Abstract:
Cotton stalks are a by-product of cotton farming. Approximately two to three tonnes of
cotton stalk are generated per hectare of cotton farmed, making available in Zimbabwe
about a million tonnes of cotton stalks every season. The cotton stalk is normally burnt to
avoid pest infestations such as pink bollworm and mealybug, but this pollutes air emitting
greenhouse gases. The current study, therefore, aims at finding an alternative use of the
cotton stalks through production of a bio-composite based on phenol formaldehyde resin.
Cotton stalks were collected from Umguza cotton farming district in Zimbabwe. The
stalks were subjected to natural retting for 3 weeks followed by manual decortications to
extract fibres. The fibre yield from extraction process was about 23%. The physical and
mechanical properties of extracted fibres were characterized and categorized according
to their relative position along the cotton stalk as top section, middle section and root
section fibres. The cotton stalk fibres had a light brownish colour and their fibre length
was determined as 8.18 cm. The moisture regain of the fibres was 11.14%, 10.68% and
10.20% for root, middle and top fibres, respectively. The fibres had an average diameter
of 0.23 mm, breaking extension of 1.5% and density of 1.45g/cm 3 . The test results were
analysed using Statistical Package for the Social Science and Minitab statistical software.
The fibres were used to fabricate a composite using phenol formaldehyde resin following
a hand layout process. The mass fraction (M f ) was increased from 0-38%and the density
maintained between 650-900 kg/m 3 . The cost of producing the bio-composite was
$5.80/m 2 which was cheaper than boards available in the market which cost
approximately $5.56/m 2 . The board tensile strength varied between 2.3 MPa to 6.8 MPa
depending on the M f while the flexural strength ranged between 46.39-170.00MPa. From
the determined properties of the fabricated composite, it can be concluded that it has
adequate mechanical properties comparable to solid wood in several applications such as
ceiling panels, partition boards and table tops. As a recommendation steam explosion for
fibre extraction can be studied as faster method to extract cotton stalk fibres. As further
study the shive that is a by-product from fibre extraction can be ground and used as a
potential composite filler material.
