Any natural fiber would consist of the following carbohydrates/ proteins in its microstructure: Cellulose, Hemi-Cellulose, Pectin, Lignin etc.
Alkali has a leaching action on the non-cellulosic components-- Hemi-Cellulose and Lignin, reduces the mass of the natural fiber and at the same there is a shrinkage of the fiber. These two effects together might increase the density of the treated fiber.
Go through the publication attached.
Article Natural fiber polymer composites: A Review
the alkali treatment role is to dissolve the non-cellulosic components such as lignin and hemicellulose, these components are characterized by low weight ( amorphos nature)
moreover, these components are also contributed to the increases of fiber volume
therefore, the elemination of these components contributed to the decreases of fiber volume, which lead to the increases of fiber density (p=m/v)
which can be attributed to the voids and pores in the surface of fibers filled with grafted molecules during alkali treatment
The increase of treated Fiber density can be explained by the morphological structure changes that occurred on the fiber following NaOH treatment. Alkali treatment contributed to the gradual elimination of micro voids existing in the fiber and hence the volume decrease and density increase.
Amorphous component in the fiber was dissolved by alkali so fiber rearrange the packing structure become dense
The density of natural fibers can increase during alkali treatment due to several factors. One reason for the increase in density is the swelling of the fibers, which can occur as the alkali solution penetrates the fibers and causes them to absorb water. The absorbed water can increase the volume of the fibers, which can lead to an increase in their density.
Another reason for the increase in density of natural fibers during alkali treatment is the removal of impurities and non-cellulosic components from the fibers. Alkali treatment can dissolve and remove impurities, such as lignin and hemicellulose, which can make the fibers lighter and more porous. The removal of these impurities can also increase the density of the fibers by reducing their volume and mass.
It is important to note that the extent of the increase in density of natural fibers during alkali treatment can vary depending on the type and concentration of the alkali solution used, the properties of the fibers, and the treatment conditions. The density of the fibers can also be influenced by other factors, such as the drying and conditioning of the fibers after treatment.
Treatment of fibers results to elimination of some compound present in the fiber moreso non cellulosic (lignin and hemicellulose). This will lead to change in fiber morphology (reduction of volume) which inturn increases the density.
Density is mass per volume giving an inverse relationship between density and volume
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