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Applications of Polyurethane Based Composites and Nanocomposites
CHAPTER 20
Applications of Polyurethane Based Composites and Nanocomposites Ajay V. Rane1, Krishnan Kanny1, Vayyaprontavida K. Abitha2, Sainath Jadhav3, Saket Mulge3 and Sabu Thomas2 1 Durban University of Technology, Durban, South Africa Mahatma Gandhi University, Kottayam, Kerala, India 3 Institute of Chemical Technology, Mumbai, India 2
Contents 20.1 Introduction 20.2 PU Composites and Nanocomposites 20.3 Conclusion References
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20.1 INTRODUCTION Thermoplastic polyurethanes provide opportunities to modern industry due to their outstanding versatility, by improving the performance of many products including shoe soles, seals, films, conveyor belts, and cables. Thermoplastic polyurethanes have high elongation and tensile strength, and their elasticity and ability to resist oils, greases, solvents, chemicals, and excellent abrasion resistance have made thermoplastic polyurethanes useful in a wide number of applications. Thermoplastic polyurethanes are classified into polyester and polyether. Polyester polyurethanes are unaffected by oils and chemicals, provide excellent abrasion resistance, offer a good balance of physical properties, and are perfect for use in polymer blends, but on other hand polyether polyurethanes are slightly lower in specific gravity than polyester polyurethanes and offer low-temperature flexibility, good abrasion, and tear resistance. Polyether polyurethanes are also durable against microbial attack and provide excellent hydrolysis resistance, making them suitable for applications where water is a consideration. Thermoplastic polyurethanes are a multiphase Polyurethane Polymers: Composites and Nanocomposites DOI: http://dx.doi.org/10.1016/B978-0-12-804065-2.00020-6
Copyright © 2017 Elsevier Inc. All rights reserved.
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block copolymer that is created when three basic raw materials are combined together in a specific way; the three raw materials are polyol or long-chain diol, chain extender or short-chain diol, and a crosslinker diisocyanate [1]. The soft block built out of a polyol and isocyanates is responsible for the flexibility and elastomeric character of thermoplastic polyurethane. The hard block constructed from a chain extender and isocyanates gives thermoplastic polyurethane its toughness (elastomeric property) and physical performance properties. Fig. 20.1 make clear the difference between a thermoplastic elastomer, thermoset, and thermoplastic types of polyurethane, the thermoplastic polyurethane elastomer does not show chemical crosslinks as compared to thermoset polyurethane rubber, in the case of thermoplastic polyurethane where physical crosslinks are broken down/melted when heated and repacked when the material is cooled. Fillers like carbon black, silica, calcium carbonate, nanosilica, nanoclay, carbon nanotubes, graphene, and CETA quantum
Figure 20.1 Thermoplastic elastomers (A), thermoset (B), and thermoplastic (C) types of polyurethanes.
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dots are used in preparation of composites and nanocomposites. Particulate fillers are usually divided into two groups, inert fillers, and reinforcing fillers. Reinforcing fillers are added in an adequate amount to increase mechanical properties and inert fillers are added to increase the bulk and reduce cost [2]. Nanofillers have an extremely large surface area and smooth nonporous surface, which could promote strong physical contact between the filler and the polymer matrix. A feature of polymer nanocomposites is that the small size of the fillers leads to a dramatic increase in the interfacial area as compared with traditional composites. This interfacial area creates a significant volume fraction of interfacial polymer with properties different from the bulk polymer even at low loadings.
20.2 PU COMPOSITES AND NANOCOMPOSITES A brief description of polyurethane composites is given in the paragraph below; Seyyed et al. (2016) prepared polyurethane/carbon black composites and studied the effect of structural and process variables on wear resistance of fabricated PU/carbon black coatings for aluminum substrate. Coatings synthesized had excellent thermal, chemical stabilities, and mechanical properties. S. S. Mirhosseini also confirmed that polyol type (polyester) and pigment concentration have the most significant effects on wear resistance due to hydrogen bonding between hard and soft segments [3]. Tartarisco et al. developed a microfabrication system and proposed a simple deposition model, wherein solid-state unimorph bender actuators were made up of polyurethane as electrolyte and a mixture of carbon black and polyurethane as electrodes. Polyurethane-based microactuators using a polyurethane/carbon black composite were driven with an electrical field of 100 V/µm and showed bending angles higher than 30˚ [4]. C. Xiong et al. (2005) proposed polyurethane/carbon black composites with high positive temperature coefficient and low critical transformation temperature in his letters to the editor, and recorded that polyurethane/ carbon black composites exhibit significant positive temperature coefficient effect, little negative temperature coefficient effect, and excellent electrical conductivity in the presence of stearic acid. The maximum intensity of positive temperature coeeficient effect recorded was 103 when the volume ratio of polyurethane/carbon black/stearic acid composite was 74/11.5/14.5. Polyurethane/carbon black composites do not have a positive temperature coefficient effect in the absence of stearic acid [5].
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Da Silva et al. analyzed and found the conduction mechanism involved in transport process based on castor oil-based polyurethane/carbon black composites, his stimulation is based on virtual composite sample generation represented by a two-dimensional model of a resistor-capacitor network. Microstructure with insulating (polyurethane) and conductive (carbon black) regions, theoretical experimental adjustment indicates that at low frequencies below 103 Hz, the transport mechanism by charges hopping in the polymeric matrix is dominant, on the other hand at high frequencies above 105 Hz electron conduction in carbon black is dominant [6]. E. Andreoli et al. (2014) in their work present a simple, reproducible, and efficient method for the preparation of uniform and compact highly conductive coatings obtained from surfactant-free waterbased mixtures of microbeads made up of crosslinked polyurethane with either carbon black or multiwalled carbon nanotube. Low percolation thresholds, 2.9 wt% with carbon black and 0.8 wt% with multiwalled carbon nanotubes, high saturated conductivities of 200 S/cm for polyurethane/carbon black composites and 500 S/cm for polyurethane/ multiwalled carbon nanotubes were achieved. This points out that highly conductive carbon-based nanocomposite coatings can be made in water without using surfactants using N-methylpyrrolidinone as dispersing agent, and secondly carbon black can be used in place of multiwalled carbon nanotubes for making cheaper coatings without sacrificing the electrical properties [7]. Furtado et al. studied thermal and electrochemical properties of polyurethane electrolyte/carbon black composites as capacitors, which were flexible and had large area collectors. The capacitors prepared with composite electrodes containing 20 wt% of carbon black exhibited a capacity of 0.7 F cm23 under 2 V, providing an energy density of 0.4 W h L21[8]. S. G. Chen et al. (2006) tried to improve the gassensing performance of carbon black/waterborne polyurethane composites with effect to its crosslinking. They analyzed that when carbon black-filled waterborne polyurethanes are exposed to organic solvent vapors, electrical resistance of the material increases rapidly and thus serves as a gas sensor. In order to improve the gas-sensing efficiency of the composites for practical applications, a crosslinking agent was added to composite latexes, forming intramolecular crosslinked networks among the polymer matrix of the composites. The method greatly increased the filler/matrix interfacial interaction and reduced the mobility of the carbon black particles. In the composites that had absorbed solvent vapors, reconstruction of conduction paths through reaggregation of the disconnected
