Reinforced Polymer Matrix Syntactic Foams

Lightweight materials are of great interest to transportation applications. Structural weight reduction directly translates into fuel saving and increased payload capacity. Porous materials can provide significant weight saving but their applications are limited by their low strength and modulus. This chapter provides an introduction to porous materials, which includes open- and closed-cell foams. The closed-cell foams can be further divided into foams containing gas porosity and the foams containing hollow particles. The hollow particle filled porous materials are called syntactic foams. These foams are also classified as particulate composites. Enclosure of porosity within a thin but stiff shell helps in obtaining low density in syntactic foams without a severe penalty on the mechanical properties. Syntactic foams possess superior properties under compression compared to foams comprising gas porosity in the matrix. Several micro- and nano-scale reinforcements have been used to improve the tensile and flexural strengths of syntactic foams. Nanoclay, carbon nanofibers, carbon nanotubes, glass fibers, and ceramic particles have been used as reinforcements in syntactic foams. Establishing structure-property correlations of reinforced syntactic foams will pave way to design effective lightweight composites for engineering structures. The chapter also discusses some of the present day applications of syntactic foams.

Syntactic foams are two component materials consisting of matrix resin and hollow particles. Reinforced syntactic foams contain an additional reinforcing material. The density of syntactic foams can be tailored based on the appropriate selection of hollow particle density and volume fraction. Glass hollow particles have been a widely used filler material because their low thermal expansion coefficient provides syntactic foams with high dimensional stability. Hollow fly ash particles, called cenospheres, have also been used as filler material. Fly ash cenospheres are inexpensive and help in developing low cost syntactic foams. However, usually defects are present in the walls of cenospheres and the mechanical properties of cenosphere-filled syntactic foams are not as high as those filled with glass hollow particles at the same density level. Enhancement of mechanical properties of syntactic foams beyond those obtained by tailoring the matrix and hollow particles can be obtained by micro- and nano-scale reinforcement of the matrix material. This chapter discusses hollow particle parameters such as wall thickness and density. Structure and properties of various reinforcements including glass fibers, nanoclay, carbon nanofibers (CNFs), carbon nanotubes (CNTs), and rubber particles are also discussed.

This chapter discusses processing methods for reinforced syntactic foams and the effect of processing parameters on the structure and properties of syntactic foams. Enhancement of the mechanical properties of syntactic foams can be achieved by incorporation of micro- or nano-scale reinforcements into the matrix material. Dispersion of nanoparticles and nanotubes and nanofibers in polymer resins is challenging. Mechanical, shear, and ultrasonic mixing techniques have been used for obtaining wetting and dispersion of nanoscale reinforcement in the matrix. Nanoparticle reinforcement may also provide unintentional effect of increased matrix porosity by stabilizing gas bubbles in polymer matrix, if the processing method is not carefully designed. The processing methods are also required to be efficient in promoting wetting of reinforcement by the matrix resin, breaking clusters without fracturing the reinforcement material, and obtaining uniform distribution of reinforcement in the matrix resin. In addition, the hollow particles should not be excessively fractured during the manufacturing process. This chapter provides an overview of various processing methods and the issues encountered during fabrication of reinforced syntactic foams.

Micro- or Nano-scale reinforcements are attractive for enhancing the tensile properties of syntactic foams. Glass and carbon fiber reinforcement was found to increase tensile strength and modulus of syntactic foams when compared to that of plain syntactic foam. The orientation of fibers with respect to the loading axis had a significant impact over the level of increase in the mechanical properties. However, reinforcement of microfibers increases the density of composite, which may be undesired in most applications; therefore, only a small volume fraction of fibers is used in syntactic foams. Carbon nanofiber (CNF) reinforcement increased the tensile strength and modulus of the syntactic foam. More specifically, addition of 0.25 wt% of CNFs increased, tensile modulus and strength of syntactic foams by 10–20 and 20–50 % (depending on the hollow particle wall thickness and volume fraction), respectively, when compared to that of plain syntactic foam. Nanoclay reinforcement was also found to increase the tensile strength and tensile modulus of syntactic foams in most studies. Nanofillers may have an undesired effect of stabilizing matrix porosity in syntactic foams during manufacturing. However, presence of nanoparticles around or across the matrix voids may provide a strengthening effect and the matrix porosity can be used to further decrease the syntactic foam density without a severe penalty on mechanical properties.

Development of theoretical models is very important for syntactic foams. Numerous parameters are involved in syntactic foam design, which include matrix and particle material, particle volume fraction and wall thickness, and reinforcement material and volume fraction. To identify the parameters that would result in syntactic foam with desired set of mechanical and thermal properties, theoretical models can be very useful and cut down the need for experimentation. Several existing models applicable to particulate composites can be modified to include the particle wall thickness effect. Multiscale models that can include the nanofibers or particles along with hollow particles are not available yet. It is challenging to model syntactic foams that contain high volume fractions of hollow particle (close to packing limit) because of particle-to-particle interaction effects. Two models are used in this chapter to estimate the properties of multiscale syntactic foams. Both models are applicable to plain syntactic foams containing only hollow particles in matrix. Therefore, semi-empirical approach is adopted and the experimentally measured properties of nanofibers reinforced polymer are assigned to the matrix. The model predictions are validated with experimental results. Finite element analysis is especially illustrative in understanding the behavior of syntactic foams under the applied load. A validated finite element study conducted on a unit cell geometry comprising a hollow particle and a fiber showed that the particle wall thickness plays an important role in determining the stress distribution in microscale reinforced syntactic foam system. For syntactic foams containing thin-walled particles, the location of the maximum von-Mises stress exists within the particle, whereas above a critical wall thickness the location of the maximum stress shifts to the fiber. This pattern illustrates that the location of failure initiation can be tailored in syntactic foams.

