| dc.description.abstract | Wild silk production is a unique eco-friendly agro-practice with enormous potential for
income generation and additional benefit of compatibility with conservation goals. African
wild silkmoths occur in three major families; Saturniidae, Lasiocampidae and
Thaumetopoeidae and have significant economic importance mainly due to their exceptional
quality fibers and aesthetic values. Silk cocoon shells from these insects are non-woven
structures majorly composed of two proteins, fibroin and sericin, and depending on their
origin, exhibit extensive variation in structure, property and composition directly influencing
their ecological adaptive functions. Hence, this study was aimed at investigating the extent of
variability in structure, composition and properties of silk cocoon shells and fibers from the
major African wild silkmoth species: Gonometa postica, Anaphe panda, Argema mimosae
and Epiphora bauhiniae. Scanning Electron Microscope (SEM) was used to study the surface
and cross sections of the cocoon shells and degummed fibers. The components on cocoon
shell surfaces were determined by Fourier Transform Infrared spectroscopy (FTIR). The
composition of major fibroin amino acids was established with High Pressure Liquid
Chromatography (HPLC). Moisture regain and weight loss of fibers and cocoons were
determined by oven dry method. The dissolution properties were studied using 9M aqueous
solutions of Caciurn chloride (CaCh), Lithium bromide (LiBr) and Sodium thiocynate
(NaSCN). Mechanical properties were measured with Instron tensile testing machine.
Thermal decomposition behaviours were determined using Thermogravimetric Analyzer
(TGA) at temperature range of 25 - 800 and 25 - 900°C for fibers and cocoon shells,
respectively.SEM micrographs showed that G. postica and A. panda cocoons had thorny spines and hairs
on their surface, while A. mimosae cocoons were highly perforated. The FTIR spectra peaks
around 1312 and 712cm-1 for outer surfaces revealed the presence of calcium oxalate crystals
on G. postica cocoons. Cocoon surfaces also showed great cross bindings, wrinkles and
networking of twisting filaments in different shapes and forms conferring rough outer
surfaces. Fiber surfaces were irregular with several fibriliar sub-structures running along the
fiber axis in A. mimosae and E. bauhiniae. The cross sectional view of degummed fibers
revealed elongated, rectangular, triangular, globular and wedge shaped fibers with variable
number and size of voids. HPLC results showed that African silk fibroins were characterized
by the presence of high proportions of the three amino acids, glycine, alanine, and serine.
These amino acids represented 71-74 and 82% of the total amino acids present in wild and
Bombyx mori silk fibroins, respectively. No significant difference (P < 0.05) was observed
among the moisture regain of the wild silk degummed fibers. However, there were significant
differences (P > 0.05) in weight loss and moisture regain among cocoons as well as shell
layers. E. bauhiniae cocoons had the lowest weight loss and moisture regain of 23.2 and
5.6%, respectively while G. postica and A. mimosae had the highest weight loss and moisture
regain, 56.8 and 9.1 %, respectively. In both A. panda and E. bauhiniae, the outer cocoon
layers had the lowest moisture regain and highest weight loss while the inner layer of E.
bauhiniae and middle layer of A. panda lost the least weight. African wild silk fibers and
cocoons also showed significant variability (P > 0.05) in their dissolution behaviours.
Degummed fibers were more readily soluble than the cocoon shells. B. mori had higher
solubility than the wild silk cocoon shells (51.5%) and fibers (59.3%). Among the wild species, G. postica cocoons and degummed fibers had the highest solubility (37.3 and 51.7%,
respectively). LiBr was the most effective dissolving agent for both the cocoons and fibers
(41.2 and 84.5%, respectively).
The tensile test measurements showed that G. postica and A. mimosae cocoons had lower
breaking stress and breaking energy and higher breaking strain than B. mori. Gonometa
postica fibers had the highest breaking strain (41.3%), while E. bauhiniae had the lowest
breaking stress (237MPa). Wild silk fibers had breaking energy ranging between 34.5-
76.4J/cm3. The TGA curves showed A. mimomsae fibers and cocoons had higher temperature
for water loss (113 and 154DC, respectively). G. postica cocoons underwent multistep
decomposition between 296-413, 483-506, and 710-730De. B. mori had the highest total
weight loss for degummed fibers (93.4%), while E. bauhiniae cocoons lost the highest total
weight (97.2%). The overall results demonstrated the extensive variability present in
structure, composition and properties among silks of the four African wild silkmoths. This
could be associated with the selection pressure for environmental adaptation experienced by
each species and the difference in molecular organization and composition of silk spun by
each species. The physical structure and chemical composition of the cocoon shells and fibers
have contributed towards the variations observed in the properties and these features have
commercial and industrial implications. Hence, the value of silks when compared across taxa
should be based on suitability of the fiber properties for specific application as well as
potential silk products. | en |