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filler particulates becomes difficult; as a result the unwanted negative vapor coefficient effect was significantly weakened, while the gas sensitivity and the performance reproducibility were enhanced [9]. J. Luo et al. (2007) prepared a polyaniline/polyurethane silica hybrid film using a sol gel process in order to improve the mechanical performance and water resistance of a waterborne film. The fabricated hybrid film showed a surface resistivity of 108 ohm, even though the conducting polyaniline loading was only 10 wt% and the mechanical performance and water resistance improved, making it suitable for antistatic application [10]. R. A. Vasudeo et al. (2015) prepared (polyester) thermoplastic polyurethane nanocomposites by melt mixing technique on the optimized conditions, and various concentrations of nanosilica were added out of which 3 phr was the optimum, which gave excellent properties to nanocomposites, the mechanical properties were increased, electrical properties were improved by adding nanosilica to the polyester thermoplastic polyurethanes, thermal stability also increased with an increase in the degradation temperature at 3 phr of nanosilica loading. Further, the samples were also tested for radiation resistance but nanosilica-based composites could stand and maintain their properties, at specified dosage and time, but there is a decrease in the tensile strength. It can also be concluded that the properties obtained by 3 phr of normal silica could not match the obtained properties of 1 phr nanosilica composites, which states that use of 1 phr nanosilica can give comparable properties obtained by 3 phr of normal silica [11]. Z. T. Mazraeh-Shahi et al. (2015) synthesized polyurethane/silica hybrid hydrogels at ambient pressure drying conditions with a specific surface area, pore volume, and pore diameter in the range of 530 850 m2 g21, 0.71 2.83 cm3 g21, and 5 13 nm, respectively; pore structure and properties of these hybrid aerogels showed dependence on polyurethane content. Improvement in structure and properties compared to native silica aerogel was achieved by adding 3 v/v% aqueous polyurethane dispersion into silica sol; a high polyurethane content led to low porosity (61%) and small surface area (535 m2 g21) [12]. L. Chen et al. (2015) prepared novel polyurethane/silica hybrid materials, without an external crosslinking agent via chemical reaction between urethane groups of polyurethane prepolymer and hydroxyl groups at the surface of the silica. The added inorganic filler-silica thus played the dual roles not only as an inorganic chain extender but also as a reinforcing agent in the preparation of the hybrid. Mechanical properties on the basis of their processing
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were compared—i.e., stirring mixing and three rollers shear mixing. The shear effect of the three roller shear process enabled the size of silica aggregates to be more uniform. Tensile strength and elongation at break of the polyurethane/silica hybrids were 51 Mpa and 590%, which indicated an increase of fourfold and 39% compared to those of neat polyurethane. Increases in thermal stabilities and lower decomposition rates for hybrids were noted [13]. M. A. Semsarzadeh et al. (2013) synthesized silicon-based particles using tetraethoxysilane as silica precursor and a low concentration of cetyltrimethylammonium bromide with polyvinyl alcohol as templating agents, synthesized silicon particles possessed higher hydrophilicity and polarity than conventional silica particles. Polyurethane and polyurethane/silica composite membranes were prepared by solution casting techniques. Gas permeation properties of membranes with different silica contents were studied for pure carbon dioxide, methane, oxygen, and nitrogen gases. The results showed an increase in the solubility and a corresponding reduction in the diffusivity of the gases through the membranes by increasing the silica content in the polymer matrix [14]. C. A. Heck et al. (2015) prepared a series of composites consisting of commercial waterborne polyurethane and silica by in situ synthesis (sol gel method) and compared with those prepared by addition of a commercial silica blending method, adhesion resistance reached a maximum at 3 wt% of added silica. Composites containing commercial silica displayed higher mechanical resistance, but adhesion was obtained with an in situ method, differential scanning calorimetry showed increasing crystallinity with increasing silica addition [15]. Yang et al. prepared multilayered optically active polyurethane/titania/manganese dioxide nanohybrids, manganese dioxide nanorods were coated by titania with the thickness around 15 nm and the amount of optically active polyurethane grafted onto titania/manganese dioxide was about 0.21 g/g inorganics. Optically active polyurethanes/titania/manganese dioxide nanorods possess well-defined multicoated core shell architectures compared with bare manganese dioxide, the infrared emissivity values decreased after the wrapping, indicating a potential for practical application. Interfacial interactions, crystallinity, and helical secondary structure of optically active polyurethane significantly affected the infrared radiation property of the composites [16]. Yang et al. prepared optically active polyurethane/titania/silica multilayered core shell composite microspheres by the combination of titania deposition on the surface of the silica spheres and subsequent polymer grafting. Optically active polyurethane/titania/silica
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exhibited clearly multilayered core shell construction, the infrared emissivity values reduced along with the increase of covering layers thus proved that the interfacial interactions has direct influence on the infrared emissivity [17]. S. Gu et al. (2013) used ester-based polyurethane with low glass transition temperature to develop shape memory nanocomposites with low trigger temperature, pristine carbon nanotubes and oxidized carbon nanotubes along with polyurethane matrix were melt processed, in order to obtain homogeneous dispersion to achieve improvement in mechanical and shape memory properties of the polyurethane matrix. The results predict that better dispersion of oxidized carbon nanotubes contributes to more stiffness effect below the glass transition temperature, with lower storage modulus above the glass transition temperature. Nanocomposites exhibit high shape fixity and recovery ratio above 98%. The oxidized carbon nanotube/polyurethane composites show higher shape recovery ratio for the first cycle, faster recovery is attributed to better dispersion of carbon nanotubes and potential applications for controlling tags or proof marks in the area of frozen food, the trigger temperature can be tailored by controlling the glass transition of polyurethane matrix or the content of nanofillers [18]. Z. Yao et al. (2013) prepared polyurethane nanocomposites containing carbon nanotubes through in situ polymerization for the creep study. The results show that the presence of carbon nanotubes leads to a significant improvement of creep resistance of polyurethanes. However, this creep resistance does not increase monotonously with increase of carbon nanotube contents because it is highly dependent on the dispersion of carbon nanotubes. Several theoretical models were then used to establish the relations between carbon nanotube dispersion and final creep and creep recovery behaviors of nanocomposites. The as-obtained viscoelastic and viscoplastic parameters of polyurethane matrix and structural parameters of carbon nanotubes further confirmed the retardation effect by carbon nanotubes during creep of the nanocomposite systems [19]. Sahoo and his group incorporated carboxylic functionalized multiwalled carbon nanotubes to segmented polyurethane block copolymer in order to demonstrate the effect of the chemical functionalization of multiwalled carbon nanotubes on thermal, morphological, and mechanical properties of multiwalled carbon nanotube-reinforced nanocomposites. Scanning electron microscopy observation indicated that homogeneous dispersion of functionalized multiwalled carbon nanotubes throughout polyurethane matrix and strong interfacial adhesion between functionalized multiwalled carbon
