging 16 scan results with a resolution of 4 cm 1. Scanning electron microscopy (SEM, SNE-3000M, SEC) was used at 15 kV to examine the surface of the calcium carbonate fillers and the cell morphologies of the PU composite foams. SEM samples were coated with gold sputter before measurements, and fifteen images were aver aged for each sample in analyzing cell and pore sizes using the Image Pro Plus software (Media Cybernetic). For measuring the sound absorption properties of PU composite foams, two impedance tube devices (SW420 and SW470, BSWA) were used with two 1/4-inch microphones (MPA416, BSWA). In order to prepare the samples for sound absorption measurements, the diameters were 100 mm for low frequencies (63–1600 Hz) and 30 mm for high frequencies (1000–6300 Hz) at the same thickness (20 mm). The results from low and high frequencies were combined using VA-Lab software (BSWA) for single-range plots. Finally, UTM (LS1, Lloyd Instruments Ltd.) was used to analyze physical prop erties of the foams following the ASTM D3574-17 under a compression mode for hysteresis loss and compression strengths. The sample dimension was 50 mm � 50 mm � 25 mm.
2. Experimental 2.1. Materials In fabrication of PU composite foams, polyether polyol (PPG-6000, Kumho Petrochemical, OH value: 28 � 2, Mw: 6000 g/mol, fav ¼ 3) and isocyanate (COSMONATE CG-3701S, Kumho Mitsui Chemicals, %NCO: 37 � 0.5, 75% polymeric diphenylmethane diisocyanate and 25% toluene diisocyanate) were used. Calcium carbonate (CaCO3) fillers were obtained from Kanto Chemical Co., Inc., and oleic acid (90%, Sigma-Aldrich) and ethanol (95%, Samchun Pure Chemical) were used for chemical treatments of the CaCO3 fillers. As catalysts, DABCO 33LV (Air Products and Chemicals, 33% triethylenediamine and 67% dipro pylene glycol) and DABCO BL17 (Air Products and Chemicals, 70% bis (2-dimethylaminoethyl) ether diluted with 30% dipropylene glycol) were used for gelling and blowing reactions, respectively. Diethanol amine (DEA, Sigma-Aldrich) was used as a cross-linking agent, and sil icon surfactant (L-3002, Momentive) was used for stabilizing cell structures by even distributions of water phases during the component mixing stage. Carbon dioxide gases, which is generated from blowing reactions between isocyanates and water molecules, are served as blowing agents in forming cellular morphology.
3. Results and discussion 3.1. Chemical treatments In polymer composites, filler surface characteristics influence the dispersion of the fillers, and the interfacial adhesion between the poly mer matrix and filler surfaces plays a critical role in achieving outstanding physical properties. By treating the CaCO3 filler surfaces with oleic acid, various organic functionalities can be attached on the CaCO3 surfaces, and thus the surface adjustability of the fillers to the polymer matrix are improved after the chemical treatments [26,27]. Consequently, the material properties of the PU composite foams can be enhanced by surface modifications of CaCO3 fillers. FTIR analysis is a useful technique to monitor the changes of surface characteristics after chemical treatments due to the strong infrared responses from the certain functional groups. In this work, after the oleic acid treatments, several noticeable peaks from the FTIR absorbance spectra are appeared, as shown in Fig. 1. The several peaks from 3005 cm 1 to 2854 cm 1 are mainly attributed to the stretching modes of carbon chains from the oleic acid, as summarized in Table 2. In addition, the peaks at 1737 and
2.2. Chemical treatments for the CaCO3 fillers Firstly, oleic acid solution (10 wt% in ethanol) was prepared for the surface modification, and then the CaCO3 was well dispersed in the oleic acid solution by vigorous stirring for 2 h at 60 � C. The treated CaCO3 fillers were filtered and washed with ethanol and deionized water sequentially to remove the by-products, and lastly dried in a convection oven for 48 h at 60 � C. 2.3. Synthesis The PU composite foams were synthesized by one-shot method following the formulations shown in Table 1. A polyol system, which consists of polyol, catalysts, cross-linking agents, blowing agents, and surfactant, was prepared in a 400 mL paper cup and mixed at 1700 rpm Table 1 Formulation details for manufacturing the PU composite foams with CaCO3 fillers. Material
Content (g)
Polyol system
100 0.72 0.08 0.60 3.00 1.32 0 (0) 44.84
Alcohol (PPG-6000) Gelling catalyst (DABCO 33LV) Blowing catalyst (DABCO BL17) Cross-linker (DEA) Blowing agent (H2O) Surfactant (L-3002) CaCO3 (wt % to polyol) Isocyanate* (COSMONATE CG-3701S)
2.00 (2.0)
**NCO Index: 1.0. 2
4.00 (4.0)
6.00 (6.0)
8.00 (8.0)
10.00 (10.0)
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Fig. 2. SEM images of CaCO3 fillers without (a) and with (b) oleic acid treatments.