Most existing applications of syntactic foams are based on their compressive properties. Hollow particles are load bearing elements in the syntactic foam microstructure under compression, which helps in obtaining a long stress plateau region in the stress–strain graphs that helps in obtaining high energy absorption. The available studies have extensively studied the effect of hollow particle wall thickness and volume fraction on compressive properties of syntactic foams. Similarly, reinforced syntactic foams have been extensively studied for compressive properties. Carbon nanofibers and nanotubes, nanoclay, crumb rubber and glass, and carbon fibers have been used as reinforcements in syntactic foams to tailor the compressive properties. The incorporation of CNFs increases the overall energy absorption capacity of the composite. Orientation of fibers with respect to the loading axis is important in obtaining strengthening effect and randomly dispersed fibers do not provide high level of increase in compressive properties. In some layered reinforced syntactic foams, fibers oriented parallel to loading direction enhanced their compressive strength by 180–220 % compared to that of plain syntactic foams. On the contrary, foams containing fiber orientation perpendicular to the loading direction showed no measurable change in strength. The strength and modulus increase with density for reinforced syntactic foams. The maximum compressive strength and modulus for reinforced syntactic foam were found to be 120 MPa and 2.2 GPa, respectively.

The flexural behavior has been studied only for a few reinforced syntactic foams. The short glass fiber reinforced epoxy matrix syntactic foams showed fiber pull out and hollow particle/matrix debonding as the main failure mechanisms under flexural loading conditions. Transition in the failure pattern was observed with the increase in the fiber content. Brittle failure was seen in syntactic foams containing less than 2 vol. % fibers, while foams containing 2–4.5 vol. % fibers showed fiber bending rather than complete failure. Silica particle (8–9 μm diameter) filled syntactic foams showed decreases in flexural strength and modulus with increasing silica content in the range 5–15 wt.%. It is found that the syntactic foams with 2 wt% nanoclay show the highest improvement in flexural properties, which include nearly 42 % and 18 % increase in strength and modulus, respectively. The flexural modulus and strength are extracted from the available studies on various reinforced syntactic foams and are plotted with respect to the density. The highest flexural strength and modulus values of any available reinforced syntactic foam are found to be 78 MPa and 3.8 GPa, respectively. Carbon nanofiber reinforced syntactic foams show high compressive and tensile properties but they are not yet tested for flexural properties.

Studies on plain syntactic foams have revealed that the fracture toughness and specific fracture toughness are found to be maximum around 30 vol. % of hollow particles. At low hollow particle volume, fraction stiffening effect and crack bowing failure mechanism was observed whereas at high volume fraction, hollow filler particle-matrix debonding is found to the dominant failure mechanism. Fracture toughness studies on reinforced syntactic foams have been performed only at a constant hollow particle volume fraction of 30 vol. %. A study on phenolic hollow particle filled syntactic foams concluded that fracture toughness increased with increasing fibers content. A maximum increase of 95 % was observed with respect to plain syntactic foam for 10 mm length fiber at 3 wt%. Carbon fibers were found to have a significantly stronger effect on the fracture toughness than glass fibers. PEEKMOH toughened epoxy matrix syntactic foams were found to have up to a 46 % improvement in fracture toughness with the addition of 5 wt% nanoclay. An increase of 37 % in fracture toughness is observed for the addition of 1.5 vol. % of carbon nanofibers in comparison to plain syntactic foams. It was also observed that microscale reinforcement (short carbon fibers) was more effective than nanoscale reinforcement (nanoclay), at similar weight fractions.

Studying the influence of temperature and loading frequency on the behavior of syntactic foams is important because of its diverse set of applications. Dynamic mechanical analysis (DMA) is a widely used technique for measuring viscoelastic properties of materials over a range of temperatures and loading frequencies. The storage modulus and loss modulus determined in a DMA experiment measure the capacity of a material to store and dissipate energy, respectively. In general, the storage modulus of syntactic foams decreases with increasing temperature. This response was consistent between plain and reinforced syntactic foams. Study of storage modulus of vinyl ester/glass hollow particle syntactic foams at three different temperatures concluded that the neat resin has higher storage modulus than the syntactic foams below glass transition temperature (T _g) but this trend is reversed above T _g. Also, the room temperature (30 °C) storage modulus of syntactic foams increases with the increase in the wall thickness of hollow particles. The addition of nanoclay increased the storage modulus of epoxy matrix syntactic foams. The effect was attributed to the toughening of matrix resin by nanoclay particles. However, increased stiffness of nanoclay reinforced syntactic foams resulted in decreased loss modulus. Cyanate ester matrix syntactic foam with 4 vol. % of nanoclay showed higher storage modulus than the plain syntactic foams, owing to the restricted movement of the polymer chains which can be attributed to the good interaction between the nanoclay and the matrix resin.

In the previous chapters, the existing literature on the mechanical properties of reinforced syntactic foams has been reviewed. This chapter summarizes the effects of the reinforcements on the mechanical properties of syntactic foams and identifies the critical areas where a lack of literature is observed relating to reinforced syntactic foams. The data for various mechanical properties such as compressive, tensile, and the flexural strength and modulus have been plotted as a function of the composites density. This helps in identifying the composite compositions that can be helpful in weight saving applications. The nanoscale reinforcements help in obtaining the highest mechanical properties without increasing the density of syntactic foams. These results show that the nanoscale reinforcements can push the boundaries of mechanical properties of syntactic foams. It is identified that understanding the effect of moisture, weathering, and ultraviolet light radiation on the composite is not well developed. The lack of theoretical models for reinforced syntactic foams, considering the interaction between the nanoscale reinforcement and the hollow particles, is also noted in the literature and should be the focus of future research efforts.