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nanotubes and the polyurethane were achieved in polyurethane nanocomposites, which brought an enhancement in mechanical properties. Multiwalled carbon nanotubes improve the thermal stability of the nanocomposites as reported by thermogravimetric analysis [20]. Po¨tschke et al. mentioned that carbon nanotubes have been shown to be attractive fillers for achieving electrical conductivity of polymers at relatively low carbon nanotube contents. Amounts lower than 1 wt% are reported to be sufficient to get conductive polymers, which is much less than for carbon black. This efficient behavior of carbon nanotubes is caused by the excellent electrical properties in combination with the very high aspect ratio, as high as 1000. In addition, mechanical properties may be enhanced due to the fiber-like shape of the filler. Thus, it is also promising to use carbon nanotubes as additives in polyurethanes for electrically conductive or antistatic applications. It could be shown that carbon nanotubes are effective fillers for thermoplastic polyurethane in order to get electrically dissipative or conductive composites. Using small-scale melt mixing, electrical percolation of as extruded strands could be reached at concentrations as low as 1.5 wt% (,1 vol%). Electrically conductive composites were obtained starting at 2 wt% addition of Nanocyl7000 industrial-grade material which has excellent dispersability and led to quite homogeneous dispersion as illustrated by atomic force microscopy. Direct multiwalled carbon nanotube incorporation compared to premanufacturing master batches having 15 wt% multiwalled carbon nanotubes indicated lower resistivity values in the latter case, which can be related to more homogeneous multiwalled carbon nanotube dispersion. The excellent mechanical properties of thermoplastic polyurethane, especially the high deformability, can be preserved in the nanocomposite. Modulus and stress at a given strain are enhanced upon addition of multiwalled carbon nanotubes which may be a combined effect of multiwalled carbon nanotube incorporation and the slightly enhanced crystallinity [21]. Verdejo et al. made flexible polyurethane foams, with loading fractions of up to 0.2 wt% carbon nanotubes by free-rising foaming using water as the blowing agent. Electron microscopy revealed an open cellular structure and a homogeneous dispersion of carbon nanotubes, although the incorporation of nanofiller affected the foaming process and thus the final foam density and cellular structure. The compressive response of the foams did not show an unambiguous improvement with carbon nanotubes content due to the variable foam structure. However, dense films generated by hot pressing the foams indicated a significant intrinsic reinforcement of the polymer,
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even at low loadings of carbon nanotubes. Most significantly, carbon nanotubes were found to increase the acoustic activity monotonically at concentrations up to 0.1 wt% [22]. H. C. Kuan et al. (2005) fabricated a novel nanocomposite consisting of multiwalled carbon nanotube/waterborne polyurethane. Carbon nanotube was modified, to be compatible with waterborne polyurethane via covalent bonding or ionic bonding. Thermal properties show that adding carbon nanotube enhanced the thermal stability by 26˚C (from 315 to 341˚C) when carbon nanotube content was 2.5 phr (parts per hundred parts of resin). After the surface modification, carbon nanotube can be dispersed effectively, and improve the interfacial strength between it and the waterborne polyurethane matrix. Consequently, the physical properties of nanocomposites are enhanced, especially in the covalent bonding system. Mechanical property tests show that adding multiwalled carbon nanotubes improves the tensile properties very significantly (370% in tensile strength). Electron microscopy microphotographs prove that carbon nanotubes can be effectively dispersed in waterborne polyurethane matrix. Rheological tests show that carbon nanotubes can increase the melt viscosity and reduce the variation of processing viscosity [23]. Electro-active shape-memory composites were synthesized using conducting polyurethane composites and multiwalled carbon nanotubes by J. W. Cho et al. (2004). Surface modification of the multiwalled carbon nanotubes by acid treatment improved the mechanical properties of the composites. The modulus and stress at 100% elongation increased with increasing surfacemodified multiwalled carbon nanotube content, while elongation at break decreased. Multiwalled carbon nanotube surface modification also resulted in a decrease in the electrical conductivity of the composites, however, as the surface-modified multiwalled carbon nanotube content increased the conductivity increased (an order of 1023 S cm21 was obtained in samples with 5 wt% modified multiwalled carbon nanotube content). Electro-active shape recovery was observed for the surface-modified multiwalled carbon nanotube composites with an energy conversion efficiency of 10.4%. Hence, polyurethane multiwalled carbon nanotube composites may prove promising candidates for use as smart actuators [24]. X. Luo et al. (2011) noted pointed out in their abstract that 1-thioglycerol-stabilized quantum dots are good candidates for performing a stepwise polymerization reaction with polyurethane prepolymers, but their use is limited by their worse photoluminescence in comparison to quantum dots stabilized by other mercapto-ligands. To overcome this problem, 1-thioglycerol-stabilized CdTe quantum dots with enhanced
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photoluminescence were synthesized in water through a N2H4-promoted growth approach. The current synthesis significantly shortened the duration of the size evolution of quantum dots, particularly for obtaining samples with orange and red emissions. It also permitted quantum dot growth at low temperatures, such as room temperature, which avoided the decomposition of 1-thioglycerol and subsequent embedment of sulfur into the quantum dots as occurs in the conventional synthesis. Most importantly, the 1-thioglycerol ligand endowed the quantum dots with hydroxyl coverage, making the quantum dots miscible with polyurethane prepolymers in dimethyl sulfoxide, and therefore overcoming the main problem in fabricating polyurethane-based nanocomposites of eliminating water. The hydroxyl coverage further allowed for the linkage of polyurethane on the surface of the quantum dots through the reaction between OH and NCO, producing CdTe quantum dot polyurethane bulk nanocomposites. Systematic characterization indicated that the quantum dots were well-dispersed in the polyurethane medium, and the sizedependent photoluminescence of quantum dots was also maintained [25]. For the first time, in 2008 Xiaodong and his group reported a simple method to fabricate a quantum dot polymer composite completely via aqueous media from synthesis of quantum dots to formation of composite, wherein a series of L-cysteine capped CdTe quantum dots with tunable emission from green to red were prepared by using hydrothermal techniques, and the corresponding average particle sizes were estimated to be from 2.5 to 4.1 nm. After incorporating these CdTe quantum dots into the waterborne polyurethane prepolymer aqueous suspension, a transparent nanocomposite film was obtained by casting and evaporating. The results recorded by UV vis and photoluminescence indicate that the quantum yield and the photochemical stability of the CdTe quantum dots in both CdTe waterborne polyurethane aqueous complex and solid composite are enhanced significantly, because of a thicker and more compact passivating layer formed on the surface of CdTe quantum dots via the reaction between the groups of NCO and NH2, transmission electron microscopy and laser scanning confocal fluorescence microscopy images show that the CdTe quantum dots with excellent crystalline structure and strong florescence emission are well-dispersed in the waterborne polyurethane matrix without obvious aggregation or agglomeration. On the basis of the versatile properties of waterborne polyurethane and the photoluminescence originating from the CdTe quantum dots, these new fluorescent composite materials could have great potential applications.
