JOURNAL OF COMPOSITE MATERIALS Article Effects of aramid fibers and colloidal particle fillers in composite ethylene propylene diene monomer rubber thermal insulators for rocket motor insulation AM El-Dakhakhny 1 , AF Ahmed 1 , M Rutkevic ˇius 2 , S El-Marsafy 1 and E Abadeer 1 Abstract In this study, we develop and characterize an asbestos-free rubber composite for potential use as a rocket motor insulator. The insulator is based on ethylene propylene diene monomer blended with aramid fiber (Kevlar) in the pulp form (KP), together with alumina and/or silica as reinforcement. Different formulations based on these fillers were prepared. The fillers were dispersed in the ethylene propylene diene monomer polymeric matrix to obtain a homogenous master batch for curing. We analyzed and compared physical, mechanical and thermal properties of different compositions (density, hardness, tensile strength, elongation, ablation resistance and mass loss on heating). Reinforcement of ethylene propylene diene monomer with Kevlar improves the performance of the material with respect to mechanical and thermal properties, while not improving well the performance with respect to ablation resistance. Using hybrid reinforcement (KP þ alumina þ silica) within ethylene propylene diene monomer improves the performance of ethylene propylene diene monomer with respect to mechanical properties, thermal properties, ablation resistance and thermal decomposition resistance. The optimum performance of the insulation material was achieved when the formulation consisted of 10 part per hundred ratio KP with even amounts of alumina and silica. A novel composite engineered for improved insulation of solid rocket motors using the hybrid reinforcements was developed for the first time, which can be applied in future space missions. Keywords Thermal insulator, fillers, solid rocket motor, ethylene propylene diene monomer, Kevlar, polymers Introduction Solid rocket motors (SRMs) are found in defense systems, model rockets and in satellite launcher boos- ters. 1 SRMs are attractive due to their simplicity and reliability; however, in the past decade there has been a lack of published research in the field. A key part of a SRM is the insulator, which protects the outer case from disintegration during the combustion of the rocket fuel. 2 During the burning of the propellant matrix, temperatures inside the rocket motor case typ- ically reach over 2000  C and interior pressures may exceed 10.35 MPa. 3 The insulator needs to withstand these conditions without burning, but instead ablate, cook and char in the presence of hot gases. Moreover, the high degree of turbulence within the rocket motor case can potentially lead to failure of the rocket itself. 4 The requirements of an SRM insulator are the fol- lowing: chemical compatibility with the propellant matrix, a long shelf life, low ablation rate, low thermal 1 Chemical Department, Faculty of Engineering, Cairo University, Egypt 2 Department of Chemical and Biomolecular Engineering, NC State University, USA Corresponding author: Ahmed Mahmoud El-Dakhakhny, Faculty of Engineering, Cairo University 1, Cairo University Rd., Giza, Cairo, 12613 Egypt. Email: a_dakhakny@hotmail.com Journal of Composite Materials 2018, Vol. 52(15) 1989–1995 ! The Author(s) 2017 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0021998317735682 journals.sagepub.com/home/jcm