dominantly produced in case of thin cavity walls due to the low wall strength compared to the cavity pressures and due to the high drainage flow rate. In addition, interfacial adhesion between fillers and matrix can affect the formed types of pores and cavity sizes depending on the compatibility. Fig. 3 shows the SEM images of PU composite foams including CaCO3 fillers without and with oleic acid treatments. Differences of cell and pore sizes depending on the chemical treatments were hardly detectable from these typical images shown in Fig. 3, and thus statistical data averaged from more than fifteen images (as explained in experimental section) were used in Fig. 4. Fig. 3d demonstrates the CaCO3 fillers agglomerated in the PU matrix, and this agglomeration might lead to lower drainage flow of PU matrix higher crack possibility during foaming process. Thus, the effect of chemical treatments of fillers on cellular structure can diminish at higher filler contents. Fig. 4 shows the average cell and pore sizes of the PU composite foams as a function of CaCO3 contents. Although the uncertainties of cell and pore sizes are very large in oleic acid treated or no-treated cases regardless of filler contents, the cell and pore sizes show opposite trends depending on the chemical treatments. For example, the average cell size was larger in the oleic acid treated filler cases than no-treated cases. It is possibly attributed to the enhanced interfacial compatibility of fillers with PU matrix which can retard the matrix breakage during CO2 gas expansion. Therefore, the chemically treated cases provide more time for cellular growth than that of no-treated filler cases, and they lead to larger cell sizes. However, in case of pore sizes, there are no big differences in various filler contents. It is because the pore formation is strongly related to the drainage flow rate of matrix, and it is considered as much
Fig. 1. FTIR absorbance spectra from CaCO3 fillers without and with oleic acid treatments.
1710 cm 1 are strongly related to the carbonyl groups attached on the CaCO3 surfaces by the oleic acid reactions. Indeed, these confirmed chemical functionalities can play a key role in promoting the compati bility of CaCO3 surfaces with the PU matrix and improving the physical properties of the PU composite foams [26]. Finally, the increments of peak absorbance at 1500 1000 cm 1 are mostly caused by overlapping of alkyl chain bending with CO23 stretching, as summarized in Table 2. In order to further examine the chemically treated CaCO3 surfaces, Fig. 2 shows the images of CaCO3 surfaces before and after chemical treat ments. After the chemical modifications, the CaCO3 surfaces revealed rather smoother than before treatments, and it is believed that the oleic acid molecules cover the CaCO3 surfaces. This coating layer can be likely more compatible with PU matrix and thus provides less interfacial damages during foaming processes and mechanical deformation of composites. 3.2. Morphology Development of cellular structures during foaming processes of PU composite foams is strongly related to physical properties of PU foams. For example, the distributions (size & number) of cells and pores affect the sound wave propagation through the cellular paths, and the ratio of partial open pores is the most important factor for sound absorption property. Additionally, they also influence on the physical properties of the foams because the number of struts and walls on specific surface area are also much dependent on the cellular morphology. Choe et al. [16] reported that the types of pores are strongly dependent on the cavity wall thickness and drainage flow rate, and for instance, open pores are Table 2 FTIR peak positions for the CaCO3 fillers and oleic acid. Band Position (cm
1
Assignment
Reference
Calcium carbonate
) 2530 1796 1420 875 713
Band for CaCO3 (calcite) Asymmetric stretching, Ca2þ and CO323 Asymmetric stretching CO23 Out-of-plane bending, CO23 Planar bending, CO23
[30] [31] [32]
Oleic acid
3005 2954 2924 2854 1737 1710 1409
–C–H Stretching, C– Asymmetric stretching, CH3 Asymmetric stretching, CH2 Symmetric stretching, CH2 –C–O Stretching, O– –O Stretching, C– Symmetric bending, CH3
[33] [34] [33]
Fig. 3. SEM images of the PU composite foams including 6 wt% CaCO3 fillers without (a) and with (b) oleic acid treatments, and 10 wt% CaCO3 fillers with (c) oleic acid treatments. The CaCO3 fillers in the yellow box in (c) image are magnified (d). The red circles represent partial open and close pores. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
[35] [33]
3
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3.3. Sound absorption property The sound absorption mainly occurs by heat dissipations caused by the collisions of sound waves with the cell walls and molecular frictions between sound waves and air molecules. Thus, high sound absorption performance through the PU foams can be achieved by long internal path length for sound wave passage and high air molecular collisions with the cell walls. In this sense, the morphology of the PU foams has a great influence on the air flow resistivity and tortuosity, which are the major factors in assessments of sound absorption of materials. Fig. 6 shows sound absorption coefficient (α) of the PU composite foams by varying the CaCO3 filler contents after oleic acid treatments. The highest value of α increases with increasing the filler contents, and the maximum value is reached at 6 wt % CaCO3 contents which is exactly matched with the lowest open porosity value (see Fig. 5). This is because low open porosity causes high air flow resistance and increases the friction of sound waves with the air molecules. Also, it leads high tortuosity which is related to the quarter-wavelength point at which energy dissipation is highest, and it lowers the frequency of maximum point [16]. In contrast, the PU composite foams with higher open porosity values than the case of 6 wt% give decreasing tendency in α due to reduced air molecular collisions and wave frictions with cell walls. In addition, the higher the maximum value of α, the lower the frequency at the maximum α peak. Fig. 7 shows the frequencies at the maximum α values and acoustic activity as a function of filler contents. Variations of frequencies in Fig. 7a revealed the same tendency with the open porosity results, and it is mainly because the air flow resistivity and tortuosity reach at the lowest open porosity, as demonstrated by Choe et al. [16]. In addition, the acoustic activity (average value of α for full frequency ranges) in Fig. 7b also results in higher values in the PU composite foams with the oleic acid treated CaCO3 fillers due to the lower open porosity. Therefore, the better compatibility between CaCO3 filler surfaces and the PU matrix produced the lower open porosity and higher sound ab sorption characteristics in PU composite foams.
