Thermal Insulation properties of Electrospun fibers

Electrospun fibrous structures with its interconnected pores have the potential for use as a thermal insulation material. Heat transfer from materials is based on convection, conduction and radiation. For electrospun structures to exhibit good thermal insulation, heat transfer through these three modes need to be examined and investigated. Preliminary tests have been carried out on electrospun membrane and block fibrous structures and they have shown thermal insulation properties.

In solid objects, heat transfer through it is via conduction. For electrospun structures, much of the volume is made of empty spaces between fibers. Conduction along the fiber as part of the whole volume is negligible unless the fibers get so compact that fiber volume becomes significant.

Heat transfer through convection is dominant where fluid or air is allowed to move where there is a temperature gradient. For electrospun nonwoven structure, the fiber entanglements reduces air flow within the structure and thus limits convection. However, it is interesting to determine the effect of slip flow of air across nanofibers on convection. Slip flow enables low pressure drop across electrospun membrane but this may be disadvantageous for thermal insulation as it allows heated air to travel across more easily.

Given the presence of numerous pockets of spaces within the porous electrospun structure, thermal radiation is possible as the photons travel through the air pockets. In membrane form, increasing density of electrospun fibers have shown better insulation property. This has been attributed to a reduction in radiative heat transfer as the photon mean free path gets reduced due to the proximity of the next fiber surface. However, beyond an optimum packing density, heat transfer via conduction through the solid fiber becomes significant and the overall insulation property drops.

Electrospun fibers have also been explored as high temperature insulation. Song et al (2021) demonstrated the potential of silica/titanium dioxide composite nanofiber as a flexible thermal insulation material. The silica/titanium dioxide composite nanofiber was fabricated by first electrospinning of precursor material comprising of tetraethyl orthosilicate (TEOS), tetrabutyl titanate (TBT), polyvinyl pyrrolidone (PVP) as the carrier material and oxalic acid as catalyst. The resultant electrospun precursor fibers are subsequently sintered at 900°C to obtain silica/titanium dioxide composite nanofibers with a TiO₂ content of 12%. The composite nanofibrous membrane exhibited a tensile strength of 3.09 MPa and a thermal conductivity 0.0899 Wm^-1K^-1 at 500 °C. At temperatures between 400 to 600 °C, the silica/titanium dioxide composite nanofibers have a lower thermal conductivity than most metal oxide fibers. A possible reason is that the pore size in the mesoporous silica/titanium dioxide composite nanofiber which is between 10 to 30 nm is smaller than the mean free path of air and this eliminates thermal convection in the fiber. The nanopores on the fibers also disrupts solid phase heat conduction at higher temperature, thermal conductivity of solid SiO₂ fibers increases rapidly while the mesoporous silica/titanium dioxide composite nanofiber thermal conductivity rises slowly. The presence of rutile TiO₂ helps to absorb energy from infrared radiation which further improves thermal insulation performance.

Early studies on electrospun fibers as insulating material are based on membrane form. One of the earliest studies is by Gibson et al (2007) using electrospun polyacrylonitrile nanofibers and compared with several other common insulating materials such as down, commerical polyester, silica aerogel-impregnated flexible fibrous insulation and meltbown pitch carbon fiber. Unfortunately, their study did not demonstrate significant thermal insulation advantage in their electrospun nanofibrous membrane over commercially available insulation materials. In fact, insulation property of the electrospun membrane is poor at low bulk density although it improves at higher density [Gibson et al 2007]. Nasouri et al (2013) investigated the influence of nanofiber diameter and bulk density on thermal conductivity of electrospun fibers. Similar to the result from Gibson et al (2007), increasing bulk density in general, increases its thermal resistance. Using electrospun PAN fibrous membrane, they found that thinner nanofibers exhibits greater thermal insulation characteristics. When the nanofiber diameter is less than 261 nm with bulk density more than 176 kg/m³, thermal conductivity drops below 0.02 W/m.k which compares well with pure SiO₂ aerogel with thermal conductivity of 0.024 W/m.k [Wu et al 2013]. This has been attributed to greater surface area to volume ratio which increases radiation absorption and scatter. Reducing fiber diameter also has the effect of reducing membrane pore size. This also reduces radiation due to less photon passing through open spaces between nanofibers.

Aerogel is commonly used as thermal insulating material however, its lack of mechanical strength is a limitation. Combining electrospun fibers with aerogel has the advantage of reinforcing the aerogel while potentially retaining its superior thermal insulation. Wu et al (2013) tested this concept with electrospun polyvinylidene fluoride (PVDF) webs within a SiO₂ aerogel. The resultant aerogel composite showed good flexibility with thermal conductivity at 0.028 W.m.k which is close to that of pure SiO₂ aerogel with thermal conductivity of 0.024 W/m.k.

Morphology and flexibility of SiO₂ aerogel composites reinforced with electrospun PVDF webs [Wu et al Journal of Nanomaterials, vol. 2013, Article ID 375093, 8 pages, 2013. This work is licensed under a Creative Commons Attribution 3.0 Unported License.]

Beyond membrane form, electrospun fibers may be made into 3D foam structure. Si et al (2014) constructed a foam made of short strand electrospun fibers. A mixture of polyacrylonitrile/benzoxazine (PAN/BA-a) electrospun fibers and SiO₂ electrospun fibers were used to made into the foam. The ultra-low density of the freeze dried foam makes it comparable to aerogel. The lowest density obtained was 0.12 mg/cm³ with porosity of 99.992%. The foam showed good thermal insulation property with prominent thermal conductivity of 0.026Wm^-1K^-1 which is close to that of air at ambient condition. Such good insulation property has been attributed to its high porosity. When the porosity reduces, thermal conductivity increases.

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