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This approach provides a simple route for preparation of various fluorescent quantum dots polymer composite materials from aqueous quantum dot solutions with neither ligand exchange nor phase transfer [26]. X. Li et al. (2011) synthesized a series of positively charged imidazoliumfunctionalized ionic polyurethanes in a one-step polymerization process by polymerization of presynthesized short-chain imidazolium-based ionic diol, polyethylene glycols with different molecular weights as long-chain diols, and toluylene-2,4-diisocyanate. The structures of ionic polyurethanes are confirmed by 1H NMR analysis, and the thermogravimetric analysis measurement indicates that the ionic polyurethanes have a high degradation temperature. Fluorescent nanocrystal polymer composites CdTe ionic polyurethanes can be prepared conveniently, by the electrostatic interaction between positively charged ionic polyurethanes and the negatively charged aqueous CdTe quantum dots. UV vis absorption and photoluminescence spectra indicate the photochemical stability and strong fluorescent emission of CdTe ionic polyurethane composites. The quantum yields of the composites are high and basically restore the quantum yields of the pure quantum dots. In addition, the transmission electron microscopy photographs show that the quantum dots in composites are uniform (about 3 nm in diameter) and monodisperse. The obtained nanocomposites are powder or elastomers with good film building. The cast CdTe ionic polyurethanes films are transparent under visible light, and the colors of the composites and their films are vivid under a UV lamp [27]. Muralidharan et al. stated that optically triggered actuators offer unique advantages like wireless actuation and remote control when compared to other types of actuators. They are extremely useful where stimuli other than electricity or heat are preferred. Thermally reduced graphene oxide/thermoplastic polyurethane composite actuators were prepared by a simple solution casting technique. The photomechanical actuation properties of the composites were studied under infrared illumination. It was found that the photomechanical response can be tuned by controlling the applied prestrain and the filler loading. Even with low filler loading of 2 wt% thermally reduced graphene oxide, the composite exhibited a very high photomechanical strain of 50.2% with an excellent stress of 1680 kPa at a prestrain of 220%. These high values were achieved at a very low light intensity of 16 mW cm22. The high values of strain obtained with very good generative forces indicate that this is a promising material for light-triggered actuators for many potential applications including robotics and biomedical devices [28].
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J. H. Park et al. (2014) fabricated shape memory behavior of crystalline shape memory polyurethane reinforced with graphene, which utilizes melting temperature as a shape recovery temperature, was examined with various external actuating stimuli, such as direct heating, resistive heating, and infrared heating. Compatibility of graphene with crystalline shape memory polyurethane was adjusted by altering the structure of the hard segment of the shape memory polyurethane, by changing the structure of the graphene, and by changing the preparation method of the graphene/shape memory polyurethane composite. The shape memory polyurethane made of aromatic 4,4´-diphenylmethane diisocyanate exhibited better compatibility with graphene, having an aromatic structure, compared to that made of the aliphatic hexamethylene diisocyanate. The finely dispersed graphene effectively reinforced shape memory polyurethane made of aromatic 4,4´-diphenylmethane diisocyanate, improved shape recovery of shape memory polyurethane made of aromatic 4,4´-diphenylmethane diisocyanate, and served effectively as a filler, triggering shape recovery by resistive or infrared heating. Compatibility was enhanced when the graphene was modified with methanol. This improved shape recovery by direct heating, but worsened the conductivity of the composite, and consequently the efficiency of resistive heating for shape recovery also declined. Graphene modified with methanol was more effective than pristine graphene in terms of shape recovery by infrared heating [29]. N. Yousefi et al. (2012) produced polyurethane-based composite films containing highly aligned graphene sheets through an environmentally benign process, where an aqueous liquid crystalline dispersion of graphene oxide is in situ reduced in polyurethane, resulting in a fine dispersion and a high degree of orientation of graphene sheets. The polyurethane particles are adsorbed onto the surface of the reduced graphene oxide, and the reduced graphene oxide sheets with a large aspect ratio of over 10,000 tend to self-align during the film formation when the graphene content is high enough, at more than 2 wt%. The resulting composites show excellent electrical conductivity with an extremely low percolation threshold of 0.078 vol%, which is considered one of the lowest values ever reported for polymer composites containing graphene [30]. G. Zhao et al. (2011) added carbon fiber or short glass fiber to polyurethane to enhance the tensile strength of the composite, so that it could be used for structural materials in high-load conditions. Polyurethane composites have been widely used to produce bearings or gears. Chemical modification of the fiber, by grafting with diisocyanatotoluene, also played an important role in reducing phase separation [31].
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H. Kim et al. (2010) developed strategies for isolating single-layer carbon sheets from graphite have enabled production of electrically conductive, mechanically robust polymer nanocomposites with enhanced gas barrier performance at extremely low loading. For the first time, we compare carbon sheets exfoliated from graphite oxide via two different processes: chemical modification (isocyanate-treated graphite oxide) and thermal exfoliation (thermally reduced graphite oxide), and three different methods of dispersion: solvent blending, in situ polymerization, and melt compounding. Incorporation of as low as 0.5 wt% of thermally reduced graphite oxide produced electrically conductive thermoplastic polyurethane. Upto a 10-fold increase in tensile stiffness and 90% decrease in nitrogen permeation of thermoplastic polyurethane were observed with only 3 wt% isocyanate-treated graphite oxide, implying a high aspect ratio of exfoliated platelets. Real- and reciprocal-space morphological characterization indicated that solvent-based blending techniques more effectively distribute thin exfoliated sheets in the polymer matrix than melt processing. This observation is in good qualitative agreement with the dispersion level inferred from solid property enhancements. Although also processed in solvents, property increase via in situ polymerization was not as pronounced because of reduced hydrogen bonding in the thermoplastic polyurethane produced [32].
20.3 CONCLUSION Properties of polyurethane composites and nanocomposites depend on the process and the processing condition, proper choice of filler and quantity of filler are also important to obtain desired properties for which a trial and error has to be performed, selection of suitable grade of polyurethane for a specified application must be taken into consideration, i.e., either ester or ether-based polyurethanes. Polyurethane composites and nanocomposites have been used in various applications including functional coatings, nanoelectronics, gas-sensing, packaging, biomedical implants, solar cells, seals, etc.
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[20] N.G. Sahoo, Y.C. Jung, H.H. So, J.W. Cho, Synthesis of polyurethane nanocomposites of functionalized carbon nanotubes by in-situ polymerization methods, J. Korean Phys. Soc. (2007) 51, 1- 7. [21] P. Po¨tschke, L. Ha¨ußler, S. Pegel, R. Steinberger, G. Scholz, Thermoplastic polyurethane filled with carbon nanotubes for electrical dissipative and conductive applications, KGK. Kautschuk, Gummi, Kunststoffe 60 (9) (2007) 432 437. [22] R. Verdejo, R. Sta¨mpfli, M. Alvarez-Lainez, S. Mourad, M.A. Rodriguez-Perez, P.A. Bru¨hwiler, et al., Enhanced acoustic damping in flexible polyurethane foams filled with carbon nanotubes, Compos. Sci. Technol. 69 (10) (2009) 1564 1569. [23] H.C. Kuan, C.C.M. Ma, W.P. Chang, S.M. Yuen, H.H. Wu, T.M. Lee, Synthesis, thermal, mechanical and rheological properties of multiwall carbon nanotube/waterborne polyurethane nanocomposite, Compos. Sci. Technol. 65 (11) (2005) 1703 1710. [24] J.W. Cho, J.W. Kim, Y.C. Jung, N.S. Goo, Electroactive shape-memory polyurethane composites incorporating carbon nanotubes, Macromol. Rapid Commun. 26 (5) (2005) 412 416. [25] X. Luo, J. Han, Y. Ning, Z. Lin, H. Zhang, B. Yang, Polyurethane-based bulk nanocomposites from 1-thioglycerol-stabilized CdTe quantum dots with enhanced luminescence, J. Mater. Chem. 21 (18) (2011) 6569 6575. [26] X. Cao, C.M. Li, H. Bao, Q. Bao, H. Dong, Fabrication of strongly fluorescent quantum dot 2 polymer composite in aqueous solution, Chem. Mater. 19 (15) (2007) 3773 3779. [27] X. Li, X. Ni, Z. Liang, Z. Shen, Synthesis of imidazolium-functionalized ionic polyurethane and formation of CdTe quantum dot polyurethane nanocomposites, J. Polym. Sci. Part A Polym. Chem. 50 (3) (2012) 509 516. [28] M.N. Muralidharan, S. Ansari, Thrermally reduced graphene oxide/thermoplastic polyurethane nanocomposites as photomechanical actuators, Adv. Mater. Lett 4 (2013) 927 932. [29] J.H. Park, T.D. Dao, H.I. Lee, H.M. Jeong, B.K. Kim, Properties of graphene/shape memory thermoplastic polyurethane composites actuating by various methods, Materials 7 (3) (2014) 1520 1538. [30] N. Yousefi, M.M. Gudarzi, Q. Zheng, S.H. Aboutalebi, F. Sharif, J.K. Kim, Selfalignment and high electrical conductivity of ultralarge graphene oxide polyurethane nanocomposites, J. Mater. Chem. 22 (25) (2012) 12709 12717. [31] G. Zhao, T. Wang, Q. Wang, Fiber-reinforced polyurethane composites, plastic research online, society of plastics engineers, 10.1002/spepro.003956; 2011. [32] H. Kim, Y. Miura, C.W. Macosko, Graphene/polyurethane nanocomposites for improved gas barrier and electrical conductivity, Chem. Mater. 22 (11) (2010) 3441 3450.