Fig. 4. Cell and pore sizes of the PU composite foams including CaCO3 fillers without and with oleic acid treatments.
similar drainage ability of the PU matrix with the same formulation except for the filler contents. Other than cell and pore sizes and distributions, open porosity is also a key parameter in analyzing the sound absorption properties of the PU foams [20,28]. Open porosity can be calculated by counting the numbers of the open, partial open, and closed pores, and the higher open porosity usually leads to lower sound absorption due to higher passage of sound waves through the foam structures. Fig. 5 shows the open porosity of the PU composite foams including CaCO3 fillers with and without oleic acid treatments, as a function of filler contents. Overall, it shows lower open porosity values in chemically treated cases than no-treated cases because higher compatibility in chemically treated cases tend to remain in partial open pores rather than fully open pores. Vice versa, more open pores can be formed in no-treated cases due to higher breakage in the interfaces between fillers and matrix. Finally, the lowest open porosity was achieved at the 6 wt% CaCO3 fillers treated oleic acid, and the highest open porosity was appeared at 4 wt% CaCO3 fillers without treatments. These results are closely related to the sound absorption coefficient of the PU composite foams.
3.4. Physical properties The PU foams are widely used in automotive compartments such as sound absorption and seat cushion applications. Other than sound ab sorption properties described above, cushioning properties like compression strength and hysteresis loss are very important to deter mine whether the materials can be applicable for the automotive products or not. Generally, the material strength of PU composites in creases with increasing filler contents regardless of the filler types since the strength of filler itself is higher than the polyurethane matrix.
Fig. 5. Open porosity of the PU composite foams including CaCO3 fillers without and with oleic acid treatments.
Fig. 6. Sound absorption coefficient (α) of the PU composite foams with oleic acid treated CaCO3 fillers by varying filler contents. 4
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Fig. 7. Frequency at maximum sound absorption coefficients (α) (a) and acoustic activity (b) of the PU composite foams with raw and chemically treated CaCO3 fillers as a function of filler contents.
However, interfacial adhesion between polymer matrix and filler sur faces is a critical factor in further enhancing the strength. Fig. 8 shows the stress at 50% strain and stress relaxation of the PU composite foams with CaCO3 fillers. The higher stress values with the chemically treated CaCO3 fillers resulted from the better compatibility than with the nontreated filler cases. Poor compatibility in non-treated cases leads to breakages between fillers and matrix, and thus the stress values become low in non-treated filler cases. For the stress relaxation measurements, the sample specimen is compressed under 2kgf for 2 h, and then the responsive force is recorded to compare with the original force. Thus, if the material is more elastic with no much deformations, the stress relaxation becomes low percentage. Fig. 8b shows the results of stress relaxation with both chemically treated and no-treated filler cases, and the treated filler cases revealed lower variations. Therefore, oleic acid treatments for CaCO3 fillers improved interfacial compatibility of filler surfaces with the PU matrix and resulted into lower deformations at the interfacial contact points. In terms of hysteresis low, it refers to the loss energy of samples during returning process to the original state after compressive defor mation of the specimen, which is also related to restoration. It can be obtained by dividing the difference between the accumulated energy (measured while loading the PU foams up to 75% of the initial height) and the released energy (measured while unloading to the original po sition). Fig. 9 shows the hysteresis loss of the PU composite foams as a function of CaCO3 filler contents. In general, hysteresis proceeds in three stages: linear elastic, plateau, and densification [29]. First, in the linear elastic region, PU foams resist compressive stress by elastic deformation of struts and cell walls. Next, in the plateau region, the struts and cell walls buckle because they cannot withstand the compressive strength. Finally, in the densification region, the compressive stress rapidly in creases due to low volume of the cavity to be compressed. During unloading, the PU foams return to its original state and undergoes plastic deformation, which results in energy loss. Thus, the lower value of hysteresis loss usually guarantees the better performance in repetitive
Fig. 9. Hysteresis loss of the PU composite foams with raw and chemically treated CaCO3 fillers as a function of filler contents.
usages. Fig. 9a shows the overall hysteresis loss, and the PU composite foams with chemically treated CaCO3 fillers tends to be lower energy loss than the non-treated cases. It also demonstrates the same reasons for the low hysteresis loss comparing with the high stress values, at the chemically treated CaCO3 filler cases with better compatibility between PU matrix and filler surfaces. If the hysteresis loss is compared at 10 wt% filler contents, the difference is about 6%. This largest difference is mostly attributed to many interfacial contact points at highest filler contents, and it also demonstrates the importance of interfacial compatibility in polymer composites to enhance the physical properties. Therefore, it is important to keep in mind that the choice of fillers and surface modifications of the fillers are crucial in achieving good physical properties of the PU composite foams.
