There are many emerging applications involving the dispersion of nanoparticles into a polymer matrix with the aim to enhance the properties of the composite material. One of the main reasons why nanoparticles are able to improve the properties of polymer resins is their large surface-to-volume ratio. The large ratio increases the number of particle–matrix interactions, thus increasing the effects on the overall material properties even at rather low filler loadings. The nanoscaled fillers are especially beneficial for foam property enhancements primarily because the foam cell walls are normally within the submicron regime which conventional fillers are incompatible in terms of size. Improvements in thermal, electrical, and mechanical properties could be achieved by synergistically combining the properties of the matrix and the fillers without altering the desired density or the foam morphology.
Traditionally three types of nanofillers of distinct geometries as being illustrated in Fig. 5 are used. 0D nanofillers are being characterized by having all the three dimensions in the nanometer scale. Typical examples include spherical silica particles, nanocrystals, and metal particles. Typical examples of 1D nanofiller are nanotubes and nanofibers which feature two dimensions in nanoscale. The last type of nanofillers, 2D, has a lateral dimension in the range of several hundreds of nanoto micrometers and nanoscale thickness. Conventional nanofillers, such as organoclay, are one of the most popular nanofillers being used for nanocomposite fabrication. However, these fillers cannot be used as cell nucleating agents in high-processing-temperature polymers due to thermal degradation of the organic modifiers. For such applications, nonconventional organicmodifiers or inorganic nanofillers should be used. Graphene has demonstrated to be an excellent candidate for its thermal stability and has shown its potential in improving the mechanical, thermal, and electrical properties. To produce single graphene layers for mass manufacturing of nanocomposite foams still poses challenges in both the fabrication method and the economic aspects. Recent development at Michigan State University on the production of robust graphene sheets of one to five layers thick and diameters ranging from less than 1 μm to over 100 μm at cost-competitive prices could be an effective solution.

Processing of Nanocomposite Foams

In general, two separate steps are involved in the processing of nanocomposite foams: the synthesis of nanocomposite and subsequent foam formation through different foaming techniques as explained in the previous paragraph. It is well understood that particles, especially in the nanorange, tend to agglomerate due to the dominant intermolecular van der Waals interactions between them. Possible incompatibility between the nanofillers and the polymer matrix makes it even more challenging to achieve uniform dispersion. Hence, surface modifications are usually carried out to promote interactions between the nanofiller and the matrix in order to overcome the strongly bonded nanofiller aggregates. Numerous studies and research were carried out to break up the nanofiller agglomerates for homogeneous nanocomposite preparation and were well documented in various academic and industrial publications and patents. A detailed survey on nanocomposite preparation is beyond the scope of this chapter. The focus of this part of the chapter is placed on the effects of nanofillers on the foaming process and foam properties. Besides the performance enhancement exhibited in nanocomposite, nanofillers may serve as heterogeneous nucleation centers facilitating the formation of bubbles, improving the cell density, homogenizing the cell size, and altering the rheological properties of the foaming polymer affecting bubble stability, foam morphology and the foam density.

Nucleating Effect

Adding nucleation fillers to improve the cell density, which is defined as the number of bubbles per cm3, is a common practice in foam manufacturing processes. It is generally understood that nanofillers with low surface energy will reduce the free energy (DF) (see Eq. 1). Reduction of free energy is required for bubble initiation in liquid by lowering the surface tension at the liquid–solid interface (Klempner and Frisch 1991):

ΔF = γ·A (1)

where γ is the surface tension and A is the total interfacial area.

Figure 6 shows how the cell size in polystyrene (PS) foam was reduced by a great extend and the cell density was increased by at least two orders of magnitude comparing to the pure foam by adding a small amount of nanofillers.

A fine dispersion of nanofillers will greatly enhance the nucleation efficiency, resulting in increased cell density and reduced cell size. This can be easily understood as more gas was being consumed by bubble nucleation at the nanoparticle sites. Simultaneously, less gas would be available for bubble growth, hence reduced the final cell size. The classical steady-state nucleation theory is often used to qualitatively describe the number of nucleation sites (see Eq. 2):

where ΔGcrit is the critical nucleation formation energy and is described as per Eq. 3. kB is the Boltzmann factor, and T is the absolute temperature. Co is the number of gas molecules dissolved per unit volume of the primary phase; fo is a kinetic pre-exponential factor.

ΔP is the pressure difference inside and outside the nucleating bubble.