Applications of Polyurethane Based Composites and Nanocomposites Ajay V. Rane1, Krishnan Kanny1, Vayyaprontavida K. Abitha2, Sainath Jadhav3, Saket Mulge3 and Sabu Thomas2 1 Durban University of Technology, Durban, South Africa Mahatma Gandhi University, Kottayam, Kerala, India 3 Institute of Chemical Technology, Mumbai, India 2
Contents 20.1 Introduction 20.2 PU Composites and Nanocomposites 20.3 Conclusion References
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20.1 INTRODUCTION Thermoplastic polyurethanes provide opportunities to modern industry due to their outstanding versatility, by improving the performance of many products including shoe soles, seals, films, conveyor belts, and cables. Thermoplastic polyurethanes have high elongation and tensile strength, and their elasticity and ability to resist oils, greases, solvents, chemicals, and excellent abrasion resistance have made thermoplastic polyurethanes useful in a wide number of applications. Thermoplastic polyurethanes are classified into polyester and polyether. Polyester polyurethanes are unaffected by oils and chemicals, provide excellent abrasion resistance, offer a good balance of physical properties, and are perfect for use in polymer blends, but on other hand polyether polyurethanes are slightly lower in specific gravity than polyester polyurethanes and offer low-temperature flexibility, good abrasion, and tear resistance. Polyether polyurethanes are also durable against microbial attack and provide excellent hydrolysis resistance, making them suitable for applications where water is a consideration. Thermoplastic polyurethanes are a multiphase Polyurethane Polymers: Composites and Nanocomposites DOI: http://dx.doi.org/10.1016/B978-0-12-804065-2.00020-6
Copyright © 2017 Elsevier Inc. All rights reserved.
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block copolymer that is created when three basic raw materials are combined together in a specific way; the three raw materials are polyol or long-chain diol, chain extender or short-chain diol, and a crosslinker diisocyanate [1]. The soft block built out of a polyol and isocyanates is responsible for the flexibility and elastomeric character of thermoplastic polyurethane. The hard block constructed from a chain extender and isocyanates gives thermoplastic polyurethane its toughness (elastomeric property) and physical performance properties. Fig. 20.1 make clear the difference between a thermoplastic elastomer, thermoset, and thermoplastic types of polyurethane, the thermoplastic polyurethane elastomer does not show chemical crosslinks as compared to thermoset polyurethane rubber, in the case of thermoplastic polyurethane where physical crosslinks are broken down/melted when heated and repacked when the material is cooled. Fillers like carbon black, silica, calcium carbonate, nanosilica, nanoclay, carbon nanotubes, graphene, and CETA quantum
Figure 20.1 Thermoplastic elastomers (A), thermoset (B), and thermoplastic (C) types of polyurethanes.
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dots are used in preparation of composites and nanocomposites. Particulate fillers are usually divided into two groups, inert fillers, and reinforcing fillers. Reinforcing fillers are added in an adequate amount to increase mechanical properties and inert fillers are added to increase the bulk and reduce cost [2]. Nanofillers have an extremely large surface area and smooth nonporous surface, which could promote strong physical contact between the filler and the polymer matrix. A feature of polymer nanocomposites is that the small size of the fillers leads to a dramatic increase in the interfacial area as compared with traditional composites. This interfacial area creates a significant volume fraction of interfacial polymer with properties different from the bulk polymer even at low loadings.
20.2 PU COMPOSITES AND NANOCOMPOSITES A brief description of polyurethane composites is given in the paragraph below; Seyyed et al. (2016) prepared polyurethane/carbon black composites and studied the effect of structural and process variables on wear resistance of fabricated PU/carbon black coatings for aluminum substrate. Coatings synthesized had excellent thermal, chemical stabilities, and mechanical properties. S. S. Mirhosseini also confirmed that polyol type (polyester) and pigment concentration have the most significant effects on wear resistance due to hydrogen bonding between hard and soft segments [3]. Tartarisco et al. developed a microfabrication system and proposed a simple deposition model, wherein solid-state unimorph bender actuators were made up of polyurethane as electrolyte and a mixture of carbon black and polyurethane as electrodes. Polyurethane-based microactuators using a polyurethane/carbon black composite were driven with an electrical field of 100 V/µm and showed bending angles higher than 30˚ [4]. C. Xiong et al. (2005) proposed polyurethane/carbon black composites with high positive temperature coefficient and low critical transformation temperature in his letters to the editor, and recorded that polyurethane/ carbon black composites exhibit significant positive temperature coefficient effect, little negative temperature coefficient effect, and excellent electrical conductivity in the presence of stearic acid. The maximum intensity of positive temperature coeeficient effect recorded was 103 when the volume ratio of polyurethane/carbon black/stearic acid composite was 74/11.5/14.5. Polyurethane/carbon black composites do not have a positive temperature coefficient effect in the absence of stearic acid [5].