Fig. 8. Compression strength at 50% strain (a) and stress relaxation (b) of the PU composite foams as a function of CaCO3 filler contents. 5
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4. Conclusions
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The CaCO3 fillers were treated with oleic acid, and the PU composite foams were fabricated with those treated and non-treated fillers to investigate various physical properties. Depending on the filler surface characteristics, interfacial compatibility is changed between PU matrix and the CaCO3 filler surfaces, and it finally determines the cellular morphology, sound absorption, and physical properties of the PU com posite foams. From the cell morphology, cell and pore sizes did not show much differences (within uncertainties) with filler contents, but the open porosity showed clear difference depending on the chemical treatments. The open porosity was lower values with chemically treated fillers than with no-treated fillers, and it resulted into the high sound absorption coefficient and the highest value at 6 wt% filler contents. The stress was also higher with the chemically treated fillers than with notreated filler cases. Stress relaxation and hysteresis loss results revealed the superior with the chemically treated fillers to the cases with no-treated cases, and it was attributed to the enhanced interfacial con tacts and compatibility between PU matrix and CaCO3 filler surfaces in polymer composites. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Hyeon Choe: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Resources, Data curation, Writing - original draft, Visualization. Jae Heon Lee: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Resources, Data curation. Jung Hyeun Kim: Conceptualization, Methodology, Writing - review & editing, Visualization, Funding acquisition, Supervision, Project administration. Acknowledgement This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Min istry of Science and ICT (NRF-2018R1D1A1A09082239). This research was also partially supported by X-mind Corps program of National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2017H1D8A1030582). References [1] G. Sung, J.H. Kim, Effect of high molecular weight isocyanate contents on manufacturing polyurethane foams for improved sound absorption coefficient, Kor. J. Chem. Eng. 34 (2017) 1222–1228, https://doi.org/10.1007/s11814-016-03616. [2] J.G. Gwon, S.K. Kim, J.H. Kim, Sound absorption behavior of flexible polyurethane foams with distinct cellular structures, Mater. Des. 89 (2016) 448–454, https://doi. org/10.1016/j.matdes.2015.10.017. [3] A. Kausar, Polyurethane composite foams in high-performance applications: a review, Polym. Plast. Technol. Eng. 57 (2018) 346–369, https://doi.org/10.1080/ 03602559.2017.1329433. [4] R. Mohammadpour, G.M.M. Sadeghi, Effect of liquefied lignin content on synthesis of bio-based polyurethane foam for oil adsorption application, J. Polym. Environ. (2020) 1–14, https://doi.org/10.1007/s10924-019-01650-5. [5] W. Patten, S. Sha, C. Mo, A vibrational model of open celled polyurethane foam automotive seat cushions, J. Sound Vib. 217 (1998) 145–161, https://doi.org/ 10.1006/jsvi.1998.1760. [6] J.G. Gwon, G. Sung, J.H. Kim, Modulation of cavities and interconnecting pores in manufacturing water blown flexible poly (urethane urea) foams, Int. J. Precis. Eng. Manuf. 16 (2015) 2299–2307, https://doi.org/10.1007/s12541-015-0295-7. [7] J.G. Gwon, S.K. Kim, J.H. Kim, Development of cell morphologies in manufacturing flexible polyurethane urea foams as sound absorption materials, J. Porous Mater. 23 (2016) 465–473, https://doi.org/10.1007/s10934-015-0100-0.
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[33] N. Wu, L. Fu, M. Su, M. Aslam, K.C. Wong, V.P. Dravid, Interaction of fatty acid monolayers with cobalt nanoparticles, Nano Lett. 4 (2004) 383–386, https://doi. org/10.1021/nl035139x. [34] M. Klokkenburg, J. Hilhorst, B.H. Erne, Surface analysis of magnetite nanoparticles in cyclohexane solutions of oleic acid and oleylamine, Vib. Spectrosc. 43 (2007) 243–248, https://doi.org/10.1016/j.vibspec.2006.09.008.
[35] L. Raghunanan, J. Yue, S.S. Narine, Synthesis and characterization of novel diol, diacid and di-isocyanate from oleic acid, J. Am. Oil Chem. Soc. 91 (2014) 349–356, https://doi.org/10.1007/s11746-013-2376-z.
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Contents lists available at ScienceDirect
Composites Science and Technology journal homepage: http://www.elsevier.com/locate/compscitech
Polyurethane composite foams including CaCO3 fillers for enhanced sound absorption and compression properties Hyeon Choe , Jae Heon Lee , Jung Hyeun Kim * Department of Chemical Engineering, University of Seoul, 163 Seoulsiripdae-ro, Dongdaemun-gu, Seoul, 02504, South Korea
A R T I C L E I N F O
A B S T R A C T
Keywords: Polyurethane foam Composites Sound absorption Hysteresis loss Compression
Polyurethane (PU) foams are versatile in automotive applications for sound absorption and seat cushioning products due to their easiness of manufacturing process and cost effectiveness. In this study, PU composite foams including CaCO3 fillers after chemical treatments with oleic acid were fabricated to examine the sound ab sorption and compressive properties. The cell and pore sizes in the PU composite foams did not show much differences with filler contents, but the open porosity revealed clear difference depending on the chemical treatments. The open porosity had lower values with chemically treated fillers than with no-treated cases, and it resulted in the high sound absorption coefficient. The compression strengths were also higher with the chemi cally treated fillers than with no-treated filler cases. Stress relaxation and hysteresis loss showed superior results with the chemically treated fillers compared with the cases with no-treated cases, and it is mainly due to the enhanced interfacial contacts and compatibility between PU matrix and CaCO3 filler surfaces. Therefore, it is much important to modify the filler surface characteristics for good interfacial compatibility to achieve improved physical properties of the PU composite foams.