However, it has to be pointed out that the data obtained by the theory shows a great deal of discrepancy from actual experiments because the nucleation formation energy is dependent on the critical nucleus size, which is an inaccessible parameter during actual foaming process. Lee et al looked into heterogeneous nucleation based on the classical steady-state nucleation theory to qualitatively describe the number of nucleation sites (Ni) generated by nanoparticles (Spitael et al. 2004; Zettlemoyer 1969; Colton and Suh 1987a, b) by introducing fi, a frequency factor of gas molecules joining the nucleus, and Ci to account for the concentration of the heterogeneous nucleation sites (see Eq. 4).

where the energy required to form a nucleus is considered proportional to the energy required in a homogeneous system by a factor dependent on the contact angle between the gas and polymer and particle surface:

A general selection guideline for nucleating agents was proposed by McClurg and Leung (McClurg 2004; Leung et al. 2008):

• The nucleation agents should be able to lower the surface energy barrier needed for bubble initiation relative to homogeneous nucleation and unintentional heterogeneous nucleation caused by contaminants in the polymer matrix.
• Ideal nucleating agents should have uniform sizes, surface geometries and surface properties and should be easily dispersible.
• A rugged surface as illustrated by Fig. 7 that contains many conical crevices of small semi-conical angles (β) is preferred.

Qualitatively, nucleating agent with small contact angle and high surface curvature causes more effective critical energy reduction and results in higher efficiency (Fletcher 1958). However, high nucleation efficiency will be achieved not only by choosing the right geometry and surface but, more importantly, through a good dispersion process (Colton and Suh 1987a, b; Lee et al. 2005). Carbon nanofibers are more effective nucleating agents compared to carbon nanotubes and nanoclay due to their increased homogeneous distribution and their favorable surface and geometrical characteristics (Lee et al. 2005).

Bubble Stabilization

By nature, liquid foams before solidification are thermodynamically unstable. Movements of the liquid cell through capillary actions and gravity further promote the collapse of the foams due to cell thinning. Bubble stabilization is generally achieved in two ways. At first the melt viscosity increases or stabilizes by the adsorbed particles on the foam cell surface. When the viscosity or melt strength of the liquid cell wall has increased, the force required to overcome pressure difference increases. This effect combats excessive liquid cell movement and hence slows down the cell thinning process and stabilizes the cell structure. There have been several recent examples of foams being stabilized by particles adsorbed at air/liquid interface forming a rigid shell that protects the bubbles against coalescence (Dickinson et al. 2004; Hunter et al. 2008; Gonzenbach et al. 2006).

Foam Properties

Thermal Properties

Well-dispersed nanofillers generally improve the thermal properties of polymer foams. Several mechanisms have been proposed to explain the phenomenon: (i) the barrier effect delayed the escape of volatile decomposition products during degradation; (ii) the nanofillers created a tortuous path for air, delaying the thermo-oxidative degradation of the material; and (iii) the thermally conductive nanofillers could ease the heat dissipation within the matrix. Figure 8 shows that the addition of 0.1 wt% of functionalized graphite sheets and carbon nanotubes shifted the onset of degradation to higher temperature and improved the char yield by about 50 % compared to the control silicone foam.

Compression Properties

The compressive properties of foam are highly dependent on their apparent density; hence, the compressive parameters are usually normalized to exclude the effect of density variations. Verdejo et al. demonstrated that the addition of CNTs and functionalized graphene sheets (FGS) into flexible silicone foams caused drastic changes in the compressive behavior (Verdejo et al. 2008) (see Fig. 9). The combination of density change and the reinforcement effect of the nanofillers increased the normalized Young’s modulus by over 200 % through the addition of only 0.25 wt% of FGS. Similar behavior was observed for the case of rigid foams; see Fig. 10 which shows the relationship of compressive parameter of rigid polyurethane foams reinforced by multiwall carbon nanotubes (MWNT) and carbon nanofiber (CNF), respectively.

It should be pointed out that not all nanofillers lead to property improvements. For the case of a rigid foam system of PU, hydrogen bonds within the structure network play a prominent role in the mechanical properties. The addition of organic fillers, such as organoclay, may interfere with the formation of hydrogen bonds, hindering the structural formation and causing property deterioration. Hu and co-workers studied the effects of nanofiller content on the compression properties of rigid phthalonitrile foams. Results, as shown in Fig. 11, indicate that the specific compressive stress improvement is dependent on the type of filler incorporated. Both MWNT and expanded graphite seem to be able to improve the compressive stress. However, excessive amount of filler resulted in negative results caused by aggregation of the nanofiller creating a point of failure initiation. On the contrary, fumed silica caused the foam property to drop regardless of the loading content caused by poor interfacial interactions between the filler and the matrix.