3.1.3. Gas Pycnometry Results

The absolute density obtained by gas pycnometry of the ubim fibers was 1.86 <sup>±</sup> 0.26 g/cm<sup>3</sup> . This relatively higher density found by the pycnometer is due to the ability of the helium gas to fill all the open pores of the material. The gaseous pycnometry provides the value of the absolute density, including all empty parts of the NLF, and as it is very porous, this higher value is justified. This higher density result by pycnometry compared to other methods was also found by Oliveira et al. [35], as 1.61 g/cm<sup>3</sup> .

Higher values for the absolute density, such as that of ubim fiber obtained by pycnometry of the order of ~1.6 g/cm<sup>3</sup> , are justified by the density of plain cellulose [21–23]. Most reported densities of NLFs [36,37], including cellulose films [24], are ~1.50 and used for technological application such as polymer composite reinforcement.

The significant amount of porosity in the ubim fiber is responsible for a lower apparent density, 0.72 g/cm<sup>3</sup> , which is good for a lighter composite reinforcement in spite of its relatively higher absolute density of 1.86 <sup>±</sup> 0.26 g/cm<sup>3</sup> .