1. Introduction Since polyurethane (PU) foams have many advantages such as light weight, inexpensive cost, and simple manufacturability, they are widely applied in diverse industries for various applications [1–4]. Especially, in the automobile industries, the PU foams are utilized as sound absorbing and seat cushioning materials to achieve comfortable driving conditions. For examples, on the purpose of sound absorptions, the PU foams have excellent performance in absorbing sound waves with fre quencies higher than 1600 Hz, generally sound generated from engine noises. In addition, in case of seat cushion, the PU foams have excellent capability in both static (posture) comfort for supporting the weight of human and dynamic (vibration) comfort for reducing vibration trans mission [5]. This is because porous PU foams allow air molecules to flow in and out and because soft PU chains cause heat dissipation by con verting the received force into kinetic energy. In the PU composite foams, the foam structures can be dependent on the types of filler and the filler surface characteristics because the interfacial compatibility of fillers with the PU matrix plays a crucial role in forming cellular morphology of the foams. Therefore, it is important to consider the type of appropriate fillers to achieve the targeted properties of PU composite
foams. Recently, many studies reported various ways to improve physical properties of the PU foams [6–9]. Basic formulations were investigated in fabricating the PU foams by changing the contents of base materials [10–15]. Also, micro- and nano-scale fillers (e.g., wood fiber [16], rice hull [17], sawdust [18], graphene oxide [19], magnesium hydroxide [20]) were added for the PU composite foams. In those studies, the interfacial compatibility between fillers and PU matrix was reported as a critical factor in improving physical properties of PU composite foams. In addition, few papers [21,22] reported the effects of hydrophilicity and hydrophobicity of fillers on the physical properties of the PU com posite foams. Amongst various fillers, SiO2 is advantageous to apply for compression purposes [23], and graphene is good for sound absorbing target products [24]. In case of the CaCO3 fillers, it greatly affects the changes of the morphology of PU composites foams [25], and thus it would be meaningful to further investigate whether it is suitable filler for the practical applications of PU composite foams. It is also important to examine the effect of CaCO3 filler surface modifications on the physical properties of the PU composite foams. In this study, the CaCO3 fillers were chosen in fabrications of the PU composite foams, and the filler surfaces were chemically treated with
* Corresponding author. E-mail address: [email protected] (J.H. Kim). https://doi.org/10.1016/j.compscitech.2020.108153 Received 5 March 2020; Received in revised form 28 March 2020; Accepted 31 March 2020 Available online 6 April 2020 0266-3538/© 2020 Elsevier Ltd. All rights reserved.
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oleic acid in certain experimental conditions. The surface treatment was confirmed using Fourier transform infrared (FTIR) spectroscopy by detecting vibrations of carbon chains from the chemically treated CaCO3 Fillers. The PU composite foams with chemically treated or no-treated CaCO3 fillers were fabricated to investigate the effects of chemical treatments on the sound absorption and physical properties. Cellular morphology (cavity and pore sizes, open porosity) of the PU composite foams were analyzed using the scanning electron microscopy (SEM) images to interpret the physical properties. For analyzing sound ab sorption property, impedance tube measurements were performed to calculate sound absorption coefficients and acoustic activities. In addi tion, compressive physical properties of PU composite foams were measured using universal testing machine (UTM) for compression strength, stress relaxation, and hysteresis loss.
for 10 min. Subsequently, isocyanate was added to the polyol system, and further mixing was carried out at 6000 rpm for 10 s. Finally, the full mixture was poured into an aluminum mold (200 mm � 200 mm � 50 mm), and the mold was then kept at 60 � C for 20 min to form the PU composite foams. After the complete reaction, the PU foams were removed from the mold, and they were stored for 3 days under room condition with a relative humidity of 50 � 10% for the following experimental measurements. 2.4. Characterizations FTIR spectroscopy (Frontier, PerkinElmer) with attenuated total reflection accessory was used to examine the chemical functionalities on the CaCO3 surfaces after the oleic acid treatments. The FTIR spectra were obtained by averaging 16 scan results with a resolution of 4 cm 1. Scanning electron microscopy (SEM, SNE-3000M, SEC) was used at 15 kV to examine the surface of the calcium carbonate fillers and the cell morphologies of the PU composite foams. SEM samples were coated with gold sputter before measurements, and fifteen images were aver aged for each sample in analyzing cell and pore sizes using the Image Pro Plus software (Media Cybernetic). For measuring the sound absorption properties of PU composite foams, two impedance tube devices (SW420 and SW470, BSWA) were used with two 1/4-inch microphones (MPA416, BSWA). In order to prepare the samples for sound absorption measurements, the diameters were 100 mm for low frequencies (63–1600 Hz) and 30 mm for high frequencies (1000–6300 Hz) at the same thickness (20 mm). The results from low and high frequencies were combined using VA-Lab software (BSWA) for single-range plots. Finally, UTM (LS1, Lloyd Instruments Ltd.) was used to analyze physical prop erties of the foams following the ASTM D3574-17 under a compression mode for hysteresis loss and compression strengths. The sample dimension was 50 mm � 50 mm � 25 mm.
2. Experimental 2.1. Materials In fabrication of PU composite foams, polyether polyol (PPG-6000, Kumho Petrochemical, OH value: 28 � 2, Mw: 6000 g/mol, fav ¼ 3) and isocyanate (COSMONATE CG-3701S, Kumho Mitsui Chemicals, %NCO: 37 � 0.5, 75% polymeric diphenylmethane diisocyanate and 25% toluene diisocyanate) were used. Calcium carbonate (CaCO3) fillers were obtained from Kanto Chemical Co., Inc., and oleic acid (90%, Sigma-Aldrich) and ethanol (95%, Samchun Pure Chemical) were used for chemical treatments of the CaCO3 fillers. As catalysts, DABCO 33LV (Air Products and Chemicals, 33% triethylenediamine and 67% dipro pylene glycol) and DABCO BL17 (Air Products and Chemicals, 70% bis (2-dimethylaminoethyl) ether diluted with 30% dipropylene glycol) were used for gelling and blowing reactions, respectively. Diethanol amine (DEA, Sigma-Aldrich) was used as a cross-linking agent, and sil icon surfactant (L-3002, Momentive) was used for stabilizing cell structures by even distributions of water phases during the component mixing stage. Carbon dioxide gases, which is generated from blowing reactions between isocyanates and water molecules, are served as blowing agents in forming cellular morphology.