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Da Silva et al. analyzed and found the conduction mechanism involved in transport process based on castor oil-based polyurethane/carbon black composites, his stimulation is based on virtual composite sample generation represented by a two-dimensional model of a resistor-capacitor network. Microstructure with insulating (polyurethane) and conductive (carbon black) regions, theoretical experimental adjustment indicates that at low frequencies below 103 Hz, the transport mechanism by charges hopping in the polymeric matrix is dominant, on the other hand at high frequencies above 105 Hz electron conduction in carbon black is dominant [6]. E. Andreoli et al. (2014) in their work present a simple, reproducible, and efficient method for the preparation of uniform and compact highly conductive coatings obtained from surfactant-free waterbased mixtures of microbeads made up of crosslinked polyurethane with either carbon black or multiwalled carbon nanotube. Low percolation thresholds, 2.9 wt% with carbon black and 0.8 wt% with multiwalled carbon nanotubes, high saturated conductivities of 200 S/cm for polyurethane/carbon black composites and 500 S/cm for polyurethane/ multiwalled carbon nanotubes were achieved. This points out that highly conductive carbon-based nanocomposite coatings can be made in water without using surfactants using N-methylpyrrolidinone as dispersing agent, and secondly carbon black can be used in place of multiwalled carbon nanotubes for making cheaper coatings without sacrificing the electrical properties [7]. Furtado et al. studied thermal and electrochemical properties of polyurethane electrolyte/carbon black composites as capacitors, which were flexible and had large area collectors. The capacitors prepared with composite electrodes containing 20 wt% of carbon black exhibited a capacity of 0.7 F cm23 under 2 V, providing an energy density of 0.4 W h L21[8]. S. G. Chen et al. (2006) tried to improve the gassensing performance of carbon black/waterborne polyurethane composites with effect to its crosslinking. They analyzed that when carbon black-filled waterborne polyurethanes are exposed to organic solvent vapors, electrical resistance of the material increases rapidly and thus serves as a gas sensor. In order to improve the gas-sensing efficiency of the composites for practical applications, a crosslinking agent was added to composite latexes, forming intramolecular crosslinked networks among the polymer matrix of the composites. The method greatly increased the filler/matrix interfacial interaction and reduced the mobility of the carbon black particles. In the composites that had absorbed solvent vapors, reconstruction of conduction paths through reaggregation of the disconnected
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filler particulates becomes difficult; as a result the unwanted negative vapor coefficient effect was significantly weakened, while the gas sensitivity and the performance reproducibility were enhanced [9]. J. Luo et al. (2007) prepared a polyaniline/polyurethane silica hybrid film using a sol gel process in order to improve the mechanical performance and water resistance of a waterborne film. The fabricated hybrid film showed a surface resistivity of 108 ohm, even though the conducting polyaniline loading was only 10 wt% and the mechanical performance and water resistance improved, making it suitable for antistatic application [10]. R. A. Vasudeo et al. (2015) prepared (polyester) thermoplastic polyurethane nanocomposites by melt mixing technique on the optimized conditions, and various concentrations of nanosilica were added out of which 3 phr was the optimum, which gave excellent properties to nanocomposites, the mechanical properties were increased, electrical properties were improved by adding nanosilica to the polyester thermoplastic polyurethanes, thermal stability also increased with an increase in the degradation temperature at 3 phr of nanosilica loading. Further, the samples were also tested for radiation resistance but nanosilica-based composites could stand and maintain their properties, at specified dosage and time, but there is a decrease in the tensile strength. It can also be concluded that the properties obtained by 3 phr of normal silica could not match the obtained properties of 1 phr nanosilica composites, which states that use of 1 phr nanosilica can give comparable properties obtained by 3 phr of normal silica [11]. Z. T. Mazraeh-Shahi et al. (2015) synthesized polyurethane/silica hybrid hydrogels at ambient pressure drying conditions with a specific surface area, pore volume, and pore diameter in the range of 530 850 m2 g21, 0.71 2.83 cm3 g21, and 5 13 nm, respectively; pore structure and properties of these hybrid aerogels showed dependence on polyurethane content. Improvement in structure and properties compared to native silica aerogel was achieved by adding 3 v/v% aqueous polyurethane dispersion into silica sol; a high polyurethane content led to low porosity (61%) and small surface area (535 m2 g21) [12]. L. Chen et al. (2015) prepared novel polyurethane/silica hybrid materials, without an external crosslinking agent via chemical reaction between urethane groups of polyurethane prepolymer and hydroxyl groups at the surface of the silica. The added inorganic filler-silica thus played the dual roles not only as an inorganic chain extender but also as a reinforcing agent in the preparation of the hybrid. Mechanical properties on the basis of their processing
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were compared—i.e., stirring mixing and three rollers shear mixing. The shear effect of the three roller shear process enabled the size of silica aggregates to be more uniform. Tensile strength and elongation at break of the polyurethane/silica hybrids were 51 Mpa and 590%, which indicated an increase of fourfold and 39% compared to those of neat polyurethane. Increases in thermal stabilities and lower decomposition rates for hybrids were noted [13]. M. A. Semsarzadeh et al. (2013) synthesized silicon-based particles using tetraethoxysilane as silica precursor and a low concentration of cetyltrimethylammonium bromide with polyvinyl alcohol as templating agents, synthesized silicon particles possessed higher hydrophilicity and polarity than conventional silica particles. Polyurethane and polyurethane/silica composite membranes were prepared by solution casting techniques. Gas permeation properties of membranes with different silica contents were studied for pure carbon dioxide, methane, oxygen, and nitrogen gases. The results showed an increase in the solubility and a corresponding reduction in the diffusivity of the gases through the membranes by increasing the silica content in the polymer matrix [14]. C. A. Heck et al. (2015) prepared a series of composites consisting of commercial waterborne polyurethane and silica by in situ synthesis (sol gel method) and compared with those prepared by addition of a commercial silica blending method, adhesion resistance reached a maximum at 3 wt% of added silica. Composites containing commercial silica displayed higher mechanical resistance, but adhesion was obtained with an in situ method, differential scanning calorimetry showed increasing crystallinity with increasing silica addition [15]. Yang et al. prepared multilayered optically active polyurethane/titania/manganese dioxide nanohybrids, manganese dioxide nanorods were coated by titania with the thickness around 15 nm and the amount of optically active polyurethane grafted onto titania/manganese dioxide was about 0.21 g/g inorganics. Optically active polyurethanes/titania/manganese dioxide nanorods possess well-defined multicoated core shell architectures compared with bare manganese dioxide, the infrared emissivity values decreased after the wrapping, indicating a potential for practical application. Interfacial interactions, crystallinity, and helical secondary structure of optically active polyurethane significantly affected the infrared radiation property of the composites [16]. Yang et al. prepared optically active polyurethane/titania/silica multilayered core shell composite microspheres by the combination of titania deposition on the surface of the silica spheres and subsequent polymer grafting. Optically active polyurethane/titania/silica
