Conventional methods such as solution mixing and melt blending have been used to disperse CNT into polymeric matrices. However, it is challenging to obtain nanocomposites with a uniform nanoparticle dispersion and high loadings (>10 wt%) because, for example, the viscosity increases more than 100 times after adding just 1 wt% CNT into the matrix due to its large specific surface area.[10,11] If the dispersion is poor, the CNTs aggregate and adversely affect the mechanical properties of the nanocomposite.
Rather than dispersing the CNTs into the polymeric matrix, starting with a pre-made nanopaper (NP) and then infiltrating the polymeric matrix into the NP is an effective way to obtain high loading, well dispersed CNT nanocomposites with improved properties. Thus, the CNT NP is used as an excellent platform to fabricate light weight multifunctional nanocomposites due to the superior properties of the CNTs. CNT NP is a porous film with a thickness ranging from 20 to several hundred microns, generally fabricated by chemical vapor deposition (CVD), [12-15] or dispersion and filtration,[8,16-18] or hydroentangling and spinning. The CVD approach could produce aligned NPs with a higher strength and electrical conductivities, but the cost of production is also higher. Thus, the dispersion and filtration method is much more feasible for mass production and commercialization at a lower cost. Besides working as a reinforcement of nanocomposites, individual CNT NP could be used for strain sensor and actuator,[21-24] gas separation membrane, gas and vapor detector, and deicing.
Resin transfer molding, solvent casting, and compression molding have been used to infiltrate different types of polymeric matrices into the NP to fabricate nanocomposites. The achieved NP/polymer nanocomposites displayed significantly improved properties such as mechanical strength and Young’s modulus, electricity conductivity, electromagnetic interference (EMI) shielding, thermal conductivity, and thermal stability, etc. [28-37] However, the permeability of CNT NP ranges from 10-17 to 10-19 m2,[16,38,39] which is several order of magnitude smaller than the glass fiber or carbon fiber preform (10-19 to 10-12 m2), leading to a longer resin infiltration time and limiting the industrial applications of CNT NP for making nanocomposites. The novel ultrasonic process is developed to greatly decrease the infiltration time and thus improves the potential for industrial applications.
The benefits of novel ultrasonic approach have been demonstrated by different polymer matrices such as epoxy, polyurethane (PU), and polydimethylsiloxane (PDMS). The thermoset epoxy, thermoplastic PU, and elastomer PDMS represent three major classes of polymers with significant differences in viscosity and end-use properties. The viscosity of the resin ranges from 0.1 to 10 Pa·s at their processing temperatures. For example, the viscosity of epoxy resin is 0.17 Pa·s at 90℃. For thermoplastic polyurethane (TPU) resin, it is 0.12 Pa·s at 70℃. For PDMS resin, it is 10 Pa·s at room temperature. These three types of NP nanocomposites have different end-use properties, thus a variety of applications can be envisioned. The solventless epoxy was selected to study the process parameters of ultrasonic infiltration process because of its wide processing window. The CNT NP/polymer nanocomposites were evaluated as coatings for glass fiber reinforced polymer composite (GFRP) providing an improved abrasion resistance for wind energy and automotive industries.[17,40,41] The TPU based NP nanocomposites can be processed by injection and extrusion molding. Because of their good biocompatibility and ductility, the PDMS based NP nanocomposites can be used as strain sensors to monitor the movement of human body parts, such as neck, arm, wrist, and leg, etc.
In this paper, novel ultrasonic assisted resin infiltration method was discussed with its utilization to prepare CNT NP nanocomposites with a uniform resin impregnation at an industrially attractive processing rate. Major characteristics were disclosed including the reduced infiltration time (4 seconds compared to more than 30 minutes for the resin transfer molding). The microstructure, morphology, mechanical properties, abrasion resistance, electromagnetic interference (EMI) shielding, strain dependent electrical response, and process flexibility of the NP/polymer nanocomposites were reviewed as well.