3. Results and discussion 3.1. Chemical treatments In polymer composites, filler surface characteristics influence the dispersion of the fillers, and the interfacial adhesion between the poly mer matrix and filler surfaces plays a critical role in achieving outstanding physical properties. By treating the CaCO3 filler surfaces with oleic acid, various organic functionalities can be attached on the CaCO3 surfaces, and thus the surface adjustability of the fillers to the polymer matrix are improved after the chemical treatments [26,27]. Consequently, the material properties of the PU composite foams can be enhanced by surface modifications of CaCO3 fillers. FTIR analysis is a useful technique to monitor the changes of surface characteristics after chemical treatments due to the strong infrared responses from the certain functional groups. In this work, after the oleic acid treatments, several noticeable peaks from the FTIR absorbance spectra are appeared, as shown in Fig. 1. The several peaks from 3005 cm 1 to 2854 cm 1 are mainly attributed to the stretching modes of carbon chains from the oleic acid, as summarized in Table 2. In addition, the peaks at 1737 and
2.2. Chemical treatments for the CaCO3 fillers Firstly, oleic acid solution (10 wt% in ethanol) was prepared for the surface modification, and then the CaCO3 was well dispersed in the oleic acid solution by vigorous stirring for 2 h at 60 � C. The treated CaCO3 fillers were filtered and washed with ethanol and deionized water sequentially to remove the by-products, and lastly dried in a convection oven for 48 h at 60 � C. 2.3. Synthesis The PU composite foams were synthesized by one-shot method following the formulations shown in Table 1. A polyol system, which consists of polyol, catalysts, cross-linking agents, blowing agents, and surfactant, was prepared in a 400 mL paper cup and mixed at 1700 rpm Table 1 Formulation details for manufacturing the PU composite foams with CaCO3 fillers. Material
Content (g)
Polyol system
100 0.72 0.08 0.60 3.00 1.32 0 (0) 44.84
Alcohol (PPG-6000) Gelling catalyst (DABCO 33LV) Blowing catalyst (DABCO BL17) Cross-linker (DEA) Blowing agent (H2O) Surfactant (L-3002) CaCO3 (wt % to polyol) Isocyanate* (COSMONATE CG-3701S)
2.00 (2.0)
**NCO Index: 1.0. 2
4.00 (4.0)
6.00 (6.0)
8.00 (8.0)
10.00 (10.0)
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Fig. 2. SEM images of CaCO3 fillers without (a) and with (b) oleic acid treatments.
dominantly produced in case of thin cavity walls due to the low wall strength compared to the cavity pressures and due to the high drainage flow rate. In addition, interfacial adhesion between fillers and matrix can affect the formed types of pores and cavity sizes depending on the compatibility. Fig. 3 shows the SEM images of PU composite foams including CaCO3 fillers without and with oleic acid treatments. Differences of cell and pore sizes depending on the chemical treatments were hardly detectable from these typical images shown in Fig. 3, and thus statistical data averaged from more than fifteen images (as explained in experimental section) were used in Fig. 4. Fig. 3d demonstrates the CaCO3 fillers agglomerated in the PU matrix, and this agglomeration might lead to lower drainage flow of PU matrix higher crack possibility during foaming process. Thus, the effect of chemical treatments of fillers on cellular structure can diminish at higher filler contents. Fig. 4 shows the average cell and pore sizes of the PU composite foams as a function of CaCO3 contents. Although the uncertainties of cell and pore sizes are very large in oleic acid treated or no-treated cases regardless of filler contents, the cell and pore sizes show opposite trends depending on the chemical treatments. For example, the average cell size was larger in the oleic acid treated filler cases than no-treated cases. It is possibly attributed to the enhanced interfacial compatibility of fillers with PU matrix which can retard the matrix breakage during CO2 gas expansion. Therefore, the chemically treated cases provide more time for cellular growth than that of no-treated filler cases, and they lead to larger cell sizes. However, in case of pore sizes, there are no big differences in various filler contents. It is because the pore formation is strongly related to the drainage flow rate of matrix, and it is considered as much
Fig. 1. FTIR absorbance spectra from CaCO3 fillers without and with oleic acid treatments.