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exhibited clearly multilayered core shell construction, the infrared emissivity values reduced along with the increase of covering layers thus proved that the interfacial interactions has direct influence on the infrared emissivity [17]. S. Gu et al. (2013) used ester-based polyurethane with low glass transition temperature to develop shape memory nanocomposites with low trigger temperature, pristine carbon nanotubes and oxidized carbon nanotubes along with polyurethane matrix were melt processed, in order to obtain homogeneous dispersion to achieve improvement in mechanical and shape memory properties of the polyurethane matrix. The results predict that better dispersion of oxidized carbon nanotubes contributes to more stiffness effect below the glass transition temperature, with lower storage modulus above the glass transition temperature. Nanocomposites exhibit high shape fixity and recovery ratio above 98%. The oxidized carbon nanotube/polyurethane composites show higher shape recovery ratio for the first cycle, faster recovery is attributed to better dispersion of carbon nanotubes and potential applications for controlling tags or proof marks in the area of frozen food, the trigger temperature can be tailored by controlling the glass transition of polyurethane matrix or the content of nanofillers [18]. Z. Yao et al. (2013) prepared polyurethane nanocomposites containing carbon nanotubes through in situ polymerization for the creep study. The results show that the presence of carbon nanotubes leads to a significant improvement of creep resistance of polyurethanes. However, this creep resistance does not increase monotonously with increase of carbon nanotube contents because it is highly dependent on the dispersion of carbon nanotubes. Several theoretical models were then used to establish the relations between carbon nanotube dispersion and final creep and creep recovery behaviors of nanocomposites. The as-obtained viscoelastic and viscoplastic parameters of polyurethane matrix and structural parameters of carbon nanotubes further confirmed the retardation effect by carbon nanotubes during creep of the nanocomposite systems [19]. Sahoo and his group incorporated carboxylic functionalized multiwalled carbon nanotubes to segmented polyurethane block copolymer in order to demonstrate the effect of the chemical functionalization of multiwalled carbon nanotubes on thermal, morphological, and mechanical properties of multiwalled carbon nanotube-reinforced nanocomposites. Scanning electron microscopy observation indicated that homogeneous dispersion of functionalized multiwalled carbon nanotubes throughout polyurethane matrix and strong interfacial adhesion between functionalized multiwalled carbon
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nanotubes and the polyurethane were achieved in polyurethane nanocomposites, which brought an enhancement in mechanical properties. Multiwalled carbon nanotubes improve the thermal stability of the nanocomposites as reported by thermogravimetric analysis [20]. Po¨tschke et al. mentioned that carbon nanotubes have been shown to be attractive fillers for achieving electrical conductivity of polymers at relatively low carbon nanotube contents. Amounts lower than 1 wt% are reported to be sufficient to get conductive polymers, which is much less than for carbon black. This efficient behavior of carbon nanotubes is caused by the excellent electrical properties in combination with the very high aspect ratio, as high as 1000. In addition, mechanical properties may be enhanced due to the fiber-like shape of the filler. Thus, it is also promising to use carbon nanotubes as additives in polyurethanes for electrically conductive or antistatic applications. It could be shown that carbon nanotubes are effective fillers for thermoplastic polyurethane in order to get electrically dissipative or conductive composites. Using small-scale melt mixing, electrical percolation of as extruded strands could be reached at concentrations as low as 1.5 wt% (,1 vol%). Electrically conductive composites were obtained starting at 2 wt% addition of Nanocyl7000 industrial-grade material which has excellent dispersability and led to quite homogeneous dispersion as illustrated by atomic force microscopy. Direct multiwalled carbon nanotube incorporation compared to premanufacturing master batches having 15 wt% multiwalled carbon nanotubes indicated lower resistivity values in the latter case, which can be related to more homogeneous multiwalled carbon nanotube dispersion. The excellent mechanical properties of thermoplastic polyurethane, especially the high deformability, can be preserved in the nanocomposite. Modulus and stress at a given strain are enhanced upon addition of multiwalled carbon nanotubes which may be a combined effect of multiwalled carbon nanotube incorporation and the slightly enhanced crystallinity [21]. Verdejo et al. made flexible polyurethane foams, with loading fractions of up to 0.2 wt% carbon nanotubes by free-rising foaming using water as the blowing agent. Electron microscopy revealed an open cellular structure and a homogeneous dispersion of carbon nanotubes, although the incorporation of nanofiller affected the foaming process and thus the final foam density and cellular structure. The compressive response of the foams did not show an unambiguous improvement with carbon nanotubes content due to the variable foam structure. However, dense films generated by hot pressing the foams indicated a significant intrinsic reinforcement of the polymer,
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even at low loadings of carbon nanotubes. Most significantly, carbon nanotubes were found to increase the acoustic activity monotonically at concentrations up to 0.1 wt% [22]. H. C. Kuan et al. (2005) fabricated a novel nanocomposite consisting of multiwalled carbon nanotube/waterborne polyurethane. Carbon nanotube was modified, to be compatible with waterborne polyurethane via covalent bonding or ionic bonding. Thermal properties show that adding carbon nanotube enhanced the thermal stability by 26˚C (from 315 to 341˚C) when carbon nanotube content was 2.5 phr (parts per hundred parts of resin). After the surface modification, carbon nanotube can be dispersed effectively, and improve the interfacial strength between it and the waterborne polyurethane matrix. Consequently, the physical properties of nanocomposites are enhanced, especially in the covalent bonding system. Mechanical property tests show that adding multiwalled carbon nanotubes improves the tensile properties very significantly (370% in tensile strength). Electron microscopy microphotographs prove that carbon nanotubes can be effectively dispersed in waterborne polyurethane matrix. Rheological tests show that carbon nanotubes can increase the melt viscosity and reduce the variation of processing viscosity [23]. Electro-active shape-memory composites were synthesized using conducting polyurethane composites and multiwalled carbon nanotubes by J. W. Cho et al. (2004). Surface modification of the multiwalled carbon nanotubes by acid treatment improved the mechanical properties of the composites. The modulus and stress at 100% elongation increased with increasing surfacemodified multiwalled carbon nanotube content, while elongation at break decreased. Multiwalled carbon nanotube surface modification also resulted in a decrease in the electrical conductivity of the composites, however, as the surface-modified multiwalled carbon nanotube content increased the conductivity increased (an order of 1023 S cm21 was obtained in samples with 5 wt% modified multiwalled carbon nanotube content). Electro-active shape recovery was observed for the surface-modified multiwalled carbon nanotube composites with an energy conversion efficiency of 10.4%. Hence, polyurethane multiwalled carbon nanotube composites may prove promising candidates for use as smart actuators [24]. X. Luo et al. (2011) noted pointed out in their abstract that 1-thioglycerol-stabilized quantum dots are good candidates for performing a stepwise polymerization reaction with polyurethane prepolymers, but their use is limited by their worse photoluminescence in comparison to quantum dots stabilized by other mercapto-ligands. To overcome this problem, 1-thioglycerol-stabilized CdTe quantum dots with enhanced
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photoluminescence were synthesized in water through a N2H4-promoted growth approach. The current synthesis significantly shortened the duration of the size evolution of quantum dots, particularly for obtaining samples with orange and red emissions. It also permitted quantum dot growth at low temperatures, such as room temperature, which avoided the decomposition of 1-thioglycerol and subsequent embedment of sulfur into the quantum dots as occurs in the conventional synthesis. Most importantly, the 1-thioglycerol ligand endowed the quantum dots with hydroxyl coverage, making the quantum dots miscible with polyurethane prepolymers in dimethyl sulfoxide, and therefore overcoming the main problem in fabricating polyurethane-based nanocomposites of eliminating water. The hydroxyl coverage further allowed for the linkage of polyurethane on the surface of the quantum dots through the reaction between OH and NCO, producing CdTe quantum dot polyurethane bulk nanocomposites. Systematic characterization indicated that the quantum dots were well-dispersed in the polyurethane medium, and the sizedependent photoluminescence of quantum dots was also maintained [25]. For the first time, in 2008 Xiaodong and his group reported a simple method to fabricate a quantum dot polymer composite completely via aqueous media from synthesis of quantum dots to formation of composite, wherein a series of L-cysteine capped CdTe quantum dots with tunable emission from green to red were prepared by using hydrothermal techniques, and the corresponding average particle sizes were estimated to be from 2.5 to 4.1 nm. After incorporating these CdTe quantum dots into the waterborne polyurethane prepolymer aqueous suspension, a transparent nanocomposite film was obtained by casting and evaporating. The results recorded by UV vis and photoluminescence indicate that the quantum yield and the photochemical stability of the CdTe quantum dots in both CdTe waterborne polyurethane aqueous complex and solid composite are enhanced significantly, because of a thicker and more compact passivating layer formed on the surface of CdTe quantum dots via the reaction between the groups of NCO and NH2, transmission electron microscopy and laser scanning confocal fluorescence microscopy images show that the CdTe quantum dots with excellent crystalline structure and strong florescence emission are well-dispersed in the waterborne polyurethane matrix without obvious aggregation or agglomeration. On the basis of the versatile properties of waterborne polyurethane and the photoluminescence originating from the CdTe quantum dots, these new fluorescent composite materials could have great potential applications.