2. Nanocomposites Processing
2.1 Solution and melt compounding
It is a major challenge to uniformly disperse CNT into a polymer matrix and to obtain high loading (>10 wt% ) using conventional methods such as extrusion and shear mixing because of the large viscosity increase resulting from the large specific surface area of CNTs.
Twin screw extrusion has been used to compound polypropylene (PP), polyethylene (PE), polystyrene (PS), polyethylene terephthalate (PET), and PU with CNT from 1 to 10wt%, and the nanocomposites exhibit improved Young’s modulus, tensile strength, electrical and thermal conductivities.[43-52] Besides extrusion, CNT has been dispersed in epoxy or TPU monomers via shear mixing and ultra-sonication, followed by polymerization, and the nanocomposites demonstrated improved mechanical, electrical and thermal properties.[53-59]
Similarly, solution mixing method has been used to disperse CNTs in epoxy to fabricate nanocomposites using solvents such as ethanol, acetone or N, N Dimethylformamide (DMF), etc. The process includes dispersing CNTs in solvent by sonication or shear mixing, then adding epoxy monomer and curing agent separately followed by degassing and curing, which takes several hours in total.[60-62] The required organic solvents in these conventional compounding approaches not only were environmentally unfriendly, but also significantly increased the processing time due to the removal of solvent.[52,60,61] The trapped solvent could reduce the mechanical properties of the nanocomposites. All of these issues significantly decrease the potential of commercialization of nanocomposites.
2.2 Nanopaper and resin transfer molding
An effective way to obtain high loadings and better dispersion is to start with a nanopaper and then to infiltrate the polymer into the porous structure. Nanomaterials such as CNT, carbon nanofiber (CNF), and cellulose nanofiber can be used to fabricate nanopapers using the dispersion and filtration method. First, the nanomaterials need be dispersed in water with the aid of a surfactant using a probe sonicator to reach uniform dispersion. Surfactant such as sodium dodecyl sulfate (SDS) has both hydrophobic and hydrophilic functional groups to interact with the nanoparticle and water molecule, respectively to achieve a good dispersion. The concentration of nanoparticle is usually maintained lower than 0.2wt%. The aqueous CNT (or CNF) dispersion is then filtered through a polytetrafluoroethylene (PTFE) membrane laid on a porous metal support in a home-made funnel (10 " × 12 ") to form the NP. The SDS was then washed away by ethanol and distilled water. Finally, the nanopaper was peeled from the membrane and vacuum dried to remove moisture.[17,41] The process is shown schematically in Fig. 1.
Fig. 1 CNT nanopaper fabrication process. Reproduced with permission.
The fabricated NP is a thin film with the thicknesses ranging from 20 to several hundred μm. Very few agglomerations of CNF or CNT from the SEM studies of the nanopapers indicated that the surfactant effectively dispersed the nanoparticles in water. The nanofibers or nanotubes were randomly entangled in the NP, and held by van der Waals forces. The pore sizes of CNT NP are much smaller than CNF NP because the CNT has much smaller dimensions compared to CNF. The porosity of CNF NP is 80%, higher than CNT NP (72%). The tensile strength of CNF NP is 1 MPa, much lower than CNT NP (4.8 MPa). Detailed information of the nanomaterials are stated as: CNF with a diameter of 100 nm and a length of 50~200 μm; multi-wall CNT with an inside diameter of 3~5 nm, outer diameter of 8~15 nm and an length of 3~12 μm; and single-wall CNT with a diameter of 1.8 ± 0.4 nm and a length of >5 μm.
CNF NP could be placed on the top of a glass fiber preform to fabricate nanocomposites using the vacuum assisted resin transfer molding (VARTM) for surface protection of products such as wind turbine or sporting goods. The sketch of the material layup and the VARTM process is shown in Fig. 2. A systematic study on the permeability of CNF NP was conducted to investigate the resin infiltration into the NP. Improvements in mechanical properties and sand erosion resistance were achieved.