1710 cm 1 are strongly related to the carbonyl groups attached on the CaCO3 surfaces by the oleic acid reactions. Indeed, these confirmed chemical functionalities can play a key role in promoting the compati bility of CaCO3 surfaces with the PU matrix and improving the physical properties of the PU composite foams [26]. Finally, the increments of peak absorbance at 1500 1000 cm 1 are mostly caused by overlapping of alkyl chain bending with CO23 stretching, as summarized in Table 2. In order to further examine the chemically treated CaCO3 surfaces, Fig. 2 shows the images of CaCO3 surfaces before and after chemical treat ments. After the chemical modifications, the CaCO3 surfaces revealed rather smoother than before treatments, and it is believed that the oleic acid molecules cover the CaCO3 surfaces. This coating layer can be likely more compatible with PU matrix and thus provides less interfacial damages during foaming processes and mechanical deformation of composites. 3.2. Morphology Development of cellular structures during foaming processes of PU composite foams is strongly related to physical properties of PU foams. For example, the distributions (size & number) of cells and pores affect the sound wave propagation through the cellular paths, and the ratio of partial open pores is the most important factor for sound absorption property. Additionally, they also influence on the physical properties of the foams because the number of struts and walls on specific surface area are also much dependent on the cellular morphology. Choe et al. [16] reported that the types of pores are strongly dependent on the cavity wall thickness and drainage flow rate, and for instance, open pores are Table 2 FTIR peak positions for the CaCO3 fillers and oleic acid. Band Position (cm
1
Assignment
Reference
Calcium carbonate
) 2530 1796 1420 875 713
Band for CaCO3 (calcite) Asymmetric stretching, Ca2þ and CO323 Asymmetric stretching CO23 Out-of-plane bending, CO23 Planar bending, CO23
[30] [31] [32]
Oleic acid
3005 2954 2924 2854 1737 1710 1409
–C–H Stretching, C– Asymmetric stretching, CH3 Asymmetric stretching, CH2 Symmetric stretching, CH2 –C–O Stretching, O– –O Stretching, C– Symmetric bending, CH3
[33] [34] [33]
Fig. 3. SEM images of the PU composite foams including 6 wt% CaCO3 fillers without (a) and with (b) oleic acid treatments, and 10 wt% CaCO3 fillers with (c) oleic acid treatments. The CaCO3 fillers in the yellow box in (c) image are magnified (d). The red circles represent partial open and close pores. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
[35] [33]
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3.3. Sound absorption property The sound absorption mainly occurs by heat dissipations caused by the collisions of sound waves with the cell walls and molecular frictions between sound waves and air molecules. Thus, high sound absorption performance through the PU foams can be achieved by long internal path length for sound wave passage and high air molecular collisions with the cell walls. In this sense, the morphology of the PU foams has a great influence on the air flow resistivity and tortuosity, which are the major factors in assessments of sound absorption of materials. Fig. 6 shows sound absorption coefficient (α) of the PU composite foams by varying the CaCO3 filler contents after oleic acid treatments. The highest value of α increases with increasing the filler contents, and the maximum value is reached at 6 wt % CaCO3 contents which is exactly matched with the lowest open porosity value (see Fig. 5). This is because low open porosity causes high air flow resistance and increases the friction of sound waves with the air molecules. Also, it leads high tortuosity which is related to the quarter-wavelength point at which energy dissipation is highest, and it lowers the frequency of maximum point [16]. In contrast, the PU composite foams with higher open porosity values than the case of 6 wt% give decreasing tendency in α due to reduced air molecular collisions and wave frictions with cell walls. In addition, the higher the maximum value of α, the lower the frequency at the maximum α peak. Fig. 7 shows the frequencies at the maximum α values and acoustic activity as a function of filler contents. Variations of frequencies in Fig. 7a revealed the same tendency with the open porosity results, and it is mainly because the air flow resistivity and tortuosity reach at the lowest open porosity, as demonstrated by Choe et al. [16]. In addition, the acoustic activity (average value of α for full frequency ranges) in Fig. 7b also results in higher values in the PU composite foams with the oleic acid treated CaCO3 fillers due to the lower open porosity. Therefore, the better compatibility between CaCO3 filler surfaces and the PU matrix produced the lower open porosity and higher sound ab sorption characteristics in PU composite foams.
Fig. 4. Cell and pore sizes of the PU composite foams including CaCO3 fillers without and with oleic acid treatments.
similar drainage ability of the PU matrix with the same formulation except for the filler contents. Other than cell and pore sizes and distributions, open porosity is also a key parameter in analyzing the sound absorption properties of the PU foams [20,28]. Open porosity can be calculated by counting the numbers of the open, partial open, and closed pores, and the higher open porosity usually leads to lower sound absorption due to higher passage of sound waves through the foam structures. Fig. 5 shows the open porosity of the PU composite foams including CaCO3 fillers with and without oleic acid treatments, as a function of filler contents. Overall, it shows lower open porosity values in chemically treated cases than no-treated cases because higher compatibility in chemically treated cases tend to remain in partial open pores rather than fully open pores. Vice versa, more open pores can be formed in no-treated cases due to higher breakage in the interfaces between fillers and matrix. Finally, the lowest open porosity was achieved at the 6 wt% CaCO3 fillers treated oleic acid, and the highest open porosity was appeared at 4 wt% CaCO3 fillers without treatments. These results are closely related to the sound absorption coefficient of the PU composite foams.
3.4. Physical properties The PU foams are widely used in automotive compartments such as sound absorption and seat cushion applications. Other than sound ab sorption properties described above, cushioning properties like compression strength and hysteresis loss are very important to deter mine whether the materials can be applicable for the automotive products or not. Generally, the material strength of PU composites in creases with increasing filler contents regardless of the filler types since the strength of filler itself is higher than the polyurethane matrix.
Fig. 5. Open porosity of the PU composite foams including CaCO3 fillers without and with oleic acid treatments.
Fig. 6. Sound absorption coefficient (α) of the PU composite foams with oleic acid treated CaCO3 fillers by varying filler contents. 4
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Fig. 7. Frequency at maximum sound absorption coefficients (α) (a) and acoustic activity (b) of the PU composite foams with raw and chemically treated CaCO3 fillers as a function of filler contents.