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This approach provides a simple route for preparation of various fluorescent quantum dots polymer composite materials from aqueous quantum dot solutions with neither ligand exchange nor phase transfer [26]. X. Li et al. (2011) synthesized a series of positively charged imidazoliumfunctionalized ionic polyurethanes in a one-step polymerization process by polymerization of presynthesized short-chain imidazolium-based ionic diol, polyethylene glycols with different molecular weights as long-chain diols, and toluylene-2,4-diisocyanate. The structures of ionic polyurethanes are confirmed by 1H NMR analysis, and the thermogravimetric analysis measurement indicates that the ionic polyurethanes have a high degradation temperature. Fluorescent nanocrystal polymer composites CdTe ionic polyurethanes can be prepared conveniently, by the electrostatic interaction between positively charged ionic polyurethanes and the negatively charged aqueous CdTe quantum dots. UV vis absorption and photoluminescence spectra indicate the photochemical stability and strong fluorescent emission of CdTe ionic polyurethane composites. The quantum yields of the composites are high and basically restore the quantum yields of the pure quantum dots. In addition, the transmission electron microscopy photographs show that the quantum dots in composites are uniform (about 3 nm in diameter) and monodisperse. The obtained nanocomposites are powder or elastomers with good film building. The cast CdTe ionic polyurethanes films are transparent under visible light, and the colors of the composites and their films are vivid under a UV lamp [27]. Muralidharan et al. stated that optically triggered actuators offer unique advantages like wireless actuation and remote control when compared to other types of actuators. They are extremely useful where stimuli other than electricity or heat are preferred. Thermally reduced graphene oxide/thermoplastic polyurethane composite actuators were prepared by a simple solution casting technique. The photomechanical actuation properties of the composites were studied under infrared illumination. It was found that the photomechanical response can be tuned by controlling the applied prestrain and the filler loading. Even with low filler loading of 2 wt% thermally reduced graphene oxide, the composite exhibited a very high photomechanical strain of 50.2% with an excellent stress of 1680 kPa at a prestrain of 220%. These high values were achieved at a very low light intensity of 16 mW cm22. The high values of strain obtained with very good generative forces indicate that this is a promising material for light-triggered actuators for many potential applications including robotics and biomedical devices [28].
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J. H. Park et al. (2014) fabricated shape memory behavior of crystalline shape memory polyurethane reinforced with graphene, which utilizes melting temperature as a shape recovery temperature, was examined with various external actuating stimuli, such as direct heating, resistive heating, and infrared heating. Compatibility of graphene with crystalline shape memory polyurethane was adjusted by altering the structure of the hard segment of the shape memory polyurethane, by changing the structure of the graphene, and by changing the preparation method of the graphene/shape memory polyurethane composite. The shape memory polyurethane made of aromatic 4,4´-diphenylmethane diisocyanate exhibited better compatibility with graphene, having an aromatic structure, compared to that made of the aliphatic hexamethylene diisocyanate. The finely dispersed graphene effectively reinforced shape memory polyurethane made of aromatic 4,4´-diphenylmethane diisocyanate, improved shape recovery of shape memory polyurethane made of aromatic 4,4´-diphenylmethane diisocyanate, and served effectively as a filler, triggering shape recovery by resistive or infrared heating. Compatibility was enhanced when the graphene was modified with methanol. This improved shape recovery by direct heating, but worsened the conductivity of the composite, and consequently the efficiency of resistive heating for shape recovery also declined. Graphene modified with methanol was more effective than pristine graphene in terms of shape recovery by infrared heating [29]. N. Yousefi et al. (2012) produced polyurethane-based composite films containing highly aligned graphene sheets through an environmentally benign process, where an aqueous liquid crystalline dispersion of graphene oxide is in situ reduced in polyurethane, resulting in a fine dispersion and a high degree of orientation of graphene sheets. The polyurethane particles are adsorbed onto the surface of the reduced graphene oxide, and the reduced graphene oxide sheets with a large aspect ratio of over 10,000 tend to self-align during the film formation when the graphene content is high enough, at more than 2 wt%. The resulting composites show excellent electrical conductivity with an extremely low percolation threshold of 0.078 vol%, which is considered one of the lowest values ever reported for polymer composites containing graphene [30]. G. Zhao et al. (2011) added carbon fiber or short glass fiber to polyurethane to enhance the tensile strength of the composite, so that it could be used for structural materials in high-load conditions. Polyurethane composites have been widely used to produce bearings or gears. Chemical modification of the fiber, by grafting with diisocyanatotoluene, also played an important role in reducing phase separation [31].
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H. Kim et al. (2010) developed strategies for isolating single-layer carbon sheets from graphite have enabled production of electrically conductive, mechanically robust polymer nanocomposites with enhanced gas barrier performance at extremely low loading. For the first time, we compare carbon sheets exfoliated from graphite oxide via two different processes: chemical modification (isocyanate-treated graphite oxide) and thermal exfoliation (thermally reduced graphite oxide), and three different methods of dispersion: solvent blending, in situ polymerization, and melt compounding. Incorporation of as low as 0.5 wt% of thermally reduced graphite oxide produced electrically conductive thermoplastic polyurethane. Upto a 10-fold increase in tensile stiffness and 90% decrease in nitrogen permeation of thermoplastic polyurethane were observed with only 3 wt% isocyanate-treated graphite oxide, implying a high aspect ratio of exfoliated platelets. Real- and reciprocal-space morphological characterization indicated that solvent-based blending techniques more effectively distribute thin exfoliated sheets in the polymer matrix than melt processing. This observation is in good qualitative agreement with the dispersion level inferred from solid property enhancements. Although also processed in solvents, property increase via in situ polymerization was not as pronounced because of reduced hydrogen bonding in the thermoplastic polyurethane produced [32].
20.3 CONCLUSION Properties of polyurethane composites and nanocomposites depend on the process and the processing condition, proper choice of filler and quantity of filler are also important to obtain desired properties for which a trial and error has to be performed, selection of suitable grade of polyurethane for a specified application must be taken into consideration, i.e., either ester or ether-based polyurethanes. Polyurethane composites and nanocomposites have been used in various applications including functional coatings, nanoelectronics, gas-sensing, packaging, biomedical implants, solar cells, seals, etc.
REFERENCES [1] J.A. Brydson, Plastics Materials, Butterworth-Heinemann, Oxford, 1999. [2] G. Wypych, Handbook of Fillers, ChemTec, Toronto, 1999.
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