Mechanical and Thermal Degradation Properties of Silk from African Wild Silkmoths Addis Teshome, 1,2 John M. Onyari, 2 Suresh K. Raina, 1 Jacques M. Kabaru, 2 Fritz Vollrath 3 1 ICIPE—African Insect Science for Food and Health, Environmental Health Division, Nairobi 30772-00100, Kenya 2 School of Physical Sciences and School of Biological Sciences, University of Nairobi, Nairobi 30197-00100, Kenya 3 Silk Research Group, Department of Zoology, University of Oxford, Oxford OX1 3PS, United Kingdom Correspondence to: A. Teshome (E-mail: additi09@gmail.com or akebede@icipe.org) ABSTRACT: Variations among silk of four African wild silkmoths, Argema mimosae, Anaphe panda, Gonometa postica, and Epiphora bauhiniae, was studied regarding their mechanical properties and thermal degradation behaviors. Cocoon shells and individual degummed fibers were examined using tensile testing, thermogravimetric analysis, and scanning electron microscope (SEM). A. mim- osae and G. postica cocoon shells had marginally higher initial moduli and strains at maximum stress. The stress–strain curves of Bob- myx mori and A. panda degummed fibers lacked clear yielding points. G. postica fibers had the highest breaking energy (76.4 J/cm 3 ) and breaking strain (41.3%). The ultimate tensile strength was the highest for B. mori (427 MPa). Fiber pull-out and detachment was predominant in fracture surfaces of both the cocoon shells and the fibers. Wild cocoon shells and degummed fibers had higher tem- perature for dehydration loss than B. mori. A. mimosae fibers (11.9%) and G. postica cocoon shells (13.3 %) had the highest weight loss due to dehydration. E. bauhinae cocoon shells and B. mori fibers had the highest total weight losses of 97.2 and 93.4%, respec- tively. The African silks exhibited variations in their mechanical and thermal degradation properties related to their physical and chemical structure and composition. V C 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 000: 000–000, 2012 KEYWORDS: degradation; yielding; fibers; fracture; thermal properties Received 15 December 2011; accepted 13 April 2012; published online DOI: 10.1002/app.37873 INTRODUCTION Natural fibers from animals and plants are gaining increasingly in importance, once again. Silk is a unique class of structural animal fiber and has long been regarded as a superb natural material due to its characteristic high strength, elongation, feel, and luster. 1 The variation in composition, structure, and prop- erties of silks from different arthropods has led to its considera- tions in wider applications expanding from more traditional textile industries to fields such as biomedical, biotechnological, and tissue engineering. The usefulness of silk fibers in these and many other applications is associated with its predominant fail- ure mechanism under the conditions of the application and its adaptability to varied environmental conditions facilitated by the silk’s molecular composition and hierarchical structure. These, in turn, are both affected by the conditions during its production such as spinning speed, relative humidity, tempera- ture, pH, ionic strength, solvent composition, and mechanical stress as well as the degumming process. 2–5 The mechanical properties of silk fibers also depend on the structure of the silk constituent proteins (sericin and fibroin) before and after silk fiber formation. 6 For example, sericin coating can add to the tensile properties of silk fibers, although these are primarily determined by their fibroin structure. 7 Sericin might also affect the transition of silk fibroin from the random coil or a-helix to the b-sheet structure and further improve the mechanical prop- erties of silk fibroin fibers. 8 High crystallinity confers the silk material greater strength by virtue of the network of hydrogen bonds between and within the proteins. 9 Temperature and moisture are among the environmental stresses influencing properties and functions of silk cocoons and fibers. Cocoon shells are important in temperature regulation, water loss reduction from the pupae, and acquisition of heat. 10 Tem- perature also influences the amount of water absorbed by silk fibers resulting in alteration of physical properties. Chemical composition and structure of silk fibers determine the thermal tolerance behavior of the cocoon shells and fibers and the resulting weight loss due to dehydration and/or decomposition. Hence, it is appropriate to deploy Thermal Analysis to V C 2012 Wiley Periodicals, Inc. WWW.MATERIALSVIEWS.COM WILEYONLINELIBRARY.COM/APP J. APPL. POLYM. SCI. 2012, DOI: 10.1002/APP.37873 1