However, interfacial adhesion between polymer matrix and filler sur faces is a critical factor in further enhancing the strength. Fig. 8 shows the stress at 50% strain and stress relaxation of the PU composite foams with CaCO3 fillers. The higher stress values with the chemically treated CaCO3 fillers resulted from the better compatibility than with the nontreated filler cases. Poor compatibility in non-treated cases leads to breakages between fillers and matrix, and thus the stress values become low in non-treated filler cases. For the stress relaxation measurements, the sample specimen is compressed under 2kgf for 2 h, and then the responsive force is recorded to compare with the original force. Thus, if the material is more elastic with no much deformations, the stress relaxation becomes low percentage. Fig. 8b shows the results of stress relaxation with both chemically treated and no-treated filler cases, and the treated filler cases revealed lower variations. Therefore, oleic acid treatments for CaCO3 fillers improved interfacial compatibility of filler surfaces with the PU matrix and resulted into lower deformations at the interfacial contact points. In terms of hysteresis low, it refers to the loss energy of samples during returning process to the original state after compressive defor mation of the specimen, which is also related to restoration. It can be obtained by dividing the difference between the accumulated energy (measured while loading the PU foams up to 75% of the initial height) and the released energy (measured while unloading to the original po sition). Fig. 9 shows the hysteresis loss of the PU composite foams as a function of CaCO3 filler contents. In general, hysteresis proceeds in three stages: linear elastic, plateau, and densification [29]. First, in the linear elastic region, PU foams resist compressive stress by elastic deformation of struts and cell walls. Next, in the plateau region, the struts and cell walls buckle because they cannot withstand the compressive strength. Finally, in the densification region, the compressive stress rapidly in creases due to low volume of the cavity to be compressed. During unloading, the PU foams return to its original state and undergoes plastic deformation, which results in energy loss. Thus, the lower value of hysteresis loss usually guarantees the better performance in repetitive
Fig. 9. Hysteresis loss of the PU composite foams with raw and chemically treated CaCO3 fillers as a function of filler contents.
usages. Fig. 9a shows the overall hysteresis loss, and the PU composite foams with chemically treated CaCO3 fillers tends to be lower energy loss than the non-treated cases. It also demonstrates the same reasons for the low hysteresis loss comparing with the high stress values, at the chemically treated CaCO3 filler cases with better compatibility between PU matrix and filler surfaces. If the hysteresis loss is compared at 10 wt% filler contents, the difference is about 6%. This largest difference is mostly attributed to many interfacial contact points at highest filler contents, and it also demonstrates the importance of interfacial compatibility in polymer composites to enhance the physical properties. Therefore, it is important to keep in mind that the choice of fillers and surface modifications of the fillers are crucial in achieving good physical properties of the PU composite foams.
Fig. 8. Compression strength at 50% strain (a) and stress relaxation (b) of the PU composite foams as a function of CaCO3 filler contents. 5
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4. Conclusions
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The CaCO3 fillers were treated with oleic acid, and the PU composite foams were fabricated with those treated and non-treated fillers to investigate various physical properties. Depending on the filler surface characteristics, interfacial compatibility is changed between PU matrix and the CaCO3 filler surfaces, and it finally determines the cellular morphology, sound absorption, and physical properties of the PU com posite foams. From the cell morphology, cell and pore sizes did not show much differences (within uncertainties) with filler contents, but the open porosity showed clear difference depending on the chemical treatments. The open porosity was lower values with chemically treated fillers than with no-treated fillers, and it resulted into the high sound absorption coefficient and the highest value at 6 wt% filler contents. The stress was also higher with the chemically treated fillers than with notreated filler cases. Stress relaxation and hysteresis loss results revealed the superior with the chemically treated fillers to the cases with no-treated cases, and it was attributed to the enhanced interfacial con tacts and compatibility between PU matrix and CaCO3 filler surfaces in polymer composites. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Hyeon Choe: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Resources, Data curation, Writing - original draft, Visualization. Jae Heon Lee: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Resources, Data curation. Jung Hyeun Kim: Conceptualization, Methodology, Writing - review & editing, Visualization, Funding acquisition, Supervision, Project administration. Acknowledgement This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Min istry of Science and ICT (NRF-2018R1D1A1A09082239). This research was also partially supported by X-mind Corps program of National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2017H1D8A1030582). References [1] G. Sung, J.H. Kim, Effect of high molecular weight isocyanate contents on manufacturing polyurethane foams for improved sound absorption coefficient, Kor. J. Chem. Eng. 34 (2017) 1222–1228, https://doi.org/10.1007/s11814-016-03616. [2] J.G. Gwon, S.K. Kim, J.H. Kim, Sound absorption behavior of flexible polyurethane foams with distinct cellular structures, Mater. Des. 89 (2016) 448–454, https://doi. org/10.1016/j.matdes.2015.10.017. [3] A. Kausar, Polyurethane composite foams in high-performance applications: a review, Polym. Plast. Technol. Eng. 57 (2018) 346–369, https://doi.org/10.1080/ 03602559.2017.1329433. [4] R. Mohammadpour, G.M.M. Sadeghi, Effect of liquefied lignin content on synthesis of bio-based polyurethane foam for oil adsorption application, J. Polym. Environ. (2020) 1–14, https://doi.org/10.1007/s10924-019-01650-5. [5] W. Patten, S. Sha, C. Mo, A vibrational model of open celled polyurethane foam automotive seat cushions, J. Sound Vib. 217 (1998) 145–161, https://doi.org/ 10.1006/jsvi.1998.1760. [6] J.G. Gwon, G. Sung, J.H. Kim, Modulation of cavities and interconnecting pores in manufacturing water blown flexible poly (urethane urea) foams, Int. J. Precis. Eng. Manuf. 16 (2015) 2299–2307, https://doi.org/10.1007/s12541-015-0295-7. [7] J.G. Gwon, S.K. Kim, J.H. Kim, Development of cell morphologies in manufacturing flexible polyurethane urea foams as sound absorption materials, J. Porous Mater. 23 (2016) 465–473, https://doi.org/10.1007/s10934-015-0100-0.
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